The succession of the Tolmin Basin forms the foothills of the Julian Alps in northwestern Slovenia. In the Jurassic, it was part of the southern Tethyan passive continental margin. The basin was located between the Dinaric Carbonate Platform in the south and the Julian High in the north. Six sections were studied that encompass the stratigraphic interval from the Toarcian to the lower Tithonian. The basinal background deposits are generally marl, siliceous limestone, and radiolarian chert, whereas in the southern part of the basin resedimented limestones occur. The studied succession lies between the Lower Jurassic Krikov Formation (resedimented and hemipelagic limestones) below and upper Tithonian to Neocomian Biancone Limestone above.

Two formations are described in this paper: (1) The Toarcian Perbla Formation was introduced by Cousin [1973] and is revised herein. It is composed of marl and subordinate calcareous shale with rare intercalated calciturbidites. The material was redeposited only within the basin or from its marginal parts. The thickness of the formation varies significantly through the basin from 2 to 135 metres; (2) The Tolmin Formation is herein defined. It is divided into two members. The lower member (Aalenian to lower Bajocian) consists of siliceous limestone and rare chert, whereas the upper member (upper Bajocian to lower Tithonian) is composed of radiolarian chert that locally in the upper part contains abundant marl intercalations. In the southern part of the basin, two intervals of resedimented limestones occur in the Tolmin Formation. The lower interval (lower Bajocian to lower Callovian) is dominated by calcarenite (rich in peloids and ooids) and limestone breccias. The upper interval (upper Kimmeridgian to lower Tithonian) is characterized by calcarenite, composed mostly of intraclasts and bioclasts. The source area of the carbonate material was the Dinaric Carbonate Platform.

The correlation of the background sediments with other basins of the western Tethys reveals that (1) the late Toarcian decrease in terrigenous input, the Bajocian change from calcareous to siliceous sedimentation and the late Tithonian onset of carbonate sedimentation are regional and (2) the silica content in the Tolmin Basin was higher than that in the western basins (e.g. the Belluno, Lombardian, Umbria-Marche and Subbetic basins) but lower than that in the Budva Basin. The Bajocian to Callovian resedimented limestones of the Tolmin Basin are much thinner than the corresponding deposits in the Belluno and Budva basins (25 metres vs. up to 600 metres) that bordered the Dinaric Carbonate Platform on the west and southwest respectively. This difference confirms the previously proposed interpretation that most of the carbonate production from the Dinaric Carbonate Platform was transported towards the southwest.

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