The origin and evolution of Dartmoor Vlei in the KwaZulu-Natal Midlands, South Africa.
Dartmoor Vlei is a 42ha un-channelled valley-bottom wetland system located in the headwaters of the Myamvubu River in the KwaZulu-Natal Midlands, South Africa. The wetland and its catchment are entirely underlain by a large dolerite sill that forms the Karkloof escarpment and plateau and the wetland terminates against a dolerite dyke ridge that has intruded into the sill. Wetlands on the Highveld and KwaZulu-Natal Midlands are thought to arise due to the lateral erosion of valleys upstream of resistant lithologies that impede vertical erosion. This is typical of valleys where Karoo sediments occur upstream of resistant dolerite dykes. Such valley widening by lateral planing is typically associated with actively migrating meanders. As a result, wetlands found upstream of dolerite intrusions are generally located on floodplains characterised by actively migrating meanders, extensive backswamps, ox bow lakes, alluvial ridges and clastic alluvial fill. However, in contrast to these floodplain wetlands, Dartmoor Vlei has evolved into an unchannelled valley-bottom wetland characterised by diffuse flow conditions, minimal channelled flow, extensive peat deposits and a general lack of floodplain features. Coring within the wetland has established that the sedimentary fill of the wetland generally comprises upward fining sequences of sediment characterised by sands and gravels near the valley floor that grade into fine organic-rich silt sediments and peat at the surface. These findings confirmed that the wetland has evolved from a floodplain wetland characterised by laterally migrating meanders to a valley-bottom wetland characterised by discontinuous streams and peat accumulation. Coring also established that the wetland is predominantly underlain by residual saprolite that extends to depths in excess of 7m. The occurrence of a large discontinuity between the residual saprolite and fresh dolerite surfaces underlying the toe of the wetland indicated that the residual saprolite surface has sagged relative to the fresh dolerite and dolerite dyke at the toe of the wetland over time. Chemical and mineralogical analyses of fresh dolerite and residual clay within the valley confirmed that the chemical transformation of the dolerite bedrock into residual clay has resulted in both volume and thickness losses in the weathered dolerite sill mass. This has in turn resulted in the sagging of the valley floor and the wetland surface over time. These findings provide an explanation for the extremely low energy conditions of Dartmoor Vlei and explain why the wetland did not evolve in the same fashion as other wetlands in a similar geological and geomorphological setting. The extensive chemical weathering of the dolerite sill underlying the wetland has been attributed to the extremely long-time period that the soils within the wetland have been saturated. The long-term saturation of soils within Dartmoor Vlei has been facilitated by the formation and preservation of the African Erosion Surface on which Dartmoor Vlei is located.