Controls on late Paleozoic and early Mesozoic eolian deposition of the western United States

Abstract

The Oregon coastal dunes and Great Sand Dunes of Colorado illustrate the dynamic relationships between factors that enhance and inhibit eolian-sand deposition. Abundant sand supply, persistent sand-transporting winds, and a well-drained, low-relief surface are the most important enhancing factors. Sand supply is a function of source, concentration, and sand availability to the wind. Abundant precipitation does not preclude deposition of eolian sand. Inhibiting factors result in sand stabilization or reduction in sand supply and include water saturation to the surface of sand accumulation, freezing of sand moisture, formation of desert pavement, and, most importantly, vegetative cover. Inhibiting factors are directly or indirectly a function of climate. Through time, climate has been a function of atmospheric circulation vis a vis continental position. Vegetative cover is a function of climate and plant evolution. Evolution of land plants, angiosperms and grasses divide the geologic history of eolian sand deposition into four intervals of decreasing latitudinal and areal distribution and increasing climatic contrast. Two types of dunefields are identified on the basis of processes responsible for sand concentration and relation of paleowind to fluvial paleoslope. Type I eolian sands are concentrated by fluvial processes only. They are relatively thin, interbedded with, and commonly overlain by, fluvial sediments. Type II sands are concentrated by fluvial and littoral processes on coasts with onshore winds. They are relatively thicker than Type I sands, intertongue at their bases but are not interbedded with fluvial sediments, and commonly are overlain by marine sediments. Both types may be initiated by tectonic uplift or climatic change in the source area. Type II sands also may be initiated by change in sea level at the site of deposition. The Pangean interval is related to low stand in world sea level and corresponding widespread deposition and preservation of terrestrial sediments. Mesozoic eolian sandstones of the western interior are Type II with distal source in the Gulf of Mexico-Atlantic rift belts. Erg development was initiated by marine regression and terminated by marine transgression. The late Paleozoic eolian sandstones of the western United States are a combination of Type I and II with proximal source in the Ancestral Rockies.