2006 Philadelphia Annual Meeting (2225 October 2006)
Paper No. 119-4
Presentation Time: 1:30 PM-5:30 PM


DUBRON, Liesbet, Dept. of Geology and Soil Science, University of Ghent, Krijgslaan 281, Ghent, 9000, Belgium, betje17@hotmail.com, GRAJALES-NISHIMURA, J. Manuel, Instituto Mexicano del Petroleo, Mexico D.F, 07730, Mexico, ALVAREZ, Walter, Department of Earth and Planetary Science, Univ of California, Berkeley, CA 94720-4767, FOUKE, Bruce, Dept. of Geology and Soil Science, University of Illinois, Urbana, IL 61801, and CLAEYS, Philippe, Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, Brussel, B-1050, Belgium

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.

2006 Philadelphia Annual Meeting (2225 October 2006)
General Information for this Meeting
Session No. 119--Booth# 120
Impact Craters: Structures, Drilling, Ages, and Geophysics (Posters)
Pennsylvania Convention Center: Exhibit Hall C
1:30 PM-5:30 PM, Monday, 23 October 2006

Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 297

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