Abstract

SEM analysis of quartz microtextures has been shown to be useful for the study of depositional environments in Quaternary strata, and more recently has been applied to ancient strata. Certain microtextures such as curved grooves, straight grooves, deep troughs, and mechanically upturned plates have been found to be distinctive of glacial processes in particular. SEM analysis of quartz microtextures from the proximal (< 1 km) lacustrine, medial (1–3 km) lacustrine, and fluvial (> 3 km) facies of the Pennsylvanian–Permian Cutler Formation where exposed adjacent to the Uncompahgre uplift, along with qualitative (histograms), semiquantitative (ternary diagram), and quantitative (nMDS) techniques were used to assess the hypothesis that this unit may reflect, in part, proglacial deposition. The proximal lacustrine facies depositionally onlaps Unaweep Canyon, a modern feature hypothesized to be an exhumed valley formed initially by late Paleozoic glaciation. The proximal lacustrine facies exhibits an abundance of high-stress microtextures consistent with a glacial influence. Grains from the distal fluvial facies exhibit fewer high-stress microtextures and more percussion fractures, which are expected to increase with transport distance a priori, owing to more prolonged entrainment in the fluvial system. Multiple approaches to data analysis, including visual inspection of raw data histograms, semiquantitative classification of textures using ternary diagrams, as well as quantitative nonparametric multivariate ordination techniques (nMDS) produce consistent results that suggest that the proximal-most Cutler Formation may reflect proglacial deposition. Despite the abundance of precipitation (diagenetic) features inherent to sedimentary strata of this age, SEM microtextural analysis is a useful tool for the study of ancient alluvial systems where other climate indicators are generally lacking.