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Deconvolving tectono-climatic signals in deep-marine siliciclastics, Eocene Ainsa basin, Spanish Pyrenees: Seesaw tectonics versus eustasy

¹ Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK

The Eocene deep-marine siliciclastic fill of the Ainsa basin, Spanish Pyrenees, gives unprecedented temporal resolution of the causes and timing of coarse clastic sediment supply to a deep-marine basin. Early Eocene tectonic subsidence linked to Pyrenean orogenesis created the Ainsa basin, with water depths of ~400–800 m above a foundered shallow-marine mixed carbonate and clastic shelf. The ~25 sandbodies or channelized submarine fans in the basin were controlled by the ~400 k.y. Milankovitch frequency, with modes at ~100 k.y. and ~41 k.y. (possibly stacked ~23 k.y.) influencing bottom-water conditions in the basin, causing periodic stratification in the water column across a submarine sill at the western basin margin (early Boltaña anticline). Intrabasinal tectonics defined and controlled the position of eight sandy systems and their constituent fans, in a process of seesaw tectonics, by (1) westward lateral offset stacking of sandy fans due to growth of the eastern side of the basin, represented today by the Mediano anticline, and (2) eastward (orogenward) back stepping of the depositional axis of each sandy system, due to phases of relative uplift of the Boltaña anticline. During basin infill, uplift of the Boltaña anticline led to increasing basin narrowing and depositional confinement. Unlike the earlier depositional systems, the youngest deep-marine system was fed from a more southern sediment source between the growth anticlines, as was the overlying deltaic system. All the older deep-marine sandy systems were fed from southeast point sources, from canyons and erosional lower-slope channels eroding the growing Mediano anticline. The depositional style outlined in this paper might be common to other active margins where siliciclastic basins evolve between active thrusts.