Formation mechanisms of Fe-Mn concretions in the Vijenac Quarry, Dinaric Ophiolite Zone

The Vijenac limestone quarry, near Tuzla in Bosnia and Herzegovina, is composed of carbonate rocks locally embedding tectonically disturbed siltite and sandstone with Fe-Mn concretions. The quarry itself represents a part of Dinaric overstep sequences (the Pogari Formation) unconformably overlying ophiolite mélange and ophiolite trust-scheets. Petrographic, chemical and mineralogical analyses had shown that the concretions may be divided into two types: (i) Mn-rich concretions with 17 wt.% of Mn and compact texture and (ii) Mn–poor with 8 wt.% of Mn and porous texture. The amount of Ca, Fe and Mg in both concretion types are similar. Nickel and Cr positively correlate with Fe and Mn, respectively. According to petrographic and mineralogical analyses, concretions are composed of calcite, dolomite, hematite, todorokite and takanelite. Petrographic study con rmed the development of concretions within three stages including two generations of calcite. Studied concretions are formed within consolidated sandstones inheriting their sedimentary textures – therefore a late diagenetic process is assumed.


Introduction
A concretion is a compact mass of mineral matter, usually spherical or disk-shaped, embedded in a host rock of a different mineralogical composition. Contrary to nodules, concretions show distinct internal structures with concentric zonation (i.e. Gasparatos, 2005). Concretions vary in mineralogical composition, size, shape, hardness, and colour. They are commonly composed of calcite, dolomite, siderite, pyrite, barite, ferromanganese oxides or gypsum. Concretions occur when a considerable amount of cementing material precipitates locally around a nucleus, which is usually some other mineral or fossil remains (Mozley, 2003).
The Fe-Mn concretions and nodules occur in a variety of geological environments like the ocean oor, river sediments, lake sediments, soils and sedimentary rocks. Extensive studies of the ocean oor Fe-Mn nodules have been conducted in order to determine their origin and potential use/exploitation (Bonatti & Nayudu, 1965;Bender et. al, 1966;Glasby, 1970). Within sedimentary rocks, one of their biggest importance is keeping records of diagenetic and post-depositional changes of the sedimentary units, since they preserve evidence of groundwater ow and the interactions on water-rock boundary resulting in dissolution and/or precipitation of minerals (Chan et. al, 2007). Concretions have been considered as unusual parts of sedimentary rocks, although they are very often found in sedimentary rocks through different geological periods (e.g., Seilacher, 2001; Mozley, 2003 and references within). Fe-Mn concretions within soils are composed of soil minerals cemented with Fe and Mn oxides, oxyhydroxide and hydroxides and their origin is a result of seasonal changes in pH and redox potential (Eh). The origin of Fe-Mn in concretions is of great interest in environmental geochemistry for their role in metal sequestration (e.g., Gasparatos, 2012; 2013 and references therein).
Within the Dinarides, there are only a few records on the concretions'/nodules' appearances (Chiari et. al, 2013;Lackovi , 1998;Photiades, 2010;Posilovi et. al, 2016). This study presented the rst evidence of the Fe-Mn concretions within sedimentary rocks of the Dinaric Ophiolite Zone (abbr. DOZ), i.e. the Vijenac Quarry, and a unique opportunity to study the corresponding diagenetic and post-depositional changes in sedimentary units. The Vijenac Quarry lies in the area situated between the Neogene basins of Tuzla and Banovi i and north-westward of the Krivaja-Konjuh Ophiolite Complex in NE Bosnia and Herzegovina. It is known for its highly pure limestone (up to 97 wt.% of calcite), with additions of quartz, hematite and ne-grained fraction (Kneževi et al., 1986). A border-zone of the quarry is composed of siltite and greywacke encompassing numerous brown-black spherical/ellipsoidal concretions.
The aim of this study is to give a better insight into the origin of Fe-Mn concretions hosted by clastic-carbonate sequences of the Vijenac Quarry DOZ, Bosnia and Herzegovina. The presented research contributed to understanding the origin and the syn/post depositional processes within the sedimentary formations associated with DOZ. This article brings results of micropetrography, X-ray powder diffraction (XRD) and atomic absorption spectroscopy (AAS) analyses.

