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Calcrete Growth in Alluvial Lowlands

I can't get any lower.

Still I feel I'm sinking.


Repeated recharge and evaporation result in the development of calcrete. We know this from soil science in general and from focused study of calcic soils worldwide (Reeves, 1976; Machete, 1985; Wood et al., 1992).

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 recent work on Pliocene-Pleistocene sediments and soils in the Saddle Mountains-Pasco Basin area. Most meter-thick (Stage III-V) calcretes that post-date sediments of the uppermost Ringold Formation formed in a lowland, basinal setting near a shallow water table or in gently-sloping alluvial fan-loess complexes low on the flanks of Yakima Fold Belt ridges.

Puzzling why this lowland association goes unmentioned in previous articles, especially when other calcrete regions of the world are considered. Many of the well-studied calcretes of Australia show a clear association with valley-fill sediments and broad alluvial plains with low gradients (Chen et al. 2002; Sofoulis, 1963; Butt et al., 1977).

One reason calcretes in Eastern Washington might not have been linked to lowland settings is the Palouse. Numerous articles on Palouse loess and its "caliche" horizons have been published in geological journals over the past four decades (Busacca, 1989; McDonald et al., 2012). In all likelihood, many Pacific Northwest geologists were introduced to caliche on field trips to the Palouse. Caliche is a colloquial term for calcrete. Caliche horizons in Palouse soils are thin, young (less than Carbonate Stage III), and unquestionably formed by soil processes on upland hillslopes. However, Palouse Slope caliche differs markedly in thickness, lateral extent, and morphology from the multi-story calcretes developed in alluvial sediments farther west. Calcrete comes into its own in the Pasco Basin.

Another reason might be linguistics, specifically the term "caprock". Calcrete in the Pasco Basin has for decades been described as caprock 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 problematic. It describes calcrete's position in the modern landscape and infers an association with plateaus and ridges. The "capping" calcretes at White Bluffs and Saddle Mountains are paleosols formed in lowland settings just above the water table and were later elevated by tectonism and fluvial incision.

Neogene lowlands of the Columbia Basin. Visualize Late Tertiary Eastern Washington 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 configuration (prior to rise of the Yakima Folds). Today's basins are small fragments of an old regional lowland. While the geographic terms "Pasco Basin", "Umatilla Basin", and "Kittitas Valley" are useful, recognize that certain valleys that today stand apart were previously interconnected. These supra-basalt depocenters (late Miocene to present) collected a wide variety of sediments shed from the surrounding highlands as well as sediments from through-flowing rivers. For example, consider the sedimentologic diversity of the Ellensburg Fm. Its lowland valley deposits and low-angle alluvial fan gravels that interfinger with them that became crusted with calcrete. The thickest calcretes occur near the center of the map, near the center of the ancient basin. Calcrete thins and hardpan soil development diminishes as one moves away from Pasco Basin in any direction. Map by USGS/Snyder and Haynes (2010, Plate 1).

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.

Calcrete in context. Geologic timescale showing common subdivisions relevant to Pliocene-Pleistocene events in the PNW.

Steep north face. 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, especially with a long lens.

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.

Calcrete-armored surface warped by tectonism. 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 or MBMG publications office.

Loess stands vertical. Some have questioned whether tilted loess would erode quickly or hold together long enough to become cemented with calcrete (e.g., develop a paleosol). The question relates to the timing of cementation. Does calcrete parallel sedimentary bedding or, being governed by soil proceses, can it overprint (crosscut) bedding in tilted sediments? In homogenous sediments like loess, horizontal calcrete may mask tilted bedding. At Lind Coulee, the Frenchman Hills Fault shoves Miocene basalt over Pleistocene loess. The deformed loess dips steeply, protected beneath the wedge of basalt. This example is admittedly an outlier, but serves illustrate a point. The faulting and the loess are both young, so a paleosol (perhaps calcrete) has not had time to form. Nevertheless, modestly-cemented loess can stand near vertical for at least a few thousand years. The coulee has existed for >10,000 years.

Lind Coulee Fault. A sketch from my investigation of faulted alluvium in lower Lind Coulee. The column 4th from the left corresponds to the photo and sketches above.

Fieldnotes. 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, questions and mysteries. 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.

CaCO3 beards. Cobble-sized clasts with very thick lower-half rinds of CaCO3 in an alluvial fan deposit. Location is the Saddle Mountains crest. Considerable time and countless cycles of evapotranspiration are preserved here. Calcrete in Columbia Basin is different from calcrete in the desert southwest (i.e., New Mexico). There, calcrete forms when soil water, charged with CO2 dissolved from the local carbonate bedrock, moves in solution with rainwater as calcium bicarbonate, precipitating in the soil profile above the water table. In Columbia Basin, loess (not bedrock) provides the calcium. Loess was certainly present in the Pliocene landscape, but dust production and loess deposition (thus a huge influx of Ca+) really ramped up during the Pleistocene in tandem with megaflooding.

Frosting. Calcrete beard on a cobble, inverted. Carbonate frosting? So postmodern.

Tubes. Silty-sandy beds between thick calcrete ledges and/or carbonate-encrusted fanglomerates contain abundant root casts and filaments (trace fossils) consistent with a low-relief, alluvial setting. Moist swales in a broad, low-angle alluvial apron. Cemented rhyzoliths like these and calcrete matrix material have been sampled for U-series for dating (Staisch et al., 2018). Saddle Mountains crest.

Crusts on bedrock. Carbonate-silica cement fills cracks in the top of a basalt flow. A sedimentary interbed used to overlie it. Saddle Mountains crest.

