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

Three extensional phases can be recognized in the northern, Anglona area of the Oligo-Miocene Sardinian Rift during a fifteen million year period whic h spanned Corsica-Sardinia continental microplate separation and Western Me diterranean back-are basin opening. In response to this multiphase rifting, a complex facies architecture involving elastic, carbonate and volcanic ro cks developed. Integrated onshore facies and structural analysis, dating an ti offshore seismic data are here used to reconstruct the tectono-stratigra phic history of the Anglona area. Initial late Oligocene extension created a half-graben geometry with syn-rift elastic deposits shed locally from fau lt-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 elastic syn-rift stratal wedges, truncated and subdivided the half-graben. The syn- rift sediments were sealed by a regionally correlated ignimbrite that in tu rn was offset by late second-phase faulting. Third-phase extensional fault movement which reactivated the original fault trend then occurred. A perche d lake developed in the resultant topography coeval with the progressive ma rine transgression of lower areas. As sea-level rose during mid-Burdigalian times, reefal carbonates and grainstones developed on fault-block highs wh ilst calcarenites and marlstones were deposited in hangingwall locations. I nitial extension was coeval with the formation of the Sardinian proto-rift and the initiation of the Western Mediterranean basin. Second-phase faultin g occurred as the Corsica-Sardinia microplate rotated to its present positi on during Western Mediterranean back-are basin spreading. Final extension c an be correlated to a second major extension phase along the Oligo-Miocene Sardinian Rift following back-are basin opening, as extension was transferr ed towards the fore-are. In Anglona, the main influence of multiphase tecto nism was on rift topography, providing accommodation space and localized up lifted source areas. Varying relative sea-level mainly controlled the broad types of facies belts that developed. Contemporaneous calc-alkaline volcan ism played a major role in the supply of basin filling material and in chan ging the topography locally.