Basins Of The Ross

Geophysical research in the Ross Sea shows that thick sedimentary sequences are present over large areas (Houtz and Davey, 1973; Wong and Christoffel, 1981; Davey et al., 1982, 1983; Hinz and Block, 1983; Sato et al. 1984; Cooper et al., 1987). Drillhole data from the Deep Sea Drilling Project (DSDP) Leg 28 (Hayes et al., 1975a, b), McMurdo Sound Sediment and Tectonic Studies (MSSTS-1) drillhole (Harwood, 1986) and Cenozoic Investigations of Ross Sea (CIROS-l) drillhole (Barrett, 1987) show that the sediments which underlie the Ross Sea are up to Oligocene in age with older rocks being found only in McMurdo Sound as erratics (Webb, 1983). The deeper sediments in the Ross Sea basins are probably at least Early Tertiary in age and possibly as old as Late Cretaceous (Houtz and Davey, 1973; Davey etal., 1982; Hinz and Block, 1983). Cooper etal. (1987) interpreted an older sedimentary sequence in the western Ross Sea as being as old as Upper Palaeozoic in age.

Three major basins have been delineated : the Eastern Basin, the Central Trough and the Victoria Land Basin (Fig. 6.6) (Davey, this volume). Multichannel seismic reflection data have defined the extent, sedimentary structure and stratigraphy of the basins in detail (Hinz and Block, 1983; Sato et al., 1984; Cooper et al., 1987; Hinz and Kristoffersen, 1987).

Eastern Basin

The Eastern Basin covers most of the eastern Ross Sea continental shelf, an area of about 100,000 km2 (Houtz and Davey, 1973 (Fig. 6.6). It contains up to about 6 km thickness of sedimentary rocks with an average of 3 m, suggesting a potential for about 6 x 108 m3 oil equivalent to be found in the basin, based on worldwide averages. The seafloor lies at an average depth of about 500 m. Reconnaissance multichannel seismic data since 1980 cover the area (Hinz and Block, 1983; Sato et al., 1984; J. Wannesson, pers. comm.) and, with the results of the DSDP drillholes, enable a preliminary stratigraphy to be derived.

Fig. 6.6. Sedimentary basins of the Ross Sea (after Davey, this volume), lightly shaded where basement > 3 km below seafloor (Polar Stereographic projection).

The basin is a simple open basin or trough, opening towards the shelf edge in the north and extending to, and under, the Ross Ice Shelf in the south (Davey, this volume). Beneath a shallow widespread unconformity, separating Middle Miocene sediments from those of Pleistocene and younger age, the sedimentary layers dip gently towards the centre of the basin and towards the shelf edge where they reach a thickness of 4 km and have the form of prograded beds. Along the margins of the basin, these Late Oligocene and Miocene sediments are folded into north-south trending broad open folds. Seismic data at the continental slope can be interpreted as showing 2 km of pre-Oligocene sediments and this thickness may continue under the main basin. Hinz and Block (1983) detected a north-south trending trough of pre-Late Oligocene sediments, up to 2 km deep, under the western margin of the Eastern trough which may continue under the southern part of the basin. The lower sequence could be either related to the separation of

New Zealand from Gondwana and therefore of Late Cretaceous age (Cooper et al., 1988) or be lowermost Tertiary in age.

The thin Oligocene sequence drilled in DSDP hole 270 suggests that the lower sequence consists of terrestrial to marginal marine sediments. The unconformity overlying the older sedimentary sequence has an age of 25-26 Ma which coincides closely with the development of the Circum-Antarctic Current (about 30 Ma), and a widespread Oligocene regression in the Southern Hemisphere. Above this older sedimentary sequence, Hinz and Block (1983) detected six depositional sequences, the upper five prograding sequences being interpreted as a series of mostly deltaic lobes, with a thin delta plain facies to the south and a prograding prodelta facies to the north on a subsiding platform. The uppermost unconformity corresponds to an age gap of 4-10 Ma (Savage and Ciesielski, 1983) and is widespread over the whole Ross Sea. It truncates the dipping reflections forming the margin of the basin in the south, therefore indicating a lack of subsidence of the basin in this region since 10 Ma ago. A thickness of several hundred metres of sediments has been removed.

Small amounts of methane and ethane were found in the Miocene glacial marine sediments of DSDP sites 271-273 (Hayes et al., 1975a, b). Mclver (1975) found that the gas extracted from the sealed cores from these drillsites had a significant fraction of higher homologues than ethane which, together with their consistent nature throughout, suggested that local organic diagenesis could have occurred and that liquid hydrocarbons may have been generated somewhere in the sedimentary section.

