Seismic geomorphology of shoal-water deltaic and mixed carbonate-siliciclastic beach-bar systems in hanging wall of rift basins: Paleogene of the Raoyang Sag, Bohai Bay Basin, China

Abstract

Recent studies of ancient rift basins focus on the tectonosedimentary models, which emphasize the control of tectonics on the distribution and evolution of depositional systems in rift basins, whereas these studies seldom address stratigraphic dominated depositional models. Compared to footwall depositional systems, hanging wall depositional systems are especially underexplored due to their fine-grained, thin-layered, and widely distributed features. We integrated seismic data, cores, and well logs to define the dispersal of hanging wall depositional systems that are related to the fluctuation of lacustrine level in the late synrift and postrift stages. We identified one third-order sequence (SQ1) on the seismic reflection data and subdivided five fourth-order sequences (SSQ1–SSQ5) based on the features of parasequence sets. The strata in SQ1 record a complete regressive and progressive lacustrine process. We interpreted shoal-water deltaic systems and mixed carbonate-siliciclastic beach-bar systems in the SQ1. Within SSQ1 and SSQ2, sediments fed from the northwest, west, and southwest, and developed shoal-water deltaic systems with an average width range of the distributary channel belts within 680–1220 m. In SSQ3, beach-bar systems gradually changed from clastic beach bars to mixed carbonate-siliciclastic beach bars in a basinward direction. Clastic beach-bar systems and mixed carbonate-siliciclastic beach-bar systems were extending north–northeast with an average length of approximately 3.5–8 km and a width of 1–4 km. For those in SSQ4 and SSQ5, shoal-water deltaic systems prograded into the lacustrine, and distributary channel belts occur with an average width range within 520–880 m. We developed a spatial and temporal quantitative evaluation of deltaic and beach-bar reservoirs within a third-order sequence on the hanging wall dip slope, and it has potential worldwide implications for other hanging wall dip slopes in rift basins.