Compositional and Taphonomic Variations in Modern Crinoid-Rich Sediments from the Deep-Water Margin of a Carbonate Bank

摘要

Multivariate analyses of the coarse-grained fraction (u3e2 mm) of sediments accumulating in deep water (419-434 m) along the western margin of the Little Bahama Bank reveal identifiable, small-scale compositional and taphonomic variations among local subhabitats (ridge crest, slope, foreslope, base of slope, pavements and scour pit) separated by meters to tens of meters. Bulk composition varies between planktic-(crest and slope) and lithic-dominated (pavements, scour pit) sediments. Local macrobenthic skeletal components also vary significantly among subhabitats, but are commonly dominated by echinoid and crinoid material; crinoid columnals contribute 9-52% of the coarse skeletal component of 17 sediment samples considered. Distributional and taphonomic analyses (abrasion, encrustation, breakage) indicate that columnals produced in dense ridge-crest assemblages of Chladocrinus decorus are transported down and accumulate along an adjacent slope. Sediments from hardgrounds supporting scattered living assemblages show columnals with the highest levels of abrasion, implying prolonged local reworking. Elevated contributions of Endoxocrinus parrae columnals to the few subhabitats where this species dominates the living assemblage suggest limited lateral transport in the absence of steep gradients. High levels of biological encrustation in areas of thin sediment cover suggest control by length of exposure of grains at the sediment-water interface. Lack of any correlation between frequency of broken columnals in samples and any observed sedimentary or environmental parameters suggests the action of predators or scavengers in this deep-water setting. Small-scale variations in sediment composition, benthic skeletal assemblages, and taphonomic characteristics are not unique to shallow-water settings, but can also be identified in deep-water carbonate bank-margin sediments. Such changes are largely a response to differences in benthic flow regime associated with small-scale topographic irregularities and may provide an important diagnostic tool for the interpretation of fossil assemblages.