Modelling of the thermal maturity of the sediments to see if conditions suitable for hydrocarbon generation have been reached has been carried out by Hinz and Block (1983) and Cook and Davey (1984). Information for the models is sparse. No measurements of heat flow or geothermal gradient have been made in the Eastern Basin and the value assumed was based on the nearest heat flow measurements on Roosevelt Island (Fig. 6.6). A basic assumption for the model is that heat flow has not varied with time. This is conservative for a basin lying close to a rifted continental margin where heat flows are probably significantly higher at breakup and slowly decay with time (Turcotte, 1980). The thermal conductivities assumed were related to the sediments predicted or shown to exist in the basin without allowing for compaction. For example, the Late Oligocene to Holocene glacial marine sequence is predominantly fine-grained shales and mudstones with glacial erratic pebbles floating in the fine-grained matrix (Hayes et al., 1975a). The fine-grained nature of the sediment places a control on its thermal conductivity with a higher value for the deeper, more sandy, marginal marine/terrestrial sequence. The ambient surface temperature from time of deposition to present was taken as 0°C, assuming high latitudes for the terrestrial and marine sequence throughout the Tertiary. The models for the Eastern Basin suggest that hydrocarbons generation is possible but only in the pre-Late Oligocene sediments.

Source rocks in the basin probably occur in the older sequence as the younger sediments comprise marine glacial sequences. Since the lower sequence has not been drilled or sampled, a measurement of the organic carbon content of the sequence has not been made. Comparable terrestrial marginal marine sequences related to breakup (i.e., Gippsland Basin) do, however, contain sufficient carbonaceous material to provide a source for hydrocarbons.

Little information exists on which to base an assessment of reservoir rocks. In the Tertiary, drillhole data through marine glacial sequences (DSDP, MSSTS-1, CIROS-1) indicate adequate porosity for some horizons but permeability is unknown. The older, preglacial rocks of probable Late Cretaceous to Oligocene age may have reservoir rocks in marine or non-marine sandstones, by analogy with coeval basins in western Tasmania and Campbell Plateau.

Many types of stratigraphic or structural traps may exist related to the tectonic history (two-phase rifting) including traps resulting from sediment drape over early rift faulting and stratigraphic traps arising from the numerous unconformities seen in the sedimentary section, particularly associated with the thick prograding sequence of glacial deposits.

Central Trough

The Central Trough (Davey, this volume) is an elongate sedimentary basin trending north-south and extending from the continental shelf edge to under the Ross Ice Shelf. It coincides approximately with 175°E longitude. The Central Trough is over 500 km long and about 100 km wide, with one major dextral offset at 74°S and a possible minor one at 73°S. Thicknesses of about 6 km of sediment are present in the deepest part of the basin and a possible oil reserve of 2.5 x 108 m3 is indicated based on worldwide averages.

DSDP site 273 (Fig. 6.6) was located over the basin and penetrated to 346 m sub-seafloor. It encountered only one major unconformity, which was at shallow depth, corresponding to the time interval 4-10 Ma.

The axis of the trough coincides with a large gravity high indicating a rift origin with associated crustal thinning and/or intrusions of high density rocks into the crust beneath the basin (Hayes and Davey, 1975). Seismic data indicate steep margins to the trough with the adjacent basement either near seafloor (Houtz and Davey, 1973) or at depths of up to 2 km where the rift contains only the oldest sediments (Davey et al., 1982). Davey (1981) postulated an Early Tertiary age, suggesting that the trough is a graben structure formed as a failed arm of the spreading centre which abutted the continental margin of the western Ross Sea at about 55 Ma (Weissel et al., 1977). Hinz and Block (1983) and Cooper et al. (1988), however, suggested that the oldest sediments may be Late Cretaceous in age.

The stratigraphy for the Central Trough is poorly known. At DSDP site 273 (Fig. 6.6), the oldest sequence penetrated was Early Miocene (Hayes et al., 1975b). This sequence was predominantly a pebbly silty claystone of marine glacial origin. The sediments in the deeper part of the trough are probably as old as Early Tertiary-Late Cretaceous (Davey et al., 1982). A coarse sandstone sequence is suggested by the complex seismic velocity-depth profile for the trough (Davey et al., 1982) and would be consistent with a graben formation and terrestrial sequence for the lower part. Two lithological models were used by Cook and Davey (1984) for maturation assessment because DSDP 273 drilled marine glacial sediments back to Early Miocene. The first model assumed a pre-Oligocene terrestrial sandy sequence overlain by marine glacial sediments. The second assumed a sandy mudstone sequence averaging 20% sand throughout.

