Naslov
Lithofacies and Conodont Based Chronostratigraphy of the Lower Triassic Shallow Marine Succession in the Gorski Kotar region, Croatia

Autori
Aljinović, Dunja ; Kolar-Jurkovšek, Tea ; Jurkovšek, Bogdan

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Albertiana 33, International Symposium on Triassic Chronostratigraphy and Biotic Recovery Part I: Program and Abstracts / Kürschner, M. Wolfram - Chaohu, 2005, 13-14

Skup
International Symposium on Triassic Chronostratigraphy and Biotic Recovery

Mjesto i datum
Chaohu, Kina, 2005

Vrsta sudjelovanja
Poster

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
Lithofacies; Conodont Chronostratigraphy; shallow marine deposits; Lower Triassic; Gorski Kotar; Croatia

Sažetak
The investigated area of the Lower Triassic sedimentary complex in the Gorski Kotar region – Croatia, is located in the External Dinarides between the Alps and the Velebit Mt. The Lower Triassic depositional environment is envisaged as shallow marine realm of a passive continental margin. Sedimentary complex differentiates in predominantly carbonate sedimentation that characterise the beginning of deposition with upward increasing trend of terrigeneous influx. Five Lower Triassic sections (Homer, Skolski Brijeg, Zelin Crnoluski, Kramarcin Potok and Dobra) have been investigated. They were lithofacially defined, but their chronostratigraphical position was uncertain due to lack of fossils or their inadequate preservation. A facial assemblage investigated in central part of Gorski Kotar region (Homer, Skolski Brijeg and Zelin Crnoluski sections) differs from those appeared in marginal parts (Kramarcin Potok and Dobra sections). In central part the shallow water carbonate facies predominate and reflect the shallow marine sedimentation of the stable rimmed shelf. Basal ca. 10 m thick cross-bedded interval of ooid grainstones (Homer and Skolski Brijeg sections) was interpreted as ooid bars that possibly formed subaqueous oolithic barrier with the landward laying lagoon. Thus the overlaying predominantly thin or medium-bedded grey dolomicrites, sandy dolomites and calcarenaceous sandstones were interpreted as lagoonal sediments. Storms and post-storm tidal reworked processes influenced the deposition of ooid bars. Ooid grainstones were late diagenetically dolomitised, while the lagoon facies reflects the early diagenetic processes. The widening of the lagoon has been conceived through overall transgressive trend. The rapid transgression caused in-place drowning of the ooid bars and formation of a new seaward oriented coast that is represented by facies (dolomites, sandy dolomites and rarely siltites) organized in typical storm sequences of Crnoluski Zelin section deposited near fair weather wave base of unrestricted shelf. The intense influx of a dominantly red siliciclastic terrigenous material has been recorded in the Lower Triassic sediments of the marginal, western and eastern parts of Gorski Kotar region (Kramarcin Potok and Dobra sections). The thick-bedded ooid barrier bars as well as overlaying lagoon dolomicrites and sandy dolomites are missing in those sections. Vertical facies successions are characterised by the interbedded oolithic limestone/dolostone, red siltite and sandstone beds with occasional appearance of flat pebble conglomerates. Beds vary in thickness from few decimetres up to 1 m and are organised in fining upward sequences that represent shifting of a laterally existed facies deposited in ooid/sandy shoals and restricted muddy bays. The lithofacies present in Kramarcin Potok and Dobra sections resemble red clastic sediments i. e. Seiser beds of the Southern Alps that make them compatible to the wide area of the External Dinarides. The different lateral facies distribution was interpreted as the result of transgression that transformed narrow rimmed shelf with ooid bars and lagoon facies to unrestricted shelf and finally to the wide epicontinental sea with its ooid/sandy shoals and restricted muddy bays. This interpretation has never been proved while the precise chronostratigraphic dating of the investigated sections were missing in previous investigations. Therefore the lithofacial investigations were supplemented by conodont studies at each of five localities. Twenty-five rock samples, with minimum weight of 3.5 kg were investigated by means of conodonts, but only fourteen samples proved to yield conodont elements. In general, the recovered conodonts are rare, except for the Pachycladina elements, but their preservation is commonly fragmented. The CAI (Color Alteration Index) of the conodonts is above 5, implying post-depositional temperatures varying between temperature ranges 300-480oC and 490-720oC (Epstein et al. 1977, Rejebian et al. 1987). The oldest investigated strata are marked by the presence of Hindeodus parvus in the Skolski Brijeg section. The first appearance datum (FAD) of this taxon has been approved to define the base of the Triassic system (Yin et al. 2001). The biostratigraphical data allow recognition of the parvus-isarcicella Zone, obliqua Zone and Platyvillosus Subzone. The conodont zonation and lithofacial definition contribute to the definition of the Lower Triassic depositional realm of the External Dinarides and prove the correlative elements for comparison with some other parts of the Western Tethys. Reference list: Epstein A.G., Epstein, J.B. & Harris, L.D. (1977) - Conodont Alteration Index - and Index to Organic Metamorphism. Geol. Surv. Prof. Pap., v. 995, 27, pp., Washington. Rejebian V.A., Harris A.G. & Huebner J.S. (1987) - Conodont color and textural alteration: An index to regional metamorphism, contact metamorphism and hydrothermal alteration. Geol. Soc. Am. Bull., v. 99, pp. 471-497, Washington. Yin H., Zhang K., Tong J., Yang Z. & Wu S. (2001) - The Global Stratotype Section and Point (GSSP) of the Permian-Triassic Boundary. Episodes, v. 24, pp. 102-114.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

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