Northern Bowen Basin Project
Interpretation of Geology

BASEMENT

Metamorphic and igneous basement to the Bowen and Drummond Basin crops out along the eastern and western margin of the study area.

The Connors-Auburn Arch to the east consists of a complex system of multiple intrusions showing both positive and negative remanence. The shape of these intrusions defines a strong east-west fabric, and is sharply truncated by the overlying Bowen Basin margin.

The Anakie metamorphic inlier to the east has an older more complex history of intrusives and volcanics. In contrast to the eastern margin of the Bowen Basin the margin with the Anakie Inlier is shallow with basement rocks visible underneath the Bowen Basin fill for many kilometres.

CARBONIFEROUS AND OLDER SEDIMENTS

Carboniferous and older sediments and volcanics are exposed in the basement rocks marginal to the Bowen Basin. These are difficult to identify on the magnetic images as they are obscured by basement texture and noise from overlying basalts. Their interpretation therefore relied strongly on existing mapping and with some support from radiometric and sattelite images

PERMIAN AND TRIASSIC BASIN FILL

As a generalisation the Permian and Triassic sequences of the Bowen Basin have low magnetic susceptibilities, resulting in a “transparent” look on the magnetic images. This is particularly apparent along the basin margin, where the strong signature of the older basement rocks continues for several kilometres underneath the sediments. Thus the magnetic images alone do not contain sufficient information to improve on current mapping.

In the western part of the basin modern alluvial sediments and Tertiary rocks cover most of the Permian units, leaving only small areas of subcrop. In the central part of the basin the Triassic Clematis group is exposed within several shallow synclines. This sandstone dominated unit forms prominent cliffs along its outcrop margin that surround low hills, making it easy to map on Landsat TM images. Further to the east the strata dip more steeply, and the Permian units are exposed in low hills and long (6-20 km) low sandstone ridges emerging from areas of subcrop. These ridges are well defined on Landsat TM, and the intervening finer-grained rocks cause weak positive anomalies on the magnetics.

In the areas of subcrop and outcrop the radiometric images differentiate well between the main stratigraphic packages. The middle Permian marine units have the highest gamma response, with the U and Th channels responding slightly stronger than K. The Collinsville coal measures of similar age also have a high overall gamma response, but with U slightly more abundant than Th. The late Permian Moranbah and Rangal Coal Measures coal crop out poorly, but small areas of subcrop suggest that their radiometric response is low. However the intervening Fort Cooper Coal Measures show a strong potassium dominated signature, reflecting the high volcanic tuff component in that formation.

The paucity of outcrop limits the use of all remotely sensed images, and the new interpretation was strongly guided by existing maps, open file borehole information and seismic data. Thus the resulting new interpretation of the distribution of the Permian and Triassic units differs from the existing maps only at a detailed scale.

INTRUSIVES

Small igneous plugs, lava flows and dykes are far more abundant within the Bowen Basin than was previously known. Several distinctive intrusion types can be identified:

Larger plugs with positive magnetic remanence. The intrusions are usually subcircular in shape and may show multiple phases of intrusions. While many have been mapped particularly by their resistant contact aureoles, many more have been mapped in the subsurface during this study. The intrusions are typically strongly magnetic but can be nearly non-magnetic in rare examples. Those intrusions that are exposed to the surface show a high potassium radiometric signature, commonly associated with a contact aureole high in potassium, uranium and thorium. Several of the intrusions have previously been dated as Early Cretaceous in age. One plug in the north of the study area is associated with extensive radial dykes.

Small plug swarms with negative or positive remanence. These small (<500m in diameter) intrusions are abundant just to the north or south of major basalt sheets. Some are positioned underneath the basalt sheets. Many of the plugs are exposed forming small hills, but many more were identified on the aeromagnetic images. Most of the plugs in the north are associated with west-northwest trending dykes of similar remanence, suggesting a direct link between the dykes and the intrusions.

DYKES

Dykes are abundant throughout the study area and both magnetically negative and positive dykes are common. Generally sykes occur in swarms of consistent orientation and remanence. Dykes intruded into the Connors Arch to the east of the Bowen Basin are positive in remanence and trend north-west, parallel to the present edge of the basin. These dykes are part of a major regional dyke swarm extending from Cairns to south of Rockhampton intruded during Mid-Permian to Early Jurassic times. Within the basin most dykes that have positive remanence trend north-south to north-northwest, some intruding Triassic thrust faults. A radial dyke swarm around a small plug in the northern basin is most prominent in its northwestern and southeastern sectors, consistent with the other dyke swarms. Notable exceptions to the northwesterly trend are widely spaced zones of northeasterly trending dykes that are associated with major gravity lineaments that also control strings of Cretaceous intrusions. It is likely that the majority of dykes with positive remanence in the basin are Cretaceous in age.

Dykes with negative remanence are much less common in the basin. Some are clearly related to small mafic intrusions that may also be related to nearby Tertiary sheet basalt flows. In this area some positive dykes are associated with positive plugs, suggesting that there are some Tertiary intrusions that are positive.

BASALT

Extensive mafic lava flows occur in the southwest of the study area, along the western margin of the basin and to the east of the Bundarra Granodiorite. The flows have strong magnetic susceptibilities, and all apart from the northernmost flow have negative remanence. Their signature on the magnetic image is strong, masking all underlying magnetic sources, while on the radiometric image the rocks have very low responses for K, U and Th.
The lava flows occur either in massive fields, or as thin flows with complex dendritic outlines. The internal texture of some of the thin flows suggests that they followed meandering palaeovalleys on the Tertiary unconformity surface. Most of the flows split and peter out toward the east, indicating the downhill slope of the unconformity.

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