An overview of geological influences on South African vineyards

The role of soils and bedrock geology has long been acknowledged as a fundamental component of terroir. In South Africa the influence of geology is misunderstood and some important geological components will be highlighted in this paper.
In South Africa’s Coastal Region the oldest rocks comprise the Late Proterozoic – Cambrian shaley sediments of the Malmesbury Group, and the Cambrian age granitic intrusives of the Cape Granite Suite. Locally these are overlain by sediments of the Klipheuwel Group. These units are unconformably overlain the Middle Ordovician–Early Carboniferous Cape Supergroup, whose basal portion comprises the sandstones of the Table Mountain Group which produce the dramatic mountain scenery of the area.
The Breede River Region covers the valley of the Breede River, to the east of the Coastal Region. The Worcester fault is the major feature defining the geology of this area. To the east of the fault the geology is essentially similar to the Coastal Region. To the west the upper portions of the Cape Supergroup, the Bokkeveld and Witteberg Groups, are present comprising sandstone and shaley sediments. Late Carboniferous–Permian age sediments of the Karoo Supergroup overly the Cape Supergroup and Upper Jurassic-Early Cretaceous sediments of the Uitenhage Group are preserved locally as unconformable remnants.
The following geological features are important for the Coastal Regions vineyards. Soils are often acidic and potassium rich, whilst granites weather to produce both saprolites and kaolin, which are possibly unique in terms of vineyard soils. River gravels are noted in two scenarios, firstly vineyards are planted in river floodplains and secondly fossil gravel terraces exist above the current river level.
In the Breede River Region river gravels are important whilst a significant portion of vineyards are planted on loam soils containing calcareous layers. These calcareous layers are formed as a result of excess evaporation over precipitation in this low rainfall region. A geological control may exist for the formation of these calcareous layers above specific bedrock strata. These soils are unique in the South African context, as they are naturally alkaline.
In addition topography resulting from differential weathering of the geological units is significant in the local terroir. Factors such as warm or cool slope orientation and the effects of altitude on mean temperatures and rainfall are important.