GEOLOGIJA 46/2, 225–230, Ljubljana 2003
Triassic beds in the basement of the Adriatic-Dinaric carbonate platform of Mt. Svilaja (Croatia)
Triasne plasti v podlagi Jadransko-dinarske karbonatne platforme na planini
Svilaja (Hrva{ka)
Vladimir JELASKA1, Tea KOLAR-JURKOV[EK2, Bogdan JURKOV[EK2 & Ivan GU[I]1
1Department of Geology, Faculty of Science, University of Zagreb, HR-10000, PO Box 223,
Zvonimirova 8, Croatia; sibila@hazu.hr, ivangusic@yahoo.com
2Geological Survey of Slovenia, Dimi~eva 14, SI-1001 Ljubljana, Slovenia; tea.kolar@geo-zs.si,
bogdan.jurkovsek@geo-zs.si
Key words: External Dinarides, basement of the Adriatic-Dinaric carbonate platform, Triassic, conodont biostratigraphy, Dalmatia, Croatia
Klju~ne besede: Zunanji Dinaridi, podlaga Jadransko-dinarske karbonatne platforme, trias, konodontna biostratigrafija, Dalmacija, Hrva{ka
Abstract
On the southwestern slope of Mt. Svilaja a Triassic sequence is exposed. It is composed of Lower Triassic carbonate siliciclastic shelf beds that are unconformably overlain by Anisian breccia. The overlying pelagic Anisian and Ladinian strata with pyroclastic rocks can be interpreted as a result of rift tectonics of Adria micro-plate. A 500 m thick sequence is capped by an emersion surface by karstification and terrigenous sediments including conglomerate as a result of Late Triassic transgression and marking the lower boundary of a new, Late Mesozoic, megasequence of the External Dinarides.
By means of conodont study, in the lowermost part of the studied Mesozoic sequence, a Lower Triassic shallow-water conodont fauna yielding Pachycladina obliqua apparatus was recorded. Pelagic limestone beds overlying the »Otarnik breccia« are marked by conodont elements of the Anisian constricta Zone. The Ladinian interval is characterized (from bottom to top): trammeri A. Z., hungaricus A. Z. and mungoensis A. Z. The uppermost part of the section below the emersion surface is identified by Pseudofurnishius murchianus, indicating the Upper Ladinian-Lower Carnian murchianus Zone.
Kratka vsebina
Na jugovzhodnih obronkih planine Svilaja je razvit profil spodnjetriasnih karbonatno siliciklasti~nih {elfnih plasti, na katerih diskordantno le‘i anizijska »Otarni~ka bre~a«. Nad njo si sledijo pelagi~ne anizijske in ladinijske plasti s piroklastiti, ki pri~ajo o riftni tektoniki Jadranske mikroplo{~e. 500 m debel profil zaklju~uje z emerzijo, za katero so zna~ilni pojavi zakrasevanja in terigenih sedimentov, vklju~no s konglomeratom, ki je rezultat zgornjetriasne transgresije in predstavlja spodnjo mejo nove mladomezozojske megasekvence Zunanjih Dinaridov.
S konodontnimi raziskavami je bila v najni‘jem delu raziskanega mezozojskega zaporedja plasti ugotovljena plitvovodna spodnjetriasna konodontna zdru‘ba z vrsto Pachycla-dina obliqua. Pelagi~ne apnen~eve plasti nad »Otarni~ko bre~o« vsebujejo konodontne elemente anizijske cone constricta. V ladinijskem intervalu si od spodaj navzgor sledijo konodontne zdru‘be: trammeri A. Z., hungaricus A. Z. in mungoensis A. Z. Najvi{ji del profila pod emerzijsko mejo vsebuje vrsto Pseudofurnishius murchianus, ki ga uvr{~amo v zgornjeladinijsko–spodnjekarnijsko cono murchianus.
226
Vladimir Jelaska, Tea Kolar-Jurkov{ek, Bogdan Jurkov{ek & Ivan Gu{i}
Fig. 1. Location sketch map of the studied Triassic section. Sl. 1. Geografska skica raziskanega triasnega profila.
Geological setting
The Upper Permian shallow marine carbonates of the External Dinarides are gradually replaced with thick succession consisting of Scythian shelf siliciclastics and carbonates. During the Early Triassic, sedimentation was characterized by strong terrigenous influx. Such conditions characterize depositional processes on wide passive continental margins, which are, depending on the predominant influence, dominated by either shelf siliciclastic or carbonate deposition.
