Pliocene Uplift & Pasco Calcretes
The sudden appearance of thick petrocalcic horizons (calcretes) in Pliocene strata of Eastern Washington got me thinking about the interplay of climate and tectonics. I drafted this figure today to illustrate my thoughts.
The Miocene was warm and wet. The Pliocene cooler and drier. Uplift in the Cascade Mountains from ~30 to ~5 Ma was probably outpaced by denudation. Consequently, summit elevations in the Cascade Range were kept low until after 8 Ma. Arrival of Pliocene dry conditions caused uplift and denudation rates to fall in line with one another, resulting in accelerated topographic rise of the Cascades to a height sufficient to form an orographic barrier and a strong rain shadow on the East Side.
The climate shifts though plate convergence and uplift rates hold steady.
A semiarid climate in Eastern Washington would have favored calcic soil development. Calcretes would grow thick there over the next couple of million years, halted only by another climatic shift to a cold and wet Pleistocene.
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