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Calcrete Field Trip 2021 - Overview

Geology Field Guide to the Rain Shadow Calcretes near Othello, WA

Northwest Geological Society

September 17-19, 2021

Skye W. Cooley, Trip Leader

skyecooley @

Mission Valley, MT

Abstract This field guide explores calcretes formed in the rain shadow of the Cascade Range near Othello, WA. The pedogenic calcretes are relict CaCO3-rich hardpan subsoils that developed in a semi-arid climate and a geomorphically-stable landscape during the Pleistocene between ~1.8 Ma and ~50 ka. Older carbonate cements occur at deeper levels of the Ringold Fm. Newly described stratigraphic sections at Paradise Flats, Crab Creek, Saddle Mountains, and White Bluffs document stacked calcretes that overprint upper Ringold Fm sediments (Pliocene), alluvial fan deposits (Pliocene and Pleistocene), Palouse loess (Pleistocene), ancestral Columbia River alluvium (late Pleistocene), and early megaflood deposits (Pleistocene). The aridity necessary to precipitate soil CaCO3 arrived no earlier than the late Miocene (after 7 Ma) and intensified during interglacials of the Pleistocene. The presence of calcrete corresponds with the rise of the Cascades to a high-elevation divide that blocked moisture-laden Pacific storms from tracking inland. The timing of uplift in the Cascades of Washington has been constrained by others to between 15-5 Ma by various chronometric methods, but the field evidence (calcic paleosols and cements) suggests topographic growth lagged significantly behind rock uplift. Uplift appears to have exhumed considerable volumes of rock, yet the ridge remained at a low in elevation through the Miocene and part of the Pliocene. A high divide (pass elevations of >1000 m) formed remarkably late - some 35 million years after the onset of plate convergence, subduction-related volcanism, and compressional uplift. The "rain shadow" calcretes highlight one of Geomorphology's classic themes: the interplay of tectonics, topography, and climate.

The map is cool. Sorry about the face.

Photo by Michael Machette at White Bluffs Overlook.

Seamless hillshade map by Ralph Haugerud.

Digital file provided to me by Jim O'Connor.

Bretz (1969) reminds us of Richmond's early work on Columbia Basin paleosols,

...Richmond has published nearly a dozen papers on the glacial history...within the northern Rocky Mountain province. That history involves three different glaciations separated by long intervals of nonglacial climate when...weathering of soils occurred...Richmond...traces these old soil markers westward into the Columbia Plateau...[finding] six "strong" soil-making intervals when glacial ice had retreated from the region and three more times of ice withdrawal...the earliest pre-Wisconsin and the following two...are of Wisconsin age.

Bretz et al. (1956) saw evidence in scabland topography for eight floods, one of which George Neff confirmed to be older than,

...a heavy caliche, whose fragments are so prominent in [younger] flood gravels.


Links to Other Stops:

Lind Coulee Fault at O'Sullivan Reservoir


The other calcrete region. Though calcretes in the Columbia Basin of Washington and Oregon were known to Pacific Northest geologists (i.e., Newcomb, 1971; Grolier and Bingham, 1978), most of the early work focused on the desert southwest (i.e., Gile, 1966). I've circled the PNW's calcrete region on this map by Machette (1985). Dryland calcretes in the Inland Pacific Northwest are as thick and widespread as many southwest examples. They have important implications for the region's climate during the Plioene-Pleistocene transition as well as its topgraphic development and tectonic history.

Classic calcretes are alluvial. Calcretes at nearly all of Machette's "classic" study sites in the U.S. Southwest developed in alluvial deposits (yellow boxes). Blue shading coresponds with carbonate stage.

Field trip stops. Black circles are stops described in the field guide, most of which we plan to visit. Open circles are other sites I've described. Black squares are the sites of others. Our September 2021 trip did not visit Lind Coulee Fault or Red Tank due to time constraints. Originally, I planned a trip focused on the crest of the Saddle Mountains, which explains why the map appears shifted.

