Petrographic Indices Research Articles

Berau coal in East Kalimantan; Its petrographics characteristics and depositional environment

To asses the characteristics of the Early to Middle Miocene Berau coal in the Berau Basin, leading to interpretation of coal depositional environments, some fresh outcrop and subcrop samples and also drill cores of the coals have been analyzed microscopically. Coal petrographic analysis was performed on twenty four coal samples from the Middle Miocene Lati Formation. Vitrinite, present in a high value, and ranging between 66.2 96.2%, is dominated by vitrinite B. On the other hand, inertinite and exinite, showing a similar value, exist in a low to moderate amount. Vitrinite reflectance, present in a low value, varies from 0.40 0.58%. Low mineral matter content is dominated by clay minerals (0.4 6.6%) with minor pyrite. Transitions from wet and very wet forested swamps to drier conditions with lower tree density are indicated by the higher content of vitrinite B, whilst a reverse trend is indicated by the lower content of vitrinite A. Petrographic indices obtained from facies diagnostic macerals show that an accumulation of the ancient peats under prevailing relatively wet limited influx clastic marsh to very wet forest swamps or moors is considered. The composition of the coal samples supports the interpretation of a system of fluvial to meandering streams in an upper delta plain environment. The original peat-forming vegetation was composed mainly of cellulose rich, shrub-like plants, tree ferns, herbaceous plant communities, with minor amount of trees. Thereby, the organic facies concept is thus applicable in basin studies context and has potential to become an additional tool for depositional environment interpretation.

Limitation of petrographic indices in depositional environmental interpretation of coal deposits

AbstractOrganic petrology based petrographic indices (Tissue Preservation Index and Gelification Index) is a widely utilized tool in the study of depositional palaeoenvironment of coal. Evaluation of these petrographic indices suggests that, at present, utilize only vitrinite/huminite and inertinite macerals to interpret depositional environment of coal. Liptinite group macerals have important depositional environment implications, but liptinite macerals have not been taken into account in earlier petrographic indices (TPI and GI) formulations. This article examines the limitation of TPI and GI, and proposes improved TPI and GI indices, including the liptinite and inertinite macerals having depositional environment significance.

Lithology of sediments and stratigraphy of glacial layers of some cliff sections on Central and Western Polish Coast

The cliffs studied reveal a high similarity in terms of geological structure and age of sediments. Differences in the geological structure of particular cliffs occur only in the upper part. They are related to the occurrence of reservoir sediments in some profiles. The thermoluminescence (TL) age of glacial sediments exposed in the lower part of cliffs shows that they are glacial sediments of the Świecie Stadial (54 400–50 800 years BP). In all profiles in which sediments of this type occur, there is a clear “rejuvenation” of their top part (33 200–38 600 years BP). The age uniformity of glacial sediments is also confirmed by petrographic analysis. The obtained values of petrographic indices explicitly indicate that in all examined profiles the till is the same and that in particular profiles, despite the variability of sediment colour, only one glacial level occurs. Keywords: stratigraphy of glacial sediments, cliff, lithology, TL age, Polish Baltic Coast

Statistical analysis of relations between petrographic and geochemical indices of oil generation potential in organic matter

