Understanding the Geomorphology of the Gobholo Granite Cave System in Eswatini

Extending some 60 m below ground, and with explored lengths of more than 1 500 m, the Gobholo cave system in Eswatini is one of the largest granite cave systems documented in the scientific literature. The cave system is found in the Gobholo valley, approximately 10 km east of Mbabane. The valley is drained by the Gobholo River, which has a subsurface flow through the cave system for more than 1,8 km. The Gobholo valley itself is located on the Mbabane granite pluton which is made up of the Mswati granite, of 2,7 Ga. in age. It is a porphyritic granite of coarse grain which forms part of the Kaapvaal Craton. The Kaapvaal Craton is an old yet stable landscape composed primarily of granitoids, gneiss, and other metamorphosed volcanic and sedimentary rocks varying in age between 2.5 – 3.6 Ga. The Mbabane Pluton is remarkably affected by tectonics as evidenced by a network of NNE-SSW and NW-SE fault lines and lineations, which account for the deep dissecting and near perpendicular valleys, with the Gobholo Valley being one of these.

A notable feature of this cave system, compared with other granite caves reported in the literature, is that it has elements of both solutional-depositional systems, as well as the ‘typical’ boulder systems in which the interstitial fines have been washed out between large, buried scree-slope boulders. The research focusses on the structure and composition of the granitic bedrock, and compares its mineralogy with the geochemistry of the water of the Gobholo River. Remote sensing and GIS are used to map the spatial arrangement of landforms, geological structure, and digital modelling of the terrain. These are further used to explain the geomorphic processes of the study area. The analysis and modelling of the Gobholo Valley is of key importance in this research, as it informs the discussions and conclusions on the genesis and dynamics of the Gobholo granite cave system.