Abstract

There is wide recognition of lacustrine sediments as excellent archives of a basin’s depositional history due to their high sensitivity to environmental changes. Among them, microbial limestones are one of the most valuable tools for paleoenvironmental reconstruction, because 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 a paleolacustrine system. We build on a multiscale comparative analysis (mesoscale, microscale, and chemical) to outline the main processes and paleoenvironmental settings that prompted this interval’s widespread and laterally nearly uniform deposition. A lamination pattern identified in different well cores was scrutinized and compared, and shows striking lateral continuity attesting to autochthonous biologically induced mineralization as 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 encompasses the 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 thinly 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.