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 INTRODUCTION

The Barito Basin is located in southeast Kalimantan. The Barito Basin is located in southeast Kalimantan, It contains a thick sedimentary succession . The basin contains a thick succession of that overlies basement rocks of Paleocene and older sedimentary rocks that are well exposed along the age. This paper presents a revised stratigraphy and eastern margin of the basin (Fig. 1). The basin is depositional model for the basin and identifies bound to the west by the Schwaner Complex, sediment source areas, based on new comprising poorly dated regionally and contact lithostratigraphic, biostratigraphic, petrographic and metamorphosed rocks and Cretaceous granitic paleocurrent data collected as part of a field-based plutons and volcanic rocks. The northern margin is study. defined by the ‘Cross Barito High’ (Moss et al., 1997), an onshore continuation of the NW-SE- The oldest sedimentary rocks of the Barito Basin trending Adang zone. This separates the Barito succession are assigned to the Tanjung Formation. Basin from the Kutai Basin to the north. Bounding They include conglomerates, sandstones, siltstones, the Barito Basin to the east is the Meratus Complex. mudstones, limestones and coal, deposited in a This forms a NE-SW-trending belt of uplifted fluvio-tidal coastal plain to marginal marine setting. ophiolitic, -related metamorphic and arc- Palynomorph assemblages indicate deposition began type rocks ranging in age from Jurassic to in the late Middle and foraminifera show Cretaceous (Wakita et al., 1998). The Meratus that it continued until latest Early . During Complex is interpreted to record a phase of collision this time, sediment was being sourced from the west and accretion along the southern margin of and southwest. The Tanjung Formation is overlain Sundaland during the mid Cretaceous, and now by the Montalat Formation in the north and the separates the Barito Basin from the smaller Asem- Berai Formation in the south. These are laterally Asem Basin and the Paternoster Platform to the east. equivalent in age and were deposited in marginal The stratigraphic similarity between these areas fluvio-deltaic to fully marine conditions suggests they were once connected, forming a single respectively. Foraminiferal assemblages indicate depocentre throughout much of the and this phase of deposition continued until the Early Early , prior to the uplift of the Meratus . The Warukin Formation overlies these Complex. formations, and includes limestones, mudstones, siltstones, sandstones and lignites deposited in a A number of models have been proposed to explain marginal marine to fluviodeltaic setting. the evolution of the Barito Basin, largely developed Palynomorph assemblages date the top of the from hydrocarbon exploration. However, due to the formation as Late Miocene. Palaeocurrent data limited number of biostratigraphic analyses and indicate sediment was being transported from the scarcity of age-diagnostic fossils, the sedimentary west for the oldest part of the formation, and partly succession has, until this study been poorly dated. from the east for the younger coal-bearing Also, there are no published studies investigating the sequences. It is suggested that this reversal in provenance of the sandstones. Consequently, the palaeoflow records uplift of the Meratus Mountains. sediment source areas have never been identified although the Schwaner Complex is often suggested * Royal Holloway University of London as the sediment source during the Paleogene (e.g. ** Palynova Limited *** University College London Rose & Hartono, 1978; Hamilton, 1979; Siregar & Sunaryo, 1980; Courteney et al.1991; van de Weerd grains older than 1000 Ma. Data were processed & Armin, 1992; Satyana et al., 1999). This paper using Isoplot™. A total of 1539 concordant U-Pb presents results from an extensive field-based study ages were obtained. 