Sheeted Clastic Dikes in the Megaflood Region

Sheeted Clastic Dikes in the Megaflood Region, Washington, Oregon and Idaho

Skye W. Cooley

Mission Valley, MT

Clastic dikes in the Channeled Scablands of Washington, Oregon, and Idaho are vertically-sheeted wedges filled with silt, sand, and gravel filled from above. For this study, I mapped dike bearing outcrops across a 30,000 km2 region and collected width measurements and sheet counts from several thousand dikes at hundreds of sites spanning the floodway between Priest River, ID and The Dalles, OR. All observed dikes exhibit identical characteristics and are exclusively found within the margins of Ice Age floodways. I interpret the dikes as sediment-filled hydraulic fractures, or 'flood injectites,' that grew incrementally in width and length through discrete, repeated loading events during the Pleistocene. Injection was triggered by glacial megafloods coursing through the landscape and inundating side valleys. This interpretation is supported by numerous lines of evidence including taper direction, the nature of their fills, their exclusive occurrence within floodways, and clear crosscutting relationships with dated flood deposits, non-flood deposits, paleosols, tephras, and various bedrock units. Contrary to expectations of tectonic influence, the dikes are not larger or more numerous near Quaternary faults of the Yakima Fold Belt. Furthermore, no evidence of liquefaction attributable to strong seismic shaking was observed in the study area, despite its transection by over a dozen faults capable of generating earthquakes in excess of M 6.0. Instead, diking appears primarily controlled by grain size, sediment thickness, and location within the floodway. The largest dikes cluster where silty slackwater sequences are thickest and where floodwaters were deepest. Crucially, these dikes are not feeder conduits to sand blows or seismites; rather, they are small-scale sand injectites formed during cataclysmic terrestrial floods. Analogous wedge-shaped dikes with sheeted fills are found in marine turbidite, subglacial, and lahar settings, where silty-sandy substrates were similarly subjected to rapid and repeated overloading.

Keywords: clastic dike, sand dike, sand injectite, hydraulic fracture, Channeled Scablands, Missoula floods, megafloods, Columbia Basin, Cordilleran Ice Sheet, Touchet Beds

Definition

The term "clastic dike" describes tabular bodies of sediment that crosscut bedding, including features formed by liquefaction (Fuller, 1912; Obermeier, 1998), forceful injection in deep sea fans and subglacial settings (Jenkins, 1930; Kruger, 1938; Anderson, 1944; Haff, 1944; Lupher, 1944; Newcomb, 1962; Fryxell and others, 1965; Potter and Pettijohn, 1977), passive filling of open cracks (Newsom, 1903; Collins, 1925; Dobie, 1926; Fackler, 1941; Shrock, 1948), intrusion involving release of gasses (Diller, 1890; Vitanage, 1894; Harms, 1958; Gonzales and Koch, 2017), freeze-thaw action in permafrost (Alwin and Scott, 1970), subaqueous slumping (Baker, 1973), and other processes (Brown and Brown, 1962; Fecht and others, 1999). 

Previous Work on Clastic Dikes

The earliest reports on clastic dikes in the geological literature date to the 1800s (Strangways, 1821; Murchison, 1827; Darwin, 1834; Gilbert, 1880; Diller, 1890; Cross, 1894; Case, 1895). Sedimentary dikes in the Pacific Northwest were first mentioned by Dana (1849). Jenkins (1925) authored the first report on sand dikes in the Channeled Scablands. Detailed reports on clastic dikes in Scabland deposits remain relatively few (Jenkins, 1925; Lupher, 1940, 1944; Black, 1979; Woodward-Clyde Associates, 1981) and reports including more than a handful of measurements are rare (Alwin and Scott, 1970; Cooley and others, 1996; Neill and others, 1997; Ward and others, 2006). Despite their widespread occurrence in Eastern Washington and neighboring portions of Oregon and Idaho, clastic dikes are frequently omitted from otherwise thorough stratigraphic studies of flood deposits (Waitt, 1985; Smith, 1988a,b; Lindsey and others, 1996; Benito and O'Connor, 2003; Sweeney and others, 2017). While numerous authors have speculated on the origin of dikes in the Touchet Beds (Flint, 1938; Newcomb, 1962; Bingham and Grolier, 1966; Jones and Deacon, 1966; Beaulieu, 1974; Carson and others, 1978; Shaw and others, 1999; Pritchard and Cebula, 2016; Reidel  and others, 2021), few have supported their assertions with maps, measurements, or models. Contributions from Baker (1973), Pogue (1998), and Howard and Pritchard (2020) offer a refreshing departure from this trend.

The document below contains my review of several dozen articles referenced in this study.

Cooley_Clastic Dike Article Review_v9.pdf

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World's first publication on clastic dikes. Strangways (1821) is the first article published on clastic dikes. In his sketch of a shoreline exposure near St. Petersburg, Russia, yellow clay veins descend from a gravel-capped bed into the blue clay below. The dikes form a polygonal network as they criss-cross the gently-sloping beach.

First study on clastic dikes in the Channeled Scablands. Field geologist Olaf P. Jenkins in 1923 examines a large clastic dike exposed in a gravel pit near Lowden, WA. The caption reads, "Clastic dikes in Touchet Beds and dust dune between Touchet and Walla Walla". The dike is sourced in light-colored slackwater sediments that overlie darker flood-laid sands. Holocene loess caps the section. Photo: Washington Geological Survey Archives (No. 00604).

Types of Clastic Dikes

Clastic dikes are found in a variety of geologic settings, vary considerably in size and shape, and form by various mechanism (primary triggers). Despite their diversity, all clastic dikes fall into one of two categories: those formed by pressurized injection and those formed by passive infilling of cracks. Dike injection involves a triggering event (load), a rapid rise in pore fluid pressure (overpressure), and the establishment of a hydraulic gradient (propagation direction). Passive infilling is less dynamic and involves open-standing cracks that fill with wet or dry sediment transported by water, wind, or gravity. Seven types of clastic dikes are recognized in the literature:

Hydraulic fracture

Overpressure of water-saturated sediment in a sealed system causes tensile fractures to open, propagate, and fill with fluidized sediment. Fractures begin to propagate when fluid pressure exceeds the resistance of the host material (fluid pressure > fracture toughness). Natural hydraulic fractures are recognized in a variety of geologic settings at sub-centimeter to kilometer scales. Manmade hydraulic fractures are commonly induced from shut-in well bores to enhance the recovery of oil and gas from tight shale (fracking). Example: Sand injectites, North Sea or Panoche Hills, CA.

Liquefaction

Strong seismic shaking of saturated sediment reduces grain-to-grain contact (granular support), induces consolidation (closer grain packing), and increases fluid pressure, which results in the upward expulsion of pore water and venting of fluidized sand at the surface (sand blows and pipes). Example: New Madrid, MO.

Rapid overloading

A vertical load rapidly imposed on a substrate of rock or sediment opens vertical fractures that fill with fluidized sediment sourced near the loaded surface. Load may be created by grounding glacial ice, debris flow, lahar, turbidite, megaflood, burial, etc. Example: Channeled Scablands, WA.

Passive fracture filling

Fractures formed in rock, consolidated sediment, or frozen soil that fill with material washed-in by water, blown in by wind, or collapsed-in by gravity. Fractures can form by various processes, including mass wasting, thermal contraction, desiccation, tectonic extension, etc. Example: Arctic permafrost regions of U.S., Canada, Russia.

Volcanic-phreatic

Pebble-, conglomerate-, or breccia-filled dikes, often associated with ore bodies, formed when hot magma from a flow, dike/sill, or vent contacts wet sediment, releasing volatile gasses and mobilizing fluids that shatter bedrock and fill fractures with rubbly material. May occur at the surface beneath erupted flows or in the shallow subsurface near vents and intrusive sheets. Example: Ouray, CO.

Syneresis

Diagenesis in clay rich sediments can cause volumetric changes that manifest as fractures that fill with a combination of transported and mineralized material. Example: White River Badlands, SD.

Impact-induced injection

Shock waves from a meteorite impact can trigger the injection of sands into surrounding bedrock. Dikes often contain shocked grains and radiate outward from the impact location. Example: Upheaval Dome, UT.

Proposed Origins of Dikes in the Channeled Scablands

Five origins for clastic dikes in the Channeled Scablands have been proposed: Earthquakes (Jenkins, 1925), ground ice (Lupher, 1944; Alwin and Scott, 1970; Black, 1979), slumping/lateral spreading (Brown and Brown, 1962; Baker, 1973; Cooley and others, 1996), desiccation (Grolier and Bingham, 1978), and hydraulic fracture (Pogue, 1998). A dubious sixth, “multigenetic” (Black, 1979; Fecht and others, 1999), suggests the dikes formed by a combination of processes. Cooley (2015) provides a concise summary of the arguments for and against each hypothesis. See Footnote 16.

This Study

I searched for clastic dikes in unconsolidated sediments, partially-lithified sediments, and flood-scoured bedrock exposed along roads, streams, and rail lines between Priest River, ID and The Dalles, OR. Field work was conducted between 1995 and 2025 using the same data collection protocols. Dozens of excavated pits, trenches, and quarries were also surveyed. Long foot traverses were made in valleys of the Columbia, Snake, Yakima, Spokane, Walla Walla, Sanpoil, Touchet, Tucannon, Umatilla Rivers, and numerous tributaries. Thick sections of non-flood sediments at Saddle Mountains, Smyrna Bench, Frenchman Hills, White Bluffs, and Palouse Hills were carefully surveyed as well as glacial deposits north of the Channeled Scablands. Holocene alluvium exposed along modern creeks inside Ice Age floodways and Pleistocene loess exposed outside the floodway were also surveyed for dikes and other soft sediment deformation features. I measured the widths of >3000 dikes at >300 exposures and counted the number of vertical sheets in >1000 dikes. The dikes observed varied in width and length from place to place, but otherwise exhibited identical characteristics throughout the region. The sheeted sand, silt, and gravel-filled dikes occur only within Ice Age floodways and at elevations no higher than Missoula flood trimlines. The conspicuous wedge-shaped structures appear to have formed by the same mechanism during the Pleistocene, not before or since.

Study sites. Locations where sheeted dikes were measured are black dots. White dots are searched locations where no dikes were found. Most of the white dots lie just outside the floodway, shown in gray. The Cordilleran Ice Sheet terminus is shown in blue. Glacial Lake Columbia, mostly north of the ice sheet margin, is shown filled to its 600m-elevation shoreline. Exposures containing dikes are more numerous in the southern part of the study area, where many streams form good outcrops. Good exposures are few between Moses Coulee and Cheney, where loose, patchy gravels lie atop scoured bedrock. The few dikes observed north of the Channeled Scablands occur in sandy outwash and some flood-laid beds. Gravels and Palouse Loess within the floodway generally contain few dikes unless interfingered with or overlain by silty Touchet Beds. No dikes cut loess anywhere above Missoula flood trimlines (local maximum stage indicators). No dikes were found east of Priest River, ID (Glacial Lake Missoula basin), east of Lewiston, ID (Snake River Valley), west of White Swan, WA (Yakima River Valley), south of Cecil, OR (Willow Creek Valley), north of Hunters, WA (Columbia River Valley), north of Bridge Creek, WA (Sanpoil River Valley), or north of Omak, WA (Okanogan River Valley). The Willamette, Wenatchee, and Methow Valleys were not part of this study. Two locations near Ellensburg (red dots) identify isolated, unsheeted dikes of Tertiary age in fluvial terraces mapped by Porter (1976) and Waitt (1979).

A typical clastic dike in Eastern Washington. A vertically-sheeted clastic dike intrudes silty-sandy late Pleistocene slackwater rhythmites (Touchet Beds) at Smith Hollow Rd in the Tucannon Valley, WA. Wall-parallel sheeting is the product of hydraulic crack-and-fill cycles. The wedge-shaped dikes were filled from the top.

Sheeted sills. Sheeted sills are more or less identical to sheeted dikes. Horizontal sills branch off from larger vertical dikes. Intrusion of both dikes and sills occurred together and at shallow depths. Because dikes greatly outnumber sills, it would appear that vertical fractures were easier to initiate under a vertical load. Perhaps horizontal slip along bedding planes, which would favor vertical pathways, required less force than would lifting the overburden against gravity, which would favor horizontal pathways. At very shallow depths, where the vertical and horizontal stresses may be approximately equal, small flaws and inhomogeneities may have controlled whether dikes or sills form. In some locations, horizontal sandy beds acted as more efficient injection pathways than did vertical flaws or fractures. Most sills observed were located relatively high in the section, suggesting sill formation may be depth-dependent. Hwy 240 at the Hanford Site.

Dike-sill-dike geometry. The dikes follow least-resistance pathways through Pleistocene megaflood sediments and, in places, terminate in the underlying bedrock. Most are vertical and crosscut bedding at high angles, but sills are not uncommon. The dike shown here cuts vertically across silty, low-permeability layers (tan) then switches to horizontal and follows the gray, higher-permeability sand. Sheeting flips between vertical in the dike segments and horizontal in the sill segments. The dike followed an efficient path along vertical fractures through the low-permeability silts and along bedding in the high-permeability sands (matrix flow). Resistive forces in the silts and sands were apparently very nearly equal during diking. Dike-sill-dike geometry is only possible with fluid-driven fractures (hydraulic fracture). Hellsgate Recreation Area near Lewiston, ID.

Stratified fills. Sediment that fills the dikes is commonly stratified with fine-scale planar to concave-up bedding consistent with fractures that opened to the surface and filled from the top. The laminated fills indicate rapid vertical stacking and a component of lateral flow in expanding fractures (flow along strike). Structureless fills, typically in sizes finer than sand, appear to signal the closure of the fracture and "freezing" of unsettled sediment. Repose angles up to 55 degrees, indicate sediments entered fractures as turbulent slurries.

Large dike. A meter-wide dike in Touchet Beds. Touchet Valley, WA.

Fill bands. This dike contains more than 40 sheets (fill bands). Stratification here is subtle and many fills appear structureless. A few pairs of distinctive sheets appear to match. The formerly wider sheets were split and separated by new injections as the dike widened.

More than a Touchet Bed story. The dikes intrude flood deposits, older partially-lithified sediments, and bedrock. Here, a sheeted dike sourced in silty flood deposits cuts reworked basaltic colluvium. The angular colluvium has been reworked and swept a short distance downstream by a Missoula flood. Alder Ridge, WA.

Huge dikes formed where slackwater lakes were deepest. Very large dikes with widths exceeding a meter often contain >100 fill bands. They are most common in the southern part of the Lake Lewis basin, where rhythmite stacks are thickest and slackwater lakes were deepest. The dike shown here strikes obliquely to the bladed cutface. Foster Wells Rd at Hwy 395 north of Pasco, WA.

Burlingame Canyon. About 40 slackwater beds are exposed at this classic locality near Gardena, WA (Waitt, 1980, 1985; Moody, 1987; Clague and others, 2003). The size and completeness of the Burlingame Canyon exposure is unique in the Scablands region, though land access is complicated and better outcrops in Walla Walla Valley and elsewhere provide superior information. Many Touchet Bed exposures outside the valley are more sedimentologically diverse and contain different features and relationships. Touchet-equivalent rhythmite sections outside the Lake Lewis basin are found at Latah Creek, Lacrosse, Cecil, Portland, Salem, and more. Photo: Washington Geological Survey Archives (1978, No. 3455).

Injection vs. Liquefaction

Most published reports on sedimentary dikes describe features formed by liquefaction triggered by strong seismic shaking. Shaking intensities above VI and M 6.0 can elevate pore fluid pressures in wet, unconsolidated beds, causing them to liquefy and vent sand to the surface (Obermeier, 1998). Feeder dikes to sand blows have unstratified to crudely-stratified sandy fills, upward-pinching shapes, and contain sediment sourced in fluidized beds deposited long before the triggering quake.

