Application of quartz sand microtextural analysis to infer cold-climate weathering for the equatorial fountain formation (Pennsylvanian-Permian, Colorado, U.S.A.)

摘要

Climatic interpretations of the upper Paleozoic (Permo-Pennsylvanian) Fountain Formation, a coarse-grained fan-delta system that formed in western equatorial Pangea, are difficult to constrain owing to a general lack of climatic indicators so typical of coarse clastic systems. We applied scanning electron microscopy (SEM) to analyze quartz grains in this system in an attempt to test the hypothesis of a glacial influence on these strata. SEM observations of first-cycle quartz grains from these strata reveal microtextures formed from fracturing during grain transport, even after diagenetic overprinting occurred under moderate burial conditions (up to 3.5 km depth and 100 °C). Transport-induced microtextures can be grouped based on inferred fracture process into: (1) high-stress fractures, consisting of fractures created through sustained high shear stress, such as grooves, deep troughs, and gouges, and are inferred to occur predominantly during glacial transport; (2) percussion fractures, consisting of fractures created by grain-to-grain contact during saltation or traction flow, such as randomly oriented v-shaped cracks and edge rounding; and (3) polygenetic fractures, such as conchoidal fractures, arc-shaped steps, linear steps, and linear fractures, that occur under a wide range of transport processes and thus possess no environmental significance. Delineation of high stress, percussion, and polygenetic fracture types demonstrate that the Fountain Formation quartz grains exhibit microtextures similar to both till and glaciofiuvial deposits, suggesting that periods of upland glaciation occurred in the source region of the Fountain Formation (Ute Pass uplift). The abundance of high-stress fractures peaks at two stratigraphic intervals. These intervals are inferred to record the presence of ice in the Ute Pass uplift and are correlative with polygonally fractured paleosurfaces in the Fountain Formation that are interpreted to reflect cold-temperature weathering. Moreover, the peak intervals are approximately coeval with inferred episodes of ice maxima from high-latitude localities, as well as other low-latitude localities. Geologically reasonable stream gradients and estimated transport distance suggest a best-estimate elevation of the ice terminus of ? 1500 m, but possibly ranging to 3000 m. These data suggest that upland glaciers episodically existed within this equatorial setting and that further use of this technique may reveal more evidence of ice in other proximal deposits of the ancestral Rocky Mountains, as well as other systems of various geologic ages.