Bioturbation in Cambrian siliciclastic shelf strata: Paleoecological, paleoenvironmental, and temporal patterns

Cambrian rocks record the morphological and behavioral diversification of early metazoans. Bioturbation was predominantly bedding-parallel during the Early Cambrian due to a combination of evolutionary and ecological factors. Consequently, trace fossils are typically preserved on bedding planes in Lower Cambrian siliciclastic strata. The overarching goal of this work was to gain a better understanding of the agronomic revolution as it occurred in shallow marine to transitional environments by studying the bioturbation preserved on Lower Cambrian bedding plane exposures. The objectives of this study were as follows: (1) develop a precise, quantitative method for evaluating bioturbation on bedding planes; (2) apply this and other methods to Lower Cambrian rocks of the Death Valley region and southern Sweden, which represent similar depositional settings; (3) compare the resulting bedding plane bioturbation data with data previously collected from the Lower Cambrian succession in the White-Inyo Mountains; and (4) study the distribution of bioturbation across exceptionally-large bedding planes in the Upper Cambrian of Wisconsin to determine whether small bedding planes effectively sample the bioturbation that is present across sedimentary horizons.;The intersection grid method was developed to estimate the percentage area of bioturbation on bedding planes. A test of the method demonstrated that results within 5-10 percent error can be obtained even at relatively coarse grid scales. This method was applied to bedding planes from the Lower Cambrian succession in the Death Valley region. Results of these analyses, which have a moderately broad distribution with a cluster between 5-25 percent bioturbation, are typical of Lower Cambrian rocks, in which bioturbation intensities are relatively low. However, an abundance of vertical burrows in coarse-grained rocks in the Death Valley region and southern Sweden indicates that vertical bioturbation, rather than horizontal bioturbation, was dominant in Early Cambrian nearshore settings. In addition, common vertical burrows in rocks of heterogeneous grain size in the Death Valley Lower Cambrian succession suggests that adaptations to burrowing vertically into fine-grained material may have first appeared in nearshore settings. Data from large Upper Cambrian exposures in Wisconsin demonstrate that ancient bedding planes, like modern intertidal zones, can exhibit considerable heterogeneity.