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doi:10.2204/iodp.sp.313.2009

Introduction

Eustasy as a global phenomenon

Understanding the history, cause, and impact of sea level fluctuations is a compelling goal of Earth system research. Not only are worldwide effects of encroaching shorelines evident today—the rate of this change is clearly increasing. Whereas global sea level rise during the previous century was ~1.8 mm/y (Church and White, 2006), today that rate is ~3.25 mm/y (Cazenave et al., 2009), in part due to anthropogenic influences (Barnett, 1990). Furthermore, in many coastal regions the rate is still higher because of the additional effect of local subsidence. The geologic record shows that global sea level has fluctuated by well over 100 m (summaries in Donovan et al., 1979) at rates as high as 20–40 mm/y (Fairbanks, 1989; Stanford et al., 2006). The importance of carefully examining the geologic record for eustatic variations goes beyond preparing for a sea level rise of 0.4 m or more during this century. Indeed, Integrated Ocean Drilling Program (IODP) Expedition 313 will not address the centennial timescale; for that, strategies synthesizing tide gauge and Holocene marsh records are required. Instead, this study leads toward a broader understanding of the long-term behavior and wide-ranging effects of the divide between land and ocean. Throughout Earth's history, the transfer of energy and material across this boundary has profoundly influenced the interactions among the lithosphere, biosphere (e.g., Katz et al., 2005), and atmosphere and continues to affect the balance of these systems today. Weathering rates, sediment distribution, stratal architecture, carbon burial, and glaciation are just a few of the myriad processes that are intertwined with eustatic change.

Despite its importance, knowledge of the basic amplitudes and rates of sea level variations on timescales of tens of thousands to millions of years is surprisingly limited. Our goal is to address this deficiency in the way endorsed by numerous study groups (e.g., Imbrie et al., 1987; JOIDES Pollution Prevention and Safety Panel, 1992): by sampling key facies across the prograding deposits of a passive continental margin at proposed Sites MAT-1, MAT-2, and MAT-3.

Unraveling eustasy from the effects of subsidence and sediment supply requires a fundamental understanding of passive margin response to sedimentation. Deposits adjacent to the shoreline are replete with stratal discontinuities on all spatial scales, including sequence boundaries and regional unconformities associated with evidence for base-level lowering (Vail et al., 1977; Posamentier et al., 1988). Sequence boundaries provide a means to objectively subdivide the stratigraphic record (Christie-Blick et al., 1990; Christie-Blick, 1990), and the intervening sedimentary sequences provide the basis for evaluating controls on sedimentary architecture and predicting sedimentary facies and societally important resource distributions (e.g., hydrocarbons and potable water) (Vail et al., 1977; Sugarman et al., 2006). Remarkably similar sequence architecture occurs on margins of widely contrasting tectonic and sedimentary histories (e.g., Bartek et al., 1991), emphasizing the fact that eustasy exerts a fundamental, worldwide control on the stratigraphic record. Nevertheless, it is clear that tectonism and changes in sediment supply also have molded the stratigraphic record (e.g., Reynolds et al., 1991); the challenge is to isolate the imprint of each of these influences.

Sequence stratigraphy provides a powerful tool for deciphering margin records, but many of its fundamental assumptions have not been tested. For example, although the facies models of Exxon Production Research Company (e.g., Posamentier et al., 1988) are widely applied, the nature of facies associated with prograding clinoforms has not been publicly documented (although Ocean Drilling Program [ODP] Legs 166 and 174A made good contributions). Furthermore, the timing and phase relationships of facies distributions with respect to sea level change have not been evaluated (e.g., Reynolds et al., 1991). More importantly, the sequence stratigraphic record has been used to extract a eustatic history, despite the fact that critical assumptions (e.g., the water depth at the lowest point of onlap; Greenlee and Moore 1988, see discussion below) have not been tested.

Eustatic unknowns: amplitude, response, and mechanism

Measuring the geologic record of amplitudes of eustatic change is a difficult task. Although deep-sea δ¹⁸O records provide precise timing of glacio-eustatic changes (Miller et al., 1991, 1996a, 2005a), eustatic amplitudes can be estimated using δ¹⁸O to no better than ±20% for the past few million years and ±50% prior to that because of assumptions about paleotemperature and application of the Pleistocene sea level/δ_w calibration of Fairbanks and Matthews (1978) to the older record (Miller et al., 2005a). Carbonate atolls have been sampled as fossil "dip sticks" (e.g., ODP Legs 143 and 144), and although this approach has been successful for the Pleistocene (Fairbanks, 1989), recovery and age control for records older than the late Pleistocene have posed very large challenges. As noted above, continental margin sediments have long been regarded as a viable source for extracting eustasy (e.g., Vail, 1977; Watts and Steckler, 1979; Haq et al., 1987; Greenlee and Moore, 1988), provided the effects of total subsidence (compaction, loading, and cooling), as well as changes in sediment supply, could be removed.

