Geological and paleobiological attributes described in a carbonate unit from the upper Silurian of the Middle Atlas (Morocco) are interpreted as the result of chemosynthetic processes fueled by fluid seepage. These attributes include: the presence of authigenic carbonate accumulation embedded in siliciclastic rocks; tightly packed, monospecific megafaunal assemblages (atrypid brachiopods); microbial-derived products; precipitation of bio-induced minerals (especially hematite); and seep-derived carbonate phases. Evidence of in-situ growth of the dense brachiopod communities is that their shells are all articulated, and show a complete range of growth sizes. Products of microbial activity are shared with other fossil seep deposits and include clotted micrite concentrations, crusts, and laminated structures, microtufts, biofilms, and endolithic filaments. The best-developed microbial features were recovered in reddish mudstone beds below and within the brachiopod concentrations, where they consist of complex microbial frames morphologically comparable to the ones produced by Beggiatoa mats in modern chemosynthesis-based environments. These remnant mats are associated with early hematite precipitation, which has permitted their extensive mineralization and, consequently, the preservation of their stratigraphic relationships with the shell accumulations. Carbonate phases such as botryoids, splayed calcite, and stromatactoids form the infilling of cavity systems and microconduits. Repeated events of mineral precipitation and corrosion are documented in botryoids and stromatactoids, as well as in more extensive corrosion surfaces, as a result of changes in the ambient chemistry related to type and changing intensity of the seeping fluids. This carbonate unit is the oldest-known seep-derived unit, with a well preserved, voluminous, and diverse microbial component.

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