Sheeted Clastic Dikes in the Megaflood Region

Here is a reprint of a short article I published in Northwest Geology v. 49 in August 2020. I've made a few fixes that were not included in the original printed version due to editorial time constraints. Northwest Geology is an annual volume of the Tobacco Root Geological Society (www.trgs.org) that is prepared by a limited staff composed mostly of volunteers. Changes include minor grammatical corrections, four new footnotes, and improved figure captions. First posted here 15 Sept 2020.

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 him 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 DNR Archives (#00604)

Touchet Beds exposed in Burlingame Canyon. Many geologists have visited this classic locality near Lowden, WA in the Walla Walla Valley. About 40 Touchet Beds are exposed here. Less clear to the visiting geologist is that these same rhythmically-bedded sediments are widely distributed across the greater Columbia Basin region (the megaflood region). Elsewhere, these slackwater flood deposits tend to appear in thinner packages with greater sedimentologic variability, reflecting the local flow conditions. Data that forms the basis of this article was collected from well known sites, such Burlingame Canyon, and from hundreds of other sites, many of which have not previously been described (or even visited). Photo source: Washington DNR Archives (1978, #3455).

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

Skye W. Cooley

Abstract

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 262 of 488 exposures. 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.

Previous Work

Clastic dikes in megaflood deposits are noted in classic papers on the Channeled Scabland (Bretz, 1929; Baker, 1973; Waitt, 1985; Smith, 1993; Atwater, 1986), but reports containing 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 mention the dikes in passing and speculate on their origin, but contain little data (Flint, 1938; Newcomb, 1962; Bingham and Grolier, 1966; Jones and Deacon, 1966; Beaulieu, 1974; Carson and others, 1978; Shaw and others, 1999; Fecht and others, 1999; Pritchard and Cebula, 2016). Seventy years of reporting on Pleistocene deposits at the U.S. Department of Energy's Hanford waste storage site (1,518 km2) provides no clarity on the dikes’ origin. The Hanford literature collectively contains few field measurements and ample speculation, much of it contradictory with respect to origin (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.

Four origins for the dikes have been proposed: earthquakes (Jenkins, 1925), ground ice (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, Click for reprint) provides a concise summary of the arguments for and against each.

This Study

I investigated unconsolidated 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 262 of 488 exposures (54%). Sites 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. 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. 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) consist primarily of unconsolidated Pleistocene-age flood and non-flood silts, sands, and gravels with mixed Neogene sediments and bedrock exposed at a few locations. 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 Scabland, where silty, sandy rhythmites were deposited. Dikes are sparse in gravel-dominated deposits in the Channeled Scabland and absent from Palouse Loess and lacustrine silts of the Glacial Lake Missoula basin.

Typical Clastic Dikes

Clastic dikes are sediment-filled fractures found worldwide in deformed sediments from the Precambrian to the Pleistocene. Most clastic dikes are soft sediment deformation features and the products of liquefaction caused 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 (small wet-sediment volcanoes). 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 sheeted, wedge-shaped structures that were filled from the top and taper downward (Figure 2). They formed by the forceful infilling of brittle tensional fractures propagated downward into sedimentary and bedrock substrates during the Pleistocene when glacial outburst floods inundated large portions of the Inland Pacific Northwest (Figure 3). Injection style and timing are consistent with overloading and surface deformation by fast-moving, rapidly deepening floods and deep, slow-draining slackwater lakes >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 - The 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. Typical features are <15 cm wide and contain fewer than a dozen sheets.

(b) Sedimentary Fill - Sedimentology of the fill material reflects the local geology. Fills contain a mix of micaceous sandy sediment carried within 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 Latah Fm gruss shed from a granitic highland to the north. Dikes in the Walla Walla Valley contain Touchet Bed sediment (Jenkins, 1925; Cooley, 2015) and oxidized material deposited by “ancient” Scabland floods (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 (reinjection). Crosscutting relationships demonstrate that sheeting did not form during a single event or several events; rather, it developed over time in response to dozens of disturbances. New fractures opened alongside older ones and were filled by sediment delivered from the top; the dikes grew by successive crack-and-fill cycles. Bladed cutslopes that expose their plan view geometry (Silver and Pogue, 2002) reveal dikes coalesced and intertwined to form polygonal networks over time. Strong grain size contrasts between adjacent sheets are evidence of a variable and changing sediment source consistent with circulating bottom currents within floods. Sheet counts are highest in floodway basins that gathered many floods and lowest in coulees that received few. The largest dikes and those that contain the most sheets occur in slackwater areas near Wallula Gap, through which all floods flowed (Bretz, 1929; Baker and others, 2016).