766 palaeocurrent conducted in the Barito Basin. A revised measurements were collected from dune cross-beds stratigraphy is presented, built on existing within channel sand bars and small-scale ripples. All nomenclature, and better dated using palynology measurements were corrected for structural dip. The and foraminiferal assemblages. Sandstone Rayleigh’s Test for a Preferred Trend was applied to petrography, U-Pb dating of zircons and all datasets. Critical values are given by Mardia palaeocurrent data suggest new interpretations of (1972). Stratigraphic logs, lithofacies analyses, trace sandstone provenance. These new data have fossils, palynomorphs and foraminifera have been significance for hydrocarbon exploration in the used to determine depositional environments of the basin and provide important information on the sedimentary succession. geological evolution of the surrounding region. EOCENE PALYNOLOGICAL ZONATION METHODS Since no published palynological zonation is Palynomorphs and foraminifera have been used to available to characterise the Middle-Late Eocene date the sedimentary succession. Palynological boundary, a reference section was compiled on analysis was conducted by Lemigas in Jakarta. No which a zonation could be based. Due to limited palynological zonation for the Eocene of the Sunda exposures, several profiles from the same area were region has been published, and so this study has joined to form a single reference section containing provided the basis for such a zonation, described in 41 samples. The palynological zones are defined as summary form below. For the Miocene, reference is follows: made to the zonation of Morley (1978, 1991). Foraminifera were analysed at University College Zone E6 - Middle Eocene London by Dr. Marcelle BouDagher-Fadel and Characterised by the presence of the Middle Eocene sediments have been dated using larger foraminifera markers Beaupreadites matsuokae and by reference to the Letter Stage scheme of van der Polygalacidites clarus in an assemblage dominated Vlerk & Umbgrove (1927) as modified by Adams by ‘Indian’ taxa such as Palmaepollenites spp, (1970), BouDagher-Fadel & Banner (1999) and Lanagiopollis spp, Lakiapollis ovatus and BouDagher-Fadel (2008) and planktonic Retistephanocolpites williamsi. All are common to foraminifera by reference to Tourmarkine & abundant in the Middle Eocene Nanggulan Luterbacher (1985) for the Eocene, and Bolli & Formation (Lelono, 2000). Saunders (1985) for the post Eocene. Letter Stages and planktonic foraminiferal zones are correlated in Zone E7 - Late Eocene BouDagher-Fadel (2008). Sandstone provenance Characterised by the first consistent occurrence of was determined from detrital modes and U-Pb Cicatricosisporites dorogensis, and by the absence dating of detrital zircons. Detrital modes were of Meyeripollis nayarkotensis, which ranges from determined from 80 sandstones. Zircons from 17 the base of the overlying zone. sandstone samples for which the stratigraphic age was known were dated at University College Zone E8 - Late Eocene London, using LA-ICPMS. The New Wave 213 Based on the regular presence of Meyeripollis aperture-imaged, frequency-quintupled laser nayarkotensis and the absence of Magnastriatites ablation system (213 nm) was used, coupled to an howardi, which ranges from the base of the Agilent 750 quadrupole-based ICP-MS. Real time overlying zone. data were processed using GLITTER™. Repeated measurements of external zircon standard Plesovic Zone E9 - Late Eocene (reference age determined by thermal ionization Characterised by the overlap of Magnastriatites mass spectrometry (TIMS) of 337.13±0.37 Ma howardi and the Eocene marker Proxapertites (Sláma et al., 2008)) and NIST 612 silicate glass operculatus, which has its top at topmost Eocene in (Pearce et al., 1997) were used to correct for Southeast Asia, India and Africa (Morley, 2000). instrumental mass bias and depth-dependent inter- element fractionation of Pb, Th and U. Data were STRATIGRAPHY filtered using standard discordance tests with a 10% cut-off. The 206Pb/238U ratio was used to determine The sedimentary succession of the Barito Basin ages less than 1000 Ma and the 207Pb/206Pb ratio for unconformably overlies basement rocks of Paleocene and older age (Sikumbang, 1986). The reference to the Florschuetzia meridionalis succession comprises five formations that record a palynological zone. full transgressive to regressive cycle (Fig. 