Tectonic Origin of the Mississippi River Using Evidence from the Cretaceous McNairy Sand.

Keywords: McNairy Sand, Detrital zircon geochronology, Mississippi Embayment, Mississippi River, Reelfoot rift. Detrital zircon U-Pb geochronology studies of the McNairy Sand in southern Illinois and northern Mississippi are being used to characterize the ancestral Mississippi River when it changed its flow direction from east-west in the Early Cretaceous, progressing to a north-south drainage pattern in the Late Cretaceous. The aim of this study is to investigate the sediment deposition pattern of the McNairy Sand units extensively along the eastern margins of the Mississippi Embayment in order to reconcile the eastward and southward drainage of the Mississippi River through time. The study will present the first extensive area coverage of the Maastrichtian (~70 Ma) McNairy Sand from northern Mississippi, western Tennessee and southwestern Kentucky. The provenance of the McNairy Sand is believed to be dominantly from the Appalachian Mountains. The sediment accumulated during the reactivation of the Late Proterozoic to Early Paleozoic Reelfoot rift system at the end of the Mesozoic era. The McNairy Sand records evidence of the tectonic activity of the Reelfoot rift system which resulted in the topographic low for the drainage system of the Mississippi River. This extensive analysis of the McNairy Sand provides evidence to reconcile the change in propagation of the reactivated rift system. Detrital zircons, yielded dominantly, ~1000 Ma U-Pb ages (Grenville; ~76%), and a smaller portion of zircons ~1250 - 1500 Ma (Granite-Rhyolite; ~9%); ~350 - 416 Ma (Acadian; ~4%), and ~445 - 540 Ma (Taconic; ~5%) suggesting an Appalachian region provenance. Further McNairy samples will be interpreted via detrital zircon U-Pb analysis. The results help characterize and further constrain the provenance and age of deposition, clarifying the drainage patterns of that period and orientation of the ancestral Mississippi River and its tributaries. The zircons used for this study are colorless, translucent, tabular to prismatic, euhedral to subhedral, subrounded to subangular, have orange and yellow iridescence and low sphericity.