Deciphering influencing processes in a tropical delta system (middle-late Eocene? to Early Miocene, Colombian Caribbean): Signals from a well-core integrative sedimentological, ichnological, and micropaleontological analysis

Deltaic depositional systems are characterized by a complex interaction of physical, chemical, and biological factors. Although fluvial-, wave- and tidal-dominated deltaic environments have been extensively studied, evaluation of the processes in tropical mixed sedimentary systems has not been fully documented. Tropical regions with active margins are tectonic environments where these multiple factors act on the development of coastal systems. An onshore well-core from this tropical setting (Sinú-San Jacinto Basin, Colombian Caribbean) revealed that a middle-upper Eocene?-lower Oligocene coarse-grained deltaic setting is replaced by a thick coal-bearing mixed-energy fine-grained deltaic succession from the Oligocene to Early Miocene. The integrated analysis of facies associations, ichnological data, and terrestrial/marine micropaleontological assemblages (palynomorphs, foraminifera, and calcareous nannofossils) of this well-core allowed us to identify changes in dominance and influence of coastal processes (fluvial-, wave- and tide) and shoreline evolution. Using this information, as well as the sediment supply and accommodation space ratio, we were able to distinguish three intervals from the bottom to the top of the siliciclastic succession: (i) middle-late Eocene?–early Oligocene, prograding, fluvial-dominated, wave- and tide-influenced coarse-grained deltas with amalgamation of hyperpycnal-dominated mouth bars with hyperconcentrated flow input, (ii) Oligocene, retrograding to prograding, hyperpycnal-dominated heterolithic delta deposits punctuated by transgressive pulses, and (iii) late Oligocene to Early Miocene, aggradational, coal-bearing fine-grained delta plain with a higher proportion of transgressive phases over the continental environment. The complete succession represents long-term (∼14 Myr) mixed-energy nearshore siliciclastic systems, showing different lithological arrangements and sedimentation styles. A long-term evolution is observed from a middle-late Eocene? steep, short and coarse-grained sedimentary system with tropical humid lowland forest and punctual development of macrobenthic tracemaker communities (Interval I) to an Early Miocene gently (poorly drained), well-developed delta plain with mangroves and wave- and storm-influence record through trace fossils assemblages (Interval III). A combination of factors, including subsidence, relief uplifting, and possible relative sea level changes, are interpreted as the main controls on the stratigraphic evolution of sedimentary styles throughout the entire succession. Minor-order sedimentary successions into each interval (e.g., prograding distributary mouth-bar channel) revealed short-term cycles presumably controlled by an internal delta dynamic. Multidisciplinary analysis is essential for recognizing the influence of fluvial, wave, and tidal processes on tropical deltas, where high spatial and temporal variability makes it difficult to determine dominant processes for long periods of time.