Age, stratigraphy, and deposition of near-K/T siliciclastic deposits in Mexico: Relation to bolide impact?

Abstract

Examination of 10 K/T boundary sections in northeastern and eastcentral 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.

Chicxulub impact predates the K-T boundary mass extinction

“…The Late Maastrichtian Chicxulub impact coincided with major Deccan volcanism (31,32), greenhouse warming (65.4-65.2 million years ago) (33), and a gradual decrease in species diversity during the last 700 ky before the K-T boundary, but no major species extinctions (11,34) (Fig. 6).…”

“…These observations made a convincing case for Chicxulub as the long-sought K-T boundary impact crater and cause for the end-Cretaceous mass extinction. But doubts persisted regarding the precise age and size of the impact crater (2)(3)(4)9), the origin of the so-called megatsunami deposits (5,(10)(11)(12), and the nature of the mass extinction (13,14). To resolve these issues, the International Continental Scientific Drilling Program (ICDP) supported the drilling of a new core within the Chicxulub crater (Yaxcopoil-1 drilled between December 2001 and February 2002) with the stated objectives to determine the precise age of the crater and its link to the global K-T boundary layer, to unravel Chicxulub's role in the K-T mass extinction, and to study the cratering event and size of the impact crater (15).…”

Magnetic characterization of Cretaceous‐Tertiary boundary sediments

“…Stinnesbeck et al (1993) reported maximum abundance of 0.8 ppb in the clay-rich interval on top of the sandstones. López-Oliva and Keller (1996) and Keller et al (1997) reported Ir abundances across the upper part of the siliciclastic pack, obtained in the far western edge of the channel. According to them, Ir abundances inside the clay-rich layer were between 0.3 and 0.5 ppb, higher than the 0.2 ppb detected in the uppermost sandstones but lower than the maximum of 0.8 ppb detected in the lowermost Velasco marls.…”

More evidence that the Chicxulub impact predates the K/T mass extinction

“…1). However, from the very beginning, a K/T age of the Chicxulub impact in these sections was inconsistent with: i) the stratigraphic position of the impact ejecta at the base of a several m-thick massive sandstone and 1-2 m of alternating sand/silt/shale layers; ii) the several horizons of bioturbation within these sediments, which indicate repeated colonization of the ocean floor by invertebrates and, hence, deposition over an extended time period (Keller et al 1997;Ekdale and Stinnesbeck 1998); iii) the presence of the iridium anomaly, which marks the K/T boundary impact and mass extinction worldwide, in the sediments above; and iv) the presence of a 20 cm-thick burrowed sandy limestone layer within the spherule deposit below the sandstone layer, which indicates that spherule deposition occurred in two phases separated by a period of time with no spherule deposition (review in Keller et al 2003a). All these factors indicate that a megatsunami could not explain the separation of the spherule ejecta deposits below and the Ir anomaly and K/T mass extinction above the sandstone and sand/shale/silt layers as proposed by Smit et al (1992Smit et al ( , 1996Smit 1999).…”