Quantitative Constraints on the Origin of Stratigraphic Architecture at Passive Continental Margins: Oligocene Sedimentation in New Jersey, U.S.A.

ABSTRACT The Oligocene of the New Jersey continental margin is divisible into as many as eight sequences and 23 lithofacies associations, documented in a series of seven boreholes across the modern coastal plain. This paper summarizes the sequence architecture of these deposits, interpreted from high-resolution biostratigraphy and Sr-isotope chemostratigraphy, and evaluates the factors that governed patterns of sedimentation, making use of previously published quantitative estimates of water-depth changes and eustasy from 2-D foraminiferal paleoslope modeling and flexural backstripping. Each sequence is markedly wedge-shaped, thinning both landward of the rollover in the underlying sequence boundary (the point at which the surface steepens into a clinoform), and seaward of the rollover in the overlying boundary. Each bounding surface is associated with evidence for offlap-onlap geometry and at least locally with benthic foraminiferal evidence for abrupt upward shoaling. Most unconformities merge up dip into a single surface marking the Oligocene-Miocene boundary. Earliest Oligocene unconformities (33.5-31.6 Ma) merge downdip as a result of sediment starvation on the deep shelf. Conventional lithostratigraphic units within the New Jersey Oligocene are highly diachronous. For example, the base of Atlantic City Formation at Cape May (a downdip borehole) is at least 6.6 Myr younger than the top of the same formation at ACGS#4 (an updip borehole). Factors controlling patterns of sedimentation include: (1) a terraced physiography, with gradients ranging from 1:1,000 (0.06°) on the coastal plain and shallow shelf and 1:500 (0.11°) on the deep shelf to < 1:100 (1.0°) on an intermediate slope; (2) generally low siliciclastic sediment flux, with in situ production of authigenic glauconite, especially during times of transgression; (3) a location landward of the hinge zone of the passive margin, with slow tectonic subsidence augmented by compaction and sediment loading; (4) low to moderate amplitudes and rates of eustatic change (10-50 m over spans of 1-2 Myr); and (5) an active wave climate that permitted efficient lateral transport and complete bypass of sediment at paleodepths of at least 20 ± 10 m. Sequence architecture in the New Jersey Oligocene differs from that of the standard "Exxon model." Sequences are highstand-dominated, in spite of deposition and preservation largely seaward of the rollover in each underlying sequence boundary. Transgressive systems tracts are thin. Recognizable lowstand units did not form because efficient transfer of sediment across the shallow shelf, combined with the absence of major river systems in the area of study, prevented the reorganization of sedimentation patterns commonly associated with point-source development, in spite of rates of eustatic fall considerably greater than the local rate of tectonic subsidence. Repeated eustatic rises and falls are expressed primarily by variations in paleo-water depth. Although 65-80% of the shallow shelf that had been flooded during each rise became subaerially exposed during the subsequent fall, well developed offlap at each sequence boundary is due primarily to marine bypassing and degradation rather than to "forced regression." Sequence boundaries correspond in time at their correlative conformities not with the onset of falling "relative" sea level, but with the start of eustatic rise.