Alternate Drilling Strategy | Table of Contents


Site Location
The drilling program includes a total of five sites, SHAT-1, 2B, 3, 4, and 5C, three on the southern high of Shatsky Rise (SHAT-1, 2B, and 3) and one each on the central (SHAT-4) and northern highs (SHAT-5C) (Fig. 9; Table 1). Paleogene sediments will be recovered at all five sites, Upper (Coniacian-Maastrichtian) and mid-Cretaceous (Barremian-Turonian) sediments will be drilled at four sites (SHAT-2B, 3, 4, 5C), and pre-Barremian sediments and basement targeted at one site (SHAT-3). The five sites provide a depth transect between 2450 and 3900 m (current depths) for the Paleogene and mid Cretaceous and 2450 to 3300 m for the Upper Cretaceous. In addition, the sites provide a transect of nearly 8° of latitude. The stratigraphy of the southern high is well established but that of the other highs is poorly known (Sliter and Brown, 1993). However, the central and northern highs appear to have thicker sediment packages, particularly in the Paleogene, and less prominent seismic reflectors. The central and northern high sites also provide additional latitudinal spread.

The entire Paleogene section should be recoverable with APC coring (Table 1). Double APC coring will increase the stratigraphic completeness of the recovered sediments. Paleogene sediments will be recovered at all five sites. The post-Santonian Upper Cretaceous should have minor chert and thus will be recovered by APC and XCB coring at all five sites. The pre Santonian record contains chert and thus will be drilled using a revised XCB/motor driven core barrel (MDCB) strategy (see below).

Drilling Problems
The Paleogene and Upper Cretaceous sequence on Shatsky Rise is interrupted by several distinct unconformities. Some of the unconformities, such as that close to the Paleocene/Eocene boundary may have paleoceanographic significance. We have attempted to locate the primary sites in places where seismic data indicate that the section is more complete than at previous sites cored nearby (e.g., Site 305) and where the seismic expression of the unconformities appears to be reduced. Previous drilling suggests that the sequence is laterally variable over rather short distances (e.g., Heath, Burckle, et al., 1985). For this reason, we will move the ship laterally by hundreds of meters after the first APC if critical stratigraphic intervals such as the PETM are missing.

The presence of chert has significantly reduced core recovery in previous coring of the mid- and Lower Cretaceous sections at Shatsky Rise. It is almost impossible to recover the softer interlayers with the drilling rates required to penetrate the hard chert layers using standard rotary core barrel (RCB) coring. We plan to maximize recovery in the cherty interval using the MDCB. The depth of prominent chert layers will be determined in the first XCB hole, then these intervals will be cored and recovered using the MDCB in the second XCB hole. MDCB coring is slower than standard XCB coring, because the core can only advance 4.5 m and the system requires an extra wireline trip; however, this tool is housed in the same coring assembly as the XCB, allowing alternation of coring tools in hard-soft units without tripping the drill pipe. Our coring program includes double XCB coring of the cherty mid-Cretaceous section at SHAT-5C and 2B; if any time remains, we will also use this strategy at SHAT-3.

Alternate Drilling Strategy | Table of Contents