2). The oldest sedimentary rocks are assigned to the The Dahor Formation was not investigated during Tanjung Formation and were deposited in a fluvio- this study. It is reported to overly the Warukin tidal coastal plain to marginal marine environment. Formation and comprises a succession of polymict The formation becomes increasingly marine- fluviatile and shallow marine sedimentary rocks influenced up section. Palynomorph assemblages (Satyana & Silitonga, 1994; Seeley & Senden, 1994; date the base of the formation as late Middle Eocene Satyana, 1995; Gander et al., 2008) derived from the by reference to palynological zone E6 (see Fig. 2). Meratus Complex during the Plio-Pleistocene. The assemblages contain common elements which relate to the Middle Eocene dispersal of plant taxa SANDSTONE COMPOSITION from India (Morley, 1998; Lelono, 2000). The major part of the Tanjung Formation is Late Eocene and Tanjung Formation Early Oligocene. The Late Eocene interval is dated palynologically by reference to the evolutionary Sandstones of the Tanjung Formation are quartz appearances of Cicatricosisporites dorogensis, arenites and sub-litharenites (Folk, 1968) and plot Meyeripollis nayarkotensis and Magnastriatites within the ‘craton interior’ and ‘quartzose recycled’ howardi. The latter taxon is often thought to have fields of Dickinson & Suczek (1979), see Fig. 3. first appeared in the basal Oligocene (Germeraad et They contain mainly angular monocrystalline and al. 1968), but is recorded commonly in the Tanjung rounded polycrystalline quartz grains with minor Formation stratigraphically below well dated Late anhedral feldspars, sub-angular radiolaria-bearing Eocene marine sediments with common planktonics, chert, and lithic fragments. Monocrystalline quartz which include Turborotalia pomeroli, has either simple or slightly undulose extinction Globigerinatheka spp and Hantkenina alabamensis, pattern and typically contains strings or bands of indicating the Late Eocene planktonic zone P15- fluid inclusions, indicating a plutonic origin. P16. The age of the top of the formation is referred Polycrystalline quartz has high angles of undulose to Letter Stage Td (late Early Oligocene) by extinction, more than three crystals per grain (often reference to the overlap of the larger foraminifera showing alignment), bimodal crystal size within a Nummulites fichteli and Eulepidina spp. single grain, and strings of fluid inclusions. These features indicate a metamorphic parentage (Smyth et The Tanjung Formation is overlain by the Berai al., 2008a). Feldspars comprise <1% of the total Formation in the south and the Montalat Formation composition, and are mainly strained plagioclase in the far north of the basin. They are laterally (undulose extinction) suggesting a metamorphic equivalent in age but are lithologically dissimilar. origin or a post-depositional deformation (Passchier The Berai Formation records fully marine & Trouw, 2005). Most of the lithic fragments are conditions, and is characterised by shallow water schistose. platform carbonate rocks. The Montalat Formation records marginal marine to braid delta deposition Montalat Formation and extends across the Barito/Kutai divide. The base of the Berai Formation has been referred to Te1 to Sandstones from the Montalat Formation are quartz lower Te5 Letter Stages (planktonic zone P21-N4) arenites, sub-arkoses and minor sub-litharenites based on the presence of Heterostegina borneensis (Folk, 1968). The Dickinson & Suczek (1979) and association with overlying samples ternary plots suggest a ‘craton interior’ provenance (BouDagher-Fadel, 2008). (Fig. 3). The sandstones are composed of angular monocrystalline and rounded polycrystalline quartz The Warukin Formation overlies the Berai and grains with minor anhedral feldspars and lithic Montalat Formations. It records a return to shallow fragments. Monocrystalline quartz grains have marine and then terrestrial, fluvio-deltaic conditions. simple or slightly undulose extinction pattern and The base of the formation shows distinct marine contain characteristics of a plutonic origin, or are influence and can be referred to upper Te5 to middle inclusion-free, exceptionally