Sound Velocity in a Thin Shallowly Submerged Terrestrial-Marine Quaternary Succession (Northern Adriatic Sea)

Abstract

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.

Inferring time averaging and hiatus durations in the stratigraphic record of high‐frequency depositional sequences

“…The increments in the second unit (gravelly shell bed, ca 5000 to 8000 years bp ) approximately coincide with the maximum ingression and can be assigned to the maximum flooding zone (MFZ). The bays and estuaries along the Istrian peninsula that were flooded during the maximum ingression were filled with prograding sediments during the latest Holocene (Faivre et al ., 2011; Felja et al ., 2015; Novak et al ., 2020a). Nonetheless, with the exception of bays, the shoreline along most of the Istrian peninsula did not prograde during the latest phase of sea‐level stabilization owing to an extremely reduced fluvial sediment supply and sediment bypassing, and sediments deposited during the highstand phase are thus typically less than 1 m thick (Tomašových et al ., 2019b; Novak et al ., 2020b).…”

“…The bays and estuaries along the Istrian peninsula that were flooded during the maximum ingression were filled with prograding sediments during the latest Holocene (Faivre et al, 2011;Felja et al, 2015;Novak et al, 2020a). Nonetheless, with the exception of bays, the shoreline along most of the Istrian peninsula did not prograde during the latest phase of sea-level stabilization owing to an extremely reduced fluvial sediment supply and sediment bypassing, and sediments deposited during the highstand phase are thus typically less than 1 m thick (Toma sov ych et al, 2019b; Novak et al, 2020b). The thickness of the bryomol unit overlying the gravelly shell bed at Brijuni approximately matches the vertical increase in the accommodation space as predicted by the relative sea-level rise estimated for Brijuni.…”

Landscapes and Landforms of Terrestrial and Marine Areas: A Way Forward

“…The outputs of research on Late Quaternary landscape changes are presented, with different approaches, in a number of papers of this Special Issue. Mattei et al [17] supported their research by means of archaeological markers; De Gioiosa et al [24], Deiana et al [27], and Savini et al [26] analyzed terrestrial and marine landforms to reconstruct past sea-level oscillations and paleolandscapes; and Novack et al [25] took advantage of the acoustic characterization of stratigraphic records.…”

“…Novack et al [25] acoustically characterized the Quaternary sedimentary sequence of an alluvial plain of the NE Adriatic Sea that was submerged after the LGM. The paper shows that the sediment grain size is the main factor influencing sound velocity in shallow areas, the overload effect being negligible.…”

New Geomorphological and Historical Elements on Morpho-Evolutive Trends and Relative Sea-Level Changes of Naples Coast in the Last 6000 Years

“…The main task is the acquisition of data that are related to the morphology of the seabed, in order to reconstruct the underwater three-dimensional landscape, using geophysical (Single Beam Echo Sounder and Side Scan Sonar) and optical (underwater cameras for photogrammetry) instruments [78][79][80][81][82][83][84][85].…”