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Geophysical Research Abstracts, Vol. 5,
14415, 2003
c European Geophysical Society 2003
THE KT TRANSITION AT YAXCOPOIL-1 DRILLHOLE.
G. Keller (1), T. Adatte (2), W. Stinnesbeck (3), D.
Stueben, (3) U. Kramar (4), M. Harting (4). (1) Department
of Geosciences, Princeton University, Princeton NJ 08540,
USA, (2) Institut of Geology, University of Neuchatel, 2007
Neuchatel, Switzerland, (3) Geological Institut University
of Karlsruhe, D-76128 Karlsruhe, Germany, (4) Institut for
Petrography und Geochemistry, University of Karlsruhe,
D-76128 Karlsruhe, Germany.
We examined the lithology, stratigraphy and mineralogy of
the KT transition in the Yaxcopoil-1 hole which was drilled
on the southern inner flank of the Chicxulub impact crater,
approximately 60 km from its center. The100m thick suevite
breccia that marks the Chicxulub impact ends abruptly with
an undulose and erosive upper contact and is overlain by
60cm of dolomitic limestones with small cross- and
flaser-bedding and numerous small disconformities. The lower
25cm of this interval are unfossiliferous due to intense
dolomitization. At the top of this interval is an erosive
contact overlain by a 3-4mm thick glauconitic layer that
indicates a significant hiatus. The first Danian planktic
foraminifera are present 2cm above the glauconitic layer (P.
eugubina) in a micritic limestone that marks an open marine
environment. Small early Danian planktic foraminifera are
abundant in the next 5cm and also indicate the P. eugubina
zone with a benthic assemblage indicative of middle to outer
neritic environments. Mottled sediments and bioturbation by
Chondrites and Thalassinoides are common. Middle to outer
shelf evironments persisted into the Danian P1c zone, when
shallow carbonate platform conditions where reestablished.
The lithologic succession above the suevite breccia reflects
normal conformable and homogenous deposition characterized
by low energy environments and sea level changes. There is
no evidence of significant detrital input, reworked breccia
or altered glass and the sediments above the breccia are
surprisingly unaffected by post impact events, such as mass
sliding and slumps, which would be expected to be
considerable immediately following the Chicxulub impact. The
age of the suevite breccia is difficult to establish in the
absence of microfossils. However, this impact breccia is
temporally and lithologically separated from the earliest
Danian by normal marine sedimentation that exhibits periods
of bioturbation, hardgrounds, erosion and the formation of
glauconite layers, all of which indicate that sediment
deposition occurred over an extended time period which may
have preceded the KT boundary. These data indicate that the
underlying suevite breccia, which marks the Chicxulub
impact, may predate the KT boundary, similar to the impact
glass spherule layers in northeastern Mexico.
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