A discourse on factors influencing the formation of sigmoidal and linear slope-geometries in the deltaic clinoforms of the calciclastic Sobrarbe Delta, Ainsa Basin, Spain

Clinoforms develop at the hearts of prograding sedimentary systems and are an integral part in understanding basin development and sediment partioning. The Eocene Sobrarbe Formation in the Ainsa Basin in Spain, displays well-developed dipping clinoform geometries. The study observes depositional facies, mineralogy, and performs curve-fitting calculations in 2D to better understanding of factors controlling clinoform geometries. The Sobrarbe Formation is a unique prograding calciclastic delta system, in which carbonates are dominant, but depositional environments and sedimentary processes are akin to silicilclastic systems. Four clinothems (CT 1–4) were selected for detailed study. The clinothems display an overall coarsening upward trend. The lowermost clinothems comprise dominantly pro-deltaic marlstone deposited from suspension and dilute hyperpycnal flows, with occasional thin fine-grained episodic turbidite beds. The uppermost clinothems are dominated by reworked remobilized topset deposits comprising bioturbated calcarenites with a variety of trace-fossils, shells and large assemblages of nummulites, and finally proximal subaqueous stacked calcarenites. Three clinothems (1, 2, 4) are characterized by sigmoidal geometry (Gaussian distribution curve), symmetrical sediment distribution, a simple coarsening upward trend, and where calculable, ascending regressive trajectories. Clinothem 3 comprises a more complex stratigraphic development and displays an overall ascending transgressive trajectory, linear slope and asymmetric sediment distribution. The results show that sigmoidal geometries develop in both the distal pro-delta and in the coarser-grained proximal delta, suggesting the sigmoidal form develops naturally granted sufficient accommodation. While speculative it appears that linear slopes might be associated with draping during transgression, and asymmetric distribution associated with high sedimentation rates vis a vis accommodation. Accommodation exerts the dominant control on the location of the rollover and the trajectory. The topset-foreset ratio indicates the relative rate of sedimentation compared to accommodation, as high sedimentation rates increase deposition on the foreset. Results indicate that accommodation appears to be the most important factor controlling foreset slope, with steeper inclination developing in deeper water. Meanwhile sedimentation rate, grain size, and the role of turbidity currents may play a secondary role influencing slope angle.