The clastic dikes described here are different. They are slender, vertically-sheeted, wedge-shaped structures that were filled from the top. They are sediment-filled hydraulic fractures propagated downward into a variety of sedimentary and bedrock substrates. They are downsized versions of sand injectites (sub-seismic) described in submarine turbidite-fan systems (Jolly and Lonergan, 2002; Hurst et al., 2011 Appendix A; Cobain and others, 2016) and sheeted dikes formed beneath glaciers (von Brunn and Talbot, 1986; Broster, 1991; Larsen and Mangerud, 1992; Dreimanis and Rappol, 1997; LeHeron and Etienne, 2005), lahars (Herriott and others, 2014), and debris flows.

Growth of composite dikes. Diking triggered by fracture, injection, and filling during three successive flood events. Since each individual sheet tapers downward, the composite dike is wedge-shaped. Figure modified from LeHeron and Etienne (2005). Similar sheeted dikes described in subglacial setting by Ravier and others (2015).

Dikes and flute casts. (A) A typical sheeted clastic dike in Missoula flood slackwater rhythmites (Touchet Beds) containing a range of grainsizes. Light-colored silt partitions (skin walls) separate sheets from one another. This example contains ~12 sheets and is filled with silty, sandy sediment resembling the host material. Location is Umatilla Basin at Cecil, OR (slackwater Lake Condon). (B) A gravel-filled dike intruding a gravelly eddy bar deposit is truncated at its top by a younger floodbed. Its fill reflects the host formation and is crudely sheeted with less prominent skin walls, reflecting a lack of fines in the gravel. Dikes in coarse-grained deposits are often stubby (lower length-to-width ratios) than dikes in fine-grained sediments. Location is near Exit 147 off I-84 at the mouth of Willow Creek. (C) Examples of flute casts that decorate the interior faces of skin walls. Upward-pointing noses are a clear directional indicator of filling from above. Flood-transported sediment entered a fracture from the top. Quarter for scale. Location is Walla Walla Valley, WA (eastern arm of slackwater Lake Lewis). (D) A dike with ~10 sheets is filled with a mix of sandy flood sediment and quartzite cobbles liberated from the underlying Ellensburg Fm. Hoe is 28 cm long. Location is Emerald Road near Granger, WA (western arm of Lake Lewis).

Dike Distribution

Sheeted dikes are not isolated features; they number in the hundreds of thousands if not millions and are distributed throughout an area exceeding 30,000 km2. Great distances separate outcrops containing dikes with identical characteristics. For example, sites near Kettle Falls, WA and Salem, OR are separated by more than 500 km. The dikes are not found above the local elevation of maximum flooding (~366m in south-central Washington and higher to the north), nor in unconsolidated sediments beyond the margins of Ice Age floodways. No dikes are known in Palouse Loess outside of flood coulees. Identical dikes occur in close proximity to mapped Quaternary faults (i.e., Wallula fault zone) and distances >150 km from them. They are largest, most abundant, and best-exposed in thick sections of silty slackwater rhythmites.

Size of Dikes

Most dikes measure <15 cm wide and contain fewer than a dozen sheets. The largest dikes exceed 2 m in width, contain >100 fill bands, and penetrate to depths >50 m. The average width of an individual sheet is around 1 cm. The widest sheets observed were 30 cm. Having examined several thousand dikes, I suspect a length-to-width limit of ~40 exists for most Touchet-type dikes.

Shape of Dikes

Dikes in silt-sand rhythmites tend to be long and slender (H >> W), while those in coarse, laminated sand, or gravel are stubby, few in number, and crudely sheeted. Dikes that penetrate bedrock (Columbia River Basalt) are slender and follow joints. In three dimensions, a dike is blade-shaped with a curved arris (i.e., PKN fracture model for fluid-driven fractures (Perkins and Kern, 1961; Nordgren, 1972; Belin and Carey, 1997; Rahman and Rahman, 2010). Fracture aperture controls dike width and scales with the volume of the fill. Dikes thin and taper in the direction their host fractures propagated (downward and outward). The shape of a dike in cross section changes depending on where the section plane is placed (i.e., where the plane of an outcrop intersects a dike).

Source of Fill

Sediment filling the dikes was derived from energetic ground-hugging bottom currents carried by megafloods and from recently-accumulated sediments at the bottom of slackwater lakes. Lupher (1944) first proposed the idea that fluctuating density "currents above the fissures" were responsible for producing stratified fills, noting that "many dikes are traceable to overlying current-bedded sand." I concur with his interpretation. It is clear that vast numbers of dikes formed during Ice Age flood events, which occur in four stages: vigorous overland flow, up-valley flow (backflooding), slackwater lake stillstand, and slackwater lake drainage. During each of these stages, ground fractures opened and were rapidly infilled with sediment entrained in the circulating flow or sediment on lake bottoms, thus dikes originate from the base, middle, and top of rhythmites.

Fills Reflect Local Geology

Though most numerous and best exposed in the Touchet Beds, the dikes intrude a dozen other geologic units (Ellensburg Fm, Latah Fm, Ringold Fm, Dalles Group, Palouse loess, fanglomerate-calcrete-loess of the "Plio-Pleistocene unit", etc.). Dike fills, therefore, often reflect the composition of both flood deposits and the local bedrock. Flood deposits are composed of quartz-plagioclase-muscovite grains (Palouse loess), grains of Columbia River Basalt, and chunks of cemented loess and calcrete. Gravel-filled dikes are less uncommon, but where found will either contain the same sediments that comprise high energy flood bars or, if atop non-basalt bedrock, a mix of basalt, quartzite, felsic volcanics, weathered mafics, and schist. In protected slackwater areas like the Walla Walla Valley, nearly all dikes are filled with Touchet Bed sediment that mineralogically resembles Palouse loess. Near the western margin of the scabland flooding, dike fills contain quartzite clasts derived from the Miocene Snipes Mountain Conglomerate abundant in there. Along the northern margin, dikes contain Miocene gruss shed from deeply-weathered granites of the Okanogan Highlands (Colville Batholith).

Stress Orientation

Most of the dikes are vertical to nearly vertical structures that crosscut bedding at high angles. Sills are less abundant and are fed by dikes. Dike length regularly exceeds 10 m, while sills tend to pinch out within just a few meters. Overall, shapes are consistent with hydraulic fracture and a maximum principle stress (O1) oriented vertically (i.e., vertical loading). Fractures opened in tension perpendicular to the load without much shear (i.e., joints not faults). Dikes strike randomly and form polygonal networks when viewed from above, a pattern consistent with the two horizontal stresses being roughly equal (intermediate principle stress, O2 = minimum principle stress, O3). Equal or nearly equal horizontal resistance explains why the dikes so often twist about their vertical axes as they descend (i.e., strike changes with depth). Stairstepping and en echelon forms at some sites is evidence for bedding-parallel shear during fracture propagation. Centimeter-scale offsets are typical in such cases. Large bedding plane slips appear to result from low-angle slumping or perhaps drag imposed by fast currents moving overland. The number and size of dikes over time, which reduced spacing between them.

Polygonal Networks

Burned fields, bladed cutslopes, and dry creek beds expose polygonal networks in plan view. Horizontal exposures often confirm crosscutting relationships observable in most vertical cuts and reveal delicate intertwined growth geometries between intersecting dikes that are rarely visible elsewhere. Whether exposed in vertical or horizontal, no field evidence indicates dike networks are influenced by joint patterns in the underlying bedrock. The distinctive polygonal joints in the Columbia River Basalt, which underlies most of the floodway region, does not translate into the overlying sediments or influence dike orientations. Polygonal dike networks in the study area developed atop jointed and unjointed bedrock formations alike. Where dikes penetrate the bedrock, however, they do follow weaknesses including joints, faults, margins of pillows, etc.

Parallelism with Drainages

Fractures opened by slumps, slides, and spreads along the banks of modern channels commonly parallel the stream itself. If the fractures were to fill with sediment, forming wedge-shaped dikes, then orientation provides. a useful tool at determining dike origin (e.g., mass wasting). Orientation data collected on clastic dikes near Pleistocene drainages in southeastern Washington roughly parallel one another (Silver and Pogue, 2002). However, narrow corridors along paleodrainages comprise a small portion of the landscape and a relatively small percentage of dikes. Polygonal networks and random orientations is the usual configuration of most dikes in the region.

Sheeting and Growth

The dikes are conspicuously sheeted structures that grew in pulses via fluid-driven crack-and-fill. Vertical sheeting develops as a dike widens and lengthens with advance of the crack tip. Coherent "stacks" of sediment within sheets, separated from one another by horizontal silt skins, record discrete increments of infilling, often with very different grainsizes. Sheeting characteristics define three types of dikes in the study area: Single-fill, compound, and composite (Hayashi, 1966). Single-fill dikes contain a single wedge of sediment between two skin walls, a form consistent with a fracture that opened and filled once. Compound dikes contain two or more sheets with skin walls between in addition to the outer walls. In compound dikes, multiple fractures opened and filled during a single diking event. Composite dikes contain multiple sheets injected during more than one diking event (reinjection over time). In composite dikes, new sediment is introduced into an older dike, single-fill or compound, during successive events separated by hiatuses. Each new set of sheets is sourced from a different horizon that may differ with respect to grainsize, composition, sorting, etc.. New fills in composite dikes are often distinct from older fills. The contrasts appear to reflect a sediment source that changes with each diking event.

Age

Field relationships constrain the timing of dike injection to between ~1.8 Ma to ~14 ka, the period coinciding with ice sheet growth and scabland flooding (Easterbrook, 1994; Baker and others, 2016; Waitt and others, 2016). While an important set of cemented dikes associated with "ancient" flooding exists, most dikes formed late, between 18–14 ka. Except for crystalline bedrock of the Okanogan Highlands, the dikes penetrate all formations exposed in the Channeled Scablands, including Miocene Columbia River Basalt, Miocene-Pliocene Ellensburg/Latah Fm sediments, Miocene-Pliocene Chenoweth Fm and other Dalles Group sediments, Pliocene Ringold Fm, a distinctive Pliocene-Pleistocene calcrete-fanglomerate-loess complex ("Cold Creek unit"), pre-Late Wisconsin cemented gravels, silt-pebble diamicts, and loess. The dikes cut the Mount St. Helens Set S tephra (16 ka), but not the Mazama Ash (6.8 ka). No dikes are known to intrude Holocene alluvium east of the Cascade Mountains.

Growth in stages. Sheeted dikes grow sheet by sheet as illustrated above. New sheets intrude alongside older ones (B parallels A) or split older ones (C2 splits B). The example dike, photo at left, contains 7 sheets formed in 4 widening stages (crack and fill pulses). Growth stages: A = 1 sheet, B = 1 sheet, C = 3 sheets, D = 2 sheets. Except in dikes with very few bands, the number of fill bands is always greater than the number of injections that split one band into two.

Stacks inside sheets record incremental fracture growth. I measured the thickness of 20 coherent packages of sediment, or "stacks", in a few dikes exposed in a roadcut at Touchet, WA. Each stack constitutes a portion of a sheet, therefore each sheet is composed of many stacks. Planar or cup-shaped silt partitions separate each stack from those above and below. Abrupt changes in grainsize and bedding angle commonly occurs at the bounding partitions. Stacks are interpreted as discrete pulses of sediment entering a fracture and represent increments of fracture lengthening (crack tip advance). The height of most stacks measured less than a meter, but one example exceeded 3 m. Stack height appears random below about 2m. Stacks taller than 2m were few and most were located high in the exposure, out of reach.

Descend and branch. Sheeted dikes cut downward (per descendum) through sandy Missoula Flood deposits at Latah Creek west of Spokane, WA. Small downward-pinching spurs mimic the form of the larger "trunk" dike. Downward pinchouts and the lack of a connection to a liquefied source bed at depth help to rule out an origin involving upward fluid escape (i.e., sand blows). The dike's irregular walls and geometry of sheeted fills are consistent with pressurized injection, not brittle fracture or open-standing cracks that can accompany lateral spreading and desiccation.

Arris and aperture. A dike is a sediment-filled fracture with a 3D shape loosely resembling an axe blade. Hydraulic fractures have a width (aperture), a volume, and a curved, irregular leading edge (arris). They thin in the directions they propagate (downward and outward). A dike's cross section changes depending on where the section line is drawn (where a dike intersects the plane of an outcrop). The figure above is simplified version of a hydraulic fracture that tapers both vertically and horizontally, as do the clastic dikes in this study. Its 2D cross section appears to taper downward if sliced at Plane C, upward if sliced at at Plane B, and both upward and downward if sliced at at Plane A. The apparent taper direction (interpreted propagation direction) can vary depending on where a dike intersects an arbitrary plane (A,B,C) or a roadcut (natural plane).

Tapered at both ends? Dikes that taper in two directions can be confusing. Did it intrude upward or downward? Recall that the dike's shape in cross section is determined by the intersection of the blade-like dike with a curved arris and the plane of the outcrop. What we see here is the leading edge of a dike propagating perpendicular to the outcrop face, that is, mostly outward and a bit downward. If you were to excavate farther into the outcrop, the dike would grow in size and become wedge-shaped.

Pleistocene dikes in older sandstone. Energetic flooding down the Columbia Gorge incised deep, v-shaped gullies into fluvial sandstone of the Chenoweth Fm (Dalles Group) and filled them with gravel. Floodwaters at this site near The Dalles filled the lower 7 km of Chenoweth Creek valley to an elevation of 340 m (O'Connor and others, 2020). A gravel-filled, parallel-sided dike exits the bottom of a flood-cut gully and descends below the road grade. The dike is filled with the same material that fills the gully. Hundreds of flood-deposited boulders mantle the bench-like surface above the roadcut. Exotic lithologies including quartzite were noted in two measured sections through the Chenoweth conglomerate nearby, at Dry Hollow and Signal Hill (Tolan and others, 1996). Chenoweth Creek Valley, OR.

Depth of Intrusion

Fractures opened to the surface and were infilled with unconsolidated sediment. Numerous outcrops contain dikes that intrude to depths exceeding 10 m. Borehole logs document dikes at depths greater than 50 m below the modern surface. At most sites, intrusion depths of 5-10 m, which is a typical thickness for flood deposits. However, at a few locations, dikes extend well beyond 10 m into either partially lithified Pliocene sediments or fractured Miocene basalt.

Rate of Injection

All of the field evidence suggests the dikes formed very rapidly. A propagation/injection velocity of ~4 m/sec appears reasonable for dikes in the Touchet Beds based on rates calculated by Levi and others (2011) for similarly-sized hydraulic fractures propagated through a weak sedimentary host.

Skin Walls

Thin silt partitions (skin walls) form the outer boundaries of dikes and separate internal sheets from one another. These silt skins develop as pore water migrates out of the fill, through the fracture walls, and into the comparatively drier surrounding material. Direct drainage into the formation appears to be the primary dewatering mechanism. Dikes that penetrate impermeable bedrock, such as basalt, lack outer skin walls but contain internal ones, indicating that pore water is diverted laterally into adjacent sheets, where it may remain temporarily before ultimately draining into the formation. Silt, not clay, rapidly coats the fracture walls, progressively thickening into a continuous layer as fine particles are filtered and concentrated against the margins. Skin walls typically reach thicknesses of 1–10 mm, sufficient to form an effective seal. Skin-wall development and fracture sealing begin immediately upon sediment entry and proceed rapidly. An analogous process occurs during the formation of concrete slurry walls used in heavy construction, such as trench-type building foundations.

While there is little doubt that a silt-clay skin wall forms effective seal, it may also be true that Touchet Bed sediment itself may have had fracture-sealing qualities, if less effective. Is it possible that a partial seal was held by the host sediment alone for a brief moment after fracturing began and before skins began to form?

Flute Casts on Skin Walls

Upward-pointing flute casts ornament the interior faces of skin walls. They unambiguously indicate sediment entered the fractures from the top. The fluting is not sparse or subtle; the forms are obvious and present in nearly all dikes.

Rip-up Fragments in Fills

Fragments of older fills, chips of skin walls, and host material are a significant component of dike fills. In places where dikes cut white tephras such as the Mount St. Helens Set S tephra, traces of ash are sometimes visible in the fill.

