EP011-11
Palaeohydraulic reconstructions of the Escanilla fluvial system, south-central Pyrenees, Spain
Tuesday, 8 December 2020: 04:30
Virtual
Nikhil Sharma1, Jean Vérité2, Stephen Watkins1, Luis Valero1, Alexander C Whittaker3, Laure Guerit4, Miguel Garces5, Cai Puigdefabregas6, Francois Guillocheau7, Thierry Adatte8 and Sebastien Castelltort1, (1)University of Geneva, Department of Earth Sciences, Geneva, Switzerland, (2)Université de Rennes 1, Gèosciences Rennes, Rennes, France, (3)Imperial College London, Department of Earth Science & Engineering, London, United Kingdom, (4)University of Rennes, Rennes, France, (5)Universitat de Barcelona, Departament de Dinàmica de la Terra i l’Oceà, Barcelona, Spain, (6)University of Barcelona, Departament de Dinàmica de la Terra i l’Oceà, Barcelona, Spain, (7)Université de Rennes, Géosciences Rennes, Rennes, France, (8)University of Lausanne, Institute of Earth Sciences, Lausanne, Switzerland
Abstract:
The origin of stratigraphic cycles in fluvial successions is highly debated, with various factors such as tectonics, sea level and climatic processes being attributed as the major cause. It has been established that upstream factors such as sediment supply, grain-size and water discharge determine the river equilibrium profile. However, downstream (base-level) changes do not modify the river equilibrium slope or do so to a much lesser extent. We hypothesise therefore, that if fluvial stratigraphy is linked with upstream factors, there should be a systematic link between stratigraphic packages and palaeoslope. In particular, aggradation should be linked to lower slopes (high accommodation availability) while less aggrading (amalgamated packages) should be linked to higher slopes.
We test this hypothesis with an ancient system, the Mid-Late Eocene Escanilla sediment routing system, northern Spain. The system consists of 1000 m alluvial deposits at the southern-margin of the Tremp-Graus Basin, south-central Pyrenees, Spain, and is characterized by remarkable cyclicity of three sequences of amalgamated and non-amalgamated units. In our hypothesis, the amalgamated units should be associated with relatively high fluvial slopes and the non-amalgamted units, with vertically stacked channel bodies, should have be associated with relatively lower slopes. We have measured, in total, 219 localities for grain size distributions and flow depths at two locations within the fluvial system, one in the proximal portion (Coll de Vent) and one in the distal portion of the sedimentary routing system (Olson) in order to examine the role of upstream and/or downstream controls on the deposition of these sequences. Our reconstructed alluvial slopes increase almost linearly up-sequence, irrespective of the amalgamated-non-amalgamated pattern, suggesting that the stratigraphic cyclicity is not linked with upstream factors but instead be linked with base-level changes.
