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Petrography and depositional environment of the No. 308 coal seam (Upper Silesian Coal Basin, Poland)—a new approach to maceral quantification and facies analysis
International Journal of Coal Geology 68 (2006) 117 – 126 www.elsevier.com/locate/ijcoalgeo
Petrography and depositional environment of the No. 308 coal seam (Upper Silesian Coal Basin, Poland)—a new approach to maceral quantification and facies analysis Jacek Misiak Department of Coal Deposits Geology, AGH University of Science and Technology, Faculty of Geology, Geophysics and Environment Protection, al. Mickiewicza 30, 30-059 Cracow, Poland Received 5 November 2004; received in revised form 26 November 2005; accepted 26 November 2005 Available online 24 February 2006
Abstract Oriented block samples of coal representing the lower layer (2.1 m) of the No. 308 coal seam were collected in the Ziemowit mine. Microscope investigations were carried out using a self-developed, fast method of maceral quantification that enables obtaining a continuous petrographical profile (“microprofile”) of a seam and computer processing of the data. The contents of macerals and maceral groups distinguished along the profile were averaged using the method of moving averages to show trends of environmental changes in a mire. The facies diagram proposed is based on an assumption that oscillations of water table in the mire affect the coal petrographical composition, while stronger water influxes into the mire increase the mineral matter content of coal. The author has identified three major types of the mire environments in which plant material was deposited during deposition of the No. 308 coal seam. These are permanently inundated = planar mire (PM) with two sub-environments—PM “margin” and PM “central”; temporarily inundated = transitional mire (TM) with two sub-environments—TM “wet” and TM “dry”; and elevated = domed mire (DM) with two sub-environments—DM “progressive” and DM “regressive”. © 2006 Elsevier B.V. All rights reserved. Keywords: Upper Silesian Coal Basin; Poland; Coal petrography; Macerals; Facies diagram; Depositional environment
1. Introduction There is a growing interest, marked particularly in the last 10 years, in facies investigations of coal seams. Investigations have used various methods of geological analyses to determine the type of plant material and environmental conditions of coal formation. Lithological investigations of coal seams, petrographical observations based on averaged samples of coal seams (often applied in the case of borehole cores), E-mail address: [email protected]. 0166-5162/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2005.11.008
lithological investigations supplemented by petrographical ones, as well as other modifications of lithological and petrographical methods, can be mentioned as examples. Interpretation of petrographical investigations are also based on a range of methods. The type of the environment was assessed from the already-known genesis of selected macerals or maceral assemblages (microlithotypes) or from various “facies formulae” and diagrams (Diessel, 1986; Veld et al., 1996). Current study investigates the genesis of coal seams, trying, on one hand, to work out a method of recording
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and storing the results of petrographical observations as a basis for their multifaceted data processing and, on the other, to determine the development history of a selected coal seam, the No. 308 seam. The coal seam is one of the most extensive seams and is mined over a large area in the eastern part of the Upper Silesian Coal Basin (USCB) in Poland. 2. Localization of the study area Geological investigations were carried out within the coal field of the Ziemowit colliery in the main basin of the Polish part of the USCB (Fig. 1). They were focused in the downthrown limb of the “Książęcy” fault (displacement is about 400 m), in the area of the “Ławecki” and “Piastowski” faults (displacements are about 70 and 60m, respectively), transverse to the former. Coal seam No. 308 was selected for lithological, petrographical and facies studies. Mining is advanced in this region; therefore, underground workings are accessible. Also, the lithology is diverse, so the seam represents a proper section to carry out the abovementioned investigations.
3. Geological characteristics of the No. 308 seam The No. 308 seam is situated in the upper part of the Mudstone Series (Pennsylvanian, Orzesze Beds s.s.; Fig. 2). In the study area, it lies 30–60m below the No. 301 seam, which terminates the sedimentation of the Mudstone Series. In this region, the Mudstone Series is about 750 m thick, with the tendency of thinning to the east (Porzycki, 1972). Coarse-grained, poorly sorted sandstones of the Cracow Sandstone Series overlie the No. 301 seam. The Mudstone Series is composed of aleuritic– pelitic sediments, with intercalations of sandstones and numerous coal seams. The horizontal variability of the sediments (lithofacies) and their thickness are the results of shifting of depositional sub-environments. These processes are typical for an alluvial plain undergoing permanent subsidence (Doktor and Gradziñski, 1985). In the study area, in the transition zone between the Mudstone Series and Cracow Sandstone Series, the No. 308 seam is one of the thickest seams (∼4 m). It comprises two layers of humic bituminous coal beds,
Fig. 1. Geological sketch map of the Upper Silesian Coal Basin (modified after Jureczka et al., 1995) with the location of the “Ziemowit” coal mine.
