Provenance Analysis Of Yan 10 In Yan'an Formation Of Jurassic In The Southeast Of Ordos Basin

Provenance analysis is an important content of basin and orogenic belt research,which is of great significance in determining the provenance location,source rocks properties,sediment transport path,sedimentation and tectonic evolution of the whole basin.Previous research on the development characteristics,sedimentary facies,sedimentary environment and reservoir characteristics of the Yan'an formation of the Jurassic system in Ordos Basin have made a lot of achievements,but the research on the source of sediment is relatively weak,especially on the southeast of North Shaan Xi Slope.Based on the observation of typical outcrops and cores in the field,through the measurement of a large number of paleocurrent data in the field,the statistics and identification of the main light minerals and debris contents and types under the thin sections of rocks,the statistics of heavy mineral assemblages,the summary and analysis of the characteristics of zircon color,CL image,the morphology and internal structure of zircon,the paleocurrent direction,the composition characteristics of light minerals and lithic debris,the assemblage characteristics of heavy minerals and the types of zircons of yan10 in the southeast of the basin are systematically studied and transportation route and source direction of the study area are discussed,which provides the basis for basin-mountain coupling in the early Jurassic and paleogeographic analysis of Yan'an formation.The following main understandings have been achieved:(1)According to the occurrence measurement of cross bedding and flat surface of gravel,the rosette map is drawn.It is considered that the paleocurrent direction of Yanan Formation in the southeast of Ordos Basin is mainly the East,North and South.(2)According to the quantitative statistics of light minerals and cuttings,the source area was divided into northern source area,southern source area and western source area.(3)According to the characteristics of heavy mineral assemblage,the provenance of the study area is divided into Western provenance,Northern provenance and Southern provenance.The combination features of Western provenance: zircon + leucosphenite + low garnet;northern provenance: zircon + garnet + tourmaline;southern provenance: zircon + garnet +tourmaline + rutile.The contour map of ZTR index of heavy minerals also reflects the similar rule.(4)According to the analysis of the CL image,reflected light,color and shape under transmission,microscope and internal structure of 400 detrital zircons from three representative samples,the Northern provenance is dominated by magmatic zircons,and the Western and Southern are dominated by metamorphic zircons.(5)Based on the analysis results of paleocurrent,light mineral,heavy mineral assemblage,ZTR index and zircon type and combined with the geological characteristics of the surrounding areas,the comprehensive indicator map of yan10 provenance is made.It is considered that yan10 provenance in the study area mainly comes from the Alashan block in the northwest,the Yinshan orogenic belt in the north and the Qinling orogenic belt in the south.In the northwest,the source rocks are mainly phyllite,granulite and other metamorphic rocks;in the northern Yinshan orogenic belt are mainly intermediate basic volcanic rocks and metamorphic rocks,and in the southern Qinling orogenic belt are mainly metamorphic rocks and granites.(6)Based on the analysis results of paleocurrent,light mineral,heavy mineral assemblage,ZTR index and zircon type,combined with the early paleogeographic characteristics of Yanan Formation,the provenance and transport path of yan10 are discussed comprehensively.It is speculated that there is a branch river flowing from Fuxian,Ganquan to Yan'an in the south.In this paper,it is named "Fu-gan Ancient River".