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doi:10.2204/iodp.sp.313.2009

Proposed drill sites

Optimal locations of Sites MAT-1, MAT-2, and MAT-3

The region between the paleo-shoreline and the paleo-inner to middle shelf is the most sensitive region for studying past sea level variations and must be sampled to obtain estimates of eustatic amplitudes. Reliability of these estimates depends on the precision of paleowater depths determined by lithologic and benthic foraminiferal criteria. Both of these are optimal indicators in nearshore to middle neritic facies but become less precise in facies deeper than middle neritic (>100 m) paleodepths (see examples in Miller and Snyder, 1997). Sections deposited in nearshore to inner neritic environments (<30 m paleodepth) are difficult to date, even though the facies associations may be clearer and the paleodepth resolution is best. Work onshore New Jersey has shown that the best results can be obtained by targeting sequences deposited between 0 and 60 m paleodepth (Kominz and Pekar, 2001). Following these guidelines, as well as concepts developed by the JOIDES Sea Level Working Group (Loutit, 1992), the ideal drilling locations are outlined in Figure F5.

Sites MAT-1 to MAT-3 target upper Oligocene to middle Miocene seismically imaged prograding clinoforms that were deposited in inner–middle neritic paleodepths (based on coeval onshore strata deposited in nearshore/prodelta settings). We have obtained excellent seismic profiles of these clinoforms (Fig. F6) across the regions that record the full amplitude of sea level change: immediately landward of and near the toes of the clinoforms (i.e., across the clinoform inflection point). Modern water depths at Sites MAT-1 to MAT-3 are ~34 m (Fig. F7; Table T1), a fortunate "crossover" depth between being too far landward for detailed control on sequence geometry (i.e., thorough seismic control on land is not possible) and too far seaward for affordable commercial drill rigs.

Site location

Three sites have been selected that lie along a dip-line transect (Fig. F7) roughly 45 to 60 km offshore New Jersey in 32–35 m water depths; primary locations are Sites MAT-1A, MAT-2D, and MAT-3A (Table T1).

Available site survey data at Sites MAT-1 to MAT-3

Three MCS surveys have passed within 1 km of proposed Sites MAT-1 to MAT-3 (IODP Site Survey Databank):

1. A reconnaissance grid using a 120-channel, 6-air gun system aboard the R/V Maurice Ewing in 1990 was the first demonstration that Oligocene–Miocene clinoforms were well developed at this location.

2. The R/V Oceanus returned with 48-channel, generator-injector gun, high-resolution seismic equipment in 1995 and collected remarkably improved images of these same features along Line 529.

3. In 1998 the R/V Cape Hatteras concentrated on three grids of 150–600 m line spacing designed to provide detailed three-dimensional control on clinoform geometries, as well as to meet the guidelines established by the JOIDES Pollution Prevention and Safety Panel (1992).
   

These data have been studied to determine the location of any subsurface features that may pose a hazard to drilling (amplitude anomalies suggesting trapped gas, faults that could serve as conduits for deep-seated hydrocarbons, or indicators of unstable settings for a jack-up rig).

A Simrad EM1000 swath-bathymetry/acoustic backscatter survey passed over the proposed drill sites during an U.S. Office of Naval Research–supported STRATAFORM study in 1996. In June 1999 Joint Oceanographic Institutions, Inc./U.S. Science Advisory Committee (USSAC) supported the collection of additional Simrad EM3000 bathymetry and seafloor grab samples across Sites MAT-1 to MAT-3. A review of all data, including reprocessing of the Cape Hatteras seismic data, was carried out by an independent site survey company (Guardline) for a gas hazard report. No significant gas hazards were found to affect any of the potential drilling locations. Finally, as a final examination for seabed hazards and characterization, additional profiling, magnetometer, and vibracore data were collected by Alpine Geophysical in Spring 2008 and each proposed site was determined safe for drilling.

Coring strategy

The "New Jersey/Mid-Atlantic Sea Level Transect" (NJ/MAT) was designed as a series of boreholes from the onshore New Jersey Coastal Plain across the shelf to the slope and rise (Miller and Mountain, 1994) (Figs. F1, F2) with the following goals:

1. Date major "Icehouse" (Oligocene–recent) sequences, a time of known glacio-eustatic change (Miller et al., 1991), and compare ages of the unconformable surfaces bracketing these sequences with times of sea level lowerings predicted from the δ¹⁸O glacio-eustatic proxy;

2. Estimate the amplitudes, rates, and mechanisms of sea level change; and

3. Evaluate sequence stratigraphic facies models (e.g., systems tracts) (Posamentier et al., 1988, Catuneanu, 2006) that predict depositional environments, sediment compositions, and stratal geometries in response to sea level change.
   

Leg 174A shelf drilling (Austin, Christie-Blick, Malone, et al., 1998) targeted similar upper Miocene–Pliocene clinoforms beneath the modern outer shelf, demonstrated that the multiple-site transect strategy is valid, and attempted to yield precise eustatic estimates across one upper Miocene sequence. Success was hampered by unstable hole conditions that led to moderate to poor recovery and the inability to reach sediments older than 12 Ma. Proposed shallow shelf Sites MAT-1 to MAT-3 (Figs. F6, F7) are ideally located to sample sequence boundaries both landward and seaward of several clinoform inflection points and to test the amplitudes of, and facies models for, late Oligocene to middle Miocene sea level changes. Prime targets include sequence-bounding late Oligocene reflection "o1" to middle Miocene "m4" (~28–14 Ma) (Monteverde et al., 2000).

Site location details

For site location details, see Tables T2, T3, and T4. Total sediment penetration for the three New Jersey margin sites is 2250 m.

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