Fluvio-Deltaic System Evolution in a Colombian Caribbean Forearc Basin (Middle-Upper Eocene to Lower Miocene)

The middle-upper Eocene to Lower Miocene sedimentary successions of the southwestern Colombian Caribbean comprise coarse- and fine-grained deposits formed within fluvio-deltaic systems developed in a forearc basin. Integrated sedimentological, ichnological, well-log, and micropaleontological analysis of outcrops and well cores from the San Jacinto Fold Belt allowed recognition of twelve facies associations and reconstruction of the spatiotemporal evolution of deltaic bodies and their stacking patterns. Middle-late Eocene deposits record coarse-grained systems dominated by hyperconcentrated and hyperpycnal flows sourced from nearby continental uplifts and reaching the coastline. Amalgamated mouth-bar successions indicate high-energy, fluvial-dominated deltas formed under limited accommodation space and poorly developed delta plains. Ichnological and sedimentological indicators, including Ophiomorpha, fluid muds, and mud drapes, document episodic marine influence linked to short-lived tidal or wave reworking during phases of reduced sediment discharge or distributary avulsion. During the earliest Oligocene, fine-grained transgressive deposits locally overlie Eocene successions despite global sea-level fall, suggesting accommodation generation driven primarily by localized tectonic subsidence within an actively deforming forearc basin. The appearance of meandering fluvial deposits, rhizolith-bearing mudrocks, fungal remains, and abundant morichal palm pollen indicates the establishment of water-logged interdistributary bays and gallery flood forests, marking the development of an expanded lower delta plain during the Eocene–Oligocene transition. Vertical changes in depositional style occurred during this interval of significant climatic and eustatic fluctuations, although local tectonic activity likely exerted the dominant control on accommodation. From the Oligocene to Early Miocene, sedimentation was characterized by repetitive coarsening-upward successions interpreted as progradational deltaic cycles dominated by fluvial processes under relatively high accommodation-to-supply conditions. Deltaic architecture reflects hyperpycnite coalescence and distributary-mouth-bar migration, with ichnological indicators (e.g., tubular tidalites and burrow size) and sedimentological features (fluid muds and mud drapes) recording short-lived tidal or wave modulation controlled by basin morphology. Increased accommodation space is interpreted to relate to fault-controlled subsidence associated with regional tectono-stratigraphic reorganization of the margin. Overall, the study demonstrates that tropical forearc deltas of the Colombian Caribbean were primarily controlled by tectonically driven accommodation and episodic hyperpycnal sediment delivery, whereas marine processes exerted only intermittent influence. These results suggest that tropical fluvio-deltaic systems in northern South America may behave more similarly to tectonically forced high- and mid-latitude systems than to monsoon-dominated equatorial deltas, emphasizing the dominant role of tectonics and orography over climate alone in shaping deltaic evolution.