THE EFFECTS OF EUSTASY, SUBSIDENCE, AND SEDIMENT SUPPLY ON MIOCENE SEQUENCES, U.S. MID-ATLANTIC MARGIN

Miocene sequences in the U.S. Mid-Atlantic margin (Salisbury Embayment) display a patchwork distribution that is best explained by the interplay of global sea-level (eustasy), subsidence, and sediment supply. Sequences can be correlated throughout the mid-Atlantic region with a Sr-isotopic chronology of ±0.6 to ±1.2 m.y. Eight Miocene sequences correlate regionally and can be related to global oxygen isotopic increases, indicating primary glacioeustatic control. Miocene sequences in New Jersey are dominated by deltaic deposits with upper delta front aquifer sands and lower prodelta aquitard clays. In contrast, Miocene sequences in Delaware and Virginia are wave dominated shoreline deposits with shoreface sands and finer-grained middle to outer neritic clays and silts. Lower to lower middle Miocene sequences are best represented in New Jersey, with thin, marginal marine upper middle to upper Miocene strata. In contrast, middle-upper Miocene sequences are thicker, temporally more complete, and more fully marine in Delaware-Virginia. We use backstripping to quantify the roles of variations in global sea-level (eustasy), subsidence, and sediment supply on the development of the Miocene stratigraphic record of the mid-Atlantic continental region. This margin is dominated by passive subsidence with little evidence for active tectonic overprints. However, differences between early Miocene sequences in New Jersey and Delaware are attributable to minor (10's m) intervals of excess subsidence. Backstripping quantifies that excess subsidence began in Delaware at ca. 21 Ma and continued until 12 Ma, with maximum rates from ca. 21-16 Ma. We attribute this enhanced subsidence to local flexural response to the progradation of thick sequences offshore and adjacent to this area. Removing this excess subsidence in Delaware yields a record that is remarkably similar to New Jersey eustatic estimates. We conclude that sea-level rise and fall is a first order control on accommodation providing similar timing on all margins to the sequence record. Movement of the crust due to tectonic changes can overprint the record resulting in large gaps in the stratigraphic record. Smaller differences in sequences can be attributed to local flexural loading effects, particularly in regions experiencing large-scale progradation.