Transgression-regression event element geochemistry records of southwestern Fujian in Late Triassic-Middle Jurassic

Abstract

Southwest Fujian area has experienced a large-scale transgression-regression cycle in Late Triassic-Middle Jurassic and the maximum transgression has taken place in Early Jurassic. The migration and enrichment of geochemical element in the continuous fine-grained sediments in the basin recorded the paleosalinity and the paleodepth. The changes of paleosalinity and paleodepth indicate the sea (lake) level relative change in every period of Late Triassic-Middle Jurassic in southwestern Fujian. The relative change curve of sea (lake) level in southwestern Fujian is established based on the m value (m=100×w(MgO)/w(Al2O3)) and the ratios of w(B)/w(Ga), w(Sr)/w(Ba) and w(Ca)/w(Mg). The curve indicates that level I sea-level relative change in southwestern Fujian is composed of the transgression in Late Triassic-Early Jurassic and the regression in the late period of Early Jurassic-Middle Jurassic. The level III sea-level relative change is frequent, which is composed by the lake level descent-lake level rise-lake level descent of Wenbin Shan formation in Late Triassic, the regression-transgression-regression of Lishan formation in Early Jurassic and the lake-level rise-lake level descent-lake level rise-lake level descent of Zhangping formation in Middle Jurassic. The transgression-regression cycle in southwestern Fujian is significantly controlled by the sea-level change in the north of South China Sea. The relative change curve trends of the level I sea-level in the north of South China Sea and the one in southwestern Fujian are the same. The maximum transgressions both occur in Early Jurassic. The level III sea-level curve reflects the fluctuation of a transgression and two regressions in the early period of Early Jurassic.