Neogene history of fluvial to shallow marine successions in the Kendari Basin, SE Sulawesi, Indonesia

Collision between Australia and SE Asia began in Sulawesi, the world's eleventh‐largest island, in the Early Miocene and subsequently Neogene sediments were deposited largely in coastal to shelf environments throughout the island. These sediments have been assigned to the Celebes Molasse, previously considered as a single post‐orogenic unit deposited unconformably on pre‐Neogene sedimentary, metamorphic and ophiolitic rocks. The most complete and extensive sequences of Neogene sediments are in the Kendari Basin, situated at the southern end of the SE Arm of Sulawesi, where an outcrop‐based sedimentological study was undertaken to interpret depositional environments, palaeogeography and stratigraphy. The oldest Neogene sediments are shallow marine carbonates and deltaic siliciclastics of the Bungku Formation. They are unconformably overlain by the Upper Miocene Pandua Formation which consists of sediments deposited in a variety of environments including braided river channels, fluvio‐tidal channels, tidal flats, mouth bar complex and shoreface deposits. A Mio‐Pliocene subaerial unconformity separates the marginal marine serpentinite‐rich sediments of the Pandua Formation from the overlying fluviatile quartz‐rich Langkowala Formation. The sediments of the lower part of the Langkowala Formation include conglomeratic channel fill, while the sediments of the upper part are transgressive deposits decreasing in maximum grain‐size, marked by a reduction in channel/overbank ratio and increasing tidal influence. The transgressive Pliocene Eemoiko Formation is characterised by transgressive lags or onlap shell beds and deposits of a landwards‐backstepping carbonate platform. The improved understanding of the Kendari Basin will aid the interpretation of the sedimentation history of frontier basins surrounding SE Sulawesi, many of which have not yet been drilled.


| INTRODUCTION
Sulawesi (previously called Celebes) is a large and unusual K-shaped island in Indonesia which has four elongate mountainous arms with elevations up to 3 km separated by deep marine inter-arm basins. The Neogene sediments in Sulawesi that unconformably overlie pre-Neogene rocks have long been assigned to the Celebes Molasse (Hasanusi et al., 2004;Kündig, 1956;Milsom et al., 1999;Surono & Sukarna, 1995), a post-orogenic unit considered to have formed after collision between Australia and SE Asia. From an economic perspective, these Neogene sedimentary rocks host abundant important resources including mineral deposits, and oil and gas (SKK Migas, 2017;van Leeuwen & Pieters, 2012). However, the sedimentary basins both onshore and offshore are underexplored and understudied. Furthermore, the sedimentology and history of the Celebes Molasse has not been studied in detail. Recent studies (Nugraha et al., 2022;Nugraha & Hall, 2018) indicate that these sediments have significant variations in age, environment of deposition, composition and history. The most complete and extensive sequences are exposed in the Kendari Basin ( Figure 1) at the southern end of the SE Arm and were deposited in terrestrial to marine settings; they provide the opportunity to improve our knowledge of Neogene stratigraphy, the sedimentary and post-collision history of Sulawesi, and palaeogeographical interpretations of the region. Kendari Basin is located in between the Bone and Buton basins and is interpreted as a pull-apart basin (Camplin & Hall, 2014;Surono & Rusmana, 1997). This study focusses on the four Neogene formations in the Kendari Basin: the Lower Miocene Bungku Formation, the Upper Miocene Pandua Formation, the uppermost Miocene-lower Pleistocene Langkowala Formation and the Pliocene Eemoiko Formation. These formations were previously considered to be parts of the Celebes Molasse (Kündig, 1956;Surono, 1994;van Bemmelen, 1949) but have now been formally defined (Nugraha et al., 2022;Nugraha & Hall, 2018) as part of a revision of the Neogene stratigraphy of Sulawesi. A detailed study of palaeodepositional environments and changes in sedimentation during the Neogene is presented here using facies and sequence stratigraphy approaches.

PREVIOUS WORK
Post-orogenic sediments in Sulawesi have been widely acknowledged to be of similar Neogene age and are famously known as the Celebes Molasse, a term introduced in the early 20th century based on comparisons to the molasse of the European Alps (Sarasin & Sarasin, 1901).

| DATASET AND METHODOLOGY
In many parts of Sulawesi Neogene sedimentary rocks are absent or exposures are limited. The most complete sequences of Neogene sediments can be observed only in the Kendari Basin where it is possible to log sections and make detailed observations of the different formations and their contact relationships. New exposures due to road construction and mining also gave fresh rock outcrops and new information. The outcrop data set collected during fieldwork includes 240 localities where sedimentological and stratigraphic observations were made and 73 sections were measured and logged (Figure 1; File S1). Biostratigraphic data are adequate to assign ages to the stratigraphic boundaries of the different Neogene formations (Figure 2; File S2). Field observations are summarised in facies that includes grain size, sorting and sphericity of matrix and clasts, colour variations, sedimentary structures, bed thickness, the nature of bounding contacts, bioturbation (Table 1) and palaeocurrent directions (File S3). Microfossils, mainly foraminifera, and pollen provided age and palaeoenvironmental data. More than 403 palaeocurrent readings were taken from cross-beds, cross-lamina sets and asymmetric ripple crests in the Pandua (71 measurements) and Langkowala (332 measurements) formations and were restored to the palaeohorizontal; they were combined with published F I G U R E 1 (A) Simplified geological map of Sulawesi, modified after Hall and Wilson (2000). The Kendari Basin is outlined in red. (B) Geological map of the SE Arm and Buton Island (modified from Rusmana et al., 1993;Simandjuntak & Surono, 1993;Sikumbang & Sanyoto, 1995). Revision made in areas where observations were made. Nugraha et al. (2022). (B) Integrated stratigraphic ranges from the Neogene sediment samples are based on the biostratigraphic range of the foraminifera, nannofossil, macrofossil and pollen taxa. The time scale is from Gradstein et al. (2012). Nannofossil biozones from GTS2012 timescale in the recent synthesis of Backman et al. (2012). Foraminifera biozones from BouDagher-Fadel (2015). The clast-supported conglomerate implies interstices filling by matrix following deposition (Miall, 1977;Miall & Gibling, 1978) Matrix-supported conglomerate  (Miall, 2010). Normal graded formed under conditions of waning flow (Steel & Thompson, 1983) Horizontal-stratified conglomerate  (Collinson, 1996;Hjellbakk, 1997;Miall, 1996 datasets from Smith (1983) and Surono (1995) for the Pandua and Langkowala formations respectively. All sedimentological observations were interpreted using a facies analysis approach to provide information about depositional environments and their changes with time. An offshore seismic profile is presented that provides sequence stratigraphic correlation between the observed stratigraphic surfaces onshore. This line was part of the 2D seismic dataset which was acquired by TGS in 2007 (Camplin & Hall, 2014). It allows the correlation of the Neogene fluvial to shallow marine successions to be tested and extends palaeogeographical interpretation offshore, where direct sampling awaits future drilling efforts.

