The modern Orinoco delta is the terminus of the world’s largest alongshore mud dispersal system, and it receives significant amount of mud (108 tons/yr) from the Amazon via the coastal Guyana Current. The influence of these huge volumes of Amazon mud on the paleo-Orinoco Delta succession has never been investigated.
New research is showing that abundant fluid-mud deposits were preserved with different styles in the paleo-Orinoco delta in storm wave-dominated, tide-dominated and wave-influenced, and tide-dominated delta lobes from the Pliocene Mayaro, Manzanilla and Lower Morne L’Enfer formations, respectively. The Mayaro Formation parasequences are characterized by alternating fluid-mud intervals (5-30 cm thick) and hummocky cross-stratified sandstones (HCS) passing upwards into amalgamated swaley cross-stratified (SCS) sandstones. The fluid-mud intervals have flat or irregular tops occasionally overlain by amalgamated SCS sandstones with mud clasts. In places, very thick (1-1.5 m) fluid-mud intervals containing small HCS are overlain by amalgamated SCS sandstones. The Manzanilla Formation parasequences exhibit majority of fluid-mud layers (2-3 cm thick) associated with bi-directional current rippled-sandstones in the uppermost part overlying interbedded fluid-mud layers (0.5-1.5 cm thick) and erosion-based sandstones with symmetrical ripples and small SCS. The Lower Morne L’Enfer Formation displays three types of fluid-mud deposits: (A) very thin (1-5 mm) layers that amalgamate to form mud bedsets or drape along ripple laminae; (B) thick (0.5-1.5 cm) layers interbedded with structureless to rippled-sandstone beds; (C) very thick (1-10 cm) deformed layers with load structures at their top and laterally variable thickness. The types A and B occur mainly in the lower parts of parasequences, and they change upwards to type C.
The fluid mud deposited in wave- or tide-dominated environments with over 1 m thick units suggest large muddy bedforms similar to mudbanks that migrate along the modern Amazon-Orinoco coast. However, there are differences between fluid-mud architectures in the wave- and tide-dominated settings. Storm waves tend to erode and re-suspend sediments in shallow water, and re-deposit fine-grained sediment into deeper water below the storm wave base (15-50 m water depth). Tide-dominated deltaic shorelines, in contrast, more likely to trap fine-grained sediments in water depth less than 10 m with less escaping offshore.
AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018