Drilling into the New Jersey shallow shelf as we propose will allow us to evaluate the several controls on the stratigraphic record at passive margins. It was known that drilling on the New Jersey continental slope during ODP Leg 150 (Mountain, Miller, Blum, et al., 1994) would yield virtually no information concerning amplitudes. By contrast, it was expected that the coastal plain drilling during ODP Leg 150X (Miller et al., 1994, 1996b) and later ODP Leg 174AX (Miller et al., 1998, 2003, 2004, 2005a; Miller, Sugarman, Browning, et al., 1998; Kominz et al., 2008) would provide valuable constraints on how high sea level rose during the last 100 m.y. Although onshore analyses have borne this out (Fig. F1), they have been based on incomplete Miocene and younger sections dating from times when the shoreline was frequently seaward of its current position (Kominz et al., 1998). By contrast, the Late Cretaceous to Oligocene shoreline was often landward of the coastal plain wells and, as a result, eustatic amplitudes from these sections have been shown to be as large or larger than those of the Miocene (Fig. F1) (Miller et al., 2005a). Analyses from ODP Leg 194 on the Marion Plateau (John et al., 2004) clearly show that the New Jersey onshore sites do not capture the full amplitude of Miocene sea level change. Though backstripping the Marion Plateau data provided a relatively precise estimate of 56.5 ± 11.5 m for a late middle Miocene fall (John et al., 2004), it did not address estimates of other Miocene events. The New Jersey continental shelf, particularly the inner to middle shelf where we propose to drill Sites MAT-1 to MAT-3, is much better suited for estimating late Oligocene–Miocene eustatic amplitudes because sediments at this location are stratigraphically more complete, record the full range of water depth variations, and provide the facies needed to estimate eustatic amplitudes.

Various facies models have been proposed to explain shelf sedimentation in response to eustatic changes (e.g., Posamentier et al., 1988; Galloway, 1989), but the fact remains the response of passive margin sedimentation to large, rapid sea level changes is not well known. One of the main reasons for this situation is the scarcity of direct sampling of well-imaged seismic sequences in the regions most affected by sea level change. Understanding the amplitude of sea level change and sedimentation response requires knowledge of the depositional setting of strata that onlap sequence boundaries, but without samples it cannot be known if this onlap is coastal, marginal marine, or deep marine (~100 m or more, as suggested by Greenlee and Moore, 1988). Furthermore, the depositional significance (e.g., shoreface versus midshelf) of the clinoform inflection point, a critical constraint in facies interpretation, has been inferred mostly through forward models, although tantalizing evidence recovered from Leg 174A Hole 1071F suggests a marginal marine setting ~3.5 km landward of one late middle Miocene clinoform inflection point (Austin, Christie-Blick, Malone, et al., 1998). Continued analysis of Leg 174A sequences will shed new light on shelf facies models and their predictions from seismic data, but these data were limited by low core recovery and penetration of only upper middle Miocene and younger strata, hampering efforts to establish reliable facies models. Drilling at proposed Sites MAT-1 to MAT-3 will provide the information needed to properly evaluate depositional facies models.

Glacioeustasy (Donovan et al., 1979) is the only known mechanism for producing the large, rapid eustatic changes that have been reported for the past 200 m.y. (Miller et al., 2005a). Previous studies of the New Jersey margin have shown that changes in ice volume are the dominant mechanism causing eustatic changes in the last 42 m.y. (Miller et al., 1996a, 1998). Most researchers have assumed that Earth was ice-free during Cretaceous to Eocene times; however, Stoll and Schrag (1996) and Miller et al. (1999, 2004, 2005a, 2005b) have argued that there were ice sheets during the Cretaceous to early Eocene. One of the sites we plan to drill, Site MAT-1, is intended to recover a Paleocene–Eocene record that will address this fundamental issue.

The importance of eustasy versus tectonism to the formation and preservation of sequences is a long-standing debate that our proposed drilling will address. Tectonism in this context includes phenomena that operate across a large range of scales in both time and space (i.e., from rapid, narrowly focused "active" processes such as faulting and salt intrusion to the slower and more laterally extensive "passive" process of flexural loading). We have backstripped seven onshore boreholes (Kominz et al., 1998, 2008; Van Sickle et al., 2004; see summary in Miller et al., 2005b) and have shown that active tectonics has played a minimal role in Cenozoic onshore deposition. By contrast, backstripping has shown that flexural loading led to ~30 m of excess subsidence at onshore Delaware wells versus those in New Jersey beginning at ~21–12 Ma. This enhanced subsidence is attributed to a local flexural response to the load of thick sequences prograding offshore Delaware (Browning et al., 2006). Based on this, we hypothesize that

• Eustatic change is a first-order control on accommodation space and provides a simultaneous imprint on all continental margins;

• Tectonic change due to movement of the crust can overprint the record and result in large gaps, though this effect is not apparent in New Jersey Miocene sequences; and

• Second-order differences in sequences can be attributed to local flexural loading effects, particularly in regions experiencing large-scale progradation.