Geological settings
Vijenac Quarry and the surrounding area is located within the north-western part of the largest ophiolite complex named the Krivaja-Konjuh, situated in the DOZ. DOZ represents the internal unit of the Dinaride-Hellenide Orogenic System, consisting of the following (i.e. Pami et al., 1998): a) Mesozoic radiolarite sequences associated with basalts (Pami , 1982); b) Ultrama c and ma c blocks featured by metamorphic sole rocks (amphibole and granulite) in their base (Lugovi et al. 1991;Pami , 1997;Šegvi , 2010); c) Ophiolite mélange composed of a shale-silty matrix in which fragments of greywacke predominate over the ultrama c, ma c and metamorphic rocks, shale, radiolarite, tuff and limestones. In some areas, the mélange displays the distinct olistrome signatures (Pami , 1982;Dimitrijevi & Dimitrijevi , 1973, 1975Tari, 2002;Šegvi et al., 2014). d) Late Jurassic / Early Cretaceous overstep sequences (Pogari Formation) transgressive overlying formation b) or c) (Pami et al, 1998;Hrvatovi , 2006). Massive reed limestone of the Vijenac Quarry belongs to this tectonostratigraphic unit. The geological map (see Figure 1) shows the location of the quarry area within the massive reef limestones, the youngest dated as Jurassic up to early Cretaceous based on fossils like Lithostrobus, Calpionella or Tintinopselae (Jovanovi , 1957). The limestones occupy the central and southern parts of the map forming a large cone .
Notable characteristics of limestones are karsti cation and well-developed fracture and crack systems as a   (Kneževi et al., 1986;Miletovi et al., 1978). The northern and eastern part of the map is predominately composed of serpentinized rocks associated with medium-grade metamorphic rocks. Contact between the massive reef limestones and serpentines is tectonic and well differentiated, with crushed contact zones ranging from 1 to 3 m (Kneževi , et al., 1986;Miletovi et al., 1978).
Ophiolite mélange associated with genetically related sedimentary sequences represents the most characteristic and widespread subunit of the Internal Dinarides (Pami et. al., 1998;Pami et. al., 2002, Karamata, 2006. This chaotic tectono-sedimentary mixture is made up of detached 'pocket' to 'mountain' size blocks and boulders of oceanic crust along with the Palaeozoic to Jurassic carbonates and clastic sedimentary rocks (Trubelja & Marchig, 1995;Tari, 2002).
A geological column (see Figure 2) of the wider quarry area presents general relations of the lithological sequences in correlation with the tectonostratigraphic characteristics of the DOZ. The earliest deposits belong to the Early-Middle Jurassic sediments (J 1,2 ) whose composition includes marls, limestones and radiolarites (Vishnevskaya et al., 2009), overthrusted by ophiolite mélange (J 2.3 ) (Šegvi et al., 2014).

Materials and methods
Eight samples were chosen for macro-and micropetrography analyses. Based on these results, three samples were further chosen for in-depth analyses using X-ray diffraction (abbr. XRD) and atomic absorption spectroscopy (abbr. AAS; Table 1). The investigation was completed in the Laboratory for analysis of geological materials (LaGEMA) at the Faculty of Mining, Geology and Petroleum Engineering, of the University of Zagreb.

Micropetrography
Thin sections of 8 concretion samples from the Vijenac Quarry were examined microscopically using a Leica DM LSP stereo-microscope under magni cations of 40, 100, 400 and 600×. The microscope is equipped with a Leica DC 100 digital camera and Leica Qwin im-   age analysis software (IM 50 v.1.20). The grain size is determined using a standard calibration micrometre scale. Samples were subjected to staining in order to determine the type of carbonates mineral present (Evamy and Shearman, 1962). Carbonates were immersed in a solution of potassium ferricyanide K 3 Fe(CN) 6 with concentrated HCl and Alizarin red S C 14 H 8 O 4 with a diluted hydrochloric acid (HCl/8) for 15 seconds and there upon being repeatedly immersed for 15 seconds in the solution Alzarin red S with a diluted hydrochloric acid (HCl/8). Finally, the samples were washed with distilled water.

