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doi:10.2204/iodp.pr.317.2010

Operational strategy

Drilling transect strategy

To understand the history of eustasy versus subsidence/sediment supply changes, borehole transects across passive continental margins are required (COSOD II, 1987). The long-term strategy developed by ODP-related planning groups (Watkins and Mountain, 1990; Loutit, 1992; Fulthorpe et al., 2008) involves drilling margins worldwide to evaluate global synchroneity, through correlation among multiple basins and with the oxygen isotopic record, and to document stratigraphic responses in diverse tectonic and depositional settings, including carbonate, siliciclastic, and mixed siliciclastic–carbonate sedimentary systems on both continental and oceanic crust. Initial investigation was to focus on the Neogene "Icehouse" period (Miller et al., 1991), when high-resolution chronological control is available and glacial cycles provide a well-understood mechanism for eustatic change, calibrated by the deep-ocean oxygen isotope record. This approach has guided ODP efforts off New Jersey (Leg 150: Mountain, Miller, Blum, et al., 1994; Miller and Mountain, 1994; Leg 174A: Austin, Christie-Blick, Malone, et al., 1998; Legs 150X and 174AX: Miller et al., 1994; Miller, Sugarman, Browning, et al., 1998) and the Bahamas (Leg 166: Eberli et al., 1997). IODP Expedition 317 represents the latest application of this global strategy.

The passive margin approach integrates seismic profiles and a drilling transect to calibrate the sequence stratigraphic model and test the global sea level model, including investigation of local controls on sequence formation. Seismic profiles provide identification of sequence boundaries, sequence architecture, seismic facies, and morphologic constraints on depositional processes and tectonism. A drilling transect is required to document (1) ages of sequence stratigraphic surfaces, including sequence-bounding unconformities (or their correlative conformities) and maximum flooding surfaces; (2) facies and lithologies comprising each sequence (stratigraphic response to sea level oscillations); (3) porosity, cementation, and diagenesis; and (4) paleowater depths from benthic biofacies. Such paleowater depth estimates are essential for determination of eustatic amplitudes. Two-dimensional modeling of these data within the sequence stratigraphic framework allows estimation of eustatic amplitudes when the form of the tectonic component of subsidence is known (Kominz and Pekar, 2001).

The ideal approach involves drilling target sequences in at least two locations. First, drilling landward of clinoform breaks or rollovers, presumed to represent paleoshelf edges, provides information on facies and water depths where paleodepth indicators are most reliable. For example, ODP Leg 174A drilling recovered valuable paleoenvironmental information diagnostic of a shallow-water lagoonal environment landward of a rollover (Austin, Christie-Blick, Malone, et al., 1998). Two-dimensional backstripping will enable the use of data from sites farther landward to constrain water depths at more basinward locations. Second, drilling on the slope where pelagic microfossils are more abundant provides sequence boundary ages. Ideally such sites would also provide the paleoenvironment and facies of the lowstand systems tract. This is also essential for eustatic amplitude estimation in the event that sea level fell below the preceding clinoform break. The ideal location for both dating and lowstand facies is near the clinoform toe, to minimize the hiatus at the sequence boundary (Christie-Blick et al., 1998), but locations higher on the slope are necessary to reduce drilling depths (e.g., the location of Leg 174A New Jersey margin Site 1072 relative to sequence boundary m1[s]) (Austin, Christie-Blick, Malone, et al., 1998). In addition, locations on the slope will provide better constrained paleowater depths, which are likely to be poorly constrained at clinoform toes (as is the case at the Clipper well). A further reason for drilling higher on the slope is that seismic correlation from clinoform toes landward to the clinoform front and shelf is difficult on all margins because the section basinward of the clinoform toes is condensed and landward divergence of reflections contributes to mis-ties.

General operations plan

The transect approach requires that drilling operations be conducted on the modern shelf to reach sequence paleoshelves landward of their rollovers, as well as the upper paleoslopes of the oldest sequences. ODP Leg 174A drilling on the New Jersey shelf illustrated the difficulty of shelf drilling with a dynamically positioned drilling vessel (Austin, Christie-Blick, Malone, et al., 1998). The main problem was the collapse of loose sands, which trapped the drill string, although station keeping in shallow water was also occasionally difficult. The proposed drilling strategy for Expedition 317 was influenced by the Leg 174A experience, which relied heavily on rotary core barrel (RCB) drilling, particularly at shelf Site 1072. As a result, the Expedition 317 precruise operations summary presumed extended core barrel (XCB) refusal at ~350 mbsf, with RCB coring in a separate hole to total depth.

In practice, it was possible to use the XCB to core to ~1000 m at shelf Site U1351 before significant difficulty was encountered. RCB drilling was therefore unnecessary at Expedition 317 shelf sites. In contrast, the greater penetration depth required at Site U1352, on the upper slope, necessitated the use of RCB coring in a separate hole.

Favorable weather conditions on arrival allowed us to follow the original drilling strategy by drilling first at shelf Site U1351 at the deepwater end (121 m) of the shelf transect (Fig. F7; Table T1). This provided us with experience in shelf-sediment drilling before we moved to sites in even shallower water. Although the target depth at Site U1351 was 1249 mbsf, actual penetration of 1030 m nevertheless exceeded expectations in view of the difficulties encountered in penetrating the section during Leg 174A. Unless otherwise noted, all depth references in meters refer to the core depth below seafloor computed by conventional method A (CSF-A) depth scale (see "IODP depth scales terminology" at www.iodp.org/program-policies/). Only the two deepest sequence boundaries targeted (U4 and U5) were not reached, although recovery was limited, averaging ~30% and generally decreasing with depth.

The ship then moved ~15 km to slope Site U1352, which was selected to provide good age control for sequences drilled on the shelf at Site U1351 (Fig. F3). An additional objective at this site was the penetration and recovery of the Marshall Paraconformity, which had been estimated to lie at ~1600 mbsf (Fig. F9).

On completion of slope drilling, we moved back to the shelf to drill the two additional shelf Sites U1353 and U1354 (Fig. F7) to provide spatial control of facies within sequences and to recover the lowermost unconformities landward of their rollovers.

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