Discussion on the concepts in paleoenvironmental reconstruction from coal macerals and petrographic indices

Marine and Petroleum Geology 73 (2016) 371e391

Contents lists available at ScienceDirect

Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Discussion

Discussion on the concepts in paleoenvironmental reconstruction from coal macerals and petrographic indices Souvik Sen a, *, Sumon Naskar b, Satyabrata Das c a

Geologix Limited, Dynasty Building, Wing A, Level 4, Andheri Kurla Road, Andheri (E), Mumbai, 400059 Maharashtra, India Geological Survey of India (GSI), Kankarbagh, Patna, Bihar 800020, India c Borehole Geophysical Research Laboratory, ESSO-MOES, Karad Patan Road, Supane, Karad, 415114 Maharashtra, India b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 January 2016 Received in revised form 7 March 2016 Accepted 8 March 2016 Available online 11 March 2016

Organic facies analyses quantify the coal constituents and plot various associations to discriminate the paleoenvironment for coal bearing successions. This allows the relation of coal composition to mire ecosystems or environments. Coal petrographic models are used extensively to reconstruct the nature of ancient peat forming environments. Many authors proposed relations between specific maceral assemblages and/or micro-lithotypes and peat forming environments. The key controlling factors which affect peat environment include hydrogeology, redox, pH, vegetation type, clastic influx, sedimentation and peat accumulation rate etc. Recent advancements in coal maceral study and organic petrology reveal the pros and cons of the available indices and models. The main reasons for the failure of the petrographic models are e oversimplification of the effects of humification on tissue preservation vs. destruction, the use of post-diagenetic processes (e.g. geochemical gelification) in determining depositional environments, changes in petrographic composition related to floral evolution, geological age, rank increase and compaction, lack of distinction between different inertinite maceral in some models. Here the widely used petrographic indices and models are reviewed based on the observations of several workers and the applicability and concepts of paleo-environmental reconstruction are discussed. A multi-disciplinary approach including petrography, palynology, chemistry etc. has been recommended, which is more logical and scientific than the exclusive use of petrographic composition for paleoenvironmental interpretation. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Coal Maceral Petrographic indices Peat forming environment

1. Introduction Several coal petrographic models reconstruct the nature of the ancient peat forming environment. Many authors have implied relationships between the peat forming environment and specific macerals/maceral assemblages and/or microlithotypes with hydrogeological/mineralogical controls (e.g. Calder et al., 1991; Diessel, 1992; Bend, 1992). Other determining factors controlling coal seam properties include original plant material, climate, geological characteristics (e.g. ash from volcanic activity, pH, redox condition, clastic influx, peat accumulation rate, sedimentation rate etc.) of the depositional environment (e.g. Cross and Taggart, 1982; Stach et al., 1982; Collinson and Scott, 1986, 1987; Chandra and Chakraborti, 1968; Scott, 1991; Shearer et al., 1995). Several

* Corresponding author. E-mail address: [email protected] (S. Sen). http://dx.doi.org/10.1016/j.marpetgeo.2016.03.015 0264-8172/© 2016 Elsevier Ltd. All rights reserved.

models were proposed to derive and interpret the nature of original peat forming environment (e.g. Smith, 1962; Hacquebrad and Donaldson, 1969; Smyth, 1979, 1984; Diessel, 1986, 1992; Bartram, 1987; Mukhopadhyay, 1986, 1989; Teichmüller, 1989; Schneider, 1990, 1995; Calder et al., 1991). Here commonly used coal petrographic models have been discussed and reviewed based on the findings and suggestions of many workers on coal macerals. We have also discussed various aspects of reconstructing palaeoenvironments from macerals and coal petrographic compositions. 2. Reviews Currently, organic facies analysis is based on the quantitative determination of the coal constituents (microlithotypes, maceral and sometimes ash content) where various maceral associations are plotted in diagrams to decipher the paleoenvironment of the mires. This has been claimed to allow the relation of coal composition to mire ecosystems or depositional systems to be interpreted

372

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

(e.g., Calder et al., 1991; Diessel, 1986; Hacquebard, 1993a, 1993b; Hacquebrad and Donaldson, 1969; Kalkreuth and Leckie, 1989; Marchioni and Kalkreuth, 1991; Mukhopadhyay, 1986). The interpretation of mire type came from the studies of minerotrophic mires (fed by surface water), based on macerals (e.g., Diessel, 1992; Kalkreuth and Leckie, 1989) or microlithotypes (e.g., Hacquebrad and Donaldson, 1969; Marchioni and Kalkreuth, 1991). For ombrotrophic modern mire systems (fed by rainwater), the distribution of diagnostic macerals, or their precursors, is poorly defined and relationships between sedimentary environment, mire type (Moore and Shearer, 2003) and maceral composition are not yet established and therefore do not provide a good basis for paleoenvironmental interpretation (Crosdale, 1993; Dehmer, 1995; Moore and Shearer, 2003; Wust and Bustin, 2001). 2.1. Diessel (1986, 1992) Diessel first discussed systematically the diagnostic value of different macerals for paleoenvironmental interpretations. He proposed a method for facies analysis based on quantitative petrographic indices. His model attempted to infer depositional environments of Australian Permian coals from maceral analyses. According to him, diagnostic macerals indicate original plant material or the biochemical conditions of preservation, permitted the description of paleoenvironmental fields in facies diagrams. The Diessel petrographic indices are Tissue Preservation Index and Gelification Index. These are the most extensively used coal petrographic indices for paleoenvironmental interpretation. These are based on vitrinite and inertinite maceral counts.

TPI ¼ ðtelovitrinite þ teloinertiniteÞ=ðdetrovitrinite þ gelovitrinite þ detro  inertinite þ gelo  inertiniteÞ (1) GI ¼ ðvitrinite þ gelo  inertiniteÞ=ðtelo  inertinite þ detro inertiniteÞ

For brown coal maceral equivalents:

TPI ¼ ðhumotelinite þ telogelinite þ fusinite þ semifusinite þ sclerotiniteÞ=ðhumodetrinite þ detrogelinite þ eugelinite þ porigelinite þ corpohuminite þ inertodetrinite þ micrinite þ macriniteÞ (3) GI ¼ ðhuminite þ macriniteÞ=ðfusinite þ semifusinite þ sclerotinite þ inertodetrinite þ micriniteÞ

(4)

Diessel's model is divided in four quadrants on the basis of high (>1) or low (<1) indices (Fig. 1). Ash content is considered to be an important factor in this model when determining environment. High GI, high TPI sector represents peat accumulation in a forested continuously wet raised bog if the ash content is low or in a forested telmatic swamp if the ash content is high. For low ash content, High GI, low TPI sector represents peat accumulation in an herbaceous dominated, continuously wet raised bog. If the ash content is high this quadrant is considered to represent telmatic to limno-telmatic peats derived from either herbaceous marsh vegetation or highly decomposed forest vegetation. Low GI, high TPI quadrant represents peat derived from a forested raised bog for low to moderate ash content, or a forested swamp subjected to occasional dry periods when high in ash. Low GI, low TPI quadrant represents peat derived from relatively dry raised bogs. Gelification Index and Tissue Preservation Index are the two coal

Fig. 1. Environmental fields on GIeTPI plot, after Diessel (1986).

petrographic indices which are extensively used for palaeoenvironmental analyses of peats and coals. Diessel's (1986, 1992) Gelification Index indicates the relative degree of wetness within mire by contrasting gelified (wetter) with non-gellified (drier) bituminous coal macerals. However this index oversimplifies a complex process since gelification in bituminous coals reflects both biochemical and geochemical gelification. Biochemical gelification is a reflection of the degree of anaerobic bacterial activity experienced by the organic matter upon incorporation into the peat, with the anoxia generally being a result of the presence of stagnant, anaerobic waters. Geochemical gelification is the part of the coalifiaction process which heralds the beginning of the bituminous coal rank and is unrelated to conditions occurring during the peat stage. Diessel's Gelification Index does not differentiate these two. Therefore it does not solely reflect water levels in the ancestral mire. Diessel's model (1986, 1992) assumes that low ash content indicates ombrogenic environment. It does not take into account the possibility of low ash content due to filtering effect of thick vegetation or isolation of parts of the mire from active fluvial Channels. Problems in the use of maceral indices such as Tissue Preservation Index and Gelification Index have been pointed out by Scott (2002a). Recent works on the petrology of modern peats has shown little correlation between maceral composition, vegetation type or environment (Moore and Shearer, 2003). The impression that the vegetational character of a peat-forming mire, its ecological structure and depositional environment can always be directly interpreted from coal petrographic analyses (either from coal macerals or coal lithotypes) is still prevalent in the literature. A variety of models based on petrography from the work of Hacquebrad and Donaldson (1969), Teichmüller (1989), Diessel (1986, 1992) and others do not stand up to scrutiny (see DiMichele and Phillips, 1994; Scott, 2002a). Diessel's model (1986, 1992) relies on the premise that humotelinite: humodetrinite ratio directly reflects the ratio of aborescent to herbaceous precursor material. However Diessel (1992) acknowledges the tissue destroying vs. preserving powers of severe vs. mid humification respectively, his model does not incorporate any possibility that herbaceous-derived material may

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

be well preserved as humotelinite, or that tree-derived woody material may be poorly preserved as humodetrinite without associated with high ash content. The ratio of is (Semifusinite þ Fusinite)/Inertodetrinite is also inferred by the model to be directly proportional to the degree of woody material contributing to the peat. However Dehmer (1995) found the highest fusinite content in Phragmites peat, which she attributed to fires. Inertinite and charcoals can be derived from outside the peat forming environment. Gelification occurs during peatification and may not necessarily relate to the original vegetation and its ecological setting (see for example comments by DiMichele and Phillips, 1994). Decay of angiosperm and gymnosperm wood is different and will affect rates of gelification (Hatcher and Clifford, 1997). Despite adverse comments on the use of petrographic indices to interpret peat-forming environments and vegetational composition, the full range of indices are still in use (e.g. Singh and Shukla, 2004). The inappropriate use of these indices have been discussed at length in the literature (see Collinson and Scott, 1987; Scott, 1989b, 1991; DiMichele and Phillips, 1994; Scott, 2002b; Moore and Shearer, 2003). These indices are fundamentally flawed. In peats, charcoal, resulting from wildfires, may result in the formation of fusinite, semifusinite, and inertodetrinite. This may happen in situ or be blown or washed into the peat from elsewhere (Scott and Glasspool, 2007; Scott, 2010; Hudspith et al., 2012). Its occurrence in the peat cannot be used, therefore, without considerable additional data, to interpret the peat-forming conditions. This, if not included in reconstruction of paleoenvironment, results can be over-simplified, erroneous and ambiguous. Glasspool and Scott (2012) used inertinite percentage in coal as paleo-atmospheric oxygen proxy, as wetter plants can burn and produce more charcoal in high oxygen atmosphere. In particular, all the macerals, used for calculating Gelification Index by Diessel (1986) may be present in a wildfire charcoal residue and their proportions may reflect transport (Scott et al., 2000; Scott, 2000). These are likely to be independent of the final environment of deposition. Likewise, TPI cannot be used to interpret tree densitydtrees do not equal wood. There are many trees that do not have wood and woody plants that are not trees (DiMichele and Phillips, 1994; Collinson and Scott, 1987; Scott, 2002a). T.P.I. includes inertinite group macerals that are formed by wildfire. There can be a peat without wood but with tree ferns and one with woody shrubs but not trees. In addition, the percentage of fusinite, semifusinite, and inertodetrinite in a sample is both a result of fire temperature and charcoal transport (wind vs. water) (Scott, 2000). Work on modern charcoal assemblages by Scott and Glasspool (2007) and Hudspith et al. (2012) have shown that the proportions of semifusinite, fusinite and inertodetrinite are controlled by a range of factors, including fire type, temperature and transport. Equally, therefore, these may tell us little about the nature of peat forming systems as such. More useful might be the occurrence of charred peat fragments showing in situ peat fires (Petersen, 1998; Glasspool, 2000). Examination of the petrography of wildfire charcoals further indicates that the use of inertinite group macerals in both the definition of Gelification Index and Tissue Preservation Index is not useful and also that these indices cannot necessarily provide data on the environment of peat formation nor on the vegetation type (Scott and Glasspool, 2007). Scott and Glasspool (2007) comment further on the widespread use of the Gelification Index (G.I.) and the Tissue Preservation Index (T.P.I.) developed by Diessel (1986) based on their observations of modern wildfire. In a significant piece of research, Wust and Bustin (2001) examine the relationship between vegetation, the resultant peat and petrography and apply the Diessel calculations. They show that while petrographic methods can contribute valuable information

373

about the palaeoecological settings of mire deposits, maceral indices have little utility. The actual degree of tissue preservation encountered in a coal is a reflection of the degree of humification suffered by the original organic material. Several or prolonged humification can mitigate the benefits of an inherently high tissue preservation potential, while minor humification can augment a lower inherent tissue preservation potential. The humotelinite: humodetrinite ratio of a peat or coal is therefore determined by both the inherent tissue preservation potential of the specific organic material, and the degree of humification experienced following death. As a consequence, this ratio reflects more than the degree of arborescence alone. 2.2. Calder et al. (1991), Calder (1993) Calder defined two indices which have been used to determine the paleoenvironment, viz. Vegetation Index (VI) and Groundwater Influence Index (GWI).

VI ¼ ðtelinite þ telocollinite þ fusinite þ semifusinite þ suberinite þ resiniteÞ=ðdesmocollinite þ inertodetrinite þ alginite þ liptodetrinite þ sporinite þ cutiniteÞ (5) GWI¼ðgelocolliniteþcorpocolliniteþmineralmatterÞ=ðtelnite þtelocolliniteþdesmocolliniteÞ (6) Converted to brown-coal terminology these become:

VI ¼ ðhumotelinite þ telogelinite þ fusinite þ semifusinite þ suberinite þ resiniteÞ=ðdensinite þ detrogelinite þ inertodertinite þ alginite þ liptodetrinite þ sporiniteÞ (7) GWI ¼ðeugelinite þ porigelinite þ corpohuminite þ mineral matterÞ=ðhumotelinite þ telogelinite þ densinite þ detrogeliniteÞ (8) The vegetation index (VI) indicates vegetation types. It contrasts the macerals derived from lignin-rich plants with macerals derived from lignin-poor plants. Lignin rich plants suggest dry environment and lignin poor plants tend to be concentrated in aquatic environments. The groundwater influence index contrasts strongly gelified macerals (corpocollinite and gelocollinite) and mineral matter, to weakly gelified macerals (telinite and telogelinite) and desmocollinite. Gelocollinite is derived from lignin-rich plants, while desmocollinite is considered to be derived in part from cellulose-rich vegetation (Calder et al., 1991). The inferred trends for these two plant types act reversely due to changes in the hydrological regime. Thus the authors placed them on opposing sides of the line in the GWI equation (Equation (8)). Mineral matters denotes flooding episodes, unless it can be shown to be of volcanic origin (tonsteins). Fig. 2 presents a graph of the two indices representing different environments and vegetation types. A high concentration of gelified material indicates high groundwater influence levels (GWI > 0.5) and so implies rheotrophic conditions. Groundwater influence indices between 3 and 5 represent a limnotelmatic environment, while a magnitude of 5 represents wetlands prone to

374

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

Fig. 2. Environmental fields on GWIeVI plot, after Calder et al. (1991).

siliciclastic inundation (Fig 2). Groundwater influence indices within 0.5 and 3 are described as mesotrophic. Low concentrations of gelified material are considered to indicate dryer conditions suggesting a more ombrotrophic environment (GWI < 0.5). Within these environments the dominant type of vegetation is indicated by the vegetation index. High vegetation indices (VI > 3) are interpreted as indicating high levels of structured tree-derived material as well as more terrestrial environment. Low values (VI < 3) have been interpreted as indicator of lower levels of preserved structured material and more limnic environment dominated by marginal aquatic or herbaceous plants (Fig. 2). Calder (1993) modified the groundwater influence index by removing the desmocollinite term:

