The terminology of peat-forming depositional environments is in a state of considerable confusion. Various authors use words such as 'mire', 'bog' and 'swamp' in a seemingly interchangeable fashion (Williamson, 1967; Gary and others, 1973; Jeglum and others, 1974; Gore, 1983; Moore, 1987; Cameron and others, 1989). Table V presents the classification of peatlands and definition of peatland terms used in this study. The generic term used in this study for a peat-forming depositional environment is 'mire'; specific peatland types include 'swamp', 'marsh' and 'fen'.
Bright banded coals dominate in all the sections studied. Bright, dull and bright, and lustrous coals are present in most sections, but dull coals are not abundant. Discrete and laterally continuous bands of clastic sedimentary rock are common throughout the mine workings (Figure 4-2). Most rock bands consist of coaly or carbonaceous mudstone. A thin band of granular mudstone is ubiquitous near the top of most sections. Silty mudstone, siltstone and fine-grained sandstone occur within the Wellington Seam in the extreme southeastern corner of the study area, but were not encountered in the three pillars which were sampled.
Lateral continuity of individual coals is moderate to high, while rock bands tend to pinch out, particularly within the Upper Wellington coal bed.
The uppermost coal bed, the Wellington Rider, is continuous and maintains a nearly uniform thickness across the study area. The Rider contains three correlatable and lithologically distinct plies of coal and rock: an uppermost bright coal, a medial granular mudstone, and a basal bright banded coal.
The upper parting is easily correlated between sections. The upper parting consists mainly of coaly mudstone; as it thickens to the southeast it progressively coarsens to carbonaceous mudstone, silty mudstone, siltstone and sandstone.
The Upper Wellington coal bed as a whole is easily correlated between sections, but its constituent lithotypes are not continuous across the study area. The Upper Wellington bed consists of interbedded coal and rock. Lustrous coal is the most abundant lithotype in the Upper Wellington bed, together with dull and bright coal and coaly mudstone. Dull and bright coals are often dirty, containing numerous thin laminae of coaly mudstone. Within the Upper Wellington coal bed there is a general upwards increase in the percentage of the duller coal lithotypes. The percentage of mudstone, both as discrete bands and as laminae within coals, gradually decreases upwards within the Upper Wellington coal bed. Lustrous coals are correlatable across distances of 60 to 100 metres, and some of the mudstone bands extend at least 180 metres.
The lower parting is continuous across the study area, but locally pinches to less than 0.1 foot (30 millimetres) thick, rendering its recognition more difficult in some sections. The lower parting consists of carbonaceous and coaly mudstone near the three sample points. In the southwestern corner of the study area the parting coarsens to silty mudstone.
The Lower Wellington coal bed consists mainly of bright banded coal, with lenses of dull and bright, dirty coal, coaly mudstone and carbonaceous mudstone. Dull and bright coals are locally concentrated near the top of the Lower Wellington coal bed, resulting in an overall dulling-upward seam profile. The bright banded coals are continuous and easily correlated, while the dull and bright coals and the mudstones are discontinuous.
Tables IX and X summarise maceral composition of the five coal and three mudstone lithotypes which were recognised in the three columnar samples of the Wellington Seam. Means and standard deviations are presented for each lithotype.
The coals consist mainly of vitrinite, low to moderate amounts of inertinite and mineral matter, and low amounts of liptinite (Figure 4-4). The low liptinite content of the coals causes the coals to cluster near the vitrinite-inertinite axis of the diagram. Figure 4-5 shows in detail the area near the vitrinite vertex of the ternary plot. Maceral compositions of the Wellington coals overlap considerably, but there is a general compositional clustering of coals of similar lithotype.
