Current Research--Bennett--page 2 of 8

Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 244, part 1
Inferring Stratigraphic Position of Fossil Vertebrates from the Niobrara Chalk of western Kansas--page 2 of 8

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Introduction

The Smoky Hill Chalk Member of the Niobrara Chalk of western Kansas is famous for the very large number of fossil vertebrates it has produced. The collection of fossil vertebrates from the Smoky Hill Chalk Member was begun in earnest by O. C. Marsh and the Yale College Scientific Expedition in 1870. That expedition and subsequent ones by Marsh and colleagues in 1871 and 1872 were so successful in the discovery of abundant remains of fish, mosasaurs, pterosaurs, birds, plesiosaurs, and turtles that Marsh hired professional collectors, including B. F. Mudge and S. W. Williston, to work the Niobrara Chalk for him each year through 1879. During this 10-year period, Marsh amassed the largest and one of the most important collections of Niobrara vertebrates. Since the 1870's, collecting of Niobrara vertebrates has been almost continuous. H. T. Martin of The University of Kansas collected many specimens, providing the core of the large collection at The University of Kansas, and selling specimens to many other museums. George F. Sternberg collected for many years and, like H. T. Martin, sold specimens widely but saved much of the best material for the Sternberg Memorial Museum [later renamed the Fort Hays State Museum (FHSM) and now called the Sternberg Museum of Natural History] in Hays, Kansas. In addition, many other individuals have made small but good collections.

The majority of fossil vertebrates were collected before the stratigraphy of the Niobrara Chalk was adequately understood, and their stratigraphic positions are unknown. The most that the collectors recorded was whether the specimen was from gray (or blue) shale or yellow chalk. The gray-yellow dichotomy is not particularly useful because it is largely a weathering phenomenon (Williston, 1897; Miller, 1968), and often the color change is found at different levels in the same outcrop.

Most studies of fossil vertebrates from the Smoky Hill Chalk Member have suffered from a lack of stratigraphic information, and authors have often had to rely on the fact that exposures along the eastern end of the outcrop area in Ellis County are low in the Smoky Hill Chalk Member, while those of the western end in Logan County are higher. Bardack (1965) located many of the old localities, but at the time the stratigraphy was not sufficiently well known to determine stratigraphic positions of those localities with any precision. The purpose of this report is to show that it is now possible to determine the stratigraphic positions of specimens from locality data. Although precise locality data are best, often even rather vague locality data are sufficient to determine the stratigraphic position of a specimen.

It is important to note that almost all of the Yale Peabody Museum (YPM) collection was collected in the 1870's, before the old Wallace County was divided into Wallace and St. John counties in 1881. St. John County was changed to Logan County in 1885 (Elias, 1931). Bardack (1965) noted that the present Logan County was formerly called St. John County, but did not mention that it was formerly a part of a larger Wallace County. The Niobrara Chalk is not widely exposed in the modern Wallace County. Many specimens in the YPM and other collections are listed as being collected in Wallace County, but almost all of them are from what is now Logan County, and a few are probably from Gove County (see below).

Stratigraphy

The stratigraphy of the Smoky Hill Chalk Member of the Niobrara Chalk in western Kansas is now well understood. Hattin (1982) described the stratigraphy of the Smoky Hill Chalk Member and identified 23 marker units. Hattin's marker units are the completion and perfection of the pioneering work of Russell (1929), and they allow quick determination of the stratigraphic position of an outcrop in the field. The invertebrate biostratigraphy of the Smoky Hill Chalk Member was examined by Miller (1968) and Hattin (1982). Stewart (1988) showed that vertebrates can also be useful for biostratigraphy in the Smoky Hill Chalk Member. Biostratigraphic zonations based on inoceramids and species of the fish Protosphyraena divide the Smoky Hill Chalk Member into four zones, but biostratigraphic zonations do not allow subdivision of the upper half of the Smoky Hill Chalk Member. This is unfortunate because the upper half of the Smoky Hill Chalk Member has produced the majority of fossil vertebrates. Stewart (1988) noted that this is not because the lower half is unfossiliferous (fossils may actually be more common in the lower parts), but rather that fossils from the upper parts were more intensively collected. Letters sent to O. C. Marsh by his collectors S. W. Williston and E. W. Guild (alias E. S. Field) indicate that they concentrated on the upper part because they believed the hunting better, particularly in regard to the birds and pterosaurs for which O. C. Marsh paid the most money. Whatever the reason, most fossil vertebrates have been collected from the upper part of the Smoky Hill Chalk Member, and biostratigraphy is of no use in determining the relative stratigraphic position of these specimens. Therefore, it is necessary to rely on the stratigraphic marker units.

Hattin (1982) and Russell (1929) both designated marker units that allow quick determination of stratigraphic position, and in some instances both authors used the same markers. Marker units identified by Russell (1929) are based almost entirely on bentonite sequences and are lettered, while those identified by Hattin (1982) also include units of unusual lithology and are numbered. In addition to the marker units, many other bentonites are readily recognizable and traceable in the field. These are particularly useful in correlating small outcrop areas with larger outcrops nearby. In this paper Hattin's marker units are used when possible, but Russell's Marker Unit H between Marker Units 17 and 18 also is used because it is readily identifiable and widely exposed.

If the exact locality of a specimen is known, one need only compare the exposure at the locality with the composite stratigraphic column of Hattin (1982). When the exact locality is not known or the exposure at the particular locality is of limited vertical extent and between marker units, a number of nearby outcrops are examined. Despite some regional variations, the bentonite sequences are remarkably uniform across the outcrop area in western Kansas. This uniformity makes possible the precise correlation of outcrops.

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Kansas Geological Survey
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