Stratigraphy of the Kr{ Gradac section (SW Serbia)

In the Kr{ Gradac section (near to Sjenica, SW Serbia), a transition of a carbonate platform to basin facies are outcropped: Norian-lower Liassic shallow-water carbonates, middle Liassic–lower Dogger Ammonitico Rosso facies, and upper Bathonian into lowermost Cretaceous deep-water radiolarites in which the carbonate graded bed and mass flow layer are intercalated. The presence of a lower Dogger condensed sequence with the Bajocian protoglobigerinid event was hitherto not evidenced. It is documented that components of a graded bed are of extrabasinal (upper Triassic–lower Tithonian carbonate platform sediments) and intrabasinal (radiolarite, meta-andesite) origin, indicating a tectonic event not older than the early Tithonian. This tectonic event caused the fracturing of the carbonate platform, also partly basinal area. Consequently, the age of the graded bed is not older than the lower Tithonian. In the uppermost radiolaritic sediments in the Kr{ Gradac section (?middle–upper Tithonian–lowermost Cretaceous), a mass flow layer appears, which contains clasts of intrabasinal origin – different radiolarites, siliceous radiolarian argillites (some of which are unconsolidated with washed radiolarians and sponge spicules in a ferruginous sediment), sandstone grains, etc. The mass flow event is estimated as Berriasian. In the Kr{ Gradac radiolarite succession, the authors recognized two deep-water formations, an older one, upper Bathonian–lower Tithonian, between hardground (Dogger) and a graded bed, and a younger formation, which started with a graded bed. This formation, according to its stratigraphic position, corresponds to ?middle–upper Tithonian–lowermost Cretaceous.


Introduction
The Kr{ Gradac section is well known in the geological literature of the SW part of Serbia.It, as one of the most well-exposed and interesting sections belonging to the older Mesozoic of the Dinarids of Serbia and has inspired geologists for many years, particularly because of the age determination of the traditional, known Diabase-Chert Formation of the Balkan Peninsula area.
Except two papers of HAMMER (1921), and PETKOVI] (1934) in which limestones from this locality were considered as Triasssic in age, in the other numerous papers (ALBRECHT 1925;KOSSMAT 1924;LEDEBUR 1941 al. 1998; etc.) different ages, based on ammonite and brachiopod fauna and microfossil associations, from upper Triassic to upper Liassic were assigned to these sediments.For the Liassic part of this section, the unformal name Kr{ pod Gradcem Formation was proposed (LJUBOVI]-OBRADOVI] et al. 1998;RADOVANOVI] et al. 2004).
During last 20 years, the radiolarian fauna from radiolarites and different siliceous rocks belonging to the upper part of Kr{ Gradac section was studied (DJERI] 2002;VISHNEVSKAYA et al. 2009;GAWLICK et al. 2009;unpublished data of [. GORI^AN, L. DOSZTÁLY).According to the results of these studied, the age of Kr{ Gradac radiolarites was documented by different radiolarian assemblages as being from the upper Bathonian to the lower Tithonian.
The aim of this paper is to present the stratigraphy of the Kr{ Gradac section in regards to: a) the condensed limestone sequence (the lowermost part of the Dogger) for which no published data exists; b) the composition of the graded carbonate bed (which is not older than middle-upper Tithonian) and its significance and c) the presence of a mass flow layer of assumed Berriasian age.The paper is based on new investigations of the authors, including data of R. RADOI^I], sampled in 1968, from the carbonate part of section, which is still lacking.In this manner, presenting data, especially those dealing with the condensed sequence, gives a more complex access to fill the lack of the stratigraphy of the area.The carbonate part of the Kr{ Gradac section was devastated during work on the road to the Jadovnik Mt.The destruction of the siliciclastics has continued to date because of stone exploitation for the construction of roads.

Geological setting
According to the last published geological map (Sheet Prijepolje 2, 1:50 000, RADOVANOVI] et al. 2004), ophiolite mélange in the western and northwestern region of Sjenica is widely distributed.In the mélange are em-bedded blocks, olistoliths and slides of carbonate rocks, gabbros, pillow lavas, ultramafics, as well as some exotic granite.According to GAWLICK et al. 2009, one of these sedimentary bodies, below the Middle Jurassic mélange, is the Kr{ Gradac tectonic slice (carbonates and radiolarites).

