Multidisciplinary studies, including stratigraphy, sedimentology,
mineralogy and geochemistry, of the new core Mullinax-1 and outcrops
along the Brazos River and Cottonmouth Creek, Falls County, Texas,
reveal the complex history of the Chicxulub impact, the event deposit
and the K-T boundary event. The K-T boundary, as identified by the
negative delta C-13 shift, first occurrence of Danian planktic
foraminifera and palynomorphs occurs 80 cm above the event deposit in
core Mullinax-1. The underlying 80 cm interval was deposited in a
shallow low oxygen environment during the latest Maastrichtian, as
indicated by high stress microfossil assemblages, small shells and
burrows infilled with framboidal pyrite. The underlying event deposit,
commonly interpreted as K-T impact tsunami, consists of a basal
conglomerate with clasts containing Chicxulub impact spherules, repeated
upward fining units of spherule-rich sands, followed by hummocky
cross-bedded and laminated sands, which are burrowed by Thalassinoides,
Planolites and Ophiomorpha and truncated by erosion. This suggests a
series of temporally separated storm events with recolonization of the
ocean floor by invertebrates between storms, rather than a series of
waning tsunami-generated waves. The lithified clasts with impact
spherules at the base of the event deposit provide strong evidence that
the Chicxulub impact ejecta layer predates the event deposit, but was
eroded and re-deposited during the latest Maastrichtian sea level
lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm
thick yellow clay layer interbedded in undisturbed late Maastrichtian
clay- and mudstones 40 cm below the base of the event deposit and near
the base of planktic foraminiferal zone CF1, which spans the last 300
kyr of the Maastrichtian. The yellow clay consists of cheto smectite
derived from alteration of impact glass, as indicated by rare altered
glass spherules with similar chemical compositions as reworked spherules
from the event deposit and Chicxulub impact spherules from NE Mexico and
Haiti. The Brazos sections thus provide strong evidence that the
Chicxulub impact predates the K-T boundary by about 300 kyr, consistent
with earlier observations in NE Mexico and the Chicxulub crater core
Yaxcopoil-1. (c) 2007 Elsevier B.V. All rights reserved.