Paleomagnetism, Stratigraphy and Geologic Structure of the Tertiary Pohang and Janggi Basins ; Geologic Structure in the Areas of Heunghae and Hyungsan River by Gravity Prospecting Method

포항 및 장기분지에 대한 고지자기, 층서 및 구조 연구; 중력탐사에 의한 홍해 및 형산강지역의 지질구조

The gravity measurement has been conducted at 327 station with an interval of 25 m along the survey lines of 1.6 km and 1.7 km traversing Hyungsan river and of 2.35 km and 2.42 km running N-S direction near Heunghae-eup in Pohang basin. Bouguer gravity anomalies were obtained, and geologic structure along four survey lines were interpreted by applying Fourier series and Talwani methods for two demensional body. A fault is in existence along the Hyungsan river, and northern block of it is displaced down by 150 m to 200 m relative to southern one. The thicknesses of Yeonil Group vary from 250 m to 550 m and from 150 m to 300 m in the northern and southern blocks of the fault, respectively. Another fault is in existence running E-W direction near Heunghae-eup, and its southern block is displaced down by about 250 m relative to its northern block. The thicknesses of Yeonil Group vary from 200 m to 400 m and from 500 m to 700 m in the southern and northern blocks of the fault, respectively. Above two faults are normal faults and make a graben structure, which results the age of rocks in the central region between the faults is younger than those of outside regions. This result coincides with that of paleontological study.