GWI ¼ðgelocollinite þ corpocollinite þ mineral matterÞ= ðtelinite þ telocolliniteÞ

(9)

Converting to brown coal terminology:

GWI ¼ ðeugelinite þ porigelinite þ corpohuminite þ mineral matterÞ=ðhumotelinite þ telogeliniteÞ (10) As desmocollinite was eliminated from the GWI formula, GWI value for the boundary between the ombrogenous and telmatic zones becomes 0.20. Model given by Calder et al. (1991) is empirically based on Carboniferous bituminous coals and the Ground Water Index (GWI) attempts to contrast strongly gelified macerals þ mineral matter, with weakly gelified macerals. This model also does not differentiate between biochemical and geochemical gelification. Gelocollinite (equivalent to the brown coal macerals eugelinite þ progelinite) and desmocolinite (densinite þ detrogelinite) are placed in opposition in the GWI equation as they are considered by authors to be derived from different vegetation types (lignin-rich vs. cellulose rich respectively) and thus to reflect opposing trends of vegetation (arborescent vs. herbaceous, respectively) controlled largely by groundwater influences. However, the association of densinite

solely with herbaceous vegetation ignores the fact that severe humification of woody material can produce significant amounts of humodetrinite, or that herbaceous material may be preserved as humotelinite e 76.6% humodetrinite in a Taxodium peat, and 56% humotelinite in a Phragmites peat (Dehmer, 1995). Furthermore, the statement by the authors that environmental change from arborscent to herbaceous vegetation is due to hydrological modifications of the mire is overly simplistic and misleading. Other factors which control the floral composition of mire and which may vary independently of groundwater fluctuation include: climate, nutrient supply and dissolved oxygen levels; while natural disasters such as ash falls may lead to recolonization. Therefore the placement of gelocollinite and desmocolinite in opposition within the GWI equation, based on their inferred botanical relationships, does not necessarily reflect the exclusive influence of groundwater. The placement of eu-ulminite (part of humtelinite), telogelinite, densinite and detrogelinite in the denominator of the GWI equation implies that these macerals indicate a low groundwater influence (Calder et al., 1991). However these macerals are strongly gelified and their presence hence indicates low levels of humification, implying oxygen poor conditions, which are generally established and maintained by the presence of covering by stagnant water (Calder et al., 1991). Use of mineral matter in the GWI ratio implies that increased groundwater influence is invariably associated with an increase in mineral content. Sediment-laden flood water may introduce clastic material into the parts of the mire; other areas may be protected from clastic input by the baffling effect of thick vegetation or by topographical features. Increased detrital mineral matter contents indicate increased groundwater activity, the absence or paucity of mineral matter in a peat does not necessarily indicate a low groundwater influence. Furthermore, the water-borne detrital mineral matter in a coal is incorporated during the peat stage; the generation of huminite/vitrinite macerals as used in the GWI occurs over a much longer time period. Thus the detrital mineral matter and generation of huminite/vitrinite macerals represent noncontemporaneous events. Inertinite formation includes: primary, fungal attack, fire, subaerial oxidation and dehydration, subaquatic oxidation by oxygen rich waters, tectonic activities and coalification. The four generic groups of inertinite macerals are e primary inertinite, pyro-inertinites, degrade-inertinites and rank inertinites and these are therefore related to different stages in the series of steps which represent the transformation from living plant material through to coal: Living plant (primary inertinites) to peatification (pyroinertinites and degrade-inertinites) to coalification (tectonic pyroinertinites, rank inertinites). Inertodetrinite may represent either severe in-situ oxidation of plant material or fragmentation of oxidized material during transportation via water and wind (Stach et al., 1982; Diessel, 1992; ICCP, 1998a). With regard to water levels, fungally and subaerially oxidized degrade-inertinites and pyro-inertinites formed by surface fires and ground fires indicate low water levels. Higher water levels are indicated by subaquatically oxidized degrade-inertinites, and inertodetrinite can indicate either low or high water levels. Thus the exclusive use of inertinite as an indicator of dry, oxidizing conditions is unreliable (Scott and Jones, 1994).

2.3. Mukhopadhyay (1986, 1989) Mukhopadhyay presents three models based on Tertiary (Palaeocene to Eocene) coals of Texas. The first two models employ ternary plot, while the third is an XeY scatter plot of two maceral ratios.

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

2.3.1. Ternary plot models (1986, 1989) It involves the concentration of a ternary diagram (Fig. 3), whose apices are A ¼ humotelinite þ sporinite þ cutinite þ suberinite þ resinite. B ¼ humodetrinite þ humocollinite (excluding corpohuminite) þ liptodetrinite þ sapropelinite þ alginite. C ¼ inertinite. In 1989 Mukhopadhyay modified Index B (Fig. 4) by substituting mixinite for humocollinite: B ¼ humodetrinite þ mixinite þ liptodetrinite þ sapropelinite þ alginite. Mukhopadhyay (1989) described mixinite and sapropelinite as follows: Mixinite e It has a grainy, grey colour with occasional framboidal pyrites. Fluorescence is dark brown or non-existent. This ‘maceral’ resulted probably from the biodegradation or mixing of fine-grained (<5 mm) humic and liptinitic materials. Sapropelinite e Mukhopadhyay approximates this component to the maceral bituminite. Index A represents a forested swamp environment situated on an alluvial plain or upper delta plain. Index B represents a reed marsh or sub-aquatic environment situated in a delta plain or backbarrier setting. The upper delta plain swampemarsh complex coals plot between Indices A and B. Index C is related to oxidation levels (Figs. 3 and 4), which is controlled by water level. The mean water level is set at C ¼ 15% (Fig. 4). Values >15% represents low groundwater levels and those >30% a predominantly oxic conditions (Fig. 4). Values <15% represent a high groundwater level, with anoxia dominating below C ¼ 5% (Fig. 4) (Mukhopadhyay, 1989). 2.3.2. Ratio plot model (1989) Mukhopadhyay (1989) defined two maceral ratios as follows:

Ratio I ¼ humodetrinite=humotelinite

(11)

Ratio II ¼ ðliptodetrinite þ sapropelinite þ alginiteÞ=ðsporonite þ cutinite þ suberinite þ resiniteÞ: (12)

375

The two ratios are plotted in Fig. 5 with ratio I plotted as the abscissa and ratio II as the ordinate axis. The plot area is divided into three environmental fields: 1. Marsh dominated: Ratio II > 1 2. Swamp dominated: l < 1 and Ratio II < 1 3. Swamp-marsh complex: Ratio I > 1; Ratio II < 1 Mukhopadhyay's model (1989) associates the formation of all inertinite macerals with aerobic oxidation entirely due to low water levels in the ancestral mire, and thereby claims the proportion of inertinite can be used as a palaeo-water level indicator. The modified version of the model (Mukhopadhyay, 1989) actually calibrates percentage of inertinite values with the position of the ancestral water table, with 15% inertinite representing the estimated mean water table, <5% inertinite indicating anoxic conditions, and >30% indicating oxidizing conditions. Use of inertinite as an indication of water level assumes that all inertinite is autochthonous and represents in-situ aerobic conditions. This assumption does not allow for the formation of inertinite via transportation in oxygen rich waters or the reworking and redesposition of previously existing inertinite from other parts of the mire during flooding events. The high reflectance of primary inertinites and rank inertinites is unrelated to oxidation levels occurring in the mire at the time of the organic materials incorporation to the peat, while the formation of pyro-inertinites by crown fires need have no connection with groundwater levels. Furthermore Tertiary coals generally contain significantly lower amounts of inertinite (usually < 5%) while compared to Carboniferous (>5%) and Permian (>10%) coals. It suggests that the levels of inertinite defining anoxia (5%), mean water level (15%) and oxidizing conditions (30%) would need to be recalibrated in order for the model to be applicable for other coals. Summarily, the increase in inertinite content with increasing rank, and general decrease through geological time (from Palaeozoic to Tertiary), means that the total inertinite content is dependent on factors other than simply aerobic oxidation related to the height of the water level in the palaeomire. Sapropelinite is described as consisting of biodegraded humic and liptinitic material with occasional (Humosapropelinite) or abundant (sapropelinite II) associated pyrite (Mukhopadhyay, 1986), although it is equated to bituminite (Mukhopadhyay, 1989). Mixinite is described as consisiting of biodegraded humic and liptinitic material with occasional pyrite (Mukhopadhyay, 1989). The only apparent difference between these “macerals” is the variability in the amount of associated pyrite. These two macerals are described as consisting of two different maceral groups, which mean the use of term “maceral” is erroneous. Depending on the size of sapropelinite or mixinite occurrences, and the proportions of the constituents, the description of these entities approximates to that of microlithotype clarite, or if some of the material has become sufficiently oxidized, a trimacerite. Ratio plot model by Mukhopadhyay (1989) fails to agree with the palynology with regard to the tree-rich horizons because it relied on (humotelinite: humodetrinite) ratio as an indicator of arborescence. 2.4. Harevey and Dillon (1985)

Fig. 3. Ternary diagram of peat-forming environments from maceral associations, after Mukhopadhayay (1986).

The Vitrinite/Intertinite ratio (V/I) by Harevey and Dillon (1985) is an indicator of the degree of aerobic oxidation. For this reason degrado-inertinite should be specified, such that the V/I ratio would be more accurately expressed as the ratio of vitrinite/ degrado-inertinite. Where, Vitrinite indicates ¼ (mild aerobic and/or molecular) oxidation þ (biochemical þ geochemical) gelification; and Degrado-inertinite indicates ¼ severe or prolonged (primary

376

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

Fig. 4. Ternary diagram of peat-forming environments from maceral associations, after Mukhopadhayay (1989).

Fig. 5. Ratio plot model, after Mukhopadhayay (1989).

aerobic) oxidation. Harvey & Dilon found the ratio of total Vitrinite to total inertinite, measured on a micrinite- and mineral matter-free basis, was related to the position of contemporaneous palaeochannel deposits of carboniferous coals of Illinois. High V/I ratio (above 12) indicate

peat accumulation near or adjacent to channels with humification dominating oxidation. Low V/I ratios (5e11) indicate peat accumulating farther away from channels (10e20 km) in more oxidizing Conditions. The model doesn't differentiate between allochthonous and autochthonous inertinite. The model excludes detrital mineral

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

matter, which is reliable indicator of flooding due to close proximity of active river channel. 2.5. Hacquebrad and Donaldson (1969) This model is based upon microlithotypes of a bituminous coal of Nova Scotia. Hacquebrad and Donaldson (1969) combined microlithotypes in following manner to denote four indices A ¼ spore-clarite þ duro-clarite; B ¼ fusito-clarite; C ¼ vitroclarite þ cutico-clarite; D ¼ claro-durite þ durite þ carbargilite Fusitoeclarite (index-B) is described as consisting of fusite in a clarite matrix. These indices are then plotted on a double triangle facies diagram (Fig. 6). If the coal has less than 20% Index D components, then the upper triangle (Bright coal) is used. If the coal has less than 20% Index D Components, then the lower triangle is used (Dull coal). 2.5.1. Upper triangle zones Terrestrial Forest Zones e It represents deposition in terrestrial zone, occurring above the high water mark. This zone is characterized by lenses of fusite in a clarite matrix. Telmatic Forest Zones e It represents deposition in the telmatic zone, occurring between the high & low water marks. This zone is characterized by high vitrinite & low sporinite content. Telmatic Reed Zones e This is also deposited in the telmatic zone, but characterized by intermediate amounts of inertinite and mineral matter, with spore content. 2.5.2. Lower triangle zones Limnic Open moor Zone: It represents deposition below the low

377

water mark (subaquatic). Limnotelmatic Forest Zone: It represents deposition in the transitional zone between limnic & telmatic environments. This zone is characterized by intermediate amounts of inertinite & vitrinite with low spore content. Limnotelmatic Reed Zone: It also represents deposition in the transitional limnotelmatic zone, but characterized by increased spore content and decreased vitrinite content. The model proposed by Hacquebard and Donaldson doesn't distinguish between spore rich and spore poor, spore-clarites. These represent different environments, and the lack of distinction between the two types could lead to erroneous classifications. Spore-Clarites in which the huminite fraction dominates the liptinite fraction also fall under the term vitro-clarite as defined in this model. Use of microlithotype fusitoclaite implies that the fusite represents the product of in situ oxidation resulting from exposures of the peat surface due to low water levels; it does not allow for the presence of wind or water-borne allochthonous fusinite. This model can't distinguish between arborscent angiosperms and gymnosperms. 2.6. Smyth (1979, 1984) This model is based on Permo-Carboniferous coals of Australia. Microlithotype composition is plotted on a ternary diagram consisting of following indices: A ¼ intermediates (duroclarite þ claroduraite þ vitrinertite); B ¼ vitrite þ clarite; C ¼ durite þ inertitte The diagram is then sub divided into five areas representing different sedimentary environments: lacustrine, fluvial, brackish, upper deltaic and lower deltaic (Fig. 7). Smyth's model is based on the “dulling-upward” sequence of Smith (1962). Coals high in vitrite and clarite (i.e. bright coals) are considered to represent deposition in fluvial dominated environments, where variable and rapid subsidence interrupts the sequence, resulting in incomplete cycles characterized by vitrinite rich coals. Conversely, slow uniform subsidence, such as experienced in lacustrine environments, allows the sequence to fully develop, producing duller coals. This model does not provide for the existence of ombrogenous environments, nor does it attempt to determine the vegetation types. The trend of dominant microlithotypes from: (vitrite þ clarite) to intermediates to (durite þ inertite), is considered by the authors to represent decreasing rate and variability of subsidence from high to medium to low. It reflects a change in the dominant depositional environment from fluvial to deltaic to lacustrine (see Scott, 1989b). The position of brackish field therefore represents a depositional environment in which subsidence characteristics are transitional in between those of fluvial and deltaic environments; it should not be considered as paleosalinity indicator. This model associates high durite content with a lacustrine environment (high proportion of transported organic matter). This ignores the spore rich durites which are considered to represent exposure of the peat surface. This model also does not incorporate mineral matter as a controlling parameter. 3. Discussions on maceral indices and coal petrographic models

Fig. 6. Microlithotype facies diagram according to Hacquebrad and Donaldson (1969).

