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Alluvial fan deposits near Stara Baška, Krk Island, Croatia

Krk Island belongs to the western part of the Outer Dinarides, more exactly it is a part of an imbricated/folded belt of Adriatic Unit (Babić, 2003, and references within). It is composed mainly of Cretaceous shallow-water deposits of Adriatic Carbonate Platform (Vlahović et al., 2005), disconformably overlain by Cretaceous to Paleogene continental and shallow-marine deposits and then by Eocene flysch (Mamužić et al., 1969 ; Šikić et al., 1969 ; Benac et al., 2013). These deposits are disconformably overlain by Paleogene and Neogene Jelar and Krk breccias (Marjanac et al., 2004). Quaternary sediments are rare and characterized by alluvial and glacial deposits (Marjanac, 2012). Here we present a geological research of a large Quaternary alluvial fan preserved in the vicinity of the Stara Baška Village. Catchment area of the fan is mainly composed of Upper Cretaceous commonly recrystallized white rudist limestones, disconformably overlain by upper Paleocene to lower Eocene foraminiferal limestones with intercalations of cross-laminated sandstones and sandy mudstones. Disconformity is marked by karst paleotopography and sporadic bauxite deposits. Eocene flysch deposits are rare. Structurally, the area is characterized by two large anticlines and an intermediate syncline, cut by two NW-SE (Dinaric) faults and numerous connecting faults. The perfect exposure of the fan deposits in cliffs (transverse section) and ravine (longitudinal cross-section) enabled a detailed sedimentological research of the alluvial fan deposits. The aim of present research was to define structure, texture and composition of these deposits and relate them to a particular sedimentary process. For this purpose 19 detailed sections in transverse and longitudinal cross-section were logged and correlated. The sediments are deposited in beds (bed thickness from few cm to meter) that are in longitudinal transection inclined toward sea with dip of few degrees. In longitudinal sections the bed contacts are mainly concordant and rarely erosive, while in transverse cross-section the bed contacts are mainly erosional surfaces. The beds are structureless or exhibit normal and inverse grading and at places also cross- bedding and imbrication. The texture of sediments ranges from matrix supported, non-sorted, angular, fine to coarse-grained sandy gravels with individual grains up to 50 cm in diameter, to well-sorted, grain- supported, angular, fine-grained gravels. The composition of the grains clearly reflects the geology of the catchment area. The most common clasts are Upper Cretaceous rudist limestones followed by Paleogene foraminiferal limestones, while flysch grains occur rarely. Analyzed size of 7414 clasts show a lognormal distribution of their areas, digitized from a selected sedimentological profile. The structure and texture of alluvial fan sediments indicate deposition by two different transport mechanisms: gravity-flow (debris flow) and fluvial. The fluvial sediments are sharp-based, well-sorted, clast-supported gravels exhibiting imbrication, cross-stratification and were deposited on bars within channels creating braided depositional forms. Deposition from numerous high-discharge events caused filling and lateral migration or avulsion of active fluvial channels on the fan. The debris-flows are characterized by poor sorting, matrix-supported fabrics and presence of outsized clasts. These sediments usually erode underlying fluvial deposits. Today, the fan is sedimentologically inactive. The fan body is currently being eroded, mainly by sea cliff erosion (Ružič et al., 2014) and in lesser content by torrental waters coming from catchment area. In this sense, the alluvial fan deposits record a time with different climatic and precipitation regimes marked by more extreme (mainly precipitation) events. References Babić, L. (2003): The geological evolution and structure of the Island of Krk: a review. In: Fossil fauna of the Island of Krk (ed.: Klepač K.), vol. 5, 1-22. Natural history library. Benac Č., Juračič M., Matičec D., Ružič I. & Pikelj K. (2013): Fluviokarst and classical karst: Examples from the Dinarics (Krk Island, Northern Adriatic, Croatia). Geomorphology 184, 64–73. Mamužić P., Milan A., Korolija B., Borović I. & Majcen Ž. (1969): Osnovna geol. karta SFRJ 1:100.000, List Rab L 33-114, Inst. geol. Istraž. Zagreb, Savezni geološki zavod, Beograd. Marjanac Lj. (2012): Glacial and periglacial sediments of Kvarner, Northern Dalmatia and Southern Velebit Mt. – Evidence of Dinaric glaciation. Ph.D. thesis, University of Zagreb, 278 p. Marjanac T., Tomša A. M. & Marjanac L. (2004): Krk-breccia, Possible Impact-Crater Fill, Island of Krk in Eastern Adriatic Sea (Croatia). Cratering in Marine Enviroments and on Ice Impact Studies, 115-134. Ružič I., Marović I., Benac Č. & Ilić S. (2014): Coastal cliff geometry derived from structure-from-motion photogrammetry at Stara Baška, Krk Island, Croatia. Geo. Mar. Lett. 34, 555-565. Šikić D., Polšak A. & Magaš N. (1969): Osnovna geološka karta SFRJ 1:100000. List Labin, L 33-101. Inst. Geol. Istraž. Zagreb, Savezni geol. zavod, Beograd. Vlahović I., Tišljar J., Velić I. & Matičec D. (2005): Evolution of the Adriatic Carbonate Platform: Palaeogeography, main events and depositional dynamics. Palaeogeogr., Palaeoclim., Palaeoecol. 220, 333-360.

Dinarides; Pleistocene; Krk Island; sedimentary processes

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