Recognition and Interpretation of Depositional Sequences and Calculation of Sea-Level Changes from Stratigraphic Data—Offshore New Jersey and Alabama Tertiary

Abstract: Tertiary depositional sequences beneath the continental shelf and slope off New Jersey and Alabama have been studied using seismic-reflection data that have been tied to available wells. These data illustrate second- (10–20 Ma) and third-order (1–5 Ma) depositional sequences in areas close to and distal to progradational siliciclastic depocenters. Paleogene deposition is characterized by sediment-starved deep-water conditions. Second-order sequence boundaries divide these sediments and are recognized by local erosion of underlying strata and deep-marine onlap. Closer to the depocenter, third-order sequences are noted by basinward shifts in coastal onlap, local erosional incision of shelf and slope strata, and planar erosion of basinal sediments. The stacking pattern of third-order sequences within the second-order sequences is similar to the stacking of systems tracts within the third-order depositional sequences modeled using a sinusoidally varying sea-level curve. Upper Oligocene to uppermost middle Miocene third-order sequences are interpreted to compose a second-order super sequence. Neogene strata in these areas are further analyzed to estimate sea levels. Subsidence is isolated using geohistory analysis and by calculating the average angular-tilt rate of the continental margin. During the early Tertiary, the subsidence rate was slow in both areas. An increase in subsidence rate in the offshore Alabama area during the Neogene is attributed to loading of the lithosphere adjacent to the study area. Short-term falls in sea level are estimated by measuring the vertical shift in onlapping paralic strata from highstand to lowstand position. Results show a lower overall Neogene sea-level position than the position represented on the Exxon curve but similar magnitudes of short-term fall.