Timescale issues. When we talk about the early Pleistocene in Washington, we're not talking about 2.6 million years ago, as the timescale would suggest. There are no Ice Age deposits that old here. No found so far, anyway. There is a difference between formalized breaks in the Geologic Timescale and what's actually on the ground in a given region. The Pleistocene record in Washington extends back to around ~2.0 Ma with the first glacial deposits (Orting Drift near Puyallup) appearing west of the Cascade Range around 1.8 Ma. Benthic oxygen isotope curves from the two ODP cores (No. 1020 and 1012) collected off the West Coast (Lisiecki and Raymo, 2005, Fig. 1, 2) terminate at 1.8-1.9 Ma. The oldest dated loess in eastern Washington is only 1.15 Ma. No major ecological or sedimentologic shift is seen in the sedimentary record of the PNW at 2.6 Ma, the formal Plio-Pleistocene boundary. Sediments do not contain the boundary; nowhere can you put your thumb on it. Fossils provide important, but somewhat fuzzy, relative ages for the top of the Pliocene in the Ringold Fm. Some of the same faunal assemblages appear above and below the unconformity between Pliocene and Pleistocene. History complicates the situation; sediments of the upper Ringold Fm were for some time considered Pleistocene, mostly by pre-WWII geologists and well drillers. And the fossil literature is not often read thoroughly by geologists. U-Series dating of calcretes constrains their growth to between 44,000-669,000 (Tallman et al., 1978; Bjornstad et al., 2001; Paces, 2014; Staisch et al., 2018). However, U/Th ages are considered by most to be minimum age estimates due to open-system behavior of vadose zone carbonates. The Hanford literature confuses the issue, too. Their so-called "Plio-Pleistocene unit" (also called the "Cold Creek unit") is a group of sediments identified beneath the Hanford Site and assessed as a hydrogeologic zone for purposes of waste monitoring and cleanup. Ostensibly, the unit spans a >1.4 million year long period (~3.2 to at least 1.8 Ma), making it both Pliocene and Pleistocene in age.

Calcic soils provide constraints on Cascades uplift. I've compiled information constraining the timing of Cascade Range uplift and arrival of a rain shadow to Eastern Washington from 40+ published articles. The gray band brackets the period of time when topographic rise established a dryland climate east of the Cascade divide. Older studies tend to push the date back in time a bit, while newer studies argue for a younger rise. The Cascades were rising and evolving long before the gray band period, but the range does not appear to have formed a high divide until quite late. This figure has been updated.

Calcrete-armored paleosurface extends beyond Pasco Basin. Stops in this field guide highlight sediments and soils from a relict Plio-Pleistocene landscape. Stacked calcretes, calcrete gravels, alluvial fan-loess complexes, and cemented silt diamicts delineate a dissected and deformed paleosurface that spans 3 geomorphic domains: flood-dominated Channeled Scabland, loess-dominated Palouse Slope, and teconism/alluvial fan-dominated Yakima Fold Belt. The paleosurface extends southwest to Eureka Flat and northern Walla Walla Valley (i.e, Rulo site), but we lose the calcrete as moisture and elevation increases. White circles are my study sites, a selection of which are in this field guide. Black circles are sites in Baker et al., (2001), Bjornstad et al. (2001), Medley (2012), Bader et al. (2016), or unpublished locations from my field notes. Bold white line delineates Pasco Basin, the colloquial name for the local portion of a once larger synclinal trough that has since been broken up by N-S and E-W faults and folds (Cheney's eggcrate model).

