Stratigraphy of the Isa Superbasin

The Isa Superbasin extends ~300 km from the eastern Leichhardt River Fault Trough through to the Murphy Tectonic Ridge (Figure 1). The basin forms part of a network of similarly aged basins that evolved across the northern Australian craton during the Palaeoproterozoic (e.g., McArthur Basin: Figure 1). The major stratigraphic packages of the Isa Superbasin are presented in Figure 2 and its distribution is shown in Figure 1.

Syn-rift sequences

Syn-rift sequences display abrupt thickness changes across normal faults, they have wedge shaped stratal geometries suggesting deposition into half graben, and their lithofacies is influenced by extensional faults. In the Mount Isa Rift syn-rift sequences are up to 3-5 km thick. Significantly thinner (< 2 km) syn-rift sequences are also preserved on the western rift flank. The lower stratigraphy of the Isa Superbasin is dominated by a east thickening wedge of fluvial to shallow marine, texturally and compositionally immature sandstone, conglomerate, minor siltstone and thin (0-40 m) trachyte volcanic flows deposited into the eastern Mount Isa Rift (Bigie Formation). Abrupt thickness changes of up to several hundred metres occur across normal faults. The Bigie Formation has a maximum thickness of 700 m but tapers rapidly westward, becoming absent in the central and western Mount Isa Rift (O'Dea et al., 1997b). The Bigie Formation was deposited into localised sub basins to the northwest of the Mount Isa Rift. It is thinner (<200 m) and is dominantly composed of proximal fluvial, coarse-grained sandstone and conglomerate (Betts et al., 1999). The Bigie Formation tapers towards the NW and is intercalated with the bimodal Fiery Creek Volcanics (Figure 2). The Surprise Creek Formation is west tapering stratigraphic package that was deposited during continued rifting (Figure 2) (Betts and Lister, in review). Along the eastern Mount Isa Rift this formation is ~2.5 km thick (Batson, 1991). It is characterised by alternating sandstone and quartzite sheets and wedge shaped siltstone horizons. The lateral continuity, the relatively large thickness, and the anoxic character of the siltstone horizons suggest deep either shallow marine or lacustrine depositional environments (Batson, 1991). The sandstone and quartzite horizons are composed of interbedded quartz arenite, quartzite and minor micaceous sandstone (Batson, 1991). Their high textural and compositional maturity suggest deposition in a distal fluvial environment or a wave dominated marine environment (Batson, 1991). Sheeted sandstone and quartzite define periods of sedimentary regression.

The Surprise Creek Formation is locally absent along the western Mount Isa Rift (O'Dea et al., 1997a, b; Derrick, 1982; Nijman et al., 1992a). Here, it overlies progessively deeper stratigraphic levels to the west, and is thinner (0-800 m) (Betts et al., 1998; Betts and Lister, in review). The lower part of the formation is characterised by proximal fluvial, coarse sandstone and conglomerate channels, which grade into near shore, mature, fine grained sandstone (Nijman et al., 1992a). These are overlain by shallow marine fine grain sandstone, siltstone, and minor carbonate horizons (Derrick et al., 1980). The overlying Torpedo Creek Quartzite and the NW-tapering Gunpowder Creek Formation are considered part of the same sedimentary package as the Surprise Creek Formation (Figure 2).

Syn-rift sedimentation continued until ~1655 Ma. The upper most syn-rift sequence is the shallow marine siltstone and fine grained sandstone of the Moondarra Siltstone. A maximum thickness of ~1300 m is preserved in the Paroo Range (Nijman et al., 1992a).

Post-rift sequences

The post-rift evolution of the Isa Superbasin began sometime before ~1653 Ma and continued until ~1595 Ma (Page and Sweet, 1998). Post-rift sequences include the McNamara Group in the northern Mount Isa terrane and by the Mount Isa Group above the Mount Isa Rift (Figure 2). The style of sedimentation changed from dominantly siliciclastic to carbonate sequences, and the basin depocentre shifted to the northern Mount Isa terrane.

The post-rift stratigraphy within the Mount Isa Rift comprises the sparsely preserved Mount Isa Group (1652±7 Ma: Page and Sweet, 1998). (Figure 2). The Mount Isa Group is dominantly composed of dolomitic and pyritic siltstone and shale, and minor conglomerate interpreted as shallow water, hypersaline lacustrine deposits with periods of sub aerial exposure (Neudert and Russell, 1981). The Mount Isa Group has a preserved thickness of ~3000 m (Figure 3).

Figure 3. Stratigraphic column

Stratigraphic column

Stratigraphic columns showing thicknesses and interpreted periods of rifting and intervening tectonic quiescence during the Mount isa Rift Event: (a) section is located ~20km to the west of the Fiery Creek Dome; (b) section is located in the Sunday Gully region along the eastern margin of the Mount Isa Rift (Batson, 1991); (c) and (d) major lithostratigrapohic fomration so fghte isa Superbasin and their depositonal ages (data adapted rom Andrews, 1998; Dunster and McConachie, 1998; Southgate et al. 1997; Page et al., 1994; Page and Swet, 1998; Neudert and Russell, 1981). Comparison of the stratigraphy of the Isa Superbasin between the northern Mount Isa terrane (c) and the Mount Isa Rift (d). FCV - Fiery Creek Volcanics; TCQ - Toperdo Creek Quartzite; WPQ - Warrina park Quartzite; GCF - Gunpowder Creek Formation; PCF - Paradise Creek Formation; EF - Esperanza Formation; LLF - Lady Loretta Formation.

The McNamara Group is divided into a lower and upper member. The lower member correlates with the Mount Isa Group (Figure 2) and has a cumulative thickness of ~3000 m. It is dominantly composed of stromatolitic and dolomitic siltstone, sandstone and mudstone, deposited in a shallow water or shallow marine environment (Dunster and McConachie, 1998). The lower McNamara Group thins towards the western margin of the Mount Isa Rift (Sami et al., 1997). Equivalent sequences of the Mount Isa Group (Figure 3d) thin towards the western margin of the Mount Isa Rift (Neudert, 1983), indicating an intrabasinal high along the western rift flank during the deposition of these formations.

The sequences of the upper McNamara Group (Figure 2) were deposited as the basin depocentre shifted to the northwest (Andrews, 1998). These sequences have a cumulative thickness of ~8 km (Andrews, 1998). There are no recognised equivalents of the upper McNamara Group above the Mount Isa Rift. This group comprises outer shelf to deeper water sandstone, siltstone and shale sequences (Andrews, 1998) overlain by turbidites (Andrews, 1998). These in turn are overlain by outer shelf sandstone, siltstone, and shale sequences (Andrews, 1998). There is abundant evidence to suggest syn-depositional fault activity occurred episodically during thermal subsidence (Andrews, 1998; Rohrlach et al., 1998). Tuffaceous horizons throughout the upper McNamara Group indicate episodic volcanism.