LATE CRETACEOUS SOURCE TO SINK DYNAMICS ALONG THE EASTERN GULF COASTAL PLAIN (USA) REVEALED BY DETRITAL ZIRCON PETROCHRONOLOGY OF THE RIPLEY FORMATION

Source to sink dynamics along the eastern Gulf Coastal Plain (USA) transitioned from regional- to continental-scale in response to drainage network reorganization between Cretaceous and Eocene time. The Late Cretaceous (Maastrichtian) Ripley Formation provides an opportunity to evaluate the spatial and temporal development of drainage network reorganization because of its lithostratigraphic relationships and shallow marine, near-shore depositional environment. Sandstone units from the Ripley Formation (N=7, n=929) were collected from eastern Alabama to northeastern Mississippi for detrital zircon U-Pb geochronology. Zircon Th/U ratios were also evaluated alongside U-Pb ages to provide a chemical-age signature that further reveals sediment provenance. Results yield similar age spectrums for all samples, with prominent peaks at 325-475 Ma and 950-1200 Ma, as well as minor constituents at ~ 800 Ma and 1200-1600 Ma. The presence of Taconic, Acadian, and Alleghanian ages suggests a southern Appalachian source; however, U-Pb ages alone cannot distinguish between foreland and piedmont provinces and similar age spectrums are present in the time-equivalent McNairy Sandstone (northern Mississippi Embayment), which is sourced from Pennsylvanian sandstone in the Illinois Basin. The Ripley Formation exhibits an average population abundance of 13.1% (range of 4.5-20.4%, n=113) for grains with Th/U ratios < 0.1, of which 80 grains correlate to Appalachian U-Pb ages. For comparison, the McNairy Sandstone exhibits an average population abundance of 3.74% (range of 2.7-6.2%, n=54), of which 12 grains correlate to Appalachian U-Pb ages. Devonian through Mississippian zircons in the Ripley Formation also correlate to Blue Ridge pluton ages in eastern Alabama and western Georgia. Because Th/U ratios < 0.1 are more commonly associated with metamorphic zircons and intrusive Blue Ridge pluton ages correlate to detrital Ripley Formation ages, our results advocate for a southern Appalachian Piedmont source. We interpret a regional drainage network that fed into the Cretaceous Interior Seaway, near the present-day Alabama-Georgia state border, followed by northwestward (littoral) transport and eventual interfingering with the prograding McNairy Sandstone deltaic system.