Architecture and Preservation in the Fluvial to Marine Transition Zone of A

MR. DANIEL S COLLINS (Orcid ID : 0000-0002-3183-2825)

Article type : Original Manuscript

Architecture and preservation in the fluvial to marine transition zone of a mixed-process humid- tropical delta: Middle Miocene Lambir Formation, Baram Delta Province, north-west Borneo

DANIEL S. COLLINS1,2*, HOWARD D. JOHNSON1, and CHRISTOPHER T. BALDWIN3 1 Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ UK 2 Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan 3 Department of Geography and Geology, Sam Houston State University, Huntsville, Texas, 77341, USA *corresponding author: [email protected]

Associate Editor – Christopher Fielding

Short Title – Mixed-process humid-tropical delta deposition

Keywords: Delta plain, fluvial to marine transition zone, humid-tropical, mixed-process, river flood, storm-flood

ABSTRACT

The interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed-process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, north-west Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed-energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/sed.12622 This article is protected by copyright. All rights reserved. distributary channels are sandstone-rich, comprising fluvial-and wave-dominated units. Successions in lateral, or interdistributary, areas, which lack active distributary channels, are mudstone-rich, comprising fluvial-dominated, tide-dominated and wave-dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well-developed turbidity maximum zones, a consequence of high suspended-sediment concentrations resulting from tropical weathering of a mudstone-rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial-dominated, tide-influenced subzone; (ii) a distal fluvial-dominated to wave- dominated subzone; and (iii) a conspicuously absent tide-dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics.

INTRODUCTION

Process regime is a fundamental control on the continuum of facies, ichnofacies and stratigraphic characteristics in coastal–deltaic depositional systems (e.g. Bhattacharya, 2006; MacEachern & Bann, 2008). The sedimentological and ichnological signatures of process interaction are increasingly recognized in ancient delta-front successions (e.g. Ainsworth, 2005; MacEachern et al., 2005; Willis, 2005; Ainsworth et al., 2008; MacEachern & Bann, 2008; Bhattacharya & MacEachern, 2009; Ainsworth et al., 2011; Bowman & Johnson, 2014; Chen et al., 2014; Legler et al., 2014; Li et al., 2015; Rossi & Steel, 2016; Collins et al., 2017b; van Cappelle et al., 2017; Collins et al., 2018b), but much less so in delta-plain successions (e.g. Rebata et al., 2006; Pontén & Plink-Björklund, 2007; Shiers et al., 2014; Ainsworth et al., 2015; Gugliotta et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b; Shiers et al., 2017). The process classification of present-day deltas is primarily based on subaerial delta-plain to delta-front morphology (e.g. Coleman & Wright, 1975; Galloway, 1975; Ainsworth et al., 2011), with geographical variations reflecting differences in the balance of river, tide and wave processes. These include the following: (i) proximal to distal variations, for example, along distributary channels (Woodroffe & Chappell, 1993; Jones et al., 2003; Dashtgard et al., 2012b; La Croix & Dashtgard, 2015; Gugliotta et al., 2017; Gugliotta et al., 2018); (ii) lateral variations between delta plain–front areas with mostly active or inactive distributary channels (Allen & Chambers, 1998; Allison et al., 2003; Goodbred & Saito, 2012; Salahuddin & Lambiase, 2013); and (iii) discharge and oceanographic differences between multiple delta lobes (Mathers & Zalasiewicz, 1999; Panin & Jipa, 2002). Furthermore, depositional processes in deltas change on various spatial and temporal scales, including the following: (i) autogenic changes during regressive–transgressive cycles (Muto & Steel, 1997; Olariu, 2014); (ii) autogenic dynamics affecting individual delta lobes (Coleman & Gagliano, 1964; Coleman, 1988; Penland et al., 1988); and (iii) seasonal changes in river dominance, notably in monsoonal systems (Thomas et al., 1987; Jones et al., 1993; Sisulak &

This article is protected by copyright. All rights reserved. Dashtgard, 2012; Dalrymple et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b; Jablonski & Dalrymple, 2016; Gugliotta et al., 2018). Several studies of ancient mixed-process deltas have demonstrated spatial and/or temporal variations in process regime in delta-plain and delta-front environments (e.g. Willis & Gabel, 2001; Ainsworth et al., 2008; Plink-Björklund et al., 2008; Pontén & Plink-Björklund, 2009; Buatois et al., 2012; Amir Hassan et al., 2013; Chen et al., 2014; Ainsworth et al., 2015; Li et al., 2015; Ainsworth et al., 2016; Amir Hassan et al., 2016; Rossi & Steel, 2016; van Cappelle et al., 2016; Vaucher et al., 2016; van Cappelle et al., 2017; Collins et al., 2018b).

Present-day delta plains are generally divided into lower and upper regions at the limit of marine saltwater incursion (Fig. 1A) (Coleman & Wright, 1971; Coleman & Prior, 1982; Posamentier et al., 1988; Bhattacharya & Walker, 1992; Gugliotta et al., 2015). The lower delta plain is influenced by river and marine processes and includes a variety of non-marine to brackish-water environments and terrestrial to brackish-water vegetation, including mangroves in humid-tropical systems. The upper delta plain is river dominated and typically includes relatively sinuous distributary channels and more extensive floodplains with freshwater terrestrial vegetation. Delta-plain areas can also be informally subdivided into axial areas with active distributary channels (‘on axis’) and lateral areas with no active or periodically active distributary channels (‘off axis’) (Gugliotta et al., 2016a). In axial areas, distributary channels with higher water and sediment discharges are relatively ‘major’ axes compared to ‘minor’ axes with lower discharges. In coastal rivers (both distributary channels or estuaries), combined river and marine processes may influence sedimentation far upstream of the marine incursion (e.g. Bhattacharya, 2006). This fluvial to marine transition zone (FMTZ) may extend several tens to hundreds of kilometres upstream of the river mouth and is subdivided into zones based on the interplay of fluvial and marine processes (Fig. 1B and C) (Bhattacharya, 2006; Dalrymple & Choi, 2007; van den Berg et al., 2007; Martinius & Gowland, 2011; Dashtgard et al., 2012b; Dalrymple et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2017; Gugliotta et al., 2018; Gugliotta & Saito, 2019). The distal reaches of the FMTZ in tide-dominated systems is typically tide dominated and fluvial influenced (Fig. 1B) but wave and/or combined wave–tidal processes may dominate in wave-dominated systems (Fig. 1C).

The interaction of depositional processes in the FMTZ of modern and Holocene systems have been recognized (Allen & Chambers, 1998; Dalrymple et al., 2003; van den Berg et al., 2007; Dashtgard et al., 2012b; La Croix & Dashtgard, 2015; Prokocki et al., 2015; Gugliotta et al., 2017; Gugliotta et al., 2018). Less common are detailed analyses of sedimentary and stratigraphic preservation in the FMTZ of ancient deltas, including differences in preserved processes within, and between, major and minor distributary channels (Dalrymple et al., 2015; Martinius et al., 2015; Gugliotta et al., 2016a; Jablonski & Dalrymple, 2016). Rarer still are ancient examples of mixed-process deltas that can be compared to directly analogous, geographically-adjacent systems (e.g. Amir Hassan et al., 2013; Collins et al., 2017b; Collins et al., 2018b).

In the Baram Delta Province (BDP), north-west Borneo, a humid-tropical, coastal–deltaic to shelf depositional system has operated for the past 15 Myr, and continues today (Fig. 2A) (Sandal, 1996; Hall & Nichols, 2002; Lambiase et al., 2003; Morley & Back, 2008; Collins et al., 2017b; Collins et al., 2018b). Exposures of the Middle–Late Miocene Belait Formation in the eastern BDP, mostly preserve delta-front deposition dominated by the coupling of enhanced storm and fluvial processes (MacEachern et al., 2005; Pattison et al., 2007; MacEachern & Bann, 2008; Bhattacharya & MacEachern, 2009; Collins et al., 2017b; Collins et al., 2018b). However, exposures of approximately

This article is protected by copyright. All rights reserved. time-equivalent coastal–deltaic deposits in the western BDP have never been subjected to detailed process-based facies and stratigraphic analysis (cf. Nasir et al., 2017).

This study aims to elucidate how variations in river, tide and wave processes are preserved in the FMTZ of distributary channels in ancient, humid-tropical, mixed-process deltas. The objectives are as follows: (i) to describe and interpret sedimentary facies, process signals and stratigraphic architecture in the Middle Miocene Lambir Formation, western BDP; (ii) to develop a depositional model that compares these observations and interpretations with existing models of the FMTZ in ancient and present-day systems; and (iii) to decipher the controls on the facies distribution, stratal architecture and preservation in humid-tropical, mixed-process delta plains, with recourse to appropriate Modern–Holocene analogues.

GEOLOGICAL SETTING

Modern

The Modern BDP ‘source to sink’ system occurs along the active continental margin of the southern South China Sea (Fig. 2A), and comprises a narrow and steep coastal plain (<100 km, ca 1°) and shelf (<100 km, ca 0.1°) (Hiscott, 2001) as well as several small-sized (102 to 105 km2) drainage basins (Fig. 2B). The humid-topical everwet climate causes intense weathering of a mudstone-rich hinterland with fluvial discharge comprising a high suspended-sediment load (Sandal, 1996; Hall & Nichols, 2002). The discharge regime is ‘flashy’ (Smith & Ward, 1998; Dalrymple et al., 2015), with maximum fluvial sediment discharge during monsoon-influenced storms (Douglas et al., 1999; Dykes, 2000). Significantly, monsoonal storms invariably occur simultaneously with coastal storms and more intense shoreline–shelf processes (‘storm-floods’; Collins et al., 2017b). The present-day open- coastline shoreline–shelf system, including the Baram Delta (Fig. 2B), is wave dominated with subordinate fluvial and tidal influence, whereas more variable mixed-energy deposition occurs in wave-protected inshore areas, such as Brunei Bay (Fig. 2B) (Lambiase et al., 2002) (Abdul Razak, 2001; Morley et al., 2003; Collins et al., 2018b).

Ancient

The Neogene succession in the BDP consists of an up to 9 to 12 km thick succession of mainly progradational to strongly aggradational shoreline–shelf deposits that were impacted by: (i) uplift (>5 km), deep tropical weathering (humid-tropical climate) and erosion of the mudstone-dominated hinterland (Rajang-Crocker Range); and (ii) high subsidence rates (up to 3000 m/Myr; Sandal, 1996) related to deltaic gravity deformation and regional compressional tectonics (Morley & Back, 2008; Cullen, 2010; Gartrell et al., 2011).

The tectono-stratigraphy of the Middle Miocene BDP records infill of a foreland basin influenced by basement-linked faults and deltaic growth faults (Tan et al., 1999; Gartrell et al., 2011; Torres et al., 2011; Jong et al., 2017). The outcropping Middle Miocene stratigraphy in the study area, equivalent to Cycles IV to lower V (Ho, 1978; Morrison & Lee, 2003; Wannier et al., 2011; Jong et al., 2017) and TB 2.4 to 2.5 (Torres et al., 2011; Balaguru & Lukie, 2012), defines the onset of coastal–deltaic progradation (Johnson et al., 1989) and comprises the interfingering Lambir, Tukau and Miri

This article is protected by copyright. All rights reserved. formations (Fig. 2D and E) (Liechti et al., 1960; Wilford, 1961; Haile & Ho, 1991; Nagarajan et al., 2015). The Lambir Formation (ca 1.5 to 2.0 km thick) is a sandstone-dominated, mixed-energy, coastal plain–shelf succession that overlies the regional Base Middle Miocene Unconformity or Deep Regional Unconformity (Levell, 1987), which is dated at 15.50 Ma with a ca 0.5 Myr time gap (location ‘0’ in Fig. 2C; Wannier et al., 2011). Planktonic foraminifera of low chrono-stratigraphic significance suggesting an early Middle Miocene age (Fig. 2E and F). The Tukau Formation (ca 2.5 to 3.0 km thick) is a relatively heterolithic, fluvial-influenced and tide-influenced, coastal plain– shoreline succession that contains a depauperate, brackish-water foraminiferal assemblage without chronostratigraphic value. The Miri Formation (at least ca 1500 m thick) is a sandstone-dominated, mixed-energy, coastal–deltaic to shelf succession with planktonic foraminifera mainly suggesting a Middle Miocene age (Fig. 2E and F) (Wilford, 1961; Haile & Ho, 1991; Banda & Honza, 1997; Mazlan, 1999; Hutchison, 2005). Given the lithostratigraphic similarities and uncertain ages, this study follows Banda & Honza (1997) in combining the Lambir and Tukau formations (Fig. 2C) (cf. Fig. 2D; Liechti et al., 1960; Nagarajan et al., 2015). The Lambir and Miri formations are genetically related, forming the first of several north-westerly-prograding regressive deltaic wedges in the western BDP that overlie and interfinger the Setap Formation (Fig. 2E) (Johnson et al., 1989). Palaeogeographic and stratigraphic data based on onshore and offshore well and seismic data, indicate that the Middle Miocene shelf edge was within ca 10 km north to north-west of the study area (Fig. 2E and F).

