A contribution to petrology of dark grey to black interbeds

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GEOLOGIJA 55/1, 77–92, Ljubljana 2012 doi:10.5474/geologija.2012.006 A contribution to petrology of dark grey to black interbeds within Upper Permian and Triassic…
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GEOLOGIJA 55/1, 77–92, Ljubljana 2012 doi:10.5474/geologija.2012.006 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 Prispevek k petrologiji temno sivih do ~rnih plasti v zgornjepermskih in triasnih karbonatnih kamninah na obmo~ju med Ljubljano in Blokami Stevo DOZET1, Tja{a KANDU^2 & Milo{ MARKI^1 1 Geological Survey of Slovenia, Dimi~eva ulica 14, Ljubljana, SI-1000, Slovenia; e-mail: stevo.dozet�gmail.com; milos.markic�geo-zs.si 2 Jožef Stefan Institute, Jamova ulica 36, Ljubljana, SI-1000, Slovenia; e-mail: tjasa.kanduc�gmail.com Prejeto / Received 22. 5. 2012; Sprejeto / Accepted 12. 6. 2012 Key words: Permian, Triassic, mudrocks, carbonates, anthracite, chemical composition, C and N isotopic composition, microscopy, Central Slovenia Klju~ne besede: perm, trias, glinavci, karbonati, antracit, kemi~na sestava, izotopska sestava C in N, mikroskopska sestava, osrednja Slovenija Abstract 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-, dolo- and 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 δ13CCaCO3 -5.7 to 1.9 ‰, whereas isotopic composition of the organic carbon varies between δ13Corg -34.7 and -21.6 ‰. The most negative δ13Corg 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. Izvle~ek V ~lanku predstavljamo rezultate makroskopskih, mikroskopskih, kemi~nih in izotopskih preiskav 12 vzorcev temno sivih do ~rnih plasti, ki nastopajo znotraj zgornjepermskih in triasnih apnencev, dolomitov in laporovcev na obmo~ju Zunanjih Dinaridov med Ljubljano in Blokami. Dodatni vzorec je vzorec antracita karnijske starosti z obmo~ja Orel pri Ljubljani. Koncentracija organskega ogljika (Corg) {tirih vzorcev je nižja kot 1 %, v sedmih vzorcih pa se spreminja med 1 in 2,3 %. Samo v enem vzorcu, z Lesnega Brda, je nekoliko vi{ja kot 5 %. Najve~ja vsebnost Corg, 30,61 %, je bila analizirana v vzorcu antracita z Orel. Kemi~na analiza glavnih prvin je pokazala, da so {tirje vzorci glinavci s 65–80 % SiO2 + Al2O3. Trije vzorci so izrazito kalcitni s podrejenim deležem dolomita. Vsi ti trije izkazujejo visoko žaroizgubo (LOI), okoli 40 %. Štirje vzorci so kalcitno dolomitni z žaroizgubo 34–43 %. Vzorec iz kamnoloma Slugovo vsebuje kremen, dolomit in kalcit, en vzorec pa je antracit z Orel, katerega anorganska snov je sestavljena prakti~no v celoti iz SiO2 + Al2O3 (gline) in nekaj železa ter žvepla, ki tvorita pirit. Izotopska sestava kalcitnega ogljika se giblje v okviru vrednosti δ13CCaCO3 med -5,7 in 1,9 ‰, izotopska sestava organskega ogljika δ13Corg pa med -34,7 in -21,6 ‰. Najbolj negativna vrednost δ13Corg, -34,7 ‰, je bila ugotovljena za vzorec, ki je najbolj z organsko snovjo bogat apnenec. Tudi izotopske preiskave du{ika niso pokazale povezav z litolo{kimi razli~ki. δ15N se spreminja med 4,6 in 9,1 ‰. Mikroskopska preiskava poliranih obruskov je jasno pokazala siliciklasti~no, karbonatno in premo{ko (antra­ citno) sestavo obravnavanih vzorcev. 78 Stevo DOZET, Tja{a KANDU^ & Milo{ MARKI^ Introduction 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 Notranj­ ska-Dolenjska Mesozoic Blocks (Buser, 1974) and geotectonically, a part of the External Dinarides (Premru, 2005; Placer 2008; and references there-in). The Dolenjska and Notranjska area was firstly mapped by Lipold (1858). Results of this mapping that Lipold realized together with Stache, were two manuscript map sheets on the scale of 1 : 75,000, namely the Vi{nja Gora–Cerknica and the Laze–^abar sheets. Fifty years later, Kossmat (1910) wrote an Explanatory guide to the Geological map of the area between Škofja Loka and Idrija. On the Basic Geological Map of Yugoslavia 1 : 100,000, our study area extends on the map sheets Ribnica (Buser, 1969,1974), Kranj (Grad & Ferjan~i~, 1974, 1976), Postojna (Pleni~ar et al., 1967, 1970) and Ljubljana (Premru, 1983 a,b). As known from the Basic Geological Map of Yugoslavia 1 : 100,000, and the monograph Geo­ logy 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 carbona­ tes. 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 carbona­ te 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 (1926, 1926/29, 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 wi­ der 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 (1979, 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 carbona­ tes 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 lea­ ding 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. (1999 a,b, 2000, 2003, 2004, 2006). A contribution to petrology of dark grey to black interbeds within Upper Permian and Triassic carbonate rocks in the ... 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 (Figs. 2a-k). 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 Ri­ bnica (Buser, 1969,1974), Kranj (Grad & Ferjan~i~, 1974, 1976), Postojna (Pleni~ar et al., 1967,1970), and Ljubljana (Premru, 1983 a,b) 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 met­ hod 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 mo­ dule. 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). Genera­ ted products were reduced in a Cu tube (600 °C), where excess O2 was absorbed. H2O was trapped 79 on a drying column composed of MgClO4. 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: δ13Csample (δ15Nsample) = ((Rsample-RRM)/RRM) Where: Rsample - ratio 13C/12C in sample (15N/14N for δ15N in sample) RRM - ratio 13C/12C in reference material, ratio 15 N/14N for δ15N in reference material 13 δ CCaCO in organic rich carbonate rocks (except anthracite sample) could only be measured in samples were beam area was above detection li­ mit. 10 mg of sample was first flushed with He and then transformed to CO2 by H3PO4 acid treatment. CO-1 and NBS 19 were used as reference mate­ rials. The carbon isotope composition of carbonate (δ13CCaCO ) 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 δ13Corg, δ13CCaCO and δ15N, respectively. 3 3 3 Description of sampling localities and investigated samples On the basis of data from the already cited geo­ logical map sheets of the Basic Geological Map of Yugoslavia (Pleni~ar et al., 1967, 1970; Buser, 1969, 1974; Grad & Ferjan~i~, 1974, 1976; Premru, 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, ooli­ tic 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- 80 Stevo DOZET, Tja{a KANDU^ & Milo{ MARKI^ A I 46° H 15° Ljubljana SLO Study area CRO 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, Fig. 1. Map of sampling sites Sl. 1. Karta odvzema vzorcev 3). Thirty metres thick Anisian lithological interval is composed of light massive dolostone. It is concordantly overlain by the Ladinian carbona­ te 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 broa­ der 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. Accor­ ding to Ramov{ (1995), conodonts indicate the Upper Fassanian age of the investigated rocks. A contribution to petrology of dark grey to black interbeds within Upper Permian and Triassic carbonate rocks in the ... 81 Fig. 2. Photographs of sample sites – for sample position and age see also Fig. 3, and for composition see Tab 1 (all photographs from the years 2005 to 2010):
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