All peats/coals contain some inherent plant ash and may also contain detrital and authigenic minerals. With regard to determining palaeoenvironments, the genetic nature of the mineral matter needs to be ascertained. While the introduction of

378

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

Fig. 7. Depositional systems (a) and related microlithotypes (b) of Australian coals, from Smyth (1984).

syngenetic minerals (both detrital and authigenic) into a peat is concurrent with peat accumulation (Diessel, 1992), epigenetic minerals are emplaced in coal after diagenesis of the peat, so they provide no information regarding the depositional conditions in the ancestral mire during peat formation. Furthermore, while the presence of waterborne detrital mineral matter indicates the influence of groundwater in a topogenous setting, low levels of detrital mineral do not necessarily imply an ombrogenous environment. Vitrinite rich clarites are traditionally associated with forest peats, while sporinite rich clarites are associated with reed peats (Stach et al., 1982). However arborescent Carboniferous pteridophytes produced vast amount of spores (some species produced significantly greater amounts than the others e Collinson and Scott, 1987), while arborescent Mesozoic gymnosperms generally produced less pollen, but vast amounts of resin and waxes (Diessel, 1992). Furthermore, Dehmer (1995) found a higher sporinite content in a modern Taxodium (conifer) peat (2.6%) than in Phragmites (reed) peat (1.2%). This implies that the spore content of a peat/coal is not exclusively controlled by the morphological habit (arborescent vs. herbaceous) of the plant. Vegetation data should be linked with coal petrographic data to make a more logical and valid interpretation (Scott, 2002b). Several authors have attempted this but each attempt has raised some difficulties. Several authors have attempted to integrate petrographic and palynological/palaeobotanical data to interpret the vegetation and

environments of Tertiary brown coals. These studies are not without their own difficulties, as can be seen from divergent interpretations (Teichmüller, 1989; Hagermann and Wolf, 1987). Figueiral et al. (1999) have shown, for example, that angiosperm and gymnosperm woods decay at different rates and macroscopic and microscopic (palynological) plant data must be used together to interpret palaeovegetation. Different plants have different physical and chemical properties which bestow upon them different resistances to decay. Evolution of vegetation through geologic time has resulted in plants with different ecological requirements and different levels of tolerance in environmental conditions (Collinson and Scott, 1987). Differences in spore/pollen production rates due to different modes of reproduction, and a deciduous vs. evergreen leaf habit of tress, will affect the composition of the peat. Variation in some environmental parameters (e.g. nutrient status) may not be reflected in the petrographic composition of a peat or coal (Collinson and Scott, 1987). Rank dependence of some macerals and potential rankdependence of microlithotypes means that petrographic composition as determined by traditional and analytical methods is not controlled exclusively by the depositional environment or the nature of the original botanical material. The physical and chemical characteristics of the ancestral mire can become obscured or transformed by post-depositional processes like compaction, geochemical gelification and bituminization occurring as part of

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

peatification and coalification (Collinson and Scott, 1986, 1987; Scott, 2002a, 2002b, 2010; Sen, 2015). Vegetation growing on the surface of a peat body will extend roots down into the peat, where they may become incorporated into the earlier peat levels (e.g. Cypert, 1972; Cohen and Spackman, 1977; Cohen et al., 1987). This material may be better preserved than organic material which was deposited at the peat surface, as it avoids exposure to the potentially aerobic surface environment. The incorporation of woody root material into a predominantly herbaceous-derived peat can result in over-estimation of the contribution of woody material to the stratigraphically older peat horizon. In his classic study, Smith (1962) examined the relationships between coal petrology and miospore assemblages. He interpreted the data in terms of ‘phases’. While this work has been widely used, reinterpretation of the data has proved difficult (Scott, 2002a). In addition to miospore data, megaspores and other palynodebris data can be added to aid vegetational analysis (Scott, 1991; Scott and King, 1981; Bartram, 1987; Pearson and Scott, 1999). The study by Hacquebrad and Donaldson (1969) on Carboniferous coals from Nova Scotia raises other issues not least the interpretation of the coal petrographic data in terms of moor types based upon modern environments; as was pointed out by Collinson and Scott (1987), ‘‘models for ancient peat-forming environments based upon modern analogues cannot have full worth unless these differences between modern and ancient plants and plant communities are taken in to account.’’ Finally, the scale of sampling often proves a problem. For example, in their study of tropical peat from Panama, Cohen et al. (1989) show that in a peat of 840 cm thick, there may be up to 16 changes in the vegetation. When compacted and coalified, this would yield a coal seam less than 1 m thick. Considering also the thickness of some of the vegetational zones, a sampling programme of between 5 and 10 cm would be needed to identify these changes. This conclusion is confirmed by the study of Bartram (1987) who showed in a Carboniferous coals that there could be as many as six vegetational zones in 50 cm of coal. Providing vegetational data for complete seams, as is commonly practiced, alone will yield only part of the story. 4. Concepts in paleoenvironment reconstruction Construction of the petrographic models and that these concepts, while often common to several models, do not necessarily remain valid under the full range of ecological and/or diagenetic conditions encountered by the original organic matter and subsequent maceral precursors. Furthermore, the original petrographic characteristics of a peat body may become modified by various post-depositional processes which can occur during the transformation from peat to coal. The level of resolution which can be attained by a petrographic model is partly dependent on the size of samples analysed; large samples represent longer time spans than smaller samples and may therefore contain several different types of mires. In this section, we discuss the following aspects:  The humotelinite: humodetrinite ratio as an indicator of the proportion of arborescent vegetation in the palaeomire flora.  Inertinite as an indicator of in situ oxidation related to groundwater levels.  Mineral matter as an indicator of groundwater activity.  The distinction between biochemical and geochemical gelification.  The evolution of peat-forming vegetation, and the effect on coal petrographic composition.  Post-depositional factors affecting petrographic composition.

379

 The importance of sample size in controlling resolution.

4.1. The humotelinite: humodetrinite ratio as an indicator of arborescence 4.1.1. Tissue preservation vs. destruction during humification The process of humification involves the oxidative degradation of mainly cellulose, hemicellulose, and lignin, and is assisted by the action of fungi (Kirk, 1971; Kaarik, 1974) and aerobic bacteria in the acrotelm. Humification is also promoted under alkaline conditions (Stach et al., 1982), which encourage bacterial activity. Humification is characterized by the progressive destruction of tissue cellular structure, with varying degrees of incomplete humification producing a variety of products, including humic acids, modified lignin compounds, and humic colloids. Some portion of the original cellulose and lignin may survive the journey through the acrotelm relatively intact (Flaig, 1968; Chaffee et al., 1984; Given et al., 1984; Taylor and Liu, 1987; Diessel, 1992). The ratio of humotelinite to humodetrinite is an indicator of the degree of remnant cellular structure, and a reflection of the degree of aerobic bacterial activity suffered by the organic material during hurnification (e.g. Stach et al., 1982; Diessel, 1992). Lignin is less easily biodegraded than cellulose by aerobic bacteria, so that highly lignified (woody) tissue often survives humification in a better state of preservation than poorly lignified (non-woody) tissue exposed to a similar degree of humification. High humotelinite contents are therefore traditionally associated with arborescent vegetation, while humodetrinite is associated with herbaceous precursors. Furthermore, gymnosperm wood is considered to be more resistant to humification than angiosperm wood, due to higher lignin and resin contents. The difference in chemical composition between plants therefore confers upon them different inherent tissue preservation potentials, such that an increasing propensity for humodetrinite formation is traditionally considered to occur in the order: gymnosperm wood to angiosperm wood to herbaceous plants (Diessel, 1992). However, while this relationship may be applicable under conditions of relatively mild or brief humification, severe or prolonged humification results in the ultimate loss. Of all cellular structure and the decomposition of organic matter into carbon dioxide and water (Diessel, 1992). Thus the ultimate extent of tissue preservation may also be determined by the degree of humification, rather than the inherent tissue preservation potential of the organic material alone. 4.1.2. Effect of varying degrees of humification on similar vegetation Studies on peat by van der Molen (1988) and van der Heijden (1994) illustrate the effect of varying degrees of humification upon the preservation of similar tissues (and thus with similar inherent preservation potentials). The study of an Irish Sphagnum peat by van der Molen (1988) found a highly decomposed layer of peat overlain by a well preserved, stratigraphically higher layer of peat. The increased level of decomposition in the lower layer of the Irish peat was attributed by the author to a period of postdepositional humification which resulted from a substantial lowering of the groundwater table due to drought, before the accumulation of the upper peat layer. This lowering of the groundwater table led to the introduction of oxygen into the upper part of the catoteim causing renewed aerobic bacterial activity in the earlier peat, resulting in further destruction of previously wellpreserved tissue. The study of a Dutch Sphagnum peat by van der Heijden (1994) found that there was a wide range in the degree of preservation of stem material, which was attributed to localized variations in the degree of aerobic bacterial activity throughout the acrotelm. The studies of van der Molen (1988) and van der Heijden

380

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

(1994) illustrate the ability of increased humification to compromise the inherent tissue preservation potential of organic matter, both during and after its incorporation into a peat body. 4.1.3. Effect of varying degrees of humification on dissimilar vegetation As well as causing variable degrees of tissue preservation in peats derived from similar vegetation (van der Molen, 1988; van der Heijden, 1994), humification may also affect the traditionally accepted order of the propensity for tissue preservation in peats derived from differing vegetation (herbaceous vegetation to angiosperms to gymnosperms) (Dehmer, 1995). The study by Dehmer (1995) of peats derived from differing vegetation and depositional environments, revealed that prolonged humification of a woody Taxodium (gymnosperm) peat in aerobic waters resulted in extensive tissue destruction, yielding a dominance of humodetrinite over humotelinite (Table 1). The inertinite content of the Taxodium peat was only 1.0% (Table 1), so that based on petrography alone, this peat would have traditionally been interpreted as being derived from herbaceous material under anoxic conditions. The same study also reported Phragmites (reed), Carex (sedge) and Nymphea (water lily) peats which were dominated by well-preserved tissue, with humotelinite: humodetrinite ratios > 1 (Table 1) (q.v. Corvinus and Cohen, 1979). Furthermore, a woody angiosperm derived peat (Alnus e alder) was found to contain a higher proportion of preserved tissue, than the Taxodium peat (Table 1). The study by Dehmer (1995) demonstrates an increasing propensity for tissue preservation in the order: gymnosperms to angiosperms to herbaceous plants, which is a reversal of the traditionally accepted order (herbaceous plants to angiosperms to gymnosperms), and suggests that the susceptibility of organic material to the effects of humification is not exclusively controlled by the nature of the precursor vegetation (q.v. Stout and Spackman, 1989). Other authors have failed to find a consistent relationship between petrography and floral composition in coals (Smith, 1962; Bartram, 1987; Miao et al., 1989). The model of Teichmüller (1989) based on the Miocene Rhenish brown coals of Germany incorporates tree dominated forest mires (Sequoia moor and Nyssa e Taxodium swamp), a shrubby angiosperm mire (Myricaceae e Cyrttlaceae moor) and an herbaceous-dominated mire (reed marsh). Coal petrographic data from samples representing each of these mire types shows that hurnodetrinite is  60% for all mire types (Teichmüller 1951; Teichmüller and Teichmüller, 1968), indicating that mire type cannot be determined on the basis of humodetrinite dominance alone (q.v. Crosdale, 1993). Furthermore, the calculation of humotelinite: humodetrinite ratios from the data in Teichmüller (1951) reveals the ratio of humotelinite: humodetrinite for angiosperm derived coals (range ¼ 0.31e0.46; mean ¼ 0.37) to be greater than that of either Taxodium moor (0.35) or Sequoia moor (0.21) derived coals. 4.1.4. Summary The actual degree of tissue preservation encountered in a coal is

a reflection of the degree of humification suffered by the organic material. Severe or prolonged humification has the ability to mitigate the benefits of an inherently high tissue preservation potential, while minor humification can augment a lower inherent tissue preservation potential. The humotelinite: humodetrinite ratio of a peat or coal is therefore determined by both the inherent tissue preservation potential of the specific organic material, and the degree of humification experienced following death. As a consequence, this ratio reflects more than the degree of arborescence alone. 4.2. Inertinite as an indicator of in situ oxidation related to low groundwater levels The origins of inertinite macerals are diverse and the group may be classified into four genetic types, viz.: primary inertinites, pyroinertinites, degrado-inertinites, and rank inertinites (Stach et al., 1982). 4.2.1. Origins of inertinite macerals 4.2.1.1. Primary inertinites. These include sclerotinite and primary fusinite and represent cellular material in which a high, inertinitic reflectance due to the black pigment melanin is already established in the parent plant during life (Stach et al., 1982; Moore et al., 1996). This type of inertinite is not a product of oxidation occurring in the mire. 4.2.1.2. Pyro-inertinites. These include pyrofusinite and pyrosemifusinite (Stach et al., 1982) and represent fossil charcoal formed by fires (e.g. Demchuk et al., 1993; Jones, 1993; Scott and Jones, 1994) or frictional heating associated with tectonic activity (Goodarzi, 1986). Different types of fires can occur in peat-forming environments (Scott, 1989), most notably:  Surface fires e involve burning of surface litter.  Ground fires e involve burning of organic material beneath the surface litter.  Crown fires e involve burning of aerial plant parts. Surface fires and ground fires require subaerial exposure of the litter and peat surfaces, respectively, although the fuel need not be dry. Local variations in the degree of waterlogging of the organic material cause variations in the (restricted) oxygen supply and temperatures of combustion. This results in the incomplete combustion of organic material to different extents, producing various levels of charring which are reflected by the formation of horizons containing pyrofusinite (high level of charring) and pyrosemifusinite (lower level of charring) (Stach et al., 1982; Staub and Cohen, 1979; Scott, 1989). Crown fires bum the aerial parts of plants (foliage, branches, stems, etc.), travelling from one tree (or shrub) to another through the canopy, and are largely independent of surface fires (Scott, 1989). Crown fires can therefore occur in areas of a mire where the groundwater table is well above the surface of the peat.

Table 1 Maceral composition of some peats formed from differing vegetation and environments. I ¼ inertinite. L ¼ liptinite, a ¼ diatoms, b ¼ sponge spicules, values in volume % (from Dehmer (1995)). Peat type

Humotelinite

Humodetrinite

Humotelinite/humodetrinite

Other huminite

I

L

Other

Taxodium Alnus Nymphea Carex Phragmites

14 27.4 49.8 52.6 56

76.6 52.8 42.4 42 42.6

0.18 0.52 1.17 1.25 1.31

0.6 12.2 0.2 0.4

1 2.8 0.2 0.2 1.2

6 4.8 4.6 4.8 0.2

1.8a 2.8b

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

4.2.1.3. Degrado-inertinites. These include degrado-semifusinite and degrado-fusinite and represent organic material which has suffered fungal attack (Moore et al., 1996) and/or aerobic dehydration and oxidation (Stach et al., 1982). Fungi require an aerobic environment in which to decompose organic matter, so fungal attack is confined to the acrotelm. Evidence for the establishment of complex symbiotic relationships between fungi and higher plants extends back to the Early Devonian, and suggests that fungi may have originated in the Precambrian (Taylor and Taylor, 1997; Scagel et al., 1965). The intimate association of fungal hyphae with inertinitic material was studied in a peat from Indonesia (Moore et al., 1996), in which the level of fungal activity, (as indicated by the abundance and length of hyphae), was found to be directly related to the proportion of oxidized material, and to diminish with depth in the acrotelrn, illustrating that fungal oxidation of organic material can be a significant contributor to the inertinite component of peat (q.v. Cohen and Spackman, 1980; Styan and Bustin, 1983; Cohen et al., 1987). Dehydration and aerobic oxidation of organic material occurs when the surface of a peat body is exposed as a result of lowered water levels (Stach et al., 1982; Diessel, 1992). Oxidation of organic material can also occur under oxygenated, non-stagnant water (Rimmer and Davis, 1988; cf. Dehmer, 1995), where motion in the water column enables replenishment of the oxygen which is consumed during aerobic decomposition processes. The presence of degrado-inertinite is therefore an indication of aerobic conditions occurring within the palaeo mire, whether due to subaerial exposure, or to the presence of oxygenated water.

4.2.1.4. Rank inertinites. These include rank: fusinite and micrinite. Rank fusinite represents organic material which does not attain its inertinitic character (high reflectance) until the geochemical coalification stage (Stach et al., 1982; Teichmüller, 1989). Micrinite is considered to represent inertinitic material produced by the disproportionation of liptinitic material during geochemical coalification (Stach et al., 1982; Teichmüller, 1989). The formation of rank: inertinites has no direct connection with the conditions which existed in the palaeomire.

