The first evidence of Permian–Triassic shallow-marine transitional deposits in northern Croatia: Samoborsko Gorje Hills

Abstract

Permian–Triassic successions occur throughout the world, but well-exposed transitional sequences are relatively rare. In Croatia, only two localities with continuous transition from Permian to Triassic have been described previously from its southern parts, but in northern Croatia the Permian–Triassic boundary remains undocumented. A succession of Permian and Triassic sub- to supratidal deposits is exposed in the Samoborsko Gorje Hills in N Croatia, on the northern margin of the Dinarides towards the Pannonian Basin. Oldest part of the sequence is composed of dolomudstones to dolopackstones containing an Upper Permian (Lopingian) biota: calcareous algae (gymnocodiaceans and dasycladales), gastropods and smaller foraminiferans (e.g. Hemigordius sp., Glomospira sp., Earlandia sp. and Ammodiscus kalhori). The middle part of the succession is characterized by the ‘Transitional breccia’—dolomitic breccia and microbreccia, with ferroan calcite cement, probably deposited during the Late Permian regression. ‘Transitional breccia’ deposits contain only disaster forms (Ammodiscus kalhori and Earlandia sp.) which are often considered as survivors, but can generally be found in both Permian and Triassic deposits, confirming environmental crises in the shallow-marine environments. Gradual recovery of the biota can be traced in the upper part of the succession, with dolomudstones to dolowackestones containing peloids and sparse, smaller foraminiferans. The presence of foraminiferan Meandrospira pusilla, which is identified for the first time in the studied area, indicates a Late Olenekian age for the youngest part of the studied deposits. This study demonstrates that the transition from Permian to Triassic can be indicated even in stressful and/or tectonized areas lacking conodonts, and contributes to the palaeogeographical reconstructions of this part of the Paleo-Tethys.