Impact of continental motion and dynamic glaciations on low-latitude climate during the Carboniferous: The record of the Wyoming Shelf (Western United States)

Abstract

The dynamic character of the Late Paleozoic Ice Age is evident from glacial deposits, but its impact on tropical climate is not well constrained. Global changes in climate are overprinted on longer-term paleogeographic variations, resulting in a complex time-space distribution of climate-sensitive lithologies. The significance of such lithologies in Carboniferous successions of the western United States has not been fully explored. In this study, we provide new interpretations for the paleoclimatic context of the Amsden and Tensleep Formations (Pennsylvanian, Northern Wyoming, USA). The Amsden Formation consists of a basal sandstone member overlain by red siltstones containing pisolites. Very large-scale (~10 m) cross-bedding within the basal sandstone indicates deposition in an erg environment. Iron pisoid-rich layers in the overlying member suggest an evolution toward more humid conditions. Persistent arid conditions during the middle Pennsylvanian are suggested by eolian sandstones and calcretes in the overlying Tensleep Formation. These formations were deposited on the karst topography that developed on top of the lower to middle Mississippian Madison Group. Although the development of karstic features implies that humid conditions prevailed during the late Mississippian, evaporites and evidence for early dolomitization within the formation suggest that it was deposited under arid conditions. These relationships argue for a long-term climate evolution from arid to humid during the Mississippian, and a return to arid conditions during the Pennsylvanian. This trend can be explained by the northward drift from 15°S to~12°N. A comparison with contemporaneous records reveals a diachronous evolution across western Pangaea, with the climatic conditions documented on the Wyoming Shelf being reached later in eastern North America. These relationships indicate that plate motion considerably overprints long-term climatic records. Departures from this trend, suggested by the presence of erg deposits in the basal Amsden Formation, record the overprinting of shorter periods of climate change.

Sequence stratigraphic framework for mixed aeolian, peritidal and marine environments: Insights from the Pennsylvanian subtropical record of Western Pangaea

“…Large‐scale cross‐bedding in the Darwin Sandstone Member indicates that it was deposited in an arid, aeolian setting (Blanchard et al ., ). The Horseshoe Shale Member records deposition in more humid settings (Blanchard et al ., ). The present paper focuses on the Ranchester Limestone Member and the Tensleep Sandstone units, which display a distinct cyclicity.…”

“…When Pangaea was assembling, forcing the uplift of basement blocks known as the Ancestral Rocky Mountains in western North America (Kluth & Coney, 1981), the Wyoming Shelf remained an area of low-accommodation sediment accumulation that was affected subtly by those changes (Sloss, 1950). It moved from latitudes ranging between ca 12°S to ca 17°S during the early Mississippian to ca 0 to 10°N during the deposition of the Tensleep Formation (Domeier & Torsvik, 2014;Blanchard et al, 2015). The asymmetrical distribution of land masses probably caused a lengthier wet season with higher rates of mean annual precipitation in southern low-latitudes (ca 0 to 12°S; Peyser & Poulsen, 2008;Horton et al, 2012).…”

“…The asymmetrical distribution of land masses probably caused a lengthier wet season with higher rates of mean annual precipitation in southern low-latitudes (ca 0 to 12°S; Peyser & Poulsen, 2008;Horton et al, 2012). The resulting belt of tropical, humid environments was mostly located in these latitudes, while subtropical, arid deposits are recorded from northern low-latitudes (ca 0 to 10°N; Blanchard et al, 2015). Mixed aeolian-marine sequences were deposited during that time across the Wyoming Shelf (Desmond et al, 1984;Kerr et al, 1986).…”

“…2; Blackwelder, 1918;Mallory, 1967). Large-scale crossbedding in the Darwin Sandstone Member indicates that it was deposited in an arid, aeolian setting (Blanchard et al, 2015). The Horseshoe Shale Member records deposition in more humid settings (Blanchard et al, 2015).…”

“…The dolomitic character of carbonate beds and the presence of silica-replaced evaporites (i.e. evaporite nodules or layers later replaced by silica) and calcretes suggest that the basal Ranchester Limestone Member was deposited in an arid setting (Blanchard et al, 2015). The boundaries between the three members that compose the Amsden Formation are clearly defined and recognizable across the basin (Fig.…”