Abstract: 

Integrating sedimentological and ichnological characteristics of the Gelasian Mayaro Formation along the SE coast of Trinidad allows recognition of a river-dominated and wave-influenced shelf-edge deltaic succession of the paleo–Orinoco River developed under strong slope instability. Four main sedimentary settings and twelve subenvironments developed on and beyond the outer shelf (i.e., at the shelf margin) have been identified. Extreme paleoenvironmental conditions make the succession rarely and sporadically colonized. Ichnologic evidence suggests that shelf-edge deltas are among the most stressful marine environments, due to a combination of physicochemical factors in response to the intrinsic sedimentary processes and the relative hierarchy of their influences specific to every subenvironment. Within any particular subenvironment, the relative dominance of the fluvial feeder system, the action of waves (and rare influence of tides), and slope instability determine the combinations and ranking of stress factors. River-dominated shelf-edge delta subenvironments demonstrate the extreme influence of stress factors related to channel activity (e.g., salinity changes, high sedimentation and erosion rates, flocculation of mud, seasonal variations of fluvial flux), whereas subenvironments dominated by waves, storms, and oceanic swells away from the axial feeder(s) are mostly influenced by barform morphodynamics and its effects on local physiography. The paleo–Orinoco delta is represented not only by the delta lobe(s) developed at the shelf edge but by delta lobe(s) formed on the outer shelf as well. Outer shelf deltaic subenvironments manifest typical ichno-sedimentological signatures of a “normal” (wave-influenced in this case) inner-shelf delta. Being susceptible to extensive gravitational instabilities, the delta system suffered from canyonization (and smaller gullying) and subsequent filling (N.B. Hereafter we use the term canyon for both canyons and smaller gullies). The shelf-edge delta system was also associated with “shelf-attached” mass-transport systems. Such depositional subenvironments dominated by slope instability and sediment-gravity-flows appear to be the most unconducive for benthic colonization, resulting in almost complete absence of bioturbation. Characterization of the stress factors, as the functions of parameters arising from specific sedimentary and/or oceanographic processes, chemical conditions, and preservational constraints specific to each subenvironment, leads to the construction of a comprehensive ichnological and depositional analog model for shelf-edge deltas in general, and for the accommodation-driven low-latitude shelf-edge deltas at an active oblique foreland setting in particular.

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