IPA12-G-164 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Sixth Annual Convention & Exhibition, May 2012 THE BOUNDARY CHARACTER OF PRE-TERTIARY AND TERTIARY ROCKS IN THE SOUTHERN END OF THE NORTH SUMATRA BASIN: BARISAN MOUNTAIN THRUST FRONT? Andang Bachtiar* Muharam Jaya Panguriseng** Hikhmadhan Gultaf* Yudi Satria Purnama* ABSTRACT 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 INTRODUCTION The Sumatran Fault is a trench-parallel strike-slip fault system that works in concert with subduction zones to accommodate obliquely convergent plate motion (Yeats et al, 1997). Splays of the Sumatran Fault are prevalent in the northeastern part of the onshore North Sumatra Basin, Conventional * PT. Exploration Think Tank Indonesia ** PT. Pertamina EP wisdom considers fault splays of the Sumatran Fault in the western margin of the North Sumatra Basin to be delineating lateral boundaries of Pre-Tertiary rock and Tertiary rock, such as described in the Barisan Moutain Thrust Front (Andreason et al, 1997; Musgrove and Sunaryo, 1998, Figure 1). The purpose of this paper is to re-examine the interpretation of a thrust boundary condition between the Pre-Tertiary and Tertiary rocks in the western margin of the southern end of the North Sumatra Basin. In this context, the study area covers the cities of Medan and Kutacane in the south, and Langsa and Lokop in the north (Figure 2). The stratigraphy of the study area (Figure 3) from older to younger rocks is composed of:  Outcrops of Pre-Tertiary rocks in the eastern part) of the study area are composed of Bohorok Formation pebbly mudstone (Early to Middle Carboniferous), Batumilmil Formation limestone (Middle Permian to Triassic), limestone and dolomites of the Kaloi Formation (Permian to Triassic), slate and phyllites of the Kluet Formation (prob. Carboniferous to Early Permian), the Situptup Formation limestones (Middle Permian to Late Triassic), and Ujeuen Formation limestones (prob. Late Permian to Triassic).  Early Oligocene Bruksah Formation composed of conglomerate  Early to Late Oligocene Bampo Formation composed of shale  Early Miocene Peutu limestone interfingering with sandstone of the Belumai Formation.  Middle to Late Miocene Baong Formation, shales.  Late Miocene to Early Pliocene Keutapang Formation sandstones .  Plio-Pleistocene Seurula Formation sandstones. Back to Session