4.2.2. Decrease in inertinite content through geological time In addition to the different modes of inertinite formation, the inertinite content of coals has significantly decreased through geological time (cf. de Sousa e Vasconcelos, 1999), with Tertiary coals generally containing less inertinite (commonly < 10%) than Palaeozoic coals (commonly > 10%) (Shearer et al., 1995). Possible reasons for the decrease in inertinite content through geological time are (Shearer et al., 1995):  Decrease in plant lignin content through geological time due to the emergence of angiosperms (angiosperms are less lignified than gymnosperms). Lignin chars more readily than cellulose, producing more pyro-inertinites.  Increased biomass production through geological time may have led to effective dilution of the total inertinite content by the non-inertinite fraction. However, this assumes that inertinite production has remained fairly constant throughout geological time.  Inertinite content has been shown to increase with increasing rank, as the more labile constituents become gradually inertinised during increasing coalification. Geologically older coals are generally of higher rank than geologically younger coals, and the higher inertinite content of the older coals may therefore reflect an increase in rank inertinites.

381

4.2.3. Summary From the above discussion it can be seen that the modes of inertinite formation include: primary, fire, fungal attack, subaerial oxidation and dehydration, subaquatic oxidation by oxygen-rich waters, tectonic activities, and coalification. The four genetic groups of inertinite macerals are therefore related to different stages in the series of steps which represent the transformation from living plant material through to coal: Living Plant (primary inertinites) to Peatification (pyro-inertinites, degrado-inertinites) to Coalification (pyro-inertinites, rank inertinites). Inertodetrinite may represent either severe in situ oxidation of plant material, or fragmentation of oxidized material during transportation via water or wind (Stach et al., 1982; Diessel, 1992; ICCP, 1998a). With regard to water levels, fungally and subaerially oxidized degrado-inertinites, and pyro-inertinites formed by surface fires and ground fires indicate low water levels. Higher water levels are indicated by subaquatically-oxidized degrado-inertinites, while the presence of inertodetrinite can indicate either low or high water levels. Thus the exclusive use of inertinite as an indicator of dry, oxidizing conditions is unreliable (Scott and Jones, 1994). 4.3. Mineral matter as an indicator of groundwater activity Mineral matter in coals may occur in either disseminated or concentrated forms (Spears, 1987) and consists of plant-derived, detrital, and authigenic minerals (Bones and Himus, 1936; Hatch and Rastall, 1969; Stach et al., 1982; Andrejko et al., 1983; McCabe, 1984; Cohen et al., 1987). 4.3.1. Plant-derived elements, trace elements and mineral matter Plants contain elements and trace elements which contribute to the total elemental composition of the peat when they die (Swaine, 1990). Some plants also contain discreet mineral bodies, e.g. siliceous phytoliths (Andrejko et al., 1983; Ruppert et al., 1985), which are incorporated into the peat when they die. As well as those elements which are essential to life, plants also take-up non-essential elements (Wilson et al., 1971), which reflect the water chemistry of the palaeomire (Diessel, 1992). 4.3.2. Detrital mineral matter Detrital mineral matter may be transported into a peat-forming environment by wind or water. Ombrogenous peats generally possess very low levels of mineral matter due to the elevation of the peat surface above groundwater influence (McCabe, 1984). The majority of mineral matter in ombrogenous peats therefore represents plant-derived minerals and wind-borne material (e.g. volcanic ash to tonsteins). The presence of water-borne detrital mineral matter in a peat or coal is indicative of groundwater activity and thereby a topogenous mire setting. The occurrence of bands or partings composed of water-borne clay, silt or sand indicate flooding events, which may be caused by crevasse-splays, migration of active fluvial channels into a mire, or run-off from surrounding higher ground (Spears, 1987). However, low levels of water-borne detrital material in a peat or coal do not necessarily imply an ombrogenous peat-forming environment. Low amounts of waterborne detrital material in a topogenous mire may reflect isolation from active drainage systems (McCabe, 1984; Cohen et al., 1987, 1989; Courel, 1989; Falcon, 1989; Grady and Eble, 1990; Diessel, 1992), a rise in groundwater level not associated with actual flooding (Hacquebrrd and Donaldson, 1969; Falcon, 1989), or the sediment trapping effect of dense vegetation (Smith, 1962; Galloway and Hobday, 1983; Styan and Bustin, 1983, 1984; Ferm and Ward, 1984; Collinson and Scott, 1986, 1987; Cameron et al., 1989; McCarthy et al., 1989; Christanis et al., 1998;

382

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

cf. Triplehorn and Bohor, 1986; Sen, 2015). While the presence of water-borne detrital mineral matter in a coal may be used to infer a topogenous setting for the ancestral mire, petrographic indices which attempt to deduce water levels in the ancestral mire by contrasting detrital mineral matter content with maceral content (e.g. GWI of Calder et al., 1991; Calder, 1993; Sen, 2015) are flawed, because they contrast entities which represent non-contemporaneous events. The distinction needs to be made between macerals and their precursors. While the emplacement of detrital mineral matter and the parent material (maceral precursor) of vitrinites within a peat are contemporaneous events, the GWI of (Calder et al., 1991) contrasts mineral matter with telinite þ telocollinite þ desmocollinite. These vitrinite macerals are a product of the peatification and coalification processes; they do not exist in the peat-forming stage. 4.3.3. Authigenic mineral matter Syngenetic authigenic mineral matter may reflect the precipitation of various minerals from solution in circulating groundwaters, e.g. formation of carbonate concretions (McCabe, 1984), or the products of diagenetic processes occurring in the mire at the time of peat formation, e.g. pyrite formed by the sulphate-reducing bacterium Desulphovibrio desulfuricans (Neavel, 1981). Epigenetic mineralization in cleats and fissures produced following diagenesis includes sulphides, carbonates and silicates (Mackowsky, 1968; Stach et al., 1982; Casagrande, 1987; Spears, 1987; Diessel, 1992; Schobert, 1995; Chyi, 1997; Sen, 2015). 4.3.4. Summary All peats/coals contain some inherent plant ash and may also contain detrital and authigenic minerals. With regard to determining palaeoenvironments, the genetic nature of the mineral matter needs to be ascertained. While the introduction of syngenetic minerals (both detrital and authigenic) into a peat is concurrent with peat accumulation (Diessel, 1992; Sen, 2015), epigenetic minerals are emplaced in the coal after diagenesis of the peat, so they provide no information regarding the depositional conditions in the ancestral mire during peat-formation (Mackowsky, 1968; cf. Casagrande, 1987). Furthermore, while the presence of water-borne detrital mineral matter indicates the influence of groundwater in a topogenous setting, low levels of detrital mineral matter do not necessarily imply an ombrogenous environment. 4.4. The distinction between biochemical and geochemical gelification A common assumption incorporated in some petrographic models, is that increasing groundwater influence leads to an increasing proportion of gelified macerals (e.g. Harevey and Dillon, 1985; Diessel, 1986, 1992), and increasing intensity of gelification (Calder et al., 1991; Calder, 1993). However, these models are based on bituminous coals in which the gelified macerals (vitrinites) are a product of geochemical gelification (Crosdale, 1993). 4.4.1. Biochemical gelification Once organic material has passed into the anaerobic catotelm the rate of degradation slows markedly (Clymo, 1984, 1987) and biochemical gelification by anaerobic microbes becomes the dominant process. Biochemical gelification is associated with decreasing cellulose content, and increasing aromaticity (Russell and Barron, 1984). Because most microbes function better when pH levels are neutral to slightly alkaline, biochemical gelification is promoted under calcareous conditions (Stach et al., 1982). The humic material which enters the catotelm consists of remnant

plant material in the form of intact cellular tissues (mildly humified), humic detritus (moderately to severely humified), and humic colloids (more severely humified) (cf. Teichmüller, 1989). 4.4.1.1. Biochemical gelification of mildly humified material. Organic material which has passed rapidly into the catotelm and consequently suffered only mild humification, subsequently experiences swelling of the tissue cell walls. This swelling causes the cell lumens to close, resulting in the loss of cell definition (cf. Russell, 1984), although the cellular structure is not actually destroyed (Diessel, 1992). This leads to the formation of humotelinites, with differing intensities of biochemical gelification producing varying degrees of cell lumen closure; complete cell lumen closure produces the humocollinite maceral telogelinite (Fig. 8). 4.4.1.2. Biochemical gelification of humic colloids and humic detritus. Colloids consist of atoms or molecules of one material dispersed within another (Atkins, 1986). The traditional view point holds that humic colloids produced during humification may either remain in situ until subsequent desiccation to a gel during biochemical gelification, or they may be transported in solution and redistributed (Liu et al., 1982). Biochemical gelification of in situ humic colloids produces humotelinites or telogelinite (Fig. 8), in which the original cellular morphology is preserved (Liu et al., 1982), although severe biochemical gelification may make this cellular structure invisible under normal transmitted-light microscopy (Diessel, 1992). Humic detritus, in which the cellular structure has been destroyed during humification, undergoes biochemical gelification to produce humodetrinites and detrogelinite (Fig. 9). The detrital humic particles may also be cemented together by varying amounts of humic colloids (Fig. 9) which have undergone transportation and redistribution so that the original cellular morphology is lost (Liu et al., 1982; Diessel, 1992). Biochemical gelification of pure humic colloids produces eugelinite (Teichmüller, 1989) (Fig. 9). 4.4.2. Geochemical gelification Geochemical gelification occurs at the beginning of the subbituminous coal stage in response to increasing temperature (primarily) and pressure, and results in the transformation of huminites into vitrinites (Stach et al., 1982; Teichmüller, 1989). Condensation and polymerization reactions lead to the continued loss of' H and O and subsequent enrichment in C, increased aromaticity, and continued loss of any remnant cellular structure. The need for increased temperature necessitates burial; with rank increasing with increased depth of burial. The rate of rank increase (rank gradient) determines the depth required for a given rank: (e.g. sub-bituminous) to be attained, and is dependent on the geothermal gradient and rate of subsidence, which determines the length of time the peat/coal is exposed to a particular temperature (Stach et al., 1982; Diessel, 1992). 4.4.3. The relationship between humification and biochemical gelification A study by Hatcher et al. (1985) found that coalified wood from a lignite which still retained high amounts of cellulose and unaltered lignin showed only minimal gelification. Wood from another lignite which contained mineral matter infilling cell lumens also displayed only minimal gelification. Conversely, wood from lignite which contained altered lignin compounds and virtually no remaining cellulose exhibited a much higher level of gelification. The authors concluded that chemical heterogeneity (presence of cellulose, unaltered lignin and mineral matter) retards gelification. The process of chemical homogenization (elimination of cellulose and alteration of lignin compounds) occurs during humification. This means

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

383

Fig. 8. Schematic diagram showing the relationship between the intensity of biochemical gelification and the resultant huminite macerals produced from mildly humified organic matter and in situ humic colloids.

Fig. 9. Schematic diagram showing the relationship between the intensity of biochemical gelification and the resultant huminite macerals produced from humic detritus and transported/redistributed humic colloids.

that organic material which has undergone minimal or no humification is still chemically heterogeneous, and thus unable to experience gelification (Hatcher et al., 1985). The presence of mineral matter infilling cell cavities is also a barrier to achieving chemical homogeneity (Hatcher et al., 1985), which suggests that the presence of mineral matter retards humification. A study of peats from the Baltic Sea area by Timofeev and Bogolyubova (1973) found that increasing mineral matter content of the peats was associated with decreasing proportions of humic acids, and concluded that the presence of mineral matter inhibits humification. The studies of Timofeev and Bogolyubova (1973) and Hatcher et al. (1985) suggest that not only is biochemical gelification dependent on prior humification (qv. Liu et al., 1982), but that humification may also be retarded by the presence of mineral matter. Furthermore, the dependence of biochemical gelification on prior humification implies that variations in the susceptibility of different plants to humification may also affect the intensity of biochemical gelification (cf. Diessel, 1992). 4.4.4. The relationship between biochemical and geochemical gelification The necessity of burial depth to initiate geochemical gelification means that geochemical gelification is not directly related to the environment conditions occurring at the peat surface. However, if geochemical gelification could be shown to be dependent upon prior biochemical gelification or humification, then it could still have relevance as an indirect palaeoenvironmental indicator. In this context artificial coalification experiments are invaluable. Earlystage (low rank) artificial coalification experiments on peats by Rollins et al. (1991) showed that after artificial coalification, porosity was reduced as a result of cellular spaces becoming infilled with corpohuminites. The authors inferred that these corpohuminites were generated during the artificial coalification process (i.e. abiogenically) and that their presence indicates that biochemical alteration of cell wall material is therefore not necessary in order to generate them. However, Rollins et al. (1991) used peat in their experiment, rather than fresh plant material. This

means that the peat could have been subjected to biochemical processes (including biochemical gelification) before-hand, so a dependent relationship between vitrinite formation and biochemical gelification in their experiment cannot be totally ruled out. An earlier experiment by Davis and Spackman (1964), produced vitrinite by artificial coalification of fresh Taxodium wood. The use of fresh material, rather than peatified material (as in the experiment by Rollins et al., 1991) precludes the occurrence of any biochemical gelification, and thus indicates that geochemical gelification (vitrinite formation) does not require prior biochemical gelification or hurnification.

4.4.5. Significance for the reconstruction of palaeoenvironments 4.4.5.1. Biochemical gelification and bituminous rank coals. Stach et al. (1982) state that biochemical gelification is dependent on the original organic material, facies, water and ion supply, pH and redox conditions. The dependency of biochemical gelification on prior humification (Liu et al., 1982; Hatcher et al., 1985) illustrates a causal link with depositional environment during the initial peat-forming stage. Biochemical coalification (which includes biochemical gelification) usually ceases at depths of less than 10 m (Stach et al., 1982), although studies have shown that anaerobic bacterial decay in peat can continue to depths in excess of 11 m (e.g. Lewis Smith and Clymo, 1984). The process of peat accumulation is slow, (e.g.~l mm/yr at the surface), and decreases with increasing depth (Ingram, 1983), so that organic material occurring at a depth of 1 m in the peat will have taken several hundred years to reach that point. Continuing degradation throughout both the acrotelm and catotelm (Clymo, 1984) means that the organic matter will have been subjected to a variety of processes occurring over hundreds of years during its journey through the peat profile, and not just the surface conditions which existed at the time of its incorporation into the peat. The use of bituminous coal maceral ratios to determine groundwater influence in the ancestral mire, relies on two assumptions:

384

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

 That the intensity of gelification (e.g. Calder et al., 1991; Calder, 1993), or the ratio of gelified to nongelified macerals (e.g. Harevey and Dillon, 1985; Diessel, 1986) is an indicator of the degree of groundwater influence experienced during the peat forming stage.  That in the bituminous coal stage (i.e. following peatification and coalification), these ratios still reflect the processes which occurred during the peat-forming stage. The continuation of anaerobic bacterial decay through the catotelm implies that progressively deeper (¼older) peat will be progressively more decayed, due to increasing residence time in the catotelrn. Peat in the catotelm is permanently water saturated and not subject to normal fluctuations in water level or aerobic decay (humification) as occur in the acrotelm (Ingram, 1978, 1983; Clymo, 1984); although severe lowering of the water table, or erosion of the acrotelm may lead to aeration or exposure of the upper part of the catotelm (Ingram, 1978; van der Molen, 1988). The very low hydraulic conductivity of the catotelm (Ingram, 1978, 1983; Clymo, 1984) also means that changes in water chemistry at the surface are likely to have increasingly less impact on biochemical processes (including biochemical gelification) with increasing depth. Thus increasing intensity of biochemical gelification (cf. GWI of Calder et al., 1991; Calder, 1993) is expected to reflect increasing residence time in the catotelm, rather than increasing wetness at the peat surface. The increase in rank from peat to bituminous coal is accompanied by geochemical gelification which leads to a loss of distinction between biochemically gelified (ulminite, densinite, gelinite) and nongelified (textinite, attrinite) huminite macerals (Stach et al., 1982). The intensity of geochemical gelification does not reflect a similar intensity of biochemical gelification, i.e. the vitrinite maceral telinite may be derived from the huminite macerals textinite, texto-ulminite, and eu-ulminite, which each display a different intensity of biochemical gelification. This means that the intensity of biochemical gelification (cf. GWI of Calder et al., 1991; Calder, 1993), may be erroneously estimated on the basis of vitrinite macerals, as geochemical gelification masks the effects of biochemical gelification. Increasing rank may also cause a change in petrographic composition, which leads to a change in the ratio of gelified to nongelified macerals. The index of Harevey and Dillon (1985) and the gelification index of Diessel (1986) both approximate to the ratio of vitrinite/inertinite. As such they are a measure of the propensity for vitrinite formation over inertinite (Crosdale, 1993). The Inertinite content of bituminous rank coals is controlled by various factors (Stach et al., 1982; Goodarzi, 1986; Shearer et al., 1995; cf. de Sousa e Vasconcelos, 1999) including:  Primary inertinites e not related to water levels in the ancestral mire.  In situ oxidation (weathering) e indicates increased dryness.  Oxidation during transportation e indicates increased wetness, rather than dryness.  Geological age e inertinite content is higher in geologically older coals.  Rank e inertinite content increases with increasing coalification.  Tectonic activities. Humic material follows either the vitrinitisation or fusinitisation pathway (Diessel, 1992), so that as inertinite content increases with increasing rank, vitrinite content must decrease accordingly. Thus the ratio of vitrinite/inertinite in bituminous rank coals is a reflection of:

Vitrinite ðVÞ=Inertinite ðIÞ Where: V ¼ (mild aerobic ± molecular) oxidation þ (biochemical þ geochemical) gelification  rank changes to inertinite. I ¼ primary inertinite þ severe or prolonged oxidation ± transportation þ geological age þ rank changes from vitrinite. The presence of rank inertinites and the relationship between inertinite content and geological age, also means that coals of different ranks and/or ages cannot be compared using these models.

