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

Laura Burrel,Antonio Teixell,Xavier Coll,David Gómez-Gras,O Lacombe,S Calassou,D Pedreira,S Tavani,A Teixell

doi:10.1051/bsgf/2021013

Laura Burrel, Antonio Teixell + Show 7 more

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https://doi.org/10.1051/bsgf/2021013

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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.

Highlights

In basins that evolve on top of thick evaporite detachment levels, differential loading of prograding sedimentary systems triggers salt mobilization towards areas with a smaller overburden load and an associated depocentre migration (Ge et al, 1997)
In this study we present a reinterpretation of the evolution of the intramontane conglomeratic basins in the northern part of the Southern Pyrenees based on new field observations, clast counting and revisiting previous structural interpretations taking into account the recent advances in salt tectonics
The results are illustrated in new palaeogeographic maps and a representative cross-section sequentially restored at four stages from the late Lutetian to the late Oligocene of the La Pobla, Gurp and Senterada conglomeratic basins and their drainage areas

Summary

Introduction

In basins that evolve on top of thick evaporite detachment levels, differential loading of prograding sedimentary systems triggers salt mobilization towards areas with a smaller overburden load and an associated depocentre migration (Ge et al, 1997). The alluvial fans progressively retrograde, displacing the apical areas to the north, shifting sedimentation from the Organyà basin northern margin towards the unrooted leading edge of the Axial Zone antiformal stack (the Nogueres têtes plongéantes; Séguret, 1972) and a Keuper evaporite province in between, which we call the Senterada salt province (Fig. 2). The Jurassic and lower Cretaceous succession of the northern limb of the Organyà syncline is shown in these works with constant thickness by the isopach data It terminates sharply against a south-dipping discontinuity interpreted as an Eocene backthrust (the Morreres backthrust) that uplifts the basin above the Senterada Keuper province (Berástegui et al, 1990; Muñoz, 1992; García-Senz, 2002). – Paleozoic granites; – Paleozoic metamorphic rocks: marbles (occasionally with pyrite and andalusite crystals), slates, gneiss, deformed quartz and quartzites

Methods

The substratum of the northern margin of the Organyà basin

Findings

Conclusions