AAS
Based on macroscopic determination, samples VJ-1, VJ-5 and VJ-6 were selected for the atomic absorption spectroscopy and prepared by means of the sequence extraction analysis (abbr. SEA) and gravimetric analysis (see Table 1). Bulk samples of the selected concretion were powdered in an agate mortar.
Sequence extraction analysis according to Rauret (1999) consists of three steps. For our purposes the rst two steps of SEA were prepared for analyses: i. Carbonate leaching: during the rst step acetic acid CH 3 COOH was used to extract the carbonate-bound fraction. The extracted solution was used for the determination of Ca and Mg through the AAS method. ii. Fe-Mn oxyhydroxide leaching: within the second step, Fe and Mn-oxide bound phases were extracted using hydroxylamine hydrochloride HONH 2 ·HCl, acetate CH 2 COOHand HNO 3 . The extracted solution was used for the determination of Cr, Fe, Mn and Ni through the AAS method. An Analyst 700 atomic absorption spectrometer equipped with an air-acetylene burner was used for the determination of Ca, Fe, Mg, Mn and Ni concentrations from the previously acquired solutions from sequence analyses. All chemicals were provided by the analyticalreagent grade (Merck). Distilled-deionized water was used throughout. Standard solutions needed for the preparation of calibration curves were made fresh-daily by diluting stock solutions of Ca, Cr, Fe, Mg, Mn, Ni. Loss of ignition (LOI) was performed at a temperature of 430°C and recalculated using gravimetry. The results are shown in Table 2.

XRD
Concretion samples VJ-1, VJ-5 and VJ-6 were powdered and measured using a Philips vertical X-ray goniometer (type X'Pert) equipped with a Cu tube and graphite crystal monochromator with the following experimental conditions: 40 kV, 40 mA, primary beam divergence 1/4º, continuous scan (step 0.02º 2 /s) three times (air-dried conditions): 1. Original, non-treated sample, 2. After carbonate leaching, 3. After Fe-Mn oxyhydroxide leaching.   Obtained spectres were analysed using X Pert High-Score plus software. The results are presented in Table 3.

Macropetrography
The observed concretions predominantly show spherical or symmetrical shapes with diameters varying from 2 to 20 cm. Some concretions show a non-symmetrical narrowing having a pear-like shape, which probably reects space de ciencies at the time of their formation. Concretion internal structure (i) shows a weak or distinct zonation, likely resulting from different concentration gradients of Fe-Mn oxyhydroxides. This zonation is visible on macroscopic samples as a variation of colour from lightly greyish brown (probably representing a Fe-Mn poor zone) to almost fully black zone (probably representing a Fe-Mn rich zone) (see Figures 3A; B) or (ii) appears as compact and uniform (see Figures 3C; D). Most concretions are surrounded or crosscut by calcite veins (see Figures 3A; B) formed by later tectonic processes. The host rock was classi ed as greywacke sandstone (classi cation after Pettijohn et. al, 1972) (see Figures 3C; D) showing internal folding and distinct grain-size grading.

Micropetrography
The host rocks of the Fe-Mn concretions are classi ed as lithic greywacke that has a clastic texture with a clearly recognizable lamination. Layering and graded bedding is present in some of the samples (see Figure 4). Concretions contain the following mineral assemblages: (1) siliciclastic host rock assemblage (primary minerals) and (2) cement composed of Fe-Mn oxyhydroxides and carbonate (secondary minerals). The primary assemblage is composed of quartz ( 60 vol%), opaque minerals, subordinate rutile ( 30 vol%), lithic fragment ( 10 vol%), and some chlorite and Fe-carbonate. The cement is composed of the Fe-Mn oxyhydroxides and calcite (see Figures 4A -D). Calcite also occurs within veins surrounding the Fe-Mn concretions, in two generations: a cm-thick calcite veins occur crosscut by the next generation of tiny calcite veinlets (see Figure 4C). Calcite in the vein and cement is subjected to partial dolomitization, as suggested by carbonate staining (see Figure  4D). This gure also shows the development of secondary Mn-dendrites within carbonate veinlets. Fe-Mn oxyhydroxides are further con rmed by XRD as hematite, takanelite ((Mn,Ca)Mn 4 O 9 · H 2 O) and todorokite ((Na, Ca,K,Ba,Sr) 1-x (Mn,Mg,Al) 6 O 12 ·3-4H 2 O) (detailed description within section 4.4.).

AAS
Results of the AAS analyses show that the bulk composition of the concretions consists of Mn (7-17 wt.%), Ca (39-42 wt.%) and Fe (4-6 wt.%) while the rest corresponds to silica residue. The concentration of Mn is considerably lower in the sample VJ-1 in contrast to samples VJ-5 and VJ-6. The abundance of Ca and Fe in all three samples did not signi cantly vary. To assess the in uence of the surrounding ophiolite rocks, the concentrations of Ni, Cr, Mg were measured (see Figures 1 and  2). Their percentage was found to be below 1 wt.% (see Table 2).

