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

The Kyokpori Formation (Late Cretaceous), southwestern Korea, consists largely of siliciclastic sediments deposited in a lacustrine basin. T he formation (ca. 500 m thick) shows two fining-upward successions, wi th its lowermost part (ca. 50 m thick) dominated by bouldery conglomer ates 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 characteriz ed by a poorly sorted coarse-grained matrix, abundant outsized and/or protruding clasts and a significant correlation (r = 0.78) between max imum particle size (up to 2 m) and bed thickness (generally < 60 cm). These features collectively suggest cohesionless debris flows. The abr upt lateral facies change may originate from either (1) freezing of th e entire flow to retain a heterogeneous theology, similar to a multipl e-plug flow, or (2) deposition of a laterally migrating, pulsatory flo w with differing rheodynamics from head to tail. The outsize-clast-bea ring 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 b oulder-grade, outsized clasts which are larger than the layer thicknes s (2-10 cm). The outsized clasts are randomly scattered within the san dstone beds, occasionally forming clusters. These features suggest dua l transport mechanisms, i.e., density flows for the sandstone beds per se and debris falls for the outsized clasts. Both the occurrence of d ebris-fall deposits and the high ''relative competence'' (($$$) over b ar MPS/($$$) over bar BTh = 0.74) of debris-flow deposits suggest a st eep slope upon which sediments can gain high downslope mobility overco ming the frictional resistance of substratum. The depositional slope a ngle is inferred to be about 5-17 degrees, based on the properties of debris-flow deposits: flow thickness is assumed to be similar to bed t hickness and debris strength is estimated as 10(3)-10(4) dyn/cm(2), us ing the parameters of protruding clasts. The steep gradient probably d eveloped on a delta slope off a valley mouth.