Is there a 400-kyr pacing of deep-marine sandbodies in the Middle Eocene Ainsa Basin, Spanish Pyrenees ?

In an attempt to understand the relative importance of climate versus tectonics in modulating coarse-grained sediment flux to a tectonically-active basin during a so-called greenhouse period, we have studied the Middle Eocene deep-marine Ainsa Basin, Spanish Pyrenees. We use orbital tuning of many spectral gamma-ray logged fine-grained siliciclastic sections, already shown to contain Milankovitch frequencies, in conjunction with a new high-resolution palaeomagnetic study throughout the basin sediments to identify polarity reversals in the basin as anchor points to allow the conversion of a depth-stratigraphy to a chronostratigraphy. We use this data, in conjunction with a new age model for the Ainsa Basin, to pace the development of the deep-marine sandy submarine fans over ~8 million years. Timing for the sandbodies shows that, unlike for the fine-grained interfan sediments, coarse-grained delivery to the basin was more complex but generally appears to coincide with long-eccentricity (400-kyr) minima and record changing climate. The position of some sandbodies is at variance with this, specifically those that coincided with a period of demonstrable intense tectonic activity within the Ainsa Basin that we propose record the time when the basin was converted from a relatively simple foreland basin into a thrust-top basin, presumably associated with rapid uplift and redeposition of coarse clastics from the staging area for sediment gravity-flows (SGFs) into deep-marine environments. This study demonstrates the complex nature of drivers on deep-marine deposition in a tectonically-active basin over ~8 Myr and suggests that even during greenhouse periods sandbodies are more likely linked with times of eccentricity minima and climate change that may be linked with lower mean global sea level, consistent with the concept of lowstand fans.