Paleoenvironmental reconstruction using biological markers for the Upper Triassic–Middle Jurassic sedimentary succession in Tabas Basin, central Iran

Abstract

The Upper Triassic–Middle Jurassic sedimentary succession in the Tabas Basin, with a thickness of about 1600 m, provides a case showing geochemical property changes through the Triassic–Jurassic boundary. The studied section (Kamarmacheh Kuh) is composed of the marine Nayband Formation (Norian–Rhaetian) overlain by siliciclastic sediments of Ab-e-Haji Formation (Lower Jurassic–Aalenian). Detailed geochemical analyses were conducted on selected samples from both formations and the results were used to infer paleo-depositional conditions. Most of the studied samples contain <1 wt% TOC composed mostly of oxidized organic matter with insignificant generative potential. Extract analysis of four representative samples indicate that the rocks also contain minor amounts of preserved algal organic matter along with a secondary contribution of higher plant organic matter from the adjacent watershed. Biomarker analyses show subtle variations in the relative contribution of land plant material that are consistent with the widespread occurrence of coal seams in the upper parts of the Nayband and basal parts of the Ab-e-Haji formations. Although the samples from the Kamarmacheh Kuh Section have low source potential, the extractable hydrocarbons indicate that conditions existed that were conducive to organic matter preservation and that regions of the Tabas Basin with higher primary productivity or lower sedimentation rates may have greater potential.