4.4.5.2. Geochemical gelification. High geothermal gradients due to igneous activity lead to increased rank gradients and anomalous coalification, e.g. Pleistocene brown coals of 0.33%Ro at <200 m depth in the Tengchong Basin, China (Kuili and Young, 1989). Rapid subsidence rates reduce the rank gradient by reducing the time spent at a particular temperature, so coal rank is lower than would be expected for a given attained temperature (Diessel, 1992). Because a minimum temperature is required for chemical reactions to take place, coals which experience only slight subsidence can remain at low ranks for very long periods of time, e.g. the Early Carboniferous brown coal of the Moscow Basin has never undergone any appreciable subsidence (Stach et al., 1982), so that the low temperatures (20e25  C) to which it has been exposed have been insufficient to raise it to a higher rank. Consequently rank gradients vary from basin to basin, so that the minimum depth of burial required to reach a given rank also varies (Stach et al., 1982). However, given a normal geothermal gradient (30  C/km) and a moderate rate of subsidence (<10 m/Ma, see Diessel, 1992) so that sufficient “cooking” time is available to reach chemical equilibrium, it would still take a depth of hundreds of metres for a coal to achieve subbituminous rank (cf. Stach et al., 1982; Diessel, 1992). The processes occurring at these depths (hundreds of metres) have no relationship to the conditions under which peat formation took place at the surface; indeed peat-formation will have ceased long before (Stach et al., 1982).

4.4.6. Summary Biochemical gelification is dependent on prior humification and may be further controlled by the nature of the organic material, pH, redox, and hydrological conditions (Glasspool and Scott, 2010). However, with increasing depth through the peat profile, the influence of surface conditions is likely to decrease, such that biochemical gelification which occurs in the catotelm is unlikely to reflect the environmental conditions at the surface. Geochemical gelification may mask the effects of biochemical gelification. Associated increases in rank may also produce changes in the ratio of gelified to nongelified macerals. The use of gelification in coals of sub-bituminous rank or higher as an indicator of water levels in the ancestral mire may thus lead to erroneous conclusions. Geochemical gelification transforms huminites into vitrinites and occurs in coals of subbituminous rank and higher in response to increasing temperature. The independence of geochemical gelification from the need for any prior biochemical gelification and/or humification, coupled with its requirement for burial to hundreds of metres depth, preclude its use as an indicator of environmental conditions existing during the peat forming stage.

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

4.5. Evolution of peat-forming vegetation and the effect on coal petrographic composition The petrographic composition of a coal is a reflection of the nature of the precursor material (woody tissue, spore, cuticle etc.), the depositional environment in the palaeomire, and the degree of coalification (Stach et al., 1982). The evolution of vegetation through geological time was accompanied by changes in the biological characteristics (e.g. mode of reproduction), physical structure (e.g. lignin: cellulose ratio) and chemical composition (e.g. nature of lignin) of the peat-forming flora, resulting in changes to the source material contributing to peats. A corollary of this, is that the petrographic composition of coals has changed through geological time in response to the evolution of mire flora, such that coals from different periods of geological time are marked by different petrographic characteristics (Stach et al., 1982; Davis, 1984; Collinson and Scott, 1987; Diessel, 1992). 4.5.1. Evolution of mire flora through geological time Three Eras may be recognized in the evolution of plants, viz. Palaeophytic, Mesophytic, and Cenophytic. The Palaeophytic Era extended from the Pre-Cambrian to the Lower Permian and was characterized by a dominance of Pteridophytes, while the Mesophytic Era extended from the Upper Permian to the Lower Cretaceous and was characterized by a dominance of gymnosperms. The Cenophytic Era began in the Upper Cretaceous and extends to the present day; it is characterized by the appearance of angiosperms which have become the dominant plants of today. The flora of Carboniferous mires was dominated by arborescent Pteridophytes such as Lepidodendron (Lycopsida, or lycopods), Calamites (Sphenopsida) and Psaronius (pteropsida e ferns) and herbaceous lycopods and sphenopsids such as Sphenophyllum. Early gymnosperms such as Cordaites were also important plants (Scagel et al., 1965; Collinson and Scott, 1987; Thomas and Spicer, 1987; Lapo and Drozdova, 1989). The Permian mire flora was characterized by a change to gymnosperm dominance (e.g. Glossopteris, Gangamopteris, cycads and ginkos). Gymnosperm dominance continued throughout the Mesozoic, with the conifer families Araucariaceae, Cupressaceae, Taxodiaceae and Pinaceae appearing in the Jurassic (Thomas and Spicer, 1987). Ferns began to increase in importance during the Jurassic, and Sphagnum first appears (Shearer et al., 1995). The Early Cretaceous was characterized by the advent of angiosperms and a continued increase in the abundance of ferns (e.g. Polypodiaceae, Osmundaceae and Cyatheaceae) and Sphagnum, although gymnosperms remained the dominant mire flora (cf. Thomas and Spicer, 1987). Mires of the Middle to Late Cretaceous, in the southern hemisphere were dominated by the southern beech e Nothofagus (Shearer et al., 1995). The Early Tertiary (Palaeocene) mire flora was similar to that of the Late Cretaceous, with the continuing dominance of gymnosperms. During the Eocene angiosperms remained subordinate to gymnosperms in the northern hemisphere, but became the dominant forms in the southern hemisphere. Miocene mires were similar to those of the Eocene and Oligocene (cf. Kvacek, 1998), but were also characterized by an increase in the importance of herbaceous angiosperms such as Ericaceae (heaths) and Grarninaceae (grasses) (Shearer et al., 1995). 4.5.2. Features of plants which influence peat formation The features of plants which influence peat-formation include: nature of the rooting system, methods of reproduction, leaf and stem biology. These in tum determine the ability of plants to occupy the specific microhabitats (e.g. marginal aquatic, open water) which may occur within a mire setting (Collinson and Scott, 1987).

385

4.5.2.1. Rooting systems. The rooting system (Stigmaria) of the Carboniferous lycopods was characterized by a shallow horizontally spreading form, in which stratigraphically higher roots rarely passed beneath lower ones (Gastaldo, 1986). The rooting system of conifers tends to form interwoven mats (Fowler et al., 1973) and penetrates deeper than Stigmaria, with neighbouring trees often overlapping their roots (Collinson and Scott, 1987). The shallowness of the Carboniferous lycopod rooting system means that they would have been far less tolerant of a lowering of the water table than the postCarboniferous conifers (Collinson and Scott, 1987). 4.5.2.2. Methods of reproduction. Reproduction in ferns involves the shedding of spores which then grow to produce a prothallus containing the male and female parts. Free water is then required for the sperm of one prothallus to swim to the egg of another for fertilization and subsequent germination to occur. The spores and prothalli are not protected by the parent plant, and are very susceptible to dryingout (Wilson et al., 1971; Collinson and Scott, 1987). Conversely, gymnosperms and angiosperms reproduce via seed production. Coniferous gymnosperms are characterized by the production of seed cones and pollen cones. Pollen is discharged from the pollen cone and borne by the wind to a seed cone where it lands in between the open scales. A fluid is then produced by the seed cone which draws the pollen further inside to fertilize the female gamete (egg). Food is supplied by the parent plant. With the food source developing before fertilization has occurred (Wilson et al., 1971). In the case of angiosperms, the pollen may be transported by wind or insects, and fertilization and germination occur within flowers attached to the parent plant. Free water is not required for fertilization, and the developing embryo is protected and supplied with food by the parent plant, with the food source being produced only after successful fertilization has occurred (Thomas and Spicer, 1987). The Pteridophyte (fern) method of reproduction necessitates the production of vast quantities of spores to ensure the development of even a small number of new plants (Collinson and Scott, 1987), while the seed method is far more efficient so that seed plants generally produce far less pollen. Furthermore, the Pteridophyte spores had no dormancy stage, so were much more susceptible to fluctuations in water levels than gymnosperms and angiosperms (Collinson and Scott, 1987). The time interval between pollination and fertilization in gymnosperms may be weeks to months (Wilson et al., 1971), while in angiosperms fertilization and germination are much more rapid; usually within hours or days (Thomas and Spicer, 1987). 4.5.2.3. Leaf and stem biology. The branching structure of arborescent species determines whether their canopies are open or closed, and thus controls the amount of light reaching lower levels. A deciduous leaf canopy will also allow more light to reach lower levels than a non-deciduous (evergreen). This in tum determines the nature and extent of any herbaceous understory which may develop (Collinson and Scott, 1987). In order to achieve an arborescent habit some method of support is required, and different types of arborescent plants support themselves by different means (Nichols, 1995). Arborescent lycopods grew up to 40 m tall and supported themselves by a thick growth of secondary cortex. The tree fern Psaronius grew up to 8 m tall and used a sheath of adventitious roots which covered the stem, while Calamites (up to 30 m tall) produced large amounts of secondary xylem (wood) (Thomas and Spicer, 1987). Wood contains lignin which provides rigidity to the cell walls (Wilson et al., 1971) and the majority of arborescent conifers and angiosperms achieved structural support by the use of wood (Collinson and Scott, 1987). However, different plants have different amounts of lignin in their main axes, while the distribution of lignin within the stem, and its

386

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

chemical composition varies between different plants (Collinson and Scott, 1987). Gymnosperm lignin is primarily of the guaiacyl type (Hatcher et al., 1989a), while angiosperm lignin is a mixture of mainly guaiacyl and syringyl types (Hatcher et al., 1989b), with the syringyl type lignin being more easily degraded than the guaiacyl type lignin (Hatcher et al., 1989b). Furthermore, Hatcher et al. (1989b) found that angiosperm wood of the same taxonomic family degraded at different rates during peatification, this they attributed to differences in the physical arrangement of lignin and cellulose, with lower levels of degradation in woods where lignin units were separated by cellulose units. 4.5.2.4. Microhabitats. Open water aquatic (floating) and marginal aquatic plants play an important role in the early stages of terrestrialisation of modern wetlands, by allowing vegetation to spread over open water, and by trapping clastic sediment. While Calamites and some arborescent lycopods may have occupied marginal aquatic sites, there is no firm evidence of open water aquatic plants existing in Carboniferous mires (Collinson and Scott, 1987). Sporeproducing plants are restricted to wet environments due to their method of reproduction. Some Pteridophytes may also have been able to propagate via rhizomes as well as via spore production, although they were probably not significant components of the Carboniferous mire flora (Collinson and Scott, 1987). 4.5.3. Vegetational influences on coal petrographic composition Vitrinite-rich clarites are traditionally associated with forest peats, while sporinite-rich clarites are associated with reed peats (Stach et al., 1982). However, arborescent Carboniferous Pteridophytes produced vast amounts of spores, (some species produced significantly greater amounts than others e Collinson and Scott, 1987), while arborescent Mesozoic gymnosperms generally produced less pollen, but vast amounts of resin and waxes (Diessel, 1992, pp.38, 39). Furthermore, Dehmer (1995) found a higher sporinite content in a modem Taxodium (conifer) peat (2.6%) than in Phragmites (reed) peat (1.2%). This implies that the spore content of a peat/coal is not exclusively controlled by the morphological habit (arborescent vs. herbaceous) of the plant. The shallow rooting system and mode of reproduction or Carboniferous Pteridophytes restricted the range of environments which they could inhabit. In contrast, later plants (gymnosperms and angiosperms) were able to occupy a wider range of habitats, including more marginal (both wetter and drier) sites and open water (Collinson and Scott, 1987; Cohen, 1973). Deciduous foliage contributes a higher proportion of leaf material to the peat than evergreen foliage (Collinson and Scott, 1987), and the rate at which deciduous plants shed their leaves will also determine rate of leaf litter input to the peat. Different species of angiosperm leaves decompose at different rates (Jensen, 1974), and the same is true for conifer leaves, which in tum are more resistant to decay that angiosperm leaves (Millar, 1974). Gymnosperm and angiosperm woods also decay at different rates due to differences in the chemical nature and internal distribution of their lignin (Hatcher et al., 1989b). This implies that tissue preservation is not exclusively determined by the depositional environment, nor is it merely a reflection of the ratio of arborescent to herbaceous flora (Wust and Bustin, 2001). Other biological constraints which control the degradation of organic material, and which may not be manifested by distinct petrographic signatures include:  Temperature. This influences both the taxonomic assemblage of micro-organisms, and their rate of activity (Kaarik, 1974).  Interactions between microbes and fungi. With regard to decomposition of the substrate, (dead organic material), both

synergistic and inhibitory relationships can occur between micro-organisms which simultaneously occupy the same substrate (Kaarik, 1974).  Nitrogen content of the substrate. Tissues with high nitrogen contents may be degraded at a higher rate than nitrogen-poor tissues (Flaig, 1968). Subtle variations in ecological conditions of the palaeomire (e.g. nutrient status) may not be reflected in changes to the petrographic composition of the peat/coal (Collinson and Scott, 1987), e.g. the difference between megaspore phase 1 (nutrient-rich, arborescent) and phase 2 (nutrient-poor, herbaceous) of Bartram (1987). The uncertainty regarding the origins of some macerals (e.g. vitrinite) and the general inability to accurately identify maceral precursors using standard petrographic techniques, means that some macerals may not be species specific, i.e. vitrinite may form from arborescent lycopods, sphenopsids, cordaites, glossopterids, conifers or angiosperms, and from the wood (secondary xylem) or bark (cortex and cork) of these plants (Collinson and Scott, 1987). This means that a floral change in the palaeomire may not be discernible by examination of the petrographic composition of the coal. A further corollary of the general inability of standard petrographic analysis to identify floral taxa, is that empirically derived petrographic models are not fully applicable to coals of different phytological Eras, i.e. a petrographic model based on a lycopod-dominated Palaeophytic coal cannot identify angiosperms in a Cenophytic coal. The above discussion suggests that there is no simple relationship between mire flora, environment, and petrographic composition which unites all coals throughout geological time (cf. Collinson and Scott, 1987). Reconstructive models based exclusively on the petrographic composition of a coal may therefore produce misleading or inconclusive results (Collinson and Scott, 1987; Crosdale, 1993; Dehmer, 1995). 4.5.4. Summary  Different plants have different physical and chemical properties which bestow upon them different resistances to decay.  Evolution of vegetation through geological time has resulted in plants with different ecological requirements and different levels of tolerance to fluctuations in environmental conditions.  Differences in spore/pollen production rates due to different modes of reproduction, and a deciduous vs. evergreen leaf habit of trees, will affect the composition of the peat.  Variation in some environmental parameters (e.g. nutrient status) may not be reflected in the petrographic composition of a peat or coal. 4.6. Post-depositional factors affecting petrographic composition As organic material passes downward through a peat body due to the continuing accumulation of dead plant material above, the peat becomes increasingly enriched in those plant tissues which are inherently more resistant to decay (Clymo, 1984; cf. Tegelaar et al., 1989). The continuing selective decomposition of organic material means that the petrographic composition of successively older peat layers, becomes an increasingly poorer indicator of the type of vegetation from which it formed (Clymo, 1984; Moore, 1987). 4.6.1. Macerals Increasing rank causes a loss of distinction among huminite macerals during the transformation to vitrinites via increasing compaction and geochemical gelification. Textinite, texto-ulrninite and eu-ulminite are changed to telinite, while densinite and