To establish reliable indices for the synthetic assessment of petroleum potential using reworked sediment from mixed datasets, multivariate statistical analysis and cross-plots were used. New guidelines for improved assessment of generative potential and thermal maturity are suggested. Based on statistical analysis and cross-plots of HI, QI and BI versus the vitrinite reflectance (%Ro) and Tmax (°C), the HImax, QImax and BImax lines are defined between the vertex of the upper and lower limits of the HI-band, QI-band and BI-band as defined by a majority of the samples. The constructed HI, QI and BI bands were broad at low maturities and gradually narrowed with increasing thermal maturity. The petroleum generation potential is completely exhausted at a vitrinite reflectance of 2.0–2.2% or a Tmax of 510–520°C. An increase in HI and QI suggests extra petroleum potential related to changes in the structure of the organic material. A decline in BI signifies the start of the oil expulsion window and occurs within the vitrinite reflectance range 0.75–1.05% or a Tmax of 440–455°C. Furthermore, petroleum potential can be divided into four different parts based on the cross-plot of HI versus %Ro. The area with the highest petroleum potential is located in section II with %Ro = 0.6–1.0%, and HI > 100. Oil generation potential is rapidly exhausted at section III with %Ro > 1.0%. This result is in accordance with the regression curve of HI and QI with %Ro based on 80 samples with %Ro = 1.02–3.43%. The exponential equation of regression can thus be achieved: curve, HI = 994.81e−1.69Ro (R2 = 0.72) and QI = 1646.2e−2.003Ro (R2 = 0.72). The worldwide organic material dataset defines two oil expulsion windows represented by the upper and lower limits of the BI band: %Ro ~ 0.75 to ~1.95% or Tmax ~ 440 to ~525°C, and %Ro ~ 1.05 to ~1.25% or Tmax ~ 455 to ~465°C, respectively. The start of the oil expulsion window occurs within the %Ro range of ~0.75–1.05 %Ro or the Tmax range ~440–455°C and the total oil window extends to %Ro = ~ 1.25–1.95 or Tmax = ~ 465–525°C.

Holocene transgression recorded by sand composition in the mesotidal Galician coastline (NW Spain)

This study confirms several inferences regarding Holocene coastal dynamics and climate through a petrographic modal analysis of 60 Holocene sand samples recovered in seven sites along the NW coast of the Iberian Peninsula. Fluvial sand can be discriminated from more mature intertidal and aeolian sand according to texture and composition. Fluvial sand contains soil products and coastal sand has significant bioclasts. Quartzofeldspathic sand appears in the western area (produced by the erosion of granite and granitoid), and quartzolithic sand occurs in the eastern area (produced by the erosion of metasediment). Changes in sand composition during Holocene deposition are manifested by an increase in modern carbonate clasts (MC) correlated with the Holocene transgression. Episodes of faster sea-level rise and subsequent erosion of surrounding cliffs are indicated by the preservation of high proportions of feldspar in intertidal sand. In contrast, fluvial sand is characterized by greater quartz enrichment. These inferences were confirmed by petrographic indices (carbonate clasts/total clasts, MC/T; total feldspars/monocrystalline quartz, F/Qm; and plagioclase/total feldspars, P/F). The different maturity of intertidal and aeolian sands is revealed by their variable quartz contents, despite similar proportions of plagioclase and K-feldspar. This suggests mechanical abrasion as the main factor controlling maturity. In contrast, fluvial sand shows depleted plagioclase contents as the result of inland weathering processes. Intertidal, beach and aeolian sands are essentially the products of the erosion of coastal cliffs and head deposits, with only the scarce contribution of fluvial drainages. The long-distance transport of Galician coastal sands is discarded based on the close relationship between their composition and that of local sand sources. Our findings indicate that short-distance transport of sediments from the west closed off coastal wetlands and occluded estuarine mouths during the Holocene transgression by deposition on sediment-trap zones along the irregularly shaped Galician coast.

Coal petrology of coal seams from the Leão-Butiá Coalfield, Lower Permian of the Paraná Basin, Brazil — Implications for coal facies interpretations

