Drilling and coring strategy

Previous work by both academia and industry on the NWS has provided abundant site data, including well completion reports, seismic data, wireline logs, cuttings, (rare) sidewall cores, and limited engineering cores in the latest Quaternary sections. Using these data, we identified a 10° latitudinal transect to investigate tropical reef and carbonate diachroneity related to Leeuwin Current intensity and ITF/Indo-Pacific Warm Pool influence.

Primary Sites NWS-1A, NWS-3A, NWS-4A, and NWS-6A (and corresponding alternate Sites NWS-7A, NWS-9A, NWS-10A, NWS-11A, and NWS-13A) (Fig. F1) will reveal insights into the timing and onset of reef development and are important targets because the pre-late Quaternary history of tropical reef development on the NWS is poorly known. The shelfal sites (all except Site NWS-4A) should allow us to obtain biogeographic evidence of ITF connectivity (Fig. F4), yielding a downstream record of this important ocean gateway. These shelfal sites are also likely to yield ooids, and coring will hopefully reveal the maximum age for these enigmatic tropical indexes in the Indian Ocean. Knowing the temporal and spatial distribution of ooids in the NWS will also provide insight into regional aridity and Indian Ocean alkalinity, and these data can also be used as paleodepth indexes. The presence of bathymetrically diagnostic facies, such as larger and smaller benthic foraminifers, at these five sites should permit well-constrained subsidence histories across 10° latitude, allowing the detailed temporal history of dynamic subsidence of the northern Australian plate to be constructed for the first time.

All of the sites are likely to yield spores and pollen, particularly during interglacials, and therefore yield a record of climate change and improve our understanding of the timing and nature of aridification in northwestern Australia. Compared to the eolian-derived floral assemblages from deep oceanic sites, material recovered from the NWS, closer to the shoreline, should contain predominantly fluvial-derived assemblages that will contribute to a more realistic assessment of regional climate. In particular, two shelfal to upper slope targets (Sites NSW-4A and NSW-5A) should provide particularly important climate records. Site NWS-4A is designed to provide insight into the onset and dynamics of the Australian monsoon and its relationship to obliquity forcing. If fully recovered, this would represent a quantum increase in our understanding of this important climate system because previous studies in the region focused only on the last 200 k.y. Site NWS-5A is not likely to yield an orbital-scale climate record because of slope erosional processes (Figs. AF4, AF5); however, it will complement Site NWS-4A because it lies south of a climatic divide between the Australian monsoon–dominated northern area and the westerly wind-driven, winter rainfall–dominated southern area (Fig. F2). Site NWS-5A will therefore provide a Pliocene–Pleistocene record of onset and variability of the southern Australian winter rainfall–dominated regime.

Our drilling and coring strategy is similar for each of the six primary (and seven alternate) sites; our primary operations plan is summarized in Table T2. For operational efficiency, we intend to core the sites in sequence from south to north (Fig. F1). The IODP Environmental Protection and Safety Panel (EPSP) approved all of the sites except for alternate Site NWS-11A. This site is pending final approval following review of additional seismic data. Our coring strategy consists of advanced piston coring (APC) in three holes (A, B, and C) at each site to ~200 meters below seafloor (mbsf) or APC refusal. APC refusal is an estimated depth that is formation dependent. It is usually defined two ways: (1) a complete stroke (determined by standpipe pressure) is not achieved because of formation hardness or (2) excessive force (>100,000 lb) is required to pull the core barrel out of the formation because it is too cohesive. In cases where a full stroke with the 9.5 m APC is not achieved, the half-length APC (4.8 m) may be used to deepen APC refusal. The decision to use the half-length APC technique will be influenced by operational timing.

Additionally in Holes B and C, the total penetration depth will be increased beyond APC refusal using the extended core barrel (XCB) technique. At two of the primary sites (NWS-3A and NWS-4A) and four of the alternate sites (NWS-11A, NWS-10A, NWS-9A, and NWS-7A), a fourth hole (D) will be required to reach the target depths using the rotary core barrel (RCB) system.

After coring is completed at each site, the final hole (C or D) will be conditioned, displaced with logging mud, and logged (see “Downhole measurements strategy”).