Dolomita y arcillas fibrosas magnésicas en microbialitas lacustres miocenas

Abstract

Sepiolite and palygorskite commonly occur associated with dolomite in the middle Miocene successions of the Madrid and Duero basins. These minerals were deposited in brackish to saline and shallow lakes, although the formation processes of this association are still not well constrained. By the study and identification of biosignatures in these minerals, the role of microbial processes in their genesis have gained attention among researchers. The study of microbial sediments in modern saline lakes offers additional support to the biogenetic origin of Mg-clays in the organic matrix of the microbial mats (Del Buey et al., 2018). Here we report a microbialite-bearing succession that form the top interval of the Intermediate Miocene Unit of the Madrid Basin. This succession covers a large geographic area of several km2 in the Mesa de Ocaña (Toledo). The succession consists of marls interbedded with smooth to gently undulated limestone beds. The marls are made up of a mixture of dolomite and minor magnesite, fibrous Mg-clays (sepiolite and palygorskite) and silt sized detrital grains, and exhibit den- sely laminated microfabrics. The mineralogical and isotopic features of dolomite (average δC13PDB values of -6.0 ‰) confirm a potential organic carbon source. Laterally, the limestones form smooth to gently undulated beds with inner microlamina- tion locally disrupted by root bioturbation. These well-preserved fossil microbialites can reinforce our understanding of the prevailing conditions during the deposition of the rocks forming the upper part of the Intermediate Unit, beyond diagenetic transformations.