Primary dolomite formation on a Carnian alluvial plain/ carbonate platform

Large amounts of dolomite occur in the Alpine Triassic units, but their origin is still poorly understood due to their late diagenetic overprint. Only a few studies so far have addressed possible primary or early diagenetic (penecontemporaneous) dolomites (e.g. Iannace and Frisia, 1994; Preto et al., 2015). However, penecontemporaneous dolomites are of great interest as they may bear signatures reflecting palaeoenvironmental conditions and, in turn, factors related to their formation pathway under earth surface conditions. We analysed dolomite beds intercalated in a clay-rich sequence of the south Alpine Raibl Group (Travenanzes Fm.) previously described by Preto et al. (2015). Large amounts of siliciclastic sediments were deposited on an alluvial plain in the aftermath of the Carnian pluvial episode (CPE). Sedimentary structures indicate soft sediment deformation, suggesting that the carbonate was originally unlithified. Sediment deformation could be due to episodes of high water energy due to high tides or storms (tempestites) during sea-level highstand, but these episodes may not be simultaneous with times of dolomite precipitation. In some beds, a 5-mm-scale lamination consisting of dolomite and clay occurs. Lack of fossilized organisms and presence of the sulphate minerals celestine and barite indicates that the depositional environment was restricted from open seawater and most likely rather saline. The unlithified characteristics indicated by the soft sediment deformation are consistent with a primary origin of the dolomicrite, most likely due to direct precipitation from hypersaline brines. Flocculation of dolomite from hypersaline brine, possibly during the dry seasons, alternating with deposition of clay during the humid seasons of the CPE is reasonably the cause of fine lamination. Each single dolomicrite crystal is, as seen by Transmission Electron Microscopy, an aggregate formed by imperfectly oriented attachment of nanocrystals, resulting in minimal orientation mismatch within the micrometre-scale individual (Preto et al., 2015). Electron backscatter diffraction (EBSD), however, indicates that each 1-5m sized dolomicrite crystal has uniform orientation. In combination, the two approaches at different scale provide insight into a particle-attachment nucleation pathway, consistent with recent experiments (Rodriguez-Blanco et al., 2015). Although an amorphous precursor may have occurred, it is not preserved. The preservation of the nano-crystal fabrics, already observed in Abu Dhabi sabkha individuals as “mottled microstructure” (Frisia, 1994) suggests that the Travenanzes dolomite has not experienced ripening during lithification.