Calcrete Growth in Alluvial Lowlands - Eastern Washington
I can't get any lower.
Still I feel I'm sinking.
Calcrete is not a caprock. Thick pedogenic calcrete is intrinsically related to the presence of a valley. Calcrete forms in stable, lowland settings where vigorous evapotranspiration occurs. Resistant calcretes, once formed, persist and often define remnant geomorphic surfaces that are later incised and dissected. Calcrete that today occupies an elevated position in the landscape (caprock) likely formed as a lowland soil - a crust on a fluvial terrace or gently-sloping alluvial fan.
Calcrete is a CaCO3-rich hardpan paleosol that forms in dry, stable landscapes of the world. Calcrete in Eastern Washington cements a 20m-thick interval that spans three geomorphic domains: Palouse Hills, Channeled Scablands, and Yakima Fold Belt. The sheet-like deposits enclose ancient Scabland flood gravels and define a regional paleosurface that has been bent and broken by Quaternary faults. Calcrete overprints primarily lowland alluvial deposits (ancestral Columbia-Snake River floodplain) and basaltic alluvial fan gravels shed from fault-bounded ridges. Thick layers of pedogenic carbonate accumulated during the Pleistocene, between about 1.8 million years ago to about 40 thousand years ago, but older cements at somewhat deeper levels date back to ~7 million years (latest Miocene). The appearance of calcrete in Eastern Washington coincides with the topographic rise of the Cascade Range and establishment of a strong rain shadow east of the divide. This study sheds new light on this lesser known part of Eastern Washington's stratigraphy.
Calcrete-armored surface warped by tectonism and incised by megafloods. North-south transects show calcrete-armored surfaces (heavy black lines) deformed across Yakima Fold Belt structures and eroded away in coulees cut by Ice Age floods. The armored surface is, however, preserved in certain places at elevations below the level of maximum flooding, here approximated by the Lake Lewis shoreline (366m). Black dots are some of my study locations. YFB ridges shown in the upper profile (Moses Coulee to Cold Creek Valley) include Badger Mountain, Frenchman Hills, Saddle Mountains, and Umtanum Ridge. Figure from Cooley (2022), Tobacco Root Geological Society Guidebook Vol. 51, Wenatchee Area Geology. Purchase from www.trgs.org or MBMG publications office.
In Eastern Washington, a robust association exists between thick pedogenic calcretes (petrocalcic horizons) and non-calceous lowland alluvial deposits. That's a key finding of my work since 2019 on Pliocene-Pleistocene geology in the Saddle Mountains-Othello-White Bluffs area. Meter-thick (Stage III-V) calcretes here post-date sediments of the uppermost Ringold Formation and have dominantly pedogenic morphologies, though some clearly were influenced by a shallow water table. Puzzling why this lowland association goes unmentioned in previous articles on Washington calcretes, especially when numerous studies from other regions of the world demonstrate their clear association with alluvial fills and low-gradient valley bottoms (i.e., Machette, 1985; Othberg et al., 1997; Chen et al. 2002; Sofoulis, 1963; Butt et al., 1977).
Herman Railcut site. Two thick calcretes, one developed in gravel the other in sandy loess, overlie a boulder gravel deposit near Othello, WA. The upper calcrete is more than a meter thick. The boulder gravel rests unconformably atop Pliocene Ringold Fm and was deposited by an ancient scabland flood. The calcretes (> Stage IV) took several hundred thousand of years to form, which means the flood gravel is likely >500,000 years old. I discovered this exposure in 2019. These same calcretes armor surfaces in the Othello-Saddle Mountains-White Bluffs-Wahluke Slope area of central Washington (northern Pasco Basin).
One reason might be the Palouse. Numerous articles on the Palouse loess and its "caliche" horizons have been published over the past century (Bryan, 1927; McDonald and Busacca, 1988; McDonald et al., 2012). Many Pacific Northwest geologists are first introduced to caliche on field trips to the Palouse. In fact, many of us equate caliche with the Palouse, though its expression there is just one of many regional variants. However, caliche in Palouse soils is generally rather weakly developed (< Stage III), laterally discontinuous, and unquestionably formed by soil processes on upland hillslopes. The landscape never permitted most to growth very thick. Meter-thick, multi-story calcretes (> Stage IV) are found west of the Palouse in the lower, drier Pasco Basin. Calcretes in Pasco Basin are at least as old as the loess at the base of the Palouse section. Stacked calcretes with advanced stage morphologies in the Othello area appear to exceed the age of the oldest dated loess (1.15 Ma). The region's thickest calcretes developed in lowland sediments of a long-lived alluvial plain that has since been deformed by YFB tectonism, today occupied by the Saddle Mountains, Smryna Bench, Wahluke Slope, and Paradise Flats.
