2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 9
Presentation Time: 4:00 PM

Quantifying Early Cambrian Bioturbation along An Onshore-Offshore Transect, Great Basin, USA

MARENCO, Katherine N., Department of Geology, Bryn Mawr College, 101 N. Merion Avenue, Bryn Mawr, PA 19010 and BOTTJER, David, Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, kmarenco@brynmawr.edu

During the Neoproterozoic-Cambrian transition, bioturbation became more plentiful, complex, and physically disruptive as a direct consequence of the Cambrian radiation of marine invertebrates. This increase in biogenic sediment disturbance transformed substrates and dramatically expanded available ecospace. In particular, microbial mats, which were ubiquitous on Proterozoic seafloors, virtually disappeared from open marine environments by the end of the Cambrian Period. Seilacher and Pflüger (1994) termed this early example of ecosystem engineering the agronomic revolution, and Bottjer and colleagues (2000) recognized the significant ecological and evolutionary effects of this transition as the Cambrian substrate revolution.

Until recently, evidence for the agronomic and Cambrian substrate revolutions had not been evaluated quantitatively. Semi-quantitative methods for assessing bioturbation intensity, such as ichnofabric indices (Droser and Bottjer 1986) and bedding plane bioturbation indices (Miller and Smail 1999), have proven effective for field scoring but lack the precision necessary for fine-scale studies. Using grid-based methods to estimate the percentage bioturbation on bedding planes helps to improve dataset consistency by reducing subjectivity. For example, the grid-based “point-intersection” method generated results within 0.50% error when applied to hypothetical bedding planes for which the exact percentage bioturbated was known (Marenco and Bottjer in prep).

In order to better understand the initial phase of the agronomic revolution as it occurred in different shallow marine environments, field studies were conducted at Lower Cambrian localities in California and Nevada that fall along a paleo-onshore-offshore transect. Because bioturbation is typically restricted to bedding plane surfaces in the Lower Cambrian, particular attention was paid to bedding plane exposures and the strata associated with them. In addition to field paleoecological methods, including visual scoring of bioturbation, the point-intersection method was applied to digital photographs of the studied bedding planes. The resulting dataset provides a new, quantitative perspective on the interaction between early animals and their environment.