Evolution model of a modern delta fed by a seasonal river in Daihai Lake, North China: determined from ground-penetrating radar and trenches

Abstract

While deltas fed by seasonal rivers are common in modern sedimentary environments, their characteristics remain unclear as compared to those fed by perennial rivers. This study identifies a small delta discharged by a seasonal stream flowing into Daihai Lake, in northern China, which is driven by ephemeral and high-energy flood events. Detailed 3D facies architecture was analyzed using ground-penetrating radar (GPR) and sedimentary logs from outcrop and trenches. Four types of radar surfaces, including truncations of underlying inclined strata, weak reflections, and depositional surface of downlap and onlap, were identified. Six radar facies (high-angle oblique-tangential, low-angle subparallel, gently plane parallel, plane-parallel, chaotic, and continuous strong reflection) were identified based on distinctive reflections, including amplitude, continuity, dip, and termination patterns. Five depositional units (Unit A to E) were documented from proximal to distal delta. Seasonal discharge signatures include significant grain-size decrease over short distance, abundant Froude supercritical flow sedimentary structures, poorly developed barforms, and small-scale scour and fill structures. Records of lake-level and sediment budget were evaluated over the past 60 years. In highstand stage (1960–1980), amalgamated channel (Units A and B), and delta front (Unit C) were deposited. In slope stage (1980–1996), the lower deposits (Units A, B, C) were eroded by Unit D with a distinct truncation surface. In lowstand stage, most eroded sediments bypassed the incised channel and accumulated in the distal part, in which a new depositional unit was formed (Unit E). The model demonstrates that deltas fed by seasonal rivers tend to accumulate large amounts of sediments carried by high magnitude floods within short periods.