BB = South Bombing Range Rd, BR = Booker Road at Canal, RR = Site 21-04, C = Connell, CB = Cummins Bridge, CCB = Cold Creek Bar, CR = Coyan Rd, EC = East Connell, FMEF = Fuels Materials and Examination Facility, FR = Field Rd, GL = George Landfill, GWB = Ringold Rd Bluffs, HL = Hatton-Lemaster Intersection, HO = Houghton Rd, HRC = Herman Railcut, HX = Hendricks Rd, LCF = Lind Coulee Fault, LF = Liesle Rd, LR = Lind Rd, LS = Leslie Rd, OFF = Offramp, OMC = Old Maid Coulee, PP = Potholes Park, PH = Poplar Heights Rd, RC = Reese Coulee, RT = Red Tank Hike, RULO = Rulo Site, Overlook, SB = Smyrna Bench, SP = Stokrose Pit, SR = Scooteney Rd, SS = Silicard Site, TPS = Taunton Power Station, WBO = White Bluffs Overlook, WC = Warden Canal, YB = Yakima Bluffs.

Above or Below. Outcrop stratigraphy (black bars) with respect to the calcrete-bearing zone. Sections entirely above are in Pleistocene Palouse loess and/or Missoula flood deposits (i.e., Warden Canal). Sections entirely below are in Pliocene Ringold Fm and Miocene Columbia River Basalt (i.e., Hatton-Lemaster).

Extent of the Ringold. The Ringold Fm (gray area) extends beyond the topographic rim of the modern Pasco Basin. That is, the Pliocene trough which Ringold sediments filled was once much larger. The Ringold represents a broad, low-relief plain at the confluence of through-going rivers including the northerly-sourced ancestral Columbia River and easterly-sourced Snake-Salmon-Clearwater system. Numerous shallow, sluggish side channels meandered through low-lying uplands that accumulated dust and soils. Alluvial fans, spilled from adjacent basalt highs, interfinger with basinal sediments. A large lake filled and spilled three times during Ringold time. The lake constituted only a portion of the gray area in the map above. The Ringold basin is interrupted by both east-west faults (YFB) and a set of subtle north-south folds. Ringold sediments are divided into a basal unit, a lower unit, a middle unit, and an upper unit. Be careful when reading about the Ringold. Most of what is written was authored by Hanford Site staff, who mainly describe borehole cuttings from small portion of the central Pasco Basin. Their task is managing leaked waste, not traditional geology as understood by geoscientists outside Hanford. Hanford geologists do applied work and lump/name units based primarily on their local hydrological characteristics (functional units not formations). If you want to see Hanford's geology, you have to leave Hanford because there are few outcrops there. Hanford staffers understand the Site's geology primarily through instruments and borehole information. The White Bluffs (actual outcrop) is located miles from the Hanford offices in Richland. Consequently, the PNL/Rockwell/Westinghouse/Hanford gray lit is copious, but generally not all that helpful. Its the consultant reports, measured sections, and articles by USGS and WADNR-DGER that matter.

Cascade divide and eastern Washington soils. Topographic rise of the Cascade Range over the past 40 million years involved compressional uplift (rock uplift, relatively constant) and erosional stripping (denudation, temporally and spatially variable). Paleosols and other lines of evidence indicate a high topographic divide formed and began to cast a rain shadow east during the Pliocene, separating the state into two climate zones. The growth of soil carbonates began after.

A.) Low topographic divide poses no barrier to wet, maritime weather systems moving inland. Late Miocene.

B.) Discontinuous divide with low passes and/or deep, cross-range valleys creates only a partial orographic barrier. Late Pliocene.

C.) High continuous divide creates a strong weather barrier and casts a rain shadow east. Arrows indicate intense erosion in the late Miocene, asymmetric erosion through the Pliocene, and strongly asymmetric erosion from early Pleistocene to present.

Caliche vs. Calcrete - Pedogenic carbonate (CaCO3) that accumulates in soil B-horizons is known as caliche or, where thick and dense, calcrete. "Calcrete", a term coined by Lamplugh (1902), first described lime-cemented gravels in Ireland (Lamplugh, 1902). "Caliche", coined the same year by Blake (1902), described calcareous desert hardpan soils in the southwest United States. Today, some recognize caliche as the thinner, less mature form of calcrete, and classify the latter by carbonate stage (Gile, 1966; Machette, 1985; Birkeland, 1999, NRCS, 2021). The term "caliche" is colloquial and should probably be abandoned in favor of "calcrete", the formal term. But old habits die hard.

