Abstract

Examination of 10 K/T boundary sections in northeastern and east-central Mexico, and new data presented from 7 sections, permit the following conclusions. (1) The globally recognized K/T boundary and mass extinction in planktic foraminifera is stratigraphically above, and separated by a thin marl layer of Maastrichtian age, from the siliciclastic deposit that is commonly interpreted as a short-term (hours to days) K/T-impact–generated tsunami deposit. A similar relationship between the K/T boundary and siliciclastic or breccia deposits is observed at Brazos River in Texas, Beloc in Haiti, and Poty Quarry in Brazil. (2) Stratigraphic control indicates that deposition of the siliciclastic member occurred sometime during the last 150 k.y. of the Maastrichtian, and ended at least several thousand years prior to the K/T boundary. (3) At least four discrete horizons of bioturbation have been observed within the siliciclastic deposit that indicate episodic colonization by invertebrates over an extended time period. (4) The glass- and spherule-rich unit, which has been linked to the Haiti spherule layer and the Chicxulub structure, is at the base of the siliciclastic deposit and thus significantly predates the K/T boundary event.

The stratigraphic separation of the K/T boundary and siliciclastic deposits and the evidence of long-term deposition between them, suggests the presence of two events: (1) a globally recognized K/T boundary (impact) event marked by Ir anomaly and the mass extinction, and (2) a Caribbean event (impact or volcanic and probably linked to the Chicxulub structure) that predates the K/T boundary and is marked by glass and siliciclastic or breccia deposits.

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