IMPLICATIONS OF SANDY PLAYA DEPOSITS, ~2.0 GA MAKGABENG FORMATION, SOUTH AFRICA

One of the oldest erg deposits is present within the 2.0 Ga Makgabeng Formation, Limpopo Province, South Africa. The upper Makgabeng Formation erg is punctuated in the middle and near the top by the interbedded medium- to fine-grained sandstones and mudstones that are interpreted as playa deposits. Clastic playa deposits have been rarely documented from the Precambrian rock record. These two playas record climatic variations and contain both evidence of evaporite dissolution preserved in macro and microscopic structures and abundant microbial life.

Three facies associations are recognized in the Makgabeng Formation playa: mudstone, sandstone and interbedded mud/sandstone. The lowermost playa is ~3 m in thickness and consists of all three facies associations whereas the uppermost is missing the mudstone dominated facies association and transitions to a coarse sandstone. Photomoasics of both playas were used to document facies stacking in the facies associations. Distinctive sedimentary features marking the former presence of efflorescent salt crusts are present including surface deformation features, such as deformed ripples and solution collapse structures represented by solution loading and growth faulting. Microbial activity is marked by the presence of biological soil crusts, displaying such features as, desiccation cracked sandstones and gas escape structures.

Stacking of facies associations reflects fluctuations in precipitation over longer time frames. In contrast, efflorescent crust features are consistent with shorter period (months or years) flooding followed by desiccation cycles. The playa deposits indicate two periods of climatic amelioration during the Makgabeng erg history. The last amelioration phase signaled the termination of the Makgabeng erg. Evidence is present of microbial activity indicating that at~ 2.0 Ga a vigorous continental ecosystem was well established.