A SILURIAN CHEMOSYNTHETIC ECOSYSTEM FROM CENTRAL MOROCCO

A Silurian, large carbonate mound, outcropping north of Khenifra in central Morocco, contains a dense brachiopod assemblage entirely made up of the atrypid Dubaria lantenoisi. The mound is included in a mainly shaly succession which contains abundant crinoids and orthocone nautiloids. Both biotic and field evidences, compared with those from other areas and ages, suggest that the genesis of this carbonate body may be related to a chemosynthetic cold-seep setting. Although there are no extant chemosynthetic brachiopods, there are numerous examples of brachiopod-bearing, fossil cold-seep sites, from Ordovician to Pliocene age. A number of biological and geological characters of the Khenifra mound are shared with other carbonate accumulations having obvious chemosynthetic cold-seep origin. These characters include i) the carbonate nature of these bodies, which are embedded in terrigenous rock units; ii) authochtonous biological communities largely made up of monospecific/oligothipic shelled taxa; iii) marked spatial changes for these faunal communities, suggesting the presence of strong environmental stress gradients. The presence of fibrous, bladed, and botryoidal calcite, undeformed brachiopod shells and pellets, associated with a wealth of stromatactis structures, suggest early cementation for the Khenifra mound. Early cementation likely occurred before burial, and carbonate crusts, iron oxides, and algal binding may have held together such deposits until diagenesis is completed.

The abundance of microbial activity is documented by microstromatolites, various endolithic structures, and a diffuse red pigmentation produced by the presence of hematite. Iron (hematite) accumulations also developed hardground-like crusts, microtufts, and various concentrations of bladed hematite grains. Apart that the microbial geochemistry leading to hematite precipitation in oxygen-poor environments (such as those from cold-seep settings) is now well described, the microbial nature for these ferric iron encrustations is also documented by peculiar features, such as the kopara-like structures. These three-dimensional, alveolar networks abound in the Khenifra mound and closely resemble the modern kopara (stromatolitic layers) described from Polynesian atolls.