NEW LITHOFACIES IDENTIFIED WITHIN SEVERAL DEVONIAN FORMATIONS IN THE CENTRAL APPALACHIANS OF PENNSYLVANIA

Geology and drainage of the Everett, Bedford, and Hollidaysburg 15-minute quadrangles were mapped by interpreting side-looking airborne radar (SLAR) imagery provided by the U.S. Geological Survey by using a technique that applies geomorphic and tectonic considerations. Mapping was conducted both stereoscopically at 1:250,000 scale and monoscopically at 1:62,500 scale. Radargeological mapping utilized a technique involving geomorphic and tectonic interpretation of textural and tonal variations exhibited in the imagery. It was discovered that several Devonian units (marine beds) revealed a series of alternating topographic lows and highs, that field checking confirmed represent fine-grained and coarse-grained facies (coarsening upward), respectively. Many of these radargeological subunits (or lithofacies) have been regionally correlated.

Radargeologic mapping has focused on Devonian-cored structural basins. In the Everett study area, previously mapped Hamilton Group (undivided) within the Dunnings Creek structural basin is subdivided into four radar units (Dm₁ through Dm₄). These have been identified through field checking as the Devonian Marcellus Shale, and 3 subunits of the Devonian Mahantango Formation (a coarse-, fine-, and coarse-grained facies). These subdivisions or depositional cycles are also recognized in the previously mapped Hamilton Group (undivided ) in the Frankstown structural basin near Hollidaysburg. The Upper Devonian Brallier and Harrell formations in the Frankstown basin are also subdivided according to their radar characteristics. Four radar units (Dm₅ through Dm₈) have been mapped and are interpreted as coarsening up sequences within the Brallier and Harrell. In the Bedford area radar units stratigraphically above these have been delineated in the core of the Pigeon Hill syncline. They apparently represent subfacies within the Upper Devonian Scherr Formation. Field checking has revealed coarsening-up sequences.

Our results show that radargeological mapping using our morpho-tectonic technique has value for geologic mapping. New lithofacies identified during this study might be regionally correlated to other areas elsewhere in the Appalachians.