Stratified Fills

Cross-lamination is a conspicuous characteristic of sandy dike fills. As a turbulent sediment slurry entered an open fracture, it began to settle and stratify almost immediately. Stratification developed under Newtonian flow during the brief interval when the fracture remained open and hydraulically connected to the surface. Laminated fills accumulated within open or widening fractures, whereas structureless fills likely record the instantaneous closure of the fracture and the resulting “freezing” of unsettled sediment. Abrupt termination of intrusion following a drop in fluid pressure has also been documented in sand injectites associated with deep-sea fans (Jonk et al., 2010; Dodd et al., 2020). Sheets of silt that appear structureless at first glance often reveal subtle laminations identical to those in sandy fills after light brushing with a soft brush. I disagree with the interpretation of structureless fills by R.L. Lupher (Lupher 1940, 1944), who proposed some surface cracks were filled by windblown or "in-falling" material without the aid of water. None of the field evidence is consistent with open-standing surface cracks or infill by dry sediment.

Leak-off Halos

"Leakoff halos" are firmer and discolored zones that extend a few centimeters beyond the outer skin walls of some dikes. Like skin walls, leakoff halos are a product of dewatering. They consist of fines that were not screened at the dike wall.

Lumpkins

"Lumpkins" are bulbous forms on the exterior walls of some dikes. They are convexities formed by leakoff. Leakoff indicates the host sediment during diking was ice-free and drier than fracture-filling slurries (i.e., vadose zone).

Truncations

Two types of truncation are identified: vertical and horizontal. Vertical truncation occurs within dike fills when a new sheet of sediment enters a fracture, crosscutting and slightly eroding an older fill. The presence of abundant rip-ups in dike fills provides clear evidence of erosive injection. Horizontal truncation happens when the top of a dike is eroded away during deposition of a younger flood bed. It is common for dike tops to be truncated along bedding contacts.

Skin walls of silt. Remnants of skin walls with flute casts cling to the outcrop. Touchet Valley, WA.

Fluted casts. Flute casts with upward-pointing noses decorate the interior silt walls of Touchet-type clastic dikes. Flutes are unambiguous directional indicators. Sediment entered from the top. Identical fluting is present in nearly all dikes throughout the study area. Quarter for scale.

Diking and leakoff. A fluid-driven fracture (hydraulic fracture) propagates via the advance of the crack tip. Tip advance in a clastic dike is jumpy and growth incremental. During a megaflood, immense pressure is generated at the ground surface. As pressure rises, fractures open and are rapidly filled by a slurry of pressurized sediment (proppant). Leakoff beings immediately, forming silty skin walls. Repeated fracturing, filling, and dewatering creates the conspicuous vertically-sheeted fills. Figure modified from Phillips and others (2013).

Leakoff halo. Fining and cementation are apparent just beyond the margin of some dikes. This is a 'leakoff halo', formed as pore water and fine sediments diffused out of the fill and through the dike walls. While the migration of fines occurs rapidly during the diking event, the associated cementation is a diagenetic process that takes place over a longer period afterward. Hwy 24 near crest of Yakima Ridge.

Leak-off lumpkins. The surface of this dike's outer skin wall is decorated with bulbous structures formed by dewatering of the fill into the drier surrounding sediment. These 'lumpkins' look very much like tiny load casts, but occur on vertical walls rather than horizontal bedding contacts. Walla Walla Valley.

Lumpkin-o-rama. Bulbous forms on delicate skin walls are easily damaged during excavation, despite use of a soft brush. White Bluffs, WA.

Truncated dikes. The sheeted sand-silt dikes intrude more than a dozen geologic units mapped acros the floodway region. The tops of dikes are commonly truncated by bedding contacts within floodbed sequences or by local unconformities between them. Dikes that penetrate non-flood sediments are invariably sourced in flood sediments above, which are typically preserved if not in the exposure, then nearby. Stratigraphic relationships that are well-exposed in southeastern Washington, representative of the larger region, indicate diking was recurrent with flooding. (A) Five Walla Walla Valley sites from Spencer and Jaffee (2002), (B) Lind Coulee site from Daugherty (1956), (C) Moxee Mammoth site from Lillquist and others (2005), (D) Hanford's FMEF site from Bjornstad and others (1990), (E) Rulo site from Bader and others (2016). A = Alluvium, C = Colluvium, CRB = Columbia River Basalt, DIA = Silt diamict, EG = Exotic-clast bearing gravel, FG = Fanglomerate/Alluvial fan gravel, L = Loess, P = Paleosol, S = Sandy, SCR = Silt-clay rhythmites, TB = Touchet Beds/Hanford Fm.

Dikes in Faulted Flood Deposits

Small normal faults, common in flood deposits, do not appear to be a primary control on diking. In most locations, dikes far outnumber faults and dip steeper. Where both are present, dikes crosscut faults, faults cut dikes, and dikes follow portions of faults. It is not unusual to find all three relationships in a single outcrop. For the most part, dikes cut cleanly through bedded sediments, creating new pathways rather than follow existing flaws, faults, or joints. While rotational slumps in slackwater sections can displace dike-bearing strata by several meters, they typically do not extend into bedrock and are limited to specific outcrops. Field sites where bedrock slumps or spreads influence the location, size, orientation, or number of dikes are rare. Bedding-parallel slip is common in slackwater sections, although it is often quite subtle. Small slips (<1 m offsets) that repeat in successive beds produce a stairstepping offset pattern in the dikes they cut. Larger slips (>10m offsets) may shift entire packages of strata laterally and truncate the dikes below. Thrust faults are uncommon in scabland deposits and their influence on diking is unclear.

Dikes near Mapped Bedrock Faults

Eastern Washington is criss-crossed by more than a dozen mapped thrust faults of the Yakima Fold Belt as well as older structures such as the Hite Fault (Schuster and others, 1997). Sheeted clastic dikes intrude folded and faulted basalt flows and interbeds at Umapine, Touchet, Rattlesnake Hills, Horse Heaven Hills, Alder Ridge, Gable Mountain, Cecil, and elsewhere. While some studies, notably Camp and others (2017, Fig. 50) and Reidel and others (2021, Fig 8.), suggest a link between faults and dikes, my own field work suggests the link is tenuous. I have traversed the post-basalt sedimentary sections preserved atop many of the fault-bounded ridges, finding fewer and smaller dikes near mapped Quaternary faults than in sections located far from them. For example, no sheeted dikes were identified in the Plio-Pleistocene section preserved at the crest of the Saddle Mountains anticline. Similarly, a 20 km traverse through the gullied and fault-bounded Smyrna Bench revealed only a few small dikes below the elevation of Missoula flooding. I surveyed the 150 m-thick section of Ringold Fm over 20 km at White Bluffs, finding clusters of dikes in sandy flood deposits and a few isolated dikes in the underlying Ringold. The portion of the bluffs surveyed lies along the strike of the Gable Mountain fault. Likewise, I found few dikes in scattered exposures of the same section at Frenchman Hills and neighboring Royal Slope. Plio-Pleistocene sediments exposed over several kilometers in lower Lind Coulee near a fault trenching site (West and Shaffer, 1988) are devoid of dikes. No dikes were observed in diatomaceous lacustrine strata exposed in an active mine located off the Beverley-Burke Road near the Frenchman Hills fault. No dikes are present in a fine-grained interbed cut by the Arlington-Shutler Butte Fault west of Arlington, OR. Read more about a roadside exposure of the fault HERE.

Water Table During and Between Missoula Floods

The presence of silt skins, leakoff halos, rodent burrows, minimal wetland soil indicators, and numerous brittle fractures in Touchet Bed sections suggest that a thick, well-drained, and ice-free vadose zone was reestablished each time floodwaters drained from the landscape. The field evidence hints at a wet-over-dry-over-wet condition that promoted brittle fracture just below the surface. Brittle fractures initiated between the base of the flood and top of the water table served as entry points for hydraulic injection through the vadose zone. The Pleistocene water table certainly fell during inter-flood periods, apparently returning to a position approximating the modern water table. Streambanks then and now reside well below the top surfaces of benches composed of flood deposits.

Radar Imaging of Dikes in the Subsurface

A few attempts to image the subsurface geometry of dikes with ground penetrating radar (GPR) have been made at the Hanford Site (Murray et al., 2001; Williams and others, 2002; Clement and Murray, 2003; Ward and Gee, 2003; Ward et al., 2006). While GPR was able to resolve the general shapes of large dikes to approximately 5 m depth, it did not reveal widening at depth or connections to source beds.

Touchet-type dikes are slender, sheeted, and wedge-shaped. Each Missoula flood rhythmite, labeled R1 through R7, corresponds to a distinct flood event. A clastic dike that extends downward through the section originates at the base of the youngest bed, R7. Nearby dikes also descend through the sequence in a similar manner. Notably, the dike cuts a clean path through the host sediment without following preexisting fracture sets or rubbly zones between laterally displaced blocks. Bedding contacts are neither offset nor tilted into the dike, and there is no evidence of a low-angle sliding surface in the outcrop. The stack of Touchet Beds remains in its original position atop the basalt bedrock, which is exposed at the base of the section. The sediment filling the dike was supplied from above, rather than from a liquefied layer below R1. The dike does not feed a sand blow (i.e., Obermeier, 1998). Both the top and bottom of the dike are clearly visible. The dike starts at the base of R7, widening gradually from a small sag. Bedding in R7 smoothly grades upward from the sag, suggesting that diking occurred early in the deposition of this bed. Both branches of the dike taper to a point. This dike is representative of thousands of others found throughout the megaflood region. Burlingame Canyon, Walla Walla Valley near Gardena, WA.

Initiation revealed in small dikes. Diking is sometimes interrupted early, producing very small dikes. These modest features often provide more insight into the initial stages of the fracture-and-fill process than their larger, more impressive counterparts. Here, two small single-fill dikes descend from the coarse-grained base of a Missoula flood rhythmite. Both dikes propagated downward and were filled from above, with no visible flaw controlling their points of initiation. Since they originate at the base of the bed, it is likely that injection was triggered by the initial surge of water into the valley. The dike on the left strikes obliquely to the outcrop face and appears wider than the one on the right, which strikes perpendicular to the face. Both dikes have similar widths of approximately 3 cm. Tucannon River Valley near Starbuck, WA.

Warden Canal. Not all dikes cut from top to bottom through stacks of rhythmites. Several beds overlie the top of this truncated dike near Warden, WA. Diking occurred many times during the Ice Age.

Lessons from Warden Canal. About 10 sandy rhythmites are exposed along a canal near Warden, WA. The exposure contains many important structures and relationships. At nearly every bedding contact, sags, load casts, contorted bedding, and sets of small wedge-shaped dikes are present. The repetitive features suggest they formed during the deposition of successive beds. A large clastic dike descends through the outcrop is truncated at its top by a prominent erosional surface, above which lies a wetland soil. Apparently, a high water table was established after deposition of bed R8. Above the truncation surface is a conspicuous 5 cm-thick gray layer, likely reworked volcanic ash, which is also present in other outcrops nearby. Near the top of the exposure, at least two wetland units are deformed. The overlying gravel, likely deposited by a late flood, appears to have swept across the wetland, causing the saturated sediment to liquefy. If the t-shaped mudsquirts and dish structures in the upper beds are interpreted as seismites, one must explain what appear to be structures formed by rapid loading and sedimentation lower down. The two different styles of soft sediment deformation are best explained by the same trigger, repeated overriding floods, rather than by two different triggers in upper and lower beds.

Injection during flooding. My conceptual model for sheeted clastic dikes in the megaflood region developed from relationships observed in the field. Downward injection occurred only during overland flood events and associated periods when slackwater lakes filled, stood, and drained. Flood loads fractured the relatively dry, brittle substrate allowing sediment circulating at the base of the flood (or soupy lake bottom) to immediately fill the fractures, forming dikes.

Flood injectites vs. sand blows. (A) The sketch illustrates differences between clastic dikes formed by liquefaction (sand blows and fluid escape structures) and those formed by floodwater loading and hydrofracture (flood injectites). Liquefaction dikes propagate upward and are sourced in wet, sandy beds deposited sometime in the past and remobilized by strong shaking. Liquefaction often produces feeder dikes that vent to the surface as sand blows (volcanic edifices of sand). Flood injectites are sediment-filled filled hydrofractures that propagate downward from the surface. The fractures are immediately filled with sediment sourced in circulating bottom currents of glacial floods. Liquefaction dikes in A cut younger strata and are filled with older sediment. Injection dikes in B cut older strata and are filled with younger sediment. (B) My dike-fill generations concept sketch explains the formation of sheeted clastic dikes in aggrading flood sediments (each bed = one flood). The four geometries represent the range of forms found in the study area: a). Unsheeted - Single-fill, b). Sheeted - Multi-fill Compound, c). Sheeted - Single-fill Composite, d). Sheeted - Multi-fill Composite. Compound = Multiple fill bands injected during a single event. Composite = Multiple fill bands injected during two or more events separated in time.

Dike abundance, shape, and grainsize. Sediment porosity (millidarcy, mD), permeability (percent, %), and dike shape are correlated. (A) The tightness of the formation, a function of grainsize in flood deposits, determines whether pore fluid pressures will build or disperse and whether slender or stubby dikes will form. Silty mixtures are tight and fracture when stressed (pore fluids move in fractures). Sandy-gravelly mixtures accommodate the same stress via matrix flow (pore fluids flush through interconnected pores). (B) Diking can occur in all phases of flooding, but dike fills are sourced at or very near the surface. Coarser fills correspond with the initial flood rush (coarse sand and gravel dikes), while finer fills correspond with the slackwater phase (silt-sand dikes). If a flood carries only finer material (no gravel), then all dikes will contain silty fills (i.e., northern Walla Valley, Skyrocket Hills, and loess islands in the Marengo-Benge area). Dikes with coarse fills were triggered by floods. Dikes with fine fills were triggered by lake loads.Dikes with coarse fills were triggered by floods. Dikes with fine fills were triggered by lake loads. (C) Numerous slender, sheeted dikes correspond with high silt content source beds, slackwater settings, prolonged loading (deep lake), and better preservation (protected valley settings). Sparse, crudely-sheeted, stubby dikes form in coarse sand, laminated sand, and gravelly bar deposits that lack silt.

Dike descends from base of bed. This dike originates within the swirled, coarse-grained base of a Missoula flood rhythmite and cuts downward through the silty top of the bed below. The dike formed early in the flood event. Tucannon Valley, WA.

Dike descends from top of bed. This dike originates within the silty, fine-grained top of a Missoula flood rhythmite and cuts downward through sandy material below. The dike formed late in the flood event. Tucannon Valley, WA.

Stubby dikes. As grainsize of rhythmites increases, dikes tend to become shorter and fatter. Starbuck, WA.

Importance of a dry vadose zone. A dry vadose zone sandwiched between the base of an overland flood and the water table may have created a wet-over-dry-over-wet situation that facilitated diking. Brittle fractures initiated at the ground surface (top of the dry interval) became pathways for hydraulic fractures to propagate downward through the dry vadose zone toward the water table.

Discrete deformation. Dikes fill tensile, Type I fractures. Shear is rarely involved. Deformation associated with diking does not extend beyond the dike wall. Material surrounding the dikes is extended slightly to accommodate the fill, but otherwise tends to remain undeformed. Clear bedding contacts and delicate bedforms in the host sediments continue across each dike in the photo. Dikes in the photo are filled with gray Touchet Bed sediment and intrude oxidized, quartzite-bearing, fluvial sandstones of the Miocene Ellensburg Fm. Snipes Mountain near Granger, WA.

Clean crosscuts. A sand-filled dike cuts cleanly across several silt-sand rhythmites at Starbuck, WA. Most dikes do not follow faults.

Touchet dikes intrude Plio-Pleistocene fanglomerate-calcrete-loess complex. A sheeted dike sourced in late WisconsinTouchet Beds cuts a thick stack of older cemented loess, calcrete, and weathered fanglomerate near Finley, WA.

Pleistocene dikes in Miocene basalt. Sheeted sand-silt dikes, intrude Columbia River Basalt in certain locations where Touchet Beds overlie flood-scoured basalt.The dikes exploit joints in the bedrock. A.) Weaver Pit near Gardena, Walla Walla Valley, B.) Hwy 12 at Alpowa Creek, Lewiston Basin, C.) Hwy 14 near Alderdale, Umatilla Basin.

