LOW-LATITUDE STRATIGRAPHIC RESPONSE TO GONDWANAN GLACIATION EVIDENT IN THE SEDIMENTOLOGY AND STRATIGRAPHY OF THE AMSDEN AND TENSLEEP FORMATIONS (MISSISSIPPIAN-PENNSYLVANIAN/NORTHERN WYOMING)

A growing number of studies have documented the imprint of the dynamic character of the Late Paleozoic Ice Age in the paleotropics, but few have examined the imprint in non-cyclothemic deposits. The goal of this project is to investigate how changes in glacial conditions influenced the stratigraphy of an arid climate paleotropical succession in northern Wyoming, USA. To evaluate changes in stratigraphic style in the late Visean to late Moscovian/early Kasimovian Amsden and Tensleep Formations, we measured 5 detailed sections from surface outcrops in the eastern Big Horn Basin, Wyoming. The Amsden Formation, which unconformably overlies platform carbonates of the Madison Formation, commences with a basal sandstone member overlain by red siltstones interpreted as floodplain deposits. The limited range of facies indicates little variation in depositional environment. Across the study area, the upper Amsden is characterized by dolomite beds containing sparse marine fossils of inferred Serpukhovian-Bashkirian age and thus spanning the Mississippian-Pennsylvanian boundary. This shallow marine interval, which represents the most distal environments in the formation, records progressive flooding of a coastal plain. As such, this interval contains a more complex record of the Mississippian/Pennsylvanian boundary than seen elsewhere. The overlying Tensleep Formation is characterized by alternations between eolian sandstones, sabkha deposits and shallow marine siltstone. These alternations indicate repeated flooding of a coastal erg, and record an increase in the amplitude of sea-level fluctuations. The fusulinid Wedekindellina attests to a Moscovian age. Facies variations could thus be related to a major phase of Pennsylvanian glaciation, corresponding to the latest Bashkirian-mid Moscovian C4 glacial of eastern Australia. The imprint of this glacial epoch has been observed elsewhere in the paleotropics. Preliminary results suggest that the Amsden and Tensleep Formations record a change from low to high-amplitude sea-level change that might be related to changing glacial conditions in the high paleolatitudes. This study provides new insight into the paleotropical response to the waxing and waning of Gondwanan glaciers, a subject of current debate.