A QUANTITATIVE METHOD FOR EXAMINING BIOTURBATION CHANGES IN THE LOWER DEVONIAN OF NEW YORK STATE AND ITS APPLICATION TO THE BULLDOZER HYPOTHESIS One aspect of the Bulldozer Hypothesis suggests that extent of bioturbation, the churning and disruption of sediment, has increased over time. The lower Devonian facies of eastern New York State represent a series of repeating onshore-offshore environments, which are ideal for examining this hypothesis. However, the existing qualitative and semi-quantitative methods of describing bioturbation are not repeatable and rigorous enough to explore this hypothesis adequately.
The majority of ichnological (trace fossil) studies have been qualitative in nature. These studies often describe size, type, and position of individual traces. Droser and Bottjer (eg. 1989) developed the first semi-quantitative method of describing the degree of disruption of primary (physical) lamination by bioturbation. Their method used an Ichnofabric Index drawn on flash cards to "quantify" large sections of outcrop rapidly. However, observations at outcrop often miss bioturbated fabrics because trace fossil details are either not visible, due to insufficient weathering, or lost, due to too much weathering.
I have developed a more quantitative method of examining the extent of bioturbation in individual rock samples; I can extend these results to entire outcrops. This method includes: taking regularly spaced samples from selected outcrops; slabbing and polishing; applying a grid system; and determining percentages of bioturbation in each sample. This method is similar to the point-count system used by mineralogists. This quantitative method has the benefit of producing data that are less subjective than the Droser and Bottjer Ichnofabric Index. My method also has the potential to apply several statistical methods to sets and subsets of samples to determine if bioturbation has changed with time and/or varies with environment.
Thayer, C. W., Y. L. Bordeaux, and C. E. Brett, 1990. Escalation in Bioturbation in the Devonian of New York State.Geol.Soc.Amer. Abstracts with Programs.,22(7),80.
