Lower Triassic ( Olenekian ) microfauna from Jadar Block ( Gučevo Mt . , NW Serbia )

Systematic study of microfossil associations on the Krivi Potok section (Gučevo Mt. area, NW Serbia) has been carried out to document and to refine the Lower Triassic stratigraphic correlations within Alpine-Mediterranean domain. Field investigation and laboratory process have enabled the identification of lowermost Olenekian (lower Smithian) conodonts, ostracodes and pyrite framboids. Two conodont zones are established in this region, in ascending order they are: Pachycladina obliqua–Foliella gardenae Assemblage Zone and Neospathodus planus Zone. A new ostracode species Paracypris ? krivipotokensis FOREL n. sp. has been described, it co-occurs with conodont Neospathodus planus within the Zone of the same name. The pyrite framboids were formed within the ostracode carapaces after their death. The size distribution of pyrite framboids supports the former suggestion that large size (>6 μm in diameter) is not suitable for the reconstruction of seawater redox conditions.


Introduction
The Lower Triassic sediments are widespread in the Jadar Block of Northwestern Serbia.Together with the Upper Permian rocks and the Permian-Triassic boundary interval, they have been intensively studied especially because represent the only such formations of this age in Serbia.Generally, the Upper Permian sediments contain diverse macro-and microbiocenoses, whereas the Lower Triassic microfossil associations are rather poor.
Gučevo Mt. is situated in the north-western part of the Jadar Block, on the eastern side of the Drina River and southern of the Loznica town (Fig. 1).It is predominantly built of Lower Triassic sediments, which the Serbian authors mostly divided in the older "Seisian" and the younger "Campilian" beds.After repeated numerous field investigations of this area, authors of the paper intended to document new geological data to refine existing stratigraphic and lithostratigraphic definitions.
Therefore, the aim of this paper was to confirm the presence of different microfossils (conodonts, foraminifera, ostracodes, etc.) and biostratigraphic data within the Lower Triassic sediments of the Jadar Block, NW Serbia.It represents the continuation of the ongoing geological study, started in 2005 year with micropaleontological/sedimentological investigations in P-T boundary interval of NW Serbia (SUDAR et al. 2007, NESTELL et al. 2009, CRASQUIN et al. 2010).

Geological settings
Geographically, the Jadar Block, is located at the southern margin of the Pannonian Basin, and belongs to the central part of the Balkan Peninsula.It occupied great parts of the northwestern Serbia, southern Srem (Vojvodina) and extends partly westward over the Drina River into eastern Bosnia (Fig. 1).
This tectonostratigraphic unit is today an exotic block, which was placed into the Vardar Zone before the late Cretaceous.It is surrounded by the Vardar Zone Western Belt, except on the farthest south-eastern part where it is in direct contact with the Kopaonik Block and the Ridge Unit (Fig. 1).The Jadar Block differs from the Vardar Zone Western Belt in lacking post-Liassic sediments as well as in the absence of ultramafites, ophiolitic mélange, and Cretaceous flysch development (FILIPOVIĆ et al. 2003).
In this area, the deposition occurred during the Variscan and Early Alpine evolution with a predomination of Dinaridic features.The later tectonic stage is characterized by sedimentation of Upper Permian and lowermost Triassic shallow-water marine carbonates, dolomites of the Anisian age, "porphyrites" and pyroclastics of Ladinian age, platform-reefal limestones of Middle and Late Triassic age and their gradual transition into Lower Jurassic limestones.
In the Jadar Block the Upper Permian is represented by the "Bituminous Limestone" Formation and the Lower Triassic by the Svileuva and Obnica formations (FILIPOVIĆ et al. 2003).

