The Thomson Orogen of the Tasman Orogenic Zone

48 (1978)

Tectonophysics, @ Elsevier

Scientific

299

299-325

Publishing

Company,

Amsterdam

-

THE THOMSON OROGEN OF THE TASMAN

C.G. MURRAY Geological

Utah Development (Received

and A.G.

Survey

in The Netherlands

OROGENIC

ZONE

KIRKEGAARD

of Queensland, Co., Brisbane,

for publication

Printed

January

Brisbane,

Queensland

Queensland

(Australia)

(Australia)

5, 1978)

ABSTRACT Murray, C.G. and Kirkegaard, A.G., 1978. Zone. In: E. Scheibner (Editor), The tions in Tectonic Style. Tectonophysics,

The Thomson Orogen Phanerozoic Structure 48: 299-325.

of the Tasman Orogenic of Australia and Varia-

The Thomson Orogen forms the northwestern segment of the Tasman Orogenic Zone. It was a tectonically active area with several episodes of deposition, deformation and plutonism from Cambrian to Carboniferous time. Only the northeastern part of the orogen is exposed; the remainder is covered by gently folded Permian and Mesozoic sediments of the Galilee, Cooper and Great Artesian Basins. Information on the concealed Thomson Orogen is available from geophysical surveys and petroleum exploration wells which have penetrated the Permian and Mesozoic cover. The boundaries of the Thomson Orogen with other tectonic units are concealed, but discordant trends suggest that they are abrupt. To the west, the orogen is bordered by Proterozoic structural blocks which form basement west of the northeast-trending Diamantina River Lineament. The most appropriate boundary with the Lachlan and Kanmantoo Orogens to the south is an arcuate line marking a distinct change in the direction of gravity trends. The north-northwest orientation of the northern part of the New England Orogen to the east cuts strongly across the dominant northeast trend of the Thomson Orogen. The Thomson Orogen developed as a tectonic entity in latest Proterozoic or Early Cambrian time when the former northern extension of the Adelaide Orogen * was truncated along the Muloorinna Ridge. Early Palaeozoic deposition was dominated by finegrained, quartz-rich elastic sediments. Cambrian carbonates accumulated in the southwest and a Cambro-Ordovician island arc was active in the north. Along the western margin of the orogen, sediments were probably laid down on downfaulted blocks of deformed Proterozoic rocks, with oceanic crust further to the east. A mid- to Late Ordovician orogeny which affected the whole of the Thomson Orogen marked the climax of its precratonic (erogenic) stage. The northeast structural trend of the orogen (parallel to its western boundary with the Precambrian craton) was imposed at this time, and has controlled the orientation of later folding and faulting. Up to three generations of folding have been recognized, and fine-grained me&sediments exhibit a prominent slaty cleavage. Metamorphism was to the greenschist and amphibolite facies, the highest grade rocks being associated with synorogenic granodiorite batholiths in the north. Following deposition of Late Ordovician marine sediments at the eastern margin,

* Referred to as Adelaide Fold Belt or Adelaide ‘Geosyncline’ sium. It has developed from an aulacogene. (Editor.)

elsewhere

in this

Sympo-

emplacement of post-tectonic Late Silurian or Early Devonian batholiths ended the precratonic history of the Thomson Orogen. The subsequent transitional tectonic regime was characterized by deposition of Devonian to Early Carboniferous shallow marine and continental sediments including widespread red-beds, and andesitic volcanics. The maximum marine transgression occurred in the early Middle Devonian. Localized folding affected the easternmost part of the Thomson Orogen at the end of Middle Devonian time, and was followed by intrusion of Devono-Carboniferous granitie plutons. However, the terminal orogeny which deformed all Devonian to Early Carboniferous rocks of the orogen was of mid-carboniferous age. It produced northeast-trending open folds and normal and high-angle reverse faults which are considered to reflect basement structures. The cratonization of the Thomson Orogen was completed with the emplacement of Late Carboniferous granites and the eruption of comagmatic voicanics in the northeast. Permian and Mesozoic sediments aceumuiated in broad, relatively shallow downwarps which covered most of the former orogen. INTRODUCTION

The name Thomson Orogen (or Fold Belt) was introduced by Kirkegaard (1974) to refer to the Early and Middle Palaeozoic rocks of the northwestern part of the Tasman Orogenic Zone. Most of the orogen is concealed beneath Mesozoic cover of the Great Artesian Basin * (Fig. l), and its geology is poorly known. Although the Thomson Orogen appears to have developed over the same time interval as the Kanmantoo and Lachfan Orogens to the south, it is not possibIe to extrapolate northwards from these orogens on the limited data available. Use of the term Thomson Orogen avoids any implied correlation and allows for a more objective evaluation of the geology. Most data on the geology of the exposed parts of the Thomson Orogen has been produced by regional mapping (1 : 250 000 scale) by the Geological Survey of Queensland and the Bureau of Mineral Resources, Geology and Geophysics. Elsewhere, information is available only from geophysical surveys and from widely scattered petroleum exploration wells which have penetrated Mesozoic and Permian cover. Because of lack of data, there have been few descriptions of the geology of the Thomson Orogen. Early attempts to outline the Early Pafaeozoic history of the southwestern and northern parts of the orogen from basement data from boreholes were made by Canaple and Smith (1965) and Lindner (1966), respectively. Kirkegaard (1974) summarized the geology of most of the Thomson Orogen, and Harrington (1974) presented a speculative reconstruction of the Early Palaeozoic tectonic development of the area. Recently, the geology of the exposed Early Palaeozoic rocks of the orogen has been synthesized by Henderson (in press). The Devonian to Carboniferous transitional basins have been described in more detail, the Burdekin Basin by Wyatt and Jell (1967) and Wyatt et al. (1970), the Drummond Basin by Olgers (1972) and Day (19’76), and the Adavale Basin by Heikkila (1965), Tanner (1967) and * Synonymous term is Great Australian Basin. It comprises Carpentaria, Eromanga, Surat and ~l~ence-Moreton basins. (Editor.)

301

Auchincloss (1976). Marsden (1972) also summarized the geology and tectonic setting of these transitions basins. The tectonic units shown on Fig. 1 are taken from the Tectonic Map of Australia and New Guinea (Geological Society of Australia, 1971), with modifications to some boundaries as a result of more recent data. In this paper, the tectonic units of the Thomson Orogen are divided into two broad categories, a precratonic or erogenic regime (which includes Cambrian and Ordovician rocks), and a transitional regime (Devonian to Carboniferous basins) in the sense that these terms are used by Rickard and Scheibner (1975). The tectonic terminology follows the recommendations of Rickard and Scheibner (1975). LIMITS OF THE THOMSON

OROGEN

The limits of the Thomson Orogen are difficult to define, but it covers most of central Queensland and extends into northeastern South Australia. Early Palaeozoic rocks considered to belong to the erogenic stage crop out as two structural blocks in the northeast, the Lolwo~h-Ravenswood Block and the Anakie Inlier (Fig. l), and form basement beneath the Drummond and Galilee Basins; similar Early Palaeozoic rocks floor the Adavale and Cooper Basins and much of the Great Artesian Basin. Two Devonian to Carboniferous tr~sition~ basins are exposed in the northeast; the Burdekin Basin lies within the Lolworth-Ravenswood Block, ‘and the Drummond Basin surrounds the Anakie Inlier. Drilling has also revealed the existence of the Devonian and possibly Early Carboniferous Adavale Basin, a concealed transitional basin in the central part of the Thomson Orogen. As originally defined by Kirkegaard (1974), the Thomson Orogen included all Early Palaeozoic rocks of the northern part of the Tasman Orogenie Zone. In this paper, a three-fold division of the northern part of the Tasman Orogenic Zone into the Thomson Orogen, the Hodgkinson-Broken River Orogen and the New England Orogen is accepted. Thus the Early Palaeozoic rocks of the Broken River Province of the Hodgkinson-Broken River Orogen are excluded from this review. They are briefly discussed by Day et al. (1978). The geology of the Lolworth-Ravenswood Block differs markedly from that of the Broken River Province (cf. Henderson, in press). Although its overall trend is not the same as that of the remainder of the Thomson Orogen, the Lolworth-Ravenswood Block is best regarded as part of the orogen because of its similar Early Palaeozoic tectonic history. The northern limit of the Thomson Orogen is the Clarke River Fault (White, 1965) along the northern margin of the Lolwo~h-Ravenswood Block (see Fig. 3). The boundary between the Lolworth-Ravenswood Block and the southern end of the Precambrian Georgetown Inlier is concealed by thin Cainozoic volcanits, but probably trends southwest in line with a major fault to the north (see Fig. 1). No definite Precambrian rocks have been identified to the east of this line, although blocks of amphibolite-grade metamorphics in the

BASIN

-‘-.....WARBURTON

-

FAULT

+

ANTICLINE

+

SYNCLINE

:’

+ +

PRECAMBRIAN EXPOSED

H a

CAMBRIAN-ORDOVICIAN

DEVONIAN-CARBONIFEROUS

--

LINEAMENT

-

BOUNDARY

OF

EXPOSED

rlf

BCUNDARY

OF

GREAT

-------

SUBSURFACE

LIMIT

OF

CAMBRIAN-ORDOVICIAN

x--x--x

SUBSURFACE

LIMIT

OF

DEVONIAN-CARBONIFEROUS

....‘...‘.‘*

SUBSURFACE

LIMIT

OF

PERMIAN-TRIASSIC

TECTONIC ARTESIAN

1 OOROONOO

S CLEEVE

OF

THOMSON

OROGEN

TRANSITIONAL BASINS OF THOMSON OROGEN

UNITS BASEMENT

BASIN BASINS BASINS

BASINS

9 GUMBARDO

PRECAMBRIAN

8

CAMBRIAN

Or 13 PANDIEBURRA

S

BORE

2 BEDOURIE

6 ETONVALE

10 BUCKABlE

14 KALLADEINA

S-D

HZS

3 PURNI

7 GALWAY

11 EuLO

15 GIDGEALPA

D

-

4 FERMOY

6 BALFOUR

12 PUTAMURDI~

16 DUlLlNGARf

417

RIDGE

Fig. 1. Major tectonic elements of the Thomson Orogen.

AGES

R

ORDOVICIAN SILURIAN SILURIAN-DEVONIAA DEVONIAN ISOTOPIC

AGE

KM

0

100

CONTOUR

100

INTERVAL

20

B

-60

Tb

-40

m

-40

TO

-20

jg

-20

TO

0

jfj?Jj

0

fl

~20

TO

200 KM MILLIGALS

+20 TO

~40

Fig. 2. Bouguer gravity anomalies, Thomson Orogen. Data from Lonsdale (1962, 1965), Gibb (1967, 1968), Darby (1969a, b) and Bureau of Mine& Resources, Geology and Geophysics (1976).

304

northern and western parts of the Lolworth-Ravenswood Block have been tentatively mapped as Precambrian because of their metamorphic grade (Wyatt et al., 1970) and structural complexity (Henderson, in press). The continuation of the southwest-trending margin of the Thomson Orogen is concealed beneath Mesozoic cover of the Great Artesian Basin. On this trend are two prominent structures within the Mesozoic sediments, the Wetherby Structure and the Cork Fault, which probably reflect basement faults. Further to the southwest this trend continues through the Diamantina River Lineament (Scheibner, 1974) which coincides with the boundary between the north to north-northwest-trending gravity and aeromagnetic anomalies of the Precambrian Mount Isa Inher and the generally northeasttrending anomalies of the Thomson Orogen (Fig. 2 and Harrison et al., in press). Data from petroleum exploration wells, summarized in Fig. 1, shows that the southwest-trending line through the Wetherby Structure, Cork Fault and Diamantina River Lineament coincides with the boundary between thin, undeformed, carbonate-dominated Cambrian and Ordovician sediments of the Georgina Basin which were deposited on the Precambrian craton, and folded, metamorphosed and intruded sediments and volcanics deposited at the same time in a marginal mobile zone to the southeast. This line therefore marks the western limit of the Thomson Orogen and the Tasman Orogenic Zone (cf. Lindner, 1966). The southwestern margin of the Thomson Orogen is more obscure. The Muloorinna Ridge, which coincides with a belt of positive gravity anomalies (Fig. 2), connects the Denison and Mount Painter Blocks, and marks the northeastern limit of the Precambrian rocks in this area. The Lake Eyre Lineament along the northern edge of the Muloorinna Ridge (Wopfner, 1964) separates a region of craton-controlled Early Palaeozoic deposition and deformation to the southwest from a belt of thick, folded Cambrian and Ordovician sediments and volcanics to the no~he~t (the eastern part of the Warburton Basin). This is considered to be more appropriate as a boundary between the Thomson and Adelaide Orogens than the parallel Lake Blanche Fault to the north. Kirkegaard (1974) defined the southern boundary of the Thomson Orogen with the Lachlan Orogen as the Darling River Lineament. One of the main criteria for distin~ishing between these two orogens on a regional scale is their differing structural trends. In the Lachlan Orogen, the dominant trend is meridional to north-northwest, whereas the structural grain of the Thomson Orogen, as indicated by basement-related structures in the cover rocks, basement topography, and gravity and aeromagnetic trends, is northeast. The eastern end of the Darling River Lineament corresponds with the position of the change in trend, but the western end does not. In particular, the north-northwest-trending Bancannia Trough (which is usually regarded as part of the Early Palaeozoic Kanmantoo Orogen) is perpendicular to the basement trends beneath the Cooper Basin to the north. A more appropriate southern limit to the Thomson Orogen, therefore, may be the curved bound-

305

ary shown by Wellman (1976, fig. 1) between crustal blocks with different gravity trends. This boundary is shown on Fig. 1. Its precise nature is unknown. The north-northwest trend of the northern part of the New England Orogen is strongly discordant to that of the Thomson Orogen, but the contact between the two orogens is concealed by the Permo-Triassic Bowen Basin. This basin can be divided into contrasting eastern and western areas showing different tectonic styles. The eastern part was a tectonically active area throughout the Permian and Triassic, as evidenced by a thick sequence of folded sediments and volcanics. These appear to have developed within the framework of the New England Orogen. By contrast, strata in the western part of the basin are gently folded shallow-marine, paralic and fluviatile clastics and minor volcanics which were deposited on a relatively stable platform assumed to be the cratonized Thomson Orogen. However, there is no clearly defined structural feature marking the eastern limit of cratonic basement. As defined above, the Thomson Orogen has a maximum length along strike of 1250 km and an average width of 750 km. PRECRATONIC

