Palaeohydrological data from the c. 2.0 to 1.8 Ga Waterberg Group, South Africa: discussion of a possibly unique Palaeoproterozoic fluvial style

摘要

Palaeohydrological data calculated from clast sizes in preserved channel-fills and from trough cross-bed set thicknesses within the Mogalakwena Formation, Main Basin, Palaeoproterozoic Waterberg Group in South Africa are presented, These data are compared to similar, published data, derived from two other Waterberg units, the Blouberg Formation (northeast of Main Basin) and the Wilgerivier Formation (Middelburg Basin, SE of Main Basin). All three data sets exhibit palaeoslope estimates that plot within the "natural depositional gap" defined by Blair and McPherson (1994) which separates alluvial fans (palaeoslopes >0.026 m/m) and fluvial systems (palaeoslopes <0.007 m/m). These data, derived from what appear to be typical braided fluvial deposits, combined with evidence for subordinate occurrences of sediment gravity-flow and sheetflood deposits, Support the presence of raised palaeoslopes (possibly restricted in time and space) for these Palaeoproterozoic fluvial systems. An abundance of fine, argillaceous material within such river systems likely resulted from warm and humid palaeoclimatic conditions, thus enabling localized debris-flow and hyperconcentrated flow processes. we postulate that temporary "floodbasin playas" (cf. Rainbird, 1992) formed localised elevated base levels and provided muddy detritus, thereby resulting in temporarily raised palaeoslopes and Waterberg sandstones with highly variable mud contents. Gravity-flow and sheetflow processes would have been possible at such sites.