Determining the relative roles of climate and tectonics in the formation of the fossil record of terrestrial vertebrates: a perspective from the Late Cretaceous of western North America

WA Museum Records and Supplements | Updated 9 years ago

Abstract - Both climate and tectonics have been demonstrated to be prominent factors in the formation of the sedimentological record. By extension, these factors must have been influential in the preservation of the vertebrate fossil record. The Late Cretaceous Foreland Basin of the Rocky Mountain region in North America is a rich source of vertebrate fossil remains and is ideally situated to investigate this issue. A sample of 185 published and unpublished vertebrate localities has been examined to determine the relative influence of regional tectonism versus climate in the formation of this fossil record.

The localities considered are from the Judith River Formation and Two Medicine Formations of Montana, and the Judith River Group of southern Alberta. These sites were classified as event, attritional, or lag accumulations. A chi-square analysis of the distribution of these types of localities along a proximal-distal transect perpendicular to the long axis of the foreland basin shows that the pattern of fossil preservation is dependent upon location, suggesting that tectonics was the dominant factor in the formation of this fossil record. However, a similar chi-square analysis of sites along a longitudinal transect in the distal regions of the foreland basin also shows that the pattern of fossil preservation is dependent on location. This pattern suggests that regional tectonism had a minor role in the formation of these deposits.

This dependency on location seems to relate closely to the sandstone/ mudstone ratios of each region. These ratios are taken as indicators of rates of f1oodplain aggradation in the individual areas, which is controlled by local tectonism, subsidence and/or climate. Of these three, climate seems the most viable alternative. Based on this statistical analysis, it appears that large-scale tectonism is not a prominent influence in the formation of the vertebrate fossil record in the Late Cretaceous of northern western North America.