engleski

Formation of Eogenetic Chlorite in Verdine Facies Sedimentary Rocks from the Gabon Basin in West Africa

Clay phases numbering odinite‐like minerals (OLM), chlorite, illite, and smectite along with interstratified odinite‐chlorite (O‐C), illite‐chlorite‐smectite (I‐C‐S), and illite‐smectite are ubiquitous in sediments of the Gamba Formation. The formation itself stems from the Gabon coast of the western African margin where multiple Cretaceous sedimentary basins came into existence prior, during and after the opening of the South Atlantic Ocean. Analyzed core material is siliciclastic and petroleum bearing, originating from the fluvio‐ lacustrine‐deltaic environment drilled at depths of ~1500 m. The OLM and O‐C occur as grain coatings but are more common as pore fillings rendering the characteristic honeycomb texture. Chlorite emerges at the contact of OLM and O‐C flakes as a particulate authigenic mineralization. This study aims to find the mineralogical and geochemical clues to explain diagenetic changes that enabled nucleation of low‐grade diagenetic chlorite in the verdine sedimentary facies. Inferring from a detailed scanning electron microscopy and X‐ray diffraction with spectra modelling, OLM appears to form in shallowly buried sediment from the alumosilicate and Fe‐oxyhydroxide debris brought as a riverine influx. Peculiar geochemical conditions induced a partial chloritization of OLM at practically syn‐sedimentary conditions through the solid‐state transformation of 7 Å to 14 Å layers. With increasing burial, at peak diagenetic conditions (50°C, ~1.5 km depth), the OLM and O‐C both recrystallized to form the eogenetic (monocomponent) chlorite most likely following the Ostwald ripening mechanism. Trioctahedral smectite in the upper part of the Gamba Formation also served as source for chloritization, where dissolution must have been complete resulting in a series of I‐S‐C intermediates. However, the I‐S‐C intermediates did not reach the composition of monocomponent chlorite.

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