The Chicxulub post-impact carbonate sequence in the Yaxcopoil-1 borehole, southern crater terrace zone

Abstract

The Chicxulub impact ∼66 Ma ago formed a large basin on the Yucatán platform, filled by sediments that provide a record of carbonate deposition, sea level and climate changes in the Gulf of Mexico. We study the post-impact sequence drilled in the Yaxcopoil-1 borehole in the crater terrace zone. The post-impact section is 792 m thick, overlying the impactites and Cretaceous carbonates. Section analyzed is between 400 and 792 m, formed by twelve units of limestones, dolomites, argillaceous/silicified limestones and calcarenites. Carbonates show cross-lamination, flow-currents, parallel lamination, cyclic graded bedding and styolitic structures. Study is based on core analyses, logging, petrography, digital scanned images, magnetic properties and X-ray diffraction and X-ray fluorescence geochemistry. The basal units U1–U4 represent low-energy deep bathyal environments and fine-grained facies varying from mudstone to wackestone. Sediment deposits, reworked and transported from the platform and crater rim, show textural and grain size changes from grainstone to packstone. Geochemical and magnetic susceptibility logs record effects of hydrothermal alteration, with secondary mineral assemblages. The SiO 2 and CaO contents display wide ranges, negatively correlated. Fe 2 O 3 , TiO 2 , Al 2 O 3 and K 2 O oxides show similar patterns downhole. The Sr and MgO show a positive correlation, except for the basal sediments. Paleocene units U1–U3 show increasing density, increasing seismic velocity and upward decreasing porosity. Upper units U5–U12 are characterized by laminated black shales and marls with microfacies varying from wackestone to packstone, with planktic and benthic foraminifera and bioclasts. Unit U9 shows low density and seismic velocity and increased porosity. Depositional environments vary from low-energy deep bathyal inside the basin to shallow neritic outside the crater rim. Sediments of the internal carbonate ramp to external neritic environments record sea level changes and platform subsidence/uplift.

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