Hydrocarbon biomarkers of different lithofacies of the Salt IV Formation of the Mulhouse Basin, France

Abstract

Forty-nine samples from an 11 m section of the Salt IV Formation of the Tertiary Mulhouse Basin (Alsace, France) covering six different lithotypes (marl, nodular, lenticular, laminated and massive anhydrite, halite) were analysed for hydrocarbon biomarkers. The hydrocarbon biomarker assemblages are dominated by acyclic isoprenoids, especially phytane (0.13 < Pr/Ph < 0.52), n-alkanes with specific distribution patterns, and steranes. Large differences in their distributions indicate significant changes in the palaeoenvironment. The largest changes observed are those in the desmethyl- and methylsterane distributions and are probably linked to an occasional reconnection of the evaporite basin with the sea, leading to an input of dinoflagellate algae. 3-Methylsteranes are present in all samples studied and their carbon number distribution mimics that of the 5-desmethylsteranes, in line with previous suggestions that they are diagenetic products of sterols with a sugar unit at C-3 biosynthesized from dietary sterols. The increase in relative abundance of the 3-methylsteranes during periods of marine incursions suggests that these precursors are biosynthesized by marine organisms. Variations in biomarker composition between the different lithofacies are minor, probably because the organic matter is mainly associated with the marl and all lithotypes contain a marl component. Detailed subsampling of two marl layers and subsequent analysis indicated that the changes on a small time scale are minor to those on a larger time scale