Krivi Potok section
On the central parts and on the eastern slopes of the Gučevo Mt. there are mostly exposed grey thin bedded to bedded limestones, in alteration with siltstones and sandstones of the upper Lower Triassic Obnica Formation (FILIPOVIĆ et al. 2003) generally thick about 200-350 m (MOJSILOVIĆ et al. 1975).These sediments, especially in surroundings of the old mine Brasina and in the source part of the stream Krivi Potok, were frequently micropaleontologically and sedimentologically investigated in the previous years (PANTIĆ 1971;BUDUROV & PANTIĆ 1973, 1974;UROŠEVIĆ & SUDAR 1980;PANTIĆ-PRODANOVIĆ & RADOŠEVIĆ 1981;SUDAR 1986, etc.).
In the Krivi Potok section, for the study presented herein, was investigated and sampled for microfauna only 11 m thick rock section near to the mouth of this stream in the rivulet Štira and on the northern side of the Loznica-Zajača road (N 19°14'13.8", E 44°28'9.8"; figs. 1, 2).The field researches were undertaken in 2010 (samples SRB-1 and SRB-2) and 2012 (samples Kp1 -Kp7) years.The section (Fig. 3) consists of: • Unit 1, bedded bioturbated limestones, 70 cm thick only with conodonts; • Unit 2, bedded, sandy, ferruginous ooidal grainstone, 1 m thick, and only with conodonts; • Unit 3, bioturbated micrites 30 cm thick with conodonts; • Unit 4, ca.5.5 m thick can be separated into the lower (4a) and upper part (4b) with covered part between.The lower part of this first entity of subunit 4a (ca. 3 m) is built of thick bedded limestones (beds are mostly 25 cm thick).Than, last 1 m is consisted of thin bedded laminated limestones (beds are less than 5 cm thick).Here in the subunit 4a are present abundant conodont and ostracode microfaunas, pyrite framboids (especially in sample SRB-1) and rare foraminifera.
After covered 6 m, also to the same unit (subunit 4b) belongs 1.5 m thick part, mostly made of change of red and grey siltstones and sandstones, but in the upper part also contains sandy, ferruginous wackestone-packstone with one characteristic 20 cm thick bed of ooidal dolomitic limestones.Ooids are rare, sometimes filled with dolomite.Beside conodonts, detritus of bivalves, gastropods, and foraminifera, also fine quartz grains and dispersed ferruginous matter characterize bed.• Unit 5, quartz sandstones thick 0.5 m; • Unit 6, thin bedded micaceous quartz sandstones 1 m thick, and • Unit 7, made of grey, thin bedded, 2 m thick limestones (thickness of beds is 2-15 cm) with ostracodes and rare pyritized foraminifera.This unit make the final part of the section.The deposition of the fine carbonate mud, characteristic for low energy regime of the shallow water, relatively restricted environment, probably shallow subtidal (traces of bioturbations in lower part, fine laminated and thin bedded limestones in middle and upper part of the section) predominates in the section.Appearance of the bed with ooids in thin bioturbated micrites in lower part (Kp6; SRB-2) and also in 20 cm bed thick in subunit 4b (Kp4/2) with change of thin bedded siltstones and sandstones in lower part of subunit indicate high water energy.Possible explanation is existing of some topographic highs (normal relief or as a result of local tectonic uplift) were is water energy like in typical tidal regime, or occassionally influence of storm (wind) currents.In upper part of section (Kp3, Kp2, K2/a) is characteristic presence of siliciclastics imput, caused by some local tectonic event or change of climate (ferruginous pigments in sediments).

Material and methods
Fourteen composite rock samples, each ca.2.5 kg, are collected from the Krivi Potok section for conodont study, and same number of thin sections were also done from each level with the conodont samples for the petrographic purposes.The laboratory work was carried out at Geological Survey of Slovenia.All samples were prepared using standard laboratory techniques: dissolved in diluted formic and acetic acid, after dissolution, the residue was collected, sieved and dried.High density liquid (ca.2.8 g/cm 3 ) is used for gravity enrichment; the density of conodont is larger than 2.8 g/cm 3 , while the density of most of the other residues is smaller than 2.8 g/cm 3 .The collected materials (samples, thin sections etc.) and collections of conodonts, ostracodes and pyrite framboids from investigated section are deposited in the Geological Survey of Slovenia under the catalogue numbers GeoZS 4768-4769 and 5104-5112, corresponding to numbers of the samples (SRB-1, SRB-2 and Kp1-Kp7) from the section.The illustrated pictures presented herein were taken with an Scanning Electron Microscope at the University of Graz, Nawi Graz, Institute of Earth Sciences.
(Fig. 3).Other three samples from the upper part of the section contain only ramiform elements and some undeterminated fragments of the conodonts (Fig. 3).
In the lower part of the presented Krivi Potok section, before the covered part of column, beside of stratigraphic ranges of determinated conodonts, two conodont zones can be recognized.They confirm the conodont sequence which has been proposed in the Idrija-Žiri area, Slovenia (CHEN et al. in preparation).These zones are: Pachycladina obliqua-Foliella gardenae Assemblage Zone and Neospathodus planus Zone and both they are of the lowermost Olenekian (lower Smithian) in age (Fig. 3).
From the source part of the Krivi Potok, SUDAR (1986) reported the results of the micropaleontological investigations of conodonts and foraminifera where it was determinated the Parachirognathus--Furnishius Z. in the Smithian and triangularis-homeri-C.R.Z. of the Spathian age.

