IPA11-G-054 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Fifth Annual Convention & Exhibition, May 2011 STRATIGRAPHY AND SEDIMENT PROVENANCE, BARITO BASIN, SOUTHEAST KALIMANTAN Duncan Witts* Robert Hall* Robert J. Morley** Marcelle K. BouDagher-Fadel*** ABSTRACT The Barito Basin is located in southeast Kalimantan. It contains a thick Cenozoic sedimentary succession that overlies basement rocks of Paleocene and older age. This paper presents a revised stratigraphy and depositional model for the basin and identifies sediment source areas, based on new lithostratigraphic, biostratigraphic, petrographic and paleocurrent data collected as part of a field-based study. The oldest sedimentary rocks of the Barito Basin succession are assigned to the Tanjung Formation. They include conglomerates, sandstones, siltstones, mudstones, limestones and coal, deposited in a fluvio-tidal coastal plain to marginal marine setting. Palynomorph assemblages indicate deposition began in the late Middle Eocene and foraminifera show that it continued until latest Early Oligocene. During this time, sediment was being sourced from the west and southwest. The Tanjung Formation is overlain by the Montalat Formation in the north and the Berai Formation in the south. These are laterally equivalent in age and were deposited in marginal fluvio-deltaic to fully marine conditions respectively. Foraminiferal assemblages indicate this phase of deposition continued until the Early Miocene. The Warukin Formation overlies these formations, and includes limestones, mudstones, siltstones, sandstones and lignites deposited in a marginal marine to fluviodeltaic setting. Palynomorph assemblages date the top of the formation as Late Miocene. Palaeocurrent data indicate sediment was being transported from the west for the oldest part of the formation, and partly from the east for the younger coal-bearing sequences. It is suggested that this reversal in palaeoflow records uplift of the Meratus Mountains. * Royal Holloway University of London ** Palynova Limited *** University College London INTRODUCTION The Barito Basin is located in southeast Kalimantan, Borneo. The basin contains a thick succession of sedimentary rocks that are well exposed along the eastern margin of the basin (Fig. 1). The basin is bound to the west by the Schwaner Complex, comprising poorly dated regionally and contact metamorphosed rocks and Cretaceous granitic plutons and volcanic rocks. The northern margin is defined by the ‘Cross Barito High’ (Moss et al., 1997), an onshore continuation of the NW-SE- trending Adang fault zone. This separates the Barito Basin from the Kutai Basin to the north. Bounding the Barito Basin to the east is the Meratus Complex. This forms a NE-SW-trending belt of uplifted ophiolitic, subduction-related metamorphic and arc- type rocks ranging in age from Jurassic to Cretaceous (Wakita et al., 1998). The Meratus Complex is interpreted to record a phase of collision and accretion along the southern margin of Sundaland during the mid Cretaceous, and now separates the Barito Basin from the smaller Asem- Asem Basin and the Paternoster Platform to the east. The stratigraphic similarity between these areas suggests they were once connected, forming a single depocentre throughout much of the Paleogene and Early Neogene, prior to the uplift of the Meratus Complex. A number of models have been proposed to explain the evolution of the Barito Basin, largely developed from hydrocarbon exploration. However, due to the limited number of biostratigraphic analyses and scarcity of age-diagnostic fossils, the sedimentary succession has, until this study been poorly dated. Also, there are no published studies investigating the provenance of the sandstones. Consequently, the sediment source areas have never been identified although the Schwaner Complex is often suggested as the sediment source during the Paleogene (e.g. Rose & Hartono, 1978; Hamilton, 1979; Siregar &