Bimodal Volcanism and Classification of the Miocene Basin Fill in the Northern Area of the Janggi-myeon, Pohang, Southeast Korea

In order to determine the new criteria on classification of the Miocene basin fill in the northern area of the Janggi-myeon and to establish its stratigraphy, the structural characteristics and petrographic and geochemical features of the volcanic materials, with the detailed mapping. were synthetically analyzed. There was a bimodal volcanism during the Early Miocene in the study area. As a result, the basin fill are composed of the earlier strata bearing abundant dacitic volcanic materials and the later strata bearing abundant basaltic volcanic materials. The earlier strata arc divided into the Janggi Conglomerate and the Seongdongri Formation, which are overlain by the Noeseongsan Basaltic rocks of the later strata. The Janggi Conglomerate consists mainly of conglomerates composing of angular to sub-angular clasts derived from adjacent basements, which are intercalated with tuffaceous (dacitic) pebbly sandstone and sandstone. The Seongdongri Formation, overlying conformably the Janggi Conglomerate, is composed of at least four dacitic lapilli tuffs and interbedded tuffaceous sediments. All the dacitic tuffs are similarly composed of welded pumices, exotic lithic fragments, and crystal fragments of quartz, plagioclase, and biotite with a number of carbonized plant materials. On the other hand, the Noeseongsan Basaltic rocks, distributed broadly in the highlands of the study area, show diverse extrusive and intrusive occurrences. which indicate various depositional and emplacement environments. Owing to the large amount of the overburdened basaltic materials, the stratal attitudes of the Seongdongri Formation have been disturbed. The results above mentioned are concordant with the biomodal volcanism and the stratigraphy reported in the adjacent Miocene basins, and they seem to become a significant scientific basis for the more rational agreement about the stratigraphy of the Early Miocene strata in southeast Korea.