The Roda Sandstone crops out at the NE-margin of the Tremp-Graus Basin, a “piggyback” basin initiated in Palaeogene times in the foreland of the Southern Pyrenees. The study area ( 30 km2) is composed of the well-understood and well-exposed parts of the Roda Sandstone cropping out along the Isabena Valley, and the less-worked and less-exposed outcrops to the north along Barranco de Ricans and Barranco de Codoñeras. Data for this study include a combination of conventional sedimentology data coupled with analyses of 3D Virtual Outcrop Models. Based on these methods 8 different facies associations have been distinguished. Their distribution suggests that the Roda Sandstone is composed of two different but coeval delta systems and their linked subenvironments; a Gilbert-type delta outcropping along Isabena Valley, and a shoal-water fan delta along Barranco de Ricans and Barranco de Codoñeras.
Along the Isabena Valley the outcrops of the Gilbert-type deltaic system are composed of at least 6 distinct packages (Roda U–Z) characterized by texturally mature, delta-front sandstones separated by offshore mudstones. The stacked basinward-stepping architecture of the Roda U–Y units here represents a minimum of 3 major phases of SW-directed progradation, capped by hardground horizons formed during the subsequent flooding phases. Tidally-modified delta-front deposits found in the distal exposures of Roda Y represent the redistribution of the delta toeset by W–NW directed ebb-tidal currents, active during advanced stages of progradation of the Gilbert-type delta. The more gentle sloping shoal-water fan-delta system found in Barranco de Ricans and Barranco de Codoñeras, is characterized by progradational to aggradational stacking of texturally immature wave-modified mouth-bar sandstone packages separated by offshore mudstones. The zone of interference between the Gilbert-type and shoal-water delta is dominated by wave-reworked sediments deposited as shoreface, barrier and spit deposits.
Biostratigraphic dating suggests that the Roda Sandstone was deposited during a 3rd order relative sea level cycle as a shelf-margin systems tract. However, the stacking of the individual Roda U–Y units demarcated by offshore mudstones, both within the Gilbert-type and the shoal-water deltaic systems, suggest that 4th order fluctuations occurred. These 4th order units are physically correlatable between the two deltaic systems, which implies that both deltas responded synchronously to a combination of regional subsidence and eustatic sea level fluctuations. The differing internal architecture, such as the steeply-dipping, textural mature, foreset beds of the Gilbert-type delta versus the more gentle sloping, textural immature mouth-bars of the shoal-water fan-delta, are controlled by different hinterland characteristics and intrabasinal topography.