Amazon fluid mud impact on tide- and wave-dominated Pliocene lobes of the Orinoco Delta

Abstract

The modern Orinoco Delta is a major sink for the world's largest alongshore (littoral) mud dispersal system; it receives some 108 tons/yr from the Amazon river delta. The influence of these huge volumes of Amazon mud on the paleo-Orinoco Delta succession is now investigated for the first time. Abundant fluid-mud deposits are preserved in outcrops from the Pliocene Orinoco Delta deposits (Lower Morne L'Enfer, Manzanilla and Mayaro formations on Trinidad) with different styles in tide-dominated, in tide-dominated and wave-influenced, and in storm wave-dominated delta lobes. Fluid-mud deposits in sandy parasequences (10–20 m thick) of the Lower Morne L'Enfer Formation change from thin layers amalgamating to form mud bedsets (up to 4 cm) or draping across ripple laminae in the lower part, to thick layers (up to 10 cm) occasionally with deformed and load structures at their top, in the upper part. The Manzanilla Formation parasequences (30–40 m thick) exhibit coarsening-to-fining-upward (CUFU) facies successions with fluid-mud deposits located mainly in the middle to upper parts of the parasequences, near the transition between the CU and FU parts of the units. Most of the fluid-mud layers are interbedded with bi-directional, current rippled-sandstones, whereas minor mud layers are associated with erosion-based sandstones with small-scale swaley strata and bi-directional symmetrical ripples. The Mayaro Formation parasequences (5–15 m thick) are characterized by alternating fluid-mud intervals and sets of hummocky cross-stratified sandstones (HCS) passing upward into amalgamated swaley cross-stratified (SCS) sandstones. The fluid-mud intervals have flat or irregular tops occasionally overlain by amalgamated SCS sandstones with fluid-mud clasts.Very thick (1–1.5 m) fluid-mud intervals occur in places in both tide- and wave-dominated strata and these are probably large muddy bedforms similar to mud banks that migrate currently along the modern Amazon-Orinoco coast. The outcrop analysis strongly suggests that the fluid-mud banks, as they approached the Orinoco delta-front area, were handled differently depending on whether the delta-front lobes were wave- or tide-dominated. Tide-dominated delta lobes tended to trap the muds near the shoreline and on the subaqueous delta-front platform in water depth < 10 m with only minor volumes escaping offshore. In contrast, storm wave-dominated delta lobes had a more dynamic impact on the approaching mudbanks. Mud accumulated in the shallow water tend to be eroded and re-deposited into deeper water near the storm wave base (15–50 m water depth).The volumes of Pliocene Amazon mud transported along the innermost shelf and arriving on the Orinoco Delta were likely influenced by high-amplitude, Pliocene glacio-eustatic sea level fluctuations. During sea-level fall and lowstand periods, increased volumes of Amazon mud would have bypassed to the deep water and thus decreased mud volumes drifted and became incorporated in the Orinoco delta front. In contrast, sea-level rise and highstand periods would have promoted increased volumes of Amazon mud both on the shelf and onto the Orinoco, similar to the Holocene and modern muddy system.