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) WT-impact-generated tsunami deposit. A similar
relationship between the WT boundary and siliciclastic or breccia
deposits is ob served 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 WT 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 WT boundary and is marked by glass and
siliciclastic or breccia deposits.