Inorganic chemistry, petrography and palaeobotany of Permian coals in the Prince Charles Mountains, East Antarctica

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Abstract

Sampled outcrops of Permian coal seams of the Bainmedart Coal Measures in the Lambert Graben, eastern Antarctica, have been analysed for their proximates, ultimates, ash constituents and trace elements. A similar series of samples has been analysed for their principle maceral and microlithotype components and vitrinite reflectance. The coals are sub-bituminous to high volatile bituminous in rank; maturity increases markedly in southern exposures around Radok Lake where the oldest part of the succession is exposed and some strata have been intruded by mafic dykes and ultramafic sills. The coal ash is mostly silica and aluminium oxides, indicating that the mineral ash component is mostly quartz and various clay minerals. The ratio of silica to aluminium oxides appears to increase in an upward stratigraphic direction. The coal macerals include a relatively high liptinite content (mainly sporinite) that is significantly higher than for typical Gondwana coals. Greater degrees of weathering within the floodbasin/peat mire environments associated with climatic drying towards the end of the Permian might account for both preferential sporopollenin preservation and increased silica:aluminium oxide ratios up-section. Correlation of the coal maceral components to adjacent peninsula India coals indicates the closest comparative coals of similar age and rank occur within the Godavari Basin, rather then the Mahanadi Basin, which is traditionally interpreted to have been contiguous with the Lambert Graben before Gondwanan breakup. The petrological characteristics suggest that either previous interpretations of Palaeozoic basin alignments between Antarctica and India are incorrect, or that environmental settings and post-Permian burial histories of these basins were strongly independent of their tectonic juxtaposition. A permineralized peat bed within the succession reveals that the coals predominantly comprise wood- and leaf-rich debris derived from low-diversity forest-mire communities dominated by glossopterid and noeggerathiopsid gymnosperms.

Introduction

Although there have been many papers describing Antarctic coals, they mainly concern coals in the Transantarctic Mountains (e.g. Schopf and Long, 1966, Rose and McElroy, 1987, Coates et al., 1990). The Transantarctic coals also tend to be highly altered due to contact thermal metamorphism (Schopf and Long, 1966). Coates et al. (1990) provided a good summary of the geology, quality, and resources of coals from the Transantarctic Mountains. They presented data from proximate and ultimate analyses but no information on their major, minor, or trace element contents. In East Antarctica the main reference to coal analysis comes from Bennett and Taylor (1972) who analysed 12 samples for chemistry and petrography from a sample set collected from the Bainmedart Coal Measures—Prince Charles Mountains (PCMs) by the Australian Bureau of Mineral Resources (BMR) in 1969. However, no information was provided on the inorganic constituents. Many of the samples were considered weathered with comparatively high ash contents averaging 21.7%. Rose and McElroy (1987) investigated a broader range of Antarctic coal beds but provided no information beyond proximate and ultimate analyses. Several authors have characterized mineral constituents of Antarctic coals (zeolites by Bradshaw, 1979, and calcite by Faure and Botoman, 1984). We found no other published information on the inorganic chemistry of Antarctic coals. Previous petrographic analyses undertaken on PCM coals by Bennett and Taylor (1972) comprised 32 samples for microlithotype counts and 13 samples with vitrinite reflectance measurements. These data have been incorporated with additional analyses undertaken for this paper.

In this paper we present the inorganic chemistry, petrographic characteristics, and vitrinite reflectance values of 15 coal samples collected from the Bainmedart Coal Measures (PCMs), East Antarctica. The inorganic chemistry results were first presented in poster form at the TSOP Washington, DC, conference in 2002 (Chiehowsky et al., 2002). The palaeobotanical content of the coals is also investigated by thin-section analysis of a siliceous permineralized peat layer preserved in the lower part of the coal measures.

Section snippets

Tectonic and palaeogeographic setting

A series of shallow dipping Permian coal measures and Triassic sediments are exposed along the margins of Radok and Beaver lakes in the northern PCMs, eastern Antarctica. The mountains themselves comprise mainly metamorphic rocks of Precambrian age that are prominent above the Antarctic ice cover in this area. The adjacent Lambert Glacier follows the course of a Palaeozoic structural depression through the mountains and debouches as the Amery Ice Shelf into Prydz Bay some 200 km north of the

Stratigraphic and sedimentological setting

The Permo-Triassic, sedimentary, Amery Group rests unconformably on Archaean and Proterozoic charnockitic granites and felsic to mafic metamorphic rocks of granulite to amphibolite grade (Tingey, 1982). The Amery Group exceeds 3000 m in thickness and incorporates three non-marine units (Fig. 2): the Radok Conglomerate, Bainmedart Coal Measures, and Flagstone Bench Formation (in ascending stratigraphic order). Age controls on some parts of the succession are imprecise due to the dearth of

Coal seam distribution within the Bainmedart Coal Measures.

Fielding and Webb (1996) and McLoughlin and Drinnan (1997a) identified nine sedimentary facies within the Bainmedart Coal Measures corresponding to major channel, floodbasin, and extensive lacustrine and lake-delta deposits within typical alluvial plain settings. Coals characterize clastic-starved depositional sites in floodbasin settings and were attributed to facies B4 of Fielding and Webb (1996). Approximately 110 identifiable coal seams occur within the Bainmedart Coal Measures but seam

Analytical methods

Exposed coal seams within the Glossopteris Gully, Dragons Teeth, Toploje, and McKinnon Members of the Permian Bainmedart Coal Measures were grab-sampled approximately in the middle of each seam (McLoughlin and Drinnan, 1997a, McLoughlin and Drinnan, 1997b). No attempt was made to systematically sample each bed due to the large number of seams and beds, and the logistical problems involved in this remote area for sample recovery. Nevertheless, the sample set used is thought to be representative

Results

The stratigraphic and geographic locations for 15 samples from the Bainmedart Coal Measures analysed for their inorganic chemistry are given in Table 1 and on Fig. 1, Fig. 3. The sources of 16 Bainmedart coal seam samples analysed for their petrography and vitrinite reflectances are also provided on Fig. 1, Fig. 3.

Burial history

Based on the reflectance data through the Amery Group (Fig. 12) the coal measure section exhibits possibly one unconformity in the upper part of the Glossopteris Gully Member in the Radok Lake area. Rank gradients significantly increase in the lower part of the Glossopteris Gully Member to the west of Flagstone Bench, whereas above this level and north of the Radok Lake the gradients remain relatively flat through the upper Glossopteris Gully, Grainger, McKinnon and Ritchie members. Based on

Conclusions

Outcrops of Permian coal seams of the Bainmedart Coal Measures (Lambert Graben, eastern Antarctica) are of sub-bituminous to high volatile bituminous rank. The rank increases markedly to the south in the Radok Lake area suggesting localised heating. The coals contain moderately high ash contents mostly silica and aluminium oxides indicating the mineral ash component is mostly quartz and various clay minerals. The ratio of silica to aluminium oxides appears to increase in the stratigraphic

Acknowledgments

Material for this study was collected by S. McLoughlin and A. Drinnan during an Australian National Antarctic Research Expedition to the Prince Charles Mountains with financial support from an Australian Research Council Discovery Grant. The Australian Antarctic Division is thanked for logistical assistance and funding of this expedition. Interpretation of the age and palaeobotanical composition of the permineralized peat and coals has benefited from discussions with Drs. Sofie Lindström

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