Sedimentary Records of Synsedimentary Tectonic Activity in Sedimentary Basins

The framework of a sedimentary basin and its evolution tend to be governed by the character and modes of synsedimentary tectonic activity which were recorded in the disposition in length and breadth of sedimentary bodies in a basin. These give expression to geodynamic settings and mechanism in the course of the evolution of a basin and to the constitution of the source area and its tectoinic characteristics. A thorough study of the sedimentary records would make it possible to expound the tectonic evolution of a sedimentary basin and the characteristics of each stage in its development. In this paper the tectonic settings of ancient sedimentary basins are discussed in four aspects.(1) Different sedimentary associations reflect variable synsedimentary tectonic activities which enable the sedimentary sequences with normal and reverse cyclic character or normal-and reverse-graded beds to be laid down. Differential uplift and subsidence of positive tectonic activities in the basinal basement bring about gravity slumping and gravity flow, thus resulting in the formation of a suite of sedimentary bodies with gravity flow structure and texture which are commonly associated with turbidites. Morever orogenic movements give rise to the steep cliffs on the basinal walls, resulting in gravity slumping and gravity flow deposits which differ from those formed by the gravity mechanism due to extensional activities. In other words, the significant difference between the two types of the deposits is that tectonic melange is present. (2) The compositions and textures of the sedimentary rocks (sediments) formed by synsedimentary tectoinc activities are immature and vary in different stages of the tectonic evoluiton. (3) Sedimentary facies and formations are restricted by the basinal framework(i.e. geometry of a basin, the distribution of the basin-controlled tectonics and the relationship between geomorphology of a basin and the source area). Differential uplift and subsidence of the basin basement caused by extensional stress lead to undulating landforms, sharp lithofacies changes horizontally and high thickness gradients, while the orogenic basins formed by compressional stress are characterized by gently sloping basement, tectonic mixing of various rocks and the burial of "new" deposits beneath the orogenic nappes. (4) Synsedimentary tectonic activities frequently account for superposition of contrasting facies such as high-energy facies upon low-energy facies, deep marine facies upon shallow marine facies, continental facies upon lacustrine facies or deeper marine facies. Attention should be paid to such discrete events in the study of palaeotectonic movements.