Pachycladina obliqua-Foliella gardenae Assemblage Zone
Originally is introduced by CHEN et al. (in preparation) in the type-section Žiri 29 of the Idrija-Žiri area, Slovenia as the Pachycladina obliqua-Foliella gardenae A. Z. in the interval between Eurygnathodus costatus and Neospathodus planus zones within the lower part of the Smithian (lowermost Olenekian).
In the Krivi potok locality the lower limit of this A. Z. lies on the beginning of the Unit 1 at the base of section.It is characterized by the first appearance of Pachycladina obliqua and Foliella gardenae.The upper limit of the Zone is marked with the first occurrence (FO) of Neospathodus planus in the Unit 3 at 1.7 m from the beginning of the section (Fig. 3).
In the investigated section of the Jadar Block the age of the Zone is the lower part of the Smithian in the lowermost Olenekian, and as associated conodont is present only Pachycladina inclinata.
In conodont zonation by BUDUROV & SUDAR (1995) exist the Platyvillosus-Foliella Beds with the range of both genera in the oldest parts of Spathian.ORCHARD (2007)   qua-Foliella gardenae Assemblage Zone in the lower part of the Smithian.

Neospathodus planus Zone
Neospathodus planus Zone was originally reported from the type-section Žiri 61 in the Idrija-Žiri region, Slovenia (CHEN et al., in preparation), where it lies between Pachycladina obliqua-Foliella gardenae A. Z. and Neospathodus robustus Zone.It is lower Smithian in age.
In the presented locality on the Gučevo Mt. their lower limit is marked with the first appearance of Neospathodus planus without elements of genus Pachycladina and Foliella gardenae, in the Unit 3 at 1.7 m above the base of the section.The upper limit is set at the end of the first part of the Unit 4 (subunit 4a), because the strata after covered part of 6 m, contain only ramiform conodonts without stratigraphic importance (Fig. 3).
In the Krivi Potok section this stratigraphic interval characterizes the lower part of the Smithian (lowermost Olenekian) only with the occurrence of the taxa Neospathodus planus and Neospathodus sp.indet.The multielement apparatus of Neospathodus planus has also been discussed here, categories of conodont elements follow SWEET (1988), they are assigned to segminate, angulate, breviform, etc., and orientation of elements follows PURNELL et al. (2000).