Rule of Thumb for Eastern Washington - In general, numerous thinner calcic horizons separated by thicker loess layers occur on the Palouse. In the Basin to the west, fewer, thicker calcrete units more thoroughly overprint a variety of mostly alluvial bottomland, outburst flood, and alluvial fan sediments.

Sources of CaCO3 - Two sources for CaCO3 (calcite) exist for aridland soils. Limestone bedrock contributes primary detrital material to soils (Ex: New Mexico or Saudi Arabia). In areas without limestone bedrock (Ex: eastern Washington), Ca, C, and O are supplied by dust and rain. Secondary soil carbonates are products of dissolution-migration-evapotranspiration-precipitation activities in the soil environment (Zamanian et a., 2016). CaCO3 is concentrated by soil processes and forms distinctive B-horizons (subsoils) that evolve over time. Pedogenic carbonates worldwide are almost exclusively found in dusty, arid to semi-arid regions on geomorphically-stable, upland surfaces.

Calcium - Ca2+ ions provided by leaching of loess, transported by rainwater/snowmelt

Carbon - CO2 generated by plant roots, microbial respiration in rooting zone, and rainwater

Oxygen - Oxygen in the form of soil water (H2O) supplied by precipitation

Silica - Silica supplied by Cascades volcanic ash, a ubiquitous component of Palouse loess

CaCO3 Accumulation in Soils - Carbonate accumulation in soils is the result of pore water movement (down or sideways), evapotranspiration (upward moisture loss), degassing of CO2 around plant roots and by rooting zone microbes. Grainsize, landscape position, and other factors influence accumulation.

- Infiltration of slightly acidic meteoric water from rainfall and snow melt leaches Ca from loess and transports it downward.

- Evapotranspiration dries the soil by transporting moisture upward, back to the atmosphere, leaving precipitated CaCO3 behind. CO2 degassing from roots and microbes facilitates carbonate precipitation.

- Precipitation, high during the growing season, drops off for much of the rest of the year. Low mean annual rainfall means the soil column is rarely fully flushed, a common occurrence in wet regions. Long dry periods enhance calcic soil development.

- Carbonate concentrates 15-50 cm below the ground surface (Bestland and Retallack, 1993; Retallack, 1994), forming a Bk horizon. Calcic subsoils can build over time to form restrictive (plugged) hardpan layers. Platy cemented calcretes and silcretes result.

- Pedogenic carbonates are products of biotic and abiotic processes, consistent with the interplay of soil forming factors: Parent Material, Time, Climate, Relief, Organisms.

Calcium carbonate accumulates in subsoils via dissolution-precipitation reactions that run either way.

Accumulation / Calcification Removal / Decalcification

Ca2+ + 2HCO3- <--> CaCO3 + H2O + CO2

Dissolution of calcite is encouraged by a.) more meteoric water flushing through soil, b.) lower pH (acidity), c.) increases in pCO2 gas by roots, and d.) decreases in Ca2+ concentration relative to amount of soil water. Reaction is driven to the left in decalcification.

Precipitation of calcite is encouraged by a.) moisture loss through evapotranspiration, b.) higher pH (alkalinity), c.) decreases in CO2 gas (lower pCO2), and d.) saturation of Ca2+. Reaction is driven to the right in calcification.

Eluviation (E = exit) is the removal of dissolved CaCO3 from upper portions of the soil profile, while illuviation is the precipitation (recrystallization) of CaCO3 lower down, if conditions are favorable (e.g., soil solution is saturated with respect to CaCO3).

