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doi:10.2204/iodp.proc.324.101.2010

Coring and drilling strategy

The goal of Expedition 324 was to core as much igneous rock as possible in the time available. Science objectives dictated that coring be distributed over a large area of Shatsky Rise, sampling all three major edifices (Tamu, Ori, and Shirshov massifs) to study geochemical, geologic, and age trends. Furthermore, because Tamu Massif is postulated as a possible plume head eruption, it was considered important to sample several sites across that edifice to examine trends within. The original proposal contained six drill sites as targets. Because of time limitations imposed by the long transit assigned to Expedition 324, this plan was cut to five sites, with three located on Tamu Massif and one each on Ori and Shirshov massifs. Proposed Site SRSH-8 (not drilled) was planned on the southwest flank of Tamu Massif, Site U1347 at the eastern flank of the summit of that edifice, and Site U1348 was planned to sample the lower north flank. The other two sites, U1349 and U1346, were planned at the summits of Ori and Shirshov massifs, respectively.

Of these five sites, four were moved from the originally proposed locations to places where sediment cover was thinner for more rapid penetration of the sedimentary cover. In addition, the drilling plan, which only included rotary core barrel (RCB) drilling, was modified to drill through the uppermost sedimentary cover without coring until ~50 m above the presumed igneous basement. Although the primary purpose of this move was to speed up drilling through the sediment cover, it was deemed acceptable because the sediments have been cored on many drilling cruises and recovery is usually very low because of cherts interfingered with soft chalk in the Cretaceous part of the sediment cap.

Because of time limitations, coring deeper than would be possible with a single bit was proposed only in one location, Site U1347, on the summit of Tamu Massif. At three of the other sites, the strategy was to core only to ~100 m. For Site U1349, only a single bit was envisioned, but we would drill to bit destruction or ~200 m, whichever came first. Site U1347 was the only multiple-bit hole planned. At this site, two bits would be used, with a free-fall funnel (FFF) for reentry, allowing penetration of ~200–300 m into igneous basement.

Although three of the holes were to be short, downhole logging was planned at all sites. In each hole, two logging strings would be run, the triple combination (triple combo) and Formation MicroScanner (FMS). The triple combo would provide density, resistivity, and radiogenic isotope logs as well as caliper measurements that would assess the diameter and condition of the hole. Following the triple combo, the FMS log would be run to provide a resistivity-based image of the borehole wall. Often this log is run in tandem with the sonic log to acquire acoustic velocity data from the formation; however, because of the short length of planned Expedition 324 holes, the sonic log was not to be regularly paired with the FMS. At the planned deep-penetration site, U1347, an additional log, the Ultrasonic Borehole Imager (UBI), an acoustic imaging device, was planned to provide an additional image of the borehole wall.

The final transit for Expedition 324 was to the south and this dictated that the drilling proceed from the northernmost site (U1346) to the southernmost site (proposed Site SRSH-8) so that transit time would be minimized. This was not the ideal arrangement because Tamu Massif sites were given highest priority, but the lost time incurred by sailing back over Shastsky Rise on the final transit was unacceptable. This merely dictated that the science party keep close track of progress during the expedition so that operations did not get far behind the plan.

Although much of Expedition 324 went as planned, actual operations differed slightly from the plan because of weather and science-related changes (see "Operations" section in the appropriate site chapter). After visiting Site U1346 as planned, passing typhoon Choi-Wan forced the ship to sail southward, away from Ori Massif. As a result, Site U1347 was the second site cored. Sites U1348 and U1349 were subsequently drilled in reverse order because the ship approached them from the south, rather than the north as planned. Finally, proposed Site SRSH-8 was not cored at all. As the expedition waned, the science party decided that cores from this site would likely give samples of sheet flows similar to those at nearby Site 1213 (~50 km away). Instead, the science party decided to drill Site U1350, a proposed alternate site on the lower east flank of Ori Massif, as the last site during Expedition 324.

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