Paleoenvironments of the Upper Pennsylvanian Ames Limestone and associated rocks near Pittsburgh, Pennsylvania

Paleoenvironmental analysis of the Upper Pennsylvanian Ames Limestone and associated rocks east of Pittsburgh, Pennsylvania, reveals a range of depositional environments from shallow marine to deltaic. The depositional history began with paludal sedimentation followed by eastward transgression of a clastic lagoon dominated by abundant molluscs and “mud-lloater” brachiopods, especially thin-shelled Neochonetes. This period was followed by the eastward transgression of a tidal belt and a shoreward, low-energy, carbonate lagoon. Tidal belt sediments were derived from (1) shoreward transportation of offshore shelf sediments (carbonate mud and fine sand, fine crinoid debris, and trilobites) by tidal currents and (2) erosion and transportation of underlying clastic-lagoon sediments (molluscs, Neochonetes, and clay minerals) in tidal channels. The carbonate lagoon contained abundant Neochonetes in carbonate mud.

Abandonment of the tidal belt allowed the eastward transgression of open-circulation, offshore-shelf carbonate sediments. The far offshore sediments were dominated by crinoids, whereas near offshore shelf sediments had a sparse fauna of Crurithyris and crinoids. In the final stage of deposition, deltaic progradation from the east established the following offshore to onshore sequence of environments: offshore carbonate shelf, near-shore clay-miner shelf, prodelta, and delta front. Nearshore, clay-mineral shelf deposits were dominated by Crurithyris, a robust, thick-shelled brachiopod indicative of higher energy conditions than were existent for the thin-shelled Neochonetes.