A sedimentologic and paleontologic study of the Sonyea Group in New York has led to a reconstruction of the environmental history of the marine shelf of the Catskill Delta during part of Upper Devonian time. By comparing the Sonyea sedimentary patterns with those reported from present-day deltas, the following environments have been recognized: open shelf, prodelta, delta platform, delta front sands, distributary mouth bars, channels, estuaries, and marshes. Each of these sedimentary environments is characterized by distinctive patterns of fossil abundance and diversity. Fossils are relatively scarce in environments located in the nearshore and outermost parts of the shelf. Between these zones, principally in the prodelta, delta front sand, and delta platform environments, an abundant shelly benthonic fauna dominated by brachiopods and bivalve mollusks flourished. Four distinctive fossil associations are present in these environments and are believed to represent the preserved remains of four benthonic communities adapted to differing dynamic conditions within the prograding deltaic sediments.

The geographic and stratigraphic distribution of the environments indicate that there were two intervals of relatively rapid delta progradation during Sonyea time. Preceding and following each of these were much longer transgressive intervals characterized by low sediment influx and slow accumulation of lutites over the entire spectrum of constructional environments formed during the preceding progradation.

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