Field studies in two sub-basins on the west coast of Korea indicate a considerable thickness of Upper Cretaceous lacustrine sediments interbedded with volcanic rocks. Most of the lacustrine strata were deposited in deep water by a variety of sedimentary processes, including turbidity currents and debris flows. The latter are exceptionally well developed in the Kyokpo area where individual clasts greater than 1 m in diameter occur in amalgamated flows. These flows are part of a subaqueous fan complex that extends over several kilometers and includes a more distal turbidite facies. In the Haenam area, turbidites and laminated shales are interbedded with shallow water and subaerial facies.

Early normal and strike-slip faulting in the Kyokpo area indicate that initial basin development was transtensional or extensional. The persistent deep water sedimentary facies imply a rapidly subsiding basin, and the abundant intermediate volcanic material suggest a back-arc or intra-arc setting. Burial depths may have exceeded 5 km based on the degree of lithification. Rapid unroofing accompanied later compression that generated thrust faults and large-scale folding. Structural basin inversion almost certainly occurred at the time of compression (Tertiary?); the inversion may or may not have involved reactivation of the original basin-bounding normal faults.

The tectonic and stratigraphic framework are conducive to the occurrence of hydrocarbon accumulations. Initial basin subsidence produced widespread deep water lacustrine environments that persisted long enough for potential source rocks to be deposited. The interbedded coarse turbidite and debris flow facies could form adequate reservoirs, and abundant siltstones and volcaniclastics should make excellent seals. Numerous potential traps should have been formed by the later compressional folding. The amount of volcanic material in the sequence and the original depth of burial may limit the exploration potential. However, the exploration potential should increase westward, coincident with an expected decrease in the abundance of volcanic rocks.

End_Pages 11 and 12---------------