Big bedding planes: Outcrop size and spatial heterogeneity influence trace fossil analyses

Abstract

Roadcuts, core, cliffs and even mountainside exposures often limit the lateral extent to which we can assess bioturbation on bedding planes, simply because exposures of such surfaces are generally rare or small. Such constraints may hamper paleoenvironmental and paleoecological interpretations drawn from facies where most bioturbation is bed-parallel or patchily distributed. To explore how acute such constraints are, we conducted a pilot study of bioturbation in three exceptionally large (>5 m2), well-exposed, paleoenvironmentally similar bedding planes from the Cambrian of Wisconsin. The goal was to test the hypothesis that trace fossils and biogenic sediment disruption can be distributed heterogeneously at the scale of meters to decameters. Mapping the size variation and distribution of trace fossils across these bedding planes reveals variable patchiness. Individual bioturbated patches tend to be large (m-scale), but the density of biogenic sediment disruption within these patches is quite heterogeneous. In addition, bioturbated areas are separated by zones of undisturbed sediment that vary in size from <0.5 m2 to >3 m2. These observations suggest that ancient bedding planes, like modern substrate surfaces, can exhibit considerable lateral bioturbation heterogeneity at a range of spatial scales. Caution is thus urged when extrapolating the distribution and diversity of trace fossils and other biogenic sedimentary structures across broad sedimentary horizons based on small (≤1 m2) bedding plane exposures.