CHARACTERISTICS AND DEPOSITIONAL PROCESSES OF LARGE-SCALE GRAVELLY GILBERT-TYPE FORESETS IN THE MIOCENE DOUMSAN FAN-DELTA, POHANG BASIN, SEKOREA

The Doumsan fan delta in the Miocene Pohang Basin (SE Korea) includes large-scale gravelly Gilbert type foresets that are more than 150 m hi gh and dip at about 20 degrees. The foresets consist of sis sedimentar y facies: medium- to thick-bedded sandy gravel deposits with variable grading patterns and bed geometries (Facies A), thin- to medium-bedded , commonly inversely graded sandy gravel deposits (Facies B), sheet-li ke layers, a few grains thick, of pebble gravel (gravel sheets; Facies C), lensoidal layers, a few grains thick, of cobble to boulder gravel (gravel lenses; Facies D), thin-bedded sand with outsized clasts (Fac ies E), and very thick-bedded (> 10 m thick) disorganized gravel depos its in the toeset area (Facies F). These facies are indicative of depo sition from cohesionless debris hows (Facies A and B), debris falls (F acies C and D), turbulent hows (Facies E), and very thick debris hows that are related to large-scale foreset failure (Facies Fl. These faci es are in close association with one another, except for Facies F, and are either vertically superposed or laterally juxtaposed within singl e sedimentation units. This suggests that these facies originated from a series of evolving sediment gravity hows. A cohesionless debris flo w generated at the topset-foreset boundary or on the middle of the for eset slope segregated its sediments into a pebble-rich lower division and a sandy upper division with sparse cobble-to-boulder clasts by pre ferential upward drift of large clasts and surface transformation of f ine-grained material. The pebbly lower division was emplaced by fricti onal freezing, forming Facies A beds, On the other hand, the sediments in the upper division continued to move as a sandy turbulent flow and bouldery debris fall, resulting in Facies E and D deposits downslope. The cohesionless debris flow occasionally transformed into a series o f thinner hows by development of roll waves along the top of the how, producing Facies B layers on the margin of a Facies A bed, Otherwise, the cohesionless debris how transformed into grain-assemblage debris f all and single-grain debris fall consecutively by removal of interstit ial sand via downward percolation and stripping into ambient water. Th e debris falls produced gravel sheets (Facies C) and gravel lenses (Fa cies D), During this dow transformation, cobble- to boulder-size clast s and sandy material were selectively transported farther downslope, r esulting in prominent textural bimodality of the prodelta deposits, wh ich comprise isolated large clasts and lensoidal deposits of cobble-to -boulder gravel set in sandy background material.