ISOLATED DEEP-WATER MAASTRICHTIAN PLANKTONIC FORAMINIFERS OF THE RESURGE BRECCIA AND SETTLING LAYER OF IODP-ICDP EXP364 HOLE M0077A IN THE CHICXULUB CRATER

Hole M0077A was drilled on the peak ring of the Chicxulub structure. Therefore the late and post-impact crater infill on the ring was not influenced by lateral sediment input like the Yax-1 core. On top of the shocked basement the direct fall-out breccia from cores 87-53 is followed by graded resurge infill from core 53 to core 40-1-110cm. A 78 cm thick graded laminated transitional interval (1G) from 40-1-110cm to 40-1-32cm has presumably settled from the turbulent waters in the waning stages of the crater basin seiche, sharply overlain by basal Paleocene background infill of marly wackestones of the P0-Pa zone. Like earlier recovered samples of the Chicxulub impact breccia (Yuc-6, Yax-1) many limestone clasts occur with enclosed foraminifers, and up-section increasing amounts of isolated benthic and planktonic foraminifers were recovered from the resurge breccias of the Chicxulub drillhole. In the basal fall-back breccia (cores 86-40) already large carbonate platform and deepwater limestone clasts were found containing a variety of upper Cretaceous large benthic foraminifers, miliolids, rudist fragments. These clasts demonstrate -perhaps surprisingly- few shock features. Upsection (cores 45-40) the amount of isolated Maastrichtian planktonic foraminifers (among others G. stuarti, C. contusa aff, heterohelicids) increases. These foraminifers are quite well preserved displaying details of the wall structure. The sub-thermocline dwelling large globotruncanids demonstrate that before the impact on the Yucatan carbonate platform a deep-water area existed, presumably the gravity low in the north¹. The presence of the well preserved foraminifers probably explains the earlier interpretation of a Maastrichtian age of the crater infill². These occurrences are documented on core-logs of Pemex, in a lithology that was not recognized as part of the impact lithologies, but regarded as background crater infill.

¹Gulick et al. 2008, Nature Geoscience, 1,131. ²Ward et al 1995, Geology, 23,873.

Co-authors: IODP-ICDP Expedition 364 Scientists