GEOCHEMICAL ANALYSIS OF SEDIMENTARY ENVIRONMENT OF LAIYANG GROUP IN TAOLIN AREA, RIQINGWEI BASIN

In the coastal area of eastern Shandong Province, the Early Cretaceous marine sedimentary strata are mainly distributed among rift continental basins. The previous study demonstrated that during the Late Mesozoic, a large-scale NE-trending rift basin was named the Riqingwei Basin. However, the sedimentary environment of the basin remains controversial. This study investigated that the provenance, tectonic setting, palaeoclimate, and palaeoredox circumstances in the study area using elemental geochemistry, thereby elucidating the sedimentary history of the Lower Cretaceous sediments and reconstructing the palaeo-environment. Fourteen siliceous sandstone samples from the Early Cretaceous Laiyang Group in Taolin were taken as the research object. The CIA value of 70 % of the samples（CIA =50-60）and the bivariate diagrams reflect the weak chemical weathering conditions in the source area. The geochemical results indicated that rocks for the region are felsic rocks and ancient sediments. The major- and trace-element discrimination diagrams indicate that rocks mainly originated from the continental island-arc and active continental margin through deciphering the tectonic history. The PIA value of the samples（PIA=36-74）, which was lower than that of PAAS, indicates that the source area's weathering degree was low. The SiO₂/Al₂O₃ ratio in the study sample fluctuated between 2.84 and 4.11, indicating that the source composition was immature. Using the SPSS statistical software, the Jiaonan and Penglai groups were most likely to be the provenance. The value of Al/Ca, Na/K, Rb/Sr, and Sr/Cu indicate that the Laiyang Group was predominantly deposited in semi-arid and sub-humidarid conditions. The Sr/Ba and CaO/(Fe+CaO) values imply it is under the brackish water environment. Furthermore, the V/(V+Ni) ratios and Ce anomalies in the rocks indicate anoxic conditions. In summary, from this study, it can be concluded that the Laiyang Group in Taolin was a Marine–Continental Transitional Facies with three-stage development characteristics, and the lower and upper sections are lacustrine facies and the middle section is marine facies.