Seismic facies and seabed morphology in a tectonically controlled continental shelf: The Augusta Bay (offshore eastern Sicily, Ionian Sea)

Abstract

Morphological and seismic–stratigraphic features of the Augusta Bay (offshore eastern Sicily) have been characterized by means of Sub-bottom Chirp profiles, Multibeam data and Side Scan Sonar images, covering an area of about 110 km 2 . The marine data set was combined to mainland stratigraphy, derived from coastal boreholes and geological maps, to achieve a land–sea correspondence of main morpho-structural and stratigraphical features. The study area is located in the western sector of the Ionian Sea which represents a key area to reconstruct the geological parameters that control the processes of convergence between Africa and Europe in the Mediterranean. In particular, the area of interest draws the attention for problems concerning high seismicity and environmental pollution from oil and chemical industries. The Augusta Bay offshore displays a marked variability of morphologies and seismic–stratigraphic characters, related to the influence of both large-scale geological processes and local variables. It consists of an irregular surfaced narrow shelf, generally tracing out the shape of the relative coastline seaward bounded by a steep continental slope. This latter is deeply engraved by gullies and shows evidences of recent withdrawal, favored by the occurrence of structural lineaments. The Augusta offshore reflects the corresponding on-land geological pattern, being characterized by a central basinal area filled with Pleistocene–Holocene successions, bounded by two relieved sectors corresponding to the seaward prosecution of the Mt. Tauro and Magnisi–St. Panagia horsts, where the Miocene substratum outcrops. The seismic–stratigraphic architecture of the basinal area not only is the result of sea-level variations related to the Late Pleistocene sea-level fall and subsequent transgressive phase, responsible for the formation of a main angular unconformity, but also depends on pre-existing substratum arrangement, which leads to a wide lateral facies variability, on short distances. Morphological and seismic–stratigraphic evidences outline the presence of structural lineaments whose orientation is consistent with that of the on-land sectors. Slight movements are testified by the dislocation of recent sedimentary covers, pointing to active tectonic processes during Holocene. Such vertical movements are also responsible for a southward differential subsidence of the basinal area, whereas the proximal portion of the shelf seems to be strictly connected to the uplifting emerged sectors. This evidence is consistent with the recognition of recently active tectonic structures where the distal offshore represents the down-faulted sector. Our results, providing a detailed knowledge of the submerged sector of a highly populated area, affected by high seismicity and environmental pollution risk may significantly contribute to an appropriate management of the area in the next future. ► We give a description of the Augusta offshore through integration of geophysical data. ► Offshore stratigraphical and structural patterns reflect those of the emerged area. ► Facies distribution is strongly controlled by preexisting morpho-structural features. ► Current tectonic activity is responsible for subsidence of the distal shelf. ► Tectonic is also responsible for the slope withdrawal.