Calcretes Arrive Around 7?
Calcretes are hard pan soils that form in semi-arid to arid climates. Calcretes - at least pedogenic CaCO3 cements - first appear in Pliocene Ringold strata at Saddle Mountains between 7 and 5 million years ago. Arrival may coincide with onset of dry climate created when the Cascade Range achieved sufficient elevation to block wet Pacific weather systems from moving inland. Eastern Washington was quite a wet place until the orographic barried formed. While convergence at the plate margin and rock uplift in the Cascades began tens of millions of years ago, it seems the Cascades achieved their storm-blocking height quite late. In short, there appears to be a disparity between rock uplift over the past 40 million years (inferred from plate convergence rates, bedrock erosion rate estimates for the Tertiary, and the volume of sediments shed into adjacent basins) and topographic relief of the late Neogene-early Pleistocene range (which is not well known from the rock record).
By the way, we're talking the bedrock guts of the range - the so-called "western Cascades" - not the high snow cone volcanoes that sit on top (Mt. Rainier, Mt. Baker, Glacier Peak, etc.).
I put the figure above together from info gleaned from an article by Staisch et al. (2017). Their paper was focused on other topics, but included 6 stratigraphic sections in Neogene-Pleistocene rocks at Saddle Mountain, which included some absolute age data.
There are local preservation issues at Saddle Mountains, as the article authors note (i.e., different units found in hanging wall vs. footwall), but the sections, despite their many covered portions and erosional surfaces, record relatively continuous deposition. Staisch did not work the eastern portion of the ridge, where more exposures occur. I've visited and described strata exposed there.
Can we invoke cessation of uplift on the Saddle Mountains anticline at ~7 Ma to explain the appearance of calcrete? Possibly. But if the argument is stability after 7 Ma permitted growth of calcretes, then how does one reconcile near-continuous deposition of Ringold seds from 9 to 3 Ma? If the crest of the fold (hanging wall block) was lifted above the level of Lake Ringold, shouldn't we find the shoreline crossing the fold hinge? How far west can you map Savage Island lakebeds? I've found them west of Corfu Rd/Saddle Crest Rd. Are sections at Smyrna Bench, located between the two thrust faults at Saddle Mtns, in the hanging wall or footwall?
Four kinds of gravels/conglomerates exist at Saddle Mountains: Ellensburg red beds sourced from the west (mostly Miocene), fluvial channel gravels deposited by ancestral versions of modern rivers (mostly Pliocene), coarse alluvial fan gravels derived from local hillslopes (Pliocene-Holocene),and outburst flood channel gravels from ancient floods and Missoula floods (Pleistocene).
Is this the correct model for the uppermost Ringold? Alluvial fans spilling from a faulted range front onto a sluggish fluvial plain? Are calcrete-overprinted Plio-Pleistocene fanglomerates in Ringold and post-Ringold sediments consistent with an amalgamated and proximal fan system (bajada)? Do the facies relationships and gentle slope angles on fan deposits support this model?
Interruption of the upper Ringold's characteristically fine grained sedimentation by coarse pulses of fan gravels gives the upper part of the formation two depositional personalities. A sluggish, low-relief fluvial to shallow lacustrine floodplain punctuated by energetic subaerial alluvial fans shed off the flanks of rising folds. Paleosols and pedogenic cements are common in the fan strata.
Are the Ringold-age and post-Ringold basaltic fan gravels syn-tectonic deposits? Some certainly are. Others could record the longer-term response of weathered hillslopes recovering from uplift (post-tectonic deposits). Basalt becomes quite incompetent when folded or tilted, even by modest amounts. Some gravels might be ancient scabland flood deposits mantling the youngest portions of the fold.
I've got a few more days of fieldwork in the Othello-Crab Creek-Saddle Mts area (measured sections mostly) and a Northwest Geological Society field trip to run (currently postponed due to COVID-19). After that submit something for review. Also, a larger summary figure that brings together tectonism, paleontology, volcanism, sed/strat, absolute age data, and soils from about 30 other studies is in the works. I'll hand out a draft of that to field trip goers, whenever the pandemic ends.