DATASETS AND METHODS

Facies analysis was undertaken on thirteen GPS-located exposures of the Lambir Formation (total stratigraphic thickness 705 m) in north-west Borneo (Sarawak, Malaysia) (Fig. 2C). Exposures are relatively small (<200 m2) but of high quality for defining millimetre to decametre-scale sedimentary and stratigraphic features. Lithofacies and ichnological characterization followed the schemes of Reineck (1963), Taylor & Goldring (1993), Taylor et al. (2003) and MacEachern & Bann (2008). Exposures were studied using integrated datasets comprising logged sections (1:50 scale), detailed photomontages and field sketches. Facies were grouped into facies associations, for which process classifications were made qualitatively based on the relative proportions and process-based interpretations of facies and ichnological characteristics (e.g. MacEachern & Bann, 2008; Ainsworth et al., 2011). The three-tiered process classification of Ainsworth et al. (2011) has been adopted, which recognizes the primary (dominated), secondary (influenced) and tertiary (affected) processes; for example, ‘Fwt’ refers to fluvial dominated, wave influenced and tide affected.

FACIES ASSOCIATIONS AND DEPOSITIONAL ENVIRONMENTS

Twelve facies defined at the centimetre to metre-scale (F1 to F12 in Table 1) are grouped into 11 facies associations defined at the decimetre to decametre-scale (FA 1 to FA 11 in Table 2), which are summarized below and supplemented by representative sedimentological logs (463 m) (Figs 3 to 5), photographs and exposure panels (Figs 6 to 14).

Description

Facies Association 1 comprises 1 to 10 m thick, mudstone-dominated units (>80% mudstone) (for example, Figs 3A, 4C and 6A to 6C; Table 2). Laminated mudstones (F3) display wavy streaks and starved ripples of siltstone to very fine-grained sandstone. Subordinate, centimetre-scale, siltstone to very fine-grained sandstone (F6 and F10a) beds generally display sharp bases, wavy–planar lamination and/or combined-flow ripple cross-lamination, and may contain allochthonous assemblages of gastropod, bivalve, crinoid and shark teeth fragments (Wannier et al., 2011).

This association is equivalent to FA 1 in the Belait Formation, eastern BDP (Collins et al., 2017b), in which bioturbation is variable [Bioturbation Index (BI) 0 to 5], increases towards bed tops, and includes (in alphabetical order) Ophiomorpha, Planolites, Palaeophycus and Thalassinoides, corresponding to a slightly impoverished expression of the Cruziana Ichnofacies (MacEachern & Bann, 2008). The top 1 m of the underlying Setap Formation (Fig. 2E, outcrop 0; Fig. 3A, ca 3.5 to 4.4 m on the log) displays a diverse ichnofauna assemblage (BI 5) including Chondrites, Cosmorhaphe, Cylindrichnus and Thalassinoides (Fig. 6C), as well as centimetre-scale Ophiomorpha and Teichichnus that extend downward from the immediately overlying FA 3 (Fig. 3A).

Interpretation

Mud deposition may occur under low (Howard & Reineck, 1981) or high energy (Schieber et al., 2007; Plint, 2014), and the laminated mudstones may record deposition by turbulent–laminar, mixed clay–silt flows, possibly related to combined river and storm discharge (Bhattacharya & MacEachern, 2009; Plint, 2014; Baas et al., 2016; Collins et al., 2017b). Deposition above storm-wave base is consistent with erosional sandstone beds displaying combined-flow ripple cross-lamination (Dott & Bourgeois, 1982; Duke, 1985; Dumas et al., 2005; Plint, 2014) and trace fossil suites indicative of a slightly impoverished Cruziana Ichnofacies (e.g. Buatois et al., 2008; MacEachern & Bann, 2008; Buatois et al., 2012). Consequently, FA 1 is interpreted to represent deposition under fluctuating energy levels in a storm-dominated and river-influenced prodelta. The ichnological suite in the uppermost Setap Formation is interpreted to record overprinting by Ophiomorpha and Teichichnus and firmground omission during a hiatus in shelf deposition preceding deltaic progradation (overlying FA 3 in the Lambir Formation).

Facies Association 2: Interbedded swaley cross-stratified, very fine-grained sandstone and mudstone to muddy heterolithic units

Description

Facies Association 2 forms 2 to 25 m thick heterolithic units (20 to 80% sandstone) of interbedded very fine-grained sandstones (F6), heterolithics (F4a and F5a) and mudstones (F2a and F3) (Figs 3A, 4C, 6A and 6B; Table 2). Sandstone beds are ca 0.25 to 1.0 m thick and form amalgamated units up to ca 1.5 m thick that mostly display: (i) basal gutter casts with common mudstone clasts; (ii) swaley cross-stratification (SCS); (iii) upward fining; and (iv) sharp bed tops (cf. FA 2 in Collins et al., 2017b). Draping centimetre to decimetre-scale mudstone layers are typically laminated or apparently

This article is protected by copyright. All rights reserved. structureless. Heterolithic intervals comprise centimetre-scale siltstone–very fine-grained sandstones with combined-flow ripple cross-lamination.

Bioturbation in sandstones is low (BI 0 to 1), increases near bed tops (BI 1 to 3), and is dominated by Ophiomorpha with subordinate Asterosoma, Palaeophycus, Planolites and fugichnia, constituting an impoverished and distal expression of the Skolithos Ichnofacies (MacEachern & Bann, 2008; Buatois et al., 2012). Bioturbation in draping mudstone layers is similarly minimal and mostly top-down Ophiomorpha. Bioturbation in the heterolithics facies is variable, BI 1 to 3 but locally up to BI 4 to 5. The dominant ichnogenera are Chondrites, Palaeophycus, Planolites and Thalassinoides, which represents an impoverished expression of the Cruziana Ichnofacies (MacEachern & Bann, 2008; Buatois et al., 2012).

Interpretation

Facies Association 2 is interpreted to have formed under episodic, erosive, waning-energy, oscillatory or oscillatory-dominated combined flows, as indicated by: (i) interbedded sandstone and mudstone-dominated facies; (ii) erosional bases and lags; (iii) normal grading; and (iv) SCS and combined-flow ripple cross-lamination (Dott & Bourgeois, 1982; Dumas et al., 2005). Bioturbation in the sandstones suggests opportunistic colonization (Pemberton et al., 1992; Pemberton et al., 1997) during and after storm-floods (Collins et al., 2017b). Draping mudstone layers may have formed by either bedload transported fluid mud or rapid mud flocculation from river-flood plumes (MacEachern et al., 2005; MacEachern & Bann, 2008). Bioturbation in the heterolithic facies suggests physico-chemical stress, most likely due to fluctuating sedimentation rates and/or salinity (MacEachern & Bann, 2008). Overall, FA 2 is interpreted to record deposition in a storm-dominated, river-influenced (storm-flood-dominated) distal delta front between fair-weather and storm-wave base (for sand) (Reading & Collinson, 1996; Buatois et al., 2012; Collins et al., 2017b).

Facies Association 3: Amalgamated swaley cross-stratified, very fine-grained sandstone-dominated units

Description

Facies Association 3 comprises 2 to 25 m thick sandstone-dominated (>80%) units with very fine- grained sandstone beds displaying SCS (F6) and subordinate hummocky cross-stratification (HCS; F7), with millimetre-scale laminae composed of mudstone and/or carbonaceous material (Figs 3A, 4C and 6D; Table 2). Sandstone bedsets up to 8 m thick consist of ca 0.2 to 1.0 m thick beds that display erosional bases and local soft-sediment deformation. Bioturbation in sandstone facies is generally low (BI 0 to 1) and dominated by facies-crossing ichnogenera including Ophiomorpha, Palaeophycus and Skolithos. Ichnological characteristics in the heterolithic (F4a and F5a) and mudstone facies (F2a and F3) resemble those in equivalent facies of FA 1 and FA 2.

The dominance of SCS, grain size range, stratification style, scale of erosional surfaces and impoverished ichnofauna suggest high sedimentation rates but with variable hydrodynamic conditions (MacEachern & Bann, 2008; Collins et al., 2017b). Compared to FA 1, the increased thickness and amalgamation of sandstone beds displaying SCS may be attributed to increased rates and magnitude of storm-wave reworking and/or increased sand availability related to the following: (i) decreased water depth; (ii) increased storm-wave energy; and/or (iii) increased proximity to the sediment source (Swift & Thorne, 1991; Thorne et al., 1991; Storms & Hampson, 2005). The frequency of mud and carbonaceous material suggests access to a mixed, partly fluvially-supplied, sediment load. Consequently, FA 3 is interpreted to record deposition in a relatively proximal, storm- flood-dominated delta front.

Facies Association 4: Interbedded swaley cross-stratified and low-angle planar cross-stratified sandstone units with rare mudstone drapes

Description

Facies Association 4 comprises sharp-based, 2 to 3 m thick sandstone-dominated units (>95% sandstone) that sharply overlie FA 6 (Figs 4 and 5F). Sandstones are very fine-grained and display interstratified decimetre-scale sets of SCS (F6) and low-angle planar cross-stratification (F8b) (Fig. 7A and B; Table 2). Less common decimetre-scale lenticular beds comprise upper fine-grained, trough cross-stratified sandstone (F9a). Mudstone and carbonaceous laminae, drapes and clasts are common throughout, but lack organization (Fig. 7C to E). Bioturbation is generally low (BI 0 to 2) but sporadically distributed and locally intense (BI 3 to 5), characteristically concentrated along specific interfaces, notably apparent dwelling structures along cross-set boundaries in SCS (Fig. 7C to E). Ichnofauna diversity is moderate and includes: (i) Ophiomorpha, Planolites, Palaeophycus and Gyrolithes; (ii) facies-crossing forms that are often overprinted by other forms (Fig. 7C to E); and (iii) various dwelling and escape structures (fugichnia).

Interstratified muddy–sandy heterolithics (F4a and F5a) are centimetres to a few decimetres thick and rich in carbonaceous material (Fig. 7C). Bioturbation is relatively high (BI 3 to 5) and dominated by simple horizontal and vertical sandstone-filled burrows (for example, Planolites and Skolithos).

Interpretation

The dominance of SCS and low-angle cross-stratification indicates the dominance of storm-related oscillatory and/or oscillatory dominated combined flows (Dott & Bourgeois, 1982; Duke, 1985; Dumas et al., 2005). However, the abundance of mudstone and carbonaceous material, and the stratigraphic association and textural similarities with FA 6 to FA 8 (Figs 4G and 5F), suggest the influence of combined marine and fluvial processes (Raychaudhuri & Pemberton, 1992; Bann & Fielding, 2004; MacEachern et al., 2005; Hansen et al., 2007; Maceachern et al., 2007; Bann et al., 2008; MacEachern & Bann, 2008; Collins et al., 2017b). Current-dominated F6 records subordinate supply of coarser-grained sediment supply and tractional reworking by river and/or tidal currents. The sharp basal surface could record partial hydrodynamic abandonment of a fluvial–tidal channel

This article is protected by copyright. All rights reserved. near the distributary river mouth, which permitted reworking by wave, storm and possibly tidal processes. The facies characteristics resemble wave-influenced and tide-influenced mouth bars in the Sego Sandstone (FA 4 in Legler et al., 2014). Ichnological characteristics are consistent with some physical and/or chemical stresses during mixed-energy inshore deposition, including possible intermittent colonization of sedimentary surfaces (Davies & Shillito, 2018). Consequently, FA 4 is interpreted as storm-reworked, fluvial-influenced mouth bars with possible tide influence.

