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Structure Supermodel 2017 - Fault Characterisation in Permian to Jurassic Coal Measures

Open Cut » Geology

Published: February 18Project Number: C24032

Get ReportAuthor: Renate Sliwa, Abbas Babaahmadi & Joan Esterle | The University of Queensland

This report provides a detailed characterisation of deformation overprinting the Permian to Jurassic coal measures of Queensland. It addresses the ACARP objective to improve understanding of key aspects of Australia's coal basins and how they impact on mining conditions, by providing a consistent structural framework across the Permo-Triassic Bowen Basin. Additional work was conducted in the outlying Triassic and Jurassic basins, but these require further work to provide a more detailed structural framework.  This report, the updated edition of the Bowen Basin Structural Geology 2017 map and the time space plot of tectonic events provide an industry reference that serves to educate current and future geologists and engineers in the industry and provide a platform for effective geotechnical assessment and mine design for both open cut and underground coal mines.

 

Data were sourced from both public and proprietary seismic, drilling and underground and open pit mapping data, integrated with potential field data, coupled with the surface geology mapping from the Geological Survey of Queensland.  Subcrop patterns of the Rangal and Moranbah Coal Measures across the Bowen Basin were refined, as were the details on faults and folds in the basin. In the new edition of the map, smaller structures from the company data compilation were included if they were significant at a scale of 1: 100,000. The map is delivered as a poster, and as a fully attributed set of GIS layers (shape file format), including: solid geology, faults, folds, fault throws, dykes, basalt, sub-surface intrusives, magnetic anomalies, relief trends, and deformation domains.  

 

The coal measure rocks consist of a strongly layered succession of rock types with different strengths (coal, siltstone and sandstone) that lend themselves to deformation by flexural slip. In flexural slip deformation, all strain is focussed along narrow shear bands that are parallel to bedding, like in a bending deck of cards. The resulting folds are either concentric parallel or kinked in shape, and invariably create space problems in the hinge area, which are usually accommodated by localised fracturing or mass flow of the weakest units.

 

Most of the small-scale folding and thrust faulting occurs in thinly interbedded rocks of contrasting strengths, such as tuffaceous coal seams and siltstone/sandstone heteroliths. This is typical for flexural slip deformation, which relies on abundant layer-parallel shear planes. The tuffaceous seams and abundant heterolithic rocks of the Rangal Coal Measures are particularly prone to small-scale deformation. In the Moranbah-German Creek Coal Measures, coal seams are thicker, and very thick channel belt sandstones are common. In this environment, strain problems near the thrust faults are accommodated by fracturing and mass flow within mudstone and coal. The thrust faults themselves also tend to exploit weak bedding planes such as at the tops of coal seams, and the base of large sandstones to propagate through the coal measures.

 

Joints were studied to provide information for local deformation, weathering, prediction of permeability, and geotechnical effects on the rock stability and strength. One of the main joint types in the Bowen Basin include large joints which are observed in thick interburden sandstones and in some locations, continue through other heterolithic units as well. One possibility for large joints is that they occur within the damage zone of major fault zones, which are intensely fractured areas. Another possibility is that large joints cut through the layers with low contrast in competency and therefore, rocks behave almost the same in response to strain.

 

Observations from high walls show that large joints control the frequency of smaller joints along various scan-lines. Joints are more abundant and more closely spaced close to large joints within both thicker and thinner beds. These observations at the regional scale for large scale faults and folds, and smaller scale joint sets, provide a context within which to investigate features that affect coal measure stability upon extraction.

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