A second reason might be linguistics, specifically the term "caprock". Calcrete in the Pasco Basin has for decades been described as the "calcareous cap on the Ringold Formation" and typically viewed in outrops at White Bluffs or along the crest of the Saddle Mountains (looking up from Crab Creek). The word "caprock" is succinct and kind of fun to say, but the concept remains problematic when it comes to calcrete. Caprocks hold a high position in today's landscape, commonly a plateau or bluff. At the time they formed, however, they were soils in lowland settings probably quite near the water table. Later they were elevated by tectonism and fluvial incision.
A third reason might be the dire warnings, which discourage new work: Here be dragons,
The dating of land surfaces presents difficult and exasperating problems for geomorphologists...fraught with problems...in many instances the essential evidence has been eliminated, so that some surfaces are...undateable.
- Twidale and Bourne (1998)
Caution is advised in...attempts to correlate duricrust types on the basis of valley exposures...only duricrust host material characteristics and not cementing materials should be considered...there is no evidence for correlation of duricrust cements between exposures. - Nash et al. (1994)
A fourth might be oversimplified figures in soil science and geomorphology textbooks. For example, Figure A below shows a slope profile where evaporation decreases downslope. In general the concept works fine, but when applied to pedogenic calcrete, it fails. Figure A implies a Bk horizon will form on the drier summit. Figure B questions this concept. What if the summit is rocky, the toeslope heavily vegetated, and the water table deep?
Time to revisit the textbooks? Calcretes at numerous locations around the world have been investigated. In what deposits does the literature suggest calcrete forms? Are calcrete-bearing sediments the stuff of convex hillslopes, planar landforms, or concave valley bottoms? Is there a single example of a thick 'caprock' calcrete forming on a high, dry ridge or mesa absent alluvium immediately below?
High or low? Do meter-thick Stage V calcretes form on top of convex loess hills? Landscape position seems to provide some answers. Landscape position is the relative topographic position of any element in the landscape to other elements nearby, independent of its elevation above sea level. For example, the summit of Little Tahoma (11,138'), a satellite peak of Mt. Rainier (14,417'), occupies a relatively high position, though it is >3200 ft lower than nearby Mt. Rainier; the peak drops away on all sides. Floodplains are low elements. Alluvial fans emerging from mountain fronts are low elements. Scabland coulees are low elements. Abandoned fluvial terraces occupy middle positions. Loess hills of the Palouse occupy a high position, though their elevation are quite modest. Windblown silt of the Palouse, like that in other loess regions, hosts multiple stacked layers of caliche, commonly Carbonate Stage III or less. Most Palouse loess is less than 1 million years old. Ledges of calcrete in the Palouse appear far less developed, thinner, laterally discontinous, but more numerous with respect to thicker, blanket-like sheets in Pasco Basin. Argillic horizons, perched water tables, and lateral throughflow in deep loess may enhance calcrete growth in Palouse soils, but to my eye the thickest accumulations of CaCO3 are not associated with wet, concave hollows between convex hills; different soils developed there. The Palouse's high sedimentation rate alone doesn't completely explain more numerous thinner Bk horizons to the east and fewer thicker Bkk horizons to the west. Landscape position and age do a better job. The oldest unit of Palouse loess, identified as 'L3' by Busacca and McDonald, along with its calcretes appears much better expressed west of the Palouse Hills in the lower, drier Pasco Basin. I interpret L3 loess in the Palouse as the eastern fringe of a unit deposited in both Pasco Basin and Palouse beginning in the Pliocene or even latest Miocene (~7 Ma) - long before Ice Age floods showed up. Cicada-burrowed L3 loess spans the Pliocene-Pleistocene boundary and is a constituent of the a.) lowest part of the Palouse Fm, b.) 'Plio-Pleistocene unit/Cold Creek unit' of the Hanford literature, and c.) upper half of the Ringold Fm. The source of L3 silt is both Ringold-age alluvial fans, the abandoned Lake Ringold surface, post-Ringold alluvial fans, overbank and bar sediments of the ancestral Columbia-Snake River floodplain, and wind-recycled Ice Age flood deposits (<1.8 Ma).
