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