See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/282604525 Geological Structures and Tectonic Reconstruction of Luwuk, East Sulawesi Conference Paper · May 2014 DOI: 10.13140/RG.2.1.3986.5042 CITATIONS READS 0 1,159 3 authors, including: Salahuddin Husein Universitas Gadjah Mada 98 PUBLICATIONS 72 CITATIONS SEE PROFILE Some of the authors of this publication are also working on these related projects: Sunda Oroclines View project stimation of S-wave Velocity Structure for Sedimentary Layered Media Using Microtremor Array Measurements in Palu City, Indonesia View project All content following this page was uploaded by Salahuddin Husein on 06 October 2015. The user has requested enhancement of the downloaded file. IPA 14-G-137 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Eighth Annual Convention & Exhibition, May 2014 GEOLOGICAL STRUCTURES AND TECTONIC RECONSTRUCTION OF LUWUK, EAST SULAWESI Salahuddin Husein* Moch. Indra Novian* Didit Hadi Barianto* ABSTRACT1 exploration is a revision of Luwuk thrust-fold belt model, which is proposed to be thin-skinned Luwuk at the eastern end of Sulawesi’s East Arm (basement uninvolved) northwestward vergeance. has been recognized to be formed under obductional tectonic where the East Sulawesi Ophiolite INTRODUCTION Complex thrusted southward over the Banggai-Sula microcontinent during Late Neogene event. At the Tectonic of eastern Indonesia is built upon southern section, a gas field of Tomori has been convergence history between three major developed with reservoirs distributed in the pre- lithospheric plates, i.e. the Australian Plate, the collisional Miocene carbonates. To date, Pacific Plate and the Eurasian Plate. One of the key explorational work over the area has been area in understanding those plate interaction and its incorporating collisional structures such as thrusts impact to geology and hydrocarbon resources is the and wrench faults into the petroleum systems, e.g. East Arm of Sulawesi, where collision has been hydrocarbon maturation and trap formation. Most of taking place since the Late Cenozoic between the the structural concept based on the existing model North Sulawesi island arc and the Banggai-Sula of southeastward vergeance thick-skinned microcontinent (Figures 1 and 2). (ophiolitic basement involved) thrust-fold belt. The East Arm of Sulawesi comprises a thrust-fold A geologic field work has been conducted belt, consisting of disrupted and tectonized transecting north-south section and east-west ophiolites, associated with Mesozoic to Cenozoic section of Luwuk. Satellite imagery interpretation oceanic sediments (Rusmana et al., 1993; Surono et and field observation suggest the existance of three al., 1993). This belt is flanked by a cover of structural compartments, i.e.: (i) the northern Neogene clastics, dominantly Pliocene sands and extensional, block-faulting, tectonic regime on the gravels, known as the Sulawesi Molasse. The thrust ophiolite complex, (ii) the central compressional, zone, called the Batui Thrust (Silver et al., 1983; northwestward vergeance thrust-fold belt, tectonic Garrard et al., 1988), separates the ophiolites from regime on the Tertiary carbonates, and (iii) the the sedimentary formations with the NW block as southern, locally gravitational sliding, tectonic upthrown part, suggesting a SE-vergent thick- regime on the Pliocene molasse. Each compartment skinned (basement-involved) thrust-fold belt exhibits distinct structural style and diverse tectonic (Figure 2). The Batui Thrust is also observed to be control. continued further eastward offshore (Watkinson et The northern extensional compartment were al., 2011). controlled by rifting of Siuna Bay along the Late- Miocene right-lateral movement of Balantak Fault. Luwuk is a coastal town near the eastern tip of The central compressional compartment were Sulawesi's east arm. This area is situated in the developed by the Late Pliocene northwestward central portion of the arcuate thrust-fold belt where shortening due to activation of Sorong Fault. The Quaternary reefs was utmost elevated. southern gravitational Molasse sliding were induced by the Late Pliocene uplift along the existing weak It is speculated that the area had suffered maximum zone of north-south transtensional strike-slip fault. compression of Banggai-Sula microcontinent, as it Implication of this research to petroleum is located at the frontal collisional zone. The distance to Peleng Island, the westernmost front of Banggai-Sula microcontinent, is just about 30 * University of Gadjah Mada kilometres as separated by a narrow Peleng Strait. the major fault zone, where kinematic analysis were The elevation of Luwuk uplifted reef terraces later performed in the laboratory. gradually