From detrital heritage to diagenetic transformations, the message of clay minerals contained within shales of the Palaeoproterozoic Francevillian basin (Gabon)

• We have explored the clay mineralogical composition of the 2.1 Ga Francevillian series.
• We find unexpected smectite-rich randomly ordered mixed layers in the black shales.
• The smectite layers are not inherited from a detrital input, but originate from early diagenesis.
• Their preservation is related to potassium deficiency, rather than organic matter content.
• Smectitic minerals cannot be used as a paleoenvironmental indicator in this basin.

Unmetamorphosed and undeformed marine siliciclastics rocks of the FB, FC and FD of the Francevillian series (Gabon) were deposited in an epicontinental basin. Clay minerals found in black shale, siltstone and sandstone are dominantly illite and chlorite except in two levels of the FB formation, which contain smectite-rich randomly ordered mixed layers. Their survival in a 2.1 Ga old sedimentary series is not related to the abundance of organic matter (total organic carbon or TOC), nor redox conditions at the time of deposition as indicated by the Fe speciation (FeHR/FeT and FePy/FeHR ratios). Rather it results from an incomplete illitization reaction that reflects potassium deficiency. The K2O/Al2O3 ratio of shale, siltstone and sandstone vary along the series, and appear to conserve the signature of the original chemical composition of the rocks. K-feldspars which are present in the FC and FD formations are missing in the FB formation. Consequently, the smectite layers do not appear to be inherited from a detrital input in the basin but must be considered as representative of an intermediate stage of the illitization reaction reached during diagenesis.