The sardic phase in the ordovician of southern sardinia and eastern pyrenees: stratigraphic, structural and magmatic constraints
- Autores: Claudia Puddu
- Directores de la Tesis: Josep María Casas Tuset (dir. tes.), J. Javier Álvaro (dir. tes.)
- Lectura: En la Universidad de Zaragoza ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: José Luis Simón Gómez (presid.), Alejandro Díez Montes (secret.), Elena Druguet (voc.)
- Programa de doctorado: Programa de Doctorado en Geología por la Universidad de Zaragoza
- Materias:
- Enlaces
- Tesis en acceso abierto en: Zaguán
- Resumen
This thesis deals with the so-called “Sardic Phase”, a major geodynamic event that caused uplift, widespread emersion and erosion of an inherited Ediacaran-Lower Ordovician basement in the south-western European margin of Gondwana. Its aftermath led to the onlapping of Upper Ordovician sediments capping an inherited and partly eroded palaeorelief. The phase provoked an intra-Ordovician stratigraphic gap, ranging geometrically from paraconformable to angular discordance, named “Sardic unconformity” and subsequently overlain by Upper Ordovician alluvial-fluvial breccias and conglomerates. The Sardic event is associated with neither foliation-related open folds affecting the pre-unconformity succession nor metamorphism. It is also contemporaneous with an important Ordovician magmatic activity whose byproducts point to an important felsic calc-alkaline plutonism intruding within the pre-unconformity sedimentary package.
The first step of this thesis was to characterize the Sardic Phase in its type area, which includes a historical reappraisal of the stratigraphic, sedimentary, structural and magmatic features that successive generations of geologists recognized in SW and SE Sardinia. A revision of the stratigraphic and sedimentary characters of the Sardic unconformity and of the pre- and post-unconformity successions in the type area has been conducted.
The second step is focused on the identification of the Sardic features in two sectors of Eastern Pyrenees. The new data obtained with this research, together to the cartographic, lithostratigraphic, biostratigraphic and geochronologic data present in the literature, led to the individuation of Sardic deformation affecting the Cambrian-Lower Ordovician rocks, and to the definition of Upper Ordovician successions cropping out in Ribes de Freser and La Cerdanya areas. These data contribute to improve the knowledge of the Palaeozoic basement of the Pyrenees, in which the occurrence of an Early-Mid Ordovician episode of uplift and erosion led to the formation of the Sardic unconformity similar to that originally described in Sardinia. Early to Late Ordovician magmatic activity accompanied and followed the uplift, which was pursued by a Late Ordovician extensional pulse that controlled the post-Sardic deposition.
The geological mapping, together to the stratigraphic and facies analysis of the units cropping out in the Alpine Ribes de Freser antiformal stack, led to the definition of the Katian El Baell Formation, which has been revised, formally erected and proposed as a lateral equivalent of the Estana Formation. The geological and structural analysis of the rocks cropping out in the El Baell and Bruguera units led to the definition of two different internal structures, and to the recognizement of a pre-Variscan (Mid Ordovician) deformational episode affecting the pre-Upper Ordovician rocks of the Bruguera unit.
Detailed geological mapping of the La Cerdanya area (Canigó Unit, Eastern Pyrenees) led to the definition of the different structural features exhibited by the pre- (Cambrian-Lower Ordovician) and post-unconformity (Upper Ordovician) successions, allowing the identification of Sardic deformations. The occurrence of Late Ordovician synsedimentary extensional faults affecting the pre-Sardic succession, the Sardic unconformity and the lower part of the post-Sardic succession has been highlighted.
Finally, a comparison between the Toledanian (Furongian-lowemost Ordovician) and Sardic (Ordovician) geochemical byproducts of felsic/acidic affinity, currently interpreted as calc-alkaline and arc-related, seemed necessary to evaluate the different geodynamic interpretations proposed so far for both events. The geochemical character of plutonic and volcanic products from several areas of the aouth-western European margin of Gondwanan suggests a similar melting of metasedimentary rocks associated with the Toledanian and Sardic events, which should help to a better understanding of the geodynamic scenario in which both tectonic phases occurred. In particular, the comparison with the Toledanian event favoured the recognition of a contemporaneity between magmatic events and stratigraphic unconformities, contrasting with the geochemical affinity of the magmatic counterparts, which suggest that the magmatism was likely induced by the underplating of hot magmas in a continental lower crust with partial melting of sediments, triggered by the stepwise opening of the Rheic Ocean.
