Oligocene shallow-water lacustrine deltas of the Baxian sag of Bohai Bay Basin, eastern China: Depositional response during rift-to-thermal tectonic subsidence transition

Abstract

Using cores, well logs, and other borehole data, the results of this study show that the shallow-water lacustrine delta has its own unique depositional characteristics of the third member of Oligocene Dongying Formation (Ed3) in the Baxian sag, Bohai Bay Basin, eastern China. During the Ed3 stage, the rift–thermal basin subsidence transition stage, the paleoslope was divided into multilevel slopes by faults along the Wen’an slope with slope angles from approximately 0.19° to 2.02°. The paleogeographic conditions, low-discharge channel, and low accommodation controlled the sedimentary characteristics. The distributions of the shallow-water delta system were controlled by multilevel flexure slopes. The delta plain was distributed on the first- and second-level slope belts, and the delta front was distributed on the third-level slope belt. The high-sinuosity fluvial channel of the delta plain was the dominant facies in the whole shallow-water delta. Most sand was deposited in these channels along the second-level slope belt. Therefore, not enough sand was present to be transported into the lake (shallow water) to form mouth bars in the delta front. Therefore, mouth bars of the shallow-water delta front were few, and the sand beds were thin. Additionally, no more sand was available to be supplied right along to deep lake, the lacustrine basin was small, and there was insufficient accommodation and sand to develop a subaqueous fan in the delta front.