COMPOSITIONAL AND TAPHONOMIC VARIATIONS IN MODERN CRINOID-RICH SEDIMENTS FROM THE DEEP-WATER MARGIN OF A CARBONATE BANK

Multivariate analyses of the coarse-grained fraction (>2 mm) of sedime nts 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 mete rs to tens of meters. Bulk composition varies between planktic- (crest and slope) and lithic-dominated (pavements, scour pit) sediments. Loc al macrobenthic skeletal components also vary significantly among subh abitats, 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 analyse s (abrasion, encrustation, breakage) indicate that columnals produced in dense ridge-crest assemblages of Chladocrinus decorus are transport ed down and accumulate along an adjacent slope. Sediments from hardgro unds supporting scattered living assemblages show columnals with the h ighest levels of abrasion, implying prolonged local reworking. Elevate d contributions of Endoxocrinus parrae columnals to the few subhabitat s where this species dominates the living assemblage suggest limited-l ateral transport in the absence of steep gradients. High levels of bio logical encrustation in areas of thin sediment cover suggest control b y length of exposure of grains at the sediment-water interface. Lack o f any correlation between frequency of broken columnals in samples and any observed sedimentary or environmental parameters suggests the act ion 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 sedimen ts. 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.