Highstand sublacustrine fans: The role of a sudden increase in sediment supply

Abstract

AbstractContrary to widely used traditional sequence‐stratigraphic models, highstand sublacustrine fans developed in the deep‐water environment in the middle of the third member of the Eocene Shahejie Formation (Es3z, 42–40.5 Ma), Dongying Sag. Using four cores and 3D seismic data, we describe the sequence stratigraphy frameworks, sediment characteristics, depositional elements, and depositional model of the Liangjialou highstand sublacustrine fan. The Es3z is a complete third‐order sequence stratigraphic, bounded by T4 at the top and T6 at the base. Between T4 and T6, there is the maximum flooding surface (MFS) distributed from proximal to distal of the system in the third‐order stratigraphic sequence. Therefore, the Liangjialou Fan at the top of the Es3z, between MFS and T4, is a typical highstand sublacustrine fan as determined from seismic data. This highstand sublacustrine fan is characterised by gravel to fine‐grained sand, abundant amalgamation surfaces, and crude stratifications common in coarse‐grained structureless‐ to normal‐ graded sandstones. Climbing ripples and thin laminae rich in plant debris are common in inverse‐then normal‐graded sandstones. Channel‐levee, channel‐lobe transition zone, and lobes developed in this highstand sublacustrine fan, with clear internal progradation stacking patterns caused by hyperpycnal flow. The sudden increase in sediment supply in an accommodation decrease situation and the narrow shelf width are the primary controlling factors for forming highstand sublacustrine fans. In the late stage of the highstand systems tract, gravity‐flow deposits caused by hyperpycnal flow are developed from another new point source to form this highstand sublacustrine fan. Hyperpycnal flow in lacustrine basins can transport coarse‐grained sediments with the help of strong flood energy and steep slopes. Gravity flow deposits in lacustrine basins are not confined by sequence stratigraphy frameworks, and many additional areas of sand caused by hyperpycnal flow could be found in lacustrine basins.