Basement-involved thrusting, salt migration and intramontane conglomerates: a case from the Southern Pyrenees

Abstract

The northern margin of the Organyà basin (Southern Pyrenees) has a complex structure in which syn-rift Lower Cretaceous carbonates flank a wide Keuper evaporite province, featuring the leading edges of the basement-involved thrust sheets of the Pyrenean antiformal stack. Recent studies show that Keuper diapirs and salt walls grew during the Cretaceous extensional episode, conditioning the development of differentiated depocenters and minibasins. The role of salt tectonics during the Pyrenean orogeny has not been addressed in previous structural studies, but present-day cross-sections indicate a Keuper evaporite-bearing vertical thickness of up to 3000 m in the Senterada-Gerri de la Sal area. We infer that salt migration was a determinant mechanism in triggering a gentle northward tilting of the Organyà basin during the Eocene-Oligocene, recorded in the La Pobla de Segur and Gurp syn-tectonic conglomerates in a large north-directed onlap, opposite to the main sedimentary influx direction. Contemporaneously, we interpret that salt migration, promoted by conglomerate differential loading, enabled the sinking and rotation of the unrooted Nogueres thrust units (têtes plongeantes). We use new and published structural data for the Lower Cretaceous margin of the Organyà basin, combined with structural and clast provenance data from the Cenozoic alluvial fan conglomerates of La Pobla and Gurp, to understand the Lutetian to late Oligocene evolution of the northern margin of the Central South-Pyrenean Unit. The tectono-sedimentary evolution of this area and the salt evacuation patterns are closely related to the exhumation history of the stacked Paleozoic thrust sheets of the Pyrenean hinterland to the north. In this study, we correlate the movements over a mobile substratum and the paleogeographic changes of conglomeratic basins at the toe of an exhuming orogenic interior.