256 by Christine J. Edgoose The Amadeus Basin, central Australia Northern Territory Geological Survey, PO Box 8760, Alice Springs, NT 0871, Australia. E-mail: [email protected] The Amadeus Basin of central Australia has a E-W and a maximum of 300 km N-S in central Australia (Figure 1). depositional history spanning the Neoproterozoic to the It overlies Paleo–Mesoproterozoic basement of the Musgrave Province to the S and Arunta Region to the N, and is overlain by Devonian/Carboniferous. It was initiated as part of the the late Paleozoic Pedirka Basin and Mesozoic Eromanga Basin Neoproterozoic Centralian Superbasin, which formed in in the SE, and by the Paleozoic stratigraphy of the Canning Basin an intracratonic setting related to the break up of to the W. The present-day Amadeus Basin is a structural remnant Rodinia. Sedimentation continued until the 580–540 Ma of a broad, shallow basin – it has been significantly tectonically Petermann Orogeny, coinciding with the assembly of modified during two major intracratonic orogenic events: the 580– 540 Ma Petermann Orogeny and the 450–300 Ma Alice Springs Gondwana, which resulted in the fragmentation of the Orogeny. superbasin into separate intracratonic basins. The The early history of the Amadeus Basin is as part of the Petermann Orogeny was focused in the Musgrave Neoproterozoic Centralian Superbasin (Walter et al., 1995), which Province and the southern part of the Amadeus Basin, also encompassed the Officer, Ngalia, Georgina, and Murraba basins (Shaw et al., 1991; Walter et al., 1995), and probably several smaller and involved significant N-directed shortening on large- basins in Western Australia (Figure 2). Formation of the Centralian scale structures that involved both the basement and the Superbasin coincided with NE-SW-directed intracratonic extension overlying Neoproterozoic sedimentary rocks. It across the Rodinia Supercontinent, which eventually led to the break- significantly transformed the basin architecture, with the up between North America and Australia at c. 830 Ma (deVries et al., development of major basin features that controlled 2000). Walter and Veevers (2000) described this phase as Centralian 1. After the break up of Rodinia, sedimentation continued locally in subsequent sedimentation. Deposition of Paleozoic the Centralian Superbasin until c. 750 Ma. In the Centralian 2 phase, successions was largely concentrated in sub-basins and renewed but localised sedimentation related to the 700–690 Ma troughs in the N of the basin, where up to 14 km is Sturtian glaciation took place. Centralian 3 is associated with the preserved. Minor events or uplifts punctuated this younger Elatina (Marinoan) glaciation. Sedimentation in the Centralian Superbasin was terminated by the 580–540 Ma Petermann depositional history and account for local Orogeny, which coincided with the final stages of the assembly of disconformities and absent sections. The 450–300 Ma Gondwana (de Vries et al., 2000), but continued locally in the now Alice Springs Orogeny was a multi-phase, intracratonic separated basins, for example in the northern part of the Amadeus event concentrated in the Arunta Region and the northern Basin. part of the Amadeus Basin. Like the earlier Petermann Korsch and Lindsay (1989) and Lindsay and Korsch (1991) recognised three main stages of basin evolution. Stage 1 was a long- Orogeny, the Alice Springs Orogeny involved both lived extensional–thermal relaxation (sag) event (c. 900–590 Ma). basement and basin sedimentary rocks, but with overall In the SW of the basin, Stage 1 sediments are underlain by a rift S-directed movement. Synorogenic sedimentation sequence (bimodal volcanics and associated sedimentary rocks (e.g., accompanied Mid–Late Devonian uplift, with Late Close et al., 2004; Edgoose et al., 2004). The relationship of this rift succession to the initiation and early evolution of the Amadeus Basin Devonian–Carboniferous basin inversion terminating is equivocal, as the rift sequence is c. 150 Ma older than the Stage 1 sedimentation, and folding the youngest successions. sediments (sag phase). Such a time gap suggests the rift and sag may The Amadeus Basin has known reserves of U, minor not be related (Shaw, 1991). However, it is also possible that a historic and recent Au production, and is prospective for significant time break occurs at a disconformity within the sag phase base metals, especially Cu, and phosphate. The sediments, and that the deposition of sag phase sediments commenced soon after rifting ceased (A. Camacho, per. comm., 2011). Stage 2 Ordovician succession supports commercial gas (c. 580–450 Ma) comprised an early compressive event (Petermann production, and the Neoproterozoic succession is Orogeny) then extension followed by a