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Fig. 2. (A) Position of the No. 308 coal seam in lithostratigraphic subdivision of the Pennsylvanian system of the USCB in Poland. (B) Generalized profile of the No. 308 coal seam.
intercalated by a claystone or locally by a mudstone layer (Fig. 2) with a thickness from 2 to 15 cm, or with a local concretions of clayey siderite, respectively. The underlying sediment is grey claystone with roots. The lower layer of the seam, with a thickness of about 2m, which represents the subject of the current investigations, consists of coal without macroscopically visible mineral intercalations. The upper layer of the coal also with a thickness of about 2m contains many clay intercalations and nests. Towards the central part of the area, over a distance of some hundred meters, the seam becomes richer in clay until the coal is completely replaced by compact, dark grey claystone with numerous carbonized plant fragments that indicate the bedding. The carbonized plants, often with the imprints typical of Sigillaria, occur in the form of thin vitrain lenses. Transition from the coal seam towards the roof rocks is smooth via coaly shale that grades into claystone. In the following part of the text, the term “No. 308 seam” is referred to the lower layer of the seam, the subject of this study. 4. Sampling and preparation of microscope mounts The No. 308 seam was sampled in the longwall face and samples were taken as sequential oriented block
samples cut along a full profile cropping out at the exposure. To protect the samples from drying out, they were soaked with varnish. The lumps of coal were polished, obtaining in this way about 50 samples to be studied under the reflected light microscope. Their total length is about 2m, corresponding to the thickness of the seam in the sampling spot. 5. Methods of investigations Petrographical investigations were carried out using an Opton polarizing microscope (“Axioskop”), using white light and fluorescent illuminations. Observations were made with an 50× immersion objective and included qualitative and quantitative analyses of bituminous coal macerals. Methodology of the analysis adapted to facies research was based on the “microprofiling”, i.e., profiling the quantitative variability of seam macerals under the microscope. The standard method of quantification of petrographical coal components with a semiautomatic counting stage (Marchioni et al., 1994; Banerjee and Kalkreuth, 2002) limits further processing of the data obtained. The standard method is not feasible for statistical analysis using a computer program because the measurements are expressed as totals referring to sections of the profile; the sections were
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selected in advance (not always independently) and are of different lengths. The method was modified in the following way. The microscope eye-piece was equipped with a special grid (Misiak, 2001; Fig. 3) constructed so that its outer size (frame) in the microscope field of view is 0.1 × 0.125 mm (100 × 125 μm). The field is divided into 20 squares with a size of 25 × 25 μm each. The amounts of petrographic components (macerals, minerals) distinguished within the frame in the form of layers, lenses and big grains were determined by adding the number of squares they occupy. The components occurring in a disseminated form, e.g., small fragments, fine grains, etc., were quantified with typical charts for visual percentage estimations. If the ground mass (matrix) was present, its content was calculated as a difference to 100%. The next measurement is done after moving the mount at the shift equal to the grid height, i.e., at 0.1mm, to the field directly next to the former. The set of such measurements recorded over the full thickness of the coal layer represents, thus, a continuous petrographical “microprofile” of the seam. The data were stored in a computer database in tabular form. Each column corresponds to a different distinguished petrographical component (maceral), and each line corresponds to the content (vol.%) of the macerals measured in a single shift of the grid, respectively. Each millimetre of the profile is covered by 10 grid measurements and recorded as 10 lines in the table. In the seam with a thickness of 2 m, it gives 20,000 measurements. The data obtained can be processed using a calculation program, such as MS Excel, STATISTICA or others.
Fig. 3. Measuring grid of the eye-piece, applied in this work (explanations in the text).
6. Variability of macerals and mineral matter in the No. 308 seam profile The petrographic composition of the coal from the No. 308 seam is presented in Figs. 4 and 5 and listed in Table 1. The profile is dominated by the vitrinite group macerals (in average about 63%), from which the most frequent are collotelinite (46.3%), telinite (8.4%) and collodetrinite (7.4%). The inertinite group is less frequent (26%), and the liptinite group makes up 7%. The average content of mineral matter in the seam is about 4%. The variability of the coal macerals and mineral matter along the profile are represented in Figs. 4 and 5. Considering the insignificant content of secretinite and the fact that liptodetrinite can be positively identified only using fluorescent illumination, these two components are not shown on the profile. On the vertical axis, the value 0 marks the roof of the seam. The contents of specific petrographical coal components have been shown as the curves representing moving averages, calculated as the arithmetic means from the contents of these components obtained in single counts along the profile. For the graphical presentation, the averages were calculated from a series of 100 discrete counts. As a result, each point of the curve represents the content of a given maceral along 1 cm of the seam profile, with the value centred (i.e., centrally situated) against the section for which the average has been calculated. 7. The proposed version of the coal seam facies diagram Facies analysis of coal seams can be based on the results of lithological, petrographical and palaeontological investigations. They are interpreted by applying various facies diagrams, applicable in the methodology selected. In this paper, an attempt to confront different sedimentary environments of organic matter in the Carboniferous peat bogs is presented. The author accepts the cyclic oscillations of the water table level in the mire as a main factor controlling coal-forming processes, which is followed by the succession of plants in the mire, while the deposited organic fragments undergo on either gelification or oxidation, depending on the mire ground water level. Sedimentary environments of coal seams are proposed by Diessel (1986) often determined on the basis of petrographical investigations using a diagram modified by Kalkreuth et al. (1991) and Veld et al. (1996). DiMichele and Phillips (1994) presented alternative methods of description of coal-forming environments on
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Fig. 4. The content of the macerals of the vitrinite and the liptinite groups in the profile of the No. 308 seam.
Fig. 5. The content of the macerals of the liptinite and inertinite groups in the profile of the No. 308 seam.
