THICK CARBONATE BRECCIA DEPOSITION IN THE WESTERN MARGIN OF THE YUCATAN PLATFORM, OFFSHORE CAMPECHE (CANTARELL FIELD), DURING THE K/T BOUNDARY CHICXULUB EVENT AND ITS OIL EXPLORATION IMPORTANCE

An important carbonate breccia interval represents a significant portion of the Cretaceous-Tertiary (K/T) boundary sedimentary succession accumulated under deep-water conditions in the western part of the Yucatan platform, offshore Campeche, during the Chicxulub impact event. The K/T boundary sedimentary succession is located 300 km westward from the center of the Chicxulub structure. At the Cantarell field and adjacent fields (e.g., Ek Balam), this sedimentary succession consists of a graded deposit made up, from base to top, of three units: (1) a basal 50 to 300 m-thick, coarse-grained carbonate breccia, (2) a 10 to 20 m-thick, fine-grained carbonate breccia, and (3) a 25 to 30 m-thick, interval of sand and silt to clay-sized constituents with abundant ejecta material. Additionally, a 10-20 m-thick, fine-grained calcareous breccia has been identified within the ejecta material-rich layer (unit 3) in some wells. The K/T boundary sedimentary succession is underlain and overlain by deep-water shaly calcareous facies of Upper Maastrichtian and Lower Paleocene age, respectively. Similar stratigraphic architecture and relationships are observed in outcrop analogs in the Sierra de Chiapas (El Guayal, Tabasco and Bochil, Chiapas). Lithoclasts of the calcareous breccias were derived dominantly from platform-interior and platform-margin environments and few from deep-water settings. Ejecta material in unit 3 includes: shock quartz, quartz with ballen structure, shock plagioclase, altered melt rock, and rare pelitic schist fragments. Wireline log data, areal distribution, and stratigraphic relationships indicate a base-of-slope apron geometry for the thick carbonate breccia deposit. The stratigraphic architecture and distribution of impact material within the K/T boundary sedimentary succession suggest the following sequence of events and products that occurred probably and uncommonly rapid after the Chicxulub impact. (1) Unusual strong seismic shaking induced the collapse of the platform margin resulting in a enormous debris flow (units 1 and 2), (2) arrival of ballistic impact ejecta that deposited the seal ejecta layer (unit 3), and (3) reworking and deposition induced possibly by tsunami currents (carbonate breccia within unit 3). Units 1 and 2 represent the most important oil reservoirs at the Cantarell field and unit 3 is the seal layer.