The sedimentary framework of Cagliari basin: A plio-quaternary underfed Rift basin in the southern Sardinia margin

Abstract

The intraslope Basin of Cagliari is located in the innermost part of the submarine depositional system of the southern Sardinian margin. The reconstruction of the main depositional features, the morphology and the architecture of the system, evolving at least during the Middle-Pleistocene-Holocene time span, was founded on the interpretation of 3.5 kHz Subbottom Profiler, 0.3 kJ Uniboom and 0.5÷3.6 kJ Sparker data, in conjunction with on-land knowledge. This small, structurally confined basin is located within the SE termination of the Oligo-Miocene Sardinian Rift. Its genesis is related to the Pliocene-Quaternary extensional tectonics of the southern Tyrrhenian Sea opening. The basin and its depositional system are outlined and controlled by the Ichnusa Seamount and Su Banghittu tectonic blocks of the continental margin. The large scale depositional features, produced during the Plio-Quaternary, are made up of a continental shelf, a prograding wedge and an intraslope basin. The inner and middle shelf are covered by moderate terrigenous sediments coming from S Sardinia and by a large amount of bioclastic material produced by the shelf itself. During the Holocene, these sediments were mainly dispersed by wave action, longshore and shelf currents, and were controlled by the growth of Posidonia oceanica seagrass and its biocoenosis. This well-defined carbonate buildup is unusual in the Sardinia shelf. It is a result of the positive interplay between tectonic stability, low terrigenous supply, eustatic changes and antecedent morphology. During the Middle-Upper Pleistocene the eustatic Milankovitch cycles led to several low stands to which the accretion of the prograding shelf-slope wedge is related. A small isolated offshore platform situated on top of Su Banghittu seamount, where terrigenous sediments are absent, is covered prevalently by carbonate-bioclastic sediments. There are two main canyons in the Cagliari Basin with several tributary channels dispersing the sedimentary charge. The channel heads are cut near the shelf break and the sediments descend mainly to the basin floor and, in minor amounts, to the deepest part of the continental margin where the Sardinian-Maghrebian deep-sea fan is located. On the shelf-basin slope and towards the base of the structural reliefs, mass transport processes occur accumulating the sediments at the base of the slope, while the basin floor is undergoing aggradation of a channel-levee system. A concave-parallel sedimentation, typical of slowly subsiding confined basins, is observed just in the most distal part of the basin. The flanks of the structural reliefs are completely covered with hemipelagic drape, whose thickness decreases as outcrops of bed rock increase. The filling rate exceeds tectonic subsidence, as testified by the slow growth of the shelf-basin sedimentary wedge together with the aggradation of the channel-levee system. For this reason the basin reached maturity during the Middle-Upper Pleistocene-Holocene time span.