Some comments on the problem of using vertical facies changes to infer accommodation and eustatic sea-level histories with examples from Utah and the southern Canadian Rockies

Some of the confusion in the literature on the history of eustatic sea level over time results from the incorrect assumption that the eustatic signal is given directly by vertical changes in water depths inferred from vertical facies patterns in stratigraphic sections. In particular, increases in water depth are often assumed to record an increase in eustatic sea level, and decreases in water depth are often assumed to record a decrease in eustatic sea level. Vertical changes in water depth, however, reflect only the local relative sea-level change, which is influenced by the balance between changes in the rate at which accommodation space forms (space available for sediments to fill) and changes in the rate at which sediment fills that space. Under certain conditions, which may not be uncommon in many areas, the balance may be such that no significant water depth change occurs, even during a relatively large third-order eustatic cycle, or such that shoaling occurs during a eustatic sea-level rise and deepening occurs during a eustatic sea-level fall. The key to sorting out the correct relation between eustatic sea level and vertical facies changes lies in first identifying the accommodation change, along with its timing and scale, and then determining whether that accommodation change (not the water depth change) occurred synchronously over a large region (continental or intercontinental). Using R2 analysis, a procedure we developed recently, we attempt to distinguish the local relative sea level from the regional or eustatic sea-level signal by recovering the accommodation history from detailed stratigraphic sections that can be correlated over large distances. We describe two examples from the late Middle Cambrian carbonate platform strata in the Cordillera of North America. In the first example, from the Pierson Cove and Trippe Formations in south-central Utah, the water depth changed little during a large (third-order?) accommodation cycle because the accumulation of sediment essentially kept pace with the change in accommodation. The form of the accommodation cycle in the Utah example is corroborated by the results of a Fischer plot of shoaling-upward meter-scale cycles in both formations. The Fischer plot is constrained by evidence, which we describe in another article (Bond et al., this volume) that the cycles are periodic (orbitally forced). In the second example, from the approximately correlative Arctomys and Waterfowl Formations in the southern Canadian Rockies, we have identified a similar-scale accommodation cycle in which the water depth decreased as accommodation increased and then increased as accommodation decreased. This complicated relation between the accommodation cycle and the water depth appears to be due to the effects of a large change in the sediment accumulation rate. The eustatic origin of the accommodation cycle observed in both examples is implied by the similarity in its timing and scale in several stratigraphic sections in the southern Canadian Rockies and in Utah. Demicco et al. (this volume) suggest a different relative sea-level history for the Arctomys and Waterfowl Formations in the southern Canadian Rockies. They suggest, mainly on the basis of water depth changes, that the Arctomys formed during a relative sea-level fall and that the Waterfowl formed during a relative sea-level rise. We do not disagree with their interpretation of the water depth change; our field data indicate the same water depth history in sections 40 km (25 mi) north of theirs. We also recognize in our R2 curves the same cycles that they describe within the Waterfowl Formation (one full cycle and part of another), but in our curves these cycles are strongly modulated by at least two lower orders of cyclicity with time scales of several millions of years to tens of millions of years. We suggest that their interpretation of the Arctomys-Waterfowl sea-level history applies only to the local relative sea-level change, probably mainly confined to the southern Canadian Rockies. Because of the effects of changing sediment accumulation rates, the local sea-level history for these strata is almost the reverse of the accommodation and, probably, the eustatic sea-level change. In addition, their field interpretations and modeling of the sea-level history for the Waterfowl Formation are limited by their emphasis on meter-scale cyclicity and the acquisition of data from a short stratigraphic section comprising only the Arctomys and Waterfowl Formations. Observations limited in this way tend to obscure the lower orders of cyclicity, which we argue from the results of our R2 analyses were important components of the eustatic signal in Middle and Late Cambrian time

Some comments on the problem of using vertical facies changes to infer accommodation and eustatic sea-level histories with examples from Utah and the southern Canadian Rockies

