Evidence for dune reactivation from GPR profiles on the Maputaland coastal plain, South Africa

Abstract

The Maputaland area of northeastern South Africa is characterized by extensive dune- fields which developed during polyphase reworking of regional aeolian cover sand from the Mid-Pleistocene to the Holocene. Extended parabolic dunes, many preserved only as wind-rift trailing limbs, as well as areas of sinuous crested dunes, hummocky dune systems and the high, composite, accretionary coastal barrier dune cordon are the dominant dune forms. There are few natural sections exposing the stratigraphic succession and unequivocal relative age relationships between dune systems are uncommon. A ground penetrating radar (GPR) survey of dunes and representative aeolian sand stratigraphic units was undertaken in order to investigate the internal structure of the different dune forms and identify stratigraphic relationships between buried sedimentary units. The GPR profiles revealed that the trailing limbs of almost all the parabolic dunes that were surveyed comprise stacked sand units, separated by low-angle reflections interpreted as bounding surfaces, which accumulated through polyphase vertical accretion. Most extended parabolic dunes are aligned north-south and the upper parts of the dunes are characterized by inclined reflections in GPR profile interpreted as large-scale sets of cross-stratification with apparent dips toward the west. A hummocky dune revealed cross-stratified aeolian sand superimposed on a truncated dune form and probably formed through deflation of pre-existing dunes. Using 100 MHz and 200 MHz antennae, it is clear that GPR is capable of imaging very fine sedimentary structures and buried erosional surfaces in the homogeneous aeolian sand of Maputaland. At some of the sites investigated, the buried sand units identified were sampled by hand-augering for infrared-stimulated luminescence dating. The age determinations on these samples suggest that vertical accretion of up to 7 m of sand occurred intermittently over variable time scales up to 25 000 years on some parabolic dune limbs during the Late Pleistocene. In some complex dunefields, adjacent dunes were mobilized at different times, suggesting that remobilization was localized. The implications of the complex internal structure and vertical accretion of extended parabolic dunes are discussed in the context of changes in vegetation cover and water table due to seasonal and short-term cyclical climate variations as well as long-term climate change patterns during the last glacial cycle and the Holocene.Peer reviewe

Facies and development of a holocene barrier spit (southern Sylt/German North Sea)

“…Corresponding to similar GPR studies measured in aeolian environments (e.g. [4], [6], [7], [8], [9], [14]) the facies of Rf-A-1 is interpreted as cross-bedded aeolian strata formed by migrating dunes of various dimensions. Bristow et al [5] found out, that small changes in capillary water reflect changes in grain size in dune sands stratification and are the main cause of the dielectric contrast picked out by the radar.…”

Building of shore-oblique transverse dune ridges revealed by ground-penetrating radar and optical dating over the last 500years on Tottori coast, Japan Sea

“…It is empirically known that these methods are effective for constraining several coastal and inland dunes (e.g., Clemmensen et al, 2001;Bailey et al, 2001;Botha et al, 2003;Havholm et al, 2004;Bristow et al, 2005Bristow et al, , 2007. There are, however, much fewer case studies using GPR and optical dating relative to a variety of aeolian dunes in diverse geographical settings.…”

The formation and evolution of the barrier islands of Inhaca and Bazaruto, Mozambique

“…1). Extensive coastal dune systems of Neogene to recent age characterize the coastal zone which is dominated by a composite accretionary coastal barrier dune cordon, containing some of the highest coastal dunes in the world (Cooper and Pilkey, 2002;Botha et al, 2003).…”

“…Due to the dune sand cover and absence of natural exposures or sections that display the stratigraphic relationships between dune sand bodies, the age relationships between these aeolian deposits have only recently begun to be understood. Extensive field investigation and geological mapping of the Maputaland coastal plain in South Africa, combined with ground penetrating radar and infrared stimulated luminescence dating studies (Botha and Porat, 2000;Botha et al, 2003), have revealed a history of aeolian deposition and polyphase reworking during the Pleistocene and Holocene. These processes have yielded a complex and dynamic coastal dune system for which the regional lithostratigraphic framework has been described by Maud and Botha (2000).…”

“…Several studies have described the evolution and main geomorphic units of Inhaca (Hobday, 1977;Cooper and Pilkey, 2002), Bazaruto (Cooper, 1991;Cooper and Pilkey, 2002) and the south-east African coastal plain (Maud and Botha, 2000;Botha et al, 2003). However, these studies contain very few absolute ages.…”