Is the standard slope-accumulation model relevant to pedogenic calcrete? A.) Generalized evaporation-erosion-deposition model shows how soil moisture, sediment type, and thickness varies along a slope (i.e., McCraw, 1967). This figure is found in all soil science, pedology, and soil geomorphology textbooks. The model predicts soil moisture will increase downslope and that greater evaporation, thus more CaCO3 in the soil, will occur higher on the hillslope. Bk horizons will form up there, not down here. B.) Real-world examples contradict the model (i.e., Foley et al., 2006). Bk horizons form at various landscape positions due to a variety of factors, but thick petrocalcic horizons tend to form at lower positions, where strong evapotranspiration moves large volumes of water through the soil column via vegetation rooted near the water table. Alluvial fans, fluvial terraces, dusty and sluggish alluvial plains, and the floors of flood coulees. At high, dry positions moisture is limited, thus the supply of dissolved constituents is limited. More water, not less, seems to create thicker pedogenic calcretes in semi-arid regions. Mountainous terrain tends to thwart tried-and-true soil science concepts developed for farmlands of middle America. No one ever said, "Go Midwest, young man." or "Go Southeast, for your health."
Neogene lowlands of south-central Washington Visualize the Eastern Washington landscape during Late Miocene-Pliocene like this. The tan areas, identified as "overburden" for purposes of this water resources map, provide a big picture perspective on what was formerly a more contiguous basin prior to rise of the Yakima Folds. Today's basins are small fragments of an older region-scale lowland, where major rivers conflued. While the geographic terms "Pasco Basin" and "Columbia Trans-arc Lowland" are useful, recognize that certain valleys that today stand apart were previously connected, at times. Three examples, Farooqui et al. (1981) correlated post-CRBG sediments across several Neogene basins in WA and OR; Tolan et al. (2002) recognized a region of interconnected lakes during 'pre-Rosalia time' (~14.5 Ma); Staisch and Sadowski (2021) confirmed the Kittitas Valley was isolated from the Pasco-Crab Creek-Quincy Basin by the rise of the Hog Ranch-Naneum anticline after 10 Ma. These post-basalt depocenters collected a wide variety of sediments shed from the surrounding highlands. The basin fill sediments often contrast with valley sediments of through-flowing rivers. For example, consider the sedimentologic diversity of the Ellensburg Fm. Its composed of quartzite-bearing fluvial gravels, mixed alluvial valley fill, and locally-derived (basaltic) alluvial fan deposits that interfinger with one another. Calcretes are thickest near the paleocenter of the ancient basin (Pasco Basin in late Miocene to Pliocene times), between Othello and Kennewick. Calcrete thins with distance away in all directions from Othello. Map by USGS/Snyder and Haynes (2010, Plate 1).
Calcrete over basalt over diatomite. All three units correspond with lowland positions in the landscape. Miocene-age diatomite beds are deposits of shallow, warm, isolated lakes atop flat-lying basalt flows. Miocene-age flood basalts fill valleys. Plio-Pleistocene-age calcrete caps alluvium. Erosional contacts separate the units, but bedding is parallel in all three. 1947 photo of diatomite mining operation near George, WA. WGS archives photo #3529.
Lind Coulee. A 2m-thick blocky calcrete developed in loess overlies mixed alluvium north of Othello, WA. The alluvium consists of wetland muck and thin channel sands with abundant soil features. It represents an overbank and sidesteam setting (ancestral Crab Creek/Columbia River floodplain) that transitions upward to a slightly drier, loess-blanketed hillslope. The water table is nearby throughout, suppling water for evapotranspiration that helped thicken the calcrete. The entire section unquestionably lies in a valley. The calcrete's caprock position and association with loess suggests it could be Pleistocene in age. Its considerable thickness and the fact that loess paleosols occur in the upper Ringold, however, indicates it sa Plio-Pleistocene age perhaps equivalent to shoaling sediments in the Ringold's uppermost lacustrine member, the Savage Island. Nearby, the top of the calcrete is reworked and overlain by younger Missoula flood rhythmites. A few thin clastic dikes descend through the ledge. Exposures also reveal basaltic crossbedded channel gravel beneath the overbank deposits. Hwy 17 bridge over Lind Coulee.
A line across the mountain. Elevation of the calcrete-bearing unit (red line) along Saddle Mountains uplift as seen from Crab Creek. The conspicuous white unit follows a mostly unbroken line for tens of kilometers atop footwall benches (Taunton Bench, Smyrna Bench) along the face of the uplift. Ridge crest topography shown in gray. Erosion and a few large landslides have removed the calcrete in places. The same unit armors Paradise Flats-Othello Bench to the west, large portions of Royal Slope to the north (not shown), and the high surface beneath the Maughan Ranch near Sentinel Gap. The east to west profile is ~75 km long.