(d) Silt Skins - Thin silt partitions (silt skins) separate sheets of sediment inside dikes (Figure 2c). Flute casts that ornament the faces of silt skins have upward-pointing noses, indicating downward transport. Silt skins form when pore water migrates laterally out of wet sediment, through the fracture wall, and into the surrounding material. Fines are screened at the fracture walls and accumulate in continuous layers, sealing the fracture. The same 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. Skins between sheets indicate the younger fills dewatered into older, drier fills nearby. Skins that line the outer walls of dikes are fluted only on their interior faces. Exterior skin walls indicate the host sediment was well-drained, ice-free, and above the water table (vadose zone) at the time of the first injection and probably during all subsequent fillings. Dikes that penetrate impermeable bedrock lack exterior skin walls altogether.

(e) Distribution - The dikes are widely distributed throughout the Ice Age floodway. Great distances separate outcrops containing nearly identical features. For example, more than 500 km separates the northernmost site in the study at Hunters, WA from the southernmost site known to contain dikes at Salem, OR (Ian Madin, photos and written communication). Glenn (1965) reports dikes in silt-dominated strata exposed along the Willamette River. Sheeted dikes intruding late Pleistocene Willamette silt, which correlates to the Touchet Beds, were discovered in the basement of the Oregon Capitol Building during a recent remodel (Ray Wells, photos and written communication). Dikes do not occur in the Palouse Loess above elevations of local maximum flood-stage indicators (trimlines), nor in unconsolidated sediments beyond the margins of the floodway. Dikes occur in close proximity to active Quaternary faults (i.e., Wallula fault zone) and at sites located far from them. They are most abundant, well-formed, and best-exposed in high silt content rhythmites (Touchet Beds) that form low benches in protected backwater valleys adjacent to high-energy flood coulees. Silty, sandy rhythmites contain dikes that are commonly long and slender with fills that contain delicate, pristinely preserved features (fine laminae, rip-ups, trace fossils, crisp truncations, etc.). Coarse-grained, high-energy flood channel gravels (“Pasco Gravels”) that contain little silt also contain few dikes. Dikes in flood gravels are typically crudely sheeted with low length-to-width ratios (Figure 4).

(f) Age - Field relationships constrain the timing of dike injection to between ~1.5 Ma to ~14 ka, the period of ice sheet growth and Scabland flooding (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 and inundated by Ice Age megafloods (Figure 5), including Late Wisconsin Missoula flood deposits, pre-Late Wisconsin scabland deposits (“ancient” flood gravels, silt diamicts, paleosols), Pliocene–Pleistocene alluvial fan gravels with thick CaCO3 cements, Pliocene Ringold Fm basin-fill sediments, Miocene–Pliocene Ellensburg/Latah Fm sediments, and Miocene Columbia River Basalt. 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 from the region reporting sheeted dikes in Holocene alluvium. The dikes did not form prior to the Pleistocene or since.

Figure 3. Conceptual sketches comparing liquefaction dikes to injection dikes.

(A) A concept sketch illustrates the 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). (B) Generations concept for clastic dikes in aggrading flood sediments. Four examples represent the typical 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 floodwater loads. Grain size governs whether pore fluid pressures will build or disperse, and whether fractures or collapses will form. (B) Dike injection appears to be primarily a slackwater phenomenon due to the necessary combination of silt deposition (low-velocity flows during megaflood events), overloading (deep water), and preservation (low erosion). (C) High-silt-content rhythmites (Touchet Beds) contain abundant clastic dikes with high length-to-width ratios. Dikes in gravelly flood deposits (bars) are typically stubby and crudely sheeted.

Pleistocene-Holocene Liquefaction

Evidence of Pleistocene to Holocene liquefaction in the study area is sporadic and confined to tight corridors along Yakima Fold Belt structures. Foundation Sciences (1980) reported finding liquefaction features in the Wallula fault zone at Finley Quarry, WA. The irregularly shaped features are small, trend with bedding, and their origin is disputed (Coppersmith and others, 2014; Sherrod and others, 2016). Liquefaction was not noted by Bennett and others (2016), who trenched the Burbank Fault near Yakima, WA or by Sherrod and others (2013), who opened trenches in the nearby Wenas Valley. A sparse set of thin, mud-filled dikes intrude certain fine-grained portions of the Ringold Fm in south-central Washington (Pasco and Othello Basins). Those features are rarely thicker than a notebook, are entirely contained in Pliocene strata, are not described in detail elsewhere, and bear little resemblance to dikes of the Pleistocene set.

