Chemical, physical, and biological depositional processes in the ~3.3 Ga upper Mendon Formation, Barberton Greenstone Belt, South Africa [electronic resource]
Keywords: Geological and Environmental Sciences.

Pre-3.0 Ga sedimentary rocks provide intriguing windows into surface conditions on the early Earth, but interpreting these rocks can be challenging. Not only have they been submitted to extended histories of tectonism and metamorphism, but they record sedimentation and diagenesis in a world in which many of the basic parameters governing these processes were very different than in the Phanerozoic world. The 3.5-3.3 Ga Onverwacht Group of the Barberton Greenstone Belt, South Africa, exemplifies many of these distinctions: an 8-10 km thick succession of basalts, komatiites, and thin sedimentary units that spans up to 270 Myr. Most of the sedimentary rocks are chert -- rocks now composed of > 85% SiO2 by weight as microcrystalline quartz. These are neither classic siliciclastic nor carbonate rocks and therefore do not fit easily into classic sedimentological interpretive frameworks. Nevertheless, examining these rocks with a first principles approach can yield many important insights into the surface environment of the early Archean Earth. This dissertation investigates chemical, physical, and biological depositional processes in the ~3.3 Ga upper Mendon Formation, the ~50-m-thick sedimentary chert unit capping the Onverwacht Group. Chapter 1 describes the occurrence of subspherical, sand-sized primary silica granules within pre-3.0 Ga cherts of the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt in South Africa and the Warrawoona Group of the Pilbara Block in Western Australia. These silica granules occur in rocks representing a variety of depositional environments, from shallow subtidal to basinal settings. Their occurrence is significant in that they provide the first clear evidence that primary amorphous silica precipitated in early Archean oceans. Chapter 2 provides additional sedimentological and geochemical analyses of Archean silica granules. The regular occurrence of silica granules as cm-scale layers within banded chert indicates that granule sedimentation was often episodic. Looking to modern siliceous sinters for insight into silica precipitation, we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres, which is consistent with estimates for Archean ocean chemistry. Chapter 3 uses in situ Si and O isotopic analyses to provide insight into chert geochemistry and the Archean silica cycle, taking advantage of the paragenetic sequence of silica precipitation provided by silica granules, silicified organic grains, and cavity-filling quartz cements in banded black-and-white chert. Systematic texture-specific [delta] 30Si differences are observed between white chert bands, which contain textures that represent primary and earliest diagenetic silica phases, and later cavity-filling cements, which we interpret to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated. Our results indicate that low temperature processes fractionated silicon isotopes in early Archean marine basins, a behavior that probably precludes the application of chert [delta] 30Si as a robust paleothermometer. Chapter 4 integrates many of the results of the preceding chapters to interpret the sedimentology of the upper Mendon Formation: sources of sediment, depositional processes, and depositional environments. The dominant mode of sedimentation was quiet settling of carbonaceous grains and fine ferruginous material, resulting in finely laminated black and grey chert. This background deposition was punctuated by volcanic inputs, silica granule formation and deposition, and reworking and deposition by high energy storm events. The oldest upper Mendon sections appear to record a deepening of depositional setting, while Mendon sections overlying younger episodes of komatiitic volcanism reflect gradual shoaling.