Some of the confusion in the literature on the history of eustatic sea level over time results from the incorrect assumption that the eustatic signal is given directly by vertical changes in water depths inferred from vertical facies patterns in stratigraphic sections. In particular, increases in water depth are often assumed to record an increase in eustatic sea level, and decreases in water depth are often assumed to record a decrease in eustatic sea level. Vertical changes in water depth, however, reflect only the local relative sea-level change, which is influenced by the balance between changes in the rate at which accommodation space forms (space available for sediments to fill) and changes in the rate at which sediment fills that space. Under certain conditions, which may not be uncommon in many areas, the balance may be such that no significant water depth change occurs, even during a relatively large third-order eustatic cycle, or such that shoaling occurs during a eustatic sea-level rise and deepening occurs during a eustatic sea-level fall. The key to sorting out the correct relation between eustatic sea level and vertical facies changes lies in first identifying the accommodation change, along with its timing and scale, and then determining whether that accommodation change (not the water depth change) occurred synchronously over a large region (continental or intercontinental). Using R2 analysis, a procedure we developed recently, we attempt to distinguish the local relative sea level from the regional or eustatic sea-level signal by recovering the accommodation history from detailed stratigraphic sections that can be correlated over large distances. We describe two examples from the late Middle Cambrian carbonate platform strata in the Cordillera of North America. In the first example, from the Pierson Cove and Trippe Formations in south-central Utah, the water depth changed little during a large (third-order?) accommodation cycle because the accumulation of sediment essentially kept pace with the change in accommodation. The form of the accommodation cycle in the Utah example is corroborated by the results of a Fischer plot of shoaling-upward meter-scale cycles in both formations. The Fischer plot is constrained by evidence, which we describe in another article (Bond et al., this volume) that the cycles are periodic (orbitally forced). In the second example, from the approximately correlative Arctomys and Waterfowl Formations in the southern Canadian Rockies, we have identified a similar-scale accommodation cycle in which the water depth decreased as accommodation increased and then increased as accommodation decreased. This complicated relation between the accommodation cycle and the water depth appears to be due to the effects of a large change in the sediment accumulation rate. The eustatic origin of the accommodation cycle observed in both examples is implied by the similarity in its timing and scale in several stratigraphic sections in the southern Canadian Rockies and in Utah. Demicco et al. (this volume) suggest a different relative sea-level history for the Arctomys and Waterfowl Formations in the southern Canadian Rockies. They suggest, mainly on the basis of water depth changes, that the Arctomys formed during a relative sea-level fall and that the Waterfowl formed during a relative sea-level rise. We do not disagree with their interpretation of the water depth change; our field data indicate the same water depth history in sections 40 km (25 mi) north of theirs. We also recognize in our R2 curves the same cycles that they describe within the Waterfowl Formation (one full cycle and part of another), but in our curves these cycles are strongly modulated by at least two lower orders of cyclicity with time scales of several millions of years to tens of millions of years. We suggest that their interpretation of the Arctomys-Waterfowl sea-level history applies only to the local relative sea-level change, probably mainly confined to the southern Canadian Rockies. Because of the effects of changing sediment accumulation rates, the local sea-level history for these strata is almost the reverse of the accommodation and, probably, the eustatic sea-level change. In addition, their field interpretations and modeling of the sea-level history for the Waterfowl Formation are limited by their emphasis on meter-scale cyclicity and the acquisition of data from a short stratigraphic section comprising only the Arctomys and Waterfowl Formations. Observations limited in this way tend to obscure the lower orders of cyclicity, which we argue from the results of our R2 analyses were important components of the eustatic signal in Middle and Late Cambrian time

Evidence for orbital forcing of Middle Cambrian peritidal cycles: Wah Wah range, south-central Utah

We have applied a new method (gamma method) for constructing high-resolution age models to peritidal cycles in the Middle Cambrian Pierson Cove Formation (13 cycles) and the Trippe Limestone (40 cycles) exposed in the Wah Wah range, south-central Utah. Spectral analyses of the time series for the gamma age model indicate the presence of significant spectral peaks (relative to a null model) in both data sets. After experimenting with different assumptions for the duration of the mean primary or measured cycle, we found that for the Trippe data set assigning the mean duration of precession to the mean primary cycle produced a reasonably good correlation between the spectrum and the early Paleozoic estimate of insolation forcing. In particular, the periods of the three significant spectral peaks in the Trippe record correspond to estimated line periods for eccentricity and precession and a combination tone of precession. A spectrum for the Trippe cycles based on the conventional assumption that time is proportional to thickness contained only one significant peak, and reasonable estimates of the duration of the mean primary cycle produced a poor fit to the insolation model. Spectral results from the Pierson Cove cycles were less compelling, possibly because of the short length of the record. The presence in the Trippe spectrum of significant peaks with periods corresponding to high-frequency orbital variations suggests that preservation of high-frequency Milankovitch signals is more common than implied by models of shallow marine cyclicity based on Pleistocene sea-level records. The results of these spectral analyses suggest that the gamma method can be used to construct age models for peritidal carbonate cycles that are accurate enough to test for periodicity and deterministic mechanisms, even in rocks as old as the Cambrian

