碎屑岩沉积从源到汇的“物-坡”耦合效应

Coupling effect of “mass-slope” from source to sink in clastic rock deposition

碎屑岩沉积体系的三维展布特征及其成因机制是建立沉积模式与预测其砂体分布的关键,更是沉积学研究的关键性科学问题。国内外绝大多数学者认为其平面形态和剖面充填型式均与其沉积的地形坡度、粒度粗细有密切的联系,但其定量关系并不清楚。建立不同类型沉积体中粒度和坡度之间的定量关系,对于预测沉积体中不同颗粒的运动方式及其分布至关重要。应用“源-汇”系统研究的思路,分析了供源区、搬运(输送)区及汇积区各自的地质特点,从单向水流对不同质量颗粒在静止/运动中的受力分析与沉积前总体地形斜坡设定2个角度,系统分析、推导并模拟了不同碎屑颗粒在纵向剖面上的运动轨迹,其表现为随着设定的坡型呈抛物线变化,反映出沉积物输送营力的变换及其随坡度变化的响应特征。建立了陆相碎屑岩沉积体系纵向剖面的流速与坡度的计算方程,明确指出沉积物的流速或流态决定其沉积方式,阐述了汇积区碎屑颗粒的加积型式与沉积物分布上的总体规律,由此提出碎屑岩沉积体系的“物-坡”耦合效应及其主要内涵,并从多个维度与源-汇的各区段,阐述了“物-坡”耦合效应在各种沉积体系的沉积物类型及其空间形态特征上的地质响应。

The three-dimensional spreading characteristics of clastic rock deposition and their genesis mechanisms are the key to establish the depositional models and predict sand distribution, which is a key scientific problem in sedimentology research. Most domestic and abroad scholars considered that the planar geometry and profile filling pattern were closely related to the slope gradient, and grain-size of their deposits. Their quantitative relationships have not been uniformly and clearly understood so far. However, establishing quantitative relationships between grain-size and slope in different types of depositional systems is essential for predicting the movement patterns and their distribution of different particles in sediment bodies. In this paper, the geological characteristics of the source, transport and sink areas were analyzed. Based on the force analysis of unidirectional water flow on different masses of particles at rest/in motion and the general topographic slope setting before deposition, the trajectories of different clastic particles in the longitudinal profile were systematically analyzed, deduced, and simulated, which shows a parabolic change with the set slope shape, reflecting the transformation of sediment-routing agent and its response characteristics with the change of slopes. The quantitative equations of flow velocity and slope of the longitudinal profile of the terrestrial clastic depositional systems were established. It is clearly pointed out that the flow velocity or regime of sediments defines their depositional mode, and general rule of the accretion pattern and sediment distribution in the sink area is clarified. In this paper, the “mass-slope” coupling effect of clastic depositional systems and its main connotation are proposed, and the geological response of the “mass-slope” coupling effect on the distribution of sediment types and their spatial morphological characteristics of various depositional systems is described from various dimensions and source-to-sink segments.