engleski

Accretionary lapilli from deep marine settings in the Middle Triassic of the External Dinarides – proof for resedimentation of the pyroclastic material?

The Middle Triassic age in the External Dinarides was characterised by a significant volcanic activity, as in other parts of the western Tethys. In the Velebit Mts, near Donje Pazarište, a 96 meter thick sedimentary succession with rather unusual occurrence of accretionary lapilli was recorded (formerly described by Marci et al., 1991). Sedimentary and pyroclastic rocks investigated in Donje Pazarište succession indicate that dominant deposition of carbonate rocks was influenced by severe volcanism lasting from the Late Anisian to the Early Ladinian (Smirčić et al., 2018). Volcanism was possibly accompanied by tectonic activity forming graben or half graben pelagic basins. Six facies were differentiated in almost continuous vertical succession: 1) Flysch Facies – FF at the beginning of the section, 2) Carbonate Shale Facies – CS, 3) Syneruptively Resedimented Pyroclastic Facies - SRP, 4) Platy Limestones with Pyroclastics Facies - PLP, 5) Limestone Breccia Facies - LB to Slumped Limestone and 6) Chert and Pyroclastic Facies – SLCP at the top. Facies characteristics indicate deposition in the open marine, pelagic environment. Nevertheless, in this pelagic deeper marine realm accretionary lapilli unexpectedly occur in limestone and tuff layers, or form distinct layers composed almost exclusively of these peculiar pyroclastic forms. Presence of accretionary lapilli in the deeper marine settings is not common and not always clear (Boulter, 1987 ; Cunningham and Bread, 2014). Accretionary lapilli from the recorded section are spheroidal forms 0, 8 to 1, 5 cm in diameter. They are large pyroclastic particles with visible internal structure organised as concentric volcanic ash rims around the core. They were formed during explosive subaerial volcanic eruptions, in the volcanic clouds by agglutination of fine ash material with presence of water vapour. Their genesis suggest terrestrial origin by volcanic explosion on land or in shallow marine environments (Boulter, 1987 ; Soh et al., 1989 ; Ayres et al., 1991 ; McPhie et al., 1993). Accretionary lapilli found in limestones have rims of coarsely crystalized sparry calcite and cores made of fine volcanic ash. However, accretionary lapilli found in tuff layers lack the calcite rimes and are completely composed of fine volcanic ash and rare fine crystalized plagioclase and quartz crystals. The occurrence of the accretionary lapilli in the deeper marine settings in the Middle Triassic of the Dinarides indicate that they primarily originated in subaerial conditions but were resedimented to deep marine areas with dominant deposition of pelagic limestones and cherts soon after their formation. This interpretation is confirmed by characteristic of the Syneruptively Resedimented Facies. It is supposed that studied accretionary lapilli originated in subaerial conditions and were primary deposited somewhere in the terrestrial environments where they could have been completely lithified. Once they became robust enough to withstand the transport they were resedimented by turbidity currents to deeper marine/pelagic basin. The depositional characteristics of the Flysch Facies - FF, Syneruptively Resedimented Pyroclastic Facies – SRP, and Slumped Limestone, Chert and Pyroclastic Facies - SLCP (horizontal- and cross-lamination and normal grading) implies possible resedimentation by turbidity currents.

Accretionary lapilli, Middle Triassic, External Dinarides, volcanism

nije evidentirano