| FACIES ANALYSIS
The Neogene sedimentary rocks for each formation were grouped into different facies on the basis of the lithology and dominant sedimentary structures (Table 1). A compilation of photographs of facies from different formations is shown in File S4.

| Facies Association Bungku 1 (FAB1):
Shallow marine carbonate Description: this facies association is based mainly on the unpublished report of van der Vlerk and Dozy (1934), and their list of foraminifera taxa was rechecked (File S2). It consists of granular limestones, conglomeratic limestones, compact limestones and grey marls. These rocks also contain grains of quartz, quartzite, tourmaline, echinoid shells, compact limestone, serpentine, chert, crystalline limestone, plagioclase, olivine and arkose sandstone, with minor glauconite and biotite.
Interpretation: FAB1 was deposited in a shallow marine reefal environment with minor siliciclastic input in the middle Burdigalian (Early Miocene) based on its foraminifera assemblages. Preserved allochthonous grains (e.g. quartz, serpentine, chert, olivine) mark the initial clastic input before the large and rapid siliciclastic input of FAB2 and FAB3 that took place in the late Burdigalian.

| Facies Association Bungku 2 (FAB2):
Foreset of delta-front Description: this facies association consists predominantly of facies Sm with minor intercalations of facies Gc, Sp, St, Sh and Fh (Figures 3 and 4A). The individual bed thicknesses range from 5 to 20 cm and beds record small cycles of fining upwards sedimentary packages. The total vertical sedimentary succession can reach 6 m thick in bedding-packages that generally dip towards the south. Overall, FAB2 shows coarsening and thickening upwards. Lenses of conglomeratic limestone appear in the lower part of succession with planar cross-stratifications. These contain shallow reefal foraminifera assemblages, fragments of bryozoa and Rodophyte spp (Samples ES15-1 and ES15-2; File S4).
Interpretation: FAB2 was deposited in a marginal marine environment, supported by its foraminifera content. There are lenses of conglomeratic limestone possibly reworked from carbonate build-ups nearby. The crossstratification and general coarsening upwards succession indicates deposition in bars or delta foresets in a shallow marine environment.

Delta-top
Description: FAB3 consists mainly of facies Sm, Sn and Gc with minor amounts of facies Fh and Sh (Figures 3  and 4A). Bedding is not as well defined as in FAB2. The individual bed thicknesses range from 30 to 200 cm. The beds of facies Sm and Gc generally coarsen and thicken upwards in the succession. Calcareous mudstone, limestone and ultramafic clasts also get coarser (from granule to cobble) and more rounded upwards. Rare clasts of chert appear in the upper part.
Interpretation: Coarsening and thickening upwards of FAB3 are interpreted to indicate progradation of delta building out into a marine shelf or basin plain. The thickest bed of poorly sorted conglomerate (Gc) in the upper part of the succession may represent an alluvial fan or a braided river forming the delta-top. The predominantly well-rounded clasts are likely to have undergone multiple cycles of erosion. A coarsening upward trend with introduction of chert clasts indicates an increasing clastic input, partly from an ophiolitic source.

| Facies Association Pandua 1 (FAP1):
Braided channels Description: FAP1 is the coarsest facies association and consists of vertically and laterally stacked facies St, Sp, Gt, Gp, Sm and rarely Sh ( Figures 4B and 5). Ultramafic and mafic rocks are still the predominant lithoclasts. Conglomerate facies contain large intra-formational and extra-formational pebbles and mud rip-up clasts. These stacked facies form single and multi-storey layers up to 6 m thick which show both fining and coarsening upwards. They have a sheet-like geometry with a lateral extent up to tens of metres. Coarser-grained facies St and Gt are observed at the base and top of the successions and are rarely associated with convoluted bedding. Cross-bedded sets of facies St and Sp are separated by sharp and relatively minor horizontal bounding surfaces. Their thickness ranges between 8 and 48 cm. Cross-stratification diminishes upwards and passes up to facies Sh, Sm and Sr. FA4 commonly scours down to underlying finer-grained deposits (facies Fh and Fm).
Interpretation: The stacked fining upwards conglomerate and sandstone units separated by closely spaced erosion surfaces typically reflect multi-storey channel fills (Allen, 1983;Bridge & Lunt, 2006;Dillinger & George, 2019;Miall, 1996). FAP1 includes a combination of major channels with multi-storey sand bodies of intercalated stacked bars. Ripple cross-laminated facies are indicative of near emergent conditions at channel margins and typically represent accretionary bar tops (Dillinger & George, 2019;Matoshko et al., 2016;Sambrook Smith et al., 2006). Convolute bedding reflects rapid sediment loading causing over steepening or dewatering (Plink-Bjölrklund & Steel, 2004). Predominantly coarse-grained channel fill deposits, extensive sheet-like geometry, largely mudstone-free facies and lack of bioturbation suggest low sinuosity, wide and multiple interweaving braided channels (Catuneanu, 2006;Flaig et al., 2016;Miall, 1996;Miall, 2014). Predominant ultramafic and mafic lithoclast suggests that ophiolite is still the main source.  Table 1). Basal erosion surfaces are scoured, flat to concave-upward and associated with granule to pebblesized conglomeratic trough cross-bedding. Overlying fine-grained sandstones with low-angle cross-lamination, bidirectional cross-stratification and current ripple crosslamination. Mud drapes commonly appear in the crossbed sets. Inclined heterolithic stratifications of facies Fh were also observed and are overlain by facies Fm and Sh that contain brackish and fresh water shells. The basal surface is overlain by the shell lags ( Figure 4D). A vertical sedimentary package reaches up to 6 m in thickness.