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Table 1 The average content (%) of coal macerals and mineral matter in the No. 308 seam Vitrinite
Liptinite
Inertinite
Telinite Collotelinite Collodetrinite Vitrodetrinite Corpogelinite Gelinite
8.4 46.3 7.4 0.2 0.5 0.2
Microsporinite Macrosporinite Cutinite Resinite Liptodetrinite
5.4 1.6 0.2 0.1 0.1
Total
63.0
Total
7.4
the basis of investigations of the Carboniferous plants. The present paper put forward a proposal to distinguish three main environments within the zone of peat sedimentation, namely, permanently inundated mire, temporarily inundated mire, and elevated mire, as well as two peripheral environments: lacustrine and of flood plain, where peat does not accumulate. The “reed moor” environment distinguished by Diessel (1986) and corresponding to the present-day reed environment according to the suggestions of DiMichele and Phillips (1994) has not been separated but was included into the area of the transitional mire. It is also possible to classify this environment as calamite reeds as done by Teichmüller (1962, 1982). But calamites occupied not only the area between a mire and a lake (where reeds grow), but also the areas close to a river, and this points to their pioneering role in the development of the mire. The oscillating water table level of the palaeomire resulted high and low positions, which caused the inundation or drying of the mire. These short periods took place within the same environment, covering ecological amplitude of the environment variations tolerated by the occupying plants. Long-lasting periods of inundation and drying of the mire enforced a gradual change of its plant assemblage. For this reason, the assemblage of the plants occupying the “temporarily inundated mire” should be treated as an ecoton between the plants of the “permanently inundated mire” and “elevated mire”. The term “temporarily inundated mire” itself suggests a succession between inundation and drying periods. Fig. 6 presents the relation between the environments mentioned above and the water table level. Within the environment of “temporarily inundated mire”, the author distinguished two sub-environments depending on the climate periods: bwetQ, temporarily inundated sub-environment, characterized by the water table level above the mire surface;
Pyrofusinite Degradofusinite Semifusinite Macrinite Micrinite Funginite Secretinite Inertodetrinite Total
Mineral matter 2.7 4.3 5.5 0.8 0.8 1.9 0.04 9.8 25.8
Clay Pyrite Carbonates
3.6 0.1 0.06
Total
3.8
bdryQ, temporarily dried sub-environment, in which the water table level is below the mire surface. In the Polish and international literature, recent coal facies studies utilize, first of all, Diessel's (1982, 1986) facies indicators and diagrams. He defines indices describing the degree of gelification (GI) and determines the degree of tissue structure preservation in a sample (TPI), as well as defines different zones in this diagram corresponding to various environments and plant assemblages occupying the given environments of the mire. Diessel's method was later discussed and modified by Calder et al. (1991), Kalkreuth et al. (1991), Calder and Gibling (1994), DiMichele and Phillips (1994), Wüst et al. (2001) and Scott (2002). The present paper agrees with the suggestions of the authors mentioned above, although the petrographical coal composition is more dependent on the level of water table in the mire (inundation) than on the type of vegetation present in it. Relatively short-lasting oscillations of the water level did not need to enforce succession of vegetation if they took place within the ecological amplitude of a given plant assemblage and the changes caused by them were minor, as reflected in the maceral composition of the deposited material. The proposed version of the facies diagram (Fig. 7) refers to the environments distinguished in Fig. 6; based on the assumption that, the oscillations of the water table level within the mire affect the coal composition, while stronger water influxes into the mire are reflected in the detrital matter content of coal. When the water level was relatively high, the process of gelification was intensive, reflected in a high content of the vitrinite-group macerals. Lowering of the water level in the mire was accompanied by an increased access of oxygen and stronger oxidation, manifested by an increasing content of the inertinite-group of macerals. Oxidation is also indicated by an increase of the liptinite-group macerals in coal during degradation of the plant material, as more resistant liptinite withstands oxidation and concentrates
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Fig. 6. Sedimentary environments in the Carboniferous peat bogs, (Misiak, 2002, modified) (DEM–detrital matter, I–inertinite, L–liptinite, V– vitrinite).
Fig. 7. Diagram of the coal facies analysis.
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at the expense of vitrinite and inertinite (domed regressive mire). Higher accumulations of the liptinite group macerals may also be water or air deposited (“planar margin mire lake”, i.e., organic layers with interbeddings of lacustrine strata). A substantial role in the coal facies analysis was attributed to the presence of mineral matter (mainly clay) of coal. It is accepted (Wüst and Bustin, 2001; Ward, 2002) that the mineral matter content not higher than 5% is associated with the decomposition of plant tissue, while the content exceeding 5% indicates a stronger influx of water into the mire. The author has assumed here an arbitrary border value of 10% as indicative of the “planar margin mire-river” environment, i.e., the environment of organic deposition with partings of extra-channel pelites–aleurites. Consequently, a value of 50% has been accepted as the uppermost mineral matter content of coal. The contents higher than 50% characterize coaly rock (coal-bearing claystones, mudstones, sandstones, coaly shales). In the proposed simplified coal facies diagram (Misiak, 2003; Fig. 7), the contents of the distinguished maceral groups (instead of individual macerals) and of mineral matter are considered. However, it must be remembered that not all the macerals are facies consistent within a given maceral group excluding micrinite from the inertinite group and alginite from the
liptinite group. The facies analysis is two-pronged and starts from the diagram centre. If the clay content exceeds or equals 10% in a sample, the environment is interpreted as PM “margin” (river). If the clay content is below 10%, the sample environment is determined on the basis of the proportions among inertinite, liptinite, and vitrinite, proceeding from the second inner circle toward the outer circles of the diagram. 8. Facies analysis of the No. 308 seam The results of the investigations carried out and plotted on the diagrams proposed form the basis of identification of the formation history of the No. 308 seam (Fig. 8). In the initial stage of the peat bog formation (the section of the profile from 1982 to 1608 mm), the palaeomire was permanently inundated—PM “central”, the water level was high and temporary rises provided the detrital material—PM “margin” (river) environment. Then, as far as the position of 1436 mm, the mire became generally more shallow and graded into the transitional mire (TM). Smaller oscillations of the water level caused the changes at this stage of the mire development within the range TM “wet” and TM “dry”. The section 1436–1399mm represents further shallowing of the mire that grades into the domed mire—DM
Fig. 8. Depositional history of the No. 308 seam.
