Organic-matter accumulation of the lacustrine Lunpola oil shale, central Tibetan Plateau: Controlled by the paleoclimate, provenance, and drainage system

Abstract

The Cenozoic lacustrine Lunpola oil shale, located in the central Tibetan Plateau, has received little research interest in its depositional history although it is estimated to be a huge resource reserve. This study investigates the palaeoclimate, provenance, and regional drainage system of the ancient lakes using a combined analysis of element geochemistry and stratigraphy to elucidate the depositional history of the Lunpola oil shale.Most of the samples in the lower part of the section have chemical index of alteration (CIA) values between 65 and 75, reflecting a moderate weathered source, while samples in the upper part the section have CIA values between 60 and 65, which indicate relatively weak chemical weathering in the source area. This stratigraphic grouping suggests a paleoclimate transition from a relatively warm or humid period to a cold or dry period. The Zr/Sc and Th/Sc ratios of the samples reveal that sorting and recycling were minor during deposition of the oil shale. The geochemical results (Al2O3–(CaO⁎+Na2O)–K2O (A–CN–K) diagrams, high contents of Th, plots of Co/Th–La/Sc and Cr/Th–Sc/Th) demonstrate that the sedimentary sources were felsic rocks. Plots of La–Th–Sc, Th–Sc–Zr/10, Th–Co–Zr/10, and Ti/Zr versus La/Sc illustrate that the source rocks were from continental arc and active continental margin. The Baingoin batholiths to the south of the Lunpola Basin consisting mainly of Cretaceous granodiorite and adamellite are likely responsible for supplying materials to the Lunpola oil shale. Similar special REE patterns further suggest the affiliation of the oil shale with the batholiths. Provenance analyses indicate a main river from the south; moreover, stratigraphic study confirms an upstream ancient Baingoin Lake between the source area and the ancient Lunpola Lake.In this drainage system, paleoclimate clearly restricted the development of rivers around the lakes. Although terrestrial organic matter (higher plants) dominated the drainage area, the main river originating from the south could only bring a certain amount of them to the lakes. Moreover, considering the filtration of the upstream ancient Baingoin Lake, minor terrestrial organic matter could be carried and deposited in the ancient Lunpola Lake. Therefore, lacustrine algae could proliferate as the main organic source and preserve in the stratified hypersaline lake to form the largest oil shale resource in Tibet.