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The Bainmedart Coal Measures
Three metre thick coal seam overlain by arkoses within the Bainmedart Coal Measures, Glossopteris Gully, Amery Oasis area, northern Prince Charles Mountains.
Photo: B. McKelvey |
Geographic and stratigraphic location
The Bainmedart Coal Measures, comprising the middle portion of the Permo-Triassic Amery Group sedimentary sequence, are exposed along the eastern side of Radok Lake, throughout Pagodroma Gorge, and along the western side of Beaver Lake. In addition, minor outcrops of Amery Group sediments located on the western side of Kamenistaya Platform, eastern shore of Beaver Lake, may also belong to this unit. The contact between the basal member of the Bainmedart Coal Measures, the Dart Fields Conglomerate, and the underlying Radok Conglomerate is interpreted to represent either a disconformity or a low angle unconformity. The upper contact of the Bainmedart Coal Measures with the Flagstone Bench Formation is conformable in the area flanking the northwest margin of Beaver Lake, but a faulted contact is evident between these units in the Dart Fields area east of Pagodroma Gorge.
Lithologies
The Bainmedart Coal Measures comprise an ~1875m thick sedimentary sequence of repetitive fining-upwards cycles which are dominated by quartzose and feldspathic sandstones, with minor carbonaceous siltstone, coal and claystone. The sandstone bodies are generally trough cross-bedded, grading upwards into finer, rippled sandstones, whereas the siltstones contain abundant rootlets. Six individual members have now been defined largely on the basis of the relative proportions of coal to sandstone packages. These are, in ascending stratigraphic order, the Dart Fields Conglomerate Member, the Toploje Member, the Dragons Teeth Member, the Glossopteris Gully Member, the Grainger Member and the McKinnon Member.
The approximately 110 bituminous coal seams, after which this sequence is named, vary in both thickness and abundance within the various members, although there is a marked decrease in their proportion and average thickness within the uppermost several hundred metres of this unit. They are generally characterised, however, by low sulphur (0.40 to 0.85%) contents, moderate to high ash (15 to 30%) contents and high mineral and volatile (29 to 37%) contents. They are also considered to be poor in maceral vitrinite and the 'bright' microlithotypes vitrite and clarite, the latter responsible for the coking properties of coal. The Amery Group coals have vitrinite reflectance values of 0.55 to 0.89 and are considered to be of lower rank than coal deposits found within the Transantarctic Mountains.
Dart Fields Conglomerate Member
The proposed type section for the basal member of the Bainmedart Coal Measures, the Dart Fields Conglomerate, is located at the southeast corner of Radok Lake. It comprises a less than 3m thick unit of pebble orthoconglomerate, the upper contact of which is interfingered with the overlying Toploje Member sediments.
Cross-bedded channel sandstone overlying a coal seam within the Toploje Member, Bainmedart Coal Measures, eastern Radok Lake, northern Prince Charles Mountains.
| Toploje Member
Exposures of the Toploje Member, the type section of which is located in cliffs bordering the southeast margin of Radok Lake, reach thicknesses of up to 303m. They display either an irregular lower contact with the Dart Fields Conglomerate or directly, disconformably to unconformably, overlie the Radok Conglomerate. The Toploje Member comprises thick sandstone packages, minor siltstone and shale beds, and thin (generally <40cm thick) coal seams, numbering about 20 in total. To the northeast of Radok Lake, this member is capped by a layer of siliceous, permineralised peat containing a diverse fossil assemblage.
Dragons Teeth Member
The type section of the Dragons Teeth Member is located along the southern side of Bainmedart Cove, although other good exposures occur near the headwaters of Cordaite Creek, and conformably overlies the Toploje Member. This unit is dominated by 5 to 12 m thick, upwards-coarsening cycles which comprise sideritic or limonitic, fine- to medium-grained sandstones, grey siltstones, dark grey to red shales and rare coals. Coal seams within the northernmost exposures of the Dragons Teeth Member measure up to 1.5m in thickness but are laterally discontinuous.
Campsite at head of Bainmedart Cove with Dragons Teeth (geographic feature only; cliffs do not comprise the Dragon's Teeth Member) in right background, Amery Oasis area, northern Prince Charles Mountains.
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Interbedded sandstones and black shales, Dragons Teeth Member, Bainmedart Coal Measures, Panorama Point, northern Prince Charles Mountains.
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Sandstones and snow, Dragons Teeth Member, Bainmedart Coal Measures, Panorama Point, northern Prince Charles Mountains.
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Glossopteris Gully Member
The best exposures of the Glossopteris Gully Member are in Pagodroma Gorge and Glossopteris Gully. However, this unit has been assigned a composite type section, the lower part of which extends northeastwards from Bainmedart Cove to Beaver Lake, through Pagodroma Gorge, and the upper part of which is exposed in cliffs adjacent to the south side of the mouth to Grainger Valley. Total thickness is tentatively assumed to approximate 670m and the Glossopteris Gully Member displays conformable upper and lower contacts. This is a coal-rich unit, containing 50 coal seams that range from 0.80 to 4.02 m in thickness. Other lithology's include a series of fining-upwards cycles dominated by feldspathic sandstones, carbonaceous shales and siltstones.
Looking down Glossopteris Gully, Amery Oasis area, northern Prince Charles Mountains.
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Coal seam overlain by interbedded mudstones and sandstones, Glossopteris Gully Member, Bainmedart Coal Measures, Glossopteris Gully, northern Prince Charles Mountains.
