In the master's thesis I describe composition and stratigraphic position of a blocky limestone breccia from the Ponikvanska planota plateau. Research area is located approximately 7 km southeast of Tolmin (NW Slovenia). It structurally belongs to the Southern Alps and is a part of Ponikva tectonic klippe, which is an erosional remnant of the Podmelec Thrust sheet, the lowermost subunit of the Tolmin Nappe. Podmelec Thrust sheet includes mesosoic successions of the Slovenian Basin which were most proximal to the Dinaric Carbonate platform. The lithostratigraphic succession in this part of the Tolmin nappe is poorly researched, especially regarding the Jurassic part of the sequence. Blocky breccias were investigated in one detailed, one partial and two schematic profiles, which were recorded based on an existing and improved geological map. Field observations were complemented with petrographic microscopy. The age of lithoclasts and matrix is based on foraminiferal assemblage.
The Ponikva Tectonic klippe consists of Upper Triassic to Upper Cretaceous deep marine successions of the Slovenian Basin in an inverted position. The Jurassic succession starts with 110 m of thin bedded micritic limestones of the Krikov Formation, which are unconformably overlain by an up to 80 m thick limestone breccia horizon – the main subject of the research. The breccias are overlain by 18 m of bedded cherty micritic limestones and resedimented limestones of the Tolmin Formation. The Jurassic succession ends with limestones of Biancone type.
Seemingly massive breccia horizon is actually composed of numerous limestone blocky breccia and subordinate calcarenitic beds. The lower boundary is erosional. Lithoclasts predominantly consist of shallow marine and subordinate deep marine carbonates. Fossils, mostly foraminifera confirm that most of the lithoclasts are Upper Triassic in age and were originally deposited on the platform margins, with some lithoclasts also Lower and Middle Jurassic in age. Due to dolomitization of the matrix, its’ age was difficult to define. But foraminifera Protopeneroplis striata Weynschenk dates the matrix to Middle Jurassic. Therefore, I consider these breccias as a part of the Tolmin Formation. Poor sorting and large bed thickness indicate that the breccias were deposited via debris flows, which might have been a consequence of tectonically induced collapse of the northern margins of the Dinaric Carbonate Platform.
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