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doi:10.2204/iodp.sp.313.2009

Scientific objectives

1. Provide a testable record of eustatic variations.

Backstripping is a proven method for extracting amplitudes of global sea level from passive margin records (e.g., Watts and Steckler, 1979). One-dimensional backstripping is a technique that progressively removes the effects of sediment loading (including the effects of compaction) and paleowater depth from basin subsidence. By modeling thermal subsidence on a passive margin, the tectonic portion of subsidence can be assessed and a eustatic estimate obtained (Kominz et al., 1998, 2008; Van Sickel et al., 2004). Backstripping requires knowing relatively precise ages, paleodepths, and porosities of sediments, and each of these criteria are best obtained from borehole transects. Such transects also allow application of two-dimensional backstripping techniques that account for lithospheric flexural effects, increasing the precision of the eustatic estimates (Steckler et al., 1999; Kominz and Pekar, 2001). The eustatic component obtained from backstripping needs to be verified by comparing sea level records with other margins and those derived from δ¹⁸O estimates.

Drilling at Sites MAT-1 to MAT-3 will allow us to make precise late Oligocene to early middle Miocene eustatic estimates using one- and two-dimensional backstripping as described above. One- (Kominz et al., 1998; Van Sickel et al., 2004) and two-dimensional (Kominz and Pekar, 2001) backstripping of onshore New Jersey sites have provided preliminary amplitude estimates of 10–60 m for million year–scale variations, but the estimates are incomplete, particularly for the Miocene, because most lowstand deposits are generally not represented (Miller, Sugarman, Browning, et al., 1998; Miller et al., 2005a, and fig. F2 therein). Amplitude estimates derived from δ¹⁸O studies require assumptions about temperature and the sea level/δ_w calibration; although the uncertainties are large, initial eustatic estimates based on δ¹⁸O records are consistent with backstripping results (Fig. F1). Sites MAT-1 to MAT-3 are precisely located to recover as nearly a complete set of late Oligocene–middle Miocene sequences as possible and, through backstripping, provide a much more direct measure of the full range of amplitudes for this time interval.

Once we have obtained precise eustatic estimates from late Oligocene to early middle Miocene records at Sites MAT-1 to MAT-3, we will be able to extend our results to the older and younger records. Middle Miocene through recent sediments record similar clinoform geometries on the New Jersey shelf; by applying calibrations of seismic profiles and facies developed as part of this work, we should be able to derive eustatic estimates for the interval 16–0 Ma. In particular, deriving a firm, independent eustatic estimate from margin sediments will

• Allow us to test temperature assumptions needed to make glacio-eustatic estimates from δ¹⁸O records (Fig. F1),

• Provide an estimate of the Tertiary sea level/δ_w calibration, and

• Evaluate the Pekar (1999) and Pekar et al. (2002) calibration of 0.09‰/10 m (versus 0.11‰/10 m for the late Pleistocene) that was based on backstripping an incomplete coastal plain record.

Whereas both backstripping and δ¹⁸O methods make inherently large assumptions, the convergence of the two methods (Fig. F1) suggests that we will be able to produce a testable eustatic model for the past 42 m.y. and perhaps for the older record as well.

2. Test models of sedimentation on siliciclastic shelves.

Shallow-water records contain unconformities observed in outcrop or in the subsurface at all spatial scales, whether they divide beds or basins. Unconformably bounded sequences are the fundamental building blocks of the shallow-water record (Sloss, 1963; Van Wagoner et al., 1990; Christie-Blick, 1991). Researchers at the Exxon Production Research Company (Vail et al., 1977; Haq et al., 1987; Van Wagoner et al., 1988; Posamentier et al., 1988) claimed that similarities in the ages of stratal unconformities pointed to global sea level (eustasy) as the overriding control. The resulting "eustatic curve" has remained controversial (e.g., Christie-Blick et al., 1990; Miall, 1991), largely because of basic assumptions about the stratigraphic response to eustatic change and because the work relies in part on unpublished data. In response to this controversy, Christie-Blick and Driscoll (1995), among others, pointed out that the fundamental enterprise of interpreting the origin of layered rocks does not really require any assumptions about eustasy. They emphasized that sequence boundaries attest to changes in depositional base level. The timing of many of the Exxon Production Research Company sequence boundaries has been validated onshore New Jersey and correlated to the δ¹⁸O proxy of eustatic change (Miller et al., 1998, 2005a), though other sequence boundaries on this and other margins may be tectonically derived. Whether sequence boundaries are caused by changes in eustasy, local tectonism, or sediment supply (Reynolds et al., 1991), disconformable surfaces irrefutably divide the shallow-water record into sequences. Whatever their cause, these stratal breaks are real and they provide an objective means of analyzing the rock record.

Facies between sequence boundaries vary in a coherent fashion, and various facies models have been proposed for shelf sedimentation (e.g., Posamentier et al., 1988; Galloway, 1989). Much work has been done by the exploration and academic communities in testing and applying these models, and much has been learned. For example, flooding surfaces (particularly maximum flooding surfaces) can be used to unravel stratigraphic stacking patterns (e.g., Galloway, 1989), whereas highstand deposits are generally regressive and commonly serve as reservoirs for oil or water resources (e.g., Posamentier et al., 1988; Greenlee et al., 1992; Sugarman and Miller, 1997; Sugarman et al., 2006). Nonetheless, predictions of facies models have not been widely successful because they are the products of many unevaluated processes (Reynolds et al., 1991).

One major reason that models are still poorly constrained is that there has been no publicly available study of continuous cores across a prograding clinoform deposit that constitutes the central element of many facies models. As a result, the water depths in which clinoforms form and the distribution of lithofacies they contain are not well known. It is widely debated whether clinoform tops ever become subaerially exposed during sea level lowstands and whether the shoreline ever retreats to (or perhaps moves seaward of) the clinoform rollover (Fulthorpe and Austin, 1998; Austin et al., 1998; Steckler et al., 1999; Fulthorpe et al., 1999). Settling these controversies will have significant implications on our understanding of how sequence boundaries develop and how much of the facies distribution within clinoforms can be attributed to eustasy. Some workers assume that the shoreline is always located at the clinoform rollover (e.g., Posamentier et al., 1988; Van Wagoner et al., 1990; Lawrence et al., 1990). Others have presented models of basin evolution that suggest the shoreline and the clinoform rollover can move independently of each other (e.g., Steckler et al., 1993, 1999). The sea level estimates of Greenlee and Moore (1988) argue that sea level falls expose an entire continental shelf and that strata onlapping clinoform fronts are coastal plain sediments deposited during the beginning of the subsequent sea level rise. Many researchers (e.g., Steckler et al., 1993) stress that if strata onlapping clinoform fronts were deposited at or near sea level, then the clinoform heights dictate that sea level occasionally fell hundreds of meters in less than a million years; such magnitudes and rates are beyond the reasonable scales of any known mechanism for eustatic change. Extracting the amplitude of sea level fluctuations from sequence architecture is critically dependent on whether the lowest point of onlap onto sequence boundaries is truly coastal or is deeper marine. Determining water depths at the clinoform edge is essential to sequence stratigraphic models and understand this basic element of the dynamic land-sea interface. It can only be established by sampling, such as proposed here.

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