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Interbedded channel sandstones and coal seams, Glossopteris Gully Member, Bainmedart Coal Measures, Pagodroma Gorge, northern Prince Charles Mountains.
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Grainger Member
The Grainger Member is also represented by a composite type section, exposed within the steeply sloping western margin of Beaver Lake between the mouths of the Grainger and McKinnon Valleys. This 349 m thick section conformably overlies the Glossopteris Gully Member. It is dominated by thick medium- to coarse-grained quartzose and feldspathic sandstone packages, which commonly incorporate a basal lag of well-rounded chert, felsic gneiss and vein quartz pebbles and cobbles. Fine- to medium-grained, and often strongly ferruginous, feldspathic to micaceous sandstones commonly form thinner packages, whereas carbonaceous and micaceous siltstones, green lithic sandstones and garnetiferous sandstones are minor components of this unit. Thin coal seams are rare.
McKinnon Member
The McKinnon Member is a 532 m thick sequence which conformably overlies the Grainger Member and comprises similar lithology's, although in different proportions, to both the Glossopteris Gully and Grainger Members. The composite type section extends for ~8 km around Whaleback Bluff, which is located along the western margin of Beaver Lake, just north of the McKinnon Valley mouth. This unit is characterised by containing a greater abundance of siderite/limonite nodules within its sandstones, than either the Glossopteris Gully or Grainger Members, as well as abundant laterally continuous coal seams of significant thickness. Of the 40 coal seams mapped within the McKinnon Member, two are more than 10m thick while at least eight others are 2 to 5m thick. In addition to containing clastic layers up to 10cm in thickness, these thick coal seams are characterised by sulphur staining, pyrite concretions up to 4 cm in diameter, intermediate lustre and macroscopic banding.
Fossil record and age of deposition
Within the coal seams of the Bainmedart Coal Measures, humification and coalification processes have resulted in the homogenisation of plant remains such that very few beds contain well-preserved plant macrofossils. Most of the recognisable fossils, including glossopterid sporangial, leaf, and cuticle remains, present within the siltstone and shale beds are fragmentary and not identifiable to species level. Coal seams are often underlain by variably oriented glossopterid root impressions (Vertebraria indica Royle) which indicate that the coals were formed largely from autochthonous plant remains within glossopterid-dominated forest mires. Ages have been assigned to the individual members of the Bainmedart Coal Measures on the basis of their stratigraphic position and/or their fossil assemblages.
Although the Dart Fields Conglomerate Member is devoid of fossils, it is stratigraphically located between the Middle to Late Permian Radok Conglomerate and the Late Permian Toploje Member. A rich fossil assemblage within the siliceous permineralised peat layer that caps the Toploje Member includes abundant remains of glossopterid gymnosperm leaves (Glossopteris Brongniart) and fruit (Plumsteadia Rigby). Less abundant macrofossils include cordaitalean (Noeggerathiopsis Feistmantel) leaves, herbaceous lycophyte axes and megaspores, fern pinnules (?Neomariopteris Maithy) and a range of fungal remains. The palynoflora include the index species Guttulepollenites hannonicus and Didecitriletes ericianus, indicating a Late Permian (stage 5 - ?Ufimian to Tatarian) age. Coprolites preserved within cavities in permineralised wood (Australoxylon, Vertebraria) provide evidence for the existence of Permian wood-boring arthropods. Two new species of Late Permian silicified gymnosperm (Australoxylon sp.) woods recently identified within the Bainmedart Coal Measures have been named Australoxylon bainii and Australoxylon mondii.
Fossil assemblages within the limonitic and sideritic shale nodules of the Dragons Teeth Member include Glossopteris leaves and fruit, as well as sphenophyte axes (Paracalamites australis) and leaf whorls (Lelstotheca or Phyllotheca sp.). However, the Late Permian age assigned to this unit is actually based on its stratigraphic position, due to the presence of Late Permian palynoflora within both the underlying and the overlying units. In addition, the overlying Glossopteris Gully Member also contains fossil impressions of glossopterid gymnosperms (Glossopteris leaves and Vertebraria roots), sphenophyte axes (Paracalamites australis), fern foliage and logs.
The stratigraphic locations of both the Grainger and McKinnon Members, combined with the presence of Glossopteris leaves, log impressions and glossopterid (Vertebraria) roots, suggest a Late Permian age. The poorly preserved palynoflora within the McKinnon Member is also typical of Late Permian southern Gondwanan assemblages, being dominated by taeniate bisaccate glossopterid pollen.
Environment of deposition
The Late Permian environment of deposition of the Bainmedart Coal Measures has been interpreted from lithological characteristics and facies associations, to have occurred within a broad alluvial valley, or parallel fault-bounded valleys, under the influence of subsidence within the Lambert Graben. The predominantly north- to northeast-directed palaeocurrent data indicate that these valleys were occupied by north flowing braided river systems which alternated, both spatially and temporally, with extensive low-energy floodbasins and forest-mire environments. These forest-mire environments were the sites of coal formation. In addition, the fossil wood-bearing silicified peat layer which caps the Toploje Member represents a drowned, glossopterid-dominated forest mire. The period of deposition of the Bainmedart Coal Measures is presumed to have coincided with a relatively wet climate, resulting in high water tables and the formation of laterally extensive forest-mires and lakes within floodbasins. Well-developed facies cyclicity in the lower part of the Bainmedart Coal Measures has been interpreted to reflect climatic fluctuations associated with precessional Milankovitch cycles. A minor lacustrine interval, during the early stages of deposition, is represented by the Dragons Teeth Member.
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