XRD
According to X-ray diffraction analysis, manganese minerals and quartz are the dominant phases in the concretions (see Figure 5). Results of micropetrography for samples VJ-1, VJ-5 and VJ-6 are presented along the results of XRD analysis in Table 3.
All three original samples show similar mineral paragenesis, consisting of quartz, calcite and Mn oxyhydroxide phases (todorokite, takanelite). Samples deprived of carbonates show analogue assemblages (without calcite), while the intensities of the peaks of Mn minerals are somewhat accentuated. Following the removal of Fe-Mn oxyhydroxides the weak re exes of different types of clay minerals (most likely kaolinite or montmorillonite), and in some cases minerals from garnet group or barite may be identi ed.

Mineralogical and chemical composition of concretions
Petrographic, chemical and XRD analyses of Fe-Mn concretions had pointed to their twofold composition. The rst type is consisted of Mn-rich concretions with 17 wt.% of Mn and compact texture (VJ-5 and VJ-6), whereas the second type is made of Mn-poor ( 8 wt.% of Mn) concretions with a porous texture (VJ-1). The amounts of Cr and Fe in both concretions are similar.
The described types of concretions, alongside with their mineralogical and chemical properties are given in Figure 6. Both concretion types are embedded within lithic greywacke inheriting previous sedimentary structures (see Figure 3B; C). The Vijenac Quarry concretions appear to be much larger in size (up to 20 cm in diameter) comparing to the available literature data for soils and sediments hosted concretions (Gasparatos 2005 Mn-poor concretions have a mineralogical composition equal to Mn-rich concretions (see Figure 6B). They are low in the Mn ( 8 wt.% of Mn), Ni (0.2 wt.%) and containing 6 wt.% of Fe.
Ni increase from 0.2 wt.% within the Mn-poor concretions until 0.8 wt.% within the Mn rich concretions. Similar trends had been observed by Gasparatos (2013 and references therein) where Ni content in Fe-Mn concretions positively correlates with a Mn-rich phase. The amounts of Cr (0.01 wt.%) and Fe (5-6 wt.%) within both Vijenac Quarry concretion types are uniform. According to the literature (Gasparatos, 2013 and references therein) an increased Cr value likely involves isomorphic substitution of Cr 3+ with Fe 3+ , and therefore positively correlates with the amount of the Fe.

Formation mechanisms of the Fe-Mn concretions
Several general types of the Fe-Mn concretion forming mechanisms are plausibly related to different scenarios for the Vijenac Quarry concretion formation.
1. Freshwater and lacustrine Fe-Mn concretions can be developed as autochthonous phases during the freshwater and lacustrine sediments diagenesis, controlled by the upward migration of the pore water (Davison, 1982 and references therein). A selective solution of the Fe and Mn, caused by changes in redox conditions, later facilitated mixing and enrichment of pore water where concretions were developing. The presence of an increased amount of organic matter, characteristic for this type of environment, affects the enrichment of the pore water as well. The main mineral of the freshwater Fe-Mn concretion is goethite, while Mn phases are represented by todorokite, psilomelane and birnessite. These concretions are characterised by variations in their Fe/Mn ratios, concentric bands of alternating Ferich (lighter) and Mn-rich (darker) laminations, increased content of organic matter and low content of microelements such as Co, Mn, Mo, Ni, Ti (Davison, 1982 and references therein). Additionally, Fe-rich laminas are associated with silica, phosphate and arsenic, whereas Mn-rich laminas associated with Cu, Co, Ni and Zn. This type of formation mechanisms was considered for the Vijenac Quarry concretions, due to the presence of goethite and todorokite and subordinate amounts of analysed microelements (Cr, Ni).
2. The Fe-Mn concretions found in soil are formed in the exchanging oxidation and reduction conditions within the soil sequence. Concretions in soil are extremely rm, rounded morphologic bodies, formed through the process of reduction, relocation and oxidation of the Fe and Mn. There are two periods of formation. During the moist period, the percentage of Fe 3+ and Mn 3+/4+ is lowered and growth of concretion is ceased. During the dry period, the percentage of oxygen is increasing, subsequently triggering the onset of Fe and Mn precipitation within the matrix. This type of concretion has an in ll of Fe and Mn oxides located between fragments of matrix, skeletons of grain, fossils, clay minerals and pores. Formation is a long-term process, speci c for some types of soil. Other minerals that can be found in concretions except Fe-Mn oxyhydroxides, quartz and clay minerals (Gasparatos, 2012). This type of formation mechanisms was considered for the Vijenac Quarry due to the presence of secondary Fe and Mn minerals and clay in the matrix and primary quartz within clastic assemblage. However, these types of concretions usually have a seed nucleus around which the mineralization started and are up to 4 cm in size. Concretions from the Vijenac Quarry are larger and our investigation did not nd traces of initial fragment. 3. The Fe-Mn concretions in sandstones occur as the result of precipitations of the Fe-Mn oxyhydroxides from groundwater. Due to circulation, the exchange of oxidation-reduction phases take place. Concretion is usually found within predominantly clastic sandstones with good circulation of groundwater or uid. These types of concretions usually lack a nucleus. A concentric structure can be observed within larger samples, whereas small concretions sometimes show massive structure (Chan et al., 2007). This formation mechanism was considered for the Vijenac Quarry concretions for the several reasons: (i) host rock lithology (sandstones); (ii) missing nucleus of the studied concretions; (iii) massive structure of the small concretions and complex structure including rinds and multiple layers of the large concretions (see Figure 3).

