内蒙古岱海现代辫状河三角洲沉积特征及沉积模式

Depositional characteristics and models of the modern braided river delta in the Daihai Lake,Inner Mongolia

古代辫状河三角洲沉积在我国含油气盆地中广泛发育,并成为油气成藏的有利条件之一,但现代湖泊辫状河三角洲却发育较少,内蒙古岱海辫状河三角洲是为数不多的、发育良好的现代实例之一。为此,实地考察、研究了该辫状河三角洲的沉积特征及沉积模式,并取得以下结论性认识:1三角洲前缘没有水下分流河道,分流河道一入湖即消失,在其前方形成朵状河口坝,沿岸流将河口坝砂再次沿湖岸搬运沉积,形成席状滩;河口坝与席状滩连片,沿湖岸连续展布,河口坝厚度比席状滩大,地形上常隆起,并垂直于湖岸延伸。2辫状河入湖可以形成2种三角洲,即分流河道呈辫状河的三角洲和分流河道呈曲流河的三角洲,前者发育于三角洲形成早期阶段,三角洲平原较小、坡度较大;后者发育于三角洲形成晚期阶段,三角洲平原变大、变平缓,皆呈朵状。3早期阶段的辫状河三角洲,分流河道两岸河岸砂发育,主要为细砂;而晚期阶段则以粉砂和泥为主。4三角洲前缘砂体比平原分流河道的细,前者以砂质沉积为主,后者以砾质沉积为主。

Deposits of ancient braided river deltas are widely developed in petroliferous basins in China and are one of the favourable condition for oil ＆ gas accumulation where major oil/gas pools have been discovered.However,modern lacustrine braided river deltas are so rare that the braided river deltas developed in the Daihai Lake are one of the rare excellent examples of well-developed modern lacustrine braided river deltas in China.Therefore,their depositional characteristics and models were investigated through field observation.The following findings were achieved.（1） Underwater distributary channels are not developed in the delta front.Distributary channels disappear immediately after entering the lake.Lobate mouthbars are formed in front of these distributary channels at first,and are re-transported by the coastal current along the lake shore to form sheet shoals.The mouthbars and sheet shoals are connected to form continuous beaches along the lake shores.The mouthbars are thicker than the sheet shoals and form underwater highs.（2） When flowing into the lake,braided rivers can lead to the formation of two types of deltas：one with braided distributary channels and the other with meandering distributary channels.The former is developed at the early stage of the delta,when the delta plain is relatively small and steep,while the latter is developed at the late stage of the delta,when the delta plain is larger and relatively flat.（3） At the early stage of the braided river deltas,bank deposits of distributary channels are mainly of fine sands,while at the later stage,they are dominated by mud and silts.（4） Delta-front sandbodies are finer than the plain distributary channel sandbodies.The former mainly consists of sands,while the latter are predominated by gravels.