top of page

Sheeted Clastic Dikes in the Megaflood Region

Sheeted Clastic Dikes in the Megaflood Region, WA-OR-ID-MT

Skye W. Cooley


Clastic dikes are sediment-filled fractures found worldwide in deformed sediments from the Precambrian to the Pleistocene. Most are soft sediment deformation features and the products of liquefaction caused by seismic shaking. The dikes described in this article are wedge-shaped, vertically sheeted, and sourced from above - characteristics that contrast with typical liquefaction dikes. This study investigated unconsolidated sediments along the path of the Ice Age megafloods between Priest River, ID and The Dalles, OR and found clastic dikes in 282 of 529 exposures (as of April 2022). Dike distribution, width, age, injection direction, absence in paleoseismic trenches excavated across Yakima Fold Belt fault scarps, and occurrence entirely wittrhin Ice Age floodways indicate the dikes are nonseismic structures formed by rapid loading and hydrofracture during glacial outburst floods. They are flood injectites, not seismites. This study is the first to map and measure sheeted clastic dikes throughout the Channeled Scablands region.

Sheeted clastic dike. A vertically-sheeted clastic dike intrudes silty-sandy late Pleistocene megaflood rhythmites (Touchet Beds) at Smith Hollow Rd in the Tucannon Valley, WA.

Dike-sill-dike. Dike follows a least-resistance pathway through a stack of Touchet Bedsat Lewiston, ID. The dike tends to cut vertically across the finer grained, low-permeability silt layers and follow bedding in the coarser, higher-permeability sands. The orientation of fill bands flips between vertical in the dike segments and horizontal in the sill segments. Geometry of the entire dike, mostly out-of-plane, is likely a branching structure with blade-like pinchouts both laterally and on bottom (i.e., KGD hydraulic fracture model).

Dikes intrude a variety of substrates. Sheeted dike intrudes hillslope colluvium composed of angular, locally-derived basaltic clasts. Prior to being reworked and transported by a catastrophic outburst flood, the material composed a scree slope on the flank of the Alder Ridge anticline. Columbia Gorge, WA.

Olaf P. Jenkins was the first geologist to report on the clastic dikes in the Columbia Basin. This 1923 photo shows Jenkins standing next to a large dike exposed in a gravel pit near Touchet, WA. The caption reads, "Clastic dikes in Touchet Beds and dust dune between Touchet and Walla Walla". This dike is sourced in light colored slackwater sediments overlying the darker sand. Source: Washington Geological Survey Archives (#00604)

Touchet Beds in Burlingame Canyon. Many geologists have visited this classic locality near Lowden, WA. About 40 Touchet Beds are exposed here (Waitt, 1980, 1985). These rhythmically-bedded flood deposits are widely distributed across Columbia Basin of WA, OR, and ID. Elsewhere, slackwater deposits occur in thinner packages with greater sedimentologic variability, reflecting the local flood/backflood/ponding conditions and valley configurations. Measurements on clastic dikes was collected from all of the well known sites documented in numerous field guides and research articles, including Burlingame Canyon, and from hundreds of others not previously described. Photo source: Washington Geological Survey Archives (1978, #3455). See Footnote 5.

Publications on clastic dikes. Ninety-four articles, abstracts, field guides, agency gray lit, and consultant reports have been published on the clastic dikes in Eastern Washington. Most simply mention the dikes in passing. Few contain measurements or data of any kind. Many of the Hanford articles derive from a single infiltration experiment conducted on one large dike located off Army Loop Road.

Way out west. Comparison of the Columbia Basin (downward-injected, wedge-shaped dikes) to the New Madrid Seismic Zone (upward-vented sand blows). Columbia Basin dikes also occur in the Columbia Gorge downstream of Wallula Gap and in Willamette Valley as far south as Salem, OR (areas outside oval). Several USGS careers have been dedicated to studying liquefaction features in the New Madrid region (Fuller, 1912; Obermeier and others, 2005). No careers have been dedicated to clastic dikes in the larger Columbia Basin. Relief basemap by USGS.

Seismic hazard map. Earthquake hazard probability based on 2018 USGS models (fault-slip rates, frequency, magnitude). Red-orange indicates a high probability for damaging quakes. Green-blue indicates a low probability. Note the difference between the Columbia Basin (Low: green-yellow) the New Madrid Fault Zone (High: dark red-red-orange).

Shaking intensity map for the 1949 Puget Sound earthquake (M6.7, VIII). Note the hit-and-miss nature of felt reports and variability in shaking intensity pattern east of the Cascade Range (I to IV). Slackwater basins show no evidence of amplification. Figure from Chleborad and Schuster, 1998 - USGS PP 1560).

Seismic exposure comparison. Seismic exposure chart for population centers in the United States. Curves show peak accelerations corresponding to a 10% probability of occurrence for the indicated exposure times (Algermissen, 1988; Rogers and others, 1998). Are major faults at New Madrid, Wasatch Front, coastal California, or Puget Sound analogous in any way to those in the Yakima Fold Belt of Eastern Washington? Fundamental differences exist between the bedrock, structure, surficial sediment, climate, and hydrology, not to mention the geometry, character, and distribution of clastic dikes preserved in unconsolidated sediments.

Basalt province boundary or floodway boundary? Sheeted clastic dikes are found in sediments overlying thick Miocene basalts in the Columbia Basin (Pasco Basin, Yakima Fold Belt, Palouse, Willamette Valley Subprovinces), but not in sediments overlying thinner basalts of the Blue Mountains Subprovince and Idaho-Nevada Graben. Subprovinces map modified from Tolan and others (2009, Figure 1).

Previous Work on Clastic Dikes

Clastic dikes in Missoula flood deposits are noted in classic papers on Channeled Scablands geology (Bretz, 1929 field notes; Baker, 1973; Waitt, 1985; Smith, 1993; Atwater, 1986), however, reports containing detailed descriptions of the dikes are few (Jenkins, 1925; Lupher, 1944; Black, 1979; Woodward-Clyde Associates, 1981), and those containing field measurements are downright rare (Alwin and Scott, 1970; Cooley and others, 1996; Neill and others, 1997; Ward and others, 2006). Several articles speculate on dike origin, but contain little to no data (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). Eighty years of reporting on Pleistocene deposits at the U.S. Department of Energy's Hanford Site (1,518 km2) has provided no clarity on the dikes’ origin. The voluminous Hanford literature contains few field measurements and lacks a regional perspective. It is rife with speculation, much of it contradictory (i.e., Bjornstad, 1980; Bjornstad and others, 1990, 2001; Bjornstad and Teel, 1993; Fecht and others, 1999; Bjornstad, 2006, Bjornstad and Lanigan, 2007). See Footnote 1.

Proposed Origins

Four origins for the dikes have been proposed: earthquakes (Jenkins, 1925), ground ice (Alwin and Scott, 1970; Black, 1979), desiccation (Lupher, 1944), and floodwater loading (Brown and Brown, 1962; Baker, 1973). A dubious fifth, “multigenetic” (Black, 1979; Fecht and others, 1999), suggests the dikes formed by a combination of these processes. Cooley (2015) provides a concise summary of the arguments for and against each hypothesis.

This Study

I investigated unconsolidated, partially-lithified sediments, and flood-scoured bedrock along the path of the Ice Age megafloods between Priest River, ID and The Dalles, OR. Clastic dikes with vertically sheeted fills were identified in 282 of 529 exposures (as of April 2022; more found since). Locations where soft sediment deformation was abundant were also mapped. Sheeted clastic dikes are identical throughout the study area with respect to sedimentology, age, structure, taper direction, and scale. All appear to have formed by the same mechanism during the Pleistocene.

Future Work

My field data collection effort could be repeated if the outcrops remain. I take excellent field notes and maintain an organized archive of information on outcrops I have visit. If a future geologist is interested in revisiting sites at which I collected measurements, they would complete the task much quicker since a map now exists to guide them. Locating and accessing dozens of new (previously unreported) outcrops throughout an unusually large study area took an enormous amount of time and effort. I have spent hundreds of days in the field and personally funded all costs associated with this project. If you are a student or geologist interested in conducting your own project on clastic dikes, please feel free to contact me. Happy to help.

Study sites. Locations with sheeted clastic dikes are shown as black circles. White circles denote locations where no dikes were observed. Stars denote locations with abundant soft sediment deformation with or without dikes. Star symbols are offset from outcrop circles for clarity. A dashed line approximates the Cordilleran Ice Sheet margin at LGM. Glacial Lake Columbia is shown at its 600 m elevation shoreline. Sediments observed at all locations (n = 488 shown on map) consist primarily of unconsolidated Pleistocene-age flood and non-flood silts, sands, and gravels (Touchet Beds, Pasco Gravels, Palouse Loess) sometimes overlying Neogene sediments and bedrock (Ellensburg Fm, Ringold Fm, CRBs, sedimentary interbeds). Thick Holocene sections were surveyed for dikes as well. Light gray areas show the extent of ephemeral slackwater lakes Lewis, Condon, Latah, Upper Columbia, and Foster that formed behind bedrock or ice constrictions (black bars A,B,C,D,E). Dikes are common in southern part of the Channeled Scablands, where silty, sandy rhythmites were deposited. Gravel deposits, Palouse Loess, and lacustrine silts in the Glacial Lake Missoula basin contain few dikes. Dozens more sites have been surveyed since this map was published.

Deformation varies along the flood route. Various kinds of soft sediment deformation structures occur in the Channeled Scablands (light blue area). The particular style of deformation is governed by grain size, saturation, lithification/cementation - factors that determine how sediment will respond to stress (loading). Different flood facies express deformation differently (i.e., Touchet Beds vs. silt-clay varves vs. bar gravel) and local flood dynamics (flow depth, flow energy) play an important role (i.e., high energy channel vs. deep slackwater lake vs. lake-filled stilling basin). The dashed blue line follows Atwater's long profile between Glacial Lake Missoula and Wallula Gap (Atwater, 1987, Figure 2). Each map symbol represents one or more outcrops.

