A CRITICAL TIME-SLICE: EARLY PENNSYLVANIAN DUST ACCUMULATION IN THE ROCKY MOUNTAINS REGION, PRESERVED IN PALEOCAVE SEDIMENTS IN THE UNDERLYING MISSISSIPPIAN CARBONATES

There have been multiple misconceptions about red clastic sediments infilling paleokarst features in the Mississippian Leadville, Madison, and Pahasapa limestones in the Rocky Mountains and Black Hills. Clastic cave deposits are important because they have a higher preservation potential than their immediate source area, which is the overlying landscape environment. In many cases, the clastic cave deposits are geologic archives of surface environments that have been subsequently eroded and lost. This appears to be the case in much of the Rocky Mountains and Black Hills, where there may be little connection between the clastic paleocave sediments in the Madison Lm. and Pahasapa Lm., and the overlying units (Amsden Fm. and Minnelusa Fm., respectively). However, in Colorado the connection between the silt-rich paleocave sediments of the Leadville Lm. and overlying loessite (eolian siltstones) of the Molas Fm. establish clear source-to-sink relationships that apply more broadly throughout this region. Preliminary results show that paleocave sediments in the Madison and Pahasapa limestones are similar to the Leadville Limestone being silt-rich, event deposits including debrites and inundites. Detrital zircon U-Pb data has shown the dust had ultimate source areas in northeastern North America, and tracked >2000 km along the paleo-equator through prevailing easterly winds. These data provide a critical time-slide of the conditions in the Rocky Mountains area immediately before the Ancestral Rocky Mountains uplifts—this was an exposed carbonate shelf that had been dissected by karst processes, accumulating surficial windblown dust, and redistributing sediment into both vadose and phreatic cave passageways, where event deposits from different hydrologic processes were interbedded with speleothems, weathered surfaces, mud drapes with desiccation structures, and paleosols. Recognition and interpretation of the paleocave sediments has implications for Early Pennsylvanian paleotopography, relief, paleoclimate, and related factors at the inception of the Ancestral Rocky Mountain uplifts.