Gravel Fabric Characteristics And Environment Significance Of Terrace Gravel Layers Of The Yellow River In North Mountain,Lanzhou Basin

Lanzhou Basin was a typical intracontinental rift sedimentary basin,where the Yellow River Terraces(T)evolved as many as nine steps.It was known as "Lanzhou pattern terrace".Terraces and their gravel layers were obvious and widely distributed in the basin.Previous studies of the Yellow River terraces mainly focused on age and origin,and were lack on the river gravel layers.Previously,the main research methods for the gravel layers were random sampling and grid sampling.However the measure discontinuities and the density differences of the methods resulted in the decrease of data representativeness.The continuous line-transect method was used to measure the diameter,lithology,shape and orientation of gravels were studied in the terraces(T3,T4,T5 and T8).Those were helpful for reconstructing the hydrodynamic conditions,material sources,transport processes and palaeocurrent directions of terrace gravel depositions,which can reveal the environmental significance.The main conclusions are as followings:1.The average diameters of the gravels of each terraces were mainly medium(10mm?100mm),and their distribution characteristics were unimodal,low kurtosis and forward peak value.The gravel diameter was the most intuitive indicator of hydrodynamic conditions.The maximum particle size was in T3.It indicated that the hydrodynamic fluctuation was larger.The maximum average diameter was in T4.It indicated that there was strongest hydrodynamic force and the maximum velocities of ancient flood were up to 3.14m/s.However,the average particle sizes and hydrodynamic of gravel layers of T5 and T8 were small comparatively.All the gravels were transported by flowing currents of river.Vertically,the gravels particle sizes decreased from the bottom to top in T3 and T4,which indicated a decreased hydrodynamics trend.Contrarily,the average gravels particle size and hydrodynamic increased in T5 and T8.2.The lithologies of all the terrace gravels were complex.They were made of mainly greyish-green siltstone,purple sandstone,granite,and quartz sandstone.Greyish-green siltstone occurred mostly in T3 and T5,purple sandstone did in T4,and granite did in T8.Granite increased obviously from low T4 to high T8.3.The weathering degrees of the gravels were moderate and the roundness was good,which were mainly subangular and subrounded.The vertical changes of T3 gravel layers were small,in flatness,sphericity and other parameters,which was indicative of comparative stable water currents.However,the vertical changes of T4,T5 and T8 were big and the water currents were unstable.The best roundness,the high shape maturity,and more oblate spheroid occurred in the gravels of T4.4.The gravels were aligned with an imbricated texture,and the ab-planes of gravels were inclined upstream.They were the good indicators of paleocurrent directions of the river water.Their directions were 280°,273°,269° and 259° from T3 to T8 respectively.These were basically consistent with the modern flow direction of the Yellow River,which is from west to east.5.The terrace gravel layers were products of the riverbed deposit of the ancient Yellow River.And there had been obvious fluctuation of dry and wet since 1410 ka B.P..T3 and T4 had gone through 3 times and 4 times of wet-dry periods respectively.The L3 of T3 and L3 of T4 indicated that super floods had taken place due to wet climate with strong precipitation and currents.While,the L2 and L6 of T5,and L2 of T8 indicated a weaken trends with a comparatively dry conditions.The gravels of T3 and T4 were deposited in wet climate against a dry trend;the gravels of T5 and T8 were formed in a dry climate against a dry trend.6.The formation of terraces and gravel layers were the results of tectonic uplift.Along with the continuous uplift of the Tibetan Plateau,the mountains in Lanzhou area were raised gradually,forcing the Yellow River to continue down-cutting and the riverbeds were moved southward.Eventually,the gravel layers of the T3,T4,T5 and T8 was formed along the river in Lanzhou basin.7.The comprehensive analysis showed that all of the gravels were not derived from the bedrocks of the locality of Lanzhou basin.They were not from the Tertiary red loose sandstone and the grey metamorphic rocks.The gravels came from the upper reaches of the Yellow River.The bedrocks from Minhe basin and Linxia basin provided the gravel sources.The gravels should come largely from Minhe Basin,without long distance transportation.