Drilling and coring strategy

Our operations plan for this expedition consists of drilling three sites to ~100 m into basement (Sites SCS-3G, SCS-6A, and SCS-4B). In order to complete these three sites within the amount of time available for the expedition, we need to drill (without coring) through the uppermost 900 m of Site SCS-6A, which is the highest priority site. If the Environmental Protection and Safety Panel (EPSP) does not permit this and full coring is necessary, our backup operations plan will be substituted. This plan includes coring at two sites where basement can be reached at shallower depths of penetration and consists of Sites SCS-3F, SCS-6A, and SCS-4C. In addition, we have proposed a number of other alternate sites that reach basement at a range of penetration depths to allow flexibility during the expedition should we be unable to achieve our objectives at any of the primary sites.

Proposed drill sites

All proposed sites are located within areas floored by oceanic crust, except for alternate Site SCS-1C, which has the potential to core the continent–ocean transition zone (COT) or attenuated continental crust. This makes the proposed sites different from those drilled previously during Leg 184 (Wang, Prell, Blum, et al., 2000), which are all located on the continental shelf and slope and targeted scientific problems primarily related to sedimentation, paleoceanography, and paleoclimate.

Sites SCS-6A (primary), SCS-6B, and SCS-6C (alternates)

Sites SCS-6A and SCS-6B are located near the northern COB and are chosen to recover the oldest oceanic crust and the oldest sedimentary rocks in the East Sub-basin to test the hypothesis that the opening of the SCS occurred here first around 32 Ma (Figs. F2, F4, F7). This part of the basin shows the deepest basement and is likely the oldest among the sub-basins based on magnetic anomalies (Taylor and Hayes, 1980, 1983; Pautot et al., 1986; Briais et al., 1993). Both sites are also located near magnetic Anomaly 11, the oldest anomaly identified by Taylor and Hayes (1980) and Briais et al. (1993) and hence will provide key calibrations between ages estimated from magnetic anomalies and those from biostratigraphy, radiometric dating, and magnetostratigraphy.

These two sites are located ~60–65 km south of Site 1148, which is located within the COT and recovered an important stratigraphic sequence spanning the Oligocene to present (Wang, Prell, Blum, et al., 2000; Li et al., 2004; Wang and Li, 2009). These sites together will help address key problems in the early tectonic transition and associated paleoenvironmental changes from rifting to drifting.

Site SCS-6C is located on a basement high between the extended continental crust and the oceanic basin (Fig. F7). Similar conspicuous basement high features can be found on the COB in many other seismic profiles and therefore represent an important tectonic structure. The true lithology and formation mechanism of this basement high is still speculative; it could be a volcanic extrusion associated with early continental breakup and the onset of seafloor spreading, extruded lower crust materials from preferential lower crust extension, exhumed mantle materials, or a basement high composed of Mesozoic rocks. Coring at this location will help pinpoint the exact nature of this structure and improve our understanding of early seafloor spreading processes.

Sites SCS-3G (primary), SCS-3F (backup primary), SCS-3E, SCS-3H, and SCS-3I (alternates)

These five sites are located in the East Sub-basin. Primary Site SCS-3G is located near the relict spreading ridge and the youngest magnetic anomaly (Figs. F2, F4, F7), and coring here will help determine the termination age of seafloor spreading in the East Sub-basin. The thick overlying sediments at this site will also provide important constraints on the evolution of the ridge and associated late-stage magmatism. The alternate sites will achieve similar goals to those for primary Site SCS-3G. Alternate Site SCS-3H, south of the spreading center, is located near identified magnetic Anomalies C5e and C6. These sites will allow correlation of ages from magnetic anomalies to biostratigraphic, magnetostratigraphic, and radiometric ages. Composition and magnetic susceptibility measurements from basement rocks will help to explain the sharp differences in magnetic amplitudes and strikes between the East and Southwest Sub-basins. Furthermore, this site will allow testing of the hypothesis that the East Sub-basin formed in an area already floored by oceanic crust (Pautot et al., 1986).

Sites SCS-4B (primary), SCS-4C (backup primary), SCS-4D, SCS-4E, and SCS-4F (alternates)

Owing to the marked contrast between the Southwest and East Sub-basins (Fig. F4; Table T1) (Yao, 1995; Jin et al., 2002; Li et al., 2007b), it is justifiable to question whether rifting and drifting within these two sub-basins was synchronous or diachronous and how they evolved in comparison to the Northwest Sub-basin. Sites SCS-4B and SCS-4C are located in the Southwest Sub-basin near the relict spreading center and magnetic Anomaly C5e identified by Briais et al. (1993) (Figs. F2, F4, F8). Together with Sites SCS-3G and SCS-3H in the East Sub-basin, coring at one of these sites will help to explain the sharp differences in magnetic amplitude and strike between the East and Southwest Sub-basins and test the hypothesis that the Southwest Sub-basin was initiated by continental rifting, a formation mechanism that might be in sharp contrast to that of the East Sub-basin (Pautot et al., 1986). Coring will help determine the age of this sub-basin and correlate ages from magnetic anomalies with biostratigraphic, magnetostratigraphic, and radiometric ages. The apparent weak magnetizations in basement rocks will be examined via studies of chemical compositions and measurements of magnetic susceptibility.