Origin of concretions within the Vijenac Quarry
Based on a detailed petrographic, mineralogical and chemical study, the formation of the Vijenac Quarry concretions is hypothesised to have proceeded in three separate stages. Here we brie y summarize the evidence supporting this assumption. According to the literature, concretions from the Vijenac Quarry show similarities to the concretions formed in soils and sandstones. Similar mineralogy (the Fe-Mn oxyhydroxides and indicated clay minerals) can be precipitated during the exchange of redox potential in soil moist vs dry periods), whereas their sedimentary host, structure and missing nucleus show similarities to the concretions formed in sandstones. Chemical analysis performed on concretions VJ-1, VJ-5 and VJ-6, enabled classi cation into two types: 1) Mnrich concretions (VJ-5 and VJ-6) having Mn 17.5 wt.%, Fe 5 wt.% and 2) Mn-poor concretion (VJ-1) having Mn 7.5 wt.%, Fe 6.3 wt.%. Both types share similar proportions of other analysed microelements as well as similar mineralogical composition. According to the pH/ Eh diagrams for the recognised Fe and Mn species, coexisting Fe and Mn minerals would precipitate under the conditions of pH= 4-6 and Eh = 0.3-1 V (Takeno, 2005). Takeno (2005) diagram is used under the assumption of saturated conditions for Fe and Mn (i.e. the total concentration of Mn of 10 at 10 mol/L and the total Fe concentration of 10 at 10 mol/L). Concretions generally show the following stages of development:

Stage 1 -Fe-Mn concretion growth
According to literature observations summarised in Section 5.2., concretion development begins with the circulation of groundwater enriched with the Fe and Mn through the porous rocks, followed by periodic precipitation of the Fe-Mn oxyhydroxides, related to the oscillation of redox potential. Periodical precipitation and variations of the Fe and Mn within groundwater led to the development of concentric structures (zones), recognisable in most of the Vijenac Quarry concretions (see Figure 3). Zones have different proportion of the Fe/Mn oxyhydroxides, generally increasing with macroscopically and microscopically visible darkening (see Figures 3; 4). According to Gasparatos (2012 and references therein) Mn precipitated at lower pH and under a more oxidised environment comparing to Fe and thus small variations of pH and Eh conditions during periodical oscillation of groundwater would largely in uence the dynamics of precipitation.

Stage 2 -Carbonatization
The second phase is characterised by the development of microtectonic-related veins and veinlets lled with secondary carbonates (mainly calcite), usually cross-cutting the concretion zonation. In some cases, secondary carbonate fractures and veinlets follow the concentric structure of concretion (see Figure 3A). Here, we had recognised two stages of veinlet development; rst generation calcite within >1 cm thick veins containing up to 1.5 mm in size is crosscut by the second generation of tiny carbonate veinlets containing crystals about 0.1 mm in size. The appearance of the secondary Mn-dendritic structures is related to the rst generation of carbonates (see Figure 4D). Carbonatization is also recognisable within the matrix of the concretions. Some manganese carbonates occur as well, as evidenced by XRD (see Table 3).