bright in thin section Tf1 Letter Stages (planktonic zone N6-N8) based on and have sharp extinction, suggesting a volcanic the presence of Miogypsinodella sp., Miogypsina parentage (Smyth et al., 2008a). Polycrystalline spp., and L. (N) brouweri and association with quartz has features indicative of a metamorphic underlying samples (BouDagher-Fadel, 2008). The origin. Feldspars comprise 2.4% of the total top of the formation is older than 7.4 Ma based on composition. These are mainly strained and unstrained plagioclase with minor K-feldspar sediment was being transported towards the indicating a predominately metamorphic parentage northwest. Palaeocurrent data from the Barabai with minor input from an acid plutonic source. Member of the Warukin Formation indicate Lithic grains include schist and igneous rock sediment was transported towards the east-southeast. fragments. This continued into the lower part of the Tapin Member. A change is recorded from the top of the Warukin Formation Tapin Member where palaeocurrent data indicate sediment was being transported towards the west. Sandstones of the Warukin Formation are quartz arenites, and rare sub-arkoses (Folk, 1968) and plot GEOCHRONOLOGY OF DETRITAL mainly within the ‘quartzose recycled’ field of ZIRCONS Dickinson & Suczek (1979), see Fig. 3. They contain a mixture of angular and rounded Tanjung Formation monocrystalline quartz and rounded polycrystalline quartz grains with minor amounts of anhedral and Detrital zircons were analysed from seven samples rounded feldspar, rounded clasts of radiolaria- collected from the Mangkook and Tambak Members bearing chert and lithic fragments. Monocrystalline of the formation. A total of 656 concordant U-Pb quartz grains contain features suggestive of plutonic ages were obtained. Ages range from Neoarchean to and volcanic origin. A number of rounded grains Cretaceous (Fig. 7). The most prominent with diagenetic quartz overgrowths were identified, populations are Cretaceous, of which 32% are Early implying multiple recycling. Most of the Cretaceous (140±5.7 Ma to 99.8±6.1 Ma) and 62% polycrystalline quartz grains are of probable Late Cretaceous (99.3±4.8 Ma to 70.7±5 Ma), and metamorphic origin. Feldspars comprise 1% of the Devonian- Carboniferous (415.8±12.6 Ma to total composition and are mainly strained 300.5±5.1 Ma) with smaller Permo-Triassic plagioclase and K-feldspar, suggesting metamorphic (295.2±10.5 Ma to 205.7±7.6 Ma), Ordovician - and plutonic provenance. Lithic fragments are Silurian (484.1±15.7 Ma to 417.6±13.2 Ma) and mainly composed of schistose material. Proterozoic (2496.7±22 Ma to 544.8±16 Ma) populations. Jurassic and Archean grains are rare. All the sandstones of the three formations are compositionally mature, yet texturally immature Montalat Formation which is relatively unusual. Tropical alteration can significantly alter the composition of sandstone by Detrital zircons were analysed from two samples, the systematic destruction of unstable lithic one from each member of the formation. A total of fragments and feldspars during transport, deposition 177 concordant U-Pb ages were obtained. Ages and storage. Borneo has been situated within from the Bentot Member are bimodally distributed, tropical latitudes and climate since the Mesozoic comprising Cretaceous (122.9±9.3 Ma to 66.9±19.9 and we believe the apparent discordance between Ma) and Proterozoic (2426.2±26.6 Ma to 906.4±44 compositional and textural maturity has been Ma) populations. The Cretaceous population is produced by intense tropical processes. The standard dominated by Early Cretaceous zircons (122.9±9.3 plots of detrital modes may therefore mislead in Ma to 99.7±21.7 Ma), and the Proterozoic identifying provenance because they were population is dominated by Paleoproterozoic grains developed in mainly non-tropical settings. (2426.2±26.6 Ma to 1600.8±26.3 Ma). Ages obtained from the Kiwa Member range from PALAEOCURRENT ANALYSIS Cretaceous to Proterozoic. The most prominent population is Cretaceous (122.6±21.8 Ma to Palaeocurrent data for the Tanjung, Montalat and 76.6±19.5 Ma). 