THE CRETACEOUS UHANGRI FORMATION, SW KOREA - LACUSTRINE MARGIN FACIES

The Uhangri Formation forms part of the Cretaceous sedimentary sequenc e deposited in a series of inland basins in the south-western Korean P eninsula. It comprises an approximately 400-m-thick epiclastic sequenc e of conglomerate, (gravelly) sandstone, cherty mudstone and black sha le. The entire sequence can be represented by 16 distinctive sedimenta ry facies organized into four facies associations. Facies association I is characterized by thick homogeneous brownish siltstone, wedge-shap ed disorganized conglomerate and thinly interlayered gravelly sandston e units. The siltstone units were formed by large floods submerging th e alluvial fan fringe (floodplain), whereas the conglomerate and grave lly sandstone units were deposited by sheetfloods and debris flows. Fa cies association II consists of stratified conglomerate - gravelly san dstone, laminated sandstone and sandstone/siltstone couplets which for m fining-upward cycles. Some facies units are low-angle trough cross-b edded and show broad channel geometries. This association represents s ubaqueous delta lobes fed by high- and low-concentration turbidity cur rents in the distal delta realm. Facies association III is characteriz ed by wedged conglomerate and gravelly sandstone facies with interfing ered massive sandstone bounded by scoured bases. It represents a delta front where distributary channels and mouth bars are dominant. Facies association IV consists of laterally continuous sequence of laminated black shale, crudely stratified sandstone and convoluted sandstone/ch erty mudstone. This facies association is suggestive of depositional p rocesses controlled by chemical equilibrium resulting from an interact ion between density inflows and lake water. The cherty mudstone result ed from inorganic precipitation from siliceous solution provided by ac idic volcanism. The Uhangri sequence generally shows a fining-upward t rend with a transition from alluvial fan fringe, coarse-grained subaqu eous delta, to shallow lake. The retrogradation was probably due to co ntinuous subsidence related to continental rifting in the oblique-slip mobile zone.