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Mixed Processes in an Asymmetic, Wave-Dominated Shelf-Edge Delta Lobe: The Pliocene Paleo-Orinoco Delta (Moruga Formation), Trinidad


Ancient wave-dominated deltas have been described with simple internal architecture, although modern examples show significant process and architecture variability. Parts of the Early Pliocene Moruga lobes of the paleo-Orinoco Delta on Trinidad provide a well-exposed window into the process mix that built the downdrift flank of the delta lobe. The 280 m thick outcrops show the downdrift to updrift delta variability. On the downdrift side, the succession (140 m thick) is muddy, rich in organic matter, lacks bioturbation, and has facies characteristics of distriburary channel, mouth bar, and delta front deposits. The distributary channel and mouth bar deposits (30 m thick) have lower-fine-grained sandstones (0.5 to 4 m thick) with trough-cross-bedding and parallel-lamination grading upwards into hummocky- and swaley-cross-stratification. In the muddy intervals (0.5 to 1 m thick), there are occasional spring-neap tidal bundles and bidirectional ripples. The overlying succession (110 m thick) consists of several coarsening-up delta front parasequences (5 to 15 m thick). The 10 to 20 cm thick interbedded mudstones and sandstones gradually change upwards into parallel-laminated to almalgamated hummocky- and swaley-cross stratified sandstones. The thin-bedded sandstones are lower-very-fine-grained, sharp-based and normally graded beds that are structureless to parallel-laminated. Contorted bedding and ball-and-pillow structures are observed in places. On the updrift side, it shows a coarsening-upward succession (140 m thick) from muddy prodelta to delta front sand ridges with low intensity but high diversity of bioturbation in the mudstones. The sand ridges are characterized by very-fine-grained, amalgamated hummocky- and swaley-cross-stratified sandstones (up to 10 m). The overall presence of wave- and storm-generated structures in the deposits is interupted by local presence of tidal (spring-neap tidal cycles) and of fluvial (delta front turbidite/ hyperpycnite) deposits that suggest spatially and temporally variable processes. The Moruga deltas had an updrift part built mainly by wave processes and a much muddier downdrift part created by fluvial, tidal and wave influences, somewhat analogous to what is seen in Mekong, Orinoco and Danube Holocene deltas. Orinoco shelf reservoirs described in the literature indicate highly variable “thin bedded” reservoirs, most likely the result of a combination of described processes and facies in this study.