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, 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 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 occur atop the Elephant Mountain basalt 27 km to the west, where tens of meters of overlying Pliocene and Pleistocene sediments remain. Corfu Landslide Overlook, Saddle Mountains.

Fine grained alluvium up high? Weathered, brownish-red sediments of the Ringold are preserved along the ridge crest of the Saddle Mountains. They contain abundant fossil cicada burrows. There are several of these beds with paleosols that thin and thicken. Lobes of basaltic alluvial fan gravels invade the section locally - gravelly channels sourced from the ridge above. This valley alluvium received material, but was not itself mobilized downslope. The sediments themselves show no evidence of internal mass wasting or failure (e.g., evidence of a steep slope). The beds resemble certain silty paleosols in the Savage Island Member at Smyrna Bench, White Bluffs, and Watt Lane that mark periods of shoaling of Pliocene Lake Ringold 3-4 million years ago. Crest of Saddle Mountains.

Cicada in modern and ancient landscapes. Do colonies of cicada populate residual soils on windy, barren ridge crests or cumulic soils in alluvial bottomlands? Rocky ridges are typically dry, degredational parts of the landscape and valleys typically wet and fertile. Cicada (like Hobbits) prefer the latter: thick, moist soils replete with plant roots - the stuff of valleybottoms and low-angle slopes nearby. Cicada feed on the fine roots of plants not on desiccated lichen or soil crusts of rocky ridges. 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 soils in four vegetation zones (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. They "interpret cicada activity as a near-surface pedogenic process whose occurence is preserved through time in soils of aggrading landscapes." According to Sandborn and Phillips (2013), sagebrush steppe habitats in the Columbia Basin host species 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.

Low-angle fan gravels. Cemented alluvial fan gravels that post-date the Ringold Fm were deposited on gently-sloping surfaces. Some lobes invaded Ringold lake beds, Ringold paleosols, and "early Palouse" loess, while others were deposited on dry hillslopes somewhere above the elevation of the shoreline. These proximal fan gravels, composed of basaltic clasts, were shed from an earlier version of the Saddle Mountains ridge, one with 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 deposit 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 basalt detritus, but 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. 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 sandstone, the top of which is baked red by the Elephant Mountain basalt. View to the west.

Weathered basalt up high. When did deep weathering of young basalt flows occur? Was it when they were flat-lying, buried, and below the water table (valley position, prior to uplift)? Or was it after uplift, when exposed to atmosphere, wind, and weather? If the former, then weathering profiles in individual Columbia River Basalt flows might indicate residence time at the surface (relatively dry) or in the subsurface (saturated), thus uplift/exposure history. If the latter, then climate - acting primarily through rainfall - was incredibly efficient at chemically degrading basalt exposed at the surface. This flow in the photo is near the Saddle Mountains crest. Flows above and below do not show the same weathering charcteristics. The depth of weathering is >4m with little vertical variation. It seems likely groundwater, which its greater capacity to degrade rock quickly, was involved. How similar is saprolite-corestone weathering in the CRBG to that in formerly-buried, spheroidally-weathered granites (i.e., Sherman Batholith at Vedauwoo, WY and Boulder Batholith at Homestake Pass, MT)? I suspect deep weathering occured prior to uplift when basalt flows were still in wet valley positions (late Miocene-Pliocene), not atop dry ridges (Pliocene-Pleistocene). The bulk of the weathering work was probably done by groundwater with minimal modification once elevated (i.e., uplift shuts off weathering). Could a flow not be traced across several YFB ridges and its weathering characteristics assessed? Could the timing and configuration of paleo-lowlands and paleo-uplands (synclines, anticlines) be reconstructed through weathering profiles?

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. Interbeds are packages of alluvial sediments and soils deposited atop basalt flows in the centuries following eruption. Interbeds are composed of valley bottom sediments that represent the reestablishment of the drainage network and hillslope sediment systems after each flow cools. The upper Ringold and PPU are about the same thickness and have a 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 valley floors. A few faults uplifting blocks of bedrock also worked against streams, diverting them. 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. The Ringold was never enclosed. If volcanism would have shut off earlier, then Ringold sediments would be stacked on top of older sediments. Neither would be interbeds. 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 is markedly higher in them than in thicker pedogenic calcretes that cap the ridge (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, thickens to more than meter, and grades laterally into an orange-white stripey unit that contains basaltic colluvium. In a few places, similar-appearing calcrete and the platy crusts occur in close proximity to one another on the ridge crest. Distinguish platy crust from pedogenic calcrete with this rule of thumb: If on basalt, then platy groundwater crust. If in gravel or loess, then pedogenic calcrete. See variants in photos below.

Crust variant. Platy crust on Tem basalt.

Crust variant. Platy crust stringers cement horizontal fractures in the weathered surface of Tem basalt. Stringers coalesce and/or grade upward into the orange-white stripey unit.

Crust variant. Orange-white stripey unit on basalt. Crust morphs into heavy stringers in detrital-alluvial 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 in troughs of secondary folds (synclines).

Calcrete-alluvium association. Alluvial units, highlighted in yellow, are commonly associated with advanced-stage calcretes (Stage III-VI) in the U.S. Southwest. Calcrete study sites in this classic paper by Machette (1985, Table 2) reveal the relationship. The Muddy Creek Fm is a Pliocene basin fill 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 similarly alluvial origins. Figure modified from Machette (1985).


A big thank you to Eric Cheney (UW Emeritus) for encouraging me to pursue this work.

Real geologists. Geology used to be blue collar work. A U.S. Geological Survey party led by Israel C. Russell, 2nd from right, works the moraines of the Malaspina Glacier, Yakutat District, Alaska. They were tougher than you.

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