Lamberson and others (1991) introduced an alternative style of ternary plot to illustrate the maceral composition of coals which contain minimal liptinite. The constituents which are plotted are structured vitrinite (SV), degraded vitrinite (DV) and inertinite. For the purposes of this study, SV is considered to consist of telinite and telocollinite; DV is considered to consist of desmocollinite. Tables XI and XII present SV, DV and inertinite contents of the coals examined in this study. Figure 4-6 is a plot of SV against DV and inertinite (volume percent, mineral-matter-free basis) for coal lithotypes of the Wellington Seam. Dull and bright coals and bright banded coals show fair clustering in adjacent fields on this plot. Figure 4-7 is a similar plot for mudstone lithotypes of the Wellington Seam. The three mudstone lithotypes show fair clustering on this plot.
On a whole rock basis (Table IX; Figure 4-8), lustrous coals have the highest mean vitrinite content (87%) of the five coal lithotypes recognised in this study, followed by bright coals (85%), bright banded coals (83%), dull and bright coals (83%) and dull coals (one sample only, 80%). Bright banded coals have the highest mean inertinite content (15%), followed by bright coals (11%), dull coals (one sample only, 7%), dull and bright coals (7%) and lustrous coals (3%). Bright coals have the highest mean liptinite content (2%), followed by bright banded coals (1%) and lustrous coals (1%). Dull and bright coals and dull coals contain only traces of liptinite. Dull and bright coals and dull coals contain the highest mean mineral matter content, at 12% each, followed by lustrous coals (8%), bright coals (2%) and bright banded coals (2%).
On a whole rock basis (Table IX; Figure 4-8), granular mudstones have the highest mean vitrinite content (70%) of the three mudstone lithotypes which were sampled, followed by coaly mudstone (60%) and carbonaceous mudstone (28%). Coaly mudstones have the highest mean inertinite content (4%) followed by granular mudstones (3%) and carbonaceous mudstones (2%). All three rock lithotypes have identical mean liptinite contents (1%). Carbonaceous mudstones have the highest mean mineral matter content (68%), followed by coaly mudstones (34%) and granular mudstones (4%).
On a mineral-matter-free basis (Table X; Figure 4-9), lustrous coals have the highest mean vitrinite content (95%) of the five coal lithotypes recognised in this study, followed by dull and bright coals (94%), dull coals (one sample only, 91%), bright coals (86%) and bright coals (84%). Bright banded coals have the highest mean inertinite content (15%), followed by bright coals (11%), dull coals (one sample only, 8%), dull and bright coals (5%) and lustrous coals (4%). Bright coals have the highest mean liptinite content (2%), followed by bright banded coals (1%), lustrous coals (1%), dull and bright coals (trace) and dull coals (one sample only, trace).
On a mineral-matter-free basis (Table X; Figure 4-9), among the mudstones, coaly mudstones have the highest mean vitrinite content (92%), followed by carbonaceous mudstones (81%) and granular mudstones (80%). Granular mudstones have the highest mean inertinite content (19%), followed by carbonaceous mudstones (18%) and coaly mudstones (8%). All three rock lithotypes have identical mean liptinite contents (1%).
The three mudstone lithotypes (carbonaceous, granular and coaly) show considerable overlap on a crossplot (Figure 4-10) of vitrinite content (volume percent, whole rock basis) versus ash content (weight percent, whole rock basis). The lithotypes can, however, be fairly well distinguished on a crossplot (Figure 4-11) of vitrinite content (volume percent, whole rock basis) against mineral matter content (volume percent, whole rock basis). Coaly mudstones have a clearly higher vitrinite content (mean 92%, standard deviation 6%) than granular mudstones (mean 80%, standard deviation 4%). The mineral-matter-free vitrinite content of carbonaceous mudstone (mean 81%, standard deviation 16%) overlaps the vitrinite content of the other two mudstone lithotypes.
On a dry basis, dull and bright coals have the highest mean ash content (22.8%) of the five coal lithotypes recognised in this study, followed by dull coals (one sample only, 18.5%), lustrous coals (16.8%), bright coals (11.2%) and bright banded coals (10.2%).
On a dry basis, carbonaceous mudstones have the highest mean ash content
(56.9%) of the three mudstone lithotypes which were sampled, followed by
coaly mudstone (45.2%) and granular mudstone (44.3%).