Abstract In the Leao-Butia Coalfield, Rio Grande do Sul the coal seams occur in the Rio Bonito Formation, Guata Group, Tubarao Supergroup of the Parana Basin, Brazil and are of Permian (Artinskian–Kungurian) age. This study is the first detailed investigation on the coal petrographic characterization of the coal-bearing sequence in relation to the depositional settings of the precursor mires, both in terms of whole seam characterization and in-seam variations. The study is based on the analyses of nine coal seams (I2, CI, L4, L3, L2, L1, S3, S2, S1), which were selected from core of borehole D-193, Leao-Butia and represent the entire coal-bearing sequence. The interpretation of coal facies and depositional environment is based on lithotype, maceral and microlithotype analyses using different facies-critical petrographic indices, which were displayed in coal facies diagrams. The seams are characterized by the predominance of dull lithotypes (dull, banded dull). The dullness of the coal is attributed to relatively high mineral matter, inertinite and liptinite contents. The petrographic composition is dominated by vitrinite (28–70 vol.% mmf) and inertinite (> 30 vol.% mmf) groups. Liptinite contents range from 7 to 30 vol.% (mmf) and mineral matter from 4–30 vol.%. Microlithotypes associations are dominated by vitrite, duroclarite, carbominerite and inertite. It is suggested that the observed vertical variations in petrographic characteristics (lithotypes, microlithotypes, macerals, vitrinite reflectance) were controlled by groundwater level fluctuations in the ancient mires due to different accommodation/peat accumulation rates. Correlation of the borehole strata with the general sequence-stratigraphical setting suggests that the alluvial fan system and the coal-bearing mudstone succession are linked to a late transgressive systems tract of sequence 2. Based on average compositional values obtained from coal facies diagrams, a deposition in a limno-telmatic to limnic coal facies is suggested.

Relationships among mechanical, physical and petrographic properties of Siwalik sandstones, Central Nepal Sub-Himalayas

Mechanical and physical characteristics of rocks generally depend on their composition and texture, because they are reflective of their environments during sedimentation, diagenesis and weathering in each location. To understand this, relationships between mechanical and physical indices and composition and texture have been analyzed using several rock samples obtained from Mio-Pleistocene sandstone sequence of the Siwalik Group, widely distributed in foothills of the Nepal Himalayas. Sandstones are quartzo-feldspathic to quartzo-lithic and quartzo-feldspathilithic. Cement, matrix and void vary from 6 to 41%, 0 to 18% and 1 to 12%, respectively. Ferroan calcitic and siliceous cements are the dominant cements, and their ratio to total cement varies from 0.2 to 1.0. Packing density of sandstones ranges from 59 to 95%. Clasts are loosely to closely packed and are weakly to strongly bonded by cement. Results of correlation analyses show that sandstones strongly depend on several indices, that is percent void, strong cement over total cement, strong over weak contacts, concavo–convex contacts and packing density, most of which are measured on thin sections. Contrarily, they depend little on grain size, shape, mineralogical maturity index and the other petrographic indices. Increased interlocking and strong cementing with reduced void tends to possess high densities, low porosity and offer good resistance against deformation. Therefore, proportion of strong cement over total cement, void content, strong over weak contacts, grain-to-void contact, concavo–convex contact and packing density are meaningful indices considering the sandstones of the Siwalik Group. Based on the multiple regression, correlation analyses and then stepwise multiple regression analysis, some regression equations with significant explanatory indices are indicated for estimating mechanical and physical properties. In other words, such relationships may be useful in estimating mechanical and physical properties using the significant petrographic indices.

Petrological and geochemical characterization of Candiota coal seams, Brazil — Implication for coal facies interpretations and coal rank

Seven coal seams, from cores of the borehole M-IV-C-101, Candiota Coalfield located in Rio Grande do Sul State, southern part of the Paraná Basin were analyzed by coal petrographical and organic geochemical methods. The seams occur in the Rio Bonito Formation, Guatá Group, Tubarão Supergroup of the Paraná Basin and are of Permian (Artinskian–Kungurian) age. The composition, rank, coal facies and depositional environment characterization were made based on lithotype, maceral and microlithotype analyses. The data were then used in the definition of different types of facies-critical petrographical indices and were displayed in diagrams of coal facies. In addition, coal seams samples from Candiota Mine were analyzed by proximate, total moisture and gross calorific values and were classified using the United Nations-Economic Commission for Europe (UN-ECE) classification. In addition, Rock-Eval Pyrolysis, Liquid Chromatography, GC and GC–MS analyses were made to obtain independent parameters on coal rank and organic matter composition and environment of deposition. The results suggest deposition in open mires, with elevated contents of detrital macerals and clay minerals and also limno-telmatic coal facies. Coal-bearing sequence of coal seams represent a back-barrier setting. According to the International Classification (UN-ECE), Candiota coals are in the subbituminous (or low rank coal A) stage. Results from geochemical analyses showed that terrestrial organic matter is predominant as indicated by hydrogen and oxygen indices, carbon preference indices and the sterane distribution. Geochemical maturity parameters such as the 20 S / (20 S + 20 R) and αββ / (αββ + ααα) ratios for C 29 regular steranes support the low level of rank (maturity) as indicated by the vitrinite reflectance measurements (0.41–0.45%) and gross calorific values (< 24 MJ/kg, m, af).

