曲流河扇相模式及应用

The facies model of a meandering fluvial fan and its application

分支河流体系的讨论有助于促进各类冲积体系的分类学研究,并可促进源—汇体系分析的定量化。博茨瓦纳的奥卡万戈(Okavango)曲流河扇是分支河流体系的典型代表,具有独特的沉积学、水文学和地貌学特征,主要特点如下:①河道形态属于单线曲流河道向下游分叉型,顺流方向产生弯度不一的分支河道网络,从顶点向下游方向,河道呈放射状,由河谷内的限制性河道变为盆地内的非限制性河道;②顺斜坡向下,河道分叉作用增强,河道的尺度和规模减小,受物源控制,无论是曲流河道还是低弯度河道,皆为砂质载荷,河道宽度、水体深度和沉积物粒度虽有系统变化但不显著,且在极低的坡度控制下,随着流量的减少,河道由曲流河逐渐变为低弯度河,河道形态转化的主要影响因素是坡度、流量、沉积物粒级和河岸强度;③根据湿地和河道分布特征,可将扇体划分为补给河谷、近源扇、中部扇和远端扇4个亚环境:补给河谷以单线曲流带和不同规模的迂回坝发育为特征,近源扇主要为泥炭限制的分支河道和河间沼泽沉积,中部扇主要为曲流河和低弯度河沉积,沼泽减少,漫滩增加,远端扇为宽浅型的非限定性河道,以沙岛林地之间的漫滩沉积为主;④沉积物主要为未固结的石英砂,主要来源于卡拉哈里盆地近代风成沉积,砂质纯净,分选和磨圆俱佳,缺乏细粒杂基,粒间细粒组分主要为生物成因的硅藻、植硅石和有机物质,亦见有方解石和二氧化硅胶结物。对现代曲流河扇体系进行调查的重要目的就是研究地下类似沉积体系的分布。通过对我国大型含油气盆地相关“内陆三角洲”沉积特征和沉积规律的重新认识,可为油气资源的勘探开发提供预测模式。鄂尔多斯盆地山西组沉积时期,沉积作用受盆地北缘物源控制,来自北部物源的碎屑物质在宽阔的湿地平原上发育了多套分支河流沉积体系,主要为曲流河扇沉积体系。顺着沉积斜坡向下,河道的尺度和规模减小,沉积物粒度变细,煤层和暗色泥岩厚度变小,缺乏明显的三角洲前缘沉积环境及稳定的前三角洲深水相。沉积组合主要表现为分支河道砂岩、漫岸细粒沉积与湿地泥岩及薄煤层的互层,为大气田的形成奠定了沉积基础。

Distributive fluvial systems may form the bulk of the continental fluvial record and may also promote systematical classification of alluvial fan systems and quantification of source—sink analysis.The Okavango Fan provides an excellent member to Distributive Fluvial Systems dominated by meandering and low sinuosity rivers.The Okavango fan may possess many unusual hydrological,sedimentological,and geomorphological characteristics.These are:(1)The dominant single-thread channel bifurcates with distance downstream,creating smaller channels with varying sinuosity,and the fluvial system is composed of a radial network of channels and associated deposits dispersed below an apex where a river emerges from valley confinement and enters a sedimentary basin.(2)The channel bifurcation increases with increasing distance from the apex,meandering and low sinuosity rivers,both of which are distinctly bedload in character,are mainly because of a restricted source of clastic sediment,and the lowest overall channel gradient measured between apex and toe and the discharge reducing with distance downstream,controls meandering and low sinuosity geometries.Potential control variables of channel pattern transitions are gradient,discharge,sediment size,and bank strength.Channel widths,Water depths,grain sizes,and bankline vegetation vary systematically but not significantly downstream,contributing to an overall downstream decrease in discharge.(3)Our field observations,combined with the results from previous studies,suggest that the fan can be subdivided into four subenvironments.These are:①the Panhandle or feeder valley characterized by high sinuosity river belts and scroll bars:②the proximal fan characterized by meander belts diverging from the fan apex,comprising peat-confined meandering channels,with interchannel swamps forming thick peats;③the middle fan with low to high sinuosity rivers and less common peats;and④the distal fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with vegetation islands.(4)The fan sediments are unconsolidated quartz sands,derived chiefly from Aeolian sands of the Tertiary to Recent Kalahari Basin,so that the supply of fine sediments onto the system is small.Sediment analyses indicate that the most abundant constituent in all the sediment examined is well rounded quartz sand,and fine matrix are mainly produced biologically and include diatoms,phytoliths and organic matter,and authigenic calcite and silica are also present.Large‘inland deltas’in Ordos basin contain river systems that are controlled by similarly channel processes.The explanations for downstream channel adjustments in the meandering fluvial fans thus may be more widely applicable.The fan in Ordos basin is a large shallowly sloping,highly vegetated subaerial fan,which can be subdivided into three subfacies:the proximal fan,the middle fan,and the distal fan.The proximal fan is characterized by fluvial gravel and sand deposits possibly formed by alluvial fan where water is carried in channels that form the major primary distributaries.Swamps develop in the interchannel areas which sustain permanent wetlands in the upper portion of the fan.The middle fan is characterized by meander belts diverging from the fan apex,comprising meandering channels,with interchannel swamps and fines forming thick floodplain deposits.the dominant channel bifurcates downstream,creating smaller channels with varying sinuosity concomitant with the arrival of the seasonal flood.Large-scale avulsion and bifurcation is evident but not prevalent.An increase in sinuosity is formed by lateral migration and point bar deposition.As gradient decreases,channels develop chuteand neck-meander cutoffs in areas where complete channel abandonment becomes apparent,thus promoting out-ofchannel flow and the scouring of new channels or reoccupation of abandoned channels.The lower fan is characterized by unconstrained shallow channel deposits and interacts with pre-existing alluvial plain and occasionally lacustrine deposits.The floodplain is intensively cultivated,and the fines are dominated.Comparable ancient examples of the fans dominated by meandering and low sinuosity rivers are recognizable,many giant gas formations in China have depositional processes associated with meandering and low sinuosity rivers.