On the Graptolite Schists Formation (Silurian–Lower Devonian) in the Carpatho–Balkanides of eastern Serbia

The fabric of the Silurian and a part of the Lower Devonian in the Carpatho–Balkanides of eastern Serbia, i.e. in two large geotectonic units, the Ku~aj Unit (Getic) and the Lu`nica Unit (West Kraishte), is reviewed in this paper. Silurian sediments of the Serbian part of the Carpatho–Balkanides are made of the Graptolite Schists Formation, from the lower part of the acuminatus Zone at the Ordovician–Silurian boundary to the transgrediens Zone at the Silurian–Devonian boundary. The presence of all parts of the Silurian is proved on the basis of graptolite faunas: Llandovery (Rhuddnanian, Aeronian, Telychian), Wenlock (Sheinwoodian, Homerian), Ludlow (Gorstian, Ludfordian) and Pridoli. Two Lower Devonian (Lochkovian) zones (uniformis and hercynicus), with which the deposition of the Graptolite Schists Formation ended in eastern Serbia is also described. Data on the geochemical characteristics of the Graptolite Schists Formation are presented as well.


Introduction
Research on Silurian sediments of eastern Serbia started in the middle of the last century with the finding of graptolites in the vicinity of Zvona~ka Banja in southeastern Serbia (ZAFIROV, 1942(ZAFIROV, , 1950) ) and between Garvanica and ^uka on the foothills of the Rtanj Mt. (PET-KOVI], 1954).Subsequently, VESELINOVI] (1959) and MIHAJLOVI] (1959,1961) published results of their research on Silurian graptolites in the vicinity of Zvona~ka Banja, while KRSTI] (l959, 1960, 1961) published results of his investigations of newly discovered localities of Silurian in the area of the Ku~aj Mts.During the next ten years, investigations were directed toward discovering as many localities with graptolite faunas as possible and the determination of the collected material.VESELI-NOVI] (1964, cum lit.) published the results of his researches on Lower Paleozoic facies of eastern Serbia and the determination of graptolite faunas which he collected himself.Results of research on the Silurian graptolites collected in the areas of the Zvona~ka Banja, the Rtanj Mt. and the Ku~aj Mts. were published by MIHAJLOVI] (1974, cum lit.), while those from the area of the Ku~aj Mts. were also published by KRSTI] (1984, cum. lit.).
At the beginning of the eighties, research on the Paleozoic sediments of eastern Serbia started within the scope of the Project A 2 of the Thematic Geologic Map 1:50 000.The research was directed toward the determination of the composition and genetic characteristics of sediments of the Caledonian-Hercynian sedimentation cycle (Ordovician-Lower Carboniferous), as well as to the reconstruction of geological columns of certain systems of beds.The column of the Silurian was already relatively well known then, due to the help of experts in graptolites, such as B. BOU^EK (Prague) and H. JAEGER (Berlin).They published results of their research particularly on the upper parts of the Graptolite Schists Formation in eastern Serbia, the Upper Ludlow, Pridoli and Lochkovian (BOU^EK et al., 1976;JAEGER in KRI@ et al., 1986).Beside graptolites, research was also aimed at Upper Ludlow conodonts (KRSTI] & SU-DAR, 1990) from the Silurian of the Ku~aj Mts.
Reconstruction of the depositional environments of the marine Paleozoic of the Ku~aj zone of the Hercynides of eastern Serbia (Getic) was published by KRS-TI] & MASLAREVI] (1990).

Ku~aj Unit
Silurian sediments have significant extension within the largest Alpine structural unit of the Carpatho-Balkanides of eastern Serbiathe Ku~aj Unit (Getic) (Fig. 1).They are found in the base of two large anticlines -the Ku~aj anticline in the north and the Kusa Vrana anticline in the southeast.They are represented by the Graptolite Schists Formation, beginning with the lower part of the acuminatus Zone at the Ordovician-Silurian boundary and ending with the Lower Devonian graptolite hercynicus Zone (Fig. 2).The sediments are folded in a system of isoclinal folds, in places broken in the hinges parallelly to the axial surfaces.After breakup, reverse movements of the limbs of the faults occurred and imbrication fabric of the Hercynian core was formed.For this reason, the column of Silurian sediments could have been reconstructed exclusively on the basis of graptolite faunas.

