Geochemistry of Middle Oligocene-Pliocene sandstones from the Nanpu Sag, Bohai Bay Basin (Eastern China): Implications for provenance, weathering, and tectonic setting

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  • LIU SHILIN
    Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Graduate School, Chinese Academy of Sciences
  • LIN GE
    Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences
  • LIU YUNHUA
    Research Institute of Jidong Oilfield Company, CNPC
  • ZHOU YE
    Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Graduate School, Chinese Academy of Sciences
  • GONG FAXIONG
    Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences Graduate School, Chinese Academy of Sciences
  • YAN YI
    Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

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Petrographic and geochemical compositions of thirty-seven Middle Oligocene-Pliocene sandstone samples from the Nanpu Sag in Bohai Bay Basin were analyzed to evaluate their provenance, weathering, and tectonic setting. These sandstones comprise mainly arkoses and lithic arenites and a few graywackes, characterized by low to moderate quartz/total feldspar ratios (1.31-6.11, average 2.72 ± 0.98) and CIA values (45.87-70.13, 57.49 ± 5.93). The Middle-Upper Oligocene sandstones have average lower SiO2 (71.72 ± 5.16%), and higher Fe2O3 (3.06 ± 1.26%) and MgO (1.65 ± 0.39%), in contrast with the Miocene samples that generally have high SiO2 (76.46 ± 4.26%) and low Fe2O3 (3.04 ± 1.51%) and MgO (0.69 ± 0.32%) contents. Following the deposition, Fe2O3 (3.17 ± 1.00%) and MgO (1.07 ± 0.38%) contents increased again in the Pliocene samples. Such a compositional variation trend is interpreted as a change of tectonic activity in the sag from Middle Oligocene to Pliocene, i.e., stronger in the Middle Oligocene, weaker in the Miocene and to stronger again in the Pliocene. Qt-F-L plot, trace element provenance discrimination diagrams (e.g., Eu/Eu*, Co/Th, and La/Sc), and REE characteristics suggest that the source rocks were mainly derived from felsic igneous rocks, possibly from Mesozoic granitoids and felsic volcanic lavas in the Yanshan Fold Belt. Tectonic discrimination diagrams (e.g., SiO2-K2O/Na2O, Fe2O3* + MgO-TiO2, and Th-Sc-Zr/10) show that these sandstones were mainly deposited in an active continental margin setting, implying a remobilized intra-continental environment.

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