Organic petrology and geochemistry of the Carboniferous coal seams from the Central Asturian Coal Basin (NW Spain)

This paper presents for the first time a petrological and geochemical study of coals from the Central Asturian Coal Basin (North Spain) of Carboniferous (Pennsylvanian), mainly of Moscovian, age. A paleoenvironmental approach was used, taking into account both petrographic and organic geochemical studies. Vitrinite reflectance ( R r) ranges from 0.5% to 2.5%, which indicates a high volatile bituminous to semianthracite and anthracite coal rank. The coal samples selected for paleoenvironmental reconstruction are located inside the oil–gas-prone phase, corresponding to the interval between the onset of oil generation and first gas generation and efficient expulsion of oil. This phase is represented by coals that have retained their hydrocarbon potential and also preserved biomarker information. Paleodepositional reconstruction based on maceral and petrographic indices points to a swamp environment with vitrinite-rich coal facies and variable mineral matter content. The gelification index (GI) and groundwater influence index (GWI) indicate strong gelification and wet conditions. The biomarkers exhibit a high pristane/phytane ratio, suggesting an increase in this ratio from diagenetic processes, and a high diterpanes ratio. This, in turn, would seem to indicate a high swamp water table and a humid climate. The maximum point of coal accumulation occurred during the regressive part of the Late Moscovian sequence and in the most humid climate described for this period of time in the well-known coal basins of Europe and North America.

Marine flooding events and coal bed sequence architecture in southern West Virginia

Eight regional Lower to Middle Pennsylvanian coal beds (Namurian B/C to Westphalian C) in southern West Virginia were examined. Beds were divided into their component sedimentary parts or benches by identifying bounding surfaces and associated siliciclastic partings within beds. Bounding surfaces represent the cessation of peat accumulation. Paleo-peat mire morphology at the bench level was determined using shape analysis, petrographic indices and coal chemistry data. Planar and raised paleo-peat mire benches were identified. All gas coal and some splint coal benches were derived from raised mires. Heterolithic coal and the remaining splint coal benches were derived from planar mires. Most bounding surfaces are thin, composed of oxidized and degraded organic materials, high in ash and sulfur content, and contain clay minerals and silt-sized quartz. Vitrinite reflectance is suppressed in the upper part of some benches. The associated siliciclastic parting materials, when thick, show evidence of tidal influence and contain brackish water to marine trace fossil assemblages. Bounding surfaces and the associated sediments represent a sudden increase in water depth and the lateral continuity of these surfaces indicates regional to basinal extent. Bounding surfaces are the up-paleoslope expression of rapid (<1-ka duration) marine flooding events. A repeating sequence was identified within six of the eight thick, regional coal beds examined. It consists in its simplest expression of (1) planar mire formation, initially occurring on interfluvial sequence boundaries, (2) probable evolution of the planar mire into a raised mire, (3) marine flooding event and bounding surface formation, (4) planar mire reestablishment, and (5) bed termination. The hiatus between the interfluvial sequence boundary and overlying coal bed, based on comparisons with modern analogues, may represent more than 100 ka of missing time. Peat accumulation and basin subsidence estimates suggest that an additional 100 ka or more may have been involved in the formation of a single coal bed sequence. The geographic extent of these sequences and the fact that they repeat indicates that an allocyclic mechanism (climate and/or glacioeustacy) was responsible for their development.

