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Triassic Volcano-Sedimentary Succession – Sekolje, Mt. Strahinjščica, NW Croatia

The Mt. Strahinjščica in NW Croatia contains volcano-sedimentary successions formed along the NE margin of Adria during period of arc-related extension in the Middle-Late Triassic. It is built of sedimentary (dolomitic rocks, radiolarites, cherts, clay, siltstones and sandstones), magmatic rocks (altered andesitic basalts and diabases), and tuffs. The Sekolje section on the southern slope of Strahinjščica Mt. represents a 100-meter thick volcano-sedimentary succession with pyroclastic and siliciclastic lithofacies types. The pyroclastic facies is recorded in the section from the beginning to 38.90 m. It consists of two different lithotypes: vitroclastic and crystaloclastic tuffs. Vitroclastic tuffs are composed of fine ash with glass shards and pumice fragments. Both glass shards and pumice fragments are devitrified into microcrystalline quartz, albite, clay minerals, calcite and chlorite respectively. Rare crystalloclasts of quartz and K-feldspar/plagioclase are still present. Crystalloclastic tuffs are composed of coarse ash to lapilli sized pyroclastic material. Dominant crystalloclasts are quartz, plagioclase and K- feldspars, with less biotite. Quartz crystalloclasts are angular with varied size, show jig-saw fit texture, and occasionally have oval- spherical cavities typical for volatile rich systems. Plagioclase and K-feldspar crystalloclasts have mainly hypidiomorphic to idiomorphic shape. Carbonate and silicified tuff lithoclasts and chert are subordinate. Matrix of the crystalloclastic tuffs is made of fine ash devitrified to chlorite, microcrystalline quartz, clay minerals, opaque minerals, with carbonate domains sporadically. The siliciclastic facies between 38.90 and 100.60 m is divided into siltstone and volcanogenic sandstone lithotypes. Dark grey to brown siltstones are homogenous, thin bedded or laminated with radiolarian-rich lamina and lenses. Grains of quartz, K-feldspar and muscovite are recognized in the siltstones, while the matrix is composed of chlorite, clay minerals, sericite, and organic matter. Volcanogenic sandstones are poorly sorted and composed mainly of pyroclastic and volcanogenic particles, with approximately equal amount of highly altered feldspars and lithoclasts, and some quartz grains. Matrix is composed of chlorite, sericite, fine quartz and feldspar grains. Described lithofacies types at Sekolje section suggest that the sedimentation of the pyroclastic material occurred simultaneously with active volcanism. Occurrence of two lithotypes in the pyroclastic facies indicate segregation of the material from the bulk pyroclastic cloud through sedimentation processes. Crystalloclastic coarse pyroclasts were transported and deposited by gravity currents, while fine ash tuffs, consisting mainly of glass shards, settled from suspensions. These processes could have happened after the pyroclastic eruption and syn-eruptive resedimentation to the deeper marine environment while the siliciclastic facies was deposited during abating volcanic activity. Siltstones were deposited in distal, probably anoxic environments. The organic matter in this lithofacies consists of charcoalified plant debrís, palynomorphs are extremely rare. The coarsening of clastic material in the upper part of the section and transition to the volcanogenic sandstones can be interpreted as shallowing of the basin areas and/or increased proximity to the source area.

pyroclastic and siliciclastic lithofacies, Triassic, Mt. Strahinjščica, NW Croatia

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