Facies Association 5: Sharp-based, trough cross-stratified, fine to medium-grained sandstone- dominated units

Description

Facies Association 5 comprises ca 3 to 10 m thick sandstone units (>95% sandstone) with sharp, erosional, flat to concave-upwards bases (Figs 4E, 8 and 9) and common lags of sandstone, quartzite and mudstone granules–pebbles (F11a and F11b). The dominant sandstone facies (F9c) are fine to lower medium-grained and form amalgamated, sharp-based, centimetre-thick to a few decimetre- thick beds displaying unidirectional, north-west to east-directed trough cross-stratification, rare mudstone and carbonaceous clasts and drapes, and very rare bioturbation consisting of diminutive Planolites associated with the mudstone drapes (Fig. 9A to C). Coset and bed boundaries are commonly low-angle sigmoidal surfaces that mostly have an apparent orientation approximately 90° to intervening palaeocurrent directions (Fig. 9A and B).

Interpretation

The basal erosional surfaces, with common lags (including extraclasts), sedimentary structures indicative of unidirectional traction currents, and overall fining upward grain-size trend, suggest that FA 5 was deposited under high sedimentation rates in river-dominated distributary channels (Miall, 1985; Flood & Hampson, 2014; Ainsworth et al., 2015; Gugliotta et al., 2016a). The depauperate bioturbation, represented exclusively by Planolites, suggests high stress, such as sporadic and short- lived brackish-water conditions, or perhaps due to storm-tides and/or incursion of a saltwater wedge. However, Planolites can also form in freshwater conditions (Lettley et al., 2007; Gérard & Bromley, 2008). The orthogonal relationship between palaeocurrent direction and the dip of low- angle sigmoidal surfaces between cosets suggest that the bars formed by lateral accretion. Consequently, FA 5 units are interpreted to preserve sinuous, laterally-migrating, river-dominated distributary channels, with subordinate saltwater incursions.

Facies Association 6: Sharp-based, trough cross-stratified, mud-draped, fine to medium-grained sandstone-dominated units

Description

Facies Association 6 comprises ca 3 to 10 m thick sandstone-dominated units (80 to 95% sandstone) with erosional, flat to concave-upward bases, commonly lined with mudstone and carbonaceous

This article is protected by copyright. All rights reserved. granules–pebbles and locally delineated by decimetre-thick, structureless mudstone beds (Figs 8, 9D to 9H and 10; Table 2). Sandstones are predominantly very fine to lower medium-grained, and form centimetre to decimetre-thick lenticular beds that display 3 to 20 cm thick cosets of trough cross bedding (F9b), which exhibit: (i) mostly north-west to north-east-directed palaeocurrents; (ii) subordinate south-west to south-east-directed palaeocurrents; (iii) scoop-shaped foresets to toesets; (iv) subtle grain-size variations within cross-sets; and (v) erratic reactivation surfaces or bed boundaries (Figs 3 to 5 and 9D to 9G). Sandstone-rich units or sub-units (ca 95% sandstone) predominantly comprise decimetre-scale beds displaying low-angle trough to planar cross- stratification (F8c and F9b). Coset and/or bed boundaries locally form low-angle sigmoidal surfaces (Fig. 8E and F). Internal erosional surfaces are common (Fig. 8A to D). Bottomsets, the lower parts of foresets and coset boundaries show common millimetre to centimetre-scale clasts and drapes of laminated or structureless mudstone and/or carbonaceous debris, which lack apparent cyclicity (Fig. 9G). Heterolithic intervals commonly occur in toesets and delineate bed boundaries and are characterized by: (i) a millimetre to centimetre-scale layering, but without definitive cyclical organization; (ii) up to 5 cm thick structureless mudstone and up to 2 cm thick laminated mudstones layers; (iii) current-ripple cross-laminated sandstones; and (iv) elevated bioturbation intensities (BI 2 to 3). There is a transitional, fining-upward change from FA 6 to FA 7, as the thicknesses of heterolithic units increase and sandstone-dominated layers decrease (Fig. 9H). Bioturbation in sandstone facies is sporadically distributed and variable (BI 0 to 3), typically concentrated at coset or bed boundaries. Bioturbation is moderately diverse and includes Planolites, Ophiomorpha, Palaeophycus and Skolithos, with complicated centimetre to decimetre-scale biogenic structures (see FA 7 for further description and interpretation). Palynological assemblages in similar facies in the Lambir Formation, including genetically-linked, mud-filled abandoned channels (see FA 8), contain spores with assemblages rich in mangrove, back-mangrove and coastal vegetation (Simmons et al., 1999). Bulk pyrolysis analysis of an F11a sample from locality 2d indicates an overwhelming dominance of land plant material over marine algal sources (Togunwa & Abdullah, 2017), as expected for paralic systems (Czarnecki et al., 2014). One perfectly preserved Turritella shell was found in FA 6 (1 to 2 m in Fig. 5C).

Interpretation

The erosional base, dominance of unidirectional cross-stratification and occasional bidirectional cross-stratification suggests FA 6 preserves deposition in fluvial-dominated, tide-influenced channels. The low intensity and moderately diverse ichnofauna of inferred trophic generalists (MacEachern & Bann, 2008) suggests: (i) a partially stressed environment, most likely reflecting low variable salinity; and (ii) brackish-water conditions, probably related to mixed fluvial and tidal processes (e.g. Pemberton et al., 1992; MacEachern & Bann, 2008). The dominance of palaeo- seaward-directed palaeocurrents with minor palaeo-landward components (Fig. 2F) suggests river currents dominated, possibly supplemented by ebb tides. Reactivation surfaces and mudstone and carbonaceous drapes without cyclicity form under purely fluvial to mixed fluvial–tidal processes (e.g. Thomas et al., 1987; Rebata et al., 2006; Pontén & Plink-Björklund, 2007; Hovikoski et al., 2008; Martinius & Gowland, 2011; Reesink et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b; Jablonski & Dalrymple, 2016). Bioturbation concentrated near coset and bed boundaries may record opportunistic colonization during upstream saltwater incursions, especially during low river

This article is protected by copyright. All rights reserved. discharge (e.g. Dalrymple et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b). Structureless or laminated mudstone layers are consistent with fluid-mud deposition (e.g. Ichaso & Dalrymple, 2009; MacKay & Dalrymple, 2011). The complex interbedding between cross-stratified sandstone and heterolithic intervals suggests variations in river versus tidal hydrodynamics and sediment supply. The mangrove-dominated spore assemblages support a close proximity to a tide-influenced lower delta plain. The Turritella shell indicates marine influence, possibly recording a relative transgressive period during channel abandonment. Consequently, FA 6 units are interpreted to preserved mixed-process channelized deposition under spatially and temporally variable fluvial–tidal conditions.

Facies Association 7: Current ripple cross-laminated and trough cross-stratified, mudstone to sandstone-dominated heterolithic units

Description

Facies Association 7 comprises ca 2 to 6 m thick units with typically sharp, erosional, flat to concave- upward bases (Figs 8A, 8B, 8E, 8F and 11A to 11C) and, less commonly, gradational bases (Fig. 10C and D; Table 2). Dominant facies are sandy heterolithics (ca 50 to 80% sand; F5b) and trough cross- stratified sandstones (ca >80% sandstone; F9b). Less common are muddy heterolithics (ca 20 to 50% sandstone; F4b) and planar cross-stratified sandstones (F8c) (Fig. 12) that may be arranged in thickening-upward and sandier-upward units (Fig. 11D).

Heterolithics are wavy to flaser-bedded and contain moderately to well-sorted, very fine to fine- grained sandstone in centimetre-thick and, occasionally, decimetre-thick beds that typically display: (i) sharp erosional bases; (ii) mostly unidirectional current ripple cross-lamination with subordinate, oppositely-dipping sets; (iii) undulatory to planar cross-stratification; and (iv) millimetre-scale mudstone and/or carbonaceous, laminated or structureless flasers and drapes (Fig. 12). Bed boundaries commonly define up to ca 5 m high, low-angle, inclined <10° planar surfaces (for example, Fig. 8E and F). Sandstone layers locally occur in groups of two to five beds, separated internally by mudstone flasers or layers (for example, Fig. 12A and D). Structureless mudstone layers several centimetres thick are infrequent (for example, Fig. 12B). Coal and amber clasts are common. Intercalation of sandstone and mudstone and/or carbonaceous layers, flasers and/or drapes occur on a wide range of scales but lack regular cyclicity (Fig. 12).

Decimetre-scale lenticular beds of moderate to well-sorted, fine-grained sandstones (Fig. 11 A, B and D) generally display: (i) an erosional base, with local, basal mudstone-intraclast breccias; (ii) tabular to lenticular geometries; and (iii) scoop-shaped trough and/or planar cross-stratification (F8c and F9b), and rare SCS (F6) (Fig. 12B). Sandstone bed bases can extend laterally over tens of metres and define flat to concave-upwards erosional surfaces (Fig. 11A and B). Trough cross-sets are 5 to 15 cm thick and typically show: (1) north-west to north-east-directed palaeocurrents with rarer south-west to south-east directions; and (ii) irregularly distributed mudstone and/or carbonaceous clasts and drapes on toesets and the lower parts of foresets (Fig. 12E).

This article is protected by copyright. All rights reserved. Bioturbation in the heterolithic and sandstone facies is generally sporadically distributed (decimetre- scale areas) and locally intense (BI 0 to 5). Elsewhere, bioturbation is more persistent and uniform, showing BI 3 to 5 (Fig. 12D). The dominant ichnogenera are Ophiomorpha, Cylindrichnus, Palaeophycus tubularis, Planolites, Palaeophycus, Skolithos, Taenidium and Thalassinoides, and possible Siphonichnus (Fig. 12A, C, D and F). Bioturbation often occurs at sandstone–mudstone interfaces. Additionally, there are sporadic, complicated biogenic structures that variably exhibit the following characteristics: (i) centimetre to decimetre-scale widths and lengths; (ii) equidimensional, downward tapering or downward widening; and (iii) chaotic, structureless or muddy infills, or partial preservation of the original layering (Fig. 12A and F to H). These biogenic structures probably capture a diverse group of structures, ranging from intrastratal repichnia, or locomotion, vertical ‘equilibrium’ adjustment, escape (fugichnia) and predation (for example, Fig. 12 A, C, D and F).

Interpretation

The typically sharp base indicates channelized deposition. The internal erosional surfaces, ubiquitous sandstone–mudstone layering, and pronounced variability in sandstone content, grain size, sedimentary fabric, and bed thickness and geometry, attests to strong spatial and temporal fluctuations in hydrodynamic energy and mixed sediment supply. The dominant current ripple cross- lamination and trough cross-stratification indicates deposition by traction currents. The large-scale, low-angle inclined surfaces resemble those within inclined heterolithic strata (IHS) (Barwis, 1978; Thomas et al., 1987; Choi et al., 2004; Dalrymple & Choi, 2007; Legler et al., 2013; Martinius et al., 2015; Gingras et al., 2016; Jablonski & Dalrymple, 2016; Timmer et al., 2016). The sporadically distributed and variable intensity bioturbation, coupled with moderate to low diversity, suggests deposition under physio-chemical stress by mostly opportunistic ichnofauna. This is consistent with spatial and temporal fluctuations in the relative strength of inferred fluvial and tidal currents (Pemberton & Frey, 1982; Buatois et al., 2005; MacEachern et al., 2005; Buatois et al., 2012; Gingras et al., 2016).

The centimetre to decimetre-thick, sharp-based sandstone beds and associated structureless mudstone layers most likely record sediment transport and deposition during relatively low- frequency, high-energy, erosive, suspended-sediment-rich river floods (e.g. Dalrymple et al., 2015; Gugliotta et al., 2015; Gugliotta et al., 2016b; Collins et al., 2018b). Predominantly unidirectional, north-west to north-east-directed (palaeo-seaward) palaeocurrents suggest river dominance, possibly augmented by ebb-tidal currents (e.g. Dashtgard et al., 2012b). However, the local south- west to south-east-directed trough-cross-sets containing mudstone drapes with apparent cyclicity, probably record deposition by mixed river and tidal processes. Sandier-upward units are consistent with preservation of lateral or down-current migrating fluvial–tidal bars (e.g. Dalrymple & Choi, 2007; Legler et al., 2013; Gugliotta et al., 2015).

