Origin of kerolite and associated Mg clays in palustrine-lacustrine environments. The Esquivias deposit (Neogene Madrid Basin, Spain)

The composition, texture and genetic evolution of kerolite and related Mg c lays belonging to the Intermediate Unit of the Miocene from the Madrid Basi n have been studied. About 400 samples from the Esquivias deposit were anal ysed by several mineralogical and/or chemical techniques. Two genetic pathw ays for the development of Mg clays during early diagenesis have been obser ved: (1) mudflat environment: AI-smectite (beidellite) --> Mg-smectite (sap onite); and (2) palustrine environment: Si-Mg gel --> kerolite --> kerolite -stevensite --> stevensite. In the mudflat deposit the transformation processes predominate, whilst in the palustrine environment, kerolite is neoformed, probably from a gel-like medium. Stevensite seems to have originated from the transformation of mix ed-layer kerolite-Mg-smectite, but also through neoformation at a later sta ge. The textural features, isotopic data and sedimentary evolution within each lithofacies are indicative of shallowing-upward sequences with development of palaeosols. A post-sedimentary origin for sepiolite, calcite, authigenic quartz, zeolites and baryte is inferred.