Michelle A. Kominz

The Phanerozoic Record of Global Sea-Level Change

We review Phanerozoic sea-level changes [543 million years ago (Ma) to the present] on various time scales and present a new sea-level record for the past 100 million years (My). Long-term sea level peaked at 100 ± 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred. Sea level mirrors oxygen isotope variations, reflecting ice-volume change on the 104- to 106-year scale, but a link between oxygen isotope and sea level on the 107-year scale must be due to temperature changes that we attribute to tectonically controlled carbon dioxide variations. Sea-level change has influenced phytoplankton evolution, ocean chemistry, and the loci of carbonate, organic carbon, and siliciclastic sediment burial. Over the past 100 My, sea-level changes reflect global climate evolution from a time of ephemeral Antarctic ice sheets (100 to 33 Ma), through a time of large ice sheets primarily in Antarctica (33 to 2.5 Ma), to a world with large Antarctic and large, variable Northern Hemisphere ice sheets (2.5 Ma to the present).

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

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 δ18O 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 δ18O 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 δ18O increases. Subsidence estimates (backstripping) indicate amplitudes of short-term (million-year scale) lowerings that are consistent with estimates derived from δ18O 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.

Late Cretaceous chronology of large, rapid sea-level changes: Glacioeustasy during the greenhouse world

We provide a record of global sea-level (eustatic) variations of the Late Cretaceous (99- 65 Ma) greenhouse world. Ocean Drilling Program Leg 174AX provided a record of 11- 14 Upper Cretaceous sequences in the New Jersey Coastal Plain that were dated by in- tegrating Sr isotopic stratigraphy and biostratigraphy. Backstripping yielded a Late Cre- taceous eustatic estimate for these sequences, taking into account sediment loading, com- paction, paleowater depth, and basin subsidence. We show that Late Cretaceous sea-level changes were large (.25 m) and rapid (K1 m.y.), suggesting a glacioeustatic control. Three large d 18 O increases are linked to sequence boundaries (others lack sufficient d 18 O data), consistent with a glacioeustatic cause and with the development of small (,10 6 km 3 ) ephemeral ice sheets in Antarctica. Our sequence boundaries correlate with sea-level falls recorded by Exxon Production Research and sections from northwest Europe and Russia, indicating a global cause, although the Exxon record differs from backstripped estimates in amplitude and shape.

Upper Cretaceous sequences and sea-level history, New Jersey Coastal Plain

We developed a Late Cretaceous sea- level estimate from Upper Cretaceous sequences at Bass River and Ancora, New Jersey (ODP [Ocean Drilling Program] Leg 174AX). We dated 11–14 sequences by integrating Sr isotope and biostratigraphy (age resolution ±0.5 m.y.) and then estimated paleoenvironmental changes within the sequences from lithofacies and biofacies analyses. Sequences generally shallow up-section from middle-neritic to inner-neritic paleodepths, as shown by the transition from thin basal glauconite shelf sands (transgressive systems tracts [TST]), to medial-prodelta silty clays (highstand systems tracts [HST]), and finally to upper–delta-front quartz sands (HST). Sea-level estimates obtained by backstripping (accounting for paleodepth variations, sediment loading, compaction, and basin subsidence) indicate that large (>25 m) and rapid (≪1 m.y.) sea-level variations occurred during the Late Cretaceous greenhouse world. The fact that the timing of Upper Cretaceous sequence boundaries in New Jersey is similar to the sea-level lowering records of Exxon Production Research Company (EPR), northwest European sections, and Russian platform outcrops points to a global cause. Because backstripping, seismicity, seismic stratigraphic data, and sediment-distribution patterns all indicate minimal tectonic effects on the New Jersey Coastal Plain, we interpret that we have isolated a eustatic signature. The only known mechanism that can explain such global changes— glacio-eustasy—is consistent with foraminiferal δ 18 O data. Either continental ice sheets paced sea-level changes during the Late Cretaceous, or our understanding of causal mechanisms for global sea-level change is fundamentally flawed. Comparison of our eustatic history with published ice-sheet models and Milankovitch predictions suggests that small (5–10 × 10 6 km 3 ), ephemeral, and areally restricted Antarctic ice sheets paced the Late Cretaceous global sea-level change. New Jersey and Russian eustatic estimates are typically one-half of the EPR amplitudes, though this difference varies through time, yielding markedly different eustatic curves. We conclude that New Jersey provides the best available estimate for Late Cretaceous sea-level variations.

