Early to middle Miocene sequences, systems tracts, and benthic foraminiferal biofacies, New Jersey Coastal Plain

Abstract

We identified and dated nine lower to middle Miocene sequences at the Island Beach, Atlantic City, and Cape May, New Jersey boreholes, integrating Sr-isotopic, lithofacies, log, and benthic foraminiferal biofacies data in a sequence stratigraphic framework. Miocene sequences typically shallow upsection, representing three major lithofacies: (1) thin, shelly, glauconite sands of the Transgressive Systems Tracts deposited in inner–middle (0- to 100-m paleodepth) neritic environments; (2) medial silty clays of the lower Highstand Systems Tracts deposited in neritic prodelta environments; and (3) upper quartz sands of the upper Highstand Systems Tracts, deposited in inner neritic, near-shore, and delta front environments. Sedimentation rates were low–moderate in the earliest Miocene (5−24 m/m.y.), increased to 40−90 m/m.y. at about 22 Ma as deltaic sedimentation began, and remained high and uniform through the middle Miocene. We develop a benthic foraminiferal biofacies model for the inner to middle neritic zones (0- to 100-m paleodepth) and show that these biofacies follow a predictable succession in the se transgressive/regressive sequences. The nomenclature and usage of the Kirkwood Formation is discussed, and the Brigantine Member is formally named. Onshore sequences result from a complex interplay of changes in eustasy, tectonics, and sediment supply. Correlation of sequence boundaries onshore with offshore and Exxon records indicate that they represent global lowerings of baselevel. Cor- relation of the sequence boundaries with δ18O increases show that they were formed during glacioeustatic lowerings. Although eustasy controls the timing of sequences, the preservation of sequences is also controlled by local tectonic effects while their architecture is strongly influenced by changes in sediment supply and provenance. For example, differences in preservation of sequences is attributed to differential tectonic subsidence due to changes in sediment loading. The depositional patterns onshore allow prediction of offshore sequence architecture, including a progressive progradational change to deltaic sedimentation in the early to middle Miocene; in addition, the absence of lowstand deposits onshore during an interval of high sediment supply may indicate the presence of substantial lowstand systems tracts offshore.