PALEOECOLOGY OF THE ANTES SHALE, PA

The Middle to Upper Ordovician, Antes Shale is a Utica-Shale like facies that crops out in the Valley and Ridge of Pennsylvania, overlying the Middle Ordovician carbonate platform deposits of the Trenton Limestone. The Antes Shale is a diachronous unit; it is oldest towards the southeast and youngest towards the northwest.

The dark color, low diversity of benthic fauna, and lack of bioturbation indicate that the Antes records an oxygen-stressed seafloor. The dominant fossils found throughout the Antes, mainly graptolites and trilobites, provide clues to the depositional environment of specific sratigraphic intervals and localities. The trilobite Triarthrus is present in relatively large numbers at all locations with the exception of the southernmost exposure, at which the oldest shale (C. bicornis zone possibly to C. americanus zone) is present (McConnellsburg, PA). Articulated, complete specimens and molts of Triarthrus indicate lack of transport and relatively rapid burial. In this low oxygen setting, Triarthrus was the only shelled, benthic organism that thrived. In fact, in one bentonite-rich, stratigraphically condensed interval, Triarthrus is present even though graptolites are not preserved, presumably lost to decomposition. This seems to indicate that the dysoxic setting had few inhabitants in the Middle Ordovician. The lack of Triarthrus in the oldest shale may indicate that these strata predate the first appearance of the trilobite in this part of the Appalachian basin. The westernmost and stratigraphically youngest exposure (Martinsburg, PA) is divided into two distinct horizons. The first is a thin (less than 1 meter) shelly interval just above the Trenton Limestone containing articulate brachiopods (Oniella? multisecta and Rafinesquina alternata in particular) and the trilobite, Cryptolithus. This fauna seems to record a dysaerobic to aerobic muddy substrate community. The shelly interval is interesting in that it records a facies that is intermediate between the Trenton Limestone and the typical Antes Shale and is not seen in any other part of the basin. The second, younger horizon lacks diverse benthic fauna but includes abundant C. spiniferus zone graptolites and Triarthrus, indicating a deeper, more oxygen-stressed setting.