| Facies Association
Interpretation: The sharp basal surface reflects erosion during channel formation and lateral migration. Trough cross-bedding indicates unidirectional river-dominated palaeocurrents, but locally opposed cross-stratification support some tidal influence. Deeper tidal-influenced channels may contain coarser cross-bedded sediment (Dalrymple et al., 2012). Abundant shell fossils at the base represent a lag of shell debris at the channel base. Such lags are commonly found in tidal channels and estuaries (Allen, 1991;Barwis & Makurath, 1978;Bridges & Leeder, 1976;Frey & Howard, 1986;Kumar & Sanders, 1974;Richards, 1994). Inclined heterolithic stratifications constitute the lateral accretion of a pointbar in a tidally influenced channel (Thomas et al., 1987). Mud is interpreted to fill this channel and is erosively overlain by coarser sand facies Sh of which mark lateral channel migration or enlargement of a channel because of increased fluvial discharge. In intertidal sand-flat areas, tidal-current speeds that can exceed 2 m/s generate extensive upper-flow-regime sand flats in shallow water (Dalrymple et al., 1990(Dalrymple et al., , 2012. The association of crossbeds with mud drapes also indicate tidal flow conditions. Mud drapes form when the tide changes direction so mud falls and drapes on the lee sides of bars (Dalrymple, 1992). A structureless formation on top of inclined heterolithic beds represents upper point bar or flood plain deposits. An abundance of mixed marine, brackish and freshwater molluscs within the succession are a useful tidal indicator. Hence, FAP2 is interpreted to reflect tide-influenced channels.
Interpretation: FAP3 is interpreted as the product of low energy sedimentation. Interbedded plant material, bioturbation and fossil-bearing beds indicate a repetitive change from marine to terrestrial environments. Millimetre to centimetre thick interlaminations of mudstones, siltstones and very fine-grained sandstones form rhythmites which indicate a tidal signature (Dalrymple, 1992;Dalrymple et al., 2012). These are commonly associated with intertidal flats which border fluvial and marine water bodies. Assemblages of pollen and spores from these facies associations suggest peat mangrove, back mangrove and other mangrove environments. The mollusc fossils indicate fresh, brackish and saline water species (File S2) and indicate a mangrove -mudflat environment with fully shallow marine intercalations.

| Facies Association Pandua 4 (FAP4):
Mouth-bar complex Description: FAP4 consists of interbedded fine-grained to pebbly sandstone facies St, Sp, Sm and Sh with minor Fm intercalation. Beds have thicknesses ranging from 0.12 to 3.8 m and are laterally continuous with a sheet-like geometry. Sandstones typically occur in thick, amalgamated packages with sharp and discrete erosional contacts. Isolated conglomeratic and pebbly sandstone intervals are common within sandstones and bounded by gradational to erosional contacts. Large scale cross bedding, both high and low-angle, plan-parallel lamination and convolute bedding were observed in this facies association. Mud drapes and herringbone cross-stratifications are locally preserved. In general, the vertical succession shows coarsening and thickening upwards ( Figures 4E and 5). FAP3 contains shells, trace fossils and plant fragments. Trace fossils of dwelling burrows Skolithos and Thalassinoides frequently appear in sandstone beds (BI: 0-2).
Interpretation: Trough cross-beds represents the dunes that formed during fair weather conditions in upper shoreface water depths (Greenwood & Mittler, 1985;  Rygel et al., 2008). Plane-parallel lamination suggests upper-flow-regime tractional flows that probably characterised the axial and proximal part of a low-relief mouthbar (Ahmed et al., 2014;Dillinger et al., 2022;Fidolini et al., 2013;Flaig et al., 2016;Martini & Sandrelli, 2015;Olariu et al., 2010). Unidirectional cross-lamination record ripple migration under decelerating turbulent currents that probably dominated mouth-bar margins and distal areas (van Yperen et al., 2020). Convolute bedding reflects rapid sediment loading causing over-steepening or dewatering (Plink-Bjölrklund & Steel, 2004). Stacked, tabular sandstone beds dipping at a low-angle represent construction of a low-relief mouth-bar by successive high-discharge, sediment-laden pulses (i.e. floods) of a feeder channel, and their subsequent waning flows (Dillinger et al., 2022;Fielding, 2010;Fielding et al., 2005;Jerrett et al., 2016;Wright, 1977). Preservation of mud drapes and bidirectional cross-stratification indicate tide-influenced processes (Dalrymple et al., 2012). This is consistent with the preserved bioturbation and plant and shell debris suggest sediment delivery to the river mouth was intermittent, allowing bottom dwelling organisms windows of opportunity. of sandstone beds. Intact bivalve fossils are rarely found in a sub-vertical position.

| Facies Association
Interpretation: FA6 indicates a lower shoreface environment characterised by hummocky crossstratification and preserved intact bivalve fossils. These features are commonly associated with wave and storm-dominated processes in a lower shoreface (Hampson & Storms, 2003). Hummocky crossstratification forms above (but near) the storm wave base (Dumas & Arnott, 2006). Intact and sub-vertically emplaced bivalve fossils represent infaunal suspension feeders that are often associated with hummocky cross-stratification (Ekdale et al., 1984;Pemberton & Frey, 1984;Reading, 1996;Vossler & Pemberton, 1988).  (Figures 6A and  7). Contact between these facies associations is gradational and sharp. It is difficult to define different beds as matrix and clast-supported conglomerates because they commonly appear to be amalgamated. Poorly developed facies Gp, Sm and Sp occur as pockets within massive facies Gm with lenticular or ribbon-shaped geometry. The minimum total thickness for this facies association is around 15 m. Quartz-rich sandstone and metamorphic clasts are the second most abundant fragments after quartz.

