SEQUENCE STRATIGRAPHY AND CONODONT BED DISTRIBUTION IN MIXED SILICICLASTIC-CARBONATE SUCCESSIONS

Sequence stratigraphy provides a predictive framework for interpreting distribution patterns of fossil concentrations. Both ecological and taphonomic factors play key roles in the recurrence of conodont beds in mixed siliciclastic-carbonate facies. The combination of shallow, clear water, and firm to hard substrates (e.g., hardgrounds, shell gravels) during siliciclastic-starved early transgressions make TSTs especially favorable to diverse stenotopic epifaunal taxa. Despite relative condensation, however, these high-diversity assemblages in shallow shelf areas, may be poor in conodonts, or contain generalized shallow-water taxa (such as icriodids in the Devonian). Conversely, basinal equivalents of these condensed carbonate beds may yield abundant, diverse assemblages of conodonts (such as polyganthids in the Devonian), together with other pelagic and dysoxic-adapted taxa. Apatite preservation appears to have been enhanced in dysoxic settings and thin bone and conodont-rich lag deposits are especially typical of surfaces of maximum starvation, as well as the minor flooding surfaces associated with small scale cycles, including bases of parasequences. Dark, organic-rich shales, typical of early highstands, carry low-diversity associations dominated by eurytopic benthic species; conversely, deeper water conodonts may be common and diverse although not as readily extractable as in condensed carbonates. These associations may exhibit significant changes in composition over relatively short intervals than do many of the higher diversity associations and important zone boundaries may coincide with maximum flooding intervals. In areas with low overall rates of siliciclastic input, regressive portions of cycles rarely may yield conodont concentrations as thin storm lag beds. However, in areas proximal to siliciclastic source areas, increased sedimentation/turbidity during regression favored low density and low to moderate diversity fossil associations. Bones and conodonts are typically rare, in part because of the strong diluting effect of increased siliciclastic input. Predictably recurring patterns of water depth and sedimentation rate change thus can be related to phases of small- and large-scale sedimentary cycles. In turn, these patterns may permit a more focused strategy for location of conodont-rich sampling units.