Eustatic Control on DeepWater Deposition Revealed by Carbon Isotope Stratigraphy: Example of the Ainsa Basin, Lower-Middle Eocene Pyrenean Foreland Basin, Spain

Abstract

The lower-middle Eocene succession (∼4km) of the Ainsa basin in the foreland basin of the Pyrenees in Spain displays several sequences made of 1) a basal erosional surface, overlain by 2) thick (∼100–200m) sandy units of sediment gravity flow (SGF) deposits, followed by 3) a thick (∼200–400m+) interval of marls and thin-bedded fine-grained sandstones. Such repeating patterns have been long interpreted as classical lowstand-highstand sequences and naturally inferred as caused by sea level changes, in consistency with sequence stratigraphic models of deep-sea deposition and correlation with global events on sea level charts. A causal relationship with eustasy remains however problematic due to well-known problems of chronostratigraphic uncertainties and correlation with eustatic charts of questionable global validity. In addition, the existence, origin, and amplitude of eustatic sea-level changes in the early-middle Eocene greenhouse also remains a matter of discussion. Finally, the tectonic nature of foreland basins also casts doubt as to the predominance of eustasy in forcing stratigraphy in such settings. We here report a high-resolution study of carbonate whole rock stable isotope composition across the lower four sand bodies of the Ainsa basin succession, spanning ∼7Ma and ca 1600m of thickness. Existing biostratigraphic and magnetostratigraphic investigations in the area provide chronostratigraphic constraints on timing of deposition. The stable isotope record in Ainsa shows a large variability compared with contemporaneous global ones and different average values. However, we find an unprecedented level of correlation between the evolution of δ13C and the Kominz et al. (2008) sea-level curve. This correlation provides an independent cross-validation of the two distant stratigraphic signals, which suggests that riverine influx of isotopically light carbon modulated by sea level changes has controlled the isotopic composition of dissolved inorganic carbon in the study area. Contrary to deep sea settings classicaly seen as ideal for global isotopic studies, the pelagic record of shallow foreland submarine fan complexes is perhaps a more sensitive recorder of past sea level variations, a counter-intuitive perspective given the tectonic activity and continental proximity of such settings. This finding is an important step towards a better understanding of past sea level and climate change which are both fundamental constraints on global earth systems models.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016