A Petrological Study of Columnar Joints with Geoheritage Value in the Cenozoic Sedimentary Basin, Korea

Abstract

A comparative petrological study was conducted on the columnar joints of Guryongpo, Pohang, Yangnam, Gyeongju, and Gangdong-hwaam, Ulsan in the Cenozoic Miocene sedimentary basin. Horizontal columnar joints, in these columnar joints, are characteristically developed, and the cross-sectional shape of each columnar joint is predominantly quadrilateral and pentagonal in Guryongpo and Yangnam columnar joints, and pentagonal to hexagonal in Gangdong-hwaam columnar joint. As a result of XRF analysis, it was confirmed that Guryongpo columnar joint is andesite, Yangnam columnar joint is basalt, and Gangdong-hwaam columnar joint is basaltic andesite, all of which are sub-alkaline. They are easily distinguished because Guryongpo columnar joint contains small-sized plagioclase (1∼2 mm), while rather large plagioclase phenocrysts (several mm∼1 cm) are uniformly distributed both in Yangnam and Gangdong-hwaam columnar joints. The value of whole rock magnetic susceptibility (WRMS) are also well classified according to the rock composition. In addition, in the tectonic discriminant diagram, these columnar joints correspond to the orogeny zone and island-arc tholeiite regions of the orogeny, but the AFM diagram and WRMS analysis show that each columnar joint was formed from the different magmas.