USING FACIES ANALYSIS TO DESCRIBE A MIGRATING DELTA LOBE OBSERVED IN THE TRIMMERS ROCK FORMATION (UPPER DEVONIAN): CENTRAL PA, STATE RT. 322 (WATTS TWP)

The Trimmers Rock Formation (Upper Devonian) in Central Pennsylvania (Watts Twp.) represents a shallow shelf environment of varying deltaic and tidal influence. In total, 55 m of rock were measured and described, and three dominant facies characterize this succession. Each observed facies can be assigned a different depositional area of a delta: The three facies represent prodelta muds, delta front mouth bar sands, and tidally influenced near shore deposits. Prodelta facies are represented by m-scale greenish-gray shales, shaly-siltstone, and fine sandstone deposited in Bouma sequences. Delta front mouth bar deposits are represented by reddish-gray sandstone bodies (50-75 cm thick) that exhibit planar and cross lamination. These sandstones are often deposited on top of finer silts and muds and contain both ball and pillow and load cast structures. The tidally-dominated, nearshore facies can be characterized by reddish-gray or greenish gray, thinly bedded, and laminated (2-3 cm thick) deposits of bioturbated siltstone and shale. This facies also contains tidal features such as flaser and lenticular bedding, and some of the delta front sand bodies are laterally discontinuous and interbed with the finer near shore sediments.

The subdivision, identification, and spatial relationship of these three facies in the succession allow for a more detailed description of the system. Each facies shows a varying degree of sediment supply, fluvial influence, and sea level change. Within all three facies, shell beds of varying thickness and lateral continuity can be observed, and given which facies they are within, the mechanism for their deposition can be more accurately determined. Applying facies analysis also helps model the different parts of a migrating deltaic lobe on a broad scale. During times of abundant sedimentation, large sand bodies are deposited which cause the observed soft sediment deformation and allow the delta front to migrate. As the areas with the highest sediment input change, the thinly-bedded, tidally influenced facies dominates the section. The abrupt pulses of sand observed in low-energy muddy environments could indicate particularly wet or stormy intervals during the Devonian that subsequently caused progradation of the Catskill Delta.