244 / 2022-03-28 13:21:28

正常湖退和强制湖退交互背景下细粒河控浅水三角洲沉积模式——沉积正演数值模拟和现代沉积实例的启示

Shallow-water meandering river delta; Channel-mouth bar complex; Sand-body architecture; Daihai lake; High-frequency lake level change

Different morphologies and stratigraphies are determined by different deltaic processes. However, most lacustrine shallow-water fine-grained fluvial deltas are formed under several phases of normal and forced lake regressions, due to the interaction between climate change and sediment feeding. Their sedimentary processes and facies distribution are barely studied. The Daihai lake at the northeastern China is a Quaternary half-graben lake basin, where shallow-water deltaic systems are widely distributed. While coarse grained braided river deltas dominate the northwestern proximal steep zone, the northeastern distal steep zone and the southeastern proximal gentle slope zone, fine-grained meandering river deltas are situated at the southwestern distal gentle slope zone. Taking the Gongba river delta at the southwestern Daihai lake as an example, satellite imagery and field geology were used to observe the deltaic planform and to collect data for the physics-based numerical modeling (Delft3D). The results indicate that the sedimentary processes of shallow-water fine-grained fluvio-deltas under the interplay of normal and forced lake regression can be divided into three stages, which are delta-front accretion (stage I), forced lake regression (stage II) and delta-plain reworking (stage III). During stage I, delta lobes, composed of crevasse channels and mouth bars, are formed controlled by the bifurcation of main distributary channels. During stage II, precedent lacustrine mud is undercut by main distributary channels without forming delta lobes. During stage III, delta lobes are reworked into channel bars by the lateral accretion of main distributary channels. Consequently, discontinuously-distributed delta lobes are connected by main distributary channels, forming a vertically-alternating pattern of “three-layer structures” and “binary structures”. As for hydrocarbon implications, the interplay of normal and forced lake regressions leads to complicated sand-body heterogeneity.