Kr{ Gradac section
The Kr{ Gradac section (Fig. 1; coordinates: x 4793454, y 7416424), is located on the western side of the road Sjenica-Nova Varo{, on the SW slope of the Gradac Hill.Generally, the geological column of this section consists of carbonatic and siliciclastic parts which are in tectonic contact with the mélange.
The oldest are massive limestones of upper Triassic age of the Dachstein type and lower Liassic shallowwater carbonates, then Middle Liassic-Lower Dogger Ammonitico Rosso and Bositra-protoglobigerinid facies which ends with hardground.The succession continues into middle Jurassic-lowermost Cretaceous radiolarites in which the middle part is intercalated with the graded bed and the mass flow layer in the upper part.
In this paper, in the stratigraphic column of the Kr{ Gradac section, only the middle Liassic-lowermost Cretaceous sediments, which are in five separated units (Fig. 2, units A-E) were studied.

Unit A
The unit A, 12.3 m thick, is of Middle and Upper Liassic limestones of Ammonitico Rosso type facies.
In the lower part of unit A (facies with Involutina liassica), a little known brachiopod of the genus Koninckella is found (Pl. 6,Figs. 11,12).
The unit ends with a 0.

Unit B
The unit B is represented by a 1.2 m thick, condensed red limestones sequence made up of: -0.4 m reddish wackestone with scarce biota; -0.3-0.4 m reddish wackestone with rare fossils; -0.35 m, wackestone with rare ammonites and brachiopod embryos, mollusk fragments and minute echinoderm grains (Pl. 1, Fig. 1).This bed in the upper part passes into dark red, ferruginous pervaded wackestone with filaments debris (Pl. 1, Fig. 2), followed by thin hardgound; -0.01-0.02m dark red, ferruginous pervaded wackestone with a few ammonite embryos and rare filaments; -0.02-0.06m pink limestones in which is clearly visible a minor discontinuity between (a) slightly deformed and altered wackestone with a rough surface containing irregularly dispersed Bositra fragments (Pl.1,Figs. 3a,4a), and overlying (b) packstone with mm laminae of more or less accumulated Bositra filaments or fragments, some ammonite embryos, a few microgastropods, numerous protoglobigerinids and rare minute Spirillina and Ophthalmidium (Pl.1,Figs. 3b,4b,[5][6][7]; -0.05-0.1 m, a very ferruginous dark red sediment with hardground.The age of the condensed sequence is lower Dogger.Limestones, 1 m thick, between middle-?upper Toarcian and the Bajocian protoglobigerinids event, according to the stratigraphic position correspond to the Aalenian.The latest, uppermost part of the condensed sequence, a very ferruginous dark red sediment with hardground, which could be ?uppermostBajocian-lower Bathonian, because it is overlain by upper Bathonian-Oxfordian radiolarites (VISHNEVSKAYA et al. 2009).

Unit C
Unit C, 9.5-10 m thick, is composed of red, green and dark radiolarites and cherts with intercalations of red radiolarian shales (Fig. 3).According to the available data based on radiolarian assemblages and UAZs (VISHNEVSKAYA et al. 2009) the age of this unit is upper Bathonian-Oxfordian.

Unit D
Unit D is represented by a 1.2 m thick graded carbonate bed (which laterally became thinner), intercalated in red radiolarite (Fig. 4a,Pl. 2,Pl. 3,.The size of the grains vary from 1-10 mm, and decrease up into 0.02-0.06mm in fine-grained, well-sorted calcarenite (Fig. 4b).Between the grains is rare sparite, at place ferruginous.The components of bed are mostly different shallow water carbonates, less present are argillites, siliceous argillites, with more or less frequent radiolarians (some filled with chlorite), radiolaritic micrites, cherts, spongolites, etc., and grains of magmatic rocks, such as meta-andesite.Quartz grains are rare.
The uppermost part of the graded bed is calcarenite with sponge spicules (Pl.3, Fig. 4).
The unit is not older than lower Tithonian, probably ?middle-upperTithonian.
Upward the 0.07-0.2m thick, loosely packed ferruginous mass flow layer occurs (Fig. 5; Pl. 4,, made predominantly of grains of dif- The studied stratigraphic column ends with 1.5 m thick argillitic and ferruginous cherts, radiolaritic cherts and radiolarites.Very interesting are radiolarians filled with chlorite or a bed disturbed in a semi-consolidated condition with rare grains of radiolaritic cherts, sand-stones, carbonized radiolarites and radiolaritic cherts (Pl.5,.
According to its stratigraphic position, unit E is not older than ?middle-upperTithonian into the lowermost Cretaceous.The mass flow layer, presumed the consequence of intrabasinal activity, can be estimated as Berriasian.
The uppermost part of the radiolaritic sequence of unit E is followed by tectonite (mélange) in which cm-dm blocks of meta-andesite (sample MS 2079) are present.