decreases towards the east tip of the peninsula and towards the southwest of Batui Town GEOLOGICAL SETTING (Rusmana et al., 1993a; Surono et al., 1993). Beyond Batui the Quaternary coastal development Luwuk is considered to be located in a collisional along the Gulf of Tolo is governed by subsidence, belt between the Banggai-Sula microcontinent and as indicated by the presence of a vast, swampy the East Sulawesi Ophiolite complex. The Banggai- coastal plain and drowned reef platforms offshore. Sula microcontinent is interpreted to be originated from western Papua (Watkinson et al., 2011) and Southwest from Batui down to Tomori Embayment had been drifted westwards along the Sorong Fault along the shoreline, a major Tomori PSC gas field (Garrard et al., 1988). Initial extensional phase on was explored and operated since mid 1980s mid Mesozoic as represented by a transgressive (Davies, 1990). Hydrocarbon exploration in this phase of continental to shallow marine Jurassic area was supported by the above-mentioned tectonic clastics was followed by passive margin concept, which was mostly derived from geologic sedimentation through Cretaceous and early observation in Luwuk area. This paper attempts to Tertiary during its drift. examine the existing concept, as a new finding during field mapping in crossing the East Arm of A major regional unconformity separated the Sulawesi, from Luwuk to the north to Pagimana. Mesozoic sequences with deposition of Oligo- Miocene carbonate. The collision of Banggai-Sula METHODOLOGY with Sulawesi probably took place from Middle Miocene (Garrard et al., 1988). It is commonly This paper presents result of geologic mapping assummed that the collision responsible for conducted by Department of Geological overthrusting the East Sulawesi Ophiolite over the Engineering Universitas Gadjah Mada (UGM) with Banggai-Sula leading edge. This compressive Pusat Survey Geologi (PSG, Center for Geological period further have uplifted the eastern arm of Survey). Particular research stages have been Sulawesi, producing molasse deposition to its developed as: (i) desk study and satellite imagery surrounding lower areas. This uplift is still taking analysis which produced tentative geologic map, (ii) place, as indicated by earthquakes and tremors field mapping where stratigraphic and structural (Watkinson et al., 2011). However, some Plio- data had been collected along selected key Pleistocene normal faulting were observed, and traverses, (iii) laboratory examination incorporated tought to be caused by relaxation of the earlier petrographic - paleontologic analysis and structural compressive phase (Watkinson et al., 2011). data analysis, and (iv) construction of geologic map and tectonostratigraphic synthesis. However, since The collision of Banggai-Sula and obduction of this paper focuses on tectonic reconstruction, East Sulawesi Ophiolite were assummed to be presentation and discussion on stratigraphic data are accommodated by the Batui Thrust zone, located just adequately given in supporting structural north of Luwuk (Watkinson et al., 2011). They aspects. formed an arcuate thrust front eastwards towards Balantak, and imbricated further southeast, had Satellite imagery used in this work was a composite caused this fault zone interpreted as SE-vergence image of Landsat and Digital Elevation Model (van Bemmelen, 1949; Hamilton, 1979). Based on (DEM), and several physiographic unit and major seismic and gravity data, Silver et al. (1983) structural elements could undoubtedly be suggested that the Batui Thrust extended offshore to established. These physiographic delineation were the east. However, with reexamination on new and intensively used in determination of lithologic unit better seismic dataset of similar line, Watkinson et and their coverage limit. Field traverse consist of al. (2011) had doubted their interpretation of a one major north-south transect from Luwuk in the north-dipping thrust plane, and suggested new south to Poh in the north, perpendicular to the East south-dipping normal faults developed in Arm structural trend and covering most of the extensional system. Another seismic line yet lithologic unit (Figure 3). Another east-west suggested the Batui Thrust probably, though poorly segmented transect was added in the northern area understood, as a north-vergent thin-skinned thrust to cover extention of those major structural system (Watkinson et al., 2011). More amusingly, elements. Structural data collected in the field these north-vergent thrusts were overlying by mostly composed of abundant minor faults along undisturbed marine sediments, which expanding contradiction to arguments on south-vergent Batui intercalated with well-bedded reddish chert. Its Thrust