Landscape evolution – a geological journey

The Australian continent has been incorporated into the massive supercontinents of Rodinia, Pangea and Gondwanaland over the past 1000 million years (Li and Powell, 2001). The breakup of the Neoproterozoic Rodina and establishment of Gondwanaland marked the development of the so called Tasman line which defined the then eastern edge of the then Australian continent. To the west of this margin lay the ancient stable Proterozoic Australian Craton. The Tasman line marked the initial position of what developed into the Tasman Geosyncline. The significance of the geosyncline in the context of this study lies in its contribution to the development of future sequences of sedimentary basins, repositories of the vast quantities of sediment eroded from a sequence of orogenies along the eastern margin of Australia. Today the eastern one third of the continent has effectively been „welded‟ to the ancient Proterozoic western craton via a sequence of accretions east of the Tasman surface (Cawood and Buchan, 2007, Cawood and Korsch, 2008). The Tasman Geosyncline effectively formed at the time of the breakup of Rodinia. Subsequent complex tectonic activity resulted in the Tasman Fold Belt System which „recorded‟ Neoproterozoic, Paleozoic, Mesozoic and Cenozoic activity, associated with initial plate convergence and subsequent divergence between the Pacific and Australian Plates (Veevers and Rundle, 1979, Scheibner and Veevers, 2000). (Geological time scale: Appendix 2).

Gondwanaland had effectively formed by the middle Cambrian (ca 505 million years ago, mya) (Li and Powell, 2001). Even at this early stage the westward subduction of the palaeo- Pacific margin along the eastern margin of Australia-Antarctica had commenced (Korsch et al., 2009). This margin, or active convergence zone, produced a long active Andean type magmatic arc. From this period until the final separation between Antarctica and Australia in the mid Oligocene 30 mya, tectonic activity on the supercontinent had major implications for the ongoing development of drainage basins and sedimentary sequences of direct relevance to the soils of the study area. Jostling of continental plates, continental rotations and drift of the supercontinent all resulted in complex extensional and contractional tectonic movements contributing to early orogenies, associated basin subsidence and sedimentary deposition (Klootwijk, 2009). The earliest basins date from the beginning of the Cambrian as the subduction zone developed along the palaeo-Pacific margin of Gondwanaland.

Internal basin development can be explained as a geochronological succession of stages (Doutch and Nicholas, 1978, Cawood and Korsch, 2008). Each of these stages of development was related to the progressive cratonisation, or accretion, out from the Tasman Line from west to east (Veevers, 1980). These are represented by the step-wise development of the Tasman Fold Belt System from the early Kanmantoo belt through the Thomson and Lachlan Fold Belts and finally to the New England Fold Belt, the youngest orogeny on the continent (Finlayson, 1993). Basin development accompanied each of these systems from the earliest Devonian-

Carboniferous period. Basins developed on the continental side of a predominantly convergent palaeo-Pacific margin, with the major period of basin formation occurring from the Mid Palaeozoic throughout the Mesozoic (Finlayson, 1993). Within the study area the Permo- Triassic period of basin development is represented by the Cooper and Galilee Basins, containing a sequence of non-marine sediments deposited in a fluviatile, lacustrine and floodplain environment. The Cooper Basin was initiated as a gentle down-warp in the Triassic marking the development of basin geography in the study area. However it is the Jurassic, Cretaceous and Cenozoic periods of basin development that are of greatest significance to the landscape and soils of the study area. The development of basins within the contemporary Lake Eyre Basin represents Gondwanan sedimentary environments (Wasson, 1982).

Epeirogenic movements associated with the initiation of the breakup of Gondwanaland during the middle Jurassic (~160 mya), resulting in crustal movement or stretching, developed the Great Artesian Basin (GAB). By the beginning of the Cretaceous (103 mya) four effective continental blocks existed: South America, Africa-Arabia, India-Madagascar and Antarctica- Australia (Quilty, c1984, Li and Powell, 2001). The Cretaceous was a critical period in the context of the study area as it resulted in much of the source material for the soils of Cooper Creek.

Until the Late Jurassic to Early Cretaceous, Australia and New Zealand constituted a „north-eastern‟ section of Gondwanaland. However, during the Late Cretaceous New Zealand split from Australia, resulting in the development of the Tasman Sea. This was accompanied by significant volcanic activity along the eastern margin of Australia. The Late Cretaceous-Early Cenozoic was a critical period in the development of the Eastern Highlands. Plate divergence contributed to the initiation of uplift which continued intermittently through the Cenozoic, reactivating erosional processes and initiating a predominantly west to south-west internal drainage system (Wasson, 1982, Williams, c1984).

The Jurassic–Cretaceous basins represent an extensive group, referred to agglomeratively as the Great Artesian Basin (Figure 4-1), comprising the Carpentaria, Eromanga and Surat Basins. The Eromanga Basin is of crucial significance to this study and shall be examined in more detail as it contains the Jurassic-Cretaceous sediments that ultimately provided most of the source materials for the Quaternary deposited clay soils of Cooper Creek. It is important to realize that the chronological sequence of basins referred to above was draped in succession over previous older basins (Doutch and Nicholas, 1978, Veevers, 1980). This had implications for the structural development of the latest basins. Folding and faulting developed on older original structures, influencing evolving erosional and drainage patterns. These „lines/areas of structural weakness‟ were subsequently re-activated during periods of epeirogenic movement or continental drift. Additionally, broad scale down warping occurred as older sequences in earlier basins were compacted (Exon and Senior, 1976).

