Sedimentary Characteristics And Controlling Factors Of Daihai Basin,Inner Mongolia

Daihai Basin,located in Inner Mongolia,China,is a typical inland depressed basin,developed with a facies association including river,lacustrine delta and lake.The conclusion in Daihai Basin will provide a good reference for analyzing the facies association in analogue basins.Three kinds of facies: river,lacustrine delta and lake are developed in Daihai Basin.Those facies are mainly controlled by tectonics and climate.In order for application on old sediments,this paper discuss the depositional characteristics of slopes,provenances and valley shapes.According to slopes and provenances,Daihai basin can be divided into four parts: nearby provenance –steep slope(NS),nearby provenance –gentle slope(NG),far provenance –steep slope(FS)and far provenance –gentle slope(FG).In each part,rivers and deltas have shown different characteristics.According to shapes,valleys can be divided into four types: V-shaped type,deep U-shaped type,shallow U-shaped type and Wshaped type.Each type has different microfacies association.Braided rivers,in Daihai basin,mainly consist of upstream and midstream.River,developed nearby the mountain,is called piedmont river,which has strong hydraulic power and mainly featured by V-shaped and deep U-shaped valleys.Channel subfacies and overbank subfacies can be identified in piedmont rivers.Channel subfacies include three microfacies: channel,channel bar and point bar.These microfacies show different features in each kind of valley.Four types of valleys appeared successively with the decreasing of elevation and slope.In a plane,piedmont rivers appeared as 4 distribution patterns: sheetflood pattern,bifurcated pattern,single channel pattern and merging pattern.Quantity of flow and slope are two main factors for these samples.Midstream has relatively lower hydrodynamic power,and mainly featured by shallow U-shaped and W-shaped valleys.Subfacies and microfacies in midstream are same to piedmont rivers,with different details however.In midstream,main channels show more apparent meandering shape in dry periods;point bars have smaller length-width ratio;channel bars show various shapes and can be divided into sedimentary channel bars and washout channel bars according to genesis.The amount,scales and valley types of rivers around the Daihai Lake changed regularly,in NS part,rivers are numerous with small scales,and mainly are V-shaped and deep U-shaped valleys;in NG part,rivers are numerous with large scales,and mainly are shallow U-shaped and W-shaped valleys;in FS part,rivers are rare with large scales,and mainly are V-shaped valleys;in FG part,rivers are rare with small scales,and mainly are shallow Ushaped and W-shaped valleys.There are 14 braided river deltas developed in Daihai Lake.Features of these deltas are related to slope and flow path.According to these features,deltas can be divided into four types,NS deltas,which are numerous with small scale,featured by small delta plain and small delta front;NG deltas,which are numerous with large scale,featured by large delta plain and small delta front;FS deltas,which are rare with large scale,featured by large delta plain and large delta front;FG deltas,which are rare with small scale,featured by large delta plain and small delta front.Deltas with the same type may have different sediments.According to distributary channels and deposit process,three types of distributary channels can be identified,sheetflood type,incision type and combination type.With the classification,sedimentary models of sheetflood distributary channel delta and incision distributary channel delta are established.Shape of delta is mainly decided by deposit efficiency and lacustrine-level fluctuation.Daihai Lake contains three subfacies,expansion lake,shore-shallow lake and semi-deep & deep lake.Beach bar and gully are developed in expansion lake.Shore-shallow lake and semi-deep & deep lake can be identified by particle size of surface sediment in lakebed.Sources and slopes of estuaries are main controlling factors for the distribution of surface sediments in Daihai Lake.According to depth-distance map,three types of filling patterns can be observed,which naturally separates a lake into three parts.For each part,the ratio of area of delta front and prodelta is similar.Generally,types of facies are regularly appeared in plane,which controlled the distribution of sediments.Specifically,grain size in river is controlled by location,slope variation and valley shape;grain size in delta is controlled by delta types;while grain size in lake is controlled by subfacies.Evolution of valley types is effected by hydrodynamic,terrace and autocyclicity.