Evidence for orbital forcing of Middle Cambrian peritidal cycles: Wah Wah range, south-central Utah

We have applied a new method (gamma method) for constructing high-resolution age models to peritidal cycles in the Middle Cambrian Pierson Cove Formation (13 cycles) and the Trippe Limestone (40 cycles) exposed in the Wah Wah range, south-central Utah. Spectral analyses of the time series for the gamma age model indicate the presence of significant spectral peaks (relative to a null model) in both data sets. After experimenting with different assumptions for the duration of the mean primary or measured cycle, we found that for the Trippe data set assigning the mean duration of precession to the mean primary cycle produced a reasonably good correlation between the spectrum and the early Paleozoic estimate of insolation forcing. In particular, the periods of the three significant spectral peaks in the Trippe record correspond to estimated line periods for eccentricity and precession and a combination tone of precession. A spectrum for the Trippe cycles based on the conventional assumption that time is proportional to thickness contained only one significant peak, and reasonable estimates of the duration of the mean primary cycle produced a poor fit to the insolation model. Spectral results from the Pierson Cove cycles were less compelling, possibly because of the short length of the record. The presence in the Trippe spectrum of significant peaks with periods corresponding to high-frequency orbital variations suggests that preservation of high-frequency Milankovitch signals is more common than implied by models of shallow marine cyclicity based on Pleistocene sea-level records. The results of these spectral analyses suggest that the gamma method can be used to construct age models for peritidal carbonate cycles that are accurate enough to test for periodicity and deterministic mechanisms, even in rocks as old as the Cambrian

Are cyclic sediments periodic? Gamma analysis and spectral analysis of Newark Supergroup lacustrine strata

Methodologies are suggested for the analysis of cyclic sediments. These include (1) linear analysis to determine whether cycles are of approximately constant duration and whether the relation between thickness and time is facies dependent and (2) multiple prolate-spheroidal windowing spectral analysis to determine whether time-series data indicate periodicities, either of the primary cycles or of higher or lower orders. The results of both methods are compared to a null hypothesis as a semiquantitative test of periodicity. Application of the methods to Newark Supergroup lacustrine cycles suggests that the primary cycles are approximately periodic and record a response to astronomical precession. The time represented by a given thickness of the different facies increases with the depositional water depth of that facies and with decreasing grain size. Precessional index cycles and long-period precessional index beats, or eccentricity, are strongly recorded in the spectra. Spectral results suggest but do not prove lengthening of the periodicities of orbital parameters since 200 Ma

Are cyclic sediments periodic? Gamma analysis and spectral analysis of Newark Supergroup lacustrine strata

Methodologies are suggested for the analysis of cyclic sediments. These include (1) linear analysis to determine whether cycles are of approximately constant duration and whether the relation between thickness and time is facies dependent and (2) multiple prolate-spheroidal windowing spectral analysis to determine whether time-series data indicate periodicities, either of the primary cycles or of higher or lower orders. The results of both methods are compared to a null hypothesis as a semiquantitative test of periodicity. Application of the methods to Newark Supergroup lacustrine cycles suggests that the primary cycles are approximately periodic and record a response to astronomical precession. The time represented by a given thickness of the different facies increases with the depositional water depth of that facies and with decreasing grain size. Precessional index cycles and long-period precessional index beats, or eccentricity, are strongly recorded in the spectra. Spectral results suggest but do not prove lengthening of the periodicities of orbital parameters since 200 Ma

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 (less than 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

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 δ18O data. Each of the nine Oligocene δ18O 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 for a min i fer al δ18O 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 (r2 = 0.807 and 0.960, respectively), and with records from ODP Site 689 (Southern Ocean; 0.13‰ per 10 m of ASL change; r2 = 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

Seafloor Spreading, Sea Level, and Ocean Chemistry Changes

High Cretaceous ocean crust production rates have been causally linked to high global sea level and global CO_2 due to increased outgassing. However, recent studies have questioned the empirical basis for high Cretaceous global seafloor spreading rates, high Cretaceous sea level (230-320 m above present), and the relationship between geochemical fluxes and spreading rates. Although this topic has been discussed at several recent international meetings, there has been little opportunity for the protagonists in the debate of constant versus variable global seafloor spreading rates to interact. However, a group of tectonophysicists, stratigraphers, and geochemists recently met at Rutgers, The State University of New Jersey (Piscataway N.J.) to discuss global seafloor spreading changes and their possible relationships to sea level and geochemical variations

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 (less than 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