Typical Clastic Dikes

Clastic dikes are sediment-filled fractures found worldwide in deformed sediments from the Precambrian to the Holocene. Most clastic dikes in the sedimentary record are soft sediment deformation features (ductile deformation) and the products of liquefaction (fluid escape) triggered by seismic shaking. Strong shaking elevates pore fluid pressures in wet, unconsolidated sediment, causing it to mobilize and vent to the ground surface, forming sand blows. Therefore, most earthquake-generated clastic dikes have upward-tapering forms, contain massive sandy fills, and serve as feeder conduits to sand blows.

Sheeted Injectites in the Ice Age Floodway

The clastic dikes described here are different. They are slender, sheeted, wedge-shaped structures that were filled from the top. They formed by the forceful infilling of hydraulic fractures propagated downward into sedimentary and bedrock substrates during periods when glacial outburst floods inundated large portions of the Inland Northwest. Injection style and timing are consistent with overloading and deformation by catastrophic glacial outburst floods and deep, slow-draining slackwater lakes that accompanied them, >200 m deep in places.

Examples of typical sheeted clastic dikes and flute casts. (A) A typical clastic dike in slackwater rhythmites (Touchet Beds) exhibits the characteristic vertical sheeting composed of darker fill bands (sheets) separated by light-colored silt skin partitions. This example contains ~12 sheets and is filled with silty, sandy sediment closely resembling the host material. Umatilla Basin at Cecil, OR (Slackwater Lake Condon). (B) A typical dike in gravelly deposits is truncated at its top by a second floodbed. Its fill is crudely sheeted and lacks silt skins. Dikes in coarse-grained deposits tend to have lower length-to-width ratio than dikes in fine-grained sediments. Umatilla Basin at Willow Creek, OR (Slackwater Lake Condon). (C) Examples of flute casts that ornament the faces of silt skins. Upward-pointing noses are clear directional indicators. Sediment entered the fractures from the top. Quarters for scale. Walla Walla Valley, WA (Slackwater Lake Lewis). (D) A sheeted dike, filled with a mix of silty, sandy flood sediment (Late Pleistocene) and quartzite-rich gravel from the Ellensburg Fm eroded from local exposures by floodwaters, intrudes micaceous, oxidized fluvial sandstones (Miocene) at Snipes Mountain. The dike contains ~10 sheets. Hoe is 28 cm long. Emerald Road at Granger, WA (Slackwater Lake Lewis).

(a) Size - Sheeted dikes penetrate more than a dozen different geologic units, including Miocene basalt. The largest examples contain >100 vertical sheets (fill bands), are >2 m wide, and penetrate to depths >40 m. Typically, the dikes are <15 cm wide and contain fewer than a dozen sheets.

(b) Fills - Sedimentology of the fill material reflects the local geology. Dike fills contain a mix of basaltic and micaceous sandy sediment swept up and carried by the floods (suspended-load). Basaltic and non-basaltic material eroded from localities along the floodway comprised the bedload. Dikes at Snipes Mountain, WA (Yakima Valley), for example, contain conspicuous clasts from the Miocene–Pliocene Ellensburg Fm that cap the ridge. Dikes at Foster Creek (upper Columbia Valley) contain Miocene gruss shed from granitic highlands to the north. Dikes near Walla Walla Valley contain Touchet Bed sediment and some ice-rafted exotics (Jenkins, 1925; Cooley, 2015). In the southeastern Palouse, dikes filled with Touchet Bed sediment overprint older dikes containing weathered, oxidized fills of sand and silt (Cooley and others, 1996; Spencer and Jaffee, 2002; Bader and others, 2016).

(c) Sheeting and Growth - The dikes are conspicuously sheeted “composite” structures (sensu Hyashi, 1966). Vertical sheeting records incremental widening by repeated crack-and-fill cycles. Dike growth involved crack-and-fill cycling during single events (compound dikes) and reinjection over time (composite dikes). New fractures opened into and alongside older ones. Strong grain size contrasts between adjacent sheets are evidence of a variable and changing sediment source consistent with circulating bottom currents within floods.

Per descendum. Sheeted dikes and an assortment of sag structures disrupt Missoula Flood deposits exposed along Latah Creek at the Qualchan Golf Course near Spokane, WA.

(d) Polygonal Networks - Burned areas and bladed cutslopes expose dikes in plan view (i.e., Silver and Pogue, 2002). The dikes form polygonal networks. Plan view exposures reveal how dikes coalesce and intertwine as they lengthen. The most well-developed polygonal networks containing the largest dikes occur near the centers of low elevation basins inundated repeatedly by glacial outburst floods. Sheet counts are highest where many floods gathered - near Wallula Gap through which all floods flowed.

(e) Silt Skins - Thin silt partitions (silt skins) form the dike walls and separate vertical sheets of sediment (fills bands) inside the dikes. Silt skins form when pore water migrates laterally out of the saturated fill, through the fracture wall, and into the surrounding material. Skins between sheets indicate younger fills dewatered into older, drier fills (older fills are exposed to new leakoff) and into dry host sediment (unsaturated sediment). Fines are screened at fracture walls and accumulate in continuous layers (filter cake), sealing the fracture. Crack sealing begins almost immediately after the fill enters the fracture and progresses quickly. An analogous filter-screening process forms slurry walls in concrete-filled trench foundations used in heavy construction. Silt skins in study area dikes are 1–10 mm thick. New sheets crosscut older sheets and remove portions of their silt seals (erosive injection). Rip-ups of older fills, skins, and host material are common in crosscutting fills. Upward-pointing flute casts that ornament the faces of silt skins unambiguously record downward infill. Skins that line the outer walls of dikes are fluted only on their interior faces. Skins on outer walls indicate the host sediment was well-drained, ice-free, and above the water table (vadose zone) at the time of injection and probably during most subsequent crackings and fillings. Outer skin formation via leakoff appears to require unsaturated conditions and porous host material. Dikes that penetrate impermeable bedrock lack outer skin walls, but contain interior partitions. "Leakoff halos", a term I coin here for lightly cemented, slightly discolored zones of altered sediment that extend a few centimeters beyond the dike wall, are fairly common. They can only form in unsaturated host sediment. Leakoff halos (and sheeting) are less common in liquefaction dikes because the sediment which mobilizes to form such dikes and the sediment from which they emerge are equally saturated.

Dike propagation. The various parts of an advancing crack and near-simultaneous filling by sediment and dewatering. Modified from Phillips and others (2013).

Cemented leak-off halo. Slight cementation of the host sediment can occur just beyond the margin of some dikes. This halo is evidence of "leak-off", the diffusion of pore water out of the fill during dike injection and probably for a short time after fracture propagation and before filling halts. Hwy 24 near crest of Yakima Ridge.

Leak-off halos formed while you wait. The dark region surrounding this clay-rich dike is wet. There are three possible explanations. 1.) Pore water in the surrounding sand is moving along the margins of the less-permeable dike and emerging at the vertical cutface. The dike's shape is highly irregular in three dimensions. The wet, dark zone that surrounds the dike indicates ground water is moving both downward and laterally through the near vertical bluff. Cemented halos around dikes may be in part diagenetic, forming long after a dike is emplaced. 2.) Pore water moving through the sand is held more tightly in the halo region due to a slightly more silt mix there. Silt particles were deposited during and shortly after dike injection via leak-off (dewatering). Silty sand holds onto pore water longer than pure sand. This is the way Hanford folks think about clastic dikes that commonly appear wetter than the sediment that surrounds them. 3.) Dike remains an active conduit. Groundwater today moves faster through the dike and diffuses outward into the sand. Conduit behavior and leak-off in the unlified dike is ongoing, though the dike is >10,000 years old. This dike is composed of and intrudes Pleistocene-age glacial sediment. It is part of a deformed zone, continuous over a half mile of shoreline bluff, thought to have formed by fluidization and mass wasting (Nelson and others, 2003; Swanson, 2007?; Tucker 2015; Troost, 2016; Knight, 2019). Location is Double Bluff on Whidbey Island, WA. The exposure was not scraped prior to taking this photograph. My photo from Feb 2023.

Bulbous forms on outer walls. Leak-off, or the dewatering of dike fills, creates bulbous forms on the vertical outer walls of some dikes. The bulbous forms resemble load casts. Hwy 397 west of Finley, WA.

Leak-off lumpkins. The outer surface of a silt skin wall is often covered with structures that look very much like tiny load casts, a type of sole marking. These "lumpkins" are consistent with outward diffusion of pore water into drier surrounding sediment at the time the fracture formed and filled. Walla Walla Valley.

(f) Distribution - Sheeted dikes are widely distributed throughout the region inundated by Ice Age floods. Great distances separate outcrops containing dikes with identical characteristics. More than 500 km separates my northernmost site near Kettle Falls, WA from my southernmost at Salem, OR Thanks to Ian Madin and Ray Wells, who provided locations and photos of dikes near Portland and Salem. Sheeted dikes intruding Willamette Silt (Touchet Bed-equivalent deposits) were discovered in the basement of the Oregon Capitol Building during a recent remodel (Ray Wells, photos and written communication). Sheeted dikes exposed along the Willamette River and reported by Glenn (1965) are identical to those in Eastern Washington. Dikes do not occur in the Palouse Loess above the local elevation of maximum flooding (i.e., above ~366m in Pasco Basin), nor in unconsolidated sediments beyond the margins of Ice Age floodways. Dikes occur in close proximity to active Quaternary faults (i.e., Wallula fault zone) and at sites located >150 km from them. They are most abundant, well-formed, and best-exposed in high silt content flood rhythmites in protected backwater valleys adjacent to high-energy flood coulees. Silty, sandy rhythmites contain dikes that are commonly long and slender. Coarse sands and gravels (little silt) typically contain few dikes. Dikes sourced in flood gravels are crudely sheeted and stubby (low length-to-width ratio). Where slender sheeted dikes penetrate coarse-grained sediment (or bedrock) below, silty-sandy rhythmites always occur above. Slender, sheeted dikes are nearly always sourced in silty-sandy material.

(g) Age - Field relationships constrain the timing of dike injection to between ~1.8 Ma to ~14 ka, the period of ice sheet growth and scabland flooding in Eastern Washington (Easterbrook, 1994; Baker and others, 2016; Waitt and others, 2016). However, most injections occurred late, during the Missoula flood cycle (18–14 ka). The dikes penetrate all formations exposed to overland megafloods, including all variants of Late Wisconsin Missoula flood deposits, pre-Wisconsin scabland deposits (“ancient” flood gravels, silt diamicts, paleosols), calcrete-capped Pliocene–Pleistocene alluvium ("Cold Creek unit"), Pliocene Ringold Fm sediments, Miocene–Pliocene Ellensburg/Latah Fm sediments and interbeds, and several Miocene Columbia River Basalt flows. The dikes cut the Mount St. Helens Set S tephra (16 ka), but not the Mazama Ash (6.8 ka). No dikes were found in Holocene deposits during this study and I am aware of no other studies reporting sheeted dikes in Holocene alluvium. Sheeted dikes did not form prior to the Pleistocene or since.

Typical Touchet-type dike. Missoula flood rhythmites R1 through R7 deposited by separate floods are intruded by a typical Touchet-type clastic dike. The dike originates at the base of R7 (within the rhythmite stack) and descends through several underlying beds. It does not crosscut the entire the section, rather it formed during one flood (flood that deposited R7) in the middle of the Missoula flood period (18-14 ka). Other dikes descend from older and younger levels in the stack. The dike cuts a clean path through the host sediment and does not follow a rubble zone between laterally-translated blocks. Bedding contacts are not offset, tilted, or laterally spread across the dike. No low angle sliding surfaces are present in the outcrop. Both branches taper downward to a point. The sediment that fills the dike was not supplied by a liquefied sandy layer at depth (a bed below R1) as would be the case if it fed a sand blow (i.e., Fuller, 1912; Obermeier, 1998), rather the source of the dike is clear. It begins at the base of rhythmite R7. Widening at the top of the dike is a small sag, not the truncated edifice of a sand blow. Bedding at the top of the dike grades smoothly upward into R7. Flute casts on the interior faces of the dike's walls provide clear evidence of downward infilling. This example exhibits the characteristics typical of sheeted Touchet-type dikes found throughout the region. Location is Burlingame Canyon, WA.

Slender, wedge-shaped injections. My conceptual model for sheeted clastic dikes in the megaflood region developed from relationships observed in the field. This particular combination of geological and hydrological factors combined to facilitate a particular style of diking that only occured during Pleistocene overland flood events. Flood loads fractured the relatively dry, brittle substrate allowing wet, circulating sediment sourced at the base of the flood to immediately fill the fractures.

Conceptual sketches comparing liquefaction dikes to injection dikes. (A) The sketch illustrates differences between clastic dikes formed by liquefaction (sand blows, 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. "Feeder dikes" tap a buried source. Flood injectites are sediment-filled filled hydrofractures that propagate downward. The dikes are "fed" by sediment sourced in circulating bottom currents of glacial floods moving overland (megafloods). 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 episodically-aggrading flood sediments. 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 during which no injection occurred.

Dike abundance and geometry differs in slackwater rhythmites vs. flood gravels. (A) Dike forms differ because fine and coarse materials respond differently to perturbation (loading by floodwater). Grain size governs whether pore fluid pressures will build or disperse, and whether slender fractures or stubby collapses will form. Loaded sediment containing abundant silt tend to respond by fracturing (pore fluids conveyed in fractures). Sediments that lack silt tend to respond by matrix flow (pore fluids flushed through interconnected pores). (B) Dike injection appears to be primarily a slackwater phenomenon due to the necessary combination of silt deposition (low-velocity flows during megaflood events), prolonged loading (deep water), and preservation (low erosion). (C) High silt content rhythmites (Touchet Beds) contain abundant clastic dikes with high length-to-width ratios (slender dikes). Sparse dikes in gravelly flood deposits (channel, bar, and sheet flood gravels) are typically stubby and crudely sheeted.

Thick, dry, brittle vadose zones. Catastrophic overland floods imposed tremendous loads on the ground surface, causing it to fracture. Backflooded valleys accumulated thick piles of unconsolidated sediments. While aggrading flood sediments raised the ground surface, they do not appear to have raised the water table. Water table reset to pre-flood levels between floods. A dry vadose zone grew thicker with each flood. Flooded substrates took on a wet over dry over wet configuration. Brittle fracture in the dry vadose zone may have been necessary for slender, sheeted dikes to form. Valleys with dense networks of Touchet-type dikes also have thick vadose zones of sand and silt (minor gravel). Liquefaction dikes, rare in the Channeled Scablands, are shown in the cartoon for comparison.

Discrete deformation. The deformation associated with a dike is limited to the crack itself. The sediments immediately adjacent to the dikes in this photo are undeformed. Clear bedding contacts and delicate bedforms continue across each dike. Dikes are filled with Late Pleistocene Touchet Bed sediment and intrude fluvial sandstones of the Miocene Ellensburg Fm. Quartzite cobbles in the fills were reworked from cobbly Ellensburg nearby. Snipes Mountain, WA.

Discrete deformation. A sand-filled dike cuts cleanly across silt-sand rhythmites at Starbuck, WA.

Pleistocene dikes in Miocene basalt. Sheeted sedimentary dikes with unconsolidated fills, sourced from above, intrude Columbia River Basalt flows exposed at the surface during Ice Age flooding. The dikes exploit older weaknesses in the bedrock. A.) Weaver Pit in Walla Walla Valley, WA, B.) Hwy 12 in Lewiston Basin, ID, C.) Hwy 14 in Umatilla Basin, OR.

Liquefaction Evidence in Paleoseismic Trenches in Eastern Washington

Evidence of liquefaction in the study area is minimal. It would be a stretch to say sporadic. In the few instances where liquefaction in Pleistocene sediments have been reported, it expresses as small, irregular, anomalous bodies. Those reports are:

- Foundation Sciences (1980) reported finding features possibly attributable to liquefaction features at Finley Quarry near Pasco, WA (Wallula Fault Zone). The blobby, ambiguous features are small, unconnected to a source bed, and partially trend with bedding. Their seismic origin (Sherrod and others, 2016) is disputed (Coppersmith and others, 2014). At best liquefaction is confined to tight corridors along some Yakima Fold Belt structures.

- Ambiguous forms in Holocene sediment revealed in a USGS trench opened near the Wallula Fault Zone were interpreted as liquefaction features by ("SUK" trench; Angster and others, 2020). The linear feature targeted for trenching has since been revealed to be an old ranch road, not a fault scarp. No fault was discovered in the SUK trench. Touchet Beds beneath the "liquefied" loess were undeformed, a finding that casts doubt on USGS's interpretation. Read my review of the SUK trench site is here:

No liquefaction was reported in other paleoseismic trenches excavated across fault scarps in Eastern Washington:

- Burbank Fault trench near Yakima, WA (Bennett and others, 2016).

- Wenas Valley trench (Sherrod and others, 2013).

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

- Buroker Fault trench southeast of Walla Walla (Farooqui and Thoms, 1980, Figure 11).

- Lind Coulee Fault trenches at O'Sullivan Dam-lower Lind Coulee (GEI/West & Shaffer, 1988).

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

- Spencer Canyon trenche near Entiat, WA. The trenched scarp is believed to have formed during the 1872 Chelan quake (magnitude ~7).

- Kittitas Valley trench (Szeliga/CWU/USGS in progress 2023).

Paleoseismic trenches in south-central Washington. Fault trenching project locations. Base map from Lidke and others (2003). The Finley Quarry site (not shown) is located just south of Kennewick. The Spencer Canyon site (not shown) is located ~60km north of the Kittitas Valley site. Paleoseismic trenching has revealed no connection between the dikes and the movements of specific faults in the Yakima Fold Belt province. I review published trench logs for most of sites shown on the map at this post: Paleoseismic Trenches in Eastern Washington


In Depth: Gable Mountain Trenches The Gable Mountain trenches are particularly instructive. Several trenches were dug and a number of serious geologists inspected them. Golder Associates/Puget Sound Power and Light (Bingham and others, 1970 Plates 8, 9; Golder Associates/PSPL, 1982) opened several trenches across two thrust faults at Gable Mountain at the Hanford Site. The South Fault displaced by ~50' Miocene bedrock and the Rattlesnake Ridge sedimentary interbed that separates Pomona and Elephant Mountain flows. Overlying Hanford Fm sediments (Missoula flood deposits) were not displaced. The Central Fault displaced the interbed by 182', but overlying Hanford Fm by only 0.2' (Reidel and others, 1992, p. 43-44). No liquefaction was found in any trench. A single clastic dike, sourced from above in flood-laid Hanford gravel, descends into the fault breccia in Elephant Mountain basalt (Trench log GT-2 in Reidel and others, 1992, Figure 39, p. 45). No other dikes were noted in the 92'- long trench. Philip S. Justus, a geologist for the Nuclear Regulatory Commission, clearly links the dikes to flood deposits not faulting in a memo summarizing his findings at the Gable Mountain trenches (Justus, 1980).

  • Flood deposits on Gable Mountain bear a close resemblance to typical Missoula flood deposits.

  • Two distinct cycles of [Ice Age flood] deposition are present on the north side of Gable Mtn; possibly three on south side.

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

  • Clastic dikes associated with each overlying [flood] cycle are found in Trenches CD-8, G-2, and G-3.

  • The youngest clastic dikes originate from the base of the coarse upper unit of flood deposits which is bounded at the top by St. Helens S ash as found in Trenches CD-4 and G-1.

  • Clastic dikes in Trenches CD-5 and G-3 are displaced by shearing on the fault plane (CD-5) and in the hanging wall (G-3).

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

  • Shears, possibly associated with the thrust fault, appear to cross and slightly displace clastic dikes in the footwall in an area of Trench CD-6.

  • Clastic dikes along fault plane in Trench CD-6 have slickensides surfaces with strikes parallel to the dip of the fault.

  • Oriented slickensides in clastic dikes parallel to slickensides in gouge on fault breccia (Trench CD-5).

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

Key take aways from the Gable Mountain trenching project are a.) clastic dikes are typical Touchet-type dikes, b.) the dikes are sourced in surficial material (flood deposits) and descend into fault-fractured basalt below, c.) dikes are not particularly large or numerous near the Gable Mountain Fault, d.) the dikes are not liquefaction structures sourced from below and formed in response to shaking, but sediment-filled fractures that propagated along a zone of weak fault breccia, and e.) slickensides and offset dikes suggest some minor faulting occurred after flooding and diking at ~12,000 years.


In Depth: Lind Coulee Fault east of O'Sullivan Dam

Paleoseismic trenches opened at Lind Coulee (south bank of O'Sullivan Reservoir) by GEI/Michael West exposed a few clastic dikes intruding the shear zone of the Lind Coulee Fault, an eastern extension of the Frenchman Hills thrust. The fault is exposed in shoreline bluffs between Rd M SE bridge and the Lind Coulee West Trench Site. It places Roza basalt over Pleistocene loess.

Lind Coulee West Trench Site. Paleoseismic trenches at Lind Coulee were opened in the 1980s as part of a dam safety study. The U.S. Bureau of Reclamation operates the nearby O'Sullivan Dam located east of MarDon Resort just north of Drumheller Channels. The dam is not shown in the photo; it is located a few kilometers to the west. The dam impounds Lower Crab Creek, which west flows through Lind Coulee, forming the vast Potholes Lake. Hwy 262 crosses the dam.

Findings in the Lind Coulee West Trench are similar to those at Gable Mountain. Geologists observed Pleistocene dikes intruding gouge zone of the Frenchman Hills fault that offsets Quaternary sediments,

The Lind Coulee trench initially presented strong circumstantial evidence for fault displacement of [basalt, reverse-magnetized loess, shear zone-fault gouge, and late Pleistocene loess with a petrocalcic horizon]. 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 modify 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 determine 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 agree with earlier speculation by Woodward-Clyde Consultants (1981), namely that the 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), despite finding little supporting evidence in their trenches,

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) over younger sediments. There are several splays. Grolier and Bingham is first identified it in their draft and final reports (Grolier and Bingham, 1971; 1978 Figures 14, 23). West and Shaffer trenched it in the 1980s. A good, easily accessible vertical exposure remains. The photo above and Sketch A show Roza basalt shoved over alluvial sediments (likely Pliocene Ringold Fm), cemented loess, and weakly-cemented Palouse loess (likely late Wisconsin age). Sketches show the observable field relationships. The fault cuts rubbly Roza basalt, creating a thin white gouge zone. Beneath the gouge is a sliver of brown mudstone (hanging wall) which is underlain by cemented buff-colored loess with light band of caliche. Faint bedding in the loess and a parallel caliche are both steeply dipping to overturned directly beneath the fault. The shattered footwall Roza is brecciated and weathered above the fault and takes on a greenish-yellow hue. The rubbly basalt grades upward to competent basalt then spheroidally weathered basalt near the flow top. The flow top is also exposed along Hwy 262 just south of the trench site. Elsewhere along the fault, the gouge zone (10-20cm wide) includes boudin-like lenses of deformed dark and light brown mudstone, rock flour, or broken basalt. Investigations of the Lind Coulee Fault Trench site and longer traverses of shoreline bluffs in the area have yielded no evidence of liquefaction. Lind Coulee Fault is part of the larger Frenchman Hills structure, 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 project from the perspective of the client (Lefevre and O'Connell, 1987; Galster, 1987). References for the Lind Coulee Fault 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 the West and Shaffer (1988, Vol. 2) report in Feb 2023.


In Depth: Willamette Valley

The Missoula floods backflooded the Willamette Valley dozens of times, depositing gravels and silt-sand rhythmites across the floor of the valley to Eugene, some 170 km south of Portland. Ice-rafted erratics numbering in the hundreds are mapped throughout the Willamette Valley (Allison, 1935; Minervini and others, 2003) and areas upstream (Bretz, 1919). In general, most exposures of Pleistocene deposits in the valley contain few dikes if any. Exposure is not great. See historical photos and unit descriptions 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). Abandoned ideas about seawater incursion into Willamette Valley and Columbia Gorge are found in Condon (1871) and Bretz (1919).

River Bend section. Glenn (1965, Figures 3, 15) reported finding a few clastic dikes in Touchet Bed-equivalent flood rhythmites (Willamette Silt) in the Willamette River Valley.

Glenn (1965) found a few sheeted dikes cutting Missoula flood rhythmites (Willamette Silt) exposed along the Willamette River at River Bend and Irish Bend. Allison (1978, Figure 14) shows a clastic dike cutting a rhythmite section near St. Paul, OR. The dikes were found in highway excavations in flood deposits near Portland (Ian Madin, 2014 written communication/photos) and in exposed dirt walls in the basement of the Capital Building at Salem, OR (Ray Wells, written communication/photos). Thurber and Obermeier (1996, unpublished) reported finding 16 clastic dikes at 7 sites along the lower Calapooia River, a tributary to the Willamette River. The largest dike measured 5m long x 10cm wide. They interpreted the dikes to be earthquake-caused liquefaction features. Sims (2002) reviewed Thurber and Obermeier's report, finding their field data set too small to support their interpretations,

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.

Peterson and others (2014) describe sand dikes and sills (unsheeted fluid escape structures) intruding Holocene overbank muds at 8 sites along the lower Willamette River. They attibute the dikes to seismicity at the Cascadia margin. Obermeier and Dickenson (2000) describe "relict liquefaction features" in low shoreline bluffs of islands in the lower Columbia River between Astoria, OR (Marsh Island) and Kalama, WA (Bonneville Dam) and in cutbanks of 10 rivers east of the Cascade divide. The thickest dikes were 30 cm. The thickest sills were 5 cm. Authors attribute the dikes to lateral spreading, hydraulic fracturing, and surface oscillations (ground shattering and warping) during earthquakes (i.e., Cooley and others, 1996; Neill and others, 1997; Pogue, 1998). Obermeier and Dickenson is a consultant's that follows USGS and DOGAMI studies on evidence of Cascadia shaking along the lower Columbia River (Obermeier, 1993; Peterson and Madin, 1997; Atwater, 1994). The report contains some of the same information as Sims (2002) and Thurber and Obermeier (1996), but no photographs or sketches of clastic dikes or vented sand. Takada and Atwater (2004) describe soft sandy sediments in the lower Columbia River gorge deformed by magnitude 8-9 Cascadia earthquakes. The features they describe are wholly different than wedge-shaped, sheeted dikes in the Touchet Beds and the sediments that host them. Their dikes of fluidized sand are outnumbered by 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."

Identical clastic dikes occur exclusively inside the floodway, mostly in Missoula flood sediments of both the dry Columbia Basin and wetter Willamette Valley. The sheeted, wedge-shaped structures are restricted to a specific interval of time (Pleistocene) and sourced only in Missoula flood deposits. Fine grained sediments outside the floodway do not contain dikes. Touchet-type dikes (sheeted, slender, wedge-shaped structures) are readily distinguishable in the field from unsheeted, younger dikes formed by liquefaction. The field evidence from the Willamette Valley supports a common origin for all sheeted clastic dikes that occur in the Ice Age floodway: fluid-driven fracture triggered by repeated flood loading.


In Depth: Toppenish Ridge

Two gravel pits near Granger, WA are located in close proximity to the Toppenish Ridge Fault, an east-west trending Yakima Fold Belt structure known to be active. Miocene Ellensburg Fm conglomerates are exposed in both pits. Sheeted dikes occur at the Lower Pit (more distant from fault), but not the Upper Pit (nearer the fault).

At the lower pit (225-250 m elevation) several large, sheeted dikes sourced in Touchet Beds intrude downward into fluvial conglomerates of the Miocene Ellensburg Fm, mapped as Mc(e) and Mcg(e). The flat-lying Touchet Beds are inset into the older conglomerates. Here, both units are essentially flat-lying. At the upper pit (265-295 m elevation), the Ellensburg dips steeply south (>50 deg). Very few dikes were found in the tilted beds despite an abundance of sandy layers and their proximity to the Toppenish Ridge Fault, mapped <200m away.

Toppenish Ridge gravel pits near Granger, WA. In the Yakima Valley, Ellensburg sediments, deposited by the ancestral Columbia River and smaller streams draining the Cascades, 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 floodplain. Sheeted dikes occur in the lower pit (minimally deformed strata), but not the upper (steeply tilted strata). Touchet Beds occur in the lower pit, but not the upper. The town of Granger is a short drive to the east. Land owned by the Yakama Indian Reservation.

Upper pit at Toppenish Ridge. Steeply-dipping, partially lithified fluvial sediments contain almost no clastic dikes and little evidence of liquefaction despite abundant sandy interbeds (light colored). The Toppenish Ridge Fault, which the USGS identifies as an active structure, is mapped less than 200 m away (Schuster and others, 1994; Lidke and others, 2003; USGS Quaternary Fold and Fault Database for the United States, accessed Dec 2022).

Lower pit at Toppenish Ridge. Flat-lying Ellensburg Fm sediments are pentrated by a number of sheeted dikes sourced in the overlying Touchet Beds. Diking appears related to flooding and deposition of the Touchet Beds, not folding and faulting of the Ellensburg. The dikes do not rise from a liquefied source bed. They post-date deposition of the Ellensburg, post-date some of the tilting, and were filled from the top by Touchet Bed sediment.


Dikes in the Miocene-Pliocene Ringold Formation (Pasco Basin)

Sheeted clastic dikes are rare in deformed Miocene and Pliocene sediments exposed near Yakima Fold Belt faults. The vast majority of Ellensburg and Ringold Formation exposures in the region are free of clastic dikes or other features that might be called "seismites".

Thin dikes do exist in the Ringold and deserve mention. A sparse set of thin, mud-filled dikes intrudes certain fine-grained portions of the Ringold Fm (9.5-3.4 Ma). The vertical dikes are rarely thicker than a notebook or longer than a meter. They are sourced and entirely contained within Pliocene strata (upper Ringold) and are often associated with a hard white claystone (thickened tephra?). These dikes are limited to specific outcrops andhave not been studied in detail to date. I have also documented a few thin, short dikes in steeply-tilted rocks at three localities along the Saddle Mountains front and at other low-elevation locations in the vicinity where the sediments lie flat.

I interpret these as incidental features typical of sandy sedimentary deposits near fault zones worldwide. They do not form networks. Their fills are unsheeted. They are only locally present and confined to specific strata. They bear little resemblance to Touchet-type dikes; dikes in the Ringold formed by a different mechanism.

Small dikes in Ringold sediments at Saddle Mountains. A few small white dikes descend into a red, sandy alluvial fan gravel that overlies Elephant Mountain basalt. Nearly identical dikes in same strata occur at Smyrna Bench.

Small dikes in Ringold Fm. Small, single-fill dikes in a white claystone at Othello Canal. I examine these dikes in a YouTube video Pliocene Clastic Dikes at Othello Canal.

Columbia River Road. Dikes from above penetrate tens of meters into the Ringold Formation.

Small dikes in Ellensburg Fm in eastern Yakima Valley. A thin clastic dike, truncated at its top, cuts fluvial-lacustrine strata at Houghton Rd north of Sunnyside, WA. These quiet water sediments closely resemble those at Snipes Mountain (mapped as Miocene Ellensburg or undifferentiated Miocene) and at many locations in Pasco Basin (mapped as Pliocene Ringold). The contact between Ellensburg and Ringold is not well defined.

Do All Clastic Dikes Indicate Paleoseismicity?

Because clastic dikes are commonly found in deformed sediments in earthquake-prone regions of the world, data on dikes are often included in post-quake damage assessments. Standardized methods for measuring and mapping liquefaction features have been developed by USGS, state geological surveys, and consultants (Gohn and others, 1984; Atwater, 1994; Obermeier, 1996, 2009; Peterson and Madin, 1998; McCalpin, 2009; Holtzer and others, 2011). Field measurements including dike width, length, distance from epicenter, etc. can help delineate the spatial extent of strong shaking (i.e., shake intensity maps). The value of such maps largely depends on the size of the dataset. A small number of measurements or measurements collected within a small area (i.e., a trench) have low value because they lack statistical power and may under-represent the wider effects of shaking.

Misinterpretation of features and field relationships can also be a problem, especially for inexperienced staff or where freatures are poorly exposure. In the absence of abundant, quality exposures in an unfamiliar region, investigators should be especially aware of their priors. The assumption that all clastic dikes form by liquefaction and are earthquake-caused, has led some to incorrectly interpret certain clastic dikes are seismites.

In 2017, an international conference was convened to coordinate proper reporting on seismites in sedimentary sequences. The conference emphasized the need for caution (Feng, 2017). It seems “seismite” (Seilacher, 1969; Montenat and others, 2007; Van Loon, 2014) has for some time been applied too liberally in journal articles to features of nonseismic or ambiguous origin, making reexamination of many "classic" seismite localities necessary. Participating geoscientists reattributed many "classic" features to nonseismic triggers, most commonly to rapid sedimentation and overloading (Moretti and Van Loon, 2014; Shanmugam, 2016 and references therein). The following three quotes capture the feelings of participants:

“Nonseismic events can create structures that are virtually indistinguishable from seismically- eformed 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

J Harlan Bretz on clastic dikes. Bretz did not pay much attention to the dikes, but does mention them from time to time. On August 7, 1928 at 'Gardena Cliffs' just south of Touchet, WA he notes, "Clastic dikes are prominent, some of them wedging out halfway down the cliff, some continuing to the bottom of the section...Wedges of the fine sand at angles of 40 degrees from horizontal penetrate or appear to penetrate up into the gravel for three or four feet" (1928 typed field notes, p. 27). On August 10, 1928 in the Walla Walla Valley, he reports, "Clastic dike of marvellous development abundant in these sections. But tho they contain the coarse black sand, they are not responsible for the pockets and lenses [ice rafted berg mounds containing non-basaltic clasts]. One clastic dike has seams of sand separated by seams of clay in the prevailing mode but seams of good brown sand" (1928 typed field notes, p. 34-35). On July 29, 1928 at Willow Creek, WA he finds, "fingers of the black sand extend from the gravel into the silt." (1928 typed field notes, p. 8). Dikes are again observed near Clarkson and Asotin in his 1928 field notes (p. 43). Bretz also describes aseismic deformation in the Touchet Beds in his typed field notes and in one article (August 14, 1923; Bretz, 1928b, Fig. 9 photo above; July 15, 1929). See Footnote 11.

Sheeted clastic dikes in the Channeled Scablands are Pleistocene phenomena. Sheeted dikes originate in Pleistocene deposits, including both “ancient” flood deposits (>35 ka, pre-Late Wisconsin) and younger Missoula Flood deposits (<35 ka, Late Wisconsin). The vertically sheeted, wedge-shaped structures number in the tens of thousands (a conservative estimate), are visually distinctive, and occur only within the boundaries of the Ice Age floodways. They penetrate to many meters depth, including to basalt bedrock. Various basalt flows and interbeds, where truncated by flood unconformities and/or overlain by flood, deposits also contain dikes - whatever flow is exposed to overland floods.

Maximum Width Method is Inappropriate for Composite (Sheeted) Clastic Dikes

Liquefaction-extent maps prepared in the wake of damaging earthquakes are based on point data collected in the field, specifically the widths and locations of surface rupture, clastic dikes, water spouts, sand boils, and related phenomena. Liquefaction dikes in the subsurface feed sand boils erupting at the surface. If cross section exposures are available, the width of the widest feeder dike can be recorded at each site and the point data contoured or otherwise summarized cartographically. The “maximum width method” is based on the notion that seismic shaking is most intense near an epicenter and drops off with distance away as energy attenuates. Larger dikes are predicted near the epicenter where ground acceleration is greater, shaking more intense, higher pore pressures in wet sediment were generated, and more lateral spreading occurred. Relationships between maximum width of liquefaction-type feeder dikes and shaking intensity are well established (Ambraseys, 1991; Galli, 2000; Zhong and others, 2022).

The most well known example of liquefaction dike mapping is the New Madrid Seismic Zone located in the lower Mississippi River Valley (Fuller, 1912; Boyd and Schumm, 1995; Obermeier and others, 2005). Magnitude 7.2–8.2 quakes with Modified Mercalli Intensities >VIII struck the region in 1811-1812, toppling structures, disrupting transportation networks, and changing local hydrology. Two earlier events are now recognized as well. Sand blows vented wet sediment to the surface over hundreds of square kilometers. Remnants of surface-vented sand are still visible on aerial photos. Obermeier (1998), using data from feeder dikes exposed in creek cutbanks, successfully demonstrated liquefaction features can be used to determine the approximate location of a paleoepicenter.

Obermeier's mapping of liquefaction features (sand blows) in the New Madrid Seismic Zone. Black circles correspond to the maximum widths of sand blow feeder dikes produced by earthquakes in the lower Mississippi Valley near the confluences of the Wabash and Ohio Rivers (Obermeier, 1998). Dashed ovals approximate the damage halos of past quakes. Obermeier recognizes three size categories: <15 cm, 15-50 cm, >50 cm.

But the USGS's methodology for measuring sand blow feeder dikes (liquefaction dikes) is inappropriate for sheeted, downward-pinching clastic dikes (Columbia Basin injectites). The maximum width method assumes dikes are single-fill structures formed during single earthquakes; dike width scales with the strength of shaking. Width captures the total amount of lateral spreading that occurred during a shaking event. By contrast, the width of a sheeted dike reflects the total amount of widening, which may have occurred during multiple events separated by decades to millennia. Sheeted dikes grow incrementally by multiple injection/widening events that can be separated by long hiatuses. The two data sets describe two entirely different phenomena, likely different failure modes (hydraulic fracture vs. fluidized escape), different triggers, and different near-surface saturation levels (water table position). The appropriate measurement, one that provides an apples-to-apples comparison, would be to record the width of the widest feeder dike at a site (single-fill structures) vs. the width of the widest individual sheet in any dike at a site (sheeted structures).

Comparing Crust Beneath Columbia Basin vs. New Madrid The seismic potential of faults in the Columbia Basin (<M 7) and faults in the New Madrid Seismic Zone (>M 7) are not comparable. The New Madrid is an ancient failed rift in crystalline basement (deep, steep faults in strong crust). The Columbia Basin is a young back-arc flood basalt province resting atop Tertiary sedimentary-volcanic sequences formed outboard of the cratonic margin (shallow, low-angle compressional faults in young, weak material). Surficial sediments in the New Madrid region are Holocene floodplain deposits of maor rivers - the Mississippi, Ohio, and Wabash. The New Madrid is a wet, low-relief, alluvial plain and has been so since at least the late Pleistocene. The water table resides near the surface throughout the region where remnants of vented sand is found (Holocene sand blows). Surficial sediments in Columbia Basin, by contrast, are a mix of Ice Age megaflood deposits, late Pleistocene loess, alluvial fan deposits, and minor Holocene alluvium that is typically confined to narrow, low order stream valleys. Large rivers of the region flow mostly in deep, bedrock-confined channels and gorges. The water table lies at significant depth beneath a thick vadose zone in backflooded valleys containing Touchet Beds. In stark contrast to the fertile Mississippi Valley, the Channeled Scablands were an agricultural wasteland prior to the construction of Grand Coulee Dam and its vast network of irrigation canals. Even in the slightly wetter Palouse Hills, dryland wheat farming is supported by soils that lie well above local water tables; the water holding capacity of Palouse silt, not shallow groundwater, supports crops.

Undeformed Holocene alluvium. Thick sections of Holocene alluvium (>4m) like this in the Dry Creek Valley near Walla Walla show no evidence of strong seismic shaking (surface ruptures, liquefaction, sand blows, convoluted bedding, seismites) despite favorable grainsize distributions and saturation levels. Thickened Mazama ash, a constituent of Holocene loess, is conspicuous in dozens of roadcuts, railcuts, and cutbanks in the Walla Walla Valley. If present, convoluted bedding, fault offsets, and fluidization structures would have long ago been identified by local geologists, farmers, and soil scientists given the sheer number of excellent exposures and the strong visual contrast between the bright white ash and darker alluvium. Holocene deformation, if present, is not widespread in south-central Washington. It is anomalous. Photo location is the intersection of Harvey Shaw Rd and Dague Rd along Dry Creek ~8 km north of Walla Walla, WA. Mapped Quaternary faults nearby include Wallula Fault Zone (21 km away), Hite Fault (33 km away), Kooskooskie Fault (23 km away), and Promontory Point Fault (6 km away). Photographed in June 2021.

Undeformed late Pleistocene-Holocene alluvium. Thick alluvial fills along Willow Creek near LaCrosse, WA are undeformed.

Undeformed late Pleistocene deposits and Holocene alluvium. Ice Age flood, reworked colluvium, and lake deposits capped by younger alluvium and Mazama Ash are well exposed along Latah Creek west of Spokane, WA. Lake beds are prone to failure along the creek, but the section remains largely undeformed. I've seen nothing resembling continuously-deformed layers (i.e., seismites). Local folds (centimeter to meter scale) are occassionally encountered. These are rollups formed where coarse bedload gravels override finer grained sediments (high energy backflood flows). Accessible cutbank exposures away from noisy Hwy 195 (near Qualchan G.C.) can be found to the south along quieter Hangman Valley Rd (near Hangman Valley G.C.).

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 shaking (Lidke and others, 2003). However, sheeted clastic dikes sourced in Pleistocene sediments are found at distances far beyond the 100–125 km outer limit for soft sediment deformation established by Galli (2000) and Zhong and others (2022).

- An epicenter placed at Wallula Gap (Wallula Fault Zone) is located >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 in the Blue Mountains (Hite Fault) is >265 km from large dikes near Granger, WA in the western Yakima Valley.

- An epicenter placed near Arlington, OR (Arlington–Shutler Fault Zone) is >230 km from dikes in the central Willamette Valley, OR.

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

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

- An epicenter placed at Spencer Canyon near Entiat, WA (unnamed fault zone) is >210 km from large dikes at Touchet, WA.

Magnitude–distance curves. Curves from six large liquefaction studies were compiled by Galli (2000) from several continents and by Zhong and others (2022) using data from China. The Zhong study builds on Qiao and others (2017). They show a strong relationship between earthquake magnitude and the distance away from an epicenter at which liquefaction features should be expected to form. Faults in the study area are believed capable of generating quakes as large as magnitude 7.0, which corresponds with an epicentral distance limit of approximately 125-150 km. Many large dikes in the study area are found at distances greater than 125 km away from Yakima Fold Belt structures, the Hite fault, the Arlington–Shutler fault zone, and the Wallula fault zone. Figure redrawn from Galli (2000).

Sedimentary Evidence of Paleoseismicity East of the Cascades is Weak

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 a likely candidate(s). However, the dikes are distributed over too large an area for a single YFB fault to be the culprit. If one or more YFB structures (i.e., Saddle Mountains Fault) acted as the trigger, then a recurrent record of seismic shaking would be evident in nearby exposures of Miocene, Pliocene, Pleistocene, and Holocene strata, not just in Pleistocene strata within scabland floodways. Thick Holocene sections in particular, where evidence of strong shaking should be widespread and best preserved, are plentiful and accessible throughout the study area. Likewise, steeply-dipping and faulted Neogene sediments at Toppenish Ridge (i.e, Tule Rd gravel pit on the Yakama Reservation), at Saddle Mountains, at O'Sullivan Dam, and bluffs west of Sentinel Gap should host numerous dikes and other seismites. Measured sections through folded and flat-lying Neogene sediments alike (i.e., Lindsey and others, 1996; Staisch and others, 2021) should contain evidence of liquefaction. Seismites should be most plentiful in roadcuts, cutbanks, and railcuts near the largest mapped faults and should decrease in size and abundance with distance.

Saddle Mountains Fault in the Lower Crab Creek Valley. Several excellent exposures of sedimentary interbeds between tilted basalt flows (Elephant Mountain seen here) are found along the Saddle Mountains front. None contain sheeted clastic dikes. The thin, light-colored fractures in the photo at right are not liquefaction features, but bleached shear bands, structures common in deformed sandstones worldwide.

Study on interbeds. Ebinghaus et al. (2012, Fig. 5) examined Miocene sedimentary interbeds (Ellensburg Fm) at 14 sites in Pasco and Quincy Basins. Minor soft sediment deformation was noted at 3 sites. At Wagon Road, Wagon Road 2 (Moses Coulee), and Mabton (Yakima Valley), deformation was observed at contacts between thin siltstones-mudstones and overlying fluvial sands. The load casts and flame structures appear directly tied to this particular sedimentary environment: crevasse splays repeatedly spilling sand onto off-channel muds. No clastic dikes were reported.

Fault zone investigations have likewise failed to reveal a pattern of strong seismic shaking in Eastern Washington:

- The Stateline earthquake of 1936, centered in the Walla Walla Valley - the quake often used as evidence for strong shaking on OWL faults - was a sub-magnitude 6.0 event that formed no sheeted dikes and caused no damage to speak of beyond its epicenter, the tiny community of Umapine, OR.

- The Hite Fault, located in the Blue Mountains southeast of Walla Walla, appears no longer seismically active. I am aware of no reports of liquefaction associated with the Hite Fault.

- Strong shaking produced by a Puget Sound fault or by the Cascadia Subduction Zone are far-fetched explanations for the dikes in Eastern Washington. Shaking generated west of the Cascade divide, would be greatly attenuated before reaching the Columbia Basin (Peterson and others, 2011; Wood and others, 2014). No evidence of megathrust shaking at 1700 AD (Atwater and others, 2005) is recognized in Eastern Washington. Likewise, no evidence of the 1918 Vancouver Island M 7.2, 1946 Vancouver Island M 7.5, 1949 Olympia M 6.7, or 2001 Nisqually M 6.8 is known.

- Widespread liquefaction was not reported following the 1872 North Cascades earthquake (~M 7) and its many aftershocks (Milne, 1956; Sherrod and others, 2015; Brocher and others, 2018) despite vast quantities of silty-sandy glaciofluvial and glaciolacustrine sediments in terraces along the Columbia, Wenatchee, Methow, Okanogan, and Sanpoil Rivers. Stories about damaging earthquakes and news onother natural disasters came from local newpapermen (see Brocher and others, 2018, Appendix B), whose job it was then and is today to amplify the spectacle and sell papers. Their reports should be taken with a grain of salt. The 1872 quake caused water spouts, ground cracks, landslides, and collapsed cabin roofs according to reports (Washington Stadard Newspaper 11 Jan 1873; Coombs and others, 1976). Let's put those reports in historical context. Wenatchee in 1872 was in every way a frontier town. Residents - all 100 of them - were 20 years away from their town being platted. The light bulb and the telephone had not yet been invented. Ulysses S. Grant was President. Washington, Idaho, Colorado, Wyoming, Utah, New Mexico, and Arizona were not yet states. Chelan County didn't exist. Just 6 rudimentary seimographs monitored ground motion for the entire PNW region, including parts of British Columbia, until the mid-1960s.

- The notion that widespread liquefaction east of the Cascades could be attributed to large, distant quakes through a "bounce of seismic of energy from the Moho" (i.e., Obermeier, 1988, p. 250) is flatly rejected.

- A long history of strong seismic shaking has not been recognized in a.) hundreds of borehole cores logged at the Hanford Site, b.) in dozens of measured sections at White Bluffs by several geologists, c.) in cores from alpine lakes in the Cascades and Okanogan Highlands, d.) in ODP cores off WA and OR, e.) in thick Ellensburg/Thorp/Latah Fm sections in Kittitas, Yakima, and Naches Valleys, f.) in Neogene sediments in the Dalles-Umatilla syncline, and g.) in thick and thin sedimentary interbeds in the CRBs across the region. A strong paleoseismic signal remains largely unrecognized despite more than a century of geological investigation by USGS, USBOR, Washington Geological Survey, various mapping crews, university researchers, graduate students, and others.

Evidence of strong shaking in central Washington is a hypothesis currently being tested through trenching and other investigations by USGS. Plans for future trenching are unclear, but I assume their work will continue. Reports are at best mixed regarding the actual threat Quaternary faults pose to citizens and infrastructure in the region. Some reports contain significant errors (i.e., Angster and others, 2020) or interpretations disputed by collaborators (Coppersmith and others, 2014). Much of the USGS's work is trustworthy, capably handled by seasoned field staff. Nevertheless, no association between YFB seismicity and sheeted clastic dikes has been established to date through trenching of more than a dozen faults or by any other method. Instead, non-seismic factors (large overland floods, rapid sedimenation) appear to control where, when, and how the dikes formed (see Footnote 6). It is entirely plausible that the YFB ridges rose one M5.9 quake at a time.

Lost exposure. About 15 Touchet Beds overlie a thick bar gravel in the Snake River canyon near Lyons Ferry. John Whitmer photo (WGS Archive #03144).

Nonseismic Analogs to Columbia Basin Dikes

Examples of sheeted, per descendum clastic dikes that closely resemble those in Columbia Basin. In all cases, overloading, rapid sedimentation, 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) and in sediments of alpine glacial lakes (Sutherland and others, 2022). Sheeted dikes intrude lahar deposits on the side of an Aleutian volcano in Alaska (Herriott and others, 2014). Dikes formed by hydrofracture propagate downward and laterally out of marine turbidites into channel levees and distal deepwater fan-lobe complexes (Braccini and others, 2008; Cobain and others, 2016). Wedge-shaped sand dikes descend from the base of debris flow deposits into the underlying sandstone at Black Dragon Canyon in the San Rafael Swell, UT (Author's field notes,

Sheeted dikes at Mount Spurr, Alaska. Sheeted dikes with all the characteristics of Touchet-type dikes 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 involved. Image courtesy of Herriott.

Sheeted dikes in a glacial setting. Descending "laminated dikes" at Voss, Norway. Laminations formed by "a repetitive process operating during formation of these types of dikes" (Mangerud and Skreden, 1972; Larsen and Mangerud, 1992).

Field Work Matters

When working on clastic dikes in earthquake country, field projects should be appropriately scaled and employ methods matched to the geologic setting.

Clastic dikes in sedimentary sequences can be ambiguous They do not necessarily require earthquakes to form. In fact, sedimentary dikes are reported in a wide variety of geological settings where nontectonic triggers operate (Shanmugam, 2016). Lessons from California or the Mississippi Valley may not apply. Dikes are threshold features that, if interpreted one way, may brand a landscape as hazardous and unfit for human occupation and industrial development. Interpreted another way, the same dikes become Ice Age relicts of little importance to anyone other than academics and the odd (really odd) megaflood enthusiast. When anchored by evidence gathered at the outcrop, investigations into the origin of clastic dikes tilt toward a correct interpretation. Lab work, office-generated theories, and probability models serve the geoscience community and society best when they are rooted in and remain subordinate to field data.

