Gradient analysis of faunal distributions associated with rapid transgression and low accommodation space in a Late Pennsylvanian marine embayment: Biofacies of the Ames Member (Glenshaw Formation, Conemaugh Group) in the northern Appalachian Basin, USA

Abstract

The Ames Sea, situated in a shallow marine embayment in the Appalachian Basin, was the result of the most extensive marine transgression of the Pennsylvanian in this region. Gradient analysis of the proportional abundance of taxa reveals an environmental continuum across the Ames embayment along which four biofacies were distributed. Biofacies distribution is interpreted to be controlled by salinity, turbidity, and oxygen gradients associated with the geometry of the Ames depositional basin and relative sea-level change. The eastern portion of the Ames Sea resembled a foredeep trough located on the detrital slope of the recently uplifted Appalachian Highlands. Biofacies 1 and 2 occur exclusively in the eastern part of the Ames depositional basin and are dominated by eurytopic molluscs tolerant of terrigenous influx. Biofacies 3 occurs in both the eastern and western portions of the Ames depositional basin. This biofacies is dominated by the opportunistic brachiopod Neochonetes, which would quickly colonize the seafloor in the wake of transgression.d Biofacies 4 occurs exclusively in the western part of the Ames depositional basin and is composed primarily of stenotopic crinoids, bryozoans, brachiopods, and epifaunal bivalves that inhabited a clear water, cratonic ramp on the eastern flank of the Cincinnati Arch. Vertical variations of faunal assemblages in the Ames Member were the result of variations in relative sea level associated with glacio–eustatic cycles. Biofacies 3, dominated by Neochonetes, was deposited at the base of the Ames Member during the initial rapid transgression in a mostly dysoxic environment with low turbidity and marine salinity. During the first regression, low-density surface water from eastern sources restricted vertical circulation, established a stratified water column, and promoted estuarine conditions in the Ames Embayment. Well-oxygenated surface waters were unable to circulate to the benthic habitat, which led to periodically dysoxic conditions and the establishment of Biofacies 1 and 2, composed of diminutive eurytopic molluscs. A second, stratigraphically-higher occurrence of Biofacies 3 is interpreted to represent a second transgression. However, unlike the first transgression, lithologic and faunal data suggest that the benthic habitat was well-oxygenated. Above the mid-Ames marine zone occurrence of Biofacies 3, Biofacies 1 and 2 were deposited in the final regressive phase as local terrigenous source areas brought increasing turbidity and fluctuating salinity to the Ames Embayment. Previous paleoecological research on the Ames Member cited freshwater and/or clastic influx as the major controls on biotic distribution, but the results of the present study suggest that oxygen availability was also a major control. Stratification of the water column during the Ames sea-level cycle promoted estuarine circulation that prevented well-oxygenated surface waters from reaching the benthic habitat. This led to the deposition of dark gray to black fissile shales that contain fossil assemblages dominated by diminutive molluscs and brachiopods that were tolerant of low-oxygen conditions.