Late-Quaternary Evolution of the Semi-Confined Alluvial Megafan of Isonzo River (Northern Adriatic): Where the Fluvial System of the Southern Alps Meets the Karst

Abstract

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.

Not another hillshade: alternatives which improve visualizations of bathymetric data

“…ripples on a dune), etc. Ogorelec et al, 1991;Busetti et al, 2010a;Busetti et al, 2010b;Trincardi et al, 2011a;Trincardi et al, 2011b;Trobec et al, 2017;Trobec et al, 2018;Novak et al, 2020;Ronchi et al, 2023 The abbreviations which are used in the figure captions refer to the: geographic location of the dataset (abbr. explained in Sect.…”

Transverse Dinaric zone of increased compression between the Kraški rob and Hrušica Regions, NE Microadria

“…Dragonja, Rižana, Glinščica/Rosandra, etc.). The youngest Late Pleistocene sedimentary sequences, deposited just before the last transgression in the eastern and central part of the area of the present-day Gulf of Trieste, show the general direction of the water currents towards the south, with one channel even running roughly parallel to the present-day coastline of the eastern part of the Gulf of Trieste (Novak et al, 2020;Ronchi et al, 2023;Trobec et al, 2017). It is very difficult to compare Late Quaternary river networks with river networks on f lysch due to the far younger geomorphology, where sedimentation plays a greater role in shaping the surface compared to erosion.…”

“…It is very difficult to compare Late Quaternary river networks with river networks on f lysch due to the far younger geomorphology, where sedimentation plays a greater role in shaping the surface compared to erosion. The shape of the river network in the Late Pleistocene was largely inf luenced by the topography of the time (Ronchi et al, 2023), since in the area of the present-day Gulf of Trieste, the terrain rose to the northwest (Trobec et al, 2018) due to the Soča megafan from the last glacial maximum (Fontana et al, 2014(Fontana et al, , 2010(Fontana et al, , 2008, which also covers the south-eastern part of the Gulf of Trieste. Possible transverse folding in the south-eastern part of the Gulf of Trieste could therefore only be determined from the structural map of the contact between carbonates and f lysch.…”

“…Dragonja, Rižana, Glinščica, itd.). Najmlajša poznopleistocenska sedimentna zaporedja, ki so se odložila tik pred zadnjo transgresijo na vzhodnem in osrednjem delu današnjega območja Tržaškega zaliva, pa kažejo generalno smer vodotokov proti jugu, pri čemer je en kanal tekel celo približno vzporedno z današnjo obalno čr to vzhodnega dela Tržaškega zaliva (Novak et al, 2020;Ronchi et al 2023;Trobec et al, 2017). Poznokvartarne rečne mreže zelo težko primerjamo z rečno mrežo na f lišu, saj gre za precej mlajšo geomorfologijo, kjer ima sedimentacija večjo vlogo pri izoblikovanju površja v primerjavi z erozijo.…”

“…Poznokvartarne rečne mreže zelo težko primerjamo z rečno mrežo na f lišu, saj gre za precej mlajšo geomorfologijo, kjer ima sedimentacija večjo vlogo pri izoblikovanju površja v primerjavi z erozijo. Na obliko rečne mreže v poznem pleistocenu je v večji meri v plivala takratna paleotopograf ija (Ronchi et al, 2023), saj se je na območju današnjega Tržaškega zaliva teren dvigal proti severozahodu (Trobec et al, 2018) zaradi Sočine megapahljače iz zadnjega glacialnega v iška (Fontana et al, 2014(Fontana et al, , 2010(Fontana et al, , 2008, ki prekriva tudi jugovzhodni del Tržaškega zaliva. Morebitno prečno gubanje v jugovzhodnem delu Tržaškega zaliva, bi bilo torej mogoče ugotoviti le iz strukturne karte stika med karbonati in f lišem.…”