THE BOUNDARY CHARACTER OF PRE-TERTIARY AND TERTIARY ROCKS IN THE SOUTHERN END OF THE NORTH SUMATRA BASIN: BARISAN MOUNTAIN THRUST FRONT?

摘要

Traditional perception considers the Sumatran splay fault act as boundary of Pre-Tertiary rock and Tertiary rock where Pre-Tertiary rock is thrusted (by thrust fault) upon the Tertiary rock, called 'the Barisan Mountain Thrust Front'. The purpose of this paper is to understand whether the boundary character of Pre-Tertiary and Tertiary rocks in the western margin of the southern end of the North Sumatra Basin is thrust fault boundary or other kind. Method used in this study consists of examination of geological cross section on published geological maps, 2D seismic interpretations, and field observation in Batang Sarangan River, nearby the seismic lines area. The fieldwork result indicates outcrops of Belumai Formation (Tertiary rocks) near the seismic lines. The seismic sections also indicate there is no thrust fault geometry. Moreover, those sections indicate onlap of the Tertiary Rocks onto the Pre-Tertiary Rocks. It is concluded the boundary character of Pre-Tertiary and Tertiary rocks in the western margin of the southern end of the North Sumatra Basin is not necessary as a thrust fault. Hence, the perception of the boundary of Pre-Tertiary rock with Tertiary rock as thrust fault is questionable