BRECCIA AND FALL-OUT SUEVITE IN THE EL GUAYAL KT SECTION (TABASCO, MEXICO)

The El Guayal (Mexico) Cretaceous-Tertiary (KT) boundary crops out some ~ 600 km southwest of the Chicxulub crater, in a depositional setting then located offshore from the shallow water carbonate platform of Yucatan. The section contains different types of impact breccia. At its base, a 35 m breccia is formed of large blocks of carbonates, which decrease in size upward. The composition of the blocks indicates that they originated from the collapse of the platform margin. Toward the top, this unit grades into a carbonate microbreccia characterized by the appearance of quartz and impact debris. Above comes about ~ 9 m of fining-upward sandstone and siltstone containing abundant impact debris mainly in the form of altered glass or melt particles, carbonate clasts and rare shocked minerals. The matrix is calcite-rich but the proportion of fine quartz increases upward. The altered glass/melt particles often have elongated shapes, show flowing texture and are now composed of vermiculite and Mg-rich smectite. They contain carbonate globules, similar to those found associated with silicate melt in the Chicxulub fall-back suevite of the Yucatan-6 well. In this unit, some 2 m below the KT boundary, a 90 cm thick siltstone bed contains accretionary lapilli < 3 cm in size. Within these lapilli, the accreted particles are arranged in regular concentric shells around a small core, made of quartz or a lithic fragment. The particles are composed of essentially of quartz, some with possible PDF, altered melt fragments, and tiny carbonate clasts. Locally, grain contacts appear fused together, which may indicate that the particles were molten when they agglutinated around the core. A SiO2-rich phase cements the accreted particles. The carbonates are recrystallized and show texture suggestive of former carbonate melts. Comparable but smaller lapilli occur in the fall-out suevite of the Ries crater (Germany) and in the ejecta of the Sudbury crater (Canada). The Loma Capiro KT boundary in Central Cuba contains pure carbonate lapilli. The El Guayal lapilli probably formed in the turbulent vapor cloud expanding over the Chicxulub crater. The whole microbreccia-sandstone-siltstone unit is interpreted as fall-out suevite. This implies a very large geographic extension of the vapor cloud expanding above the Chicxulub crater.