Steep nested clinoforms in the mixed siliciclastic-carbonate Sobrarbe Deltaic Complex (Eocene, Aínsa Basin, Spain)

This study investigates the processes controlling the development of nested clinoforms in outcrops of the Eocene Sobrarbe Deltaic Complex of the Aínsa Basin, highlighting their significance as archives of basin-margin evolution. Small-scale clinoforms record the higher-frequency cycles of delta progradation and the coetaneous development of a carbonate factory, while large-scale clinoforms record longer-term high-energy processes on the shelf/slope, including the reworking of previous deltaic/shelf deposits. High-resolution lithological variations were characterized across three of the high-frequency cycles, revealing a downdip transition along the small-scale clinothems from subaqueous channels to distal delta front/shelf settings. The coarsest grain sizes are restricted to the proximal delta plain environments evidencing limited coarse sediment transfer to deeper waters in this part of the deltaic complex, enabling the development of a coeval carbonate factory resistant to moderate clastic input and water turbidity. The small-scale clinoforms display steeper slopes than expected for siltstone to fine sandstones, which is explained by early cementation of the carbonate-rich horizons preserving relatively steep clinoform slopes and preventing their collapse. The study emphasizes the importance of recognizing heterogeneity in such mixed siliciclastic-carbonate environments, which has implications for understanding reservoir quality and connectivity in subsurface systems.