TYLER, ROGER, and NOEL TYLER, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Cyclic Evolution of a Lower Proterozoic Carbonate Platform, Malmani Dolomite Subgroup, South Africa

The Malmani Dolomite Subgroup of the Chuniespoort Group, Transvaal Supergroup, is a 1,000-m-thick succession of supratidal to deep subtidal and shelf-edge dolostones that record the cyclic evolution of a Lower Proterozoic (2.4 billion yr old) carbonate platform. Despite the age of the platform, the influences of metamorphism and structure are minimal, and excellent exposures of the Subgroup in the Eastern Transvaal, South Africa, allow delineation of the cyclic evolution of the succession. Several scales of cyclicity are recorded in this laterally extensive (>1,000 km), largely aggradational carbonate platform.

Two major cycles, a lower 650-m-thick retrogradational cycle and an upper 350-m-thick aggradational cycle, compose the carbonate platform succession. Depositional style changes progressively through the retrogradational cycle from basal thin-bedded siliciclastics and grainstones with minorstromatolites to giant stromatolites (15- to 20-m relief) of deep subtidal to shelf-edge origin. This cycle comprises four intermediate scale cycles. The basal siliciclastic member (Black Reef Quartzite) is gradational with overlying fluvio-marine sandstones (100-m-thick Oaktree Formation) and the overlying intertidal to subtidal dolostones of the 330-m-thick Lower and Upper Monte Christo Formation. These, in turn, grade into deep subtidal to shelf-edge bioherms of the 170-m-thick Lyttelton Formation Cycle caps are karst-produced chert/shale breccias or thin shallow-water siliciclastic sandstones and mudstones.

In the upper aggradational cycle (Eccles Formation), intermediate-scale cyclicity is present on the scale of 100 m or less. Subtidal to shelf-edge carbonates aggrade to thinly bedded grainstones and shallow-water siliciclastics. Cycles are capped by siliciclastic mudstones that host significant precious-metal mineralization. Throughout the Malmani Dolomite Subgroup, higher frequency cycles at 1-m and 10-m scales are reflected by progressive changes in stromatolite dimension and morphology. Multiscale cyclicity in this carbonate succession, one of the oldest platform successions known, is very similar to that recorded in much younger sequences throughout the Phanerozoic.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.