Fault control on sedimentation in the southern gentle slope of the Dongying Sag, Bohai Bay Basin

Abstract

Located in eastern China, the Dongying Sag is characterized as a prototypical rift basin with intricate fault systems. While research efforts have traditionally honed in on the geometry and kinematics of the northern steep slope zone, the structural characteristic of the southern gentle slope zone (SGS) have received comparably less attention, with a notable gap in the understanding of the fault's geometry and kinematics. The sedimentary facies distribution of the rift basin is unequivocally linked to structural activity, but previous studies have treated sedimentary facies and structural characteristics in isolation, leading to an unclear comprehension of the factors influencing sedimentary facies distribution. This study leverages 3D seismic data encompassing 650 km2 along with 20 well logs, directing focus on the key fault zones within the SGS (i.e. Chenguanzhuang (CGZ) and Liangjialou (LJL) fault zone). Using methodologies such as seismic-well integration, planar attribute extraction, and throw-stripping, the research studies the geometric and kinematic characteristic of these fault zones, scrutinizes their activity features, and elucidates the interplay between structural and sedimentary processes in both temporal and spatial dimensions. The results show that (1) the CGZ fault zone is characterized by an array of east-west oriented, en-echelon faults, which nucleated during the nascent phases of rifting (Es4). Within the fault zone, the F1 fault exhibits the greatest strike extension, which can be further segmented into three distinct sections: F1-1, F1-2, and F1-3. These segments have undergone a kinematic evolutionary process of “separation-connection-extension” during the Es. Concurrently, the SGS is subject to a supply of sources from an east-west orientation, leading to the development of a delta-gravity flow sedimentary system within the hanging wall of the CGZ fault zone. (2) Conversely, the LJL fault zone exhibits a broom-like , comprising a series of NEE-oriented faults. The predominant faults (i.e. F10, F11, and F13) nucleated during the late rifting phase (Es3u). Concurrent with the Es3u interval, the northern part of the study area developed shallow-water delta within the hanging wall of the LJL fault zone. (3) During the Es3m period, the expansion index of the CGZ fault zone attained its zenith, this has facilitated the formation of gravity flow deposits arising from slumping at the delta front. Subsequently, fault activity within the study area experienced an overall decline, the gentle paleo-topography promotes a large-scale shallow water delta developed in the Es3u. The decreasing activity of the CGZ fault zone and the increasing activity of the LJL fault zone in Es3u drove the depocenter shifting towards the NW.