Carboniferous–Permian transgression/regression mechanisms in the Eastern Ordos Basin and their sea-level spatiotemporal variability: Insights from source-to-sink systems

Abstract

Developed on the North China Craton, the intracratonic Ordos Basin contains a complete Paleozoic-to-Cenozoic sediment record, allowing for long-term paleoenvironmental and climate change investigation. During the Carboniferous–Permian period, convergence between the North China block and the paleo-Yangtze plate to the south resulted in a general marine regression characterized by a series of second-order transgression/regression cycles diachronous along the eastern margin of the Ordos. However, the detailed mechanisms that induced these cycles, as well as the associated paleoecological changes, remain unknown. In this study, we integrate multiple indices, including δ18O and δ13C, rare earth element (REE), paleontological assemblages, and clay content (w (chlorite + kaolinite)/w((Illite) ratio) planar distribution, to restore the paleo-water depth distribution. These parameters are then used to reconstruct the paleo-sea level from the Pennsylvanian to the middle Permian. We conclude that the direction of second-order transgression/regression was mainly toward the east during the Pennsylvanian–early Permian and switched clockwise toward the north during the middle Permian. We suggest that the second-order cycles, diachronous in space and time, are mainly linked to local variations in sediment supply and regional uplift. Using detrital zircon U–Pb data and the REE and trace element content and heavy mineral assemblages (HMA), we estimate the sediment provenance area. The sediment volumes deposited in the basin through time are obtained using 2D seismic data. During the Carboniferous, the coarse-grained sediments deposited in the eastern Ordos were derived from the uplifting Helan Mountain (Qiandam–Qilian orogenic belt). By the middle Permian, the detrital material became multi-sourced, thus issuing the Yinshan range to the north and the Qinling range to the south. We then integrate the description of numerous core samples with electric log and 2D seismic data to reconstruct the sediment facies associations across the first-order regression from the Carboniferous tidal flat depositional system to the middle Permian prograding fluvial delta system. According to the transfer of the glacial epoch, the sedimentation rate, and the transgression/regression rate above, we classify the evolution process into three patterns: low-transgression rate and less-sediment supply pattern (the late Carboniferous), high regression rate and mass sediment supply pattern (the north block during the early Permian), and low regression rate and mass sediment supply pattern (the south block during the Sakmarian stage). Lastly, with the quantitative calculation of the source-to-sink (S2S) parameters, including the S2S system volume and the elevated height of regional uplift with the 2D seismic data, we propose the mechanism of transgression/regression in the Ordos Basin, responding to the above three pattern s. The first pattern was controlled by regional uplift, whereas the second pattern was controlled by sediment supply. As to the third one, uplift and sediment supply could affect the transgression/regression process.