Optimal biostratigraphic resolution is achieved in deep-water settings of the continental slope and rise, where open-ocean marine fauna are best represented in pelagic sediments (i.e., seaward of the hypothetical profile shown in Fig. 6). However, geometric evidence for the existence of a sequence boundary (offlap and onlap) is best preserved near the offlap break (Site MAT-8B), which in the case of the Oligocene to Miocene sequences of the New Jersey margin is located beneath the modern shelf (Fig. 3). Individual sequences tend to thin seaward as a result of condensation (sediment starvation) in deeper water. This, and locally marked erosion in the vicinity of the continental slope, lead to uncertainties in the tracing of sequence boundaries from the shelf to the slope, and hence to uncertainties in the calibration of those surfaces independent of inherent limitations in biostratigraphic resolution.
For this reason, an additional site (Site MAT-9B) was identified near the clinoform toe of sequence boundary m1 beneath the shelf (Fig. 3 and MAT-9B site summary). Drilling locations near the clinoform toe for each sequence boundary (i.e., center vertical line in Fig. 6) sample an expanded section that is sufficiently close to the offlap break for the stratigraphic position of the sequence boundary to be well established. Furthermore, reflection geometry indicates that any hiatus associated with sediment bypass along this surface, or erosion into it, is minimized. As a practical matter, such a site is also selected as far as possible landward of any "lowstand" sands resting on the sequence boundary, because these are likely to contain fauna that were reworked from shallower water.
Estimating amplitudes of sea-level change is best undertaken by "backstripping" a profile across the shelf (Steckler and Watts, 1982). This requires information from multiple sites about stratigraphic thickness, age, composition (to account for the effects of flexural loading and compaction), and paleobathymetry (from paleoecology and the interpretation of lithofacies). As the amplitude of the sea-level signal is small compared to the thickness of the strata in which it is recorded, the potential errors are large. Estimates of sea-level change are especially sensitive to errors in paleobathymetry.
Key drilling locations are therefore (1) close to the offlap break for each sequence boundary (i.e., lefthand vertical line in Fig. 6), which provides the most complete record of paleobathymetric change in the underlying sequence; and (2) in the vicinity of the corresponding clinoform toe (i.e., center vertical line, Fig. 6), for the most complete record of the overlying sequence. In some cases, it may also be possible to obtain information from sites in between. Future drilling at such sites would help to resolve whether any of the onlap against clinoforms is "coastal onlap" (e.g., Greenlee et al., 1992), implying exposure of the offlap break during sequence-boundary development, or whether it is entirely marine onlap. Existing data on sedimentary facies and a consideration of likely amplitudes of glacial-eustatic change tend to favor the latter view.
The MAT ultimately requires drilling into the slope, shelf, and coastal plain (Fig. 1); all are underway (Fig. 5). Slope drilling by Leg 150 has provided the "deep-water" age control. Onshore drilling (Legs 150X/174AX and related) has supplied updip facies control at multiple locations (Miller et al., 1994, 1995; Miller and Sugarman, 1995), and is continuing. Drilling on the intervening shelf, the primary focus of Leg 174A, is critical both to the dating of sequence boundaries at geometrically favorable sites and to the estimation of amplitudes of eustatic change.
Sites MAT-8B and MAT-9B
Proposed shelf Sites MAT-8B and -9B are the focus of Leg 174A operations (Table 1); these are located to optimize sampling in the vicinity of the offlap break and clinoformal toe of sequence boundary m1 ("Tuscan" of Greenlee et al., 1992; upper middle Miocene of Mountain, Miller, Blum et al., 1994; Figs. 3 and 5). These sites will also sample the updip and somewhat condensed shallow-water portions of younger sequences and the deeper-water portions of older sequences. A key priority is to sample at least to the level of surface m3 (middle middle Miocene) at both sites. If time and conditions permit, drilling will continue through the Miocene section and perhaps into the lower Oligocene (horizon o1) at one or both of the shelf sites. Our primary goal, however, is to achieve stratigraphic precision rather than to reach some pre-approved target depth (TD) below m3. It is vitally important to be able to tie seismic and borehole data as far as possible without needing to resort to arguments about sedimentary facies. This is necessary to test existing sequence stratigraphic models (e.g., Vail, 1987; Posamentier et al., 1988; Greenlee et al., 1992) for the New Jersey margin. Our intent therefore, in addition to attempting to achieve complete core recovery in multiple holes, is to undertake Logging While Drilling (LWD) and a check-shot survey/Vertical Seismic Profile (VSP) at each site. Wireline logging at least at one site, probably Site MAT-8B, for comparison with the LWD results, is also envisioned.
Sites MAT-13A to D
Proposed slope Sites MAT-13A to D are secondary sites (Table 2) that will be occupied only in the event of safety problems and/or adverse weather conditions too inclement for shallow-water drilling operations. These sites are designed to date the middle Miocene to Pleistocene sequence boundaries (particularly m1 and younger horizons; Fig. 3) and to evaluate Pleistocene sequence stratigraphy. We propose VSPs and LWD at one or more of these sites, if for some reason these operations are not conducted at Sites MAT-8B/-9B.
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