Detailed stratigraphic studies encourage geostructural reinterpretation of the eastern Southern Alps

The general geological structure of the eastern Southern Alps and the transition to the Dinarides is generally well understood. The main structural boundaries run in a west-east direction and coincide with the margins of the main Mesozoic paleogeographic units. The eastern Southern Alps are divided into two large-scale composite thrust units. The lower ones are the Tolmin nappes, which consist of deep-marine Slovenian Basin successions. They are further subdivided into the lowermost Podmelec, the middle Rud and the uppermost Kobla Nappe. Above these are the Julian nappes composed of the Julian Carbonate Platform successions. They are traditionally divided into the Krn Nappe and the Slatna thrust sheet. However, detailed stratigraphic investigations within the Bohinj range of the Julian Alps in the NW Slovenia, provide an alternative solution. In the Kobla Nappe, significant lateral variations can be observed throughout the basinal succession, especially within the best-studied Rhaetian Slatnik Formation. In the west, this formation shows a distal development dominated by hemipelagic limestone. Towards the east, the resedimented limestones become progressively abundant. Near the Soriška planina ski resort, they already dominate the sequence and the formation is characteristic of the lower slope sedimentary environment. According to the existing geological maps, further east the successions of the Slovenian Basin of the Kobla Nappe suddenly disappear and the area is dominated by Norian-Rhaetian platform carbonates, which often contain marginal reef limestones. This entire area (Jelovica Plateau) was traditionally considered part of the Krn Nappe. The described geological situation was the reason for a detailed geological mapping of the Soriška planina ski resort area. Preliminary results indicate that the Kobla Nappe does not wedge to the east. Instead, the succession of the Slovenian Basin (including the Slatnik Formation) passes laterally into the succession of the Julian Carbonate Platform within the same overthrust unit, namely within the Kobla Nappe. This is also supported by a thin, newly mapped Mačji potok thrust sheet composed of Jurassic basinal rocks that lies beneath the basinal successions in the west, but can be traced all the way to the platform limestones to the east. In such a structural reinterpretation, large part of the southeastern Julian Alps, previously considered part of the Krn Nappe, actually belongs to the Kobla Nappe. Therefore, the Krn Nappe is located exclusively in the western and central Julian Alps. The contact between the two nappes is clear in the west, where the nappes consist of successions belonging to a different paleogeographic unit. In the east, the structure was probably overlooked because it runs between the platform limestones. We emphasise that the structure is further complicated by post-thrusting stike-slip displacements. The proposed reinterpretation also opens new perspectives for some other regional problems, such as the occurrence of isolated Oligocene deposits in the central Julian Alps and the emplacement of the Bled Basin (paleogeographically distal Adria margin) in the northwestern Julian Alps.