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 mou
After analyzing the diatoms and silicoflagellates in the 1787 of east area and the 2437 of west area from CC Zone, the East Pacific, 31 genus and 81 species and varieties of diatom and 7 genus and 12 species of silicoflagellates are identified. There are 8 diatom zones in Core 1787 and 1 diatom Zone in Core 2437. According to the characteristics of diatom zones and the occurring and extinct ages of some diatom and silicoflagellate species, the strata since Oligocene in the region are subdivided. Lastly, com-bined with the data of radiolarians and paleomagnetism, some problems for Core 1787 are discussed, including where is the lower boundary of Quaternary, whether Pliocene is absent or not and whether Oligocene is present or not.
Lan Dongzhao, Chen Chenghui (Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China)
Submarine landslides occur frequently on most continental margins. They are effective mechanisms of sediment transfer but also a geological hazard to seafloor installations. In this paper, submarine slope stability is evaluated using a 2D limit equilibrium method. Considerations of slope, sediment, and triggering force on the factor of safety （FOS） were calculated in drained and undrained （4=0） cases. Results show that submarine slopes are stable when the slope is 〈16° under static conditions and without a weak interlayer. With a weak interlayer, slopes are stable at 〈18° in the drained case and at 〈9° in the undrained case. Earthquake loading can drastically reduce the shear strength of sediment with increased pore water pressure. The slope became unstable at 〉13° with earthquake peak ground acceleration （PGA） of 0.5 g; whereas with a weak layer, a PGA of 0.2 g could trigger instability at slopes 〉 10°, and 〉3 ° for PGA of 0.5 g. The northern slope of the South China Sea is geomorphologically stable under static conditions. However, because of the possibility of high PGA at the eastern margin of the South China Sea, submarine slides are likely on the Taiwan Bank slope and eastern part of the Dongsha slope. Therefore, submarine slides recognized in seismic profiles on the Taiwan Bank slope would be triggered by an earthquake, the most important factor for triggering submarine slides on the northern slope of the South China Sea. Considering the distribution of PGA, we consider the northern slope of the South China Sea to be stable, excluding the Taiwan Bank slope, which is tectonically active.
