Patterns of Coal Sedimentation in the Ipswich Basin Southeast Queensland

Autor: Chern, Peter Kyaw Zaw Naing
Rok vydání: 2004
Předmět:
Popis: The intermontane Ipswich Basin, which is situated 30km south-west of Brisbane, contains coal measures formed in the Late Triassic Epoch following a barren non-depositional period. Coal, tuff, and basalt were deposited along with fluvial dominated sediments. The Ipswich Coal Measures mark the resumption of deposition in eastern Australia after the coal hiatus associated with a series of intense tectonic activity in Gondwanaland during the Permo-Triassic interval. A transtensional tectonic movement at the end of the Middle Triassic deformed the Toogalawah Group before extension led to the formation of the Carnian Ipswich Coal Measures in the east. The Ipswich Coal Measures comprise the Brassall and Kholo Subgroups. The Blackstone Formation, which forms the upper unit of the Brassall Subgroup, contains seven major coal seams. The lower unit of the Brassall Subgroup, the Tivoli Formation, consists of sixteen stratigraphically significant coal seams. The typical thickness of the Blackstone Formation is 240m and the Tivoli Formation is about 500m. The coal seams of the Ipswich Basin differ considerably from those of other continental Triassic basins. However, the coal geology has previously attracted little academic attention and the remaining exposures of the Ipswich coalfield are rapidly disappearing now that mining has ceased. The primary aim of this project was to study the patterns of coal sedimentation and the response of coal seam characteristics to changing depositional environments. The coal accumulated as a peat-mire in an alluvial plain with meandering channel systems. Two types of peat-mire expansion occurred in the basin. Peat-mire aggradation, which is a replacement of water body by the peatmire, was initiated by tectonic subsidence. This type of peat-mire expansion is known as terrestrialisation. It formed thick but laterally limited coal seams in the basin. Whereas, peat-mire progradation was related to paludification and produced widespread coal accumulation in the basin. The coal seams were separated into three main groups based on the mean seam thickness and aerial distribution of one-meter and four-meter thickness contour intervals. Group 1 seams within the one-meter thickness interval are up to 15,000m2 in area, and seams within the four-meter interval have an aerial extent of up to 10,000m2. Group 1A contains the oldest seam with numerous intraseam clastic bands and shows a very high thickness to area ratio, which indicates high subsidence rates. Group 1B seams have moderately high thickness to area ratios. The lower clastic influx and slower subsidence rates favoured peat-mire aggradation. The Group 1A seam is relatively more widespread in aerial extent than seams from Group 1B. Group 1C seams have low mean thicknesses and small areas, suggesting short-lived peat-mires as a result of high clastic influx. Group 2 seams arebetween 15,000 and 35,000m2 in area within the one-meter interval, and between 5,000 and 10,000m2 within the four-meter interval. They have moderately high area to thickness ratios, indicating that peat-mire expansion occurred due to progressively shallower accommodation and a rising groundwater table. Group 3 seams, which have aerial extents from 35,000 to 45,000m2 within the one-meter thickness contour interval and from 10,000 to 25,000m2 within the four-meter interval, show high aerial extent to thickness ratios. They were deposited in quiet depositional environments that favoured prolonged existence of peat-mires. Group 3 seams are all relatively young whereas most Group 1 seams are relatively old seams. All the major fault systems, F1, F2 and F3, trend northwest-southeast. Apart from the West Ipswich Fault (F3), the F1 and F2 systems are broad Palaeozoic basement structures and thus they may not have had a direct influence on the formation of the much younger coal measures. However, the sedimentation patterns appear to relate to these major fault systems. Depocentres of earlier seams in the Tivoli Formation were restricted to the northern part of the basin, marked by the F1 system. A major depocentre shift occurred before the end of the deposition of the Tivoli Formation as a result of subsidence in the south that conformed to the F2 system configuration. The Blackstone Formation depocentres shifted to the east (Depocentre 1) and west (Depocentre 2) simultaneously. This depocentre shift was associated with the flexural subsidence produced by the rejuvenation of the West Ipswich Fault. Coal accumulation mainly occurred in Depocentre 1. Two types of seam splitting occurred in the Ipswich Basin. Sedimentary splitting or autosedimentation was produced by frequent influx of clastic sediments. The fluvial dominant depositional environments created the random distribution of small seam splits. However, the coincidence of seam splits and depocentres found in some of the seams suggests tectonic splitting. Furthermore, the progressive splitting pattern, which displays seam splits overlapping, was associated with continued basin subsidence. The tectonic splitting pattern is more dominant in the Ipswich Basin. Alternating bright bands shown in the brightness profiles are a result of oscillating water cover in the peat-mire. Moderate groundwater level, which was maintained during the development of the peat, reduced the possibility of salinisation and drowning of the peat swamp. On the other hand, a slow continuous rise of the groundwater table, that kept pace with the vertical growth of peat, prevented excessive oxidation of peat. Ipswich coal is bright due to its high vitrinite content. The cutinite content is also high because the dominant flora was pteridosperms of Dicroidium assemblage containing waxy and thick cuticles. Petrographic study revealed that the depositional environment was telmatic with bog forest formed under ombrotrophic to mesotrophic hydrological conditions. The high preservation of woody or structured macerals such as telovitrinite and semifusinite indicates that coal is autochthonous. The high mineral matter content in coal is possibly due to the frequent influx of clastic and volcanic sediments. The Ipswich Basin is part of a much larger Triassic basin extending to Nymboida in New South Wales. Little is known of the coal as it lacks exposures. It is apparently thin to absent except in places like Ipswich and Nymboida. This study suggests that the dominant control on depocentres of thick coal at Ipswich has been the tectonism. Fluvial incursions and volcanism were superimposed on this.
Databáze: Networked Digital Library of Theses & Dissertations
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