Heterolithic facies are interpreted to record transport and deposition during relatively high- frequency, low-magnitude river floods and interflood periods with a background tidal influence. The prevalence of unidirectional palaeocurrents supports river dominance, but minor bidirectional palaeocurrents and increased bioturbation in muddier facies suggests subordinate tidal influence. Laminated mudstones probably record low-energy suspension fallout during falling-stage river flow

This article is protected by copyright. All rights reserved. and/or tidal slackwater (Gugliotta et al., 2016b). Structureless mudstone layers probably record fluid mud deposition from turbulent–plug flows (Dalrymple & Choi, 2007; Ichaso & Dalrymple, 2009; Baas et al., 2016) that shielded underlying sandy substrates from bioturbation (MacEachern & Bann, 2008). The absence of definitive cyclical organization of mudstone and/or carbonaceous layers and/or drapes makes a purely tidal origin for the heterolithic sedimentation equivocal. Instead, mud deposition is likely to record mud flocculation and deposition under elevated suspended-sediment concentrations during and after river floods, independent of tidal rhythmicity. Additionally, the generally unordered variations in thickness of sandstone and mudstone layers suggests that: (i) tides were incapable of modulating river-flood dynamics or significantly reworking river-flood deposits; and (ii) river flood discharge was erratic and highly variable, which is typical of flashy, monsoon- dominated fluvial systems with small, steep catchment areas (Smith & Ward, 1998; Dalrymple et al., 2015).

The complex interaction of river and tidal processes inferred for FA 7 are directly comparable to IHS in the Lower Cretaceous McMurray Formation, Alberta Basin (Smith, 1988; Musial et al., 2012; Martinius et al., 2015; Jablonski & Dalrymple, 2016; Timmer et al., 2016), and comparable facies in the Jurassic Lajas Formation, Neuquén Basin (e.g. Dalrymple et al., 2015; Gugliotta et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b; Rossi & Steel, 2016), Middle Neslen Formation (Dalrymple et al., 2015) and the Belait Formation, eastern BDP (Collins et al., 2018b). Overall, it is concluded that FA 7 records deposition in channelized barforms in a fluvial-dominated, tide- influenced lower delta plain.

Facies Association 8: Sharp-based laminated and structureless mudstones to muddy heterolithic units

Description

Facies Association 8 comprises 1 to 4 m thick units with erosional, flat to concave-upward bases that abruptly overlie FA 6 (Figs 8 and 13). Dominant facies are interbedded laminated (F3b) and structureless (F2a) mudstones (ca <20% sandstone; Table 2). Laminated mudstones contain the following: (i) very fine to fine-grained sandstone and/or carbonaceous laminae, which lack systematic vertical organization; (ii) lenses of starved current ripple cross-lamination; and (iii) coalified fragments (Fig. 14A and B). Mangrove palynomorphs are the dominant spore type (e.g. Togunwa et al., 2015). Structureless mudstone layers are centimetre to decimetre-scale and commonly overlain by carbonaceous laminae. Rare centimetre-scale, fine-grained sandstone layers display sharp bases, planar to trough cross-stratification and common mudstone and carbonaceous laminae (Fig. 14B). Bioturbation intensities are very low (BI 0 to 1) and comprises small-scale sand- filled Planolites and Palaeophycus.

Interpretation

The cross-sectional geometry, mud-dominated infill and apparent co-genetic relationship to underlying FA 6 confirms deposition in partly abandoned fluvial–tidal channel with significantly reduced sand supply and hydrodynamic energy. Laminated mudstones likely record deposition by relatively low-energy suspension settling and minor traction currents. Structureless mudstones more

This article is protected by copyright. All rights reserved. likely record fluid mud deposition, which would suggest continued, perhaps periodic, of freshwater– saltwater mixing and high suspended sediment concentrations (Wright et al., 1988; Uncles et al., 2006). Abundant, mangrove-rich carbonaceous material probably attests to fluvial–tidal current reworking of lower delta plain vegetation along channel margins. Rare sandstone layers probably record infrequent river floods. The depauperate ichnological suite confirms an inhospitable environment (Beynon et al., 1988; Buatois et al., 2005; MacEachern & Gingras, 2007). Consequently, FA 8 is interpreted to record lower delta plain deposition in partly abandoned fluvial–tidal channels with significantly reduced sand supply.

Facies Association 9: Variably bioturbated mudstone to muddy heterolithic units

Description

Facies Association 9 comprises ca 1 to 20 m thick, sharp or gradational-based units, dominated by laminated (F3b) mudstones and muddy heterolithics (F3a or F3b) (Figs 13, 14C and 14D; Table 2). Laminated mudstones commonly show very fine to fine-grained sandstone and carbonaceous laminae, as well as coaly fragments. Lenticular to wavy-bedded muddy heterolithics contain centimetre-scale, sharp-based, very fine to fine-grained sandstone layers displaying horizontal lamination and current ripple cross-lamination, with minor combined-flow ripple cross-lamination. Bioturbation is of variable intensity (BI 1 to 5) and sporadically distributes, with suites including Skolithos, Planolites, Palaeophycus and Thalassinoides (Fig. 14C and D). Subordinate structureless mudstones include scattered carbonaceous detritus and sparse bioturbation (BI 0 to 1). The carbonaceous material is typically rich in mangrove-derived pollen (e.g. Togunwa et al., 2015). Subordinate sandstone beds are decimetre-scale, sharp-based beds and variably display trough cross-stratification (F9b), wavy to planar cross-stratification (F8b and F8c) or SCS (F6). Sporadic bioturbation (BI 0 to 2) includes Gyrolithes, Ophiomorpha and occasional complex biogenic structures.

Interpretation

Horizontal laminated mudstones without starved current ripples (cf. FA 8) suggest deposition by suspension fallout, whereas structureless mudstones suggest episodic fluid mud deposition. Muddy heterolithics record combined fluvial–tidal processes (see FA 7). Carbonaceous debris suggests reworking of mangrove-dominated vegetation. The sporadic, variable bioturbation suggests high but variable physio-chemical stresses. The ichnological suite is typical of reduced salinity settings (Gingras et al., 1999; Buatois et al., 2005; MacEachern & Bann, 2008; Gingras et al., 2016). Sandstone layers represent river-flood and storm deposition. The variable facies characteristics are consistent with river-influenced and tide-influenced, lower delta plain deposition in a marginal-marine embayment under variable but generally low wave energy.

Description

Facies Association 10 comprises ca 2 to 5 m thick units dominated by intensely and relatively uniformly bioturbated (BI 4 to 5) mudstones (F2c) and muddy sandstone to sandy mudstones (F11), which contain the following: (i) abundant carbonaceous debris, including common millimetre- to centimetre-scale coaly fragments; (ii) rarely preserved horizontal lamination and lenticular- to wavy- bedding; and (iii) very rare centimetre-scale coal layers (F1) and coalified roots (Fig. 14E; Table 2). Palynological assemblages in similar facies in the Middle Miocene Belait Formation, eastern BDP, are dominated by mangrove pollen (Simmons et al., 1999). The trace fossil suite is largely indistinct but contains probable Ophiomorpha, Planolites and Thalassinoides.

Interpretation

The intense bioturbation, abundant organic material, coal layers, coalified roots and dominance of mangrove pollen in analogous, time-equivalent facies in the eastern BDP (Simmons et al., 1999), suggest deposition on a mangrove-fringed lower delta plain, probably in a low-wave-energy coastal embayment (Amir Hassan et al., 2013). Sharp basal contacts with FA 6 (Fig. 5F) also suggest that FA 10 may locally represent the infill of abandoned fluvial–tidal channels in mangrove swamps (Simmons et al., 1999).

Facies Association 11: Intensely bioturbated muddy sandstone to sandy mudstone units

Description

Facies Association 11 comprises ca 2 to 8 m thick units dominated by intensely bioturbated muddy sandstone to sandy mudstone with common carbonaceous material (Fig. 14F–H; Table 2). The intense (BI 4 to 5) and largely uniformly distributed bioturbation is mostly characterized by indistinct mottling but locally contains possible Psilonichnus, Thalassinoides, Palaeophycus, Teichichnus, Chondrites, Asterosoma, Phycosiphon and decimetre-scale unidentified biogenic structures. Bioturbation may be locally dominated by several decimetres long and several centimetres wide, sandstone-filled vertical shafts with irregular walls and similar diameter, sandstone-filled horizontal burrows (Fig. 14E to H). Apparent branching may be overlap of inclined vertical shafts (Fig. 14E) and rare J-shaped burrows with a heterolithic infill (Fig. 14F to H). Together, these features are more consistent with Psilonichnus rather than Ophiomorpha, Skolithos or Thalassinoides (Fig. 14F to H). Heterolithic backfill in these dwelling structures reflects physical sedimentation into the open burrows, similar to ‘tubular tidalites’ (Gingras & Zonneveld, 2015). Rarely preserved primary sedimentary structures are highly variable and include muddy to sandy heterolithics, trough cross- stratification, SCS, and/or low-angle planar cross-stratification.

Partial preservation of several sedimentary structures and variable association with FA 6, FA 7 and FA 9, suggest inshore, mixed-process deposition. The intense bioturbation characterized by a moderate diversity trace fossil suite probably reflects a combination of salinity stress and reduced sedimentation rates (Buatois et al., 2005; MacEachern & Bann, 2008). The probable large-scale Psilonichnus attest to deep-tiering and dwelling by crustaceans and suggest that the substrate was partially dewatered and semi-consolidated, consistent with reduced sedimentation rates (Frey et al., 1984; Nesbitt & Campbell, 2006; MacEachern & Bann, 2008). Deposition may have occurred in an embayment setting (MacEachern & Bann, 2008), perhaps analogous to parts of modern Brunei Bay (Collins et al., 2018b).

STRATIGRAPHIC ARCHITECTURE

The gross stratigraphic architecture of the Lambir Formation reflects a ca 3.4 Myr period of large- scale deltaic regression (Fig. 2E). At outcrop-scale, this architecture is manifested by the following distinctive vertical facies association relationships: (i) gradationally-based sandier-upward units (SU1, SU2 and SU3; ca 10% of the gross preserved stratigraphy) (Fig. 15A); and (ii) erosively-based, muddier-upward units (C1, C2 and C3; ca 90% of the gross preserved stratigraphy) (Figs 15B to 15E, 16 and 17).

Gradationally-based, sandier-upward units

Three types of sandier-upward (SU) units (Fig. 15A) are distinguished by their different environment and process regimes: (i) SU1 are up to 15 m thick, comprise FA 1 to FA 3 (bottom to top) (Fig. 4C), and are interpreted as storm-dominated delta front successions; (ii) SU2 are up to 10 m thick, variably comprise (bottom to top) FA 1 or FA 9, through FA 4 and/or FA 7, to FA 6 (Fig. 5F), and are interpreted as mixed wave-influenced, tide-influenced and river-influenced delta front successions; and (iii) SU3 are 2 to 6 m thick, comprise FA 9 (bottom) to FA 7 (top) and individual FA 7 units (Figs 3D and 5F), and interpreted as fluvial–tidal bars within lower delta plain distributary channels.

Successions comprising SU1 units (locality 0, Fig. 3A) dominant in the partially lateral equivalent Miri Formation (Fig. 2E) but are effectively erosional remnants of delta front deposits adjacent to active distributary channels in the Lambir Formation (Figs 17 and 18).

Erosively-based, muddier-upward units

There are three main types of erosionally-based, muddier-upward units, which range from sand- dominated (C1), through heterolithic (C2) and into mudstone-dominated (C3) (Figs 15B to E). They are interpreted as single-storey and multi-storey channel bodies (Fig. 16).