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“progressive”. Next, to the position 1180mm, the water level rose again and the mire character varied in the range of the TM “wet”—TM “dry” and PM “central” environments. The period corresponding to the section 1180–1120mm was dry and the mire evolved towards a domed mire—DM “regressive”. Such an environment has been inferred as a result of degradation of plant material due to oxidation: the macerals of the liptinite group, being more resistant, were preserved under such conditions and concentrated in the section at the expense of other coal components. Point 1120mm marks the direction of further changes in the mire. The water level started to rise, a conclusion corroborated by a higher amount of clays. The conditions became more wet, and as far as the position 459 mm, the mire periodically changed, getting shallower or deeper, grading from the environment temporarily inundated due to water level oscillations (TM “wet” and TM “dry”) to the environment permanently inundated (PM “central”). The near-roof section of the seam between the positions 459 and 18 mm represents the environment with oscillations of the water level–transitional mire (TM), getting progressively dry and grading into the domed mire (DM). A thin layer of coaly clay (188–177) points to an incursion of a broading river into the mire. The mire terminates its existence in the position 18 mm, when it became finally inundated and buried under a substantial amount of detrital material transported by fresh waters. 9. Summary The No. 308 coal seam from the Upper Silesian Coal Basin (USCB) in Poland have been used to reconstruct paleomire. Methodology of the analysis adapted to facies research was based on the “microprofiling”. The author modified the method of microscope analysis. The microscope eye-piece was equipped with a special grid. The data were stored in a computer database in tabular form and the data obtained can be processed using a calculation program. The contents of specific petrographical coal components have been shown as the curves representing moving averages, calculated as arithmetic means from the contents of these components obtained in single counts along the profile. The proposed version of the facies diagram (Fig. 7) refers to the environments distinguished in Fig. 6 and was based on the assumption that the oscillations of the water table level within the mire affects the petrographical coal composition, while stronger water influxes into the
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mire reflect in the detrital matter content of coal. The results of the investigations carried out and visualized on the diagrams proposed formed the basis of identification of the formation history of the No. 308 seam (Fig. 8). Acknowledgements I express my gratitude to Prof. Dr Ireneusz Lipiarski who supervised my D.Sc. dissertation; it forms the basis of the current paper. The author also acknowledges the financial support of the AGH University of Science and Technology in Cracow (statutory research No. 11.11.140.47).
References Banerjee, I., Kalkreuth, W.D., 2002. Sedimentology, sequence stratigraphy, organic petrology, geochemistry and palynology of Mannville coals in south-central Alberta. Geological Survey of Canada Bulletin 571 (64 pp.). Calder, J.H., Gibling, M.R., 1994. The Euramerican coal province: controls on late Paleozoic peat accumulation. Palaeogeography, Palaeoclimatology, Palaeoecology 106 (1–4), 1–21. Calder, J.H., Gibling, M.R., Mukhopadhyay, P.K., 1991. Peat formation in a Westphalian B Piedmont setting, Cumberland Basin, Nova Scotia: implications for the maceral-based interpretation of rheotrophic and raised paleomires. Bulletin Societe Geologique de France 162 (2), 283–298. Diessel, C.F.K., 1982. An appraisal of coal facies based upon maceral characteristics. Symposium Proceedings-Geological Society of Australia Inc., Coal Group 4, 474–483. Diessel, C.F.K., 1986. On the correlation between coal facies and depositional environments. Advances in the Study of the Sydney Basin. Proc. of the 20th Newcastle Symposium, pp. 19–24. DiMichele, W.A., Phillips, T.L., 1994. Paleobotanical and paleoecological constraints on models of peat formation in the Late Carboniferous of Euramerica. Palaeogeography, Palaeoclimatology, Palaeoecology 106, 39–90. Doktor, M., Gradziński, R., 1985. Srodowisko depozycji aluwialnych utworów węglonosnych serii mułowcowej (górny karbon Zagłębia Górnośląskiego). Studia Geologica Polonica. Wyd. Geologiczne, vol. LXXXII. Warszawa, pp. 1–67. Jureczka, J., Aust, J., Buła, Z., Dopita, M., Zdanowski, A., 1995. Mapa Geologiczna Górnośląskiego Zagłębia Węglowego (odkryta po karbon), skala 1:200 000, PIG, Warszawa. Kalkreuth, W.D., Marchioni, D., Calder, J., Lamberson, M., Naylor, R., Paul, J., 1991. The relationship between coal petrography and depositional environments from selected coal basins in Canada. International Journal of Coal Geology 19, 21–76. Marchioni, D., Kalkreuth, W.D., Utting, J., Fowler, M., 1994. Petrography, palynology and geochemistry of Hub and Harbor seams, Sydney Coalfield, Nova Scotia, Canada—implication for facies development. Palaeogeography, Palaeoclomatology, Palaeoecology 106 (1–4), 241–270. Misiak, J., 2001. Metodyka wykonywania “mikroprofilu” pokładu węgla. Mat. XXIV Symp. nt. Geologia formacji węglonośnych Polski. Wyd. AGH, Kraków, pp. 65–68.
