Paleoenvironmental Implications of Authigenic Magnesian Clay Formation Sequences in the Barra Velha Formation (Santos Basin, Brazil)

The characterization of Mg-clays in rock samples (well P1) from the Barra Velha Formation (Early Cretaceous) allowed establishment mineral assemblages on basis their kerolite and Mg-smectite (stevensite saponite) content. Kerolite-rich (A B) rarely contain saponite. Assemblage B is composed kerolite-stevensite mixed layers, while assemblage A consists more than 95% kerolite. Mg-smectite-rich (C CB) are made up both Mg-smectites. predominance stevensite lower interval stratigraphic succession suggests evaporative conditions, higher salinity pH, which would favor its authigenesis by neoformation. In upper portion, occurrence thick kerolite-rich intervals regular water inputs, contributing with a decreasing favoring neoformation later layering. saponite be result transformation Al-smectite into Mg2+ rich medium. results indicate that lake hydrochemical processes have basic depositional sequence, base to top, as follows: (i) initial expansion stage marked saponite, (ii) neoformation, (iii) formation layer increasing salinity, (iv) stevensite, marking final maximum (evaporation) alkalinity lake.