Type C1: Sandstone-dominated channel units

Sandstone-dominated channel units (C1) are at least ca 10 m thick, erosionally overlie FA 6 to FA 9, and almost exclusively comprise FA 5 (for example, Figs 8A, 8B, 9A, 9B, 15B and 16A). This suggests preservation of laterally accreting point bars and downstream accreting bars in fluvial-dominated distributary channels. The sharp contacts between facies associations in C1 units may preserve

This article is protected by copyright. All rights reserved. stepwise infill of a single channel (Fig. 16A) or lateral erosional contacts between multiple channels (for example, Figs 8C, 8D , 9A and 9B).

Type C2: Sandstone-dominated to heterolithic channel units

Sandstone-dominated to heterolithic channel units are between ca 1.5 to 12.0 m thick (average ca 6 m thick) and are dominated by FA 6 (C2.1) and/or FA 7 sub-units (C2.2), with subordinate overlying FA 4 (C2.3) and FA 8 to FA 10 (C2.1 to C3) (Figs 15C and 16B). The abrupt vertical transitions between facies associations (for example, Fig. 8) may record: (i) changes in sand versus mud supply and/or fluctuating hydrodynamic energy in single-storey channel bodies; or (ii) multi-storey horizontal and/or vertical stacking of separate channel bodies (Fig. 16B).

Comparable vertical facies successions have been observed in the following examples: (i) tide- dominated Holocene–Modern Fly River delta (Dalrymple et al., 2003); (ii) tide-dominated Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) (Legler et al., 2013); (iii) mixed wave- influenced and tide-influenced deltaic successions in the Cretaceous Bearpaw–Horseshoe Canyon Formation (Ainsworth et al., 2015); (iv) mixed wave-influenced and tide-influenced deltaic successions in the Cretaceous Lower Sego Sandstone (Legler et al., 2014; van Cappelle et al., 2016), and (v) fluvial-dominated, tide-influenced deltaic successions in the Middle Jurassic Lajas Formation (Gugliotta et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b; Rossi & Steel, 2016).

Type C3: Mudstone-dominated channel units

Mudstone-dominated channel units (C3) are up to 3.5 m thick and comprise combinations of FA 8 to FA 10 with subordinate FA 6 to FA 7 (Figs 15D and 16C). These units are interpreted to preserve mostly subtidal (C3.1 and C3.2) and intertidal (C3.3), partly abandoned fluvial–tidal channels. Units dominated by FA 8 more commonly record river- and tide-influenced deposition compared to FA 9, which also preserves local evidence of wave and storm influence. Abandoned river distributaries in subtidal to intertidal delta regions become estuaries that are subject to complex biological and physical feedbacks, especially increased reworking by marine processes (Coleman, 1988; Penland et al., 1988; Allen & Chambers, 1998; Temmerman et al., 2007; Murray et al., 2008; van Maanen et al., 2015).

DEPOSITIONAL MODEL AND PROCESS VARIATIONS

The Lambir Formation mainly preserves the lower delta plain of a mixed-process delta characterized by: (i) variable interactions of river and tidal influence (Fig. 18); and (ii) simultaneous modification by monsoon-related, river flood and storm processes (‘storm-floods’; Collins et al., 2017b). Channel units are the dominant architectural element, with their deposition and preservation reflecting: (i) a FMTZ setting; (ii) spatial variability, including proximal to distal trends, along distributary channel axes (axial areas) and between axial and lateral delta-plain areas; and (iii) temporal variability linked to high and low river stage (e.g. Dalrymple et al., 2015; Gugliotta et al., 2016a; Gugliotta et al., 2016b). Consequently, facies-association relationships and stratigraphic architectures are considered in terms of: (i) proximal to distal deposition along major distributary channel axes with higher discharge; (ii) deposition in axial areas along major and minor distributary axes (‘on axis’) versus lateral areas (‘off axis’); and (iii) turbidity maximum distribution (following Gugliotta et al., 2016a).

Preserved deposits along major distributary channels comprise thick (10 to 30 m), sandstone-rich channel belts, with C1 and C2 units dominant and subordinate C3 (mostly C3.1) units (Figs 17 and 18). Sandstone-dominated C1 units record deposition in river-dominated axial zones, which is supported by unidirectional, palaeo-seaward-directed palaeocurrents and absence of tide- modulated features. Subordinate bidirectional palaeocurrents and increased mud draping suggest minor tidal influence, while scoop-shaped cross-sets and non-cyclical mudstone drapes are equivocal evidence for tidal processes. Hence, these units suggest river-dominated deposition with minor tidal influence (Gugliotta et al., 2016a).

Heterolithic FA 7 in C2.1 and C2.2 units record the dominance of river flood–interflood deposition. Rare, but compelling, evidence of tidal processes include bidirectional cross-stratification, while more ambiguous tidal indicators include sporadically distributed bioturbation, low diversity trace fossil suites and non-rhythmic sandstone–mudstone layering. The increased mud content in FA 7, including common fluid-mud beds, may reflect closer proximity to the turbidity maximum zone (TMZ), more distal deposition along major channel axes, and/or deposition along relatively minor channel axes (Fig. 18) (Sisulak & Dashtgard, 2012; Gugliotta et al., 2016a; Gugliotta et al., 2017).

The inferred dominance of fluvial deposits in C2.1 and C2.2 units implies a narrow or no FMTZ along major distributary axes and/or strong preservational bias in FMTZ deposits. Overprinting by river processes would be anticipated during high river stage related to monsoonal storms (Fig. 19A and B). In relatively small, steep-gradient and monsoon-influenced drainage basins, such as the BDP, monsoonal precipitation is linked to coastal storms during ‘storm-floods’ (Pattison et al., 2007; Bhattacharya & MacEachern, 2009; Collins et al., 2017b), which are characterized by: (i) simultaneously increased wave energy and river discharge; (ii) seaward extension of the fluvial- dominated zone; and (iii) significantly reduced or absent tide-influenced zone (Fig. 19A and B). Interstratified SCS in C2 and C3 units mostly likely record mixed fluvial–wave deposition in distributary-channel mouth bars during storm-floods. Between successive larger river floods, tidal and fair-weather-wave processes probably influenced sedimentary processes in the lower delta plain and upper delta front, respectively. However, these were probably strongly overprinted or obliterated during succeeding storm-floods. Indeed, the lack of unequivocal tidal indicators in fluvial–tidal distributary channels may reflect: (i) strong bias in hydrodynamic regime; and (ii) skewed preservation of high fluvial discharge events (Ainsworth et al., 2016).

Major and minor distributary channel axes versus lateral areas

Reduced water and sediment discharge along relatively minor distributary channels is manifested in heterolithic to mud-dominated deposition (C2.2 and C3 channel units) (Figs 17 and 18). Facies and architectural characteristics of C2.2 units and SU3 are consistent with fluvial-dominated, tide- influenced deposition in bank-attached or mid-channel bars. Channelized C3 units display sharp or gradational contacts with C2 units, attesting to significantly reduced water and sediment discharge that allowed more mixed fluvial–tidal preservation (Allen & Chambers, 1998; Salahuddin & Lambiase, 2013). Variations in sandstone versus mudstone in fluvial-dominated zones may reflect a proximal (sandier) to distal (muddier) variability (Fig. 19C and D) and/or temporal changes in sand supply, principally related to avulsion dynamics (102 to 103 year timescale) (Törnqvist & Bridge,

This article is protected by copyright. All rights reserved. 2002). Storm-flood processes dominate across the FMTZ in major and minor axes. However, lower fluvial energy in minor axes, especially during low river stage, probably resulted in a relatively wider marine-influenced zone (Figs 19A to D). During river floods, the extent of tide influence is inferred to vary with the magnitude of high river stage and may be entirely overridden during the largest flood events, especially in major distributary axes (Fig. 19).

Significantly diminished fluvial discharge in lateral areas means deposition is inferred to be mud dominated and of lower energy (FA 9 to FA 11) (Figs 17 and 18) (Jones et al., 2003; Salahuddin & Lambiase, 2013). Transition from axial to lateral delta plain deposition may result from the following: (i) autogenic processes, such as channel abandonment and subsequent marine reworking; (ii) allogenic processes, such as increased tectonic subsidence causing along-strike relative transgression (e.g. modern Brunei Bay; Collins et al., 2018b) or eustatic sea-level rise causing regional transgression; or (iii) a combination of autogenic and allogenic processes (e.g. Shiers et al., 2014). However, lateral areas may contain tidal channels that were connected up-dip to distributary channels (for example, Mahakam Delta; Salahuddin & Lambiase, 2013), and may receive fluvial discharge, especially during high river stage (Fig. 19E and F). Sedimentological complexity (FA 9 to FA 11) indicates variable interactions between fluvial, tide and wave processes, mainly due to variations in: (i) the degree of sheltering from storms and fair-weather waves; (ii) tidal amplification versus frictional dissipation; and (iii) sediment supply characteristics (Fig. 19E and F). Mangrove-influenced deposition (FA 10) may have occurred in proximal and elevated lateral, wave-protected, intertidal to upper-subtidal areas (Figs 18B and 19E).

Turbidity maximum distribution

Abundant mudstone drapes and/or layers, including fluid muds (Ichaso & Dalrymple, 2009; MacKay & Dalrymple, 2011), in both axial and lateral lower-delta-plain deposits (FA 4 and FA 6 to 9) suggests that: (i) high suspended-sediment concentrations (SSCs) were widespread; and (ii) a turbidity maximum zone (TMZ) was well developed near the landward limit of saltwater incursion in distributary channels (Fig. 19) (Postma, 1967). In the Miocene BDP, persistently widespread and high SSCs are consistent with intense tropical weathering of a mudstone-rich hinterland in an ever-wet climate. In this setting, very high SSCs (>10 gL-1) and fluid-mud deposition probably characterized storm-floods due to: (i) elevated suspended- sediment supply and remobilization; (ii) seaward expansion and shift of the TMZ; and (iii) flow variability of river, tidal and possibly wave processes (North et al., 2004).

DISCUSSION

Controls on lower delta plain geomorphology

Three important controls on delta plain morphology are: (i) the relative importance of river, tidal and wave processes (Galloway, 1975; Ainsworth et al., 2011); (ii) backwater-zone dynamics (Paola & Mohrig, 1996; Colombera et al., 2016); and (iii) tectonically-driven changes in base level. The stratigraphic architecture and interpreted geomorphology of the Lambir system (Fig. 18) is a balance between several competing controls on a range of spatial–temporal scales in the Miocene Baram Delta Province (BDP).

This article is protected by copyright. All rights reserved.  Tectonic uplift combined with an erodible hinterland subjected to intense tropical weathering, monsoon-influenced ever-wet climate, and a short (<300 km) ‘source to sink’ distance resulted in an exceptionally efficient fluvial sediment supply system.  High tectonic subsidence resulted in a strongly aggradational shoreline–shelf system (Atkinson et al., 1986; Collins et al., 2017b; Collins et al., 2018b), increasing the likelihood of channel and overbank aggradation, super-elevation, and avulsion on a thousand-year timescale (Törnqvist & Bridge, 2002; Jerolmack & Swenson, 2007).  Evidence for storm-wave reworking suggests high, long-term (≥103 year) wave energy that could have suppressed mouth-bar formation, channel bifurcation and the abundance of smaller-scale channels (Wright & Coleman, 1973; Bhattacharya & Giosan, 2003; Swenson, 2005; Jerolmack & Swenson, 2007). However, the interaction of multiple distributary channels along the multi-river sourced coastline would have limited the effect of waves on avulsion by decreasing the maximum length scale for alongshore transport (Swenson, 2005; Jerolmack & Swenson, 2007).  The paucity of unequivocal evidence of tidal processes suggests a minor tidal effect on delta-plain geomorphology, including: (i) more stable major distributary channels (Dalrymple et al., 1992; Tanabe et al., 2003; Geleynse et al., 2011; Hoitink et al., 2017); (ii) development of funnel- shaped, sinuous tidal channels (D'Alpaos et al., 2005; Lentsch et al., 2018); and (iii) increased coastline rugosity (Eisma, 1998; Hughes, 2012; Rossi et al., 2016; Hoitink et al., 2017). Ebb tides can enhance river currents and sediment transport (Fagherazzi, 2008; Rossi et al., 2016; Lentsch et al., 2018), which is commonly manifested as unidirectional-dominated, basinward-directed palaeocurrent trends in ancient fluvial–tidal deltaic successions (Willis et al., 1999; Willis & Gabel, 2001; Legler et al., 2013; Chen et al., 2014; Legler et al., 2014; Gugliotta et al., 2015; Eide et al., 2016; Gugliotta et al., 2016a; Rossi & Steel, 2016; van Cappelle et al., 2016; van Cappelle et al., 2017). Tides also affect river flow processes and delta plain morphodynamics, even during high fluvial discharge in microtidal (<2 m tidal range) regimes (Pugh, 1987; Leonardi et al., 2015; Hoitink & Jay, 2016). Tidal stabilization of fluvial channels may explain the high vertical channel connectivity and amalgamation in the interpreted axial areas compared to lateral areas in the Lambir Formation (Fig. 17) (Rossi et al., 2016; Lentsch et al., 2018).  The dominant preservation of mangrove palynomorphs strongly suggests deposition in a wave- protected, tide-influenced, tropical, lower delta plain, with tidal range principally controlling the extent of mangrove swamps (Woodroffe, 1992).  Mangroves and other dense tropical vegetation dampen tidal velocities, promote sediment deposition and enhance initiation and branching of multi-scale fluvial–tidal channels (Wolanski et al., 1992; Furukawa et al., 1997; van Maanen et al., 2015).  A relatively narrow (hundreds of kilometres), steep (ca 0.3 to 1.0°) and tectonically-influenced coastal–deltaic plain may have resulted in increased avulsion frequency (Törnqvist & Bridge, 2002). Modelled low mesotidal conditions (ca 3 m, 15 to 11 Ma) (Collins et al., 2017a; Collins et al., 2018a) suggest a relatively narrow (several kilometres) zone of tidal influence in the backwater reaches of coastal rivers. For the present-day Baram River, assuming an average channel depth of ca 10 to 15 m and nearshore coastal plain gradient of 0.05°, the backwater zone is ca 11 to 17 km (Paola & Mohrig, 1996).