| Langkowala Formation
Interpretation: This poorly bedded association of predominant poorly sorted and matrix-supported facies (Gm and Gi) with poorly developed facies Gn, Gp, Sp and Sm suggest mixed depositional products of debris and traction flows. The proximity of sediment sources causes a very high concentration of sediments to be deposited. The mixture of a higher concentration of sediment than water forms typical debris flows (Nichols, 2009). These characteristics suggest deposition in alluvial fans during very high rainfall or monsoon seasons (Blechschmidt et al., 2009). Sandstone lenses within massive conglomerate beds may indicate stream channel fans. Alternatively, rivers that emerge from the feeder canyon or gorge can also produce alluvial fanlike deposits with prominent sedimentary structures of a braided river, including imbrication and crossstratification (Reading, 1996). Increasing quartz and metamorphic clasts indicate a newly emergent source that is different from the ophiolitic source of the Pandua Formation.  (Figures 6C and 7) and is up to 7 m thick with minimum lateral extent of about 25 m wide. Bed geometry commonly shows a concaveup scoured base and horizontal top with lenticular body. Conglomerate facies commonly overlie erosion surfaces that concentrate intra-formational and extraformational clasts and mud rip-up clasts. Beds of planar to sigmoidal stratified conglomerate have thicknesses around 2 m and dip at angles of 11-25°. Sigmoidal stratified conglomerate beds are bounded by planar or slightly curved and irregular surfaces. They fine-upward into trough and planar cross-stratified, well bedded, and massive sandstone facies (facies Sp, St, Sh, Sn and Sm). Channel margins are rarely preserved and channel fills include cross-stratified facies (facies Gt and St) that laterally pass into massive conglomerates or sandstones. Lateral accretionary surfaces were rarely observed. Inclined planar sets of facies Sp dip in the same downstream direction. The thickness of the facies St and Sp units generally decreases upwards. Geometrically, they are ribbon-like bodies and may be over 200 m wide and 1-5 m thick. Flat to irregular bounding surfaces and erosive to slightly concave bases are present between the sets. No trace fossils were observed in FAL2.
Interpretation: The combination of tabular horizontal, planar and cross-stratified sets is typical of compound bars (Allen, 1983). The migratory dune-scale bedforms indicate deposition either in flanks of pointbars or alternatively on mid-channel bars (Allen, 1963(Allen, , 1983Bridge & Lunt, 2006;Cant & Walker, 1976Capuzzo & Wetzel, 2004;Ghazi & Mountney, 2009Jackson, 1976aJackson, , 1976bMiall, 1985;Nijman & Puigdefabregas, 1977). The cross-stratified facies that commonly grade laterally and vertically into relatively massive or horizontal units represent deposition in longitudinal and transverse bars within a low sinuosity multichannel river (Miall, 1977;Ori, 1982;Rust, 1977). The lack of lateral accretion surfaces suggests predominant longitudinal, laterally or downstream accreting midchannel bars in braided channels (Di Celma et al., 2016;Dillinger & George, 2019;Martinsen et al., 1999;Miall, 2014). Stacked fining upward conglomerate and sandstone units separated by closely spaced, pebble-rich erosion surfaces typically reflect multistorey channel fills (Allen, 1983;Dillinger & George, 2019;Miall, 1985Miall, , 1996. Basal pebbly lags with mudstone clasts represent the armoured bases of channels (Ielpi, 2012). The absence of bioturbation supports a high energy, unstable fluvial setting. Interpretation: The nature of fine-grained sediments, wide lateral extent and sheet-like geometry reflect low energy deposition over a wide area. Fining upward successions in such facies may also result from migration/avulsion of the nearby channel, whereas the coarsening upward trend might indicate a progradation of splays. Plant debris and root traces in the heterolithic facies indicate a low energy, nonmarine environment with episodic sub-aerial exposure (Dillinger & George, 2019). This is consistent with the presence of red sediment layers that indicate an oxidising terrestrial environment. The black layers suggest organic-rich, probably plant material, accumulation. Non-channelised sandstone beds (facies Sr and Sh) with unidirectional currents and high sedimentation rates may reflect crevasse-splay deposits from overbank spills deposited on low-relief floodplains (Dillinger & George, 2019;Ielpi, 2012;Michaelsen & Henderson, 2000).  Figures 6D,E and 7). This succession can be up to 10 m thick with general fining upward units and basal erosion surfaces. FAL4 is characterised by the presence of bidirectional stratification and bioturbation. Crossstratified sandstones (facies St and Sp) predominantly occur in the lower part, while facies Sh and Sm occur in the upper part. Flat to irregular bounding surfaces are present between cosets and beds. Bedsets contain abundant reactivation surfaces. Foresets and bottomsets are typically mantled by thin mudstone laminae. Opposite dipping strata are locally present in planar cross-stratified beds. Cross-stratified sandstones are typically bioturbated by trace fossils. Bioturbation intensity is low to high (BI 1-5). Predominant Skolithos and minor Ophiomorpha were observed in sandstone beds, while Planolites commonly occurs in mudstone beds. Fine-grained sediments (Fwl and Fh) appear as thin bounded beds adjacent to cross and horizontally stratified sandstone beds.

