Geochemical constraints on depositional environment of Late Mesozoic sedimentary rocks from the Ri-Qing-Wei Basin in the East Shandong

Ri-Qing-Wei (Rizhao-Qingdao-Weihai) Basin in the eastern Shandong Peninsula, which was developed in the Sulu Orogen, is a large-scale rift basin formed since Late Mesozoic. The unique sedimentary features in the basin are different from adjacent terrestrial rift basins in the same period, which have received great attention from sedimentologists recently. To study the paleosalinity, paleoclimate, and redox conditions in the deposition period (the Laiyang and Qingshan Period) in the Ri-Qing-Wei Basin, the geochemical (major, trace and rare earth elements (REEs)) characteristics of sedimentary rocks samples from five field typical profiles situated in the Lingshan Island, Laoshan, Taolin, Dingzi Bay, and Zhougezhuang areas, respectively, were analyzed. The samples are characterized by relatively low content of SiO₂ and Al₂O₃ (avg. 52.13% and 13.06%, respectively) and high contents of Fe₂O₃ (avg. 6.14%). ΣREE contents of the samples (avg. 199.15×10⁻⁶) are higher than those of UCC. The patterns of REE are characterized by LREE enrichment, HREE depletion, and weakly positive Eu abnormity. Compared with UCC, the samples are enriched in LILEs and depleted in transitional elements. The values of Sr/Ba and Mg/Ca indicate that the water body of deposition during the Laiyang-Qingshan period was from saline/semi-saline to weakly saline water. The value of Sr/Cu, ∑(Fe+Mn+Cr+V+Ni+Co)/∑(Ca+Mg+K+Na+Sr+Ba), and bivariate SiO₂-(Al₂O₃+K₂O+Na₂O) diagram show that the deposition period was in a dry climate environment, and V/(V+Ni) and Ce abnormity indicate that the deposition period was dominated by a reducing condition. The above comprehensive studies show that the transformation of palesalinity, dry climate and eductive environment of the basin have important geological significance for revealing the process of sedimentary evolution in this area.