The calcite dissolution-precipitation reaction is: Ca2+ + H2O + CO2 = CaCO3 + 2H+

Increasing the amount of CO2 gas in soil increases the partial pressure of CO2 and causes calcite to dissolve, making it mobile again: CaCO3 + CO2 + H2O = Ca2+ + 2HCO3-

Does More or Less Rainfall Make Thicker Calcrete? - Thick calcretes are often attributed to hyper-arid conditions (i.e., drier deserts have thicker petrocalcic horizons). We don't know exactly the seasonality/timing of precipitation in eastern Washington during the latest Pliocene and early Pleistocene. However, sediments, paleosols, pollen, and fossils tell us that the Pasco Basin climate was possibly a more intense version of what we see today, with colder winters, short hot summers, altered timing of rainfall events. Based on what I see in the field, it seems highly-evaporative, dusty summer periods alternated with cold, dry winters. Most of the annual moisture probably arrived during shoulder seasons, similar to today. The rain shadow calcretes appear to indicate long periods of drying between brief, soaking rainstorms. The landscape was a filling with sagebrush and pine after ~5 Ma and became a cool, sagebrush steppe after ~3 Ma. Influence of a shallow water table is seen, too. Thic calcretes may not signify a hotter, drier desert (hyper-arid conditions), rather it may indicate soils formed just above a stable water table (lowland setting, capillary fringe) and received most of their moisture from below. Only a small of soil water was contributed by short, heavy downpours that moved Ca, C, and O rapidly into the soil column.

Growth Rate of Calcrete - Soil carbonate builds at variable rates. Several studies suggest formation of a thoroughly-cemented carbonate hardpan tens of centimeters thick requires 10,000 to 1,000,000 years (Arkley, 1963; Machette, 1985; Lal and Kimble, 2000). Vincent et al. (1994) estimated rate for the growth of 0.6 mm/10 kyr for carbonate coatings on clasts in Idaho. Others have estimated the rate of calcrete accumulattion at 0.03-0.80 mm/year, which means that a 100 cm-thick calcrete would form in 1,250 to 33,300 years. Slower rates seem to describe the Eastern Washington calcretes.

Dates on Calcrete in Washington - Systematic dating of calcrete in eastern Washington has not been done, but some age data has been published. Paces (2014) applied U-series (U-Th) techniques to carbonate-silica rinds on cobbles and cemented fault gouge collected near Richland, WA. Minimum ages fell between 17 ka (caliche) and >500 ka (calcrete, cemented gouge), consistent with Late Pleistocene cataclysmic flooding and Middle Pleistocene alluvial fan growth. The Th-230/U-234 technique, typically used on materials which build concentric layers such as teeth or stalagmites, has an upper limit of reliability of 600,000 kyr, an age exceeded by some samples. The study authors make clear theirs was not a systematic accounting of the entire calcrete-bearing section and "was not designed to address the history of pedogenesis or climate change". Dates helped to constrain Quaternary deformation rates in the Yakima Fold Thrust Belt. The Paces study is consistent with other U-series ages (44,000 to 669,000 yr) on soil carbonates sampled elsewhere in Pasco Basin and Saddle Mountains (Tallman et al., 1978; Bjornstad et al., 2001; Staisch et al., 2018).

Ages at Smyrna Bench. U-series dating of caclrete-bearing sediments in Ringold Fm (Staisch et al., 2018) indicates rainshadow conditions and dryland soils in the Othello-Saddle Mountains area are younger than ~7 Ma with most ages clustered between ~400-100 ka (inset chart). The field evidence (calcrete morphology) suggests the clustered dates are too young. Open system problems associated with vadoze zone calcite are suspected. I've redrawn her figure to emphasize stacked calcretes (paleosols) and cemented zones (position of paleo-water table?). Pedogenic calcrete looks very different than carbonate-silica cements in outcrop. In recent months, having observed many more exposures, I've begun to doubt some of the deeper "calcic paleosols" actually formed as soils. Some appear to be cemented zones formed in the subsurface sometime after burial. If so, older cements may not have anything to do with the formation of a Cascade rain shadow. Important to note: The structure and stratigraphy of Saddle Mountains and Smyrna Bench by Lydia Staisch builds on work by Steve Reidel (1980s-90s) who followed John Bingham (1960s-70s). Bingham et al. (1970) introduced most of what is contained in articles published 50 years later.

