Testing Patterns of Association between Brachiopod Skeletal Concentrations and Stratigraphic Discontinuities in Devonian Carbonate-Dominated Records of North America

Densely-packed bioclast accumulations, typically dominated by molluscs, are commonly associated with marine stratigraphic discontinuities in post-Paleozoic siliciclastic records, providing a means of recognizing otherwise obscure hiatuses and predicting the distribution of potential reservoir zones. But are such patterns general to Paleozoic carbonate records, typically dominated by other benthic taxa? Non-depositional hiatuses favor accumulation of abundant skeletal material because sedimentary dilution of bioclast production is minimal. However, bioclasts must be durable and/or bioclast production high enough to accrue despite the repeated events of seafloor exposure, reworking, bypass, and/or starvation associated with the formation of hiatuses. Detailed field descriptions of stratigraphic sections and skeletal concentrations in Middle-Upper Devonian brachiopod-rich carbonate sedimentary records from the continental margin of Nevada and the cratonic interior of Iowa reveal a surprising lack of abundant skeletal material mantling stratigraphic discontinuities. Rather, the distribution of bioclast-rich intervals tends to be constrained by the deposition of normal-marine subtidal lithofacies.

Most brachiopod deposits are bounded by bedding planes and bedset boundaries, less commonly bounded on top, and only rarely bounded at the base by parasequence boundaries. Where systems tracts could be identified within third-order depositional sequences, brachiopod deposits are most common within transgressive systems tracts (TST) and maximum flooding zones where subtidal normal-marine facies dominate. Less than 10% of transgressive surfaces marking bases of TST’s encountered in stratigraphic sections are mantled above by densely packed brachiopod deposits. Moreover, the taphonomic state of brachiopod material does not consistently vary with inferred magnitude of duration of their bounding discontinuities. Most brachiopod deposits reflect within-habitat time averaging, which places a constraint on bioclast residence time at or near the sediment-water interface. The inability of brachiopod deposits to accumulate during non-depositional hiatuses may be due to relatively low bioclast production and/or high destruction rates characterizing Paleozoic carbonate-dominated seafloors. These findings have implications for Phanerozoic trends in the thickness and abundance of bioclast-rich facies, and the distribution and connectivity of potential reservoir zones.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California