한반도 서남부에 분포하는 백악기 능주분지의 기원지

Abstract

The Neungju Basin is a Cretaceous nonmarine basin located in the southwestern Korean Peninsula. Basinfill of the Neungju Basin consists mainly of conglomerate, sandstone, siltstone, and tuff. The strata of the Neungju Group consist of Oyeri Fm., Manweolsan Tuff, Jangdong Fm., Yeonhwari Fm., Yeonsan Fm., Jeokbyeok Tuff, and Ongam Conglomerate up sequence. The Jangdong Tuff, one of the strata comprising the Neungju Group, bears dinosaur tracks and its depositional environment is interpreted to be a lake margin. However, sedimentological characteristics of the whole Neungju Basin have not been studied yet. Thus, we carried out petrography and detrital zircon geochronology analyses on the Neungju Basin sedimentary rocks to constrain their depositional periods and provenance. Petrographic analysis on the Neungju Basin sediments reveals that the sandstones and matrices of the conglomerate are comprised mainly by angular grains of quartz, feldspar, and metasedimentary and volcanic rock fragments. The conglomerates are generally matrix-supported and contain angular clasts such as quartzite, schist, granite, and tuff. The amount of rock fragments increase up sequence. Zircon grains in tuffs in the Neungju Basin show ages of 96~88 Ma and indicate depositional period of the basin. Conglomerates and sandstones mainly contain detrital zircons of Jurassic (187 Ma) and Paleoproterozoic age (1853 Ma), but no Cretaceous zircons. Oyeri and Yeonsan Fm., located in the northernmost part of the basin contain Jurassic (187 Ma) and Paleoproterozoic zircon grains (1854 Ma). Jeokbyeok Tuff and Ongam Conglomerate, located in eastern of the basin contain only detrital zircons of Paleoproterozoic (1848~2476 Ma). Jandong Fm., located in southwestern of the basin mainly contains Jurassic zircons (187 Ma). Deposition of the Neungju Basin was begun at about 96~94 Ma, earlier than the eruption time of the Mudeungsan Tuff (86~84 Ma) covered the Neungju Basin sediments except for the Ongam Conglomerate. Sediments comprising clastic rocks of the Neungju Basin were mainly supplied from nearby basement rocks such as Jurassic granite, Paleozoic metasediments, and Paleoproterozoic gneiss. Considering that the clastic sedimentary rocks do not contain Cretaceous zircons, the pyroclastic material was supplied intermittently from the southwest of the basin during the deposition of the Basin.