Limits on Calcrete Distribution - Though extensive between Pasco and Moses Lake, calcretes are not found everywhere in eastern Washington. Factors that retard calcrete growth include 1.) rapid sedimentation, 2.) erosion, 3.) high rainfall.

- Continuous sedimentation (aggradation) suppresses horizonation in dust-dominated soils. Soil processes beneath a newly-buried surface are reset to the new surface.

- Erosion lowers the ground surface by stripping material, which also resets soil development to a lower position deeper in the profile or below the formerly active soil zone altogether.

- Rainfall, specifically when above about 700 mm/year (28 in/yr), effectively flushes CaCO3 from soil profiles. Calcrete is therefor restricted to drier basin center and the lowest flanks of the Cascades, Blue Mountains, and Okanogan Highlands.

Some calcretes occur on vegetated upland hillslopes of the Palouse. Windblown dust accumulation rates there have remained high, if variable, over millennia. Consequently, numerous caliche horizons are more common in Palouse soils (i.e., Thatuna, Oliphant, Santa Series soils). In the Pasco Basin, thicker calcretes form due to higher aridity and possibly lower landscape position, sparser vegetation cover, and lower dust accumulation rate. Calcretes and related paleosols appear to achieve their fullest, most concentrated expression in the Saddle Mountains and Wahluke Slope rather than on the Palouse, where substantial amount of research on paleosols has been conducted. In the Basin, we find fewer, thicker calcretes thoroughly overprinting a variety of mostly alluvial sediments, not just loess.

Calcretes Associated with Regional Unconformities - In the western U.S., calcic paleosols are associated with unconformities. Soil growth follows tectonic upheavals. Prominent unconformities between packages of terrestrial rocks define "synthems" (Wheeler and Mallory, 1970; Cheney, 2016). The post-Ringold unconformity is the bounding surface between Walpapi Synthem rocks (20-4 Ma, pronounced WALL-puh-pie) and High Cascades Synthem rocks (4 Ma-present, pronounced kass-KAYDS). The ages of sequence-bounding unconformities in WA, OR, and MT cluster at ~30 Ma, ~20 Ma, and ~4 Ma (Hanneman et al., 2003). The ~4 Ma unconformity is conspicuous in south-central Washington, separating Pliocene basin-fill deposits from Ice Age deposits.

Pliocene to present climate. The Pliocene was a time of incredible change. Major mountain ranges were rising. Silicate weathering was considerable. The Isthmus of Panama closed, altering global ocean circulation patterns and opening new routes for migrating mammals. Hominid species were emerging. The Arctic was forested. Figure by Burke et a. (2018).

Ancient Scabland Flooding - Scabland surfaces, bedrock coulees, streamlined hills, perched erratics, divide crossings, weak soils, enormous gravel bars, MSH tephras, slackwater rhythmites, varved-and-sand bed sections, and major freshwater influxes into the Pacific constitute an indisputable record of late Wisconsin megaflooding (e.g., the Missoula floods, 18-14 ka).