Missoula flood gravel fills dikes in Chenoweth Formation. Unconsolidated flood gravel fills a clastic dike in tuffaceous sandstone of the Chenoweth Fm at The Dalles, OR.

Pleistocene dikes in Ellensburg sandstone. Silt-sand dike cuts a crossbedded fluvial sandstone of the Ellensburg Fm at West Foster Creek near Bridgeport, WA. The location was overridden by glacial ice several times.

Touchet dikes with sheeted fills intrude fractured basalt. Sheeted sand dike fed from above cuts Columbia River Basalt at Prosser, WA. An old fracture in the rock appears to have been widened a combination of floodwater erosion (incipient block topple similar to Finley Quarry) and by incremental injections of pressurized sediment during the slackwater period.

Early to Middle Pleistocene dikes cut Pliocene Ringold Fm. Cemented, downward-tapering dikes are sourced in and truncated by ancient scabland flood deposits at Ringold Road, WA.

Touchet dike cuts Pliocene fan gravel. Gray dike sourced in unconsolidated flood-laid sediment cuts older, reddened fanglomerate shed from the north flank of the Saddle Mountains anticline. Smyrna Bench, WA.

Insignificant Liquefaction in Eastern Washington

To date, no liquefaction features have been identified in more than a dozen trenches excavated across fault scarps in Eastern Washington and northeastern Oregon. A review of all trench logs listed below is HERE.

• Ahtanum Ridge-Burbank trench near Yakima, WA (Bennett and others, 2016).

• Horned Lizard trench in the western Boylston Mountains, WA (Barnett and others, 2013).

• Toppenish Ridge trenches above Pumphouse Rd, WA (Campbell & Bentley, 1981; Campbell & Repasky, 1995).

• Wenas Valley trench, WA (Sherrod and others, 2013).

• Saddle Mountains trenches at Smyrna Bench, WA (Bingham and others, 1970, Plates 4,5,6).

• Buroker roadcut southeast of Walla Walla, WA (Farooqui and Thoms, 1980; Foundation Sciences, 1980).

• Lower Lind Coulee trenches east of O'Sullivan Dam, WA (GEI/West & Shaffer, 1988).

• Gable Mountain trenches at the Hanford Site, WA (Bingham and others, 1970; Golder Associates/PSPL, 1982).

• Spencer Canyon trench near Entiat, WA (Sherrod and others, 2015).

• Finley Quarry west of Wallula Gap, WA (Sherrod and others, 2016; Coppersmith and others, 2014).

• Starthistle trench east of Wallula Gap, WA (Angster and others, 2020, 2023; Mahan and others, 2022).

• Kittitas Valley trench, WA (Huddleston, 2022; Dr. Walter Szeliga, personal and written communications, 2023).

• Gate Creek trench near The Dalles, OR (Bennett and others, 2021; Madin and others, 2021).

• Gales Creek trenches near Portland, OR (Horst and others, 2021; Redwine and others, 2017).

Two investigations claim to have found liquefaction features in sediments near Wallula, WA (Wallula Fault Zone). The first investigated Finley Quarry located west of Wallula Gap (Sherrod and others, 2016). A dike-like feature crosscutting "loess" was interpreted as a liquefaction feature caused by seismic shaking. A team of co-investigators working at the quarry disputed the interpretation in a separate report (Coppersmith and others, 2014). The second investigation by Angster and others (2023), found liquefaction in dry Holocene "loess" overlying 13 ka Glacier Peak G tephra in the Starthistle trench located east of Wallula Gap. The surface lineament they trenched turned out to be an old ranch road, not a fault scarp. Remnants of old pavement are clear in the trench wall and in historic aerial photographs of the site. No fault was discovered at Starthistle and Touchet Beds beneath the ash and the "liquefied" loess remain undeformed. At both Finley and Starthistle, the purported liquefaction features are few, small, and anomalous. Features at Finley bear no resemblance to those at Starthistle and nearby outcrops do not contain similar features. I believe the features have been misinterpreted in both cases. A windblown origin for beds at Finley interpreted as "loess" is incorrect; they are water-laid silts. Claims of "widespread liquefaction" in the Wallula Fault Zone by Mahan and others (2022) are premature if not spurious.

Trenched faults in the Yakima Fold Belt. Locations of fault trench investigations by USGS, Washington Geological Survey, DOGAMI, USBOR, and others. Trenching targeted prominent scarps in Columbia River Basalt and younger sediments, except for the trench at Spencer Canyon. Spencer Canyon, epicenter of the 1872 North Cascades quake, is located west of the Columbia River in older crystalline rocks of the Cascade Range. The Gales Creek trench, near Portland, is not shown on the map. Collectively, trench studies have established no connection between Pleistocene clastic dikes in the Channeled Scablands and the movement of Quaternary faults east of the Cascades. Basemap by Czajkowski and Bowman (2014).

Liquefaction rare in geological reporting over the past century. Just 2 of 107 studies on sediments that contain clastic dikes in Columbia Basin found evidence of liquefaction. The two studies on the Wallula Fault Zone were authored by staff from the same USGS office in Seattle (Sherrod and others, 2016; Angster and others, 2023).

Gable Mountain Fault Trenches Several trenches were opened across two thrust faults at the Gable Mountain on the Hanford Site by Golder Associates in the 1970s (Bingham and others, 1970 Plates 8, 9; Golder Associates/Puget Sound Power and Light, 1982). The South Fault displaces Miocene Pomona and Elephant Mountain basalt flows and the Rattlesnake Ridge sedimentary interbed by about 15 m. The fault is overlain by the unfaulted Hanford Fm (Missoula flood deposits). The Central Fault displaces the Rattlesnake interbed by 55 m, but the Hanford Fm by only 6 cm (Reidel and others, 1992, p. 43-44). No liquefaction was found in any trench at Gable Mountain. A single clastic dike, wedge-shaped and filled with flood-laid gravel, descends into a narrow zone of brecciated basalt (Trench log GT-2 in Reidel and others, 1992, Figure 39, p. 45).

J.A. Blume and Associates Engineers (1970) summarized what was revealed in the Gable Mountain trenches,

A well defined thrust fault with about 70 feet of displacement was exposed by trenching at Gable Mountain. This fault is known to be older than 10,000 years and probably is 40,000 years old. It was concluded that faults there are inactive. A second thrust fault was also identified at Gable Butte and similar conclusions were reached.

Philip S. Justus, project geologist for the Nuclear Regulatory Commission who also inspected the trenches, clearly linked the dike observed to flooding, not faulting (Justus, 1980). Agreeing with the engineers, he suggested most if not all of the faulting occurred prior to the Late Wisconsin flooding of the Hanford Plain and that movements on Gable Mountain faults post-date flooding were minor. The key findings of his report are,

• Flood deposits atop the Gable Mountain anticline are typical Missoula flood deposits.

• Two to three distinct cycles of Ice Age flood deposition are present at Gable Mtn.

• Clastic dikes on Gable Mountain are similar to those found elsewhere in Pasco Basin.

• Clastic dikes associated with several flood cycles were observed in Trenches CD-8, G-2, and G-3.

• Youngest clastic dikes originate from the base of a coarse grained flood bed capped by St. Helens S ash.

• Clastic dikes in Trenches CD-5 and G-3 are displaced by young shearing on the bedrock fault.

• Faulted flood deposits in Trench G-3 appear to post date the youngest clastic dike.

• Shears appear to cut and slightly displace clastic dikes in the footwall of Trench CD-6.

• Clastic dikes along fault plane in Trench CD-6 have slickensides surfaces that strike parallel to the fault's dip.

• Slickensides in clastic dikes parallel to those in fault breccia in Trench CD-5.

• Wherever fine-grained material is present along fault plane, slickensides are present.

Lower Lind Coulee Fault Trenches

Paleoseismic trenches opened across the Lind Coulee Fault by Michael West/GEI exposed a few clastic dikes in sheared basalt. The Lind Coulee Fault is an eastern extension of the larger Frenchman Hills thrust. The fault, exposed in a shoreline bluff along the south shore of O'Sullivan reservoir west of the Rd M SE bridge, places the Miocene Roza flow over Pleistocene Palouse loess.

Lower Lind Coulee Trench Site. Trenches at Lind Coulee were opened along the south shore of Potholes Reservoir in the 1980s as part of a seismic safety study of O'Sullivan Dam. The U.S. Bureau of Reclamation operates the dam, which impounds the reservoir and spans the head of Drumheller Channels near MarDon Resort. Lower Crab Creek (Lind Coulee Wasteway) empties from the west into the reservoir. The trenches were located near the center of the photo.

Findings in the Lind Coulee West Trench are similar to those at Gable Mountain, but investigators engaged in a lengthy debate before arriving at an interpretation acceptable to all (Grolier and Bingham, 1971, 1978; Galster/USBOR memo, 1987, "Area No. 2"; Lefevre and MCConnell memo, 1987; West and Shaffer, 1988; Shaffer and West, 1989; Reidel and Campbell, 1989, "Stop 21-A", Figure 14; Geomatrix Consultants Inc., 1990, "East Fault Exposure"; Reidel and Fecht, 1994; Schuster and others, 1997; Lidke and Haller, 2016).

Pleistocene dikes intruding fault gouge initially excited the crew,

The Lind Coulee trench initially presented strong circumstantial evidence for fault displacement of [basalt, a very old reverse-magnetized loess, and a younger less cemented loess]. The evidence for displacement was magnified by the protruding knob of brecciated basalt [seven meters from west end of NE-trending trench], the apparent overturned contact with flood deposlts on the north side of the knob, flood sands injected along a shear plane in the fault zone and discontinuity of the petrocalcic horizon and infiltration of loess north of Station 7 [near west end of trench]. The geometry of flood deposits overlying the paleosol on the footwall block was also suggestive of colluvial wedge geometry.

Upon further investigation, West and Shaffer modified their interpretation,

In spite of this body of circumstantial evidence, we could find no evidence of shearing, tectonic displacement or colluviation characteristic of surface fault rupture. The [flood-deposited] sands along the shear plane appear to have been injected hydraulically along the plane rather than dragged along it.

They ultimately conclude that dike injection post-dated faulting,

The lacustrine silt [that separates “intermediate” flood deposits from “youngest” flood deposits] could be traced as a continuous, uninterrupted horizon across the main fault zone, indicating with 100% certainty that the fault had not moved since deposition of the silt layer. Careful excavation of Unit 5B [“intermediate”] flood deposits disclosed no evidence of shearing or tectonic colluviation. These deposits were in intimate contact with the eroded basalt surface on the hanging wall and exhibited an open-work fabric that we attribute to high energy flood deposition.

Busacca and McDonald (Appendix V) conclude the flood deposits exposed in the trench are not related to the most recent episodes of flooding (about 12 to 16 Ka) but are older…based on soil development and stratigraphic position that the age of flood deposits in the Lind Coulee West area is 40 to 50 Ka. The last surface fault displacement, therefore, occurred before 40 to 50 Ka.

The apparent injection of flood sands along a shear plane in the fault zone is more difficult to explain. We are of the opinion that the sand was injected hydraulically from the top down. The sands filling the shear however do not appear to be continuous with flood deposits mapped as Unit 5 [“intermediate” flood deposits].

Similar injection of flood sands along shear planes was noted in fault trenches excavated on Gable Mountain (DOE/Westinghouse, 1987b).

The authors retreat to an earlier interpretation by Woodward-Clyde Consultants (1981), namely the few dikes formed by,

…either hydraulic injection associated with catastrophic flooding or hydraulic injection resulting from fault movement and liquefaction offer reasonable interpretations for the origin of clastic dikes including the feature in the Lind Coulee West trench.

They dutifully entertain an alternative origin for the dikes (liquefaction), though supporting evidence was not found,

Another possibility is that the sands were injected from below and are part of an older flood deposit preserved deeper in the footwall. The exposures in both cross-cut trenches suggest older flood deposits are indeed involved in faulting and could be preserved at depth in the footwall and locally along shear planes…

Lind Coulee Fault at O'Sullivan Reservoir. The Lind Coulee Fault is a south-dipping thrust that places Miocene basalt (Wanapum Roza flow) over younger sediments. There are several splays. Grolier and Bingham first identified the fault (Grolier and Bingham, 1971; 1978 Figures 14, 23). West and Shaffer trenched it in the 1980s. Easily accessible exposures remain. The photo and sketches above show Roza basalt shoved over alluvial Ringold Fm sediments and at 2-3 generations of loess. The fault shatters the Roza basalt. A thin white gouge zone is observable in places. Gouge is 10-20cm wide and associated with boudin-like lenses of deformed dark and light brown mudstone, rock flour, or broken basalt. Beneath the gouge is a sliver of brown mudstone (hanging wall) and cemented loess. Faint bedding in the loess confirms its vertical to overturned tilt beneath portions of the fault. The shattered footwall Roza is weathered above the fault and takes on a greenish-yellow hue. The rubbly zone grades upward to competent basalt then to spheroidally weathered basalt at the flow top. The boulder-sized spheroidal forms are also exposed along nearby Hwy 262. My own investigations of the Lind Coulee Fault Trench site and all nearby bluffs have yielded no evidence of liquefaction. Lind Coulee Fault is part of the larger Frenchman Hills thrust, known to have Quaternary movement (Reidel and Fecht, 1994; Schuster and others, 1997; Lidke and Haller, 2016). USBOR memos recount some details of West's trenching work from the perspective of the client (Lefevre and O'Connell, 1987; Galster, 1987). Much of the work on the Lind Coulee Fault was done pre-Internet, which makes written reports difficult to find. Relevant articles include Grolier and Bingham (1971, 1978), Galster/USBOR memo (1987, "Area No. 2"); Levfevre and MCConnell memo (1987), West and Shaffer (1988), Shaffer and West (1989), Reidel and Campbell (1989, "Stop 21-A", Figure 14), Geomatrix Consultants Inc. (1990, "East Fault Exposure"), Reidel and Fecht (1994), Schuster and others (1997), Lidke and Haller (2016). A big thanks to Brian Sherrod for sending me a scanned version of West and Shaffer (1988, Vol. 2).

Toppenish Ridge Fault Trenches and Borrow Pits

Two large gravel pits at Toppenish Ridge expose tilted and untilted sections of Miocene conglomerate separated by an active fault. The Toppenish Ridge Fault is located between the two pits. At the Lower Pit (225-250 m elevation) several large, sheeted dikes sourced in Touchet Beds descend into the flat-lying cobble conglomerate below. At the Upper Pit (265-295 m elevation), located <200m from the fault in steeply-titled conglomerate tilts steeply south (>50 deg). Very few dikes were found in the tilted beds.

Seventeen kilometers to the west, four seismic trenches were opened across faults of Toppenish Ridge by Newell Campbell and Ted Repasky. The work was supported by the Yakama Indian Tribe and USGS (Campbell and others, 1995). Trench 1 was excavated across the Mill Creek Thrust (see below). Trenches 2, 3, and 4 crossed nearby extensional structures. No liquefaction features or clastic dikes were observed in the trenches.

Toppenish Ridge near Granger, WA. Gravels deposited by the ancestral Columbia River interfinger with and overlie the Columbia River Basalts. Green areas of the map are Miocene Ellensburg Fm sediments. Light brown areas are basalt flows. Gray areas are late Pleistocene Touchet Beds. Yellow is recent alluvium of the Yakima River. Sheeted dikes and Touchet Beds occur in the Lower Pit (minimally deformed strata), but not the Upper Pit (steeply tilted strata). Landowner is the Yakama Indian Reservation.

Dikes missing from Upper Pit at Toppenish Ridge. Steeply-dipping, partially lithified fluvial gravels contain no liquefaction features or clastic dikes, despite its proximity to the fault Toppenish Ridge Fault, mapped less than 200 m away (Schuster and others, 1994; Lidke and others, 2003; online USGS Quaternary Fold and Fault Database for the United States). Sheeted dikes are present in a thin remnant of tilted(?) Touchet rhythmites at the top of the exposure (~295m elevation).