Alternate Sites SCS-4D, SCS-4E, and SCS-4F are located northwest of Sites SCS-4B and SCS-4C. Together they form a sampling transect in the Southwest Sub-basin (Fig. F9) and would achieve similar objectives to those of the primary and backup primary sites in this sub-basin. These alternate sites are all located on basement highs, with sedimentary cover no thicker than ~300 m. Based on our interpretation of seismic and regional magnetic data, we conclude that these basement highs are not younger volcanic extrusions; they show clear magnetic anomalies typical of other oceanic crust (e.g., Fig. F4). In particular, Sites SCS-4E and SCS-4F are located on the uplifted shoulders of the relict spreading center, and rock samples cored here will place immediate constraints on cessation of seafloor spreading of the Southwest Sub-basin and on the terminal processes of oceanic crustal accretion and postspreading volcanism.

Sites SCS-2C and SCS-2D (alternates)

As mentioned above, two striking magnetic anomalies (M1 and M2) in the East Sub-basin further divide the sub-basin into a central part with strong magnetic strengths bounded by M1 and M2 and two weakly magnetized parts (C1 and C1′, respectively) near the two conjugate continental margins (Fig. F4). Based on traditional interpretations (Taylor and Hayes, 1980, 1983; Briais et al., 1993), these two anomalies correspond to magnetic Anomaly C8 (~26 Ma), which is most likely linked to a large-scale magmatic and compressional event all along the Chinese continental margin (Wang, Prell, Blum, et al., 2000; Li et al., 2004; Su et al., 2010; Li and Song, 2012). Ridge jump is another favored explanation, although Briais et al. (1993) suggested that a minor ridge jump occurred after magnetic Anomaly C7 rather than during C8.

Alternate Sites SCS-2C and SCS-2D are located near magnetic Anomalies C7 and C8 (Figs. F2, F4). At these sites, we will test various hypotheses regarding the age and mechanism of the presumed ridge jump through an integrated age calibration with magnetic anomalies, biostratigraphy, radiometric dating, and magnetostratigraphy and test alternative hypotheses on regional transformations in magmatism and stress field (and consequently in spreading rate). Magnetization measurements will help us to investigate the causes of the sharp increase in the magnetic amplitude of magnetic Anomaly C8.

Site SCS-1C (alternate)

This site is located in magnetic Zone A of the northeasternmost SCS near the Manila Trench (Figs. F2, F4). This northeastern area is characterized by 11–15 km thick crust that is either thinned and magmatically modified continental crust (Nissen et al., 1995; Wang et al., 2006) or thick oceanic crust (Hsu et al., 2004; Yeh et al., 2010). Furthermore, part of the area interpreted as thick oceanic crust may be as old as 37 Ma, some 5+ m.y. older than other oceanic crust of the SCS (Hsu et al., 2004), whereas an adjacent area is possibly a remnant of the proto-SCS (Yeh et al., 2010). The impact of coring here could be very substantial if initial spreading is confirmed to have begun around 37 Ma, which could eliminate some of the existing models proposed for the SCS opening. Conversely, if thin continental crust is cored, it would document a globally significant 150–200 km wide zone of extended crust. This result would also offer better understanding of the rifting history of this margin, including the transition from Mesozoic subduction to Paleogene rifting and subsequent spreading.

In addition to answering questions about the fundamental nature of the northeastern SCS, coring in this area would also test contrasting regional tectonic models that suggest different sizes for the initial SCS. Most tectonic reconstructions argue that the northern SCS passive margin extended northeast of present-day Taiwan, substantially larger than at present (Teng, 1990; Hall, 2002; Clift et al., 2003); however, Sibuet and Hsu (1997) and Sibuet et al. (2002) argued that prior to 15 Ma the Ryukyu subduction zone extended south to the northeasternmost SCS. In this interpretation an abandoned portion of the Philippine Sea plate, formerly subducting beneath the southwest extension of the Ryukyu arc, may exist in this area southwest of Taiwan. Drilling at Site SCS-1C would directly test this hypothesis.

