Delineation of palaeochannels of the Northern Cape Province using remote sensing: Implications for mineral targeting

Palaeochannels are unconsolidated or semi-consolidated sediments deposited in ancient, currently inactive river and stream channel systems causing linear or sinuous depressions. In order to reconstruct the palaeodrainage system, a good understanding of the geomorphology is required. The study area is located within the Namaqualand Metamorphic Province in the Northern Cape Province, South Africa, covering an area of about 20446 km2 on the Bushmanland Plateau. The arid to semi-arid climatic conditions of the Northern Cape contribute to the preservation of the palaeochannels. This study explores the effectiveness of satellite remote sensing techniques to map out the spatial distribution and characteristics of preserved palaeochannels. Detecting palaeochannels using remote sensing techniques is a cost and time effective way of narrowing down target areas, thereby assisting in exploration programs. Various remote sensing datasets, such as Landsat 8, Aster Night Time Thermal Infrared (NTIR) and SRTM digital elevation model (DEM) have been utilized for detection and delineation of palaeochannels and associated geomorphological features. Target areas associated with surficial uranium within palaeochannels are mainly in calcretes. These were mapped using their characteristic absorption bands between 1.70 μm, 2.16 μm, 2.21 μm, 2.33 μm and 2.44 μm wavelength regions. Image enhancements such as image fusion, linear stretching and Normalized Difference Vegetation Index (NDVI) were also computed. Results from processed Landsat 8 data shows a high correspondence to surficial regolith landform and allowed spectrally homogenous units to be equated with terrain units, the bedrock tends to be the granite or gneiss of the Namaqua Natal Metamorphic Province. A number of band combination, i.e. False Colour Composite(FCC), band ratios for separating wet areas from dry areas Band 12 (Band 5/Band7) were also computed. Results from the DEM data showed indirect associations between modern and ancient relief attributes. Modern day topographic lows corresponded to the Tertiary palaeochannels and related depressions (playas), with topographic highs corresponding to basement interfluves characterized by irregular mountain ranges and dunes. A Topographic Wetness Index (TWI), a combined measure of upper catchment and slope, provided a good indication of the spatial distribution of soil moisture, surficial saturation and erosional landforms (low TWI). Another dataset that was used to delineate the palaeochannels was ASTER night time data, which has five bands in the thermal infrared region. The ASTER Night Time Thermal Infrared is effective at discerning the difference in thermal properties between the palaeochannels fill sediments and the surrounding bedrock. Validation using spatial profiling through a DEM was done, where profiles for different sections were plotted to distinguish surface channels from subsurface channels and also to attempting to interpret the thickness and nature of palaeochannels fill.