| Facies Association Langkowala 5 (FAL5): Tidal flats
FAL 5 is dominated by heterolithic beds of facies Sm, Sr, Fwl, Fh, Fm and Fmf with low to high (BI 1-5) bioturbation intensity (Figures 6F and 7). The individual beds have millimetre and decimetre thickness, while the total observed thickness ranges from 0.22 to 6 m. These successions may coarsen or fine upward and have sharp to weakly scoured basal surfaces and iron-rich top surface. Locally, flaser and sandstone beds show lenticular geometry. Bidirectional cross-laminations are locally present in facies Sr. Observed trace fossils include Thalassinoides, Ophiomorpha, Skolithos, Palaeophycus and Cylindrichnus. High bioturbation intensity disrupted bedding and caused mottling. Iron-rich 'ball' concretions were commonly observed within the red and grey heterolithic laminations.
Interpretation: Superimposed planar and current ripple cross-laminated sandstone with mudstone drapes indicate deposition during upper flow regime, plane-bed conditions and accelerating/waning flows, respectively, under a tidal influence (Dalrymple, 2010;Dillinger & George, 2019). It is consistent with the presence of bidirectional laminations that formed when the reverse-flow tidal current was strong enough to rework sediments deposited during the dominant phase (Dillinger & George, 2019). Mudstone drapes that were settled out of suspension during slack-water periods also strongly suggest a tide-dominated environment (Dalrymple, 2010;Plink-Björklund, 2005). Vertical trace fossil assemblages are attributed to the Skolithos ichnofacies, whereas horizontal burrows more typical of Cruziana ichnofacies. The red colour of concretions indicates a sideritic iron carbonate composition (Boggs, 2012). All these features, including iron-rich surfaces, suggest brackish water conditions with a notable marine influence (Boggs, 2012;Carmona et al., 2009), consistent with palynological analysis that suggests supratidal to subtidal environments. Sand-prone and mud-prone heterolithic facies are thus interpreted as tidal flats that accumulated in the upper intertidal to supratidal zone flanking active tidal-fluvial channels (Dalrymple, 2010) It typically shows a general coarsening and thickening upward from (1) the basal mudstone dominated-facies to (2) alternating millimetre to centimetre thick heterolithic facies and (3) centimetre thick to massive sandstonedominated facies (Figures 7 and 8A). The lower succession consists predominantly of mudstone of facies Fmf. Disconnected and convoluted sandstone beds were also observed in this unit ( Figure 8B,C). Sandstone and mudstone contacts are gradational and sharp. Within the mud-dominated interval are isolated pebbles and boulders of sandstones. Massive mudstone facies typically grade into the millimetre to centimetre thick heterolithic deposits. The upper unit of sandstone-dominated facies includes thickly bedded sandstone and mudstone and diminution of bioturbation. The lower bounding surfaces are typically sharp. Sedimentary structures in sandstone comprise plane-parallel lamination, cross-lamination and convolute bedding. Beds may be amalgamated via sharp to slightly erosional surfaces or separated by heterolithic beds. There are locally preserved low-angle clinoforms in dip-oriented sections. A normal fault was observed cutting this facies association ( Figure 8A).

| Facies Association Langkowala 7 (FAL7): Shoreface
Description: FAL7 consists of heterolithic beds of facies Sm, Sr, Shc, Fwl, Fh and Fmf at the base. Primary sedimentary structures are planar lamination, symmetrical to nearly symmetrical ripples with rounded tops, climbing ripples, hummocky and swaley cross-stratifications (Figures 7 and 8A,B,C). Concretionary layers of sideritecemented sandstone, soft sediment structures and convoluted laminations in heterolithic beds were locally observed. Bioturbation intensity is generally low to moderate (0-3) from the mud-dominated to the heterolithic facies. Bioturbation includes Thalassinoides, Helminthoidichnites and escape traces. Heterolithic beds overlie sharply the foraminifera-rich mudstone bed.
Skeletal wackestones (Lw), packstones (Lp) and grainstone (Lg) form thin to massive beds (from 3 cm to more than 1 m thick) composed of large benthic foraminifera, red algae, gastropods, bivalves, echinoid spines, small coral fragments and rare quartz grains. Coral floatstone (Lf) and rudstone (Lr) are massive beds (over 1 m) and contain broken platey, domal and branching corals. Their matrix ranges from wackestone to grainstone and fills the interstices between large coral fragments. Massive coral framestone beds predominantly consist of domal and head corals with Lp and Lg matrix occupying interstices. Mixed-carbonate siliciclastic (Ls) beds are composed of fine to medium-grained sand, rounded quartz, mafic minerals and fragmented bioclasts of broken shells, gastropods, echinoid spines, large benthic foraminifera, and red algae, crossed by sub-vertical burrows. A condensed shell bed (Lc) is dominated by disorganised and partially disarticulated mollusc shells (predominantly bivalves and minor gastropods) varying in size from 4 to 64 mm.

| Depositional settings and palaeogeography
The facies associations described above and their bounding stratigraphic boundaries (Figure 2 and File S2) have been integrated to define depositional environments for each of the Neogene formations in the Kendari Basin. Limited exposure and high variability of bedding orientations makes lateral stratigraphic correlation problematic.

| Bungku Formation
The carbonates (FAB1) of the Bungku Formation overlie a major erosion surface and were deposited in a shallow marine reefal environment during the Early Miocene (middle Burdigalian). These pre-date the ultramafic-rich siliciclastic upper parts of the Bungku Formation which are mainly late Burdigalian. The coarsening-up succession of FAB2 and FAB3 represents delta-front to delta-top progradation in a marginal marine setting. It is suggested that the depositional environment changed from shallow marine (carbonates) to deltaic marginal marine (siliciclastics) in response to the elevation of a mountainous region following collision and ophiolite emplacement.
This uplifted land was a major source for the Bungku Formation siliciclastic sediments.

