A CONTINUING ION GEOCHEMISTRY AND SR ISOTOPE STUDY OF THE EXTENT OF THE K/T EJECTA BLANKET IN THE YUCATAN PENINSULA, MEXICO

Groundwater geochemistry of the NW and north central part of the Yucatan Peninsula, Mexico has been shown by previous studies to be effective in elucidating the subsurface stratigraphy of rocks that contain significant quantities of soluble evaporite minerals. The study reported here extends that work to two adjacent regions where little geologic and geochemical information is available: 1) southern Campeche and Quintana Roo and 2) NE Quintana Roo. Groundwater geochemistry of strontium and major ions, as well as O, H, and Sr isotopes, is used as a geochemical tracer to help determine the extent of the Chicxulub impact ejecta blanket that is referred to as the K/T Albion Formation where it crops out in SE Quintana Roo. Although sulfate minerals have not been reported in the weathered surface exposures of the Albion Formation, drill core of correlative impact breccia (and associated lower Paleocene evaporite) in south central Yucatan state contains abundant evaporite fragments with an Upper Cretaceous-Eocene Sr isotope signature. Where exposed, near the banks of the Rio Hondo, the poorly consolidated, clay-rich Albion Formation forms a blanket, mantling a karstified surface formed on Cretaceous limestone. In NE Quintana Roo we are seeking to determine from groundwater geochemistry whether evaporite-bearing impact material is present. If no evaporite is present there it may never have been deposited. Alternatively, it may have been removed by high groundwater flow through a regional fracture system draining into the Gulf of Mexico. In southern Campeche, evaporite mapped as Paleocene is present, and sinking streams of the area are evidence for a partially breached perched water table, perhaps floored by Albion Formation clays. Regional groundwater of S Campeche has low Cl concentrations (no saline intrusion) and average equivalent SO4/Cl ratios of 2.6 and a maximum ratio of 13.2 compared to a seawater ratio of 0.1. Determination of the extent of the impact ejecta-evaporite unit is important for regional hydrogeology, stratigraphy, and geomorphology and for water management in a region with an exponentially growing population.