PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Ninth Annual Convention & Exhibition, October 2003

IPA03-G-036 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Twenty-Ninth Annual Convention & Exhibition, October 2003 THE CENOZOIC GEOLOGY OF THE LARIANG AND KARAMA REGIONS, WESTERN SULAWESI: NEW INSIGHT INTO THE EVOLUTION OF THE MAKASSAR STRAITS REGION Stephen J Calvert* Robert Hall** ABSTRACT Orogenic deformation, a regional unconformity and input of orogenic sediment are Pliocene. Deformation The Lariang and Karama regions of Western offshore Western Sulawesi dates from the Pliocene Sulawesi, an area of approximately 10,000 km2, were whereas deformation offshore Eastern Kalimantan the subject of a field-based investigation with the aim dates from the Early Miocene. of understanding the Cenozoic evolution of the eastern Makassar Straits. Western Sulawesi was INTRODUCTION influenced by the development of the Makassar Straits to the west, and the collision of continental, The earliest known exploration in the Lariang and ophiolitic and island arc fragments to the east. The Karama regions was in the years 1898-1900 when the timing of these events has been the subject of Doda Oil Company drilled about four wells in the considerable debate and it has been suggested that southern part of the Lariang region. The company’s Neogene collisions in Sulawesi caused inversion in failure to find commercial quantities of hydrocarbons Borneo. A new stratigraphy for the Lariang and led to its liquidation and no further interest was Karama regions of Western Sulawesi, based on shown in the area until the arrival of BPM geologists fieldwork, is presented here and provides new and from Balikpapan in 1927. One hundred years later we significant insights into the evolution of the Makassar find ourselves in a comparable situation. Since the Straits region. The oldest sediments are non-marine 1970’s several oil companies have investigated the and could be as old as Paleocene; they include coals, region only to conclude that the area contains no sandstones and mudstones. Rifting had started by the commercial quantities of hydrocarbons. However, Middle Eocene and continued into the Late Eocene. now that deeper water areas of the west Makassar Eocene sediments were deposited in graben and half Straits are being explored the need to understand graben in both marine and marginal marine potential hydrocarbon systems towards the east at the environments. The Eocene Makassar Straits rift was conjugate margin cannot be ignored. highly asymmetrical; the Kalimantan margin was approximately twice the width of the Sulawesi This paper presents the findings of a Ph.D. study margin. Thermal subsidence had started by the Late (Calvert, 2000a) and the stratigraphy presented Eocene and by the end of the Oligocene most of replaces that of a previous IPA paper (Calvert, Western Sulawesi was an area of shelf carbonate and 2000b). The overall aims of the study were: to map mudstone deposition. Carbonates and mudstones were and establish the stratigraphy of the Lariang and deposited throughout the Early Miocene and in places Karama regions of Western Sulawesi (Figures 1 until the Middle or Late Miocene. During the and 2); to make palaeogeographic interpretations of Pliocene the character of sedimentation changed the Cenozoic sequences (Figure 3) and to determine significantly. Uplift and erosion was followed by the the effects on Western Sulawesi of Paleogene rifting deposition of coarse clastics derived from an orogenic in the Makassar Straits and Neogene collision events belt to the east of the study area. Early Miocene in Eastern Sulawesi. collisions to the east had little effect on Western Sulawesi. * Midland Valley Exploration Limited, U.K. Four months fieldwork in 1996 and 1997 involved ** SE Asia Research Group, Royal Holloway, University of London detailed logging, field mapping, and the collection of 550 rock samples. Data were collected from a series STRATIGRAPHY of logging roads in the deforested coastal lowlands, and from river traverses into the mountainous jungle A summary of the stratigraphy follows. Key place interior. Samples were dated using nannofossils, names are shown on Figures 1 and 2. planktonic foraminifera and pollen (courtesy of SE Asia Research Group, UCL and LEMIGAS). Remote Mesozoic Basement sensing data (courtesy of PT North Mining, PT Rio Tinto, Caltex and Chevron) have been interpreted and The Mesozoic basement in the Lariang and Karama integrated with the field and biostratigraphic data to regions consists of metamorphic rocks unconformably produce a new geological map of the region (Figure overlain by less deformed Upper Cretaceous dark 2) with a significant revision of the pre-existing shales and volcanic rocks. This basement crops out stratigraphies of the study area (Hadiwijoyo et al., mainly in the highlands where there are peaks up to 1993; Ratman and Atmawinata, 1993; Sukamto, 3000m high. Previous workers (Hadiwijoyo et al., 1973). Detailed sedimentary facies analyses combined 1993; Ratman and Atmawinata, 1993; and Sukamto, with information from the microfossil assemblages 1973) made comparisons between the Mesozoic of allowed interpretation of Cenozoic environments. central Western Sulawesi and equivalent basement to Thin section analysis identified provenance trends in the south described by Djuri and Sudjatmiko (1974) the Neogene sequences. The new map (simplified in and Sukamto (1982). The Cretaceous dark shales Figure 2) along with interpretation of onshore seismic observed in this study are similar to those described data (courtesy of Caltex), has enabled a structural by van Leeuwen (1981) and are therefore considered interpretation of the region. laterally equivalent. These rocks are interpreted to be the deposits of a forearc basin situated to the west of a Reasons for this study west-dipping subduction zone (Hasan, 1991). At the beginning of this study it was clear that the The Mesozoic rocks in the South Arm of Sulawesi geology of the Lariang and Karama regions of have affinities with Mesozoic rocks in Eastern Western Sulawesi was poorly known yet should offer Kalimantan (Hamilton, 1979; Hasan, 1990; Moss and significant insights into the evolution of the Makassar Chambers, 1999; Parkinson, 1991; Sikumbang, 1990; Straits area. van Leeuwen, 1981). Palaeomagnetic data from both areas is also similar (Fuller et al., 1999). These are Several models have been proposed for the evolution two lines of evidence that suggest Western Sulawesi of the region and are driven by different insights from and Eastern Kalimantan were positioned closer both Eastern Kalimantan and Eastern Sulawesi (Ali et together prior to the Cenozoic. al., 1996; Bergman et al., 1996; Charlton, 2000; Guntoro, 1999; Parkinson, 1998; McClay et al., 2000; Simandjuntak and Barber, 1996; Walpersdorf et al., The Toraja Group 1998). These models have never been compared or amalgamated with one other and data from the critical The Middle Eocene to lower Upper Oligocene Toraja area in the middle (the Lariang and Karama regions) Group (new name) is exposed between the northern has never been included in them. Cenozoic Quarles Mountains and the northeastern Molengraaff reconstructions of the Eastern Makassar Straits / Mountains. The Toraja Group rests unconformably on Western Sulawesi area have assumed that it was Upper Cretaceous rocks. Although the upper contact affected firstly by Paleogene extension, which with the Neogene Lisu Formation was not seen it is eventually led to the formation of the Makassar believed to be conformable. The group is separated Straits, and secondly by Neogene contraction and into two formations: the thicker marginal marine / uplift driven principally from the east. Therefore, the terrestrial sedimentary rocks of the Kalumpang study area would seem to be in an ideal location to Formation and the more extensive marine observe the effects of Paleogene extension and is sedimentary rocks of the Budungbudung Formation. directly in front of a postulated Neogene orogenic The Toraja Formation 70 km to the south as originally belt. The investigation of the geology of this region defined by Djuri and Sudjatmiko (1974) is quite was therefore carried out to help constrain the timing similar to the Budungbudung and Kalumpang of Cenozoic tectonic events in this part of SE Asia. Formations. Therefore the Toraja Formation of 502 Ratman and Atmawinata (1993) has been elevated to by mudstone. This lithofacies change is interpreted as group status in recognition of this similarity. marking a relative rise in sea level. Lisu Formation a. Kalumpang Formation The Lisu Formation (new name) is a sequence of The Middle to Upper Eocene Kalumpang Formation interbedded mudstone, greywacke and pebbly is a sequence of shales, coal beds and metre thick greywacke. The type locality of the Lisu Formation is quartzose sandstones that outcrop in the northern in the Budungbudung River (Figure 2). In 1997 the Quarles Mountains. The type locality of the type locality was reached by a logging road up to the Kalumpang Formation is in the Karama River (Figure Lisu tributary and by boat along the Budungbudung 2). The formation is approximately 3200m thick and River. However, in 2001 the logging road was unconformable on Upper Cretaceous rocks. This overgrown and a series of rapids on the river formation is thought to be analogous to the prevented access. The formation is approximately ‘Coalfields’ that were first described in this region by 2000m thick and has a late Early Miocene to Early Reyzer (1920). The formation passes laterally into Pliocene age based on nannofossils

Load more