Lower Triassic stromatolites in Luodian County, Guizhou Province, South China: evidence for the protracted devastation of the marine environments.

Stromatolites are one of the oldest and most intriguing organosedimentary deposits. In contrast to stromatolites of the Precambrian to Early Ordovician, Phanerozoic equivalents occurred episodically under specific conditions. A group of previously undescribed stromatolites in composition occur in the Lower Triassic (Olenekian) at the Dajiang section in the Luodian region of Guizhou Province, South China. We described the textures of these stromatolites with the aim of determining the genetic mechanisms and revealing the nature of interactions between micro-organisms and marine environments. Mesoscopic features show that the stromatolites consist of several sets of stacked slices, and that they are embedded in alternating beds of fine and coarse microsphere packstones that include aggregates of microspheres, forming grapestones and lumps. Microscopically, the stromatolites consist of spar- and dolomite-infilled microspheres (average diameter, 100 μm), micrites, peloids, small-sized pyrite framboids (average diameter, 5.8 μm) and fenestrae. Micrite-dominant intercalations accentuate laminated textures at a mesoscopic level and are laterally continuous with micrite-rich parts in surrounding interstromatolites, indicating the simultaneous, widespread deposition of these layers. The microspheres and associated micrites were the products of in situ microbial activity, probably sulphate-reducing or anoxygenic phototrophic bacteria, which led to the formation of these unusual stromatolites. Even during a protracted period of harsh marine conditions, the micrite-rich carpets were deposited intermittently on the stromatolites and their surroundings under severely anoxic/sulphidic conditions. The presence of Early Triassic stromatolites and their subtle but important vertical variations in texture provide a record of temporal changes in marine conditions during geobiologically critical intervals.