Impact-Induced Sediments at the K-T Boundary: Offshore Campeche and Chiapas-Tabasco Region, Southeastern Mexico
J.M. Grajales-Nishimura, 2007. "Impact-Induced Sediments at the K-T Boundary: Offshore Campeche and Chiapas-Tabasco Region, Southeastern Mexico", The Paleogene of the Gulf of Mexico and Caribbean Basins: Processes, Events and Petroleum Systems, Lorcan Kennan, James Pindell, Norman C. Rosen
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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 approximately 300 km westward from the center of the Chicxulub structure. This sedimentary succession consists of a single graded deposit subdivided into three main units that from base to top includes: (1) a basal 50 to 300 m-thick, coarsegrained carbonate breccia; (2) a 10 to 20 m-thick, finegrained carbonate breccia, and; (3) a 25 to 30 m-thick, interval of sand and silt to clay-sized constituents, mostly abundant ejecta material. Additionally, a 10-20 m-thick, fine-grained calcareous breccia is recognized within the ejecta material-rich layer (unit 3) in some wells. The K-T boundary sedimentary succession is bounded at its base by a deep-water, shaly-calcareous facies of Upper Maastrichtian age and at its top by similar rocks of lower Paleocene age. Similar sedimentological characteristics and stratigraphic relationships are observed in analog outcrops in the Sierra de Chiapas (El Guayal, State of Tabasco; and Bochil, Chilil and Soyalό, State of Chiapas). Lithoclasts of the calcareous breccias are derived dominantly from platform-interior and platform-margin environments and only a few from deep-water settings. Ejecta material in unit 3 includes: shocked quartz, quartz with ballen structure, shocked plagioclase, altered melt rock, and rare pelitic schist fragments. Wireline log data, 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 probably occurred within a very short time span following the Chicxulub impact:
Unusually strong seismic shaking induced the collapse of the platform margin, resulting in an enormous debris flow (units 1 and 2),
Arrival and deposition of ballistic impact ejecta (unit 3), and
Reworking and deposition, possible induced by tsunami currents (carbonate breccia within unit.
Units 1 and 2 represent the most important oil reservoirs at the Campeche Bay oil fields, and unit 3 is the seal layer. Unit 4 is a dark clay bed deposited during a global decrease in ocean productivity following the meteorite impact.