Rivers in the humid-tropical region of south-east Asia and Oceania supply approximately 40% of terrigenous sediment to the global coastal ocean (Milliman & Syvitski, 1992; Nummedal et al., 2003; Milliman & Farnsworth, 2011) and several well-studied deltas in the region are partial modern analogues for the Lambir Delta.

The present-day Mekong River Delta displays the following spatial and temporal variations in process regime: (i) more landward regions of the FMTZ of major distributary axes are sandstone-rich and river-dominated; (ii) tide influence increases in muddier seaward regions of the FMTZ; (iii) wave processes strongly affect the shoreline and delta front in sandier axial areas; (iv) muddier lateral areas are strongly tide influenced; (v) during the summer monsoon wet season (high river stage), river dominance extends throughout the FMTZ of distributary channels but north-east-directed alongshore transport is reduced; and (vi) during the winter monsoon dry season (low river stage), the FMTZ expands landward and south-west-directed alongshore transport is enhanced (Nguyen et al., 2000; Ta et al., 2002; Ta et al., 2005; Hanebuth et al., 2011; Gugliotta et al., 2017; Gugliotta et al., 2018) (Szczuciński et al., 2013; Xue et al., 2014; Thanh et al., 2017). Compared to the Mekong Delta, interpreted spatial variations in depositional processes were similar but temporal variations were more episodic and non-seasonal in the Lambir Delta.

The Trusan River Delta is a fluvial-dominated, wave-influenced and tide-influenced bayhead delta within Brunei Bay and shares the following features with the interpreted Lambir Delta: (i) a single major active distributary channel with several bifurcations in its distal reach related to mouth-bar formation; (ii) several abandoned distributary channels that have evolved into tidal channels; (iii) wave-reworked mouth bars in basinward positions; and (iv) extensive intertidal mangrove swamps (Fig. 20A and B) (Lambiase et al., 2003; Collins et al., 2018b). Active distributary channel facies are comparable to interpreted river flood–interflood facies in C2 units in the Lambir Formation (Collins et al., 2018b). However, unlike the open coastline Lambir Delta (Fig. 2F), the Trusan Delta is relatively protected from waves and storms. Furthermore, the Lambir system probably had a larger drainage basin, perhaps closer to that of the present-day Baram River (ca 20,000 km2), rather than that of the much smaller Trusan River (ca 2,500 km2) (Sandal, 1996; Storms et al., 2005).

The Mahakam River Delta is a mixed fluvial-influenced and tide-influenced delta (Fig. 20C and D) (Galloway, 1975; Ainsworth et al., 2011) and shares the following features with the interpreted Lambir Delta: (i) multiple distributary channels with variable sediment and water discharge; (ii) more tidally-influenced areas containing higher sinuosity channels with variable fluvial discharge; (iii) a densely vegetated, mangrove-dominated lower delta plain; and (iv) abandoned fluvial distributaries that have evolved into tidal channels and wide-mouthed estuaries (Allen et al., 1977; Caratini & Tissot, 1988; Allen & Chambers, 1998; Storms et al., 2005). By contrast, the Mahakam River system differs from the interpreted Lambir system in five important ways: (i) it has a larger drainage basin (ca 75,000 km2) (Storms et al., 2005); (ii) channels are not dominated by flood discharge, meaning that distributary channels were created by channel bifurcation rather than avulsion (Storms et al., 2005); (iii) sandstone–mudstone layers in distributary channel deposits record spring–neap tidal cyclicity (Allen et al., 1977; Gastaldo et al., 1995; Storms et al., 2005); (iv) the setting displays lower wave energy and minimal morphological or sedimentological evidence of wave processes (Allen & Chambers, 1998; Storms et al., 2005); and (v) the system shows considerably diminished fluvial

The Holocene–Modern Mitchell River Delta (Fig. 21), classified as tide-dominated, fluvial-influenced and wave-affected (Nanson et al., 2013), shares the following features with the interpreted Lambir Delta: (i) a small number of major distributary channels; (ii) several minor distributary channels that may form secondary deltas; (iii) a multitude of relatively sinuous, variously sized tidal channels; (iv) fringing mangroves mostly along minor distributary and tidal channels (Fig. 21C and D); and (v) mesotidal conditions (ca 3 m) similar to modelled Middle Miocene tides along the Lambir shoreline (Collins et al., 2017a; Collins et al., 2018a). However, the Mitchell River system differs in the following ways: (i) the larger catchment area (71,000 km2); (ii) the semi-humid, semi-arid and monsoon-influenced climate; (iii) extensive saline flats in the lower delta plain (Fig. 21C and D) (Ridd et al., 1988; Wolanski, 1993); and (iv) extensive beach and chenier ridges that reduced lateral migration of delta-plain channels (Jones et al., 1993).

In the Burdekin River Delta, most sediment delivery occurs during short (few days) but extreme monsoonal discharge periods, which drives rapid accretion of river mouth and channel bars (Fielding & Alexander, 1996; Fielding et al., 2005). The far smaller area of tidal bars and beach ridges indicates minor tide and wave influence (Fielding & Alexander, 1996; Fielding et al., 2005). However, Holocene stratigraphic units preserving widespread tide-influence and wave influence suggest that combined river and marine processes played an important role in longer-term delta construction, similar to the Lambir Delta. Likewise, mangroves preferentially occur in abandoned lower-delta-plain areas of the Burdekin system, despite the drier and more-arid climate (Fielding et al., 2005).

Process preservational bias in mixed-process delta plains

There is always potential for preservational bias in mixed-energy delta plains due to spatial and temporal variations in the rates of sedimentation and burial versus erosion. Delta-plain channels have the highest rates of sediment supply, accommodation creation (Muto & Steel, 2000) and sedimentation (Törnqvist & Bridge, 2002), with channel erosion removing previously deposited sediment. Overbank sedimentation requires channel overtopping, and aggradation rates decrease rapidly away from channels (Pizzuto, 1987). Overall, sand bars deposited in the deeper, topographically lower parts of channels will have highest preservation potential. In contrast, topographically higher areas of delta plains and upper bar deposits have lower preservation potential. In the Lambir Formation, this is consistent with the dominance of relatively sandy barforms in the basal parts of erosionally based channel units. The paucity of vegetated floodplain deposits, especially in situ mangroves, may reflect a combination of: (i) erosion due to channel migration, avulsion and/or re-occupation; and (ii) transgressive erosion.

In the distal backwater zone during high discharge events, water surface drawdown and resultant erosion would disproportionately affect preservation of more marine-influenced deposits (Lamb et al., 2012; Nittrouer et al., 2012; Chatanantavet & Lamb, 2014; Trower et al., 2018). Drawdown length depends on flood discharge and can exceed half the backwater length (Lamb et al., 2012). In the Lambir Formation, the overall net progradational parasequence architecture and internal erosional surfaces in C1.1 and C2.1 units (Figs 15 to 17) share similarities with emerging stratigraphic models of distributary channels in the backwater zone (Dashtgard et al., 2012a; La Croix &

Preservational bias challenges the long-held assumption that internal facies and stratigraphic framework predicts external plan-view delta morphology, and vice versa (Galloway, 1975; Fielding et al., 2005; Gani & Bhattacharya, 2007; Ainsworth et al., 2011; Vakarelov & Ainsworth, 2013; Ayranci & Dashtgard, 2016). For example, the arcuate–cuspate morphology of the Cretaceous Raptor delta suggests wave dominance, but the facies architecture records the dominance of river-flood deposition with significant tidal reworking (Gani & Bhattacharya, 2007). Similarly, despite the cuspate morphologies of the Brazos and Burdekin River deltas, sedimentary dynamics are primarily controlled by river floods (Rodriguez et al., 2000; Fielding et al., 2005). In the Lambir Formation, preserved delta plain successions are dominated by channelized units comprising river flood– interflood deposits, with variable tidal interaction; wave influence is indicated by storm-wave reworking in channel-mouth and delta-front units (Lambiase et al., 2003; Collins et al., 2017b; Collins et al., 2018b). Hence, the interpreted delta morphology is relatively lobate and rugose with a few major distributaries, several minor active and inactive channels, and shoreline spits and beach ridges (Fig. 18). Alternatively, the delta may have had a more cuspate morphology, perhaps similar to the Mitchell River Delta (Fig. 21). However, as suggested by physical and numerical models, the distributary channel network and delta front morphologies of deltas can change substantially through time (Kim et al., 2009; Geleynse et al., 2011).

CONCLUSIONS

 Analysis of sedimentary facies, including ichnofacies, and stratigraphic architecture of the Middle Miocene Lambir Formation, western Baram Delta Province, demonstrates progradational to strongly aggradational deposition in a large-scale, mixed-energy deltaic clastic wedge. Fluvial processes with superimposed tidal influence characterized the lower delta plain, while combined fluvial and wave processes (‘storm-floods’), with subordinate tidal influence, dominated the delta front.  The stratigraphic architecture of this mainly lower delta plain succession was controlled by sediment-supply axes: vertically-stacked and variably-sized active distributary channels dominated axial areas (‘on axis’), while smaller, less active to inactive channels, with increased tidal influence, characterized lateral interdistributary areas (‘off axis’).  Mudstone deposition and preservation was widespread in both axial and lateral areas, which implies widespread high suspended-sediment concentrations and well-developed turbidity maximum zones in distributary axes. This is consistent with expected high suspended-sediment load of rivers, which emanated from a mudstone-rich hinterland undergoing deep tropical weathering.  Axial areas preserve relatively sandstone-dominated, sandier-upward successions comprising sharp-based, muddier-upward, single- or multi-storey, fluvial-dominated and fluvial–tidal channel-fill deposits. Subordinate units comprise sandier-upward or mudstone-dominated fluvial–tidal and storm-reworked deposits. Sedimentary transport, deposition and preservation in axial distributary channels were dominated by river floods.