Other more subtle lines of evidence argue for a pre-Missoula flood record, often called the "ancient" or "pre-late Wisconsin" record. Deeply weathered clasts, petrocalcic horizons, high-relief erosional surfaces in Palouse loess, and anomalous deposits beneath Missoula flood deposits constituent the older record (Bretz, 1956; Richmond et al., 1965; Baker, 1973; Baker and Nummedal, 1978; Patton & Baker, 1978; Rigby, 1982; McDonald & Busacca, 1988; Busacca, 1989; Baker et al. 1991; Kiver et al., 1991; Bjornstad et al., 2001; Spencer and Jaffee, 2002; Pluhar et al., 2006; McDonald et al. 2012; Medley, 2012; Bader et al., 2016). Discoveries made at 14 sites more than 50 years ago including Marengo, Old Maid Coulee, Revere, McCall, Ritzville, and George established a partial framework for the older flood argument (see summary in Baker et al., 2016 or Appendix E in Coppersmith et al., 2014). Supporters often lament the paucity of unambiguous field evidence for the ancient old floods.

Some go so far as to suggest Ice Age flooding began 2.0-2.5 million years ago, coincident with the opening of the Pleistocene (i.e., Smiley et al., 1991), but a tempered view of the literature and the field evidence support a first-flood arrival no earlier than ~1.8 Ma. The oldest glacial deposit in Washington is the Orting Drift at 1.8 Ma (Easterbrook, 1994). Below I briefly summarize often-referenced articles that address ancient scabland flood deposits.

Patton and Baker (1978) - A flood gravel capped by three loess units each with its own paleosol and lying beneath Missoula flood deposits at Marengo was correlated with strata at Revere, Macall, and Old Maid Coulee. The gravel was interpreted to be pre-late Wisconsin in age. Similar to Baker and Nummedal (1978).

Myers and Price (1981) - Boreholes at Hanford's "Reference Repository Location (RRL) in the Cold Creek syncline reveal two coarse-to-fine graded "sequences" incised into Ringold. In both graded units, a poorly-sorted, exotic-bearing, pebble-to-boulder gravel (>30cm clasts) with a sand matrix grades upward to a sandy cobble gravel, then to silt and sand. A thick "calcic horizon" cements the lower of the two flood "sequences", making it (likely both) pre-late Wisconsin. No one but the Hanford dorks use the term "sequence" to describe glacial outburst flood beds.

Baker et al. (1991) - Sites described included Kiona Quarry, Leslie Road, Macall, Marengo, Old Maid Coulee, Yakima Bluffs Poplar Heights, South Bombing Range Road, FMEF, Cummings Bridge, Revere, and Washtucna.

Bjornstad et al. (2001) - Compilation of radiometric age data on flood deposits sampled from boreholes and outcrops. Sites included Kiona Quarry, Leslie Road, Macall, Marengo, Old Maid Coulee, and Yakima Bluffs.

McDonald et al. (2012) - This article is a culmination of work that began with McDonald and Busacca (1988) establishing links between glacial outburst floods and Palouse loess. The authors use several lines of evidence collected from various sites to argue for the presence of a pre-late Wisconsin flood record there. One representative site is Winona, WA, where a 38m-deep core was collected in 1996 (WIG-1). The core contained a stack of ~20 calcic paleosols developed in loess deposited atop basalt. Magnetic polarity in the upper 28m of core was normal, but reverse in the lower 10m. The polarity change was correlated to the Matuyama-Brunhes boundary at 780 ka making the age of early Palouse loess, therefore Ice Age flood deposits from which it was derived, at least 1 million years old.

Pluhar et al. (2006) - Borehole samples of flood deposits at the Hanford Site exhibit a pattern of normal and reversed polarity signatures that can be correlated to the published polarity timescale. The authors argue the sediments of eastern Cold Creek Bar are pre-late Wisconsin flood deposits laid down between 1.07 Ma and 780 ka, between the Jaramillo subchron and the Brunhes-Matuyama reversal.