On an as-received basis, bright banded coals have the highest mean FSI (3.6), followed by bright coals (3.3), lustrous coals (3.3), dull and bright coals (3.0) and dull coals (one sample only, 2.0).
The TPI as defined by Diessel (1986, page 19) is:
(telinite + telocollinite + semifusinite + fusinite) / (desmocollinite + macrinite + inertodetrinite)
The TPI of a coal is the ratio of material with remnant cellular structure to material lacking cellular structure. Coal with a high TPI contains abundant well preserved plant tissue. High TPI values may therefore indicate increased contribution of arboreal vegetal material to a peat, or they may be due to concentrations of semifusinite and fusinite, which were formed by burning of plant tissues (Lamberson and others, 1992).
The GI as originally defined by Diessel (1986, page 19) is:
(vitrinite + macrinite) / (semifusinite + fusinite + inertodetrinite)
The GI of a coal is the ratio of gelified material to non-gelified material. Coal with a high GI contains abundant gelified material, which may be due to the presence of a high water table with limited oxygen supply; a decrease in the GI indicates increased oxidation of plant tissues (Lamberson and others, 1992).
Crossplots of GI of whole seam sections against TPI were introduced by Diessel (1986) to distinguish coals which originated in different depositional environments. Diessel recognised eight different depositional environments: limno-telmatic, telmatic, marsh, fen, limnic, wet forest swamp, dry forest swamp and terrestrial.
Figures 4-12, 4-13 and 4-14 are crossplots, constructed according to Diessel's model, of GI and TPI for the Wellington coals sampled in this study. One lustrous coal from the Upper Wellington coal bed at locality WM222, near the southeastern corner of the area, has an extremely high TPI (8.1) and therefore plots outside the limits of the diagram.
Figure 4-12 is a crossplot of GI and TPI of the various coal lithologies sampled in this study. Bright banded coals and banded coals tend to plot together, with lower GI values than those of dull and bright coals, dull coals and lustrous coals.
Figure 4-13 is a crossplot of GI and TPI for the same data set, keyed to stratigraphic position within the Wellington Seam. Coals from the Wellington Rider, Upper Wellington and Lower Wellington subdivisions of the Wellington Seam overlap on this plot. At the scale of entire coal beds there is no obvious stratigraphic control on GI and TPI values.
Figure 4-14 is a crossplot of GI and TPI for the same data set, keyed to sample location within the study area. There is considerable overlap of the GI and TPI values for coals from all three locations. Coals from locality WM220, near the northwestern corner of the underground study area, tend to have slightly lower TPI values than coals from locality WM100, in the middle of the mine.
Figure 4-23 is a crossplot of free swelling index (FSI) against ash content (by weight, dry basis) of the Wellington coals. There is a general tendency for coals with lower ash contents to have higher free swelling indices. In an attempt to clarify the relationship between FSI and ash content, the coals were grouped by lithotype. FSI values of coals in the first group (Figure 4-24), consisting of bright and bright banded coals, are strongly depressed by increased ash content. FSI values of coals in the second group (Figure 4-25) show no clear relationship to ash content.
Figure 4-26, 4-27 and 4-28 are crossplots of FSI against vitrinite content (by volume, whole rock basis) of the Wellington coals. Considerable scatter is evident on these plots. Separation of the coals into two groups as above does not resolve the scatter; it therefore appears that changes in vitrinite content have no major effect on the FSI of the Wellington coals.
Bright banded coals and bright coals from the Wellington Seam have moderate to high TPI and GI value, suggesting that they originated mainly in wet forested swamps and less commonly in fens or marshes (Figure 4-16).
Dull and bright coals and dull coals from the Wellington Seam have high TPI and high GI values, again suggesting that they originated in wet forested swamps (Figure 4-17). Very high GI values (over 20) suggest that the coal-forming peat was either rapidly buried or perhaps accumulated under very wet conditions.
