The Chicxulub Impact Melt-Rock

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.