现代乌伦古湖滨岸沉积环境与沉积体系分布及其控制因素

通过现场实地踏勘、拍照、开挖探槽、利用卫星图解译等方法,对乌伦古湖环布伦托海区域和吉力湖北部乌伦古河现代三角洲地区的湖泊滨岸沉积环境和沉积体系进行了现代沉积调查。研究表明乌伦古湖滨岸沉积环境可以划分为基岩型湖岸、砾质湖岸、砂质湖岸、泥质湖岸等4种类型,发育山前基岩型湖岸、侵蚀基岩型湖岸、砾质冲积扇-扇三角洲、砾质辫状河三角洲、砾质滩坝、砂质滩坝、砂质三角洲、风成沙丘和泥质沼泽等9种滨岸沉积体系。山前基岩湖岸分布在布伦托海的北部,主要发育小型塌积扇、倒石锥和狭窄的湖滩。侵蚀型基岩湖岸位于布伦托海西岸和东北角地区,发育湖滩宽20~40 m。砾质冲积扇-扇三角洲沉积体系分布在布伦托海西北部25.8 km狭长区域,表现为一系列冲积扇-扇三角洲体系在山前形成裙边状展布的辫状平原,顺流向长5~15 km。砾质辫状河三角洲体系发育在布伦托海西部,砾质滩坝发育在砾质三角洲前缘,沉积物一般为中砾和粗砾,泥质含量低。现代乌伦古河三角洲位于吉力湖北部,沙丘广泛分布在布伦托海东部的三角洲平原。砂质滩坝发育在布伦托海东岸南部地区,滩坝带宽30~100 m,发育大量障碍痕、冰划痕。泥质沼泽占据湖岸总长度29.22 km,沼泽地带植物繁茂,水动力微弱,泥质和有机质含量高。根据卫星照片推测乌伦古湖水位可能发生过3次较大的下降,现代乌伦古河三角洲可能经过了4个发育阶段,但目前缺乏地质年代学证据。构造格局控制了湖泊边界的地形地貌特征,平行构造线走向容易形成规模较大的沉积体系,垂直构造走向形成的沉积体系规模较小。寒旱地区湖泊周缘入湖河流较少,具有季节性和暂时性特点,洪水泥石流、塌积扇等重力沉积体系比较发育。湖泊封冻是寒旱区湖泊区别于温暖地区湖泊的重要特征。在相同气候背景下,源汇地区的高差和河流的流程、流量决定了沉积物的供给总量和沉积体系的特征。湖盆边界形态影响沿岸流的发育,也影响湖泊风动力方向和强度。乌伦古湖滨岸沉积体系的多样性对研究古代湖泊滨岸沉积体系具有重要的启发,开展湖泊滨岸沉积环境和沉积体系调查对完善陆相湖盆沉积体系模式,对发现新的储层类型,对重建湖泊古地理环境具有重要的意义。

MODERN SEDIMENTARY ENVIRONMENT AND DEPOSITIONAL SYSTEM ON THE COAST OF ULUNGUR LAKE:THEIR CLASSIFICATION,DISTRIBUTION AND CHARACTERISTICS

Modern present deposits investigation has been done on the coast of Ulungur Lake in the area of around Buluntuohai and the northern Jili Sublake by field sedimentological observation(including photographing and trenching)and satellite sensing interpretation.Four types of sedimentary environments including(1)bedrock shoreline,(2)pebbly shoreline,(3)sandy shoreline and(4)muddy shoreline have been found on the coast of Ulungur Lake with nine kinds of depositional systems as(1)piedmont bedrock shoreline system,(2)eroded bedrock shoreline and beach system,(3)gravelly alluvial fan-fan deltaic system,(4)gravelly braided river deltaic system,(5)gravelly beach bar system,(6)sandy beach bar system,(7)sandy delta system,(8)aeolian dune system and(9)muddy swamp system developed in these environments.Piedmont bedrock shoreline system distributed at northern Buluntuohai with mainly of small colluvial fans and talus.The eroded bedrock shoreline and beach system located in the west bank and northeast corner of Buluntuohai with a beach of 20~40 meters wide.The gravelly alluvial fan-fan deltaic system formed a braided plain ranging about 25.8 km along the northwest bank of Buluntuohai with the fan radius in 5~15 km.The gravelly braided river deltaic system situated at the west of Buluntuohai with gravelly beach bar system developed in front of the deltas,sediments of these systems are pebble and cobble with fewer mud contents.The present Ulungur River delta system developed at the north of Jili Sublake.The aeolian dunes system spreaded widely on the former delta plain areas at the east bank of Buluntuohai.Sandy beach bar system distributed at the south of the east bank of Buluntuohai with a zone as wide as about 30~100 m.Muddy swamp system occupies 29.22 km shoreline of the lake with heavy vegetation and weak current,muddy and organic plenty sediments deposited in these areas.Three stages of lake level fall and the four stages of delta migration may be estimated by remote sensing interpretation,but there no geochronological evidence can be used in present.Basin structure controls the topographical and geomorphic features of the lake.Larger sedimentary systems are likely to be parallel to the structural strikes while the shorter system may developed to the dip direction.In dry and cold areas,rivers are usually seasonal and temporary river,fewer channels can flow into the lake,thus the flooding induced debris flow and colluvial fan depositional systems are very developed.Freezing is an important feature of lakes in cold areas.Elevation difference between source and sink areas,as well as the river discharge determined the sediment supply and the characteristics of a depositional system.The lake boundary configuration controls the longshore current and the wind field of the lake.The diversity of the shoreline depositional system of Ulungur Lake is of great significance to the study of the ancient lake shoreline depositional system.The modern present investigation for the coastal sedimentary environment and shoreline depositional system of Ulungur Lake is not only great helpful for building the sedimentary depositional models of the terrigenous lake basin,paleogeographic environment reconstruction and oil and gas reservoir prediction in the terrigenous depositional system.