Composition and Provenance of Sandstones and Siltstones in Paleogene, Huimin Depression, Bohai Bay Basin, Eastern China

Abstract

This study was conducted to distinguish the compositions and provenance of sandstones and siltstones in the E k 1–E s 3 members of Huimin depression. The samples have been analyzed for petrographic, major element and selected trace element compositions (including REE). The results show that the sandstones from the first member of Kongdian Formation (E k 1) have higher quartz compositions than those from the fourth member (E s 4) and the third member (E s 3) of Shahejie Formation. The alkali feldspar/plagioclase ratio in the A-CN-K diagrams decreases in the order E k 1>E s 4>E s 3, which suggests that the E k 1 member had a more alkali feldspar-rich granitoid source and more intense weathering of the source than the E s 4 and E s 3 members. The mineral distributions in the A-CN-K diagrams also indicate that the sandstones and siltstones in the three members underwent K-metasomatism. Extrapolation of the sandstones and siltstones back to the plagioclase-alkali feldspar line in the A-CN-K diagram suggests a high average plagioclase to alkali feldspar ratio in the provenance (tonalite to granite). In addition, the chemical index of weathering (CIW) and chemical index of alteration (CIA) parameters of the sandstones and siltstones suggest that the weathering of the first cycle material was intense, and the CIW decreases in the order E k 1>E s 4>E s 3. Trace element ratios suggest all the sedimentary rocks were mostly derived from granitoids. Elemental ratio plots (e.g., Th/Sc vs. Eu/Eu*) of sandstones and siltstones suggest a mix of a granodiorite-tonalite source as a source of the sandstones and siltstones. The sandstones and siltstones of E s 3 and E s 4 members also contain higher Eu/Eu* and lower Th/Sc ratios than the sandstones and siltstones of E k 1 member. As is consistent with the petrography, it suggests that the sandstones and siltstones of E s 3 and E s 4 members are derived from a source with less granite and more granodiorite-tonalite than the sandstones and siltstones of E k 1 member.