Struktury sedymentacyjne powierzchni dna wybranych rejonów południowego Bałtyku

Abstract

Na podstawie ciągłego, poziomego profilowania akustycznego typu side-scan-sonar, przeprowadzonego na obszarze Ławicy Słupskiej i w północno-wschodniej części Ławicy Odrzanej, określono występujące na powierzchni dna struktury sedymentacyjne. Wyróżniono duże ripplemarki, wstęgi piaszczyste i pólka piaszczyste oraz ustalono zasięg ciągłej pokrywy piaszczystej, wielkość tych form, a także podjęto próbę określenia ich genezy. Wyróżnione formy dna porównano z formami występującymi w Morzu Północnym i w Cieśninach Duńskich.  SEA-FLOOR SEDIMENTARY STRUCTURES IN SELECTED REGIONS OF THE SOUTHERN BALTIC SEA On the basis of continuous horizontal acousting profilling of the side-scan-sonar type, sea-floor sedimentary structures of the Słupska Bank and north-eastern part of the Odra Bank were analysed. The survey was carried out with the use of sonar Transit Sonar MS, 43 MKJ, made by Kelvin Hughes. Echo sounder profiles made within the range 0–550 m in the Słupska Bank area are 138 NM (256 km) long, and those, made within the range 0–275 m in the Odra Bank area – 65 NM (120 km) long. The main beam angles was equol 3–5° (horizontal) and 51° (vertical) in the profilling.The operations were accompanied by simultaneous vertical echo sounder profilling. The ship speed was about 4 knots and its positions during profilling were determined with the use of radio navigation system Decca by readings in 10 min. intervals.On the basis of the obtained sonographs, the following sea-floor forms were identified: megaripples, sandy ribbons and sand patches, as well as the extent of continuous sandy cover (Fig. 1).In the zone of occurrence d discontinuous sandy cover in the Słupska Bank area, sand patches predominate. In places, they are accompanied by sand ribbons which indicate predominance of processes of erosion of sea-floor deposits. In the Odra Bank (Table I, Fig. 3), sand ribbons are NNW – SSE oriented, about 500 m long, 30 to 50 m wide and their spacing is varying from 40 to 100 m. The overlap of sand ribbon and patches (Table I, Fig. 4) implicates relic nature of some of the recorded forms, whereas the forms displayed by Table II, Fig. 5, presumably represent initial phase of formation of sand ribbons. Fields of megaripples, often varying in orientation, were found in the zone of occurrence of continuous sand cover, built of medium- and fine-grained sands in the Słupska Bank. The south-eastern region is characterized by the occurrence of megaripples, usually with rectilinear crests, up to about 800 m long and 0.5 m high, and with spacing ranging from 90 to 100 m (Table III, Figs. 8, 9). Transversal section of megaripples, most often asymmetrical, with steeper slope inclined at about 20 and the gentler – at about 30', and only sometimes symmetrical, indicates current-wave genesis of these forms. Megaripples known from the Great Belt and North Sea are similar in length of crests and spacing, differing in larger height and steeper inclined slopes which may indicate partial destruction of the forms recorded in the Słupska Bank. In the central and south-western parts, megaripples are smaller than those known from the south-eastern part: crests of the former are up to 200 m long and the spacing is varying from 10 to 40 m (Table II, Fig. 7; Table III, Fig. 10). Orientation of longer axes of crests is also varying. Megaripples occurring in the Odra Bank area are also small in size: their crests are up to 40 m long and spacing is up to 5 m. Cooccurrence of sea-floor forms varying in orientation indicates their origin under different hydrodynamic conditions.The studied showed, that the origin of sea-floor forms of the southern Baltic was mainly determined by waving-wind currents. In the case of the North Sea and White Sea, such forms are usually formed by strong, regular tidal currents, and in the case of the Great Belt – by seasonal, strong bottom currents flowing to the Baltic. The recorded differences between forms found in the studied area and those known from the North Sea, White Sea and Great Belt appear related to different hydrodynamic regime of the southern Baltic.