Capping calcretes formed low. View looking east along the crest of the Saddle Mountains anticline. The ridge today stands some 600m above Crab Creek Valley. The ledgy white layer that caps the ridge is calcrete-cemented alluvial fan with loess beds. The calcrete-bearing unit lies in angular relationship with the underlying Elephant Mountain Basalt and grow to >25m thick in a synclinal trough (one of several secondary structures along the hinge of the larger fold). The calcretes didn't form on top of the ridge. They formed in a low-elevation, low-relief alluvial plain (alluvial parent materials) or on genntle slopes (fan-loess complexes) prior to ridge uplift. Tectonism brought them here - at least some of them. Beneath the fan-loess unit is a a dark red, heavily-cicada burrowed sandy silt unit that thickens locally to more than 5m. In places it is punctuated by pods of coarse fan gravels. Two thin alluvial units directly overlie the Elephant Mountain basalt, a.) an orange-white striped unit with a strong horizontal fabric and b.) a non-descript brown bed of unstratified alluvium (basaltic sandstone) containing locally-derived basalt clasts, some weathered. In places, it contains cicada burrows to 1-3cm in diameter. Beneath the Elephant Mountain basalt is a crossbedded tan sandstone baked red by the heat. At its base is the Cougar Point Tuff (11.8 Ma), a bright white airfall ash reworked and locally thickened.
Thick calcrete in context. Geologic timescale showing common subdivisions relevant to Pliocene-Pleistocene events in the PNW. CaCO3 cements occur in sediments older than 1.8 Ma, both with pedogenic and groundwater-influenced morphologies. For example, Pliocene-age cicada-burrowed loess paleosols in the upper Ringold Fm are carbonate cemented.
Steep north face. Sediments between basalt flows are alluvial deposits - the stuff ovalley bottoms, flowing streams and, to a lesser extent, wind. Interbeds are sometimes just baked soils a meter or so thick. Look closely for soil features or other information that indicates landscape position. Do most baked soils in the Columbia River Basalt province indicate formation on a high, rocky ridgecrest (colluvium, regolith, dust, minimal transport), bare basalt surface (deeply weathered regolith), alluvial fan (repetitive gravel lenses, dusty swales), colluvial mountain slope (angular clasts, unsorted seds), or mucky bottomland (gleyed, clay-rich, trace fossils)? The hike from Crab Creek up the north flank of Saddle Mountains only gains about 500m, but its a stiff go. Luckily, the view from the valley floor isn't too shabby. Calcrete-bearing strata at Saddle Mountains post-dates the 10.5 Ma Elephant Mountain flow (Tem).
Elevated calcretes. Selected sites (colored circles) are distributed near a topographic profile line (gray curve) drawn between the Waterville Plateau and White Bluffs. The profile approximates the lay of the land from north to south. Plio-Pleistocene calcretes are offset by the Saddle Mountains Fault and others in the region.
Previous studies on calcrete in Pasco Basin-Palouse. Relatively few studies have been published on calcrete-capped "ancient" flood deposits in Eastern Washington. I've redrawn the key figure from Bjornstad et al. (2001), who sampled pre-late Wisconsin flood and flood-related deposits at locations listed in Baker et al. (1991). At all sites, calcrete (yellow) predates the Missoula floods and is developed in loess or a flood gravel parent (low-relief, undulating terrain or coulee bottoms). East is at left. Datum is the Bruhnes-Matuyama magnetic reversal at 780,000 year ago.
Calcrete-alluvium association in U.S. Southwest. Alluvial units, highlighted in yellow, are commonly associated with advanced-stage calcretes (Stage III-VI) in the U.S. Southwest. Calcrete study sites shown in this figure are from a classic paper by Machette (1985, Table 2). They reveal a key relationship: Calcrete is intrinsically related to the presence of a valley. All formations in the figure are dominantly alluvial. The Muddy Creek Fm is a well-studied Pliocene basin fill unit composed of clay, silt, conglomerate, sand (bajada and playa deposits in a low-relief landscape). The Ogallala Fm is a Miocene-Pliocene fluvial sand and gravel shed east off the Rocky Mts (fan spilling to a low-relief plain). The Gatuna Fm is a Pleistocene sandstone, conglomerate, siltstone, and limestone (braided stream deposits, terrace, floodplain, playa sediments). Other units and surfaces mentioned have alluvial origins. I hope Michael will forgive my modifications to his figure (colors, labels). Figure below shows similar information, but with age estimates and rates, from Birkeland et al. (1991).
