DEPOSITIONAL CYCLES WITHIN THE UPPER DEVONIAN CATSKILL FORMATION OF NORTHEASTERN PENNSYLVANIA

Bedrock mapping projects during the 2015-2017 field seasons were designed to provide a regional characterization of the enigmatic Upper Devonian Catskill Formation in northeastern Pennsylvania. Although this did result in re-affirming the transition from the underlying marine Lock Haven Formation to the repetitive, fining-upward fluvial cycles commonly associated with the Catskill, there was no observable datum to subdivide the terrestrial sequences into areally extensive, mappable members.

Based on outcrops and cored boreholes, the Catskill Formation in Wayne and Susquehanna Counties is approximately 520 m (1700 ft) thick and displays a pattern of stacked depositional cycles ranging from 60 to 75 m (200 to 250 ft) in thickness. These cycles tend to be incomplete due to lateral facies changes or new cycles cutting out and replacing older ones.

The ideal cycle records an erosional base, marked by the presence of a relatively hard, calcareous, intraformational conglomerate or “agglomerate.” It is apparent that these poorly sorted mixtures of fine to coarse quartzitic sand grains, lithic clasts and pebbles, calcareous nodules, and plant detritus, have commonalities with adjoining lithotypes, particularly paleosols, and support an interpretation as storm-water runoff or flood deposits. The agglomeratic beds grade upward into low angle (<15 degrees on average), non-calcareous, medium- to coarse-grained, 1-2 m thick, trough cross-bedded channel sandstones. These sandstones in turn grade into progressively finer-grained, laminated sheet-deposit sandstones punctuated by regularly spaced (3 to 8 cm) bedding partings. Fragmented, and often carbonized, fossil-plant debris is commonly observed within these laminar pulses.

Silt and mud-sized sediments represent the waning deposits of the cycle, filling topographic lows and covering inter-channel floodplains. These silty shale and claystone units show an observable increase in preserved fossil plant roots and root systems and mark the transition to “redbed” lithologies.

Paleosols mark the top of the cycle representing soils developed on stream embankments, inter-channel lowlands, or fluvial plains that have undergone stream abandonment through the migration of distributary channels.