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doi:10.2204/iodp.proc.345.101.2014

Drilling strategy

The highest priority for drilling at Hess Deep was to sample one or more 100 to ≥250 m long sections of primitive gabbroic rock. The Expedition 345 Scientific Prospectus (Gillis et al., 2012) identified three primary drill sites (proposed Sites HD-01B–HD-03B), with the intention that if coring was proceeding well in the first or second sites, it would be continued as long as possible in order to capitalize on good drilling conditions and obtain the longest possible continuous sample. Site survey data in conjunction with on-bottom observations collected during the expedition were used to plan specific hole locations in the vicinity of proposed Sites HD-01B–HD-03B.

We anticipated that drilling, coring, and logging operations in the Hess Deep Rift could be quite challenging. Thus, the operations team prepared a range of potential operational approaches that could be applied to address challenges that included water depths in excess of 4500 m, very thin sediment cover, initiating/maintaining a hole through the basement contact, and coring deeply through potentially unstable basement. These approaches included the possible deployment of a standard reentry cone, deployment of a free-fall funnel (FFF) or “nested” FFFs, bare rock hole without installation of a seafloor structure, and bare rock installation of a reentry cone with a short entry pup. In addition, the experiences of previous hard rock drilling expeditions were reviewed for best practices (see “Operations plan” in the Expedition 345 Scientific Prospectus [Gillis et al., 2012] for more details).

The initial focus for drilling during Expedition 345 was a flat-lying east-west–trending sedimented bench at ~4850 mbsl along the southern slope between the intrarift ridge and Hess Deep in an area previously shown to be dominated by primitive gabbro (Fig. F7). During the early stages of the expedition, permission was granted by the Environmental Protection and Safety Panel to expand the initial approved area of operations to the north of the bench to maximize drilling options along the southern slope of the intrarift ridge.

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