Cenozoic global sea level, sequences, and the New Jersey Transect: Results From coastal plain and continental slope drilling

Abstract

The New Jersey Sea Level Transect was designed to evaluate the relationships among global sea level (eustatic) change, unconformity‐bounded sequences, and variations in subsidence, sediment supply, and climate on a passive continental margin. By sampling and dating Cenozoic strata from coastal plain and continental slope locations, we show that sequence boundaries correlate (within ±0.5 myr) regionally (onshore‐offshore) and interregionally (New Jersey‐Alabama‐Bahamas), implicating a global cause. Sequence boundaries correlate with δ 18 O increases for at least the past 42 myr, consistent with an ice volume (glacioeustatic) control, although a causal relationship is not required because of uncertainties in ages and correlations. Evidence for a causal connection is provided by preliminary Miocene data from slope Site 904 that directly link δ 18 O increases with sequence boundaries. We conclude that variation in the size of ice sheets has been a primary control on the formation of sequence boundaries since ∼42 Ma. We speculate that prior to this, the growth and decay of small ice sheets caused small‐amplitude sea level changes (<20 m) in this supposedly ice‐free world because Eocene sequence boundaries also appear to correlate with minor δ 18 O increases. Subsidence estimates (backstripping) indicate amplitudes of short‐term (million‐year scale) lowerings that are consistent with estimates derived from δ 18 O studies (25–50 m in the Oligocene‐middle Miocene and 10–20 m in the Eocene) and a long‐term lowering of 150–200 m over the past 65 myr, consistent with estimates derived from volume changes on mid‐ocean ridges. Although our results are consistent with the general number and timing of Paleocene to middle Miocene sequences published by workers at Exxon Production Research Company, our estimates of sea level amplitudes are substantially lower than theirs. Lithofacies patterns within sequences follow repetitive, predictable patterns: (1) coastal plain sequences consist of basal transgressive sands overlain by regressive highstand silts and quartz sands; and (2) although slope lithofacies variations are subdued, reworked sediments constitute lowstand deposits, causing the strongest, most extensive seismic reflections. Despite a primary eustatic control on sequence boundaries, New Jersey sequences were also influenced by changes in tectonics, sediment supply, and climate. During the early to middle Eocene, low siliciclastic and high pelagic input associated with warm climates resulted in widespread carbonate deposition and thin sequences. Late middle Eocene and earliest Oligocene cooling events curtailed carbonate deposition in the coastal plain and slope, respectively, resulting in a switch to siliciclastic sedimentation. In onshore areas, Oligocene sequences are thin owing to low siliciclastic and pelagic input, and their distribution is patchy, reflecting migration or progradation of depocenters; in contrast, Miocene onshore sequences are thicker, reflecting increased sediment supply, and they are more complete downdip owing to simple tectonics. We conclude that the New Jersey margin provides a natural laboratory for unraveling complex interactions of eustasy, tectonics, changes in sediment supply, and climate change.