Many dikes in Lower Pit at Toppenish Ridge. Flat-lying Touchet Beds are the unambiguous source for clastic dikes cutting older gravels off Tule Rd. Diking at this lower elevation fall within the Ice Age floodway and appear to be related to flooding, not faulting or seismicity. The dikes do not rise from a liquefied source bed. They post-date deposition of the Ellensburg and most, if not all, of the tilting.

Trenching at Toppenish Ridge. The last movement on the Mill Creek Thrust offset the modern soil, L-1 loess, Mt. St. Helens Set S tephra (13 ka), and a dark-brown paleosol (~40 ka) by 3.5 m. The latest movement was estimated at 500-700 years BP, consistent with previous reporting (Campbell and Bentley, 1981) and local Indian legend. Evidence for two older ruptures indicate the fault has been active three times in the past 40,000 years. No liquefaction was found.

Dikes in the Columbia Gorge west of the Cascade divide

Floodwater spilling out of the Columbia Gorge ponded in the Willamette Valley, blanketing its floor with silt-sand rhythmites from Portland to Eugene. Ice-rafted boulders of "granite and schist" were first(?) noted by Diller (1896) and later mapped in the hundreds throughout the valley by others (Bretz, 1919; Allison, 1935; Minervini and others, 2003).

While the Willamette Silt does contain clastic dikes, the number of dikes is far fewer than in the Columbia Basin, located some 350 km to the east. PhD student Jerry L. Glenn ( Glenn, 1965) documented a few sheeted dikes in Willamette Silt at his River Bend and Irish Bend sites near Corvallis, OR. A photo by Ira Allison (Allison, 1978, Figure 14) shows a clastic dike cutting slackwater rhythmites near St. Paul. Photos of dikes exposed in highway excavations near Portland were sent to me by Ian Madin in 2014. Dikes exposed in the basement of the Oregon State Capital Building at Salem were sent by Ray Wells in 2012.

Thurber and Obermeier (1996) reported finding 16 clastic dikes at 7 sites along the lower Calapooia River, a tributary to the Willamette River. The largest features measured 5 m long x 10 cm wide. They attributed the dikes to liquefaction triggered by a Holocene earthquake. The Calapooia report contains no photos of dike fills or sketches detailing crosscutting relationships between the dikes and the sediments they intrude. Consultant John Sims (2002) reviewed the Thurber and Obermeier report, concluding their data set too small to support their interpretation,

The limited area surveyed by [Thurber and Obermeier] in the Willamette Valley does not allow for a high level of confidence in determining if the features result from large subduction events or local intracrustal events. The age of the structures is somewhat in doubt as few radiocarbon dates are available for the host deposits and Thurber and Obermeier (1996) do not report any radiocarbon dates as part of their study. They also do not mention any evidence for liquefaction in post Pleistocene deposits of which there are many in the banks of the Willamette River and its tributaries. Thus, with incomplete coverage and lack of dating of paleoliquefaction features, the question of source zones is moot. Earthquake source determination can only be addressed with broader coverage of liquefaction features and better age data to constrain timing of events and to allow regional correlations of liquefaction features. In addition, we need a more complete picture of the size distribution of similar-aged features for the purposes of evaluating the magnitudes of prehistoric earthquakes.

River Bend section. Glenn (1965, Figures 3 and 15) found a few clastic dikes in Touchet-equivalent flood rhythmites along the Willamette River. Note the undeformed beds. Outcrop photos of various other sites taken prior to urban development of the valley as well as serviceable descriptions of the Willamette Silt can be found in Bretz (1925, 1928), Allison (1932, 1933, 1936, 1953, 1978), Piper (1942), Treasher (1942), Lowry and Baldwin (1952), Baldwin and others (1955), Allison and Felts (1956), Wells and Peck (1961), Trimble (1957, 1963), Balster and Parsons (1969), Hampton, (1972), Robert (1984), McDowell (1991), Yeats and others (1996), and McDowell and Roberts (1987).

Obermeier and Dickenson (2000), working in the nearby Columbia River Valley west of the Cascade divide, found "relict liquefaction features" in low shoreline bluffs of sandy islands between Astoria, OR (Marsh Island) and Kalama, WA (Bonneville Dam) and in cutbanks of 10 tributary streams in the Hood River area. The thickest dikes they measured were 30 cm wide, on par with dikes in Missoula flood rhythmite sections I investigated farther upstream (i.e., Sixprong, Glade, Rock, Old Lady, Arlington, Chenoweth, Willow, etc.). The authors attributed the dikes to lateral spreading, hydraulic fracturing, ground shattering, and warping triggered by earthquakes. Similar investigations by USGS and DOGAMI were conducted in the Columbia gorge (Obermeier, 1993; Peterson and Madin, 1997; Atwater, 1994) and contain some of the same information.

Atwater (1994) and Takada and Atwater (2004 + Appendix A Supplement) describe sandy riverbank sediments in the lower Columbia River gorge deformed by the 1700 AD Cascadia earthquake. They note Holocene-age clastic dikes filled with fluidized sand and sills that,

...mostly follow and locally invade the undersides of mud beds. The mud beds probably impeded diffuse upward flow of water expelled from liquefied sand. Trapped beneath mud beds, this water flowed laterally, destroyed bedding by entraining (fluidizing) sand, and locally scoured the overlying mud.

Peterson and Madin (1997) and Peterson and others (2014) describe unsheeted sand dikes and sills in Holocene overbank muds at sites near the mouth of the Willamette River and in bluffs along Pacific beaches near the mouth of the Columbia. They interpret the dikes as features triggered by seismicity at the Cascadia margin, possibly the 1700 AD event. A field guide to their study locations was prepared for a Friends of the Pleistocene outing (Peterson and others, 1993).

All of the dikes and sills described by Atwater (1994), Thurber and Obermeier (1996), Obermeier and Dickenson (2000), Sims (2002), Takada and Atwater (2004), and Peterson and others (2014) are fluid escape features that formed west of the Cascade divide. They are not sheeted, wedge-shaped injection dikes found in scabland deposits. Their seismite interpretation is reasonable, though none of the reports clearly document a source bed for the dikes. I wonder if some sections they called Holocene are actually Pleistocene. Perhaps a deeper dive into their unpublished field notes would clarify.

Holocene liquefaction dikes in the lower Columbia gorge. Caption for Figure 13b in Atwater (1994) reads, "Dikes with raised edges at upper Wallace Island [near Longview, WA]...The dikes transect mud beds that extend parallel to shoreline." This is the same dike pictured in Peterson and Madin (1997, Fig. 11b) and probably the largest example seen by all parties involved. Guessing that's Atwater's shovel in the photo.

Liquefaction features exposed along Pacific beaches. Caption for Figure 2 in Peterson and Madin (1997) reads, "Drawing of subsurface fluidization features including clastic dikes and sills and flames. Internal structures include intruded contacts with host deposit and disoriented mud blocks in sandy matrix. Fluidization features such as clastic sills are often enhanced under thin capping deposits of mud overlying thick source beds of sand." A nearly identical cartoon in Obermeier (2005, Fig. 2) shows the various components of an idealized liquefaction dike.

Clastic Dikes and Seismic Hazard Maps

Clastic dikes are commonly observed in earthquake-prone regions of the world and often highlighted in post-quake damage assessments (i.e., Walsh and others, 1995). Methods for describing deformation caused by shaking have matured over the past century thanks to dedicated staff at USGS, state geological surveys, and consulting firms (McCulloch and Bonilla, 1970; Gohn and others, 1984; Atwater, 1994; Obermeier, 1996, 2009; Peterson and Madin, 1998; McCalpin, 2009; Holtzer and others, 2011). Maps of liquefaction features often help geologists delineate the extent of deformation. The value of such maps largely depends on the intensity of the field effort (size of the dataset). A small number of measurements or measurements collected within a small area (i.e., one valley or one trench) have low value because they lack statistical power and may not reflect the actual pattern of damage.

Misinterpretation of features and field relationships can also be a problem, especially for inexperienced staff or where exposure is poor. The assumption that all clastic dikes form by liquefaction triggered by earthquakes has led many to call features formed by aseismic processes seismites. Where outcrops are sparse or the geology unfamiliar, investigators should be especially aware of knowledge gaps, their own biases, and those of their managers. Experienced authors encourage caution when interpreting paleoseismic information (Borradaile, 1984; Bonilla and Lienkaemper, 1990; Holtzer and Clark, 1993; Moretti and van Loon, 2014).

Seismic hazard in the Columbia Basin vs. New Madrid. Earthquake hazard probability map generated by the 2023 USGS model (fault-slip rates, frequency, magnitude). The map shows the 2% in 50-year probability of exceedance for fixed VS30 760 m/s. Red-orange indicates a high probability for damaging quakes. Green-blue indicates a low probability. Note the stark difference between the Columbia Basin (green-yellow) the New Madrid Fault Zone (dark red-red-orange). Dikes in the Columbia Basin are wedge-shaped and filled from above. Dikes in the New Madrid Seismic Zone are feeder conduits to sand blows. Columbia Basin dikes are Pleistocene age and occur entirely within the Ice Age floodway. New Madrid dikes are Holocene features that occur in floodplains of the Mississippi River and a few large tributaries.

In 2017, an international conference was convened to review reporting on seismites in sedimentary sequences. Participant emphasized the need for caution (Feng, 2017). It seems “seismite” (Seilacher, 1969; Montenat and others, 2007; Van Loon, 2014) has for some time been assigned too liberally to features of nonseismic or ambiguous origin, making reexamination of "classic" seismite localities necessary. Clear-eyed geoscientists who participated reattributed many features formerly identified as seismites to nonseismic processes, most commonly to rapid sedimentation and loading (Moretti and Van Loon, 2014; Shanmugam, 2016 and references therein). The following quotes capture the feelings of some participants:

Nonseismic events can create structures that are virtually indistinguishable from seismically-deformed sediments, or seismites. Therefore, paleoseismologists must correlate candidate seismites over regions and rule out nontectonic origins before concluding that an earthquake occurred.

– L.B. Grant

A great progress has been made in researches [sic] of soft-sediment deformation structures (SSDs) and seismites in China. However, the research thought was not open-minded. About the origin of SSDs, it was almost with one viewpoint, i.e., almost all papers published in journals of China considered the beds with SSDs as seismites. It is not a good phenomenon.

– Z-Z. Feng

At present, there are no criteria to distinguish...soft-sediment deformation structures formed by earthquakes from SSDs formed by the other 20 triggering mechanisms...the current practice of interpreting all SSDs as “seismites” is a sign of intellectual indolence.

– G. Shanmugam

Obermeier's Maximum Width Method is Inappropriate for Sheeted Dikes

Relationships between liquefaction and earthquake intensity are well established (Ambraseys, 1991; Galli, 2000; McCalpin, 2009; Zhong and others, 2022). Shaking intensity maps prepared in the wake of damaging earthquakes are constructed from sensor data and field observations (i.e., locations of surface ruptures, toppled structures, water spouts, sand blows/boils, foundered slopes, widths of clastic dike, etc.).

A method developed by Steve Obermeier of USGS, here called the "maximum width method", involves measuring the width of the widest sand blow feeder dike at a number of locations and contouring the values in order to produce a map that hopefully reveals an epicenter. Since seismic shaking is often most intense near the epicenter. The idea being the largest dikes should occur where shaking is most intense.

Obermeier applied this method to the New Madrid Seismic Zone (Obermeier, 1998; Obermeier and others, 2005). He measured the widths of sand blow feeder dikes triggered by magnitude 7.2–8.2 quakes with Modified Mercalli Intensities >VIII that struck the region in 1811-1812. Sand vents distributed over hundreds of square kilometers are still visible on aerial photos. The resulting map identified potential epicenters of pre-historic earthquakes, an improvement over earlier efforts (Fuller, 1912; Russ and others, 1978; Boyd and Schumm, 1995).

Clustering of large clastic dikes at New Madrid. Obermeier (1998) used the widths of sand blow feeder dikes to delineate the extent of liquefaction and identify the paleoepicenter. Black dots correspond to dike width categorie, where dot sizes correspond to the largest dike measured at each site (<15 cm, 15-50 cm, >50 cm). Six dashed ovals are the interpreted damage halos associated with historic quakes. I find it odd that Obermeier identifies six separate clusters of large dikes in his study area. Clustering of large dikes could just as easily be related to local sediment response than to proximity to the epicenter. Likewise, dike widths appear to vary as much along the length of each stream valley as they do across the entire map area; large dikes are mapped at the western and eastern edges of the study area, near St. Louis, MO and Columbus, OH. The fact that more dikes were found near Vincennes, IN could be attributed to stream network density (more outcrops), rather than to stronger shaking. Also, the map would benefit from the addition of topographic contours and clearly delineated interfluve areas (dry uplands between wet valleys). Dike measurements were only collected in floodplain alluvium that regularly exceeds 30m thickness (Saucier, 1964). No measurements were collected in interfluve/upland areas as no sand blows formed there.

Obermeier's "maximum width method", while appropriate for sand blows in modern floodplains, is in appropriate for sheeted injectites formed in other settings. The method assumes dikes are single-fill structures that rose from a liquified source bed at depth during a single triggering event. It uses dike width measurements (fracture aperture) to predict the shaking intensity. But the dikes in the Channeled Scablands require different assumptions. Width of these dikes grows incrementally over time by the addition of subparallel fillings with apertures of varying widths. Compound dikes widen sheet by sheet during the course of a single event. Composite dikes widen during two or more separate events separated by decades to millennia. The widths of single-fill dikes and sheeted dikes are simply not comparable. One involves the pressurized injection of sediment into hydraulic fractures propagated downward, while the other involves the upward escape and venting of fluidized sand at the ground surface. An apples-to-apples comparison of the sand blow feeder dikes to sheeted injection dikes, would be to measure the widest liquefaction dike at each site vs. the widest sheet in any dike at each site.

Columbia Basin Crust vs. New Madrid Crust The tectonic settings of the two regions, composition of the crust beneath each, and the potential for faults to generate strong shaking are not comparable. The New Madrid is an ancient failed rift in crystalline basement. Seismicity >M 7.0 is generated by deep, steep faults in strong crust. The Columbia Basin in WA and OR, by contrast, is a young back-arc flood basalt province resting atop extended Tertiary crust capable of ~M 7.0 quakes (Madin and others, 2021). At New Madrid, Holocene floodplain deposits liquefied during shaking and vented sand upward. In Columbia Basin, dikes were injected downward into various substrates during Ice Age megaflood events.

Bedrock geology or floodway processes? Sheeted clastic dikes are abundant where Ice Age floods swepth through basins floored by basalt bedrock. No dikes are found where basalts are overlain by sandy-silty non-flood sediments, such as the Blue Mountains or Idaho-Nevada Graben. Cataclysmic flooding, not bedrock lithology, appears to control where dikes formed. Map modified from Tolan and others (2009, Figure 1).

Missing Holocene Deformation in Eastern Washington

Sections of thick, unconsolidated alluvium preserved in the valleys of creeks across Eastern Washington lack clastic dikes, liquefaction features, and other soft sediment deformation structures commonly found in regions subjected to strong shaking. The absence of such features in wet, fine-grained Holocene floodplains is difficult to explain if local faults have been generating earthquakes with magnitudes >6 M (Intensities >VII) every 500-1000 years since the Miocene. It is difficult to believe modern floodplain sediments would deform differently than Missoula flood sediments or that earthquakes of exceeding 6 M have not occurred in the past 10,000 years, or that deformation features in floodplain sediments have been erased by channel processes or vigorous bioturbation while Pleistocene sediments remain pristine.

Alluvium of Dry Creek. Thick Holocene alluvium (>4m) like this section along Dry Creek near Walla Walla, WA shows no evidence of strong shaking, pre- or post-Mazama ash. If present, convolute bedding, soft sediment deformation features, and faults would have long ago been identified by local geologists, farmers, and soil scientists given the strong visual contrast between the reworked ash and darker overbank alluvium. Mapped Quaternary faults in the vicinity include the Wallula Fault Zone (21 km away), Hite Fault (33 km away), Kooskooskie Fault (23 km away), and Promontory Point Fault (6 km away). Intersection of Harvey Shaw Rd and Dague Rd ~8 km north of Walla Walla. Photographed in June 2021.

