Regional stratigraphic studies indicate a minimum of 23 eustatic sea-level fluctuations in the Appalachian region from New York to Alabama. A eustatic fluctuation is interpreted if the stratigraphic and sedimentologic records on two or more sides of the Appalachian basin show evidence of a similar simultaneous shift in relative sea level within the limits of temporal resolution by fossils, intertonguing facies, or bentonite chronology. Simultaneous sea-level shifts affecting different lobes of ancient delta complexes built into the Appalachian basin from eastern sources also are considered eustatic. For analysis of a hypothetical single basin, a eustatic sea-level change is one which affects the entire basin. The cause may be large-scale tectonics of the continental area ontaining the basin, or a true sea-level shift related to glaciation or rate of sea-floor spreading.

Timing on the sea-level variation curve is related closely to ages of strata with hydrocarbon production in the Appalachian basin. Changes in sea level result in shifting of sand deposition along shorelines, solution porosity in carbonate rocks exposed along basin margins, or modifications of reef growth. The clearest relations to hydrocarbon production are in the well-explored oil and gas fields in the Devonian and Silurian, where the sea-level shifts can be used to explain permeability distribution. Superposing sea-level shifts onto sedimentary tectonics in a basin of known shape allows prediction of exploration trends.

The largest fluctuations of sea level are at the base of the Sauk sequence (Cambrian transgression), the Owl Creek discontinuity (base of the Middle Ordovician), the Wallbridge discontinuity (end of the Early Devonian), and the discontinuity at the base of the Absaroka sequence (beginning of Pennsylvanian). Fluctuations associated with the Wallbridge discontinuity are related to deposition of the Oriskany Sandstone, which is the largest Appalachian gas producer. Lesser sea-level changes are related to other production, notably oil and gas from the Upper Devonian fields which were the birthplace of the American petroleum industry, and gas from the Silurian Newburg sandstones.

The eustatic sea-level curve from one basin such as the Appalachian area should be compared with other basins to identify worldwide patterns and to help to focus petroleum exploration in distant basins. The major level drops at the end of the Early Ordovician, end of the Early Devonian, and the beginning of the Pennsylvanian seem established. The sea-level drop in the Appalachians at the end of the Ordovician appears related to continental glaciation centered in the African Sahara. The rise in sea level at the end of the Precambrian is possibly a result of an increase in rate of sea-floor spreading as the proto-Atlantic Ocean opened. Recognition of true global changes should permit more precise intercontinental correlations because eustatic sea-level change is not related to distribut on of faunal provinces or local tectonic processes.

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