Queensland Geology 12

Geology of the Auburn Arch, southern Connors Arch and adjacent parts of the Bowen Basin and Yarrol Province, central Queensland

IW Withnall, LJ Hutton, RJ Bultitude, FE von Gnielinski & IP Rienks

This report describes the geology of part of central eastern Queensland from Mackay to Mundubbera covering twenty 1:100 000 map sheet areas with a total area of 27 500km². The major structural provinces within the area are the Connors and Auburn Arches and the Gogango Overfolded Zone, which are parts of the northern New England Orogen. A narrow strip of rocks of the Bowen Basin flank them on the eastern side and separate them from the Yarrol Province. They are flanked on the west by the main part of the Bowen Basin.

The Connors Arch in the north and Auburn Arch in the south both consist of early Carboniferous to Permian mafic to felsic volcanic and plutonic rocks that are herein interpreted to have formed an Andean volcanic arc west of a forearc basin represented by the Yarrol Province.

In the Auburn Arch, a metamorphic basement of unknown age is intruded by deformed early Carboniferous granitic rocks. The lower part of the overlying succession, the Torsdale Volcanics, consist mainly of felsic ignimbrite with SHRIMP dates in range 325 to 300Ma. These are overlain by the Camboon Volcanics, which pass upwards through a succession of alternating felsic ignimbrite and mafic lava packages into dominantly basaltic rocks. Unconformities marked by conglomerates that locally overlie small granitic bodies have been observed, but these may be local and/or diachronous and the dominantly mafic and felsic successions of the Torsdale and Camboon Volcanics may overlap in age. Numerous plutons ranging from gabbro to granite were emplaced synchronously with the volcanic rocks.

SHRIMP U-Pb zircon dating suggests at least three major magmatic episodes in the Connors Arch. An early Carboniferous (older than ~330Ma) mafic to felsic magmatic event is represented by volcanic and minor plutonic rocks. A late Carboniferous felsic magmatic event at around 310 to 320Ma is represented by extensive ignimbritic units and small granitic plutons. These are separated by a regional unconformity from a more heterogeneous assemblage of volcanic and sedimentary rocks that contain some local unconformities. SHRIMP U-Pb zircon ages range from 300 to 285Ma. Most of the plutonic rocks (gabbro to granite) of the Urannah Batholith were probably also emplaced during this interval. The oldest rocks in the interval (Leura Volcanics) show a complete range of compositions from basalt through andesite to rhyolite, but the younger rocks (Lizzie Creek Volcanic Group) tend more towards bimodality.

Most of the Yarrol Province lies outside the study area, but in the north-east, a succession of Late Devonian to early Carboniferous mafic to felsic volcanic and sedimentary rocks is the northern continuation of the Yarrol Province, and is herein referred to as the Campwyn Subprovince. Farther south, volcanic and sedimentary rocks ranging from the Late Devonian to early Permian comprise the Stoodleigh Subprovince. The Late Devonian to Carboniferous volcanic rocks in these subprovinces are more oceanic in geochemical affinities than those in the Connors and Auburn Arches and probably represent an intra-oceanic island arc. Along the eastern edge of the study area in the south, successions of mainly mafic volcanic and related clastic rocks assigned to the Nogo Subprovince may include some Late Devonian rocks, but most are probably early Permian.

The basal part of the Bowen Basin consists of shallow marine sedimentary rocks, but in many places, these are separated from the dominantly volcanic successions of the Connors and Auburn Arches by discontinuous sequences that contain fluviatile and lacustrine sedimentary rocks and bimodal volcanic rocks. These have been interpreted as the early extensional or rift phase in the formation of the Bowen Basin. Within the adjacent Yarrol Province, the Rookwood Volcanics, a sequence of mainly submarine basalt with MORB-like geochemical affinities was possibly erupted as part of this extensional phase. The overlying marine sediments of the Bowen Basin were deposited during the thermal sag phase of basin development. They are overlain by non-marine sedimentary rocks that include coal measures and represent a foreland basin succession formed during the onset of the Hunter-Bowen Orogeny. These rocks are overlain by Early Triassic redbed and sandstone units.

