Evolution of deepwater turbidite bedforms in the Huaguang channel-lobe transition zone revealed by 3D seismic data in the Qiongdongnan Basin, South China Sea

摘要

The theoretical model of turbidite bedform evolution has been verified based on laboratory tests and outcrop sur-veys. However, the evolution of the bedforms associated with flow regime variation within the channel-lobe transition zone (CLTZ) is poorly documented and generally extremely poorly understood. In this study, a combination of seismic sedimentological and topographic analyses, based on high-resolution 3D seismic data within the Qiongdongnan Basin of the South China Sea, was used. Continuous bedforms varied with the depositional elements within the CLTZ of the Huaguang leveed channel system along the flow direction. The depositional elements within the CLTZ change from the main channel to the distributary channels as well as to the sub distributary channels. Numerous bifurcation and confluence points of gravity flows illustrated the complexity of the drainage network within the CLTZ. The average asymmetry index of the bedforms increased from 0.8 to 1.6, with the average wavelength increasing from 803 m to 1545 m along the thalweg of the channels. Crevasse splays at the starting reach of the CLTZ are likely to form the initial state of the new CLTZ in the future, because the slope gradient starting from the avulsion point was higher than that inside the channel. The deep scours at the bifurcation points and high steps at the confluence points demonstrate the critical roles of supercritical flows and hydraulic jumps in reworking the bedforms of the CLTZ. The high steps at confluence points were simultaneously formed along the margin and thalweg of the main channel. The acceleration mechanisms of flow velocity forming the high steps were the increase of the elevation difference along the internal slope of the main channel margin and promotion of discharge along the main channel thalweg. This paper presents newly completed deep water observations of the present seafloor bedforms based on the turbidity currents within the CLTZ. These observations update the evolution model of local supercritical flows and hydraulic jumps under the global decreasing trend of the Froude numbers within CLTZs. (C) 2020 Elsevier B.V. All rights reserved.