Sedimentary Morphodynamic and Ichnologic Characteristics of Paleo-Orinoco Shelf-edge Delta – Example of Pleistocene Mayaro Formation

Sudipta Dasgupta
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
[email protected]

Quaternary Orinoco delta is an accommodation-driven one requiring relative sea-level fall to reach the shelf-edge. The Mayaro Formation outcrops, being the remnants of the first phase of its growth-fault bound shelf-edge delta, are exposed along the SE coastline of Trinidad. The Mayaro Formation, being a ‘fourth-order’ sequence, at least 2000 m thick and with high sediment accumulation rate (approximately 10 m/Ka), was deposited coeval to the Pretiglian time or the first cold episode of Pleistocene. It can be subdivided into several ‘fifth-order sequences’ separated by canyon-incision surfaces and their correlative conformity as the sequence boundaries. The smaller sequences consist of 6 major ‘sixth-order’ facies tracts – five of them being complex combinations of sustained and hybrid gravity-flow dominated canyon-fills and the other facies tract, the most dominant one accounting around 80% of the thickness, belongs to the shelf-edge delta-front. Extreme ecological conditions provided by high slope instability, very high sedimentation and erosion rate, high turbulence and turbidity, chemical and organo-chemical fluctuations, associated with low preservation potential, pertain the sediments almost devoid of trace fossils. Rare and sporadic occurrences of colonization, with low ichnodiversity and simple ichnofabrics, mark the deviations of the delta-front and proximal prodelta deposits towards more wave-dominated depositional environment from more river influence or gravity flow dominance. While the lateral facies variations reflect wave-induced deltaic asymmetry and avulsion of architectural elements, the stacking patterns of the facies tracts manifest the interaction between growth-tectonics and contemporaneous eustasy, acting as a switch of sand-delivery into the deep marine environment.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects