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 vertically sheeted and have other 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, relative age, and association with syndepositional deformation 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 systematically map and measure large numbers of sheeted clastic dikes at a regional scale (>25,000 km2) in the Inland Pacific Northwest.

Keywords: megafloods, injectite, clastic dike, channeled scablands, missoula floods, washington geology, hydraulic fracture


Author's note - Here is an updated reprint of an article I published in Northwest Geology v. 49 in August 2020. Northwest Geology is the annual volume of the Tobacco Root Geological Society ( This web-based article contains new text, figures, and photos not included in the original printed version. Numbered figures are from the original article. I update this version from time to time as new information becomes available. First posted here 15 Sept 2020.

UPDATED: 29 Nov 2022


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.

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". 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). Rhythmically-bedded sediments like these are widely distributed across the Columbia Basin of WA, OR, and ID (the megaflood region). Elsewhere, slackwater deposits appear in thinner packages with greater sedimentologic variability, reflecting the local flood/backflood conditions and valley configurations. Data that forms the basis of this article was collected from all of the well known sites, including Burlingame Canyon, and from hundreds of others not previously described. Photo source: Washington Geological Survey Archives (1978, #3455). See Footnote 5.

Way out west. Comparison of the Columbia Basin (downward-injected clastic dikes) to the New Madrid Seismic Zone (upward-vented sand blows). Columbia Basin dikes also occur in the Willamette Valley as far south as Salem,OR. 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 map 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 (green-yellow) the New Madrid Fault Zone (dark red-red-orange).

Basalt province boundary or floodway boundary? Sheeted clastic dikes are found in sediments overlying thick Miocene basalts in the Columbia Basin (dark gray), but not in sediments overlying thinner basalt in the Blue Mountains Subprovince and Idaho-Nevada Graben. Basalt subprovinces map modified from Tolan and others (2009, Fig. 1).

Previous Work

Clastic dikes in Missoula flood deposits are noted in classic papers on Channeled Scablands geology (Bretz, 1929; Baker, 1973; Waitt, 1985; Smith, 1993; Atwater, 1986), however detailed descriptions of the dikes are few (Jenkins, 1925; Lupher, 1944; Black, 1979; Woodward-Clyde Associates, 1981), especially those containing field measurements (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). Seventy years of reporting on Pleistocene deposits at the U.S. Department of Energy's Hanford Site (1,518 km2) provides no clarity on the dikes’ origin. The voluminous Hanford literature contains few field measurements and generally 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 and partially-lithified sediments along the path of the Ice Age megafloods between Priest River, ID and The Dalles, OR (Figure 1). Clastic dikes with vertically sheeted fills were identified in 282 of 529 exposures (as of April 2022). Locations where soft sediment deformation was abundant were also recorded (starred locations on map). Dikes observed throughout the study area are identical with respect to sedimentology, age, structure, taper direction, and scale. All appear to have formed by the same mechanism during the Pleistocene. See Footnote 2.

Figure 1. Study area map.

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 soft sediment deformation structures, including clastic dikes, occur in the Channeled Scablands (light blue area). The dashed blue line is follows Atwater's long profile between Glacial Lake Missoula and Wallula Gap (Atwater, 1987, Fig. 2). 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). I surveyed >550 locations over 20 field seasons to produce this draft map. Each 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 (Figure 2). 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 (Figure 3). 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.

Figure 2. 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) Sedimentary Fill - Sedimentology of the fill material reflects the local geology. Dike fills contain a mix of micaceous sandy sediment carried by floods (suspended-load) and coarser bed-load material eroded from localities along the floodway. For example, dikes at Snipes Mountain, WA (Yakima Valley) contain clasts from the Miocene–Pliocene Ellensburg Fm. Dikes at Foster Creek (upper Columbia Valley) contain Miocene gruss shed from granitic highlands to the north. Dikes in the Walla Walla Valley contain Touchet Bed sediment and some exotics (Jenkins, 1925; Cooley, 2015). In places, dikes filled with Touchet Bed sediment overprint older dikes containing weathered, oxidized fills of sand and silt (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.

(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 (Figure 2c). 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.

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 filling halts. Hwy 24 near crest of Yakima Ridge.

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.

(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) (Figure 4). 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 (Figure 5), 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.

Figure 3. 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. Flood injectites are filled hydrofractures that propagate downward and are filled by sediment sourced in circulating bottom currents of glacial outburst floods (megafloods). Liquefaction dikes in the figure cut younger strata and are filled with older sediment. Injection dikes cut older strata and are filled with younger sediment. (B) Generations concept sketch explains the formation of sheeted clastic dikes in aggrading flood sediments. The four examples represent the range of forms found in the study area (single-fill, multi-fill compound, and multi-fill composite).

Figure 4. Dike abundance and geometry differs in slackwater rhythmites & 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.

Wet over dry over wet.

Overland floods imposed tremendous loads on the ground surface, causing it to fracture. In most places sediments compose the flooded substrate. Fracture and sediment injection into a dry, brittle vadose zone created the dikes. A wet over dry over wet configuration is necessary for Touchet-type dikes to form.

Pleistocene and Holocene Liquefaction?

Evidence of past liquefaction in the study area is minimal and sporadic. In the few instances where liquefaction has been reported, it occurs as small irregular bodies and appears confined to tight corridors along some Yakima Fold Belt structures. 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 and partially trend with bedding. Their seismic origin (Sherrod and others, 2016) is disputed (Coppersmith and others, 2014).

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

No liquefaction was not found in paleoseismic trenches across the Burbank Fault near Yakima, WA (USGS/Bennett and others, 2016) and in the nearby Wenas Valley (USGS/Sherrod and others, 2013), in trenches across the Saddle Mountains Fault at Smyrna Bench (Bingham and others., 1970 Plates 4,5,6), or in a trench through the Buroker Fault southeast of Walla Walla (Farooqui and Thoms, 1980, Fig. 11).

Smyrna Bench paleoseismic log for Trench #2, north flank Saddle Mountains. Conspicuous vertical features are loess-filled tension cracks (Bingham and others, 1970 Plate 6). They are not vertically-laminated clastic dikes nor products of liquefaction, but features formed by passive (gravity) infill of cracks formed by low-angle block sliding (mass wasting) in the Ringold Fm. Block sliding at Smyrna Bench is a local phenomenon, a product of its peculiar geology. Shear displacement is not associated with the loess-filled cracks, only tensile opening (Mode I). According to the trenching project geologist John Bingham, "In both trenches [3N and 3S], the fanglomerate is broken by the separation cracks similar to those in trench 2. Some of these are filled with loess; others contain fragments derived from the walls of the crack. Several of the cracks show some stratigraphic offset, but no gouge zones or slickensides were found." I labeled units in red text for clarity.

Smyrna Bench paleoseismic log for Trench #1, north flank Saddle Mountains. No clastic dikes or liquefaction evidence in loess, paleosol in old loess, or colluvium overlying sheared basalt. Sketch from Bingham et al. (1970 Plate 4) redrawn by me in 2022.

Finley Quarry (Wallula Fault Zone) paleoseismic investigation, north flank of Horse Heaven Hills.

No clastic dikes were found in the quarry exposure through a portion of the Wallula Fault Zone south of Pasco, WA. The exposure was investigated in the late 1970s by Foundation Sciences, Inc. Consultants (Foundation Sciences, 1980) and by several field geologists before. Miocene basalts, a Miocene sedimentary interbed, and pre-Missoula flooding colluvium are faulted.

Gable Mountain Fault paleoseismic log for Trench #3 East Wall, Hanford Site, WA. No liquefaction evidence or clastic dikes found.

Buroker Fault paleoseismic log for trench, Walla Walla Valley. No liquefaction evidence or clastic dikes found atop this minor fault located in the Russell Creek valley southeast of Walla Walla, WA. The fault was investigated in the late 1970s by Shannon & Wilson Consultants for Washington Public Power Supply System (WPPSS). This is one of seven faults they evaluated in the field. The Buroker Fault offsets Miocene basalt and oxidized Pleistocene loess. Fault offset of the tan Holocene loess (<11,000 years) is unlikely. Elevation of the site (1315', 400m) is above the maxiumum level of Lake Lewis (~366m).

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. No liquefaction was found in any trench. The South Fault displaced the Miocene rocks and the Rattlesnake Ridge sedimentary interbed that separates Pomona and Elephant Mountain flows was displaced ~50'. 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). 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 at Gable Mountain are typical Touchet-type dike common in the region, b.) the dikes are sourced in flood deposits and descend into fault-fractured basalt, c.) dikes are neither large nor numerous near the Gable Mountain Fault, d.) the dikes are not liquefaction structures formed in response to shaking, but sediment-filled fractures that propagated into and exploited fault breccia, and e.) minor faulting deformed some dikes.

Dikes in the Miocene-Pliocene Ringold Formation

A sparse set of thin, mud-filled dikes intrudes certain fine-grained portions of the Ringold Fm (9.5-3.4 Ma). The Ringold is best exposed in the Pasco and Othello Basins (Lindsey and others, 1996). The vertical dikes are rarely thicker than a notebook or longer than a meter. They are sourced and entirely contained in Pliocene strata (upper Ringold), are often associated with a hard white claystone (paleosol?), and have not been described in detail to date. I have documented a few thin, short dikes in steeply-tilted rocks at three localities along the Saddle Mountains front. I interpret these as incidental features typical of sedimentary units and fault zones worldwide. They do not form networks, are unsheeted, are only locally present, and bear little resemblance to Touchet-type dikes. The Ringold dikes 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.

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

Small dikes in Ellensburg Fm (possibly Ringold-equivalent sediments). Small dike, truncated at its top, cuts fluvial-lacustrine strata at Houghton Rd north of Sunnyside, WA.

Clastic Dikes as Paleoseismic Indicators

Because clastic dikes are so commonly found in deformed sediments in earthquake-prone regions of the world, data on dikes are often included in post-quake damage assessments and seismic hazard reports. Field measurements (width, length, distance from epicenter, etc.) are used to delineate the spatial extent of earthquake damage halos and to construct 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. Standardized methods for 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).

Misinterpretation of features and relationships observed in the field can be a problem, especially for inexperienced staff or where exposure is poor. In the absence of quality exposures, familiar explanations and faulty logic are too often employed and can derail an investigation. The misguided notion that all clastic dikes form by liquefaction, therefore all liquefaction features are earthquake-caused, has led some to incorrectly conclude all clastic dikes are seismites.

In 2017, an international conference was convened to coordinate proper reporting on seismites in sedimentary sequences (Seilacher, 1969; Montenat and others, 2007; Van Loon, 2014). The conference emphasized the need for caution (Feng, 2017). It seems “seismite” 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 of the features to nonseismic triggers, most commonly to rapid sedimentation and loading (Moretti and Van Loon, 2014; Shanmugam, 2016 and references therein). The following three quotes express 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

Figure 5. 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 margins of the Ice Age floodway. They penetrate to many meters depth, including to basalt bedrock. Various basalt flows are contain dikes - whatever flow is exposed to overland floods.

Maximum Width Method is Inappropriate for 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 clastic dikes or sand boils. Liquefaction dikes in the subsurface feed sand boils which erupt at the surface. If cross section exposures are available, the width of the widest dike is recorded at each site and the point data contoured or otherwise summarized cartographically. The “maximum width method” is based on the recognition that seismic shaking is most intense near an epicenter and becomes less intense as energy attenuates radially outward. It follows that large dikes will form in close proximity to the epicenter where greater ground accelerations were felt, higher pore pressures were generated, and more lateral spreading occurred. Relationships between maximum dike width and shaking intensity are well established (Ambraseys, 1991; Galli, 2000).