OR OROGENIC

Lolworth-Ravenswood

REGIME

OF THE THOMSON

OROGEN

Block

The Lolworth-Ravenswood Block consists of Early Palaeozoic sediments and volcanics intruded by two large composite batholiths of Ordovician to Devonian age (Fig. 3.) It is separated from the Broken River Province of the Hodgkinson-Broken River Orogen to the north by the Clarke River Fault (White, 1965). This fault is largely concealed by Permo-Carboniferous volcanic and intrusive complexes which also border the Lolworth-Ravenswood Block to the east, Rocks of the Lolworth-Ravenswood Block pass beneath Late Devonian to Early Carboniferous strata of the Drummond Basin t,o the south, and Permian and Triassic sediments of the Galilee Basin to the west. The block forms basement to the Middle Devonian to Early Carboniferous Burdekin Basin and much of its western half is covered by a veneer of Cainozoic sediments and basalt. A thick east to east-northeast--trending sequence of tuffaceous arenite, siltstone and black shale (Cape River Beds) and calcalkaline volcanics occurs along the southern margin of the Lolworth-Ravenswood Block south of Charters Towers. The proportion of interbedded volcanics increases from west to east, and the thickest sections have been mapped separately as the Mount Windsor Volcanics (Wyatt et al., 1971). Acid volcanics are dominant, but the sequence is characterized by rapid variations in lithology along strike, and andesite and basalt are locally abundant. Early Ordovician graptolites have been found in sediments of the Cape River Beds overlying the volcanics (Dear, 1974; M&lung, 1976; Henderson, in press). This age is consistent with a Late Cambrian or Early Ordovician Rb-Sr date from the easternmost exposures of unmetamorphosed acid volcanics (510 t; 100 m.y.; Webb, 1974).

306

fwENswocmY

cz

CAINOZOIC

JK

JURASSIC-CRETACECUS

Pk

FERMiAN

PV

PERMIAN

COVER

DEYONlAN

BURDEKIN

XL,

BUNDOCK

Dee

CLARKE

10

EASlPfS

I

L

DCd

-%!YAN

CftDOVlClAN

SEDIMENTS

VOLCANICS TO

OF

ARTESIAN

EASIN

BASIN

BASIN

VOLCANICS

AND

INTRUSIVE

COMPLEXES

BASIN B~IN RIVER

DRUMMOND

x

x

SILURIAN

+

BASIN

BASIN

+

ORDOVICIAN

TO

FLYSCH

DEVONIAN TO

UNDIFFERENTIATED

CO1

RUNNING

coo

ARGENTINE

METAMORPHICS BEDS

RIVER

COC

CAPE

COW

MOUNT

RIVER

e.z

PRECAMBRIAN

WINDSOR

OF

GRANITIC

DEVONIAN

CO

I

OF GREAT OF GALILEE

BOWEN

PERMIAN

ORDOVICIAN-DEVONIAN

O-D

r

SEDIMENTS

-TRIASSIC

CARBOMFEROUS fDC’

+T+

BROKEN

RIVER

RAVENSWOOO

GRANODIORITE

METAMORPHKS METAMORPHICS

VOLCANKS

FAULT

Fig. 3. SimpIified geological map, Loiworth-~avenswood

PROVINCE

ROCKS

Block.

COMPLEX

307

A belt of metasediments which trends west-northwest along the southwestern margin of the Lolworth-Ravenswood Block has been correlated with the fossiliferous sediments south of Charters Towers (Wyatt et al., 1971; Paine et al., 1971). Rock types include phyllite, quartzite, mica schist, amphibolite and garnet-biotite gneiss of greenschist to amphibolite facies. Up to three generations of folding have been recognized in these rocks (Baker, 1974), which appear to have had a much more complex history of deformation than the unmetamorphosed sediments. A Rb-Sr isochron on four samples of metamorphics yielded an Early to Middle Ordovician date (483 i 25 m.y.; Paine et al., 1971) which probably is the age of the final episode of metamorphism and deformation. The metamorphosed Cape River Beds are similar in structure and metamorphic grade to the Running River Metamorphics at the northern edge of the Lolworth-Ravenswood Block, and to the Argentine Metamorphics which occur as basement to the Burdekin Basin. The Running River and Argentine Metamorphics have been assigned a tentative Precambrian age (Wyatt et al., 1970), and it is possible that the metamorphosed Cape River Beds are also of this age (Henderson, in press). The Mount Windsor Volcanics and unmetamorphosed Cape River Beds are readily interpreted as remnants of a Cambro-Ordovician volcanic island arc and its associated sediments which were separated from the Precambrian Georgetown Inlier by a marginal sea of unknown extent. If the Running River and Argentine Metamorphics are of Early Palaeozoic rather than Precambrian age, it is possible that they are deformed deposits of the marginal sea, since they include amphibolite and minor serpentinite. Quartz-rich sediments in the metamorphics may have been derived from the Precambrian craton to the northwest. Quartzose flysch of the southwestern Broken River Province (Arnold and Henderson, 1976; see Day et al., 1978) may have been contemporaneous with or younger than the island-arc deposits. Two main episodes of granite emplacement are recognized in the Lolworth-Ravenswood Block. The Ravenswood Granodiorite Complex is a large composite batholith which crops out mainly east of Charters Towers where eight subunits were mapped by Clarke (1971). Rb-Sr isotopic dating has yielded Middle or Late Ordovician (454 + 30 m.y.) and Late Silurian or Early Devonian (394 k 30 m.y.) ages (Webb, 1969a). Both intrusive epochs appear to have commenced with emplacement of a main ~anodiorite phase followed by intrusion of later more acid phases (Wyatt et al., 1971). Diorite and gabbro are widespread but not abundant. All rock types of the Ravenswood Granodiorite Complex locally are strongly foliated. Gneissic granodiorite which intrudes the metamorphosed Cape River Beds is foliated parallel to the schistosity (Paine et al., 1971). This suggests that t.he initial intrusions of the Ravenswood Granodiorite Complex were synorogenic with the final mid-Ordovician phase of metamorphism and deformation of the Cape River Beds (and possibly of the Running River and Argentine Metamorphics also). The Lolworth Igneous Complex, which crops out west of Charters Tow-

