重力流沉积:理论研究与野外识别

Gravity Flow Sedimentation:Theoretical Studies and Field Identification

重力流沉积是(半)深海和深湖环境中一种重要的沉积现象,因此准确识别重力流沉积对恢复古代沉积环境具有重要意义。从沉积物重力流的基本理论出发,介绍四类重力流沉积的特点和野外鉴别特征。碎屑流沉积表现为颗粒大小混杂,底面平坦,板条状砾石平行层面排列;超高密度流沉积的砂岩呈厚层状或块状,砂岩内部经常出现较大砾石或泥岩碎片,泄水构造和液化构造常见,具逆粒序的颗粒流也属于超高密度流;高密度流沉积兼具超高密度流和浊流沉积的特点;鲍玛层序Ta-d段是一次浊流事件的产物,但只有Ta段为浊流沉积,Tb-c段为牵引流沉积,Td段为悬浮沉积。在重力流发展过程中可出现各种流动转换。底流和过路流对重力流沉积的改造也是一种普遍现象。

As an important sedimentary phenomenon,gravity flow sedimentation is widespread in deep-marine and deep-lacustrine environments.Obviously,differentiation of different types of gravity-flow sediments is crucial for reconstructing paleo-depositional environment.Four distinct types of gravity flows and their diagnostic sedimentary structures are introduced,which can be used as guide for field identification.Debris flow depositions are characterized of their disorganized internal texture,flat bed base,and parallelism of planar clasts to bedding.Thick-bedded and massive sandstones are usually formed by hyperconcentrated density flow;typified by occurrence of out-sized clasts,rip-up mudstones,water escape structures and other liquefaction-related structures.Grain flow is a subtype of hyperconcentrated density flows,and inverse grading is characteristic of its deposits.Concentrated density flow possesses the depositional characteristics of both hyperconcentrated density flow and turbidity current.Ta-d divisions in Bouma sequence are the products of one turbidity current event,but only Ta is formed from turbidity current.Tb-c divisions are actually generated from traction flow,whereas Td division is fallout deposits.Flow transformation can occur in the processes of gravity flows,and the gravity-flow sediments are often reworked by bottom currents and bypass flows to varying degrees.