We used geophysical and sedimentological analysis to study the high-resolution acoustic stratigraphy and sedimentology of the submerged Late Quaternary alluvial plain in the present-day Gulf of Trieste which was transgressed by the post-LGM sea-level rise. We interpreted six acoustic facies from sub-bottom sonar profiles and sampled the sediments of the five topmost acoustic facies with a gravity corer. The characteristics of the acoustic facies, the results of granulometric analysis, and radiometric ages of the sediment show that the studied sequence was deposited in three sedimentary environments. Alluvial sediments were firstly deposited as Last Glacial cross-stratified and graded deposits of braided or wandering rivers. After the Last Glacial Maximum alluvial sediment were deposited until the Younger Dryas as fine-grained overbank deposits. Transgressive sediments deposited in the Early Holocene and were later replaced by Holocene shallow marine sediment. Each of the described units has a characteristic acoustic facies. On the geophysical profiles cross-stratified alluvial sediments can be recognised by low-amplitude chaotic reflection geometries. Graded deposits exhibit characteristic subhorizontal high-amplitude and high-frequency reflections. Overbank sediments are represented by a transparent acoustic facies with individual discontinuous low-to-medium amplitude reflections. Onlapping or concordant medium-to-low amplitude reflections are characteristic for transgressive sediments. Marine sediments can be recognised on geophysical profiles by their completely transparent acoustic facies. Our work provides a reference for future studies of the Late Pleistocene-Holocene transition on well-preserved low-gradient mid-to-low latitude continental shelves.
COBISS.SI-ID: 1515102
We used geophysical data (sub-bottom sonar profiles) and litho-sedimentological borehole descriptions to determine the average sound velocities through the youngest terrestrial-marine Quaternary seabed sediments in the Bay of Koper. The sound velocities through clay-dominated successions average around 1530 m/s. The velocity values of silt-dominated successions extend between 1550 and 1590 m/s. The maximum sound velocity (approximately 1730 m/s) was determined for a succession which contained sandy sediment. The minimum sound velocity (approximately 1250 m/s) was calculated for gas-charged sediments. We showed that the main factor influencing sound velocity in shallow areas with thin Quaternary successions is the predominant sediment type (i.e. grain size). In these settings the influence of overburden is negligible. Furthermore, the presence of gas within the sedimentary column can reduce the velocity for a few hundred meters per second. Our study provides reference values for sound velocity 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 comprise a large percentage of present-day offshore coastal environments.
COBISS.SI-ID: 1551454