thermal relaxation event and considered prospective for oil and gas. resulted in shallow-marine to terrestrial deposition across the basin. Stage 3 (c. 450–300 Ma) was the final phase of basin evolution and was a dominantly compressional phase (Alice Springs Orogeny). Introduction Events relating to this stage account for most of the obvious structures The Amadeus Basin is a large (c. 170,000 km2) elongate intra- (folds, domes and thrusts) evident in the present-day surface geology cratonic Neoproterozoic–Devonian basin that extends c. 800 km (Figure 3). March 2012 257 zircon data points to different source areas for the two basins (Haines and Wingate, 2007). The Amadeus Basin shows evidence of having been open to the E and sourced material from eastern Australia. Marine sedimen- tation ceased at the end of the Ordo- vician in response to broad, regional uplift. From this time to the Early Devonian, only limited deposition occurred in the basin, in aeolian and lesser fluviatile systems. The Silurian was largely a period of hinterland erosion, followed by fluvio-lacustrine to paralic conditions in parts of the basin in the late Early Devonian. The middle–late Devonian is charac- terised by an upward-coarsening fluvial system, deposited in response to uplift related to the Alice Springs Orogeny, and sedimentation culmi- nated in the Late Devonian/early Carboniferous as a result of final orogenic processes and basin inversion. Permian glaciation in the area suggests that highlands persisted Figure 1 Location of Amadeus Basin and surrounding tectonic regions. until this time, and that the area was at relatively high paleolatitudes. Shaw (1991) subdivided the tectonic development of the basin The units of the Amadeus Basin are discussed in more detail below in more detail, and described at least nine tectono-stratigraphic (see also Figure 4). intervals of basin history, separated by regional unconformities, which strongly influenced basin shape and evolution. This tectono- Neoproterozoic stratigraphic record is interpreted to have largely resulted from far-field forces at plate margins well outside the region. Basement Cryogenian: Supersequence 1 structures also appear to have placed long-term and repeated controls on subsidence patterns. Reactivation of pre-basin Paleo- A sequence stratigraphic analysis has identified 4 supersequences proterozoic and Mesoproterozoic fault zones beneath the Amadeus in the Neoproterozoic stratigraphy of the Amadeus Basin (Walter et Basin, during both the Petermann and Alice Springs orogenies, has al., 1995). Supersequence 1 forms part of the initial sag phase, been a first-order control on basin architecture and the location equivalent to Stage 1 basin evolution of Lindsay and Korsch (1991) of basement highs and depocentres (Shaw, 1991; Munroe et al., and Interval 1 of Shaw (1991). The sequence comprises a relatively 2004). thin, basal clean sand sheet deposited in intertidal and fluviatile During the early–mid Paleozoic (Stage 2 of Lindsay and Korsch, environments (Heavitree and Dean quartzites), followed by a thicker 1991), Australia was located at low paleolatitudes, and at times of succession of marine carbonate rocks, fine siliciclastic sedimentary high sea level, transcontinental seaways may have existed (Walley et rocks and evaporites (Bitter Springs Formation and Pinyinna al., 1991; Lindsay, 1993). During the Cambrian, the seaway axis lay beds). to N of the Amadeus Basin (across the Georgina,Wiso and Daly The characteristics of the Heavitree and Dean quartzites imply basins). Most paleogeographic reconstructions of Ordovician Australia an abundant supply of sediment deposited in a high-energy shelf-like (e.g., Nicoll et al., 1988; Walley et al., 1991) show an open E-W environment. Sedimentological and stratigraphical analyses by seaway (Larapintine Seaway) across the continent. The Ordovician Lindsay (1993, 1999) show that quartz sandstone sedimentation was a period of widespread deposition across Australia, with deeper occurred in a shallow, low-gradient ramp setting. Detrital zircon data water continental margin systems established in eastern Australia, for the Heavitree Quartzite suggests a maximum deposition age of and shallow-marine to paralic conditions prevailing in several inland 1050–1000 Ma (Zhao et al., 1992), with detrital zircon age data and northern basins. Thick successions accumulated in the Amadeus indicating a source dominated by the Arunta Region. Maidment et and Canning basins, and most reconstructions show the two basins al., (2007) provided the youngest zircon age at 1.12 Ga, probably linked during this time, effectively operating as a large, single sourced from the Musgrave Province. depositional system. However, differences in stratigraphy, depositional