Effects Of Lake Level Fluctuation On The Different Orders Archetecture Elements Of River Dominated Delta

Because the lake area is relatively small,affected by season,climate and other factors,the lake water level fluctuation is significant compared to the stability of sea level.As a result,water level fluctuation plays an important role in the formation of the lake river dominated delta.Controlled by different factors,water fluctuates with different levels.Different orders water level fluctuations control different orders architecture elements from complete delta which extends tens of meters to several centimeters set,coset.Different research methods have different research accuracy.Modern sediment study is the most accurate,it is possible to observe the internal deposition units in the delta micro facies.The accuracy of outcrop observation is the second,but it is easy to sample for the research of single sand and hyperplasia of single sand.Although the anatomy of the underground work area has the most direct core data,but the quantity is limited.It is usually researched by using the seismic and logging data and more suitable for the composite sand and single sand.As a result,it is difficult and not scientific to discuss the effects of water level fluctuation on different orders architecture elements by using only one method.Based on the unified architecture grading scheme,using modern sediment study,outcrop observation,underground work area anatomy for main methods,taking the modern river dominated delta,river dominated delta outcrop and underground ancient river dominated delta for research objects,this article targeted at the internal deposition units in single sand,single sand and hyperplasia of single sand,composite sand and single sand,made system work on the effects of water level fluctuation on different orders river dominated delta architecture elements.Effects of water level fluctuation on the internal deposition units in single sand reflect on that it can be recognized new deposition units in traditional delta distributary channel and mouth bar.In the straight section of delta front under water distributary channel,transgressive retrogradation sand ridge and shallow beach formed during water level rising period,strip channel sand formed during low water period can be recognized.The convex bank of delta plain distributary channel mainly develop point bar and riverbed bottom deposits.Rising wedge-shaped sand formed during water level rising period,water level drop side-blown sand formed during water level dropping period and fallen silt layer formed during high static water period can be recognized in point bar further.The three form complete third order architecture element together of point bar.It will experience a series of depositions after the formation of the river mouth bar and develop levee,splay,crevasse channel,sheetflood deposits,sheet sand,mud deposits,et al.The development of the river mouth bar compomer can be divided into four stages: triangle river mouth bar compomer,V river mouth bar compomer,irregular V river mouth bar compomer,diara-like river mouth bar compomer.The ratio of the length of river mouth bar compomer and the width of river channel before bifurcation can be used as important parameter to divided the development stages of river mouth bar compomer.During the high water level conditions,the development of the river mouth bar compomer performance to the deposits of far sand bar,prodelta,lake mud and the development of splay,swamp,interdistributary bay as the result of the river overflew the levee.During the low water level conditions,the development of the river mouth bar compomer performance to the development of river mouth bar and levee.Water level fluctuation have effects on the lithofacies combination in single sand,the compositional maturity and structure maturity of river channel single sand and hyperplasia of single sand,et al.According to characteristics of lithofacies combination,Fangniugou section was divided into 5 layers.Through the count of characteristic elements ratio and water depth functions,water depth of every layer when deposition from deep to pale is followed by layer 1,layer 2,layer 5,layer 3,layer 4.In the Fangniugou section,facies percentage of thickness represented river dominated strata increases with the drop of water level;Facies percentage of thickness represented wave dominated and still strata decreases with the drop of water level.Taking the delta front underwater river channel for example,clay content and feldspar/quartz of single sand and hyperplasia of single sand,which represent the compositional maturity,increase with the rise of water level;The selection coefficient in the single sand positive turn,which represent the structure maturity,decrease with the rise of water level,while in hyperplasia of single sand positive turn,it increase with the rise of water level.Water level fluctuation have effects on composite sand body and single sand body of river dominated delta.It reflects in the aspects of development type,distribution patterns,distribution extent,stacked style,development scale,et al.For the composite sand body,with the lake level rise from low to high,the distributary channels branch more and more frequently from dendritic to netlike,the crevasse splay,mouth bar and sheet sand develop increasingly,the form of the delta change from small plane large front to large plane small front.The relationship between the change amount of water level and delta distribution distances is controlled by basin slope angle and delta front slope angle.For the single sand body,with the lake level rise from low to high,in the slope belt close to the sources,its stacking patterns change from cutting in vertical,isolated in plain to cohering and isolated in vertical,cohering in plain.In the sag far from the sources,its stacking patterns change from cohering to isolated in vertical and plain.During the same water level condition,more far away from the sources is,the ratio of width and thickness is bigger.With the same distance from the sources,the higher the water level is,the ratio of width and thickness is bigger.