Importance of mixed energy in the Pliocene development Orinoco Delta lobes, and the impact of large volumes of Amazon fluid mud

This research focuses on how river-, wave- and tidal current energy is interpreted to have mixed and been preserved in well-exposed Pliocene strata of both wave-dominated and tide-dominated Orinoco delta lobes. A new method of process facies analysis has been adopted, allowing a more quantitative interpretation. In addition, the very large volumes of Amazon-derived mud that impact the front of the Orinoco Delta today are followed back through the Pliocene Orinoco record, and documented here for the first time. The detection of mixed energies and the additional impact of large mud volumes on the delta front are important because most previous studies on ancient systems have commonly attempted to focus on the dominant process and tended to overlook the complexity of process mixing. The research was carried out in the Pliocene Orinoco Delta on Trinidad using four datasets: an outcrop dataset of 1190 m of measured sections from the topset segments of shelf margin clinoforms of the Moruga Formation; a 260 m outcrop segment from the outer shelf to upper slope environments of the Moruga Formation; a 125 m thick outcrop segment from a tide-dominated delta lobe of the Manzanilla Formation; and three selected outcrop examples (15-80 m thick) from the Morne L’Enfer, Manazanilla and Mayaro formations. The results demonstrate how fluvial and wave signals have mixed as the shelf margin clinoform developed in a strong storm wave-dominated setting. The data suggest that the fluvial signals (including parts of distributary channel-fills and delta fronts) were preserved because of unusually high subsidence rates (averaging >1 km/My on the topsets) and rapid sediment burial probably with a sheltered coastal morphology on this early Atlantic shelf margin, preventing some of the fluvial deposits from being completely reworked by storm waves. Some of the tide-dominated segments of the Pliocene Orinoco Delta are shown to have had a compound deltaic clinoform that reveals detailed interaction of river, wave, and tide processes with the impinging fluid mud. This part of the research suggests that the abundant fluid mud caused wave damping and the preferred preservation of river and tidal signals on the subaqueous delta platform due to the reduced wave action. The Amazon-derived mud tended to accumulate on the subaqueous platform in tide-dominated delta lobes, whereas it was more likely to be eroded and re-deposited into a deeper setting on storm wave-dominated delta lobes.