The Submarine Landslide Types and the Response With BSRs in Dongsha Sea Area, South China Sea

Abstract

In 2013 gas hydrate samples (average saturation of 45%~55% of pore volume) were obtained in Dongsha Sea Area (GMGS2). The high resolution 3D seismic data here shows there are small various landslide bodies in huge mass transport deposits. Referring to the landslides in outcrops of other areas, these bodies are divided into 7 types by analyzing the shape, architecture, and genesis, namely the slide, collapse, and deformation above BSRs, as well as the slump wedge, lens, block, and sheet below BSRs. The features are listed to identify them in seismic profiles and multi beam bathymetric submarine geomorphology maps. Based on this classification detailed studies are completed, including the measurements for landslide body scales, and slope gradients, the depiction for three-dimensional characteristics, and the research on landslide distribution. The results display that slump sheet and wedge have obvious long axes while slide, slump lens, and block have similar length and width. The slope gradients imply the collapse and slump sheet develop when the slope gradient is below 5° while the slide, slump block and lens occur when the slope gradient higher than 10°. In space the slump wedge is a lobe-like shape while the slump sheet seems like a spade. Above BSRs, the deformation constitute the main part of slump fan while below BSRs, the slump lens, and blocks make up the slump fan and slump sheet appears in fault belts. In order to determine the genetic differences for these 7 types, this paper discussed the possibility of vertical flow-pattern-transformation and specific mechanism for submarine landslides. In the study area the trigger mechanisms for these landslide bodies include gas hydrate decomposition, sediments overload, steep slope, sand-beds interbeds, and tectonic activities. The conclusions are that the dominant factor is free gas caused by gas hydrate decomposition and transformation between flow patterns exists probably, which can be verified by the complex configuration in slump lens. Finally there are 2 kinds of response between landslides and BSRs since the free gas either influences the landslide bodies below BSRs or those above BSRs, leading to A and B, 2 kinds of landslide body associations. A type is characterized by slide above BSRs and slump lens below BSRs while B type has collapse or deformation above BSRs and slump block below BSRs. A type is preferred because it indicates less decomposition and better capping of gas hydrate layers.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015