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 an earthquake’s damage halo 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 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 and observations, 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 conclude that all clastic dikes are seismites.

An international conference called to coordinate proper reporting with respect to the suite of features called seismites (Seilacher, 1969; Montenat and others, 2007; Van Loon, 2014) illustrates the need for caution (Feng, 2017). It seems “seismite” had been applied too liberally to features of nonseismic or ambiguous origin in journal articles, making it necessary for sites around the world touted as classic seismite localities to be reinspected. Participating geoscientists inside and outside the paleoseismology community reattributed many of the features at these sites to nonseismic processes, 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- deformed sediments, or seismites. Therefore, paleoseismologists must correlate candidate seismites over regions and rule out nontectonic origins before concluding that an earthquake occurred.”

– L.B. Grant

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

– Z-Z. Feng

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

– G. Shanmugam

Figure 5. Sheeted clastic dikes in the Channeled Scabland are a Pleistocene phenomenon. 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.

Maximum Width Method 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. 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 attenuates radially outward; therefore, large dikes will form in close proximity to it and smaller dikes away from it. Wider dikes indicate more lateral spreading and more intense shaking. 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 perhaps 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. Sand blows vented wet sediment to the surface over hundreds of square kilometers. Obermeier (1998), using data from sand blows, successfully demonstrated liquefaction features can be used to determine the approximate location of a paleoepicenter.

But the USGS methodology for sand blows is inappropriate for downward-pinching, sheeted clastic dikes in the megaflood region and elsewhere. The maximum width method assumes dikes are single-fill structures formed during single quakes (e.g., event structures). Measurements on composite (sheeted) dikes would not capture the amount of lateral spreading during the event that formed them, as sand blow measurements do, but the total amount of widening that took decades to millennia to develop. The data would describe two entirely different phenomena, reflect two different fracture modes, different geologic environments, and result in entirely different maps. The appropriate measurement, one that provides an apples-to-apples comparison, would be the width of the widest dike at a site (single-fill) vs. the width of the widest sheet in any dike at a site (sheeted).

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 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 Finely, WA (Wallula fault zone) is located >225 km from the northernmost dikes at Hunters, 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 zone) is >265 km from dikes near Granger, WA in western Yakima Valley.

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

If the dikes are the products of seismic shaking, then one of the Yakima Fold Belt structures would be the most likely candidate. However, the dikes are distributed over too large an area to attribute to a single structure (Figure 7). Seismicity at the Cascadia plate margin, located several hundred kilometers to the west, also seems far-fetched, as shaking would be greatly attenuated before reaching the Columbia Basin.

Silt Skin Seals 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 fractures propagate outward. Pressurized proppant (sand + water + chemicals) immediately fills the fractures and holds them open, permitting hydrocarbons to flow back to the well.

The geometry of sand-propped hydrofractures closely resembles that of the natural clastic dikes found in the Channeled Scabland. Hydrofracturing in the Touchet Beds was suspected by Pogue (1998), whose article summarized earlier studies that established the dikes as wedge-shaped, fracture-filling structures that penetrate to bedrock (Cooley and others, 1996; Niell and others, 1997). See Footnote 3.

Silt skins appear to have facilitated hydrofracturing of substrates during megafloods. Loading by floodwater raises fluid pressure in the formation that lasts 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 become fractures that open a few centimeters and sediment rapidly enters, dewaters, and forms a silt skin. The entering water-sediment slurry (natural proppant) is sourced from the base of the overriding flood. The skin-sealed crack now behaves as a pressure vessel. Continued loading of the sealed fracture raises pore fluid pressures (Pf) inside. The point at which fluid pressure exceeds the confining strength of the formation (Pf > 03), breakout occurs, the fracture tip propagates, and a dike forms 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. Each breakout causes a forward jump of the fracture tip and temporarily relieves fluid pressures in fractures (volume increase). 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 (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. Fluid escape along a normally-oriented pressure gradient forms dikes that pinch upward.

Fast-moving currents in main flood channels (coarse-grained deposits) suspended silt and sweep it downstream, making it unavailable to fill cracks opening in the substrate. Slower-moving currents in off-channel and slackwater areas dropped their silty sediment loads, making it available to fill fractures. Dike injection appears to be primarily a slackwater phenomenon due to the necessary combination of silt deposition (low-velocity flow), overloading (deep water), and 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.

