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

Abstract

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.