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doi:10.14379/iodp.sp.356.2014

Background

Tectonic setting

The NWS is a rifted margin that has existed since late Paleozoic time, when Australia was part of eastern Gondwana (Etheridge and O’Brien 1994; Exon and Colwell, 1994; Longley et al., 2002). Ribbon-like microcontinents separated from this part of the margin in multiple rifting events, with the latest phase of rifting occurring in the Late Jurassic (Heine and Müller, 2005; Exon and Colwell, 1994; Metcalfe, 1988).

Two of our primary sites (NWS-5A and NWS-6A) are in the northern part of the Perth Basin (Fig. F1). The remaining sites lie in the Northern Carnarvon Basin (NCB) and Roebuck Basin with an 8 km minimum stratigraphic thickness (Longley et al., 2002; Goncharov, 2004). Here, the earliest rifting occurred during the late Permian (initial breakup of eastern Gondwana; Sengor, 1987). Subsequent rifting episodes occurred during the Late Triassic–Early Jurassic and Late Jurassic (von Rad, Haq, et al., 1992; Driscoll and Karner, 1998) with the final rifting in the latest Jurassic also culminating in earliest Cretaceous separation of greater India from Australia (Boote and Kirk, 1989; von Rad, Haq, et al., 1992; Heine and Müller, 2005). The post-rift thermal subsidence history of the margin has been affected by mild shortening, generally attributed to plate boundary forces resulting from plate reorganization (Romine et al., 1997; Driscoll and Karner, 1998; Sayers et al., 2001; Cathro et al., 2003; Dyksterhuis et al., 2005). The most recent major nearby tectonic event was the late Miocene collision between northern Australia and the Banda arc (Audley-Charles et al., 1988; Lee and Lawver, 1995; Richardson and Blundell, 1996). Although occurring several hundred kilometers to the north of the drilling area, intraplate stresses associated with this ongoing collision have resulted in localized reactivation and inversion of extensional faults in the NCB (Malcolm et al., 1991; Struckmeyer et al., 1998; Cathro et al., 2003).

Oceanographic setting

The Indo-Pacific Warm Pool is a region of warm surface waters (average temperature = 28°C) that covers most of the tropical western Pacific and eastern Indian Oceans (Fig. F2). The Indo-Pacific Warm Pool plays a major role in heat transport from low to high latitudes and is subject to decadal scale variability due to the El Niño Southern Oscillation (de Garidel-Thoron et al., 2005). The intensity of the Indo-Pacific Warm Pool functions as a switch in the climate system, and it is consequently a key influence on long- and short-term global climate change (Xu et al., 2006). Therefore, the history of the Indo-Pacific Warm Pool is crucial to our understanding of the global climatic and oceanic systems, as well as their regional effects on the NWS.

Climatic cooling since 15 Ma and an evolving tectonic configuration created appropriate boundary conditions to generate “near-modern” oceanic conditions in the Indo-Pacific. The Indo-Pacific Warm Pool (Fig. F2) is trapped by the Indonesian archipelago and released into the Indian Ocean via the ITF (Gordon, 2005). The ITF transports 10–15 Sverdrups (1 Sv = 10⁶ m³/s) of low-salinity warm water via the Indonesian archipelago (Kuhnt et al., 2004) to the Indian Ocean, forming an important switching point in the global thermohaline conveyor.

The extratropical shelf regions of the Indo-Pacific are strongly influenced by shallow (50–300 m) currents that originate in the Indo-Pacific Warm Pool region. For example, the NWS oceanography from 5° to 15°S is dominated by the South Equatorial Current (Collins, 2002; Fig. F2). South of 15°S, the shallow and narrow Leeuwin Current (Fig. F2) (<100 km wide, <300 m deep) transports warm, low-salinity nutrient-deficient water southward along the west coast of Australia (Pattiaratchi, 2006). It is driven by long-shore winds and an upper-ocean pressure gradient (upper 250–300 m) (Tomczak and Godfrey, 1994) that overcomes equator-ward wind stress and upwelling to flow south (Pattiaratchi, 2006). Another driver for this current is the steric height difference between the low density and salinity Timor Sea and the cooler, denser, saline waters off the coast of Perth. It is the only south-flowing eastern boundary current in the Southern Hemisphere and has an enormous effect on the climate of the region. The Leeuwin Current extends modern coral reef development to 29°S (the Houtman-Abrolhos reefs, Fig. F1) (Collins et al., 1993) and the tropical–subtropical transition as far south as Rottnest Island (33°S; Greenstein and Pandolfi, 2008). Although the Late Pleistocene record and modern oceanography of the Leeuwin Current are well understood (see Cresswell, 1991; Pearce, 2009, and references therein), the pre-Late Pleistocene history of this current is not well known (Kendrick et al., 1991; Wyroll et al., 2009). James et al. (1999) suggested that the Leeuwin Current ceased during glacial periods and restarted during interglacials. Kendrick et al. (1991) used fossil mollusks to suggest that onset of the Leeuwin Current occurred <500 ka, whereas Sinha et al. (2006) and Karas et al. (2011) suggested onset at 2.5 Ma, and McGowran et al. (1997) proposed a Late Eocene (40 Ma) onset age. However, Gallagher et al. (2009; in press) used subsurface well cutting data from the NWS to suggest that the “modern” Leeuwin Current is younger than 1 Ma.

