Introduction-Modern Hydrography | Table of Contents

INTRODUCTION (continued)

Geological Setting
Broad Phases of Cretaceous and Cenozoic Deposition
The Tasmanian region lay within the continent of Gondwana until breakup started during the Late Cretaceous (Fig. 5). Rifting related to the separation of Antarctica and Australia may have started as early as the Late Jurassic, and by the Early Cretaceous there was a well-developed east west rift system along the southern margin of Australia that passed north of Tasmania through the Bass Strait (Willcox and Stagg, 1990). The rift sequences in outcrop and petroleum exploration wells are volcaniclastic fluviatile and lacustrine sediments thousands of meters thick in places. The volcanism was basic to andesitic, and the Lower Cretaceous sediments are dominantly immature lithic conglomerates, sandstones, and mudstones, with some better sorted quartz-rich sandstone bodies. They were probably derived from volcanism along what is now the east coast of Australia.

During the beginning of the Late Cretaceous, the early rifting in the Bass Strait failed and a NW-SE zone of strike-slip faulting, west of Tasmania, absorbed motion related to the continuing east-west rifting (Fig. 5). This motion eventually separated Australia and Antarctica (Fig. 6, Fig. 7). During the Late Cretaceous, the sea intruded into the rift from the west, along the gulf between Australia and Antarctica, here named the Australo-Antarctic Gulf (Fig. 1). Data from petroleum exploration wells show that coastal plain to shallow-marine detrital sediments were deposited along the east-west rift (Smith, 1986; Lavin, 1997; McKerron et al., 1998) and also along the northernmost part of the zone of strike-slip faulting (Moore et al., 1992). In depocenters near Tasmania, these sediments are relatively mature, quartz-rich and 1000-2000 m thick (Moore et al., 1992). However, southwest of Tasmania, dredging has recovered immature, shallow-marine lithic sandstones and mudstones (Hinz et al., 1985; Exon et al., 1992) of Late Cretaceous age that are reminiscent of the Lower Cretaceous rocks farther north. Seismic reflection profiles show that these sequences are frequently prograded and deltaic (Hill et al., 1997b).

Australia's Eastern Highlands were uplifted at the end of the Early Cretaceous at ~95 Ma (O'Sullivan et al., 1995), and rifting commenced between Australia to the west and the Lord Howe Rise and the Campbell Plateau to the east. During the Campanian (75 Ma, Chron 33), drifting of the latter elements to the east-northeast was well established (Royer and Rollet, 1997), and the eastern margin of Australia/Tasmania/STR started to collapse.

In the latest Cretaceous to Eocene, the east-west rift continued to fill with prograding shallow marine detrital sediments and coal-bearing strata. The depression along the strike-slip zone also filled with prograding sediments, and seismic interpretation suggests the depocenter moved southward with time relative to Tasmania, with Paleocene sedimentation dominating in the north and Eocene in the south (Hill et al., 1997b). Paleogene sediments are as thick as 1500 m in places. In the Oligocene, Australia cleared Antarctica, its margins subsided, and deposition of relatively thin hemipelagic, pelagic, and shallow-water carbonate predominated thereafter.

The Tasmanian Offshore Region
Today, the Tasmanian offshore region consists of continental crust of the Tasmanian margin (Moore et al., 1992; Hill et al., 1997b), the STR (Hinz et al., 1985; Exon, et al., 1997b), and the ETP (Exon et al., 1997a) and is bounded on all sides by oceanic abyssal plains (Fig. 3). Oceanic crust to the east was created by the seafloor spreading that formed the Tasman Sea in the Late Cretaceous and Paleogene. The crust to the south and west was formed during the Cenozoic, and perhaps the latest Cretaceous, by the seafloor spreading that led to the separation of Australia and Antarctica.

The continental shelf around Tasmania (Fig. 3) is mostly nondepositional at present. The continental slope west of Tasmania slopes fairly regularly, at ~4°, from 200 to 4000 m. The continental rise lies at 4000-4500 m, and the abyssal plain is generally 4500-5000 m deep. Sampling cruises have shown that the slope is underlain by continental basement and that Upper Cretaceous and Paleogene shallow-marine sandstone, siltstone, and mudstone are widespread in deep water west of Tasmania, overlain by Oligocene to Holocene pelagic carbonates. Seismic interpretation shows that basement is generally overlain by several kilometers of sediments (Fig. 8-P1).