Calcrete chronosequence on Boise River terraces. Othberg et al. (1997) investigated pedogenic calcretes developed in a flight of terraces in the Boise River Valley, ID. Ages of the seven alluvial surfaces increased with elevation and Carbonate Stage from ~15 ka for the lowest terrace (B = Boise terrace) to ~1.7 Ma for the highest (T = Tenmile terrace). Study suggests plugged CaCO3 horizons require a minimum of ~130,000 years to form in west-central Idaho. Approximate ages shown.
Gravel over calcrete over alluvium. Coarse flood gravel truncates a thick, welded calcrete developed in overbank alluvium of the Ringold Fm. Valley deposits in the Pliocene. A flood channel during the Pleistocene. The sediments exposed here are not tectonically tilted, though the site is located on the north flank of Saddle Mountains anticline. At the base of the exposure, a deformed zone (soft sed def) is sandwiched between flat-lying strata above a paleosol. Similarly deformed zones are found elsewhere in the region. Taunton Bench, WA.
Calcrete ledge 40-50cm thick overlies green-gray-brown alluvium, slightly deformed with abundant soil features. Hwy 262 roadcut on north flank of Frenchman Hills west of MarDon Resort.
Liesle Road. Slackwater rhythmites (Touchet Beds) unconformably overlie a calcrete ledge. Possible displacive fabric in calcrete. Quality exposures of rhythmites are not common in the this part of the scablands. Sand tends to dominate here. Location is north of Warden, WA in eastern Quincy Basin.
Calcrete on Beaver River terraces. Machette (1985) investigated pedogenic calcretes developed in alluvial terraces in the Beaver River Valley, UT. Ages of the seven surfaces increased with elevation and Carbonate Stage from ~13 ka for the lowest terrace (Qbv) to ~750 ka for the highest (TG = Table Grounds). Approximate ages shown.
Coyote Creek. Calcrete overlies an exotic-bearing stream gravel. The gravel rests atop bedrock and all indications are it is alluvial (not a megaflood deposit). Its location, however, is suspicious. Exotics in the gravel suggest a former version of this stream reworked older glacial material - certainly till, outwash, and possibly flood-laid sediment. The abundant granitic clasts are not unusual, but light green Belt-like quartzite clasts are. Belt rock does not occur to the north, thus quartzite was not delivered here by the Okanogan Lobe. Quartzite was likely brought into Moses Coulee from the east by an ancient flood. Estimated age on the calcrete is >200,000 years. Gombiner's field area on the Waterville Plateau west of Moses Coulee. SWC photo May 2022.
Fancher Bat at East Wenatchee. "Nice festoon-bedded flood gravel capped with multi-layered petrocalcic horizons, totaling at least 4m thick by more recent measurements nearby." Photos c.1991 and description courtesy of Jim O'Connor. This gravel pit, located along a bedrock-confined reach of the Columbia River, is now mostly gone. Upland setting or valley setting?
Harder Road. Thick calcrete overprints a high-energy gravel and loess. Was the gravel deposited low or high in the landscape? A lowland channel or loess upland? North of Benge, WA.
CaCO3 beards. Cobble-sized clasts with very thick lower-half rinds of CaCO3 in an alluvial fan deposit. Location is the Saddle Mountains crest, 1000' above the Crab Creek Valley. Some interpret lower-half coatings on clasts as a sign of early-stage calcrete development. These rinds are remarkably thick, suggesting that interpretation might not be valid in all cases. Considerable time and countless evapotranspiration cycles are preserved in these beards. In Columbia Basin, loess (not weathered limestone bedrock) provides the calcium. Rainwater provides the CO2. Together, they combine to form CaCO3 in the shallow subsurface. Loess was certainly present in the Pliocene landscape, too. Loess paleosols are found in the upper Ringold Fm and mark shoalings of Lake Ringold. Dust production - a huge influx of calcium - really ramped up during the glacial Pleistocene when outburst floods from proglacial lakes delivered huge volumes of fine grained sediment that the wind recycled into loess hills.
Frosting. Calcrete beard on a cobble, inverted. Carbonate frosting? So postmodern.
Tubes and filaments. Silty-sandy beds between thick calcrete ledges and fan gravels typically contain abundant root casts and filaments consistent with soil development in a moist swale in an alluvial fan. Cemented rhyzoliths like these and denser calcrete have been sampled for U-series for dating (Staisch et al., 2018). Saddle Mountains crest.
Crusts in cracked bedrock. Carbonate-silica cement fills cracks in the top of the Elephant Mtn basalt flow. This crust occurs in the uppermost meter of the basalt and within a thin body of alluvial Ringold sediment above. Not all white crusts in south-central Washington are pedogenic. Saddle Mountains crest.
