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 glaciofluvial deposits, suggesting that periods of upland glaciation occurred in the source region of the Fountain Formation (Ute Pass uplift).