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In the La Sierrita area of Nuevo Leon, Mexico, three spherule layers are present and separated from the overlying siliciclastic deposits by up to 6 m of pelagic marls. The marls are of latest Maastrichtian age (Plummerita hantkeninoides (CF1) Zone, Micula prinsii Zone) and deposited under normal pelagic conditions with no significant evidence of reworking or slumping. Original deposition of the spherule layers occurred during the last 300 ka of the Maastrichtian and well prior to the Cretaceous-Tertiary (K–T) boundary event. Thus, if the spherules in northeastern Mexico provide critical evidence of an impact at Chicxulub, this impact predates the K–T boundary.

Examination of two new expanded K–T transitions and reexamination of road outcrops near Beloc, Haiti, reveals that deposition of the glass spherule-rich deposit (SRD) occurred within the early Danian Parvularugoglobigerina eugubina (Pla(1)) Zone, followed by an Ir anomaly, and 50 cm above it, a second Pd-dominated platinum group element (PGE) anomaly. The K–T boundary is at an erosional unconformity between the base of the SRD and underlying Maastrichtian limestone, where an interval representing about 100–250 thousand years appears to be missing (juxtaposition of planktic foraminiferal zones (Pla(1)) and Plummerita hantkeninoides (CF1)). It is possible that the spherule layers are reworked from original deposits at or below the K–T boundary. The Ir anomaly is of roughly chondritic-type and compatible with an impact event, whereas the Pd-dominated PGE anomaly is a more basalt-type and compatible with a magmatic origin. This suggests a multi-event scenario consistent with one impact followed by a major volcanic event in the Caribbean or elsewhere.

Abstract-Yaxcopoil-1 (Yax-1), drilled within the Chicxulub crater, was expected to yield the final proof that this impact occurred precisely 65 Myr ago and caused the mass extinction at the Cretaceous-Tertiary (K/T) boundary. Instead, contrary evidence was discovered based on five independent proxies (sedimentologic, biostratigraphic, magnetostratigraphic, stable isotopic, and iridium) that revealed that the Chicxulub impact predates the K/T boundary by about 300,000 years and could not have caused the mass extinction. This is demonstrated by the presence of five bioturbated glauconite layers and planktic foraminiferal assemblages of the latest Maastrichtian zone CF1 and is corroborated by magnetostratigraphic chron 29r and characteristic late Maastrichtian stable isotope signals. These results were first presented in Keller et al. (2004). In this study, we present more detailed evidence of the presence of late Maastrichtian planktic foraminifera, sedimentologic, and mineralogic analyses that demonstrate that the Chicxulub impact breccia predates the K/T boundary and that the sediments between the breccia and the K/T boundary were deposited in a normal marine environment during the last 300,000 years of the Cretaceous.