Geology of the New Jersey Continental Margin and Suitability for Sea-Level Studies

Figure 1 shows part of New Jersey and adjacent areas, a classic passive continental margin. Rifting began in the Late Triassic (Grow and Sheridan, 1988), and seafloor spreading commenced by the Middle Jurassic (~165 Ma; Sheridan, Gradstein et al., 1983). Subsequent subsidence has been governed primarily by lithospheric cooling and by flexural loading and compaction of accumulating sediment (Watts and Steckler, 1979; Reynolds et al., 1991). In the vicinity of proposed Leg 174A sites, the Jurassic section is composed of 8-12 km of shallow-water limestones and shales. A barrier reef complex fringed the margin until the mid-Cretaceous (Poag, 1985). Accumulation rates were generally low during latest Cretaceous to Paleogene, when the margin became starved of sediment and the shelf subsided to a depth of as much as several hundred meters below sea level (Poag, 1985). An abrupt increase in sediment supply in the late Oligocene led to the deposition of a series of unconformity-bounded wedges that built seaward during the Miocene and produced a shelf with a terraced morphology (Figs. 2 and 3). The cause of this change is not known, although it may reflect hinterland tectonics (Poag and Sevon, 1989; Sugarman et al., 1993).

This part of the U.S. Atlantic margin is well suited for the study of sea-level changes during the Oligocene to Holocene "Icehouse" interval for several reasons:

Progress on the Mid-Atlantic Transect

Before ODP could move to the adjacent New Jersey shelf, a grid of high-quality seismic data was needed to frame the objectives and locate optimal targets. Based on reinterpretation of Exxon Production Research multichannel seismic (MCS) data and well logs, Greenlee et al. (1992) published a refinement of previously identified Oligocene and Miocene depositional sequences and bounding surfaces (Greenlee and Moore, 1988). An MCS seismic program carried out aboard the R/V Maurice Ewing in November 1990 collected 3700 km of single-channel seismic (SCS) and MCS profiles (see MAT-13A site summary). The Ewing MCS profiles roughly doubled the number of prograding upper Paleogene-Neogene wedges that could be resolved using older seismic data (Fig. 3). Furthermore, this grid included dip lines that extended from the inner shelf to a position seaward of the shelf break. For the first time, "Icehouse" sequence boundaries could be mapped across the shelf to the slope. In 1995, the Ewing profiles were augmented by higher resolution MCS profiles, including detailed hazards grids, collected aboard R/V Oceanus (see Figs. 1 and 4). Locations of all but one site (MAT-13A; see Fig. 3 and site summary) proposed for drilling during Leg 174A were refined on the basis of analysis and interpretation of the new Oceanus data.

Leg 150 (June-July, 1993; Mountain, Miller, Blum, et al., 1994) capitalized on the Ewing shelf-to slope imaging and drilled four locations on the slope (Sites 902-904 and 906) at water depths of between 445 and 1134 meters below sea level (mbsl) (Fig. 1). These sites document the age and facies of sediments associated with a total of 22 lower Eocene to mid-Pleistocene reflecting surfaces tentatively interpreted as, or correlated landward with, sequence boundaries (see Fig. 5 for a synthesis of Oligocene-Miocene results). Integrated bio-, magneto-, and strontium-isotopic stratigraphy yield temporal resolution approaching several hundred thousand years (Miller et al., 1995). In most cases, interpreted sequence boundaries are associated with little or no temporal hiatus; many are expressed by a slight coarsening of sediment transported to the slope, it was presumed, during low stands of sea level.