The Ordovician-Silurian boundary Llandovery
These sediments are located in the lower course on the river Bogovina, between Stra`a and Bogovinski Kamen, about 6 km WNW of Bogovina.They outcrop in the classical Silurian locality of the Ku~aj Mts.(KRS-TI], 1960; VESELINOVI] & KRSTI], 1970; KRSTI], 1984).In addition to the Upper Ordovician (Hirnantian), the same section exposes Lower Llandovery sediments in a length of tens of meters.The oldest stratigraphic unit is made of grey-green foliated to thin-bedded meta-shales (equivalent to the Cerecel beds of Bulgaria) in a visible thickness of about 30 meters.It is overlain by a set (about 8 m) of grey or 30-50 cm thick dark grey fine-to-medium-grained quartzose metasandstone beds, interstratified by thin metasiltstones.The sandstones exhibit gradation: medium-grained metasandstone-metasiltstone, and load casts with distinct lower surfaces.The metasandstones contain Upper Ordovician acritarchs: Lophosphaeridium citrinum, L. parverarum, L. cf.papilatum, Lophosphaeridium sp., Brochopsophosphera cf.uralica, Trachipsophosphaera sp., Leiosphaeridia sp.type C, Leiomarginata simplex, Priscogallea sp., ?Tylotopallia sp., Michrystridium palidum (ERCEGOVAC & \AJI], 1996).In the uppermost part of the metasandstone set of beds, there are fragments from deeper Ordovician levels: metasandstones, metasiltstones and metashales.The metasandstones are overlain by graphitic metapelites (0.5 m) bearing graptolite Glyptograptus sp. and graphitic metapelites and lydites of the acuminatus Zone (association: Climacograptus The set of sandstone beds underlying the graptolite schists of the acuminatus Zone is definitely at the top of the Upper Ordovician, Hirnantian, and it is correlatable with similar deposits of the Kosovo Formation of the Czechish Barandium (STORH, 1986) or the Sirman Formation of the Sofia Stara Planina in Bulgaria (SA-^ANSKI, 1993, 1994).
Little information is available about the depositional environments of the Upper Ordovician sediments.The Upper Ordovician is likely to be equivalent to glacial marine deposits.Evidently, a sudden change in material, i.e. rapid input of detrital materials, and probably a climate change occurred in the Upper Ordovician.
The Llandovery in the cross-section in the river Bogovinska Reka ends with the first zone of the middle part of the Llandovery (Aeronian) -the triangulatus Zone, with the following association: Climacograptus Cl.Younger zones of the Telychian are found on the section of the Silurian in the river Vejska Reka on the Ku~aj Mts., but they are not sufficiently studied.This part of the Upper Llandovery is made of grey thin-bedded metaclaystones with Spirograptus spiralis cf.contortus and Globosograptus singularis singularis, ac-cording to which the part of the column between the crispus and crenulata zones can be determined with a certain reserve.Several meters further, there are Retiolites geinitzianus angustudens and Demirastrites cf.pragensis, which indicate the presence of the griestoniensis Zone (KRSTI], 1966(KRSTI], , 1984)).
The upper part of the Lower Silurian -Telychian is well documented on the section of Ba~i{te in Zvona~ka Banja, from where MIHAJLOVI] (1974) proved the presence of the following zones of this part of the Silurian turriculatus, crispus, griestoniensis, crenulata, spiralis and grandis.

