The Moodies Group is classically described as a synorogenic sequence of quartz-rich sandstone, conglomerate, fine-grained sediments with subordinate ferruginous sediments and volcanic components. A prominent basaltic lava (MdL) approx. mid-section marks BGB-wide changes in facies trends, tectonic regime, and sandstone petrographic composition. This unit is the surface expression of the large Moodies Igneous Complex (MIC) which, aside from the basaltic lava and associated tuffs, also consists of plutonic sills (e.g., Lomati River Intrusive, LRI) and a subvolcanic stockwork complex. The interaction of this (sub)volcanic component with the coastal sedimentary setting has not been studied to-date. The stockwork, which likely acted as feeders to MdL, includes at least one diatreme, surrounded by tidal bars. Numerous widespread outcrops show peperitic textures that suggest that hydrothermal activity affected the alluvial, fluvial, deltaic and tidal environments.
Microbial mats, best preserved in the Saddleback Syncline of the central BGB and thought to be restricted to coastal environments, also exist in other environments, all of which appear related to volcanic activity. Preservation potential appears to have been best near the LRI.
Continued geochemical and geological research will resolve the complex near-contemporaneous, close spatial association of magmatism, sedimentation, deformation, and hydrothermal alteration in the Moodies Group. This, in turn, will allow us to better constrain the paleoenvironmental setting of the microbial mats. We speculate that the elevated temperatures and ready the supply of nutrients, combined with the estuarine setting of the middle Moodies Group, provided an ideal laboratory for the evolution and preservation of surface-adapted microbiological life forms.
Deon Janse van Rensburg (1), Sebastian Reimann (2), Christoph Heubeck (1), Andreas Zametzer (2) & Paul Fugmann (3)
FSU Jena, Germany (1); Curtin University, Australia (2); FSU Jena, Germany (1); Curtin University, Australia (2); Université de Liège, Belgium (3)