Microbial primary dolomite from a Norian carbonate platform: northern Calabria, southern Italy

摘要

The origin of fine-grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High-relief thrombolites and stromatolites characterize sub-tidal facies, and low-relief and planar stromatolites, with local oncoids, typify the inter-supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub-tidal facies. Some 75% of the succession consists of fabric-preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end-members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size < 5 and 5-60 mu m, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter-coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic-rich micritic laminae and clots of the inter-supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron-sized coccoid bacteria and spheroidal-ovoidal nanometre-scale dwarf-type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive delta O-18 values. The occurrence of fossilized bacteria and organic matter in the fabric-preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn-sedimentary dolomite.