To complement the Leg 150 results, MAT proponents launched a land-based drilling program with support from ODP, the National Science Foundation (NSF), and the U.S. and State of New Jersey geological surveys (Miller et al., 1994). Primary objectives of these onshore boreholes (Leg 150X and related; Fig. 1) have been to date Late Cretaceous to Cenozoic sequences, including the Paleocene-Eocene "Doubthouse" section, and to evaluate facies arrangements in an updip setting. Thus far, four holes have been cored and logged, all at sites close to the modern shoreline (Fig. 1). Oligocene to middle Miocene sequence boundaries in both onshore and Leg 150 boreholes appear to correlate with prominent d18O increases, consistent with the hypothesis that these surfaces developed during global lowering of sea level (Mountain, Miller, Blum, et al., 1994; Miller, Mountain et al., 1996a, b). The ages of these sequence boundaries also compare well with the timing of the Haq et al. (1987) "global" boundaries (Miller et al., 1996a,b). Recently completed drilling at Bass River, New Jersey (to be designated Leg 174AX) has recovered a complete Cretaceous/Tertiary boundary section (K.G. Miller, per. com., 1996).

Facies successions onshore are generally well developed for the Paleocene through middle Miocene, with a transgressive shell bed or glauconite sand at the base of each sequence and quartz sand at the top (upper part of the highstand systems tract; Miller et al., 1994). Onshore drilling has, thus, provided important data for regional profiles, but all of the boreholes are landward of the Oligocene-Miocene clinoforms imaged in seismic reflection data beneath the shelf (Figs. 2 and 3). The shelf sites, to be tackled for the first time as the focus of Leg 174A, are the ones most critical for estimating amplitudes of sea-level change during the "Icehouse" interval (see below).

Available Data
The New Jersey margin is one of two study areas recently selected by the U.S. Office of Naval Research (ONR) for a multiyear initiative it has termed "Strata Formation on Margins" (STRATAFORM). Together with studies of the contrasting margin off Northern California, the goal is to understand the range of factors affecting the deposition and preservation of shelf and slope stratigraphy (Nittrouer and Kravitz, 1995). Off New Jersey, the missions of STRATAFORM and MAT coincide.

As a result, both ONR and JOI supported a consortium of investigators from the University of Texas Institute for Geophysics (UTIG), Lamont Doherty Earth Observatory (LDEO), and Rutgers University to collect, analyze, and interpret high-resolution MCS data on the New Jersey shelf and upper slope in support of proposed drilling in summer 1995 (Fig. 1). These data, which include a series of detailed "hazards" seismic grids mandated by ODP (Fig. 4), augmented a substantial set of regional geophysical and geological data that includes (Fig. 1): (1) 60-fold MCS profiles collected by the R/V Maurice Ewing from the inner shelf to the rise (see Fig. 3 and MAT-13 Site Summary); (2) 2D and 3D single-channel seismic (SCS) seismic grids (using a Huntec deep towed system) and associated vibracores collected by UTIG in 1989 and 1993; (3) commercial MCS profiles collected during the 1970's in a dense grid (~2.5 km line spacing) across the outer shelf and upper slope (Fulthorpe and Austin, in press); and (4) multibeam bathymetry/backscatter coverage of the whole area using a commercial Simrad EM-1000 system (Goff et al., 1996).

The 1995 MCS survey consisted of two interwoven track plans and missions: (1) hazards-type survey grids at eight proposed shelf sites to meet MAT goals set by the JOIDES Pollution Prevention and Safety Panel (hazards surveying was funded by both JOI and ONR); Sites MAT 8B and -9B (Fig. 4) were approved for drilling by both JOIDES and ODP/TAMU safety panels in September 1996; and (2) a regional grid (Fig. 1) across the outer shelf and upper slope (funded by ONR), to achieve both STRATAFORM objectives and MAT goals by tying Leg 150 sites to the shallower shelf stratigraphy to be sampled by Leg 174A.

In conjunction with the earlier seismic data (Fig. 1), these profiles will allow us to: (1) determine the configuration of buried stratal surfaces and their accompanying acoustic characteristics across a wide range of depositional environments; (2) establish links among the various elements of the STRATAFORM initiative; and (3) tie well-dated Leg 150 sequences and other upper slope data to coeval shelf (Leg 174A) and onshore (Legs 150X/174AX and related) sections.

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