SEQUENCE STRATIGRAPHY, FOSSIL ASSOCIATIONS, AND DIVERSITY IN THE PALEOZOIC APPALACHIAN BASIN

Bathymetric and sedimentologic factors are two of the most significant controls on the distribution pattern of shallow marine benthic organism, resulting in benthic assemblages zoned both along and across depositional strike of ancient epeiric/foreland ramps. Sequence stratigraphy provides a predictive framework for interpreting distribution patterns in Paleozoic fossil assemblages in the Appalachian foreland basin. In some carbonate and mixed siliciclastic-carbonate cycles with low rates of terrigenous input, stratigraphic patterns of community replacement may be nearly symmetrical throughout transgressive-regressive cyclothems reflecting species tracking of temporally-consistent, but laterally shifting environments. However many siliciclastic-influenced depositional sequences show distinctive patterns of biotic expression in different systems tracts. Seafloor erosion and availability of over-compacted substrates favored firm-ground burrowing assemblages and cobble encrusters at forced regression surfaces. The combination of shallow, clear water and typically hard substrates (hardgrounds, shell gravels) during sediment-starved early transgressions made them especially favorable for colonization by diverse stenotopic epifaunal taxa (corals, bryozoans, and crinoids). Widespread horizons of skeletal buildup (coral-algal bioherms, thrombolites) typify maximum flooding zones, because the combination of deepening and clear water favors rapid rates of upward growth. Conversely, increased sedimentation/turbidity favor low diversity associations of sediment-tolerant, eurytopic taxa (quasi-infaunal brachiopods, bivalves) during highstand to falling stage systems tracts. These patterns imply not only shore-perpendicular, but also lateral, along-strike migration of biotas. However, the concomitant expansion/contraction of habitable area does not appear seriously to affect species or their associations. Obviously stenotopic organisms did not entirely disappear during regressions but merely became confined to a smaller proportion of the basin distal to sediment sources. Likewise there must be higher sedimentation refugia during transgressions. A key implication is that stable high overall biotic diversities may be favored in foreland basin settings with siliciclastic- and carbonate-dominated sides.