Petrography and diagenesis of pre-salt microbialite in the Kwanza Basin, Angola

Petrographic and preliminary geochemical analyses have been conducted on a core from the pre-salt microbialite succession of the Kwanza Basin, Angola. The core is characterised by shrubby boundstones and spherulitic grainstones, with several intermediate facies. Magnesian clays are present in small amounts, this may be in part due to later diagenesis. The removal of Mg clays would make some facies classifications non-representative of the unit at the time of deposition. Scanning electron microscopy has also revealed the presence of microbial filaments within the shrubby calcite fabrics, but it is not certain if microbes contributed to calcite precipitation via organomineralization, or if they were merely a passive substrate. Chert preserves a variety of relict calcite textures. Some silica textures are similar Magadi Type microbial chert. Diagenesis was complex and multi-phase, with evidence of low and high temperature (possibly hydrothermal) fluids.