Calcrete Field Trip - Offramp
Directions - Offramp stop is located just east of the Hwy 26-Hwy 17 interchange SE of Othello. There are two ways to access it depending on how many vehicles you have. If your group is small, drive south from Othello on Hwy 17 and exit via the offramp to Hwy 26 toward Pullman. Before the overpass, pull into a gravel lot on left (north side). Walk beneath the overpass and along a fence and dusty two-track between the northbound offramp road and the hay storage area. Walk a few hundred meters south, cross the wire fence, and drop below the rim to canal level and the cutbank exposure. Footing can be loose, brushy, and seasonally wet.
If your group is large (>6 vehicles), access is better via a short track accessible from the northbound lane of Hwy 17 south of the interchange. From Othello, drive S on Reynolds Rd to Bench Rd and turn L (north) onto Hwy 17. Before reaching the northbound offramp onto Hwy 26, look for short driveways leading to an unmarked spur road on the right just past the white "Othello Truck Route" sign and before the green "Othello/Vantage" sign. Beware of fast-moving traffic behind you. Drive the grassy track north for a few hundred meters, paralleling the highway, to its end near Potholes Canal. Follow a messy, informal path north a few hundred meters through sagebrush to the outcrop, crossing a wire fence (5 min hike). Drop down to outcrop at its north end.
Outcrop Description - Two thick calcrete ledges are exposed beneath irregular lenses of reddish, fine grained sediment containing abundant soil features and trace fossils between. The reddish beds are wind deposited, closely resembling Palouse loess. The sediment was colonized by plants, insects, and rodents. Flat-lying Savage Island lake beds are present at the base of the exposure. Lakebed parent material is clearly visible through the lower ledge except for the upper 20cm, where it becomes obscured by a blocky, fully-plugged calcrete.
Close, but not that close - Offramp is near the road, but not that easy to get to. Decide between parking options, then hike, cross a wire fence, and drop below the rim to the cutbank exposure. Its worth the trouble.
Early work on old soils - Numerous researchers have described the ambiguous package of carbonate-cemented sediments sandwiched between the Ringold Fm and Missoula flood deposits, but few in detail. The Plio-Pleistocene interval has gone by various names, including "early Palouse soil", "pre-Missoula gravels", "Plio-Pliestocene unit", "stratified fines", and a "locally derived subunit" (Merriam and Buwalda, 1917; Bryan, 1927; Beck, 1936; Culver, 1937; Bretz and Horberg, 1949; Bretz et al., 1956; Newcomb, 1958; Brown, 1959, 1960, 1970; Tallman et al., 1979; DOE, 1988, Bjornstad 1990; Delaney et al., 1991; Lindsey et al., 1992; Lindsey et al, 1994; Slate, 1996, 2000; Lindsey et al., 2000; Wood et al., 2000; Wood et al., 2001). A notable early effort to map and characterize the unit was by Grolier and Bingham (1971, 1978). Since then, there has been considerable naming, renaming, lumping, and splitting of the deposits at the Hanford Site based on information from wet borehole cuttings (i.e., Bjornstad et al., 2010).
A two-ledge exposure - The Offramp stop provides a good look at a "two-ledge" exposure, common in this area. The carbonate ledges are traceable units, while the sediments they overprint change from place to place.
Lower ledge - The lower calcrete ledge is developed in Savage Island lakebeds. Look close and you'll see the parent material. Does the red sediment on top look like flood sediment to you?
