QUANTITATIVE ANALYSIS OF BEDDING PLANE BIOTURBATION: NEW INSIGHTS FROM THE UPPER CAMBRIAN OF WISCONSIN

Horizontal bioturbation, particularly common in shallow marine strata of Cambrian age, generates little visible disruption of primary bedding and thus can be difficult to recognize in vertical outcrop exposures. Bedding plane exposures are a superior medium for studying the nature and quantity of horizontal bioturbation in siliciclastic strata. In most deposits, however, exposed bedding planes are few in number and generally small, rarely exceeding one square meter in size. In central Wisconsin, quarries of Upper Cambrian (Dresbachian-Franconian) sandstones expose bedding planes that commonly extend for many tens of square meters. These well-preserved surfaces, reflecting deposition in an intertidal setting that experienced intermittent subaerial exposure, contain an ichnofauna that is predominantly horizontal and characterized by moderate diversity and moderate to high abundance (Hagadorn et al., 2002). Individual traces vary in diameter from a few millimeters (Gordia-type) to more than ten centimeters (Climactichnites). Extensive bedding plane exposures such as these provide a rare opportunity for detailed study of bioturbation intensity and patchiness at a variety of scales. As part of an ongoing project to develop new ways of assessing bedding plane bioturbation, several of these large bedding planes were analyzed in detail using grid-based methods. For example, a 48 m² bedding plane surface containing Gordia-type traces was gridded using a 25 cm²-sized subunit. On this surface, no true patches (defined as one or more adjacent bioturbated subunits surrounded entirely by unbioturbated subunits) are present. Instead, the meandering Gordia-type traces form an apparent network across the bedding surface such that each bioturbated subunit is at least diagonally adjacent to one other bioturbated subunit. If this network is examined at a finer scale, however, patches smaller than 25 cm² in size emerge. The results of this work illuminate the complex problem of scale for quantitative analysis of horizontal bioturbation, particularly in Cambrian shallow to marginal marine deposits.