Conference Paper

Hybrid Turbidite Beds in the Guarico Formation (Eocene), Venezuela

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Abstract

The Guarico Formation in the north of Venezuela corresponds to an up to 8,000 ft thick clastic succession of turbidites and associated deep water deposits. These deposits accumulated in an extended foredeep developed as a consequence of the oblique collision of the Caribbean plate against the South American plate during the Paleogene. Marine deep water deposits comprise a complete suite of coarse to fine grained deposits related to extrabasinal and intrabasinal turbidites. Intrabasinal turbidites include cohesive debris flow (also known as wild flysch), hyperconcentrated flow, concentrated flow and turbidity current deposits. Extrabasinal turbidites compose graded to complex beds with abundant plant remains, and are mostly related to hyperpycnal fluvial discharges. In the Rio Panapo area, a 195 m thick succession of turbidites is exposed. Sandstone beds are up to 2 meters thick, and internally display normal to inverse grading. Most beds are composed of coarse to fine grained sandstones, with massive, laminated and climbing ripples as dominant sedimentary structures. Deformational and water escape structures are common, as well as mica, plant remains, clay chips and charcoal clasts. Main paleocurrents are from west and northwest. Turbidite beds compose typical finning upward cycles up to 20 metres thick. Of particular interest are several intervals of hybrid turbidity current deposits. These packages are composed of two clearly differentiated intervals. The lower interval is composed of massive coarse grained sandstones with clay clasts, showing dish and deformational (load and water escape) structures. This lower interval is sharply overlaid by massive to parallel laminates fine grained sandstones, with large (up to 15 cm) clasts of charcoal towards the top. Elongated charcoal clasts often show imbrication. The upper sandstone bed shows a very irregular basal boundary, product of overloading over a water-saturated substrate. Internally, the sandstones shows a cyclic recurrence between massive and laminated sandstones, suggesting flow fluctuations in the original discharge. It is interpreted that these packages were accumulated by an intrabasinal inertia-dominated concentrated flow followed by an extrabasinal hyperpycnal flow. Field evidences suggest that concentrated flows could have been triggered in slope areas by passing-by hyperpycnal currents. Since concentrated flows travel faster, their deposits were found in the lower part, and were rapidly covered by hyperpycnal flow deposits when the previous level was still saturated in water, resulting in a strong basal deformation.

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