Climate and paleoclimate setting

The arid to semiarid conditions of the Australian interior extend to the west coast of Australia. In the north, rainfall is erratic but predominantly monsoonal (Fig. F2) with the summer rainfall dominance declining sharply toward the south (Sturman and Tapper, 2005). Warm, moist, equatorial air is the major source of monsoonal and cyclonic rain in the north but is replaced in the south by tropical air from the Indian Ocean, which is also known as the “pseudomonsoon” (Gentilli, 1972). The marine pollen record from 6-1.8 Ma at Ocean Drilling Program (ODP) Site 765 (15°S; Fig. F1) shows progressive aridity through the replacement of sclerophyll forest (dominated by Casuarinaceae and grassland) with increased saltbush from 5-3 Ma, and particularly since 1.8 Ma (McMinn and Martin, 1992; Martin and McMinn, 1994). By contrast, <1 Ma strata yield sufficient Eucalyptus species to indicate woodland cover, but it is not clear whether this is related to increased rainfall or an evolutionary change associated with higher burning levels. A marine core (GC17) (Fig. F1) from offshore Cape Range shows marked changes in total and seasonal rainfall in the last 100 k.y. with changing monsoon intensity (van der Kaars and De Deckker, 2002). Climate variation has been quantified for this record using transfer functions from core-top pollen samples in the region (van der Kaars et al., 2006).

Sedimentation history

The northward drift of Australia led to a transition from siliciclastic to predominantly carbonate deposition on the NWS. Carbonate sedimentation was already dominant by the Eocene, although a siliciclastic component persisted (Hull and Griffiths, 2002). This drift brought the NWS into tropical latitudes (Veevers et al., 1991; Müller et al., 2008a); the region had reached 36°–40°S by the early Oligocene and is now at 18°–22°S. Prograding carbonate clinoforms developed in the early Oligocene and continued to the Miocene (Hull and Griffiths, 2002; Cathro et al., 2003). Late early Oligocene–early late Miocene carbonate sediments are heterozoan (i.e., derived from light-independent organisms) and include benthic foraminifers with subordinate bryozoa and rare coral fragments (Cathro et al., 2003). Such sediments develop unrimmed platforms lacking reefs. Resulting clinoformal sequences comprise fine-grained calcilutites on the slope, a mixture of calcisiltites and calcarenites near clinoform rollovers (equivalent to paleoshelf edges), and calcarenites on paleoshelves (Hull and Griffiths, 2002; Moss et al., 2004). Evidence for pre-Quaternary reef development is limited; seismically identified reefs or reef mounds occur in the Oligocene–Miocene section (Romine et al., 1997; Cathro et al., 2003; Ryan et al., 2009; Liu et al., 2011). Rare reefs also occur in the Pliocene–Quaternary section (Ryan et al., 2009), and conditions were favorable for late Quaternary reef development even farther south to 28°S (Collins, 2002). However, sedimentation rates, even in temperate water carbonates, can be high (>40 cm/k.y.), comparable to the lower end of the spectrum of tropical carbonate platform growth rates (James and Bone, 1991).

Previous drilling in the region

The NWS has been extensively drilled by industry over the last 40 y (Longley et al., 2002). As a result, well (cuttings) and existing seismic data assisted with our site choices. However, cores that sample the upper kilometer (Miocene to Holocene carbonate section) are extremely rare. The only continuous cores that exist are from engineering boreholes, typically sampling Late Pleistocene carbonates in the upper 100 m (Fig. F3), and intermittent sidewall cores (Fig. F4). Expedition 356 primary sites redrill six pre-existing industry wells (Table T1) where hydrocarbons are known to be absent from the targeted section. Alternate sites are also located near the primary sites (Fig. F1). All supporting site data for Expedition 356 are archived at the IODP Site Survey Data Bank.