Alluvium of Union Flat Creek. No soft sediment deformation has been found in the floodplain of Union Flat Creek near Dusty, WA.

Alluvium of Touchet River. Thick deposits of alluvium along the modern Touchet River contain no evidence of deformation consistent with strong seismic shaking.

Alluvium of Willow Creek. Thick alluvial fills along Willow Creek near LaCrosse, WA are undeformed.

Alluvium of Latah Creek. A mix of sandy Ice Age flood deposits, reworked colluvium, and varved lake beds capped by younger alluvium containing Mazama Ash is exposed along Latah Creek west of Spokane, WA. While the Pleistocene lake beds at the Qualchan Golf Course are especially prone to landsliding and the overlying Touchet Beds contain flood-formed folds and numerous clastic dikes, the Holocene sediments remain undeformed. I've seen nothing upstream of the Hatch Rd bridge resembling liquefaction. Photo of a cutbank below Hangman Valley Rd northwest of Hangman Valley Golf Course.

Shaking Intensity-Liquefaction Distance Relationships Fail

Shallow, intraplate faults in the study area are believed capable of producing magnitude 6.5 earthquakes and MMI VII–VIII intensities (Lidke and others, 2003). However, large clastic dikes in the study area are found at distances far beyond what local faults are presumed capable (~150 km from epicenter), according to analyses by Galli (2000) and Zhong and others (2022). In a review of sand blows produced by 28 large earthquakes, Castilla and Audemard (2007) found only subduction zones capable of producing liquefaction features beyond ~150 km. None of the quakes reviewed in the aforementioned articles occurred in flood basalts and there is no indication that the Cascadia Subduction Zone is in any way related to clastic dikes in Eastern Washington. In summary, intensity-liquefaction curves do a poor job of predicting where dikes occur in the study area. Large dikes are routinely found at distances twice those predicted. For example,

• An epicenter placed at Wallula Gap (Wallula Fault Zone) is >285 km from large dikes near Kettle Falls, WA.

• An epicenter at Burbank, WA (Umtanum–Gable Mountain Fault) is >260 km from large dikes in Lewiston Basin, ID.

• An epicenter on the Hite Fault is >265 km from Granger, WA in the western Yakima Valley.

• An epicenter near Arlington, OR (Arlington–Shutler Butte Fault Zone) is >230 km from the central Willamette Valley, OR.

• An epicenter at Smyrna, WA (Saddle Mountains Fault) is 225 km from Kettle Falls, 205 km from Tammany Creek, ID, 135 km from Cecil, OR, and 120 km from Bridgeport, WA.

• An epicenter at Wyeth, OR (Mount Hood Fault Zone) is 250 km from Warden, WA, 375 km from Lewiston, ID, and 395 km from Latah Creek, WA.

• An epicenter at Spencer Canyon near Entiat, WA (1872 North Cascades Earthquake) is >210 km from Touchet, WA.

Magnitude–distance curves compiled from many studies. Liquefaction outer-distance limits compiled from studies on several continents (Galli, 2000; Qiao and others, 2017; Zhong and others, 2022). A robust relationship exists between earthquake magnitude and the radial distance away from an epicenter liquefaction features will form. Liquefaction produced by a M 6.5 quake is predicted to occur out to ~75 km. For a M 7.5 quake, the limit approaches 150 km. My field work throughout the Scablands region shows that many large dikes are located at distances exceeding 150 km from mapped Quaternary faults and potential epicenters.

Distances from an assumed epicenter. Distances measured from an assumed epicenter at Wallula Gap (Wallula Fault Zone) to outcrops containing clastic dikes are shown (circles). Most dikes occur within 150 km of the assumed epicenter, but many are found at distances far beyond liquefaction limits established by Galli (2000), which suggests the dikes are not seismites. Black bars represent the boundaries of subbasins along the Ice Age floodway. Outcrops containing dikes are abundant in the 10 subbasins located within 150 km of Wallula Gap, but become less abundant beyond 150 km due to a reduced subbasin count. Subbasin count, at bottom of figure, is a proxy for exposure. Outcrops are more numerous in valleys near Wallula Gap, where several rivers converge and the road network is extensive. The dikes themselves are largest and most abundant in exposures immediately upstream and downstream of Wallula Gap, though very large dikes are found in a number of distant exposures. Subbasins: CC = Crab Creek Valley, GT = Gorge Tributary valleys downstream of Wallula Gap to The Dalles, LB = Lewiston Basin, OK = Okanogan Valley, PB = Pasco Basin, RP = Rathdrum Prairie, QB = Quincy Basin, SR = Snake River Valley, TV = Tucannon River Valley, UB = Umatilla Basin, UC = Upper Columbia River Valley, WC = Willow Creek Valley, WW = Walla Walla Valley, WV = Willamette Valley, YV = Yakima Valley.

Quite a few large dikes are located hundreds of kilometers from Wallula Gap. Very large clastic dikes in the Upper Columbia River gorge are too distant from Quaternary faults to have been created by shaking and liquefaction. Colville River mouth south of Kettle Falls, WA.

Thin Record of Strong Shaking East of the Cascades

If the dikes in the Channeled Scablands are the products of seismic shaking, then one or more of the Yakima Fold Belt structures would be the likely trigger. However, the dikes are distributed over too large an area for a single fault to be the culprit. If movement on the Saddle Mountains Fault, for example, triggered diking, then we should see evidence of repeated dike injection dating to the Miocene and on some interval consistent with its recurrence. Since the Saddle Mountains have been rising for at least the past 15 million years, dikes and other seismites should be present in Miocene, Pliocene, Pleistocene, and Holocene strata. The Ellensburg, Latah, and Ringold Formations should host numerous soft sediment deformation features. A radial pattern of strong shaking by YFB faults should be preserved Eastern Washington, yet no such pattern has been recognized.

Fault zone investigations have likewise failed to reveal a pattern of strong shaking in Eastern Washington. The often-referenced Stateline earthquake of 1936 that struck the Walla Walla Valley was a sub-magnitude 6.0 event that formed no sheeted dikes and caused no damage to speak of beyond the immediate epicenter, the tiny outpost of Umapine, OR. The Hite Fault, located in the Blue Mountains southeast of Walla Walla, shows no indication of Quaternary movement and appears to be inactive (Foundation Sciences, 1980; Brocher and others, 2018). I am aware of no reports of liquefaction or other seismites associated with the Hite Fault or any other. Lupher (1940) seems to have had it right, "Strong evidence shows that the fissures are not the result of earthquakes".

Simply put, geologic evidence of strong shaking is absent in fine-grained Neogene sediments of the region, despite more than a century of investigation. No regional pattern has been recognized:

• Hundreds of borehole cores logged in post-basalt sediments at the Hanford Site

• Dozens of measured stratigraphic sections through the Pliocene Ringold Fm at White Bluffs

• Ringold-equivalent sediments in the Dalles-Umatilla syncline and Columbia Gorge

• Cores from alpine lakes in the Cascades and Okanogan Highlands

• Numerous exposures of Ellensburg/Thorp/Latah Fm fills in Kittitas, Yakima, and Naches Valleys

• Numerous exposures of sedimentary interbeds in the Columbia River Basalts

• No evidence of the 1700 AD megathrust event (Atwater and others, 2005) east of the Cascade divide

• No reports of sheeted clastic dikes following the 1918 Vancouver Island M 7.2, 1946 Vancouver Island M 7.5, 1949 Olympia M 6.7, or 2001 Nisqually M 6.8 quakes

Seismographs record modest shaking east of the Cascades. Map of historic-period earthquake epicenters recorded between 1970-2015 (Brocher and others, 2017, Fig. 2). East of the Cascade divide, historic quakes have mostly been small, shallow, and weakly clustered (Gomberg and others, 2012). Most epicenters do not fall along the prominent scarps and mapped faults of the Yakima Fold Belt (heavy gray lines). No spatial correspondence exists between the epicenters and locations of large clastic dikes in the study area as the dikes are neither larger nor more numerous in areas where epicenters cluster (i.e., eastern Saddle Mountains, Entiat area, White Bluffs, MSH-MH corridor, etc.). YFTB = Yakima Fold Thrust Belt, D = The Dalles, El = Ellensburg, GRZ = Goat Rocks fault zone zone, HA = Hanford Site, MA = Mount Adams fault zone, MH = Mount Hood fault zone, Pa = Pasco, SHZ = St. Helens fault zone, UL = Umtanum lineation, P = Portland, W = Wenatchee, WRZ = Western Rainier fault zone.

Review of the 1872 North Cascades Earthquake

Reports on the 1872 Chelan/North Cascades quake deserve scrutiny and a bit of historical context. The event is the largest on record for Washington, but it is important to note that nearly all contemporary reports are from newspapers. Witnesses to the quake observed water spouts, ground cracks, small landslides, and the collapse of a cabin roof (Washington Standard Newspaper 11 Jan 1873; Coombs and others, 1976; Brocher and others, 2018, Appendix B). Certainly, it was a memorable event, but what actual science was reported? No geologists were interviewed following the calamity. And recall, Wenatchee in 1872 was an incorporated frontier town constructed of wood and unreinforced masonry. The city would not be platted for another 20 years. In fact, Chelan County didn't exist at the time. Neither did the light bulb or the telephone. Ulysses S. Grant was President. Washington, Idaho, Colorado, Wyoming, Utah, New Mexico, and Arizona were not yet States of the Union. In 1872, just 6 rudimentary seismographs monitored ground motions for the entire region, including parts of Canada. The rupture at Entiat occurred in old crystalline rocks, not Columbia River Basalt. The fault is unrelated to activity in the Yakima Fold Belt.

The 1936 Milton-Freewater/Stateline earthquake record was reprocessed in 2018 and its magnitude subsequently down-rated to below 6.0 (Brocher and Sherrod, 2018). The quake is no longer considered a major event. Now 1936 data is pretty old, but what about data from 1872? Would you trust seismic sensor data from the 1800s? I know I wouldn't. Knowing the antiquity of the sensors and processing methods, the prudent geologist should flag such data as "raw" or "provisional". Another reasonable policy would be to down-rate the magnitude of old quakes to the lowest value in the range in the original report. Report it this way, "the quake had a magnitude of at least 6.5", instead of this way, "the 7.2 temblor sent shockwaves across a vast region". I am also struck by the ingenuity of the USGS team in drawing a connection between the long term aftershock hazard following the 1872 Entiat event and that following an M 7.5 subduction zone quake at Nobi, Japan (Brocher and others, 2017). The two ruptures share absolutely nothing in common - not geologic setting, not rock type, not stress regime, not rupture length. I have to wonder why USGS is publishing retrospectives at all? Have staff run out of fundable projects or are they no longer capable of doing fieldwork? I scoff at the "widespread liquefaction" the team reported finding in trenches at Wallula and Finley, WA (Mahon and others, 2022; Angster and others, 2023; Sherrod and others, 2016). Their evidence is flimsy, yet they publish anyway. The fact is no comparable liquefaction features have been observed in hundreds of other exposures of the same strata nearby. This group of overenthusiastic, desk-bound researchers consistently punches up the spectacle regardless of the datasets they revisit, the faults they trench, or the field evidence they observe. Read their stuff; its driven by narrative and analogy, not field work. While newspapers are free to splash "Earthquake!" across their front pages whenever they choose (spectacle sells newspapers), no one is asking for USGS to sell us anything more than science. If a dash of spectacle is necessary to keep hazards program administrators employed, then its time to redirect funding to the field and trim the office staff to one post-doc running an M5 and a few AI agents.

Saddle Mountains Fault. The Saddle Mountains Fault has been active since the Miocene and remains one of the most active in the region. Excellent exposures of sandy interbeds between tilted basalt flows are found along its north flank where railway, ranch roads, and trails are abundant. No Miocene sections above the level of Missoula flooding contain sheeted clastic dikes. The thin, light-colored fractures in the photo at right are not liquefaction features, but bleached, unfilled shear bands, structures common in deformed sandstones worldwide.

Saddle Mountains crest. I've worked methodically along the entire crest of the Saddle Mountains, Smryna Bench, and Taunton Bench, examining Pliocene and Pleistocene sediments preserved there (Cooley, 2023). I found no sheeted clastic dikes above ~360 m elevation. In fact, surprisingly little evidence of strong shaking is found in numerous tilted alluvial sections exposed along the 85 km-long anticline. Is it possible that Saddle Mountains and its neighboring YFB ridges rose to their current heights one M 5.9 quake at a time?

Minimally deformed interbeds. Ebinghaus and others (2012) examined Miocene-age sedimentary interbeds in the Columbia River Basalt at 14 sites in Pasco and Quincy Basins near the Saddle Mountains and Frenchman Hills Faults. Small flame structures and load casts were noted at certain contacts between mudstones and overlying sands. Such features are common where channel sands spill through levees onto overbank muds. No clastic dikes were observed. The Ebinghaus team's findings are consistent with those of previous workers (Schmincke, 1964; Hays and Schuster, 1983; Smith, 1988a,b; Humphrey, 1996) and my own.

Mostly undeformed White Bluffs. The White Bluffs of the Columbia River expose Pliocene Ringold Fm and Pleistocene flood deposits over some 50 km. The nearly continuous section is gently tilted, but otherwise undeformed. Recent landslides along the river, locally exacerbated by irrigation practices, create fresh exposures. Most of the White Bluffs is on public land and accessible by foot.

Lost near Lyons Ferry. About 15 Touchet Beds overlie a thick bar gravel in the Snake River canyon near Lyons Ferry. John Whitmer photo (WGS Archive No. 03144).

Sheeted Dikes Without Earthquakes

Several examples of sheeted, wedge-shaped dikes formed by aseismic processes in a variety of geologic settings are reviewed below. In all cases, overloading, rapid sedimentation, silty sediment, and hydrofracture were involved. Sheeted, wedge-shaped dikes intrude muddy deposits beneath tidewater glaciers in Sweden (Von Brunn and Talbot, 1986; Jolly and Lonergan, 2002; Le Heron and Etienne, 2005; Phillips and others, 2013), New England (Kruger, 1938), and alpine lake sediments in New Zealand (Sutherland and others, 2022). Sheeted dikes intrude lahar deposits on the side of an Aleutian volcano in Alaska (Herriott and others, 2014) and ash flows inside an Guatemalan caldera (Brocard and Moran-Ical, 2014). Sand intrusions formed by hydrofracture during turbidite-fan deposition propagate downward, upward, and laterally in deep water clastic systems (Jenkins, 1930; Duranti and Hurst, 2004; Huuse and others, 2007; Monnier and others, 2015; Cobain and others, 2015, 2016). Wedge-shaped sand dikes descend from the base of thick debris flow deposits at Black Dragon Canyon in the San Rafael Swell, UT (author's field notes).

Lahar at Mount Spurr, AK. Sheeted dikes with characteristics identical observed to dikes in the Touchet Beds were discovered by Herriott (2014) in sandy lahar deposits on the side of an Aleutian volcano. Rapid deposition, surface loading, wet over dry sediments, and hydraulic fracturing were all involved. The red arrows are Herriott's and point to silt skins. Image courtesy of Herriott.

Subglacial dikes at Voss, Norway. Clastic dikes in subglacial settings often indicate high fluid pressures developed in muddy substrates by ice loading. Laminated dikes identical to dikes in the Touchet Beds descend into outwash sand at Voss, Norway. Wall-parallel laminations formed by "a repetitive process" (Mangerud and Skreden, 1972; Mangerud and others, 1981; Larsen and Mangerud, 1992). Similar downward-injected dikes formed in glacial settings are reported in Scandinavia, Iceland, British Columbia, Quebec, Ontario, and New England (Kruger, 1938; Dionne and Shilts, 1974; Amark, 1985; Boulton and Caban, 1995; Brunn and Talbot, 1986; Broster and Clague, 1987; Dreimanis and Rappol, 1996; Rijsdijk and others, 1999; Crossen, 2014; Ravier and others, 2015).

Subglacial dikes at Hat Creek, British Columbia. Gravel dike from above penetrates underlying outwash sand in British Columbia (Broster and Clague, 1987).

