Deep-lacustrine sediment gravity flow channel-lobe complexes on a stepped slope: An example from the Chengbei Low Uplift, Bohai Bay Basin, East China

Abstract

Many stepped slopes are present at deep-lacustrine basin margins in China, especially in the Paleogene rift basin margins of East China. Sediment gravity-flow deposits on these stepped slopes have the potential to form excellent reservoir quality sediments that are targeted for hydrocarbon exploration, but still remain poorly understood. In this study we use the Ed3 member of Oligocene-aged Dongying Formation deposited over the Chengbei Low Uplift, as an example for investigating facies types, distribution and main controlling factors of sediment gravity-flow deposits on stepped slopes at deep-lacustrine basin margins. Paleotopographical reconstructions indicate that the study area displayed a three-level stepped slope that were dissected by three large canyons and many associated smaller-scale channels prior to the deposition of the Ed3 member. On the basis of facies identification, six facies associations are further distinguished: (1) fan valley fills, (2) confined channel deposits, (3) poorly confined channel deposits, (4) levee deposits, (5) interchannel deposits, and (6) lobe deposits. These facies associations are grouped into four systems, namely fan valley fills, confined channel-levee, poorly confined channel-levee and lobe systems, as architectural elements constituting two types of channel-lobe complexes in the study area, namely anastomosing channel-lobe complexes and meandering channel-lobe complexes. The interaction between paleotopography and lake level fluctuations likely represented the direct control on the morphology, pattern and spatial-temporal distribution of channel-lobe complexes in the study area. Moreover, lake level fluctuations can also influence the volume and grain size of sediment supply to the stepped slope systems. Hyperpycnal flood events, as trigger factors, caused the generation of subaqueous sediment gravity flows that fed the stepped slope systems. Climate variations appear to be a contributing driver of lake-level fluctuations, and also indirectly control sediment supply into the deep lake and development scale of the stepped slope systems. These controls therefore determined the sediment supply, trigger and spatial-temporal variations of the stepped slope systems. This study improves the understanding of facies types and their distribution across stepped slopes at deep-lacustrine basin margins, as well as the controlling factors on the development of these sediment gravity-flow deposits.