The Wenlock
Sediments of the Lower Wenlock (Sheinwoodian) are documented in the vicinity of Zvona~ka Banja and on the Ku~aj Mts.
Sediments of the Upper Wenlock (Homerian) were found on numerous localities on the Ku~aj Mts. and in the vicinity of Zvona~ka Banja.
In the area of the Ku~aj Mts., sediments of the Upper Wenlock are found in the river Bogovinska Reka (NW from Bogovinski Kamen), in the areas of the origin of the rivers Bogovinska Reka (east of Bro{}e) and Radovanska Reka, in Vejska Reka (spring Klencu{ki Potok), and in the river Kr~eva Reka.The following fossils are most often found: Cyrtograptus lundgreni lundgreni, C. hamatus, Monograptus flemingi flemingi, M. testis testis and Pristiograptus pseudodubius, which points to the Upper Wenlock -the lundgreni-testis Zone.In the area of Zvona~ka Banja, sediments of the Upper Wenlock were found on several localities.They are represented by platy metasiltstones and black graphitic metaclaystones with associations of graptolites of the lundgreni-testis and nassa zones, which have not been separated, even the fossiliferous level is about 6 m thick.MIHAJLOVI] (1974) cited the following association of graptolites from the cross-section of the Upper Wenlock in the river Blatanica, south of Zvona~ka Banja: Monograptus flemingi, M. subflexilis, M. testis, Monoclimacis flumendosae, Cyrtograptus hamatus, C. lundgreni, Pristiograptus ex gr.dubius, P. pseudodubius and Diversograptus gracilis, as well as Plectograptus praemacilentus, Monograptus testis inornatus, Cyrtograptus radians.They are replaced by an association with Gothograptus nassa, Monograptus flemingi, M. testis, Cyrtograptus hamatus, C. lundgreni.A similar association has also been found on Ba~i{te, in the springs Dupski Potok, [u{kovica, Jasenov Do, and in the spring Blatna Dolina.

The Ludlow
The Lower Ludlow (Gorstian) is present in all localities where sediments of the Upper Wenlock, which gradually transit into sediments of the Lower Ludlow, are documented.This part of the Ludlow is proved on the Ku~aj Mts. and in the vicinity of Zvona~ka Banja.It is represented by associations of graptolites of the nilssoni and scanicus zones, which have not been separated.This part of the Silurian is most completely developed in Zvona~ka Banja, from where MIHAJLOVI] (1974) cited the following association, from the valley of the river Blatanica: Monograptus uncinatus, Neodiversograptus nilssoni, Pristiograptus bohemicus, P. vulgaris, Colonograptus colonus, Saetograptus himmaera.There are also Globosograptus crinitus, Lobograptus scanicus, Neodiversograptus nilssoni, Pristiograptus bohemicus, P. butovicensis, P. dubius ludlovensis, Saetograptus himaera, Lobograptus scanicus, Linograptus posthumus tenuis, Pristiograptus bohemicus, P. dubius ludlovensis.A somewhat more varied association was found in the vicinity of Zvona~ka Banja, on the line of a former mine track, where, besides the above mentioned species, there are also: Plectograptus macilentus, Spinograptus spinosus, Monograptus uncinatus, Globosograptus crinitus, Lobograptus scanicus, Neodiversograptus nilssoni, Pristiograptus gotlandicus and others.
In the area of the Ku~aj Mts., at the well-known cross-section of Silurian sediments in the river Vejska Reka (spring Klencu{ki Potok), above the schists with graptolites of the nilssoni-scanicus Zone, there is about 60 cm of thick stratified limestones with rich association of conodonts, composed of the following multielements: Kockelella variabilis, Ozarkodina confluens, Ozarkodina excavata excavata, as well as of the "form species" forms: Polygnathoides siluricus, Ligonodina salopia, Lonchodina walliseri and Pseudooneotodus beckmanni.This association of conodonts comes from the siluricus Zone (KRSTI] & SUDAR, 1990), which is in the base of the Upper Ludlow (Ludfordian) -the equivalent of the graptolite leintwardensis Zone.
Only one, but very significant and guide zonal species -Neocuculograptus inexspectatus inexspectatus, which appears in association with Bohemograptus cf.tenuis and Pristiograptus sp.(BOU^EK et al., 1976), is described from the Upper Ludlow (Ludfordian) of Zvo-na~ka Banja.

