Results of Studies of Ordovician Rocks East of Allegheny Front in Pennsylvania: ABSTRACT

Abstract

The Ordovician rocks adjacent to the Allegheny Front consist of 6,500 feet of limestones, dolomites, and shales. The rocks become increasingly clastic upward, reflecting the growing movement that culminated in late Ordovician orogeny on the southeast. The Lower Ordovician consists of 2,775 feet of dolomite with two relatively thin limestone members. The homogenous lithology does not permit as detailed zoning as middle Ordovician limestones. However, a few lithic and faunal zones persist over fairly wide areas and are useful as mapping units. Some units may be helpful in subsurface geology in the adjacent Allegheny Plateau area. Middle Ordovician rocks, 1,375 feet thick, are divided into 15 major units which may be subdivided. Upper Ordovician clastics, 2,350 feet thick, c nsist largely of thick shales, claystones, siltstones, and sandstones which are not readily subdivided into characteristic zones. Favorable reservoir rock in the Ordovician is limited. However, coarse calcarenites of the Trenton and Black River may offer some promise. The foetid odor of hydrocarbons is noticeable throughout much of the Ordovician, especially in some of the clastic limestones. Coarse-grained limestones and dolomites are generally darker than the denser carbonates, the darkness of color roughly proportional to increasing grain size. The relationship of the hydrocarbon content to grain size in these rocks deserves further study. Beekmantown and Upper Cambrian dolomites show small carbonate-lined cavities having foetid odor, a few showing oil stains. The Upper Cambrian Gatesburg contains numerous coarse sandstone beds which probably offer better potential reservoir rock than Ordovician units. Several conclusions regarding the nature of the Ordovician rocks beneath the Allegheny Plateau may be based on outcrop data and paleogeographic probabilities. Gatesburg sandstones likely thicken and coarsen westward beneath the plateau. The Beekmantown dolomite probably thickens for a short distance beneath the plateau, with the limestone phases becoming more dolomitic. The Chazy probably thins and disappears in the central plateau area. Lower Black River units thin while upper Black River units thicken beneath the plateau for a short distance. Most Middle Ordovician units become more magnesian westward beneath the plateau. Upper Ordovician units become less clastic and thin westward. End_of_Article - Last_Page 2161------------