Staff Scientist: Mitch Malone
Co-Chief Scientists: James Austin, Jr. & Nicholas Christie-Blick
Abstract
Leg 174A will core four holes off the coast of New Jersey to investigate the Oligocene Holocene history of sea-level change. The New Jersey shelf margin is an ideal location for this research for several reasons: rapid sedimentation, tectonic stability, good chronostratigraphic control, and abundant reconnaissance-quality seismic, well log, and borehole data (Miller and Mountain, 1994). The general goals of the New Jersey Mid Atlantic Transect (MAT) are to determine the geometry and age of Oligocene to Miocene depositional sequences, and to evaluate the role of relative sea-level changes in developing this record. When completed, the New Jersey Mid-Atlantic Transect will evaluate possible causal links between glacioeustatic changes inferred from the deep-sea 18O record and depositional sequences from this "Icehouse World."
The Leg 174A transect (Fig. 1) is designed to sample several Oligocene to Holocene shelf sequences (unconformity-bounded depositional units) in three locations: (1) immediately landward of each clinoform inflection point; (2) at the toe of each clinoform; and (3) at a distal setting well seaward of the clinoforms (Fig. 2). The first location provides facies and paleodepths of the toplapping portion of a target sequence as well as the thickest record of the underlying sequence. The second location at the clinoform toe establishes the paleodepth and facies of the lowstand systems tract. Drilling both locations allows estimation of the amplitude of relative sea-level change associated with each sequence boundary using methods similar to Greenlee et al. (1988). Oligocene to Holocene clinoforms are beneath the modern New Jersey shelf and none of these shelf boreholes have been drilled to date (Fig. 3). The third, distal location for transect boreholes can be reached beneath the modern continental slope where samples have more pelagic microfossils and provide optimal conditions for dating the age of sequence boundaries. Furthermore, properly chosen slope sites promise a more continuous record than can be expected on the adjacent shelf. Leg 150 drilled four slope sites that provide the age control needed for most of the Oligocene-Holocene sequences (e.g., Miller et al., in press). Only one additional slope site (proposed and previously approved Site MAT-13) is needed to complete this task.
Two primary goals of Leg 150 were to date major Oligocene to Holocene sequences on the New Jersey slope and rise and to evaluate their correlation with glacioeustatic-age estimates obtained from the 18O record. Multichannel and single channel seismic grids allow the tracing of seismic sequences from the shelf (e.g., the Tuscan, Yellow, Blue, Pink, and Green sequences of Greenlee et al., 1992) to the slope (Miller and Mountain, 1994). To evaluate sequence ages, Leg 150 drilled four sites on the New Jersey continental slope that allow direct dating of seismic reflectors traced from the continental shelf (the Greenlee et al. [1992]) shelf reflector nomenclature has been superseded by names developed by the Leg 150 party and will be used from here on). Leg 150 met its major goal by integrating Sr isotopic stratigraphy (Miller et al., in press) with planktonic foraminifer biostratigraphy (Snyder et al., in press), nannofossil biostratigraphy, and magnetostratigraphy (Van Fossen and Urbat, in press); this provided a chronology of Oligocene to middle Miocene sequences (Miller et al., in press). In addition, Leg 150 evaluated the response of slope deposition to changes in sea level. For example, Mountain et al. (in press) evaluated the incision, maintenance, and demise of a buried middle Miocene slope canyon.
