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Rangal Supermodel 2015: The Rangal-Baralaba-Bandanna Coal Measures in the Bowen and Galilee Basins

Open Cut » Geology

Published: February 17Project Number: C22028

Get ReportAuthor: Renate Sliwa, Joan Esterle, Laura Phillips & Steven Wilson | The University of Queensland

This project compiled over 7000 company boreholes supplemented by more than 1000 open file petroleum (mostly CSG) wells to develop a regionally consistent stratigraphy for the Rangal-Baralaba-Bandanna Coal Measures in the Bowen Basin and the Galilee Basin.

 

The results are presented as a series of cross sections and maps that illustrate the regional scale distribution of major coal seams and their interburdens. This exercise resulted in a revised stratigraphy and confirmation of the relationship between the Betts Creek beds and the Late Permian coal measures in the Bowen Basin, confirmed through age dating. The Yarrabee Tuff was correlated regionally and samples from mine sites and drill holes returned a consistent CA-IDTIMS date of 252.88 ± 0.07 Ma. More detailed dating from a single location confirmed reproducibility and suggests that the Yarrabee Tuff event was akin to a small super volcano that could have been instantaneous, or else erupted over a 40,000 to 70,000 year time period. The geochemistry was quite evolved but the silicic nature of the tuff could also reflect its deposition and alteration in acidic peat mires. Regardless, it corroborates the long-lived periods of low siliciclastic sediment input and basin stability resulting in laterally extensive peat mires that form the Rangal and equivalent coals.

 

The late Permian Rangal-Baralaba-Bandana coal measures formed in less than 1 MY, and their thickness preserved in the Nebo Synclinorium is less than 120m whereas that in the eastern Taroom Trough is over 220m. Isopach maps of the underlying Fort Cooper Coal Measures (including the Black Alley Shale) show a shift in depocentre from west along the strike of the basin to southeast in the Taroom Trough in response to foreland loading. This loading continued, resulting in the accumulation of over 1600m of the overlying Permian to Triassic Rewan Group sediments. Isopach maps of the Late Permian and Triassic corroborate early interpretation of the basin axis extending due north, parallel to the Marlborough Thrust that would have been active at that time.

 

The base of the Rewan in Galilee and western limb of the Bowen Basin is floored by a 1 to 10m thick carbonaceous mudstone that transitions upward into the thinly interbedded, red and green siltstones and sandstones commonly associated with the Rewan Group. This sequence continues to coarsen upward into thicker, lithic sandstones that collect on the western margin to form the wide and thick belt of the erosively based, often conglomeratic Sagittarius Sandstone. This coarsening upwards transition suggests a base level rise that drowned the coal measures regionally, perhaps even across the Anakie Inlier and Springsure Shelf, most likely in response to down-warping of the foreland basin before the massive influx of sediment from the rising hinterland and the change to more arid climate culminated in the global "Coal Gap".

 

The distribution, thickness and splitting character of the coals follows the basin paleo depocentre. The net coal is highest, > 30m, in the Baralaba Coal Measures in the south-eastern Taroom Trough but is distributed into a greater number of thinner seams separated by thick but almost regular interburdens. Along the western edge, roughly parallel to the Roma, Springsure and Collinsville shelves, the Bandana and Rangal Coal Measures thin, coal net to gross increases and coal seams tend to be thicker (>6m). In these lower accommodation areas of the basin, multiple seams can converge to form crab-like geometries. These coal crabs are rare but significant in locations like Scotia, Fairview, Ensham, Burton and South Walker Creek, among others. Within the Nebo Synclinorium, thick crabs occur along the Collinsville Shelf in the older Fort Cooper and Moranbah Coal Measures. These "peat islands" were stable locations of peat accumulation for long periods of time, ~100,000 years or more, and fluvial channel courses divert around them.

 

The nature of the interburden sediments are basically continental throughout the Late Permian, although responding to local (compaction) and regional subsidence. Similar to previous interpretation, basin drainage is generally southward but thick (60m), stacked trunk channels also traverse the basin. The channelised belts, as defined by the seam splitting patterns, show local compensational stacking, but are thicker and wider in the Nebo Synclinorium. In the south, as the basin subsides more rapidly, smaller scale channels drain westward into the trough. In the Baralaba Coal Measures, the channels exhibit steeply inclined master bedding known as Inclined Heterolithic Strata, which could also reflect tidal or estuarine influence through the section. This follows for increasing subsidence in the basin towards the Permo-Triassic boundary.

 

Subsidence driving coalification is evident in the thermal maturity or rank of the coal seams, but elevated rank towards the east of the Nebo Synclinorium, and west of the Marlborough Thrust, suggests deeper more rapid burial in a higher heat flow. However, these same areas are also structurally more deformed, and evidence of hydrothermal overprinting is suggested by a combination of the thermal maturity and clay mineralogy.

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