Plio-Pleistocene Calcretes & Age of the Cascades Rain Shadow
Calcrete overprints a regional unconformity of Pliocene-Pleistocene age in south-central Washington. The unconformity truncates lacustrine sediments of the uppermost Ringold Formation (8.0 to 3.4 Ma) and is overlain by glacial outburst flood deposits (<2 Ma). The aridity necessary for calcrete formation post-dates drainage of the last Pliocene lake system from Pasco Basin and pre-dates Ice Age cooling. The calcretes suggest a strong rain shadow first formed in Eastern Washington around 4-5 Ma coincident with the topographic rise of the Cascade Mountains. Ten new stratigraphic sections from the eastern Pasco Basin describe a set of stacked, stage III-V calcretes and the lacustrine, eolian, outburst flood, mass wasting, sluggish fluvial, and alluvial fan deposits into which the paleosols are developed.
In the Spring of 2020, I may be leading a geology field trip to the Othello/Saddle Mountains area for the Northwest Geological Society (nwgs.org). A packet of information will be handed out to field trip participants. A formal research article is in the works.
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Thick, advanced-stage calcretes in Eastern Washington - The calcrete pedocomplex consists of multistory and composite (welded) calcretes developed into a variety of sedimentary parent materials, from loess to boulder gravels. It lies unconformably atop the upper Ringold Fm and is truncated by cataclysmic outburst flood deposits of the latest cycle (Missoula floods). Cross-cutting relationships, fossils, and paleomagnetics bracket calcrete development to between 3.4 Ma and 34 ka.
Calcretes delineate a folded, sedimentologically-diverse paleosurface - A calcic pedocomplex overprints a regional unconformity in eastern Washington. Calcretes occur atop basalt bedrock along the crests of Yakima Fold Belt ridges and cement coarse, basalt-dominated alluvial fan gravels (fanglomerates) on their flanks. Calcrete also caps flat-lying Ringold sediments at White Bluffs and in folded Ringold beneath the Hanford Site (Cold Creek Syncline). The surface projects beneath the Palouse loess to the east, but calcrete is not known at that stratigraphic position there. Thick calcrete may never have formed on the higher and slightly wetter Palouse. The calcrete-bearing unit has received relatively little attention from geologists over the past century despite its widespread occurrence, considerable thickness (up to 20m), relevance to the topographic rise of the Cascades, and establishment of rain shadow conditions in Eastern Washington.
Late topographic rise of the Cascades - Did a climate shift at ~5 Ma from wet-warm to cool-dry conditions trigger topographic rise of the Cascades?