Paleosalinity significance of occurrence and distribution of methyltrimethyltridecyl chromans in the Upper Cretaceous Nenjiang Formation, Songliao Basin, China

A group of methyltrimethyltridecyl chromans (MTTCs) was found in core samples of Member 1 (K(2)n(1)) and Member 2 (K(2)n(2)) of the Nenjiang Formation (Upper Cretaceous) from the SK-l southern borehole (Songliao Basin, China). They are assigned for the first time in sediments older than the Tertiary in China. Their composition and distribution are indicative mainly of the redox and salinity conditions in the depositional environment. The delta-MTTC isomer is in relatively higher abundance in samples from enhanced salinity and reducing conditions, but was not detected in samples from aerobic and low salinity environments, while alpha-MTTC appears to be present in samples from environments with a broad range of salinity and redox conditions. The alpha-MTTC/gamma-MTTC (alpha/gamma) ratio has a similar indication as that of the alpha/delta ratio and could be used as a corroborative ratio of paleosalinity. A combination of biomarker parameters suggests that the massive lacustrine petroleum-prone source rocks associated with the K(2)n(1) interval were likely developed under a stratified water column with enhanced salinity and an anoxic bottom water layer, associated with a much less saline (fresh to brackish) upper water layer. (C) 2011 Elsevier Ltd. All rights reserved.