Multiscale characterization of an extensive stromatolites field: a new correlation horizon for the Crato Member, Araripe Basin, Brazil

Abstract

There is wide recognition of lacustrine sediments as excellent archives of a basin's depositional history due to their high sensibility to environmental changes. Among them, microbial limestones are one of the most valuable tools for paleoenvironmental reconstruction, as the biological agents responsible for their genesis tend to respond to short-lived variations of the depositional setting creating specific precipitation patterns. We here document and investigate the sedimentary features of a specific sedimentary layer, remarkable by the extraordinary lateral continuity of its textural attributes over kilometer distances. This marker horizon occurs among the first carbonate layers of the Crato Member (Aptian, Araripe Basin, NE Brazil), commonly assigned as a paleolacustrine system. We build upon a multiscale comparative analysis (mesoscale, microscale, and chemical) to outline the main processes and paleoenvironmental settings that prompted this interval's widespread and laterally almost uniform depos ition. A lamination pattern identified in different well cores was scrutinized and compared, proving the striking lateral continuity of the layer and attesting that autochthonous biologically-induced mineralization was the primary mechanism of the formation of the microbialites. Compositional and stable isotope results also show similar trends throughout the well cores, where minor differences represent the influence of local processes. The studied interval encompassed a relatively swift transition of organic shales rich in ostracod valves to planar stromatolites, where both developed in the anoxic benthonic zone of a freshwater lake. The precipitation of the overlying finely laminated limestones is related to a change in the carbonate genetic mechanism as a response to a more stable lacustrine stratification. The widespread formation of microbialites preserving an almost identical textural pattern must be related to a regional event, constituting a rare example of a preserved ancient biostrome. Moreover, the investigation of this sedimentary layer can further contribute to determining the roles of different biotic and abiotic processes in microbialite precipitation over large areas.