The most well known example of liquefaction feature mapping is the New Madrid Seismic Zone located in the 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 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. Obermeier (1998), using data from dike-fed sand blows, successfully demonstrated liquefaction features can be used to determine the approximate location of a paleoepicenter.

But the USGS's methodology for measuring sand blows (e.g., maximum width of liquefaction dikes) is inappropriate for sheeted, downward-pinching clastic dikes (sheeted 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 reflect two different fracture modes, and different geologic environments. The appropriate measurement, one that provides an apples-to-apples comparison, would be to record the width of the widest dike at a site (single-fill structures) vs. the width of the widest sheet in any dike at a site (sheeted structures).

Different Geology, Water Table, and Seismic Potential 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 old failed rift in crystalline basement (deep, steep faults in old, strong material). The Columbia Basin is a young back-arc flood basalt province resting atop sedimentary and volcanic sequence formed outboard of the cratonic margin (shallow, low-angle faults in young, weak material). Surficial sediments in the New Madrid region are Holocene floodplain deposits of the Mississippi River and several large tributaries. The New Madrid is a wet, low-relief, alluvial landscape and has been so since the late Pleistocene. The water table resides near the surface throughout the region where sand blows are found. Surficial sediments in Columbia Basin, by contrast, are a mix of Ice Age megaflood deposits, late Pleistocene loess, and minor Holocene alluvium in narrow, low order stream valleys. Large rivers of the region flow mostly in deep, bedrock-confined channels. In slackwater basins of the Channeled Scabland where sheeted dikes are numerous (Yakima, Walla Walla, Pasco, Lewiston, etc.), the water table lies at significant depth beneath a thick vadose zone. In stark contrast to the fertile Mississippi Valley, the scablands of eastern Washington were an agricultural wasteland prior to the building of Grand Coulee Dam and its vast network of irrigation canals. Even in the slightly wetter Palouse Hills, dryland wheat farming dominates (i.e., rainfall-reliant agriculture).

Undeformed Holocene alluvium. Thick sections of Holocene alluvium like this one in the Walla Walla Valley (>4m thick) show no evidence of strong seismic shaking (faults, liquefaction dikes, sand blows, convoluted bedding, other seismites) despite favorable grainsize distributions and saturation levels. Thickened Mazama ash, typically associated with young loess, is conspicuous in dozens of roadcuts, railcuts, and cutbanks in the Walla Walla Valley. 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, but 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), Kooskooskie Fault (23 km), and Promontory Point Fault (6 km). Photographed in June 2021.

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, the dikes in the Ice Age floodway are found at distances far beyond the 100–125 km outer limit for soft sediment deformation established by Galli (2000; Figure 6).

An epicenter placed at Wallula Gap (Wallula fault zone) is located >280 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 dikes at Kettle Falls, 207 km from Tammany Creek, ID, and 137 km from Cecil, OR, and 123 km from Bridgeport Hill Rd, WA.

Paleoseismic Evidence 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 (Figure 7). If one or more YFB structures (i.e., Saddle Mountains Fault) acted as the trigger, then a recurrent record of seismic shaking would be preserved in nearby Miocene, Pliocene, Pleistocene, and Holocene strata, not just 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 (i.e., Valley Grove Rd-Hwy 125 intersection near Walla Walla). Likewise, steeply-dipping and faulted Neogene sediments at Toppenish Ridge (i.e, Yakama Reservation gravel pit) and along the Saddle Mountains front (i.e., Smyrna-area sections by Staisch and others, 2021) should host many dikes and blows. Seismites should be plentiful in roadcuts, cutbanks, and railcuts near mapped faults. A radial decrease in the size of mapped liquefaction features away from the offending fault zone should be observable.

