Shallow-water drilling—a new beginning

The formation of the Deep Sea Drilling Project (DSDP) in 1968 began an era of scientific exploration that has revolutionized our understanding of how Earth works. With bold ambitions, a converted drillship, and an innovative planning structure, geologists and geophysicists set out to retrieve hard facts from wherever their scientific questions led them. Four decades later we are still mining knowledge from the ocean basins with objectives, tools, and management features that have direct ties back to those prescient architects of the DSDP.

Despite successes in deep water, the difficulty in ensuring the ability to drill safely and effectively in shallow water left a large blank spot. This was not a matter of oversight or inadequate technology; DSDP purposely set out to explore the rich frontier of the deep ocean floor in the best manner possible, and that required using a drillship that was inappropriate for shallow-water objectives. Until 2003, earth scientists were confronted with a situation similar to what they had faced prior to 1968: we had uncovered tantalizing questions (concerning decadal climate change, land-sea links across shoreline systems, and factors that control the discontinuous buildup of shallow-water sediments, to name just three), but until that time there was no international means of organizing such a pursuit of these questions.

The Integrated Ocean Drilling Program (IODP) was conceived, in part, to address this need for more mission-oriented technology. This strategy has proved very successful, leading to breakthroughs in understanding previously unattainable objectives in complex, deeply buried seismogenic zones (Nankai Trough; IODP Expeditions 314/315/316, 319, and 322); ice-covered regions (the Arctic Ocean; IODP Expedition 302); and environmentally sensitive modern reefs (Tahiti; IODP Expedition 310) (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Cuerwitz, Masago, Moe, et al., 2009; Saffer et al., 2009; Underwood et al., 2009; Backman, Moran, McInroy, Mayer, et al., 2006; Camoin, Iryu, McInroy, et al., 2007). The New Jersey margin, because its geology is known and is easily accessible, is an especially attractive location for documenting sedimentation during times of large sea level change. Equipped with mission-specific platforms, the time is now for the ocean science community to make fundamental discoveries concerning the workings of this complex and fundamental set of Earth processes.

Eustasy as a global phenomenon

Understanding the history, cause, and impact of sea level change is a compelling goal of Earth system research. There is worldwide evidence of encroaching shorelines today, with the added concern that the rate of this change has been increasing over the last 50 y: ~1.8 mm/y in the last half of the twentieth century (Church and White, 2006) and ~3 mm/y at present (Cazenave et al., 2008). In many coastal regions the rate of rise is even higher because of the additional effect of local subsidence. Although this may be in part a natural pattern, there is overwhelming scientific evidence indicating this rate increase is due to human activities. The geologic record shows that global sea level has fluctuated by well over 100 m at rates as high as 20–40 mm/y (summaries in Donovan et al., 1979; Fairbanks, 1989; Stanford et al., 2006) at various times in Earth's history. Clearly we must learn about these past events if we are to prepare in any sensible manner for the sea level increase ahead of us.

IODP Expedition 313 does not address the centennial scale of eustatic variation; for that, strategies synthesizing tide gauge histories and Holocene marsh records are required. Instead, this study leads toward a broader understanding of the long-term behavior and wide-ranging effects of changes in the divide between land and sea. Throughout Earth's history, the transfer of energy and material across this boundary has profoundly influenced the interactions between the lithosphere, biosphere, 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 many processes 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 several study groups (e.g., Imbrie et al., 1987; JOIDES SL-WG, 1992; Quinn and Mountain, 2000): by sampling key facies across the prograding deposits of a passive continental margin, such as New Jersey.

Distinguishing eustasy from the effects of subsidence and changing 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, 1991; Miall, 1991; Catuneanu et al., 2009), and the intervening sedimentary sequences provide the basis for evaluating controls on sedimentary architecture and for predicting sedimentary facies and societally important resource distributions (e.g., hydrocarbons and potable water; Vail et al., 1977; Sugarman et al., 2005). 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, and many recent papers show the interest of the community in developing this approach (Catuneanu et al., 2009; Embry, 2008–2009; Neal and Abreu, 2009). But the fact remains that many of its fundamental assumptions have not been tested. For example, although the facies models of the Exxon Production Research Company (EPR) (e.g., Posamentier et al., 1988) are widely applied, the nature of facies associated with prograding clinoforms has not been documented (although participants of Ocean Drilling Program [ODP] Legs 166 and 174A made contributions to understanding facies associated with clinoforms). 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 proven.

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, 1996b, 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/δw 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; Camoin, Iryu, McInroy, et al., 2007), recovery and age control for records older than the late Pleistocene have posed very large challenges. 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), though the effects of total subsidence (compaction, loading, and cooling) as well as changes in sediment supply must be estimated.

Various facies models have been proposed to explain shelf sedimentation in response to eustatic changes (e.g., Posamentier et al., 1988; Galloway, 1989a, 1989b; and Embry, 2008–2009; among others), but the fact remains that the response of passive margin sedimentation to large, rapid sea level changes is poorly known. One of the main reasons for this lack of information 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 the sedimentary response requires knowledge of the depositional setting of strata that onlap sequence boundaries. 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 element in facies interpretation, has been inferred mostly through forward models, although tantalizing evidence recovered at ODP Hole 1071F (Leg 174A) suggests a marginal marine setting ~3.5 km landward of one upper middle Miocene clinoform inflection point (Austin, Christie-Blick, Malone, et al., 1998). Although continued analysis of Leg 174A sequences may shed new light on shelf facies models and their predictions from seismic data, these drilling results were limited by low core recovery and penetration of only upper middle Miocene and younger strata, hampering efforts to establish reliable facies models. Drilling in Holes M0027A–M0029A provides the information needed to properly evaluate these depositional facies models.

The importance of eustasy versus tectonism to the formation and preservation of sequences is a long-standing debate addressed by Expedition 313. 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). Backstripping analyses at 11 onshore boreholes (Kominz et al., 1998, 2008; Van Sickle et al., 2004; see summary in Miller et al., 2005b) have shown that active tectonism has played a minimal role in Cenozoic onshore deposition. By contrast, backstripping has shown ~30 m of excess subsidence at onshore Delaware wells versus corresponding wells in New Jersey due to the flexural load of 21–12 m.y. old sediments offshore Delaware (Browning et al., 2006). From this and other evidence they concluded the following:

  1. Eustatic change is a first-order control on accommodation space and provides a simultaneous imprint on all continental margins;
  2. Although tectonic movement of the crust can result in large stratigraphic gaps, no evidence of this effect is detected in Miocene sequences from New Jersey; and
  3. Second-order differences in sequences can be attributed to local flexural loading, particularly in regions that experienced large-scale progradation.