79% of which are Late Cretaceous. Warukin Formations are shown in Figs 4 to 6. The There is a small Devonian- Carboniferous data indicate that during the deposition of the population (398.6±30.5 Ma to 313.2±26.7 Ma), but Tambak Member of the Tanjung Formation (which zircons of other ages are rare. accounts for approximately 80% of the formation), sediment was transported by rivers towards the Warukin Formation north. Orientations of small-scale ripples within the tidal facies indicate a SW-directed tidal flood. This Detrital zircons from seven samples collected from implies the coastline was located towards the the Warukin Formation were analysed. Both northeast. Palaeocurrent measurements from the members were represented. A total of 492 Kiwa Member of the Montalat Formation indicate concordant U-Pb ages were obtained, ranging from Mesoarchean to Paleogene. The largest populations ages from these areas have been compiled. The are Cretaceous (143.9±9.3 Ma to 68.6±5.1 Ma), Lower Cretaceous Schwaner granites range in age Permo-Triassic (295.7±18.5 Ma to 204.1±14.5 Ma) from 130 Ma to 100 Ma (Williams et al., 1988) and and Proterozoic (2493.0±45.1 Ma to 546.8±20.4 form the main pluton in the Schwaner Complex, Ma). Jurassic ages form a small population based on K-Ar analysis of hornblende and biotite. (190.9±18.1 Ma to 145.8±25.6 Ma). Grains of other Zircons from a smaller Upper Cretaceous pluton ages are rare. yield ages of 87 Ma and 80 Ma (van Hattum et al., 2006). The Pinoh Metamorphic rocks form an E-W- ZIRCON MORPHOLOGY trending belt in the north of the Schwaner Complex. Their exact age is unknown. All that is known for Zircon morphology (expressed in terms of width-to- certain is they are older than the Cretaceous granites length ratio and prismatic character) is considered to that intrude them. In the Meratus Complex a Lower reflect the physical and chemical conditions present Cretaceous granite has a K-Ar age of 115 Ma during crystal growth (Schafer & Dorr, 1997). For (Heryanto et al., 1994) and most metamorphic rocks example, elongate or needle-like crystals are have Cretaceous K-Ar ages that range from 140 to commonly associated with rapidly ascending 105 Ma (Parkinson et al., 1998). Wakita et al. magmas, high-level granites and volcanic eruptive (1998) reported 2 Jurassic K-Ar ages of 165 and 180 deposits (Corfu et al., 2003), whilst crystals with a Ma from metamorphic rocks. low width-to-length ratio are produced during slow- cooling of plutonic intrusions. Representative Palaeocurrent data suggest a southern source for the images showing zircon morphology are presented in Tanjung Formation. The Karimunjawa Arch located Fig. 8. Zircon types and their associated ages are to the SW of the Barito Basin (Fig. 1, inset) is described below. reported to have been elevated throughout the Late Eocene to Late Miocene (Bishop, 1980; Smyth et The Tanjung Formation contains euhedral, al., 2008b) and supplying sediment into the East and elongate grains (Cretaceous age); euhedral, non- West Java Basins (Smyth et al., 2008b). It contains elongate grains (Cretaceous and Jurassic ages); abundant exposures of quartz-rich sandstones, euhedral, stubby grains (Cretaceous and Permo- metasedimentary rocks, and quartz- and mica-rich Triassic ages) and well-rounded and angular grain phyllites and schists (Smyth, 2008). Zircons were fragments (all ages from Neoarchean to Cretaceous). analysed from Karimunjawa Island situated in the Zircons from the Montalat Formation include centre of the arch. A total of 186 concordant U-Pb euhedral, elongate grains (Cretaceous and ages were obtained (Fig. 9, top). These range from Proterozoic ages); euhedral, stubby grains Proterozoic to Triassic. The dominant populations (Cretaceous age); non-elongate, mostly rounded are Permo-Triassic (294.9±10.6 Ma to 218.5±5.4 grains (Cretaceous and Triassic-Devonian ages); Ma), Carboniferous-Devonian (401.8±9.3 Ma to angular grain fragments (Cretaceous and Proterozoic 317.3±10.2 Ma) and Proterozoic (2439.7±14.8 Ma ages) and rounded grain fragments (all ages from to 852.1±24.3 Ma). There is also a small population Cretaceous to Proterozoic). Zircons from the of Silurian and rare Ordovician ages. Warukin Formation include elongate grains (Cretaceous, Permo-Triassic