The Albian – Cenomanian Kotroman Formation of Mokra Gora ( western Serbia )

Cretaceous deposits in the wider area of Mokra Gora village (western Serbia) were studied previously during basic, thematic mapping and detailed metallogenic investigations. These former studies neither distinguished the clearly defined formations, nor defined the lower rank lithostratigraphic units. The oldest Cretaceous formation in the area of Mokra Gora, the “Basal series”, lies above peridotite and serpentinite and below the “Hemipelagic series” (RADOI^I] 1995, and references therein). This study preliminary defines the “Basal series” as a formation and proposes the lithostratigraphic term “Kotroman Formation”. The stratigraphic column of the Kotroman Formation consists of three separate members of lower rank. The ferruginous sandstone and conglomerate of the Kami{na Member occupy the lower part, nodular bedded marly limestone of the Uro{evi}i Member are in the middle part and thin bedded bioclastic limestone of the Jatare Member made up the upper part of the formation. Fossils are represented by sporadic assemblages of mollusks, foraminifers, algae, brackish water charophytes and ostracodes, which indicate an Albian–Cenomanian age of the formation.


Introduction
Cretaceous sediments have a limited extent in western and south-western Serbia.However, in the wider area of Mokra Gora, there exist well defined Creta-ceous exposures, which have been studied by numerous authors, e.g., MILOVANOVI] (1933, cum. lit.), PEJO-VI] & RADOI^I] (1971), RADOI^I] (1995), BANJAC (1994) and DULI] (2003).Some detailed mapping of the formations and lower rank units was made during The Albian-Cenomanian Kotroman Formation of Mokra Gora (western Serbia) Exposures at Vardi{te in Bosnia (5 km southwestward from Mokra Gora, Fig. 1), with rocks similar to Basal Series, were object of detailed lithological investigations by BORTOLOTTI et al. (1971).Their level 1 and 2 could be correlated with lower part of Kotroman Formation, while their level 3 (with abundant fossil associations), could be correlated with upper part of Kotroman Formation, or with Jatare member as lateral equivalent.On the contrary, the authors refer to Upper Jurassic age of these rocks.

Distribution and main features of the Kotroman Formation ("Basal Series")
The "Basal Series" is best exposed in the close vicinity of the Kotroman village (43°46 '3.81'' N, 19°28'22.05'' E), which can be regarded as the type locality (Fig. 1).We propose and define the new lithostratigraphic term "Kotroman Formation" for the former "Basal Series" of JOVANOVI] et al. (2004).The aforementioned locality by the Kotroman village is adopted as the type section of the Kotroman Formation.Other localities are distributed on the surrounding of Mokra Gora, the Rzav syncline, and include: the Valley of the Beli Rzav River and Kr{anje, Vardi{te, Me|edova Ljeska and Jezdimiro-vi}i villages.Some limited exposures are present at a larger distance, i.e., Jago{tica, Pozder~i} and Ljuto Polje hamlets.
In the western part of the study area, in the Kotroman and Uro{evi}i localities, the Kotroman Formation directly overlies serpentinite, while in the eastern part of the study area it is located over a few meters thick weathering crust.It is of nontronite type with nickel concentrated in a smectite zone.There is relative enrichment of Fe 2 O 3 and Al 2 O 3 and some trace elements in the uppermost part of the crust MAKSI-MOVI] (1996).The Kotroman Formation is overlain by the Hemipelagic Series (Formation).
The Kotroman Formation is about 50 m thick and consists of a clastic sequence in the lower part and limestone beds in the upper part of the stratigraphic column.The lower limit is a sharp transgressive boundary with serpentinite or a few meters thick weathering crust, while upper limit is a gradual transition to the Hemipelagic Series.
Three separate members can be distinguished: The Kami{na Member, the Uro{evi}i Member and the Jatare Member, even though the last one can be consider as lateral equivalent of Kotroman Formation.
The Kami{na Member is represented by a clastic sequence consisting of reddish conglomerates and thin- bedded oolitic iron-rich sandstones with cherty nodules.The lower segment of the Kotroman Formation is adopted as the type section of the Kami{na Member.This clastic sequence can also be found close to Vardi{te, at the northern slope of Me|e|ova ljeska and Jezdimirovi}i.Here, the polymict basal conglomerate is composed of serpentinite, gabbro, chert, diabase and limestone fragments in a sandstone matrix.These conglomerates are interbedded with gradational sandstones.The upper part of the clastic sequence is characterized by red chert and alevrolite.There is a smooth transition to the overlain Uro{evi}i Member.Conglomerates and clastites are increasingly replaced with calcareous fine grained sandy limestone with ostreids, algae and microfauna.An economic potential of the Fe-Ni ore has been reported for the lower part of the clastic sequence (FOTI] 1965, JANKOVI] 1990).
calations, as well as macro and microfossils associations.Smooth transition to the upper part of the Kotroman Formation is marked by decreasing thickness of beds and less nodular limestone.Upper segment of Formation, which is represented by thin bedded bioclastic limestone, is here adopted as Jatare Member.
Schematic stratigraphic columns of the Uro{evi}i, Kotroman and Jatare localities are presented in Fig. 2. The Kotroman section is designated as the type section of the formation.

