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Calcrete Field Trip 2021 - Red Tank

Time did not allow our NWGS group to visit the Red Tank area in September 2021. The Saddle Mountains crest is an incredibly photogenic place. The landscape is dramatic, the shoulder season light incredible, and the geology first rate. I wonder why, after whole careers have been spent out here, there isn't a coffee table book highlighting its beauty and geology. Talking to you, Mr. Reidel.

One portion of Smyrna Bench near the Red Tank (exploration well BN 1-9)

Red Tank area map to outcrops at the rim of Smyrna Bench.

One outcrop, which I call the "SSD Outrcop", shows off thick, cemented fan gravels and Pliocene lakebeds beneath.

Eight deep boreholes penetrate the basalt in Columbia Basin including BN 1-9. Information from these wells has greatly increased our understanding of the sub-basalt geology and the Basin's oil and gas potential. Figure from Reidel et al. (2002, Fig 2.16).

According to Google Earth, the length of Saddle Mountains is 85 km as measured along its ridge crest (A to A'). An as-the-crow-flies measurement would be slightly shorter. The range changes structurally west of Badger Gap and becomes the Boylston Mountains. The range terminates near Scooteney Reservoir/Hwy 17 (western edge of Palouse Slope). For some reason, geological reports from past decades indicate the length of the Saddle Mountains is much greater (>100 km).

Contact between Savage Island lakebeds and the overlying cemented fan gravels. Do these CaCO3 stringers predate or postdate the fanglomerate?

Several sandy beds containing soft sediment deformation structures certainly predate deposition of the fanglomerate and appear syn-sedimentary (soupy lake bottom sediments).

I counted 8 separate deformed layers in Savage Island sediments exposed at the SSD Outcrop. Each set of deformation structures is truncated at its top along a clean, horizontal surface and flat-lying lakebeds. Deformed zones are slightly sandier, but not by much. Cementation in the deformed zones comes and goes.

Truncated tops of centimeter-scale SSD zones.

Other exposures nearby reveal interesting details about Ringold stratigraphy.

Fan gravels and loess always occur together at Saddle Mountains. "Fanglomerate-Loess-Calcrete complex" is a useful term. Fan gravels, mass wasting of accumulated colluvium, is response to uplift. Fanglomerates indicate hillslope instability. Loess beds with soil profiles indicate just the opposite: hillslope stability. Saddle Mountains and the surrounding Pasco Basin was a dusty place throughout the Pleistocene and probably for much of the Pliocene, too. No one has investigated Pliocene-age loess paleosols in detail. Cicada-burrowed loess and fine grained sandstones occur with other soils between sections of Savage Island lakebeds at White Bluffs Overlook and elsewhere in the vicinity.

A few short clastic dikes occur in Ringold sediments, descending from a light-colored bed into red, basaltic, cemented gravels located some 10m below the contact between fanglomerate and Savage Island lakebeds.

I redrew Crane's fault geometries for Saddle Mountains.

Fanglomerate and loess deposits on the north flank of Saddle Mountains. Thinning and thickening along strike is typical and may reflect the irregular, gullied surface being filled by the alluvial fan material.

Fanglomerate-loess-calcrete complex on Saddle Mountains ridge.

I'm beginning to wonder if named loess paleosols described in the Palouse Hills (Washtucna Soil, Old Maid Soil, Devils Canyon Soil, etc.) aren't better developed at Saddle Mountains. Imagining the title of a future journal article, "Palouse Extended 100 km West".

Angular relationship between dipping CRBs and gently tilted fanglomerate-loess-calcrete.

Cougar Point Tuff 11.8 Ma.

Cobbles in synclinal hinge along ridgeline. Tilted fanglomerate in distance.

Folded and faulted calcrete-silcrete layer along Saddle Mountains ridgeline. This platy unit is more or less continuous along this portion of the Saddle Mts crest. Elsewhere its dip is shallow. The unit lies at the contact between Elephant Mtn basalt (10.5 Ma) and the overlying Ringold (9.5-3.2 Ma). It is not part of the pedogenic calcrete zone that occurs at the top of the section (fan-loess-calcrete complex). This is a groundwater deposit that lies atop impermeable basalt.

Super thick lower half rinds on cobbles in the fan-loess-calcrete complex.

Devils Canyon Soil in the Saddle Mts?

Basalt flows and sedimentary interbeds on north flank of Saddle Mountains. View to east.

Wahatis Peak summit area.

Hazy view south to the Hanford Reach from road to Wahatis Peak.

Wahatis Peak. View to the west.

Sentinel Bluffs above Sentinel Gap. View to west.

Links to Other Stops:

Lind Coulee Fault at O'Sullivan Reservoir


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