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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 Jeletzkytes 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

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