Mineralogia y sedimentologia del yacimiento de saponita de Yuncos (Toledo)

Abstract

Mineralogy and textures of saponite-bearing bentonites and related facies have been studied in quarries located southwest of Yuncos village (Toledo) in the Madrid Neogene Basin. Results allow us to establish three stages related with the evolution of sedimentary environments, where bentonite deposits (middle stage) are at the boundary between lacustrine (lower stage) and distal alluvial facies (upper stage). Lower stage: Regressive mud flat deposits with paleopedogenic features, made up of green clays facies with variable detrital content and hydromorphic mottling at topo Clay mineralogy association: Smectite (tri-dioctahedral)-illite-(kaolinite). Middle stage: Transitional stage of shallow flooding with variable development of paleopedogenic features. The facies association is made up of saponitic brown clays with several textures (massive, fissile-Iaminated, brecciated) and occasional carbonates as calcite ooids or dolomite nodules. Clay mineralogy association: Smectite (trioctahedral)-illite-sepiolite-( kaolinite) . Upper stage: Progradation of distal alluvial fan deposits. The facies association is made up by reddish to brownish mudstones often with bioturbation features and hydromorphic mottling, with sporadic sandy inserts. Clay mineralogy association: Smectite (tri-dioctahedral)- illite-sepiolite-(kaolinite-chlorite). Illite, dioctahedral smectite, kaolinite and chlorite are interpreted as inherited clay minerals, whilst saponite and sepiolite are of authigenic origino Saponite origin is early diagenetic both by dioctahedral smectite transformation and neoformation, being an inhomogeneous genetic process so that the purest bentonite is located far away from distal alluvial progradation where the flooding is longer in time. From a genetic point of view, sepiolite is later than trioctahedral smectite sometimes being in short fibers on saponite sheets, at others in longer fibers as a cement, both cases related to salinity-alkalinity changes in the environment.