Nonseismic Analogs to Columbia Basin Dikes

Per descendum clastic dikes that form by rapid overloading and hydrofracture in reports by others appear analogous to features in this study. Sheeted, downward-pinching dikes intrude muddy deposits beneath tidewater glaciers (Von Brunn and Talbot, 1986; Jolly and Lonergan, 2002; Le Heron and Etienne, 2005; Phillips and others, 2013). Sheeted dikes with prominent silt(?) skins intrude lahar deposits on the side of an Aleutian volcano (Herriott and others, 2014). Dikes formed by hydrofracture propagate downward and laterally out of marine turbidites into channel levees and distal fan-lobe complexes (Braccini and others, 2008; Cobain and others, 2016). Wedge-shaped sand dikes descend from the base of debris flow deposits at Black Dragon Canyon in the San Rafael Swell, UT (Cooley, unpublished field notes and photos; https://commons.wikimedia.org/wiki/File:Dikes_in_black_dragon_canyon_UT.JPG). In all cases, overloading, rapid sedimentation, and hydrofracture produced clastic dikes with characteristics remarkably similar to those observed in the Columbia Basin.

Field Work Matters

The preceding examples teach a valuable lesson: when working in earthquake country, field projects should be appropriately scaled and employ methods matched to the geologic setting.

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. Paleoseismic investigation results (fault slip rates, event dates, and shaking effects) are critical inputs to building codes, community planning documents, and land use policies. Data collected in the field inform and often drive policymaking. Unlike technical trench wall 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.

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 primarily 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 geoscience and society best when they are rooted in and remain subordinate to field data.

Careful, comprehensive field work that involves a sufficient number of outcrops (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.

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. 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, formed deep, slow-draining slackwater lakes, and deposited thick sections of silty, sandy rhythmites.

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 reinjection. 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 involving 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 active widening (floods) were separated by longer periods of inactivity (hiatus), a pattern that follows 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, and 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 downward injection within an inverted pressure gradient. Sediment from surficial materials 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 cannot sustain pore pressures, 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 control how fractures propagate in flood-loaded strata in the study area.

(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 feature 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 count data suggest that fewer than 10 sheets form in any given dike during any given flood, depending on local conditions (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.

Conclusions

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: repeated floodwater loading and hydrofracture. Hydrofracture was suspected by earlier workers (Baker, 1973; Pogue, 1998), but not tested. Based on the field data, I interpret the dikes as nontectonic structures. Fracture response of silty-sandy rhythmites and other substrates to repeated overloading by floodwaters best explains why, when, and where the dikes occur. Dike widths reflect local flood counts and sheeting reflects reinjection over time. Their field-observable characteristics distinguish them from sand blows and related liquefaction structures formed elsewhere by strong recurrent earthquakes, strongly suggesting that clastic dikes in the megaflood region are flood injectites, not seismites. See Footnote 4.

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 guidebooks on Columbia Basin geology. The primary focus of his professional career was the flood deposits underlying Hanford Nuclear Site (Pasco Basin, WA) and, to a lesser extent, sediments of the Neogene Ringold Formation beneath. Despite his lengthy publications list, Bjornstad has never published new, peer reviewed work on the clastic dikes in Eastern Washington, though he is listed as a coauthor on Fecht and others (1999). The list of quotes below show that his views on the origin of the dikes has changed over time. What explains the changes remains unclear. His views somewhat track ideas contained in articles by others, but 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 did not submit this manuscript to other journals.

Footnote *3* In our work as Whitman College geology students (Cooley and others, 1996; Niell and others, 1997), we imprecisely stated that dikes penetrate from top to bottom through the entire stack of rhythmites, thus were late-flooding to post-flooding features. While many dikes do cut from top to bottom through large exposures containing dozens of flood rhythmites, 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 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). As geologists, we call identify a dike as a single structure, but sheeted dikes are actually composite structures. While the overall dike may crosscut the exposure, each sheet traverses only a portion of it. The dikes grew by the addition of new sheets sourced from different rhythmites. Dike growth occurred in tandem with Ice Age flooding and floodbed deposition, a period which spanned thousands of years. As students, we regularly observed truncated sheets (and entire dikes), routinely commented to one another about them, photographed them, and sketched various truncation relationships in our field books. We did not, however, emphasize the fundamental importance that sheet truncations played 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 (after deposition of many 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.

Footnote *4*

The term "injection" has no directional implication. Injected material may have moved upward, downward, or sideways. Injection describes fracture-filling material that has been mobilized and inserted into sediment or rock. The usage of injection and injectite in this article is consistent with the geoscience literature. 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.

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