The eastern side of the Bowen Basin in the study area is strongly deformed in the Gogango Overfolded Zone (GOZ), a thrust belt that consists of strongly cleaved sedimentary and some volcanic rocks. It separates the Connors and Auburn Arches and continues south along the eastern side of the Auburn Arch, separating it from the Yarrol Province. The boundary between the GOZ and the Yarrol Province is a system of major thrust faults.

The northern termination of the thrust belt also marks the northern edge of the Marlborough Block, a belt of ultramafic and mafic rocks interleaved with deformed and metamorphosed sedimentary rocks that are probably part of the Yarrol Province. The mafic-ultramafic rocks probably formed in a Neoproterozoic oceanic spreading centre and later interacted with magmas of island arc composition in a supra-subduction zone in the Late Devonian. The ophiolite complex so-formed was emplaced into its current position as an out-of-sequence thrust sheet in the Late Permian.

Intruding the Auburn Arch and southern part of the GOZ is the Rawbelle Batholith that consists of granitoids that mostly have late Permian to Early Triassic ages and can be divided into two main geochemical suites. Two other suites of early Permian granitic rocks show probable S-type and I-type affinities respectively. Minor areas of Late Triassic felsic volcanic rocks and some granitic plutons crop out in the southern part of the study area.

The Surat and Mulgildie Basins formed in the Jurassic when sediments were deposited in fluvial, lacustrine and some coal swamp environments. In the Styx Basin, an Early Cretaceous coal-bearing sequence was deposited. Early Cretaceous ignimbritic volcanic rocks crop out on the coast near Mackay and are probably related to the Whitsunday Province, a felsic-dominated large igneous province that is mainly preserved on islands offshore of central Queensland. Several plutons in the Urannah Batholith and adjacent area were emplaced in the Cretaceous and some subvolcanic intrusions and extrusive rocks of this age also occur farther south.

In the Eocene, several narrow, north-north-westerly trending basins developed as half grabens with faulted westerly margins developed in central Queensland. They were filled by fluvial to lacustrine sediments including oil shale. In the study area, the basins are the Biloela, Duaringa and Herbert Creek Basins. Deposition was followed by the development of a deep weathering profile over much of the area during the Oligocene.

Basalt lavas were erupted sporadically through the study area from the Late Cretaceous to the Oligocene. Some felsic rocks of probable Oligocene age also occur, particularly in the north, and include lavas and plugs as well as some high-level plutonic rocks near Mackay. Cainozoic sedimentary deposits are extensive and include high level alluvial fans and talus, flood plain alluvium, colluvial and residual deposits and various coastal deposits.

 

Area of the Connors Report

Maps available (as pdf files) with Queensland Geology 12

Auburn Special 1:100 000 Geology Map
Banana 1:100 000 Geology Map
Bungaban 1:100 000 Geology Map
Carmila 1:100 000 Geology Map
Connors Range 1:100 000 Geology Map
Cracow 1:100 000 Geology Map
Duaringa 1:100 000 Geology Map
Eidsvold 1:100 000 Geology Map
Mackay 1:100 000 Geology Map
Marlborough 1:100 000 Geology Map
Mirani 1:100 000 Geology Map
Mount Bluffkin 1:100 000 Geology Map
Mount Morgan 1:100 000 Geology Map
Mundubbera 1:100 000 Geology Map
Nebo 1:100 000 Geology Map
Rawbelle Special 1:100 000 Geology Map
Rookwood 1:100 000 Geology Map
Scoria 1:100 000 Geology Map
St Lawrence 1:100 000 Geology Map
Theodore 1:100 000 Geology Map

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Last Updated 22 June 2009