Subglacial dikes in southwest BC. Sheeted gravel-sand dikes filled from above in British Columbia (Broster, 1991, Fig. 9b).

Subglacial dikes in Wrangell-St. Elias National Park, AK. Sheeted dike intrudes sandy outwash of the Bering Glacier. Photo by Crossen (2014).

Subglacial and sheeted. A sheeted, wedge-shaped clastic dike formed by downward injection in a subglacial setting. The dike is nearly a meter wide and extends >16 m through late Wisconsin drift exposed along the shoreline of Lake Erie near Bradtville, Ontario. Its laminated fill is composed of diamict, pebbly sand, silt, and clay. Figure by Dreimanis and Rappol (1997, Fig. 5).

Sub-debris flow dikes at Black Dragon Canyon, UT. Wedge-shaped clastic dikes sourced in an overriding debris flow are injected downward into cross-bedded sandstone below. San Rafael Swell.

Downward dikes at Big Pumice Cut, CA. Gravel- and sand-filled dikes sourced in a boulder gravel at the top of the exposure descend through the Bishop Tuff and pinch out in the underlying Sherwin till. (Sharp, 1968; Lipshie, 1976; Wahrhaftig, 1965). Hwy 395, Eastern Sierras.

In Patagonia. Sheeted dike cuts varved glaciolacustrine sediments in the northern Patagonian Andes (Perucca and Bastias, 2008, Fig.11). Pocket knife in shadow.

Subglacial dikes in Sweden. A grounding tidewater glacier forces wedge-shaped, till-filled dikes into the muddy substrate below. Figure by Von Brunn and Talbot (1986, Fig. 16).

Polish coal mine. Two sets of wedge-shaped, sheeted dikes fill extensional fractures at the crest of an active anticline near Kleszczow, Poland (Haluszczak and others, 2007, Fig. 6e). This example is nearly a meter wide and sourced in undeformed Quaternary deposits that overlie folded Miocene-Pliocene bedrock. A slightly older set of dikes also cuts the underlying bedrock, but is sourced in slightly older, folded Quaternary deposits.

Idaho vs. Utah. On the left is a sketch of a Touchet-type clastic dike at Lewiston, ID that I measured in several places (sheeting not shown). On the right is a cartoon of a typical deep sea sand injectite from a slideshow by Dr. Lansing Taylor, formerly of the University of Utah's Energy & Geosciences Institute. I flipped the injectite image upside down. In both examples, fractures follow the most efficient pathway that alternate between horizontal and vertical in response to changes in grainsize (porosity, permeability).

Rubbly Injectites at Indian Creek, WA

In November 2017, I discovered and measured several breccia-filled dikes that cut varved Glacial Lake Columbia beds along lower Indian Creek Rd (Hawk Creek) east of Lincoln, WA. The intrusions formed in sediments deposited in a protected side canyon in response to subaqueous slumping of house-sized blocks of varved sediment. A highstand lake was present at the time. Several blocks were exposed in high tractor-bladed slopes, now covered by erosion control matting. Fills are unsheeted and contain broken, stratified clasts torn from the host material during injection. The dikes intrude the lower portion of the >20 m-thick section of at least 24 rhythmites (alternating lake varves and flood sand intervals).

Indian Creek. Pleistocene injectites cut varved lakebeds along the northern margin of the floodway region.

Rubbly fills. Rubbly injectite crosscuts clay-rich varves at a low angle.

Rip-ups. Rubbly, unsheeted fills contain stratified rip-up clasts - chunks liberated from the surrounding material.

Parallel and crosscutting. A light-colored sand-filled injectite intrudes clayey varves of Glacial Lake Columbia. Both crosscutting (stair-stepping base) and bedding-parallel (separates bedding) relationships are clear. Indian Creek is located north of Davenport, WA and also goes by Olson Canyon and Hawk Creek.

Field Work Matters

The origin of clastic dikes in sedimentary sequences can be ambiguous. Earthquakes, though often involved, are not required. In fact, clastic dikes are reported in many settings where active seismicity played no role whatsoever (Shanmugam, 2016). Lessons learned from coastal California or the Wabash Valley do not apply universally. Only when anchored by evidence gathered at the outcrop will an investigation into the origin of clastic dikes tilt toward a correct interpretation. Office-generated theories and probability models serve society best when they are rooted in and remain subordinate to field observations.

Dikes are threshold features that, if interpreted one way, may prompt policy makers to brand a landscape hazardous and unfit for occupation and/or future development. Interpreted another way, the same dikes become Ice Age relicts of little importance to anyone other than academics and megaflood enthusiasts.

Careful field work that involves a significant number of observations, measurements, descriptions, samples and a study area scaled to the geological phenomenon under investigation should be de rigueur. Overuse of "seismite", shoddy field documentation, and the application of methods poorly suited to the region are unacceptable practices.

Project planning is the responsibility of the Field Geologist. The subdiscipline Paleoseismology will hopefully remain a field-based discipline going forward, one focused on determining the timing and effects of prehistoric earthquakes, not getting one's name in the newspaper (or on NPR). Data gathered in the course of a paleoseismic investigation (fault slip rates, event dates, and shaking effects) are critical inputs to building codes, hazard planning documents, and land use policies. Data from the field informs and often drives policymaking, which affects the lives of real people. Unlike journal articles and tables of recurrence probabilities, maps constructed from field measurements are easily understood by all audiences. They are uniquely influential and tend to find their way into land use policy documents, which persist for decades.

Dike geometries in outcrop. (A) Twin-tapering forms that do not look like typical dikes. They are axe blade-shaped fractures propagating laterally and emerging from the face of the outcrop. A trick of geometry in the third dimension (see Arris and Aperture figure earlier in article). (B) Three dikes that lack a taper direction are truncated at their tops by erosional surfaces (bedding contacts). (C) Buried sediment remobilized in response to shaking vents sand upward to a higher stratigraphic position (sill) or to the surface via a feeder dike (sand blow). May be sourced from above or below. (D) Upward and downward tapering dikes. Local shearing may have offset a single dike, causing it to appear as two with opposite tapers. A trick of limited exposure. Excavate features or keep looking to find more conclusive relationships. (E) Downward tapering dike-sill geometry with upward-curving intersections are sourced from above. (F) Upward tapering dike-sill geometry with upward (tree branch-like curving intersections are sourced from below.

Know your SSDs. Many soft sediment deformation features are distinctive, but many others can look alike. This is because ductile material is involved, more than one process may be at work, and features at an early stage of development may morph into very different shapes over time. Careful observation is usually the key to sorting things out correctly.

Deformation caused by rapid sedimentation. The swirls, flames, and dike-like features pictured here formed during a Missoula flood, not during an earthquake. The deformed sediments were first laid down flat, but only partially consolidated. At some later date, the soupy substrate was overridden by a flood and thoroughly reorganized. Combined loading by water and sediment as well as viscous drag by the fast-moving current produced the features seen here at White Bluffs, WA and numerous other locations in the Channeled Scablands.

Deformation during deposition. Flame structures in the light-colored mud formed during a flood. The dense, sand-choked current moved left to right over the unconsolidated, silty bed, sweeping some of the sediment upward and into the flow, forming these spectacular flames. White Bluffs, WA.

T-shaped mudsquirts are syndepositional structures. Rapid deposition of a sand bed on top of soupy lake bottom muds triggered the rise of t-shaped mudsquirts. Note how sand swirls with the mud dikes. The deformation is a result of rapid deposition and loading during a flood, not strong shaking. The gray sand on top, dumped by a Missoula flood, temporarily disrupted quiet-water deposition in Glacial Lake Columbia. Sanpoil Valley, WA.

Flood and repeat. Three flood rhythmites exhibiting an identical bedform progression provide clear indication of repeated, syn-depositional fluidization in the soupy, upper portions of Touchet Beds. Ringold Rd, WA.

Wrinkled lakebeds. Typical mass wasting features, not evidence of worrisome seismicity. Priest River, ID.

Silt-sealed Cracks and Hydrofracture

Sand-propped hydraulic fractures are used to stimulate oil and gas reservoirs, a procedure commonly known as "fracking". Fracturing is induced by shutting in a portion of the well, adding a proppant slurry (sand + water + chemicals), and using pumps to jack up the fluid pressure until the formation yields. When the rock surrounding the wellbore fails fluid-driven fractures propagate outward. The pressurized proppant slurry fills the expanding fractures and holds them open, permitting hydrocarbons to flow back to the well. The network of propped fractures exponentially increases the surface area of a well.

Unlike fracked formations at depths of hundreds to thousands of meters, the sediments that host the dikes are surficial deposits, unconsolidated and sandy that lack low-permeability layers that might act as a seal. Two key factors explain the formation of the dikes in Missoula flood deposits: high strain rate (rapid loading) and the rapid formation of silt skin walls on fracture walls (sealed pressure vessels).

• High strain rate - When loaded by a catastrophic flood, pressure in the shallow subsurface built so rapidly that hydraulic fracturing was induced. The normally loose (ductile) material failed in the brittle mode at the high strain rate. Pressure rose above that required for fracture and exceeded the sediment's capacity to dissipate pressurized fluid through its pore network. Rapid loading alone appears adequate to initiate fracture in a low tensile strength material with no true seal such as the Touchet Beds.

• Silt skin walls - Once fractures began to form and fill, silt skin walls began to build. The sealing effect of the low-permeability skins delayed leakoff and facilitated further fracture. New fractures as well as natural flaws (cracks, soil macropores, burrows, etc.) provided low-resistance routes for new fractures to follow. Fractures immediately filled with sediment, the injected slurry (a natural proppant) sourced from within the overriding flood. Dewatering (leakoff), integral to the formation of the skin wall, begins the moment sediment enters a fracture. The skin-sealed crack begins to behave as a pressure vessel almost immediately. Pressure inside of the sealed fracture (pore fluid pressure, Pf) rises until it exceeds the confining strength of the material (Pf > 03) and the crack tip advances or, if leakoff loss exceeds Pf, the fracture closes. The fracture propagates in the 01–02 plane (vertical), widening against 03. As the fluid pressure equilibrates to the confining pressure (Pf = 03), the fracture tip halts, filling ceases, the crack closes down on its proppant, and pressure begins to build again if the load is still present. Each time Pf exceeds 03, the tip jumps forward or a new fracture initiates nearby. With each increment of widening, fluid pressure drops (volume increase = pressure decrease), but soon rebuilds. This loading-driven crack-fill-seal cycling created the dikes’ vertically sheeted fabric. 

Sheeted infill illustrated. Fluid pressure-driven crack and fill (crack volume cycling) is shown at the scale of a dike (nearfield scale) during a flood loading event. Time steps 1 through 12 in the pressure-time curve correspond with crack tip locations. During overloading, dike growth corresponds with pressure-volume cycling where fluid pressure remains between the minimum and maximum principal stress values. Silt-sealed fractures become sheeted dikes in my study area and in other geologic settings where silt is present and similar overloading has occurred (lahar, grounding glacier, debris flow, etc.). Diking seems to have occurred twice during a flood-load event. The first is the initial onrush of the overland flood (or backflood). The second occurs once a slackwater lake has formed (sustained load). Gravelly or sandy dikes are likely produced by the overland flood, while silty-sandy dikes result from the lake.While hydraulic fracture is well understood, the development of wall-parallel laminae (sheeting) in clastic dikes by a combination of rapid loading, hydraulic fracture, and silt wall seals, as I've illustrated here, has not previously been described in detail. Figure 9 in LeHeron and Etienne (2005) is the closest I've seen. My illustrations are not copied from anyone; I created them to clarify my thoughts and convey my argument to others.

Sheeting forms pulse by pulse. Cyclic fluid-driven fracture results in the growth of dikes with vertically-sheeted fills during flood events (a single hydrofracture event). The cross sections correspond to the gray shaded portion of the pressure-time curve in the figure above. During hydraulic fracture, new fractures open, propagate, and fill. Here I show 4 pulses corresponding to 4 episodes of adjacent diking (2a, 3a, 3b-c, 4b, 4c) and nonadjacent diking (1a, 2b, 4a). Incremental growth of dikes involves the cycling of fluid pressure, the repeated opening of new fractures, and near simultaneous infilling by sediment carried by a flood. Evidence of repeated flooding (stacks of rhythmites) and repeated fracture injection (sheeted fills) is a pattern observed throughout the Channeled Scablands.

Near field and far field fracture. I modified the stress-strain curve for hydraulic fracture to illustrate my concept of sheeted diking during floods. The curves relate floodwater loads imposed over a broad area (far field flood load) to cyclic pressure pulses that occur at the local scale (near field injections). The fracking/leak-off test framework captures most of the important elements. Leak-off begins when a fracture opens (not at closure) and continues after the fracture closes, but that point, somewhat secondary, is not well captured in this diagram. See Footnote 15 for relevant equations.

Flood counts and the development of vertical sheeting. Stacks of rhythmites (Touchet Beds) accumulated to different thicknesses in different parts of the Channeled Scablands. Many scabland floods followed different paths from others, therefore, rhythmite counts vary depending on location in the floodway. The most complete rhythmite sections occur in protected slackwater valleys inundated by Lake Lewis, Lake Condon, and Lake Allison. All floods flowed through Wallula Gap and slackwater lakes that ponded there were the deepest anywhere in the region (>200m). Consequently, full rhythmite sections in the southern Pasco Basin, easternmost Umatilla Basin, and western Walla Walla Valley contain the largest composite dikes. Dikes widen by the addition of new sheets of sediment, so their widest portions occur lower in the section rather than near their tops. Large dikes can appear to taper upward because younger fills tapping successively younger flood beds intruded alongside older fills. Sheet counts on composite dikes do not match flood counts, but scale proportionally with them. Sheets counts up to ~10 are common for compound dikes (single flood, one rhythmite). Sheet counts for large composite dikes can exceed 100 (multiple floods, many rhythmites). Lake Lewis filled the Walla Walla Valley (Waitt, 1980; 1985), Lewiston Basin (Bretz, 1929 field notes; Webster and others 1982), and Tucannon Valley (Smith, 1993) to ~366 m elevation. Lake Condon filled the Umatilla Valley (Benito and O'Connor, 2003), Willow Creek Valley (Cooley, 2015), and Sixmile Valley. Lake Allison filled the Willamette Valley (Glenn, 1965). Lake Columbia filled the Upper Columbia Valley (Kiver and Stradling, 1982; Hanson and Clague, 2012), Sanpoil Valley (Atwater, 1986), Latah Creek Valley (Rigby, 1982; Kiver and Stradling, 1982; Waitt, 1983; Meyer, 1999), and Foster Coulee (Russell, 1893). Floods that ponded in glacial Priest Lake deposited rhythmites today exposed at Peninsula Road near Priest River, ID (Walker, 1967; Breckenridge, 1989). Varved beds in glacial Lake Missoula are found at Lightning Creek near Clark Fork, ID (Breckenridge and Othberg, 1998), Clark Fork Valley (Smith, 2004, 2017), Missoula area (Chambers, 1971; Hanson and others, 2012), and Mission Valley (Levish, 1997). An excellent summary of varved deposits is provided by Waitt and Atwater (2023, Fig. 17).

Evaluating Proposed Origins

In this section, I evaluate seven proposed origins based on my observations and the literature.

(A) Desiccation hypothesis - Little evidence supports a desiccation origin. Dike geometry, distribution, size, sedimentology, and internal characteristics are fundamentally at odds with an origin involving the passive infilling of meters-deep, open-standing cracks. The dikes are not filled mudcracks.