Formerly low, now elevated basalt surface. Along the Saddle Mountains crest east of Wahatis Peak, calcrete-cemented alluvial fan-loess sediments commonly lies directly atop 10.5 Ma Elephant Mountain Basalt. The Elephant Mountain is the youngest basalt flow in the range. Sometimes another unit is present between the basalt and the fan-loess complex: the Ringold Fm. At its base, one finds a fine grained unit with an orange-white striped lower portion and a fine grained brown-gray upper portion composed of weathered, oxidized basalt detritus. The sediment appears alluvial, slightly reworked, often burrowed by cicada, and contains chunks of angular, weathered basalt (locally derived colluvium). Bedding in the alluvium is flat-lying and parallels the underlying bedrock, consistent with a low-energy floodplain developed atop a more or less flat basalt surface (i.e., overbank-floodplain setting prior to ridge uplift ). The unit appears to represent a sluggish drainage system reestablished after the Elephant Mountain basalt erupted and cooled. The unit exhibits no characteristics of a mountain slope or ridge-crest deposit.
Extra crusty. White carbonate-silica crust atop 12 Ma Pomona basalt thickens downhilll and thins to nothing just uphill of here. It appears to be the base of the Ringold - the local expression anyway. Cements of identical character and position occur atop the 10.5 Ma Elephant Mountain basalt 27 km to the west, where tens of meters of overlying Pliocene and Pleistocene sediments remain, preserved in structural lows. The crust did not form high on an airy ridge crest. The crust overprints an unconformity formed during uplift, which bevels both the Pomona and Elephant Mountain flows. Uplift and beveling can be no older than latest-Miocene, but is probably younger. Uplift of Saddle Mountains seems to have occurred in two stages: gentle warping early, rapid uplift late. Corfu Landslide Overlook, Saddle Mountains.
Fine grained alluvium way up here? Weathered, brownish-red sediments mapped as Ringold are preserved along the ridge crest of the Saddle Mountains 1000' above Crab Creek. Brown beds often contain abundant fossil cicada burrows. Others clearly used to be wetland muck. Lobes of basaltic alluvial fan gravels invade some beds - gravelly channels sourced from the ridge above. That ridge, that source no longer exists; we're just a few meters down from the crest. Valley alluvium like this received fan and debris flow material from above, but was not itself transported downslope. The receiving sediments show no evidence of internal mass wasting (e.g., evidence of a steep slope). The beds closely resemble portions of paleosols in the Savage Island Member of the Ringold Fm at Smyrna Bench, White Bluffs, and Watt Lane - paleosols that record periods of shoaling of Pliocene Lake Ringold 3-4 million years ago. All sedimentary beds and paleosols that sit atop the Elephant Mountain flow thin and thicken with the folds in the bedrock. Crest of Saddle Mountains west of Wahatis Peak.
Cicada in modern and ancient landscapes. Do colonies of cicada populate residual soils on windy, barren ridge crests or cumulic soils in vegetated, alluvial bottomlands? Rocky ridges are typically dry, degredational parts of the landscape. Valleys are typically wet and fertile. Cicada are like Hobbits. They prefer valleys where thick, moist soils sustain healthy plants. Cicada feed on the fine roots of plants, not on desiccated lichen or soil crusts of rocky ridges. In fact, many cicada species show a strong affinity for specific plant communities. In their study of modern cicada in the Columbia Basin, O'Geen et al. (2002) examined cicada-burrowed soils in four plant communities (sagebrush steppe, bunchgrass steppe, meadow steppe with pine stands, coniferous forest). Only beneath sagebrush rooted in loess (Haploxeroll and Haplocambid soils) were significant populations of genus Okanagana found. Okanagana is one of the most common cicada found in Eastern Washington. They "interpret cicada activity as a near-surface pedogenic process whose occurence is preserved through time in soils of aggrading landscapes." Sandborn and Phillips (2013) confirm sagebrush steppe habitats in the Columbia Basin host many cicada species, including O. occidentalis, canadensis, fratercula, luteobasalis, gibbera, napa, and utahensis. O. striatipes, vanduzeei, lurida, and yakimaensis are found in the Palouse Hills (prairie). O. vanduzeei is common to both sagebrush steppe and the Palouse prairie. Photo shows a fossil cicada feeding burrow in Pleistocene loess at Rulo, WA. Whatever organic matter the thing was eating has long since decomposed, but we know it (roots of sagebrush) was there in the past.