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Misiak, J., 2002. Środowiska depozycji materii organicznej w torfowiskach karbońskich. Mat. XXV Symp. nt. Geologia formacji węglonośnych Polski. Wyd. AGH, Kraków, pp. 105–108. Misiak, J., 2003. Projekt diagramu do analizy facjalnej pokładów węgla kamiennego. Mat. XXIV Symp. nt. Geologia formacji węglonośnych Polski. Wyd. AGH, Kraków, pp. 109–113. Porzycki, J., 1972. Seria mułowcowa piętra westfalu dolnego Górnośląskiego Zagłębia Węglowego. Karbon Górnośląskiego Zagłębia Węglowego, Prace Instytutu Geologicznego, T. LXI, pp. 467–508. Scott, A.C., 2002. Coal petrology and the origin of coal macerals: a way ahead? International Journal of Coal Geology 50, 119–134. Teichmüller, M., 1962. Die Genese der Kohle. C.R. 4 Congres Int Stratiggr. Geol. Carbonifere. Heerlen 1958 (3), 699–722. Teichmüller, M., 1982. Origin of the petrographic constituents of coal, In: Stach, E., Mackowsky, M.Th., et al. (Eds.), Textbook of Coal Petrology, 3rd ed. Borntrager, Stuttgart, pp. 219–294.
Veld, H., Fermont, W.J.J., David, P., Pagnier, H.J.M., Visscher, H., 1996. Environmental influence on maturity parameters in Carboniferous coals of the Netherlands. International Journal of Coal Geology 30 (1–2), 37–64. Ward, C.R., 2002. Analysis and significance of mineral matter in coal seams. International Journal of Coal Geology 50, 135–168. Wüst, R.A.J., Bustin, R.M., 2001. Low-ash peat deposits from a dendritic, intermontane basin in the tropics: a new model for good quality coals. International Journal of Coal Geology 46, 179–206. Wüst, R.A.J., Hawke, M.L., Bustin, R.M., 2001. Comparing maceral ratios from tropical peatlands with assumption from coal studies: do classic coal petrographic interpretation methods have to be discarded? International Journal of Coal Geology 48, 115–132.
Petrography and depositional environment of the No. 308 coal seam (Upper Silesian Coal Basin, Poland)—a new approach to maceral quantification and facies analysis Jacek Misiak Department of Coal Deposits Geology, AGH University of Science and Technology, Faculty of Geology, Geophysics and Environment Protection, al. Mickiewicza 30, 30-059 Cracow, Poland Received 5 November 2004; received in revised form 26 November 2005; accepted 26 November 2005 Available online 24 February 2006
Abstract Oriented block samples of coal representing the lower layer (2.1 m) of the No. 308 coal seam were collected in the Ziemowit mine. Microscope investigations were carried out using a self-developed, fast method of maceral quantification that enables obtaining a continuous petrographical profile (“microprofile”) of a seam and computer processing of the data. The contents of macerals and maceral groups distinguished along the profile were averaged using the method of moving averages to show trends of environmental changes in a mire. The facies diagram proposed is based on an assumption that oscillations of water table in the mire affect the coal petrographical composition, while stronger water influxes into the mire increase the mineral matter content of coal. The author has identified three major types of the mire environments in which plant material was deposited during deposition of the No. 308 coal seam. These are permanently inundated = planar mire (PM) with two sub-environments—PM “margin” and PM “central”; temporarily inundated = transitional mire (TM) with two sub-environments—TM “wet” and TM “dry”; and elevated = domed mire (DM) with two sub-environments—DM “progressive” and DM “regressive”. © 2006 Elsevier B.V. All rights reserved. Keywords: Upper Silesian Coal Basin; Poland; Coal petrography; Macerals; Facies diagram; Depositional environment
1. Introduction There is a growing interest, marked particularly in the last 10 years, in facies investigations of coal seams. Investigations have used various methods of geological analyses to determine the type of plant material and environmental conditions of coal formation. Lithological investigations of coal seams, petrographical observations based on averaged samples of coal seams (often applied in the case of borehole cores), E-mail address: [email protected]. 0166-5162/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2005.11.008
lithological investigations supplemented by petrographical ones, as well as other modifications of lithological and petrographical methods, can be mentioned as examples. Interpretation of petrographical investigations are also based on a range of methods. The type of the environment was assessed from the already-known genesis of selected macerals or maceral assemblages (microlithotypes) or from various “facies formulae” and diagrams (Diessel, 1986; Veld et al., 1996). Current study investigates the genesis of coal seams, trying, on one hand, to work out a method of recording
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and storing the results of petrographical observations as a basis for their multifaceted data processing and, on the other, to determine the development history of a selected coal seam, the No. 308 seam. The coal seam is one of the most extensive seams and is mined over a large area in the eastern part of the Upper Silesian Coal Basin (USCB) in Poland. 2. Localization of the study area Geological investigations were carried out within the coal field of the Ziemowit colliery in the main basin of the Polish part of the USCB (Fig. 1). They were focused in the downthrown limb of the “Książęcy” fault (displacement is about 400 m), in the area of the “Ławecki” and “Piastowski” faults (displacements are about 70 and 60m, respectively), transverse to the former. Coal seam No. 308 was selected for lithological, petrographical and facies studies. Mining is advanced in this region; therefore, underground workings are accessible. Also, the lithology is diverse, so the seam represents a proper section to carry out the abovementioned investigations.
