Orbital forcing as principal driver for fine-grained deep-marine siliciclastic sedimentation, Middle-Eocene Ainsa Basin, Spanish Pyrenees

Abstract

The relative importance of climate, tectonics and/or autocyclicity in driving deposition of laminated deep-marine fine-grained siliciclastic sediments in tectonically active basins remains controversial. Consequently, the recognition of Milankovitch periodicities in such successions remains poorly documented. This study evaluates the cyclostratigraphy and its likely drivers in the Middle Eocene Ainsa Basin, Spanish Pyrenees, a basin that is known to have been associated with major syndepositional tectonics. This basin was infilled by a thick succession of siliciclastic sediments, characterised by an alternation of coarse-grained sandbodies interpreted as submarine fans and finer grained packages of interfan, fan lateral-margin, and channel levée–overbank sediments. This study presents time-series analysis of outcrop spectral gamma-ray and cumulative sandstone turbidite abundance data from eight fine-grained, essentially interfan sections located between coarse-grained sandbodies (submarine fans). The ~1.4km of cumulative stratigraphy studied show that Milankovitch cyclicity can be recognised throughout the entire stratigraphic evolution of the Ainsa Basin, even though the basin was tectonically active and suggest that the accumulation of the interfan deposits was orbitally controlled. Orbital parameters most likely paced the cyclic delivery of the fine-grained sediments mainly by river- and delta-supplied turbidity currents from the non-marine and shallow-marine Tremp Basin immediately to the east. Sediment accumulation rates determined from spectral analysis show a gradual decrease throughout the deep-marine stratigraphy from ~57 to ~24cm/kyr. This study shows that orbitally-induced climate change acted as a principal driver on deep-marine terrigenous sediment accumulation within a tectonically active basin. High-frequency climate variability, therefore, should be considered when studying controls on siliciclastic sedimentation in tectonically active regions.