Careful, comprehensive field work that involves a sufficient number of observations (measurements, samples) and a study area scaled to the geological phenomenon under investigation should be de rigueur. Slothful reporting on clastic dikes and other features highlighted by Shanmugan (2015), shoddy work, and methods successfully developed in one region, but uncritically applied to another (i.e., maximum width method on sheeted dikes) are unacceptable practices.

Project planning is the responsibility of the paleoseismologist. Paleoseismology is primarily a field-based discipline focused on determining the timing and effects of prehistoric earthquakes. 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 collected in the field inform and often drive policymaking. Unlike technical trench logs and tables of recurrence probabilities, maps constructed from field data are easily understood by technical and non-technical audiences alike. They are uniquely influential and readily migrated into land use policy documents, which tend to persist for decades.

Distances from an assumed epicenter (Wallula Gap) and dike-bearing outcrops in the Channeled Scablands. Distances from outcrops containing clastic dikes (circles) measured from an assumed epicenter at Wallula Gap (Wallula Fault Zone). Most dikes occur within 150 km of the assumed epicenter, but many occur at distances far beyond limits established by Galli (2000) for earthquake-caused liquefaction. Black bars represent the boundaries of subbasins along the Ice Age floodway. Subbasin count, at bottom of figure, is a proxy for outcrops availability. Outcrops are more numerous at shorter distances because most slackwater basins and deposits are located near Wallula Gap. Also, roadcuts, railcuts, and streamcuts in slackwater deposits are, in general, larger and more numerous in southeastern Washington, where several rivers converge. Dikes are most abundant in exposures immediately upstream and downstream of Wallula Gap, though large dikes commonly occur in 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.

Same dikes hundreds of kilometers apart. One of several large Touchet-type clastic dikes in the Upper Columbia River region (part of Ice Age floodway), more than 285 km north of Wallula Fault Zone. Lake Roosevelt near Kettle Falls, WA.

Key Characteristics of Clastic Dikes Assessable in the Field

Three key physical characteristics of clastic dikes, (a) vertical sheeting, (b) taper direction, and (c) truncation by bedding, speak directly to dike origin and are readily assessable in the field.

(a) Vertical Sheeting - Sheeting is the result of repeated fracturing and sediment injection. Dikes in the study area grew in staccato fashion by filling of newly opened fractures by new pulses of sediment. Sheeting in dikes found elsewhere likely records a similar pattern of reinjection along preexisting weaknesses. New sheets erode older ones; rip-ups derived from adjacent bands should be present in abundance. Small packages of parallel sheets, typically up to six or so in the case of study area dikes, commonly crosscut older packages and reveal variations in the way fractures propagate. If new sheets tap progressively younger source beds, small collapses should be present above fractures opening below. At some of my study sites, multiple collapses appear in successive rhythmites, each initiating a dike. Brief periods of dike widening alternate with longer periods of inactivity (hiatus), a pattern that tracks with cyclic filling and spilling by proglacial lakes.

(b) Taper Direction - Taper direction is strongly tied to dike origin and, whether upward or downward, must be assessed correctly, even when exposure is limited. Taper direction reveals how fractures propagate, the mode of fracture, and the orientation of the pressure gradient. Taper often reveals the source of dike fills. Upward-tapering dikes indicate a buried sediment source and correspond with upward fluid escape under a normal pressure gradient. Fluid pressure in the mobilized bed exceeds the confining stress of the overburden. Downward-tapering dikes indicate injection from the surfacewithin an inverted pressure gradient. Sediment sourced at the surface fills descending dikes. Downward-tapering (per descendum) dikes have few triggers. Where formed, the local stratigraphy will commonly contain low-pressure zones, such as openwork gravels or layers of coarse sand with abundant pore space. Such layers are resistant to compaction and too dry to sustain fluid pressure, thus may provide low-resistance pathways that mimic unconfined layers located near the ground surface. Coarse layers, commonly the basal portions of rhythmites, appear to act as efficient pathways that control how fluid-driven fractures propagate in flood-loaded strata.

(c) Truncations - Understanding how dikes are truncated is a key to understanding how many dikes formed. Truncation by bedding contacts or other surfaces is common in Touchet Bed rhythmite sections. Truncations are not just another characteristic in a checklist. Dikes with bedding-truncated tops connect the timing of deposition to the timing of injection. Tops truncated by multiple bedding contacts in a stack of rhythmites (or other strata) are powerful evidence for repeated deformation, reinjection, and dike growth over time.

Flood counts and the development of vertical sheeting in the dikes. Stacks of rhythmites (Touchet Beds) deposited by Pleistocene megafloods accumulated to different thicknesses in different parts of the Channeled Scabland. Rhythmite counts vary depending on location. The most complete rhythmite sections occur in slackwater basins repeatedly filled by Lake Lewis, Lake Condon, and Lake Allison. 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). 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). Glacial Lake Columbia filled the Sanpoil Valley (Atwater, 1986), Upper Columbia Valley (Kiver and Stradling, 1982; Hanson and Clague, 2012), Latah Creek Valley (Rigby, 1982; Kiver and Stradling, 1982; Waitt, 1983; Meyer, 1999), and Foster Coulee (Russell, 1893). Rhythmites also occur in the Glacial Priest Lake basin (Walker, 1967; Breckenridge, 1989). The rhythmite count at a site approximates the flood count. All floods flowed through Wallula Gap and slackwater lakes that ponded there were the deepest anywhere in the region (>200m). Consequently, the largest dikes occur in full rhythmite sections in the southern Pasco Basin, eastern Umatilla Basin, and western Walla Walla Valley. Fill band counts (sheet counts) record repeated flood-loading, substrate failure, and sediment injection. Sheet counts (injections) roughly scale with rhythmite counts (flood counts), though a one-sheet-per-flood pattern is not robust. The sheet count data suggest that up to about 10 sheets may form in a given dike during a flood. Local conditions seem to play a role(flow regime, water depth, valley configuration, grainsize, slackwater lake residence time, etc.). Because dikes widen by the addition of new sediment via reinjection, their widest portions occur near the base of rhythmite stacks (lower in the section) rather than near their tops (higher in the section). Dikes can appear to taper upward because newer fills that tapped successively younger flood beds intruded alongside older fills, forming composite injection dikes

Truncated dikes descend from multiple levels. Previous reports that describe dike-bearing sediments at sites located within the Ice Age floodway. Figures redrawn to facilitate an apple-to-apples comparison. Clastic dike geometry simplified for clarity. Not to scale. A.) Walla Walla Valley sites from Spencer and Jaffee (2002). B.) Lind Coulee Archaeological Site from Daugherty (1956). C.) Moxee Mammoth Site stratigraphy from Lillquist and others (2005). D.) Stratigraphy at Hanford's FMEF Site from Bjornstad and others (1990). E.) Rulo Site stratigraphy in northern Walla Walla Valley 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.

Silt-sealed Cracks Facilitate Hydrofracture

Sand-propped hydrofractures are commonly used in the petroleum industry to stimulate tight gas reservoirs ("fracking"). Hydrofracturing is induced by shutting in the well bore and using pumps to jack up the fluid pressure inside until the formation yields and fluid-driven fractures propagate outward. Pressurized proppant (sand + water + chemicals) immediately fills the fractures and holds them slightly open, permitting hydrocarbons to flow back to the well.

Silt skins appear to have facilitated hydrofracturing of substrates during megaflooding (rapid loading events). Loading by floodwater raised fluid pressure in the formation. Elevated pressure state that lasted for a period of seconds to minutes. Shallow natural weaknesses such as frost cracks, soil macropores, burrows, and joints provide nucleation planes for new fractures. During flooding, some of the weaknesses became fractures that opened a few centimeters and were rapidly filled by sediment. Dewatering (leakoff) forms a silt skin at the dike wall. The entering water-sediment slurry (natural proppant) was sourced from the base of the overriding flood. The skin-sealed crack behaved as a pressure vessel. Continued loading of the sealed fracture raised pore fluid pressures (Pf) inside. The point at which fluid pressure exceeded the confining strength of the formation (Pf > 03), breakout occurred, propagating the fracture tip, and a forming dike in the 01–02 plane (vertical). As the fluid pressure equilibrates to the confining pressure (Pf = 03), the fracture tip halts, the crack completely fills, and pressure begins to build again if flood load is still present. Each breakout causes a forward jump of the fracture tip and temporarily relieves fluid pressures in fractures (volume increase, pressure decrease). This load-crack-fill-seal cycle is responsible for the dikes’ vertically sheeted fabric.

The direction of fracture propagation appears controlled, in part, by the orientation of older sheets and bedding contacts (weakness planes), in part by the vertically-oriented water load, and in part by the orientation of the local fluid pressure gradient during flood loading (downward-tapering dike = inverted pressure gradient). In sand blow systems, by contrast, the pressure gradient is normal and decreases upward, toward the ground surface (free face). Sand blow feeder dikes are filled with sediment escaping from a fluidized layer at depth and pinch upward (normally-oriented pressure gradient).

Diking within a flood seems to occur at two different times. One, at initial onrush of floodwater. Two, during slackwater. Gravelly or sandy dikes are likely produced by the first. Silty-sandy dikes are likely produced by the second. Dike injection in the Touchet Beds appears to be primarily a slackwater phenomenon due to the necessary combination of silt deposition (lower-velocity flow), overloading (deep water), and off-channel preservation (low erosion).

Formation of sheeted dike fills. Fluid pressure-crack volume cycling at the scale of the fracture (nearfield scale) explains vertical sheeting in clastic dikes during a loading event. Time steps 1 through 12 in the pressure-time curve correspond with crack tip locations in the illustration below. During rapid overloading, dike growth (cracking and filling) corresponds with pressure-volume cycling where fluid pressure remains between the minimum and maximum principal stress values. Fluid-driven fracture is also responsible for producing slender, descending, sheeted dikes in other geologic settings where rapid overloading has occurred (lahar, glacier, debris flow, etc.). While the general principles of hydraulic fracture were well established in practice and in the literature prior to WWII, the development of sheeting in clastic dikes by hydraulic fracture, as I've illustrated here, is new and published here for the first time.