The anomalously-high TPI of the lustrous coal (sample WM222/3) from the Upper Wellington coal bed at locality WM222 is due to its high combined telinite and telocollinite content (87% by volume On a mineral-matter-free basis). This coal may have originated as individual logs. The other samples of lustrous coal show markedly lower TPI values, suggesting that they originated in wet forest swamps or marshes (Figure 4-18).
The basal coal of the Upper Wellington bed at locality WM100 has a markedly higher GI, suggesting the return of wetter conditions; the overlying coal has a lower GI, suggesting that conditions subsequently became drier. The TPI of both coals is similar to that of the uppermost coal of the Lower Wellington bed, suggesting that the relative abundance of woody and non-woody plants remained stable while the Upper Wellington peat was deposited. The brief increase in GI for the Upper Wellington may reflect the presence of standing water in channels at the top of the underlying Lower Wellington peats, only partially infilled by muds of the lower parting (Figure 4-2).
Following deposition of the upper parting mud, increased GI and TPI of the basal coal of the Wellington Rider bed suggest that wetter conditions again prevailed, and that trees became more dominant as a source of organic debris. The upper coal of the Wellington Rider has higher TPI, indicting perhaps a greater component of woody material, but slightly lower GI, indicative of slightly drier conditions.
Following deposition of the muds of the lower parting, the basal Upper Wellington peat was deposited. The TPI and GI of this unit are similar to those of the middle coal of the Lower Wellington bed, suggesting a return to somewhat drier conditions within a wet forest swamp environment. The overlying two coals are markedly duller (Figure 4-2) and have successively higher GI values, suggesting that levels of oxidation declined and conditions again became wetter, probably in a marsh environment. The uppermost two coals of the Upper Wellington bed are also dull, but display a gradual increase in TPI. Slight upward increase of the GI of the coals suggests an initially lower level of oxidation followed by a slightly higher level of oxidation, perhaps related to initially drier then slightly wetter conditions. The precursor peats of these coals were probably deposited in an environment which was transitional from a marsh to a wet forest swamp.
Following deposition of the upper parting mud, the Wellington Rider peats were deposited. The coals of the Wellington Rider bed are brighter than the upper coals of the underlying Upper Wellington Bed. The basal bright banded coal of the Wellington Rider has a moderately low TPI and GI, indicating a decline in the contribution of structured plant tissue and increased oxidation of the precursor peat; this peat was perhaps deposited in a fen environment. The TPI and GI of the overlying bright coal are moderate, indicating a higher content of structured plant tissue and slightly lower levels of oxidation in the precursor peat, which was probably deposited in a wet forest swamp environment.
Following deposition of muds of the lower parting, the Upper Wellington peat was deposited. The Upper Wellington bed consists of a single ply of lustrous coal, with very high TPI (8.1) and GI (47.7) values. This coal probably originated as a deposit of logs, branches and twigs.
The upper parting at WM222 is represented by a bed of carbonaceous mudstone; immediately to the southeast of this locality the parting thickens and coarsens rapidly. It is therefore probable that WM222 was on the distal edge of a crevasse splay deposit.
Following deposition of the upper parting mud, the Wellington Rider
peats were deposited. The coals of the Wellington Rider bed are, as elsewhere
at Wolf Mountain, brighter than the underlying coals. The basal bright
banded coal of the Wellington Rider has a moderately high TPI and a high
GI, suggesting that it was deposited in a very wet forest swamp environment.
The overlying bright coal has a very low TPI and moderately high GI, indicating
that its precursor peat consisted mainly of degraded, moderately oxidised
plant tissue. This coal's GI and TPI values, together with its high content
of ash (23%) and desmocollinite (61%, mineral-matter-free) suggest that
it originated in a marsh which was subject to inundation by mud-rich water.
The uppermost coal of the Wellington Rider is also bright, but has a relatively
low ash content (9%), moderately high TPI and moderate GI, indicating a
markedly greater input of structured plant tissues and slightly higher
levels of oxidation, consistent with an origin in a wet forested swamp.