Impact-induced sediment deposition on an offshore, mud-substrate continental shelf, cretaceous-paleogene boundary, Brazos river, Texas, U.S.A.

摘要

Data from twenty Brazos River stratigraphic sections of the K-Pg event deposit are compiled to provide a record of changing depositional processes on the Texas continental shelf that left a record of disturbance triggered by the Chicxulub impact. Foraminiferal paleobathymetric depth control of enclosing sediments indicates paleo-water depths of 75-125 m with gradual deepening above the K-Pg boundary. Outcrop-scale observations indicate that the most complete deposits have a basal unit of mudstone clast-bearing mass flows indicative of movement by noncohesive laminar flow. These mudflows were mobilized by impact-generated seismic shaking of seafloor sediment. The material of the mass flows consists of reworked shell-rich mud matrix containing outsize clasts of consolidated mudstone and cemented concretions. The upper parts of the K-Pg event deposit consist of units of high-energy upward-fining sandstones, each with a basal lag layer of spherule-rich coarse sandstone lag overlain by poorly or moderately sorted hummocky cross-bedded sandstone. The last deposited sandstone is capped with a siltmud suspension settling layer that as much as 10 cm thick where cemented with calcite, but is much thinner elsewhere. The provenance of these K-Pg Brazos River event deposit sediments is offshore marine muds (lower units) and shallow-water sands (higher units) with admixed impact spherules and no identifiable land-derived component. Deposition was the result of secondary processes activated by energy from seismic disturbance or by atmospheric disturbance, with no deposits directly attributable to tsunami wave control.