TECTONIC AND CLIMATIC FORCING ON DEPOSITION OF DEEP-MARINE CLASTIC SEDIMENTS, MID-EOCENE, SPANISH PYRENEES

The Eocene Ainsa basin, Spanish Pyrenees, contains ~4 km of deep-marine deposits that accumulated in ~10 M.yr., as two angular-unconformity-bound tectono-stratigraphic units, in which the younger unit is structurally less deformed and shows a WSW shift in depositional axis, showing a first-order tectonic control on accommodation and deposition. The basin evolved with mainly nonmarine and marginal marine environments in the eastern sectors, whilst farther west there was an overall change from fluvio-deltaic to deep-marine systems. The deep-marine deposits are overlain by ~0.5 km of fluvio-deltaic and related sediments fed mainly from the south and east. Micropaleontology suggests water depths in the Ainsa basin were in the range 400-800 m. The two "tectono-stratigraphic" units contain 8 coarse clastic systems, each in the order of 100-200 m thick, and vertically separated by up to several tens of meters of mainly marls with lesser amounts of thin- to very thin-bedded sandstone turbidites. Each system typically contains 2-7 individual sandbodies (amounting to at least 25 throughout the basin), from 30-100 m thick, separated by 10s m of mainly thin- and very thin-bedded sandstones with subordinate marls. Within each sandbody, submarine channels show a consistent WSW offset stacking pattern away from the deformation front that acted as one of the confining basin lateral-margins. In contrast to the longer-term 4-5 million-year tectonic driver, the number of deep-marine sandbodies in the Ainsa basin (~25) that accumulated in ~10 M.yr, suggests the operation of a faster, non-tectonic driver, possibly related to the 400 k.yr. long-period eccentricity. Spectral analysis of bioturbation intensity in thin-bedded, fine-grained sediments in one of the cored intervals shows a likely 41 k.yr. and ~100 k.yr. Milankovitch cyclicity. The age model for the Ainsa basin yields an average sediment accumulation rate of ~40 cm ka-1, that is consistent with that inferred from the spectral analysis (~30 cm ka-1) for fine-grained sedimentation. A detailed analysis of the Ainsa basin sediments has permitted the deconvolution of tectonic and climatic signals at several time scales within a tectonically active basin.