(B) Ground ice hypothesis - The dikes have been interpreted as fossil ice wedge casts based on their arrangement in polygonal networks, vertically-laminated fills, and age (Alwin and Scott, 1970, Lupher, 1944, and Black, 1979). While the dikes do bear some resemblance to certain ice wedge casts at mid-latitude sites on other continents, including England (Briant and others, 2004), France (Antoine and others, 2005), The Netherlands (Van Huissteden and others, 2000), Poland (Zoller and others, 2022), Germany (Grube, 2012), Mongolia (Owen and others, 1998), Niger (Denis and others, 2010), Patagonia (Perucca and Bastias, 2008), as well as the Canadian Arctic (Van Vliet-Lanoe, 2005). However, other indications of periglacial cold are not present in the landscape. While frost-cracks are common in loess paleosols of the Palouse and Umatilla Plateau and rock glaciers linger in cold hollows east of the Cascade divide (Lillquist and Weidenaar, 2021), deep Pleistocene cold sufficient to form permafrost, ground ice, or an active layer never materialized. Loess units that date to nearly 2 Ma contain abundant evidence of soil life (plant roots, rodent burrows, cicada burrows) and, therefor, were never deeply frozen. Backfilled burrows that riddle the Touchet Beds attest to rapid recolonization between outburst flood events. No mention of soil wedges, frost stirring, or gelifluction is made in NRCS Soil Surveys for the Colville Indian Reservation (NRCS, 2002), Okanogan County (NRCS, 2010), Chelan County (USDA, 1975), Douglas County (NRCS, 2008), Grant County (USDA, 1984), or Lincoln County (USDA, 1981). Frost wedges found in varved lake beds in western Montana (Chambers, 1984; Chambers and Currey, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021) have not be found in varved lake beds of northeastern Washington (i.e., Lake Roosevelt, Lake Rufus Woods, Banks Lake). Fossil soil wedges such as those in the Lemhi Range of Idaho (Butler, 1984; D.R. Butler photos and written communication), on the Snake River Plain (Dort, 1968; Butler, 1969), in terrace gravels near Lewistown, MT (Schafer, 1949), and in prairie soils near Browning, MT (unpublished field notes by the author) and Laramie, WY (Grasso, 1979; Mears, 1981, 1987; Nissen and Mears, 1990; Munn and Spackman, 1991; Dillon and Sorenson, 2007) did not form in the Columbia Basin or Okanogan Highlands. Pollen samples from lake bottom cores indicate cold-tolerant plant species and conifers persisted across the region throughout the Late Wisconsin (Blinnikov and others, 2002; Whitlock and Brunelle, 2006). Mammoth in Eastern Washington were nourished by steppe-grassland forage, not tundra plants (Fry, 1969; Last and Barton, 2014). Pleistocene cirque elevations in the Rocky Mountains (Pierce, 2003, Fig. 1) project well above the crests of Yakima Fold Belt ridges. Silt mounds (mima mounds), abundant in the Scablands, are not unique to periglacial landscapes or diagnostic of deep cold (Busacca and others, 2004). Mima mounds are found from central Mexico to the Arctic and some mound fields in Eastern Washington clearly date to the Holocene. Mima mounds indicate abundant wind, dust, and aridity, not cold.

No permafrost in Washington. Compiled climate-proxy information indicates permafrost never formed in Columbia Basin during the Late Wisconsin. The "alpine permafrost zone" drawn by Pewe (1983) was defined on the basis of loess, not ground ice features. Event the periglacial zone that should have accompanied the former margin of the Okanogan Lobe is remarkably narrow and only loosely delineated (Murton, 2020). 

Ice wedge forms. Hundreds of studies have been published on fossil ice wedges, ice wedge casts, and soil wedges formed in permafrost (Lachenbruch, 1962; Pewe, 1973; Romanovskij, 1973; Mears, 1987; Yershov, 1998; Bockheim, 2002; Murton, 2020, etc.). All climate-proxy archives indicate the Columbia Basin was never glaciated and remained free of permafrost even during the very coldest parts of the Pleistocene. Eastern Washington's clastic dikes are not fossil ice wedge casts, despite their laminated fills, organization in polygon networks, and speculation by authors (Lupher, 1944; Alwin and Scott, 1970; Black, 1979). At its coldest, the Pleistocene Columbia Basin was "tundra-like" (Cooley, 2008) and perhaps best described as a "cold steppe" with widespread sagebrush and pockets of pine forest supporting species commonly "found in alpine and sub-alpine valleys in the [present-day] Cascade Mountains of Washington...cool-to-cold, moist, open-park conditions...consistent with the presence of continental ice to the north" (Spencer and Knapp, 2010). While the use of "periglacial" may persist among certain groups (O'Geen and Busacca, 2001; Gaylord and others, 2003), Eastern Washington was never tundra and always contained trees.

Ice wedges active and relict. Left: Ice wedge in thick silt at the USACE Permafrost Tunnel Research Facility near Fairbanks, AK (www.erdc.usace.army.mil/CRREL/Permafrost-Tunnel-Research-Facility). Right: A fossil ice wedge cast in sandy alluvium in northern Europe. Photo by Richter/Freiberg Instruments.

Wedges in western Montana. Small wedges descend from numerous horizons in Glacial Lake Missoula "varve" beds exposed in the Clark Fork River Valley, MT. Specific sites include Rail Line (A,B), Jocko River (C), Crow Dam, and Garden Gulch sections (Chambers, 1971, 1984; Chambers and Curry, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021). These structures are though to have formed during lowstand periods (shoaling, subaerial exposure) and resemble wedges commonly found in the Arctic. A combination of desiccation and shallow ice wedging seem to be at play. Photos A and C by the author. Photo B by Michelle Hanson.

Soil wedges in eastern Montana. Soil wedges formed in treeless prairie soils south of Browning, MT. Relict frost wedges like these help define the former limits of perennially frozen ground associated with the Laurentide Ice Sheet (i.e., Murton 2020, French 2017). Soil wedges are not found along the former margin of the Cordilleran Ice Sheet in Eastern Washington, some 450 km to the west. Roadcut along Hwy 89 between the Two Medicine River and Badger Creek south of Browning, MT.

(C) Lateral spreading hypothesis - Lateral spreading involves liquefaction and can form wedge-shaped cracks. Surface cracks open when a block of earthen material slides sideways along a low-angle slip plane. The figure below shows three scenarios where wedge-shaped cracks may form in thick sediments and gentle terrain given a 'free face'. Space is needed to accommodate spreading. According to Cruden and Varnes, tension fractures open where steep escarpments fail. Fractures that fill with sediment may form dikes. Dikes formed in this way are passively infilled by material slumped or washed in. The dikes would be oriented parallel to the headscarp. However, most dikes in the study area occur in terrace-like benches and on the floors of broad valleys, not near steep escarpments. Incised channels that might create steep breaks necessary for lateral spreading are simply not found in the Touchet Beds. And while Touchet Beds do erode in the typical ways, they show little evidence of low-angle sliding and no block topple as depicted by Cruden and Varnes.

Lateral spreading. The standard landslide classification system in use today was developed by Varnes (1978) and updated by Cruden and Varnes (1996). The system formalizes terminology associated with the various modes and features of mass wasting, including lateral spreads. Two classic examples of lateral spreading are shown above. A.) Blocks of sandstone sliding atop a weak shale layer tilt and separate along vertical fractures containing rubble (Zaruba and Mencl, 1969). B.) Spreading of clay-rich glaciomarine sediments at Anchorage, AK triggered by a massive earthquake in 1964 (Seed and Wilson, 1967). By this model, clastic dikes would rise from the liquefied slide layer and fill vertical fractures between the blocks.

Lateral spreading triggered by a major earthquake. Large portions of the Turnagain Heights neighborhood located near downtown Anchorage slid into Cook Inlet following the magnitude 9.2 Great Alaska Earthquake that lasted nearly 5 minutes and produced a rupture >950 km long. Note the NE-SW orientation of the large fractures and scarps. The 1964 quake remains the largest ever recorded in North America. Nothing comparable has struck Eastern Washington. Aerial photo from the W.G. Sprowls archive at University of Alaska (Number UAA-HMC-1464-F1-41).

Lateral spreading vs. Liquefaction vs. Loading. (A) Lateral spreading scenario - Channel incision removes support and creates a free face that facilitates lateral spreading. Sliding and block separation atop a weak layer at depth creates wedge-shaped, vertical fractures that fill with sediment. (B) Liquefaction scenario - Seismic shaking triggers liquefaction in a wet, sandy layer and clastic dikes that taper upward. Some dikes reach the surface and vent liquefied sand in cone-like sheets (sand blows). (C) Loading-triggered hydrofracture scenario - A large vertical load imposed by a megaflood (overland flood, slackwater lake) increases pore fluid pressures in the underlying sediments and triggers hydraulic fracture. Fractures propagate downward through the vadose zone and immediately fill with sediment sourced from the base of the circulating flow. Fractures are propped by sandy fills become wedge-shaped clastic dikes. Internal sheeting develops during each flood event (pressure-volume cycling) and new dikes form over time during subsequent floods. New dikes follow and merge with older ones.

Dikes actually formed by lateral spreading. A set of wedge-shaped, gravel-filled dikes that unambiguously formed by lateral spreading to a free face is exposed near Hunters, WA. Unstable glacial lake sediments capped by outwash gravel comprise steep shoreline bluffs and show evidence of block topple into the Columbia River channel. Spreading opened wedge-shaped fractures between blocks that were passively filled from above with outwash gravel. These dikes check all the boxes for lateral spreading - free face accommodation, block translation, and a slide plane at depth. The dikes at Hunters are a local phenomenon. They are unusual and not representative of most dikes in the Channeled Scablands. More about the geology of the shoreline bluffs near Hunters Campground HERE.

(D) Rebound following slackwater lake drainage - Ice Age floods imposed enormous loads on the crust. In the southern Pasco Basin, the depth of Lake Lewis depth exceeded 200m. Each flood imposed a transient load for hours. Each lake a load for days to weeks. While we can assume the crust was depressed a bit during flooding and rebounded as the water drained away, we don't know the of total amount of depression, the rate of depression and recovery, or whether the effects of loading and unloading are preserved in the geologic record. No one has studied the effects of floodwater loading on surficial sediments.

Belly up to the bar. Wedge-shaped fractures might result from up-bending and rebound following the removal of floodwater load by drainage.

(E) Seismic shaking and liquefaction hypothesis - According to some, earthquakes created the clastic dikes in the Channeled Scablands. This hypothesis requires the dikes to have formed by liquefaction, in whole or in part. The dikes are therefor feeder conduits to sand blows. However, the dikes bear no resemblance to liquefaction features described in textbooks (Allen, 1982; McAlpin, 2009), review articles on soft sediment deformation (Dzulzynsky and Walton, 1965; Van Loon and Brodzikowski, 1987; Nichols and others, 1994; Obermeier, 1996; Montenat and others, 2007; Owen and others, 2011; Shanmugam, 2017), or >200 research articles on liquefaction published in journals over the past two centuries. No liquefaction features have been found in more than a dozen seismic trenches excavated across active faults in Eastern Washington (see review above). Liquefaction-related clastic dikes in Washington have only been found in Holocene floodplains and coastal bluffs west of the Cascade divide (Dickenson, 1997; Obermeier and Dickenson, 1997; Peterson and Madin, 1997; Atwater, 2000; Atwater and others, 2005, 2015).

(F) Flood-generated vibration hypothesis - The Missoula floods would have produced a tremendous rumble as they coursed through the landscape. The repeated cataclysms must have terrified humans and animals who witnessed their passage. In addition to an audible roar, a vibratory resonance may have been established in the bedrock and sedimentary cover, perhaps well int front of an advancing flood. Fracturing of the substrate caused by vibration is purely speculative at this point. Icelandic jokulhlaups of the past century do not appear to create damaging vibrations and no evidence of flood vibrations has been identified in the geologic record. Research into seismicity generated by large floods is purportedly underway at Université de Grenoble-Alpes, France by Kristen Cook, Florent Gimbert, and Alain Recking.

(G) Hydraulic fracture triggered by rapid floodwater loading hypothesis - This is my preferred origin. The field evidence is most consistent with massive, rapid overloading of bedrock and cover sediments by cataclysmic floods. Fracturing was triggered by floods, an aseismic process.

Rapid overloading, sedimentation, and fracturing. Clastic dikes often form in response to rapid sedimentation and overloading. Dikes are widely documented in both seismically-active and low-seismicity areas and in a variety of geologic environments and deposits. While earthquakes commonly trigger liquefaction and can a produce clastic dikes, they are most commonly unsheeted, upward-tapering, and small. Dike morphology indicates whether liquefaction was involved. Evaluate taper direction, internal sheeting, and the connection to source bed to distinguish each type, its trigger, its origin. Not all clastic dikes are seismites. Figure modified from Shanmugam and others (2016, Figure 16).

Dike-sill-dike. Evidence of both fluid-driven fracture (hydraulic fracture) and Darcian flow (matrix flow) is present in clastic dikes in the Touchet Beds. An understanding of how hydraulic fractures initiate and propagate is necessary to understanding the clastic dikes in the megaflood region. Loading-triggered hydraulic fracture can be taught at the undergraduate level, even by professors who must learn it for the first time themselves. Read Jolly and Lonergan (2002), Ravier and others (2015), Bons and others (2022), and articles on sand injectites from the folks at Aberdeen. Students will pick it up. If you avoid the physics, then you will forever flounder about with stories involving earthquakes, liquefaction, and the OWL.

Style of Deformation Changes Along the Floodway

The same floods produced different types of deformation features depending on the grainsize and rheology of the sediment they encountered. Same stimulus, different response. In silt-sand rhythmites near Walla Walla, Lewiston, Cecil, and Zillah, we find sheeted, wedge-shaped dikes in the hundreds. In varved beds of the upper Columbia Valley, we find abundant t-shaped mud squirts, a few rubbly injectites, and features associated with mass wasting. In eddy bars near Umatilla and Washtucna, we find a few stubby, gravel-filled dikes with crude vertical sheeting. In silty, gravel-free silt rhythmites near the upper limit of flooding (i.e., Palouse Hills), a few thin dikes appear here and there. Where basalt was exposed at low elevation to energetic flows (Snake and Columbia gorges), a few sheeted dikes cut the bedrock. Coarse, laminated pebble-sands like those at at Qualchan, the mouth of Rock Creek, and the big quarry north of Corfu are nearly devoid of dikes. Same floods, same forces, different substrates, different features.

Deformation style varies with host sediment type. The type of soft sediment deformation at a site will differ depending on its location with the floodway (light blue area). Each map symbol represents multiple outcrops. Wedge-shaped sheeted dikes are common in slackwater deposits in the southern half where sand-silt rhythmites dominate, while t-shaped mud squirts and flame structures are common in the northern half, where abundant varved lacustrine sediments occur. The divide between north and south is approximately Moses Lake, WA. The dashed blue line follows a longitudinal profile of the Columbia River with the study sites of Atwater (1987, Figure 2) and O'Connor and others (2020, Figure 8) labeled in blue text.

Conclusions

This article summarizes my work on sheeted clastic dikes in the Channeled Scablands over the past 30 years. I've closely documented and measured thousands of dikes at hundreds of outcrops throughout the floodway region. All of the dikes occur within the margins of Ice Age floodways and are identical with respect to size, shape, sedimentology, and age. All formed by the same mechanism: loading and hydraulic fracture triggered by megafloods moving overland and ponding in slackwater valleys. Overland floods and slackwater lakes imposed enormous loads on sedimentary and bedrock substrates, opening wedge-shaped fractures that rapidly filled with sediment sourced from circulating bottom currents and unconsolidated lake bottom deposits. Vertical sheeting reflects crack-and-fill cycling that occurred during each flood event (producing compound dikes) and during successive flood events over time (producing composite dikes). Fluted skin walls indicate fractures were filled from the top and leakoff began the instant sediment slurries entered fractures. The largest dikes occur where floodwaters were deepest and rhythmite stacks thickest. Dike widths scale with rhythmite counts and sheet counts. Unlike most clastic dikes in the literature, the features described here did not form by liquefaction or seismic shaking; they are not feeder conduits to sand blows. Clastic dikes in the Channeled Scablands are flood injectites, not seismites. This study confirms a hydrofracture origin first proposed by Pogue (1998), but finds little support for an origin involving slumping as proposed by Baker (1973) or lateral spreading as defined by Cruden and Varnes (1996).

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This article expands on one I published in Northwest Geology v. 49 in August 2020. Northwest Geology is published annually by the Tobacco Root Geological Society in conjunction with the TRGS field conference. TRGS is a Montana-based group of geoscience professionals. I update this online version from time to time as new information becomes available. Online version was first posted here 15 Sept 2020.

LAST UPDATED: 25 Feb 2026

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