TECTONIC REMOTE-SENSING FOR HYDROGEOLOGICAL MODELING IN TABLE MOUNTAIN GROUP AQUIFERS, WESTERN CAPE PROVINCE, SOUTH AFRICA

The lower part of the Late Ordovician-Early Devonian Table Mountain Group (TMG) contains an extensive (~100 000 km²) ultraquartzose formation, probably the thickest (1.5-2 km) of its kind anywhere. The structural geology of the Peninsula Aquifer was the main subject of a recent regional study, funded by the South African Dept. of Water Affairs and Forestry (DWAF) and known locally as the “Citrusdal Artesian Groundwater Exploration” (CAGE) Project. The name comes from a small town, ~200 km NE of Cape Town, along the upper Olifants River, the course of which is structurally controlled by a major synclinal fold.

Using Landsat-5 and SPOT imagery over a high-resolution (30 m pixel) digital elevation model of mountainous terrain, the CAGE Project accurately determined boundaries and internal structures of the TMG fractured-rock aquifers, quantified patterns of fracture orientation and spatial density, and thereby mapped the principal hydraulically conductive structures (“hydrotects”). Quantitative structural analysis from satellite-based remote sensing was supplemented by aerial photo interpretation of selected well-exposed terrains, follow-up fieldwork, and 3-D stereographic analysis.

In balanced section, large confined volumes of the Peninsula Aquifer are located up to 3 km below sea-level in a boxfold-like Olifants River Syncline (ORS), and its N/S fold-axial trend is kinematically related to a dominant NW/SE to NNW/SSE direction of sinistral strike-slip faulting. Four megafault systems transect the ORS on this trend, and are linked by connecting splays, cross-faults, and innumerable joints in five principal sets. Fracture-spatial density and -connectivity relations show that the fault-fracture system is a percolating network for kilometres-deep movement of groundwater from high recharge zones in the SE to valley and coastal-plain discharge zones in the N and W.

Perennial hot springs (e.g., 42.9°C flowing 30 l/s at “The Baths” spa) occur at key system nodes, and several deep boreholes of thermal artesian nature are sited on the hydrotect basis. Preliminary 3-D mass-balance modelling of surface- and groundwater flow regimes indicates conservatively that ~45 million m³/yr of the natural discharge to the ocean is potentially available for sustainable exploitation.