Low temperature, authigenic illite and carbonates in a mixed dolomite-clastic lagoonal and pedogenic setting, Spanish Central System, Spain

The mechanism for low temperature illitisation involves enhanced layer charge resulting from Mg2 + substitution for Al3 + (or Fe3 +) and Fe3 + to Fe2 + reduction. Mg2 + enrichment may have occurred principally in saline lagoons or lakes, while Fe3 + to Fe2 + reduction occurred as a result of wetting and drying in a pedogenic environment. So far as it has been possible to establish, this dual mechanism has not previously been documented. This study indicates clearly that the dolomite and calcite are authigenic cements that precipitated in a clastic sediment, probably soon after deposition. Dolomitisation and Mg enrichment of the clay may have occurred at the same time. Seawater is the most probable source of Mg.