SAND INJECTITE COMPLEX WITHIN THE UPPER CASSELMAN FORMATION AT MT. NEBO POINTE, PITTSBURGH, PA
The wall is 320 m long and up to 20 m high. Exposure includes flat-lying strata of Casselman Formation (Conemaugh Group). We studied the undisrupted stratigraphic interval of sandstone, siltstone, claystone and coal at a nearby roadcut on I-279. We then applied our expectation of the stratigraphic interval to the Mt. Nebo Pointe highwall. At the base of the wall is the uppermost Birmingham Sandstone overlain by laterally displaced and sheared Wellersburg Coal and claystone.
Sand dikes ranging from a few cm to 1.0 m wide intrude these lower strata. Smaller dikes tend to cut horizontally while the largest dikes intrude vertically. About 20 clear vertical dikes can be identified, with approximately 80% being concentrated in the western half of the wall. Dikes are strongly bedded parallel to orientation, melding into bedding of cross-cutting dikes at intersections. Dike shapes resemble pillars rather than V’s. Clasts are common and oriented either concurrent with the host rock or parallel to injectite direction and displaced upwards. These indicate vertical sediment flow coupled with rapid brecciation.
The upper highwall is typical Morgantown Sandstone (cross-bedded, feldspathic and comparatively well-sorted) interstratified with slurry-flow sandstone (massive, lithic and poorly sorted). This slurried lithic sand was intruded upward from the Birmingham Sandstone interval beneath the parking lot level. The overall depth of the injectites is unknown, however, a Birmingham source would imply a minimum of 6 m.
This exposure presents a unique opportunity to study sand injectites directly. It is our hope that this exceptional injectite complex will provide valuable insights into injectite formation and associated processes.