Early Cretaceous Tectonic And Sedimentary Analysis Of The Southeast Coast Of China

The Dickson' principle and geochemical analysis of sandstone was carried on 34 medium-sized sandstone samples from the volcanoclastic rock and tuffaceous sandstone analyzed respectively from the bottom to the mid-upper Shipu Group,The samples of volcanoclastic rock in the lower section plot inthe zone of continental island arc with some in thezone of active continental margin;samples of tuffaceous sandstones in the middle-upper section plot in the zone of active continental margin,which indicates that volcanic arc was moveing gradually away from Shipu area during 135Ma-100Ma and the tectonic setting changed from continental island arc to active continental margin.such results coincide with the analysis of sand skeleton,and based on detailed study on the macroscopic and microscopic features of the Shipu Group in the Xiangshan,19 micro facies are recognized,including 14 clastic microfacies and 5 carbonate microfacies.According to the sedimentary interpretations of lithofacies,the sedimentary structures,and microfacies associations,we recognized three major facies for the Shipu Group(i.e.,fan delta,tidal flat,and the offshore-shallow sea).According to the unconformity surface and discontinuity surface of deposition trend,5 intact T-R sequences and 1 poor-preserved T-R sequence which only contains the regressive systems tract are recognized in the Shipu section,and the regional change of relative sea level is established according to the superposition mode of systems tract and types of facies.The results indicate that during the period from the Aption to the Albian,the second-order changes of the relative sea level in Shipu area is characterized by persistent transgression,but the third-order changes of relative sea level fluctuate frequently.Based on the isotopic age framework,the changes of relative sea level in the Shipu section coincided with the changes of eustatic sea level during 123-106Ma.But there was obvious variation between them during 106-100Ma where the eustatic sea level rise in 106-102Ma and then drop in 102-100Ma,however,the relative sea level rise in 106-104Ma,and drop in 104-102Ma,then rapidly rise in 102-100Ma.Considering the characteristic of the volcanic rocks,it is inferred that the changes of the relative sea level in the Shipu section were controlled by the eustatic sea level and regional tectonic activity.