Sites MAT-1 to MAT-3 (Fig. F2) provide the crucial link in the onshore–offshore transect (Fig. F3) required to evaluate eustasy versus local lithospheric flexure on the development of prograding late Oligocene–Miocene sequences.

The New Jersey margin: its suitability, results, and promise

The New Jersey margin is an ideal location to investigate the history of sea level change and its relationship to sequence stratigraphy for several reasons: rapid depositional rates, tectonic stability, and well-preserved, cosmopolitan fossils suitable for age control characterize the sediments of this margin throughout the time interval of interest (see summary in Miller and Mountain, 1994). In addition, there exists a large set of seismic, well log, and borehole data with which to frame the general geologic setting from the coastal plain across the shelf to the slope and rise (Miller and Mountain, 1994) (Figs. F2, F3).

Drilling into the New Jersey slope (ODP Sites 902–904 and 1073) and the Coastal Plain (Island Beach, Atlantic City, Cape May, Bass River, Ancora, Oceanview, Bethany Beach, Millvile, Fort Mott, Sea Girt, and Cape May Zoo) has provided a chronology for sea level changes over the past 100 m.y. (Miller and Snyder, 1997; Miller et al., 1998, 2005a). Sequence boundaries from 10 to 42 Ma have been shown to correlate (within ±0.5 m.y. both regionally (onshore–offshore) and interregionally (New Jersey–Alabama–Bahamas), as well as globally, with glacio-eustatic lowerings inferred from the δ¹⁸O record (Fig. F4). These correlations establish a firm link between late middle Eocene to middle Miocene glacio-eustatic change and margin erosion on the million year scale. Oxygen isotopic studies of slope Site 904 provide prima facie evidence for a causal connection between Miocene δ¹⁸O increases (inferred glacio-eustatic falls) and sequence boundaries (Miller, Sugarman, Browning, et al., 1998). Results of these studies are consistent with the general number and timing of Late Cretaceous to middle Miocene sequences initially published by Exxon (Vail and Mitchum, 1977), although the Exxon group's sea level amplitudes are substantially higher than those derived in New Jersey studies (Miller et al., 1996b, 2005a; Miller, Sugarman, Browning, et al., 1998; Van Sickel et al., 2004).

Aided by easier access to older strata than is found downdip/offshore, New Jersey Coastal Plain drilling (Miller et al., 1994, 1996b; Miller, Sugarman, Browning, et al., 1998) has sampled "Greenhouse" (Cretaceous to Eocene) sequences and addressed their relationship to global sea level changes. One surprising result has been to extend the history of ice sheets back to a time previously considered to be ice-free (Late Cretaceous–middle Eocene, Browning et al., 1996; mid-Maastrichtian, Miller et al., 1999, 2005a, 2005b). Late Cretaceous to middle Eocene comparisons of onshore hiatuses/sequence boundaries and δ¹⁸O indicate that growth and decay of small ice sheets (<30 m sea level equivalent) also occurred in this supposedly ice-free world (Browning et al., 1996; Miller et al., 1998, 2003, 2005a, 2005b).

ODP drilling in the Bahamas (Leg 166 and supplementary platform drilling; Eberli, Swart, Malone, et al., 1997) complements the results from New Jersey by providing a chronology of base-level lowerings (Fig. F4) and an evaluation of prograding carbonate sequences.

Published results of drilling at the New Jersey and Bahamas margins validate the approaches outlined by COSODII (Imbrie et al., 1987) and the JOIDES Seal Level Working Group (Loutit, 1992). In particular,

• Each region has proved the age of sequence boundaries on margins can be determined to better than ±0.5 m.y.;

• Both regions have validated the "transect" approach of drilling passive continental margins (arrays of holes: onshore, shelf, and slope);

• The siliciclastic New Jersey margin and the carbonate Bahamas margin yield correlatable records of base-level change, as deduced from definitions of the chronostratigraphy of seismically observed stratal discontinuities; and

• Orbital-scale stratigraphic resolution has been achieved on continental slopes and carbonate platforms.

Despite these advances in dating sequences and linking them to glacioeustasy, there are major gaps in our understanding of amplitudes, response of sedimentation, and mechanisms that drive eustatic change. Only by drilling in the region most sensitive to sea level change, the paleo-nearshore zone to inner shelf region of a passive margin, can these gaps be filled.

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