The Pridoli
Sediments of the Pridoli have been proved only in the vicinity of Zvona~ka Banja (Zajednica i.e.Dupski Potok).This part of the Silurian is made up of black silty metaclaystones with anthracite, which contain an association of graptolites that form the parultimus Zone from the base of the Pridoli: Monograptus parultimus, M. fragmentalis, Formosograptus formosus, Pristiograptus cf.kolednikensis and Linograptus posthumus (BOU^EK et al., 1976;JAEGER in: KRI@ et al., 1986).

The Silurian-Devonian Boundary
The uppermost zone of the Pridoli -the transgrediens Zone was found on the hill Varni~e above Zvona~-ka Banja.The top of the Silurian is made up of black metaclaystones which contain Pristiograptus transgrediens (JAEGER in KRI@ et al., 1986).The Silurian-Devonian boundary is also present on this cross-section, because the transgrediens Zone is continually replaced by graptolite associations of the following, Lower Devonian (Lochkovian) uniformis Zone: Monograptus uni-M.uniformis angustidens, M. aequabilis aequabilis, M. microdon microdon, Linograptus posthumus posthumus, Abiesgraptus sp.(BOU^EK et al., 1976).Above the uniformis Zone, there is the last graptolite zone in eastern Serbia, marked with occurrence of the species Monograptus hercynicus (VESELINOVI], 1973), with which the deposition of the Graptolite Schists Formation ends (Akidograptus acuminatus -Monograptus hercynicus).After the hercynicus Zone, the conditions of sedimentation rapidly changed and sediments of a balanced basinal facies prograde upwards in an association of unbalanced preflysch sedimentation (lack of balance between sedimentation and deposition).Preflysch is represented by siliceous metaclaystones, laminated lydites and cherts, metasiltstones, dolomitic limestones, thick beds of sandstones and, occasionally by turbidites.These sediments are about 100 meters thick and they are located between the graptolite schists and the Upper Devonian-Lower Carboniferous flysch (Famennian-Viséan) (KRSTI] & MASLAREVI], 1990).

The Lu`nica Unit
The Silurian Formation of Graptolite Schists has also been found in the Lu`nica structural unit (West Kraishte), which is located west of the Ku~aj Unit (Getic) (Fig. 1).It should be emphasized that the Paleozoic sediments of the Lu`nica Unit were deposited in an independent sedimentary basin.This basin was located west of the Getic one from which it was separated by the Ozren-Baikalian paleostructure (land) (KRSTI], 1984).There was no sedimentation on this land during the Lower Paleozoic and, in the whole territory of eastern Serbia, over the Upper Proterozoic crystalline schists there are lacustrine sediments of the Upper Carboniferous or the Permian Red Sandstones.
Sediments of the Lower Paleozoic of the Lu`nica Unit are today in a zone of nappes.In most part of their extension, they make a tectonic footwall of Paleozoic schists of the Supragetic, which are metamorphosed under conditions of greenschist facies and thrusted over them.As they themselves were moving eastwards and thrusting over a large number of geological units of different age, the Lower Paleozoic sediments of the Lu`nica Unit are very much altered kinematically and significantly metamorphosed in places.

Geochemistry
The Graptolite Schists Formation are deep water sediments (LJ.MASLAREVI], unpublished data).DT and TG analyses proved a content of 3-4% of bituminous organic matter of coaly-anthracitic character, which originates from the graptolites.An increased content of uranium, from 7 to 16 g/t, was determined by chemical analyses.High V/Cr (12-20) and V/Mo (20-120) ratios, according to KUKAL (1971), represent a characteristic of deepwater sapropel.The content of Ba rises, however, with salinity, so the found quantity, according to this author, corresponds to the salinity 34-35%, while the low Cr/V (0.05-0.08) and Cr/Ni (0.7-0.8) ratios point to sedimentation far from the coast.The sediments originated by a calm and slow deposition in an environment poor in free oxygen.The sea bed was poorly fed and flows along the sea bed were weak so they occasionally moved just mud and organic matter for laminar sedimentation.

Fig. 2 .
Fig. 2. Correllation of the graptolite zones of eastern Serbia with the graptolite biozones according to HARLAND et al., 1989 and subdivision of the Silurian from the International Stratigraphic Chart of ICS(GRADSTEIN et al., 2004).