The input parameters for maturation modelling are poorly controlled. Assuming a constant sedimentation rate and an Early Tertiary age of formation, the central part of the basin, where a total sedimentary thickness of 4,200 m has been measured, contains 200 m of Plio-Pleistocene, 2,100 m of Oligocene to Miocene and 1,900 m or more of Eocene-Upper Palaeocene sediment (Davey et al., 1982). Hinz and Block (1983) inferred up to about 3,500 m of Late Oligocene or younger sediments in the deepest part of the basin. The heat flow measurements on the south-eastern margin of the trough (Sato et al., 1984) are unusually high for the tectonic regime and may be affected by seasonal bottom water temperature variations. The value assumed for the Eastern Basin was taken to be more realistic. The thermal conductivities used were based on the inferred or observed litholo-gies.

The maturation models for the Central Trough give unprospective results with maximum maturation values well below the onset of hydrocarbon generation, even if the higher heat flow values are used. On present knowledge therefore, the hydrocarbon potential of the Central Trough is considered to be poor.

Reservoir rocks are not well defined for the Central Trough. However, if the older sedimentary sequences are of early rift terrestrial sandy sediments, then moderate to high porosity and permeability may be expected provided no diagenesis has occurred.

Probable traps are less well defined than for the Eastern Basin as the lack of prograding sequences downgrades the possibility of stratigraphic traps.

Victoria Land Basin

The Victoria Land Basin covers an area of some 80,000 km2 and contains up to 6 km of young (?Late Cretaceous to Tertiary) sediments (Davey, this volume). Probable oil reserves would total some 4 x 108 m3, based on worldwide averages.

The Victoria Land Basin is a broad basin some 150 km wide extending along the western margin of Ross Sea from Ross Island to Cape Washington, a distance of over 500 km (Fig. 6.6). The basin contains two main sedimentary sequences (Cooper et al., 1987). The upper sequence has a thickness of about 5-6 km and is underlain by a stratified, presumably low grade, metasedimentary sequence up to 6 km thick with a seismic velocity of about 5.5 km sec.-1. The western flank of the basin contains a sedimentary sequence which dips towards the centre of the basin and is truncated at or near the seafloor. Basement underlying this western flank is apparently blockfaulted down to the east reflecting the uplift of the Trans-antarctic Mountains. A rift depression about 20 km wide lies along the axis of the basin linking the volcanic provinces of Ross Island and Cape Washington. The rift is marked by numerous faults, especially on the eastern flank, which apparently have been recently active. The older sedimentary sequence thins over the eastern flank of the basin, onlapping against basement. Back-tilted blockfaulting and igneous activity also occur over the eastern flank. The crust under the Victoria

Land Basin is thin for continental regions, being about 22 km under the basin and thickening to about 28 km at the coast and in the Ross Sea to the east of the basin (McGinnis et al., 1985; Davey and Cooper, 1987), increasing to probably 35-40 km under the Transantarctic Mountains (Davey and Cooper, 1987).

The two main sedimentary sequences suggest two main episodes of basin formation. The older episode indicates a broad downwarp and sediment deposition in probably post-mid Jurassic (Beacon Supergroup) and pre-Late Cretaceous (New Zealand-Antarctic separation) time. Tectonics associated with continental breakup led to the major unconformity between the two main sedimentary sequences. Subsequent downwarping, presumably since Late Cretaceous times, has led to the deposition of the younger sequence. The tectonic activity increased in the Neogene with the formation of the active rift, which links the active Late Cenozoic volcanic centres of Ross Island and Cape Washington, and is also reflected in the uplift of the Transantarctic Mountains which increased markedly (15 m/Ma to 90 m/Ma) about 50 Ma ago (Gleadow and Fitzgerald, 1987).

The MSSTS-1 drillhole, sited on the western flank of the basin in McMurdo Sound, penetrated a thin sequence (226 m) of Late Oligocene and younger sediments (Harwood, 1986). Correlation of these data with seismic reflection data suggests that about the upper 1-2 km of sediments in the deeper part of the basin are Late Oligocene or younger in age (Davey and Christoffel, 1984; Cooper and Davey, 1985). The remainder and thickest part of the younger section is therefore probably Palaeogene and perhaps Late Cretaceous (Cooper et al., 1987) with deposition following the rifting of New Zealand from Antarctica. The older sequence is considered to have an age range commencing after the end of Beacon sedimentation (mid-Jurassic) and finishing before continental breakup in the Late Cretaceous. Sedimentological studies (Barrett, 1981) indicate that the Ross Sea was an elevated region during Beacon sedimentation. The tectonic activity during continental fragmentation provides a mechanism for the inferred major break in sedimentation and low grade metamorphism of the older sequence.