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

detrogelinite are changed to desmocollinite; eugelinite and porigelinite are changed to gelocollinite (Teichmüller, 1989). The gelification of vitrinites includes both biochemical and geochemical gelification. While biochemical gelification is related to peatification and reflects the activity of anaerobic bacteria, geochemical gelification is a function of coalification (i.e. burial depth, temperature, pressure) and is unrelated to conditions existing in the original mire (cf. Crosdale, 1993). Rank induced changes in liptinites (up to high volatile bituminous rank) and inertinites (up to anthracite rank) are less severe than for huminites/vitrinites, and the same terminology is used for these macerals in both brown coals and hard coals (Stach et al., 1982). However, at low volatile bituminous coal rank sporinite (Rimmer and Davis, 1988; cf. van Krevelen, 1981), cutinite (Stach et al., 1982) and resinite can no longer be distinguished from vitrinite. During the process of coalification the secondary liptinite macerals bituminite, fluorinite and exsudatinite may form Teichmüller (1989). Secondary inertinite macerals include micrinite and rank fusinite. Micrinite generally appears at the bituminous coal rank and is traditionally viewed as a by-product of the disproportionation of liptinites (including sporinite, resinite and bituminite) into fluid bitumen and micrinite (Stach et al., 1982). However, there is also some evidence to support a micrinite origin from the bacterial degradation of algal material (Stasiuk, 1993). Rank fusinite forms during geochemical coalification (Stach et al., 1982). 4.6.2. Microlithotypes Microlithotypes are arbitrarily defined (Stach, 1968) on the basis of the concentration of specific macerals occurring within an area of 50 mm  50 mm on the coal sample surface (ICCP, 1963). Macerals which constitute <5% of this area do not contribute to the classification of the microlithotype. The concentration of macerals occurring within the 50 mm  50 mm area is a function of their size and distribution. Macerals are distributed throughout the coal seam with varying degrees of heterogeneity, such that a particular maceral may be more concentrated in one horizon than in another. The transformation of a lignite into a bituminous coal is accompanied by compaction. Consequently, a microlithotype within a sample of lignite will be vertically compressed in the corresponding bituminous coal (assuming negligible horizontal compaction). Although estimates of the amount of compaction which accompanies the change from peat to coal vary among different authors (Ting, 1977; Stach et al., 1982; Winston, 1986; Courel, 1987), the value of 3: 1 given by Stach et al. (1982) for the transformation from lignite to bituminous coal is used to illustrate the following point. By definition a microlithotype must cover a 50 mm  50 mm area (ICCP, 1963). Assuming a compaction ratio of 3:1, the 50 mm  50 mm area of a lignite rank microlithotype would, upon compaction to bituminous rank, become 16.7 mm  50 mm. Thus the 50 mm  50 mm area of a bituminous rank microlithotype would possibly represent an equivalent lignite rank area of 150 mm  50 mm. The varying heterogeneity of maceral distribution throughout the seam means that the distribution of macerals in the compacted bituminous-rank area may potentially differ from that previously encountered in the lignite. Furthermore, if the proportion of macerals is significantly altered this will result in the necessary reclassification of the microlithotype. For example, a 50 llm  50 llm area of lignite consisting of 71% huminite, 25% liptinite and 4% inertinite would be classified as the microlithotype clarite. Subsequent compaction associated with an increase in rank to the bituminous stage might lead to a composition of 74% vitrinite, 20% liptinite and 6% inertinite occurring within the 50 llm  50 llm area, in which case the microlithotype would be reclassified as a duroclarite.

387

Compaction of peat and coal continues up to at least the bituminous coal rank (Stach et al., 1982). Therefore as compaction and rank increase due to increasing overburden and depth of burial, the concentration of macerals within a particular microlithotype may potentially change. If a previously minor (<5%) maceral group is subsequently increased to > 5% this will result in the necessary reclassification of the microlithotype, which implies that microlithotype composition is potentially rank dependent (cf. ICCP, 1998b). Furthermore, given the potential rank dependence of microlithotypes, the restriction of their defining area to a constant size (50 mm  50 mm) means that the ability to make meaningful comparisons between microlithotypes of different ranks may become compromised. 4.6.3. Introduction of post-depositional organic material Vegetation growing on the surface of a peat body will extend roots down into the peat, where they may become incorporated into the earlier peat levels (e.g. Cypert, 1972; Cohen and Spackman, 1977; Gunther et al., 1979; Cohen et al., 1987). This material may be better preserved than organic material which was deposited at the peat surface, as it avoids exposure to the potentially aerobic surface environment (Cohen et al., 1987). The incorporation of woody root material into a predominantly herbaceous-derived peat, can result in an over-estimation of the contribution of woody precursor material to the earlier peat horizon. 4.6.4. Significance for the reconstruction of palaeoenvironments The rank-dependence of certain macerals and potential rankdependence of microlithotypes has significant consequences for the application of petrographic models. As coal rank increases, petrographic composition is susceptible to change via factors which are unrelated to the original conditions prevailing in the ancestral mire at the time of peat-formation. This implies that the application of empirical models to coals of differing rank involves a potential risk of misinterpretation regarding inferred depositional environments. 4.6.5. Summary Rank-dependence of some macerals and potential rankdependence of microlithotypes means that petrographic composition as determined by traditional analytical methods is not controlled exclusively by the depositional environment or the nature of the original botanical material. The physical and chemical characteristics of the ancestral mire, can become obscured or transformed by post-depositional processes (compaction, geochemical gelification and bituminization occurring as part of peatification and coalification). Furthermore, penetration of roots down into older peat levels introduces younger material into stratigraphically older peat and can result in misleading petrographic composition. 4.7. The non-genetic Stopes-Heerlen classification system The Stopes-Heerlen classification system is based primarily on the morphology of macerals (Stach et al., 1982), and as such is nongenetic. Although correlations have been made between certain macerals and botanical components (most notably the liptinite macerals), the genetic precursors of other macerals remain either poorly understood or unknown (cf. Diessel, 1992). The use of certain maceral varieties, e.g. cordaitotelinite ¼ telinite derived from the wood of Cordaites (Stach et al., 1982), reflects the attempts which have been made to establish a genetically based aspect to the classification system where excellent structural preservation allows the identification of maceral precursors. However, it is rarely possible to assign the

388

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

individual petrographic entities encountered under the microscope to specific genetic precursors, due to the usually poor preservation of diagnostic features (Schneider, 1995). The primarily non-genetic nature of the Stopes-Heerlen classification system means that some of the environmental interpretations based on the presence, absence, quantity, quality, or interrelationships of macerals can be difficult to support. For example, the ternary plot model of Mukhopadhyay (1989) associates the formation of all inertinite macerals with aerobic oxidation caused by low water levels in the ancestral mire. However, this allegation ignores the fact that the formation of certain inertinites (primary, rank, and tectonically-induced inertinites) is unrelated to atmospheric oxidation, and thus does not reflect oxidation levels occurring within the ancestral mire at the time of organic matter accumulation. The placing of telogelinite, detrogelinite and eugelinite in the same category (levigelinite) implies a relationship exists between these three macerals. However, telogelinite is genetically related to the humotelinite macerals. While detrogelinite and eugelinite are genetically related to the humodetrinite macerals (Diessel, 1992). The primarily non-genetic nature of the Stopes-Heerlen classification system thus poses problems when attempting to determine the genetic history of a coal seam from its petrographic composition. 5. Conclusions and suggestions for further research 5.1. Petrographic models Recent advancements in coal maceral study and organic petrology reveal the pros and cons of the available indices and models. Petrographic models rely on the assumption that distinct processes acting upon distinct precursors produce diagnostic petrographic signatures. Main reasons for the failure of the petrographic models are ascribed to:

samples. However, calculation of the mean compositional value of a particular mire type conceals the degree of variability (standard deviation) in petrographic composition which is exhibited by individual samples of that particular mire type. 5.1.1. Mean petrographic composition values and resolution levels While there is a tendency for some petrographic components to show relationships with the palynology, these relationships only appear when mean compositional values are calculated for each mire type; no relationships exist for individual samples. However, calculation of the mean compositional value of a particular mire type conceals the degree of variability (standard deviation) in petrographic composition which is exhibited by individual samples of that particular mire type. 5.2. Biochemical gelification as an indicator of water levels in the ancestral mire Biochemical gelification is used by many petrographic models as an indicator of water levels in the ancestral mire. However, anaerobic microbial processes continue to occur throughout the catotelm. The surface conditions existing during the time of peat deposition mainly affect the acrotelm. It would therefore be prudent to attempt to determine the relative amounts of biochemical gelification occurring within both the acrotelm and catotelm. If biochemical gelification is primarily restricted to the catotelm, then its usefulness as a palaeoenvironmental indicator of surface conditions would be reduced. Petrographic analysis of modern peat bodies existing in a variety of rheotrophic and ombrotrophic settings should provide information about the extent of biochemical gelification which occurs in the acrotelm, relative to the catotelm. 5.3. The potential rank dependence of microlithotypes

▪Over-simplification of the effects of humification on tissue preservation vs. destruction. ▪The lack of distinction made by some models between the different inertinite macerals and their modes of formation. ▪The inability to recognize rheotrophic mires with low mineral matter contents. ▪The use of post-diagenetic processes (e.g. geochemical gelification) in determining depositional environments. ▪Changes in petrographic composition related to floral evolution, geological age, rank increase and compaction. ▪The provisional nature of petrographic models. The petrographic composition of coal cannot be used exclusively to determine the original peat forming vegetation. Variations in the severity of humification can compromise the inherent resistance of plant tissues to decay, such that the original vegetation cannot be determined on even a broad basis (i.e. arborescent vs. herbaceous origin). Only those models which incorporate palynology, histology and/or geochemistry (biomarker analysis) can determine the nature of the vegetation which contributed to the ancestral peat. Therefore, the models which rely on exclusively petrographic composition cannot fully reconstruct the nature of environments which occurred in the ancestral mire. The petrographic composition of coal is also dependent on rank and geological age. This means that petrographic models which are empirically based on the coals of a single rank (e.g. bituminous) and/or geological age (e.g. Carboniferous) cannot be reliably applied to coals of differing rank and age. While there is a tendency for some petrographic components to show relationships with the palynology, these relationships only appear when mean compositional values are calculated for each mire type; no relationships exist for individual

The defining 50 mm  50 mm size of microlithotypes is not only arbitrary, but also remains constant, regardless of coal rank. Compaction due to burial means that bituminous coal rank microlithotypes represent a much larger area of compacted coalified peat compared to coals of lower rank. This factor is not corrected for in standard micro petrographic analysis, and means that the composition of microlithotypes is a reflection of all coal forming factors, including burial depth (rank). A study to assess the impact of compaction on microlithotype composition could be designed as follows: Assuming a compaction ratio from brown coal to bituminous coal of 3: 1 as given by Stach et al. (1982), microlithotype analysis would be performed on a brown coal using microlithotype areas of 50 mm  50 mm and 50 mm  150 mm, with the 50 mm side parallel to the coal bedding. The 50 mm  50 mm area represents the bituminous coal microlithotype, while the 50 mm  l50 mm represents the equivalent uncompact brown coal microlithotype. Comparison of the two analyses would then reveal the extent to which compaction has affected the microlithotype composition. 5.4. Genetic approach to the determination of palaeomire vegetation Different plant tissues respond to the decay process in different ways, as a result of different inherent preservation potentials. However, under mild or severe humification the effectiveness of inherent preservation potentials may be enhanced or reduced (respectively), such that under mild or brief humification nonwoody precursor material may form humotelinite, while under severe humification woody precursor material may produce

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

humodetrinite. Thus, the degree of tissue preservation (as measured by the humotelinite/hurnodetrinite ratio) is not always a reliable indicator of the woody or non woody nature of a precursor. Attempts to reconstruct the nature of the vegetation which grew in the ancestral mire require a genetically based approach, which is not facilitated by the non-genetic Stopes-Heerlen classification system. Histological analyses are not constrained by arbitrary size limitations (microlithotypes), nor by the imposition of a nongenetic classification system (macerals). While the amount of anatomical information which can be acquired from the organic material via standard light microscope observations declines with increasing coal rank. low rank coals often still retain a great deal of genetically related anatomical structure, which can be used to determine the origin and (in some cases) the taxonomy of the precursor material (e.g. Lapo, 1975, 1976; Ting, 1977; Scott and Collinson, 1978; Schneider, 1978, 1980, 1984, 1995; Lapo and Drozdova, 1989; Moore and Hilbert, 1992; Shearer and Moore, 1994; Upchurch, 1995). 5.5. Multidisciplinary approach to palaeoenvironmental reconstruction Coal petrography, and the definition of coal macerals, will no doubt remain important. We must learn, however, the limitations of coal petrology and to make future progress, we need to engage in a multidisciplinary multi-technique approach to understanding coal. With regard to the overall approach to paleoenvironmental reconstruction, one should employ all the available information, i.e. petrography, palynology, chemistry etc. in the development of a reconstructive model and not restrict oneself to a single type of data (e.g. the exclusive use of petrographic composition). Models based on a single type of data/input may subsequently prove to be insufficiently robust or inaccurate when applied to peats or coals which differ in age or rank from those which they are empirically derived (e.g. Crosdale, 1993; Dehmer, 1995). Hence a multidisciplinary approach including petrography, palynology, chemistry etc. has been recommended, which is more logical and scientific than the exclusive use of petrographic composition for paleoenvironmental interpretation. Acknowledgements Authors express their sincere gratitude to Thomas Gentzis, Associate Editor and Octavian Catuneanu, Editor-in-Chief of Marine and Petroleum Geology for their timely support, coordination and communication. Authors thank the reviewers for their constructive reviews and suggestions, which helped in the betterment of the manuscript. Authors thank Satabdi Banerjee aka Pandu, Dipanjan Maiti, Nayani Das, Rajib Dhar (Essar Oil Limited, Raniganj CBM Project, India) and Soumen Sarkar (Cairn Energy India Ltd.) for their immense support, encouraging words and suggestions. SS is thankful to Geologix Limited for giving the permission to submit this work. References Andrejko, M.J., Cohen, A.D., Raymond Jr., R., 1983. Origin of mineral matter in peat. In: Raymond Jr., R., Andrejko, M.J. (Eds.), Mineral Matter in Peat: its Occurrence, Form and Distribution. Proceedings of a Workshop Held, 26e30 September, 1983 at Los Almos National Laboratory, Los Almos, NM, pp. 3e24. Atkins, P.W., 1986. Physical Chemistry, third ed. Oxford University Press, Oxford. Bartram, K.M., 1987. Lycopod succession in coals: an example from the Low Barnsley Seam, Yorkshire, England. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata: Recent Advances, Geological Society Special Publication, 32, pp. 187e199. Bend, S.L., 1992. The origin, formation and petrographic composition of coal. Fuel 71, 851e870. Bones, W.A., Himus, G.W., 1936. Coal: its Constitution and Uses. Longmans, Green and Co. Ltd., London. Calder, J.H., 1993. The evolution of a ground-water-influenced (Westohalian B) peat-