Organic facies and depositional palaeoenvironment of lignites from Rio Maior Basin (Portugal)

The Rio Maior Basin (Portugal) is a tectonic depression, filled by a Pliocene sequence that comprises, from floor to roof: (i) kaoliniferous fine sands, (ii) diatomites and lignites, (iii) recent deposits of sandstone and clay. The diatomites and lignites form a small dissimetric syncline with alternating seams. Ten lignite seams were identified and named from floor to roof as F, E, D, C.2, C.1, C, B, A, a and a′. Seams A, D, E and F are considered to be the main seams. The organic fraction consists mainly of macerals of the huminite group, with small percentages of inertinite and liptinite groups. However, the petrographic composition of each seam is distinct, particularly with regards to macerals of the huminite and liptinite groups. Calculation of petrographic indices permitted to plot the coals in facies and palaeoenvironment diagrams. Five facies have been defined: (i) aquatic, (ii) herbaceous swamp, (iii) mixed swamp with forest and herbaceous vegetation, (vi) forest swamp (wetter) and (v) forest swamp (drier). These lignites are humic coals formed from organic matter of terrestrial origin. The peat biomass at the origin of these coals formed from a very diverse vegetation comprising gymnosperms and angiosperms. In seams F, and occasionally in seams E and D, Botryococcus algae have also contributed to the biomass. Peat deposition corresponded to a rheotrophic hydrological regime: the water level always remained above the topographic surface of the basin. Nevertheless, during the deposition of seam A in the northern part of the basin, the water level was slightly below the topographic surface. The organic matter was preserved in anaeorobic conditions.

Weathering indices and their applicability for crystalline rocks

In the recent past, several weathering indices have been proposed to characterize the extent of weathering and weatherability depending upon the nature and requirement of the study. The weathering index provides a quantitative measure of the extent of weathering of rock; hence it can provide input to the prediction models to assess the strength and deformational properties of rocks and classifications of weathered rock material. In the present study some of the important weathering indices, broadly categorized as chemical, (micro) petrographical and engineering weathering indices, are reviewed and studied experimentally for three common rocks of India, namely granite of Malanjkhand, basalt of Nagpur and quartzite of Delhi, along with results of other rocks reported by other researchers. The study reveals that none of the existing chemical weathering indices is valid for genetically different common rock types and useful for engineering purposes. However, loss on ignition (LOI) may provide an approximate estimation of altered minerals (clays and hydroxides) in tested rocks. It has also shown good correlation with petrographic indices and engineering index properties. Among the petrographic indices, crack density (ρcr) and unsound constituent (an input to the micropetrographic index) indices provide good correlation with engineering index properties. To quantify the extent of weathering in terms of strength degradation due to weathering in rock, an index is suggested – strength ratio (Rs) – which is the percentage of uniaxial compressive strength (σc) of weathered rock with respect to σc of fresh rock. Its significance is shown statistically through the relationships with other indices for several rock types including sedimentary and metamorphic rocks.

The effects of source rocks and chemical weathering on the petrogenesis of siliciclastic sand from the Neto River (Calabria, Italy): implications for provenance studies

