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Pleistocene(?) Alluvial Stratigraphy - Lind Coulee, WA

Non-flood sediments of Pleistocene age are well exposed along Crab Creek in Lind Coulee east of O'Sullivan Dam. Very little has been written about Pleistocene alluvium in the Channeled Scabland. A few cryptic comments by mapping geologists working in the area 50 years ago is about all. West and Shaffer (1988) note the odd Pleistocene alluvium and loess encounteered during the paleoseismic trenching work, but provide few details. The valley bottom deposits pictured below are partially Ringold Fm, but mostly post-Ringold Columbia River floodplain deposits and old Palouse loess. A calcrete ledge caps much of the section. Missoula flood deposits unconformably overlie strata shown in photos below. Its remarkable how much of Eastern Washington's geology is hidden in plain view.

A 2" thick white tephra occurs in the loess unit here and there, sometimes accompanied by cobbles.

Dark cemented blobs appear to be angular basalt cobbles at first glance.

Swale filled with curious buff-tan sediment capped by calcrete truncates the underlying alluvium and its orange paleosol. Flood sediment or alluvium?

The outcrops are located along Crab Creek about a mile east of O'Sullivan Dam and the upper end of Drumheller Channels.

Original aerial photo from paleoseismic trenching study by GEI/West and Shaffer (1988). View to SE.

Modern aerial photo oriented to match the historic photo above. Locations of two abandoned test trenches, the final trench, fault escarpments, and old road routes are highlighted. View to SE.


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