A contribution to petrology of dark grey to black interbeds within Upper Permian and Triassic carbonate rocks in the area between Ljubljana and Bloke, Central Slovenia

This paper presents results of macroscopic, microscopic, chemical, and isotopic investigations of 12 samples of dark grey to black coloured interbeds occurring within Upper Permian and Triassic lime-, doloand marlstones in an area of Outer Dinarides between Ljubljana and Bloke in Central Slovenia. An additional sample is anthracite of the Carnian age from the Orle locality. Concentration of Corg in four samples is below 1 %, and in seven samples it varies between 1 and 2.3 %. Only in one sample, in the black Carnian limestone of the Lesno Brdo area, it is somewhat higher than 5 %. The highest Corg content, 30.61 %, was analysed in the Orle anthracite. Chemical analysis of major elements (as oxides) showed that four samples are clearly siliciclastic mudrocks, with 65–80 % SiO2 + Al2O3. Three samples are typical calcite rich – dolomite poor rocks, with high loss on ignition (LOI about 40 %) derived from calcite decomposition. Four samples are calcite – dolomite characterized rocks with LOI of 34–43 %. One sample, from the Slugovo quarry, is composed of quartz, dolomite and calcite. The anthracite sample from Orle has inorganic matter composed almost exclusively of SiO2 + Al2O3 (clays), and some iron and sulphur which form pyrite. Isotopic composition of the calcite carbon ranges from δCCaCO3 -5.7 to 1.9 ‰, whereas isotopic composition of the organic carbon varies between δCorg -34.7 and -21.6 ‰. The most negative δCorg value of -34.7 ‰ was analysed in a sample, which is the most organic-rich limestone. Isotopic investigations of nitrogen, expressed by δ15N values, also did not express notable differences in respect to lithology. They vary between 4.6 and 9.1 ‰. Microscopy of polished surface samples showed clearly fine grained siliciclastic, carbonate and coal composition of the treated rocks.


ntroduction
The aim of this study was to investigate basic petrologic, mainly microscopic and chemical characteristics of dark-coloured interbeds within Upper Permian and Triassic rocks in the territory south of Ljubljana, towards Bloke. More precisely, the investigated area extends between Ljubljana in the north, Postojna in the west, Velike Bloke in the south and Ribnica in the east (Fig. 1). From the geographical point of view, this area belongs to the Notranjska and Dolenjska karst area (Melik, 1959). Tectonically, it is a part of the Notranjska-Dolenjska Mesozoic Blocks (Buser, 1974) and geotectonically, a part of the External Dinarides (PreMru, 2005;Placer 2008; and references there-in).
As known from the Basic Geological Map of Yugoslavia 1 : 100,000, and the monograph Geology of Slovenia (Eds.: Pleni~ar et al., 2009) the area of External Dinarides south of Ljubljana is built up mostly of Mesozoic carbonates, underlain by Permian and Carboniferous rocks. Occurrences of Tertiary rocks are "fragmental" and will not be discussed in this paper. Within Mesozoic rocks, our study is mostly restricted to the Triassic carbonates. We include only one locality from the Upper Permian rocks. The term "Triassic carbonates" refers mostly to limestones and dolostones which are more or less bedded and massive, respectively. In general, Triassic carbonates are prevailingly grey in colour. Limestones and dolostones sporadically contain thinly bedded bed-sets of either carbonate or non-carbonate (Si-Al) mudrocks, which are quite often dark grey to black in colour. As already mentioned, exactly these dark inter-beds, at some localities having appearance of coaly rocks and even true coals, were target lithologic varieties of our investigations.
