Stratigraphy of the Basal Paleocene Carbonate Sequence and the Impact Breccia-Carbonate Contact in the Chicxulub Crater: Stable Isotope Study of the Santa Elena Borehole Rocks

Abstract

We present results of a stable isotope study of the carbonate sedimentary sequence recovered in the Santa Elena borehole (located 110 km from the crater center) of the UNAM Scientific Drilling Program, immediately above the impact breccia-carbonate contact at a depth of 332 m. Sixty samples spaced in a 17 m thick section representing the basal Tertiary sequence were analyzed for oxygen and carbon isotopes. The δ13C ranges from about 1.2 to 3.5‰, and the δ18O ranges from about -1.4 to -4.8‰. We correlate the isotopic records for bulk carbonate for the Paleocene with the DSDP Hole 577, assuming a tie point for the Cretaceous/Paleocene (K/T) boundary. The variation pattern in the Santa Elena borehole correlates well with the global pattern defined for marine carbonate sediments. In the Santa Elena borehole, δ13C values are slightly more positive; δ13C values reach ~3.5‰ and there is a gradual increase from the Lower to the Middle Paleocene strata. The trend of δ18O values from Santa Elena borehole are also similar to δ18O DSDP Hole 577, however values in the Chicxulub Tertiary sequence are more negative; δ13C values reach ca. -4% in the Lower Paleocene rocks. Paleomagnetic data for the study section document four geomagnetic polarity zones for the impact breccias and basal sedimentary sequence, which span from chron 29r (K/T boundary) to chron 28n. The combined stable isotope and magnetic polarity data provide a stratigraphic framework for the first 2.5 Ma after the K/T boundary, and permit inferences about the paleoceanographic conditions following the Chicxulub impact and into the Early Paleocene.