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Periglacial aeolian– alluvial interaction: Pleistocene of the Island of Hvar (Eastern Adriatic, Croatia)

he Island of Hvar is located along the NE coast of the Central Adriatic, Croatia. Along its W– E oriented longer axis it stretches for almost 70 km, while it is approximately 5 km (only in the western part up to 10 km) wide. It is composed mostly of Cretaceous carbonate deposits (stratigraphic range Neocomian– Maastrichtian), originating from the ancient Adriatic Carbonate Platform, one of the largest and best preserved Perimediterranean carbonate platforms [1]. The structure of the Island of Hvar is characterized by asymmetrical anticline composed of Cretaceous deposits (limestones and dolomites) which is, along the southern margin of the island, in the tectonic contact with the youngermost Cretaceous limestones and Eocene deposits (foraminifera limestones, transitional marls, and flysch). Structure of the island (oriented W– E) differs from the general regional strike (so called Dinaric strike – NW– SE). Major outcrops of the Pleistocene deposits [2] are situated in the western, widest part of the island where U-shaped valley developed. Studied deposits crop out at the southern side of the valley. The outcrop is 35 m long and up to 12 m high, and is composed of sands and sandstones, and subordinate breccia. Lower part is dominated by a planar cross-bedded unit composed of sand and sandstone. Its thickness is up to 2.5 m, varying laterally because of erosion. Cross-beds are steep (20– 31°), mostly tangential, sigmoidal at places. Reactivation surfaces are frequent within the unit. Breccias occur as irregular lens-shaped bodies with scoured bases at the level of the upper bedding plane of the cross-bedded unit. Medial part is more complex. The sands and sandstones are represented by smaller units showing generally tangential cross-bedding of lower angle, and trough cross-bedding. Breccia lens-shaped bodies are more frequent, and occur both within cross-bedded deposits and at the level of the upper bedding planes of the cross-bedded units. Some isolated carbonate clasts can be found within sands. In the upper part of the outcrop low-angle small-scale cross-bedded units prevail, and scouring is rare. The cross-bedding in whole cross-section documents general palaeotransport to the west. Breccia lens-shaped bodies are of different geometry, from one-clast thick lenses to 70 cm deep irregular erosional bodies, and are up to 3 m wide. Breccias are clast- to matrix-supported, consisting of carbonate fragments of local origin with sandy matrix. The grain-size distribution is bimodal. Grain-size of the breccia fragments extremely varies from granule to boulders up to few dm long. The largest fragments are usually related to deepest erosion, i.e. they occur in more narrow but deeper lenses, where content of sandy matrix is low. Some bodies show channel-fill. Sands and sandstones are mostly fine- to medium-grained, well sorted, rich in carbonate content (70% in average). The sandstones, determined as calclithites, mostly consist of limestone extraclasts. Rock fragments (cherts, quartz– sericite schists, quartzite) and quartz dominate in siliciclastic component of the sandstones. The sands, both in cross-bedded units and in breccia lenses, are of the same composition as the sandstones. Amphiboles, pyroxene, garnets and epidote are their most abundant translucent heavy minerals. The dominance of chemically unstable rock fragments and heavy minerals in the composition of clastic material of the Island of Hvar suggests its mineralogical and petrological immaturity. This immaturity is related to the intense mechanical and less intense chemical weathering of the source rocks. The composition of the material indicates their provenance from carbonate and siliciclastic sedimentary and metamorphic rocks. The structural maturity of material is probably linked to the long distance transport and/or to the transport by wind. The cross-bedded sand and sandstone units are of aeolian origin, and represent dunes. The dunes migrated by influence of strong eastern winds generated by cold and arid climate. The sand source area were probably river flood-plains within Dinarides which are composed of carbonate, siliciclastic, magmatic and metamorphic rocks. The breccia lens-shaped bodies represent alluvial deposits of local origin, which accumulated at the foot of a steep hill slopes possible as a consequence of snow melting during short less cold and humid periods. The occurrence of heavy minerals in its sandy matrix can be explained by erosion and redeposition of aeolian deposits. Isolated limestone clasts within cross-bedded units are probably a consequence of subaerial rock-fall.

aeolian deposits; alluvial deposits; Pleistocene; Hvar; Croatia

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