Concerning coals, the most known locality in the study area is that at the Orle-Klen , where an anthracite-rank coal (Rm%: 4.5) (haMrla, 1987) occurs in three lenticular beds (up to 0.5, 0.5 and 1.0 m thick) within Carnian (Upper Triassic) organic-rich limestone beds (sedlar et al. (1948). Between 1878 and 1948, this coal was mined underground in small quantities (below 1500 tons/a) (^e{MiGa, 1959). Some other coal layers, that are currently considered to occur mostly in the Carnian beds, but are of lower extent than those at the Orle-Klen locality, were mentioned in the works of kossMat (1902), kraMer (1905), ŠPoljari} (1917), and Petrascheck (1926and Petrascheck ( , 1926and Petrascheck ( /29, 1927, and finally summarized to a great part by rakovec (1955) in the book History of Ljubljana. In this book, rakovec (1955) described coal occurrences in the borderland of the Ljubljana Moor and wider surroundings. He worked out occurrences of coal and anthracite, respectively, in Podlipska dolina, Drenov Gri~, Lesno Brdo area, Klen (at Orle), Orle, eastern borderland of the Ljubljana Moor, Lipalnica south of Horjul, Vnanje gorice, and near Dule. ŽleBnik & Grad (1953) mapped »Wengen and Raibl beds« between Drenov Gri~, St. Jo{t and Butajnovo. Pleni~ar et al. (1970) quoted coal lenses at Dule near Škofljica, Gri~ at Ligojna and Lipalnica south of Horjul. Buser (1974) described Carnian beds with coal at Orle. dozet (1979Orle. dozet ( , 2002 studied lito-and biostratigraphy of the Carnian beds south of Ljubljana and described a paralic shallow-water coal-bearing formation termed as the Grosuplje-Orle Formation. Within this formation, bituminous coal and anthracite, respectively, is interpreted to occur within limestones of the lower part of the Julian stratigraphic sequence. dolenec & jelen (1987) studied isotopic composition of carbon and oxygen of the Carnian beds in the Lesno Brdo quarry, and three years later jelen (1990) published his study on litostratigraphy, bivalves and their paleobiological significance in the Carnian carbonate-clastic beds as exposed in two Lesno Brdo quarries. At the same locality, oBlak (2001) studied Carnian Foraminifera. In 1990, a short contribution on fossil lamellibranch fauna from the Carnian beds at Orle was published by jurkov{ek & jelen (1990).
Although coal resources in the Triassic beds, and also in the Pre-Triassic as well as in younger Mesozoic beds, are presently recognised as out of any economic value in Slovenia, dark varieties of Mesozoic rocks did invoke some attention as potential source rocks for hydrocarbon generation. In the area of External Dinarides in Slovenia, these rocks were for the last time under more detailed investigations in the 1980s. A published work about the oil and gas potential of carbonates of External Dinarides in Slovenia, based on almost 200 samples, is that of oGorelec et al. (1996). Maturity of organic matter versus clay mineralogy of Carboniferous to Tertiary sediments was regionally studied on nearly 1000 samples by rainer et al. (2002).
An overview study of isotopic composition of O and C of Mesozoic carbonates on almost 300 samples was carried out by oGorelec et al. (1999). Isotopic composition of different geological materials and media, and geochemical processes leading to their characteristic isotopic composition is described in Pezdi~ (1999). Basic research work in Slovenia referring to C and O isotopic composition at the transition from Permian to Triassic strata was made and published by dolenec & oGorelec (2001), dolenec & raMov{ (1998), and dolenec et al. (1999a,b, 2000, 2003, 2004, 2006.

Methods
Our study is based on regional geological mapping (1 : 10,000 and 1 : 25,000) of the area south of Ljubljana towards Bloke, and field sampling of dark coloured (organic-rich) rock varieties, either siliciclastics or carbonates. By the field outlook, the samples resembled to different organic matter rich rocks, as coaly, sapropelic, oil and/or gas sourcing, and oil shale rocks, respectively. All sites of sampling are shown on the map in Fig. 1 and were photographed . Litho-stratigraphic column of the rocks investigated and positions of samples 1 (bottom) to 13 (top) are shown in Fig. 3. The column was made by compilation of regional geological data from the map sheets Ribnica (Buser, 1969(Buser, ,1974, Kranj (Grad & Ferjan~i~, 1974, 1976, Postojna (Pleni~ar et al., 1967), and Ljubljana (PreMru, 1983 of the Basic Geological Map of Yugoslavia 1 : 100,000, and by regional geological mapping of the first author of this paper in recent years. A representative fragment of each sample was photographed to show dark colour of the samples and their structure (Fig. 4). Colour was defined using the Rock Colour Chart (RCCC, 1970).
