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doi:10.2204/iodp.sp.342.2011

Drilling strategy

Expedition 342 will begin with a 2 day transit to ODP Site 1073 to conduct a 2 day engineering test of the newly developed Motion Decoupled Hydraulic Delivery System (MDHDS), a device that should enhance the quality of formation temperature and pressure measurements (see “MDHDS tool testing”). After an additional ~4 days of transit, the vessel will arrive at the main operations area (Fig. F14). An overview of primary and alternate drill sites, including links to figures showing site positions along the single-channel, high-resolution seismic survey lines, is given in Table T1. Additional operational planning detail is given in Tables T2 (primary sites) and T3 (alternate sites).

Our primary goal is to drill a depth transect in high deposition rate, middle Eocene–?upper Eocene and ?Oligocene drift sites. Our depth transect consists of six drill sites, each of which will be cored three times to obtain continuous spliced sections (Tables T1, T2). We propose to start with the deepest end of the depth transect on J Anomaly Ridge to pin the deep end of the depth transect (Fig. F1) This first site (JA-1A) will be drilled 400 meters below seafloor (mbsf) through the middle and ?upper Eocene sediment package into the lower Eocene and Paleocene (Figs. F4, F6, F10). Two additional sites (JA-14A and JA-5A) will be drilled to ~250 mbsf on this same drift higher on the flank of J Anomaly Ridge (Figs. F4, F6, F10). These sites are intended to sample a very expanded record of the ?upper Eocene and upper middle Eocene. We will then move to Southeast Newfoundland Ridge to drill the ?upper Eocene and ?Oligocene boundary sections at Site SENR-16A to 400 mbsf (Figs. F4, F7, F11). Next, we will move to Site SENR-11A, where we will drill the upper part of one of the plastered drifts on the north flank of one of the seamounts to sample the ?upper Eocene and middle Eocene (300 mbsf) (Figs. F4, F15). We will then finish drilling operations by coring the ?upper Eocene to middle Eocene pelagic cap of the seamount (Site SENR-19B; 250 mbsf) (Figs. F4, F16). All holes will be triple cored, with the option to extend two holes to 400 m subbottom and log them. Advanced piston corer (APC) penetration will be maximized by “drilling over” the APC barrel until this is no longer safe or practical, at which point the extended core barrel (XCB) will be used to achieve the depth objective.

Two alternative scenarios have also been considered and may be implemented in some form depending on the results from Sites JA-1A and SENR-16A. In both alternate scenarios we would drill both Sites JA-1A (targeting the Paleocene–Eocene drift section at the deep end of the transect) and SENR-16A (to target the EOT).

In the first alternative scenario, we may find that drilling at Site JA-1A makes a compelling case to prioritize Paleocene–lower Eocene sections over upper Eocene drift deposits. A Paleocene–lower Eocene depth transect could be obtained by drilling a 400 m section at alternate Site JA-15A, a 400 m section at alternate Site SENR-18A, and a 200 m section at alternate Site SENR-1B (Figs. F4, F6, F17, F18). This strategy would yield a four-point depth transect only slightly (280 m) shorter than the transect focused on middle Eocene–?upper Eocene sediments. However, we would likely encounter more unconformities in the section (likely near the lower Eocene/middle Eocene boundary) and more chert.

In the second alternative scenario, we may find that drilling results make a compelling case to prioritize capturing a continuous high deposition–rate section in the middle Eocene–Oligocene record over the depth transect aspect of our plan. This would involve drilling Site JA-1A (400 m penetration) and then a combination of Site JA-14A (250 m penetration) in the middle Eocene–?upper Eocene, followed by ~200 m penetration at alternate Site JA-15A (to the lower Eocene/middle Eocene boundary) (Figs. F4, F6, F10). The combined record of Sites JA-14A and JA-15A would provide a complete record of the acoustically transparent drift package on J Anomaly Ridge. Coring would then switch to Site SENR-16A to obtain the 400 m thick Oligocene–upper Eocene section, followed by Site SENR-11A to 400 m to obtain the lower part of the middle Eocene (Figs. F4, F7, F10, F15). If time allowed, we would core the pelagic cap at either Sites SENR-19B or SENR-1B (both ~200–250 m penetration; Figs. F4, F16, F17, F18). We would obtain a four-point depth transect with this strategy in the middle Eocene and a three-point transect in the ?upper Eocene as long as there were time to core both pelagic caps. A further possibility under this alternative scenario targeted at capturing a continuous high deposition–rate section in the middle Eocene to Oligocene would involve drilling alternate Site SENR-10A to 200 m, depending on the results of drilling at Site SENR-16A.

Our drilling strategy depends on maintaining a balance between (1) multiple coring at sites to obtain continuous sections, (2) collecting long enough records to capture numerous “critical boundaries,” (3) maximizing the total depth range captured by our coring transect, (4) obtaining logging records to fill gaps in discontinuously cored sections, and (5) providing log data to create synthetic seismograms to tie cores to seismic records. Two additional considerations are (1) to produce logs that will help us tackle the problem of differential compaction in cores and (2) to maximize the logged record relative to hole depth. These objectives are not fully compatible because all require trade-offs in operations time. We rank these priorities in favor of (1) obtaining the depth transect, (2) maximizing sedimentation rate in local sections instead of targeting long time series records, (3) collecting spliced records of physical core, (4) obtaining logging records to bridge core gaps, and (5) constructing synthetic seismograms for core-seismic integration. We rank the effort to tackle the differential compaction problem and acquire logs in shallow holes as a lower priority.

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