Stratigraphy - This is how I typically depict layering at calcrete-bearing outcrops. It works well enough. The amount of detail I show is appropriate for geology and geomorphic interpretation. Soil scientists might do it differently. Let them. The gains are minimal and their notation tends to be pretty abstract. Moving on. There are two important things to note at the Offramp site. 1.) The upper ledge overlies and overprints silty sediment containing abundant cemented root casts and burrows of insects and rodents (i.e., all signs of biological colonization and land surface stability). Sand between the ledges is minimal and gravel absent. What's exposed here hardly resembles a flood deposit. These fine grained deposits, based mostly on their stratigraphic position, could be distal equivalents to coarser, more obvious flood deposits nearby. 2.) The lower calcrete ledge is developed in Savage Island lakebeds. This parent material has a strong horizontal fabric and, though thoroughly cemented by CaCO3, bedding is still clearly visible in all but the upper 20cm of the ledge, which is a fully plugged, blocky calcrete. This overprinting demonstrates calcrete overprints the scaffolding provided by the parent sediment. Unlike loess, which can aggrade smoothly and drag the Bk horizon upward as the ground surface rises, these lake beds were deposited, beveled by erosion, then cemented. There was no upward aggradation coincident with cementation. The Bk horizon did not migrate upward. The carbonate front descended through the parent material.
Cemented tubes - Cemented tubes of different diameters occur in old loess. Here, small diameter root casts and insect burrows. They are relict soil structures that indicate a stable, lowland setting. Larger root casts of woody plants are also common (3-4cm diameter) in the silty sediment between the two ledges.
Traces of cicada - Cicada nymph feeding burrows are a distinctive trace fossil in soils of Eastern Washington, both old and young. They are B-horizon features, typically a couple centimeters in diameter that form stubby cylinders 3-15cm long with fine, arcuate ridges. People tend to associate them with loess, but they occur in coarser sediment, too. Again, a mark of surface stability.
Pliocene at the bottom - Green and tan Savage Island siltstones (<4 Ma), present at the base of the exposure, are lake bottom and sluggish fluvial sediments of upper Ringold Fm (8.5-3.0 Ma). The unit is flatlying in the area. The Othello area has been a relatively low elevation and low relief location for quite some time. Local relief is the result of erosive megaflooding, primarily down Crab Creek Valley and Koontz-Ringold-Eagle Lake Coulees.
Large group option - If the hay storage area is busy, you can pull off the northbound lane of Hwy 17 onto a short grassy track that ends at the canal. Pullouts located near the white "Othello Truck Route" sign. Hike north to the outcrop near stacked bales.
Nearby outcrop - A second outcrop is visible to the south along the canal. Poor footing requires some excavation. Savage Island is well exposed and overlain by the thick carbonate ledges. The intrepid geologist will find other exposures along the canal, but they are more difficult to access without attracting attention.
A word on Hanford's "Cold Creek unit" - Geologists at the Hanford Site, 30km SW of Othello, call their Plio-Pleistocene strata the "Cold Creek unit (CCU)" and divide it into upper and lower units. Lower unit "calcrete and sidestream deposits" (late Pliocene) are differentiated from the "eolian and alluvial silt-dominated facies" of the upper unit (early-middle Pleistocene) along a sharp, distinct contact (Reidel and Chamness, 2007).
The lower CCU consists of basaltic to quartzitic gravels, sands, silt, and clay cemented by multiple layers of secondary calcium carbonate interpreted to be pedogenic locally influenced by the paleo-water table. It includes a variety of relict soil structures, cemented root casts, and burrows by insects and animals.
The upper CCU consists of generally massive, unconsolidated, brown-red-yellow silt and sand of fluvial and eolian origin. It is typically loose, friable, and includes the "early Palouse soil" and other pre-Missoula Pleistocene sediments believed correlative. The upper unit includes relict soil structures, root casts, and burrows. The contact between the upper unit and overlying megaflood deposits is indistinct and possibly gradational. At Hanford, it is unclear whether pre-Missoula flood deposits overlie or interfinger with the CCU.
Slate (1996, 2000) employed a soil-stratigraphic approach to cores from a Hanford study site. She recognized lateral continuity, low permeability, and variable thickness of CCU carbonates. She found evidence of superimposed carbonate development (welding), some modification by groundwater, and an enigmatic absence of soil horizons that are typically found above pedogenic carbonate horizons (cambic horizon, etc.). Of particular interest is her recognition of carbonate morphologies unlike those described at classic localities in the U.S. Southwest (i.e., Gile et al., 1966).