Mixed Energy Process Interactions Read From a Tide-Dominated Shoreline: The Paleo-Orinoco Delta, Trinidad

Abstract

Although most deltaic research still tends to describe particular examples in terms of river-, wave- or tide-dominated, most deltas are subject to intense mixed-energy influence which remains understudied. The Manzanilla Formation, a segment of the Pliocene Orinoco Delta on Trinidad, reveals details of the interacting river, wave and tide processes at dm and larger-scales in the deposits, despite overall progradation of what would be considered a tide-dominated shoreline. Based on sedimentary structures, grain size, bioturbation and organic-matter content, each measured bed or set was assigned a percentage reflecting probability of a certain dominant process, as judged from extensive literature review of previous work, and process probabilistic curves were generated by connecting percentage values of beds from bottom to top. Characteristic river signals in beds are outsize grain sizes, structureless or normal graded beds and abundant organic-matter content. Wave and storm-wave signals include symmetrical ripples and hummocky/swaley stratification commonly associated with a diverse ichnofauna population. Tide signals are stacked, orderly cross-bedded sands with mud drapes, abundant mud layers, bi-directional ripple laminae, ponded rhythmites, spring-neap tidal bundles with restricted ichnofauna species. The study segment of the paleo-Orinoco succession (125 m thick) records a muddy open-coast tidal flat, through delta front into distributary channels in the subaerial delta plain, and demonstrates process variations as follows: (i) at the scale of the entire regressive unit, the intensity of river and tide signals increases and the intensity of wave signals decreases in a landward direction, and there are high-frequency process changes with wave-, tide- or river-currents in alternating dominance at the dm scale; (ii) wherever unbioturbated, unlaminated, squishy fluid mud layers occur, river and tide signals become stronger, whereas wave signals become weaker at both large and small scales. The observation of decreased wave energy and increased river and tide energy in a shoreward direction, not unknown on other open-coast tidal flats, is interpreted here in another way. We suggest that the fluid mud, brought in huge quantity to the Orinoco littoral zone by the Guyana current (ultimately from the mouth of the Amazon delta) tended to dampen the nearshore wave activity, leading to preferred preservation of river and tide signals nearest to the shore.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016