308

ers, is the same age as the younger intrusions of the Ravenswood Granodiorite Complex (Late Silurian or Early Devonian, Rb-Sr isochron 401 ir 7 m.y.; Webb, 1971). It consists chiefly of biotite adamellite and granodiorite with banded pegmatitic and garnetiferous muscovite granite. The Late Silurian to Early Devonian post-erogenic intrusions stabilized the Lolworth-Ravenswood Block. From Late Devonian to Early Carboniferous time, the emergent block was a source area separating the largely marine Burdekin Basin on its northeastern margin from the continental Drummond Basin to the south (Hill, 1960). Anakie Inlier The Anakie Inlier is a north-northwest-trending block of Early to Middle Palaeozoic rocks surrounded by Late Devonian to Permian strata of the Drummond and Bowen Basins. Its geology has not been comprehensively studied and is poorly known. The inlier contains rocks of at least five distinct ages: pre-Late Ordovician metamorphics, Late Ordovician sediments, Middle Devonian volcanics and sediments, Late Devonian granodiorite, and Late Carboniferous granite. In the south, the inlier comprises generally low-grade regional metamorphics, intruded by Late Devonian granodiorite. Present data suggest that the Anakie Metamorphics can be subdivided into two units. The older unit consists dominantly of albite-muscovite-quartz schist with quartz-albite-chloriteactinolite-epidote schist which appears to be metamorphosed basic volcanics and is closely associated with small lenses of serpentinized harzburgite. Structurally the metamorphics are complex, and have been deformed by more than one generation of folding. A Middle or Late Ordovician age for the folding and metamorphism is indicated by a K-Ar date of 458 m.y. from mica schist (Webb and McDougall, 1968). Syntectonic plutonic activity is suggested by similar dates from massive granodiorite intruding schist in the core of the Telemon Anticline in the Drummond Basin south of the Anakie Inlier (444, 445 and 452 m.y.; Webb, 1969b), but no Ordovician granites are known from the inlier itself. Within the metamorphosed rocks are belts of unmetamorphosed quartz arenite and mudstone with lenses of crystalline limestone. These sediments have not been mapped in detail, but appear to be unconformable on the schists. They may be equivalent to a Late Ordovician sequence of quartzrich siltstone, mudstone, limestone and minor calcarenite which crops out in the crest of an anticline in Permian strata of the Bowen Basin about 5 km east of the Anakie Inlier (Anderson and Palmieri. 1977). Fold axes, bedding and schistosity within the Anakie Metamorphics, particularly the older unit, commonly have a northeast trend (Kirkegaard, 1974). Thus the mid-Ordovician folding was about northeast axes, parallel to the overall trend of the Thomson Orogen, and the north-northwest orientation of the Anakie Inlier must have been due to later tectonism. The Middle Devonian volcanics and sediments of the Anakie Inlier are

309

considered to belong to the transitional gen, and will be described in a following

tectonic section.

stage of the Thomson

Oro-

data

Subsurface

West of the Diamantina River Lineament (Fig. l), all samples of isotopitally dated basement collected from boreholes have given Precambrian ages. The data is summarized in Table I. Petroleum exploration wells drilled through the Mesozoic and Permian cover of the Great Artesian, Galilee and Cooper Basins revealed the Adavale Basin and penetrated basement rocks described as low-grade metamorphies, volcanics, intrusives and steeply dipping sediments. Isotopic age determinations have given Early to Middle Palaeozoic dates for most of the basement east of the Diamantina River Lineament. The only Precambrian rocks are Late Proterozoic volcanics intersected beneath the Gidgealpa gas field and in the Kalladeina 1 well. Table II lists basement ages determined by isotopic dating and palaeontology from wells within the area of the Thomson Orogen. Southwest Thomson Orogen Voleanics which underlie Cambrian ~~bonates of the Warburton Basin in the southwest of the Thomson Orogen are petrologically similar to Late Proterozoic volcanics of the Adelaide Orogen (Harrison and Ngginbotham, 1966b, c). Thus they may be remnants of a northern extension of the AdeTABLE

I

Isotopic ages of Precambrian -..--Location Borehole

Ooroonoo Bedourie

1

Purni 1 Marduroo Canary

1 1

Elizabeth Springs 1 Beantree 1

Netting Fence

1

basement

in boreholes -___-.__-____Isotopic age Basement rock type

_--.--~...---Reference

see Fig. 1 see Fig. 1

granite ?

860 (K-Au) 798 (K-Ar)

see Fig. 1

shale

24’02% 139’54’E 23’15% 140°22’E 23O21 ‘S 140’36’E 22O43’S 140°06’E

shale

Late Proterozoic (K-Ar) 720 + 130 (Rb-Sr)

Evernden and Richards, 1962 Canaple and Smith, 1965 Harding, 1969

shale

710 + 250 (Rb-Sr)

Harding,

1969

shale

710 f 250 (Rb-Sr)

Harding,

1969

amphibolite foliated adamellite ?

1495 (K-Ar) 1444 (K-Ar)

Harding,

1969

1665 (K-Ar)

Bennett 1975

et al.,

22’56% 138*02’E

Webb et al., 1963

310 TABLE

II

Ages of basement Borehole

rocks from boreholes Basement

Fermoy

1

within

rock type

schist

Cleeve 1

granite

Etonvale

1

granite

Galway

1

shale

Balfour

1

feldspar

Gumbardo Buckabie

alt.ered olivine

1

basalt

phyllite

1

Putamurdie

1

Pandieburra

1

Kalladeina

porphyry

1

Gidgealpa

Gidgealpa and 3

2

Dullingari

1

Cothalow 1 25O44’S 144’23’E Yongala 1 25”3O’S 143O56’ Purbrook 1 24’37’5 1&48’E Innamincka 1 27”29’S 140’55’E Merrimelia 1 27O46’S 140°10’E

siltstone and silty sandstone quartzite and black shale. Dip 70” mainly carbonate sediments overlying voIcanics limestone, black shale, tuffaceous siltstone, quartz sandstone, agglomerate. Dip 50’ trachytic, andesitic and rhyolitic tuff and lava with thin dolomite and shale interbeds shale and sandstone with dolomite bands altered

basaltic

altered schist

basalt,

andesite

mica

interbedded greenish sandstone, carbonaceous siltstone and shale interbedded red mudstone and shale and red to green sandstone red-beds similar to rocks in fnnamincka 1

the area of the Thomson

Age _--_. .~

Orogen Reference

-.~~---..“-.----

Cambrian-Ordovician 518 -t 22 m.y. (Rb-Sr) 541 + 14 m.y. (K-AI+) Silurian-Devonian 418 m.y. (K-Ar) Silurian 429 m.y. (Rb-Sr) Ordovician 479 + 15 m.y. (K-Ar) Silurian-Devonian 420 + 20 m.y. Ordovician 480 t 50 m.y. (K--Ar) Silurian-Devonian 417 ? 25 m.y. (K-Ar) Ordovician Early Ordovician (graptolites) Cambrian overlying Proterozoic volcanics Middle to Late Cambrian (trilobites)

Harding,

1969

Webb et al., 1963 Galloway,

1970

Bennett et al., 1975 Bennett et al., 1975 Galloway, 1970 Krueger,

1962

Canaple and Smith, 1965 Canaple and Smith, 1965 Wopfner, 1972

Harrison and Higginbotham, 1966a

Late Proterozoic to Early Cambrian Base 700 m.y. Top 400 m.y. Early to early Late Ordovician (graptolites) ?Ordovician

Harrison and Higginbotham, 1966b,c Canaple and Smith, 1965

? Ordovician

Galloway, 1970

Late Devonian (plants)