Primary objectives of the New Jersey onshore boreholes (Leg 150X, Fig. 1) were to date Cenozoic sequences (including Paleocene-Eocene Doubthouse sequences) and to evaluate facies models in an updip setting. Drilling at Island Beach, Atlantic City,, and Cape May, NJ, exceeded expectations (Miller et al., 1994, in press). In particular, Oligocene to middle Miocene sections at Atlantic City and Cape May represent two of the best-dated sections for sea-level studies (Miller and Sugarman, in press), whereas the Paleocene-Eocene at Island Beach provides a remarkably complete and well-dated section (Browning et al., in prep.). Oligocene to middle Miocene sequence boundaries on shore correlate well with major 18O increases, suggesting that these unconformities were cut by global sea-level lowerings. In addition, these unconformities correlate with the slope sequence boundaries drilled during Leg 150, suggesting a causal link between onshore and slope sequences (Mountain, Miller, Blum, et al., 1994; Miller et al., in press). Facies successions (systems tracts) are generally well developed for the Paleocene through middle Miocene onshore sequences, with a shell bed or glauconite sand at the base (=transgressive systems tract) and quartz sand at the top (=highstand systems tract; Miller et al., 1994). Thus, onshore drilling provided sections needed to evaluate systems tracts successions, although the boreholes are updip of the Oligocene to Miocene clinoforms and do not provide information on clinoform facies or constraints on the amplitudes of relative sea-level variations.
The Oligocene to Holocene is an interval in which continental margin sequences may be directly compared with eustatic estimates obtained from 18O studies (e.g., Miller et al., 1991). The Leg 174A transect will evaluate possible causal links between ice-volume (glacioeustatic) changes inferred from the deep sea 18O record and depositional sequences from this "Icehouse World." The goals of this leg are to:
Site MAT-7B
Proposed Site MAT-7B (Fig. 4) will focus on recovering Paleocene-Eocene strata for two important reasons: (1) to calculate amplitudes of Oligocene-Holocene sea-level changes, and (2) to examine mechanisms of sea-level change in the Paleocene-Eocene.
1) Amplitudes
Accurate knowledge of basin subsidence is an essential element in modeling the amplitude of sea-level change. Sediment compaction is a major component of total subsidence history, and to measure it accurately, one must have porosity information as far into the sub-bottom as possible, ideally well below the actual clinoform that is used to measure sea-level change. Equivalent strata without the concerns of drilling in water depths less than 75 m can be reached at reasonable depths on Ew9009 MCS Line 1002 where we propose Site MAT-7B. We have preserved the "7" designation in this site to emphasize that this location is the equivalent of Site MAT-7 in determining the facies across the m3 sequence boundary.
2) Paleocene-Eocene "Doubthouse"
Penetration below the Icehouse interval at MAT-7B will also address a secondary goal of the transect by providing valuable information about the Paleocene-Eocene "Doubthouse." This is a time for which glacioeustatic forcing is a questionable factor in sequence development. This aspect of sequence stratigraphic studies has been highlighted by results from the onshore Island Beach borehole that obtained an excellent record of Paleocene-Eocene sequences (Miller et al., 1994; Browning et al., in prep.) and by recent onshore drilling of an Eocene-Oligocene dip transect on shore in Alabama and Mississippi (Miller et al., 1993; Baum et al., 1994; Dockery et al., 1994). Additional sites are needed to evaluate Paleocene Eocene sequences on this and other margins; Site MAT-7B will be the only site that recovers this section off the shore of New Jersey.
Sites MAT-8B and MAT-9B
Proposed Sites MAT-8B and -9B will focus on upper Miocene clinoforms m1 and younger (Tuscan of Greenlee and Moore, 1988; Greenlee et al., 1992). LWD at both sites is proposed.
Site MAT-13B
Proposed Site MAT-13B (Fig. 5) is a slope site designed to date Miocene to Pleistocene sequence boundaries (particularly m1 that is still poorly dated) and to evaluate Pleistocene sequence stratigraphy. We propose LWD at this site.
Available Data
The New Jersey margin is one of two study areas recently selected by the U.S. Office of Naval Research for a multi-year initiative that it has termed "Stratal Formation" (STRATAFORM). Together with studies of the contrasting margin off Northern California, the goal is to understand the range of factors affecting shelf and slope sedimentation (Nittrouer and Kravitz, 1995). Modern processes will be linked to the seismically imaged and sampled (preserved) record through an evolution of increasingly sophisticated models. The key will be the collection of "nested" geophysical and geological data using a variety of tools whose individual temporal and spatial scales overlap to form a wide-ranging continuum of measurements. Clearly, the missions of STRATAFORM and the Sea Level Transect coincide offshore New Jersey.
