Study On The Events Of Turbidity And Earthquake In Depression Lacustrine Basin

Having surveyed the field sections, cores of wells, observed manymicrostructures and analysed various relevant documents, with the guide of the theoryof sedimentology, petroleum geology and seismogeology, the paper reacher the eventsof turbidity and earthquake in the Chang6-Chang8 of Yanchang Formation, UpperTriassic, Ordos Basin.Types of petrology, characteristics of granularity, sedimentary structures ofturbidite of Chang6-Chang8 of the Yanchang Formation are researched. The turbiditeof Chang6-Chang8 mainly composed of lithic feldspathic sandstone, feldspathiclithic sandstone and feldspathic sandstone. Which have low compositional maturity,textural maturity and high matrix. The results of grain-size analysis show that thegrain-size parameters, distribution, compositions are evidently turbidite sediment.Sedimentary features observed in the cores include unidirectional graded sequence,burthen structures, fluidization structures, breccia, rhythm and cyclic sequence andBouma sequence. There are three type of turbidite which are lamina turbidite, mediumthickness turbidite and thick turbidite, they are mostly slumped turbidite from deltafront.Base on those characteristics, sedimentary faces are identified, The characteristicof sedimentary sub-facies are described detailed. It is confirmed that the distributionof turbidite is located in Qingyang, Heshui, Gucheng, Anzhinongchang, Ningxian,Tongchuan region. Outside trigger mechanism, enough sources and slope landform arethe most important conditions to form turbidite.Seismites were identified by soft-sediment deformational structures. Manyearthquake-related Seismites were discovered in the Chang6-Chang8 of theYanchang Formation. Having surveyed the cores of wells, many typical deformationstructures were discovered, Such as syn-sedimentary micro factures, stone veins, vibrational liquefaction, deformation structure, earthquake breccia and load-casts andballs-and pillows types, cyclic bedding etc. They were formed in const rained horizonsbounded by undeformed strata, suggesting that they are syndepositional. Thecomplete vertical sequence of seismite of Ta 17 well, which include shattered rock bed,ladder-shaped faulted bed, vibrational liquefaction deformation structure bed,liquefied sandstone vein bed, rubble bed and homogenized bed. The association isdiscussed between the character of seismites with earthquake intensity.Deformational structure of earthquake genesis is different from slump genesis.The discovery of seismit can be used to interpret the tectonic evolvement of thebasin. In Late Triassic, with the Closeing of Qinling and the sea slot of SouthernQilian, The south-north thrust band occurrence Strong activity. It was the directinducing factors of the seimo-deposition in Yanchang Formation.and provided theevidence for the turbidite of Chang6-Chang8 of the Yanchang Formation,southwestern Ordos Basin, the outsider trigger mechanism of the turbidite isearthquake activity.