PALEOECOLOGICAL ANALYSIS OF SHELL BEDS FROM THE TRIMMERS ROCK FORMATION (UPPER DEVONIAN- FRASNIAN) ALONG PA STATE ROUTE 322, CENTRAL PENNSYLVANIA

Shell beds from the Trimmers Rock Formation (Upper Devonian) in Central Pennsylvania (Watts Twp.) were deposited by two controlling factors: density driven currents in deep water, prodelta deposits and storm activity in shallow, delta front deposits. Prodelta muds are greenish-gray and composed of fine sandstone and fossiliferous shaly-siltstone couplets that exhibit graded bedding. Delta front deposits are reddish-gray and composed of multistoried, medium-grained sandstones with planar lamination and low-angle cross stratification. Some of these deposits appear influenced by tidal activity. Soft sediment deformation including both small and large ball and pillow structures and load casts occur in all depositional environments. Other tidal, nearshore deposits contain abundant bioturbation and rare shells.

Shell hash samples were collected from 12 beds throughout the changing prodelta and delta front environments. Some samples were collected from the same bed to determine lateral continuity within the shell hash. Shell beds from the prodelta environment exhibit normal graded bedding, the shells are sparsely distributed with abundant, fine matrix, and are generally composed of crinoid columnal segments, large, branching bryozoans, and brachiopods with some tentaculites and carbonized plant fragments. Shell beds from the delta front are laterally discontinuous, shells appear randomly distributed, and they are densely packed with skeletal grains consisting of crinoid pieces, brachiopods, small bryozoans, tentaculites, fish bone, and carbonized plant fragments.

This paleoeological analysis integrates the types of organisms, their sizes and distribution, and taphonomy to characterize the energy and community dynamics in the Trimmers Rock Formation. Prodelta communities are composed of sessile, benthic suspension feeders with some bioclasts transported from shallower water environments by density currents. Shallow water environments contained a similar community, but may have had a climax community consisting of bryozoans. Coastal fishes may have also occupied this environment. Closer examination of shells beds from different environments provide evidence for two different mechanisms of shell deposition in the shell beds of the Trimmers Rock Formation.