In addition to the field/stratigraphic/tectonic positioning of the samples, further aim of this study was to analyse these samples more in detail, using microscopic and bulk chemical analyses.
For microscopic investigation, polished blocks with reflective surfaces, as in ore and/or coal micro-petrography, were prepared -since more coaly materials were expected at the beginning of the investigation. Polished blocks were inspected under normal white reflected polarized light.
Preparation of samples for chemical analysis was done at the Geological Survey of Slovenia according to well established procedure as practiced by the survey's geochemical group. Samples were dried and pulverized, 10 grams in weight, and sent to the ACME (Canada) laboratory (www. acmelab.com -Acme Labs Schedule of Services & Fees 2009-2010. They were analysed on major "rock-forming" elements (as oxides) by the method of inductively coupled plasma (ICP) -emission spectrometry (ACME Group 4A). Total carbon and sulphur, graphite carbon and organic carbon were analysed by Leco (combustion infrared detection technique) (ACME Group 2A). Results of major elements analysis, together with loss on ignition (LOI) at 1000 °C, and forms of carbon are given in Tab. 1.
The isotopic composition of carbon and nitrogen was determined using a Europa 20-20 continuous flow IRMS ANCA-SL preparation module. 20 mg of homogenized sample was weighed in a tin capsule for nitrogen and 1 mg for carbon analysis. Samples for carbon analysis were pretreated with 3 molar HCl to remove carbonates. The isotopic composition of nitrogen and carbon was determined after combustion of the capsules in a hot furnace (temperature 1000 °C). Generated products were reduced in a Cu tube (600 °C), where excess O 2 was absorbed. H 2 O was trapped on a drying column composed of MgClO 4 . Gases were separated on a chromatographic column and ionized. NBS 22 (oil) and IAEA N-1 (ammonium sulfate) reference materials were used to relate the analytical results to the VPDB -Vienna Pee Dee Belemnite (karbonat fosilne {koljke Bellemnitela americana), and AIR standards as follows: Where: R sample -ratio 13 C/ 12 C in sample ( 15 N/ 14 N for δ 15 N in sample) R RM -ratio 13 C/ 12 C in reference material, ratio 15 N/ 14 N for δ 15 N in reference material δ 13 C CaCO 3 in organic rich carbonate rocks (except anthracite sample) could only be measured in samples were beam area was above detection limit. 10 mg of sample was first flushed with He and then transformed to CO 2 by H 3 PO 4 acid treatment. CO-1 and NBS 19 were used as reference materials. The carbon isotope composition of carbonate (δ 13 C CaCO 3 ) was measured with a Europa Scientific 20-20 continuous flow IRMS ANCA -TG preparation module.
Stable isotope results are expressed in the conventional delta (δ) notation, defined as per mil (‰) deviation from the reference standard VPDB. Precision of working standards was ± 0.2‰ for δ 13 C org , δ 13 C CaCO 3 and δ 15 N, respectively.

Description of sampling localities and investigated samples
On the basis of data from the already cited geological map sheets of the Basic Geological Map of Yugoslavia (Pleni~ar et al., 1967(Pleni~ar et al., , 1970Buser, 1969Buser, , 1974Grad & Ferjan~i~, 1974, 1976PreMru, 1983 a,b), and self observations, we submit the following description of the sampling localities that cover the study area in Fig. 1.
