ABSTRACT

The evolution of the Callabonna Sub-basin (central Australia) is described in terms of sediment provenance, stratigraphy, sedimentology, 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^TM (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, mottled, 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^TM show that the youngest sandy clay unit exhibits the lowest reflectance intensities because of its lower diagenetic maturity. Weathering of primary minerals within the basement produced an illite-kaolinite-rich saprolite whose clays exhibit strong reflectance intensities and well-developed crystallinity. The micromorphology of all secondary products reflects 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 resulting in better-developed secondary products.

This study distinguishes between preburial weathering of the basement, detrital mineralogy, postdepositional alteration of the sedimentary cover, and subsequent weathering of previously formed mineral assemblages. This has widespread implications for paleotopographic(-weathering) reconstitution and landscape evolution thus: (1) the extent 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 distribution and preservation of sediments and weathered bedrock, and hence regional landscape morphology.