Drilling and coring operations

The overall primary operations plan and time estimates for Expedition 349 are summarized in Table T3. We have also developed a backup operations plan, which is summarized in Table T4. Multiple alternate sites have been selected for each of the primary sites to allow for the best chance to achieve the expedition objectives within the amount of time available. Proposed operations plans for all sites available for the expedition are summarized in Table T5. Time estimates are based on formation lithologies and depths inferred from seismic and regional geological interpretation, including prior drilling in the area during Leg 184 (Wang, Prell, Blum, et al., 2000). After departing from Manila, Philippines, we will transit for ~1 day to the first site and prepare for drilling operations.

Primary operations plan

In order to address the expedition objectives to date the onset and cessation of seafloor spreading in the SCS and to examine the differences between the East and Southwest Sub-basins, the primary operations plan includes coring 100 m into basement at three sites: SCS-3G, SCS-6A, and SCS-4B (Table T3). We propose a basement penetration of 100 m at each site to guarantee obtaining the amount of fresh material necessary for studying lithology, geochemistry, physical properties, and affiliations of basement rocks. Additionally, drilling ~100 m into basement should recover enough lava flows (20–45) to average out secular variations in the paleomagnetic measurements and to establish the paleolatitude of each site.

Site 1148 (ODP Leg 184) and Sites SCS-6A and SCS-3G (Expedition 349) together form a sampling transect from the COT to the relict spreading ridge (Fig. F7). Reaching basement at Sites SCS-3G and SCS-6A would allow dating of the oldest and youngest oceanic crust in the East Sub-basin. This drilling strategy is strongly endorsed by the participants of the 2012 SCS international workshop held at Tongji University (People′s Republic of China; Li et al., 2012). Coring to basement at a site in the Southwest Sub-basin (Site SCS-4B) will address questions regarding the evolution of different sub-basins of the SCS, which show distinct differences in magnetic character (Fig. F4). Together, coring basement rocks at these three primary sites would sample a range of distinct mantle and crustal compositions in different sub-basins, offering a means to investigate such important issues as the geochemical evolution of the mantle from the onset to the cessation of seafloor spreading, temporal and spatial scales of mantle convection, and lithosphere-mantle interactions.

There is unanimous support from the 2012 SCS international workshop for a deep hole to obtain the oldest ocean crust near the northern COB (Li et al., 2012). Sites SCS-6A and SCS-6B (alternate) are designed to achieve this goal. These sites require a thick penetration of up to ~1930 mbsf. In order to recover the oldest oceanic basement basalts and the oldest sediments deposited at the onset of seafloor spreading, but at the same time retain reasonable core recovery and on-site time, the primary operations plan adopts the following strategy:

  1. Complete coring at Site SCS-3G first. This site requires a total penetration of 1061 m (961 m of sediments and 100 m of basement), with the oldest sediments estimated to be ~15 Ma. At this site we plan a single hole, with advanced piston corer (APC) coring to refusal (~200 mbsf), followed by XCB coring to refusal. A free-fall funnel (FFF) will be dropped to allow for reentry before changing to the RCB, which should reach the target depth of 100 m into basement.

  2. At Site SCS-6A, drilling without coring to ~900 mbsf and then coring the remaining ~1030 m with the RCB. A reentry system including three strings of casing will be required at this site for deep coring and logging. Drilling through the top 900 m requires approval by the EPSP; if this strategy is not approved, then complete coring of the section will be required (see “Backup operations plan”). Drilling without coring is proposed because Site SCS-3G will record essentially the same shallow sedimentary section that will be drilled through at Site SCS-6A. Furthermore, Site 1148 is only ~65 km north of Site SCS-6A, and the sedimentary section recovered at Site 1148 can provide some guidance to the shallow lithologies of Site SCS-6A (Figs. F2, F4, F7).

  3. Complete coring of Site SCS-4B, which requires a total penetration of 965 m (865 m of sediment and 100 m of basement). A single hole is planned for this site, with APC coring to refusal (~200 mbsf), followed by XCB coring to refusal. After dropping a FFF, the hole will be reentered with the RCB to reach the target depth of 100 m of basement.

Nonmagnetic core barrels, drill collars, and shoes/core catcher subs (if available for the RCB) will be used for APC and RCB coring operations at all sites. In addition, the FlexIT tool will be deployed above the core barrel during APC operations so that the cores can be oriented.

Backup operations plan

Drilling through the top ~900 m of sediment at Site SCS-6A, as proposed in the primary operations plan (Table T3), requires approval from EPSP and the Texas A&M Safety Panel, which has not yet been granted. In case either of these panels denies this request, we have developed a backup operations plan (Table T4), as complete coring at Site SCS-6A will take a substantial amount of the operations time available for the expedition. Because reaching basement at Site SCS-6A (or alternate Site SCS-6B) is one of the highest priorities of the expedition, full coring will significantly reduce the amount of time available to reach basement at two other sites. Thus, the backup operations plan includes sites where basement can be penetrated at shallower depths than the sites included in the primary operations plan. The backup operations plan includes coring at Sites SCS-3F, SCS-6A, and SCS-4C. The backup operations plan adopts the following strategy:

  1. Complete coring at Site SCS-3F, which requires penetration of 197 m of sediment and 100 m of basement. This may be completed in one hole, with APC coring to refusal, which should recover the entire sedimentary section, although XCB coring will be used if APC coring does not reach basement. A FFF may be dropped to allow for reentry with the RCB to core 100 m of basement.

