Northern Bowen Basin Project
Interpretation of Structure

NORMAL FAULTS

Normal faults are generally steeply or moderately dipping, transecting part or all of the mine high wall exposures with throws ranging from centimetres to 80 m. Individual fault segments are up to several hundred metres in strike-length, combining to form regionally significant fault zones that can be traced for >12 km. Apparent mismatch of geology on either side of the faults, and complex termination structures near the top of the high walls suggest a component of strike-slip movement. These faults can be interpreted in the context of basement geometry and structural evolution. Reverse reactivation is common and lead to local broadening of the fault damage zone. All normal faults, including reactivated ones are truncated by the Tertiary unconformity.

THRUST FAULTS

The project area is is dominated by the Jellinbah thrust fault system, a regional structure with a cumulative throw of 600-800 m, which transects the Bowen Basin from Collinsville to south of Blackwater. Individual faults can be identified by “older over younger” map relationships and breaks in magnetic patterns. Faults also can be traced on the images where they are intruded by magnetic dykes.

The faults system generally trends northwest with an east-over-west throw, but locally picks up NS basement structures. The fault geometry typically includes sub-horizontal thrust flats, particularly within or at the top of coal seams, and moderately to steeply dipping thrust ramps with well developed hanging wall anticlines. Complex thrust duplex structures and small back thrusts are common and are usually particularly well developed where thrust faults overprint and interfere with existing normal faults.

The thrust system is interpreted as a thin-skinned thrust belt where all component shallow out at depth, although a deep regional-scale detachment is unlikely.

FOLDS

Folds were generally interpreted from outcrop patterns, particularly in the northern part of the study area. They are open, very gentle, folds with bedding dips rarely exceeding 10°. The trend of the fold axes is generally northwest, even close to fault sections trending north-south, against which they truncate.
Timing of the folds in interpreted to be older than or contemporaneous with the big thrust faults.

FOLD-THRUST BELT

Magnetic patterns resembling tight folding with common truncations is interpreted as a fold-thrust belt in the southeastern part of the study area. This area has no outcrop at all, so no other evidence exists to support this interpretation.

The structural position is equivalent to the Gogango Overfolded Zone in the southeastern part of the Bowen Basin.

CROSS SECTIONS

10 cross-sections were constructed across the northern Bowen Basin. Depth control was mostly given by deep gas exploration wells and seismic lines.

The sections highlight the shallow, synclinal nature of the Bowen Basin, and the shallow easterly dips on the thrust fault systems.

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