This evaluation of the Miocene–Modern Baram Delta Province (BDP) depositional system provides: (1) a rare case study of outcrop observations that can be directly compared with a closely comparable and geographically adjacent modern analogue; (2) new insights into how deposition and preservation occur across a range of process regimes in a highly aggradational tectono-stratigraphic setting; and (3) an example of a well-exposed mixed-influenced shoreline–shelf depositional system, displaying variable interaction of fluvial, wave, and tidal processes. The exceptionally close relationship between the present-day BDP source-to-sink system and its ancient (Miocene–Pliocene) counterpart is because the climatic (humid-tropical, ever-wet, monsoon-influenced), tectonic (active foreland margin), hydrological (multiple, relatively short rivers), and gross depositional (shoreline–shelf) settings have remained consistent over the past c. 15–20 Myr. This study compares exposure-based analyses of facies and stratigraphic architecture in the middle Miocene Belait Formation (eastern BDP) with process-based geomorphological and sedimentological analyses of coastal–deltaic depositional environments in the present-day BDP. The Belait Formation comprises three distinct types of vertical facies-succession sets: (1) aggradationally-stacked, upward-sanding units (10–50 m thick), dominated by erosionally based sandstone beds showing swaly cross-stratification and gutter casts, record deposition during simultaneously high storm-wave energy and storm-enhanced fluvial discharge (“storm floods”); these are interpreted as analogs for deposits along the present-day open coastline in the BDP (e.g., the present-day, open-shelf Baram delta and flanking strandplain); (2) aggradationally stacked, heterolithic, upward-sanding units characterized by interbedded swaly cross-stratified sandstone and combined-flow-rippled heterolithics, record deposition by time-varying storm and storm-flood processes under relatively high fluvial influence; these compare favorably with deposits along the present-day low-wave-energy, embayed deltaic coastlines in the BDP (e.g., Trusan delta and other bayhead deltas in eastern Brunei Bay); (3) heterogeneous successions comprising heterolithic, variably bioturbated and carbonaceous-rich, mudstone-dominated and muddier- to sandier-upwards units, document highly variable mixed-process deposition; these are similar to the present-day fluvio-tidal deposits along embayed deltaic coastlines in the BDP (e.g., northern Brunei Bay and Inner Brunei Bay in the southwest). The principal controls on large-scale (100–1000s m) ancient stratigraphic architecture and changes in process regime in the BDP were: (1) sediment-supply variation along the multiple river-sourced coastal plain, mainly caused by tectonically driven drainage-basin switching, and (2) formation of tectonically controlled embayments.

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