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

This study investigates and discusses the processes controlling the development of nested clinoforms in outcrops of the Eocene Sobrarbe Deltaic Complex of the Aínsa Basin, and demonstrates that early cementation in fine-grained sediments can help preserving steep deltaic foresets. Small-scale clinoforms (few 10s of m thick, few 100 s of m long) record the higher-frequency cycles of delta progradation and the coetaneous development of a carbonate factory, while large-scale clinoforms (several 10s of m thick, few km long) record longer-term high-energy processes on the shelf and slope, including the reworking of previous deltaic/shelf deposits. Detailed lithological analysis across three of the high-frequency cycles reveals a downdip transition along the small-scale clinothems from subaqueous channels to distal delta front and shelf settings. The coarsest grain sizes are restricted to the proximal delta plain deposits evidencing some periods of limited coarse sediment transfer to deeper waters, enabling the development of coeval carbonate production on the delta front/shelf. Early cementation of carbonate-rich horizons preserved the relatively steep, yet fine-grained clinoform slopes and prevented their collapse. Observation of steep cemented foresets in fine-grained lithologies challenges a univocal relation between delta slope and grain size. In addition, we propose that early cementation is linked to in-situ carbonate production, which implies faunal adaptation probably favored by hiatuses in sedimentation between successive stages of delta progradation. The study also emphasizes the importance of recognizing heterogeneity in such mixed siliciclastic‑carbonate environments, and contributes to the understanding of depositional processes and sedimentary architecture in shallow-marine prograding systems.