A case study of microbial mat-related features in coastal epeiric sandstones from the Paleoproterozoic Pretoria group (Transvaal supergroup, Kaapvaal craton, South Africa) : the effect of preservation (reflecting sequence stratigraphic models) on the relationship between mat features and inferred paleoenvironment

Abstract

The Magaliesberg Formation sandstones within the upper part of the Pretoria Group are interpreted as a second-order highstand systems tract within an overall clastic epeiric marine setting, where episodic braided fluvial systems fed sediment directly into a tidally dominated coastline setting. Mat-related features are abundantly preserved within the inferred tidal flat and supratidal deposits. The Daspoort Formation sandstones at a lower stratigraphic level are ascribed to a closely analogous palaeoenvironmental setting, yet have hardly any preserved mat features at all; they are ascribed to a second-order transgressive systems tract in a sequence stratigraphic framework. During highstand the depositional systems are inferred to have aggraded, thus preserving the mat-related features formed in Magaliesberg intertidal-supratidal environments. During second-order transgression, the formation of the wave/tidal ravinement surface in the subtidal/intertidal environments largely obliterated the features of the Daspoort intertidal and supratidal environments, leading to a paucity of mat-related structures in the preserved Daspoort succession. This case study thus supports the application of mat-related features to integrated palaeoenvironmental and sequence stratigraphic analysis, while emphasizing the role of preservation.