Consequently, both ONR and JOI are supporting investigators from LDEO, UTIG, and Rutgers to collect and interpret high-resolution MCS data on the New Jersey shelf and upper slope in June-July, 1995. The new data will build on a substantial set of regional geophysical and geological data that includes: (1) 60-fold MCS profiles collected by the Maurice Ewing (Figs. 1, 3, 4, 5) from the inner shelf to the rise; (2) Huntec 2D and 3D seismic grids and associated vibra-cores collected by UTIG in 1989 and 1993; and (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 that are now undergoing analysis and interpretation at UTIG.
A survey in 1995 consisted of two interwoven track plans and missions: (1) hazards-type survey grids at proposed shelf sites (to meet Transect goals, funded by JOI and by ONR); and (2) a regional grid across the outer shelf and upper slope (to meet STRATAFORM goals funded by ONR, and to meet ODP goals by tying the shallow stratigraphy together).
Together these profiles will: (1) determine the nature of buried stratal surfacesand their accompanying acoustic characteristics across a wide range of depositional environments; (2) provide links among the various elements of the STRATAFORM initiative; (3) tie well-dated ODP Leg 150 sequences and other upper slope data to coeval shelf and onshore sections; and (4) pave the way for ODP to drill a number of shelf sites off the shore of New Jersey.
The aforementioned data should assess the degree of safety of New Jersey shelf drilling with respect to trapped "shallow" gas and migration of thermogenic hydrocarbons upward along faults. Still to be resolved is the issue of seafloor hazards (e.g., pipelines, shipwrecks, etc.). Proponents have already coordinated the proposed site locations with cable location services at AT&T, and there are no cable hazards at any site. The ONR/STRATAFORM schedule calls for swath mapping and associated backscatter coverage to be collected during the spring summer of 1996 (by Larry Mayer and his group at the University of New Brunswick). These data will be incorporated into the overall hazards assessment prior to drilling.
The very good to excellent core recovery at slope sites on Leg 150 (88% mean) was due largely to the abundance of fine-grained sediments; however, problems arose whenever sands were encountered. Sand is likely to be much more prevalent at the shelf sites, and logging will consequently take on a particularly important role in meeting the objectives of facies characterization. Even in mudstones, Leg 150 operations relied exclusively on the Side-Entry Sub (SES) technique of wireline logging, which left the pumps online during the logging operation so that fluid circulation was available to clear downhole obstructions. Unfortunately, SES cannot be used at sub-bottom depths greater than the water depth, and hence will not be available at any of the proposed Sites MAT-7 through 9. Logging While Drilling is a cutting-edge technology still on a steep development curve in the oil industry. It was used successfully on Leg 156 on the Barbados accretionary wedge, and it will be used again on the Costa Rica and Barbados accretionary prisms on Legs 170 and 171B, respectively. Although LWD has drawbacks (cost; lack of sonic, FMS, and geochemical log data), it is rich in positives: (1) in borehole conditions typical of ODP operations, it is likely to provide the best gamma-ray, density, porosity, and caliper logs possible by measuring these data within minutes of being drilled (the sensors are a few meters above the bit); (2) it is nearly certain to save time over wireline logging, which requires drilling to TD, and then logging a potentially unstable hole from there back up to ~100 mbsf; and (3) LWD provides log data right up to the mudline. LWD will provide logging details of 0-100 mbsf that will be crucial to tying the Pleistocene of Site MAT-13 to that of Sites MAT-9 and -8B (Fig. 6). In lieu of missing sonic, FMS, and geochemical data, and as a "log-log integration" plan, we propose to conduct standard wireline logging and LWD at the deepest site drilled (MAT-7B). We are confident that the velocity data provided by this one "sonic calibration hole" will yield sufficient detail for extrapolating to the other shelf sites, provided each of them have LWD porosity and density logs. To Leg 174A Proposed Site Information