The O Grao turbidite system is part of the infill of the piggy-back stage of evolution of the Ainsa basin, and a portion of the Ainsa Slope Complex. The outcrops are on the flanks of the Buil syncline, on a 15 km long... [ view full abstract ]
The O Grao turbidite system is part of the infill of the piggy-back stage of evolution of the Ainsa basin, and a portion of the Ainsa Slope Complex. The outcrops are on the flanks of the Buil syncline, on a 15 km long polygonal line of section, which facilitates notions on the 3D system architecture.
A dataset was grown from conventional outcrop data by adding interpretations of Digital Outcrop Models and a research well. The well drilled the system at an axial position and gave continuous core and well logs. Integrating the dataset allowed readdressing some of the fundamental architectural and depositional interpretations of the O Grao turbidite system.
The well data reaffirm that the system is up to about 90 m thick, up to 45% sandstones, and bounded below and above by similarly thick and tabular units of laminated mudstones (low density turbidites and/or hemipelagites). Also that the architectural elements making the system are: muddy slumps, channel-fills, thin-bed elements, and lobes. Muddy slumps dominate in proximal outcrops (SE), where observations indicate that slumping often removed pre-existing channel-fill and thin-bed elements. In distal outcrops (NW), a lower architectural interval is distinguished from an upper one. The lower interval includes slumps and turbidites, with the slumps deriving from a tectonically-active flanking slope in the NE, and their topography influencing the locus for the basal turbidites: thoroughly rippled lobe-fringe thin-bed elements nearing the slumps, and thick-bedded amalgamated massive sandstone lobes depositing away, on the axis of the system. Towards the SW, the opposed system margin is characterized by a rapid transition to mudstones, and a NE-facing carbonate slope. The overall stacking pattern in the lower interval indicates system backstepping. The upper interval develops on the upper third of the system. In distal outcrops, this upper slice commences with erosional channel-fills, rich in mud-clast conglomerate, with complex cross-cutting bedding architectures. This reflects that the system had reached a maximum basinward expansion. The upper architectural interval is also backstepping, but in this case, the upper thin-bed elements include gullied erosional features, abundant scours, mudstone clasts, accessory cross-bedding, and maximum grain size kept almost as coarse as that of the underlying channels. This late thin-bed element is interpreted as retreating channel-lobe transition deposits, prior to system abandonment.
The authors acknowledge financial support to ExxonMobil and project SEROS (CGL2014-55900-P). ROXAR is thanked for RMS licenses used in well-data integration.
