A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas

Abstract

At sites near the Brazos River, Texas, an iridium anomaly and the paleontologic Cretaceous-Tertiary boundary directly overlie a sandstone bed in which coarse-grained sandstone with large clasts of mudstone and reworked carbonate nodules grades upward to wave ripple-laminated, very fine grained sandstone. This bed is the only sandstone bed in a sequence of uppermost Cretaceous to lowermost Paleocene mudstone that records about 1 million years of quiet water deposition in midshelf to outer shelf depths. Conditions for depositing such a sandstone layer at these depths are most consistent with the occurrence of a tsunami about 50 to 100 meters high. The most likely source for such a tsunami at the Cretaceous-Tertiary boundary is a bolidewater impact.

Rapid short-term cooling following the Chicxulub impact at the Cretaceous–Paleogene boundary

“…As a result, previous studies have resulted in a wide range of different estimated settling times, ranging from <10 y up to >10 kyr, although these latter values seem improbable because mechanisms to keep particles suspended for such prolonged periods of time appear lacking (31). With estimated water depths of 75-200 m (21,22), settling time of PGE-bearing impact dust will have been in the order of 1-100 y at the Brazos River site. The scattered nature of the Ir record, combined with the dinocyst assemblages, implies that the complex of Units E, F, and G reflects a mixture of earliest Paleocene and reworked uppermost Maastrichtian materials.…”

“…2), suggesting that it was formed in a rapid depositional event (19). This material was probably initially deposited as the settling tail-end of the impact-induced tsunami/ seiche (21,22) and subsequently resuspended during postimpact storms (23,29). Such storms might have been triggered by the enhanced contrast between warm oceans and cold atmosphere during an impact winter (13).…”

“…1). In the Late Cretaceous and early Paleogene, the Brazos area was characterized by nearly continuous and predominantly siliciclastic sedimentation on the shallow shelf of the northern Gulf of Mexico, close to the entrance of the Western Interior Seaway (19,20), at estimated depths of 75-200 m (21,22). The sedimentary successions in this region comprise the Maastrichtian Corsicana (Kemp Clay) Formation and the Paleocene basal and upper Littig members of the Kincaid Formation.…”

“…2) (21,22). The upper Maastrichtian fossiliferous shales of the Corsicana Formation (Unit A) (22,23) are overlain by the basal part of the Paleocene Kincaid Formation, consisting of a sequence of sandstone layers yielding multiple types of clasts and shell debris (Units B, C, and D) that have been interpreted as impact-triggered tsunami deposits (21)(22)(23)(24)(25).…”

地層中に存在する古津波堆積物の調査

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