REPTILE TRACKWAYS PROVIDE A TRIASSIC DATE FOR ENIGMATIC ROCKS AT VALLEY QUARRIES FAIRFIELD OPERATION, PENNSYLVANIA
WEEMS, Robert E.1, VAN SCYOC, Randy2, GANIS, G. Robert3 and BENDER, Brad2, (1)U.S. Geological Survey, Mail Stop 926A, Reston, VA 20192, (2)Valley Quarries, 297 Quarry Road, Chambersburg, PA 17202, (3)Consulting Geologist, Southern Pines, NC 28387, RVanScyoc@valleyquarries.com
The age and origin of structurally isolated, metamorphosed carbonate rocks quarried by Valley Quarries near Fairfield, Pennsylvania has long been in doubt. The sequence consists of alternating stacks of massive, clast-supported carbonate conglomerate (lacking Blue Ridge volcanic or cover rocks) and thin, well-bedded, finer-grained carbonate laminites. Typical Triassic border fanglomerates in the Gettysburg Basin contain both Blue Ridge Proterozoic siliciclastic clasts and Paleozoic carbonate clasts derived from the west, so the rocks in the Fairfield quarry have an atypical depositional origin. A playa environment, sensu lato, is indicated by mudcracked bedding planes and footprints found on shaley bedding surfaces. The conglomerate units originated as episodic alluvial fan debris covering shallow lacustrine lakes and fringe areas. The lack of westerly derived Blue Ridge clasts and a similarity of the carbonate clasts to lower- middle Paleozoic units (Vintage, Kinzers, Ledger, and Conestoga Formations) in the York valley to the east suggest an easterly source. Frederick valley carbonates to the southeast (Frederick and Grove formations) also are a possible source area. The rocks quarried near Fairfield are intruded by basal Jurassic diabase dikes and also located near one or more large sills, both of which are part of the Central Atlantic Magmatic Province (CAMP). These intrusive rocks have thermally metamorphosed their surrounding rocks pervasively. This local thermal overprint on the conglomerate clasts masks regional metamorphic effects seen in their parent lithologies.
Until recently, no fossils were known from these rocks. Several of the carbonate laminite horizons are now known to have reptile footprints that include two kinds of small dinosaur (including Grallator tuberosus), two kinds of aetosaur (including Brachychirotherium parvum), the small lizard-like Rhynchosauroides, and parasuchian (phytosaur) swimming scrapes. This assemblage provides definitive evidence that this rock sequence is no older than late Ladinian (latest Middle Triassic) and no younger than Rhaetian (latest Late Triassic). Based on the presence of cyclic lake beds, the rocks quarried at Fairfield most likely are an isolated part of the Upper Triassic (uppermost Carnian to Norian) Gettysburg Formation.