Bedrock Origins from Petrology and Geochemistry: Volcanic Gravel Clasts from the Rawhide Terrace in the Pleistocene Ancestral Mississippi River Pre-Loess Terrace Deposits

Situated throughout the southeastern United States within the Laurentian craton are occurrences of various aged deposits (Late Proterozoic to Early Paleogene) that contain volcanics spanning from lamprophyres to carbonatites and basalts to rhyolites. Several are intrusive, while others have been reworked detritally, deposited as river gravels out onto the Gulf Coastal Plain. The earliest occurrence of igneous gravel clasts in the coastal plain of the lower Mississippi Valley lie along the Mississippi River’s eastern valley wall in the ancestral Mississippi River’s pre-loess terrace deposits (PLTDs). The coarse clastics of the PLTDs are dominantly chert gravels derived from Paleozoic carbonate bedrock, but also include clasts of Precambrian Sioux Quartzite, glacially faceted and striated stones, and ice-rafted boulders, which indicate a direct relationship between the PLTDs and glacial outwash during the cyclic glaciation of the Pleistocene Epoch. The PLTDs also contain the oldest known examples of igneous gravels exposed at the surface in Mississippi. An understanding of their igneous bedrock provenance and the timing of their contribution to the sedimentary record of the lower Mississippi River Valley sheds a valuable light onto the geologic history and evolution of the ancestral Mississippi River during the Pleistocene Epoch. The use of fusion inductively coupled plasma mass-spectroscopy (ICP-MS) in the identification of the igneous suites of one of the pre-loess terraces, well-delineated by geologic mapping, adds important geochemical source data from the gravel constituents for the further interpretation and correlation of the individual PLTD allounits. Gravel constituent geochemistry also offers a better understanding of the evolution of the ancestral Mississippi River watershed and the contributions of bedrock sources during Pleistocene glaciation. This petrological study suggests that the igneous gravels sampled from within the Rawhide PLTD allounit originated from the St. Francois Mountains (SFMs) in southwestern Missouri, with the implications that the SFM igneous terrain was in the direct path of the Independence “Kansan” glaciation. This could indicate a glacial extent further southwest than previously documented.