Stage 3 -Dolomitization
Finally, large calcite grains within rst generation veins are subjected to partial dolomitization, as proven by the method of carbonate staining (see Figure 4D). Dolomitization is also recognized within the matrix.

Concretions age and spatial correlation within the Dinarides
The studied concretions are formed within consolidated sandstones inheriting their sedimentary textures. Macro and micropetrography of the concretions shows bedding, internal folding and micro-faulting in lled with carbonates. Observed structural features continue without a break to the host greywacke sandstone pointing to a late origin of concretions, most likely related to the uplift of the Pogari Formation toward a level of meteoric water system. According to Pami et al. (1998) compressional events throughout Eocene and Oligocene were accompanied by the several uplift episodes of the Dinarides. So far, Vijenac Quarry concretions are a single report of the Fe-Mn concretions within the Internal Dinaride units.
The Upper Cretaceous limestones within the External Dinaride formations have reported records of the Mn. It appears that Late Cretaceous backshore limestones from the island of Dugi Otok, Croatia, host an unusual occurrence of Mn-hydrated oxide mineralization in the form of botryoidally and globular, ne-laminated concentric aggregates consisting of todorokite and accessory MnO·OH (Lugovi et al., 2008). The authors suggested that Mn has been leached and mobilized from the Late Pleistocene sea oor sediments located around 50 nautical miles south of Dugi Otok, therefore the Mn occurrence is related to the uplift of buried sediments into a meteoric water system. Furthermore, (Posilovi et al. 2016) discovered the variously shaped and internally zoned siliceous-carbonate concretions in Southern Croatia (Šubir hill tunnel; Pojezerje municipality area) within the Upper Cretaceous carbonate deposits (latest Cenomanian / Early Turonian). Their carbon isotope composition corresponds to the globally known Cenomanian-Turonian 'Oceanic Anoxic Event', therefore corresponding to early (syn) diagenetic origin. Concretions are of different size, from less than 1cm to more than 20 cm in diameter. Another late diagenetic appearance of carbonate concretions is recorded within the Early Cretaceous limestones and dolostones of the Tounj Cave, near Ogulin, in Central Croatia (Lackovi , 1998).

Conclusion
In this study, we had investigated the Fe-Mn concretions embedded within the border zone of the Vijenac Quarry, composed of tectonically disturbed siltite and sandstone. The quarry is a part of Dinaric overstep sequences (the Pogari Formation) of Late Jurassic / Early Cretaceous to Late Cretaceous age.
According to petrographic, chemical and mineralogical analyses, concretions may be grouped into Mn-rich and Mn-poor concretions having 17 wt.% and 8 wt.% of Mn respectively, 5 6 wt.% of Fe and consisting of quartz, calcite, dolomite, takanelite and todorkite. The content of Ni in concretions positively correlates with Mn abundances whereas Cr correlates with Fe.
According to the pH/Eh diagrams, coexistence of the Fe-Mn oxyhydroxides would precipitate under the conditions of pH= 4-6 and Eh = 0.3-1 V (Takeno, 2005). Petrographic and mineralogical study points to the development of Fe-Mn concretions in three stages: 1) The Fe-Mn concretion growth and development of zonal structures related to the oscillation of redox potential and variation of Fe and Mn within circulating groundwater through the porous Vijenac Quarry grey wackes.
2) Two generations of carbonates developed within microtectonically-related veins and veinlets crosscutting surrounding the concretions or within the concretion matrix. First generation veins contain secondary Mn-dendritic structures. 3) Large calcite grains within rst generation veins are subjected to partial dolomitization, also recognized within the concretion matrix. The Vijenac Quarry concretions are similar to the Fe-Mn concretions within sandstones occurring as a result of precipitations of Fe-Mn oxyhydroxides from oscillating groundwater. This formation mechanism was considered for the Vijenac Quarry concretions for several reasons: (i) host rock lithology (sandstones); (ii) missing nucleus of the studied concretions; (iii) the massive structure of the small concretions and complex structure including rinds and multiple layers of the large concretions. The studied concretions are formed within consolidated sandstones inheriting their sedimentary textures most likely related to the uplift of the Pogari Formation toward a level of the meteoric water system. Formation of the concretions might have occurred during the Eocene to Oligocene uplift of the Dinarides.