Preservation of compound delta in the rapidly subsiding Pliocene Orinoco Delta: Insights from quantitative fluvial, tidal and storm-wave signals

Despite the documentation of compound clinoform morphology in modern deltas over the past two decades, there is still a scarce recognition of these in the stratigraphic record. Among the ancient cases that have been recognized, tide- and wave-dominated compound clinoforms have been documented. Here we present a fluvial- and wave-dominated compound clinoform delta in the Pliocene Orinoco Moruga Delta on Trinidad. The outcrops exhibit lateral variation with wave-dominated and mixed-influence deltaic clinothems preserved in the proximal (i.e., upstream) part (SSW) of the compound system and wave-dominated deltaic clinothems in the distal (i.e., downstream) part (NNE). The shoreline clinothems are 5–15 m thick, sandy coarsening-upward units, and they commonly comprise HCS/SCS and wave-rippled sandstones generated by storm wave processes in both upstream and downstream parts. In the upstream part where the deposition location was close to main sediment supply, the shoreline clinothems comprise an upper unit of fluvial-dominated mouth bar and channel deposits overlying a lower unit of mixed-influence mouth bars and wave-to tide-influenced channel deposits. The subaqueous clinothems are 30–50 m thick and are characterized by coarsening-upward units changing from thick, bioturbated siltstones and mudstones at lower levels, through interbedded siltstones/mudstones and thin hummocky/swaley cross-stratified (HCS/SCS) sets, to occasional amalgamated HCS/SCS beds. The subaqueous clinothems in these proximal and upstream parts are somewhat sandy and notably influenced by wave processes. In contrast, the distal parts of the subaqueous clinothems are muddy with repetitive, frequent thin beds of both HCS or wave-rippled beds and especially wave-enhanced sediment gravity flow deposits. The compound clinoform delta in the Moruga Formation thus exhibits a spectrum of fluvial, tidal, and storm-wave signals, along with diverse channel types. This variability provides a valuable opportunity to document the interactions among river, tidal, and storm-wave processes as well as delta building processes driven by the various types of channels within the compound clinoform delta.