Acta Petrologica et Mineralogica

In order to investigate the sedimentary environment, controlling factors and formation pattern of the shale of Longtan Formation of the Upper Permian in eastern Sichuan Basin, systematic tests of organic petrology, organic geochemistry and elemental geochemistry were conducted on the shale samples from Huayingshan profile as the research object. The results indicated that the source rock of the Longtan Formation shale in the studied area is complex. Diagrams of Co/Th-La/Sc, La/Yb-ΣREE and TiO₂-Al₂O₃/TiO₂ suggested that the source rocks of the Longtan Formation shales are mainly Emeishan basalt with high Ti content and intermediate acidic volcanic rocks. The trigonometric diagrams of Th-Sc-Zr, Th-Co-Zr and La-Th-Sc showed that the source rocks were mainly formed in the tectonic background of passive continental margin environment. The average chemical alteration index (CIA) of the Longtan Formation shale was 80.64, indicating moderate to high intensity weathering; the Sr/Cu values ranged from 0.48 to 8.08, with an average of 2.22, and the MgO/CaO values ranged from 0.62 to 52.58, with an average of 14.36, reflecting warm and humid climate condition during the sedimentation period of the Longtan Formation overall. The Sr/Ba values ranged from 0.37 to 1.86 and 100 MgO/Al₂O₃ values ranged from 3.27 to 5.45, indicating a semi-saline transitional sedimentary environment. The mean value of Zr/Al was 7.97×10^-3 and that of 100 Mn/Fe was 0.68, indicating that the sedimentary water in the Longtan period of the Huayingshan area was relatively shallow and was also consistent with the geological background of shallow continental shelf reported by previous studies. The V/(V+Ni) values ranged from 0.62 to 0.98, averaged 0.89, Ni/Co values ranged from 0.47 to 13.81, averaged 5.49, and Ce_anom values had averaged value of 0.03, suggesting that the sedimentary water of the Longtan Formation was mainly in a weakly reductive to reductive environment. In terms of sedimentary environment, organic matter characteristics and geological background, it could be inferred that the organic-rich shale of the Longtan Formation was formed in a warm and humid climate environment. In the background of strong weathering, after the death, the higher plants entered the water body accompanied with the numerous input of terrestrial debris formed by basalt and intermediate acidic volcanic rocks, and high plants rapidly deposited, buried, and effectively preserved under the background of the condition of rapid deposition, and finally that high-quantity source rock with inclined kerogen type was formed.

Longtan Formation; elemental geochemistry; sedimentary environment; sedimentary model; eastern Sichuan