Sedimentary Record And Paleoenvironmental Response Of Cretaceous Lumuwan Formation In Baisha Basin

The cretaceous period is a typical representative of extreme climate.The study of cretaceous period can help us to predict and analyze the future greenhouse climate.At present,stratigraphic timescale of the cretaceous period is mainly built on the basis of marine stratum.While in China,continental stratum was developed well.Cretaceous stratum is well deposited in Baisha Basin(Hainan island)and this stratum is also well exposed.For the lack of study on the cretaceous stratigraphy,we choose Baisha Basin to be the target area.And by analyzing the sedimentary facies and ancient climate,we could perfect the strata system in cretaceous period.In this study,the Daohancun Section(250m in length)was selected for field measurement.Based on the identification of lithology,lithofacies and lithofacies associations,the sedimentary characteristics were interpreted and analyzed in detail.The Lumuwan formation in the target area could be divided into 10 types of lithofacies,they are respectively Massive gravel facies(Gm),Trough cross-bedding gravel facies(Gt),Massive sandstone facies(Sm),Horizontal-laminated sandstone facies(Sh),Diamicton sandstone facies(Sd),Sandstone facies(Ss),Flaser-bedding sandstone facies(Sf),Lenticular-bedding sandstone facies(Sl),Diamicton fines facies(Fd)and Massive fines(Fm).According to the integrated lithofacies analysis above,the sedimentary period of Lumuwan formation is mainly deposited braided river delta facies and lake facies.Further,it could be subdivided into five subfacies(semi lacustrine deposits,shallow lake facies,shore lake facie,delta front and delta plain in braided river)and six microfacies(braided channels,mouth bars,distal bars,mudflats,mixed beaches and gravelly beaches).The braided river deltas and shallow lake facies are appear alternately may be related to the changes in sedimentary supply,sedimentary environment and the paleoclimate.Two detrital zircon samples from the Daohancun section were tested,and the dating data was 106.5±0.68 Ma.The age of the section belongs to the Albian sedimentary period,and the sedimentary supply is mainly from the Indosinian-Hercynian orogenic belts.The handheld XRF tester was used to test the main trace elements,and K,Fe,Mn,Ti and other elements as well as their ratio changes were used to indicate the paleoclimate.The Daohancun section was divided into five stages,And three anti-climate cycles(warm-humid,arid and reduction-oxidation)were identified.Compared with the Songliao Basin and Jibei region in the same period,we found coal beds were poorly deposited in the study area because of the different climate condition.And the red clay strata were extensively developed may be related to the dry climate.The integrated analysis of sedimentary characteristics and paleoclimatic response in Lumuwan formation will be beneficial to the future study of cretaceous continental strata.