| Pandua Formation
The absence of Middle Miocene sediments indicates an unconformity due to erosion or a break in deposition (Nugraha et al., 2022) preceding the Pandua Formation. FAP5 and FAP4 records deposition in mixed-processes shoreline environments comprising a wave and storm-dominated shoreface (FAP5) and shallower tidally influenced river mouth-bar (FAP4). The storm-dominated interval reflects lower shoreface sedimentation under fluvial influence in a tropical setting, perhaps analogous to the Miocene and modern Baram Delta shoreline-shelf deposition (Collins et al., 2018). The coarse-grained FAP5 were possibly deposited when storm-wave energy and fluvial discharge were simultaneously elevated (Collins et al., 2017). Analyses of the coarsening-up sandstone-rich facies in FAP4 suggest deposition in a tidally influenced river mouth-bar and/ or delta-front. Palaeocurrent analysis indicates predominant south-easterly and north-westerly directed flows trending from the complex channel and mouthbar bodies ( Figure 10A). These are interpreted to record flood and ebb tidal flows, within distributary channels ( Figure 11A).
Interdistributary facies comprise heterolithic facies of FAP3 with millimetre-scale to centimetre-scale lenticular to wavy bedding, common organic debris and bioturbation. Abundant mangrove pollen in FAP3 suggest subtidalintertidal deposition on a mangrove-fringed coastal plain (Simmons et al., 1999). Present-day mangroves are most prevalent on the fringes of low-wave-energy embayments and abandoned fluvial-tidal channels in humid-tropical delta systems (e.g. Mekong, Ganges-Brahmaputra and Baram deltas; Collins et al., 2018;Woodroffe et al., 2016). Subordinate FAP2 are deposits of tidal channels with erosional bases, including bidirectional foresets, inclined heterolithic stratifications, mud drapes and abundant shell debris.
It is proposed that the coarsening -upward sandstone beds of FAP1 that sharply overlie the mixed to muddy tidal flats mangroves of FAP2 constitutes the record of a braided-river on a broad, low gradient coastal deltaplain (Browne & Naish, 2003;Holbrook, 1996;Martinsen et al., 1999). The fluvial influence is implied by the coarser grain size, dominant basinward palaeocurrent direction, lack of systematic mud drape bundling and low bioturbation intensity (Collins et al., 2018;Gugliotta et al., 2016;MacEachern et al., 2005;MacEachern & Bann, 2008). To sum up, the Pandua Formation environments include braided rivers, tidal channels, subtidal-intertidal mangrove-fringed coastal plain, tidal bars and a storminfluenced shoreface ( Figure 11A). It is inferred that deposition occurred during gradual spatial and temporal shifts in delta and embayment with mixed-energy shorelineshelf depositional systems (cf. the Baram deltas of Brunei).

| Langkowala Formation
The Langkowala facies associations show a wide range of depositional environments from terrestrial to marine (Figure 7). Both FAL6 and FAL7 are the oldest deposits of the Langkowala Formation based on their foraminifera F I G U R E 1 0 (A) Palaeocurrent analysis of the Pandua Formation indicates predominant south-easterly and north-westerly directions from mostly tidally influenced channel and mouth-bar complex. (B) Palaeocurrent analysis from FAL1 and FAL2 suggests predominantly south-eastward transport, whereas palaeocurrents from FAL4, FAL6 and FAL 7 record north-easterly and northwesterly palaeocurrent directions.
assemblages. FAL7 comprises storm-wave-dominated shoreface sediments over an outer neritic mudstone. Complex hummocky cross-stratifications that include rounded tops climbing ripple, swaley cross-stratifications and soft-sediment sedimentary structures indicate shallowmarine sandstone storm deposits (tempestites) that were deposited under highly unsteady storm-wave-generated oscillatory flows or oscillatory-dominated combinedflows in a prodeltaic setting (Jelby et al., 2020). The FAL6 mouth-bar succession abruptly overlies prodeltaic mudstone deposited far from the terminal distributary channel (Schomacker et al., 2010) and/or subaqueous erosion took place prior to mouth-bar deposition (Dillinger et al., 2022;Edmonds & Slingerland, 2007;Overeem et al., 2003;Schomacker et al., 2010;Zavala et al., 2006). Mudstone to heterolithic interbeds record prodeltaic to lower-delta front sedimentation during a prolonged low river stage, lateral accretion and/or temporary abandonment of the mouth-bar. Loading of this substrate by rapid deposition of the mouth-bar sand led to failures of the upper deltafront slope, caused deformed sand masses, and developed growth faults (Fielding, 2010).
The scoured base of channel-fill deposits FAL2 on top of the Pandua Formation marginal marine deposits FAL5 suggests a subaerial unconformity (Catuneanu, 2006). The fluvial incision was filled by the coarsest sediment fractions of FAL2 multi-storey braided channel fills that lack floodplain deposits. The prevalent coarse-grained conglomerates and absence of floodplain fines suggests broad, high energy braid plains with significant bypass of finegrained sediments ( Figure 11B; Dillinger & George, 2019). The upper part of FAL2 is characterised by smaller channel size, finer-grained channel fills, and increasingly finegrained floodplain deposits. Carbonaceous floodplain deposits of FAL3 are relatively well developed with the presence of rootlets and red oxidised beds. The preservation of carbonaceous material indicates highly vegetated environments, high groundwater levels, and a humid climate (Catuneanu, 2006;Dillinger & George, 2019;Olsen et al., 1995).
Predominant matrix-supported conglomerates, subangular to sub-rounded metamorphic and quartz clasts in FAL1 suggest sediment deposition in an alluvial fan next to an uplifted metamorphic complex bounded by faults with significant dip-slip (Blair & McPherson, 1994). Couplets of horizontal planar-laminated conglomerates interstratified with pebbly sandstone beds are interpreted as the product of sheetfloods (Blair & McPherson, 1994). Wedge, lenticular and lens geometries of conglomerate and sandstone facies reflect lag, gully and channel fills that are common in alluvial fans (Blair, 1999). Sediments were deposited under sediment-deficient flows concentrated into gullies and minor channels across the surface of sheetflood deposits.
FAL4 and FAL5 record deposition along tide-dominated shorelines in which there are tidal channels, tidal bars and mangrove-rich coastal plains. The erosional base, extensive tabular and bipolar compound cross-stratifications, coarse grain-size, mud-drapes, reactivation surfaces and moderate to intense bioturbation indicate that FAL4 was deposited in tidal bars and/or channels. Extensive tidal bars are characteristic of the seaward portions of most macrotidal environments (Hayes, 1975;Harris, 1988;Dalrymple & Zaitlin, 1989;Dalrymple et al., 1990). Tidal sand bars have been described from estuaries, tide-dominated delta-fronts to tide-dominated shallow-marine settings (Dalrymple et al., 1990;Swift & Heron, 1967). Heterolithic FAL5 that contain mainly mangrove pollen is interpreted as mixed sand and mudflats that accumulated in the supratidal, intertidal and subtidal deposition on a mangrove-fringed coastal plain (Simmons et al., 1999). Tropical intertidal regions in the Pliocene Langkowala Formation would have been vegetated by mangroves, as seen in analogous modern environments (Giri et al., 2011).
Palaeocurrent directions are dominantly southeasterly trending in FAL1 and FAL2 reflecting the prevalent unidirectional current measured from fluvial channel bodies ( Figure 10B). Conversely, FAL4, FAL6 and FAL 7 record north-easterly and north-westerly palaeocurrent directions. North-eastward palaeocurrents reflect landward flood-tidal flows, while north-westward palaeocurrents indicate sediment transport subparallel to the palaeoshoreline.

