General Geological Setting

The c.2.06-1.88 Ga (SACS, 1980; Jansen, 1982; Walraven and Hattingh, 1993; Eglington and Armstrong, 2004; Hanson et al., 2004) Waterberg Group is amongst the sedimentary units globally marking the first appearance of red beds (sensu stricto) and large erg (sand sea) deposits, at c.1.9-1.8 Ga (e.g. Eriksson and Cheney, 1992; Eriksson and Simpson, 1998); this was a period also marked by large-scale paleoenvironmental changes related to the supercontinent cycle (e.g., Eriksson et al., 2004). Waterberg sedimentation (preserved in a Main and Middelburg Basins; Fig. 2) was mainly fluvial, with lesser alluvial, lacustrine and paleodesert deposits (Callaghan et al., 1991; Simpson et al., 2002, 2004).

The Main Basin is a fault-controlled, continental depository (Callaghan, 1987). The stratigraphy of the Waterberg Group in this basin comprises 11 formations (Fig. 2). Following deposition of marginal, essentially protobasinal Waterberg alluvial deposits (Blouberg in the north; Swaershoek-Sterkrivier-Alma Formations in the south) in the Main Basin, are three correlated pairs of formations with the respective double-sets of formations arranged essentially SW and NE of a synsedimentary fault system (the Vaalwater Fault), running NW-SE across the preserved Main Basin (Fig. 2). The first set of these, comprising the SW Skilpadkop and the NE Setlaole Formations, reflects mainly lithic to arkosic, locally pebbly sandstones and lesser conglomerates, all ascribed to a fluvial braidplain depositional setting (Callaghan et al., 1991). This 450-600 m thick succession is succeeded by the second pair of correlated units, the SW Aasvoelkop and NE Makgabeng Formations; the former is conformably-based with the latter being locally unconformably-based. The Aasvoelkop succession, of basal mudrocks-medial lithic sandstones-uppermost immature pebbly sandstones (with intercalated tuffaceous eruptive beds) is interpreted as the deposits of a through-flow lake followed by fluvial sediments (Callaghan, 1987). The correlated Makgabeng Formation comprises pale-odesert deposits containing predominant paleodune sediments, with evidence for an interaction of eolian and wet-desert processes (such as wadi-fluvial, interdune and saline pan influences) within this setting (Simpson et al., 2002, 2004). It is the interdune deposits that contain the microbial mat-related structures discussed in this paper (e.g., Eriksson et al., 2000). This combination of arid and locally/impersistently wetter conditions applies also to the entire Waterberg Group, which is seen as having enjoyed a semi-arid

Figure 2. (A) Location of Main and Middelburg Basins of the Waterberg Group in South Africa. (B) Geological sketch map of the Main basin, showing the component formations making up the Waterberg Group; note three pairs of formations arranged NE and SW of the NW-SE orientated Vaalwater Fault. (C) Location of study area with interdune beds and microbial mat features, within the Makgabeng Formation. (Modified after Simpson et al. 2002, 2004.)

Figure 2. (A) Location of Main and Middelburg Basins of the Waterberg Group in South Africa. (B) Geological sketch map of the Main basin, showing the component formations making up the Waterberg Group; note three pairs of formations arranged NE and SW of the NW-SE orientated Vaalwater Fault. (C) Location of study area with interdune beds and microbial mat features, within the Makgabeng Formation. (Modified after Simpson et al. 2002, 2004.)

paleoclimate (Callaghan et al., 1991; Simpson et al., 2002, 2004). The third set of correlated formations, respectively SW and NE of the Vaalwater Fault, are the Sandriviersberg (1,250m thick; arenites, granule-rich arenites and conglomerates) and Mogalakwena (similar thickness and lithology) (Fig. 2). A large fluvial braidplain, proximal to the NE, is envisaged (Callaghan, 1987; Callaghan et al., 1991).

Uppermost more mature sandstones of the Cleremont and succeeding Vaalwater Formations complete the succession in the Main Basin, and were deposited across the trend of the Vaalwater Fault, testifying to a smaller tectonic control, lower gradient fluvial systems and significant reworking, with the possibility also of littoral influences (Callaghan et al., 1991). In the much smaller Middelburg Basin, a single formation, the Wilgerivier, reflects western-sourced arenaceous and lesser conglomeratic braidplain deposits (Van der Neut and Eriksson, 1999), which are thought to be coeval with the basal protobasinal formations in the Main Basin.

0 0

Post a comment