| Sedimentary basins and orogenic belts are the most important tectonic units on the continental crust.The study of the coupled deposition process of the earth surface system has become one of the leading hot topics in the field of earth system science.Sedimentary strata are the product of the regional tectonic-geographic-climate-water system evolutionary coupling process,which records rich surface process information.As the lower limit of the Quaternary in the Songnen Plain,the Baitushan Formation is of great research value for studying the coupling relationship between Great Xing’an Range and the Songliao Basin,restoring the surface process,and rebuilding the regional palaeogeographic environment.Therefore,we have carried out sedimentology,mineralogy and geochemistry research on the standard Baitushan Formation in Longjiang County.The results show that:（1）The lithology of the sediments of the Baitushan Formation is gray-white and yellow-brown sand and gravel accumulation,mixed with gray-white clay and fine sandy lenses.Gravel sorting,grinding roundness is poor,and has undergone strong weathering.Gravel lithology mainly includes rhyolite（30.65%）,gangue quartz（21.51%）and tuff（20.97%）,as well as a certain proportion of terrestrial clastic rock（15.59%）and granite（11.29%）.The oblique development of mud-silty lens is obvious.The sedimentological characteristics show that the Baitushan Formation is formed under the influence of flood and river.（2）Heavy minerals are mainly ilmenite（32.28%）,white titanite（29.05%）,chlorite（19.77%）and zircon（6.21%）,followed by ilmenite（3.58%）,hematite+limonite（2.02%）,magnetite（0.69%）,and Sphene（0.05%）,the content of other minerals is less.Unstable minerals（e.g.,pyroxene,amphibole,and sphene）were not detected,indicating the high degree of chemical weathering of the sediments of the Baitushan Formation.（3）Geochemical element composition,the sediment constant elements of the Baitushan Formation are mainly Si O2,Al2O3,K2O and Fe2O3,totalling 96.73%.Compared with UCC,Al2O3and Ti O2are slightly enriched,Fe2O3,Mg O,Ca O,Na2O,Mn O,P2O5has obvious losses.Compared with UCC,Cs,Ga,Zn,Y,Th,U,Rb and Ba are generally enriched.Sr,Sc,Ni,Cu,Nb and Ta are generally losses.V,Cr,Co,Zr,Hf and Pb fluctuate greatly;the total number of rare earth elements is higher than UCC and PAAS,and the distribution mode of rare earth elements is similar to that of UCC.Light rare earth elements are enriched,heavy rare earth elements are lost,and the samples show negative anomalies of Eu and Ce.（4）Higher CIA and CIW values and A-CN-K and A-CNK-FM graphs of sediments from the Baitushan Formation indicate that the sediments have undergone moderate chemical weathering.The ICV index and Th/Sc-Zr/Sc and CIA-WIP diagrams indicate that the sediments of Baitushan Formation are relatively mature,which is the product of the first cycle.（5）Gravel lithology,distribution pattern of rare earth elements,ratio of inactive elements La/Sc-Th/Co,La/Th-Hf,Co/Th-La/Sc,and discriminant function analysis of major elements show that the sediments of Baitushan Formation are mainly middle-acid magmatic parent rocks from the Great Xing’an Range.Si O2-（Al2O3+K2O+Na2O）and Ga/Rb-Sr/Cu indicate that the sediments were formed in a cold-semi-arid climate and underwent moderate chemical weathering after deposition.La-Th-Sc and Th-Sc-Zr/10tectonic background discrimination diagrams indicate that the tectonic background is a passive continental margin. |