This article is protected by copyright. All rights reserved.  Lateral areas preserve relatively mudstone-rich successions including sandier-upward fluvial–tidal successions, sharp-based and mudstone-dominated channel-fill deposits and subordinate storm- reworked and tide-dominated units. These areas preserve a much more diverse spectrum of processes that combined storm, tidal and, possibly, subordinate river processes. Combined storm and river flood processes dominated in seaward axial and lateral areas.  During major storm-floods in axial areas, fluvial processes likely dominated the fluvial to marine transition zone and probably caused significant overprinting or obliteration of preceding tide- influenced and wave-influenced deposits. In lateral areas, marine processes were more effective because of subordinate or absence fluvial connections.  The abundance of heterolithic, variably bioturbated channel deposits and reworked mangrove organic material, suggests that the lower delta plain was mangrove vegetated and tide influenced. However, the lack of preserved in situ mangroves suggests: (i) erosion due to channel migration, avulsion and/or re-occupation; and (ii) transgressive erosion.  The paucity of unequivocal tide-dominated process indicators, most notably cyclical sandstone– mudstone layering, may reflect a combination of: (i) preservational bias towards storm-flood events; (ii) a short backwater length and weak backwater dynamics; and/or (iii) microtidal to low mesotidal conditions.

ACKNOWLEDGEMENTS

This work was funded by a NERC PhD scholarship (DSC). Fieldwork was supported by Shell International Exploration and Production, Houston, USA (DSC and HDJ). We thank M. van Cappelle for fieldwork assistance and extensive discussion, and further discussion with P. Allison, M. Gugliotta, G. Hampson, D. Hodgson, Wan Hasiah Abdullah and T. Tamura. The authors thank extensive, thought-provoking reviews by L. Colombera and J.A. MacEachern, especially the insightful discussion on the ichnological analysis, and constructive comments by the Associate Editor C.R. Fielding.

Fig. 1. (A) Schematic plan-view subdivision of major deltaic environments adjacent to and encompassing the fluvial to marine transition zone (FMTZ). (B) and (C) Generalized process-based model for the FMTZ in distributary channels in relatively tide-dominated (B) and wave-dominated (C) shoreline systems, indicating the balance of fluvial and marine (tidal and wave) processes, sediment transport directions, salinity and suspended-sediment concentrations. The spatial and temporal variability in the balance of processes can vary significantly between systems. Modified after Dalrymple & Choi (2007) and Gugliotta et al. (2016a).

Fig. 2. (A) Location of the Baram Delta Province (BDP) and Mahakam delta (pink box; Fig. 20C and D) in south-east Asia. (B) Simplified topographic and bathymetric map of the present-day BDP, including the major rivers, onshore study area (pink box; Fig. 2C and D), seismically interpreted Middle Miocene to Pliocene shelf-edge positions (coloured lines with ages from subsurface biostratigraphic studies; Cullen, 2010), and position of the Trusan delta in Brunei Bay (BB) (pink box; Fig. 20A and B). (C) Geological map of the study area (modified after Banda & Honza, 1997) showing the position of studied exposures within the Lambir Formation (pink labels 0 to 12) and different mapped positions for the West Baram Line (WBL; see Fig. 2D). (D) Alternative geological map of study area (after Liechti et al., 1960) that also recognizes the Tukau Formation and, in Nagarajan et al. (2015), the Miri Formation (asterisk) in the area of the studied Lambir Formation. Mapped positions for the WBL are from (1) Kessler (2010), (2) Cullen (2010), and (3) Clift et al. (2008) (after Cullen, 2014). (E) Schematic proximal (south-east) to distal (north-west) cross-section, showing the stratigraphic and depositional relationships between the broadly contemporaneous Lambir, Miri and Setap formations. The positions of studied outcrop and the Miri Field (labelled pink) are approximate. Available biostratigraphic data indicate the Lambir and Miri formations are Middle Miocene in age, equivalent to Cycles IV to lower V (Ho, 1978; Morrison & Lee, 2003; Wannier et al., 2011; Jong et al., 2017) and TB 2.4 to 2.5 (Torres et al., 2011; Balaguru & Lukie, 2012), underlain by the Middle Miocene Unconformity (MMU) dated at ca 15.5 Ma with a ca 0.5 Myr gap (Wannier et al., 2011) and approximately overlain by the Lower Intermediate Unconformity (LIU) dated at ca 12.1 Ma (Torres et al., 2011; Balaguru & Lukie, 2012) (see also Levell, 1987). (F) Interpreted time-averaged gross depositional maps for the western BDP at approximately 15.0 Ma, 14.1 to 12.7 Ma, 12.7 to 12.1 Ma and 12.1 to 9.6 Ma (modified after Mazlan, 1999; Hutchison, 2005; Cullen, 2010; Jong et al., 2017). Abbreviations: BB–Brunei Bay; BF–Bakong Fault; CHF–Canada Hill Fault; LF–Lambir Fault. Informal outcrop location names are as follows (used in local field guides and some publications): (0) Entulang; (1) Sungai Lian; (2) Golden Hill Memorial Park; (3) Duck Pond; (4) Bukit Song; (5) Bukit Song North; (6–8) Miri-Bintulu Roadcuts; (9) Miri Bintulu Road Quarry (‘Double Decker Quarry’); (10) and (11) Brick Factory; and (12) Sungai Rait.

Fig. 3. Stratigraphic logs for locality 0 (A), locality 10 (B), locality 11 (C) and locality 12 (D) illustrating facies (F), sand versus shale (S-Sh), bioturbation index (BI) and facies associations (FA). In Fig. 3A, Facies Association 1 (FA 1) preserves lateral and/or distal shelf deposition in the Setap Formation (denoted by an asterisk). See Table 1 for facies classification, Table 2 for facies-association

Fig. 4. Map (A) and stratigraphic logs (B) to (G) for locality 2, illustrating facies (F), sand versus shale (S-Sh), bioturbation index (BI), and facies associations (FA). Three stratigraphic logs (Logs 1 to 3) for locality 2c are correlated at major erosional surfaces. See Table 1 for facies classification, Table 2 for facies-association classification and trace fossil abbreviations, Fig 3 for stratigraphic log legend, and Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 5. Map (A), summary log (B), and stratigraphic logs for locality 4 (C) to (F) and locality 3 (G) illustrating facies (F), sand versus shale (S-Sh), bioturbation index (BI), and facies associations (FA). Logs at locations 4a and 4c have been arranged from top to bottom; log 4b overlaps both. See Table 1 for facies classification, Table 2 for facies-association classification and trace fossil abbreviations, Fig. 3 for stratigraphic log legend, and Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 6. Representative photographs illustrating facies characteristics of FA 1 to FA 3. (A) Outcrop photograph and (B) interpreted stratigraphic architecture of a sandier-upward unit (SU1) and overlying units illustrating facies association type and position of interpreted erosional surfaces (ES), which may have formed by autogenic or allogenic processes (stratigraphic heights from log in Fig. 4C). (C) Bioturbated mudstone in the Setap Formation (FA 1 shelf equivalent) within 1 m of the overlying Lambir Formation (ca 4 m, Fig. 3A). Ichnogenera include Chondrites (Ch), Cosmorhaphe (Cr), Cylindrichnus (Cy), Ophiomorpha (Op), Planolites (Pl), Teichichnus (Te) and Thalassinoides (Th). (D) Hummocky to swaley cross-stratified sandstone (F6 to F7), including muddy carbonaceous laminae and coalified fragments (ca 8 m, Fig. 3A). Refer to Table 2 for the facies-association classification shown in B (above). See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 7. Facies characteristics of FA 4. (A) Stratigraphic architecture and facies association interpretation at locality 2a (Fig. 4B). (B) Intermediate facies between swaley cross-stratified (F7) and trough cross-stratified (F9b) upper very fine to fine-grained sandstone, including muddy carbonaceous layers, clasts and burrow infills. (C) Swaley cross-stratified very fine-grained sandstone (F7), displaying decimetre-scale cross-sets with exquisitely preserved sub-millimetre to millimetre- thick laminae with common carbonaceous drapes or partings. Subordinate centimetre-scale carbonaceous mudstone layers may occur at bed boundaries. (D) Line drawing of photograph in (C), illustrating sedimentary fabric and ichnological characteristics, including Ophiomorpha (Op), Planolites (Pl) and fugichnia (fu), possible Siphonichnus (Si?), and common biogenic disturbance (for example, labelled 1) and collapse features (for example, labelled 2). (E) Detailed photograph showing aspects of sedimentary and ichnological fabric (cf. Fig. 7C and D). See Fig. 2C for outcrop localities (circled pink text) and names.

This article is protected by copyright. All rights reserved. Fig. 8. (A) Outcrop photograph, and (B) interpreted stratigraphic architecture at locality 2c, showing the location of stratigraphic logs 1 to 3 (Fig. 4E to G). (C) Detailed outcrop photograph, and (D) interpreted stratigraphic architecture of part of locality 2c (cf. Fig. 8B), including a high-angle erosional margin of an interpreted C1 unit (FA 6) (arrows). (E) Outcrop photograph and (F) interpreted stratigraphic architecture of locality 2d with stratigraphic heights from log in Fig. 4D. Note the low-angle inclined surfaces in FA 6 and FA 7 between 12.5 m and 27 m. Red stratigraphic surfaces are interpreted erosional surfaces underlying channel units. See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 9. (A) Outcrop photograph, and (B) interpreted stratigraphic architecture between ca 31 to 40 m in Fig. 3C. Note the low-angle inclined surfaces in FA 5. Pink arrows and labels indicate channel (C) stratigraphic units (Fig. 15). (C) Trough cross-stratified sandstone (F9c) in FA 5. (D) Interpreted stratigraphic architecture between ca 109 to 111 m in Fig. 5F. The basal erosional surface (ES) of FA 6 appears partly gradational, due to thinning of beds downwards along inclined surfaces into toesets within FA 7. (E) Outcrop photograph and (F) interpreted stratigraphic architecture of FA 6, FA 7 and FA 9 between ca 0 to 10 m in Fig. 5C, which includes a complex arrangement of multiple stacked FA 6 units with apparently opposing internal inclined surfaces in the basal 1 m. (G) Trough cross- stratified sandstone with scoop-shaped cross-sets and mudstone drapes along the lower parts of foresets and topsets (F9b) in FA 6, sharply overlain by more thinly bedded sandy heterolithics (F5b) with centimetre-scale massive mudstone layers (FA 7). (H) Intercalation of sandy heterolithics (F5b) and mudstone-draped trough cross-stratified sandstone (F9b), illustrating the spectrum of facies arrangements possible between FA 6 and FA 7 (see Tables 1 and 2). See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 10. (A) Outcrop photograph, and (B) interpreted stratigraphic architecture between ca 30 to 45 m in Fig. 5D, where an onlapping FA 7 unit infills a prominent, inclined erosional surface above FA 6. (C) Outcrop photograph and (D) interpreted stratigraphic architecture at between ca 113 to 134 m in Fig. 5F. Note the following: (i) sandier-upward FA 9 to FA 7, which forms an SU3 sandier-upward (SU) unit (Fig. 15); (ii) the inclined, lateral accretion surfaces within FA 6; and (iii) the sharp upper and lower contacts of FA 10, which forms a C2.1 channel (C) unit with the underlying FA 6 (Fig. 15). See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 11. (A) Outcrop photograph, and (B) interpreted stratigraphic architecture between ca 38 to 40 m in Fig. 5E, showing a sharp-based heterolithic FA 7 unit with internal erosional surfaces, and an overlying erosionally-based FA 6 unit. Pink arrows and labels indicate stratigraphic units (Fig. 15). (C) Sharp-based heterolithic FA 7 unit with a basal carbonaceous (including coal) and mudstone-clast breccia, and inclined mudstone-lined surfaces that downlap onto the basal erosional surface. (D) Sandier-upward FA 7 units, showing a stepwise upward change from basal muddy heterolithics to sandy heterolithics and minor trough cross-stratified sandstones with inclined internal surfaces (black lines) (stratigraphic heights from the log in Fig. 5F). See Fig. 2C for outcrop localities (circled pink text) and names. SU—Sandier Upward; C—Channel.