The current-swept STR is a large, north-northwest-trending bathymetric high that rises to <1000 m below sea level (mbsl) and is separated from Tasmania by the west-northwest-trending South Tasman Saddle >3000 m deep (Fig. 3). The STR is a continental block, and seismic profiles show it is cut into basement highs and deep basins with several kilometers of sedimentary section (Fig. 8-P2 and P3). The overlying sequences in faulted basins include known Oligocene to Holocene pelagic carbonates and Paleogene marine mudstones, and seismic evidence suggests they also contain Cretaceous sediments. The top of the rise is a gentle dome with low slopes, but slopes are generally steeper between 2000 and 4000 m. The western slope is more gentle to 3000 m, but below that there is a very steep scarp trending 350°, which drops away to 4500 m as part of the Tasman Fracture Zone.

The ETP is a nearly circular feature, 2500-3000 m deep, separated from southeast Tasmania by the 3200-m-deep East Tasman Saddle (Fig. 3). Slopes are generally low, but considerably greater on the plateau's flanks. Atop the plateau is the Cascade Seamount guyot, which formed as the result of hot spot volcanism and has yielded Eocene and younger shallow-water sandstone and volcanics. Seismic profiles show that the plateau has as much as 3 s two-way traveltime (TWT) of sediment cover (Fig. 8-P4), which are believed to comprise mainly Oligocene to Holocene pelagic carbonates and Cretaceous to Eocene siliciclastic sediments. These are underlain by continental basement rocks.

The structural setting of Sites 1168-1172 is shown in Figure 9. The west Tasmanian margin is cut by strike-slip faults, trending north-northwest or northwest, that were most active in the latest Cretaceous to mid-Paleocene (Hill et al., 1997b). They were generated by the northwest movement of Australia away from Antarctica. The STR is cut by these early faults, and also by younger, middle Eocene- to late Oligocene-age faults. These younger faults are strike-slip faults trending north-south, and on the northwestern STR, related normal faults trending east-west (Exon et al., 1997b). The three sets of faults all have throws reaching as much as 3 km. Sites 1168-1172 were all located in depocenters to ensure that thick Cenozoic sections with high sedimentation rates would be cored.

Plate Tectonics
Early extension between Australia and Antarctica began in a northwest-southeast direction during the Late Jurassic (Willcox and Stagg, 1990), and this motion created much of the western margin off Tasmania. Subsidence studies along the southern Australian margin, as well as the conjugate pattern of magnetic anomalies off Australia and Antarctica, suggest that the breakup between Australia and Antarctica propagated toward Tasmania from the Great Australian Bight (Mutter et al., 1985). Seafloor spreading may have started west of Tasmania during the Late Cretaceous and continued at a slow rate until the early Eocene, when fast spreading began. The trajectory of the central STR since Australia-Antarctic separation, and its paleolatitudes since the Campanian, are shown in Figure 4. Royer and Rollet (1997) reexamined the seafloor magnetic anomaly data and satellite-derived gravity data in the region along with plate tectonic reconstructions and concluded the following about the region south of Tasmania (Fig. 10):

  1. The STR is composed of two distinct domains of different origin: a western terrane, lying between the Tasman Fracture Zone and a N170°E oriented boundary at 146.5°E, was initially part of the continental shelf of north Victoria Land, Antarctica (and adjacent to west Tasmania), whereas an eastern terrane, east of the 146.5°E boundary, rifted from Tasmania and the ETP.
  2. The western terrane rifted from Antarctica during the late Paleocene to early Eocene and was welded to the eastern terrane. Then, until the early Oligocene (Chron 13), when the STR cleared the Antarctic margin, the western domain underwent severe wrenching and left-lateral shearing between the Antarctic shelf break and the 146.5°E boundary. Deformation continued, but perhaps to a lesser extent, along the transform margin until the early Miocene.
  3. The western margin of the STR became active as a transform in the late Paleocene to early Eocene; the SEIR axis was in contact with the margin rim from the early Eocene (~Chron 24) until the early Miocene (~Chron 6B, 23 Ma), after which the transform margin became passive.
  4. Seafloor spreading initiated in the Tasman Sea in the Campanian (Chron 34), north of the ETP. A spreading center also probably initiated between the STR and the ETP during the Campanian (Chron 33) and failed shortly afterward during the Maastrichtian (~Chron 30).

