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Characteristics of some Middle Triassic volcaniclastic rocks in the External Dinarides (Croatia and Bosnia and Herzegovina)

Several volcaniclastic-sedimentary successions of Middle Triassic age have been investigated in Croatia (vicinity of Sinj and Knin) and in Bosnia and Herzegovina (vicinity of Bosansko Grahovo), in the External Dinarides, Volcanic and volcaniclastic rocks in the External Dinarides represent Middle Triassic syn-rift phase in which along deep structural fractures graben-like depressions had been formed. The Middle Triassic rift phase is in the External Dinarides is marked by basaltic extrusions to acidic explosive activities that resulted in the deposition of pyroclastic and volcaniclastic deposits (ŠĆAVNIČAR et. al., BELAK, 2000 ; MARJANAC, 2000). Volcanic and volcaniclastic successions near Sinj, described mineralogically and petrographycally by the same authors, were interpreted as vitro- to crystaloclastic tuffs and ignimbrites deposited as pyroclastic flows in intrashelf environment, not far from subaerialy located caldera. In our investigation we examined several lithotypes of volcanic and volcaniclastic and in the interpretation we considered the new palaeontological data. Near Sinj a thick volcaniclastic beds (called „pietra verde“) are interbeded with marine bioclastic, well bedded limestones, cherts and dolomites. Bioclastic limestone and dolomites are characterised by presence of calcareous algae, foraminifers, gastropods, bivalves, brachiopods, crinoids, serpulids as well as ammonoids, radiolarians and conodonts suggesting open marine (pelagic) associations. (JAECKS et al., 2003 ; BALINI et al., 2006). Limestone beds are strongly silicified and recrystalised. Dolomites exhibit macrocrystaline anhedral texture suggesting a secondary dolomitisation. On the base of conodonts and amonoids the Lower Ladinian age was inferred (BALINI et al., 2006 ; JELASKA et al ; 2003) Volcaniclastic beds (tuffs) near Sinj are massive or evenly laminated. Cross lamination occur at the top of some beds. Several volcaniclastic lithotypes of tuffs dominantly contain formerly glassy fragments which are cuspate, platy or udeformed bubble-wall shards. Pumice wisps occur in some lithotypes in random orientation, showing relict tube vesicle texture. Other components are sparse feldspar and quartz crystal fragments. The formerly glassy shards are recrystalised to a fine quartz-rich mosaic. Shards are occasionally well preserved due to carbonate microcrystaline calcite that outlines them. The dominance of juvenile pyroclasts (shards and quartz crystal fragments) suggest that they were sourced from explosive, acidic eruptions. The interlaying with carbonates containing pelagic (open marine) fossil assemblages suggests that they have been deposited farther offshore. In this circumstances are these subaqueous deposits not likely to by strictly primary. Bedforms indicate rapid, possibly mass-flow, deposition in offshore environment. Nevertheless the abundance of texturally unmodified pyroclasts suggests that pyroclastic material was delivered more of less directly to sedimentary transport and deposition systems. Although it has been suggested that the origin had been primary pyroclastic surge deposits from subaerial depositional settings, we thought that pyroclastic material generated from explosive eruptions at marine submerged vents. The dominance of juvenile pyroclasts suggests that fragmentation occurred when lavas emplaced into marine unlithified sediments allow explosive vaporisation of pore fluids or when superheated water flashes to steam that rapidly expands. Thus we interpret deposits near Sinj as pyroclast rich offshore mass-flow deposits. Near Bosansko Grahovo there are occurrences of blocky peperites. In the angular peperite type jigsaw-fit texture can be observed suggesting quenching of basaltic lava on the contacts with unconsolidated pelagic limestones. The Ladinian age of the succession was discerned according to conodonts found in limestones intercalations. References: BALLINI, M., JURKOVŠEK, & B., KOLAR-JURKOVŠEK, T. (2006): New Ladinian ammonoids from Mt. Svilaja (External Dinarides, Croatia). Rivista Italiana di Pal. E Strat., 12/1, 383-395 BELAK, M. (2000): Postaja 2: profil Sutina-Zelovo Sutinsko ; Kristaloklastični i vitoroklastični tufovi (pietra verde) s proslojcima silicificiranih dolomita, vapnenac, tufita i rožnjaka. U: JELASKA, V., BENČEK, Đ., MATIČEC, D., BELAK, M., & GUŠIĆ, I: Geološka povijest i strukturna evolucija Vanjskih Dinarida.- Vodič ekskurzija (VLAHOVIĆ, I. & BIONDIĆ, R. Eds.). 2. hrv. gol. kongres, Cavtat-Dubrovnik, 6-9. Cavtat-Dubrovnik. JEACKS, G.S., LEIGHTON, L.R., KOLAR-JURKOVŠEK, T., & JURKOVŠEK, B., JELASKA, V. (2003): Phylogenetic analysis of a likely productide from the Triassic, and the implications for survival through the Permo-Triassic extinction: Geological Sociaty of America, Seattle Annual Meeting, Abstracts with Program, 66-1. 230, Seattle. JELASKA, V., KOLAR-JURKOVŠEK, T., JURKOVŠEK, B, & GUŠIĆ., I. (2003): Triassic beds in the beds in the basement of the Adriatic-Dinaric carbonate platform of Mt. Svilaja (Croatia). Geologija, 46/2: 225-230. MARJANAC, T. (2000): Triassic of Dalmatia – Evidence of a failed rift (Muć section). PANCARDI 2000, Vijesti Hrv. geol. dr., 37/2: 117-126, Zagreb ŠĆAVNIČAR, B., ŠĆAVNIČAR, S., & ŠUŠNJARA, A. (1984): The volcanic-sedimentary Middle Triassic in the Suvaja brook area (Mt. Svilaja, Outer Dinarides). Acta Geologica, 14/2, 35-82.

Middle Triassic; rifting phase; volcaniclastic rocks; External Dinarides; Croatia; Bosnia and Herzegovina

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