Other data should corroborate a seismic trigger in the YFB. But evidence of repeated strong shaking in the hundreds of borehole cores logged at the Hanford Site, in measured sections at White Bluffs, in cores from alpine lakes in the Cascades and Okanogan Highlands, in Missoula flood deposits in offshore ODP cores, in Ellensburg/Thorp/Latah Fm sections in Kittitas and Yakima Valleys, in Neogene sediments in the Dalles-Umatilla syncline, and in thick sedimentary interbeds in the CRBs remains unreported despite more than a century of geological investigation of the region's rocks and sediments by federal and state agencies, university researchers, and others.

The Stateline earthquake of 1937, 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 the tiny community of Umapine, OR, its epicenter.

Th Hite Fault, located in the Blue Mountains southeast of Walla Walla, appears no longer seismically active.

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) despite vast, nearby quantities of silty-sandy glaciofluvial and glaciolacustrine sediments in terraces along the Columbia, Wenatchee, Methow, Okanogan, and Sanpoil Rivers. Just 6 rudimentary seismometers recorded earthquakes in the Pacific Northwest prior to c. 1966. Pre-WWII reports of earthquakes came primarily from local newpapermen, whose job it was (is) to report on spectacle and sell newspapers.

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

Evidence of strong shaking in central Washington remains a hypothesis currently being tested through trenching by USGS. Their work is far from over. Early reports are 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). Others are disputed (Coppersmith and others, 2014). No association between YFB seismicity and sheeted clastic dikes has been established by trenching. Non-seismic factors appear to control where, when, and how the dikes formed. See Footnote 6.

Lost exposure.

About 15 Touchet Beds overlie a thick bar gravel with foresets in the Snake River canyon. John Whitmer photo (WGS Archive #03144).

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 (Figure 8).

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 produced by the first. Silty-sandy dikes are produced by the second. Dike injection 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).

Figure 6. Magnitude–distance curves.

Compiled curves from six studies by Galli (2000) show a similar relationship between earthquake magnitude and the distance away from an epicenter at which liquefaction features will form. Faults in the study area are believed capable of generating quakes as large as M6.5 (possibly up to M7.0), which corresponds with an epicentral distance limit of approximately 125 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).

Nonseismic Analogs to Columbia Basin Dikes

Per descendum clastic dikes that form by rapid overloading and hydrofracture are reported by others and appear to be in part analogous to the dikes described in this study. Sheeted, downward-pinching 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 with prominent skins 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 (Cooley, unpublished field notes and photos; In all cases, overloading, rapid sedimentation, and hydrofracture produced clastic dikes with characteristics remarkably similar to those in the Columbia Basin.

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 are often ambiguous structures that do not necessarily require earthquakes to form. In fact, dikes are widely reported in a variety of geological settings where nontectonic triggers operate (Shanmugam, 2016). Dikes are threshold features that, if interpreted one way, will 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.

Figure 7. Distances between dike-bearing outcrops and the Wallula fault zone.

Distances from outcrops containing clastic dikes (circles) were measured from an assumed epicenter located on the Wallula fault zone (Wallula Gap). 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. Outcrops within the 150 km limit are far more numerous than areas beyond and sediments there are thicker and more continuous. Black bars show the span of various subbasins along the Ice Age floodway. Dikes are most abundant in valleys immediately upstream and downstream of Wallula Gap, specifically Walla Walla Valley, Pasco Basin, Umatilla Basin, and Willow Creek Valley. All megafloods flowed through these basins and formed deep, slow-draining slackwater lakes in which thick sections of silty, sandy rhythmites were deposited.

CC = Crab Creek Valley, WA

GT = Gorge Tributary valleys downstream of Wallula Gap, WA-OR

LB = Lewiston Basin, ID

OK = Okanogan Valley, WA

PB = Pasco Basin, WA

RP = Rathdrum Prairie, WA

QB = Quincy Basin, WA

SR = Snake River Valley, WA

TV = Tucannon River Valley, WA

UB = Umatilla Basin, OR

UC = Upper Columbia River Valley, WA

WC = Willow Creek Valley, OR

WW = Walla Walla Valley, WA-OR

WV = Willamette Valley, OR

YV = Yakima Valley, WA

Key Characteristics 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.