and Archean ages); DISCUSSION euhedral, non-elongate grains (Paleogene); euhedral, stubby grains (Cretaceous, Permian and Proterozoic The Barito Basin overlies basement rocks of ages); rounded, non-elongate grains and rounded distinctly different character. To the west is the and angular fragments (all ages from Cretaceous to Schwaner Complex, comprising plutonic, volcanic Mesoarchean). Many of the older, rounded zircons and metamorphic rocks that represent part of the from each formation have etched and pitted pre-Cretaceous continental basement of Sundaland. surfaces, interpreted as features of multiple To the east is the Meratus Complex. It records recycling. suturing along the Sundaland margin in the mid Cretaceous, but its subsequent history is poorly POSSIBLE SOURCE AREAS known due to limited exposures, most of which are located within the Meratus Mountains. The complex It is commonly believed that the Schwaner Complex comprises Jurassic and Cretaceous metamorphic was providing sediment to the Barito Basin area rocks, Cretaceous granitic, ophiolitic and arc rocks during much of the Cenozoic, and the Meratus (Sikumbang, 1986; Wakita et al., 1998). Upper Complex provided additional sediment into the Cretaceous and Paleocene volcanic rocks and basin during the Neogene. Therefore, radiometric coarse-grained clastic sedimentary rocks of the Manunggul Formation (Sikumbang, 1986) are also from these distant areas would require a complicated considered here as part of the basement. and long drainage pattern in conflict with previous provenance studies (e.g. Clements, 2008). A The Tanjung Formation overlies these basement plausible southern source area is the Karimunjawa rocks unconformably. The oldest rocks above the Arch and basement areas beneath the East Java Sea. unconformity are conglomerates and sandstones Hall et al. (2009) and Granath et al. (2011) suggest with palynomorph-bearing mudstone interbeds of this region is underlain by Australian basement, the Mangkook Member. The mudstones are late overlain by a thick Permo-Triassic sequence. This Middle Eocene age. The member records the erosion could have produced the quartz and metamorphic and infilling of irregular basement topography and debris observed in the sandstones, and would also was deposited by debris flows and braided rivers. account for the zircons of Permo-Triassic and older These are overlain by sandstones, siltstones, age. Furthermore, only a simple and relatively short mudstones and coals (Tambak Member) deposited (<300 km) drainage pattern would be needed to across a wide intertidal floodplain constructed by transport sediment northeast from the Karimunjawa rivers flowing towards the north along the present Arch into the Barito Basin area where flow was then western flank of the Meratus Complex. The directed towards the north (Fig. 10). We suggest floodplain extended from the Schwaner Complex in both the Schwaner Complex and the Karimunjawa the west, to the Paternoster Platform in the east, and Arch were supplying sediment to the Barito area from the area now submerged under the Java Sea to from the late Middle Eocene to late Early the northern margin of the present Barito Basin. Oligocene. Overlying the Tambak Member are fine grained sedimentary rocks of the Pagat Member, deposited During Late Oligocene to Early Miocene, extensive in a marginal to shallow marine setting until the late shallow water platform carbonates were deposited Early Oligocene. Subsurface data and surface across much of the Barito Basin area, and deep mapping show the Tanjung Formation thins to the water sediments were being deposited to the north, west, east and south, with the greatest thicknesses in the southern part of the Kutai Basin (Moss & north of Tanjung (Siregar & Sunaryo, 1980). These Chambers, 1999). Across the northern margin of the observations suggest a N-S orientation for the axis Barito area, a large braid delta formed (Kiwa of a wide Eocene depocentre, with the thickest part Member, Montalat Formation) in which deposition of the sequence approximately in the position of the continued into the Early Miocene. Based on this present-day Meratus Mountains. This configuration