Figure 4-1 The Great Artesian Basin (GAB)

An indication of the complex tectonic and drainage histories of the Lake Eyre Basin which combine to contribute to the current dune fields of the region is provided by Pell et al. (2000). Their research determined that the sands of the southeastern Simpson, Strzelecki and Tirari deserts were originally sourced from a number of regionally dispersed, major basement rock provenances. These include, in order of decreasing contributional significance, the Tasman Orogenic System (New England and Lachlan Fold Belts), the Georgetown Inlier, the Musgrave and Arunta Blocks, and the Gawler and Curnamona Cratons.

Their research provides an indication of the shifts in drainage orientation that have occurred through geological time within the Lake Eyre Basin. It also indicates the nature of connection between sedimentary basin systems and adjacent orogenic systems. Pell et al (ibid). use the term „protosource‟ to describe the parent igneous/metamorphic rock in which particular primary minerals are originally crystallized. The more general term „source‟ they use to describe the large number of intermediate „reservoirs‟ within the sedimentary system through which these minerals have cycled over geological time from „protosource‟ to current location (Pell et al., 2000). The sands of these dune fields are derived mainly from the erosion of sedimentary sequences that themselves are a result of the erosion, transport and deposition of material from „protosources‟.

Exon and Senior (1976) provide an excellent, definitive overview of the geological development of the Eromanga and Surat basins. The following account draws heavily from their work. The Eromanga Basin formed largely as a result of broad intracratonic subsidence or sag over underlying older Permo-Triassic basins (Wake-Dyster et al., 1983). During the Jurassic sedimentation was non-marine. At the close of this period over 1000m of quartz-rich, clastic sediments had been deposited. The region consisted of broad alluvial plains draining mainly to the north. During the Early Cretaceous, tensional tectonic activity between Antarctica and Australia resulted in increased subsidence and increased sedimentation rate in the basin. From the east, vast amounts of volcanogenically derived detritus further increased the basin load further encouraging subsidence.

The Late Aptian to Cenomanian (Appendix 2) marked the gradual inundation and eventual regression of a vast inland sea, a period during which the Rolling Downs Group of sediments, highly significant in the context of this study, was sequentially deposited (Turner, 1993). Appendix 3 provides detailed descriptions of the characteristics of the Rolling Downs sediments. Importantly for the eventual synthesis of the clay mineral montmorillonite, the Rolling Downs Group is dominated by andesitic volcanogenic sediments derived from volcanic activity that extended along the north-eastern margin of the Eromanga basin. These vocanogenically derived sediments cover around 2,000,000 km2 of the basin to an average thickness of around 500 m. Exon and Senior (1976) speculate that this would have required an original thickness of around 2000 m of volcanics transported rapidly by wind and stream action from ranges 150 km wide and 300 km in length.

Following the period of Cretaceous deposition an erosional phase from the Late Cretaceous to Early Tertiary removed vast amounts of the Cretaceous sedimentary sequence, leading to regional peneplanation (Twidale and Campbell, 1988, Twidale, 1994, Twidale and Campbell, 1995). A subsequent extended period of deep weathering of the Cretaceous sequence took place, altering profiles to a depth of as much as 120 m thick in places. This Tertiary weathering period was an extremely significant precursor to the subsequent development of the contemporary landscape of the study region (Ludbrook, 1980). The weathering chemically altered the profile, creating a thick bleached saprolite. The previously unweathered marine sediments contained significant quantities of sodium, chloride and sulfate conducive to the creation of an acidic environment and the mobilisation and/or conversion of minerals (Gunn and Fleming, 1984). Warm wet conditions prevailed in the Paleocene, leading to the conversion of montmorillonite within the mantle to deep kaolinitic profiles, rich in gypsum. Following this period, climatic conditions changed from the Eocene through the Oligocene. Alternating wet and dry periods led to the creation of pedogenic silcrete in a vadose groundwater environment. This involved cycles of leaching, infiltration and illuviation alternating with evaporative conditions resulting in the precipitation of silica mineral (Simon - Coincon et al., 1996, Thiry et al., 2006). As drier conditions prevailed the acid/saline rich groundwater level continued to fall

during the Miocene, creating ideal conditions for the maximum development of a deep, bleached, kaolinitic profile with distinctive ferruginous, mottled and pallid zones.

By the end of the Cretaceous only Antarctica and Australia remained connected and by the Late Jurassic separation between the two continents had commenced (Li and Powell, 2001). During the Tertiary broad regional structural deformation led to the development of broad anticlines and synclines in the Eromanga Basin. This established the spatial framework for the development of the current land surface and associated drainage direction and orientation of the Cooper Creek drainage system (Maroulis et al., 2007, Nanson et al., 2008).

The continued down warping of the Lake Eyre Basin, combined with epeirogenic uplift in the north and east, reactivated extensive regional erosion. This resulted in the extensive stripping of the deeply weathered mantle during the mid-Tertiary (Alderman, 1973, Moss and Wake-Dyster, 1983, Gurnis, 1992, Turner, 1993, Alley, 1998). These tectonic movements additionally altered the slope of drainage and established the contemporary drainage towards Lake Eyre. The stripping was most extensive around the margins of the basin, in the upper catchments of the main river systems, resulting in almost complete removal of the weathered mantle and exposure of the older unweathered („fresh‟) fine-grained Cretaceous sediments of the Rolling Downs Group (Gunn, 1974, Exon and Senior, 1976). In mid-catchment regions the stripping was not complete. Erosive dissection of the landscape left remnants of the weathered mantle on interfluves or raised areas within the drainage pattern, protected by a cap of resistant silcrete. These protected areas are now expressed in the region as plateaus and mesas where ongoing erosion exposes scarps of the weathered pallid mantle below.

As a result of the above geological and geomorphological process, a range of grey, brown and red clay soils are found extensively throughout the catchment of Cooper Creek.