IMPLICATIONS OF NEW CORRELATION OF MIDDLE DEVONIAN STRATA IN EASTERN NORTH AMERICA: THE ROLES OF TECTONICS VS. EUSTASY AND A REFINED SEA LEVEL CURVE FOR THE GIVETIAN STAGE

The well-exposed Givetian age (upper Middle Devonian) rocks of the Appalachian foreland basin (Hamilton Group, Tully Formation, and the Genesee Group of New York State) preserve one of the most detailed records of high-order sea level oscillation cycles for this time period in the world. Detailed examination of coeval units in the distal portions of the Appalachian Basin, as well as portions of the Michigan and Illinois basins, has revealed that the pattern of high-order sea level oscillations documented in the New York section is not restricted to the New York area, but can be positively identified at all scales in most areas where coeval units are preserved.

The correlation of small-scale depositional sequences across eastern North America has been conducted within a framework based on conodont, brachiopod, and ammonoid biostratigraphical data in hopes of elucidating the nature of biotic change across a wide area during the Givetian interval. In the course of this study, it has become apparent that various tectonic features (local arches and basins) did not impose great control on deposition until the latest portion of the Givetian, a time closely associated with increased tectonism (tectophase II of Ettensohn) during the Acadian Orogeny. In such instances where tectonic features did exert control on deposition, it was mainly manifested in the nature of local unconformities and sediment condensation.

The persistence of the pattern of high-order sea level cycles across such a wide geographic area suggests that these cycles are allocyclic in nature with primary control on deposition being eustatic sea level oscillation, as opposed to more autocylic controls such as sediment supply, which would be more local in their manifestation. These new correlations lend strong support to a revised high-resolution sea level oscillation curve for the Givetian Epoch for the eastern portion of North America. In contrast with the widely cited sea level curve of Johnson, Klapper, & Sandberg, which recognizes only one major transgressions in the early-mid Givetian (If) prior to the major late Givetian Taghanic onlap (IIa, Geneseo Shale), we recognize at least six 3rd order sea-level fluctuations corresponding respectively to the Oatka Creek, Skaneateles, Ludlowville, Moscow and lower and upper Tully formations of New York State.