Traves, 1965; Day, 1976

? Devonian (brachiopod ? Devonian

Galloway, 1970

Ryan,

1961

fragments) Canaple and Smith, 1965; Battersby, 19’76

311

laide Orogen, and predate the tectonic development of the Thomson Orogen. According to Thomson (1970), the former noshes extension of the Adelaide Orogen was truncated along the Muloorinna Ridge by folding, granite intrusion, and thrusting of the basement to the south and southwest along steep reverse faults in the Late Proterozoic or Early Cambrian. North of the Muloorinna Ridge, the Warburton Basin was the central part of the CircumDenison Arc (Wopfner, 1972), which was the locus of maximum Middle Cambrian subsidence and which extended from the Officer Basin in the west to the Bancannia Trough in the east. Middle to Late Cambrian sequences of the Warburton Basin were presumably deposited on relatively shallow platforms or shelves close to the Precambrian craton. Agglomerates and tuffs interbedded with the carbonates may have been derived from a shoreline with active volcanoes to the south or west (Harrison and Higginbotham, 1966a). The Cambrian seas continued north to the Georgina Basin, where the sequence is thinner and lacks volcanics. The Delamerian Orogeny deformed the Adelaide and Kanmantoo Orogens and the Willyama Block in Late Cambrian and Early Ordovician time (Thomson, 1970). In the Warburton Basin, the only effect of the orogeny was the withdrawal of the sea at the end of the Cambrian. However, the subsequent Ordovician transgression was marked by a change of sediment type from a carbonate-dominated sequence to terrigenous elastics. Quartz sandstone was deposited adjacent to the basin margins, grading into interbedded quartz sandstone, siltstone and shale (e.g. in Pandieburra, Putamurdie and Dulling& wells) with increasing distance from the Precambria~l craton (Wopfner, 1972). Some of the Ordovician elastics show features typical of turbidite deposits. This change in sediment type probably reflects both a deepening of the depositional area and elevation of the Precambrian source region. Ordovician sediments appear to cover an extensive area in the southwestern part of the Thomson Orogen, since the basement rocks of several wells have been correlated with the known Ordovician because of lithological similarities. The Cambrian and Ordovician rocks of the southwest Thomson Orogen are steeply dipping to vertical and must be structurally.very complex. Folds and faults in the overlying Permian Cooper Basin sequence trend northeast. Seismic data suggest that some faults are reverse faults which may be related to basement thrusts (Stuart, 1976). Large-scale thrusting or overturning of the basement is indicated by repetition of Cambrian strata in Gidgealpa 1 (Harrison and Higginbotham, 1966a). It is probable that basement structures also trend northeast. The age of the folding and faulting cannot be determined precisely. Permian and probable Devonian sediments are unconformable on the Early Palaeozoic rocks. Wopfner (1972) considered that. the ccssation of Ordovician deposition was caused by the deformation, which was therefore of Late Ordovician age.

312

Northern

and Central

Thomson

Orogen

The age of deposition of sediments of the northern and central Thomson Orogen which form basement to the Great Artesian Basin is unknown. Most are terrigenous elastics which have been metamorphosed to slate, phyllite, quartzite and schist. Isotopic dates (of which there are very few) presumably record the age of deformation and metamorphism. The sediments may be Late Proterozoic or Early Palaeozoic; a Cambrian and Ordovician age is considered most probable. If they are Early Palaeozoic, they represent a different facies from that of their time equivalents in the Georgina Basin, and have been much more severely deformed. Whether the Early Palaeozoic sediments were deposited in an oceanic area east of the Prec~brian craton or on Proterozoic crust which is now deeply buried is uncertain. Seismic results reveal a substantial thickness (at least 10 000 m) of low-grade metamorphic rocks to the southeast of the Cork Fault (Harrison et al., in press). The oldest isotopic date was obtained from Fermoy 1, and is Late Cambrian or Early Ordovician. It suggests that tectonism of the same age as the Delamerian Orogeny may have occurred at least locally along the northwestern margin of the Thomson Orogen. Steeply dipping slate, phyllite and quartzite which may be the same age as the basement rocks in Fermoy 1 were intersected in several wells in the area east of the Cork Fault. The Middle Ordovician date of basalt in Gumbardo 1 may be the age of extrusion, since it was determined on pyroxene phenoerysts. The nearby Yongala 1 and Cothalow 1 wells also penetrated basement consisting of basaltic andesite, basalt and schist which are probably the same age as the basalt in Gumbardo 1. These three wells lie on the projected strike of the Anakie Metamorphics and contain similar rock types to those in the older unit of the metamorphics. Basement in Galway 1 is described as silty shale and also yielded a Middle Ordovician date. The similarity of these dates to those obtained from the metamorphosed Cape River Beds and mica schist of the Anakie Metamorphics suggests that mid-Ordovician tectonism affected an extensive area in the central and northern part of the Thomson Orogen. Structures in the basement rocks beneath the Adavale Basin are presumably similar to those of the Anakie ~e~mo~hics. Dates from granitic basement in the Cleeve, Etonvale and Balfour wells suggest an episode of intrusive activity in Late Silurian to Early Devonian time. The indicated ages are intermediate between those of the older and younger phases of the Ravenswood Granodiorite Complex. The date from Cleeve 1 is unreliable and may be considerably less than the true age (Webb et al., 1963). Thus the granite in Cleeve 1 may belong to the same intrusive epoch as the Late Ordovician granodiorite in the Telemon Anticline at the southern end of the Drummond Basin and the older phase of the Ravenswood Granodiorite Complex. The granitic rocks in Etonvale 1 and Balfour 1 could be correlated with either phase of the complex. The significance of the single Late Silurian to Early Devonian metamorphic age from phyll~te below strata of the Adavale Basin in Buckabie 1 is un-

313

known. gen.

No tectonism

Subsurface

of this age is recognized

elswhere in the Thomson

Oro-

structure

A northeast trend for the structural grain within the Thomson Orogen can be inferred from several features: (1) Most fold axes within the Permian and Mesozoic cover trend northeast, apparently reflecting basement structures. (2) Although the Devonian to Carboniferous Drummond Basin is a northtrending feature, fold axes in the southwestern part of the basin trend northeast, again probably reflecting basement structures. (3) The main elevated areas of basement beneath the Great Artesian Basin, the Nebine and Eulo Ridges, trend northeast. (4) In the Adavale Basin, the northeast trending Yaraka Shelf appears to have been an important depositional margin. (5) The dominant trend of gravity anomalies in the Thomson Orogen is northeast (Fig. 2; also see fig. 1 of Wellman, 1976). (6) Aeromagnetic anomalies trend northeast (see Harrison et al., in press). The dominant northeast trend within the Thomson Orogen is parallel to the Diamantina River Lineament, the probable margin of the Precambrian craton. The trend is discordant to trends of the adjoining Adelaide, Kanmantoo, Lachlan and New England Orogens and the Precambrian Georgetown and Mount Isa Inliers. In most areas the change in trend appears to be abrupt (e.g. along the Diamantina River Lineament, at the northern end of the Bancannia Trough, and beneath the Bowen Basin), suggesting that the Thomson Orogen is bounded by major fault zones. TRANSITIONAL

