A Holocene–late Pleistocene aeolian record from lunette dunes of the western Free State panfield, South Africa

Abstract

The greatest concentration of pans in southern Africa occurs in the western Free State province, South Africa. A feature of many Free State pans is their fringing lunettes, located on the southern and south-eastern margins. Lunette dunes associated with pans in the neighbouring and presently drier Kalahari region show depositional ages, determined by optically stimulated luminescence (OSL) dating, primarily in the Holocene. However, to date, the precise timing of Free State lunette accumulation has not been investigated. The morphology, sedimentology and age of lunettes at five pan sites in the western Free State panfield are reported here. The lunettes form distinct topographic features, with heights up to 5 m above the pan floor, and all have been dissected by gully erosion. Sediment in the sand size class dominates in the lunettes, often overlying clay-rich basal or pan floor sediments. The intra- and intersite data consistency of 46 OSL ages is interpreted as reflecting regional causal factors responsible for lunette accretion, with phases of lunette building at 12—10 ka, 5.5—3 ka, 2—1 ka and 0.3—0.07 ka ago. These are in good agreement with the findings from pan-fringing lunettes in the southwest Kalahari and consistent with established records of palaeocirculation and wind direction over central southern Africa during the late Pleistocene. Lunettes in the western Free State are currently not in a major accretion phase. They are subject to degradation by localized fluvial erosion, with sediment being recycled into the pans.

Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300 years

“…In addition to these relatively long records, there are also a number of semi-continuous or shorter paleoclimate records that corroborate the Lake Nhauhache record. Lunette dune accumulation in western Free State, South Africa, indicates dry conditions within the time intervals AD 1-1000 and AD 1300-1700 (Holmes et al 2008). Isotopic analyses of archaeological fauna from the Shashe-Limpopo River Basin show variable semi-arid conditions at AD 880-1220, trending towards wetter conditions, interrupted by a dry spell at AD 1200, followed by much wetter conditions that persisted until AD 1415, whereas drier conditions reappear until the end of the record at AD 1685 (Smith et al 2007).…”

Depositional environments of the Florisbad Spring site and surrounds: a revised synthesis

“…This entails fluvial reworking of sediment and topsoil downslope towards the pan (see discussion above on conductivity values). This phenomenon is clearly evident at, for example, Morgenzon Pan and Deelpan (Holmes et al 2008). Such reworking can result in older aeolian sediment being contaminated (from an age determination perspective) by younger, reworked sediment.…”

“…First, they may imply that the lunette was an active aeolian feature between 500 and 200 hundred years ago. Holmes et al (2008Holmes et al ( , 2012 consistently found evidence of recent sediment mobility in what they termed capping dunes from a number of western Free State pan-fringing lunettes. However, the morphology of the south-western lunette does not suggest (other than a topsoil) capping.…”

“…Second, we suggest that geological controls have facilitated the accumulation of sediment within the Soutpan depression, and encouraged the formation of dunes. The largely ubiquitous presence of lunette dunes, or evidence of at least some sediment accumulation on the downwind margins of pans in the Free State Panfield, has been documented (Holmes et al 2008). In the case of Soutpan, the dolerite intrusions (particularly to the south west) have facilitated the entrapment of wind-blown sand.…”

Grain-size characteristics of surface sands and sandstones: indicators of potential sediment sources for the southern Tuli Basin, South Africa

“…Le Baron et al Carranza-Edwards 2005). In southern Africa, grain-size analysis has been applied in numerous geomorphological studies of surface sands, inter alia the characterization of dune granulometry (Lancaster 1986, Schlegel et al 1989, Livingstone et al 1999, examinations of the cover sands of Botswana and the Northern Cape Province, South Africa (Baillieul 1975, Bateman et al 2003, investigations of the nature and transport history of dune and pan sediments (Binda and Hildred 1973, Moore and Dingle 1998, Ringrose et al 1999, Holmes et al 2008, Telfer et al 2009), descriptions of floodplain sediments (Tooth et al 2002, Nash et al 2006, inferences on paleowind strength (Pichevin et al 2005), and assistance with paleoenvironmental reconstructions (Marker and Holmes 1993). Although, in most of these studies, grain-size analysis was not necessarily the only methodological approach, the method has nevertheless served as an important component of such past investigations.…”