SEQUENCE CHAMELEONS: THE MIOCENE OF NEW JERSEY VS. DELAWARE

Correlation of sequences among basins on passive continental margins is the first step in using sequence stratigraphy as a global correlation tool. Correlations of Miocene strata among basins on the US east coast were limited by reliance on provincial diatom zonations until the advent of integrated Sr-isotopic and sequence stratigraphy. A firm Sr-isotopic chronology was established for Kirkwood-Cohansey Miocene sequence boundaries in seven continuously cored boreholes in New Jersey. Miocene sequence boundaries in NJ are associated with moderately long hiatuses (~0.2-1 m.y.), with the hiatuses increasing updip. Lower/middle Miocene NJ sequences are comprised of deltaically influenced successions of transgressive sands, medial silty clays, and upper regressive highstand sands. Sequences became progressively shallower during the Miocene; upper Miocene sequences were deposited in estuarine environments. Recent drilling at Bethany Beach provides the first continuously cored record of Miocene strata from a more downdip location in Delaware and provides numerous shell beds for Sr-isotopic dating. The resultant well-dated record of Miocene sequences in Delaware shows a relatively complete lower and middle Miocene section with short hiatuses at sequence boundaries. The upper Miocene is thicker (140 vs. 70 m) and more fully marine than coeval NJ sections. The Delaware section is generally free of the deltaic influence seen in the NJ Miocene and is instead dominated by shelfal fine sands and silts. Lower and middle Miocene sequences are characterized by shelly, sometimes glauconitic, basal transgressive lag deposits overlain by offshore to lower shoreface laminated silts that shallow upward to clean, shelly upper shoreface sands. The upper Miocene section changes upward from silt-dominated shelfal sequences to sandier nearshore and estuarine sequences. Because of its down dip location Bethany Beach apparently records sequences and sea-level variations not recovered in NJ. Although differences in sediment input, sedimentary environment, and tectonics result in different sedimentary patterns, the ages of sequence boundaries and the internal stratal architecture are similar between regions, testifying that relative sea level is a dominant control on sequence development.