This article is protected by copyright. All rights reserved. Fig. 12. Facies characteristics of FA 7. (A) Interbedded heterolithics comprising: (i) decimetre-scale sandy heterolithic (F5b) bedsets containing very thin to thinly bedded sandstone layers (lighter colours) displaying undulatory bed tops and mudstone layers and flasers (dark grey–black); and (ii) centimetre to decimetre-scale muddy heterolithic (F4b) layers, locally forming unbioturbated wavy- bedded intervals with structureless mudstone layers and/or drapes (red arrow). Rare, low diversity trace fossil suites include (in alphabetical order) Cylindrichnus (Cy), Teichichnus (Te) and Thalassinoides (Th), and the additional features: (i) sandstone-filled sub-vertical burrows, possibly Skolithos or Siphonichnus; and (ii) a composite burrow complex including sandstone-filled and mudstone-lined burrows (cf. Fig. 11D). (B) Interstratified centimetre-scale massive to planar laminated sandstone layers (F8c) with sharp bases and tops, and massive mudstone layers (light grey; F2b) containing minor bioturbation (BI 0 to 1). (C) Wavy-bedded muddy heterolithics without systematic ordering of sandstone–mudstone layering and sporadically distributed bioturbation (BI 2 to 4), which is dominated by facies-crossing elements such as (alphabetical order) Ophiomorpha (Op), Planolites (Pl), Thalassinoides (Th), with Asterosoma (As), Cylindrichnus (Cy), Taenidium (Ta), and an unidentifiable biogenic structure that suggest escape behaviour (fugichnia) or intrastratal locomotion (purple arrow). (D) Sandy heterolithics with variable bioturbation intensity (BI 1 to 2 ranging up to BI 5 at the bed scale). Trace fossils include (alphabetical order) Cylindrichnus (Cy), Ophiomorpha (Op), Planolites (Pl), Skolithos (S), Thalassinoides (Th) and Taenidium (Ta). (E) Subordinate dune-scale cross stratification (F9a) dipping towards the north-east (basinward) within sandy heterolithics (see Fig. 11A and B). (F) Erosional surface (inclined red line – ES) between FA 7 (below) and FA 6 (above). In FA 7, flaser to wavy-bedded sandy heterolithics include structureless mudstone layers that are locally broken into mudstone chips (blue arrow). Bioturbation in FA 7 includes Asterosoma (As), Cylindrichnus (Cy), Ophiomorpha (Op), Palaeophycus tubularis (Pt) and Planolites (Pl), with intrastratal repichnia (ir), or locomotion, and ‘equilibrium adjustment’ (ea), or fugichnia. Above the erosional surface, FA 6 includes a basal lag of mudstone clasts and coal fragments (green arrows) (see Fig. 11A and B). See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 13. (A) Outcrop photograph, and (B) interpreted stratigraphic architecture between ca 15 to 75 m in Fig. 3B. Note the prominent C2.2 channel units displaying sharp bases and muddier-upward profiles (white triangles). (C) Outcrop photograph, and (D) interpreted stratigraphic architecture between ca 7 to 27 m in Fig. 3D. Note that FA 6 correlates laterally with FA 7 and both are sharply overlain by FA 9 between ca 15.0 to 18.5 m. See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 14. (A) Laminated mudstone with abundant millimetre to centimetre-scale carbonaceous laminae and layers and sporadically distributed centimetre-scale coal fragments (green arrows) in FA 8. (B) Wavy to lenticular-bedded muddy heterolithics in FA 8 displaying sandstone layers with combined-flow ripple cross-lamination, synaeresis cracks and a low diversity, sparsely distributed ichnological suite, which includes Ophiomorpha (Op), Planolites (Pl) and Teichichnus (Te). (C) Bioturbated muddy heterolithics with local coal fragments (green arrow). Bioturbation includes Asterosoma (As), Chondrites (Ch), Palaeophycus tubularis (Pt), Phycosiphon (Ph), Planolites (Pl) and

This article is protected by copyright. All rights reserved. Thalassinoides (Th), with a chaotic assemblage of sandstone-filled burrows (red arrows), which is probably an admixture of Planolites and Thalassinoides. The possible Thalassinoides (Th?) could also be bivalve adjustment structures. (D) Muddy heterolithics in FA 9 with a moderately diverse ichnological suite, which includes Chondrites (Ch), Ophiomorpha (Op), Planolites (Pl), Rosselia (Ro) and Thalassinoides (Th). Possible reworked Rosselia mud-balls (blue arrows) occur as rip-up clasts along the base of sandstone and fluid-mud layers. (E) Massive carbonaceous mudstone, with abundant mangrove palynomorphs (e.g. Togunwa et al., 2015), stained greenish–yellow due to reduction of iron-bearing material (Fig. 5F, ca 95 to 96 m), which likely records sediment deposition and destratification in mangroves. (F) Thoroughly bioturbated muddy sandstone in FA 11 with a very low diversity ichnological suite. The dominant trace fossils are centimetre to decimetre-scale, sandstone-filled, curved, vertical and horizontal burrows (white arrows) with irregular walls, indicating either branched forms or a structure that changes from vertical to horizontal along its length. These structures are most likely Psilonichnus and/or Thalassinoides. (G) Thoroughly bioturbated muddy sandstone in FA 11. Bioturbation includes J-shaped and straighter burrows, probably Psilonichnus, with a heterolithic backfill. Because the trace fossils are dwelling structures, the backfill was formed by passive infill of the open burrow, similar to ‘tubular tidalites’ (Gingras & Zonneveld, 2015). The facies also includes possible Asterosoma, Chondrites and Phycosiphon but none are sufficiently well expressed to warrant being labelled. (H) Thoroughly bioturbated muddy sandstone to sandy mudstone in FA 11. The ichnological suite includes possible Asterosoma (As), Psilonichnus (Ps), Teichichnus (Te) and Thalassinoides (Th), and also includes probable Chondrites, and Ophiomorpha, Phycosiphon and Planolites but none are obvious enough to label. See Fig. 2C for outcrop localities (circled pink text) and names.

Fig. 15. Synthesis of the genetic classification of interpreted vertical facies association (FA) trends into (i) sandier-upward (SU) units, and (ii) erosively-based-based channel (C) units in the Lambir Formation. The schematic logs show primary sedimentary structures and simplified grain size and bioturbation index (BI) trends. For BI, solid grey bars indicate typical BI value range and dashed lines indicate local BI range (between 0 to 2, 2 to 4 or 4 to 6). (A) Sandier-upward unit types 1 to 3 (SU 1 to 3). (B) Sandstone-dominated channel units (C1) comprising sub-units C1.1 and C1.2. (C) Sandstone- dominated to heterolithic channel units (C2) comprising three sub-units C2.1 to C2.3. (D) Mudstone- dominated channel units (C3) comprising three sub-units C3.1 to C3.3. (E) Bioturbated channel units comprise existing channel units (for example, C2.1 and C3.2) containing FA 11. See Table 2 for facies association classification and Fig. 3 for stratigraphic log legend.

Fig. 16. Generalized vertical and lateral stratigraphic architecture and approximate dimensions for single-storey and multi-storey channel-fill successions. (A) sandstone-dominated channel units (C1). (B) Sandstone-dominated to heterolithic channel units (C2). (C) Mudstone-dominated channel units (C3). Red stratigraphic contacts indicate erosional surfaces. See Table 2 for facies association classification and Fig. 3 for stratigraphic log legend.

This article is protected by copyright. All rights reserved. Fig. 17. Summary of the vertical and lateral stratigraphic architecture for outcrop successions of the Lambir Formation, which is interpreted to preserve upper to lower delta plain deposition in major and minor axial (‘on axis’) to lateral (‘off axis’) areas (see Fig. 18). Stratigraphic logs are colour-coded with respect to the facies association classification (Table 2); see Fig. 3 for stratigraphic log legend. The inferred relative stratigraphic positions and lateral relationships are shown, except for closely adjacent and correlatable localities 10 and 11 (ca 300 m along strike separation) and internal correlation between successions at locality 2 (Figs 4 and 8). Dimensions and geometries of stratigraphic surfaces are approximate. Arrows with black arrowheads show interpretable trends from more major to minor axial deposition. Curved arrows with white arrowheads indicate grain size and/or sand–mud trends. Examples of channel (C) and sandier-upward (SU) delta front units are labelled (see Figs 15 and 16). At locality 0, a sharp basal stratigraphic contact separates the Lambir Formation (LF) from the underlying Setap Formation (SF). This contact has been interpreted as the Middle Miocene Unconformity (MMU) and has been dated at ca 15.5 Ma with a ca. 0.5 Myr time gap (Wannier et al., 2011).

Fig. 18. (A) Schematic process depositional model for the Lambir Formation comprising a multi- sourced, mixed-energy coastal–deltaic plain with spatial variations in river, tidal and wave processes. (B) Reconstructed delta-plain environments within the fluvial to marine transition zone (FMTZ). Axial areas (‘on axis’) contain: (i) major distributary channels; and (ii) minor distributary channels. Lateral areas (‘off axis’) contain inactive distributary channels converted to tidal channels, estuaries or embayments, which may have a minor fluvial connection. Mangroves were the dominant lower delta plain vegetation. (C) to (E) Examples of three schematic progradational successions from prodelta to alluvial plain: (C) major distributary channels (axial); (D) minor distributary channels (axial); and (E) tidal channels, estuaries or embayments (lateral). Process classification follows Ainsworth et al. (2011), for example, Fwt refers to fluvial-dominated, wave-influenced and tide- affected. See Table 2 for facies association classification and Fig. 3 for stratigraphic log legend.

Fig. 19. Reconstructed fluvial to marine transition zone (FMTZ) for the Middle Miocene, Lambir Delta system along the following: (A) and (B) major distributary channel axes in axial areas during (A) low river stage and fair-weather conditions, and (B) high river stage and storm conditions (coupled ‘storm-floods’); (C) and (D) minor distributary channel axes in axial areas during (C) low river stage and fair-weather conditions and (D) high river stage and storm conditions; and (E) to (F) inactive or periodically active distributary channels in lateral areas during (E) low river stage and fair-weather conditions and (F) high river stage and storm conditions. Diagrams show the distribution of the following: (i) FMTZ; (ii) process zones (see Fig. 18 for legend); (iii) facies associations, where ‘/’ means ‘and/or’ and those placed in brackets are subordinate and variably present; and (iv) the turbidity maximum zone (TMZ) (partly based on Gugliotta et al., 2016a). See Table 2 for facies association classification.

This article is protected by copyright. All rights reserved. Fig. 20. Partial modern analogues for the Middle Miocene Lambir Delta system. (A) Satellite image of the Trusan Delta, western Brunei Bay (see location in Fig. 2B) (Bing). (B) Approximate distribution of sedimentary environments and vegetation zones in the Trusan Delta (Abdul Razak, 2001; Lambiase et al., 2003; Collins et al., 2018b). (C) Satellite image of the Mahakam Delta, Indonesia (see location in Fig. 2A) (Google Earth). (D) Approximate distribution of sedimentary environments and vegetation zones in the Mahakam Delta (Allen & Chambers, 1998; Storms et al., 2005).

Fig. 21. The modern Mitchell Delta, Gulf of Carpentaria, Australia as a partial analogue for the Middle Miocene Lambir Delta system. (A) Satellite image of the Mitchell Delta (Google Earth imagery). (B) Depositional environment (element) map of the Mitchell Delta that distinguishes environments dominated by wave, tidal or fluvial processes (from Nanson et al., 2013). (C) Satellite image on an area of the Mitchell River delta plain (Google Earth). (D) Approximate distribution of distributary and tidal channels and flanking mangroves and saline supratidal flats in the satellite image in Fig. 21C.

TABLE CAPTIONS

Table 1. Facies (F) classification for studied outcrops of the Lambir Formation. Ichnogenera identified include Asterosoma (As), Chondrites (Ch), Cylindrichnus (Cy), Diplocraterion (Di), Gyrolithes (Gy), Macaronichnus (Ma), Ophiomorpha (Op), Palaeophycus herbeti (Pa), Palaeophycus tubularis (Pt), Phycosiphon (Ph), Planolites (P), Psilonichnus (Ps), Rhizocorallium (Rz), Rosselia (R), Schaubcylindrichnus coronus (Sh), Siphonichnus (Si), Skolithos (S), Taenidium (Ta), Teichichnus (Te), Thalassinoides (Th) and fugichnia (fu), which are listed in the approximate order of relative abundance. Grain-size abbreviations: mst–mudstone; slst–siltstone; sst–sandstone; vf–very fine- grained; f–fine-grained; m–medium-grained; mL–lower medium-grained. Other abbreviations: BI– Bioturbation Index; cm–centimetre; dm–decimetre.

Table 2. Facies association (FA) classification for studied outcrops of the Lambir Formation. Refer to Table 1 for facies classification and legend.

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