Earliest High-Latitude Coal Beds from the Middle Devonian of South Africa Suggest Extent of the Silurian-Devonian Terrestrial Revolution to Polar Regions

The Silurian-Devonian Terrestrial Revolution (SDTR) is a bioevent that took place during the Middle Palaeozoic marked by the relatively rapid colonization and diversification of land plants in the terrestrial realm. The afforestation of land during the SDTR allowed for the creation of new environments and ecosystems on land; creating new pathways for carbon cycling through the creation of carbonaceous deposits particularly coals, terrestrial mud production as well as advent of meanderbelt and floodplain alluvial environments. The SDTR has exclusively been documented from low-latitude areas, with knowledge of the event being unknown at high-latitudes. Although terrestrial plant megaspores, pollen and fossils are known from several Ordovician-Devonian locales in West Gondwana, these are often allochthonous and without context of their immediate environments from which they were derived. This research presents evidence for the earliest known in-place coal deposits and their associated alluvial environments from high latitudes filling an important gap in the global record of the SDTR. Reconnaissance fieldwork in the northern Cederberg of South Africa has revealed the presence of at least three coalified plant bearing horizons in the Bokkeveld Group spanning the Tra-Tra-Wuppertal interval. Relative age constraints suggest a late Eifelian-earliest Givetian (Middle Devonian) age for the interval. Coalified plant material typically presents as compressions of simple irregular (branched and unbranched) phytoclast fragments with surface ornamentation being rarely preserved in hand samples. Those deposits from the uppermost Boplaas Formation are of interest in this study as they contain a sufficient plant biomass to manifest as discontinuous carbonaceous lenses of up to 40 cm thick. These are interbedded among multi-storey channelized sandstone bodies which show evidence for lateral accretion, plant rootlet horizons and subaerial biogenic activity. These deposits were laid down in a paralic subaerial bayhead delta environment during the late highstand regressive to earliest transgressive phase of an incised valley fill complex. Coally deposits associated with the Wuppertal Formation manifest as thinly bedded coaly stringers in a coarse-grained sandstone unit (provisionally named the Voëlvlei member). Internally, the architecture of alluvial sandstones of the Voëlvlei member manifest as sharp-based plant-laden sandy bedforms and show a high degree of amalgamation and is interpreted to have accumulated under falling-stage forced regressive conditions. Rapid transport and burial of plant material in this scenario might explain the mechanism by which sufficient plant material accumulated to form coals. Organic petrographic studies of the Middle Devonian coal beds suggest that they are of anthracitic-meta-anthracitic rank. The coalified plant fragments exhibit high reflectance and display both isotropic and anisotropic coke textures, indicating prolonged thermal alteration under low-oxygen conditions. The palynoassemblage of the seams are dominated by amorphous organic matter and have a low-diversity assemblage of mega- and miospores, scolecodonts and chitinozoans in addition to structured phytoclast tissues. Collectively, this suggests that high-latitude regions must have supported an appreciable amount of plant material by the Middle Devonian to have formed coal at a time when these deposits became widespread globally implying that at least this phase of the SDTR was a near synchronous event.