| The Qaidam Basin is rich in coal and hydrocarbon resources,and has long been an important energy base in Qinghai Province and even in Northwest China.The Jurassic in the northern Qaidam Basin(NBQ)is distributed continuously with relatively good outcrops,which is a favorable carrier for the research of paleoenvironment,geological events,sequence stratigraphy and paleogeography.In recent years,it has gradually become a hot area for study of Jurassic continental stratigraphic sedimentology.In this paper,the lithofacies,sedimentary system,sequence stratigraphy,paleogeography and source-to-sink system budget of Early to Middle Jurassic in the northern Qaidam Basin have been systematically studied by outcrops and borehole cores description,detrital components and macerals identification as well as database analysis.Five rock types including conglomerates,sandstones,siltstones,mudstones and organic rocks were identified in the Lower to Middle Jurassic,northern Qaidam Basin,which can be divided into 22 lithofacies types.Seven lithofacies associations were identified,which can be interpreted as alluvial fan,fan delta,braided fluvial,meandering fluvial,braided fluvial delta,meandering fluvial delta and lacustrine deposits,respectively.The main coal-forming environments include flooded swamp,interdistributary swamp,interdistributary-bay swamp and lacustrine-bay swamp.Based on borehole cores,outcrops,well logging and seismic section data,five types of sequence boundary of Lower to Middle Jurassic in the northern Qaidam Basin were identified,including regional unconformity,incised valley,abrupt changes of lithology and color,inversion of sedimentary cycle and paleo exposed surface.The Lower to Middle Jurassic in the northern Qaidam Basin can be subdivided into ten third-order sequences from S1 to S10.Based on typical single outcrop or borehole section and correlation cross section,the sedimentary evolution and distribution characteristics of coal seams in sequence stratigraphic framework were analyzed.In the Lower Jurassic,coal accumulation from sequence S4 to S6 gradually becomes favorable.In the Middle Jurassic,coal accumulation in sequence S8 is the best,while coal accumulation in upper sequences S9 and S10 gradually becomes unfavorable.Generally speaking,the Middle Jurassic is main coal accumulation period in the northern Qaidam Basin,and coal accumulation in this period is obviously better than that in the Early Jurassic.According to the distribution of coal seams in sequence stratigraphic framework of Lower to Middle Jurassic in the northern Qaidam Basin,coal seams were mainly developed in lacustrine transgressive systems tract,and there are few coal seams in highstand systems tract and lowstand systems tract.Based on the alternated development of coal seams and detrital deposits,the content variation of macerals and minerals in vertical direction,five key surfaces were identified,including terrestrialization surface,erosional subaerial exposure surface,paludification surface,give-up transgressive surface and accommodation reversal surface.A multi-peat superposition genetic model defined by key surfaces is established,and the genetic mechanism of the anomalously thick coal seams is revealed.The high-resolution sequences contained in the Middle Jurassic thick coal seams were discovered and divided into four fourth-order sequences.The key surfaces that can be used as the fourth-order sequence boundary and the diachronism surfaces in anomalously coal seams were discussed.Combined with paleocurrent direction,sandstone detrital component,heavy mineral analysis and detrital zircon dating results,it shows that the Early to Middle Jurassic provenance of the northern Qaidam Basin were mainly distributed in north and south sides of the basin.Qilian mountains are main source area of the northern part in sedimentary period,while Altun mountains and Saishiteng mountains have not yet been uplifted at this time and tend to accept deposits.The eastern Kunlun mountains and Qaidam archicontinent in the south also provide some provenance for the basin during sedimentary period.At the same time,small-scale paleouplifts in the basin provide limited provenance for the depositional zone.Based on outcrop and borehole section database,single factor contour maps were obtained by big data compilation method.Combined with results of sedimentary environment analysis and provenance analysis,the paleogeography of Early to Middle Jurassic in the northern Qaidam Basin were reconstructed.The Lower Jurassic were mainly distributed in the southern region of Lenghu-Nanbaxian.A series of braided fluvial and braided fluvial delta developed in the north and south sides,and gradually flowed into the central lacustrine environment.The Middle Jurassic migrated to the northeast of the study area,meandering fluvial and meandering fluvial delta began to developed.Palaeogeographic units of this period include fluvial,upper delta plain,lower delta plain,delta front,shore and shallow lake and semi deep lake.The favorable coal accumulating palaeogeographic units are interdistributary swamp in the delta plain.Coal accumulating centers are concentrated in Tuanyushan,Yuqia and Dameigou areas,and the coal accumulating center has the characteristics of northeast migration from Early to Middle Jurassic.Combined with the paleohydrologic equation,sediment volume transported by trunk channel was obtained and compared with the sediment volume in the sedimentary area.Source-to-sink budgets were analyzed comprehensively,and key parameters of source-to-sink system were reconstructed.The quantitative analysis of source-to-sink system budgets indicate that the trunk river of Middle Jurassic in Yuqia area was 3.1-3.3m deep and 69-77m wide,carried mainly fine-to medium-grained sand and flowed over a low-gradient paleoslope of 0.0002046-0.0002178.The drainage area is about 3209.8-3781.6km~2,and drainage length was 177.8-196.2km.The bankfull discharge of the trunk river is estimated to be 239.9-286.2m~3/s;the bedload sediment discharge,estimated by the Chezy Coefficient method,ranges from 0.043m~3/s to 0.048m~3/s;and the suspended-load sediment discharge estimated by the Van Rijn equations ranges from 0.083m~3/s to0.094m~3/s.Annual total sediment load is estimated to be in the range of 158862.4-179242.3m~3,including both bedload and suspended load.Within 2.2Ma in the sequence S8,the river is estimated to have transported 349.5-394.3km~3of sediment into the basin,which is consistent with the 322km~3of sediment documented in the sink area.The high value of the input volume of sediment obtained from the paleohydrologic equation is about 1.22 times of the statistical volume of the downstream sedimentary area of the fulcrum.If this analysis result is accurate,it indicates that there is a certain degree of sediment escaping,and the gravity flow deposits in the study area may be the main way of sediment escaping.The selection of modern rivers for analogy,the use of paleohydraulic equations more in line with the actual geological background,and the direct measurement of basic parameters of source-to-sink system,will significantly reduce uncertainty and error in the process of source-to-sink system budget analysis. |
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