Low-angle fan gravels. Cemented alluvial fan gravels that post-date the Ringold Fm were deposited on gently-sloping surfaces. Some of the gravelly lobes invaded Ringold lake beds and paleosols (stuff earlier workers called "early Palouse" loess). Other gravel-filled troughs are found on formerly dry hillslopes located somewhere above the elevation of the lake's shoreline. These are proximal fan gravels composed of basaltic clasts shed from an earlier version of the Saddle Mountains ridge, one with a bit less topographic or structural relief. Range front faults and smaller cross-range structures tilt and offset the fan gravels locally.
Range front tilting. A young alluvial fan gravel truncates an older, tilted soil profile (brown-beige-purple) that is underlain by coarse, angular basalt regolith. Bedrock is <1m below the not-far-traveled basaltic detritus, off the photo. Repose angle of the young fan is steep, but not so steep that one can conclusively say it too has been tilted. The range-bounding thrust is to the left (north), nearby, but hidden. These youngest fans are not cemented by calcrete and closely conform to the steeper angle of modern hillslopes. North flank of Saddle Mountains a few hundred meters south of Lower Crab Creek Rd near rail line.
North flank viewpoint. The 11.8 Ma Cougar Point Tuff is overlain by tan crossbedded fluvial sandstone (Columbia River), the top of which is baked red by the Elephant Mountain basalt. Remember that everything seen here - CRBG lava flows and their sedimentary interbeds - was confined by or deposited in a structural trough or river canyon (Camp, 1981; Tolan et al., 1984). View to the west.
Weathered basalt up high. What controls the geographic distribution of deeply weathered zones in individual CRBG flows? Spheroidally weathered Priest Rapids flow in the photo is located high on the Saddle Mountains ridge. It crumbles away in your hands. When did deep weathering occur? Before uplift or after? When the basalt was flat-lying, buried, and below the water table (low in the landscape) or after uplift (high in the landscape)? Is groundwater to blame or atmosphere, wind, and weather? If the former, then weathering profiles in individual Columbia River Basalt flows might indicate residence time in the subsurface (low position, saturated), thus say something about the timing of uplift. If the latter, then climate - acting primarily through rainfall and freeze-thaw action - was incredibly efficient at chemically degrading basalt flows exposed on ridges. Flows above and below the Priest Rapids do not show the same weathering charcteristics. Considerable variation in the degree of weathering exists in the same flow as you move east to west across the Basin. Here, the depth of weathering is >4m with little vertical zoning. It seems likely groundwater, with its greater capacity to degrade mafic rocks quickly, was involved. I suspect deep weathering in some flows mostly occured prior to uplift, when flows occupied valley positions (Miocene-Pliocene). I don't believe deep weathering occurred after uplift atop high, dry ridges (Pliocene-present). Minimal modification/soil profile formation occured once elevated (i.e., uplift shuts off deep weathering). Could a flow not be traced across several YFB ridges and its weathering characteristics mapped using some simple classification system (weathered, fresh, mixed)? Do deeply-weathered portions of flows correlate to synclines? Could the timing and configuration of the paleo-landscape be reconstructed through weathering profiles in CRBG flows?
Water-lain ash. Pure white volcanic ash at the base of the Cougar Point Tuff (Miocene Ellensburg undifferentiated, Tel) fell into a shallow, quiet body of water. The ash almost looks varved, but I suspect that's just bedding produced by slow-flowing water. Following the eruption 11.8 million years ago, the airfall ash blanketed a valley bottom - a former Crab Creek valley. The ash grades upward into crossbedded sands of a fluvial channel of some size (not a mountain gully). The exposure in the photo is located high on the north flank of Saddle Mountains, just below the ridge crest, some 500m above modern day Crab Creek.
Cougar Point ash. The rhyolitic airfall ash, erupted 11.8 million years ago from the Owyhee-Jarbidge volcanic center on the ID-NV border, blanketed parts of central Washington. The slug of ash was dumped on the landscape and subsequently reworked by water. Here, the ash is several meters thick and overlain by a tan crossbedded sandstone and the Elephant Mountain flow, which bakes the sand red. The depositional setting was not a ridge crest, but the low-elevation floodplain of the ancestral Columbia River. The white lower portion is almost pure ash up to several meters thick. It contains laminae that resemble varves, but are not. Tan sediments above, part of the Ellensburg Fm, is a sandstone with crossbeds and other ripple forms, evidence of a lowland channel of some size. The rather abrupt color change from white to tan to red is often conspicous, but is not seen everywhere. The white to tan transition may represent a lateral shift in the stream channel (fluvial sediments over pond sediments). Or perhaps it marks the point when the ash-choked channel, temporarily defeated, recovered and resumed its usual flow. Western Saddle Mountains. Camera for scale.