| Eemoiko Formation
The Eemoiko Formation consists of thinly to massive bedded carbonates interpreted to have been deposited on a reefal carbonate shelf (FA18). The Eemoiko Formation is laterally equivalent to, and contemporaneous with deposition of the Langkowala Formation. The carbonate platform was established on the shelf possibly during the Early Pliocene. This interpretation is supported by the macro and microfacies analysis that suggested lagoonal backreef, inner platform, reefal area (including reef flanks), reef core and forereef environments. The landwardsindented distribution of Middle Pliocene limestones suggests transgressive carbonates developed by backstepping, either forming a carbonate platform or leaving isolated pinnacle reefs ( Figure 11C).

| Stratigraphic analysis
Ideally, sequence stratigraphic analysis integrates many types of data from the study of outcrops, cores, well logs and seismic volumes. However, the Kendari Basin is within a frontier region allowing only limited study of the outcrops. Furthermore, the locally sparse outcrops together with the heavily vegetated nature of land in Sulawesi make construction of correlation panels for systematic interpretation of up-dip to down-dip parasequences impractical. Fortunately, 2D regional seismic lines from Bone Bay that extend to the southern end of the SE Arm were available to be integrated in this study. Thus, a sequence stratigraphic framework (Catuneanu, 2006) based on the observed stratigraphic surfaces, palaeoenvironmental reconstructions and their specific facies associations, and offshore seismic stratigraphy can be used to provide genetic context in which event-significant surfaces, and the strata they separate, are placed into a coherent model that accounts for all temporal and spatial relationships.

| Stratigraphic surfaces
The most important stratigraphic contact that was observed from the field is an erosional contact between the base of Langkowala Formation channel fills and underlying marginal marine deposits of the Pandua Formation ( Figure 12A,B). This is a subaerial unconformity (Catunenanu, 2006) and is marked by the significant sediment change from the ultramafic-rich Pandua Formation to the overlying quartz-rich Langkowala Formation. Furthermore, a lowering of the base level below the shelf edge is also observed from the offshore seismic lines, where the unconformity surface truncates the clinoforms in strata below (13). This unconformity is related to the exhumation of the basement rocks in SE Sulawesi including metamorphic and Mesozoic rocks in the Mio-Pliocene (ca 6-5.3 Ma; Nugraha et al., 2022). Uplift of these rocks, bounded by the Kolaka and Lawanopo faults in the central part of the SE Arm, blocked the pathway of ultramaficrich sediment from the north and provided a lithologically different quartz-rich source.
The sharp based-swaley cross-stratified shoreface deposits of FAL7 that directly overlie outer shelf mudstones mark a regressive marine erosion surface at the contact between a forced regressive shoreface (above) and the outer shelf (below) deposits ( Figure 8D; Plint, 1991). This regressive surface of marine erosion was possibly formed during a forced regression (Hunt & Tucker, 1992) or falling stage (Catuneanu, 2006) in a wave-dominated shelf setting.
A maximum flooding surface is difficult to pinpoint in outcrop as it is not associated with a lithological contrast (Catuneanu, 2006). However, the position of the maximum flooding surface is inferred from the youngest outer neritic mudstone (location 122; sample ES13-194; File S4) that yields Early Pleistocene foraminifera assemblages.

