Testing Patterns of Association between Brachiopod Bioclast Deposits and Stratigraphic Discontinuities: A Case Study in Middle–Upper Devonian Carbonate-Dominated Settings of North America

摘要

Nondepositional hiatuses tend to favor the dense accumulation of skeletal material because they are associated with minimal sedimentary dilution of bioclast input. However, bioclast durability or production must be sufficient to counterbalance the rigors of delayed burial and especially the repeated events of seafloor exposure and reworking that bioclasts can experience during sedimentary bypass or starvation associated with the formation of hiatuses. Detailed field investigations of skeletal concentrations in Middle–Upper Devonian brachiopod-rich carbonate stratigraphic records from the continental margin of Nevada and the cratonic interior of Iowa reveal a surprising lack of abundant skeletal material mantling major stratigraphic discontinuities. Rather, the distribution of densely packed brachiopod deposits tends to be determined by physical processes that operate within the timescale of deposition of a single lithofacies, producing skeletal concentrations that are mostly associated with bedding planes and bedset boundaries. Although a small proportion of small-scale depositional-cycle boundaries and third-order depositional-sequence boundaries do have associated brachiopod deposits, the taphonomic characteristics of those brachiopod deposits do not vary systematically with the inferred magnitude of duration of the associated discontinuity. These brachiopod deposits overwhelmingly reflect time averaging limited to a single habitat, suggesting strong limits on bioclast residence time at or near the sediment-water interface. The inability of brachiopod deposits to accumulate over the entire duration of formation of small-scale depositional-cycle boundaries and third-order nondepositional hiatuses in these Devonian carbonates likely reflects a combination of relatively low bioclastic production (low fecundity) and relatively high destruction rates (low inherent durability or postmortem conditions more unfavorable than those in siliciclastics). This study suggests t