The Mid-Albian unconformity, a key to understand the geodynamics of the North Pyrenean Trough

Abstract

The Pyrenees are the result of the structural inversion of a rift of general direction N110E, established at the articulation of the European and Iberian plates. This rift was created by left-lateral transtension with severe crustal thinning during the latest Early Cretaceous and Mid-Cretaceous. Its inversion, which led to the uplifting of the Pyrenean chain, took place gradually, by transpression from east to west, during the Late Cretaceous followed by an Eocene collision. From historical fieldwork, this note mainly focuses on the rifting period at the scale of the Pyrenean domain. It clearly shows the fundamental role of the stratigraphic approach in structural interpretation. Although synchronous at the scale of the Pyrenees, the Cretaceous extension is more important in the western part of the chain, near the Bay of Biscay. It is therefore at this location, within the Tardets-Mauléon Basin, that the various stages of creation and evolution of the rift could be clearly recognized with, successively: (1) an epirogenic pre-rifting stage, during which the facies lines, submeridian during the Jurassic, gradually change into N110E during the Neocomian before giving way, in the Early Cretaceous (Barremian-Gargasian), to a moderate acceleration of subsidence and, consequently, to the gentle uplift of a southern and western margin, future High Primary Belt (also often called Axial Zone) and Landes Plateau, initiating the next stage; (2) a major, brittle, newly identified rifting stage in the Latest Aptian (Clansayesian) and Early Albian. It is characterized by the installation of elongated lows on the downthrown side of transverse N20 normal faults, active diapirism of the Triassic evaporites and central detachment of the post-Triassic cover. During the Mid-Albian, a major surface of stratigraphic unconformity, often associated to uplift, erosion, and local centripetal gliding of the basin borders, marks a new structural organization, a rapid mantle ascent and the end of this episode. Although obscured by the later Pyrenean compression, the reconstructed basin geometry is interpreted as a “pull-apart” basin associated to a modest left-lateral motion; (3) a late rifting stage characterized by the collapse of the internal zones by rapid mantle cooling. The erosion of the coeval uplift of the southern High Primary Belt and the Landes Plateau feeds the accumulation of the thick, deep and transgressive unconformable formation of the Black Flysch of Mid-Albian to Early Cenomanian age in a larger coalescing basin. Faulting associated with a simple NS distension is only active on the last major border faults; (4) finally, a post-rift stage of Late Cretaceous age illustrated by the establishment of a passively subsiding basin, devoid of major deformations and whose heterogeneity of the previous depocenters is gradually erased. The Pyrenean transpression, later and less marked than in the eastern and central Pyrenees, allowed the conservation of most of these Cretaceous transtensive structures. Within the Tardets-Mauléon Basin, the Pernes and Col d’Urdach sector, the only Pyrenean location where the Mid-cretaceous transtension led to a proven underwater mantle exhumation, illustrates from outcrops the various stages that have marked the history of the Pyrenean Trough. Towards the east, along the Internal Metamorphic Zone, the Lourdes-Bagnères, Baronnies, Ballongue, Aulus and Saint-Paul de Fenouillet-Boucheville Basins, or those, further north like the Camarade and Quillan Basins, successively examined, testify to the same sedimentary and structural evolution. As in the case of the Tardets-Mauléon Basin, we deal here again with “pull-apart” type basins established on a thinned crust, arranged in transtensive or locally transpressive relays and separated by zones of thicker crust and shallower facies successions. The Mid-Albian unconformity is always present, heralding a general drowning of the North Pyrenean Trough and cessation of the extensional activity. The Pyrenean deformation, however, constitutes an obstacle for a precise geodynamic reconstruction, in particular in the eastern and central Pyrénées. Towards the west and north-west, in the South Aquitaine, Parentis and Basque-Cantabrian Basins, where Pyrenean deformation has far less obscured the rifting events, a very comparable Cretaceous geodynamic evolution is observed thus strengthening the present interpretation.