Tectono-sedimentary evolution of the Paleogene Qikou Sag, Bohai Bay Basin, NE China

Abstract

The Qikou Sag is a petroliferous tectonic unit within the Bohai Bay Basin, northeast China. The depositional system evolved temporally, spatially, and episodically in five rifting stages (stages 1–5) during the Paleogene and Neogene, with sedimentation responding to regional subsidence. This study focuses on the tectono-sedimentary evolution through stages 2–4, while the main oil-bearing sedimentary rocks were deposited in the Bohai Bay Basin. The depositional system exhibited retrogradation and then progradation during the rifting stage 2 (43–36.7 Ma), retrogradation in stage 3 (36.7–32.8 Ma), and upward coarsening and progradation in stage 4 (32.8–24.6 Ma). This vertical superposition corresponded to three episodic stages of rising and falling lake levels during the Paleogene, which were equivalent to a transgressive with regressive (stage 2), transgressive (stage 3), and then regressive (stage 4) cycle. The sediment fills coincided with the tectonism and the regional subsidence history of the Qikou Sag, which is composed of uplift areas and sub-sags. In this respect, the boundary fault activity around the sub-sags decreased from stages 2–4, leading to the migration of sub-sag depocenters from near faults to the center of sub-sags, which suggests that faults released control over the sedimentation. Secondly, tectonism and the total subsidence rate decreased from SQ1 to SQ7 (stages 2–3) and then accelerated at the end of stage 4 (SQ10 to SQ11). The tectonic subsidence rates decreased quicker than the total subsidence rate; this demonstrates that major rift faulting was the dominant geological process in the early Paleogene, whereas overall subsidence was dominant in the late Paleogene.