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doi:10.2204/iodp.sp.341.2011

Drilling and coring strategy

We propose to drill a depth transect in Miocene–modern continental margin clastic strata (continental margin glacimarine, turbidites, and hemipelagic and diatomaceous clastic strata) using a combination of sites in water depths from 178 to 4218 m and drilled to ~150–1158 m depths. The operations plan and time estimates for the primary and alternate sites are summarized in Tables T2 and T3, respectively. Time estimates are based on formation lithologies and depths inferred from seismic and regional geological interpretations, including prior drilling in this area (Leg 18 and industry; Risely et al., 1992). After departing from Victoria, British Columbia, Canada, we will transit for ~4 days to the Gulf of Alaska and prepare for drilling operations.

The proposed drilling strategy begins with drilling at proposed Site KB-2A followed by proposed Sites GOAL-15C, GOAL-17B, GOA16-1A, and GOA18-2A.

At Site KB-2A, three holes will be cored with the advanced piston corer (APC) to refusal or ~200 meters below seafloor (mbsf). A fourth hole will be cored with the APC/extended core barrel (XCB) ~400 mbsf. Nonmagnetic core barrels will be used in all APC sections, and the orientation of APC cores will be measured with the FlexIt tool in Hole A.

At Sites GOAL-15C and GOAL-17B, the first hole at each site (Hole A) will be APC cored to refusal (~250 mbsf) followed by XCB coring to ~1112 mbsf (GOAL-15C) or ~1032 mbsf (GOAL-17B). We are approved by the Environmental Protection and Safety Panel (EPSP) to drill 10% deeper than these depths in case the velocity estimates are incorrect. Nonmagnetic core barrels will be used in all APC sections. The orientation of APC cores will be measured with the FlexIt tool at Site GOAL-17B.

At Site GOA16-1A, three holes will be cored with the APC to ~200 mbsf. A fourth hole will be APC/XCB cored to ~978 mbsf. At Site GOA18-2A, two holes will be cored with the APC to ~200 mbsf. A third hole will be APC/XCB cored to ~780 mbsf. A fourth hole will be APC/XCB cored to ~425 mbsf. While drilling/coring in Hole A for both these sites, a number of advanced piston coring temperature tool (APCT-3) and sediment temperature tool (SET) measurements will be made, as formation conditions permit, to complement existing data. Nonmagnetic core barrels will be used in all APC sections, and the orientation of APC cores will be measured with the FlexIt tool in Hole A.

After coring is completed at each site, holes will be conditioned, displaced with logging mud, and logged as per the logging plan (see “Downhole measurements strategy”).

Drilling operations in shallow water require special precautions to ensure safety for crew and equipment. Weather conditions (particularly sea state and resulting heave behavior of the vessel) are critical (see “Risks and contingency”).

Risks and contingency

There are a number of risks to achieving the objectives of this program. Weather and the nature of the sediments in the Gulf of Alaska may present operational risks that could negatively impact coring/logging operations, hole stability, core recovery, core quality, and rate of penetration. Expedition 341 has been scheduled to take place during the summer, the optimum weather season; however, severe weather may still occur and could adversely impact operational efficiency and transit speed. Additionally, the possible presence of ice rafted debris, chert layers, and glacial deposits could lead to bent core barrels and damaged APC shoes and XCB bits. A series of alternate sites are available for contingency operations.

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