Discussion
The lower part of the red condensed limestone sequence (unit B), between the middle (?partly upper) Toarcian and Bajocian protoglobigerinid layer, correspond to the ?latestToarcian-Aalenian time interval.There is no discussion about to which part of the Bajocian the protoglobigerinid event could be ascribed.The top of the condensed sequence, i.e., dark red sediments with hardground, partly ?Bathonian, is overlain by a red argillite-radiolarite succession.In this lower part of the sequence, the radiolarian assemblage of upper Bathonian to lower Callovian (co-existence of Pterotrabs marculus and C. carpathica ) and the middle Callovian to Oxfordian (UAZz 8-9 with Archaeodictyomitra minoensis, E. unumaense sl. and Z. ovum) are documented (VISHNEVSKAYA et al., 2009).
The components of the graded bed (unit D), according to data from this paper, are of extrabasinal and intrabasinal origin.The extrabasinal grains (upper Triassic-  -lower Tithonian) indicate deposition in carbonate platform/ramp environments (presently, not known in situ in large adjacent Dinaridic area).Subordinate are grains of intrabasinal origin, such as siliceous argillites, with or without radiolarians, radiolarites (some of them are ?Triassic), radiolarian micrites, cherts and magmatic rocks.
The mass flow layer, which occurs in unit E, contains different fragments of deep basinal sediments, including those of unconsolidated radiolarite.This indicates a ?latestTithonian-Berriasian event which caused the destruction of a part of the deep basin sequence.The mass flow event is considered as Berriasian.
In the radiolarites from the lowermost parts of the unit E according to the former investigations (VISHNEV-SKAYA et al. 2009) the youngest radiolarians of middle Oxfordian to early Tithonian age (UAZs 9-11 with the species A. minoensis, Z. ovum and T. brevicostatum) are documented.
The Kr{ Gradac section represents a typical transitional succession from a carbonate platform into a basin: the uppermost Triassic platform of Dachstein type is followed by Lower Liassic shallow-water carbonates.Furthermore, from the middle Liassic to Bajocian, an Ammonitico Rosso facies of a drowned platform and Bajocian Bositra-protoglobigerinid limestones were sedimented, which ends in a dark red sediment and hardground.Intensive basin deepening is characterized by sedimentation of deep basinal radiolarites through the late Bathonian into the earliest Cretaceous.The slow basin sedimentation was interrupted during the ?middle-upperTithonian (carbonate graded bed), as a consequence of a tectonic event, i.e., the fracture of the carbonate platform and also the adjacent area of the basin.This important tectonic event can not be older than ?latestKimmeridgian-early Tithonian; consequently, unit E can not be older than the ?middle-upperTithonian.
Siliciclastics over limestones in the upper part of Kr{ Gradac succession was considered mostly to be a part of the Diabase-Chert Formation, i.e., volcanogeno-sedimentary series or an ophiolitic complex of different ages: According TO RAMPNOUX (1974), the breccia intercalations inside the siliciclastics (= in this text graded bed, unit D and mass flow layer in unit E) contain Liassic-Portlandian biota.Therefore, RAMPNOUX (op.cit., p. 46) concluded that an important stratigraphic gap existed between the middle Toarcian with hardground and the volcanogeno-sedimentary formation, which was dated "au moins du Malm supérieur".Consequently, the siliciclastics below the breccia intercalations, has also been ascribed to the upper Malm.Between the underlying limestones and the Diabase-Chert Formation, ]I-RI] (1984,1996) observed certain discordances, which indicate some tectogenetic movements on the boundary between the Liassic-Dogger, i.e., to the influence of the late Kimmerian phase in the Dinarides.RADOVANOVI] et al. (2004) assigned the siliciclastics to one unformal Zlatar Formation of upper Triassic-Tithonian age.GAWLICK et al. (2009) treated them as a Middle to Upper Jurassic/?Lower Cretaceous part of the Upper Triassic-?Lower Cretaceous tectonic slice below the radiolaritic-ophiolitic mélange.Besides, the whole complex succession of Kr{ Gradac is interpreted as "a tectonic window or as a tectonically incorporated sliver scraped off the footwall due to younger tectonic shortening " (op. cit., p. 299).
The authors of this paper, also did not include the mentioned siliciclastites into tectonite (mélange) and inside of them recognized two deep-water argillite-radiolaritic formations: an older, upper Bathonian-lower Tithonian (below the graded bed) and a younger, ?middle--upperTithonianlowermost Cretaceous, which commenced with the graded bed.

Suplementary note (R. RADOI^I])
The distribution of the Liassic sediments of the proximal basinal facies (limestones with Involutina farinacciae) in the eastern Zlatibor Mt. (Dre`nik) and Sjenica area (Vrelo) is a fact that should be mentioned.

Fig. 3 .
Fig. 3. Siliciclastics of the western part of the Kr{ Gradac section.The arrows show the position of the graded bed (unit D).
Fig. 4. A, Detailed view of the different units (C, D, E) from the Kr{ Gradac section; B, Polished sample from the basal part of the graded bed (horizontal section).