Gorenji Lazi quarry (sample 1) The Gorenji Lazi quarry is situated about one kilometre to the NW of Žlebi~ (Fig. 1). The quarried rocks (Fig. 2a,b) are the Upper Permian carbonates ( Fig. 3) of the Žažar (Bellerophon) beds (raMov{, 1978) (or the Žažar Formation). The Upper Permian carbonates in the Gorenji Lazi quary consist of dark dolomites, limestones, oolitic limestones and marls. From the Gorenji Lazi quarry, sample 1 was taken from a greyish black (N2) dolostone (Figs. 3,4). rni potok (sample 2) rni potok locality ( Fig. 1) (S of Velika Slevica) refers to a 600 m high hillock at Jazbine, which is built up of Scythian, Anisian and Cordevolian beds. For our study, especially interesting were the Scythian beds ( Fig. 3) that are composed of rosy and yellowish grey, sandy (micaceous), platy and thin-bedded dolomite succession with some interberds of a dark platy limestone. In the upper-ig. 1. Map of sampling sites Sl. 1. Karta odvzema vzorcev most part of the Scythian beds occurs 3.5 to 4.5 m thick bedset of dark grey to black (coaly like) carbonate rocks (mainly dolostone). The uppermost Scythian sedimentary succession is overlain by about 60 m thick sequence of pale yellowish grey bedded and massive Anisian dolomite, covered by thick-bedded medium light grey biointrasparitic and biolithitic limestones of the Cordevolian age. Sample 2 was taken from greyish black Scythian dolostone (Figs. 3,4).

Podpoljane quarry (sample 3)
The Dolenje Podpoljane quarry is situated at the road Velike La{~e -Ortnek ( Fig. 1) along which variegated Lower Triassic (Scythian) clastic sediments and dolostones are exposed. Sample 3 was taken from the greyish black dolostone (Figs. 3,4). Scythian beds in the Podpoljane quarry ( Fig. 2c) are tectonically highly deformed. They form a syncline dipping approximately to the north. Ten metres thick synclinal core is built of black platy carbonate sediments (limestone, dolostone, marlstone) intercalated by up to 20 cm thick seams of dark grey to black (coal-like) mudstone (Fig. 2c). This dark grey to black organic matter enriched sequence, underlain by bedded and platy yellowish grey to grey sandy dolostone with a distinctive parallelepiped cleavage (Fig. 3) belongs most probably to the Brinje (Cencenighe) member of the Vi{nja Gora Formation of the Uppermost Scythian age (dozet, 2000).
Ortnek (sample 4) At Ortnek, at the cross-road Velike La{~e -Žle-bi~ and Ortnek -Velike Poljane (Fig. 1), exposed are Anisian and overlying Ladinian rocks (Fig. 2d, 3). Thirty metres thick Anisian lithological interval is composed of light massive dolostone. It is concordantly overlain by the Ladinian carbonate sequence that may be separated in two parts. The lower part is composed of greyish black limestones alternating with dark olive grey marlstones (Fassanian), whereas the upper part is composed of rosy and reddish brown bedded intrasparitic limestones with red marlstones, claystones and black limestones (Langobardian). In the described sequence, Ladinian conodonts have been determined. Within the Anisian dolomite succession, a 7.5 m thick horizon of black marlstone and coaly claystone containing several intercalations of black dolomite, dolomitic breccia and up to 25 cm thick seams and bodies of hard coal occurs.

Slugovo quarry (sample 5)
The Slugovo quarry is situated in the Cer k-ni{~ica valley N of Bloke (Fig. 1). Ladinian dark coloured bedded limestone with dark, organic matter enriched interbeds of platy micritic limestone (mudstone), marlstone and claystone is exposed in this quarry (Fig. 2e). Sample 4 (Figs. 3, 4) was taken from the micritic limestone. In a broader frame, these rocks belong to the lower part of the so called Slugovo Formation (raMov{, 1994/95;dozet & Buser, 2009). The upper unit of the Slugovo Formation is built of medium-grey, grey and medium dark grey, sometimes banded biomicritic, micritic and intramicritic limestone. The uppermost part of the upper unit consists of platy and bedded, dark micritic limestones and interbeds of reddish marlstones and shaly claystones. According to raMov{ (1995) Mala Stara vas (sample 6) At this locality at Grosuplje (Figs. 1, 2f, 3) were observed alternating black to grey and light strata of limestones of the Carnian age. The black lithological varieties, from which sample 6 was taken (Fig. 4), were found to be prevailingly non-carbonate. Due to tectonic effects, the rock is highly crushed at this locality.