palaeogeography, the sandstones of the Kiwa suggests that sediment could have been derived Member were likely sourced from the Schwaner from the east, west or south. Complex and/or uplifted sedimentary successions of northwest Borneo (e.g. Rajang-Crocker Group). An eastern source (SW ) would have These would have produced a mixture of texturally provided texturally immature sediment, rich in immature and mature fragments, quartz and lithic- volcanic material. This is not supported by our data. rich, including radiolaria-bearing chert and plutonic In addition, there was mid-Eocene extension and metamorphic debris. The sandstone between the Paternoster Platform and SW Sulawesi, compositions and textural features suggest the and by the Early Oligocene, much of the area to the Schwaner Complex was the dominant source, which east of the basin was below sea level and the site of would also account for the large number of carbonate deposition. A western source (Schwaner Cretaceous zircons. It is unlikely that significant Complex) as previously suggested (e.g. Hamilton, numbers of zircons of older age were derived from 1979) would have supplied texturally immature, the Pinoh Metamorphic Group as these rocks quartz-rich sediment with a significant plutonic typically lack zircons. Alternatively, the small component, and zircons of dominantly Cretaceous number of older grains reflects a less important age. These characteristics are observed in all source to the northwest. Palaeocurrent sandstones of the Tanjung Formation. However, a measurements conflict with this interpretation, but Schwaner source does not explain the zircons older were collected from a relatively small area (<6 km2) than Cretaceous. Older zircons may have been and may record local flow. derived from the Pinoh Metamorphic Group (in the Schwaner Complex). However, this seems unlikely By the Early Miocene, deposition of the Warukin as the rocks are typically zircon-poor, and their Formation had begun, signifying the onset of a position is inconsistent with palaeocurrent data. regression, probably driven by regional tectonic Extensive outcrops of Permo-Triassic granites are changes (e.g. Hutchison et al., 2000; Hall, 2002). present in the Thai-Malay Peninsula, but transport Carbonate deposition was replaced by fluvio-deltaic sedimentation. The Schwaner Complex and • The oldest sedimentary rocks of the Barito Karimunjawa Arch were emergent to the west and Basin are late Middle Eocene. This is south respectively, and marine conditions existed to considerably younger than some previous the east. This palaeogeography suggests sediment estimates (e.g. Campbell & Ardhana, 1988; could have been transported from the northwest, Kusuma & Darin, 1989; Bon et al., 1996). west or south. A northwestern source would be • The sandstones of the Tanjung Formation were expected to supply texturally mature quartz- and probably derived mainly from the Schwaner lithic-rich material and radiolaria-bearing chert Complex in the eas, and the Karimunjawa Arch fragments. Material of this character is present in the southwest. within the Warukin sandstones, and the age spectra • During the Late Oligocene, an extensive braid of zircons are very similar to those of north Borneo delta formed across the northern margin of the (Fig. 9, bottom). The abundance of first cycle Barito Basin area, probably fed by material shed plutonic and volcanic quartz and a prominent from the Schwaner Complex in the west and the Cretaceous zircon population strongly suggests Rajang-Crocker Group in the northwest. Schwaner derivation, and is supported by • By the Early Miocene carbonate deposition was palaeocurrent data. Sediment derived from the replaced by marginal marine to fluvio-deltaic Karimunjawa Arch may be represented by schistose deposition, represented by the Warukin lithic fragments and polycrystalline quartz, but this Formation. The top of the formation is assigned could also have a Pinoh Metamorphic origin. There to Late Miocene. is no evidence that the Meratus Complex was • The sandstones of the Warukin Formation were emergent at this time. mainly derived from the Schwaner Complex and to a lesser extent the Rajang-Crocker