Figure 8. 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; 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

Figure 9. Lateral spreading?

(A) Channel incision and creation of a free face removes support and facilitates lateral spreading. (B) Seismic shaking, liquefaction in a sandy layer at depth results in mobilization of in situ sediment and venting of a slurry to the surface (sand blows). Volcanic edifices of sand are formed. (C) A large vertical load imposed by a megaflood (or slackwater lake) increased pore fluid pressures in the substrate (sediment or rock), initiating hydraulic fracture. Fractures immediately fill with sediment sourced from circulating currents at the base of the flood (or newly-deposited lake bottom sediments). Fractures are propped by the sandy fill and become clastic dikes. Internal sheeting develops during single events and over time as new dikes merge with older ones. Repeated flooding creates composite clastic dikes (merged dikes sourced in floodbeds of different ages). The three scenarios above assume a near-level ground surface and thick sediment, which comports with known locations of large clastic dikes (basin centers). A free face cliff is required for spreading to occur, specifically because valley floors have so little slope; blocks of sediment have nowhere to go without one. Is there evidence of free faces (i.e., filled troughs) in rhythmite exposures containing large sheeted dikes? Is there evidence of widespread liquefaction? Is there evidence of block translation? Why are the largest dikes not found on the sloping sides of valley (areas more prone to sliding), but in flat valley bottoms?

Figure 10. Formation of sheeted dike fills.

Fluid pressure-crack volume cycling at the scale of the fracture (nearfield scale) seems to explain vertical sheeting in clastic dikes. 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.

Non-seismic triggers. Clastic dikes commonly form in response to rapid sedimentation and overloading. Dikes are widely documented in both seismically-active and low-seismicity areas alike. While earthquakes commonly trigger liquefaction and produce clastic dikes, liquefaction dikes are readily distinguished from dikes formed by other processes by their morphology, injection direction, distribution, and other characteristics assessable in the field. Not all clastic dikes are seismites. Figure modified from Shanmugam and others (2016, Fig. 16).


This study documents sheeted dikes throughout a vast inland region located far from the plate margin and swept repeatedly by Ice Age megafloods. Sedimentology, age, structure, and scale of the dikes indicate they all share a common origin: floodwater loading and hydrofracture. Hydrofracture and flood-triggered slumping was suspected by earlier workers (Baker, 1973; Pogue, 1998), but not tested. New field data supports both of their suspicions. I interpret the dikes as nontectonic structures. They are the products of fluid-driven fracturing of substrate materials triggered by forces generated by catastrophic overland flood. Dike widths reflect local flood counts and sheeting reflects crack-and-fill cycling during flood events (Figure 10). Reinjection occured during successive floods (Figure 8, 9). Field-observable characteristics distinguish the dikes from earthquake-triggered sand blows and related liquefaction structures in other regions (i.e., New Madrid Seismic Zone). Clastic dikes in Eastern Washington's megaflood region are flood injectites, not seismites.


Footnote *1* Bruce Bjornstad, a retired career Quaternary geologist at Hanford, has, more than any of his PNNL colleagues, mentioned the clastic dikes in his writing, beginning with his university work in the late 1970s and continuing with his recent guidebooks on scabland geology. The primary focus of his professional career was the hydrogeological behaviour of megaflood deposits at the Hanford Nuclear Site and, to a lesser extent, sediments of the Ringold Formation. Despite his authoring numerous agency reports, Bjornstad has never published original or peer reviewed work on clastic dikes. He is listed as a coauthor on Fecht and others (1999) - a mystifying publication - which I believe he compiled from Fecht's notes. The list of quotes below reveals his drifting opinion on the origin of the dikes through time. What accounts for these changes remains unclear. I consider Bjornstad and his former Hanford colleagues as casual observers of the dikes. Prior to 1996 few articles contained any data on the dikes.

a.) Bjornstad (1980)

