Abstract

In the Gulf of Mexico region, facies related to the Cretaceous/Paleogene boundary (better known as Cretaceous-Tertiary [K-T] boundary) are important key horizons for hydrocarbon exploration. For example, in the Campeche Sound, some of the main oil reservoirs are interpreted as K-T boundary deposits. However, because of the complexity of these sediments, chronologic uncertainties may arise due to difficulties in recognizing the most significant mineralogical, geochemical, and micropaleontological markers: the ejecta layer in the basal part of the K-T boundary deposit, and the planktic foraminiferal mass extinction horizon. K-T boundary marine facies observed in outcrops and boreholes in the Gulf of Mexico region are characterized by a distinctive Complex Clastic Unit (CCU) that contains impact-derived materials originated from the Chicxulub impact area on the Yucatan Platform. Overall, the CCU is a fining-upward sequence, which displays different primary sedimentary structures and lithologies, depending on their distance from the Chicxulub impact site and their paleogeographic context. In proximal areas (e.g., southeastern Mexico and Cuba) the CCU may begin with a thick carbonate or polymictic breccia, and ends with fine-grained sandstones, capped by a thin, Ir-rich silty layer. In more distal and/or deeper regions (e.g., Haiti, northeastern Mexico), the CCU has a basal coarse microtektite bed and in some localities also terminates with a thin Ir-rich clay layer. High-resolution planktic foraminiferal biostratigraphy in a dark clay bed above the CCU in southeastern Mexico sections has allowed the identification of the first Danian biozone (Zone P0), further confirming the biochronostratigraphic position of the CCU as is in fact K-T in age.