Orle-Klen (sample 7) The Orle-Klen locality is situated about ten kilometres SE of Ljubljana (Figs. 1, 2g). It is a part of the Grosuplje-Orle Formation, a variegated sucession of paralic and shallow marine sedimentary rocks between the Cordevolian Di-plopora limestone or/and dolomites and the Main Dolomite Formation. The Grosuplje-Orle Formation is therefore of the Julian-Tuvalian age and is composed of colourful succession of limestones and marlstones interbedded by conglomerates, brec cias, sandstones, shales and tuffs (Fig. 3). A par ticularity of this rock succession is coal of the anthracite rank (Rm%: 4.5 -by haMrla, 1987) that lies in the lower part of the Grosuplje-Orle Formation, i.e. in the Julian beds. Coaly matter, black shale and black limestone strata are visible in ness can locally enlarge up to 4 m. Coal seams pass laterally and vertically in coaly shales and claystones. At the Klen locality near Orle, three anthracite seams (50, 50 and 120 centimetres thick) were mined in the past (until 1948). Entrance into the mine is still visible (Fig. 2h). Sample 7 -anthracite (Fig. 4) -was taken from a waste dump site close to the mine.
centuries by good quality building and statuary natural stone (vesel et al., 1992;Mirti~ et al, 1999). The three quarries extend approximately in the W-E direction. The western one is opened in the Cordevolian limestone of a rosy colour, micritic, and with lenses and nests of red, pur-ple and greenish claystone (paleo-carstification). Lateral ly out of the quarry, and vertically downwards, this limestone transits to a light grey Cordevolian sparitic dolostone and limestone with a typical alga Diplopora annulata Schafhaütl. The two quarries to the east are opened in a prevailingly dark grey to black micritic limestone beds (Figs. 2i, 3) from which samples 8 and 9 were picked out. In a jargon, these two quarries, which are not actively exploited at the present, were called "the black quarries". Dark limestone was defined to be of the Julian (middle Carnian) age. However, the basement of this black limestone succession, which lies discordantly upon the Cordevolian dolomite, is composed of yellowish grey, somewhere poorly oolitic claystone passing locally into the iron-rich oolitic bauxite or a transgressive limestone comglomerate with a red clayey and limonitic groundmass. Upwards follows a clastic rock unit, which is about 15 m thick. It is composed of dark marlstone, mudstone and claystone (sample 10) with several seams of anthracite. Between 1900 and 1905, three of four known anthracite seams have been ascertained there with exploration trenches and a shaft. Individual seams were from 1.0 to 1.5 m thick. Between 1901 and 1902, the anthracite was excavated but very soon later the exploitation was stopped. Namely, due to numerous faults and displacements along the fault-surfaces, the coal seams disappeared already at short distances. A great problem was also underground-water. The anthracite-bearing unit is overlain by about 20 m thick sequence of a dark thick-bedded limestone, intercalated by shaly marlstone and claystone.

Results of chemical and microscopic analyses and discussion
Geologic position of investigated samples is given in Fig. 3. Chemical analysis of major elements (as oxides), is given in Tab. 1. As visible from Fig. 3, only one sample is Permian, whereas all others are Triassic.
Colour of samples varies from black (N1) to medium dark grey (N4) (Fig. 4). Structure of samples is massive to laminar. All samples are well lithified.