"The assemblage of sedimentary structures within the Touchet Beds comparable to turbidites ...suggest periodic, rapid, subaqueous deposition of successive rhythmites by turbidity-like currents created by flood surges during a single flood. Additional evidence suggesting that flood surges rather than separate floods were responsible for rhythmite formation [includes]...the possible association relating clastic dikes with soft sediment deformation."

b.) Bjornstad (1990)

"These dikes are thought to represent dewatering structures that developed during compaction and settling of cataclysmic flood deposits during or soon after floodwaters drained from the Pasco Basin (Bergeron and others, 1987)." "Most clastic dikes, ubiquitous in flood deposits throughout the Pasco Basin, appear to have formed through forcible injection during waning stages of flooding (Black, 1979; WCC, 1981) during this time."

c.) Bjornstad and Teel (1993)

"In the Pasco Basin, clastic dikes are believed to be dewatering structures associated with lake draining following cataclysmic floods." d.) Bjornstad and others (2001)

"The dikes signify soft-sediment deformation during or soon after flooding, perhaps associated with flood-induced seismicity (Cooley and others, 1996; Fecht and others, 1999)."

e.) Bjornstad (2006)

"Clastic dikes formed during or soon after Ice Age flooding, perhaps because of ground shaking during earthquakes...If earthquakes occurred more frequently, we might expect to see more dikes in sequences of flood beds with truncations atop flood beds. But this is not the case..."

f.) Bjornstad and Lanigan (2007)

"Clastic dikes may be the result of ground shaking, which caused the wet sediments to liquefy and flow along paths of weakness down into or up along vertical earthquake-generated cracks in the flood deposits."

Footnote *2*

I have not submitted this manuscript to an academic journal. An earlier version was published in Northwest Geology v. 49 (Tobacco Root Geological Society 2020). Supporting their excellent annual field conference is far more important to me than whatever prestige is to be gained through publication in a traditional academic journal. Thousands have read this free online article as compared to dozens if published by some for-profit journal. I don't pay others to publish my work. The manuscript was reviewed by Mike Stickney, Director of Earthquake Studies Office at Montana Bureau of Mines and Geology and Jeff Lonn, Research Geologist also at Montana Bureau of Mines and Geology.

Footnote *3* In our work as Whitman College geology students (Cooley, 1996; Cooley and others, 1996) and follow on studies (Niell and others, 1997; Pogue, 1998), we imprecisely stated that the dikes penetrate from top to bottom through the entire stack of rhythmites, thus were late-flooding and/or post-flooding features. While many dikes do cut from top to bottom through the stack, their internal structure - their vertical sheeting - preserves a more nuanced history of incremental growth coincident with flooding. Vertical sheets of sediment that comprise large dikes (sheets = dikelets = fill bands), are often truncated at their tops by depositional contacts between rhythmites and surfaces within rhythmites that correspond to abrupt changes in flow regime (i.e., upvalley flow, slackwater, downvalley drainage). The dikes do not descend from the top of the rhythmite stack.

As geologists, we think of a "clastic dike" as a single structure, but sheeted dikes are actually compound (multiple parts) and composite (new parts addedover time) structures. While the "dike" may crosscut the exposure, each sheet (or packages of sheets) traverses only a portion of it. The dikes grew as single fills andsheet packages during single events, and by the addition of new sheets/packages through time, each sourced from a different rhythmite. Dike growth occurred in tandem with Ice Age flood cycles, which punctuated the Pleistocene. As students, we regularly observed truncated sheets (and entire dikes) and were somewhat puzzled by them. We routinely commented to one another about them, photographed them, and sketched various truncation relationships in our field books. We did not, however, fully recognize, much less emphasize the fundamental importance sheet/package truncation plays in dike growth. The dikes are composite structures that grew wider and deeper by reinjection during dozens of catastrophic glacial outburst flood events. Truncated sheets indicate the dikes are not single-event structures injected at the tail end of Missoula flooding (crosscutting features that post-date deposition of all or most floodbeds), as our clumsy early interpretations suggest. Rather, they are long-lived structures that grew in pulses during floods over time. Many large dikes grew by repeated sheet injection over thousands of years. Their growth recurrence interval is the flood interval.