Earlier Drilling (DSDP) Results
During DSDP Leg 29, four partially cored sites were drilled in the Tasmanian region (Kennett, Houtz, et al., 1975) (Fig. 3; Table 1). The three sites most relevant to the goals of Leg 189 are Site 282 on the west Tasmanian margin, Site 281 on the STR, and Site 280 on the abyssal plain immediately south of the STR (Fig. 3). The Leg 29 sites were generally located on regional highs to minimize the depth of penetration necessary to reach older strata, and hence much of the succession was cut out by hiatuses. Furthermore, during the first scientific drilling in the area, core recovery was relatively poor. Total sediment recovery for the three critical sites was fairly low (Table 1).

Site 282 was drilled to 310 mbsf on a basement high in deep water west of Tasmania. This sequence includes much of the Cenozoic but contains four major unconformities. The sequence consists of a veneer of Pleistocene ooze, underlain by upper Miocene ooze, lower Miocene marl, lower to mid-Oligocene mudstone, and upper Eocene mudstone. The sediments rest on a pillow basalt of presumed Tertiary age. There is little in the sediments to suggest that the site was located in deep water until the margin began to subside during the Oligocene. Calcareous microfossils are present throughout, and total core recovery was 20%.

Site 281 was drilled to 169 mbsf on a basement high of quartz-mica schist of latest Carboniferous age southwest of the crest of the STR. The sequence consists of Pliocene-Pleistocene foraminifer-nannofossil ooze, Miocene foraminifer-nannofossil ooze, upper Oligocene glauconite-rich detrital sand, and upper Eocene basement conglomerate and glauconitic sandy mudstone. Evidence from the recovered intervals suggests that the site subsided into deep water after the Miocene. Calcareous microfossils are present throughout, and total core recovery was relatively high (62%).

Site 280 was drilled to 524 mbsf, on a basement high in deep water southwest of the STR (Fig. 3), and bottomed in an "intrusive basalt," almost certainly associated with oceanic crust. The site penetrated a veneer of upper Miocene to upper Pleistocene clay and ooze, underlain (beneath a sampling gap) by 55 m of siliceous lower Oligocene sandy silt, and 428 m of middle Eocene to lower Oligocene sandy silt, containing chert in the upper 100 m and glauconite and manganese micronodules in the lower succession. The lower 200 m is rich in organic carbon (0.6%-2.2%). The younger part of the lower Oligocene to upper Eocene sequence contains abundant diatoms, but the lower part is almost completely devoid of pelagic microfossils. All sediments were probably deposited in abyssal depths. A brown organic staining suggests that reducing conditions were present in parts of the upper Oligocene and lower Miocene. Total core recovery was only 19%.

Site 281, in particular, assisted with the development of a broad, globally significant history of Cenozoic paleoceanographic events. Shackleton and Kennett (1975) produced composite foraminiferal oxygen and carbon isotope curves from the late Paleocene to the Pleistocene from Sites 277, 279, and 281. This record, although of relatively low resolution, exhibits the now classically known general increase in oxygen isotopic values, reflecting a decrease in bottom- and surface-water temperatures and/or ice buildup during the Cenozoic. A general increase occurred in isotopic values following the early Eocene, with a rapid increase during the early Oligocene reflecting major cryosphere expansion and cooling. Average oxygen isotopic values remained steady but oscillatory until the middle Miocene, when there was another rapid oxygen isotopic increase as the Antarctic cryosphere expanded farther. This was followed by further increase in oxygen isotopic values reflecting the development of the West Antarctic ice sheet during the late Miocene and the Northern Hemisphere cryosphere during the late Pliocene (Fig. 2). Isotopic studies were conducted at Site 281 (STR) for the interval from the early Miocene to the Pliocene. In contrast, at Site 277 (Campbell Plateau) isotopic analyses were conducted on the earliest Miocene to late Paleocene.

Introduction-Modern Hydrography | Table of Contents