THE MIDDLE TO UPPER DEVONIAN GENESEO SHALE TO PENN YAN SHALE SUCCESSION OF NEW YORK: INTERPLAY BETWEEN EUSTATIC SEA LEVEL RISE AND TECTONICALLY DRIVEN SEDIMENT SUPPLY

The Middle to Upper Devonian Geneseo Shale and Penn Yan Shale of New York were deposited as part of the westward prograding Catskill Delta complex. Acadian uplift supplied the sediment for delta growth, and coincides with a general rise of eustatic sea level.

Depositional parameters were derived from core description, hand specimen and thin section examination. The Geneseo Shale is a 34 m succession of black and dark gray shales. It overlies the Tully Limestone (surface of maximum starvation), and contains a benthic fauna suggestive of dysaerobic conditions. The lower Geneseo is a black shale with relict lamination and indications of surficial sediment mixing by benthos. Upsection, the Geneseo exhibits multiple black to gray shale cycles (1 to 25 cm thick). Both black and gray shale intervals show bioturbation. Cycles decrease in thickness upsection and grade into dark gray shales with scours, wave and current ripples, and macroscopically visible bioturbation. This interval suggests decreasing sediment supply, dysoxic setting, and continued transgression and sea level rise.

Above the Geneseo follows the Lodi Limestone, a thin concretionary carbonate/calcareous siltstone unit that marks an interval of strongly reduced clastic input. The Lodi Limestone displays three internal cycles (0.75 to 1.5 m thick) consisting of a sharp basal contact with the previous cycle, followed by dark gray shales that grade into bioturbated calcareous muddy siltstone. This interval marks probably the farthest eastward shoreline retreat.

The Upper Devonian Penn Yan Shale is a 30 m succession of dark gray variably bioturbated silty shale. The lower portion of the Penn Yan is a dark gray silty shale with wave and current ripples. Silt content and current produced features increase upsection, suggestive of a general shallowing trend.

In the context of global eustasy in the Devonian, it appears the Geneseo Shale represents a budding regression (growing sediment input from the east) that was overpowered by sea level rise (accommodation outstrips sedimentation). The time of Lodi deposition marks the maximum eastward “push-back” of the shoreline. With the onset of Penn Yan the balance between sediment input and newly generated accommodation is reversed. In spite of continued sea level rise water depth decreased and the shoreline migrated westward.