Wave-Dominated Deltaic Sedimentation in the Devonian Bokkeveld Basin of South Africa

Abstract

ABSTRACT In the Devonian Bokkeveld Basin on the southwestern periphery of Gondwana, over 3,000 m of shales and sandstones were deposited as a southward-prograding clastic wedge. These rocks are organized into five thick regressive sequences that record processes on arcuate deltas along a mixed wave and tidal energy coastline. Variations between each of the upward-coarsening sequences are the result of differential preservation and transgressire marine reworking caused by uneven rates of relative sea-level rise. Each npward-coarsening sequence consists typically of shelf-prodelta, distributary mouth bar, tidal flat, and interdistributary bay deposits and is usually capped by tidal- and wave-reworked delta-front sands. The base of each sequence is represented by dark shelf-prodelta shales which grade upward through mudstones to graywackes and lithic arenites deposited as river mouth bars. This upward-coarsening deltaic package is topped by two types of intertidal and shallow subtidal sequences deposited on the delta plain: tidal-flat facies consist of sandstones, mudstones, and shales deposited in upward-fining units up to 10 m thick; bayfill deposits comprise small upward-coarsening subdelta sequences. The delta-plain deposits are extensively bioturbated and contain brackish invertebrate taxa of the Malvinokaffric Province. Reworked delta-front sands are present largely as tidal-channel and tidal-inlet deposits of abandoned distributaries. These consist of upward-fining, cross-bedded quartz arenites that occur as lenticular bodies up to 8 m thick, and that erosively overlie both the distributary-mouth-bar facies and in places the delta-plain facies. Thin, flat-bedded sheetsands which occur locally at the base of the quartz arenite units are attributed to storm washover processes. It is suggested that deposition of the entire Bokkeveld Group occurred on arcuate deltas similar to the modern Brazos and Niger deltas. Rates of relative sea-level rise were high and were punctuated by periods of decreased rates or near stillstand. The duration of these stable periods increased through time, culminating in stable shelf conditions that characterized Witteberg deposition.