Lacustrine Sedimentation and Paleolimnology in an Early Cretaceous Backbulge-basin Lake

Trees, Mark

The Lower Cretaceous foreland basin of the Cordilleran Orogenic Belt in western North America includes the fluvial and lacustrine deposits of a backbulge deposystem located in present-day South Dakota. The Lakota Formation of the Inyan Kara Group represents Lower Cretaceous strata found in the southern Black Hills. Individual members of the Lakota Formation include the fluvial Fuson and Chilson Members and the lacustrine Minnewaste Limestone Member. Previous studies of the Lakota Formation revealed little about the depositional character and lithology of the Minnewaste Limestone. Field observations, petrographic, provenance and stable isotopic analysis presented in this study of the Minnewaste Limestone reveal cyclic changes in hydrologic lake evolution and extent.

The Minnewaste Member of the Lakota Formation is a nearly pure limestone that outcrops almost exclusively along the southern and southeastern margins of the Black Hills in South Dakota with typical thicknesses between 3-6 meters. Fossils such as ostracods and stromatolites were only rarely encountered throughout the study area, but pervasive tufa development suggests that the outcrops we investigated of Minnewaste Paleolake may have been a saline and possibly spring-fed shallow hydrologic system throughout the span of its existence. Lacustrine and paleowetland deposystems related to the Early Cretaceous Sevier Orogeny can provide a well-constrained record of climate history and the shallow crustal response to far-field tectonic forcing events. The relationships between δ18O and δ13C in the Minnewaste data sets were evaluated to determine hydrological changes in lake development. The variance of Minnewaste Limestone δ18O and δ13C values suggests an evolving system dominated by ground-water input during the Minnewaste Paleolake development. Other factors that may have affected the oxygen and carbon isotopic composition include vapor exchange, lake productivity and total CO2 concentration. The sequence stratigraphic variation within the unit indicates occasional rapid fluctuation in lake level and sedimentary input. The examination of the Minnewaste Limestone through several lines of evidence, including detrital zircon geochronological analysis of provenance and stable isotope data suggest that hydrologic inflow to the Minnewaste lake system during this timeframe was predominately from the south and southeast and not from the west across the Sevier forebulge.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013