Controls on stratigraphic architecture in contemporaneous delta systems from the Eocene Roda Sandstone, Tremp-Graus Basin, northern Spain

Abstract

The Eocene Roda Sandstone of the Spanish Pyrenees is comprised of two coeval coarse-grained delta systems which share correlatable surfaces but show a markedly different internal facies architecture. The two deltas developed contemporaneously with the differences in internal architecture attributed to basin morphology and the role of two, distinct sediment sources, while the correlatability of the larger-scale packages indicates an external, allocyclic control. As such the Roda Sandstone provides the possibility to address the relative roles of different parameters known to interact and control stratal architecture. 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 km 2) 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.