The sites lie along a latitudinal transect designed to sample the lateral variability in subsidence and tropical conditions along the NWS over the last 5 m.y. (Fig. F1). The wells adjacent to the primary sites have gamma wireline logs (mostly to the seabed) (Fig. F5) and cutting samples. Additionally, there are two continuously cored engineering bores (BHC4 and BHC1) near the Angel-1 well (19.5°S; Figs. F1, F3). Facies data (%CaCO₃) from these are directly comparable to the LR2004 oxygen isotope record (Lisiecki and Raymo, 2005). For example, the lower carbonate marly facies (with relatively high gamma response) were deposited during interglacial highstands, and the high-carbonate calcarenites (with ooids) were deposited as the sea level fell during glacials. It is likely the presence of increased siliciclastics (gamma peaks) on the NWS is related to increased precipitation and terrestrial runoff across the shelf during the interglacial periods from an enhanced Australian monsoon. The decrease in terrestrial input during glacials was due to increasingly arid conditions, starving the shelf of siliciclastics (Gallagher et al., in press). This sedimentation model is likely applicable throughout the Pliocene–Pleistocene shelfal carbonate section (Figs. F5, F6) because regional subsidence has likely facilitated the preservation of the majority of the eccentricity/obliquity-controlled eustatic cycles throughout this period (Fig. F6).

In addition, the gamma log pattern clearly changes through time: Pliocene strata are more gamma-rich and there is a secular shift to lower values during the Pleistocene (Figs. F5, F6). This could reflect progressive changes in shelf geometry; however, this effect can be “filtered” out using paleobathymetric estimates in each section. Furthermore, the prevalence of this upward decreasing gamma pattern across 10° of latitude suggests a more regional mechanism, such as an increase of aridity through time and decreasing influence of the Australian monsoon.

The shelfal sections also yield well-preserved Globigerinoides ruber and Cibicidoides spp. (plus many other benthic foraminiferal species) in the interglacial marly facies. The G. ruber and Cibicidoides spp. exhibit isotope values close to deep-sea Pleistocene values in the region (Wells and Wells, 1994), providing an opportunity to use proxy data for salinity and temperature variations (such as paired Mg/Ca and δ¹⁸O analyses) for each glacial cycle to investigate the shallow-water (<200 m) influence of the Indo-Pacific Warm Pool and ITF on the region and their relationship to the Leeuwin Current.

Chronostratigraphic framework

A robust chronostratigraphic framework is particularly important for this expedition. This should be possible for each site, and a wealth of information is already available.

Biostratigraphy

Planktonic foraminifers and nannofossils are common in NWS shelfal sections, especially the highstand and outer-shelf to upper-slope facies, but they are absent in the oolitic facies and poorly preserved in the coarse calcarenites. They are abundant in all the facies of the West Tryal Rocks-1 well (near Site NWS-4A; see data and zonation in Gallagher et al., 2009). There are also a few useful dinoflagellate datums (McMinn, 1992, 2002) in the Pliocene–Pleistocene section to assist age calibration.

Magnetostratigraphy

Typically, this technique is used to provide a chronostratigraphy for deepwater siliciclastics or carbonates. However, Sakai and Jige (2006) successfully demonstrated that it can also be useful in calibrating Pleistocene shallow-water tropical carbonates in the Ryukyus (Japan), so that it should work well in the finer grained, more marly facies of the NWS.

Correlation to oxygen isotope curves

With a few biostratigraphic calibration points, it is possible to correlate log gamma maxima to interglacial cycles (Gallagher et al., in press), similar to the approach adopted by Carter and Gammon (2004). These authors correlated the gamma profile from ODP Site 1119 to oxygen isotope data to achieve a millennial-scale record of New Zealand upper slope sedimentation for the last 3.9 m.y. This type of wireline analysis has also revealed Pliocene–Pleistocene 40 k.y. scale climate variability in the Japan Sea (deMenocal et al., 1992).

Carbon isotope dating

There are sufficiently well preserved mollusks in the upper parts of the sections to permit carbon isotope dating. This technique has already been applied successfully near the top of Core BHC4 (Fig. F3).

Strontium isotope dating

With thorough petrographic screening for diagenetic effects, such as subaerial exposure, it is possible to use strontium isotopes from bioclasts to construct age-depth profiles. Ehrenberg et al. (2006) have successfully dated calcitic bioclasts, such as bivalves, red algae, and large benthic foraminifers, from Miocene platform carbonates. It is likely that we will encounter such macrofossils in most of the shelfal sequences.

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