BIOFACIES ANALYSIS ALONG A MIDDLE DEVONIAN PALEOGRADIENT: WATER DEPTH AS A PRIMARY CONTROL ON BIOFACIES FORMATION IN THE STAGHORN POINT CORAL BIOSTROME

Recent investigations of biofacies within the Middle Devonian of eastern North America have focused primarily on the recurrence of biofacies through time. The issue of biofacies response to sea level fluctuations has been analyzed to a large extent in vertical stratigraphic successions; the matter is complicated by the interplay of controls such as sediment input and water depth on the formation of biofacies. In order to disentangle the effect of these two dominant factors, it is necessary to examine biofacies changes along a single time-parallel gradient, where one of these factors can be assumed to be nearly constant. One such possibility would be a gradient that runs parallel to the direction of incoming sediment supply and perpendicular to depositional shoreline strike, where water depth is known to change. Such a gradient would factor out the control of sediment supply upon the distribution of biofacies along the gradient.

Just such a case exists in the Middle Devonian of the Appalachian Basin in the coral beds of the Otisco Member of the Ludlowville Formation of central New York State. Exposures of the Staghorn Point submember along the shores and tributaries of Skaneateles Lake run roughly perpendicular to the main gradient of sediment supply in the basin and display a deepening trend to the northwest. The Staghorn Point coral biostrome is a dense thicket of mainly solitary rugose corals that sits atop a siltstone platform at most localities and is interpreted to represent a period of sediment starvation at the beginning of a small-scale sea level oscillation cycle within the larger 3rd-order Ludlowville Sequence. This coral biostrome comes to an abrupt edge at a buried submarine escarpment, but the horizon, marked by phosphatic pebbles can be traced for nearly 10 km down ramp into distal facies.

Biofacies found along the gradient preserved within the Staghorn Point submember range from shallow water associations dominated by abundant rugose and rarer tabulate corals through into deeper water associations dominated by athyrid and leiorhynchid brachiopods. The biofacies spectrum preserved within the single time-plane of the Staghorn Point submember is analogous to the suite of biofacies associated with a sea level oscillation cycle in areas where water depth change dominates over sediment input as a biofacies control.