Neospathodus
Multielement apparatuses of conodont play an important role in the conodont taxonomy system (e.g., ORCHARD 2005), thus it is expected that conodont workers illustrate these ramiform elements together with P 1 elements.Conodont clusters and nature assemblages have demonstrated that there are three types of apparatuses in the geological record of earth history, they comprise of 15, 17 and 19 elements respectively (ALDRIDGE et al. 2013).Early Triassic conodont composed mainly three families: Gondolellidae, Anchignathodontidae, and Ellisoniidae (ORCHARD, 2007), nature assemblages or clusters have shown that apparatuses of all these three families consist of 15 elements (RIEBER 1980;ORCHARD & RIEBER 1999, KOIKE et al. 2004, ORCHARD 2005, GOUDEMAND et 2012) revise these S 1 and S 2 elements of Gondolellidea apparatuses as occupied S 2 and S 1 position, the S 3 and S 4 elements of subfamily Novispathodinae (family Gondolellidea) occupied S 4 and S 3 position.
The monospecies Neospathoides planus in Zone of the same name provides a chance for the reconstruction its apparatus.Apart from P 1 elements, P 2 , S 1 , S 2 , S 3-4 , and M elements are also identified, as they are shown in Fig. 5.3-8.However, S O element has been found from neither Krivi Potok section, Serbia nor the Idrija-Žiri section, Slovenia (CHEN et al., in preparation).
Elements of Neospathodus planus are characterized as follow: P 2 element is angulate, relatively slender, has a cusp in the middle part of the unit.S 1 element is breviform digyrate, with well-developed rostral process, but lacks caudal-ventral process, the largest denticle located on the caudal end of the unit and caudally pointed.These characters of S 1 elements are similar to that of many Triassic conodont species which are illustrated by ORCHARD (2005).S 2 element is breviform digyrate, with a large rostral process but a very short caudal-lateral process which only bears 2 to 3 denticles.Only caudal part of the S 3 or S 4 element is found, as it is illustrated in Fig. 5.7, character of rostral part in not clear so far.M elements is also breviform digyrate, similar to S 1 element, however, it has a short ventral process which bears one denticle.Over all, elements of Neospathodus planus show high similarity with elements of Triassospathodus homeri (BENDER) which are figured by ORCHARD (2005), especially for P 2 and S 1 elements.Since we lack well preserved S 3 and S 4 elements, their relationship can not be decided presently.
Neospathodus sp.indet.Fig. 4.7 In the sample SRB-1 from the Neospathodus planus Zone in the Krivi potok section exists one speci-Lower Triassic (Olenekian) microfauna from Jadar Block (Gučevo Mt., NW Serbia) men showing characters between Neospathodus planus and Neospathodus robustus.The ventral (anterior) part of the element lost some denticels, thus it is possible the specimen has seven or more denticles which probably should be identified as Neospathodus robustus.In this moment, because of the bad preservation of the

Elements of the Pachycladina-Foliella conodont microfauna
The elements determinated within this conodont microfauna, Pachycladina obliqua, Pachycladina inclinata and Foliella gardenae were first reported from the middle to upper part of Campil Member in North Italy (STAESCHE 1964) with higher stratigraphic distributions than Eurygnathodus costatus STAESCHE.

Conodont Colour Alteration Index
The Conodont Colour Alteration Index (CAI values sensu EPSTEIN et al. 1977) of the conodonts from the presented Krivi Potok section are in the range of CAI from 5 to 6-7 (5, 5.5, 6, 6-7), with the tendency that conodonts from the middle parts of the section (Neospathodus planus Zone) have the highest values of CAI.It is in correspondance to the earleir data of CAI values from the Krivi Potok section which are 5.5 (unpublished information for the conodonts determinated by SUDAR 1986).All these mentioned data where CAI values are between 5 and 6-7 correspond to temperatures from 300-490 °C and characterize very low to low grade metamorphism (GAWLICK et al. 1994, SUDAR & KOVÁCS 2006).

