Abstract. We use various geophysical datasets (multibeam and singlebeam echosounder
data, sub-bottom profiling Chirp and sonar data and very high-resolution
boomer seismic data) along with published sedimentological data and depth
data from nautical charts in order to create models of the depth of the
seafloor and the base of Holocene marine sediment in the Gulf of Trieste. The
two models are later used in order to calculate the thickness of marine
Holocene sediment which has been depositing on the Late Pleistocene alluvial
plain since the Holocene transgression in the Italian and Slovenian parts of
the gulf. Thicker Holocene marine sedimentary sequences averaging at around
5 m are characteristic for the southeastern part of the gulf. In other parts
of the gulf the Holocene marine sedimentary cover is very thin or even
absent, except in close proximity to the shoreline and fluvial sediment
sources, in the area of the Trezza Grande paleodelta and above topographic
depressions of the Late Pleistocene base. The presented datasets available
from the OGS SNAP data repository (http://doi.org/cpz2) represent a
valuable reference for a wide variety of research disciplines dealing with
the dynamic Earth system in the Gulf of Trieste and could be used as a
valuable tool for designing sampling and geophysical campaigns in the studied
area.
Estimating sound velocity in seabed sediment of shallow near-shore areas submerged after the Last Glacial Maximum is often difficult due to the heterogeneous sedimentary composition resulting from sea-level changes affecting the sedimentary environments. The complex sedimentary architecture and heterogeneity greatly impact lateral and horizontal velocity variations. Existing sound velocity studies are mainly focused on the surficial parts of the seabed sediments, whereas the deeper and often more heterogeneous sections are usually neglected. We present an example of a submerged alluvial plain in the northern Adriatic where we were able to investigate the entire Quaternary sedimentary succession from the seafloor down to the sediment base on the bedrock. We used an extensive dataset of vintage borehole litho-sedimentological descriptions covering the entire thickness of the Quaternary sedimentary succession. We correlated the dataset with sub-bottom sonar profiles in order to determine the average sound velocities through various sediment types. The sound velocities of clay-dominated successions average around 1530 m/s, while the values of silt-dominated successions extend between 1550 and 1590 m/s. The maximum sound velocity of approximately 1730 m/s was determined at a location containing sandy sediment, while the minimum sound velocity of approximately 1250 m/s was calculated for gas-charged sediments. We show that, in shallow areas with thin Quaternary successions, the main factor influencing average sound velocity is the predominant sediment type (i.e. grain size), whereas the overburden influence is negligible. Where present in the sedimentary column, gas substantially reduces sound velocity. Our work provides a reference for sound velocities in submerged, thin (less than 20 m thick), terrestrial-marine Quaternary successions located in shallow (a few tens of meters deep) near-shore settings, which represent a large part of the present-day coastal environments.
The evolution of alluvial megafans has mainly been investigated in unconfined settings; however, at the boundary of these large depositional systems, the development of fluvial channels can be affected by structural constraints with regional extent. Here we present the study of the eastern sector of the megafan of Isonzo River, in the Gulf of Trieste, where this system fed through the southern Alps is constrained by the Karst and Istria cliffs. Although this area is now submerged under the northern Adriatic Sea, stratigraphy from seismo-acoustic profiles, drill cores and multi-beam bathymetry allows us to reconstruct the paleochannel system of the Isonzo River in detail, which was likely active within the period of 21–17.5 ka cal BP, at the end of LGM. This was reconstructed for over 50 km and currently represents the longest abandoned fluvial channel in the Mediterranean seabed. The occurrence of the mountain fringe and competition with nearby alluvial systems forced the paleochannel to follow the present coastline and conditioned the slope of its thalweg to decrease almost to null, resulting the transformation from the megafan to the undifferentiated alluvial plain.
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