Plutonic and gneissic rocks of the Sila Massif in the uppermost portion of the Neto drainage basin (Calabria, Southern Italy) weather and erode under a humid Mediterranean climate. During the development of weathering profiles, a combination of chemical weathering and granular disintegration processes occurred. Chemical weathering involves a loss of both plagioclase (mainly during grus generation) and K‐feldspar (mainly during soil formation). This loss is attributed to transformation of plagioclase to clay minerals and to leaching and dissolution of K‐feldspar. Sand composition is quartzofeldspathic and nearly homogeneous along the main channel of the Neto River, even where the river cuts across a blanket of sedimentary cover. Thus, fluvial transport does not alter sand composition within the Neto drainage basin. Petrographic indices are effective in (1) discriminating between contributions from similar (granite and gneiss) source rocks (Qm/F); (2) relating the provenance of plutoniclastic and gneissiclastic sand found in the headwaters to grus horizons (Qm/F; Q/Rg); and (3) distinguishing between upstream first‐cycle and downstream multicycle sand (Q/Rg). This last distinction is further emphasized by considering both aphanitic and phaneritic varieties of rock fragments (RgRmRs diagram). Chemical weathering is the main sand producer within the regolithic environment in northern Calabria. In addition, rapid erosion resulting from steep slopes removes weathered products, and rapid and short transport leads to minimal sediment maturation. In general, the F/Q index is climate and relief dependent; thus, it should be used in conjunction with palaeoclimatic and palaeophysiographic evidence for provenance interpretations of ancient quartzofeldspathic sandstones.

Weathering of gneiss in Calabria, Southern Italy

The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.

Composition of modern stream sand derived from a mixture of sedimentary and metamorphic source rocks (Henares River, Central Spain)

The Henares River, central Spain, flows westward from the Iberian Range (Mesozoic sedimentary rocks) under semiarid climatic conditions. In the middle and lower reaches, the Henares River receives sediment from three tributaries (Canamares, Bornova and Sorbe rivers) that drain the Central System (Paleozoic crystalline rocks, low-grade metamorphic rocks, and minor amounts of Mesozoic sedimentary rocks). Modern sands from the Henares River and its tributaries offer an excellent opportunity to evaluate the importance of lithology and physiography in determining detrital modes from mixed metamorphic and sedimentary source terrain. Sand modes from the Henares River and its tributaries are quartzolithic. They plot in a restricted area on a QmFLt diagram, with low contents of feldspar and variable amounts of quartz, and carbonate, metamorphic and minor siliciclastic lithic fragments. Higher feldspar abundances in the Canamares and Bornova stream sands are related to the erosion of gneissic rocks. By contrast, sands from Sorbe River are low in feldspar, reflecting the absence of coarse metamorphic sources. Thus, proportions of bedrock lithologies in the drainage sub-basins are the main control on detrital modes of the tributaries. In addition, slope acts on the sand productivity of source rocks. Sedimentary source rocks in the upper reaches of the three tributaries have a poor productivity in the carbonate lithic fragments (Lsc), compared with the Lsc productivity of sedimentary sources located downstream, with higher relief. The percentage of bedrock types in the source area versus petrographic indices (Lm/L, Lss/L and Lsc/L) provide a useful contrast between source and sand composition in the tributaries. Thus, the high content of metamorphic lithic grains in the lithic grain population over-represents this lithology (slate plus schist) at the source terrain. Lithic grains from sedimentary clastic sources generate few recognizable grains (Lss) in the lithic grain population. Local supplies from carbonate sources may produce important increases of Lsc in short reaches of the channels. Sand compositions in the Henares River seem to be very homogeneous in a QmFLt diagram, with only minor differences caused by the supplies from the tributaries. These differences can be modeled in terms of end-member mixing processes between Henares detritus and detritus from the relevant tributary. These mixing processes are more evident when lithic grain contents (LmLvLs and LmLssLsc diagrams) are compared. Changes produced by inputs from the Canamares, Bornova and Sorbe rivers are not permanent along the Henares course. An important homogenization of Henares sand composition takes place by the mixing of tributary deposits with previously deposited Henares River terraces. Abrasion during transport does not appear to cause significant changes in the sand composition along the Henares River.

