Evolución tectónica y sedimentaria durante el Priaboniense superior-Mioceno inferior, en el frente de cabalgamiento de las Sierras Marginales occidentales

J.G. SENZ, M. ZAMORANO

Abstract


Four systems tracts have been distinguished in the uppermost Eocene-lower Miocene continental deposits of the northern margin of the Ebro basin. 1) The Satinar systems tract 5pper Pnabonian-Stampian) consists of an alluvial system (Salinar Fm) located in the north, that passes to playa-lake evapontes (lower Barbastro Fm) towards the foreland. The alluvial sediments are represented only by distal fan and alluvial plain facies which are in the footwall of the Sierras Marginales thrust. The top of the Salinar systems tract is inarked by lacustrine limestones that represerit the alluvial fan retre,at and a change to less saline conditions. 2) The Peralta systems tract (upperPriaboniiin?-Stampian) consists of srnall alluvial fans attached to the Sierras Marginales (Peralta Fm) that pass to playa-lake evaporites towards ;he foreland (upper Barbastro Fm). The alluvial facies are represented by calcareous conglomerates, that grade in a few kilometers to thin-bedded sandstones and mudstones. Evaporitic facies are represented by nodular and larninated gypsum. The top of the Peralta systems tract is also marked by an extensive body of lacustrine carbonates that represent a new alluvial fan retreat. 3) The Peraltilla systems tract (Stampian-Chattian) consists of small alluvial fans (Calasanz Fm), that show i;imilar characteristics to those of the Peralta I'rn but they interfinger distally with a large fluvial systern sourced in the axial Pyrenees (Peraltilla Fm). This fluvial fan consists of red overbank deposits with paleosoils, and channelized bodies, which are mostly conglomeratic in the Azanuy syncline and become sandy towards the southern limb of Barbastro anticline. This granulometric change is accompanied by a change in channel type, from typical braided to sinuouir or straight. The Salinar and Peralta systerns are contemporaneous with the emplacernent of the Sierras Marginales thrust sheet. The clastic sedirnents accumulated in a flexura1 trough that widens while the thrust propagates. As a consecuence, the saline lake facies expand towards the foreland. During the Peralta systerns tract a blind detachment located in the base of the Barbastro evaporitic formation propagated and started to fold the foreland. The Peraltilla systems tract is contemporaneous with the end of displacement of the Sierras Marginales thrust sheet and the rnain growth of the Peralta detachment anticline. However, these tectonic events were accompanied by a significant forelandward shift of the basin depocenter and a more homogeneous subsidence. 4) The Sarinena systems tract (Chattian-Aquitanian) is very similar to the Peraltilla one but it shows a marked southwards shift of the proximal facies. It consists of srnall fans of local provenance (Baells Fm) that interfinger distally with alarge fluvial fan (Sarinena Fm). The sedimentation of the Sariñenasystems tract is contemporaneous with the continued southward propagation of the detachment through the foreland evaporites, that creates new detachment folds (Barbastro and Altarriba anticlines, Azanuy syncline). An episode of out-of sequence thrusting generates small thrust-top basins on the Sierras Marginales, filled by the Baells conglomerates. The Sariñena systems tract accumulated in a wide foreland trough, with a strongly subsiding located in the Azanuy syncline.



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