engleski

Facies properties and provenance of glaciofluvial sediments deposited in the Eastern Adriatic coast (Croatia)

The Eastern Adriatic coast is built predominantly of Mesozoic and Cenozoic carbonate rocks that extend over 90% of its length. Less resistant Eocene flysch and Pleistocene sedimentary rocks and sediments cover approximately 6% of the coast. Aforementioned sediments are in research focus and they were investigated in the central part of the Eastern Adriatic coast. Pleistocene sediments usually make small patches along the Eastern Adriatic coast and form outcrops whose thickness rarely exceeds 10 m. They are represented by various types of sediments which include: fluvial, glaciofluvial and aeolian sediments. These sediments occur also on Adriatic islands, including the Vrgada Island which is located 2 km off the coast in the central part of the Adriatic. Other two localities are Vrsi – beach cliff situated at the coast, north of the Vrgada Island and Smilčić – abandoned sand pit which is situated 8 km inland. Vrgada deposits are sands with significant percentage of silt in the lower part of the outcrop and numerous, intercalated gravel layers and lenses from the top to the middle of the outcrop. Smilčić locality is dominanted by sand deposits which consist of fine to medium grained sand with intercalated gravel layers, while almost regular sand and gravel interchange in the form of horizontal gravel layers and medium to coarse grained sand is present in Vrsi deposits. In all three outcrops sand is dominant fraction, while gravel and silt make up to 40% together. Average sorting (So) values are 2.32 for Smilčić, 2.46 for Vrgada and 2.70 for Vrsi. Heavy mineral analysis showed that sediment from the Vrgada Island and Vrsi are much alike, with high garnet percentage and low epidote percentage, while pyroxene percentage is similar, with exception of the lower part of the Vrgada outcrop where augite percentage is unusually high. Inversely, sediment from the Smilčić outcrop displays lower garnet percentage and higher epidote percentage. Mineral assemblage in Vrgada and Vrsi outcrops indicate that the siliciclastic material was derived from the erosion of local source rocks in the northern Dalmatia, which are mostly Eocene sandstones. Mineral assemblage from Smilčić indicate certain influence of northern Italian rivers which supplied most of the material for Northern Adriatic Pleistocene sediments. SEM images of quartz sand grains from all three outcrops display following surface features: conchoidal fractures, V-shaped percussion marks and parallel striations. Combination of these surface textures indicates a short, fluvial transport of sediment. First results suggest complex origin of the studied sediments from three localities. Fine to coarse sand and gravel with horizontal laminations and layers, which is dominant facies in all outcrops, coresponds to high flow regime in minor channels. Grain size distribution as well as sedimentary structures indicate poorly sorted, stratified sediment which is similar to sediment deposited in modern day sandurs which are usually not bounded by valley walls and commonly form in coastal areas. We suggest that two major depositional forces, both climate controlled, may have formed the sediment: glaciofluvial and in case of Vrgada and Smillčić, glaciofluvial with certain aeolian impact.

facies, glaciofluvial, pleistocene, Adriatic coast, sediment

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