Coring and drilling strategy

Site selection

The primary drilling strategy for Expedition 330 is to core 350 m (or deeper) into the volcanic basement of four Louisville seamounts in order to retrieve the largest possible number of lava flows to cancel out secular variations in the paleomagnetic measurements. By scientific preference we will drill the Louisville seamounts from oldest to youngest (proposed Sites LOUI-1C to LOUI-4B) because we expect to see the largest paleolatitude shift in the oldest targeted seamounts. Our principal science objectives are to acquire accurate paleolatitudes, geochemistry and isotope compositions, and 40Ar/39Ar age dates for four Louisville seamounts that have formation ages similar to those of the Emperor seamounts drilled during Leg 197. In addition, we have generally selected drill sites where seismic lines cross midway between the center of the targeted flat-topped seamounts (i.e., guyots) and their margins, a compromise designed to avoid eruptive centers (which may harbor fewer lava units) while at the same time avoiding marginal sites with potentially thicker volcaniclastic units and higher potential for tectonic disruption. The Louisville seamounts targeted for drilling are relatively small structures (particularly proposed Sites LOUI-2B, LOUI-3B, and LOUI-4B) that have typically thin sediment cover and few subsidiary peaks and pinnacles that we interpret as late-stage (posterosional) lavas.

Four primary sites (Fig. F1; Table T1) on four flat-topped seamounts at the oldest end of the Louisville Seamount Trail were selected. Based on the new 40Ar/39Ar ages from the AMAT02RR site survey, we estimate the ages of these dormant volcanic structures to be 75–77 Ma (proposed Site LOUI-1C on the 26.5°S guyot), 58.5 Ma (proposed Site LOUI-2B on the 33.7°S guyot), 54.0 Ma (proposed Site LOUI-3B on the 36.9°S guyot), and 50.1 Ma (proposed Site LOUI-4B on the 168.6°W guyot).

Seven alternate sites (proposed Sites LOUI-1B, LOUI-6A, LOUI-7A, LOUI-7B, LOUI-8A, LOUI-8B, and LOUI-9A) were selected (Fig. F1; Table T2). Except for the 36.9°S guyot, which has a second set of crossing lines close to primary proposed Site LOUI-3B, most of the alternate sites are located on different but closely neighboring seamounts (also surveyed during site survey Cruise AMAT02RR) in order to stay as near as possible to our original age-selection criterion and our overall drilling strategy.

Coring plan

We propose to drill four sites into shield-building lavas at the summits of four Louisville seamounts and guyots (Table T3). Drilling and logging plans for each of these sites are similar, with basaltic basement penetration of 350 m planned for proposed Sites LOUI-1C, LOUI-2B, and LOUI-4B and nearly that much (~330 m) for proposed Site LOUI-3B. All primary and alternate drill sites are capped with a thin layer of pelagic sediments (on average 10–20 m thick, but as thick as 50 m in the case of alternate Site LOUI-8B) that will be cored with the RCB using a gravity-push technique (if applicable) to maximize sediment recovery for paleoceanographic studies. This soft sediment is possibly underlain by a sequence of volcaniclastics that are perhaps interspersed with some basaltic lava flows and/or carbonates. Although it is unlikely that thick sequences of coral rocks were formed between the paleolatitude at which the oldest sediments were deposited (close to the present-day ~50°S latitude of the Louisville hotspot) and the northernmost location of the seamounts today (~26°–38°S), we will still account for this possibility because dredging has occasionally recovered coral debris. From the seismic cross sections we estimate that this sequence is maximally ~55 m thick (0.025–0.035 s two-way traveltime) below each primary drill site and ~110 m below each alternate drill site before transitioning into a seismically opaque zone that we interpret to represent basaltic basement, our principal 350 m total drilling and coring objective.

Table T3 summarizes estimated drilling and logging times, showing a total operation time (on site time) of 45.1 days. Penetration of several hundred meters into basement will require multiple bit changes and reentries. We assume basalt penetration rates of 1.5–3.5 m/h depending on drill depth (consistent with average 3.1 m/h rates achieved during Leg 197; Tarduno et al., 2002) and the use of three RCB drill bits per hole (two bit trips) for proposed Sites LOUI-1C, LOUI-2B, and LOUI-4B. Because of time constraints, only one bit replacement is currently planned for proposed Site LOUI-3B.

The time estimates shown in Table T3 are based on the assumption that we will use free-fall funnels (FFFs) for reentry. The use of FFFs carries the risk that we may have to terminate drilling early if reentry fails or the sidewalls collapse. As a backup, reentry cones and single-string casing may be used (depending on availability onboard).

The number of lava flow units encountered during Expedition 330 drilling will determine whether 350 m of basement penetration is adequate, because we seek to recover a sufficient number of these flow units to average out secular variation and attain nominal 2σ uncertainties of <5° in the paleolatitudes (Fig. F8). Approval of additional penetration of basement (>350 m), however, will need to be reviewed by the operator (USIO) during drilling operations. On the other hand, depending on sufficient coring results, we may decide to terminate a hole before reaching the 350 m basement penetration target, permitting us to begin drilling elsewhere early. We will apply real-time monitoring of onboard paleomagnetic data, geochemical data, and physical volcanology to recognize basaltic (independent cooling) flow units and determine when the most effective termination depth has been reached.