MIDDLE AND LATE DEVONIAN RHYNCHONELLID DOMINATED ASSEMBLAGES AS A SENSITIVE TOOL TO DELINEATE THE DYSAEROBIC BIOFACIES

As oxygen levels decrease in modern marine settings, there is a predictable corresponding decrease in benthic marine diversity and abundance as well as depth of burrow penetration and burrow width. However, low diversity fossil assemblages associated with sedimentological features characteristic of low oxygen conditions, such as laminated fine-grained sediment with limited bioturbation, are typically lumped into a single broadly defined dysaerobic biofacies. With the exception of the dysaerobic trace fossil model of Savrda and Bottjer (1986), variations within this biofacies have rarely been systematically recorded from the stratigraphic record. Using a microstratigraphic approach utilizing macrofaunal associations, ichnofabric indices, and pyrite framboid diameters we have examined Middle and Late Devonian black to gray shale units of upstate New York to attempt to further delineate the dysaerobic zone.

Black to gray shales were examined through seven stratigraphic intervals from 12 localities across the paleobasin that represent a gradient from aerobic to anaerobic depositional settings. These strata represent deposition is a broad, dynamic eperic sea-way. Leiorhynchid brachiopods are common and occur in monospecific assemblages within laminated black shale ranging to more diverse assemblages in dark gray shales, suggesting that there is a tractable gradient from lower dysaerobic to fully oxygenated conditions. At each locality, samples were collected continuously through the section for analysis on a bed by bed (cm to mm) scale. Along bedding planes, diversity, relative abundance, and taphonomy of the shells was recorded.

The composition and relative abundance of fossils on bedding planes record variations through intervals of constant ichnofabric indices and unvarying signal in pyrite framboid diameters, suggesting that the macrofaunal signal is more sensitive than lithologic proxies for determining relative oxygen levels.