389

forming ecosystem in a piedmont setting: the No. 3 seam, Springhill coalfield, Cumerland Basin, Nova Scotia. In: Cobb, J.C., Cecil, C.B. (Eds.), Modern and Ancient Coal-forming Environments, Geological Society of America Special Paper 286, pp. 153e180. Calder, J.H., Gibling, M.R., Mukhopadhyay, P.K., 1991. Peat formation in a Westphalian B piedmont setting, Cumberland Basin, Nova Scotia: implication for the maceral-based, interpretation of rheotrophic and raised paleomires. Bull. Soc. Geol. Fr. 162 (2), 283e298. Cameron, C.C., Esterle, J.S., Palmer, C.A., 1989. The geology, botany and chemistry of selected peat-forming environments from temperate and tropical latitudes. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies and Depositional Models. Int. J. Coal Geol. 12, 105e156. Casagrande, D.J., 1987. Sulphur in peat and coal. In: Scott, A.C. (Ed.), Coal and Coalbearing Strata: Recent Advances, Geological Society Special Publication No. 32, pp. 87e105. Chaffee, A.L., Johns, R.B., Baerken, M.J., de Leeuw, J.W., Schenck, P.A., Boon, J.J., 1984. Chemical effects in gelification processes and lithotype formation in Victorian brown coal. Org. Geochem. 6, 409e416. Chandra, D., Chakraborti, N.C., 1968. Coalification trends in Indian coals. In: Lyons, P.C., Alpern, B. (Eds.), Coal: Classification, Coalification, Mineralogy, Trace Element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol. 13, 413e435.
ndez-Turiel, J.L., Bouzinos, A., 1998. Christanis, K., Georgakopoulos, A., Ferna Geological factors influencing the concentration of trace elements in the Philippi peatland, eastern Macedonia, Greece. Int. J. Coal Geol. 36, 295e313. Chyi, L.L., 1997. Groundwater transformation of a low-sulfur to a high-sulfur coal, the Harlem coal of the northern Appalachian basin. Int. J. Coal Geol. 33, 317e331. Clymo, R.S., 1984. The limits to peat bog growth. Philos. Trans. R. Soc. Lond. B303, 605e604. Clymo, R.S., 1987. Rainwater-fed peat as a precursor of coal. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata: Recent Advances, Geological Society Special Publication No. 32, pp. 17e23. Cohen, A.D., 1973. Petrology of some Holocene peat sediments from the Okefenokee Swamp-Marsh complex of southern Georgia. Geol. Soc. Am. Bull. 84, 3867e3878. Cohen, A.D., Spackman, W., 1977. Phytogenic organic sediments and sedimentary environments in the Everglades-mangrove complex; part II. The origin, description and classification of the peats of southern Florida. Palaeontographica (Abt. B) 162, 71e114. Cohen, A.D., Spackman, W., 1980. Phytogenic organic sediments and sedimentary environments in the Everglades-mangrove complex of Florida, part III. The alteration of plant material in peats and the origin of coal macerals. Palaeontographica (Abt. B) 172, 125e149. Cohen, A.D., Spackman, W., Raymond Jr., R., 1987. Interpreting the characteristics of coal seams from chemical, physical and petrographic studies of peat deposits. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata: Recent Advances, Geological Society Special Publication No.32, pp. 107e125. Cohen, A.D., Raymond Jr., R., Ramirez, A., Morales, Z., Ponce, F., 1989. The Chaguinola peat deposit of northwestern Panama: a tropical back-barrier, peat (coal)forming environment. Int. J. Coal Geol. 12, 157e192. Collinson, M.E., Scott, A.C., 1986. Factors controlling the organization and evolution of ancient plant communities. In: Gee, J.H.R., Giller, P.S. (Eds.), Organization of Plant Communities, Past and Present. 27th Symposium of the British Ecological Society, Aberystwyth. Blackwell Scientific Publication, Oxford, pp. 399e420. Collinson, M.E., Scott, A.C., 1987. Implications of vegetational change through the geological record on modes for coal forming environments. In: Scott, A.C. (Ed.), Coal and Coal Bearing Strata: Recent Advances, Geological Society Special Publication No. 32, pp. 67e85. Corvinus, D.A., Cohen, A.D., 1979. Pre-maceral characteristics of carbonaceous sediments from Snuggedy Swamp, South Carolina. In: Cross, A.T. (Ed.), Compme Congre s International de Stratigraphie et de Ge ologie de terendu neuvie re, Economic Geology: Coal, Oil and Gas, vol. 4, pp. 171e181. Carbonife Courel, L., 1987. Stages in the compaction of peat: examples from the Stephanian and Permian of the Massif Central, France. J. Geol. Soc. Lond. 144, 489e493. Courel, L., 1989. Organics versus clastics: conditions necessary for peat (coal) development. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies and Depositional Models. Int. J. Coal Geol. 12, 193e207. Crosdale, P.J., 1993. Coal maceral ratios as indicator of environment of deposition: do they work for ombrogenous mires? An example from the Miocene of New Zeeland. Org. Geochem. 20, 797e809. Cross, A.T., Taggart, R.E., 1982. Causes of short-term sequential changes in fossil plant assemblages: some considerations based on a Miocene flora of the northwest United States. Ann. Mo. Botanical Gard. 69, 676e734. Cypert, E., 1972. The origin of houses in the Okefenokee prairies. Am. Midl. Nat. 87, 448e458. Davis, A., 1984. Chapter 3. Coal petrology and petrographic analysis. In: Ward, C.R. (Ed.), Coal Geology and Coal Technology. Blackwell Scientific Publications, Melbourne, pp. 74e112. Davis, A., Spackman, W., 1964. The role of the cellulosic and lignitic components of wood in artificial coalification. Fuel 43, 215e224. de Sousa e Vasconcelos, L., 1999. The petrographic composition of world coals. Statistical results obtained from a literature survey with reference to coal type (maceral composition). Int. J. Coal Geol. 40, 27e58. Dehmer, J., 1995. Petrological and organic geochemical investigation of recent peats with known environments of deposition. Int. J. Coal Geol. 28, 111e138.

390

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391

Demchuk, T., Cameron, A.R., Hills, L.V., 1993. Organic petrology of an early Paleocene coal zone, Wabamun, Alberta: palynology, petrography and geochemistry. Org. Geochem. 20, 135e148. Diessel, C.F.K., 1986. On the correlation between coal facies and depositional environments. In: Advances in the Study of the Sydney Basin, Proceeding of the 20th Symposium, Newcastle, NSW, Australia, pp. 19e22. Diessel, C.F.K., 1992. Coal-bearing Depositional Systems. Springer-Verlag, Berlin, p. 721. DiMichele, W.A., Phillips, T.L., 1994. Palaeobotanical and palaeoecological constraints on models of peat formation in the Late Carboniferous of Euramerica. Palaeogeogr. Palaeoclimatol. Palaeoecol. 106, 39e90. Falcon, R.M.S., 1989. Macro- and micro-factors affecting coal-seam quality and distribution in southern Africa with particular reference to the No. 2 seam, Witbank coal field, South Africa. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies and Depositional Models. Int. J. Coal Geol. 12, 681e731. Ferm, J.C., Ward, C.R., 1984. Chapter 5. Geology of coal. In: Ward, C.R. (Ed.), Coal Geology and Coal Technology. Blackwell Scientific Publications, Melbourne, pp. 151e176. Figueiral, I., Mossbrugger, V., Rowe, N.P., Ashraf, A.R., Utescher, T., Jones, T.P., 1999. The Miocene peat-forming vegetation of northwestern Germany: an analysis of wood remains and comparison with previous palynological interpretation. Rev. Palaeobot. Palynol. 104, 239e266. Flaig, W., 1968. Biochemical factors in coal formation. In: Murchison, D.G., Westoll, T.S. (Eds.), Coal and Coal-bearing Strata. Oliver & Boyd, Edinburgh, pp. 197e232. Fowler, K., Edwards, N., Brett, D.W., 1973. In situ coniferous (Taxodiaceous) tree remains in the Upper Eocene of southern England. Palaeontology 16, 205e217. Galloway, W.E., Hobday, D.K., 1983. Terrigenous Clastic Depositional Systems: Applications to Petroleum, Coal and Uranium Exploration. Springer-Verlag, N.Y. Gastaldo, R.A., 1986. Implications on the paleoecology of authochthonous lycopods in clastic sedimentary environments of the Early Pennsylvanian of Alabama. Palaeogeogr. Palaeoclimatol. 53, 191e212. Given, P.H., Spackman, W., Painter, P.C., Rhoads, C.A., Ryan, N.J., Alemany, L., Pugmire, R.J., 1984. The fate of cellulose and lignin in peats: an exploratory study of the input to coalification. Org. Geochem. 6, 399e407. Glasspool, I., 2000. A major fire event recorded in the mesofossils and petrology of the Late Permian, Lower Whybrow coal seam, Sydney Basin, Australia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 164, 357e380. Glasspool, I.J., Scott, A.C., 2010. Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal. Nat. Geosci. 3, 627e630. Glasspool, I.J., Scott, A.C., 2012. Identifying past fire events. In: Belcher, C.M. (Ed.), Fire Phenomena in the Earth System e an Interdisciplinary Approach to Fire Science, pp. 179e206. Goodarzi, F., 1986. Anisotropic fragments in strongly folded and faulted coals from the Rocky Mountain area in British Columbia. Can. J. Earth Sci. 23, 254e258. Grady, W.C., Eble, C.F., 1990. Relationship among macerals, minerals, miospores and paleoecology in a column of Redstone coal (Upper Pennsylvanian) from northcentral West Virginia (U.S.A.). Int. J. Coal Geol. 15, 1e26. Gunther, P.P., Casagrande, D.J., Chipman, M., Considine, R., 1979. The Nymphaea rhizome as a model of subsurface peat formation in the Okefenokee Swamp, me congress international de Georgia. In: Cross, A.T. (Ed.), Compte rendu neuvie ologie du Carbonifere, Washington and Champaignstratigraphie et de ge Urbana, Economic Geology: Coal, Oil and Gas, vol. 4, pp. 161e170. Hacquebard, P.A., 1993a. Petrology and facies studies of the Carboniferous coals at Mabou mines and inverness in comparison with those the Port Hood, St. Rose and Sidney coalfields of Cape Breton Island, Nova Scotia, Canada. Int. J. Coal Geol. 24, 7e46. Hacquebard, P.A., 1993b. The Sydney coalfield of Nova Scotia, Canada. Int. J. Coal Geol. 23, 29e42. Hacquebrad, P.A., Donaldson, J.R., 1969. Carboniferous coal deposition associated with flood-plain and limnic environments in Nova Scotia. In: Dapples, E.C., Hopkins, M.E. (Eds.), Environments of Coal Deposition, Geological Society of America, 114, pp. 143e191. Hagermann, H.W., Wolf, M., 1987. New interpretations of the facies of the Rheinish brown coal of West Germany. Int. J. Coal Geol. 7, 337e348. Harevey, R.D., Dillon, J.W., 1985. Maceral distributions in Illinois coals and their paleoenvironmental implications. Int. J. Coal Geol. 5, 141e165. Hatch, F.H., Rastall, R.H., 1969. Textbook of Petrology. In: Petrology of the Sedimentary Rocks, vol. 2. Thomas Murby & Co., London fourth ed., revised by Greensmith, J.T. Hatcher, P.G., Clifford, D.J., 1997. The organic geochemistry of coal: from plant materials to coal. Org. Geochem. 27, 251e274. Hatcher, P.G., Romankiw, L.A., Evans, J.R., 1985. Gelification of wood during coalification. In: Proceedings of the International Conference on Coal Science, Sydney, Australia, 28e31 October, 1985, pp. 616e619. Hatcher, P.G., Lerch III, H.E., Verheyen, T.V., 1989a. Organic geochemical studies of the transformation of gymnospermous xylem during peatification and coalification to subbituminous coal. In: Lyons, P.C., Alpern, B. (Eds.), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry and Oil and Gas Potential. Int. J. Coal Geol. 13, 65e97. Hatcher, P.G., Wilson, M.A., Vassallo, A.M., Lerch III, H.E., 1989b. Studies of angiospermous wood in Australian brown coal by nuclear magnetic resonance and analytical pyrolysis: new insights into the coalification process. In: Lyons, P.C., Alpern, B. (Eds.), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry and Oil and Gas Potential. Int. J. Coal Geol. 13, 99e126.

