Sedimentation and volcanism linked to multiphase rifting in an Oligo-Miocene intra-arc basin, Anglona, Sardinia

Abstract

Three extensional phases can be recognized in the northern, Anglona area of the Oligo-Miocene Sardinian Rift during a fifteen million year period which spanned Corsica–Sardinia continental microplate separation and Western Mediterranean back-arc basin opening. In response to this multiphase rifting, a complex facies architecture involving clastic, carbonate and volcanic rocks developed. Integrated onshore facies and structural analysis, dating and offshore seismic data are here used to reconstruct the tectono-stratigraphic history of the Anglona area. Initial late Oligocene extension created a half-graben geometry with syn-rift clastic deposits shed locally from fault-bounded highs, passing laterally to lacustrine marlstones. Calc-alkaline volcanic activity subsequently predominated as volcanic centres developed along one half-graben bounding fault. Voluminous pyroclastic and epiclastic material was supplied to the adjacent half-graben accommodation space and was deposited in marginal to marine conditions. Second-phase mid-Aquitanian–early Burdigalian extensional faulting, recognized from localized clastic syn-rift stratal wedges, truncated and subdivided the half-graben. The syn-rift sediments were sealed by a regionally correlated ignimbrite that in turn was offset by late second-phase faulting. Third-phase extensional fault movement which reactivated the original fault trend then occurred. A perched lake developed in the resultant topography coeval with the progressive marine transgression of lower areas. As sea-level rose during mid-Burdigalian times, reefal carbonates and grainstones developed on fault-block highs whilst calcarenites and marlstones were deposited in hangingwall locations. Initial extension was coeval with the formation of the Sardinian proto-rift and the initiation of the Western Mediterranean basin. Second-phase faulting occurred as the Corsica–Sardinia microplate rotated to its present position during Western Mediterranean back-arc basin spreading. Final extension can be correlated to a second major extension phase along the Oligo-Miocene Sardinian Rift following back-arc basin opening, as extension was transferred towards the fore-arc. In Anglona, the main influence of multiphase tectonism was on rift topography, providing accommodation space and localized uplifted source areas. Varying relative sea-level mainly controlled the broad types of facies belts that developed. Contemporaneous calc-alkaline volcanism played a major role in the supply of basin filling material and in changing the topography locally.