Sediment mixing of extrabasinal and intrabasinal sources in a narrow elongated half-graben? Constraints on provenance and seismic geomorphological analysis of the Upper Eocene Liaoxi Sag in the offshore northern Bohai Bay Basin (China)

Abstract

The interaction of provenance systems is very complicated in narrow elongated half-graben basins, such as some small sags in the Liaodong Bay region, offshore northern Bohai Bay Basin (China). Province identification was conducted through an integrated analysis of petrographic data, heavy minerals, detrital zircon U-Pb data, and three-dimensional (3D) seismic data from the upper Eocene Liaoxi Sag, offshore Bohai Bay Basin. The (light or heavy) mineral indices together with their spatio and temporal distribution show that the Mesozoic-dominated extrabasinal source and the Neoarchaean-dominated intrabasinal source contributions to provenance mixing can be distinguished in the oilfield area. The detrital zircons sampled from the potential sediment mixing area have ages from intrabasinal and extrabasinal sources, suggesting the potential evolution of the hanging wall drainages between the syn-rift and post-rift stages. The overlapping deltaic area between the western shoal margin braided-river delta and the eastern border-fault fan deltas, providing direct visualization of provenance mixing, is well imaged, particularly during the SQ3 lowstand period, but it is not apparent from seismic geomorphological evidence throughout the entire SQ4. Further investigation also reveals that this absence may be attributed to the gradually submerged Liaoxi Low Uplift and the more frequent delta autogenic shifting due to lake-level rise. Shallower water and more efficient wave transport, however, could be another cause of sediment mixing given the presence of widely distributed lacustrine beach bars between transverse deltas from both margins during the post-rift stage. This study enhances an integrated approach to provenance analysis for distinguishing extrabasinal and intrabasinal sources as well as provenance mixing in a narrow half-graben basin on an oilfield scale, even with limited cores and cuttings for instrumental analysis in the offshore region. The study also provides further implications for reservoir properties and connectivity from different source-to-sink systems.