Bacterial mediation, red matrices diagenesis, Devonian, Montagne Noire (southern France)

Two Devonian led carbonate rock sections are studied in the Montagne Noire, at Coumiac (Frasnian/Famennian) and at the Pic de Vissou (Eifelian/Givetia n). The sediments are,grey-red mudstones and wackestones rich in pelagic fo ssils. They are characteristic of an outer ramp. The Coumiac sequence is co ndensed with numerous hardgrounds and hiatuses. The Pic de Vissou successio n is more complete and of shallower origin. In both cases, the origin of th e red coloration of the micritic matrix is probably linked to bacterial act ivity which produced submicronic hematite. Both iron and manganese concentr ations are low (average 0.2%). Bacteria form ferruginous microstromatolites , blisters, microtufts, 'hedgehogs' filling sponge perforations and thin co ntinuous mineralized films (probably biofilms), Hardgrounds are underlined by ferruginous microstromatolites. The origin of the matrix color is probab ly related to the destruction of these bacterial constructions, the submicr onic hematite ultimately coating the crystal faces of the calcite mosaic. D uring early lithification, microfissures appeared and were invaded by micro bial colonies. Scanning electron microscopy (SEM) shows that these colonies are composed of spheroidal beads, This suggests continuity of the bacteria l activity during early diagenesis. Later on, these early fissures were cut by burrows. Subsequently a secondary fissure network transected all the pr eviously mentioned sedimentary structures. This late fissure network is cha racterized by diagenetically remobilized hematite and/or calcite. The lates t alterations rue stylolites and ultimate tectonic fractures. (C) 1999 Else vier Science B.V, All rights reserved.