Description of the stratigraphic columns A -Uro{evi}i
The cumulative thickness of the Kotroman Formation in the Uro{evi}i locality is 48 m.Here, the basal conglomerates of the Kami{na Member (A-1 in Fig. 2) transgressively overlie serpentinite rocks.Iron rich dark green chamosite ooides and serpentinite particles can be frequently found in conglomerate fragments.The grains are cemented with calcareous or clay-ironstone cement.In addition to conglomerates, loose dark gray sandstone predominated in this Member (Fig. 3).Small cherty fragments as well as sand particles of different size are bound by clayey or limonitic red or brown cement.The bedding surfaces in the entire lower Member are not well expressed.Floral remnants, such as fine dispersed plant particles and fragments of branches and tree trunks, are common.In addition, bisect particles of conifers, dominated by Pinus and rarely Podocarpus, Cedrus can be found.
The Jatare Member of the Kotroman Formation is here overlain by bituminous, thin bedded, marly limestone of hemipelagic series.The bedding surfaces of the above series are plain and well expressed.

B -Kotroman
The cumulative thickness of the Kotroman Formation at its type section (Kotroman Village) is 48 m.The stratigraphic column at the Kotroman locality (B-1 in Fig. 2) has a lithology which can be compared with the previous column.Similarly to Uro{evi}i, the lowermost Kami{na Member is transgressive iron rich sandstone.The type section of the Member is at same locality as the Kotroman Formation (43°46 '3.81'' N, 19°28'22.05'' E).This Member here is characterized by well rounded pyritized grains and fragments of serpentinite, without any fossils.Subsequently, sediments of the Uro{evi}i Member can be found.It is well bedded sandy reddish nodular limestone with bivalve and gastropod shells (Figs. 4, 5).These beds contain rare ostracode remnants, gyrogonytes, charophytes, algae Radiocicelapses sp., etc. Rarely, cm-scale lenses of calcirudite and calcarenite rich in bio-and lithoclasts, fragments of mollusks, peridotites and siliceous rocks can be found.These are replaced by nodular reddish limestone with rare macrofauna (B-2 in Fig. 2).Above these, thin bedded yellowish biomicrite -wackstone (B-3 in Fig. 2) of the Jatare Member can be found.These beds contain ostracodes, bivalves and the algae fragments.The palynomorph association is similar to that at the Uro{evi}i locality.

C -Jatare
The cumulative thickness of the Jatare stratigraphic column is 26 m.Here, nodular limestones of the Uro{e-vi}i Member are superimposed on top of a weathered crust.Field observations indicated that no clastic section comparable to that at the Kami{na Member is present.
Here, gray and yellowish marly limestone is the dominating lithological constituent of the stratigraphic column (C-1 in Fig. 2).The beds contain mollusk shells with geopetal fillings, foraminifers, ostracodes and rare charopohyte remnants.The latter ones indicate intermittent fresh water influx.The uppermost part of the stratigraphic column (C-2 in Fig. 2) is represented by thin bedded marly limestone of the Jatare Member (Fig. 6).The type section of this Member is situated southward of the Jatare Hamlet (43°47'45.60''N, 19°31'4.45''E).Numerous foraminifers (Miliolidae, Nezzazatidae, Glomospira sp.etc.), ostracodes and cyanobacteria Decastroporella inconstans are present.Rarely Rhapydionina dubia and some orbitolins can be found.The abovementioned microfuna association indicate an Albian/Cenomanian age of the Kotroman Formation.
The Kotroman Formation at Jatare is, like at some other localities, overlain with thin bedded, marly, bituminous limestone of the Hemipelagic series with plain and well expressed bedding surfaces.

