ABSTRACT

Provenance of the Lower Cretaceous Hayang Group in the Gyeongsang Basin, Korea, was investigated using sandstone petrography and the composition of detrital plagioclase and clinopyroxene grains in sandstone and mudrock. A magmatic-arc tectonic setting is inferred for the source on the basis of sandstone composition. Detritus derived from granitic/gneissic rock and incipient continental-arc volcanic rock were mixed in a variable ratio in the Hayang sedimentary rocks. Before deposition of the Hayang Group, the source rock was composed mainly of granitic/gneissic rocks. This condition persisted in the north, the Euiseong subbasin, during deposition of the lower part of the Hayang Group. In the Milyang subbasin, the southern part of the Gyeongsang Basin, source-area volcanism occurred just before deposition of the Hayang Group. Compositions of detrital pyroxene and plagioclase revealed the source volcanic rock to be calc-alkaline, volcanic-arc basaltic andesite to dacite. Later, granitic/gneissic rocks as well as volcanic rocks supplied detritus to both subbasins, and the source volcanic rocks, inferred from plagioclase composition, might have been of silicic composition.

The Hayang Group was deposited in a transitional setting, changing from strike-slip movement with little volcanic activity to active subduction-related volcanism in the eastern continental margin of Asia. During this transitional stage, the source area was occupied by granitic/gneissic rocks and within the source area intermediate to silicic volcanic rocks of dominantly calc-alkaline affinity erupted sporadically.