Source rocks are unlikely to occur in the younger, marine glacial sediments. Marine sediments, possibly interbedded or underlain by non-marine sediments probably occur in the lower part of the sequence as suggested in the recycled marine microfossils of Late Cretaceous and Early Tertiary age. These marginal marine sediments have been suggested as the source of the oil found in the CIROS-1 hole (Cook and Woolhouse, 1989). Coal measures occur in the Beacon Supergroup rocks and, if these are not as heavily metamorphosed by contact with volcanic sills as they are in the Transantarctic Mountains, they could also be potential source rocks.

The maturation levels for the Victoria Land Basin have been modelled by Cook and Davey (1984) and Cooper et al. (1988). Cook and Davey (1984) assumed a sedimentation rate based on the uplift rates for the Transantarctic Mountains in the region (Gleadow and Fitzgerald, 1987) to obtain the age of the sediments. A total sediment thickness for the younger sedimentary section in the Victoria Land Basin was used (Davey et al., 1982; Cooper and Davey, 1985). Cooper et al. (1988), on the other hand, used the stratigraphy for the basin (Cooper et al., 1987) to define the age-depth relationships for the whole of the sedimentary section of younger (post-Late Cretaceous) rocks and older (pre-Late Cretaceous) rocks. The MSSTS-1 and DSDP 270-273 drillholes encountered a regional hiatus which occurred during the Late Miocene and Pliocene (Hayes et al., 1975a) which has been included in the model for the basin.

Heat flow measurements in the basin are highly variable. Most of the measurements were taken in the McMurdo area, adjacent to active volcanism and plutonic granites and are therefore associated with high local heat flows. However, the MSSTS-1 drillhole gave a value of 60 mW/m2, assuming marine glacial sediments with thermal conductivity of 1.7 W/m°C (Sissons, 1980), and values of about 100 mW/m2 were obtained in Terra Nova Bay at the northern end of the Victoria Land Basin (Blackman et al., 1984). The sedimentary sequence in the MSSTS-1 hole is glaciomarine (Barrett and McKelvey, 1986) and an appropriate thermal conductivity value was used by Cook and Davey (1984) in the modelling. However, Cooper et al. (1988) assumed marine sediments for the pre-38 Ma sediments with possible non-marine sediments in the older part (pre-85 Ma) of the sedimentary section corresponding to pre-breakup sedimentation.

The maturation calculations for the Victoria Land Basin indicate that, even in the low heat flow model of Cook and Davey (1984), the lowermost 1 km or more of the younger sedimentary sequence of the basin have reached maturation values corresponding to the onset of oil generation. The thicker, younger sequence in the deepest part of the Victoria Land Basin would have a thicker mature section and the older sedimentary sequence would also be a potential source having passed through the "oil window". These results are consistent with the drillhole data from the CIROS-1 drillhole in western McMurdo Sound where a 2 m thick sandstone containing traces of residual oil was detected (Barrett, 1987). The sandstone is considered to contain a residue of a hydrocarbon generated at greater depth and migrated through the sandstone. Detailed chemistry of this oil residue indicates that its source was a nearshore marine sediment (Cook and Woolhouse, 1988).

The organic content of the sediments in the MSSTS-1 hole reached a maximum of 0.18% wt (Collen and Froggatt, 1986) which is below the industry-accepted minimum source rock requirement. Similar sediments measured in DSDP holes 271-273 where the sedimentation rate is higher have organic contents of 0.25-0.7% (Mclver, 1975). Possible reservoir rocks are again poorly defined but are indicated by high porosity sediments sampled in the drillholes and by analogy with older rocks from coeval basins on Campbell Plateau and Tasmania.

Possible hydrocarbon traps would include stratigraphic and structural forms. Vertical deformation, especially Late Tertiary deformation associated with the Terror Rift system and its associated intrusive structures, may be expected to give rise to many late rift structural and stratigraphic traps associated with numerous unconformities.

The older sedimentary sequence, if deposited during early rift formation prior to continental breakup, may contain synrift sediment of terrestrial or shallow water, alluvial plain origin, similar to those found along the southern Australian margin and therefore include significant source material.

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