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doi:10.2204/iodp.proc.313.101.2010

Background

Several features of the New Jersey margin make it an ideal location to investigate the late Cenozoic history of sea level change and its relationship to sequence stratigraphy: rapid depositional rates, tectonic stability, and well-preserved cosmopolitan fossils suitable for age control 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. F1, F2).

Geologic setting

The U.S. middle Atlantic margin (New Jersey–Delaware–Maryland) is a classic passive margin. Rifting began in the late Triassic (~230 Ma; Sheridan and Grow, 1988; Withjack et al., 1998), and seafloor spreading commenced by the Callovian (~165 Ma; middle Jurassic). Subsequent tectonics have been dominated by simple thermal subsidence, sediment loading, and flexure (Watts and Steckler, 1979; Reynolds et al., 1991). In the region of the Baltimore Canyon Trough, the Jurassic section is composed of thick (typically 8–12 km) shallow-water limestones and shales. A barrier reef complex fringed the margin until the mid-Cretaceous (Poag, 1985). Accumulation rates were generally low during Late Cretaceous to Paleogene siliciclastic and carbonate deposition (Poag, 1985). A major switch from carbonate ramp deposition to starved siliciclastic sedimentation occurred in the late middle Eocene onshore to earliest Oligocene on the slope in response to global and regional cooling (Miller and Snyder, 1997). Sedimentation rates increased dramatically in the late Oligocene to Miocene (Poag, 1985; Miller and Snyder, 1997). The cause of this large increase is unknown, although it may reflect tectonics in the hinterland (Poag and Sevon, 1989; Sugarman et al., 1993).

Previous drilling

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, Ocean View, Bethany Beach, Millvile, Fort Mott, Sea Girt, and Cape May Zoo) has provided a chronology for sea level events over the past 100 m.y. (Miller et al., 1996a, 1998, 2005a). Sequence boundaries from 10 to 42 Ma defined on the basis of onshore facies successions, erosional criteria, and hiatuses have been shown to correlate (within ±0.5 m.y.) offshore to packages of seismic reflections linked to drill cores on the continental slope and, most importantly, to the history of glacio-eustatic lowerings inferred by the global δ¹⁸O record (Fig. F3). These correlations establish a firm tie 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 direct 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 Oligocene to middle Miocene global sequences published by EPR (Vail and Mitchum, 1977; Haq et al., 1987), although amplitudes of the accompanying sea level changes derived by the EPR group 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 the evidence for ice sheets back to a time previously considered to be ice-free: comparing Late Cretaceous to middle Eocene onshore hiatuses/sequence boundaries to the global δ¹⁸O record indicates that small ice sheets (<30 m sea level equivalent) waxed and waned in this supposedly ice-free world (Browning et al., 1996; Miller et al., 1998, 2005a, 2005b).

ODP drilling in the Bahamas (Leg 166 and supplementary platform drilling; Eberli, Swart, Malone, et al., 1997) has also provided a chronology of base-level lowerings in prograding carbonate sequences during similar time periods (Fig. F3). These findings represent complementary and supporting evidence to the New Jersey reports of global sea level change during the Miocene.

These independent data at the New Jersey and Bahamas margins validate the approach outlined by COSOD II (Imbrie et al., 1987), a JOIDES Sea Level Workshop (Watkins and Mountain, 1990), and the JOIDES Sea Level Working Group (JOIDES SL-WG, 1992). In particular,

• Both regions show that the age of sequence boundaries on margins can be determined to better than ±0.5 m.y.;

• Both regions have demonstrated the value of the "transect" approach to drilling passive continental margins (arrays of holes spanning onshore, shelf, and slope settings); and

• 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.

Despite these accomplishments, drilling on the New Jersey margin prior to Expedition 313 has not provided a complete history of sea level change for the Oligocene–Miocene interval. Although sequences tied to regional reflectors were cored and dated on the continental slope (ODP Leg 150; Mountain, Miller, Blum, et al., 1994), these efforts provided virtually no information about the amplitudes of past sea level change. Likewise, coastal plain drilling (ODP Legs 150X and 174AX; Miller et al., 1994, 1996b; Kominz et al., 2008, and references therein) led to valuable constraints on how high sea level rose during the last 100 m.y. (Fig. F4), but because of their updip locations, sites from these legs provided little information concerning the extremes of sea level lowstands. This was made clear when evidence of a prominent mid-Miocene eustatic fall drilled during ODP Leg 194 (at the Marion Plateau, northeast Australia; John et al., 2004) provided an estimate of 55 ± 15 m versus ~40 m from onshore backstripping (Kominz et al., 2008). This confirmed the suspicion that, because of their updip location, the New Jersey onshore sites do not capture the full range of Miocene sea level change.

Drilling into the Australian margin also had a serious limitation: it was in a dominantly carbonate province without the benefit of prograding, aggrading packages of siliciclastic sequences that record the cyclic nature of sea level rise and fall. All indications from offshore seismic data point to a lengthy and relatively complete record of sea level change in the shallow-shelf sediments of the New Jersey margin, making this an ideal location to take core samples. It has required incremental advances from initial attempts at drilling with the JOIDES Resolution on the outer shelf to finally using the L/B Kayd to bring this to fruition during Expedition 313.

Site selection

Setting a 173 ton lift boat down on the seafloor requires seabed assessment to establish sediment type, local topography, and proximity to any seafloor artifacts or natural subseafloor anomalies. Data relevant to each of these were collected by several groups with scientific grants from the U.S. National Science Foundation and the U.S. Office of Naval Research (ONR); additional data were acquired by the European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO) in March–April 2008.

Three multichannel seismic (MCS) surveys have crossed directly over Holes M0027A–M0029A (Fig. F1) (see the site chapters). A reconnaissance grid using a 120 channel, 6 air gun system aboard the R/V Ewing in 1990 was the first demonstration that Oligocene–Miocene clinoforms were well developed at this location (Fig. F2); the R/V Oceanus returned with 48 channel generator-injector (GI) gun and HiRes equipment in 1995 and collected remarkably improved images of these same features along line 529 (Fig F5). The R/V Cape Hatteras used identical HiRes gear in 1998 to concentrate on three grids of 150–600 m line spacing designed to provide detailed control on clinoform geometries, as well as to meet the guidelines established by the JOIDES Pollution Prevention and Safety Panel (Fulthorpe and Blum, 1992). A Simrad EM1000 swath-bathymetry/acoustic backscatter survey passed over the drill sites during an ONR-supported STRATAFORM study in 1996; Joint Oceanographic Institutions, Inc. (JOI)/United States Science Advisory Committee (USSAC) supported the collection of additional Simrad EM3000 data over each site in June 1999 (J. Goff and N. Driscoll, pers. comm., 1999). Grab samples within a few hundred meters of each site were collected during this same cruise (J. Goff, pers. comm., 1999). These site-assessment data were reviewed by the IODP Environmental Protection and Safety Panel (EPSP) for comment and input, and an evaluation of hazards posed by subsurface gas was completed for ESO by an independent contractor.

Reaching a long-standing goal

The overriding reason to return to the New Jersey margin is that drill cores on the shallow shelf can recover the lowstand sediments that (1) are missing in the coastal plain, (2) have been dated at slope boreholes, and (3) can be tied to the arrangement of composite siliciclastic packages seen in seismic profiles. Continuous coring in Holes M0027A–M0029A can provide estimates of eustatic amplitudes, a testable record of eustatic variations, and an opportunity to evaluate models that predict the nature and distribution of facies in passive margin strata. It is a rewarding accomplishment that goals developed over years of study during the ODP era and later incorporated in the IODP Science Plan have now been realized.

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