ENVIRONMENTAL CHANGE ASSOCIATED WITH FORELAND BASIN DEVELOPMENT IN THE LATE ORDOVICIAN (SANDBIAN-KATIAN) NEALMONT, DOLLY RIDGE AND REEDSVILLE FORMATIONS AT DOLLY RIDGE, WEST VIRGINIA

The Late Ordovician was a time of environmental change in southeastern Laurentia associated with the Taconic Orogeny that was accompanied by changes in regional faunas. As the Taconic foreland basin deepened, cool, oxygen-poor water flooded onto carbonate platforms and siliciclastic input increased as sediment was shed from the uplifting highlands. Following the environmental change in shallow subtidal environments, diverse faunas become depauperate and contain distinctive elements such as Prasopora and Cryptolithus. The Dolly Ridge Formation was deposited in a deep subtidal environment in which faunal diversity was consistently low.

The underlying Nealmont Formation, consisting of lime mudstone, grainstone and rudstone with abundant rip-up clasts, was likely deposited between fair-weather and storm wave bases. Faunal diversity is high including bryozoa, ostracodes, crinoid columnals and trilobites. The Nealmont grades upward into dark, fine to sublithographic lime mudstone of the lower Dolly Ridge. The dark color and lack of bioturbation indicate deposition in a low-oxygen environment, and the fine grain size suggests that the environment was deeper than in the Nealmont. The fauna is limited to sparse Cryptolihtus and graptolites. This lithofacies grades up into mainly light to medium grey lime mudstone with intermittent thin (cm-scale) layers of fine shell material, likely deposited near storm wave base. Cryptolithus becomes more abundant and Prasopora and a low diversity brachiopod assemblage are added to the fauna. Shale content and grain size increase upward into the overlying Reedsville Formation, but faunal diversity does not rebound to levels present in the Nealmont.

Lithofacies and faunal changes from the Nealmont through the Reedsville formations at this locality correlate well with previously published oxygen isotope shifts.