Typical Submarine Dynamic Landforms And Their Formation Mechanisms In NW Palawan, Southern South China Sea

The offshore Northwest Palawan in the southeastern South China Sea hosts various submarine dynamic features,including submarine canyon systems,large sediment waves,pockmarks,mud volcanoes,volcanoes,and faults.The submarine canyon system and pockmarks are the most typical dynamic landforms on the continental slope offshore Northwest Palawan.Although a large number of geological and geophysical investigations have been conducted previously in this region to resolve scientific questions on the origin of the Reed Bank-Palawan terrane,deep lithospheric tectonics,sedimentary sequences,and carbonate development and evolution,however,there are few studies on the identification and cause analysis of submarine dynamic landforms in the offshore Northwest Palawan.In this thesis,based on multi-beam bathymetry data,chirp sub-bottom profiles data,and 2D multichannel seismic profile data,guided by statistics,we present a detailed investigation of the typical submarine dynamic features(i.e.,submarine canyon system and pockmarks)offshore northwest Palawan.We characterized the morphology and distribution of canyons and pockmarks along the southeastern margin of the South China Sea,and further presented a detailed morphosedimentary investigation.The formation processes and mechanisms of canyons and pockmarks were analyzed in the context of geologic overview and oceanography of the study area.Three main insights were obtained as follows:(1)The modern canyon system’s erosional state on Palawan’s northwest slope accounts for 71% of the canyon morphology.It is still in an active state of sedimentary processes in the present day of sea level rise.The turbidity current is the main reason for maintaining the activities of the canyon.According to the regular spacing of canyon tributaries and gullies,the lack of slump scars of head-ward erosion,and the aggradational status and the sediment waves of the banks of the channels,it is speculated that the turbidity current is a thick and unrestricted sheet-shaped current.The source of the turbidity current is presumed to be the resuspension effect of the typhoon on the continental shelf sediments.(2)The canyon system in the study area contains three parts,Canyon A,Canyon B,and Group of channel C.Three network patterns exist within the canyon system according to morphology: dendritic canyon,braided channels,and slope-confined regular interval gullies.The dendritic morphology is caused by tectonic movements in the eastern part of the study area,the braided morphology is caused by nucleated seabed topography where gas-liquid seepage occurs,and hydrodynamic changes of sheet turbidity currents at the shelf break cause the regular interval gullies.Canyon A is a dendritic canyon where sediments accumulate in the inner bend of the entire mainstem and are eroded in the upstream outer bend and deposited in the downstream outer bend.The tributaries of Canyon B belonged to braided channels where gas-liquid seepage activities promote erosion in the middle mainstream and deposition at the distal end of the mainstem of Canyon B,respectively.Some of the tributaries of Canyon B and Group of channel C,all of which are slope-confined channels.In slope-confined channels,slope morphology affects sedimentation,and a reduction in gradient can lead to slower turbidity flow,leading to sediment deposition.(3)Two primary factors control fluid migration and overpressure generation beneath the pockmarks in the study area.The first is volcanism,which occurs within the tectonic highlands of the study area.The tectonic compression of the volcanism generates faults that act as the main fluid migration paths in the pockmarks.The magma intrusion will cause local overpressure at the root of the pockmarks.The second is gas hydrate decomposition,which occurs at the distal end of slope-confined channels in the study area.Gas pipes/chimneys and paleo-channels/paleo-pockmarks act as the main migration pathways in this type.Five fluid migration pathways were identified throughout the study area: faults,mud volcanoes/mud diapirs,concentrated fluid flow structures(gas pipes/chimneys),paleo-channels and paleo-pockmarks,and mass transport deposits.Two currents remodel the pockmarks: contour currents focus on erosion processes,and turbidity currents focus more on deposition to the pits.This study improves the genetic model of the global typical submarine dynamic features of the seafloor and provide more information on the geological process and tectonic evolution of the southeastern margin of the South China Sea.This study could provide more sedimentological support for the research of the southeastern South China Sea’s geological processes and tectonic evolution,help China further grasp the geological structure and resources in the southeastern part of the South China Sea,provides essential services for the exploration and development of oil and gas in China,and contributes to geological hazard assessment,thus avoiding the risks and challenges of offshore operations.