Ostracode microfauna
From the fourtheen samples treated for conodonts extraction, two were productive with ostracodes (SRB-1 and Kp1; Fig. 3).A total of fifty-nine specimens have been recovered, illustrating a new species described below.Most of the specimens are represented by complete carapaces, testifying the absence or limitation of post-mortem transportation with low wave energy and/or rapid burial by high sedimentation ratio (OERTLI 1971).
During the earlier phases of micropaleontological investigations of the Lower Triassic and P-T boundary interval in the Jadar Block were published only few papers dealing with ostracodes: PANTIĆ-PRODANOVIĆ (1979), KRSTIĆ (1980) and CRASQUIN et al. (2010).In paper of PANTIĆ-PRODANOVIĆ (1979), only five species of ostracodes were cited (three in open nomenclature) from "Campilan" substage in the Valjevo area.In the southern and eastern slopes of the Gučevo Mt., KRSTIĆ (1980) in detail described and illustrated rich ostracode microfauna of the "Campilan" age: Judahella tsorfatia, "Cultella" cf.laevis, Spinocypris nepalensis, "Bythocypris" aff.bijieensis, Bythocypris cf.pricei, etc. Studying very abundant ostracode assemblages not found earlier in the P-T beds of this region, CRASQUIN et al. (2010) also introduced three new species.All together, were the first record of the youngest Upper Permian age microfaunas not only from NW Serbia, but also from the whole Serbia and the central part of the Balkan Peninsula.
In the present study, we follow the systematic classification of MOORE (1961)  Material.Fifty-nine complete carapaces and several fragments.
Diagnosis.A species attributed with doubt to the genus Paracypris, with AB and PB maximum of convexity symmetrically located in the lower 1/3 rd of the carapace, upper portion of AB straight.
DB long (50-65% of L max ), regularly convex, smoothly sloping to PB; PB rounded with narrow radius of curvature, maximum located in the lower 1/3 rd of H max ; posterior half of the carapace slightly wedge-shaped; VB flat to slightly concave at both valves; upper part of AB long and straight to slightly convex, resulting in a shouldering of the transition to VB larger than PB: anterior maximum of convexity located symmetrically to PB in the lower 1/3 rd of H max .

Pyrite framboids
Plentiful studies indicate that this kind of pyrite can be a proxy for redox conditions in the paleoseawater and sediments (e.g., WILKIN & BARNES 1996;SHEN et al. 2007;WIGNALL et al. 2005;TIAN et al. 2014).They form in dysoxic and sulfidic conditions which is close to the redox boundary (WILKIN & BARNES 1996).The diameters of pyrite framboids are very sensitive to the water depth between the redox boundary and the sediments, as they sink down rapidly after formation if they are formed in the water column and thus smaller (< 6 µm in diameter) and has a narrower size variation range (WILKIN & BARNES 1997;WIGNALL & NEWTON 1998;WIGNALL et al. 2005).However, larger sized and wider range of size variation of pyrite framboids indicate a formation within the sediments, as they have longer time and slower rate for their formation (WILKIN & BARNES 1996, 1997).
Assemblage Zone CHEN et al. (in preparation) roughly correlate with the stratigraphic range within the lower part of Smithian belonging to the Parachirognathus-Furnishius conodont fauna by SWEET et al. (1971) and BUDUROV & SUDAR (1995).They change only the name of the Zone according to the dominating presence of the species of the genera Pachycladina STAESCHE and Foliella BUDUROV & PANTIĆ what is also obvious in the Krivi potok section.The same interval, characterized by dominating Pachycladina obliqua joined with elements of Hadrodontina STAESCHE, was in External Dinarides attributed to the Pachycladina obliqua Zone (KOLAR- indicated that species of the genera Parachi-rognathus CLARK, Furnishius CLARK, Hadrodontina, Pachycladina or Foliella, belonging to the family Ellisoniidae, are not present in the Spathian, but nearly only in the same time interval of the lower and middle Smithian.These facts were applied by CHEN et al. (in preparation) when define Pachycladina obli-
planus CHEN & KOLAR-JURKOVŠEK Fig. 4.1-6, 8-10; Fig. 5.1, 2 P 1 elements of Neospathodus planus is characterized by 3-6 denticles and sometimes a large basal cavity that occupies almost the whole length of the unit (e.g., Fig. 4.9).The outer side of the basal cavity is always greatly expanded.It is similar to Neospathodus robustus KOIKE, but Neospathodus robustus commonly has more denticles, which are 6-10 (KOIKE 1982).Neospathodus planus can be differentiated from Triassospathodus hungaricus (KOZUR & MOST-LER) by much wider and inflated basal cavity, and by relatively longer and stronger unit.So far, it is only recognized from Slovenia and Serbia, and it could be a species controlled by local environment.Multielement apparatus of Neospathodus planus CHEN & KOLAR-JURKOVŠEK Fig. 4.1-6, 8-10; Fig. 5.1-8
fossil and since occur only one, ilustrated, specimen it is determinated in the open nomenclature like Neospathodus sp.indet.