Foothills of Mt. Svilaja (Mu} area) is a well known locality of the Lower Triassic, described since the Hauer geologic map (1868). Kittl’s (1903) monograph comprised description of ammonoids collected from the Upper Scythian of the Mu} area. K e r -
n e r (1916) mapped the area under consideration. Within Scythian deposits, he distinguished Seisian and Campilian beds. The carbonate breccias, so-called “Otarnik breccia” Kerner also ascribed to the Scythian. As distinct from that, Ivanovi} et al. (1978), considered that “Otarnik breccia” belongs to the Anisian. Krystyn (1974) recognized 3 horizons with ammonoids in the Upper Scythian (he took samples near Miji}i village (east of Mu}). Herak et al. (1983) proposed the Lower Triassic of Mu} as a standard stratigraphic section of the Upper Scythian. [}avni~ar et al. (1984) carried out sedimentological and petrographic examinations of the Lower Triassic of the Zmijavac Valley (behind of Mu}).
The Lower Triassic deposits of External Dinarides usually can be divided into two lithostratigraphic units: (1) The Lower unit
Triassic beds in the basement of the Adriatic-Dinaric carbonate platform of Mt. Svilaja ...             227
– composed of reddish and violet micaceous clastics (this unit corresponds to Seiser beds of the Southern Alps, and (2) The Upper unit – consisting of grayish to brownish limestones and marls, and corresponding to the so-called Campilian beds.
The thickness of the entire Lower Trias-sic deposits of Mu} area is up to 300 m.
(1)  The Lower unit consists of thin bedded, calcareous and micaceous, fine-grained sandstones, siltstones and silty or calcareous pelites. Approximately in the middle part of the Lower unit, there is an interval composed of somewhat thicker bedded grainstones and mudstones (the former consist of voids, ooids, skeletal remains and intraclasts). The top part of the Lower unit contains predominantly terrigenous rocks again. The most common cement is calcite. In the whole unit, various structures can be observed, but intraformational deformed structures prevailed.
(2)  The Upper unit consists of constantly alternating skeletal grainstones, silty and/or argillaceous wackestones to packstones, and marls. Silty admixtures are sporadically present. Thin bedding prevails (up to 3 dm), with the increase of the clayey component the bedding becomes thinner (up to few cm). The upper bedding planes are, as a general rule, flat, whereas the lower bedding planes of grainstones have sharp contact. These fos-siliferous grainstones are marked by gradation resembling the so-called allodapic limestone. Deformation structures are not frequent, but various bioturbations are abundant.
The Middle Triassic of the External Dinarides mostly consists of shallow marine carbonate successions. This widespread environment could be characterized as an epeiric sea and therefore the carbonates could be attributed to a phase of “attached” (or shelf) carbonate platform deposits.
The boundary between the Scythian “upper unit” and the Middle Triassic “Otarnik” unit of the Mu} area is marked by a local disconformity (Zelovo locality). The “Otar-nik” intraformational carbonate breccias and mud-pebble conglomerate laterally pass into the so-called “Diplopora Limestone” (or “Klimenta Limestone” according to K e r -n e r , 1916).
Zelovo is a well known locality of the Middle   Triassic   outcrops   of   pyroclastic
rocks. Vitrocrystalloclastic tuffs (“pietra verde”) are accompanied with platy pelagic limestones, with or without cherts, whereas sandstones and shales are less abundant. Limestones contain abundant bioclasts, which originated from the adjacent areas. This py-roclastic rocks can be interpreted as a result of rift tectonics (Jelaska, 2003) and accompanying basalt magmatism ([}avni-~ a r et al., 1984; Belak , 2000).
The above described “pietra verde” interval contains bioclastic limestones characterized by an abundance of calcareous algal skeletons, but also particles of corals, mol-lusks, and brahiopods (Jaecks et al., 2003). This limestones represents the termination of the Middle Triassic at the foothills of Mt. Svilaja. The topmost surface is intensely karstificated during an emergence phase. On the karstified surface there are some occurrences of bauxites and terrigenous materials, including conglomerates due to the Late Triassic transgression.
