Areas W07-12 to W07-15

Canning Basin

Basin Geology

The Offshore Canning Basin is a northeast-trending passive margin of Late Palaeozoic and Mesozoic age (part of the Westralian Superbasin), overlying a major northwest-trending intra-cratonic basin of mainly early Palaeozoic age (Colwell and Stagg, 1994). The temporal boundary between the intra-cratonic and passive margin systems is in the Late Carboniferous at the time of the Alice Springs Orogeny.

Structural framework

The Offshore Canning Basin can be divided into a number of major structural provinces (Figure 2). The offshore Fitzroy Trough (hereafter referred to as the Oobagooma Sub-basin) and offshore Willara Sub-basin (including the Samphire and Wallal embayments) are depocentres of the Palaeozoic intra-cratonic system. These are separated by the west-northwest-trending, largely unfaulted, structural high of the Broome Platform and are disconformably overlain by ‘Westralian’ passive margin sediments of the Bedout and Rowley sub-basins. These latter depocentres comprise the outboard Roebuck Basin, which in this report is distinct from the inboard Offshore Canning Basin. The release areas are primarily located over the Oobagooma Sub-basin and Broome Platform; therefore these depocentres will be described in more detail.

The Oobagooma Sub-basin is approximately 190 km in length and 100 km in width. A major change in Palaeozoic structural orientation, from northwest–southeast to north–south, occurs at approximately 17°S in the sub-basin (Figure 2). At this location a weakly developed graben structure also extends to the west-southwest creating a triple-point junction. The change in structural orientation occurs around an asymmetric intruded high (the Oobagooma High), which marks the western limit of the Oobagooma Sub-basin.

The style of bounding faults along the sub-basin margin changes from asymmetrical half-graben extension onshore (with the northeastern margin acting as a hinge and the southwestern margin faulted down on a shallow dipping detachment; Drummond et al, 1991), to symmetrical graben extension in the near offshore (Smith et al, 1999; Figure 3), to thin skinned detachment along a northeast-dipping surface and its southwest dipping antithetic in the outer offshore (Colwell and Stagg, 1994). The style of extension appears to be controlled by the location of the more rigid elements of the basin, with normal faults developing near the stable bounding Kimberley Craton and Leveque Shelf to the northeast and the Broome Platform to the southwest (Smith et al, 1999). Major offsets along the boundaries of the onshore Fitzroy Trough are interpreted as orthogonal transfer or accommodation zones. These zones have exerted a major control on the pattern of sediment dispersal and accumulation, especially in the Devonian (Kennard et al, 1994).

The Oobagooma High is a 25 km wide north–south-oriented elongate feature with approximately 3 km of relief from the surrounding basement and separates the Oobagooma Sub-basin from the Rowley Sub-basin at the Palaeozoic level (Smith et al, 1999; Figure 4). It consists of an asymmetric northeastward-dipping intruded high, which is fault-bound on it southern flank. At basement level the Oobagooma High trends from the Oobagooma Sub-basin to the Bedout High, truncating the Broome Platform. The Oobagooma High appears to be associated with volcanic lithologies, as evidenced by the presence of a strong magnetic anomaly associated with its western flank. The structural formation of the Oobagooma and Bedout highs appear to be related, although the mechanism is uncertain. The highs likely developed over areas of change in orientation of a zone accommodating differential northeast–southwest Palaeozoic extension (the North West Shelf Mega-shear). Orientation changes in these areas resulted in transpression along the mega-shear. Minor reactivation on the Palaeozoic detachment system and underplating (due to pure shear removal of the lower crust and simple shear) contributed to uplift of the highs. Development of the highs appears to have commenced in the Early to Middle Carboniferous to Late Permian (when sediments of that age are observed to onlap the feature), with reactivation during major tectonic events throughout the Late Carboniferous to Late Permian (major erosion during the Late Permian prevents the delineation of any finer tectonic history for the structure; Smith et al, 1999).

The Broome Platform is a mid-basinal swell or arch covered by less than 2 km of sediments (Kennard et al, 1994). It is a long-lived uplifted area of shallow basement, capped by a thin succession of Ordovician, Devonian and Permian rocks (around 1–2 km thick) gently dipping to the southeast. The platform is flanked on its northern margin by fault-bounded terraces tens of kilometres wide that preserve a thicker section (2–4 km) of mostly Ordovician and Devonian platform carbonates (the Jurgurra Terrace is the western-most of these terraces, which extends into the offshore; Figure 2). Fault bounded terraces up to 50 km wide also flank the Leveque Shelf along the northern margin of the Fitzroy Trough. These terraces also contain mostly Ordovician to Devonian carbonates capped by thin Permian sections (the Pender Terrace is the western-most of these terraces, which extends into the offshore; Figure 2).


