Calcrete Growth in Alluvial Lowlands

I can't get any lower.

Still I feel I'm sinking.


A robust association between thick pedogenic calcretes (petrocalcic horizons) and non-calceous lowland alluvial deposits is a key finding of my recent work in south-central Washington. This lowland association is hardly mentioned in previous articles. Most, if not all, meter-thick (Stage V) calcretes that post-date sediments of the uppermost Ringold Formation (Savage Island Member, >3.4 Ma) 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.

Were all calcretes in Eastern Washington formed in a lowland setting? No. Many calcrete ("caliche") horizons in the Palouse Hills, some of which are quite thick and developed beyond carbonate Stage III, clearly formed by similar soil processes on upland hillslopes in loess (Busacca, 1989; McDonald et al., 2012). However, the morphology of Palouse calcretes differs markedly from those developed in alluvial sediments in Pliocene-Pleistocene basins farther west.

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 a formerly more-contiguous basin configuration - prior to rise of the Yakima Folds. Today's basins are small fragments of a once larger lowland. While the geographic terms "Pasco Basin", "Umatilla Basin", and "Kittitas Valley" are useful, it is important to recognize that certain valleys were previously interconnected. These supra-basalt depocenters (late Miocene to present) collected a wide variety of sediments shed from the surrounding highlands and deposits from through-flowing rivers (think Ellensburg Fm). Many lowland valley deposits and dusty fan gravels on adjacent hillslopes stabilized and became crusted with calcrete, a hardpan soil composed of calcium carbonate. Today we find the thickest calcretes in the lowest basinal areas near the center of this map (northern Pasco Basin). Calcrete development diminishes as one moves away from Pasco Basin. May by USGS/Snyder and Haynes (2010, Plate 1).

Capping calcretes. 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 with various common subdivisions and events in the Pacific Northwest.

Steep north face. The hike up the north flank from Crab Creek only gains about 500m, but its stiff. 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 through south-central Washington show calcrete-armored surfaces (heavy black lines) deformed by Yakima Fold Belt structures and eroded awy in coulees cut by Ice Age floods. Note surface preservation at elevations below the level of maximum flooding, here approximated by Lake Lewis highstand (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. Purchase from

CaCO3 beards. Cobble-sized clasts with very thick lower-half rinds of CaCO3 in an alluvial fan deposit. Saddle Mountains crest.

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. Perhaps stable swales in a broad, low-angle alluvial apron. Saddle Mountains.

Crusts. Carbonate-silica cements fill cracks in basalt bedrock where sedimentary interbeds overlie them (or used to) or where bare bedrock has been exposed for a long period (i.e., bare crests of anticlines). Saddle Mountains crest.

Elevated basalt surface. In the Saddle Mountains east of Wahatis Peak, a complex of calcrete-cemented alluvial fan-loess sediments commonly lies directly atop 10.5 Ma Elephant Mountain Basalt (Tem). The Elephant Mountain is the youngest basalt flow in the range. Sometimes another unit is present between the basalt and the fan-loess complex: a fine grained unit with an orange-white lower portion and a brown-gray upper portion composed of weathered, oxidized basalt detritus that appears reworked in a sluggish alluvial setting. In places, it contains 2 cm-diameter burrows (trace fossils) and angular basalt clasts (colluvium). Bedforms are always flat-lying, parallel to the underlying bedrock, and consistent with a low-energy depositional environment akin to a overbank-floodplain setting. The unit appears to represent the reestablishment of the local drainage system atop the recently-cooled, flat-lying surface of the Elephant Mountain basalt. The unit is in no way a mountain slope or ridge-crest deposit.

Extra crusty. White carbonate crust atop Pomona basalt thickens downhilll and thins to nothing just uphill of here. Corfu Landslide Overlook, eastern Saddle Mountains.

