Sedimentación lacustre-palustre, formación de ópalos y otras silicificaciones en el Mioceno medio al sur de Villaluenga

Abstract

The petrological and sedimentological analysis of Middle Miocene carbonate sediments and siliceous rocks , outcropping North Toledo, provides new ideas on the interpretation of these lacustrine and paludine sedimentation. The main lithofacies found are: 1) micrite with crytalgal (?) laminite, caleite gypsum pseudomorphs, and trace of diatoms; 2) biomicrite with varying amount of lacustrine mollusks, ostracodes and charophytes, which form laminar structure; 3) biomicrite with the same skeletal components (charopytes, ostracodes, etc .) and sometimes diatoms but disorganized, 4) pseudospar and microspar calcite with lenticular gypsum pseudomorphs; 5) sepiolitic marls. The sedimentation environment was very shallow, with low salinity, calcareous, highly productive with abundant vegetation (sometimes diatoms) and periodic subaerial exposures. All type of sediments (paludine or lacustrine) have silicifications although they are different. An early discontinuos diagenetic silicification process, which forms only silica minerals (mainly opal C-T) and where the silica probably comes from the diatoms dissolution, has been observed. Also early in the diagenesis, but later than the former, a silicification process takes place, originating the continuous opal C-T and clay minerals (mainly sepiolite) horizonts, which can be homogeneous or present Liesegang bands (rings and laminations). This second silicification seems to happen in the interface water-sediment, during periods of evaporation, when the sha1low lake is near disecation. The silica cements in the different porosities of the rock (fenestral karst dissolution,…etc.) indicate that after the second silification, there is a remobilization of the primary silica, which sometimes produce other silicifications of quartz and silica cementations.