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

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.