Group. By the Late Miocene, the upper part of the Tapin Towards the top of the formation, it is probable Member was being deposited in a brackish, that material was also sourced from the Tanjung fluviatile environment. Emergent areas included the Formation as a result of Meratus uplift. Rajang-Crocker Group, Schwaner Complex, and • Palaeocurrent data suggest uplift of the Meratus possibly the Meratus Complex and Karimunjawa Complex may have begun in the Late Miocene. Arch. The sandstones of the Tapin Member contain many features suggesting a Schwaner Complex and ACKNOWLEDGEMENTS Rajang-Crocker Group provenance. These include their compositions, mixed textural maturities and We thank the following people for their assistance zircon ages. If uplift of the Meratus Complex had and contribution to this study: B. Sapiie from the begun, debris of Meratus basement rocks and Institut Teknologi Banding for his support and help Tanjung Formation would be expected in the in organising fieldwork; A. Rudyawan and Y. sandstones. However, material of Meratus character Sindhu for their valued assistance in the field; for is absent, and despite the abundance of recycled discussions concerning the geology and evolution of material, differentiating between recycled Rajang- Borneo and the Barito Basin we are grateful to J. Crocker Group and recycled Tanjung Formation Howes, D. Le Heron, G. Nichols, M. Cottam, I. material is difficult. Uplift of the Meratus Complex Watkinson, J.T. Van Gorsel, I. Sevastjanova, B. could explain the change to west-directed Clements, Y. Kusnandar, S. Pollis and E. Deman . palaeocurrents at the top of the Tapin Member, and This research was funded by the SEARG . the absence of Meratus debris may be due to fact that the Meratus basement rocks were not yet exposed. We suggest that the sandstones of the REFERENCES Warukin Formation were mainly derived from the Schwaner Complex and to a lesser extent the Adams, C.G. 1970. 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Figure 1 – Cenozoic geology of the Barito and Asem-Asem Basins (after Supriatna et al. (1994)).

Figure 2 – Generalised stratigraphy of the Barito Basin. Age diagnostic fauna and flora from this study are shown. East Indian (EI) Letter Stages correlated with planktonic zones by BouDagher-Fadel (2008). Numerical ages from GTS2004 (Gradstein et al., 2004).

Figure 3 – Ternary diagrams showing detrital modes of sandstones from the Tanjung, Montalat and Warukin Formations. (Left) sandstone classification according to Folk (1968). (Right) tectonic setting according to QmFLt ternary plot of Dickinson & Suczek (1979). Q: quartz, Qm: monocrystalline quartz, F: feldspar, L: lithics, Lt: total lithics (including polycrystalline quartz).

Figure 4 – Palaeocurrent data for the Tambak Member of the Tanjung Formation. Individual locations with fluvial data are shown on map. Total fluvial data are shown at top left. In addition, palaeocurrent data collected from small-scale ripples within tidal facies are shown in blue, centre left. Bin size is 10° on all plots.

Figure 5 – Palaeocurrent data for the Kiwa Member of the Montalat Formation. Total fluvial data is shown top left. Bin size is 10° on all plots.

Figure 6 – Palaeocurrent data for Warukin Formation. Total fluvial data for the Barabai Member are shown bottom left. Total fluvial data for the lower part of the Tapin Member are shown centre left. Total fluvial data for the top of the Tapin Member are shown top left. Bin size is 10° on all plots.

Figure 7 – U-Pb ages of zircons analysed from sandstones of the Tanjung, Montalat and Warukin Formations.

Figure 8 – Examples of zircon morphologies present in the samples analysed. (A) euhedral, elongate, (B, C) euhedral, non-elongate, (D) rounded, non-elongate. Scale bar is 100µ in all images.

Figure 9 – (Top) U-Pb ages of zircons from Karimunjawa Arch samples. (Bottom) U-Pb ages of zircons from sandstones of the Rajang-Crocker Groups, northern Borneo (after van Hattum, 2005).

Figure 10 – Map showing suggested flow into the Barito Basin area during the Late Eocene based on data of this study. Flow into west Java from the Schwaner Complex, reported by Clements (2008) is also shown.