Areas W07-18, W07-19, W07-20 and W07-21

Beagle Sub-basin, Carnarvon Basin

Release Area Geology

Tectonic setting

The Beagle Sub-basin is a complex Mesozoic wrench depocentre in the northern part of the Carnarvon Basin, and is somewhat transitional between the Dampier Sub-basin to the southwest, and the Rowley and Bedout Sub-basins of the Roebuck Basin to the northeast and east. It is bounded to the east by the shallow basement areas of the Lambert Shelf, and covers an area of approximately 30,000 km2. The sub-basin forms part of the Late Palaeozoic–Early Cretaceous rift margin of northwestern Australia, and is bordered to the southeast by the Pilbara cratonic block, and flanked to the northwest by the northern portion of Exmouth Plateau and oceanic crust of the Argo Abyssal Plain.

Sub-basin evolution and structural elements

The Beagle Sub-basin can be sub-divided into two tectonically controlled regions (Blevin et al, 1994a, 1994b; Figure 2):

  1. An east-northeast-trending region of shallow basement in the south and east, encompassing the Lambert Shelf and flanking marginal fault blocks of the Bruce Terrace, North Turtle Terrace and De Grey Nose.
  2. A Mesozoic to Cainozoic depocentre comprising the bulk of the sub-basin to the north and west of the North Turtle Fault Zone.

The structural terraces along the southeast basin margin probably formed during an early extensional phase in the Palaeozoic. The Bruce Terrace is a board, down-faulted, relatively shallow basement terrace adjacent to the wide shallow Precambrian basement area of the Lambert Shelf. The terrace is overlain by a moderately thick section of Late Palaeozoic and Triassic sediments, and a thinner section of Jurassic–Cainozoic sediments that onlap and pinchout across the Lambert Shelf.

The North Turtle Terrace is an arcuate north-northeast-trending zone of ?Permo-Carboniferous fault blocks that are progressively down-thrown to the northwest. The outer boundary of the terrace is defined by a narrow zone of listric northwest-dipping, growth faults (North Turtle Fault Zone). Periodic reactivation of these faults has provided a sediment source for fan systems that prograded into the main basin depocentre during the Late Jurassic to early Neocomian, and that have continued to be active into the Late Cretaceous–Early Tertiary.

The De Grey Nose forms the boundary between the main Beagle Sub-basin depocentre and the Dampier Sub-basin to the south. It is a Palaeozoic (?Permian) shallow basement fault block that is cut by a series of north-northwest-trending transfer faults (Blevin et al, 1994a). Late Triassic structuring virtually closed off the Beagle Sub-basin from the Dampier Sub-basin, and generated rotated fault blocks beneath the De Grey Nose. The isolation of the Lewis Trough (Dampier Sub-basin) from the Cossigny and Beagle troughs had a profound effect on subsequent depositional patterns throughout the region.

The main basin depocentre is characterised by a series of parallel, north-trending, Triassic–Jurassic fault blocks that are separated by intervening structural lows or troughs (Figure 2). The fault blocks dip gently north and appear to narrow and bifurcate to the south, whilst broadening and becoming more highly faulted to the north. Although this fault block and trough geometry resembles rift-related Jurassic extensional horsts/graben elsewhere on the North West Shelf, the principal fault movement preceding Callovian breakup was predominantly lateral in the Beagle Sub-basin with only local components of extension and compression (Blevin et al, 1994b). A major Late Triassic compressional event (Fitzroy Transpression) formed a series of topographic highs and lows, and subsequent Jurassic fault movement occurred along a series of sub-parallel, north-northeast-trending wrench zones. The Triassic–Jurassic fault blocks in the main depocentre are interpreted to have acted as relatively stable structures and wrench movement was focussed within the intervening troughs. The fault blocks are structurally high due to uplift in combination with the collapse of the surrounding wrench zones followed by a period of thermal subsidence.

During the Late Jurassic to Early Cretaceous, collapse of the wrench zones and post-breakup subsidence resulted in minor post breakup depocentres between the uplifted blocks. During this period, prograding fan systems developed down-dip of the main basin-margin fault and along the flanks of the central basin fault blocks. Subsidence continued throughout the Cretaceous and Early Tertiary and was followed by Miocene reactivation of the basin-margin fault and along the outer margin of the De Grey Nose.

