----- Original Message -----
From: "Phil Bigelow" <bigelowp@juno.com>
To: <dinosaur@usc.edu>
Sent: Friday, May 27, 2005 1:09 AM
Subject: Re: "Meteor theory gets rocky ride from dinosaur expert"
If the southern end of the seaway was closed, ....
PALEONTOLOGY OF THE K/T BOUNDARY INTERVAL: BADLANDS NATIONAL PARK, SOUTH
DAKOTA
JOHN A. CHAMBERLAIN, JR .(1), DENNIS O. TERRY, JR. (2), PHILIP W. STOFFER
(3), MARTIN A. BECKER (4)
Abstract-We have documented a distinctive zone of disrupted sediments over
300 km2 in the Badlands area of South Dakota. This Disrupted Zone (DZ) is
located in the Maastrichtian Fox Hills Formation about 20 meters above its
transitional contact with the Pierre Shale, and ranges from 0.5 to 5
meters
in thickness. Based on Sr age dates, sedimentary features, and impact
ejecta, we interpret the DZ as a distal manifestation of the
end-Cretaceous
Chicxulub Impact Event. The DZ occurs in an interval with uncommon, but
significant fossil content. Scaphitid ammonites characteristic of the
Jeletzkytes nebrascensis ammonite zone occur below and within the DZ, but
not above it. In addition, the sections, including the DZ itself, contain
nuculid and inoceramid bivalves, osteichthian and chondrichthian remains,
crustacean crawling traces, leaf fragments, and disseminated, and
sometimes
carbonized, plant debris. Compared to earlier late Cretaceous faunas of
the
Western Interior, the Badlands K/T fauna is impoverished in that it
contains
relatively few species. This impoverishment is unrelated to the end-K
impact, and is more likely the result of environmental conditions in the
Seaway coupled with the high-stress conditions of local environments
within
which the Badlands fauna lived.
____________________
INTRODUCTION
Figure 1. Paleogeography of North America at the end of the
Cretaceous
Period based on recent ammonite biostratigraphy. WIS _ Western Interior
Seaway. BNP _ Badlands National Park. Modified from Fig. 1 of Terry et al.
(in review). Paleogeographic data from Kennedy et al. (1998).
During late Cretaceous time the Western Interior Seaway, an epeiric sea of
considerable extent, covered much of the mid-continent region of North
America. Kennedy et al. (1998) suggest that during the Jeletzky[i?]tes
nebrascensis zone of the Late Maastrichtian the Seaway extended northward
from the Cretaceous Gulf of Mexico to North Dakota (Figure 1). However,
the
absence of marine sections that can be unequivocally dated to the latest
Maastrichtian or to the Cretaceous/Tertiary transition (e.g. Obradovich,
1993) has generally been interpreted to mean that the Seaway had retreated
completely from the northern plains region of the United States by the end
of the Cretaceous. In this scenario, marine rocks of early Tertiary age,
the
Cannonball Formation for example, necessarily imply the temporary return
of
marine conditions during the early Tertiary.
The late Cretaceous succession exposed in and near Badlands National Park
in
southwestern South Dakota contains evidence that challenges this view. In
two recent papers (Stoffer et al. 2001; Terry et al., in review), we
describe a zone of distorted bedding in the marine rocks of the Badlands
area (referred to here as the DZ) which resembles the highly contorted,
chaotic bedding seen in some Gulf of Mexico K/T boundary sections
(summarized in Smit et al. 1996). Moreover, the Badlands DZ interval
contains impact ejecta and fossil occurrences which suggest association
with
biotic and geologic events that mark the end of the Cretaceous. The
Badlands
DZ is conformably overlain by a considerable thickness of what thus would
be
earliest Paleocene marine sediments. In suggesting continuous marine
deposition across the K/T boundary in the Badlands area, this
interpretation
implies that the Seaway had not disappeared from the northern plains at
the
end of the Cretaceous, but was a significant paleogeographic feature in
this
area well into the Paleocene[.]