Determination of the top boundary strata of the Taiyuan Formation in the southeastern Ordos Basin-Constrainted by U-Pb geochronology of the Shanxi Formation-Taiyuan Formation in the Yichuan Area

Key words: 
• Ordos Basin /
• Taiyuan Formation /
• clastic rocks /
• zircon U-Pb age /
• sequence classification

Abstract: Abstract: [Objective] In this study, to redetermine the stratigraphic boundary of the Carboniferous-Permian Taiyuan and Shanxi formations in the Yichuan area of the Ordos Basin and to solve the problem of sedimentary age attribution of coal-bearing natural gas mudstone section near the boundary of the Shanxi-Taiyuan Formation. [Methods] We selected the sandstone and mudstone samples of the transition layer between the Taiyuan and Shanxi formations in the typical coring well Yi 120 to conduct the U-Pb dating of detrital zircon and we used the maximum depositional age ( MDA ) estimation method, combined with previous research results on the zircon age of the Taiyuan Formation-Shanxi Formation in the North China Plate, to constrain the depositional age of the study interval. [Results] Experiments show that the maximum likelihood age (MLA) of the sandstone and mudstone at the top of the Taiyuan Formation is 298±2 Ma, which represents the age of the latest deposition of the strata. This interval was deposited in the Early Asselian period, which is consistent with the Asselian sedimentary age determined by conodont. The MLAs of the sandstone samples at the bottom of the Shanxi Formation is 295±1 Ma, representing the age of the earliest deposition, which was deposited in the Middle-Late Arthurian period. [Conclusions] Based on the above research and analysis, we conclude that the boundary between the Taiyuan and Shanxi formations in the study area should be above the sand-mudstone section at the top of the Taiyuan Formation; that is, the marine sand-mudstone layer at the top of the Taiyuan Formation in the study area should belong to the Taiyuan Formation, which provides a certain basis for the stratigraphic division of the Shanxi-Taiyuan Formation in the study area.