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A thick package of sedimentary carbonate breccias accumulated under deep-water conditions in southeastern Mexico and offshore Campeche in the latest Cretaceous. The origin of these breccias has been linked to the Chicxulub impact event that occurred on the Yucatan Peninsula at the Cretaceous/Tertiary (K/T) boundary. This sedimentary succession was examined at three locations: outcrops at El Guayal and Bochil (in the Sierra de Chiapas) and at the Cantarell field in the Bay of Campeche. The succession fines upward and is composed of (1) a basal very coarse-grained carbonate breccia followed gradationally by (2) a fine-grained carbonate breccia and (3) an ejecta-rich layer. (4) A very thin shaly layer caps the sedimentary sequence in the exposed sections and has not been documented in wells. In some wells, another fine-grained carbonate breccia is found in the ejecta-rich layer. Thickness of the carbonate breccia succession ranges from 50 to 300 m. Areal distribution, stratigraphic architecture, and stratigraphic relationships indicate deposition of the K/T sedimentary succession under deep-water marine conditions. Carbonate microfacies analysis of the lithoclasts that comprise the K/T boundary calcareous breccias was carried out to identify the source. Microfacies identified in the carbonate breccias are typical of three depositional environments: inner platform (lagoon and tidal flats), platform margin, and deep-water settings. Inner-platform microfacies include: (1) miliolid-peloid wackestone and packstone, (2) alveolinid wackestone and packstone, (3) orbitolinid peloidal-skeletal packstone, (4) macroforaminifer-algal packstone, (5) lime mudstone and wackestone with fenestrae and cryptmicrobial laminites, (6) Microcodium-bearing lime mudstone and packstone, (7) dolostone, and (8) dolomitized evaporite. Platform-margin carbonate microfacies include: (1) skeletal-peloidal grainstone, (2) macroforaminifer skeletal grainstone, and (3) rudist fragment microfacies. The deep-water carbonate microfacies consist of pelagic lime mudstone and wackestone with common planktonic foraminifers.

Lithoclast composition in the calcareous breccias varies from place to place, but in general the most abundant microfacies types were derived from inner-platform and platform-margin environments. Deeper-water microfacies fragments are less common, mainly in the Cantarell field. The microfacies types provide clear evidence for a high-energy platform-margin facies tract that sheltered a lagoonal setting in which diverse facies were deposited in the western Yucatan Peninsula and the Sierra de Chiapas.

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