Drilling and coring strategy

To achieve the scientific objectives of Expedition 351, the chosen site location had to fulfill the following criteria:

  • There must be remnants of drillable oceanic crust that existed in the region immediately before arc inception.
  • The initial IBM magmatic record should be preserved and include geological evidence for inferring the tectonic setting at subduction initiation.
  • Temporal variations of magmatism in the rear IBM arc should be preserved as a sequence of volcaniclastic sediment and tephra.
  • To highlight the temporal evolution of the IBM arc crust, the effect of along-strike variation should initially be minimized or well understood.

The selected primary site (IBM-1) in the ASB, between the KPR and the Amami Plateau (Fig. F2), best meets the above criteria.

The overall operations plan for Site IBM-1 is summarized in Table T2. The operations time estimates are conservative and based on formation lithologies and depths inferred from seismic and regional geological interpretations, including prior drilling in the region at DSDP Sites 296, 445, and 448 and ODP Site 1201. We have approval from the Environmental Protection and Safety Panel (EPSP) to drill at Site IBM-1 to a total depth of 1600 meters below seafloor (mbsf) in case the velocity estimates are incorrect. We used this approved total depth during operations planning to ensure we would have enough time to drill to and core the igneous basement, which is one of the scientific objectives of this expedition. If the velocity estimates are correct, then the total depth of penetration will likely be closer to 1450 mbsf.

After departing from Yokohama, Japan, we will transit for ~2 days to Site IBM-1 and prepare for drilling operations. Our primary operations plan consists of drilling at Site IBM-1 to recover ~1300 m of sedimentary cover and ~150 m of basement. Based on previous sites in the area (Sites 296, 445, 448, and 1201), the sedimentary layers overlying basement are anticipated to consist of Pliocene–Pleistocene pelagic sediment, Miocene turbidites, Oligocene/Eocene volcaniclastic turbidites, and pelagic sediment of Eocene or older age (Table T1). The oceanic basement is likely to be Early Cretaceous age or older (i.e., Neotethyan).

First, Hole A will be cored to refusal using the APC followed by the extended core barrel. A jet-in test (Hole B) will then be conducted to determine a casing depth for the 20 inch conductor casing to be washed in with the reentry cone. After changing the bottom-hole assembly, we will then establish a new pilot hole in Hole C and core with the rotary core barrel (RCB) to ~900 mbsf to establish surface casing points and to determine actual basement contact depth. In Hole C, we will also drop a free-fall funnel (FFF) as part of our contingency plan (see “Risks and contingency”) to allow us to potentially reenter the hole. Finally, Hole D will be a reentry hole that includes penetrating ~150 m or more into the basement with the RCB. Based on drilling experience from ODP Leg 126 (Sites 787, 792, and 793), the sedimentary sections older than the Oligocene are likely to be well lithified and may not require casing below that depth (~600 mbsf). Actual casing points (casing string lengths) required will be determined while coring the RCB pilot Hole C.