Depositional Characteristics and Source to Sink Systems of Gravity Flow of the Third Member of Shahejie Formation in Gudao West Slope Zone of Zhanhua Sag, Bohai Bay Basin, China

Key words: 
• debris flow /
• hyperpycnal flow /
• slump /
• source-to-sink system /
• slope zone /
• the third member of Shahejie Formation /
• Zhanhua sag

Abstract: By employing the main techniques of core observation, grain size analysis, thin section examination, log data and seismic data interpretation etc., the major types and sedimentary characteristics of sediment gravity flows in the third member of Shahejie Formation in Gudao west slope zone of Zhanhua sag in the Bobai Bay Basin were studied in this paper. Their control factors under different triggering mechanisms and tectonic activities over the sedimentary process of gravity flows and sand body distribution were discussed, the source-to-sink coupled systems were summarized and the development mode of gravity flow sand bodies in the slope zone were established. The results indicated that five types of sediment gravity flow, including hyperpycnal flows, debris flows, turbidity currents, fluidized flows, and slides-slumps were developed during the sedimentary period of the third member of Shahejie Formation in the study area, and involving two triggering mechanisms, namely, flood and slump. The fluid evolution was in general at the early stage of evolution from debris flow to turbidity current, and it is predicted that in the deep waters to the north of the study area, debris flow sediments were still developed and turbidity current sediments started to develop extensively. Tectonic action exerts significant control over fluid property and evolution, development and distribution of contemporaneous deformed structures and sand bodies of gravity flow origin in the third member of Shahejie formation in the study area. In general, there are four major source-to-sink systems occurred, including fault trough gully, fault slope break, fault strike slope, and gentle slope gully, respectively. Vertically, the (semi) deep lake, nearshore subaqueous fan and slump fan deposition were evolved into shore-shallow lake, braided delta and fan delta deposition from the bottom up in the third member of Shahejie Formation. On the plane, fan delta front and nearshore subaqueous fan from Gudao uplift are mainly developed in the east part of the study area whereas braided delta fronts are mainly developed in the west part of the study area, and slump fans in bead string pattern are mainly developed in the middle part of the study area.