REGIME

OF THE THOMSON

OROGEN

The transitions tectonic regime of the Thomson Orogen is represented by three Devonian to Carboniferous basins, the Burdekin, Drummond and Adavale Basins, which overlie Early Palaeozoic metamorphics and intrusives of the orogen. Isolated occurrences of Devonian strata are known from several additional localities. Early to Middle Devonian volcanics and sediments crop out within the Anakie Inlier and in the core of the Nogoa Anticline at the southern margin of the Drummond Basin. A large area encomp~sing the Roma Shelf and the southern part of the Nebine Ridge is underlain by poorly known low-grade Devonian metasediments referred to as the “Timbury Hills Formation”. Thick red-bed sequences of probable Devonian age have been encountered in some wells beneath the Cooper Basin; these rocks may be related to Devonian sediments of the Adavale Basin and/or the Bancannia Trough. Burdekin Basin The Burdekin Basin formed as a shallow but subsiding basement of Early Palaeozoic intrusives and metamorphics

marine basin on a of the Lolworth-

314

Ravenswood Block when the sea transgressed southwards from the Broken River Province in Middle Devonian time. Its subsequent depositions history was similar to that of the Bundock and Clarke River Basins to the west (see Fig. 3). Fluctuating shallow marine to continental sedimentation continued into the Early Carboniferous (Wyatt and Jell, 1967). Periodic mild tectonism during deposition accompanied the extrusion of calcalkaline lavas and generated growth faults which caused abrupt changes of facies and thickness (Heidecker, 1974). The basin was uplifted and gently warped in the mid-Carboniferous, so that the succeeding Late Carboniferous calcalkaline volcanics and terrestrial sediments overlie the older rocks unconformably or disconformably. Following deposition of the Late Carboniferous strata, the rocks of the Burdekin Basin were folded into irregular open basins and domes whose axial trends were controlled by the proximity of the Early Palaeozoic basement or by basement faults. Dips are shallow to moderate. Two prominent sets of faults strike northwest to west and southwest; the former set shows considerable strike-slip movement, whereas the latter are mainly normal faults (Wyatt, 1968; Wyatt et al., 1970). Intrusion of plutons of Carboniferous to Permian granite and adamellite in the Burdekin Basin and surrounding areas completed the cratonization of the Lolworth-Ravenswood Block. Most of the granites were comagmatic with the Late Carboniferous volcanics. Anakie Inlier The northern part of the Anakie Met~o~hics consists of weakly metamorphosed quartz-rich elastic sediments. Schistosity appears to be parallel to bedding, fold axes trend north, and dips are steep (Malone et al., 1964). The presence of crenulation cleavage in some rocks suggests more than one generation of folding. These rocks can be traced northwards with no apparent break into sediments of the Ukalunda Beds (Malone et al., 1966) which contain limestones with Early to Middle Devonian corals (Jell and Hill, 1969). The Ukalunda Beds and Late Carboniferous granites constitute the northern end of the Anakie Inlier. Several small outcrops of Early to Middle Devonian volcanics and sediments occur along the eastern side of the Anakie Inlier (Veevers et al., 1964a, b) and in the core of the Nogoa Anticline at the southern end of the Drummond Basin (Fordham, 1976). Andesitic volcanics are dominant, with minor rhyolite, shale and limestone lenses. These rocks appear to overlie the older unit of the Anakie Metamorphics unconformably. The restriction of Early to Middle Devonian outcrops to the eastern margin of the Anakie Inlier suggests that the inlier was emergent at this time, and that the Devonian rocks were deposited close to the shoreline. The small size and poor exposure of most outcrops prevents determination of structure, except in the Nogoa Anticline, where andesitic volcanics strike northeast and dip almost vertically (Fordham, 1976).

315

The Early to Middle Devonian rocks of the Anakie Inlier were deformed in the late Middle Devonian and are unconformably overlain by Late Devonian strata of the Drummond Basin (Malone, 1967; Olgers, 1972; Fordham, 1976). This orogeny also affected most of the northern part of the New England Orogen (Day et al., 1978), but no folding occurred in the Adavale Basin to the southwest. The Anakie Inlier probably developed as a distinct northtrending tectonic unit at this time since it controlled stream directions in the Late Devonian to Early Carboniferous Drummond Basin. Drummond

Basin

The Drummond Basin contains Late Devonian to Early Carboniferous strata which were deposited mainly west but also east of the Anakie Inlier on a basement of Early Palaeozoic metamorphics and granite. The western, southern and eastern limits of the basin are concealed beneath Permian and Triassic sediments of the Galilee and Bowen Basins. Vine (1972) considered that evidence from petroleum exploration wells suggests that the Drummond Basin did not extend west or southwest of the existing outcrops, and that its western margin coincided with the fault-generated Belyando Feature (Fig. 1). However, the fluviatile elastic sediments penetrated at the bottom of the Lake Galilee 1 well west of the Belyando Feature are more typical of the Drummond Basin sequence than of deposits of the Adavale Basin; with which they were correlated by Vme. Palynological studies on poorly preserved spores could not define their age more precisely than late Middle Devonian to Early Carboniferous (Pemberton, 1965). Interpretations of seismic traverses show Drummond Basin strata thinning to the west and pinching out 20 km west of the Belyando Feature (Harrison et al., 1975). However, these interpretations do not include the basal sediments of the Lake Galilee 1 well as part of the Drummond Basin sequence. Sedimentation in the Drummond Basin was continental, except for a brief marine incursion in the north in Late Devonian time. Fluviatile elastics were deposited in a narrow, subsiding basin west of the emergent Anakie Inlier by a north-flowing river system. Olgers (1972) recognized three distinct cycles of sedimentation separated by minor epeirogenic events associated with volcanism . Folding by compression from the east in the mid-Carboniferous produced open folds. The overall orientation of fold axes is north to north-northwest, parallel to the boundary with the Anakie Inlier, but variations from this trend are common, particularly in the southern and northern parts of the basin. Anticlinal crests are cut by normal or high-angle reverse faults with a zone of thrusting immediately west of the inlier. Post-erogenic Late Carboniferous granitic plutons were emplaced in the area between the LolworthRavenswood Block and the Anakie Inlier, accompanied by extrusion of genetically related acid volcanics.

316

Adavale Basin Terrestrial and marine sediments and volcanics of Early Devonian to (?) Early Carboniferous age were deposited in the Adavale Basin on a basement, of Ordovician schist and basalt, Silurian granite, and Siluro-Devonian metamorphics (Heikkila, 1965). Throughout the history of the basin, shorelines and facies boundaries appear to have been arcuate in shape, trending north to northeast approximately parallel to the Canaway Fault and the Yaraka Shelf (Auchincloss, 1976). However, the arcuate shape may be in part a result of later tectonism (R.J. Paten, personal communication, 1977). The northeast-trending Yaraka Shelf, which forms the northwest limit of the basin (Fig. l), was a depositional margin (Heikkila, 1965). In the Early Devonian, continental andesitic volcanics were erupted in the west, and possibly marine arkosic sandstone and conglomerate accumulated in the east. The marine transgression progressed west with time, and at the when marine conditions were most beginning of the Middle Devonian, widespread, elastics were deposited near the shoreline and carbonates in deeper water to the east (Tanner, 1967; Auchincloss, 1976). The subsequent late Middle Devonian and Late Devonian regression was marked by red-bed deposition with evaporites in the eastern part of the basin. The upper age limit of these red-beds is uncertain. Heikkila (1965) and Auchincloss (1976) suggested that they may include rocks as young as Early Carboniferous. However, Vine (1972) considered it possible that deposition was confined to the early part of the Late Devonian, .and that the entire Adavale Basin sequence is older than the basal units of the Drummond Basin. The presence of a late Middle Devonian unconformity beneath strata of the Drummond Basin lends support to this view. Minor tectonism occurred during deposition, but the major deformation was of mid-Carboniferous age, and produced northeast-trending normal faults and open folds. Their orientation was probably controlled by preexisting basement faults (Auchincloss, 1976). “Tim bury Hills Formation

