DEGREE OF ENVIRONMENTAL STABILITY AS A PRIMARY CONTROL ON BIOTIC DISTRIBUTION: GRADIENT ANALYSIS OF FOUR MARINE ZONES WITHIN THE GLENSHAW FORMATION (CONEMAUGH GROUP, UPPER PENNSYLVANIAN) IN THE APPALACHIAN BASIN

The Upper Pennsylvanian Glenshaw Formation contains a series of marine zones that were deposited on the detrital slope of the Appalachian highlands during the last major transgressions from the Midcontinent Sea in the Paleozoic of eastern North America. These marine zones contain distinctive fossil assemblages characterized as biofacies that inhabited a variety of shallow-marine, carbonate- and clastic-dominated environments. Biofacies that contain a high abundance of opportunistic, paleoecologic generalists are characterized as eurytopic. These biofacies are dominated by infaunal and epifaunal deposit- and suspension-feeding molluscs tolerant of periodic environmental fluctuations in salinity and turbidity associated with the activity of terrigenous source areas that contribute to environmental instability. Biofacies that contain a high abundance of paleoecological specialists are characterized as stenotopic. These biofacies contain a high percentage of epifaunal suspension-feeding echinoderms, articulate brachiopods, bryozoans, and corals that preferred stable, normal marine conditions.

The degree of environmental stability fluctuated during the establishment of marine faunas in the Glenshaw Formation due to the rate of relative sea level change, the relative extent of the four major marine incursions, geographic position relative to the source of marine influence from the Midcontinent Sea, and variations in total accommodation space within the Appalachian Basin. Rapid transgression, a more extensive marine incursion, and close proximity to the source of marine influence increased environmental stability by buffering the benthic habitat from environmental fluctuations commonly associated with terrigenous sources. Whereas slow transgression, a less extensive marine incursion, and decreasing proximity to the marine source decreased environmental stability by increasing the influence of terrigenous sources on the benthic habitat. Greater accommodation space led to the deposition and/or preservation of a more complete sequence of biofacies. Whereas low total accommodation space hindered the deposition and/or preservation of eurytopic biofacies at the top of each marine zone.