Depositional setting and cyclic development of the lower part of the Witteberg Group (Mid- to Upper Devonian), Cape Supergroup, Western Cape, South Africa

Abstract

Mid-to Upper Devonian strata of the lower Witteberg Group in the Cape Fold Belt of South Africa comprise three principal facies deposited in the shoreface and contiguous inner shelfalong a storm-dominated coastal zone. Weakly burrowed mudrock with variable minor proportions of wave-rippled sandstone (linsen, flasers, hummocky cross stratification [HCSJ) (facies 1) accumulated in the inner shelf below, near, and just above storm wave base. Very fine-and finegrained sandstone that displays flat and wavy lamination and HCS, as well as compound wave-ripple lamination (facies 2), was deposited in the lower shoreface above fair-weather wave base. Fine-to coarse-grained sandstone containing crosslamination, anisotropic HCS, and compound wave-ripple lamination (facies 3) formed in the mid to upper shoreface. Regionally, lower Witteberg strata change from proportionally more facies 2 and 3 at proximal (northern) sites to mostly facies 1 at distal (southern) sites. The lower Witteberg Group consists of approximately 40 shallowing-upward facies sequences (parasequences) whose average thickness is approximately 12 m. These are grouped into progradational parasequence sets of up to 10 parasequences each for a total of approximately 3.5 parasequence sets in the lower Witteberg. The lowest parasequence set extends from the base of the Karoopoort Formation (top of underlying Bokkeveld Group) and culminates at the top of the Blinkberg Formation in the Witteberg Group, and approximately 2.5 parasequence sets make up the overlying Swartruggens Formation. The characteristics and stratigraphic architecture of the lower Witteberg Group are consequences of the interaction oftwo different orders of global sea-level fluctuation. The lower Witteberg parasequences had a periodicity between 250 and 300 ka, within the Milankovitch range, and their character and mean thickness are very similar to cyclic patterns recorded in contemporaneous Givetian-Frasnian siliciclastic and carbonate strata on four other continents. The progradational parasequence sets had a mean recurrence interval in the order of 3.0 Ma and can be correlated with specific third-order fluctuations of global sea-level proposed for contemporaneous Upper Devonian strata in Euramerica. This reinforces assignment of the age of the lower Witteberg as extending from the mid-Givetian Stage to the end of the Frasnian Stage.

The Cape Basin, South Africa: A review

“…He noted the probable deepening associated with the Bokkeveld sedimentation and commented on the anoxic nature of the Bokkeveld sea floor. Metre-scale coarsening-upward sequences in the lower Witteberg Group have been linked to global sea level changes by Cotter (2000), who suggested that the regional architecture of the lower Witteberg Group can be explained by an hierarchical series of global sea level fluctuations. Thomas et al (1992b) used aeromagnetic data and the occurrence of ''Beattie-set'' anomalies to suggest that two different types of Mid-to Late-Proterozoic crust (inferred from the magnetic data) controlled sedimentation and the later deformation (or lack of deformation) of sediments in the Cape Basin.…”

“…The Witteberg Group strata have been described in detail by only a handful of researchers, amongst them Loock (1967), Hiller and Dunlevy (1978), Theron and Loock (1988), Hiller and Taylor (1992), and Cotter (2000).…”

“…In the eastern part of the fold belt, near Grahamstown, Hiller and Taylor (1992) describe carbonaceous black shale units containing fossilized fish and plant material, in association with thinly-bedded quartzites and shales. Cotter (2000) recognized a typical coarsening-upwards facies sequence in the lower part of the Witteberg Group which is repeated at least 40 times in the stratigraphic interval he investigated. The sequence, from the bottom upwards, consists of a silt-streaked mudrock grading into mudrock containing isolated wave-ripple lenticles, through wave-rippled flaser-bedded sandstone, and into hummocky cross-stratified sandstone.…”

“…The Witteberg Group is widely believed to be the result of sedimentation in a shallow marine environment (Loock, 1967;Rust, 1973;Cotter, 2000) although distal delta settings and freshwater lagoonal depositional environments have been suggested by some researchers for specific units in the Witteberg succession (Gardiner, 1969;Johnson, 1976Johnson, , 1991. The abundance of hummocky stratified sandstone and wave-rippled, flaser-bedded siltstone and mudrock suggests that much of the Witteberg was deposited close to ordinary wave base in a shallow marine depositional milieu.…”

Stratigraphic variation of complex impurities in platform limestones and possible significance of atmospheric dust: a study with emphasis on gamma-ray spectrometry and magnetic susceptibility outcrop logging (Eifelian-Frasnian, Moravia, Czech Republic)

“…House 1995;Olsen 1990;Cotter 2000;Crick et al 2001). Fundamentally, the correct analyses need uninterrupted linear time series (even though wavelet analysis can slightly reduce this requirement; Prokoph and Barthelmes 1996).…”