Petrography, palynology and depositional environments of the early Miocene Oberdorf lignite seam (Styrian Basin, Austria)

An early Miocene (Ottnangian) lignite seam up to 36 m thick occurs in the Oberdorf trough at the northwestern margin of the Styrian Basin. It forms part of the fluvial Köflach–Voitsberg Formation. Petrographic and palynologic data, together with ash yield and sulphur content of samples from two boreholes and two surface profiles have been used to study vertical and horizontal coal facies variations. Numerous partings and a high ash yield show that the lignite originated in a topogenous mire. Petrographic indices and palynological data suggest a wet-forest swamp paleo-environment. A fluvial main parting with channel sediments at the eastern margin of the Oberdorf trough and overbank sediments in its central part splits the lignite into a lower and an upper seam. The lower seam consists of a lower high-ash part with frequent splittings and a higher low-ash part. The upward decrease in number and thickness of silty–clayey partings, interpreted as crevasse splay deposits, reflects the withdrawal of the fluvial sediment source. Coal composition is characterized by an upward increase in (detro-)xylitic coal and in tissue preservation. A good correlation between tissue preservation and percentages of Taxodiaceae–Cupressaceae pollen suggests that this is mainly a result of an increasing contribution of decay-resistant gymnosperms (conifers). The upper seam is also ash-rich. Abundant inertinite at the eastern margin of the Oberdorf trough is related to fire episodes inside or outside the mire. Main features of the upper seam in the central basin are an upward increase in tissue preservation and in percentages of Taxodiaceae–Cupressaceae pollen. High sulphur contents in the fresh-water lignite are a result of a concentration of sulphate-rich waters above the impermeable floors of the lower and upper seams. Carbonate-rich surface waters from the Graz Palaeozoic and Raasberg-Sequence raised the pH value and controlled sulphur (and ash) contents in those parts of the lower seam, which are uninfluenced by the basal sulphur enrichment. Palynomorph spectra are dominated by Taxodiaceae–Cupressaceae pollen and pollen from plants living in a mixed mesophytic forest. Pollen from aquatic and reed plants are rare. The ratio between arctotertiary and paleotropic pollen is close to one and supports the early Miocene age of the lignite. The detected pollen spectra point to warm–temperate subtropic and humid conditions.

The petrology, organic geochemistry and palynology of Tertiary age Eureka Sound Group coals, Arctic Canada

Tertiary age lignites from Arctic Canada were analysed by petrographical, palynological and organic geochemical methods. Results from petrographic analyses indicate the predominance of wood-derived macerals (humotelinite group) in seams from the Strathcona and Stenkul Fiord areas, whereas in the Bache Peninsula area many seams are characterized by the predominance of detrital macerals (humodetrinite) and have also higher inertinite contents. Palynological assemblages are dominated by Taxodiaceae pollen, whereas Pinaceae pollen and pteridophyte spores are less common. Angiosperm derived pollen contents are quite variable. Organic geochemical results indicate that norisopimarane and pimarane are the most abundant diterpenoids. Cadalene was present in the aromatic fractions of all samples, with other components such as simonellite and retene in variable abundance. Rock-Eval data obtained from Stenkul Fiord area samples indicate low petroleum generation potential for this predominantly woody material. The depositional environment at Strathcona and Stenkul Fiords was that of a forested swamp, which formed in a lagoonal/alluvial plain setting. Palynomorph assemblages, petrographic indices and the abundance of diterpenoids suggest that the mire flora was dominated by gymnosperms, although locally angiosperms may also have been abundant as indicated by the occurrence of tetra- and pentacyclic aromatic hydrocarbons. At Bache Peninsula the mire was less forested as indicated by palynological assemblages typical for a marsh environment and petrographic characteristics such as low tissue preservation and the abundance of detrial macerals.

Petrographic composition and depositional environments of the Pliocene Velenje lignite seam (Slovenia)

