Physics
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p43a..03s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P43A-03
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 9350 North America, 0400 Biogeosciences, 3030 Micropaleontology (0459, 4944), 3099 General Or Miscellaneous
Scientific paper
No Evidence for Autochthonous Maastrichtian Sediment Above Chicxulub Suevitic Ejecta in the Yaxcopoil-1 Core: The Chicxulub Impact is the K/T Boundary Impact. J. Smit, Dept. of Sedimentology, Faculty of Earth and Life Sciences FALW, Vrije Universiteit, de Boelelaan 1085, 1081HV Amsterdam, Netherlands Recently(1), it was suggested that the Chicxulub crater might precede the K/T boundary by about 300kyrs. Initially, the idea was based on multiple layers of (mostly altered) impact glass spherules that were found up to 11 m below and at the base of a sandstone interval and an iridium anomaly occurs at the top. Normal hemipelagic deposits of the Mendez shale would separate the multiple spherule layers. However, those layers are slumped and discontinuous, and separated by displaced layers of Mendez shale. The sandstone deposits in between spherules and iridium anomaly betray very rapid deposition (thick climbing ripple units), probably deposited in a few days at most, compatible with a single impact (at Chicxulub). The Yaxcopoil-1 core in the Chicxulub crater recovered a complete core interval of the same time period, and a similar controversy erupted. The critical interval in the Yaxcopoil-1 core is a short, 64cm long core-segment overlying the highest (displaced) suevitic ejecta (794.75m), and underlying uncontested deep-water crater-fill sediments of basal Paleocene age (794.11m). The interval includes an interval of cross-bedded dolomitic sand, alternating with fine-grained parallel laminated dolomitic sand, overlain by a 9cm thick bored dolomitic hard ground. A 2cm thick, dark clay-layer with dissolution features overlies the hard ground, followed by micritic basal Paleocene wackestones. The clay layer is often linked with the K/T boundary However, the evidence suggesting that the 64 cm interval is an authigenic, pelagic, micritic sediment of upper Maastichtian age is seriously flawed. This idea is based on four arguments: 1) presence of abundant latest Maastrichtian foraminifers, 2) presence of multiple layers of glauconite, 3) presence of pelagic micrite, 4) presence of an stable isotope shift, comparable to an identical shift at the K/T boundary elsewhere. We have made four thin sections of each of the 13 samples (yax306-318) of that contested interval, and subjected these slides to SEM backscatter, microprobe, XRD, Cathode luminescence and microscopic analysis. In these thin sections we were unable to distinguish 1) Even a single specimen of a Maastrichtian planktic foraminifer, and found: 2) Smectite instead of glauconite in all layers except the dissolved clay layer at the top, and found: 3) No evidence for micrite, and found out that 4) the shift in stable isotopes across the clay layer is due to measurements of diagenetic sparry calcite pore-filling below, and pelagic micrite above the isotopic shift. Thus, in conclusion, we reject all evidence for a Maastrichtian age of sediments overlying the Chicxulub ejecta in the Yaxcopoil-1 core. 1. Stinnesbeck, W.,et al., 2004. Yaxcopoil-1 and the Chicxulub impact. Int. Jour. of Earth Sci., 93. 1042-1065.
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