Fine grained sediments up high? The weathered, brownish-red sediment, preserved along the ridge crest of the Saddle Mountains, is fine grained and often thoroughly burrowed (cicada). There are multiple beds of the stuff that thicken, in places, to several meters. The unit is invaded by lobes of basaltic alluvial fan gravels. Its deposition is consistent with a gently-sloping, low-relief setting and relatively low landscape position; it periodically received fan gravels shed from higher on the same slope, but itself shows no evidence of internal mass wasting or failure. The beds resemble silty paleosols in the upper Ringold Fm (Savage Island Member) that mark periods of shoaling of Pliocene Lake Ringold between ~3-4 m.y. and to certain Pleistocene deposits in the region sometimes called "early Palouse soil".

Low-angle fan gravels. Cemented alluvial fan gravels, even where coarse grained, were deposited on gently-sloping surfaces. Some lobes invaded Lake Ringold sediments while still wet, 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 bedrock ridge on which they reside today, in this case Saddle Mountains. Near range-bounding faults the fan gravels are tilted, sometimes steeply.

Range front faulting. 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.

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 Cougar Point ash, erupted 11.8 million years ago from the Owyhee-Jarbidge volcanic center located along the ID-NV border, is believed to be an airfall ash that blanketed parts of central Washington. The slug of ash dumped on the landscape was subsequently reworked by water. Here, the ash is several meters thick and overlain by the Elephant Mountain flow, which bakes it or the fluvial sediments that overlie it red. The depositional setting was not a ridge crest, but a lowland plain. White lower is almost pure ash up to several meters thick that was deposited in a quiet water setting. It contains laminae that resemble varves, but are not. Tan sediments above, part of the Ellensburg Fm, comprise several meters of sandstone with crossbeds and other ripple forms, evidence of a lowland channel of some size. The rather abrupt color change from white to tan is conspicous here, but is not visible everywhere. It may represent a lateral shift in the channel (fluvial seds over lacustrine). Or perhaps the change marks the point when the ash-choked, temporarily defeated channel recovered and resumed its usual flow. 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".

Deeper cements. I've become convinced that the calcrete-bearing interval in south-central Washington (up to 25m thick), the so-called "Plio-Pleistocene unit", is nearly identical to Miocene sedimentary interbeds in the Columbia River Basalt Group, like the one shown atop a basalt flow here. Its about the same thickness and has a similar geographic extent as many of the named interbeds. That's a new take. CRB volcanism simply shut off before late Ringold and the Plio-Pleistocene unit could be enclosed by another basalt flow. Saddle Mountains.

Cemented bedrock on the cheap. Cements form crusts directly on bedrock in places. Big fan of cheap sunglasses. Saddle Mountains.

Red gravel. The red gravelly deposit with a fine grained matrix is a debris flow shed from high on the north flank of the Saddle Mountains ridge. Here, it overrides and truncates a green-gray wetland mudstone shot through by vertical and horizontal stringers of calcrete (a mature lowland paleosol). The debris flow deposit formed in the blink of an eye (instability). The cemented soil surface took tens of thousands of years to develop (stability). Sediments and soils preserved on and around the Saddle Mountains speak to stability and instability - slope angle, landscape position, triggers, source and sink concepts - that help geologists reconstruct the former landscape.

Calcrete-alluvium association. Alluvial units, highlighted in yellow, are commonly associated with advanced-stage calcretes (Stage III-VI) at study sites from Colorado, New Mexico, California, and Utah in this classic paper by Machette (1985, Table 2). Several of the other formations and surfaces, not highlighted, are also alluvial.

- Muddy Creek Fm: Pliocene basin fill of clay, silt, conglmerate, sand. Bajada and playa deposits in a low relief landscape.

- Ogallala Fm: Miocene-Pliocene fluvial sand and gravel shed east off the Rocky Mts, deposited in a low relief landscape.

- Gatuna Fm: Pleistocene sandstone, conglomerate, siltstone, and limestone. Braided stream deposits, terrace, floodplain, playa sediments.

Real geologists. U.S. Geological Survey geologist Israel C. Russell, 2nd from right, leading his party across the moraines of the Malaspina Glacier, Yakutat District, Alaska. They were tougher than you. Geology used to be blue collar work. Go outside.

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