THE UPPER TUSCARORA FORMATION - LOWER ROSE HILL FORMATION CONTACT IN NEW EXPOSURES NEAR ALTOONA, BLAIR COUNTY, PA

Recent excavation for a shopping mall near the base of Brush Mountain, a Tuscarora-capped ridge of the Valley and Ridge province in central Pennsylvania, has revealed approximately 20 m of beds representing the transition from the Silurian Tuscarora Formation into the Silurian Rose Hill Formation. The study area is underlain by bedrock that composes the NW limb of the SW-plunging Sinking Valley anticline. The units are homoclinal with a general attitude of 220° / 20°W.

The Tuscarora Formation is typically a distinctive white orthoquartzite. We define the upper Tuscarora (progressing up section), to include fine-grained massive white sandstone, thin shale beds (up to 0.5 m thick), and a green highly-bioturbated sandstone (Castanea?). The overlying Castanea Member when present is a red sandstone but may be green. In addition to typical Rose Hill Formation lithologies (light olive-gray to tan shale, with some siltstone) in the study area, a complex clastic sequence exists, ranging from claystones and shales to siltstones and fine- to medium-grained sandstones with rare conglomeratic facies.

Wedge faults and back-thrusts exists, but do not disrupt the depositional sequence substantially. Late-stage, NW-trending (270° - 295°) high-angle normal faults represent the latest (Post Alleghenian) deformation. Displacement on normal faults is minor (up to 1 m) and many are mineralized (quartz, marcasite, pyrite, sphalerite, galena, carbonate, and barite. These may be linked geometrically and temporally to sulfide-bearing vein-faults (312°) that were mined for galena approximately 25 km to the NE near Fort Roberdeau. SLAR mapping of the Altoona region reveals a strong NW-trending (305°) set of lineaments with a rather regular spacing of ~2 km and may be related to the late-stage faults and sulfide mineralization.