Sheeting and cyclic pressure-volume pulses. Another depiction of fluid-driven fracture and the growth of sheeting during a single flood (single hydrofracture event). The cross sections correspond to the gray shaded portion of the pressure-time curve. During the hydraulic fracture period, new fractures form and fill, but not all at once. Here I show 4 pulses corresponding to 4 episodes of diking, which manifest as new single-fill dikes (1a, 2b, 4a) or additional sheets (2a, 3abc, 4bc). Either way, we get rapid, yet incremental growth of the dike network (eg., total volume of the fracture network) during a flood-loading event. Each time a new fracture opens, sediment circulating at the base of the flood fills it. Floodwater loading is some combination of the inital overland flood, backfloods, and slackwater lakes. In most large exposures dikes descend from at least two flood beds. Repeated flooding and repeated diking is a consistent pattern observed in the Channeled Scablands.

Near field and far field fracture. I came up with this concept to explain how loads imposed over a broad area (far field flood load) relate to localized deformation (near field clastic dikes). The fracking/leak-off test framework captures most of the important elements. Leak-off begins once fractures begin to propagate, which is not well show in fracture propagation diagrams like this.

Relevant equations. Elastic Pressure (Pe) describes properties of the fractured material. Specifically, the stress perpendicular to a crack required to keep it open, where h is crack width, L is crack length, G is the shear modulus, and v is Poisson's ratio. Source Pressure (Pr) is fluid pressure of the source, considered here to be the pressure of the sediment-water slurry at base of a megaflood or slackwater lake, measured at the opening (top) of the crack. Source Pressure is assumed to be constant during diking. E is Young's modulus of the host sediment or rock, delta p is the density difference between host and the injected fill, g is gravity, Q is the volumetric flux of material injected into the crack, and u is the average injection velocity. Viscous Pressure Drop (Pv) is the pressure change along its length from crack opening to crack tip, where n is the slurry viscosity. Fracture Pressure (Pf) is the pressure required to propagate the crack tip forward, where Kc is the critical fracture toughness. Hydrostatic Pressure (buoyancy) is ignored for near-surface sedimentary dikes at atmospheric temperatures. The equations above are basic elements of undergraduate-level courses in Fracture Mechanics. Good YouTube channels for learning include Nicholas Espinoza, Scott Ramsay, Taylor Sparks, and others.

Evaluating Proposed Origins

Proposed origins can be evaluated based on data and observations collected from hundreds of dike-bearing outcrops within the floodway and dike-free formations beyond the floodway margin.

Origin Hypothesis #1: Desiccation - Little evidence supports a desiccation origin. Dike geometry, distribution, size, and internal sedimentary characteristics are fundamentally at odds with an origin involving the passive infilling of meters-deep, open-standing mudcracks.

Origin Hypothesis #2: Ground ice - Ice wedge casts are wedge-shaped bodies of sediment common to permafrost lowlands of North America, Europe, and Asia. Ice wedges grow by seasonal freeze-thaw action and coalesce to form conspicuous polygonal networks (plan view). Many display vertically-laminated fills (Lachenbruch, 1962; Romanovskiy, 1973). Their occurrence at middle latitudes is far less common (Horber, 1949; Dylik, 1966; Burbidge and others, 1988; Stone and Ashley, 1992; Demoulin, 1996). Alwin and Scott (1970), Lupher (1944), and Black (1979) interpreted clastic dikes in the Pasco Basin as fossil ice wedge casts based on their arrangement in polygonal networks, sheeted fills, and age. While the dikes in some ways resemble ice wedges of the modern Arctic, considerable evidence argues against a ground ice origin. The Columbia Basin is located at the transition between maritime and continental climate zones. Its average elevation is quite low. No periglacial evidence is know in the Blue Mountains of SE Washington. Cirque elevation contours project far above the crests of all Yakima Fold Belt ridges (Pierce, 2003, Fig. 1). The Basin lies significantly west of the relict permafrost zone in the Rocky Mountains (French, 2018). Small wedges that formed on exposed surfaces between drainings of Glacial Lake Missoula deposits in much colder areas of western Montana (Chambers, 1984; Chambers and Currey, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021) are fundamentally different than clastic dikes described in this report. Periglacial wedges akin to those found in the Lemhi Range of Idaho (Butler, 1984; D.R. Butler written communication), the Owl Cave-Wasden Site on the Snake River Plain (Dort, 1968; Butler, 1969), terrace gravels near Lewistown, MT (Schafer, 1949), and prairie soils east of Glacier National Park (unpublished photos and field notes by the author) are not known in Washington. Fully-developed fossil ice wedge networks in high-elevation basins of Wyoming (Grasso, 1979; Mears, 1981, 1987; Nissen and Mears, 1990; Munn and Spackman, 1991; Dillon and Sorenson, 2007) are unheard of in the Pacific Northwest.

Wedges in the Glacial Lake Missoula basin. Numerous small wedges like this descend from several horizons in Glacial Lake Missoula "varve" beds exposed at Rail Line (A,B), Jocko River (C), Crow Dam, and Garden Gulch sections (Chambers, 1984; Chambers and Currey, 1989; Levish, 1997; Hanson and others, 2012; Hanson, 2013; Smith, 2014, 2021). These structures formed during lowstand periods (shoalings) in the lake basin and resemble wedges commonly found in periglacial regions. A combination of desiccation and shallow ice wedging are likely.

The terminus of the Cordilleran Ice Sheet is well mapped across northern Washington (Porter and others, 1983; Atwater 1986; Cheney, 2016), but a corresponding periglacial zone remains loosely delineated. Murton (2020) delineates a conspicuously narrow permafrost zone south of the Okanogan Lobe; the southern limit of permafrost occupied essentially the same position. Abundant relict periglacial features are found in a 200 km-wide zone south of the Laurentide Ice Sheet (Pewe, 1983; Clark and Ciolkosz, 1988), but few such features are reported south of the Cordilleran Ice Sheet (Orme, 2002; French and Millar, 2013; French, 2017).

The landscape never supported a tundra plant community. Tundra biomes are identified by their lack of trees. Tundra plants are adapted to cold conditions, short growing seasons, and shallow rooting zones limited by shallow bedrock or permafrost. Pleistocene mammoth were nourished by steppe-grassland forage (Fry, 1969; Last and Barton, 2014). While pollen data from Columbia Basin lake cores indicate the presence of some cold-tolerant, low-growing species, the region never lost its tree component (Blinnikov and others, 2002; Whitlock and Brunelle, 2006).

Ice wedge forms. Numerous studies have been published on ice wedge and ice wedge casts in Arctic regions (Lachenbruch, 1962; Pewe, 1973; Romanovskij, 1973; Mears, 1987; Yershov, 1998; Bockheim, 2002; Murton, 2020). Columbia Basin was never glaciated and barely periglacial during the coldest parts of the Pleistocene. Its clastic dikes are not fossil ice wedge casts, despite their vertical sheeting and speculation by Lupher (1944), Alwin (1970), and Black (1979).

No mention of relict ground ice features, frost stirring, or gelifluction is made in NRCS Soil Surveys for the Colville Indian Reservation, Okanogan County (NRCS, 2010), Chelan County (USDA, 1975), Douglas County (NRCS, 2008), Grant County (USDA, 1984), or Lincoln County (USDA, 1981). Palouse loess and scabland deposits contain abundant phytoliths, rodent burrows, and insect burrows - features incompatible with perennial ground ice. Rodents recolonized the landscape between floods throughout the Pleistocene. Frost shattering in Columbia River Basalt, exposed over thousands of square kilometers, was not unusually intense (Pidwirny, 2006). Thick loess and mima mounds, often erroneously attributed to periglacial processes, are not diagnostic of periglacial conditions. Identical silt mounds are found from central Mexico to the High Arctic and certain mounds in south-central Washington are clearly Holocene age. Mima mounds indicate that its a windy place, little else.

Late Pleistocene conditions in the Columbia Basin were not periglacial as some suggest (i.e., O'Geen and Busacca, 2001). At its coldest, the landscape was "tundra-like" (Cooley, 2008) and perhaps best described as a "cold steppe" (Spencer and Knapp, 2010, p. 50) with sagebrush and forest refugia that supported species,

...found in alpine and sub-alpine valleys in the [present-day] Cascade Mountains of, moist, open-park conditions...consistent with the presence of continental ice to the north

Soil wedges east of Glacier National Park. These soil wedges formed under periglacial conditions of the Pleistocene east of the Continental Divide (465 km east of Grand Coulee Dam). Wedges like these help define a former ground ice region mapped by Murton (2020) and French (2017). Location is along Hwy 89 between the Two Medicine River and Badger Creek south of Browning, MT. Columbia Basin, though often miserable in January, never suffered Continental cold like this.

Origin Hypothesis #3: Lateral spreading - Lateral spreading requires liquefaction and lateral extension, typically involving blocks of material sliding sideways on a low-angle plane. The figure below shows three scenarios that assume wedge-shaped dikes and a gently sloping ground surface above a thick body of sediment, consistent with the settings in which dense networks of clastic dikes in Eastern Washington are found. The free face, formed by channel incision, accomodates the lateral extension and spreading. The effects of a free face are local; tension fractures form near the bluff edge (within 200m?). Several elements in the model are not present in outcrops. High-relief sediment-filled channels, which create the space required for spreading, are not found in the Touchet Beds. Valley floors, where the largest dikes are found, are broadly compressional in the horizontal plane, not extensional. The floors of slackwater basins are flat. Slopes appear insufficient to translate blocks of sandy sediment sideways, much less downhill. Slide blocks have no reason to form and nowhere to go. Any spreading that did occur was localized along linear bluffs (diking restricted to narrow swaths along former channels). They do not. There is no evidence of widespread liquefaction or block sliding on low-angle surfaces in the dozens of large rhythmite exposures I visited during this study. The dikes do not occupy rubbly zones between slide blocks.