Organic material accumulation of Carnian mudstones in the North Qiangtang Depression, eastern Tethys: Controlled by the paleoclimate, paleoenvironment, and provenance

摘要

The Late Triassic mudstones are considered to be the most significant hydrocarbon source rocks (TOC: 0.54%-3.29%) in the North Qiangtang Depression, eastern Tethys. Here, we present geochemical data from the Woruo Mountain Carnian mudstones, in order to investigate their paleoclimate, paleoenvironment, and provenance and to analyze the mechanism of organic material accumulation. The paleoclimate condition was warm and humid during the Carnian mudstones deposition, as indicated by moderate chemical index of alteration (CIA; 73-76), which may be connected with the Late Triassic Carnian stage global climate event in the Tethys. The low U/Th (0.17-0.25) and C-org/P-tot (7-33) ratio values and moderate manganese contents, reflect the oxidizing conditions during the Carnian mudstones deposition. The relatively high primary productivity in this study is supported by the relatively high P concentrations. The Al2O3-(CaO* + Na2O)-K2O ternary plot and Th/Sc-Zr/Sc crossplot reflect that the source areas have undergone a medium chemical weathering with weak sedimentary recycling. The TiO2-Zr, Co/Th-La/Sc, La/Th-Hf, and La/Yb-Sigma REE bivariate diagrams indicate that the provenance of Carnian mudstones was primarily from felsic igneous rocks. The collision setting has been identified based on the multi-major elements discriminate plots in the present study. The Riwanchaka and Mayigangri masses to the southwest and south of the study area consisting mainly of Middle-Triassic granodiorite and Late Triassic granite are likely responsible for supplying provenance to the Woruo Mountain Carnian mudstones, which have similar REE patterns. The relatively high TOC contents of Carnian mudstones are related to high paleoproductivity and fast sedimentation rates, which will lead to preservation of some organic matter even when bottom waters are completely oxidizing. The detrital input during the Carnian mudstones deposition would result in dilution of organic matter. (C) 2017 Elsevier Ltd. All rights reserved.