Characteristics and origins of middle Miocene mounds and channels in the northern South China Sea

Numerous elongated mounds and channels were found at the top of the middle Miocene strata using 2D/3D seismic data in the Liwan Sag of Zhujiang River Mouth Basin(ZRMB)and the Beijiao Sag of Qiongdongnan Basin(QDNB).They occur at intervals and are rarely revealed by drilling wells in the deepwater areas.Origins of the mounds and channels are controversial and poorly understood.Based on an integrated analysis of the seismic attribute,palaeotectonics and palaeogeography,and drilling well encountering a mound,research results show that these mounds are dominantly distributed on the depression centres and/or slopes of the Liwan and Beijiao sags and developed in a bathyal sedimentary environment.In the Liwan and Beijiao sags,the mounds between channels(sub)parallel to one another are 1.0–1.5 km and 1.5–2.0 km wide,150–300 m and 150–200 m high,and extend straightly from west to east for 5–15 km and 8–20 km,respectively.Mounds and channels in the Liwan Sag are parallel with the regional slope.Mounds and channels in the Beijiao Sag,however,are at a small angle to the regional slope.According to internal geometry,texture and external morphology of mounds,the mounds in Beijiao Sag are divided into weak amplitude parallel reflections(mound type I),blank or chaotic reflections(mound type II),and internal mounded reflections(mound typeⅢ).The mounds in Liwan Sag,however,have the sole type,i.e.,mound type I.Mound type I originates from the incision of bottom currents and/or gravity flows.Mound type II results from gravity-driven sediments such as turbidite.Mound typeⅢis a result of deposition and incision of bottom currents simultaneously.The channels with high amplitude between mounds in the Beijiao and Liwan sags are a result of gravity-flow sediments and it is suggested they are filled by sandstone.Whereas channels with low-mediate amplitudes are filled by bottom-current sediments only in the Beijiao Sag,where they are dominantly composed of mudstone.This study provides new insights into the origins of the mounds and channels worldwide.