Characteristics of some Middle Triassic volcaniclastic rocks in the External
Dinarides (Croatia and Bosnia and Herzegovina)
Aljinović, D.,1 Kolar-Jurkovšek, T., 2 Jurkovšek, B., 2 Hrvatović, H. 3
1
University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering, Pierottijeva 6, 10 000 Zagreb,
Croatia, dunja.aljinovic@rgn.hr
2
Geological survey of Slovenia, Dimičeva 14, 1 000 Ljubljana, Slovenia, tea.kolar@geo-zs.si,
bogdan.jurkovsek@geo.zs.si.
3
Federal Bureau of Geology, Ilidža, Ustanička 11, 71 210 Sarajevo, Bosnia and Herzegovina
zgeolbih@bih.net.ba
Key words: Middle Triassic, rifting phase, volcaniclastic rocks, External Dinarides, Croatia,
Bosnia and Herzegovina
Abstract
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.
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