Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: A restudy of DSDP Leg 77 Sites 536 and 540

Abstract

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.

Origin and Clay-Mineral Genesis of the Cretaceous/Tertiary Boundary Unit, Western Interior of North America

“…They found that the impact-glass particles at these marine sites are partly altered to smectite. We feel that the glass particles discovered by Alvarez et al (1992) are probably similar to those originally deposited in the K/T lower claystone layer but were subsequently altered to mostly kaolin minerals in the ancient peat swamp.…”

“…Specifically, the K/T lower claystone layer contains mostly homogeneous types ofneoformed clay minerals, little quartz and feldspar, and no detrital illite or muscovite: a mineral assemblage very similar to those that make up altered, vitric, volcanic ash-fall beds and tufts (Minato and Utada, 1969;Schultz, 1978;Hoffman and Hower, 1979;Sudo et al, 1981;Bohor et aL, 1978;Pollastro and Scholle, 1986) and strongly implies a glass precursor for the KiT boundary layers. Furthermore, Alvarez et al (1992) recently discovered several fragments of impact-ejecta glass stringers, shards, droplets, and spherules preserved at the K/T boundary in DSDP Leg 77, Sites 536 and 540, southeast Gulf of Mexico. They found that the impact-glass particles at these marine sites are partly altered to smectite.…”

Scientific Prospectus: FY97 Program

“…Haiti and Mimbral, as well as gravity measurements and drill core data that imply the existence of a 180 km diameter structure of Maastrichtian-earliest Paleocene age beneath the Yucatan (Sigurdsson et al, 1991b;Margolis et al, 1991;Alvarez et al, 1991;Hildebrand and Boynton, 1990). Re-analysis of DSDP Sites 536 and 540 in the Gulf of Mexico has led to the discovery of thick deposits of reworked carbonates of diverse ages.…”

“…These deposits contain uppermost Maastrichtian nannofossils and occur immediately below lowermost Paleocene sediments. Alvarez et al (1991) have interpreted these K/P boundary deposits as part of the ejecta blanket. Size distributions of spherules found in boundary beds on Haiti and in the DSDP sites indicate that the impact occurred within the Caribbean (Sigurdsson et al, 1991ba).…”

The Great Chicxulub Debate -Synchronicity of the Chicxulub impact and the Cretaceous/Tertiary boundary-

“…200 m , Sharpton et al, 1996;Pierazzo et al, 1998Bougeois et al, 1988Alvarez et al, 1992;Smit et al, 1992Smit et al, , 1996Smit, 1999;Takayama et al, 2000;Matsui et al, 2002;Tada et al, 200240 K/T 1990Smit et al, 1992, 1996Keller et al, 1993Keller et al, , 1997Keller et al, , 2002aSmit, 1999;Soria et al, 2001 K/T 1 3 m Fig. 2; Smit et al, 1992 1 Fig.…”