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Geological evolution and tectonic development

The Offshore Canning Basin is a long-lived, multiple phase basin which has undergone a complex structural evolution (Kennard et al, 1994; Smith et al, 1999). A series of basinwide tectonic events have been identified in the Onshore and Offshore Canning Basin and used to divide the basin-fill into megasequences (Figure 5):

  1. Cambrian–Carboniferous. Earliest sedimentation occurred following northeast–southwest extension as the Chinese blocks separated from Gondwana (Samphire Marsh Movement). Intracratonic extensional and sag related deposition were periodically interrupted by folding and regional uplift or tilting and fault block movement associated with tectonic pulses in the earliest Devonian (Prices Creek Compressional Movement), Late Devonian (Van Emmerick Extension) and Early Carboniferous (Red Bluffs Extension).
  2. Sedimentation was terminated in the mid-Carboniferous during uplift and erosion associated with inversion on Devonian faults during the northeast–southwest oblique slip Meda Transpressional Event, which probably represents the peak of the Alice Springs Orogeny in central Australia.
  3. Late Carboniferous–Permian. The onset of formation of the Westralian Superbasin in the Early Permian defines the boundary between the northwest-oriented structures associated with the Canning Basin and the northeast-oriented structures associated with the Roebuck Basin. This was a period of transition from a regime of northeast–southwest extension to one of northwest–southeast extension during separation of the Sibumasu blocks from Gondwana. Landward of the Rowley Sub-basin synrift, sedimentation was still occurring along pre-existing intracratonic fractures formed during the northeast–southwest extension. A major tectonic episode produced extensive uplift, faulting and volcanism in the Late Permian to Early Triassic (Bedout Movement).
  4. Triassic–Early Jurassic. The Triassic was dominated by a period of thermal sag with transgressive marine sedimentation, punctuated by a series of Triassic to Early Jurassic northwest–southeast transpressional events (Fitzroy Movement). Here the Fitzroy Movement can be subdivided – Fitzroy Movement I in the Ladinian (Middle Triassic), Fitzroy Movement II in the Norian (Late Triassic – not widely documented offshore, but seen as an unconformity in the wells to the west; eg Phoenix), and Fitzroy Movement III in the Sinemurian (Early Jurassic). This last event marks a major change in gross stratal geometries within the basin from predominantly back stepping to prograding and aggrading.
  5. Middle Jurassic–Early Cretaceous. Middle Jurassic structural development occurred as a result of thermal subsidence after Early Jurassic uplift and erosion. A broad prograding wedge system developed across the shelf. Callovian breakup resulted in a second phase of prominent uplift and erosion that marked the end of active rifting adjacent to the Roebuck Basin. Thermal relaxation resulted in rapid transgression and development of a condensed section until the Early Cretaceous, when rejuvenation of the source province occurred during uplift associated with the rifting of India away from the western margin of Australia during the Valanginian
  6. Early Cretaceous–Recent. Thermal relaxation of the crust soon after the Valanginian led to the development of a passive margin. Full ocean circulation was established by the end of the Aptian. The separation of Antarctica and Australia reactivated older Palaeozoic features causing inversion and oblique slip movement, especially in the Oobagooma Sub-basin. Collision of the Australian and Eurasian plates in the mid-Miocene resulted in transpressional inversion of older north-northwest–south-southeast-trending Palaeozoic faults in the northeastern Oobagooma Sub-basin.

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Basin fill

Basin fill in the Oobagooma Sub-basin consists of approximately 5.5 km of Palaeozoic section and 4.5 km of Mesozoic and Cainozoic section (Smith et al, 1999). Little is known about the nature of Palaeozoic sedimentation offshore, as few wells have penetrated this succession. Most authors believe that the Palaeozoic sequence in the offshore is merely an extension of that seen in the Onshore Canning Basin (Passmore, 1991; Lipski, 1993; Colwell and Stagg, 1994; Smith, 1999). Ordovician to Middle Carboniferous sediments in the onshore primarily comprise alternating sequences of marine clastics and carbonates (Kennard et al, 1994). Of particular note within this succession are widespread evaporitic halite deposits, which form highly effective seals in parts of the onshore Canning Basin, and the Devonian reef complexes that have been the target of numerous exploration wells.

A thick (approximately 4 km) sequence of probable Late Carboniferous and Permian sediments comprising interbedded sandstones, siltstones and claystones with some dolerite sills, minor coal and thin limestone beds, forms the bulk of the basin fill (Colwell and Stagg, 1994).

The Palaeozoic sequence onlaps the Leveque Shelf and Broome Platform and is overlain by a strong regional unconformity that marks the Bedout and/or Fitzroy movements (Colwell and Stagg, 1994). The Triassic–earliest Jurassic section is absent and Early–Middle Jurassic sediments unconformably overlie the Permo–Carboniferous succession in each of the offshore wells. The Early–Middle Jurassic section forms a broad, sag ‘blanket’ less than 1.5 km thick across the trough and onlaps the Leveque Platform to the northeast. The sequence shows little structuring, other than minor reactivation of faulting, and comprises interbedded fluvio-deltaic sandstones, siltstones, shale and coal. A regional unconformity produced by continental breakup along the northwestern margin of the Canning Basin (Argo Breakup) separates Jurassic sediments from the overlying, mainly Cretaceous and Tertiary.

The Tertiary sequence is dominated initially by fluvio-deltaic and shallow marine clastics, but includes, with increasing margin subsidence, an increasing proportion of carbonates in its upper part (Colwell and Stagg, 1994).


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