Pleistocene calcareous aeolian-alluvial deposition in a steep relief karstic coastal belt(island of Hvar, eastern Adriatic, Croatia)

摘要

Pleistocene aeolian and alluvial deposits occur on the island of Hvar, belonging to the eastern Adriatic karstic coastal belt along the north-eastern Mediterranean margin. The depositional mechanism of the aeolian, alluvial and talus sediments are interpreted based on facies analysis and mineralogical composition. Aeolian deposits are represented by cross-bedded, cross-laminated, and sub-horizontally laminated fine- to medium-grained calcareous sands. The cross-bedded units form tabular sets stacked into cosets. Sets and cosets are separated by distinct low-angle bounding surfaces which mostly dip towards the east interpreted as the upwind direction. Cross-laminated units form isolated sets within the cross-bedded and sub-horizontally laminated units. Cross-bedded and cross-laminated units represent dunes dominated by grain fall and wind ripple deposition, respectively. Dunes produced by sand flow also occur, but are less common. The cross-bedding is truncated by reactivation surfaces. Transverse dunes and some small dome-shaped dunes were also present. Sub-horizontally laminated sandy units represent aeolian sand sheets developed by wind ripple migration in the interdune area. A few wind-rippled dune apron deposits also occur. Trace fossils are locally very abundant within the aeolian deposits, mostly produced by plants. Soft-sediment deformation, such as contorted cross-bedding and pocket structures occur scattered, and rare reddish horizons show pedogenesis. Unlikely most Quaternary coastal aeolian sands, these sands do not include marine bioclasts in composition. The sands are composed of extraclasts dominated by limestone with subordinate chert, quartz-sericite schist, quartzite and quartz. Amphiboles, pyroxene and epidote are the most abundant translucent heavy minerals. The mineralogical composition and palaeoflow directions indicate that the Dinarides were the main provenance of the sand transported by primary easterly and northerly continental winds causing migration of dunes towards the palaeo-Adriatic Sea. Alluvial deposits are intercalated with the aeolian sands. They are composed of breccia derived from a nearby steep hill-ridge, and by resedimented aeolian sand deposited by traction currents during flash floods. Talus fragments reached the aeolian sands only sporadically. Climate strongly affected aeolian, alluvial and talus depositions. During arid conditions dunes migrated forming a dune field, whilst more humid climate triggered heavy rains and generated erosion of aeolian deposits, alluvial sedimentation, colonisation of plants and pedogenesis. Deposition was in a topographic basin, and was controlled by capacity of source area and wind. However, local orography characterised by developed and steep karstic relief strongly affected wind directions, and in that way had specific controls on the dune field evolution. Dominating winds caused seaward dune migration differing from most Mediterranean Quaternary coastal aeolian dune localities characterised by landward migration.