JOURNAL OF SEDIMENTARY RESEARCH,VOL. 73, NO. 6, NOVEMBER, 2003, P. 1036–1050 Copyright  2003, SEPM (Society for Sedimentary Geology) 1527-1404/03/073-1036/$03.00 COMPOSITION, DIAGENESIS, AND WEATHERING OF THE SEDIMENTS AND BASEMENT OF THE CALLABONNA SUB-BASIN, CENTRAL AUSTRALIA: IMPLICATIONS FOR LANDSCAPE EVOLUTION ERIC TONUI 1, 2, * AND PATRICE DE CARITAT 2,§ 1 Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), c/- University of Canberra, Canberra, ACT 2601, Australia 2 Cooperative Research Centre for Landscape Evolution and Mineral Exploration (CRC LEME), c/- Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia ABSTRACT: The evolution of the Callabonna Sub-basin (central Aus- tralia) is described in terms of sediment provenance, stratigraphy, sed- imentology, and postdepositional mineral and chemical alteration. Six mineral exploration drillholes in the Mundi Mundi Plain were studied using a combination of qualitative and quantitative XRD, PIMA (portable infrared mineral analyzer), XRF, normative analysis, and SEM. The sedimentary cover consists of 160–210 m of diagenetically modified fluvio–lacustrine Tertiary clay, silt, and sand comprising sandy, mot- tled, and gray clay units overlain by Quaternary soil. The basement consists of weathered Proterozoic schist, gneiss, siltstone, and shale. Quartz, kaolinite, smectite, and muscovite are the dominant minerals within the sediments, whereas the weathered bedrock contains quartz, feldspar, kaolinite, illite, and muscovite. Characteristic wavelength absorption features from PIMA show that the youngest sandy clay unit exhibits the lowest reflectance inten- sities because of its lower diagenetic maturity. Weathering of primary minerals within the basement produced an illite–kaolinite-rich sapro- lite whose clays exhibit strong reflectance intensities and well-devel- oped crystallinity. The micromorphology of all secondary products re- flects their environment of crystallization. Low porosity and anoxic conditions within the sediments resulted in poorly developed clays of variable textures, colors, and compositions. Conversely, the weathered bedrock developed over protracted periods of in-situ weathering re- sulting in better-developed secondary products. This study distinguishes between preburial weathering of the base- ment, detrital mineralogy, postdepositional alteration of the sedimen- tary cover, and subsequent weathering of previously formed mineral assemblages. This has widespread implications for paleotopographic (-weathering) reconstitution and landscape evolution thus: (1) the ex- tent of weathering of basement relative to diagenesis and maturity of sediments and gradational nature of their unconformity suggests that most of the alteration of basement occurred prior to, and was sustained after, deposition of sediments, and (2) morphotectonic evolution of the region controlled the composition of transported materials by changing the style and patterns of fluvial systems, further influencing the distri- bution and preservation of sediments and weathered bedrock, and hence regional landscape morphology. INTRODUCTION The purpose of this study is to describe the evolution of the Callabonna Sub-basin (central Australia) in terms of sediment provenance, stratigraphy, sedimentology, and diagenesis, and to establish links, if any, between the postdepositional processes and weathering of basement. Variations in sed- iment compositions and diagenesis in modern exposure surfaces may serve * Present address: California Institute of Technology/Jet Propulsion Laboratory, Division of Geological and Planetary Sciences, Mail Code 170–25, Pasadena, Cal- ifornia, 91125, U.S.A. § Present address: Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME), c/- Geoscience Australia, GPO Box 378, Can- berra, ACT 2601, Australia. as analogs for buried paleoweathering surfaces and are therefore a key to unraveling regional basin paleotopographic, geomorphic, and tectonic his- tories. These studies, however, are generally uncommon (e.g., Velbel et al. 1994). Much of the Australian landscape is of considerable antiquity and has developed under conditions that have varied with time and location. Parts of its cratons and tectonically stable continental margins have been sub- jected to subaerial conditions for up to hundreds of millions of years (e.g., Pillans et al. 1999). Much of this basement is buried by younger sediments that have undergone protracted diagenesis (and weathering) under a variety of climatic conditions ranging from humid to arid, tropical and/or subtrop- ical to temperate, and even sub-Antarctic. The basement may also host rich mineral resources, and hence studies of the nature, provenance, and com- position of sediments are invaluable in developing more effective explo- ration techniques in areas of extensive cover (e.g., Butt et al. 1998; Tonui et al. 2003). The study area (Thunderdome Prospect) is situated ca. 60 km northwest of Broken Hill (Fig. 1), and ca. 20 km due west of the Mundi Mundi range front, a prominent north–south trending feature that extends for a distance of about 100 km. It is located within the Mundi Mundi Plain, which is part of the Callabonna Sub-basin, the southeastern depocenter of the larger Lake Eyre Basin. This intracratonic basin extends into parts of South Australia, the Northern Territory, Queensland, and New South Wales in central Aus- tralia. The basin has evolved through extensive Cenozoic sedimentation that covered the Mesozoic Eromanga Basin sequence and the Proterozoic basement. In this study, we combine modern mineralogical techniques like portable infrared mineral analyzer (PIMA) and quantitative X-ray dif- fraction (XRD) with conventional micromorphological and chemical ap- proaches in order to: (1) examine the nature, composition and distribution of diagenetically modified sediments; (2) determine the relationship, if any, between the postdepositional alteration of the sediment and that of the sub- sedimentary basement; and, (3) provide an evolution framework of the basin fill based on (1) and (2) for application in regional paleotopographic reconstruction and landscape evolution studies. GEOLOGICAL SETTING The geology of the basement below the Mundi Mundi Plain consists predominantly of a sequence of Broken Hill equivalent Proterozoic rocks belonging to the wider Curnamona Province, which extends into South Australia (Leyh and Conor 2000). Cenozoic sediments generally cover this basement, except in the areas of outcrop of the Broken Hill Domain to the east and the Olary Domain to the south. Detailed accounts of the geology and tectonic history of the Broken Hill Domain are given by Stevens (1980, 1986), Willis et al. (1983), Page and Lang (1990), and Stevens and Corbett (1993). The main components of the Broken Hill Domain are: (1) the complexly deformed high-grade metamorphic rocks of the Early Protero- zoic Willyama Supergroup; (2) the more gently folded, mostly lower greenschist grade metasediments and minor volcanic rocks of the Late Pro- terozoic Adelaidean sequence; and (3) products of Cenozoic sedimentation and weathering. The Willyama Supergroup comprises aluminous metasedimentary gneisses, plus locally abundant albite-rich rocks, lesser quartzofeldspathic gneisses, and substantial bodies of deformed pegmatite and leucocratic