3. Geological characteristics of the No. 308 seam The No. 308 seam is situated in the upper part of the Mudstone Series (Pennsylvanian, Orzesze Beds s.s.; Fig. 2). In the study area, it lies 30–60m below the No. 301 seam, which terminates the sedimentation of the Mudstone Series. In this region, the Mudstone Series is about 750 m thick, with the tendency of thinning to the east (Porzycki, 1972). Coarse-grained, poorly sorted sandstones of the Cracow Sandstone Series overlie the No. 301 seam. The Mudstone Series is composed of aleuritic– pelitic sediments, with intercalations of sandstones and numerous coal seams. The horizontal variability of the sediments (lithofacies) and their thickness are the results of shifting of depositional sub-environments. These processes are typical for an alluvial plain undergoing permanent subsidence (Doktor and Gradziñski, 1985). In the study area, in the transition zone between the Mudstone Series and Cracow Sandstone Series, the No. 308 seam is one of the thickest seams (∼4 m). It comprises two layers of humic bituminous coal beds,
Fig. 1. Geological sketch map of the Upper Silesian Coal Basin (modified after Jureczka et al., 1995) with the location of the “Ziemowit” coal mine.
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Fig. 2. (A) Position of the No. 308 coal seam in lithostratigraphic subdivision of the Pennsylvanian system of the USCB in Poland. (B) Generalized profile of the No. 308 coal seam.
intercalated by a claystone or locally by a mudstone layer (Fig. 2) with a thickness from 2 to 15 cm, or with a local concretions of clayey siderite, respectively. The underlying sediment is grey claystone with roots. The lower layer of the seam, with a thickness of about 2m, which represents the subject of the current investigations, consists of coal without macroscopically visible mineral intercalations. The upper layer of the coal also with a thickness of about 2m contains many clay intercalations and nests. Towards the central part of the area, over a distance of some hundred meters, the seam becomes richer in clay until the coal is completely replaced by compact, dark grey claystone with numerous carbonized plant fragments that indicate the bedding. The carbonized plants, often with the imprints typical of Sigillaria, occur in the form of thin vitrain lenses. Transition from the coal seam towards the roof rocks is smooth via coaly shale that grades into claystone. In the following part of the text, the term “No. 308 seam” is referred to the lower layer of the seam, the subject of this study. 4. Sampling and preparation of microscope mounts The No. 308 seam was sampled in the longwall face and samples were taken as sequential oriented block
samples cut along a full profile cropping out at the exposure. To protect the samples from drying out, they were soaked with varnish. The lumps of coal were polished, obtaining in this way about 50 samples to be studied under the reflected light microscope. Their total length is about 2m, corresponding to the thickness of the seam in the sampling spot. 5. Methods of investigations Petrographical investigations were carried out using an Opton polarizing microscope (“Axioskop”), using white light and fluorescent illuminations. Observations were made with an 50× immersion objective and included qualitative and quantitative analyses of bituminous coal macerals. Methodology of the analysis adapted to facies research was based on the “microprofiling”, i.e., profiling the quantitative variability of seam macerals under the microscope. The standard method of quantification of petrographical coal components with a semiautomatic counting stage (Marchioni et al., 1994; Banerjee and Kalkreuth, 2002) limits further processing of the data obtained. The standard method is not feasible for statistical analysis using a computer program because the measurements are expressed as totals referring to sections of the profile; the sections were
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selected in advance (not always independently) and are of different lengths. The method was modified in the following way. The microscope eye-piece was equipped with a special grid (Misiak, 2001; Fig. 3) constructed so that its outer size (frame) in the microscope field of view is 0.1 × 0.125 mm (100 × 125 μm). The field is divided into 20 squares with a size of 25 × 25 μm each. The amounts of petrographic components (macerals, minerals) distinguished within the frame in the form of layers, lenses and big grains were determined by adding the number of squares they occupy. The components occurring in a disseminated form, e.g., small fragments, fine grains, etc., were quantified with typical charts for visual percentage estimations. If the ground mass (matrix) was present, its content was calculated as a difference to 100%. The next measurement is done after moving the mount at the shift equal to the grid height, i.e., at 0.1mm, to the field directly next to the former. The set of such measurements recorded over the full thickness of the coal layer represents, thus, a continuous petrographical “microprofile” of the seam. The data were stored in a computer database in tabular form. Each column corresponds to a different distinguished petrographical component (maceral), and each line corresponds to the content (vol.%) of the macerals measured in a single shift of the grid, respectively. Each millimetre of the profile is covered by 10 grid measurements and recorded as 10 lines in the table. In the seam with a thickness of 2 m, it gives 20,000 measurements. The data obtained can be processed using a calculation program, such as MS Excel, STATISTICA or others.
Fig. 3. Measuring grid of the eye-piece, applied in this work (explanations in the text).
