engleski

Obrovac moraine - new evidence of extent of glaciation in Dinarides

The road cut along the Obrovac-Zadar road, just south from the northern Dalmatian city of Obrovac, reveals a diamictite in a section nearly 10 m high and about 50 m long. The Obrovac city is situated on the left bank of the Zrmanja river, that flows westwards through the canyon and meets the Novigrad Sea about 6 km downstream. The Obrovac environs is built of well-bedded Promina-conglomerates (Eocene/Oligocene age) which are underlain by the Upper Cretaceous limestones, and form a NW-SE syncline with rather steep flanks (40-45°) (Ivanović & al. 1973). The diamictite (debrite-like sediment) is very poorly sorted conglomerate that consists of large clasts “ floating” in poorly cemented coarse-grained (gravel-size) matrix. The clasts are commonly medium- to well rounded, elongated to subsphaerical, and reach from 20 cm to blocks of several meters across, with no particular orientation. Lithologically, the clasts and blocks are represented by the Promina-conglomerates. The contact with bed-rock is not visible, and the sediment is additionally tectonized, with several minor subvertical faults visible. All large clasts and blocks have striations developed on all their surfaces. The striae are differently oriented, some are long and straight, some are short, curved or straight. They are mm-size shallow grooves, commonly tightly spaces, commonly organized in groups or parallel- to sub-parallel striae. Some groups of striae are being crossed at sharp angles by striae of other orientations. The curved striae indicate rotation of the block, whereas the straight ones indicate block sliding over hard substrate. Their characteristics are typical for glacial striae on clasts and blocks in basal and lateral moraine. This diamictite is interpreted in terms of glacial lateral moraine due to its position, clast size and distribution, and in particular - striated coarse debris. The lack of preferred orientation of striae, and their occurrence on all faces of the clasts, rules out the hypothesis that they are tectonical in origin. Our previous research documented glacial and periglacial sediments around Novigrad Sea and outreach of a large glacier coming from the Knin area through Ervenik and Žegar poljes all the way to Obrovac, Novigrad Sea and Karin (Marjanac & Marjanac 2004). Lacustrine clays with cold ostracod fauna of Pleistocene age have been documented near Obrovac, as well as in Ervenik and Žegar poljes (Malez & Sokač 1969) and the Novigrad Sea (Marjanac et al. 1990). Basal and terminal moraines were found at Novigrad Sea (Marjanac & Marjanac 2004). Therefore, newly found Obrovac moraine represents additional evidence of glacial advance in the region of Ravni Kotari and will be further studied.

glaciation; Dinarides; Dalmatia

nije evidentirano