Nick always finds it. Crossbedding in sandstone that overlies the Cougar Point ash. Screenshot from Nick Zentner's YouTube video "Saddle Mountains - Volcanic Ash from Yellowstone Explosion".
Nearly an interbed. I've become convinced that the Ringold and overlying calcrete-bearing unit (up to 25m thick), the so-called "Plio-Pleistocene unit", is very similar to Miocene sedimentary interbeds in the Columbia River Basalt Group; they are valley fills. Interbeds are packages of alluvial sediments and soils deposited atop nearly flat basalt flows in the centuries following eruption. Valley bottom sediments that represent the reestablishment of the drainage network and hillslope sediment systems after each flow cools. The upper Ringold is about the same thickness and similar geographic extent as many of the named interbeds. The northern Pasco Basin during the Miocene wanted to be a broad floodplain where large rivers of the region converged, but business-as-usual fluvial activity kept being interupted by invading flood basalts that covered floodplains. Faults warped the bedrock, diverting streams. The only reason we don't call the Ringold Fm an interbed is because CRB volcanism shut off around 6 Ma. That last basalt flow never came to bury the Ringold. If volcanism would have shut off earlier, then Ringold sediments would be stacked on top of older sediments of the Ellensburg. Along the margins of certain basalt flows, Ringold may rest on Ellensburg, but maps don't clearly distinguish one from the other. Photo: Saddle Mountains.
Crusts on bedrock. Platy crusts atop young basalt flows in the Saddle Mountains are a combination of calcrete and silcrete. Silica content, though never dominant, is markedly higher there than in thicker pedogenic calcretes higher in the section (fan-loess-calcrete complexes). A prominent bedrock crust at Saddle Mountains occupies the contact between Elephant Mtn basalt and Ringold Fm. The crust appears to be groundwater-formed (not soil formed), thickens through a zone of about a meter, and grades laterally into an orange-white stripey unit that contains basaltic colluvium. If understanding the lay of the paleolandscape, its important to distinguish the two types of duricrusts and their parent materials. Distinguish white platy crust from white pedogenic calcrete with this rule of thumb: If on basalt, then look for weathered bedrock and groundwater features. If in gravel or loess, then look for soil features (root casts, burrows, horizons, etc.) and evidence of surface aggradation (addition of dust). See variants in photos below.
Crust variant. Platy crust on Elephant Mountain basalt lies at the base of the Ringold Fm. Unlikely that this deposit is pedogenic.
Crust variant. Platy crust stringers cement horizontal fractures in the weathered surface of Elephant Mountain basalt. Stringers coalesce and/or grade upward into the orange-white stripey unit. Weathered bedrock regolith (C-horizon), but not much in the way of pedogenic features. Where's the paleo-water table?
Crust variant. Orange-white stripey unit on basalt. Platy wrust morphs into heavy stringers in oxidized, fine grained sediment containing angular chunks of basalt (colluvium).
Crust variant. Thickened groundwater calcrete-silcrete crust with distinctive ribboning. Sometimes directly on basalt, sometimes on a thin gray shale overlying basalt. Typically found near axes of secondary folds - N-S trending synclines along crest of the larger E-W trending Saddle Mountains anticline.
A big thank you to Eric Cheney for encouraging me to pursue this work. Others that have provided tremendous help include Jim, Michael, Kevin, Kathleen, Richard, and Bruce. The work continues.
Real geologists. Geology used to be blue collar work. Let's not forget our roots. A U.S. Geological Survey party led by Israel C. Russell, 2nd from right, working the moraines of the Malaspina Glacier, Yakutat District, Alaska. They had it tougher than you.
Field notes matter. Some general field notes on post-Elephant Mountain sediments I observed along the Saddle Mountains crest east of Sentinel Peak. If you watch Nick Zenter's YouTube video "Saddle Mountains with Skye Cooley", this is some of what you see me writing. Fieldnotes are a combination of facts and measurements, thoughts and ideas, answers and questions. You often only get one chance to visit a place, so make it count by taking good notes. Notetaking is a bit of a dance. Your skills improve with practice. Record what the rocks are telling you, but listen to that voice in your head, too. Get those thoughts, those sketches, those questions down on paper before you leave the outcrop. The field is often ambiguous and physically uncomfortable, factors that keep many geologists indoors. You don't control the weather or the terrain. You'll always miss something. There's always more to see. Smarter people came before and will come after. Get used to it. A central goal of the Field Geologist is to efficiently characterize and document what matters, missing as little as possible in the process. Sketches are the center of my notetaking. They help organize my thoughts and fill about half the pages of my notebooks.