Medley (2012) - The author revisited 14 previously documented sites with ancient flood deposits and reports 11 new ones. Previously visited sites were those of Baker and Nummedal (1978), Patton and Baker (1978), Busacca et al.(1989), Baker et al. (1991), Kiver et al. (1991), Fecht et al. (1999), Bjornstad et al. (2001), Spencer and Jaffee (2002), Bjornstad (2006), Cordero (1997), and Gastineau (2011). No sites were described in detail as the focus of the project was primarily a region-wide sampling effort. She measured CaCO3 content using a Chittick apparatus (Dreimanis, 1962; Machette, 1985) and determined carbonate Stage for each sampled horizon following the criteria of Gile et al. (1981) and Birkeland (1999). Carbonate Stage of III or greater requires more than 15,000 years to develop, thus are older than the Missoula floods. Results of her Stage analyses:

Stage II (8 sites): Benge, Collier Coulee, Connell, East Callaway Rd, Leslie Rd, Palouse, Potholes Coulee, Rulo.

Stage II+ (5 sites): Benge, Brown Rd, Connell, Leslie Rd, Reese Coulee.

Stage III (14 sites): Canal Outcrop, East Callaway Rd, East Connell, Frenchmen Coulee, Macall, Othello Canal, Poplar Heights, Potholes Coulee, Reese Coulee, Ritzville, Rulo, The Dalles, Winans Rd 1, Yakima Bluffs.

Stage III+ (1 site): Othello Canal, with 35.0-49.9% CaCO3, is the same site as Offramp in this field guide.

** Bjornstad (1990, Table 6) shows weight % CaO measurements for 6 subsurface samples collected from the Plio-Pleistocene unit at Hanford's 200 West Area. Average CaO = 12.48% (SD 8.24). Max CaO = 27.07%. Four well logs in Appendix A indicate the PPu and occasionally one lower horizon contained calcium carbonate at 30% or higher. The article does not address the apparent discrepancy.

Bader et al. (2016) - The Rulo site exposes a 30m-thick section of loess, exotic clast-bearing diamicts, a fluvial sandstone, tephras including Newbury, more than a dozen paleosols, and numerous Irvingtonian vertebrate fossils. Five unconformity-bound sequences and truncated clastic dikes provide additional evidence for pre-Wisconsin loess accumulation and glacial outburst flooding in northern Walla Walla Valley. The section is normal polarity except for a micaceous, fossiliferous fluvial sandstone at the base of the exposure with reversed polarity. This work built on Spencer and Gilk (1999), Spencer and Jaffee (2002), and Gastineau (2011).

Some remain unconvinced by the evidence for ancient scabland flooding in eastern Washington. Detractors attribute it to alluvial action of local streams. While the skeptics' arguments do have some legitimacy, a growing body of geologic evidence is accumulating, some of which is in this field guide. What remains of the ancient record is subtle and "indirect" (Spencer and Jaffee, 2002).

Ancient flood sites. The map shows a few of the more well known outcrops from the literature (yellow). Sites in blue are new locations I have discovered or revisited. Calcrete overprints flood gravels, old loess, alluvial fan deposits, and the upper Ringold Fm. My "Offramp" site is Medley's "Othello Canal" site. The "Lind Coulee Fault" site was trenched by M.W. West, but I've not seen those reports. "Cold Ck Bar" is a subsurface site drilled by Hanford crews.


Alonso-Zarza, A.M.; Tanner, L.H., 2010, Carbonates in Continental Settings: Geochemistry, Diagenesis and Applications, Alonso-Zarza, A.M.; Tanner, L.H. (editors), v. 62, p. 1-319

Arkley, R.J., 1963, Calculation of carbonate and water movement in soil from climatic data, Soil Science, v. 96, p. 239-248

Armstrong, R.L.; Leeman, W.P.; Malde, H.E., 1975, K-Ar dating quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho, American Journal of Sciences, v. 275

Bader, N.; Spencer, P.K.; Bailey, A.M.; Gastineau, K.R.; Tinkler, E.; Pluhar, C.N.; Bjornstad, B., 2016, A loess record of pre-Late Wisconsin glacial outburst flooding, Pleistocene paleoenvironment, and Irvingtonian fauna from the Rulo site, southeastern Washington, USA, Palaeogeography, Palaeoclimatology, Palaeoecology. v. 462

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