”

Numerous petroleum exploration wells with targets in the Mesozoic and Permian strata of the Great Artesian and Bowen Basins have penetrated lowgrade metasediments of the southeast part of the Thomson Orogen. Little attention has been paid to these basement rocks, which cover a large area south of the Drummond Basin including the Roma Shelf (Fig. 1). They have been named the “Timbury Hills Formation”, and consist of indurated, steeply dipping shale, siliceous siltstone and quartzose sandstone. Commonly the rocks are sheared and have a phyllitic appearance (Traves, 1965). Devonian plants have been found in one core, and Late Devonian rocks intersected in Purbrook 1 well at the southeast margin of the Drummond Basin are very similar to metasediments of the “Timbury Hills Formation” (Traves,

317

1965; Day, 1976). However, it is by no means certain that all rocks referred to the unit are of Devonian age; some may be Ordovician metamorphics of the erogenic regime of the Thomson Orogen. On the Roma Shelf, the “Timbury Hills Formation” is intruded by adamellite plutons which have given Carboniferous radiometric ages ranging from 298 to 348 m.y. (Webb et al., 1963; Harding, 1969). These dates must be regarded as minimum ages because they were obtained from weathered material and all the plutons appear to be part of a single batholith (Houston, 1964). The true age of the adamellite may be close to 360 m.y. (Devono-Carboniferous), which is the date obtained from granodiorite at the southern end of the Anakie Inlier (359 f 2 m.y.; Webb and McDougall, 1968) and granite intruding metamorphic basement rocks of the Eulo Shelf (363. m.y.; Evernden and Richards, 1962). Descriptions of the adamellite plutons and their thermally metamorphosed contact rocks (Houston, 1964) suggest that they are post-erogenic. If so, folding of at least part of the “Timbury Hills Formation” predates the mid-Carboniferous deformation of the Drummond and Adavale Basins. It may have been synchronous with the late Middle Devonian orogeny which affected the northern part of the New England Orogen and the Early to Middle Devonian rocks of the Anakie Inlier. Thus there could be an unconformity within the “Timbury Hills Formation” between pre-late Middle Devonian rocks and possible equivalents of the Late Devonian sediments and volcanics of the Drummond Basin. Until the structure, lithology and metamorphic grade of the “Timbury Hills Formation” are better known, precise correlation of this unit with other tectonic elements of the Thomson Orogen is not possible. South west Thomson

Orogen

Thick red-bed sequences which unconformably underlie Permian sediments of the Cooper Basin were penetrated in the Innamincka and Merrimelia wells (Ryan, 1961; Canaple and Smith, 1965; Battersby, 1976). They have been correlated with the uppermost unit of the Adavale Basin and Late Devonian fluviatile sediments of the Bancannia Trough, and assigned a Late Devonian to possibly Early C~boniferous age. Meagre p~aeontolo~c~ evidence from Innamincka 1 well supports a Devonian age (Ryan, 1961). These isolated occurrences are presumably remnants of a formerly extensive sheet of continental sediments which covered much of the central and southwest parts of the Thomson Orogen. The red-beds were folded along northeast axes in mid-Carboniferous time, and now dip at moderate angles. The age of deformation is indicated by an overprinted isotopic date of 330 m.y. obtained from volcanics underlying Cambrian carbonates in Kalladeina 1 well (Wopfner, 1972).

318 TEeTONIC

HISTORY

Precrutonic (erogenic)

OF THE THOMSON OROGEN

regime

The tectonic framework of the Thomson Orogen developed in Early Cambrian time, when the former northern extension of the Adelaide Orogen was truncated along the Muloorinna Ridge. The initial sediments of the orogen were deposited in a deep marine basin bordered to the west, southwest and northwest by Precambrian crustal blocks. Cambrian and possible Cambrian rocks are known from three separate areas along the western margin of the orogen, each of which exhibits markedly different litholo~es and deposi~on~ environment. In the southwest (the W~bu~on Basin), Middle and Late Cambrian carbonates interbedded with detritus derived from an active volcanic arc accumulated on shelf areas built of Proterozoic volcanics. Further north, near Fermoy 1 well east of the Diamantina River Lineament (Fig. l), a thick sequence of dominantly finegrained elastics was presumably derived from the Precambrian craton to the west. Close to the lineament, these sediments may have been laid down on a downfaulted basement of deformed Proterozoic rocks, with an oceanic area to the east. Late Cambrian calcalkaline volcanics of the Lolworth-Ravenswood Block at the northern end of the Thomson Orogen probably were deposited in an island arc separated from the Precambrian Georgetown Inlier by a marginal sea. Isotopic dating of low-grade me~morphics from Fermoy 1 suggests that these rocks were deformed in latest Cambrian time. Folding of this age is not recognized elsewhere in the Thomson Orogen, although regression at the end of the Cambrian in the Warburton Basin was followed by a pronounced change from shallow-water carbonate deposition to deep-water elastic sedimentation reflecting downwarping of the basin and uplift of the basin margins. These movements were approximately synchronous with the Delamerian Orogeny of the Adelaide and Kanmantoo Orogens. Ordovician deposition was widespread in the Thomson Orogen. Quartzose turbidite sequences and black graptolitic shales of Early to early Late Ordovician age overlie Cambrian carbonates of the Warburton Basin. Metasediments and me~volcanics of possible Ordovician age occur in the Anakie Inlier and as basement to the Adavale Basin. The association of basic metavolcanics with serpentinite lenses in the Anakie Inlier suggests that deposition took place on oceanic crust. In the Lolworth-Ravenswood Sock, Early Ordovician tuffaceous sediments overlie the Late Cambrian volcanics. The Cambrian and Ordovician sediments and volcanics and small areas of possible Precambrian metamorphics were deformed in mid- to Late Ordovician time in an orogeny which represented the climax of the precratonic or erogenic regime of the Thomson Orogen. This tectonism was probably synchronous with folding of the quartzose f’lysch of the southwestern part of the Broken River Province (Arnold and Henderson, 1976; see Day et al.,

319

1978). Isotopic dates from metamorphics of the Adavale Basin basement, Anakie Inlier, and Lolworth-Ravenswood Block and synorogenic granites of the Lolworth-Ravenswood Block range from 454 to 483 m.y. On the basis of observed structures in the Anakie Inlier and correlation with basementrelated structures in strata overlying Early Palaeozoic rocks of the Thomson Orogen, it is suggested that the domin~t northeast trend of the orogen was first imposed by this mid- to Late Ordovician tectonism. The only discordant area is the Lolworth-Ravenswood Block, characterized by an overall eastwest trend. The cause of this anomalous trend is unknown. Virtually no detailed structural studies have been carried out, even in the Anakie Inlier and Lolworth-Ravenswood Block. However, more than one generation of folding is recognized in the metamorphic rocks, and their st~ctural history appears to be complex. This also seems to be true of basement rocks penetrated by petroleum exploration wells. A prominent slaty cleavage has been developed in all argillaceous sediments, and large-scale thrusting or overturning is required to explain the sequence of trilobite faunas in Gidgealpa 1. Met~orphism was mainly to the greenschist facies, with local development of ~phibolite facies. An upper limit for the age of deformation is given by Late Ordovician sediments of the Anakie Inlier, which are probably unconformable on the complexly deformed Anakie Metamorphics. The available data therefore suggest that the orogeny was mid-Ordovician. Local folding of this age is recognized in the Lachlan Orogen (Crook and Powell, 1976), but tectonism was earlier than the widespread Late Ordovician-Early Silurian Benambran Oro-

geny. There is no evidence of Silurian deposition in the Thomson Orogen. In this respect, the Thomson Orogen differs markedly from the Lachlan Orogen, where several troughs and volcanic rifts developed during the Middle and Late Silurian. The precratonic regime of the Thomson Orogen is considered to have ended with emplacement of post-erogenic Siluro-Devonian batholiths in the northern part.

