Recent investigations of modern deltas permit a refined interpretation of sedimentation in the Catskill Deltaic Complex. This complex demonstrates two separate patterns related to delta growth and migration in a framework of basin-source area tectonism.

The Middle and Upper Devonian stratigraphy of New York can be broadly separated into lithologic phases that can be related to distinct environments of deltaic sedimentation which recur during the clastic deposition of this period. These include:

Table

The initial pattern of delta growth in Middle Devonian time was produced during a period of constant subsidence in which a progressive increase of Hamilton clastics eventually exceeded the rate of basin downwarping and established the growth and migration pattern of the deltaic environments across the state. During periods of negligible source contribution, regional subsidence of the sub-basin, together with local compaction, caused a landward shift in the marine environments on the delta platform. These transgressive migrations permitted rhythmic deposition of limestones and enabled the shallow seas and interdistributary bays on the proximal delta platform to encroach and rework the nearshore of alluvial delta platform deposits. Following Hamilton time, renewed compaction and strong ubsidence in the eastern part of the sub-basin permitted formation of the Tully Limestone on the distal delta platform at the beginning of the Upper Devonian.

Continued basin subsidence and renewed clastic deposition during this period established the second major deltaic pattern. This pattern contrasts with that displayed by the Hamilton delta in that it formed under nearly continuous sedimentation throughout Late Devonian time with frequent changes in the stratigraphic succession of deltas during times when subsidence predominated over deposition.

Each period of dominant subsidence in the Upper Devonian delta was marked by the formation of a black shale (delta toe environment) over a previously deposited gray shale or siltstone (prodelta slope or distal delta platform deposit) and initiated a new sub-phase of delta deposition and a new delta. The amount of subsidence controlled the thickness and relative position of the black shale on the previous delta slope or distal delta platform deposit, and was also reflected nearshore by the emplacement of marine tongues into the eastern redbed deposits on the alluvial delta platform.

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