Abstract

A segment of the Pliocene Orinoco Delta on Trinidad preserved a progradational compound clinoform that reveals detailed interaction of river, wave, and tide processes with fluid mud at a scale of tens of meters to centimeters in the deposits. Based on sedimentary structures, grain size, bioturbation, and organic-matter content, each measured bed or bed set was assigned a probability percentage of having been dominated by a particular process and process probabilistic histograms were generated. Characteristic river signals in these deposits are rare, and are represented by outsized grain sizes, normally to inversely graded beds, and abundant organic matter, though organic matter can also be derived from vegetated supratidal areas without fluvial influence. Wave and storm-wave signals are recorded by symmetrical ripples, hummocky–swaly cross stratification, and a diverse ichnofauna. Tidal signals are represented by stacked, orderly cross-bedded sandstones with mud drapes, abundant mudstone layers, bidirectional ripple laminae, spring–neap tidal bundles, and a restricted ichnofauna. The study gave the following results: (i) at the scale of regressive parasequences (30–50 m), the intensity of preserved wave signals increased from the prodelta towards the outer delta-front platform and decreased to the inner delta-front platform, whereas there was a reciprocal trend in the intensity of tidal signals; the fluvial signals are generally not strong, but sometimes they become irregularly stronger towards the upper levels of a parasequence, (ii) at the scale of individual spring–neap tidal bundles (centimeter to decimeter), thick tidal rippled sandstones with thin to partially eroded fluid-mud layers were deposited during spring tides, and thin tidal rippled sandstones or silt laminae with thick fluid-mud layers were deposited during neap tides on the middle-inner delta-front platform; storm waves reworked the deposits mainly on the outer delta-front platform. The observations suggest that the abundant fluid mud (transported by the wave-driven littoral Guyana Current from the Amazon river mouth) on the delta-front platform caused wave damping and the preferred preservation of river and tidal signals on the middle-inner delta-front platform and distal subaerial delta.

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