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 fine-grained 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 cross-lamination, 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.