Sedimentary characteristics and stratigraphic implications of the Kusan-dong Tuff, Cretaceous Gyeongsang Basin, Korea

Abstract

The Cretaceous Gyeongsang Basin, SE Korea,developed on a continental arc massif, is filed with 6-9 km thick,nonmarine (aluvial, fluvial, and lacustrine) deposits and subduc-tion-related volcanogenic rocks. The Kusandong Tuff is a thin (2-4 m thick) but lateraly extensive (ca. 200 km long) ignimbrite sheet,rhyodacitic in composition and rich in crystals (40-60 vol.%). Thetuf has been used as an important key bed for stratigraphic clas-sification and correlation of the basin fil. Overall sedimentary andcompositional characteristics of the tuf suggest, however, that thenorthern and southern parts of the tuff represent different ignim-brite units. The northern part of the tuff (NKT), lying above anerosional contact, consists of a massive division and an overlyingstratified division, whereas the southern part (SKT), lying con-formably above the substrate, has a basal layered division beneatha massive division. The NKT has distinctively higher crystal (51-61 vol.%) and lithic (7.0-7.8 vol.%) contents compared with theSKT, which contains 35-48 vol.% crystals and 0.2-2.5 vol.% lithicfragments. Major and trace element compositions are also differ-ent, suggesting that the NKT and the SKT originated from com-positionally different magmas. These results suggest that the use ofthe Kusandong Tuf as an event stratigraphic unit needs to bereconsidered and the formations adjacent to the Kusandong Tuffneed to be redefined, especially in the Milyang subbasin.