Authigenic Mg-clay assemblages in the Barra Velha Formation (Upper Cretaceous) from Santos Basin (Brazil): The role of syngenetic and diagenetic process

The compositional and textural characterization of authigenic Mg-clays from the Barra Velha Formation (Early Cretaceous) samples (well P1) was performed. The Mg-clay mineral assemblage is comprised by Mg-smectite (stevensite and saponite) and kerolite. Based on clay fraction XRD data, was possible to establish four assemblages rich in kerolite (A and B) and Mg-smectites (C and CB). Saponite and stevensite were identified on the basis of its behavior in XRD patterns after thermal treatment and corroborated by EDS chemical analysis under SEM examination. The vertical distribution of Mg-clay minerals shows the predominance of assemblages rich in Mg-smectite (C and CB) in its basal older zone, while assemblages rich in kerolite (A) or showing mixed layering of kerolite-stevensite (B) occur preferentially in the younger upper zone of the sedimentary succession. The predominance of stevensite in the lower part of the stratigraphic succession would indicate evaporative conditions (higher salinity and pH), while the upwards intercalations with saponite suggest detrital inputs that would favor the dilution of the lacustrine system giving rise to hydrochemical conditions of high pH but lower salinity favorable for the precipitation of kerolite and later kerolite-stevensite mixed layering. Kerolite-rich samples (>90%) presents ordering ("crystallinity") higher than that observed in other worldwide deposits presenting FWHM values ranging between 0.52 and 1.23 (2 theta). The formation of kerolite and stevensite is interpreted as authigenic by neoformation, with transformation between them by mixed layering. The formation of saponite would be authigenic by transformation of previous aluminum-rich phases, but without ruling out the possible presence of inherited saponite. There are petrographic evidences of diagenesis affecting both the Mg-clays minerals and carbonates. Especially Mg-clays show replacement by silica (quartz, chalcedony) and carbonates (dolomite, magnesite). Homogenization of the Mg-clay textures into laminar texture appears to have occurred during the diagenetic compaction process.