A Pliocene lignite seam up to 160 m thick occurs in the Velenje depression in northern Slovenia. It forms part of a non-marine transgressive sequence. Petrographic data, together with ash and sulphur content of samples from two boreholes (one located in the central, and the other in the marginal part of the seam) have been used to study vertical and lateral coal facies variations. In the central part, the lignite seam overlies fluvial sediments. Facies critical macerals and petrographic indices suggest that the lower (ash rich) part of the seam (unit I) accumulated in a wet forest swamp in an alluvial plain setting. Later this environment was replaced by a dry (telmatic) forest swamp (unit IIa). According to our interpretation, a decrease in tree density in units IIb and IIc reflects a change to a bush moor. The transition from unit II to unit III is characterized by a rapid water table rise, which resulted in the establishment of a wet (limnotelmatic) forest swamp. Within unit III, the tree density decreases and the degree of gelification increases towards the top of the seam suggesting a transition to a fen type environment. Another rapid water table rise ended peat formation in the eutrophic, topogenous mire, and established open lacustrine conditions. A calcium rich environment resulted in high pH values and is responsible for high sulphur lignites (up to 4% on a dry basis). In the upper part of the seam, the pH values were controlled by the amount of the inflowing lacustrine water. Only thin, ash-rich topotelmites accumulated along the northern margin of the mire. There, the seam originated in a wet forest swamp and was separated by limnic and fluvial sediments from other organic rich horizons. Due to its closer vicinity to the pre-Tertiary carbonates, the depositional environment was probably even more alkaline. Therefore, the lignite shows a higher degree of gelification and is even more sulphur rich (up to 5.5% on a dry basis).

Petrography and depositional environment of coal seams in the Carboniferous Morien Group, Sydney Coalfield, Nova Scotia

The Morien Group (Westphalian A – Stephanian) of the Sydney Basin, Nova Scotia, contains numerous coal seams, many of which can be traced across the onshore part of the basin. Depositional environments during coal formation range from proximal, fluvially dominated to intercalated distal fluvial, to restricted marine. Most of the coal samples analyzed in this study come from the Bras d'Or section, a near-continuous coastal exposure of the Sydney Mines Formation. Petrographic compositions of the samples are discussed in the context of the depositional setting of the enclosing strata. The coal seams have a high vitrinite content (70–90 vol. %), with moderate inertinite and low liptinite. Petrographic indices suggest that most coals of the Sydney Mines Formation are similar to seams deposited in interdistributary or lower delta plain settings in other basins and provide support for the interpreted sedimentological setting of this sequence. The coal seams of the Bras d'Or section show little variation in gross petrographic composition relative to their stratigraphic position, although a slight trend of increasing vitrinite content towards the upper part of the sequence may indicate peat accumulation in a more distal setting. Coal seams that developed in the lower part of the section have relatively high amounts of detrital macerals, indicative of higher water levels and a higher degree of water circulation within the mires. The common occurrence of red beds in the upper part of the section and the increase of structured macerals, including fusinite, suggest somewhat drier conditions during peat accumulation. Petrographic compositions of underlying coals of the South Bar Formation (Gardiner and Tracy seams) indicate deposition in a somewhat more proximal setting than most coals from the Sydney Mines Formation, reflecting the overall retrogradational development of the Morien Group.

Petrographic evaluation of the depositional environments of the Cretaceous Obi/Lafia coal deposits in the Benue trough of Nigeria

Paralic coal beds occur intercalated within the Cretaceous systems in the area around Obi/Lafia of the middle Benue trough of Nigeria. The petrographic compositions of the coal beds indicate three ptrographic facies with different depositional environments. The coal beds at the bottom of the stratigraphical sequence, termed the vitrinite-fusinite coal facies, are rich in vitrinite, poor in liptinite with variable amounts of inertinite and low mineral matter content. Those at the top, the trimaceritic coal facies, have well represented amounts of vitrinite, liptinite and inertinite. Their mineral matter content is also low. Below, between the above these two coal facies occur the shaly coal facies with a very high content of mineral matter and variable amounts of vitrinite, liptinite and inertinite. Correlation of the distribution of microfossils in the interseam sediments with the maceral associations, mineral matter contents and the petrographic indices, indicates that the vitrinite-fusinite coal facies was deposited in a telmatic wet forest swamp subenvironment along and within lagoons; the trimaceritic facies in a limno-telmatic clastic marsh subenvironment in lower delta plains. The shaly coal facies shows characteristics of different subenvironments, but tends to be more marine.