Chemical analysis shows that the samples are composed predominantly of either SiO 2 + Al 2 O 3 or Fig. 4. Photographs of 1-13 samples -note their dark colour (N1 -N4) and structure from masive to laminated. Sl. 4. Fotografije vzorcev 1-13. Zna~ilna je njihova temna barva (N1-N4) ter tekstura, ki se spreminja od masivne do laminirane. learly limestones are samples number 4, 8 and 9, intensively reacting with 1:10 diluted HCl acid (Tab. 1, Graph 1). These samples are characterized by highly predominant CaO content and high loss on ignition (LOI) -between 40 and 43 %. Their CaO + MgO content is 42-52 %. In the Upper Tria ssic samples 8 and 9, MgO content is below 1%, whereas MgO content is relatively much higher in the Middle Triassic sample 4 in which MgO : CaO ratio is close to 1 : 2. All three samples are black to greyish-black (Fig. 4). Sample 9 is outstanding by its C org content which amounts to over 5 %. Microphotographs of samples 4 and 9 are shown in Figs. 5a and 5b. Sample 4 in Fig. 5a shows granular structure with more or less grown-together carbonate grains (bright grey colour), which still show euhedral forms up to 10-20 µm in size. Fossils were not found in this sample. Black colour (C org ca. 2 %) and pyrite (Fig. 5a -right side) clearly indicate anoxic environment of formation of this black limestone. Black fields of approximately the same size as individual carbonate grains, or somewhat smaller, are pores. They seem to be more or less empty and quite well connected. Porosity, considerably effective, can be estimated to about 30 %. This characteristic can classify this limestone as a good reservoir rock. On the other hand, it might also be a source rock for oil and/or gas generation. Graph 1. CaO + MgO versus SiO 2 + Al 2 O 3 ratio Graf 1. Razmerje CaO + MgO proti SiO 2 + Al 2 O 3 Sample 9 (Fig. 5b) can be classified as a micritic limestone with broken fragments of molluscs. As already mentioned, it contains more than 5 % C org . Organic matter is finely dispersed within dark micritic matrix. In addition, fragments of blownin coal (vitrinite) particles can be found (see highly magnified insert picture in Fig. 5b), but they are very rare (below 1 %). Such rocks might be source rocks for oil and/or gas generation at sometime in the geological past after their deposition.
On the contrary to the above described limestones, clearly mudrocks are samples number 6, 10, 11, 12 (all Upper Triassic) (Tab. 1, Graph 1). All these samples contain 65-80 % SiO 2 + Al 2 O 3 . SiO 2 : Al 2 O 3 ratio varies between 2.3 and 4.3 : 1. They are predominantly composed of quartz and clay minerals. Among clay minerals, remarkable MgO and K 2 O contents may indicate small occurrences of chlorite and illite, or, feldspars. They are very poor in organic matter, except for sample 12, which is black (N1) and contains 2.3 % C org which is almost equal to C tot . Other samples contain less than 1 % C org . Samples 10 and 12 are shown in microphotographs in Figs. 5d and 5e.
Sample 5 -Anisian dolomite (silicified) -is the most Mg characterized sample (Graph 1). Its chemical composition (considerable shares of SiO 2 , and MgO, very subordinate CaO and negligible Al 2 O 3 ) indicates presence of quartz, dolomite and subordinately calcite.