Conodont fauna
Rock samples, with minimum weight of 2.5 kg, from the Svilaja section were collected and treated for conodonts. A standard conodont technique was used, followed gravity enrichment. Only 16 out of 40 samples proved to yield conodont elements. The conodont material is catalogued and deposited at Geolo{ki zavod Slovenije / Geological Survey of Slovenia, under the designations GeoZS 3436-3446, 3454-3476, 3483-3487, 3498.
The most important characteristics are summarized below, in a stratigraphic order.
Lower Triassic
Samples SSP 1/1–1/4: obliqua Zone. Most numerous are elements of the Pachycladina obliqua Staesche apparatus. This zone is also characterized by rare occurrence of Hadro-dontina sp. (biserialis type) and Parachirog-nathus ethingtoni Clark. The examined con-odont fauna contains characteristic Lower Triassic shallow-water elements. It is comparable with the assemblages obtained from some Slovenian localities (K olar-Jurkov -{ e k , 1990a; Kolar-Jurkov{ek & Jur-k o v { e k , 1995, 1996; Jurkov{ek   et al.,
228
Vladimir Jelaska, Tea Kolar-Jurkov{ek, Bogdan Jurkov{ek & Ivan Gu{i}
Fig. 2. Stratigraphic succession of Triassic beds with conodont zonation in the basement of Adriatic-Dinaric carbonate platform of Mt. Svilaja.
Sl. 2. Stratigrafsko zaporedje triasnih plasti s konodontno conacijo v podlagi Jadransko–dinarske
karbonatne platforme na planini Svilaja.
Triassic beds in the basement of the Adriatic-Dinaric carbonate platform of Mt. Svilaja ...             229
1999). P. obliqua is a geographically widely distributed species, ranging, according to Perri and Andraghetti (1987), from the upper Nammalian to the Spathian. As reported by Solien (1979), in the Thaynes Formation of Utah, Pachycladina and Hadrodontina characterize the Smithian.
Anisian
The Anisian–Ladinian boundary is marked by the first appearance of P. trammeri. The definition of this boundary was proposed by K r y s t y n (1983) and it coincides with the boundary between the Parakelnerites and Nevadites Zones. As this boundary is easily applicable in conodont biostratigraphy, it has been accepted later by some authors (N i c o r a & K o v a c s 1984, K o v a c s et al. 1990). It should be noted that in the last decade, Middle Triassic strata have been the object of intensive multidisciplinary studies; therefore the base of the Ladinian stage is the main topic of several debates (B r a c k et al, 2003; V ö r ö s et al., 2003; M i e t t o et al., 2003).
Samples SSP 4/1, 4/2: constricta Zone is provided by the occurrence of the nominate species that is accompanied by Paragondo-lella excelsa Mosher group and Neogondo-lella aff. excentrica Budurov & Stefa-nov.
Ladinian
Conodont zonation of the Ladinian part of the section corresponds well with the results obtained from the pelagic sequence of Epidaurus, Greece by Krystyn (1983). Accordingly, the recognized Ladinian zones of Svilaja were stated based on the first appearance of the nominate species: Paragon-dolella trammeri (Kozur), Budurovignathus hungaricus (Kozur & Vegh) and Budurovig-nathus mungoensis (Diebel).
Samples SSP 4/3, 4/4: trammeri A. Z. – Neogondolella constricta (Mosher & Clark) is still present, but the first appearance of P. trammeri is recorded in the sequence.
Samples SSP 4/5, 4/6, 4/6, 4/7, 4/8, 4/9, 4p: hungaricus A. Z. Specimens of B. hunga-ricus were obtained starting with the sample SSP 4/5, along with P. trammeri, and rare exemplars of Paragondolella alpina (Kozur & Mostler) group.
Samples 5/9, 5/8: mungoensis A. Z. is marked by the first appearance of B. mungo-ensis, that is accompanied by B. hungaricus and P. trammeri.
Upper Ladinian–Lower Carnian Sample 5/5: murchianus Zone. The youngest conodont zone is indicated by the presence of a single element of Pseudofurnishius murchianus Van den Boogaard. The genus Pseudofurnishius is very typical element of the Southern Tethys and its marginal seas (Kozur , 1993). The most characteristic Upper Ladinian to Lower Carnian species, P. murchianus, is the marker of the murchianus Zone (Kolar-Jurkov{ek, 1990b).
The uppermost part of the sequence measured and sampled was barren of conodonts, but it yielded abundant holothurian fauna (Theelia sp.). A detailed study of the con-odont fauna, as well as other microfossils, is in progress and will be presented in a forthcoming paper.
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