QUANTIFYING THE BIRTH OF THE MISSISSIPPI RIVER: EVIDENCE FROM THE CRETACEOUS MCNAIRY SAND

The McNairy sand is a member of the Ripley Formation (~67 Ma; Late Cretaceous), exposed intermittently around the margins of the Mississippi Embayment in Tennessee, Illinois, Arkansas, and Mississippi (MS). The Mississippi Embayment is a southwest plunging syncline which began developing during the rifting period during the breakup of Rodinia in the Neoproterozoic creating the northeast trending Reelfoot Rift. Through time the area subsided forming the MS Embayment and the drainage patterns of the ancestral MS River in the Laurentian craton were rerouted to drain southward into the MS Embayment.

The formation of the MS Embayment is crucial to understanding the depositional history of the McNairy sand. The McNairy is thought to have been deposited intermittently following transgressive cycles through the Late Cretaceous into the Paleogene with heaviest deposition occurring in the Maastrichtian period (72.1 Ma to 66.0 Ma) with the significant source areas from the Appalachian Mountains associated with Grenville and Taconic-Acadian aged sediments. Through eustatic sea level changes and tectonic activity the deposition of the McNairy has alternated between shallow marine sediments and fluvial dominated depositional environments. As subsidence occurred and drainage networks adapted, the McNairy was deposited along the eastern and western lengths of the embayment. Current theories suggest the orientation of the MS adjusted from an east-west orientation to a north-south orientation during this time of reorganization, however further analysis of sediments of the MS Embayment may provide insight into the orientation and drainage patterns during that period.

Detrital zircon U-Pb analysis of the McNairy sand will provide us with information to address the ages and sources of the sediment. Most of the zircons used for this study are colorless, translucent, sub-rounded, have low sphericity, and are anhedral. This process will not only help determine the maximum age of deposition for the unit, but will clarify the drainage patterns of that period demonstrating the distribution and orientation of the ancestral Mississippi River and its tributaries. Combining the data from this experiment and previous work, the timeline of the Mississippi River’s drainage patterns can be extrapolated from the depositional history.