engleski

Tectonic controls on lacustrine sedimentation on Mt. Medvednica (Ottnangian, Pannonian Basin System, North Croatia)

The Ottnangian (late Early Miocene) succession in the Mt. Medvednica (North Croatia) consists of a lower, alluvial portion and an upper, lacustrine portion. The lower portion overlies pre-Neogene basement, and may attain a thickness of approximately 100 m. It is dominated by poorly stratified, pebble to cobble conglomerates, which reflect deposition within braided-stream systems. General direction of streams was toward the NE. The coarse-grained character of these systems reflects the activation of faults at the closely situated active tectonic margins of the depositional basin. Coarse-grained alluvium is sharply overlain by upper, lacustrine portion of Ottnangian succession, which consists of three different parts. The sharp transition from coarse-grained alluvium to the fine-grained swamp and peat deposits is regarded to have been a consequence of extensional tectonics, which caused subsidence, enlargement of the depositional area, i.e. the activation of new faults situated farther from the previous basin margins. The swamp and peat deposits are overlain by carbonate deposits of lake margin, and rhytmites and thin turbidites in the deeper lake realms, which reflect continued subsidence. In this part of the succession there are rare thin intercalations of pyroclastics. Lower part of lacustrine deposits may be some 55 m thick. The second part of the lacustrine deposits, which may be more than 45 m thick, shows a coarsening-upward trend with an increase of the proportion of clastic deposits. Sands and gravels reflect a delta progradation ending with coarse-grained material. These gravels are sharply overlain by the third part of the lacustrine Ottnangian succession, which is dominated by silts. The relevant change must have been caused by renewed tectonic activity, i.e. the activation of extensional faults. The silts contain fresh-water gastropods, and may contain rare intercalations of gravels deposited from gravity flows. The fresh-water deposition was ended by Karpatian (latest Early Miocene) marine transgresssion and deposition of marine silts. This transgression is known from other parts of the Pannonian Basin System and has been explained by tectonic events.

Croatia; Pannonian basin system; lake; tectonic controls

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