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Middle Permian fan-delta complex of the Gorski Kotar (Croatia)

Intensive uplift occurred at the end of Lower or at the beginning of Middle Permian resulted in molasse type sedimentation and the of vast quantities of siliciclastic sediments were accumulated in the Gorski Kotar area (central part of Republic Croatia). The presence of autochthonous marine fossils (ammonoids, brachiopods, crinoids), found in black shales that intercalate with conglomerates and sandstones, allow the assumption of fan delta type sedimentation. The successions described in the northern part (successions in the vicinity of Tršće) and in central part of Gorski Kotar (vicinity of Mrzla Vodica) envisioned the two different fan delta type sedimentation which reflect the different depth of the sedimentary basin. Deeper and shallower part of the delta assume the different basin floor morphology and possibly resulted as a consequence of a different subsiding processes due to subbasinal faulting in a tectonically active area. The sedimentation processes in central part of Gorski Kotar were determined by steep delta slope. Sliding as well as formation of chutes have been noticed. The following fan delta facies were recognized: 1) delta-slope facies and 2) prodelta-shelf facies. The delta slope facies is characterized by several types of conglomerates and usually coarse grained sandstones that are: matrix supported massive conglomerates (Gms), clast supported massive conglomerates (Gmc), normally graded conglomerates (Gg), inversely to normally graded conglomerates ((in)g2g1G), gravelly mudstones (GyM), calcrudites (D) and massive sandstones (Sm). The prodelta-shelf facies is determined by: sandstone-shale intercalations (SM), shales (M) and planar bedded calclithites (C). In shale intercalations the autochtonuous ammonoid fauna has been found. Sedimentation processes record the slope-type delta of ETHRIDGE & WESCOTT (1984). In northern part of Gorski Kotar the sedimentation was determined by low inclined relief with coarse clastic sedimentary rocks described as 1) mouth bar facies and 2) subareal or coastal facies. The mouth bar facies association consists of: normally graded conglomerates (Gg), inversely graded conglomerates ((in)gG), inversely to normally graded conglomerates ( (in)g2g1G), massive matrix supported conglomerates (Gms), massive clast supported conglomerates (Gmc) planar cross bedded conglomerates (Gp), trough bedded conglomerates (Gt), gravelly sandstones, where gravels faintly mark shallow troughs, (GyS) and parallel laminated sandstones (lS). Subareal or coastal fan delta facies is characterized by irregularly bedded sandstones and gravelly sandstones (A) and micaceous sandstones (B) containing coarse plant detritus. Shales (M) and sandstone-shale intercalations (SM) are interpreted as prodelta-shelf facies. The delta type sedimentation corresponds to shelf-delta of ETHRIDGE and WESCOTT (1984). Sandstone-shale intercalations, interpreted as prodelta-shelf facies, have similar characteristic in central and northern part of Gorski Kotar and were interpreted as unique “back-ground” sediment deposits connecting the two fan delta types. According to fossiliferous sediments associated with sandstone-shale intercalations (found only in the central part), the same age is proposed for the clastic fan delta sequence of the Gorski Kotar. Analyses of the fossil assemblage in calclithites (C), although considered as resedimented detritus, permit the conclusion of Middle Permian age (SREMAC & ALJINOVIĆ, 1997). Carbonate sedimentation, equivalent of Bellerophon formation, is missing in Gorski Kotar, thus assuming the hiatus between Permian and Lower Triassic sedimentary rocks. During Upper Permian the intensive inundation lowered the uplifted relief and the Lower Triassic sedimentation was characterised by shallow marine deposition on flat relief.

Fan-delta facies assemblage; Middle Permian age; Gorski kotar; Croatia

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