| Systems tracts
The underlying fluvial to shallow-marine strata of the Pandua Formation below the subaerial unconformity may be either normal regressive (landward from the shoreline position at the onset of base-level fall) or forced regressive (within the area of forced regression), whereas the overlying fluvial deposits of the Langkowala Formation may be either normal regressive (lowstand) or transgressive, depending on landscape gradients and the degree of development of lowstand normal regressive strata (Catuneanu, 2006). The predominant coarse-grained amalgamated deposits filling channels cutting poorly preserved floodplain deposits in the Pandua Formation are interpreted as the late highstand system tract (HST, Figure 14). The late phase of the highstand stage is defined by much lower rates of base-level rise compared to its early phase, and hence is prone to an increase in channel clustering and a lower ratio of floodplain deposits and channel fill (Aitken & Flint, 1994;Catuneanu, 2006;Legaretta et al., 1993;Shanley & McCabe, 1993). Overlying fluvial deposits of the Langkowala Formation are characterised by the coarsest sediment fractions within multi-storey braided channel fills that lack floodplain deposits (base of FAL2). Their deposition on top of a subaerial unconformity reflects the early stages of renewed sediment accumulation of a lowstand system tract (LST) when the amount of fluvial accommodation was still limited.
The wave and storm-dominated shoreface FAL7 that abruptly overlies the outer shelf mudstone is interpreted as forced regressive deposits of a falling-stage system tract (FSST, Figure 14). The lowering of fair-weather wave base during base-level fall results in the erosion of the formerly aggrading lower shoreface to inner shelf areas, which enables the deposition of swaley cross-stratified upper to middle shoreface sandstones directly over a scour surface cut in inner to outer shelf mudstone-dominated deposits (Plint, 1991). From the seismic line, the key to recognising forced regressive deposits is their offlapping (seaward downstepping-offlap character) due to a fall in the relative sea level below the shelf edge ( Figure 13).
The coarsening upward profile, slump scars and associated extensional growth fault of the oldest prograding deltaic FAL6 are characteristic of a LST in a shallow marine section that formed during early base-level rise when the rate of rise was outpaced by the sedimentation rate (Figure 14; case of normal regression, Catuneanu, 2006). Coeval lowstand fluvial deposits include amalgamated channel fills of basal FAL2 overlying a subaerial unconformity ( Figure 12A,B). They accumulated in topographic lows, and it is commonly assumed that incised valleys are at least in part filled with such deposits (e.g. models developed by Gibling & Bird, 1994;Shanley & McCabe, 1991Wright & Marriott, 1993). The LST in the offshore seismic line is depicted as downlap and onlap of a shelf-edge delta with topset and fluvial onlap ( Figure 13). The topset is interpreted to be reworked by a transgressive ravinement surface and the lowstand fluvial deposits are inferred to fill F I G U R E 1 3 2D seismic transect showing the interpreted system tracts in the Bone Gulf (modified from Camplin & Hall, 2014, location shown in Figure 1B). The red line marks the interpreted Mio-Pliocene unconformity surface. Regressive deposits (both lowstand and falling-stage system tracts) downlap the sea floor, whereas the transgressive deposits onlap the youngest prograding clinoform ('healingphase' deposits). Forced regressive deposits are associated with offlap. Coeval retrograding (backstepping) geometries are interpreted as a transgressive backstepping carbonate platform. F I G U R E 1 4 Stratigraphic model for the upper Neogene sedimentary rocks in SE Sulawesi. This model provides a complete scenario, in which all system tracts are preserved, and is correlatable with the offshore 2D seismic transect. the concave-up erosional relief on top of the subaerial unconformity.
A shallow marine transgressive system tract (TST) is characterised by the landwards-indented development of the Eemoiko limestones from Early to Middle Pliocene ( Figure 11C). They possibly correlate with the high amplitude backstepping reflections that were observed on the seismic line ( Figure 13). An upward increase of tidalinfluenced deposits, decrease in maximum grainsize and lowering of channel/overbank ratio indicate transgression in the upper part of the Langkowala Formation following a lowstand normal regression. It is consistent with the Lower Pleistocene outer neritic mudstones (location 122; sample ES13-194; File S4) that were found inland. Tidal channels (FAL4) and flanking tidal flats (FAL5) are interpreted as parts of transgressive coastal deposits. Equivalent offshore transgressive siliciclastic deposits are characterised by the onlap 'healing-phase wedges' on the seismic line ( Figure 13). Further east, well Abuki-1 to the east of the SE Arm penetrates inner to middle neritic transgressive sediments deposited during the Early Pliocene to Middle Pleistocene, based on foraminifera and nannofossil analyses (Amoseas Indonesia Inc., 1990), which are the equivalents of the Langkowala Formation. Collectively, the upper portion of the Langkowala Formation and carbonates of the Eemoiko Formation record transgression by a TST (Figure 14). This transgression was caused by subsidence of the basin contemporaneous with progressive uplift and unroofing of low to high grade basement metamorphic rocks of the Mekongga and Rumbia complexes onshore (Helmers et al., 1989;Mawaleda et al., 2016).
Onshore, a transgressive ravinement surface is overlain by a rudstone transgressive lag and thinly bedded grainstones to packstones of the Eemoiko Formation ( Figure 12G). They show a fining and deepening upward associated TST and are overlain by tidal flat FAL7 strata of the Langkowala Formation. A change from shallow marine carbonate to tidal flat mudstones and sandstones indicates shallowing-upwards as a high clastic sediment supply caused base level to rise with possible aggradation of coastal-plain deposits before the system started to prograde. Thus, the overlying tidal flat deposits can be interpreted as the HST. The absence of its HST onshore may be due to the subaerial unconformity. Alternatively, an increasing rate of sedimentation might have been related to a punctuated transgression in which temporary phases of slow relative sea-level rise or relative sea-level stillstand favour shoreface or deltaic progradation and/or aggradation (Swift et al., 1991;Zecchin et al., 2019). A high-frequency cyclicity of rising and lowering sea levels in the transgressive package developed in this case, as for example during the post Last-Glacial Maximum transgression (Berné et al., 2007; Hernández- Molina et al., 1994;Labaune et al., 2008;Maselli et al., 2011), and post-Miocene and Pliocene optimum (Miller et al., 2020). A continuous transgression recorded from the Pliocene backstepping carbonates seen on the seismic line and in the Abuki-1 well to at least Middle Pleistocene support the latter interpretation.

| SUMMARY AND CONCLUSION
The Neogene successions in SE Sulawesi began with deposition of Lower Miocene shallow marine carbonates that were succeeded by significant clastic input which formed the deltaic marginal marine Bungku Formation during a regressive cycle. They are unconformably overlain by sediments of the Upper Miocene Pandua Formation that were deposits of braided river channels, fluvio-tidal channels, tidal flats and mangroves, mouth-bar complex and storm influenced-shoreface. It is inferred that deposition occurred within deltas and embayments with mixed-energy shoreline-shelf depositional systems during a late highstand phase. Uplift of basement metamorphic and Mesozoic rocks in the Mio-Pliocene produced an unconformity that separates the Pandua and Langkowala formations. This uplift lowered the base level below the shelf edge causing fluvial erosion or bypass and downstepping regression of the shelf edge. It is reflected by deposition of forced regressive wave and storm-dominated shoreface sediments of the Langkowala Formation that abruptly overlies an outer shelf mudstone (sensu Catuneanu, 2006). Onshore, a subaerial unconformity separates the marginal marine serpentiniterich sediments of the Pandua Formation and the fluviatile quartz-rich Langkowala Formation. The lower part of the Langkowala Formation includes conglomeratic braided channel fills and coarsening upward deltaic deposits of a LST. The upper part records an upward increase of tidalinfluenced sediments, decrease in maximum grainsize, and reduction of the channel/overbank ratio that indicate a TST. Coeval carbonates of the Eemoiko Formation also record a transgressive phase that is characterised by the transgressive lags or onlap shell beds and landwards-backstepping reefal carbonates. This study, based on a comprehensive study of Neogene successions in SE Sulawesi, offers an onshore and offshore sequence stratigraphic model that may be applied in a similar depositional context to frontier basins surrounding SE Sulawesi, many of which have not yet been drilled.

AUTHOR CONTRIBUTIONS
Abang Nugraha was involved in conceptualisation, methodology, formal analysis, investigation, visualisation, writing-original draft and writing-review and editing. Robert Hall was involved in conceptualisation, supervision, funding acquisition and writing-review and editing. Marcelle BouDagher-Fadel and Jonathan Todd were involved in investigation and writingreview and editing. Adam Switzer was involved in review and editing.