  2. Complete coring at Site SCS-6A in three holes. The first hole will be cored to refusal with the APC (~200 mbsf), followed by XCB coring to refusal. The hole will then be conditioned and logged with the triple combo and FMS-sonic tool strings (see “Downhole measurements strategy”). The second hole will be to detect the depth of the seafloor and to perform a jet-in test for the 20 inch casing string that will be deployed as part of the reentry system in the third hole. A reentry system with three strings of casing will be deployed in the third hole to the depth reached in the first (APC/XCB) hole. The RCB will then be used to reach the target depth of ~1930 mbsf, with one bit change planned at ~1830 mbsf before coring basement.

  3. Complete coring at Site SCS-4C, which requires penetration of 689 m of sediment and 100 m of basement. This will be completed in one hole, with APC coring to refusal (~200 mbsf), followed by XCB coring, which should recover the remainder of the sediment section. A FFF will be dropped and the hole reentered with the RCB to recover ~100 m of basement.

Nonmagnetic core barrels, drill collars, and shoes/core catcher subs (if available for the RCB) will be used for APC and RCB coring operations at all sites. In addition, the FlexIT tool will be deployed above the core barrel during APC operations so that the cores can be oriented.

Table T5 summarizes the proposed operations for all 16 sites (primary and alternate) approved for Expedition 349. This table also summarizes the different operational options for Sites SCS-6A and SCS-6B (drilling through the top ~900 m or full coring). Multiple alternate sites were identified near each of the proposed primary sites because reaching and coring 100 m of basement at the primary sites is operationally challenging (see “Risks and contingency”). Many of these alternate sites would reach basement at shallower penetration depths, offering options during the expedition should reaching basement at any of the primary sites not be possible.

Risks and contingency

A number of undersea telecommunications cables cross the SCS. Although none are located in the vicinity of the proposed sites, a camera survey will be performed at each site prior to spudding. The highest cable density occurs in the Luzon Straight and the northeasternmost SCS, where alternate Site SCS-1C is located.

The time available to complete coring operations at the three preferred primary sites during the expedition is very limited, and this may well be the biggest risk to achieving all of the expedition objectives. Several steps have been taken to mitigate this problem, but if any of the primary sites consume more time than has been allocated, it may become necessary to substitute less desirable alternate sites that require less operations time into the schedule. If too much time has been used at any of the primary sites, it may become impossible to achieve all of the objectives at the remaining primary sites.

If Site SCS-6A is drilled to the planned depth, it will be the third deepest hole in Deep Sea Drilling Project/ODP/Integrated Ocean Drilling Program history. The depth of this hole and the basement objective at the bottom of the hole present several challenges for successful drilling. Hole stability is always a risk during coring operations, and the longer the open-hole sections, the higher the risk. Casing has been planned to 900 mbsf to mitigate the risk of hole collapse and to provide a smaller annulus for improved annular velocity for hole cleaning. Hole cleaning also becomes a problem in the deeper sections of the hole, particularly when dense basement material is cored. Additional mud sweeps with larger volumes of mud will be planned for this section. The same problems apply to the other primary sites, but no casing has been planned to achieve the depth objectives. Lower annular velocities will make hole cleaning more difficult in the deeper sections of these holes. Increasing flow rates to ensure hole cleaning will likely result in washed-out sections of sediment in the upper part of the hole. This can also cause hole stability problems toward the end of the drilling process.

FFFs have been planned for primary Sites SCS-3G and SCS-4B (as well as many of the alternate sites) to decrease the amount of time allocated to reaching the planned objective. There are several risks associated with FFF deployment. The FFF can be dislodged while pulling out of the hole. The FFF can become buried or impossible to use for reentry. The use of the FFF leaves the open-hole section open longer, which can contribute to hole instability. The contingency plan is to redrill the APC/XCB section with an RCB bit with a center bit installed. This will require additional time and will put more strain on achieving expedition objectives in the time allocated.

A stuck drill string is always a risk during coring operations and can consume expedition time while attempting to free the stuck drill string or in the worst case, severing the stuck drill string. This can result in the complete loss of the hole, lost equipment, and lost time while starting a new hole. The JOIDES Resolution carries sufficient spare drilling equipment to enable the continuation of coring, but the time lost to the expedition can be significant.