Debris Flow and Hyperconcentrated Flood‐Flow Deposits in an Alluvial Fan, Northwestern Part of the Cretaceous Yongdong Basin, Central Korea

The alluvial‐fan deposits in the Cretaceous Yongdong Basin, Korea, consist of conglomerates with a muddy or sandy matrix and sandstones with thick or thin laminations. The conglomerates and sandstones occur commonly in couplets, constituting apparently single sedimentation units. Facies transitions in the deposits can be summed up in a tripartite facies sequence: (1) a clast‐supported conglomerate with a muddy or sandy matrix and parallel clast alignment (facies A and B), (2) a matrix‐supported coarse‐tail normally graded conglomerate with random clast orientation (facies C) and thickly stratified pebbly sandstone (facies D), and (3) laminated sandstone (facies E). The clast‐supported conglomerate is interpreted as deposits of debris flows dominated by frictional grain interactions. Development of pervasive parallel clast alignment, lacking large floating clasts and inverse grading, suggests deposition via incremental aggradation rather than en masse freezing. The matrix‐supported conglomerate and thickly stratified sandstone are interpreted as deposits of dense inertia layers or traction carpets developed beneath a high‐concentration bipartite flow. The laminated sandstone indicates traction sedimentation associated with dilute flows. The facies sequence therefore suggests deposition from a composite sediment flow that comprises a preceding debris flow, a trailing watery flow, and an intermediate flow between. The intermediate flow is regarded as a hyperconcentrated flow on the basis of its bipartite nature because a hyperconcentrated suspension has a meager yield strength and is prone to be density stratified. The measured section comprises three depositional sequences, decameters thick and separated by thick mudstone beds, which could be interpreted in terms of fan evolution (progradation and retreat) under an influence of changing sediment supply from a drainage basin. Close association of sediment type with constituent facies in the three sequences suggests that composite sediment‐flow deposits are favorably developed by sand‐matrix debris flows drained from large and less rugged catchments.