|Age ||Neoproterozoic - Middle Cambrian |
|Area in South Australia ||43 000km2 (16 600 su. miles) subsurface, 12 000km2 (4630 sq. miles) outcrop. |
|Depth to taget zones ||<1000-1700 m. |
|Thickness ||1000-3500 m. |
|Hydrocarbon shows ||Oil shows in Wilkatana wells; trace free oil (Moodlatana Formation), slightly gas-cut, very saline water (Wirrealpa Limstone) in Moorowie 1; gas shows from water bores in Moorowie and Yalkalpo synclines (anecdotal). |
|First commercial discovery ||None |
|Identified reserves ||Nil |
|Undiscovered rexources (50% prob.) ||Not determined |
|Production ||Nil |
|Basin type ||Extensional, closed by subsequent collision. |
|Depostional setting ||Marine shelf-ramp and shelf-basin complex; reefs |
|Reservoirs ||Dolomite, limestone karst, sandstone. |
|Regional structure ||Anticlines, some detached, others diapiric; local thrust-wrench complexes with pop-up structures. |
|Seals ||Micritic limestone, redbeds, anhydrite. |
|Source rocks ||Micritic limestone, shale. |
|Depth to oil/gas window ||1300-2000 m in Moorowie Syncline. |
|Number of wells ||4 deep geothermal wells, 8 petroleum, 3 stratigraphic, ~20 deep mineral wells. |
|Seismic line km ||3398 2D. |
Arrowie Basin carbonates and clastics were deposited in tropical latitudes on the eastern seaboard of the Gawler Craton from the Early to mid Late Cambrian. These deposits disconformably overlie Neoproterozoic strata of the Adelaide Geosyncline.
The Arrowie Basin comprises four major structural components: thin flat-lying cover on the Stuart Shelf; faulted slices of unknown thickness in the structurally complex Torrens Hinge Zone between the Stuart Shelf and Flinders Ranges; thick but dispersed outcrops in the ranges; and a synclinorium bisected by the Proterozoic Benagerie Ridge between the Flinders and Barrier ranges in western New South Wales.
More than 5 km of sediment accumulated in the central Arrowie Basin, now exposed in synclines in the Flinders Ranges. Thinner Cambrian cover (~300 m) on the Stuart Shelf is situated west of a complex of N–S striking faults (Torrens Hinge Zone). In the eastern Arrowie Basin, up to 2300 m of Cambrian strata occur at depth in the Moorowie and Yalkalpo synclines which are separated by a structurally elevated Proterozoic ridge.. Total subcrop area east and west of the ranges with hydrocarbon potential, is ~43 000 km2.
Depositional continuity between the Arrowie Basin and the eastern Warburton Basin to the north was severed late in the Early Cambrian by uplift which preceded a possible microcontinental collision in the Middle and Late Cambrian; the collision site being the Wonominta area in western New South Wales. Depositional links with the Stansbury Basin to the south may have continued until the Middle to Late Cambrian collisional Delamerian Orogeny, but evidence has long since disappeared. The original area of sedimentation was thus quite different from the remnant now preserved.
Structural style in the Torrens Hinge Zone, west of the Flinders Ranges, is poorly known due to an absence of useful seismic coverage. Judging by the >1000 m depth difference to the top of Wilkawillina Limestone in existing wells, fault displacements are large and faults are likely to be compressive. On structural ridges, as at Old Motpena 1, the Lake Frome Group (1200+ m thick at Brachina Gorge), has been completely eroded. Gravity interpretations also indicate major faults at shallow depth in the Motpena area. The fold pattern of domes and basins mapped in outcrops of the Flinders Ranges does not convey the extent of thrust faulting, particularly on the eastern and western margins of the ranges.
The Yalkalpo Syncline east of the Benagerie Ridge appears to be a simple north-plunging depression. The Moorowie Syncline is terminated to the north by faulted basement; beneath Lake Frome a major N–S wrench complex (Poontana Fracture Zone) extends through the syncline.
Three sequence sets (supersequences) Є1, Є2 and Є3 constitute the Cambrian Arrowie Basin fill
. The lowest two, subdivided into six third order sequences (‘Uratanna’ sequence, Є1.1, Є1.2, Є1.3, Є2.1, Є2.2), have hydrocarbon potential.
Above the basal clastic Uratanna sequence, sequences Є1.1–Є1.3 (Hawker Group) are dominated by carbonates ranging from an initial shelf–ramp complex (Є.1.1) to a reef-rimmed platform passing north to a deep marine basin. Distinct phases of reef growth, platform exposure and karsting, onlap and offlap of slope sediments, and a final progradational–aggradational phase of basin infilling are recorded. The uppermost sequence Є1.3 is punctuated by episodes of volcanism and tectonic activity, the latter eroding channels through the reef complex and shedding sediment down the palaeoslope to the north. The Benagerie Ridge was uplifted at the same time, effectively separating the Moorowie and Yalkalpo synclines. Widespread regression in upper Є1.3 is reflected by development of evaporitic sabkhas characterised by stromatolitic dolostone and gypsum deposits. The evaporite facies is preserved as an 8 m thick anhydrite cap in Moorowie 1 at a depth of 2180 m.
Sequences Є2.1 and Є2.2 were deposited under the waning influence of base-level readjustments following the tectono-volcanic episode known as the Kangarooian Movements. Billy Creek Formation redbeds were succeeded by a transgressive–regressive phase of carbonate deposition (Wirrealpa Limestone) that is also recorded in contemporary or slightly younger deposits across Australia (e.g. Warburton, Amadeus, Georgina, Daly, Bonaparte basins).
The third sequence set Є3 is represented by redbeds of the Lake Frome Group. This is a package of arkose with minor carbonate which in general becomes more quartzose and coarser grained up-section. Sequences have not yet been differentiated in this essentially deltaic to shallow subtidal succession. Fluvial influence is evident in upper beds of the Grindstone Range Sandstone which represents the youngest Cambrian exposure in the Arrowie Basin. Age is probably no younger than early Late Cambrian. More information on the Arrowie Basin Key references