HIGH RESOLUTION STRATIGRAPHY AND PARASEQUENCES OF THE DEVONIAN, ORISKANY SANDSTONE, MAPLETON, PENNSYLVANIA

The Lower Devonian, Oriskany Sandstone of the Appalachian Valley and Ridge (the Ridgeley Sandstone in PA), a quartz arenite, is one of the most economically important formations in the Northeast. It is one of the finest glass sands in the world and is one of the two principal gas producing formations in Pennsylvania. Despite this economic importance, little had been published about its internal stratigraphy or its fine-scale depositional history.

To begin to resolve this issue, we examined Oriskany exposures in an abandoned glass sand quarry near Mapleton, Pennsylvania. The quarry contains semi-continuous exposures over a distance of 1.25 km and facilitates detailed correlation and examination of lateral continuity of stratigraphic patterns. The upper 71.6 meters of Oriskany are accessible in the quarry, and conglomeratic, fossiliferous, cross-stratified, and bioturbated intervals can be traced or correlate along the length of the quarry with little variation or compositional shifting. Body fossils include predominantly spiriferid brachiopods and distinct platycerid gastropod snails.

The overall pattern within the Oriskany is upward-deepening with the majority of the lower strata comprising unfossiliferous, cross-stratified, upper shoreface to foreshore deposits. In the upper Oriskany, fossiliferous and bioturbated sands dominate, recording middle to lower shoreface settings. This upward-deepening pattern reaches an acme above the Oriskany: black shale of the Esopus Formation rests on the Oriskany. Cobbles of Oriskany quartz arenite delineate the base of the Esopus.

Within this upward-deepening pattern, there are 8 or 9 upward-shallowing parasequences, averaging about 8 m in thickness. The general pattern of the parasequences is fairly consistent. The bases of parasequences are sharp and contain quartz pebbles and/or abundant broken shells. These are transgressive lags associated with the marine flooding surfaces (mfs). Strata directly above mfs typically contain well preserved fossils, many of which are wave- and current-oriented. These fossiliferous horizons grade upward into bioturbated sandstone, typically lacking body fossils. The bioturbated sandstone, in turn, grades upward into unfossiliferous, cross-stratified sandstone. Each parasequence is truncated by the overlying mfs.