2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 3
Presentation Time: 2:10 PM

The Chicxulub Impact Melt-Rock

CLAEYS, Philippe, Analytical, Environmental and Geo-Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, Brussels, 1050, Belgium, phclaeys@vub.ac.be

The ICDP Yaxcopoil hole was drilled in the structural low zone between the inner peak-ring and the rim of the ~ 200 km KT boundary Chicxulub crater in Yucatan. It encountered only a ~ 100 m thin sequence of suevite and impact-melt breccia, which compositions are strongly influenced by the upper ~ 3km of sediment present in the Yucatan target rock. The Yucatan 6 well (Y6) drilled many years ago by PEMEX on the side of the peak-ring encountered a thicker (up to 500 m?) impactite sequence. Despite the sporadic sample preservation, its composition is estimated from top down as ~ 170 m of suevite of different types, followed by at least ~ 120 m impact melt breccia, possibly extending down for another 250 m (no sample available). It is composed of solid and melted basement clasts (gneiss) dispersed in a microcrystalline matrix of plagioclase and pyroxene. The contribution of the Yucatan sedimentary target decreases with depth in Y6. So far true impact-melt rock is only present in the Pemex core Chicxulub 1 (C1) drilled near the center of the peak-ring. The only two fragments preserved from this core originate from depths around ~ 1400 m. They are composed of a majority of melt-fragments floating in a coarse matrix of pyroxene, plagioclase and k-feldspar. In C1, the melted material is derived from the deep basement, with little contribution from the overlying carbonate and evaporates. The absence of unmelted fragments and the coarse-grained matrix do support a rather slow cooling process within the inner part of a probably thick melt-sheet. Based on seismic data this melt-sheet could extend down to a depth of ~ 4 km. Consequently, it probably fractionated and slowly evolved in a way similar to the impact melt-sheet of the Sudbury crater.