[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: "Meteor theory gets rocky ride from dinosaur expert"

I think this is the paper that reports the last ammonite -- ten centimeters below the K-Pg boundary.

Entire message repeated below because it came with the "REMAINDER OF MESSAGE TRUNCATED" tag as an attached (!) .txt file.

----- Original Message -----
From: "jrc" <jrccea@bellsouth.net>
To: <bigelowp@juno.com>
Cc: <dinosaur@usc.edu>
Sent: Friday, May 27, 2005 4:21 PM
Subject: Re: "Meteor theory gets rocky ride from dinosaur expert"

----- 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, ....



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.


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[.]