Transition from siliciclastic to carbonate sedimentation near the base of the Transvaal supergroup, northern Cape Province, South Africa

Abstract

The transition from the Black Reef to the Schmidtsdrift Formation takes place through a number of upward-fining siliciclastic cycles (quartz arenite → argillite), mixed siliciclastic-carbonate cycles (argillite → quartz arenite → dolarenite → cryptalgal dolomite) and finally, carbonate cycles (oolitic dolomite → cryptalgal dolomite). The siliciclastic cycles represent progradational subtidal-tidal flat deposits, and the carbonate cycles progradational tidal flat deposits. These cycles are bounded by sharp erosional contacts which represent rapid transgressive ravinements. The mixed cycles consist of transgressive siliciclastic tidal flat deposits (argillite → quartz arenite) which are overlain by progradational carbonate tidal flat deposits (dolarenite → cryptalgal dolomite). The association of both the clastic and carbonate tidal flat deposits in the mixed cycles suggests that during rapid transgression carbonate precipitation was suppressed due to the influx of cold marine water. During periods of relatively stable sea level (warm and clear water) metasaline conditions developed which enhanced carbonate deposition and resulted in the progradational carbonate tidal flat subcycles. Eventually the epiclastic source areas were eliminated and carbonate deposition became established. Several of the cycles display a gradation from subtidal to high tidal flat deposits. In these cycles the thickness of the intertidal facies corresponds to the paleotidal range and that of the subtidal facies to water depth. The thickness of an intertidal facies added to that of the overlying subtidal facies correspond to the rise in sea level or magnitude of basin subsidence during a transgression. Water depths of between 0.3 and 0.5 m combined with a tidal range of between 1 and 1.5 m prevailed during the deposition of the mixed siliciclastic-carbonate cycles. Basin subsidence during transgressions was of the order of 2 m and was most probably related to faulting.