摘要： 河流平衡是指河流在输送沉积物的过程中达到了无净沉积、无净侵蚀、沉积物过路不留的平衡状态。它代表了沉积物加积与侵蚀的临界状态,因此是地貌学、地质学和地层学的基本概念。传统观点认为,在基准面、物源供给等外部条件保持不变的情况下,河流总会自发地向理论平衡剖面去逼近,最终达到平衡状态。这种传统观点适用于河流的局部平衡,并不适用于冲积河流的整体平衡。这是因为传统观点并未充分考虑源汇系统尺度下的冲积河流整体具有自由边界的特性(河流上游与下游端可以自由延长)。如在冲积河流系统的末端,三角洲的进积使得河流剖面延长,这一过程必然伴随着河流的加积。这表明在自由边界条件下,冲积河流实现整体平衡的必要条件是基准面下降,而非稳定。物理与数值模拟实验进一步证明了冲积河流达到系统整体平衡的3种情形: (1)强制平衡。发生于河流末端无法延长的固定边界条件下(如蓄水盆地坡度很陡、水深很大等情形),河流平衡于稳定基准面。(2)自成因平衡。发生于自由边界条件、河流剖面的坡度与河流末端下超面坡度(盆地坡度)相等的条件下,河流平衡于稳定下降的基准面。(3)他成因平衡。发生于自由边界条件、河流剖面的坡度小于河流末端下超面坡度的条件下,河流平衡于以特定方式减速下降的基准面。注重地层叠置过程的成因地层学理论应当充分考虑这3种河流平衡的方式,才能更为准确解释地层叠置的成因。

关键词: 河流平衡, 基准面, 自成因与他成因, 层序地层学, 成因地层学

Abstract: Grade refers to the state of a river where sediment is conveyed without net deposition or net erosion but completely bypassed through the river. The concept of grade is fundamental to geomorphology,geology and stratigraphy,because it represents the critical condition between river aggradation and degradation. According to conventional views,rivers will spontaneously approach the theoretical graded profile and finally reach it,given constant external forcings such as stable base level and steady sediment supply. The conventional notion of grade is valid for local scale of the alluvial system,but not for system scale of the whole alluvial system from a source-to-sink perspective. This is because the conventional notion seldom considers the free boundary condition of the alluvial system(i.e.,the upper and lower boundary of the alluvial system can move freely). For example,extension of the alluvial system by delta progradation at the downstream end of the river is inevitably accompanied by alluvial aggradation. This indicates that in moving boundary conditions,the necessary condition for the alluvial river to attain the state of grade at system scale is base level fall rather than base level standstill. Physical and numerical experiments further verified that the alluvial river can reach system-scale grade by three ways. (1)Forced grade,which can be realized in fixed downstream boundary conditions,for example,the downstream end of the alluvial river cannot extend freely by extreme deep basin water depth. In this case,the alluvial river grades to stationary base level. (2)Autogenic grade,which is realized in full moving boundary conditions and the basin floor slope equals that of the alluvial system. In this case,constant rate of base level fall is necessary. (3)Allogenic grade,which is realized in full moving boundary conditions and the basin floor slope is steeper than that of the alluvial system. In this case,slowing down of the base level fall in a particular pattern is necessary. Genetic stratigraphy which highlights the stacking process of strata should fully take the three ways of grade into account,by which the interpretation of stacking of strata will be clearer.

Key words: alluvial grade, base level, autogenesis and allogenesis, sequence stratigraphy, genetic stratigraphy

中图分类号: 

• P532