Stratigraphy

The Beagle Sub-basin contains a thick sequence of Palaeozoic to Cainozoic sediments that reach an estimated maximum thickness of 10 km in the inboard Beagle and Cossigny troughs and in the outer portion of the sub-basin. Little is known of the Palaeozoic sediments in the sub-basin. In a reassessment of the Poissonier 1 well, Purcell (Ingram, 1990) identified Visean age palynomorphs in a 10 m thick porous sandstone that unconformably overlies recrystallised calcilutite and claystone, which in turn overlies sandstone and igneous basement.

The Mesozoic stratigraphy of the sub-basin is summarised in Figure 3 (modified after Blevin et al, 1994a, 1994b and Woodside, 1999). Representative seismic transects across the sub-basin are shown in Figures 4, 5 and 6.

The Early Triassic section is marked by a widespread marine transgression and deposition of the Locker shale in shallow shelf and shoreline facies. The overlying Mungaroo Formation consists of numerous fining-upward cycles of thick fluvio-deltaic sandstones with minor interbeds of siltstones, shales and coals. In outboard areas (eg Delambre 1) the upper facies of the Mungaroo Formation consist of claystones and interbedded sandstones deposited in terrestrial to marginal marine environments. The Middle Triassic Cossigny Member comprises paralic to marine siltstones, claystones and limestone, and forms an important transgressive regional seismic marker throughout the sub-basin.

The Early Jurassic Brigadier and North Rankin formations are poorly documented in the Beagle Sub-basin, as they are either thin or absent along the basin margin and rarely fully intersected in the deeper basin depocentre. The Brigadier Formation comprises numerous fining- and coarsening-upward cycles of thinly bedded sandstones, siltstones and claystones deposited in low energy, marginal to shallow marine environments. The North Rankin Formation comprises thick clean sandstones deposited in a high energy shallow marine setting, and minor interbedded siltstone and claystone units.

The Early–Middle Jurassic Legendre Formation is characterised by a series of stacked coarsening-upward fluvio-deltaic cycles of sandstones, siltstones, shales and minor coals, capped by massive sandstones. The cycles are up to 60 m thick and are interpreted as sand-dominated north-to-northwest prograding delta-front deposits separated by fine-grained interdistributary and overbank deposits. The Athol Formation and Murat Siltstone represent marine facies outboard of the Legendre delta. At Picard 1, an informal member of the Athol Formation known as the Picard Shale (early Pliensbachian, approximately upper C. torosa zone; Partridge, 1988) comprises restricted marine shale with potential source quality.

The Callovian Calypso Formation disconformably overlies the Legendre Formation and consists of glauconitic claystone and thinly interbedded sandstones and siltstones. It represents a widespread transgressive marine unit associated with the onset of continental breakup and sea-floor spreading in the Argo Abyssal Plain to the north. The top of the unit is marked by the Oxfordian and/or amalgamated base Cretaceous unconformity.

Late Jurassic sediments are generally thin or locally absent in the Beagle Sub-basin, but thicken markedly into the Dampier Sub-basin to the southwest where they are dominated by organic-rich restricted marine claystones of the Dingo Claystone. Prograding fan sands of Tithonian age (Angel Formation), and locally more restricted fans of Oxfordian age (Eliassen Formation), occur along the flanks of horst blocks in the southern portion of the sub-basin, and down-dip of the main basin-margin fault. These sands consist of good quality reservoir sands and thin claystone interbeds.

The Berriasian to early Valanginian Forestier Claystone is a marine claystone that drapes and onlaps the horst blocks and basin margin. The claystone thickens into the troughs between the uplifted blocks, and forms a regional seal across the sub-basin, although seal quality deteriorates towards the south-eastern margin.

The Valanginian–Aptian Muderong Shale is a thick marine claystone that occurs throughout the sub-basin, and provides an important regional seal where the underlying Forestier Formation is thin or absent.

The mid-Cretaceous to Cainozoic section comprises claystones, marls and calcilutites deposited on a progradation, increasingly carbonate-dominated, open marine passive ramp margin.

 

 


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