Calibration between eustatic estimates from backstripping and oxygen isotopic records for the Oligocene

Eustatic estimates from the backstripping of Oligocene sections are compared quantitatively with d 18 O data. Each of the nine Oligocene d 18 O events (maxima) identified in previous studies correlates with a stratigraphically determined sea-level lowstand. Oxygen isotopic records from planktonic foraminifers from western equatorial Atlantic Ocean Drilling Program (ODP) Site 929 indicate an isotopic increase of 0.16‰ per 10 m decrease in the depth of the ocean (apparent sea level, ASL). Amplitudes of ASL change also correlate with moderate- and high-resolution benthic foraminiferal d 18 O records from ODP Sites 803 (western tropical Pacific) and 929 and from Deep Sea Drilling Project (DSDP) Site 522 (South Atlantic Ocean), with an isotopic change of 0.22‰ per 10 m of ASL change (r 2 5 0.807 and 0.960, respectively), and with records from ODP Site 689 (Southern Ocean; 0.13‰ per 10 m of ASL change; r 2 5 0.704). This correlation suggests that Southern Ocean deep-water temperature changes were smaller than tropical sea-surface temperature changes between million year‐scale glacials and interglacials. It also suggests that the deep-sea Southern Ocean records may provide the best means to calibrate sea level to oxygen isotopes.

Oligocene eustasy from two-dimensional sequence stratigraphic backstripping

Estimates of the magnitudes of changes in third-order (0.522 m.y.) eustasy were obtained for Oligocene sequences defined by a suite of largely onshore boreholes of the New Jersey coastal plain. Benthic foraminiferal biofacies and multiple age constraints in a sequence stratigraphic framework formed the database for this study. The geometry of the margin through time was determined using two-dimensional backstripping. The depth ranges of benthic foraminiferal biofacies were determined from a combination of standard factor analysis techniques and the backstripped geometries. Benthic foraminiferal biofacies were then used to determine the depths of the Oligocene margin profiles obtained from backstripping. The water depths of 16 of these horizons were confirmed by independent benthic biofacies determinations from at least two wells. This internal consistency indicates that the data and the twodimensional backstripping approach were robust. Where benthic biofacies require a vertical shift of the horizons generated by two-dimensional backstripping, eustatic changes were required and were readily calculated. Results indicate a major eustatic fall from the end of the Eocene to the first record of Oligocene deposition. Subsequent long-term shoaling of sea level through the Oligocene was ;30 m in 10 m.y. Superimposed on this long-term trend were higher frequency (third-order) variations in eustasy with amplitudes of ;40 m or less.

Quantification of the effects of eustasy, subsidence, and sediment supply on Miocene sequences, mid-Atlantic margin of the United States

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 margin of the United States (New Jersey, Delaware, and Maryland). Eustasy is a primary infl uence on sequence patterns, determining the global template of sequences (i.e., times when sequences can be preserved) and explaining similarities in Miocene sequence architecture on margins throughout the world. Sequences can be correlated throughout the mid-Atlantic region with Sr-isotopic chronology (±0.6 m.y. to ±1.2 m.y.). Eight Miocene sequences correlate regionally and can be correlated to global δ 18 O increases, indicating glacioeustatic control. This margin is dominated by passive subsidence with little evidence for active tectonic overprints, except possibly in Maryland during the early Miocene. However, early Miocene sequences in New Jersey and Delaware display a patchwork distribution that is attributable to minor (tens of meters) intervals of excess subsidence. Backstripping quantifi es 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 fl exural 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 fi rstorder control on accommodation providing similar timing on all margins to the sequence record. Tectonic changes due to movement of the crust can overprint the record, resulting in large gaps in the stratigraphic record. Smaller differences in sequences can be attributed to local fl exural loading effects, particularly in regions experiencing largescale progradation.