Footnote *4*

The term injection has no directional implication. Injected material may have moved upward, downward, or sideways. Injection describes fracture-filling where wet or slurried material is mobilized and moves into fractured sediment or rock. The usage of injection and injectite in this article is consistent with the relevant geoscience literature (sediments and structure of petroleum reservoirs), not general textbooks on sedimentology and stratigraphy. For example, injection wells move water from the surface to the subsurface. Hydraulic injection involves the lateral propagation of fractures and proppant from the well bore into the formation. Fluidized injection is commonly used to describe both upward-pinching clastic dikes and dikes that were filled from the top.

Footnote *5* Burlingame Canyon is on private land is not accessible to the public without formal permission from landowners.

Footnote *6* In the course of my investigation of calcrete-bearing sediments of Plio-Pleistocene age near Othello, WA, I have not observed evidence of extensive soft sediment deformation consistent with strong, recurrent shaking in the two dozen sections I have described. I recently correlated 28 detailed stratigraphic columns from White Bluffs by Kevin Lindsey (Lindsey and others, 1996 Appendix A), finding no evidence of repeated, widespread shaking. Soft sediment deformation in Pleistocene sediments is plainly syn-depositional. Local deformation and some small, unsheeted fluidization structures in Ringold sediments are found along Saddle Mountains' frontal thrust. Trenching by Michael West and others decades ago documented young faulting higher on the mountain. Steve Reidel never mentioned seismites in reports on Yakima Fold Belt uplifts or in map unit descriptions accomanying his geologic maps maps. Paleoseismic trenches logged by USGS have not shown liquefaction to be widespread.

Footnote *7*

A package of fill bands injected during a single flood. Three fill bands comprise a composite clastic dike in the idealized example above. Each band formed at a slightly different time during a megaflood event. Each fracture opening corresponds with a slightly different flow regime, taps a slightly different stratigraphic level within a rhythmite as it forms, and accesses sediment of a different grainsize. Substitute different flood beds for stratigraphic position within a single bed to explain reinjected dikes in the region. At a larger scale, dike fills reflect the caliber of the sediment available to them. Grainsize in flood deposits is primarily determined by the local flow regime - high-energy channel, backflooded valley, slackwater lake. Since the configuration of most valleys and bedrock water gaps were not radically changed by flood erosion, successive floods produced more or less the same flow regimes and deposited the same grainsizes in the same places over and over. For example, the protected Touchet Valley received mostly medium to fine sand and silt. Dikes there are filled with the same. The Starbuck area, situated close to high-velocity coulees, received more gravelly sand. Dikes there are filled with coarser material. A page from one of my field books.

Footnote *8*

Figures, photos, and text have been added to this online version. This article superceeds the printed version (Cooley, 2020 in NORTHWEST GEOLOGY v. 49). Numbered figures are the same as in the printed article. I update this version from time to time as new information becomes available. This seems a modern way to work. I feel its appropriate to report results of ongoing research as it comes in rather than wait for some journal to publish it a year later.

Footnote *9*

If my work informs yours, you should cite this web-based article or the original print article (Cooley, 2020). What is presented here is new work and original work. It is entirely my own. Please include the date you accessed it in your citation.

Online version

Cooley, S.W., date accessed, Sheeted clastic dikes in the megaflood region, WA-OR-ID-MT,

Print version

Cooley, S.W., 2020, Sheeted clastic dikes in the megaflood region, WA-OR-ID-MT in Lonn, J; English, A.; McDonald, K.; Hargrave, P. (editors), Northwest Geology: Journal of the Tobacco Root Geological Society, 45th Annual Field Conference - Geology of the Bitterroot Region and Other Papers v. 49, p. 1-17



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