On the origins of eogenetic chlorite in verdine facies sedimentary rocks from the Gabon Basin in West Africa

The verdine facies clay assemblages are regularly encountered in shallow marine sediments of equatorial latitudes where Fe of terrigenous origin is readily available. There, the 7 Å Fe-rich green clay (odinite) of syn-sedimentary origin commonly occurs. Previous findings suggested that odinite serves as a precursor of diagenetic chlorite whose importance in porosity conservation has been widely acknowledged. The investigation of Fe-rich clays is, therefore, helpful to constrain the specific conditions and mechanisms, which give rise to chlorite growth, thus illuminating the history of early burial. In this contribution, we studied Cretaceous sediments from the Gabon coast where multiple sedimentary basins came into existence prior, during and after the opening of the South Atlantic. Analyzed core is siliciclastic and petroleum bearing stemming from the fluvio-lacustrine-deltaic environment recovered from depths of ∼1500 m TVD. Electron micro-beam investigation coupled with X-ray diffraction on the clay fraction and spectra fitting revealed odinite, interstratified odinite-chlorite (O-C), and to a lesser extent, illite-chlorite-smectite (I-C-S) and illite-smectite occur as grain coatings and pore-filling phases with a characteristic honeycomb texture. Embryonic chlorite emerges at the contact of odinite and O-C flakes as a particulate authigenic mineralization. Formed from aluminosilicate and Fe-oxyhydroxide debris at sediment-water interface, odinite becomes intrinsically unstable, possibly due to the activity of Fe-reducing bacteria or decaying organic matter creating a prevalently reducing environment that promotes Fe reduction. Such condition facilitated a two-step chlorite growth, which first included a partial chloritization of odinite through the in situ solid-state transformation of 7 Å to 14 Å layers. Then, with increasing burial, at peak diagenetic conditions (50 °C, ∼1.5 km depth), odinite and O-C both recrystallized to form eogenetic chlorite most likely following the Ostwald ripening mechanism.