SEDIMENT DISPERSAL ALONG THE WESTERN MARGIN OF THE YUCATAN PLATFORM DURING THE K/T BOUNDARY CHICXULUB EVENT

Stratigraphic field data and offshore subsurface information document the occurrence of a thick carbonate breccia of Cretaceous-Tertiary (K/T) boundary age in the western part of the Yucatan platform (Campeche Sound) and southeastern Mexico (Chiapas-Tabasco platform). The calcareous breccia is associated with ejecta material such as: glass, shocked minerals and accretionary lapilli. The ejecta components were derived from the K/T boundary impact event which occurred at the Chicxulub site. The K/T boundary sedimentary succession is located 300 to 500 km westward from the center of the Chicxulub structure. It consists of a graded sedimentary deposit made up, from base to top, of four units: (1) a 40 to 300 m-thick coarse-grained carbonate breccia, (2) a 2 to 10 m-thick medium to fine-grained carbonate breccia mixed with little ejecta material, (3) a ~9 m-thick ejecta material-rich calcareous sand, and (4) 1 to 2 m-thick silt to clay material composed essentially of ejecta material. Units 1 and 2 in the Campeche Sound are strongly leached and dolomitized and represent the main oil reservoir stratigraphic interval. The glassy material in units 3 and 4 is altered to clay minerals (smectite) and represents the main seal layer. The graded architecture of the K-T sedimentary sequence seems to represent one single giant debris flow deposit, followed by ballistic sedimentation and reworking by tsunami currents. Wireline log data, areal distribution, and stratigraphic relationships suggest a base-of-slope apron geometry for the thick calcareous breccia deposit. The stratigraphic architecture and distribution of impact-generated materials within the calcareous sedimentary succession suggest that the following sequence of events and products occurred probably within minutes, hours or days after the Chicxulub impact. (1) Mega-seismic shaking inducing the collapse of the platform margin resulting in deposition of calcareous breccias (units 1 and 2) and (2) arrival and reworking of ballistic impact ejecta that formed the seal layer (units 3 and 4).