CSDP core Yaxcopoil-1 was drilled to a depth of 1,511 m within the
Chicxulub crater. An organic-rich marly limestone near the base of the
hole ( 1,495 to 1,452 m) was deposited in an open marine shelf
environment during the latest Cenomanian ( uppermost Rotalipora cushmani
zone). The overlying sequence of limestones, dolomites and anhydrites (
1,495 to 894 m) indicates deposition in various carbonate platform
environments ( e. g., sabkhas, lagoons). A 100-m-thick suevite breccia (
894 - 794 m) identifies the Chicxulub impact event. Above the suevite
breccia is a dolomitic limestone with planktic foraminiferal assemblages
indicative of Plummerita hantkeninoides zone CF1, which spans the last
300 ky of the Maastrichtian. An erosional surface 50 cm above the
breccia/dolomite contact marks the K/T boundary and a hiatus. Limestones
above this contact contain the first Tertiary planktic foraminifera
indicative of an upper P. eugubina zone P1a(2) age. Another hiatus 7 cm
upsection separates zone P1a( 2) and hemipelagic limestones of planktic
foraminiferal Zone P1c. Planktic foraminiferal assemblages of Zone Plc
to P3b age are present from a depth of 794.04 up to 775 m. The
Cretaceous carbonate sequence appears to be autochthonous, with a
stratigraphic sequence comparable to late Cretaceous sediments known
from outside the Chicxulub crater in northern and southern Yucatan,
including the late Cenomanian organic-rich marly limestone. There is no
evidence that these sediments represent crater infill due to megablocks
sliding into the crater, such as major disruption of sediments, chaotic
changes in lithology, overturned or deep dipping megablocks, major
mechanical fragmentation, shock or thermal alteration, or ductile
deformation. Breccia units that are intercalated in the carbonate
platform sequence are intraformational in origin ( e. g., dissolution of
evaporites) and dykes are rare. Major disturbances of strata by the
impact therefore appear to have been confined to within less than 60 km
from the proposed impact center. Yaxcopoil-1 may be located outside the
collapsed transient crater cavity, either on the upper end of an
elevated and tilted horst of the terrace zone, or even outside the
annular crater cavity. The Chicxulub site thus records a large impact
that predates the K/T boundary impact and mass extinction.