The Catskill clastic wedge is composed of four units in central Pennsylvania: turbidites, slope deposits, alternating shallow marine and nonmarine sediments, and alluvial plain fining-upward sequences.
In the east (Lehigh Valley area), the vertical upward succession is distal turbidites grading up into proximal turbidites, very thin (3 m) shales representing the slope, and a rapid passage into thick alluvial plain fining-upward sequences with a high proportion of sand. By contrast, in the west (Susquehanna Valley area), the upward succession is distal turbidites, grading up into thick slope shales (about 60 m) without development of proximal turbidites, 600 m of alternating marine and nonmarine beds, and alluvial plain fining-upward sequences.
The marine-nonmarine alternations are characterized by a short facies sequence, termed a "motif," which is repeated 20 to 25 times. It begins with abrupt marine transgression, non-deposition, winnowing and bioturbation, and is followed by a marine shoaling phase which is dominantly shaly. Sandstones are conspicuously absent near the shoreline, and the upper part of the motif consists of nonmarine red siltstones and mudstones with root traces and desiccation cracks. Each motif developed by progradation of a quiet, muddy shoreline.
The establishment of the Catskill clastic wedge in the east took place by the advance of a sandy shoreline on the top of proximal turbidites which were probably deposited, at least in part, on a slope. The shoreline built forward rapidly, and resulted in the permanent establishment of an alluvial plain. As the alluvial plain widened during westward progradation, more and more sand was trapped on the alluvial plain, the shoreline became muddier and prograded more slowly. Switching of position of the major rivers led to progradation in some places, but winnowing and transgression in others. The reduced amount of sand reaching the coast resulted in fewer and smaller turbidity currents being generated, resulting in a lack of proximal turbidite facies in the west. The Catskill shoreline in central Pennsylvania is therefore more comparable to the quiet prograding muddy coast of western Louisiana, rather than a typical sandy delta as previous work has implied.