Seismic Expression of the Mass-Transport Complex with Crescent-Shaped Compressional Ridges in the Xisha Trough, South China Sea
A multi-phase inherited suite of Mass-transport complex (MTC) overlaid on and partly infilled the Tertiary Qiongdongnan-deep-sea-channel on the lower slope of Xisha trough, Qiongdongnan basin, South China Sea. The deep water meandering channel was formed as a result of global sea level drop during early Pliocene in the linear canyon at the paleo-suture-line, incised into the Tertiary sediments from east to north with inclination of 0.39 degree along the channel, creating the accommodation space for the continental debris. With the progradation of the shelf margin, the early MTC units infilled the flat base of the sand-prone canyon from north and ceased at the south scarp as the terminal wall; and the later MTC units continued to propagate and covered the south scarp. The resulting MTC units were imaged as chaotic composite waves with the erosional relief of 100m-350m, which were separated by six to seven near-flat sliding surfaces (possible the condensed section) expressed as high amplitude reflection events.
The abundant MTC units show a gradual transition from chaotic expression (possibly under seismic resolution in some instance) to small scale (30m in horizontal distance between two faults, 100m in erosional relief) to large scale (450m in distance, 350m in erosional relief) compressional ridge. Two MTC units could be captured by a later thicker MTC unit of much higher energy, and avulsion could break the lobe front, resulting as smaller branches with closer compressional ridge group. The external and the internal architecture of the MTC in Xisha trough was clearly imaged on the newly acquired seismic data: 1). Lobate and branching geometry. 2) MTC capture. 3) Frontal splay. 4) Crescent-shaped compressional ridge (plan view). 5) Thrust fault at approximately 13-16 degree dip. 6) Chaotic or blank internal reflection. 7) Concave upward (on the brim) and flat (in the main part) decollement surface. 8) Large pockmark on top surface of some lobes.
Continued sedimentation of deep water MTC with crescent-shaped compressional ridges in Xisha trough from the end of Miocene to present indicates the environment need not have to be lowstand, and the sliding need only minor inclination. Shorter-term eustatic change seems to lack the control on the mass-failure feature.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California