Yk. Sohn et al., Debris flow and hyperconcentrated flood-flow deposits in an alluvial fan, northwestern part of the Cretaceous Yongdong basin, central Korea, J GEOLOGY, 107(1), 1999, pp. 111-132
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 couple
ts, constituting apparently single sedimentation units. Facies transitions
in the deposits can be summed up in a tripartite facies sequence: (1) a cla
st-supported conglomerate with a muddy or sandy matrix and parallel clast a
lignment (facies A and B), (2) a matrix-supported coarse-tail normally grad
ed conglomerate with random clast orientation (facies C) and thickly strati
fied pebbly sandstone (facies D), and (3) laminated sandstone (facies E). T
he clast-supported conglomerate is interpreted as deposits of debris flows
dominated by frictional grain interactions. Development of pervasive parall
el clast alignment, lacking large floating clasts and inverse grading, sugg
ests deposition via incremental aggradation rather than en masse freezing.
The matrix-supported conglomerate and thickly stratified sandstone are inte
rpreted as deposits of dense inertia layers or traction carpets developed b
eneath a high-concentration bipartite flow. The laminated sandstone indicat
es traction sedimentation associated with dilute flows. The facies sequence
therefore suggests deposition from a composite sediment flow that comprise
s 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 ha
s a meager yield strength and is prone to be density stratified. The measur
ed section comprises three depositional sequences, decameters thick and sep
arated by thick mudstone beds, which could be interpreted in terms of fan e
volution (progradation and retreat) under an influence of changing sediment
supply from a drainage basin. Close association of sediment type with cons
tituent 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.