Two-Dimensional Paleoslope Modeling: A New Method for Estimating Water Depths of Benthic Foraminiferal Biofacies and Paleoshelf Margins

ABSTRACT A new method developed for estimating water depths of benthic foraminifers was implemented for Oligocene foraminiferal biofacies from the New Jersey Coastal Plain. We combined benthic foraminiferal biofacies and two-dimensional flexural backstripping to construct a two-dimensional paleoslope model. The original depositional geometry of New Jersey Oligocene strata was reconstructed by calculating the effects of sediment accumulation and flexural loading. Paleodepth estimates for benthic foraminiferal biofacies were calibrated to inner neritic facies along the resulting nonlinear profile (e.g., with clinoforms). Paleodepth estimates for ten foraminiferal biofacies ranged from 20 ± 10 m to 115 ± 30 m and were consistent with more qualitative estimates based on other methods (e.g., foraminiferal abundances and species diversity). Applying this method to the New Jersey coastal plain for the Oligocene showed clear distributional patterns of benthic foraminiferal biofacies. This enabled paleobathymetry to be determined for the reconstructed New Jersey margin as well as significant stratal surfaces within sequences (i.e., systems tracts and condensed sections). Paleodepth estimates ranged from nearshore (20 ± 10 m) up to 85 ± 25 m for deposits landward of the clinoform rollover of the underlying sequence boundary and from middle neritic to outer neritic (50 ± 20 m to over 100 ± 30 m) on the slopes of the clinoforms. During earliest transgressive systems tracts, paleodepths immediately seaward of the clinoform slope ranged from inner neritic ( 100 m) at downdip sites. During early highstands, paleobathymetry ranged from 45 ± 10 to 85 ± 25 m landward of the clinoform rollover to 85 ± 25 m to over 100 ± 30 m on the slope (seaward of the developing prograding sedimentary wedge), while during late HSTs, paleodepths shoaled to as shallow as 25 ± 10 m near the clinoform rollover. This method can be applied to other continental passive margins to reconstruct the stratal geometry of and estimate water depths on the shelf. Furthermore, by extending this process, relative sea level and eustatic timings and amplitudes could be evaluated.

Evaluating the stratigraphic response to eustasy from Oligocene strata in New Jersey

Previously published Oligocene eustatic records are compared with observed stratigraphic architecture at the New Jersey continental margin in order to evaluate the stratigraphic response to eustatic change. Lower to mid-Oligocene sequence boundaries (33.8‐ 28.0 Ma) are associated with relatively long hiatuses (0.3‐0.6 m.y.), in which sedimentation in many places terminated during eustatic falls and resumed early during eustatic rises. Upper Oligocene sequence boundaries are associated with relatively short hiatuses (,0.3 m.y.), and provide the best constraints on phase relations between sea-level forcing and margin response. The interval represented by each upper Oligocene sequence varies in dip profile. At updip locations, landward of the clinoform rollover in the underlying sequence boundary, sedimentation commenced after the eustatic low and terminated before the eustatic high (with partial erosion of any younger record). At downdip locations, sedimentation within each sequence was progressively delayed in a seaward direction, beginning during the eustatic rise and terminating near the eustatic low. Combining data from all available boreholes, ages of sequence boundaries (correlative surfaces) correspond closely with the timing of eustatic lows, and ages of condensed sections (intervals of sediment starvation) correspond with eustatic highs.

Reconstructing the stratal geometry of latest Eocene to Oligocene sequences in New Jersey: resolving a patchwork distribution into a clear pattern of progradation

Nine latest Eocene to Oligocene (34.2‐23.9 Ma) sequences were identified and dated from eight sites situated on the onshore New Jersey Coastal Plain and nearshore region. These sequences show a patchy distribution, with more complete lower Oligocene sections updip and more complete upper Oligocene sections downdip. We projected these sequences onto a dip profile and reconstructed their original thicknesses and distributions by 2-D flexural backstripping. This demonstrated that depocenters migrated offshore during the Oligocene, indicating that the patchy distribution can be best explained by progradation of generally thin and spacially limited clinoforms over an Eocene carbonate ramp. During the late Eocene to Oligocene, the New Jersey passive margin underwent a major morphologic change. Reconstructions indicate that the margin was a relatively steeply dipping carbonate ramp (1:500 paleoslope gradient) during the Eocene and was transformed into a siliciclastic margin characterized by a gentler gradient of 1:1000 and prograding clinoforms by the Miocene. Clinoform progradation probably began during the latest Eocene. Increased sediment supply during the Oligocene resulted in the further progradation of sediments across the antecedent carbonate ramp. The heights of the clinoforms ranged from ,20 m during the latest Eocene and earliest Oligocene to nearly 50 m during the late Oligocene. Most sediment accumulated within clinoform wedges, with little or no sediment being preserved behind the clinoform inflection point. q 2000 Elsevier Science B.V. All rights reserved.