Sedimentary Mechanism Of The Gravity Flow In The Chang 7 Member Of The Ordos Basin And Its Response To The Climate

During the sedimentary period of the Chang 7 Member of the Yanchang Formation in the Ordos Basin,the lake basin was in the expansion stage,and a large number of deep-water gravity flow deposits developed.According to the triggering mechanism,the gravity flow can be divided into two main types,namely,the gravity flow formed by the direct supply of river floods outside the basin,and the gravity flow formed by the collapse and re-transported of sediments at the delta front inside the basin.At present,the relevant theories about slump-related gravity flow have been widely used in the Ordos Basin,but the important contribution of flood-related gravity flow to deep-water sedimentation has not been paid much attention.In addition,the differences in the depositional scales,sedimentary processes and depositional mechanisms of these two types of gravity flow,and whether and how they respond to climate,have not yet been clarified,all of which have become hot issues in the field of sedimentology and have important scientific significance.In this thesis,firstly,comprehensively using field outcrop,core and logging data,seven basic lithofacies and five lithofacies combinations of gravity flow deposits in the study area are identified.Five types of architecture elemnts including erosive channel,transitional channel,sedimentary channel,levee and lobe were determined,and their quantitative-semi-quantitative identification standard were established,which are considered to reflect the transformation of erosion-type to sedimentary-type bed in the"channel-lobe"sedimentary system.Through investigation,collation and comparison of a large number of documents,it is believed that the Triassic Ordos lacustrine turbidite system represents the largest modern and ancient lacustrine flood turbidite system in the world.Then,the sedimentological research on the slump-related and flood-related gravity flows in the Chang 7 Member of the Ordos Basin was further carried out,and the changes in the source,fluid kinetic energy,flow velocity and kinematic viscosity of the gravity flows with the two trigger mechanisms were clarified,and the different causes of gravity flow were established.Grain-size identification chart of turbidite.Flood-related and slump-related turbidites vary greatly in the particle size peak-sorting scatter plot,and the relative change rate between D₉₉ and D₅₀（that is,the slope value of the one-dimensional linear regression of the subsample）,so the deposition The grain size sorting-slope discriminant diagram can be used to identify the triggering mechanism of turbidity currents.Finally,combined with dating and geochemical data,the turbidites in the Ordos Basin were correlated with the Carnian flood event（CPE）.It is believed that the three humidification events inside the CPE promoted the generation of river floods,brought a large amount of terrigenous clastic materials into the deep lake environment,and formed a large area of flood-type turbidites.Based on the above,a geological model was established to describe the deposition law of turbidites during the beginning and peak periods of CPE.The turbidites of the Chang 7 Member in the Ordos Basin showed a trend of increasing in size throughout the depositional period,but basically lagged behind the CPE wet event,indicating that the arrival of the wet climate on land may be slower than that on the ocean.This study highlights the potential for lacustrine turbidite deposition under the influence of extreme wet events and enriches the CPE theory of continental strata.More detailed studies will be carried out in the future to quantify the scale of response of turbidity current deposition to CPE in terrestrial environments.It is hoped that the research in this paper can enrich the theory of lacustrine gravity flow deposition,and provide reference for the development of paleoclimatology,deep-water oil and gas exploration and development.