Hudspith, V., Scott, A.C., Collinson, M.E., Pronina, N., Beeley, T., 2012. Evaluating the extent to which wildfire history can be interpreted from inertinite distribution in coal pillars: an example from Late Permian, Kuznetsk Basin, Russia. Int. J. Coal Geol. 89, 13e25. ICCP (International Committee for Coal and Organic Petrology), 1963. International
Scientifique, Paris. Handbook of Coal Petrology. Centre National de la recherche ICCP (International Committee for Coal and Organic Petrology), 1998a. Inertinite, 8th Draft, 55 p. ICCP (International Committee for Coal and Organic Petrology), 1998b. Microlithotype, 3rd draft, June 1998. (unpublished). Ingram, H.A.P., 1978. Soil layers in mires: function and terminology. J. Soil Sci. 29, 224e227. Ingram, H.A.P., 1983. Hydrology. In: Gore, A.J.P. (Ed.), Ecosystems of the World 4A. Mires: Swamp, Bog, Fen and Moor e General Studies. Elsevier, Amsterdam, pp. 67e158. Jensen, V., 1974. Decomposition of angiosperm tree leaf litter. In: Dickinson, C.H., Pugh, G.J.F. (Eds.), Biology of Plant Litter Decomposition, vol. 1. Academic Press Ltd., London, pp. 69e105. Jones, T.P., 1993. New morphological and chemical evidence for a wildfire origin for fusain from comparisons with modern charcoal. In: Collinson, M.E., Scott, A.C. (Eds.), Studies in Paleobotany and Palynology in Honour of Professor W.G. Chaloner, F.R.S, Special Papers in Palaeontology No. 49. The Palaeontological Association, London, pp. 113e123. Kaarik, A.A., 1974. Decomposition of wood. In: Dickinson, C.H., Pugh, G.J.F. (Eds.), Biology of Plant Decomposition, vol. 1. Academic Press Ltd, London, pp. 129e174. Kalkreuth, W.D., Leckie, D.A., 1989. Sedimentological and petrographical characteristics of Cretaceous strandplain coals: a model for coal accumulation from the North American Western Interior Seaway. Int. J. Coal Geol. 12, 381e424. Kirk, T.K., 1971. Effects of microorganisms on lignin. Annu. Rev. Phytopathol. 9, 185e210. Kuili, J., Young, Q., 1989. Coal petrology and anomalous coalification of Middle and Late Pleistocene peat and soft brown coal from the Tengchong Basin, Western Yunnan, People's Republic of China. In: Lyons, P.C., Alpern, B. (Eds.), Coal: Classification, Coalification, Mineralogy, Trace-element Chemistry, and Oil and Gas Potential. Int. J. Coal Geol. 13, 143e170. Kvacek, Z., 1998. Bilina: a window on Early Miocene marshland environments. Rev. Palaeobot. Palynol. 101, 111e123. Lapo, A.V., 1975. Paragenetic associations of microcomponents in the Jurassic coals of Tuva. Doklady Akademii NaukSSSR (Earth Sci. Sect.). 221, 210e213. Lapo, A.V., 1976. Phyterals of Jurassic coals of Tuva. palaeobotanist 25, 205e216. Lapo, A.V., Drozdova, I.N., 1989. Phyterals of humic coals in the U.S.S.R. Int. J. Coal Geol. 12, 477e510. Lewis Smith, R.I., Clymo, R.S., 1984. An extraordinary peat-forming community on the Falkland Islands. Nature 309, 617e620. Liu, S., Taylor, G.H., Shibaoka, M., 1982. Biochemical gelification and the nature of some huminite macerals. Aust. Coal Geol. 4, 142e152. Mackowsky, M.-Th, 1968. Mineral matter in coal. In: Muchinson, D.G., Westoll, T.S. (Eds.), Coal and Coal-bearing Strata. Oliver & Boyd, Edinburgh, pp. 309e321. Marchioni, D., Kalkreuth, W., 1991. Coal facies interpretations based on lithotype and maceral variations in Lower Cretaceous (Gates Formation) coals of Western Canada. Int. J. Coal Geol. 18, 125e162. McCabe, L.C., 1984. Depositional environments of coal and coal-bearing strata. In: Rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences, International Association of Sedimentologists Special Publication No. 7. Blackwell Scientific Publications, Oxford, pp. 13e42. McCarthy, T.S., McIver, J.R., Cairncross, B., Ellery, W.N., Ellery, K., 1989. The inorganic chemistry of peat from the Maunachira channel-swamp system, Okavango Delta, Botswana. Geochim. Cosmochim. Acta 53, 1077e1089. Miao, F., Qian, L., Zhang, X., 1989. Peat-forming materials and evolution of swamp sequences e case analysis of a Jurassic inland coal basin in China. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies and Depositional Models. Int. J. Coal Geol. 12, 733e765. Millar, C.S., 1974. Decomposition of coniferous leaf litter. In: Dickinson, C.H., Pugh, G.J.F. (Eds.), Biology of Plant Litter Decomposition, vol. 1. Academic Press Ltd., London, pp. 105e128. Moore, P.D., 1987. Ecological and hydrological aspects of peat formation. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata: Recent Advances, Geological Society Special Publication No. 32, pp. 7e15. Moore, T.A., Hilbert, R.E., 1992. Petrographic and anatomical characteristics of plant material from two peat deposits of Holocene and Miocene age, Kalimantan, Indonesia. Rev. Palaeobot. Palynol. 72, 199e227. Moore, T.A., Shearer, J.C., 2003. Peat/coal type and depositional environment are they related? Int. J. Coal Geol. 56, 233e252. Moore, T.A., Shearer, J.C., Miller, S.L., 1996. Fungal origin of oxidised plant material in the Palangkaraya peat deposit, Kalimantan Tengah, Indonesia: implications for ‘inertinite’ formation in coal. Int. J. Coal Geol. 30, 1e23. Mukhopadhyay, P.K., 1986. Petrography of selected Wilcox and Jackson group Lignites from the Tertiary of Texas. In: Finkelman, R.B., Casagrande, D.J. (Eds.), Geology of Gulf Coast Lignites. 1986 Annual Meeting of Geological Society of America. Field Trip, pp. 126e145. Mukhopadhyay, P.K., 1989. Organic Petrography and Organic Geochemistry of Texas Tertiary Coals in Relation to Depositional Environment and Hydrocarbon Generation. Texas Bureau of Economic Geology. Report of Investigations No.188. Neavel, R.C., 1981. Origin, petrography and classification of coal. In: Elliot, M.A. (Ed.),

S. Sen et al. / Marine and Petroleum Geology 73 (2016) 371e391 Chemistry of Coal UtilizationJohn Wiley & Sons Ltd, N.Y, pp. 91e158, 2nd Supplementary Volume. Nichols, D.J., 1995. The role of palynology in palaeoecological analyses of tertiary coals. Int. J. Coal Geol. 28, 139e159. Pearson, A., Scott, A.C., 1999. Large palynomorphs and debris. In: Jones, T.P., Rowe, N.P. (Eds.), Fossil Plants and Spores: Modern Techniques. Geological Society, London, pp. 20e25. Petersen, H.I., 1998. Morphology, formation and palaeoenvironmental implications of naturally formed char particles in coals and carbonaceous mudstones. Fuel 77, 1177e1183. Rimmer, S.M., Davis, A., 1988. The influence of depositional environments on coal petrographic composition of the Lower Kittanning seam, western Pennsylvania. Org. Geochem. 12, 375e387. Rollins, M.S., Cohen, A.D., Baiely, A.M., Durig, J.R., 1991. Organic chemical and petrographic changes induced by early-stage artificial coalification of peats. Org. Geochem. 17, 451e465. Ruppert, L.F., Cecil, C.B., Stanton, R.W., Christian, R.P., 1985. Authigenic quartz in the Upper Freeport coal bed, west-central Pennsylvania. J. Sediment. Petrol. 55, 334e339. Russell, N.J., 1984. Gelification of Victorian Tertiary soft brown coal wood, I. Relationship between chemical composition and microscopic appearance and variation in the degree of gelification. Int. J. Coal Geol. 4, 99e118. Russell, N.J., Barron, P.F., 1984. Gelification of Victorian Tertiary soft brown coal, II. Changes in chemical structure associated with variation in the degree of gelification. Int. J. Coal Geol. 4, 119e142. Scagel, R.F., Bandoni, R.J., Rouse, G.E., Schofield, W.B., Stein, J.R., Taylor, T.M.C., 1965. An Evolutionary Survey of the Plant Kingdom. Wadsworth Publishing Co. Inc., Belmount, California, U.S.A. Schneider, W., 1978. Zu einigen Gesetzm€ aßigkeiten der faziellen Entwicklung im 2. €z. Z. Angew. Geol. 24, 125e130. Lausitzer Flo €obotanische Faziesanalyse in der Weichbraunkohle. Schneider, W., 1980. Mikropala Neue Bergbautech. 10, 670e675. Schneider, W., 1984. Angewandte Pal€ aobotanik and Braunkohlenpetrologie e pflanzliche Gewebe als Gefügebildner in der Braunkohle. Freiberger Forschungsh. C381, 14e19. Schneider, W., 1990. Floral succession in Miocene bogs of Central Europe. In: Proceedings of the Symposium of Pale Floristic and Palaeoclimatic Changes in the Cretaceous and Tertiary, Prague 1989, pp. 205e212. Schneider, W., 1995. Palaeo histological studies on Miocene brown coals of Central Europe. Int. J. Coal Geol. 28, 229e248. Schobert, H.H., 1995. Lignites of North America. In: Coal Science and Technology, vol. 23. Elsevier, Amsterdam. Scott, A.C., 1989. Observations on the nature and origin of fusain. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies and Depositional Models. Int. J. Coal Geol. 12, 443e475. Scott, A.C., 1989b. Deltaic coals: an ecological and paleobotanical perspective. In: Whateley, M.K.G., Pickering, K.T. (Eds.), Deltas: Sites and Traps for Fossil Fuels, Geological Society Special Publication, 41, pp. 309e316. Scott, A.C., 1991. An introduction to the application of paleobotany and palynology to coal petrology. Bull. Soc. Geol. Fr. 162, 145e153. Scott, A.C., 2000. The pre-Quternaery history of fire. Palaeogeogr. Palaeoclimatol. Palaeoecol. 164, 281e329. Scott, A.C., 2002a. Coal petrology and the origin of coal macerals: a way ahead? Int. J. Coal Geol. 50, 119e134. Scott, A.C., 2002b. Hydrogeologic factors affecting gas content distribution in coal beds. Int. J. Coal Geol. 50, 363e387. Scott, A.C., 2010. Charcoal recognition, taphonomy and uses in paleoenvironmental analysis. Paleogeogr. Paleoclimatol. Paleoecol. 291, 11e39. Scott, A.C., Collinson, M.E., 1978. Organic sedimentary particles: results from scanning electron microscope studies of fragmentary plant material. In: Whalley, W.B. (Ed.), Scanning Electron Microscopy in the Study of Sediments. Geo-Abstracts, Norwich, pp. 137e167. Scott, A.C., Glasspool, I.J., 2007. Observations and experiments on the origin and formation of inertinite group macerals. Int. J. Coal Geol. 70, 53e66. Scott, A.C., Jones, T.P., 1994. The nature and influence of fire in carboniferous ecosystems. Paleogeogr. Paleoclimatol. Paleoecol. 106, 91e112. Scott, A.C., King, G., 1981. Megaspores and coal facies: an example from the Westphalian A of Leicestershire, England. Rev. Palaeobot. Palynol. 34, 107e113. Scott, A.C., Cripps, J., Nichols, G., Collinson, M.E., 2000. The taphonomy of charcoal following a recent heathland fire and some implications for the interpretation of fossil charcoal deposits. Palaeogeogr. Palaeoclimatol. Palaeoecol. 164, 1e31. Sen, S., 2015. Review on coal petrographic indices and models and their applicability in paleoenvironmental interpretation. Geosci. J. 1e11. Shearer, J.C., Moore, T.A., 1994. Botanical control on banding character in two New Zealand coal beds. Palaeogeogr. Palaeoclimatol. Palaeoecol. 110, 11e27. Shearer, J.C., Moore, T.A., Demchuk, T.D., 1995. Delineation of the distinctive nature of tertiary Coal Beds. Int. J. Coal Geol. 28, 71e98. Singh, M.P., Shukla, R.R., 2004. Petrographic characteristics and depositional

391

conditions of Permian coals of Pench, Kanhan, Tawa Valley Coalfields of Satpura Basin, Madhya Pradesh, India. Int. J. Coal Geol. 42, 315e356. Smith, A.H.V., 1962. The paleoecology of Carboniferous peats based on the miospores and petrography of bituminous coals. Proc. Yorks. Geol. Soc. 33,423e33,474. Smyth, M., 1979. Hydrocarbon generation in the Fly Lake- Brolga area of the Cooper Basin. J. Aust. Pet. Explor. Assoc. 19, 108e114. Smyth, M., 1984. Coal microlithotypes related to sedimentary environments in the Cooper Basin, Australia. In: rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences, pp. 333e347. Spears, D.A., 1987. Mineral matter in coals, with special reference to the Pennine Coalfields. In: Scott, A.C. (Ed.), Coal and Coal-bearing Strata: Recent Advances, Geological Society Special Publication No. 32, pp. 171e185. Stach, E., 1968. Basic principles of coal petrology: macerals, microlithotypes and some effects of coalification. In: Murchison, D.G., Westoll, T.S. (Eds.), Coal and Coal-bearing Strata. Oliver & Boyd, Edinburgh, pp. 3e17. Stach, E., Mackowsky, M.-Th., Teichmüller, M., Taylor, G.H., Chandra, D., Teichmüller, R., 1982. Stach's Textbook of Coal Petrology, third ed. Gebruder Borntrager, Berlin, p. 125. Stasiuk, L.D., 1993. Algal bloom episodes and the formation of bituminite and micrinite in hydrocarbon source rocks: evidence from the Devonian and Mississippian, northern Williston Basin, Canada. Int. J. Coal Geol. 24, 195e210. Staub, J.R., Cohen, A.D., 1979. The Snuggedy Swamp of South Carolina: a backbarrier estuarine coal-forming environment. J. Sediment. Petrol. 49, 133e143. Stout, S.A., Spackman, W., 1989. Peatification and early coalification of wood as deduced by quantitative microscopic methods. Org. Geochem. 14, 285e298. Styan, W.B., Bustin, R.M., 1983. Petrography of some Fraser River Delta peat deposits: coal maceral and microlithotype precursors in temperate-climate peats. Int. J. Coal Geol. 2, 321e370. Styan, W.B., Bustin, R.M., 1984. Sedimentology of Fraser River delta peat deposits: a modern analogue for some deltaic coals. In: Rahmani, R.A., Flores, R.M. (Eds.), Sedimentology of Coal and Coal-bearing Sequences, International Association of Sedimentologists Special Publication No. 7. Blackwell Scientific Publications, Oxford, pp. 241e271. Swaine, D.J., 1990. Trace Elements in Coal. Butterworths, London. Taylor, G.H., Liu, S.Y., 1987. Biodegradation in coals and other organic-rich rocks. Fuel 66, 1269e1273. Taylor, T.N., Taylor, E.L., 1997. The distribution and interactions of some Paleozoic fungi. Rev. Palaeobot. Palynol. 95, 83e94. Tegelaar, E.W., de Leeuw, J.W., Derenne, S., Largeau, C., 1989. A reappraisal of kerogen formation. Geochim. Cosmochim. Acta 53, 3103e3106. Teichmüller, M., 1951. Vergleichende mikroskoposche Untersuchungen verme congre s de steinerter Torfe des Ruhrkarbons und der entsandenen. In: Trisie ologie du Carbonife re, vol. 2, pp. 1e87. Stratigraphie de ge Teichmüller, M., 1989. The genesis of coal from the viewpoint of coal petrology. In: Lyons, P.C., Alpern, B. (Eds.), Peat and Coal: Origin, Facies, and Depositional Models. Int. J. Coal Geol. 12, 1e87. Teichmüller, M., Teichmüller, R., 1968. Cainozoic and Mesozoic coal deposits of Germany. In: Murchison, D.G., Westoll, T.S. (Eds.), Coal and Coal-bearing Strata. Oliver & Boyd, Edinburgh, pp. 347e379. Thomas, B.A., Spicer, R.A., 1987. The Evolution and Palaeobiology of Land Plants. Croom Helm Ltd., Beckenham, Kent. Timofeev, P.P., Bogolyubova, L.I., 1973. Diagenesis of organic substance in peat-bogs of the Rioni Intermontane Depression (Transcaucasian). In: Tissot, B., Bienner, F. (Eds.), Advances in Organic Geochemistry, pp. 571e585. Ting, F.T.C., 1977. Microscopical investigation of the transformation (diagenesis) from peat to lignite. J. Microsc. 109, 75e83. Triplehorn, D., Bohor, B., 1986. Volcanic ash layers in coal: origin, distribution, composition and significance. In: Vorres, K.S. (Ed.), Mineral matter and Ash in Coal, ACS Symposium Series 301. American Chemical Society, Washington, D.C., pp. 90e98 Upchurch Jr., G.R., 1995. Dispersed angiosperm cuticles: their history, preparation and application to the rise of angiosperms in Cretaceous and Paleocene coals, southern western interior of North America. Int. J. Coal Geol. 28, 161e227. van der Heijden, E., 1994. A Combined Anatomical and Pyrolysis Mass Spectroscopic Study of Peatified Plant Tissues (Ph.D. thesis). University of Amsterdam. van der Molen, P.C., 1988. Palaeoecological reconstruction of the regional and local vegetation history of Woodfield Bog, Co. Offaly. Proc. R. Ir. Acad. 88B, 69e97. van Krevelen, D.W., 1981. Coal: Typology-Chemistry-Constitution. Elsevier, Amsterdam. Wilson, C.L., Loomis, W.E., Steeves, T.A., 1971. Botany, fifth ed. Holt, Rinehart & Winston, N.Y. Winston, R.B., 1986. Characteristic features and compaction of plant tissues traced from permineralized peat to coal in Pennsylvanian coals (Desmoinsian) from the Illinois Basin. Int. J. Coal Geol. 6, 21e41. Wust, R.A.J., Bustin, R.M., 2001. Low-ash peat deposits from a dendritic, intermontane basin in the topics: anew model for good quality coals. Int. J. Coal Geol. 46, 179e206.