Transitional regime The sea transgressed over the Thomson Orogen from the east in Early and Middle Devonian time. The maximum extent of the transgression was reached in the early Middle Devonian, when the shoreline was located along the eastern side of the Anakie Inlier and continued to the southwest close to the northwest margin of the Adavale Basin. Deposition of elastic sediments and limestone was accompanied by eruption of andesitic volcanics along a no~heast-trending continental margin. At least some of the dominantly fine-grained elastics of the “Timbury Hills Formation” may have been laid down at this time in deeper water to the southeast of the Adavale Basin. The Burdekin Basin was separated from the depositional area to the south by the slightly emergent southern Lolworth-Ravenswood Block, and marine sedi-

320

mentation did not commence until late Middle Devonian time, when the sea trangressed southwards from the Broken River Province. The Early to Middle Devonian strata of the Anakie Inlier, and possibly the “Timbury Hills Formation”, were folded in the late Middle Devonian. Folding was dominantly along north-trending axes, and the Anakie Inlier was probably formed as a north-northwest-orientated structural unit at this time. Tectonism of this age is also recognized in the northern part of the New England Orogen and has been correlated with the Tabberabberan Orogeny of the Lachlan Orogen. No folding occurred in the Adavale or Burdekin Basins, but uplift caused the withdrawal of the sea at the beginning of the Late Devonian. Post-tectonic granites were emplaced in the southern part of the Anakie Inlier, the Roma Shelf, and the Eulo Ridge. Late Devonian red-beds and similar continental sediments were deposited over much of the Thomson Orogen. The thickest deposits accumulated in the Drummond Basin, where sedimentation and sporadic volcanism continued into the Early Carboniferous. Volcanics were also erupted in the Burdekin Basin, which was the site of fluctuating continental to marine conditions from Late Devonian to Early Carboniferous time. Part of the “Timbury Hills Formation” may be of Late Devonian age. The uppermost unit of the Adavale Basin is a thick red-bed sequence, remnants of which occur to the west. Sedimentation may have been confined to the Late Devonian, or have continued into the Early Carboniferous. Widespread fold movements affected virtually the whole of the Thomson Orogen in the mid-Carboniferous. In most areas, open folds and faults strike northeast, probably reflecting the dominant structural trend imposed during the mid-Ordovician orogeny. The north--south orientation of the structures in the Drummond Basin is attributed to the influence of the north-northwest trending Anakie Inlier. Variable trends within the Burdekin Basin were controlled by basement topography and faults. This mid-Carboniferous folding, which ended the transitional tectonic regime of the Thomson Orogen, was of similar age and style to the Kanimblan Orogeny of the Lachlan Orogen and also of similar age to the Alice Springs Orogeny of the Amadeus Basin. Posttectonic Late Carboniferous granites and associated continental calcalkaline volcanics were emplaced in the northeastern part of the Thomson Orogen. Subsequently, the Thomson Orogen has behaved as a stable cratonic region. Broad downwarping initiated deposition in the Permian and Triassic Bowen, Galilee and Cooper Basins in Permo-Carboniferous time, and most of the area was covered by the essentially undeformed Jurassic and Cretaceous sediments of the Great Artesian Basin. CONCLUSIONS

The Thomson Orogen forms the northwestern part of the Tasman Orogenie Zone. It developed at the end of the Early Cambrian to the east of the faulted margin of the Proterozoic craton, and was the site of thick sedimen-

321

tation, orogenesis and plutonism until Carboniferous time. Most of the orogen is now concealed beneath Permian and Mesozoic platform cover. The evolution of the Thomson Orogen spans the same time range as that of the combined Kanmantoo and Lachlan Orogens to the south. However, there are significant differences between the three orogens. Chief among these are: (1) The northeast structural grain of the Thomson Orogen is sharply discordant to trends within the Kanmantoo and Lachlan Orogens, and also to trends within the Georgetown Inlier, Mount Isa Inlier, Adelaide Orogen and New England Orogen. (2) The major mid-Ordovician deformation in the Thomson Orogen was intermediate in age between the Delamerian and Benambran Orogenies of the Kanmantoo and Lachlan Orogens, respectively. (3) In the Lachlan Orogen, several volcanic troughs or rifts developed during the Middle and Late Silurian, but this pattern is not typical of the Thomson Orogen, where no Silurian deposits have yet been recognized. The transitional tectonic history (Devonian-Carboniferous) of the Thomson Orogen is similar to that of the Lachlan Orogen, with widespread deposition of red-beds which were folded by a terminal orogeny in the mid-carboniferous. ACKNOWLEDGEMENTS

We are grateful to Dr. R.A. Henderson (James Cook University of North Queensland) for providing a preliminary copy of his paper and for criticism of the manuscript, and to Mr. R.J. Paten (Mines Administration Pty. Ltd.) for discussions and criticism. This paper would not have been written without the encouragement of Dr. E. Scheibner (Geological Survey of New South Wales). Gravity data shown in Fig. 2 is reproduced with the approval of the Director, Bureau of Mineral Resources, Geology and Geophysics. C.G.M. publishes with the permission of the Under Secretary, Department of Mines, Queensland. REFERENCES Anderson, J.C. and Palmieri, V., 1977. The Fork Lagoons Beds, an Ordovician unit of the Anakie Inlier, central Queensland. Queensl. Gov. Min. J., 78: 260-263. Arnold, G.O. and Henderson, R.A., 1976. Lower Palaeozoic history of the southwestern Broken River Province, north Queensland J. Geol. Sot. Aust., 23: 73-93. Auchincloss, G., 1976. Adavale Basin. In: R.B. Leslie, H.J. Evans and C.L. Knight (Edi. tors), Economic Geology of Australia and Papua New Guinea, 3, Petroleum. Monogr. Australas. Inst. Min. Metall., 7: 309-315. Baker, E.M., 1974. Geology of the Cape River-Gorge Creek area. Unpubl. Hons Thesis Dep. Geol., James Cook Univ. Battersby, D.G., 1976. Cooper Basin Gas and Oil Fields. In: R.B. Leslie, H.J. Evans and C.L. Knight (Editors), Economic Geology of Australia and Papua New Guinea, 3 Petroleum. Monogr. Australas. Inst. Min. Metall., 7: 321-368.

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