Depositional environment and stratigraphical problems
During the Early Cretaceous period, the area of the Inner Dinarides was a landmass composed of mafic and utramafic rocks with widespread weathering crust, due to intensive and deep weathering under humid climate conditions.Transgression, which started at the mid-Albian, initiated erosion and redeposition of sediments that include the basal series of the Cretaceous succession, i.e., the Kotroman Formation.This also includes the deposition of nickel/iron rich oolithic sandstones.
The sedimentation occurred in littoral and sublittoral environments.The water was shallow, warm, with limited circulation.Depending on the morphology of the sea-bottom, the water table was characterized by low   energy conditions in isolated and uneven depressions.The presence of charophytes and ostracodes in some layers strongly suggest irregular fresh water influx from the aforementioned land masses.There were periods of time when rapid input of clastic sediment ceased and calm environmental conditions allowed the fast growth of mollusk fauna.The origin of the nodular limestone texture is the product of later differential compaction, diagenesis and pressure dissolution of the sediment due to the clay content (JENKYNS 1974).
Detailed stratigraphic analyses in the area of Mokra Gora were not easy because of the somehow restricted character of the fossil assemblage.The collected lagoonal gastropod fauna, with Pseudomesalia teniucostata, P. multicostata, Pirenella cf.levadhiae, Paraglauconia lujani, Bicarinella bicarinata and Cassiope kotromanensis, indicates isolated sublittoral areas with limited aerial coverage, and with some features of endemism.This makes the comparison and correlation with other faunal assemblages from surrounding localities problematic and doubtful.This is supported by the fact that the gastropod family Cassiopidae is characterized by rapid ontogenical and phylogenical changes, e.g.MILOVANOVI] (1933), CLEEVELY & MORRIS (1988), MENNESSIER (1984), AKOPYAN (1976), KOLLMANN (1979).

Conclusion
This preliminary research paper investigates a Cretaceous formation in western Serbia known as the Basal Series.A new lithostratigraphic term is proposed for this unit, the "Kotroman Formation" after the type locality and type section at the Kotroman village.
The stratigraphic columns from the described localities imply three members within the Kotroman Formation: (1) The Kami{na Member consist of clastites, up to 8 m thick, (2) The Uro{evi}i Member consist of nodular limestone up to 27.5 m thick, and (3) the thin bedded marly limestone of Jatare Member up to 12.5 m in thickness.Nodular limestones are more or less present in all horizons of the Formation, although they predominate in the Uro{evi}i Member.Paleontological analyses indicate an Albian-Cenomanian age.Deposition probably started during the mid-Albian, and continued up to Cenomanian.
Numerous beds of terrigenous clastics with gradual transitions to nodular limestones and fossiliferous strata point to frequent changes in the local environmental conditions during the mid Cretaceous time.We believe that, in some future work, the recognition and correlation of the depositional sequences of the investigated strata will lead to the assemble of a convenient model of the sequence stratigraphy for these deposits, which will be helpful in defining the stratigraphy of a wider area in western Serbia.The present study shows that in spite of the limited data, a definition of the formations and a detailed mapping of these lithostratigraphic units is possible.Tada su izvr{ena i prva stratigrafska ra{-~lawavawa.Detaqna istra`ivawa izvedena su tokom izrade listova OGK 1:100 000.MOJSILOVI] i dr.(1978), OLUJI] i KAROVI] (1986).JOVANOVI] i dr.(2004) izdvojili su vi{e jedinica ni`eg reda, koje stratigrafski pripadaju albu, cenomanu i turonu.Najstarija tzv "Bazalna serija" le`i di-rektno preko serpentinita ili kore raspadawa.Asocijacija prisutne mikrofaune detaqno je opisana od strane RADOI^I] & SCHLAGINTWEIT (2007).