6. Variability of macerals and mineral matter in the No. 308 seam profile The petrographic composition of the coal from the No. 308 seam is presented in Figs. 4 and 5 and listed in Table 1. The profile is dominated by the vitrinite group macerals (in average about 63%), from which the most frequent are collotelinite (46.3%), telinite (8.4%) and collodetrinite (7.4%). The inertinite group is less frequent (26%), and the liptinite group makes up 7%. The average content of mineral matter in the seam is about 4%. The variability of the coal macerals and mineral matter along the profile are represented in Figs. 4 and 5. Considering the insignificant content of secretinite and the fact that liptodetrinite can be positively identified only using fluorescent illumination, these two components are not shown on the profile. On the vertical axis, the value 0 marks the roof of the seam. The contents of specific petrographical coal components have been shown as the curves representing moving averages, calculated as the arithmetic means from the contents of these components obtained in single counts along the profile. For the graphical presentation, the averages were calculated from a series of 100 discrete counts. As a result, each point of the curve represents the content of a given maceral along 1 cm of the seam profile, with the value centred (i.e., centrally situated) against the section for which the average has been calculated. 7. The proposed version of the coal seam facies diagram Facies analysis of coal seams can be based on the results of lithological, petrographical and palaeontological investigations. They are interpreted by applying various facies diagrams, applicable in the methodology selected. In this paper, an attempt to confront different sedimentary environments of organic matter in the Carboniferous peat bogs is presented. The author accepts the cyclic oscillations of the water table level in the mire as a main factor controlling coal-forming processes, which is followed by the succession of plants in the mire, while the deposited organic fragments undergo on either gelification or oxidation, depending on the mire ground water level. Sedimentary environments of coal seams are proposed by Diessel (1986) often determined on the basis of petrographical investigations using a diagram modified by Kalkreuth et al. (1991) and Veld et al. (1996). DiMichele and Phillips (1994) presented alternative methods of description of coal-forming environments on
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Fig. 4. The content of the macerals of the vitrinite and the liptinite groups in the profile of the No. 308 seam.
Fig. 5. The content of the macerals of the liptinite and inertinite groups in the profile of the No. 308 seam.
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Table 1 The average content (%) of coal macerals and mineral matter in the No. 308 seam Vitrinite
Liptinite
Inertinite
Telinite Collotelinite Collodetrinite Vitrodetrinite Corpogelinite Gelinite
8.4 46.3 7.4 0.2 0.5 0.2
Microsporinite Macrosporinite Cutinite Resinite Liptodetrinite
5.4 1.6 0.2 0.1 0.1
Total
63.0
Total
7.4
the basis of investigations of the Carboniferous plants. The present paper put forward a proposal to distinguish three main environments within the zone of peat sedimentation, namely, permanently inundated mire, temporarily inundated mire, and elevated mire, as well as two peripheral environments: lacustrine and of flood plain, where peat does not accumulate. The “reed moor” environment distinguished by Diessel (1986) and corresponding to the present-day reed environment according to the suggestions of DiMichele and Phillips (1994) has not been separated but was included into the area of the transitional mire. It is also possible to classify this environment as calamite reeds as done by Teichmüller (1962, 1982). But calamites occupied not only the area between a mire and a lake (where reeds grow), but also the areas close to a river, and this points to their pioneering role in the development of the mire. The oscillating water table level of the palaeomire resulted high and low positions, which caused the inundation or drying of the mire. These short periods took place within the same environment, covering ecological amplitude of the environment variations tolerated by the occupying plants. Long-lasting periods of inundation and drying of the mire enforced a gradual change of its plant assemblage. For this reason, the assemblage of the plants occupying the “temporarily inundated mire” should be treated as an ecoton between the plants of the “permanently inundated mire” and “elevated mire”. The term “temporarily inundated mire” itself suggests a succession between inundation and drying periods. Fig. 6 presents the relation between the environments mentioned above and the water table level. Within the environment of “temporarily inundated mire”, the author distinguished two sub-environments depending on the climate periods: bwetQ, temporarily inundated sub-environment, characterized by the water table level above the mire surface;
Pyrofusinite Degradofusinite Semifusinite Macrinite Micrinite Funginite Secretinite Inertodetrinite Total
Mineral matter 2.7 4.3 5.5 0.8 0.8 1.9 0.04 9.8 25.8
Clay Pyrite Carbonates
3.6 0.1 0.06
Total
3.8
bdryQ, temporarily dried sub-environment, in which the water table level is below the mire surface. In the Polish and international literature, recent coal facies studies utilize, first of all, Diessel's (1982, 1986) facies indicators and diagrams. He defines indices describing the degree of gelification (GI) and determines the degree of tissue structure preservation in a sample (TPI), as well as defines different zones in this diagram corresponding to various environments and plant assemblages occupying the given environments of the mire. Diessel's method was later discussed and modified by Calder et al. (1991), Kalkreuth et al. (1991), Calder and Gibling (1994), DiMichele and Phillips (1994), Wüst et al. (2001) and Scott (2002). The present paper agrees with the suggestions of the authors mentioned above, although the petrographical coal composition is more dependent on the level of water table in the mire (inundation) than on the type of vegetation present in it. Relatively short-lasting oscillations of the water level did not need to enforce succession of vegetation if they took place within the ecological amplitude of a given plant assemblage and the changes caused by them were minor, as reflected in the maceral composition of the deposited material. The proposed version of the facies diagram (Fig. 7) refers to the environments distinguished in Fig. 6; based on the assumption that, the oscillations of the water table level within the mire affect the coal composition, while stronger water influxes into the mire are reflected in the detrital matter content of coal. When the water level was relatively high, the process of gelification was intensive, reflected in a high content of the vitrinite-group macerals. Lowering of the water level in the mire was accompanied by an increased access of oxygen and stronger oxidation, manifested by an increasing content of the inertinite-group of macerals. Oxidation is also indicated by an increase of the liptinite-group macerals in coal during degradation of the plant material, as more resistant liptinite withstands oxidation and concentrates
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Fig. 6. Sedimentary environments in the Carboniferous peat bogs, (Misiak, 2002, modified) (DEM–detrital matter, I–inertinite, L–liptinite, V– vitrinite).