Sample 7 is anthracite from Orle. This anthracite occurs in the Carnian limestone. It was sampled on the waste dump because there was no possibility to gain fresh samples from the interior of the abandoned mine. As visible from Figs. 5f and g, this anthracite is composed of detro-and telovitrinite. Typical are dark oval structured "bodies" filled with clay (note almost exclusive SiO 2 + Al 2 O 3 composition of the inorganic matter for this sample in Graph 1). Graph 3. C org and C tot contents, and loss on ignition (LOI) Graf 3. Vsebnosti organskega (C org ) in skupnega (C tot ) ogljika ter žaroizguba (LOI). Because both relatively significant organic matter and carbonate matter were expected to be present in the samples, C tot and C org were analysed. In most samples, LOI was found to be primarily dependant on carbonates (Graph 2). Relation between forms of carbon (C org and C tot ) and LOI is shown in Graph 3. Percent of C org in relation to C tot is shown in Graph 4. In general (nine of thirteen samples), it is below 20-25 %. In two samples, it is around 40 %, and in two samples (7 and 12) it is 98 and 100 %, respectively. C org varies in all samples from 0.25 to 2.30 %. Only in sample 9 (Carnian limestone), it is 5.36 %, and in the anthracite sample 7, it is 30.61 % (Tab.1, Graph 3). Carnian limestone is regionally well known by its black colour. Due to Alpine tectonics, it is thoroughly folded and fissured, the fissures being filled with calcite. It contains black laminated interbeds and locally hosts even thin beds and lenses of coaly matter. The Orle anthracite (sample 7) (see also introduction) occurs in such beds. Similar occurrences of bituminous -25.5 -1.0 Carbonate Graph 5. Isotopic composition of organic and of calcite carbon expressed by δ 13 C org and δ 13 C CaCO 3 values. Graf 5. Izotopska sestava organskega in kalcitnega ogljika, izražena z vrednostmi δ 13 C org in δ 13 C CaCO 3 . Graph 6. Relation between C org content and δ 13 C org value (marked are three outstanding samples). Graf 6. Relacija med vsebnostjo C org in δ 13 C org vrednostjo (ozna~eni so trije izstopajo~i vzorci).
coal to anthracite are also known at some other localities (Ligojna, Drenov Gri~). Further-on, also from previous investigations, Carnian limestone is well known by its outstanding C org content. Namely, oGorelec et al. (1996), in their regional study about the Permian and Mesozoic carbonate rocks of W Slovenia as potential source rocks for hydrocarbon generation, clearly show the highest C org content in the Carnian limestone beds. According to their data (oGorelec et al., 1996 -their Fig. 2), C org content in the Carnian limestone considerably exceeds 2 % and is markedly the highest in comparison to all other Permian, Mesozoic and even Paleocene (Liburnian) formations, where it does not exceed 1%.
In organic matter of the investigated samples, δ 13 C org varies between -34.7 and -21.6 ‰ (Graph 5). In most carbonate mudrock samples, except for samples 8 and 13, δ 13 C org is below -25.5 ‰. In more Si-Al mudrocks and the anthracite sample, δ 13 C org is somewhat above -25.5 ‰. Maybe, this slight partition could be attributed to different diagenetic effects of organic matter in different geochemical media, namely carbonate (alkaline) and silico-aluminous (acid), respectively. Alkaline environment is well known to be an enhancer of biochemical transformation of organic matter (e.g. taylor et al., 1998). Another reason could be different types of original organic matter. The most negative δ 13 C org value of -34.7 ‰ was analysed in sample 9, which is the most organic-rich limestone sample. Depletion in 13 C org (decreasing in δ 13 C org values to more negative values) correlates quite remarkable with increasing C org content (Graph 6).
δ 13 C org values for our carbonate samples, which are mostly below -25.5 ‰, are comparable to those in dolenec & oGorelec (2001).
Isotopic composition of the calcite carbon ranges from δ 13 C CaCO 3 -5.7 to 1.9 ‰, but mostly between -1.0 and 1.9 ‰ (Graph 5). There is almost no dependence in isotopic composition of calcite carbon referring to lithology and bulk chemical composition of the investigated samples. In comparison to results of isotopic composition of calcite carbon in Mesozoic carbonate rocks published by oGorelec et al. (1999), our samples correspond to their organic-rich limestones, dedolomites and diagenetically altered dolomites, respectively.
Isotopic investigations of nitrogen, expressed by δ 15 N values, also do not express distinctive differences in respect to lithology. They vary between 4.6 and 9.1 ‰. Both extreme values refer to Si-Al rich rock samples 6 and 12 (Tab. 1). The whole range of isotopic composition of nitrogen is comparable to soil material investigated in the watershed of the Idrijca River (kandu~ et al., 2008).

