Multibeam sonar technology and geology to interpret ancient harbor subsidence off Crotone Peninsula, Italy

Multibeam sonar (MBS) technology was developed to examine in great detail large stretches of the seafloor surface, providing accurately positioned and excellent 2-D and 3-D images of features as small as a few centimeters or covering areas as large as hundreds of square meters. In the present study, such high quality images obtained in plan and oblique views are used primarily for archaeological purposes as related to geological (sedimentological and tectonic) parameters. Presented here as example is an area on Calabria’s Ionian coastal margin, the Capo Colonna-Punta Scifo shelf platform located off the Crotone peninsula. We show how multibeam sonar can help solve geoarchaeological problems with respect to identification, origin, and subsidence of seafloor structures that were once positioned near a former coastline but now rest at a considerable depth of 12.5-13 m below sea level. MBS images, coupled with diver observations, indicate the structures are of human construction, not of natural origin, and were once part of a now-submerged Greek harbor facility. These structures are positioned on a seafloor that subsided by ~10 m since late or post-Greek time; submergence occurred not by mass gravity flow processes, but by a number of tectonic pulses over time that most probably involved lowering by extensional faulting and possible strike-slip events. Linear structural features observed on the seafloor show axial trends similar to those mapped on land, indicating that both subaerial and submerged areas of this Calabrian Arc sector were modified tectonically to the present time. Multibeam data show that this Crotone shelf platform has been subject to considerable structural offset and subsidence on Calabria’s Ionian margin since ancient Greek time.

La technologie du sonar multifaisceaux a été développée pour examiner en détail les fonds marins. Cette méthode offre une précision excellente de quelques centimètres et couvre des surfaces de plusieurs centaines mètres carrés. Dans notre étude, ces images sont utilisées à des fins géoarchéologiques en relation avec le contexte géologique. L’exemple présenté correspond à la marge ionienne de la Calabre, la zone de Capo Colonna-Punta Scifo située au large de la péninsule de Crotone. Nous montrons comment le sonar multifaisceaux peut aider à résoudre des problèmes d’identification et d’affaissement du fond marin jusqu’à une profondeur de 13 m au-dessous du niveau actuel de la mer. Ces images, couplées avec les observations des plongeurs, indiquent des structures d’origine humaine qui peuvent correspondre à une installation portuaire grecque maintenant submergée. Ces structures sont positionnées sur un fond marin qui s’est affaissé d’environ 10 m depuis la fin de l’antiquité. Cette immersion traduit un certain nombre de pulsations tectoniques en relation avec l’arc de Calabre.