Fig. 7. Diagram of the coal facies analysis.
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at the expense of vitrinite and inertinite (domed regressive mire). Higher accumulations of the liptinite group macerals may also be water or air deposited (“planar margin mire lake”, i.e., organic layers with interbeddings of lacustrine strata). A substantial role in the coal facies analysis was attributed to the presence of mineral matter (mainly clay) of coal. It is accepted (Wüst and Bustin, 2001; Ward, 2002) that the mineral matter content not higher than 5% is associated with the decomposition of plant tissue, while the content exceeding 5% indicates a stronger influx of water into the mire. The author has assumed here an arbitrary border value of 10% as indicative of the “planar margin mire-river” environment, i.e., the environment of organic deposition with partings of extra-channel pelites–aleurites. Consequently, a value of 50% has been accepted as the uppermost mineral matter content of coal. The contents higher than 50% characterize coaly rock (coal-bearing claystones, mudstones, sandstones, coaly shales). In the proposed simplified coal facies diagram (Misiak, 2003; Fig. 7), the contents of the distinguished maceral groups (instead of individual macerals) and of mineral matter are considered. However, it must be remembered that not all the macerals are facies consistent within a given maceral group excluding micrinite from the inertinite group and alginite from the
liptinite group. The facies analysis is two-pronged and starts from the diagram centre. If the clay content exceeds or equals 10% in a sample, the environment is interpreted as PM “margin” (river). If the clay content is below 10%, the sample environment is determined on the basis of the proportions among inertinite, liptinite, and vitrinite, proceeding from the second inner circle toward the outer circles of the diagram. 8. Facies analysis of the No. 308 seam The results of the investigations carried out and plotted on the diagrams proposed form the basis of identification of the formation history of the No. 308 seam (Fig. 8). In the initial stage of the peat bog formation (the section of the profile from 1982 to 1608 mm), the palaeomire was permanently inundated—PM “central”, the water level was high and temporary rises provided the detrital material—PM “margin” (river) environment. Then, as far as the position of 1436 mm, the mire became generally more shallow and graded into the transitional mire (TM). Smaller oscillations of the water level caused the changes at this stage of the mire development within the range TM “wet” and TM “dry”. The section 1436–1399mm represents further shallowing of the mire that grades into the domed mire—DM
Fig. 8. Depositional history of the No. 308 seam.
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“progressive”. Next, to the position 1180mm, the water level rose again and the mire character varied in the range of the TM “wet”—TM “dry” and PM “central” environments. The period corresponding to the section 1180–1120mm was dry and the mire evolved towards a domed mire—DM “regressive”. Such an environment has been inferred as a result of degradation of plant material due to oxidation: the macerals of the liptinite group, being more resistant, were preserved under such conditions and concentrated in the section at the expense of other coal components. Point 1120mm marks the direction of further changes in the mire. The water level started to rise, a conclusion corroborated by a higher amount of clays. The conditions became more wet, and as far as the position 459 mm, the mire periodically changed, getting shallower or deeper, grading from the environment temporarily inundated due to water level oscillations (TM “wet” and TM “dry”) to the environment permanently inundated (PM “central”). The near-roof section of the seam between the positions 459 and 18 mm represents the environment with oscillations of the water level–transitional mire (TM), getting progressively dry and grading into the domed mire (DM). A thin layer of coaly clay (188–177) points to an incursion of a broading river into the mire. The mire terminates its existence in the position 18 mm, when it became finally inundated and buried under a substantial amount of detrital material transported by fresh waters. 9. Summary The No. 308 coal seam from the Upper Silesian Coal Basin (USCB) in Poland have been used to reconstruct paleomire. Methodology of the analysis adapted to facies research was based on the “microprofiling”. The author modified the method of microscope analysis. The microscope eye-piece was equipped with a special grid. The data were stored in a computer database in tabular form and the data obtained can be processed using a calculation program. The contents of specific petrographical coal components have been shown as the curves representing moving averages, calculated as arithmetic means from the contents of these components obtained in single counts along the profile. The proposed version of the facies diagram (Fig. 7) refers to the environments distinguished in Fig. 6 and was based on the assumption that the oscillations of the water table level within the mire affects the petrographical coal composition, while stronger water influxes into the
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mire reflect in the detrital matter content of coal. The results of the investigations carried out and visualized on the diagrams proposed formed the basis of identification of the formation history of the No. 308 seam (Fig. 8). Acknowledgements I express my gratitude to Prof. Dr Ireneusz Lipiarski who supervised my D.Sc. dissertation; it forms the basis of the current paper. The author also acknowledges the financial support of the AGH University of Science and Technology in Cracow (statutory research No. 11.11.140.47).
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