超临界沉积物重力流形成演化及特征

超临界沉积物重力流是指在深水环境中弗洛德数大于1,沿水下斜坡向盆地中心搬运、在水力跳跃机制作用下发生超临界流与亚临界流频繁转化、形成易于保存的大型波状旋回坎的高密度流。超临界沉积物重力流作用的沉积识别标志主要包括牵引毯作用形成的分层构造,即后积层理、短波状上攀交错层理、假前积层等层理构造;水力跳跃作用形成的侵蚀和软沉积物变形构造等局部识别标志和旋回坎综合识别标志。理想条件下,超临界沉积物重力流作用在垂向上由下至上形成砾质旋回坎、砾质流槽-凹坑沉积充填、砂质不稳定逆行沙丘沉积、砂质分层沉积和沙纹沉积及正常沉积的有序组合。旋回坎可分为侵蚀型和沉积型,从盆地边缘向盆地中心,呈现侵蚀型向沉积型的演化。超临界沉积物重力流作用沉积演化过程主要受流体沉积物浓度、流体流量、沉积物粒度、沉积速率等内部因素和地形坡度、坡折带位置、水深及可供侵蚀的沉积物特征等外部因素的综合控制。超临界沉积物重力流作用的相关研究加深了对沉积物重力流水道形成机理、深水块状粗碎屑成因、重力流沉积演化过程的认识;现阶段关于超临界沉积物重力流的流体动力学特征、沉积识别标志及形成演化过程的系列问题还有待深入研究。

Formation,evolution and sedimentary characteristics of supercritical sediment gravity-flow

Supercritical sediment gravity-flow refers to the high-density flow with Froude number more than 1, transporting from underwater slope towards basin center, able to frequently transform between supercritical flow and subcritical flow under hydraulic jump mechanism, so as to form large wave-shaped cyclic steps easy to be preserved. The sedimentary identification marks of supercritical sediment gravity-flow mainly include the cyclic step integrative identification mark and the local identification marks including the stratification structures caused by traction carpets, the lamination structures such as backset bedding, short-wavelength climbing cross bedding and pseudo-foreset bed, and the erosion and soft-sediment deformation structures caused by hydraulic jump. Under the ideal conditions, supercritical sediment gravity-flow leads to a sequential assemblage composed of pebbly cyclic steps, gravely chutes-pool sedimentary fillings, unstable antidune sediments, stratified sandy sediments, sandy rippled sediments and normal sediment from top to bottom in the vertical direction. Cyclic steps can be divided into erosional type and sedimentary type. From basin edge to center, the erosional cyclic step is evolved to sedimentary type. The sedimentary evolution process of supercritical sediment gravity-flow is jointly controlled by internal factors, such as fluid sediment concentration, flow discharge, sediment granularity and sedimentary rate, and external factors, such as topographic slope, slope break position, water depth and the characteristics of sediments available for erosion. The related research on supercritical sediment gravity-flow has deepened the understanding of the formation mechanism of sediment gravity-flow channels, the genesis of deep-water massive coarse-grained sandstones and the evolution process of gravity flow. In current stage, a series of problems require further research, including the fluid dynamic characteristics, sedimentary identification marks and formation-evolution processes of supercritical sediment gravity-flow.