Research On The Sedimentary Facies Of Late Mesozoic Rift Basin In The Offshore Area Of Eastern Shandong

A set of sedimentary strata represented by terrigenous clastic rocks, volcaniclastic rocks and volcanic lava were developed among the metamorphic rocks and Yanshan granites in Laoshan Mountain, Lingshan Island and Tangdaowan Bay area along the Qingdao coastline. They are not well researched and much disputed. The author’s research group took the above outcrops as the study area, combined with the latest gravity and magnetic data, and first proposed the concept of late Mesozoic rift basin in the offshore area of Eastern Shandong. Therefore, it is important to find out types of the sedimentary facies and volcanic facies and figure out their characteristics, regularities of distribution and forming time, which can provide basic information for the late prototype basin restoration and basin-mountain evolution analysis.On the basis of field work and lab analysis, the author finds that there are marine turbidite facies, microbial facies, fan delta and volcanic facies in the study area. From bottom to top, there are(bathyal-abyssal) fluxoturbidite facies, neritic facies, turbidite facies volcanic overflow facies, fan delta facies and volcanic eruption / overflow facies in the Lingshan Island and it reflects the gradual reduction of water depth and gradual filling of the basin. Twelve soft-sediment deformation structures triggered by earthquakes were recognized in the turbidite of Lingshan Island. With the analysis of slump folds, asymmetric load structures and soft dual structures, the author concluded that there existed a paleoslope with the southeast higher and northwest lower, and the sediment gravity flows came from the southeast. In the upper part of Lingshan Island were discovered fan-delta front sub-facies and there are three layers of basalt and eight layers of pyroclastic flow deposits sandwiched in them. Epimetamorphic marine turbidite was discovered in the Laoshan Mountain area and there exist volcanic vent facies and volcanic slope in the Tangdaowan Bay.Taking previous studies, facies analysis and zircon dating results into consideration, the author found that the marine turbidite facies in Lingshan Island and Laoshan Mountain both developed in the Cretaceous which corresponded with that of Laiyang group in the Jiaolai Basin and speculated that they were different facies formed in the same time in the late Mesozoic rift basin in the offshore area of Eastern Shandong. Volcaniclastic rocks and volcanic lava formed in the same time with Qingshan group. Sedimentary rocks sandwiched in the volcanic rocks in Lingshan Island correlated with the purple glutenite in the Qingshan group formation of Jiaolai basin. Based on these evidence, the author concluded that Jiaolai basin and the study area were in the similar tectonic environment in the Cretaceous and they both evolved from sedimentary basins to volcanic basins. Widely distributed volcaniclastic rocks and volcanic lava proved that there were multiple craters and multistage volcanism which played an important controlling role in the basin evolution.