Depositional Architecture And Formative Mechanism Of Mouth Bar In Lacustrine Fan Delta Front

Fan delta,as an important sedimentary type at the margin of basin,is an important place for the enrichment of petroleum,natural gas and other geological resources.Although many studies about sedimentary characteristics of fan delta front in lacustrine basin have been done in the past decades,it is poorly understood that the depositional architecture,development degree and formation mechanism of mouth bar.Therefore,taking the Sangyuan outcrop within Xiguayuan Formation of Lower Cretaceous in Luanping Basin as an example,this thesis aims to analyze the sedimentary characteristics of lacustrine fan delta front and to discuss the formation mechanism of development degree and vertical grain-size trend of mouth bar by the integration of field survey,unmanned aerial vehicle(UAV)observation and flume experiment.The main results and conclusions are as follows.The genetic units of fan delta front include distributary channels,tongue-shaped debris deposits,mouth bars and sheet sand.The mouth bar with flat bottom and convex top mainly consist of sandy and gravelly accretions.The sandy accretion shows an overall coarsening upward trend,and gravelly accretion shows the fining upward trend.The vertical grain-size trend of mouth bar is controlled by superposition patterns of the accretion bodies,which shows either coarsening upward trend or fining upward trend.The mouth bar is widely developed in study area,which accounts for 36.88% of glutenite bodies in Sangyuan outcrop.The distributary channels,tongue-shaped debris deposits and sheet sand account for 53.42%,8.39% and 1.31%,respectively.The development degree of mouth is negatively related to the slope of the substrate layer,discharge,sediment/water ratio,and grain size of sediment,but positively related to water depth.The vertical grain-size trend of accretion within mouth bar is controlled by the grain size of sediment,sediment/water ratio,discharge,slope of the substrate layer and water depth.Experiment with coarser sediment,higher sediment/water ratio,greater discharge and steeper slope of the substrate layer induce a fining-upward trend of accretion when the water depth is shallower,and with the increase of water depth,the accretion tend to show the coarsening upward trend.Otherwise,the mouth bar accretion exhibited a coarsening-upward trend controlled by the finer sediment,lower sediment/water ratio,smaller discharge and gentler slope of the substrate layer.The vertical grain-size trend of mouth bar is related to the superposition pattern of accretion within mouth bar,and the latter is controlled by the variations of supply and water depth.The normal grading of mouth bar is easily formed with the decreasing or constant supply conditions,or with minor increasing supply conditions when the mouth bar accretions show the fining-upward trend.And the inverse grading of mouth bar formed with major increasing supply conditions.Whereas,the inverse grading of mouth bar is easily formed with the increasing or constant supply conditions,or with minor decreasing supply conditions when the mouth bar accretion show the coarsening-upward trend.And the normal grading of mouth bar formed with major decreasing supply conditions.With the increase of water depth(lake lavel rises),the grain size of the accretion is finer vertically,the mouth bars show the fining-upward trend,whereas the mouth bars showing coarsening-upward trend are developed with the decrease of water depth(lake level falls)because the grain size of the accretion is coarser vertically.