Linking the high-resolution acoustic and sedimentary facies of a transgressed Late Quaternary alluvial plain (Gulf of Trieste, northern Adriatic)

Abstract

We use geophysical and sedimentological data to study the high-resolution acoustic stratigraphy and sedimentology of a Late Quaternary alluvial plain (located in the Gulf of Trieste, northern Adriatic) which was transgressed during the post-LGM sea-level rise. With sub-bottom sonar profiling we determined six acoustic facies, of which the top five acoustic facies (A-E) were sampled with a gravity corer. We performed core descriptions, radiocarbon dating and granulometric analysis with a laser particle analyser. The acoustic facies and their corresponding sediments are associated with three general sedimentary environments: Last Glacial to Younger Dryas alluvial (E-B), Early Holocene transgressive (Ab) and Holocene shallow marine (Aa). Acoustic facies E with low-amplitude chaotic reflection geometries is represented by cross-stratified sandy mud deposited by braided or wandering rivers. Acoustic facies D with high-amplitude and high- frequency sub-horizontal reflection geometries is represented by graded deposits (sandy mud and sandy clay grading into clay) of braided or wandering rivers. Acoustic facies C and B with individual low to middle amplitude reflections in an otherwise transparent facies are represented by overbank fine-grained sediments deposited by a meandering river systems. The top surface of AF B presents the Younger Dryas paleosurface in the geophysical record. Acoustic facies Ab with onlapping and concordant middle to low amplitude reflection geometries is represented by bioclastic transgressive sandy mud containing brackish mollusc assemblages. Acoustic facies Aa is acoustically transparent and contains bioclastic sandy mud with shallow marine mollusc assemblages. 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.