Sedimentary facies and stacking patterns of fluvio‐deltaic and ‐estuarine deposits from the Palaeoarchean Moodies Group in the Stolzburg Syncline (Barberton Greenstone Belt, South Africa)

Abstract Investigating fluvio‐marine depositional environments from early geological times is of great importance, as they provide key insights into the palaeoenvironmental conditions of the early Earth. The present study aims to reconstruct the deposition and evolution of these systems from the Palaeoarchean Moodies Group in the Stolzburg Syncline of the Barberton Greenstone Belt, South Africa. It is based on a detailed investigation of the new, ~490 m‐long research drillcore BASE‐5B, using sedimentological logging, facies analysis and palaeocurrent measurements. Thirteen sedimentary facies were recognised and grouped into five facies associations. The stratigraphically lower section of the core, from ~490to ~294 m drilled depth, with an overall coarsening‐upward trend, is mainly characterised by unidirectionally cross‐stratified sandstones with occasional conglomerates and silicified sulphate concretions; this section was interpreted as a regressive, river‐dominated fluvio‐deltaic environment. The stratigraphically upper section, from ~294 to ~17 m, with an overall fining‐upward trend, is characterised by cross‐stratified sandstones with mudstone drapes and bidirectional palaeocurrents; this section was interpreted as a transgressive, fluvio‐estuarine environment dominated by combined tidal and riverine processes and with a well‐defined turbidity maximum. The vertical stacking pattern implies an evolution from a fluvio‐deltaic to a fluvio‐estuarine system with an overall regressive to transgressive trend due to an increase in basin subsidence and/or decrease in sediment supply. Two orders of transgressive–regressive cycles are superimposed on this general trend, reflecting relative sea‐level fluctuations, most probably driven by changes in sediment supply. These results help in understanding the sedimentary dynamics of Archean fluvial‐marine systems and their response to external forcings.