engleski

Geological control of beach sediment in the Kaštela-Ploče flysch zone (Adriatic Sea, Croatia)

Eastern Adriatic Coast (EAC) is characterized by its karstic nature. Most of the island and mainland coast is composed of the Mesozoic carbonates (>90% in length). Eocene Flysch is the second prevalent rock assemblage characterized by alternation of breccias, sandstones, siltstones and marls. It stretches along the EAC of approximately 6% in length. Unlike carbonates, flysch is soft and prone to mechanical weathering. Accordingly, most of the longest gravel and sandy beaches along the EAC are developed within flysch and associated deposits. Together with other coastal structures, flysch basins follow the same NW-SE, so-called Dinaric direction. As a result, many of flysch zones are narrow, facing the sea perpendicular to the Dinaric direction. Only one significantly longer zone of flysch deposits facing the sea in the Dinaric direction stretches from the Kaštela Bay to Ploče. Coastal landscape in this area is a typical fold and thrust structure, characterized by older carbonates thrusted over younger flysch. Along this zone numerous gravel beaches have been developed. In order to get better acquainted with the role of the coastal geologic material in beach formation, three representative natural beaches were selected to be investigated. First beach is a mixed gravely-sandy beach in front of the typical marl-sandstone cliff in Split urban zone. The most common pebbles/cobbles derive from the Palaeogene foraminiferal limestones, while minor portion derives from siliciclastic sandstones and siltstones. Median diameter (Md) of both gravel populations ranging usually between 5-15cm. Moderately sorted gravelly sand (Md~0.6mm) is winnowed from the gravel and transported towards NW. In this mostly carbonaceous sand (90-95%) Palaeogene particles prevail, containing fragments of benthic fossil community from Palaeogene foraminiferal limestones and planktonic foraminifera from Eocene marls. Non-carbonate particles are less represented ; comprising chert and siltite fragments, as well as dominating opaque minerals and garnets, typical for adjacent flysch. Both gravel and sand is the product of the natural slope degradation of the flysch cliff, further reworked by waves. Cobbles and pebbles are gradually rounded and weathered to produce sand. Their size decreases towards NW while share of the sand increases in the same direction, both showing the prevalent alongshore transport. The finest particles originated from marl are constantly removed by waves and eventually transported to the deep sea. The second beach is a pure gravel beach in Dugi Rat surrounded by the Upper Cretaceous carbonate breccias. Well rounded carbonate pebbles and cobbles are of Md usually ranging from 3-10 cm. Two-thirds of the analyzed gravel originated from the surrounding breccias ; while the smaller part of originate from Palaeogene foraminiferal limestones, indicating their proximity. The third is pure sandy beach in Duće area, NW of the Cetina River mouth. Beach sediment is moderately well sorted slightly gravelly sand (Md~0.45mm), provided by the Cetina River and deposited by the longshore drift. Sand particles are mostly carbonaceous (97-98%) rock and fossil fragments, heavily weathered due to reworking by waves. Many of recognizable particles are of Palaeogene (Eocene) age, while particle of Cretaceous age are present in smaller quantities. Non-carbonate particles are dominated by garnets. Both sand populations have been derived from the Cetina River drainage basin, prevalently made of Triassic, Jurassic and Cretaceous carbonates, and of Triassic, Palaeogene, and Neogene clastics, being more susceptible to fluvial weathering. Along the Kaštela-Ploče flysch zone mostly gravel beaches have been developed, regardless the composition of the surrounding flysch. Sand fraction is usually removed by waves and deposited in intertidal. The exception is the sandy stretch of the coast, loaded by the Cetina River. The finest muddy material has been carried to the deep sea.

beach; Eastern Adriatic Coast; flysch; gravel

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