Platform to basin transitions: mapping observations at the Krvavica Mountain, and Čemšeniška Planina, in the Sava Folds region.

The Krvavica Mountain and Čemšeniška Planina are situated on the northern limb of the Trojane Anticline as part of the Sava Folds region in middle Slovenia. This Cenozoic fold belt is situated in the transition zone of the Alps and Dinarides. This area was part of the Adriatic rifted margin of the Neotethys during the Middle-Late Triassic. Repeated rifting phases created the Slovenian Basin, which subsided until the Late Cretaceous. The extensional phase was followed by contraction in the Paleogene and Neogene during the Dinaric and Alpine phases (Placer 1998a; Schmid et al., 2020). The interplay of two-phase thrusting led to specific young-on-older tectonic contact between the Dinaridic Carboniferous-Permian clastics (“softbed” of Placer 1998b) and Mesozoic formations of uncertain origin (Placer 1998b).

The study area SW from Celje, near the Krvavica Mt. has good outcrops. According to previous studies (Buser, 1978, Premru, 1983, Dozet & Buser 2009) the Krvavica Mt. is composed of the platform Schlern Formation while the Čemšeniška Planina is partly composed of Bača dolomite, a characteristic Slovenian Basin formation. Our new observations show that through the Krvavica Mt. three formations can be traced from S to N: latest Ladinian to Carnian siliciclastic basin sediments (shale, sandstone, siltstone, micritic cherty limestone, breccia, and pyroclastics), all composing the Pseudozylian Formation, Triassic platform carbonates (Schlern Fm.) and latest Jurassic to Early Cretaceous carbonates and mixed carbonate-siliciclastic rocks. The formations are repeated at least two times by a major thrust. On the other hand, a platform progradation into the clastics basin can also be suggested; this feature is typical in central Slovenia (Gale el al., 2020).

However, 500 m west of the Krvavica in the eastern side of Čemšeniška Planina and on the Flinskovo ridge, recent mapping showed a typical but condensed Slovenian Basin-type sequence. The succession starts with the pelagic Bača dolomite, followed by the Hettangian–Pliensbachian calciturbiditic Krikov Formation. This was followed, after a potential gap in the Toarcian by the recently described Bajocian-Bathonian Ponikve Breccia as part of the Tolmin Formation (Rožič et al., 2018, and 2022 submitted). After a possible gap in the early late Jurassic, the Late Jurassic-Early Cretaceous Biancone Formation represents the youngest exposed member.

The proximity of fundamentally different lithological sequences can shed light on the platform to basin transition and describe the border of Slovenian Basin with the Dinarides. However, the structural geometry is complex, and could also involve normal faulting to achieve young-on-older contacts. Alternatively, post-folding, gently dipping thrusts could dismember the pre-existing northern limb of the Trojane Anticline. The displacement of the tilted (folded) Mesozoic Slovenian basin succession can also be a post-folding thrust which led to the contact of the pelagic formations to the underlying folded Carboniferous-Permian rocks of the Dinarides.

The research was supported by the National Research, Development and Innovation Office of Hungary (134873).