Geological constraints on Archean (3.22 Ga) coastal-zone processes from the Dycedale Syncline, Barberton Greenstone Belt

Abstract

Geological constraints bearing on Paleoarchean surface environments are rare but critical, establishing the physiological consequences of a very different physical environment for early life, including a different tectonic style, closer Earth-Moon interaction, different chemical composition of atmosphere and hydrosphere, higher weathering intensity, and to validate geochemical proxy data. Strata relevant to these topics can be studied in uniquely accessible, subvertically dipping beds of the Moodies Group (ca. 3.22 Ga) within the tightly folded, steeply plunging western Dycedale Syncline of the Barberton Greenstone Belt. They display a large variety of environment-specific sedimentary structures in a small (ca. 1 km 2 ) but excellently exposed area, including microbial mats, abundant shoreline and tidal structures and early diagenetic pedogenic nodules. These are set among conglomerates, sandstones, minor lava flows and banded-iron formation within an initially deepening-, then shallowing-upward sequence, including (from base to top) braided-fluvial floodplain, sandy shoreline, and estuarine and protected tidal environments, some of which were temporally choked by volcanic ash. Shorelines were (semi-)arid, had fluctuating groundwater levels and moderate to high tides. Nevertheless, diverse microbial communities gained footholds in this challenging setting, including sulfate reducers in the shallow subsurface, photosynthesizers along high-energy shorelines, and photoferrotrophs or other Fe-metabolizing chemolithautotrophs in lagoonal or offshore settings. Facies analysis and regional structural geology suggest that the Dycedale area formed a minor, structurally controlled basin within a southward-facing fold-and-thrust belt which became intensely deformed during subsequent steepening. Overall, the datasets derived from this area allow the detailed description of interactions between early Earth’s volcanism, atmo- and hydrosphere and place the emergent biosphere into physical settings that are observable to such detail in only few other places worldwide.