Conclusions
This study was performed as a preliminary study to investigate some varieties of so called black mudrocks occurring as more or less thick bed-sets within Upper Permian and Triassic carbonate rocks in the area between Ljubljana and Bloke, recently geologically mapped by the first author. For this purpose, 13 samples of dark grey to black interbeds within mainly carbona te rocks were collected. For all sites of sampling, detailed textual and photographic descriptions with representative citations of previous researchers are given in this paper. The samples were investigated chemically, microscopically and by isotopic composition of carbon and nitrogen. At the first glimpse, on the field, the samples resembled to coaly materials, but in fact, only one sample was really coaly -the Orle anthracite -in fact authracitic minerite. All other 12 samples were clearly grouped into Si-Al mudrocks with 65-80 % SiO 2 + Al 2 O 3 and into carbonate mudrocks with less than 25 % SiO 2 + Al 2 O 3 , more than 35 % CaO + MgO and ca. 35-45 % loss on ignition (at 1000 °C) derived from decomposition of carbonates. C org of four samples (mostly siliciclastic mudrocks) was below 1 %. It was somewhat higher in carbonate mudrocks, up to 2.3 %, and the highest in sample 9 (HCl reacting limestone) which contained 5.36 % C org . According to the schluMBerGer Oil Field Glossary, rocks with 1 to 4 % C org can be termed as fair to good oil/gas source rocks, and those with more than 4 % as very good source rocks. It can be concluded that carbonate mudrocks are better source rocks than Si-Al mudrocks.
Detailed field observations at localities of sampling, broader regional geological information and results of microscopic and chemical analyses indicate in general that the Upper Permian as well as Triassic organic matter enriched carbonate and noncarbonate (siliciclastic) sediments accumulated in relation to transgressive-regressive cycles in restricted shallow lagoonal environments (with more or less intensive water and organic matter influxes from the hinterland terrains).
Isotopic composition of organic carbon slightly differs between Si-Al mudrocks and carbonate mudrocks. In first case, δ 13 C org is somewhat above -25.5 ‰, and in the second case somewhat below -25.5 ‰. The whole range of δ 13 C org values for all samples varies from -34.7 to -21.6 ‰. The reason could be isotopic differentiation due to different diagenetic effects in organic matter in different original litho-geochemical environments -either Si-Al (relatively acid) or carbonate (relatively alkaline).
Isotopic composition of calcite carbon varies between δ 13 C CaCO 3 -5.7 and 1.9 ‰. The results did not show dependence on lithological and/or chemical composition of the investigated samples, but are well comparable to organic-rich limestones, dedolomites abd diagenetically altered dolomites as thoroughly defined by oGorelec et al. (1999). Isotopic investigations of nitrogen also did not show distinctive differences in respect to litho logy. δ 15 N values varied between 4.6 and 9.1 ‰. Both extreme values were analysed in Si-Al mudrocks.
This study showed some approaches and contributions to previous investigations, especially those of oil and gas potential of carbonate rocks having been carried out by oGorelec et al. (1996) and rainer et al., (2002), as well as to previous isotopic studies of Mesozoic carbonate rocks by oGorelec et al. (1999).

cknowledgements
This study was done in the framework of the scientific programme Sedimentology and Mineral Resources P1-0025 (D) financed by the Slovenian Research Agency. A wealth of fundamental geological information of previous investigations was summarized from the Basic Geological Map of Yugoslavia 1 : 100.000 (sheet maps and textual guides for Slovenia). Among our technical collaborators at the Geological Survey of Slovenia, we express our great thanks to Mrs. Du{ka Živanovi}, and the young colleagues Sniježana Mileti}, and Matjaž Budkovi~, to all of them for their graphical support, and to Mrs. Bernarda Bole for the final technical editing of the text and graphics. We are also highly thankful to the reviewers of the paper and their constructive, positive and welcome comments.