Transgressive system tract and continental shelf morphology: an example from the Hyblean foreland ramp offshore. 35th International Association of Sedimetologists

Coastal deposits of the Transgressive System Tract (TST) have a wide range of characters that reflects different process-regimes and genetic environments. The morphology of the shelf, often resulting from tectonic activity, plays an important role in controlling the character and the preservation potential of TST coastal deposits. The reconstruction of the influence of shelf morphology on TST deposits, which are often characterized by a very reduced thickness, can be accomplished only with the aid of high-resolution data. Such an approach, through the interpretation of multibeam and high-resolution Sparker profiles (acquired in the frame of the SIMIT-THARSY Project Interreg V-A Italy – Malta), is adopted in the present study to investigate an offshore portion of the eastern sector of the Hyblean Plateau (Sicily, Italy). Within this context, our research is particularly aimed at the reconstruction of the influence of the tectonic grain of the continental shelf on TST deposits. An acoustic substratum, characterized by wavy reflectors, is correlated with the calcarenite units outcropping on land. Its upper boundary appears as an evident unconformity with a marked irregular trend and is interpreted as the subaerial erosional surface developed during the last falling stage and lowstand of sea level. It formed a wide depression and two narrow, v-shaped incisions that can be interpreted as incised valleys. The incised valleys present a complex infill that records the different environments of the successive transgressive System Tracts (TST). A first unit, present only in the lower part of the incised valleys, consists of high amplitude reflectors with an inclined attitude and sometimes a laterally accreting pattern. They correspond with fluvial deposits likely formed during the early stage of the TST. They are followed by a transparent unit that is here interpreted as representing the infill of a low-energy environment such as an estuarine basin or a lagoon. An overlying or interfingering unit, with higher amplitude reflections, can be the expression of bay head deltas, locally deposited in the low-energy environments. Finally, a seismic unit with high amplitude and very good lateral continuity, and a large areal extension is interpreted as corresponding with highstand deposits that form an extensive drape on the seabed. Several tectonic discontinuities, interpreted as normal faults characterized by little throws (< 10 m) are present in the study area. They likely represent the extension towards the eastern Sicilian coastline of the Hyblean-Malta Escarpment, a regional system of extensional faults oriented NW-SE/NNW-SSE, which control the tectonic subsidence of the eastern Sicilian offshore and, consequently, also the accommodation space of the study area. These extensional faults interrupt the seismic reflectors of the Pleistocene calcarenite substratum and control the development of the paleo-fluvial incisions. Therefore, they are the main control on the distribution of the TST environment and deposits. The palaeogeographic reconstruction obtained through the present study shows that coastal barrier-lagoon systems, with an ultimate tectonic control, dominated the sedimentation in the area, from the TST to the present-day development of the unique pond areas in the “Vendicari” Natural Reserve.