YAXCOPOIL-1 DRILLHOLE IN THE CHICXULUB IMPACT CRATER

The ICDP-funded Yaxcopoil-1 hole was drilled on the southern, inner flank of the Chicxulub crater, ~60km from its center. A continuous sequence of cores was recovered from 400 to 1511m. Down-hole logging shows an increase in clay at ~200m, probably representing the transition from Miocene-Oligocene platform carbonates to an Eocene-Paleocene pelagic limestone-marl sequence. Rhythmic dm-scale alternations dominate the crater-fill sequence from 400-794m. Organic-rich laminated marls alternate with bioturbated micrites near the Eocene-Paleocene transition. Gravity flows increase from 680 to 794m, where the contact with the subjacent impactite sequence occurs. A sample 40cm above the contact yielded basal Paleocene G. eugubina zone faunas; no Cretaceous planktic foraminifers were observed in that interval. The basal 2 cm of the Paleocene consist of a black clay containing very small planktic foraminifers, presumably correlatable to the P0 zone. The absence of Cretaceous forms supports the conclusion by Olsson and Smit that Cretaceous foraminifers found in basal Paleocene sequences elsewhere, are reworked from underlying strata, and are not survivors, except for small, unornamented cosmopolitan forms (Heterohelix, Globigerinelloides, Hedbergella). From 794-894m a series of very melt-rich suevitic breccias and layers of impact melt breccias have been cored. The top 15m of these breccias are reworked, presumably by the backflow of ocean water into the crater. The uppermost 0.9m is a fine-grained, crossbedded suevite composed primarily of sand-sized carbonate grains and occasional green (altered glass) fragments. From 894-1511m shattered platform limestones, dolomite and anhydrite was cored, that may be a displaced megablock possibly from the overturned flange of the ejecta curtain. The lower part of this megablock is dissected by numerous pseudotachlite veins and dikes of suevite and of melt breccia. No fossils were found that constrain the age or stratigraphic sequence of this interval.