252 / 2019-06-30 17:54:32

Paleoenvironment reconstruction of the Upper Paleozoic in the Linxing area, northeastern Ordos Basin, China: evidence from elemental geochemistry and sedimentary facies

Trace elements; REEs; sedimentary environment; Paleoenvironment; n eastern Ordos Basin

The interpretation of the depositional environment, provenance, and paleoclimate provides important information for understanding the formation and distribution of conventional and unconventional petroleum and natural gas systems. The concentrations of trace elements and rare earth elements (REEs) in 100 mudstone samples from the entire Upper Paleozoic section (Carboniferous Benxi and Taiyuan formations; Permian Shanxi, Xiashihezi, Shangshihezi, and Shiqianfeng formations) in the Linxing area of the northeastern Ordos Basin were determined via an inductively coupled plasma–mass spectrometry test. The study area has clear light REE enrichment, but the degree of enrichment is generally poor. The Ce content is basically normal or has a slightly negative anomaly, and the Eu has a negative “V” type anomaly gradually weakening upwards. Redox proxies (V/Ni, V/(V + Ni), and δCe) indicate that most of the mudstones were deposited in relatively dysoxic/oxic conditions. A warm and humid climate (represented by the Sr/Cu and Rb/Sr ratios) prevailed during the Benxi to Xiashihezi periods, even though an extremely dry climate occurred in the Benxi and Taiyuan periods. The Shiqianfeng period has a completely dry climate. Discriminant diagrams, consisting of La-Th-Sc and Th-Co-Zr/10, suggest that the mudstones were deposited in an active continental margin and passive margin, with a few samples from the Ben and Taiyuan formations from the continental island arc. Additionally, the ΣREE vs La/Yb discrimination diagram shows that the property of the provenance for the mudstones consists mainly of sedimentary rocks and certain alkaline basalt and/or granite areas, whereas the Shiqianfeng Formation was only influenced by sedimentary rocks. The increase in ΣREE concentrations and the decrease in the Sr/Ba ratios above the Benxi Formation indicate that a terrigenous clast grew significantly and the sea level dropped, which is related to Permian tectonic movement. Sedimentary provenance from bivariate plot diagrams of Hf versus La/Th and La/Sc versus Co/Th shows mixed felsic/basic source influences for all the formations, with the Shanxi Formation displaying certain influences from acidic sources. A comprehensive REE analysis shows that the provenance of the Linxing area is from the Alashan–Yinshan paleocontinent to the north of the Ordos Basin, with the two main invading fluvial directions being recognizable.