Sedimentary Characteristics And Evolution Rule Of Modern Shore Beach Bar By The Effect Of Wind And Waves In Poyang Lake,South China

Poyang Lake formed along with the changing geological environment in Quaternary as a continental faulted basin.There are plenty of typical modern coastal deposits at the Songmenshan Island shore in the center of the Poyang Lake,while the sedimentary outcrops of the unconsolidated sandy strata in the island indicate the process of sedimentary evolution in the Holocene.Modern coastal deposits and Holocene outcrops are researched comprehensively in this article by geological surveying,observing of scanning electron microscope and experimenting of particle size analysis.The controlling factors of modern coastal sedimentary system and transformation relationships of different microfacies are analyzed.The sedimentary evolution process of Holocene unconsolidated sandy strata is confirmed.It is found that modern coastal deposits include two kinds of microfacies that the lower wave-built beach bar divided into 5 types which are forward cuspidal beach cusps(TYPE 1),oblique short beach cusps(TYPE 2),oblique long beach cusps(TYPE 3),rhythm topography on inner bar(TYPE 4)and crescentic bar(TYPE 5)according to the different shapes of sand body and upper aeolian dune.Wave-built beach bar and aeolian dune show 3 different sedimentary characteristics in the bedding,impact trace in quartz grain surface and particle granularity.Characteristics of the beach bar are low-angle bedding,fanshaped,conchoidal and disciform fracture,small crooked V-shaped impact crater on the face of quartz grain,more roll and lump components,curves show the type of two jumps.Characteristics of the aeolian dune are large scale high-angle bedding,dished,crescent and V-shaped with the macroaxis in the same direction mpact crater on the face of quartz grain,less roll and suspension components.It is the wind and the waves formed by the wind that are the main controlling factors of the modern coastal deposits according to the evidence of environment,climate and wind in Poyang Lake.In the case of lake level constant,there is one wave model for each of 5 types to keep its shape unchanged,the sand body is showed overall destruction or construction,some parts are corroded and others parts are deposited and die out or become another shapes when becomes another kind of wave model;in the case of wave constant,5 types are showed whole smaller or bigger when the lake level raise or decline.There are 35 kinds of transformation relationship among the 5 kinds of beach bar by the waves of different direction and scale or lake level.According to the variation of particle size,two complete sedimentary cyclicities are recorded in the outcrop of unconsolidated sandy strata in the upper Holocene and are the result of large scale fluctuation of lakelevel caused by recent two climate cyclicities in the Holocene.The strata is uplifted after the deposition and forming the present landscape.So,the unconsolidated sandy strata of upper Holocene is formed by the effect of early climate change and late tectonic uplift.It is representative for the sedimentary model and evolution of the continental downfaulted lake basin and provides some references that the modern coastal sedimentary model which are beach bar influenced by waves and aeolian dune influenced by wind in Poyang Lake,transformation relationship among the 5 kinds of beach bar and the sedimentary evolution of unconsolidated sandy strata in the upper Holocene influenced by climate change and tectonic uplift.