HYBRID EUSTATIC AND FLEXURAL EFFECTS WITHIN A SYNOROGENIC CLASTIC WEDGE: ESTABLISHING TEMPORAL LINKS BETWEEN THE LATE MIDDLE DEVONIAN TULLY FORMATION AND THE NEARSHORE GILBOA SANDSTONE IN EAST-CENTRAL NEW YORK

In central and western New York, the late Middle Devonian is represented, in part, by anomalous shelf carbonate deposits of the late Givetian Tully Formation. This limestone succession, recording key aspects of the global Taghanic Bioevent, contrasts markedly with bounding detrital facies of the syntectonic Catskill Delta Complex. The upper part of the Tully (Bellona, Moravia, Fillmore Glen beds) commences a major transgression (Taghanic Onlap Event) which corresponds partly to the major eustatic highstand IIa of Johnson, Klapper, and Sandberg, and partly to a flexural basin thrust-loading event (Tectophase II) of Ettensohn. In central and western New York, this hybrid transgression is marked by the post-Tully Geneseo basinal black shale; in east-central New York, it is represented by clastic neritic and nearshore facies (respectively Unadilla and Gilboa formations), but correlational details are as yet poorly understood.

The present authors, establishing numerous bed-scale correlational links from the Tully Limestone into its eastern clastic equivalent deposits in east-central New York, show that a detailed sequence stratigraphic framework can be extended eastward into the vastly thicker detrital facies belt. Recent mapping indicates that a thin lag layer of quartz pebble conglomerate as well as shell-rich, chamositic beds, respectively marking probable lowstand and flooding surface horizons, are present within the western offshore part of the Gilboa Sandstone; the quartz pebble bed, yielding corals and large brachiopods, rests upon a major erosion surface (sequence boundary?) that may correlate to the lowstand horizon at the base of the Bellona Bed in western New York which directly underlies strata marking the beginning of the Taghanic transgression event. If this correlation is valid, the quartz pebble bed could serve as a temporal datum for assessing the timing and character of latest Tully- and Geneseo-age depositional events associated with the earliest syntectonic, basin-fill phase of Tectophase II in eastern New York.