Molecular indicators of palaeosalinity and depositional environment of small scale basins within carbonate platforms: The Late Triassic Hauptdolomite Wiestalstausee section near Hallein (Northern Calcareous Alps, Austria)

摘要

Microfacies and biomarker analyses are used to determine the environmental conditions during deposition of marly dolomicrites from the Hauptdolomite Wiestalstausee section near Hallein (Salzburg, Austria), part of the Late Triassic Hauptdolomite/Dachstein carbonate platform of the Northern Calcareous Alps. Phytoplankton and photosynthetic bacteria are considered to be the major primary producers of the organic matter in these immature (similar to 0.5% equivalent vitrinite reflectance), carbonate-rich rocks. Contributions from vascular plants are suggested from the n-alkane distribution pattern and high content Of C-29 steranes. Enhanced microbial activity in the depositional environment is proposed on the basis of the hopanoid content. The occurrence of aryl isoprenoids, probably derived from carotenoids of the photosynthetic green sulfur bacteria (Chlorobiaceae) indicates the presence of free H2S in the bottom water. Mesohaline conditions in the upper part of the water column are deduced from the presence of dimethylated and trimethylated 2-methyl-2-(tridecylchromans) (MTTCs). Co-variation in the distribution of MTTCs and pristane/phytane ratio and the concentration of a C-16 aryl isoprenoid argue for photic zone anoxia due to salinity stratification. The biomarker composition points to a high productivity marine environment with limited water exchange and a stratified water column confirmed by microfacies analysis. A small scale basin formed within the restricted inner lagoonal area of the Hauptdolomite/Dachstein carbonate platform through erosion by currents, followed by a sea level rise is considered the most appropriate depositional environment of the dolomicrites of the Wiestalstausee section. (c) 2006 Elsevier Ltd. All rights reserved.