Bouldery deposits in the lowermost part of the Cretaceous Kyokpori Formation, SW Korea: cohesionless debris flows and debris falls on a steep-gradient delta slope

Abstract

The Kyokpori Formation (Late Cretaceous), southwestern Korea, consists largely of siliciclastic sediments deposited in lacustrine basin. The formation (ca. 500 m thick) shows two fining-upward successions, with its lowermost part (ca. 50 m thick) dominated by bouldery conglomerates and outsize-clast-bearing sandstones.The bouldery conglomerates are either disorganized, normally graded, inversely graded, or crudely stratified with common abrupt lateral facies changes, and characterized by a poorly sorted coarse-grained matrix, abundant outsized and/or protruding clasts and a significant correlation (r = 0.78) between maximum particle size (up to 2 m) and bed thickness (generally < 60 cm). These features collectively suggest cohesionless debris flows. The abrupt lateral facies change may originate from either (1) freezing of the entire flow to retain a heterogeneous rheology, similar to a multiple-plug flow, or (2) deposition of a laterally migrating, pulsatory flow with differing rheodynamics from head to tail.The outsize-clast-bearing sandstones are either stratified or thin bedded with alternation of massive/inversely graded and laminated layers. Each bed consists of moderately to well sorted, fine to coarse sandstone, with pebble- to boulder-grade, outsized clasts which are larger than the layer thickness (2–10 cm). The outsized clasts are randomly scattered within the sandstoe beds, occassionally forming clusters. These features suggest dual transport mechanisms, i.e., density flows for the sandstone beds per se and debris falls for the outsized clasts.Both the occurence of debris-fall deposits and the high “relative competence” ( MPS/ BTh= 0.74) of debris-flow deposits suggest a steep slope upon which sediments can gain high downslope mobility overcoming the frictional resistance of substratum. The depositional slope angle is inferred to be about 5–17°, based on the properties of debris-flow deposits: flow thickness is assumed to be similar to bed thickness and debris strength is estimated as 10 3–10 4 dyn/cm 2, using the parameters of protruding clasts. The steep gradient probably developed on a delta slope off a valley mouth.