VARIATION IN TAPHONOMIC SIGNATURES BETWEEN BIOFACIES AND SYSTEMS TRACTS IN THE BROMIDE FORMATION, OKLAHOMA

Modern taphofacies analysis often focuses on the loss of information and biases that are produced by local environmental conditions at the time of deposition. Less well understood is how taphonomic signals integrate with sequence stratigraphic architecture at a macroscale. We use Non-metric Multidimensional Scaling (NMMDS) and hierarchical cluster analysis to assess the similarity of sample sites based on taphonomic signature, lithofacies, and inferred sequence regime (TST vs HST). Previous biofacies and diversity analyses on Bromide Formation trilobites have shown that in most biofacies, rarefied alpha diversity and gamma diversity of middle and outer ramp HST deposits are greater than in the TSTs, with biofacies replacement occurring in a downramp direction. Diversity patterns do not agree with model predictions and other data sets that indicate low beta and high alpha diversity in the TST, likely because of taphonomic degradation. We tested the taphonomic degradation hypothesis by combining a series of trilobite-specific taphonomic variables into ranked distances, including abrasion, fragmentation, encrustation, and sclerite size/shape ratios. The results of the multivariate analyses suggest that samples within the TST are characterized by abrasion and fragmentation, likely from strong winnowing effects in condensed grainstone and rudstone. During early HST conditions, well-articulated trilobite exoskeletons preserved by obrution deposits are lightly imprinted by taphonomic processes. Trilobite-brachiopod shell pavements of the late HST show little fragmentation, but have high levels of post-mortem encrustation. This suggests exposure on the sea floor but at lower average energy levels than in the TST.