Stratigraphic context and timing of sand supply to deep-marine Ainsa-Jaca basin, middle Eocene, Spanish Pyrenees: Constraints from geochemistry and sedimentology

摘要

The sediment flux to a basin is controlled by a complex combination of tectonics, climatic variability and stochastic events, thereby creating a cryptic geological record. Deconvolving the effects of individual factors controlling sedimentation can often be challenging, due to a variety of reasons including a lack of outcrop, a poor understanding of the regional tectonic framework and insufficiently detailed geological maps. However, many of these deficiencies can be overcome within the deep-marine Ainsa basin, South Central Pyrenees where detailed research over the last decade has provided an extensive knowledge base. The Ainsa basin comprises ~4 km of middle Eocene deep-marine sediments. Basin stratigraphy consists of a succession of ~25 discrete sandy submarine fans and inter-fan deposits belonging to the Hecho Group. Recently it has been hypothesised that the supply of coarse-clastic sediment to the basin was paced by orbitally induced climate and/or sea level variability, whilst tectonics controlled the locus of deposition. This hypothesis is tested within the Upper Hecho Group using a refined basin age model and the creation of floating orbital time scales between submarine fans. Using calcareous nannofossil and large shallow benthic foraminifera, deposition of the Upper Hecho Group took place over a 6.0-8.3 Myr period between ~40.5-48.4 Ma, giving an average sediment accumulation rate (SAR) of 43.2±10.5 cm/kyr. Stratigraphic time series analyses of inter-fan fine-grained sediments indicate the presence of short eccentricity, obliquity and precession Milankovitch cycles. These floating time scales provide average SARs of 36, 28 and 25-33 cm/kyr for the Banaston, Ainsa and Guaso systems respectively. Applying these age models to the three systems suggest that submarine fan deposition potentially corresponds to specific eccentricity minima. As in the Pleistocene, such Milankovitch forcing could be linked with ephemeral glacio-eustatic low-stand conditions, associated with increased coarse sediment flux to the deep-marine Ainsa basin.