THE RAYMONDVILLE FLUVIAL SYSTEM: A SEDIMENTARY UNIT WITH AN IDENTITY CRISIS

Extending through Willacy, Cameron, and Hidalgo Counties in south Texas, the Raymondville Fluvial System(RFS) is bounded by the Pleistocene Lissie Formation to the west, the Holocene age South Texas Sand Sheet to the north, the Rio Grande Delta to the south and coastal sediments of Laguna Madre to the east. This ~2,000 km²fluvial deposits is scarredwith relict meandering channels, point bars, andmud rich floodplain basins. The RFS is ill studied and poorly understood, most notable is the lack of absolute ages. Geologic maps ascribed it as the southernmost extension of the Beaumont Formation in Texas, based on sediment texture, stratigraphic position and proximity to the Gulf Coast. Absolute ages of the Beaumont vary widely, but it is generally considered a Marine Isotope Stage 5 deposit (70-130 ka). Here we revisit recently published Optically Stimulated Luminescence (OSL) ages from the RFS obtained to date a community of giant fossiltortoises (Gopherus hexagonatus), as well as other unpublished OSL ages, to frame the time of deposition of the RFS. The average of five OSL analyses yields 52.3±4.2 ka, which falls within Marine Isotope Stage 3 (27-60 ka). This figure suggests the RFS postdates the time of deposition for the Beaumont and is instead the younger Deweyville Formation, which occurs primarily as river terraces throughout the Texas Gulf Coast.