Relationships between deformation and basin evolution in the intracratonic Amadeus Basin, central Australia

Elsevier

Science Publishers

B.V., Amsterdam

- Printed

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Relationships between Reformation and basin evolution in the intracratonic Amadeus Basin, central Australia R.J. KORSCH and J.F. LINDSAY Didion

of Continenral Geology, Bureau of Mineral Resaurces, G.P. 0. Box 378, Canberra, A. C. T. 2601 (Australia)

(Received

February

2,1987;

revised and accepted

August

28,1987)

Abstract Korsch,

RJ. Lindsay,

Basin, central Tectonic

subsidence

deformational during

J.F., 1989. Relationships

Australia.

at about

(Stage 2), and was shortened

volcanics

previously

northern

margin

Extension time a major

the sediment

Petermann

Ranges

Formation,

northward

During formed basin

Stage 3 a foreland

in the northern margins

structural

Orogeny,

rapidly

towards

the presence

only in the northern the northern of a major

of the sediments

with southward-directed

Horizontal

shortening

than depositional.

above

pile, and formation

bounding the salt

Arunta during

Complex

the southern

at the same

margin.

In the

and then thins dramatically fault. horizon

In the southwest, in the

of major basement-cored

Bitter

during

at the

Springs

nappes.

sheets of the Alice Springs

that developed

and

on the

this event.

part of the basin;

was of the order of 50-100

The structures

period

with rift basin sediments

along

margin

overthrust

developed

to another

event on the southwest

In the metamorphic

was occurring

and to indicate

(Stage 3).

to have been emplaced

Ranges

of the sediment

basin associated rather

succession.

Amadeus

The basin

(Stage l), was subjected

Carboniferous

is considered

and

detachment

folding

part of the basin.

are structural

pattern

caused

transport,

structure

Australia.

of the basin was associated

with Stage 2 occurred

event, the Petermann

Orogeny

comers

dyke swarm

associated

a half-graben

Basin in central

time when there was also a shortening

to the Amadeus

fill at this time firstly thickens

implying

are used here to study basin evolution

Amadeus

the Late Devonian-Early

and northwest

as basement

direction

compressional

the margin,

during

of the basin, a dolerite

in a N-S

boundary

in the intracratonic

TectQnophys~cs, 758: 5-22.

Rocks.

at about 900 Ma in the Late Proterozoic

in the southwest regarded

and basin evolution

of CrustaI

record

of the intracratonic

Proterozoic-Cambrian

Stage 1 extension

north,

in the history

an initial period of extension

margin

deformation

Deformation

curves based on the sedimentary

periods

of extension

between

In: A. Ord (Editor),

Orogeny

km and hence the present this stage dominate

the

of the basin.

Introduction The development of the concepts of sequence stratigraphy, which grew out of seismic stratigraphy (Vail et al., 1977a, 1977b), has shown that many unconformities are not related to deformational events but are actually related to eustatic sea level changes or to basin dynamics. Also the sedimentary record itself is an extremely sensitive

indicator of deformational events, and can be used to determine basin history. The Amadeus Basin in central Australia (Fig. 1) is a large (800 km x 300 km) intracratonic basin that developed on a time span of over 600 Ma from the Late Proterozoic to the Devonian (Fig. 2). It has a complicated stratigraphy and its development is linked to several major episodes of crustal deformation. In this paper we will examine

6

r

21 3

n

i @wk NT_.J._____.____I_____-~SA

_L.--__-_-__l_.-_____---

1

1 UNTIE

PO” *“g”Sfa

Fig. 1. Map showing location of the Amadeus Basin in Central Australia and the main morphological features such as the northern sub-basins and troughs, the western and southern platforms and the northernmost limit of the Central Ridge. Basement surrounding the basin is conveniently divided into the Arunta Block to the north and the Musgrave Block, including the Petermann Ranges to the southwest.

the evolution of the Amadeus Basin in terms of thermomechanical models of basin evolution, and show how this evolutionary history can be used to predict deformational events which have contributed to basin development.

dence associated with the cooling phase can be offset from the rift phase. During the rift phase the upper crust behaves in a brittle manner, below which there is ductile extension, and the two mechanisms are separated

by a detachment

of extension

zone (Coward,

Basin models

1986). During the following thermal recovery phase, subsidence proceeds essentially unaccom-

There are currently two main models for the thermomechanical evolution of sedimentary basins, involving fundamentally different mechanisms (Figs. 3 and 4): (1) Extension (e.g. by thinning of the crust or by dyke intrusion) followed by thermal relaxation (cooling and loading) (e.g. Sleep, 1971; McKenzie, 1978; Royden et al., 1980; Dewey, 1982; Neugebauer, 1987). This mechanism often leads to a symmetrical cross section shape to the basin (De-

panied by faulting. Structures produced during the rift phase become buried during thermal subsidence (unless the stretching of the lithosphere below the upper crust is sufficiently displaced that thermal subsidence occurs beside the rift, rather above it-see Wernicke, 1981, 1985) and can only be examined by seismic reflection profiles or where exhumed by later tectonic inversion and erosion. (2) Subsidence induced by advancing thrust sheets which load the crust (Fig. 4) (e.g. Price, 1973; Beaumont, 1981; Jordan, 1981; Quinlan and Beaumont, 1984). The thrust sheets can overrun the edge of the basin, causing deformation in the

wey, 1982) in continental interiors, but at a continental margin only half the basin is preserved. If asymmetrical extension occurs (Fig. 3), the subsi-

7

FORMATION

EVENT

AGE NORTH

CENTRAL

iOUTHWEST

Brewer

EA.% AllW

Conglomerate

Springs LATE Hermannsburg

DEVONIAN

Parke

Sandstone

SlItstone PertnJara

EARLY DEVONIAN

SILURIAN Rodingan CarmIchael

Sandstone

LATE 3RDOVICIAN

SlItstone

Stokes

Sandstone

sta1rwav EARLY 3RDOVICIAN

Horn

Valley

Pacoota

Slitstone

Sandstone

-

Formation

Govder

p

LATE Peterm.jnn

Sst

Deception

Fm

Jav Cleland Sandstone

f :

lllara

Sst

Namatjlra

Ft.

Glles Creek Dolomite

Fm

Chandler EARLY

Shannon

Lst

Hugh Rtver Shale

MIDDLE Tempe

Creek

FormatIon Todd River Dolomltc

Fm

vlount Curr1e ‘onglomerate

Arumbera

Sandstone Petermanr

-

Ranges Maurice

Julie

FormatIon W~nnall

Sir Frederick Conylomer
h

Beds Pertatataka

Plotleer Olymptc Carnegw Et Board Formation

FormatIon

Format~or~

Silnd5tonc, Furmdt~on

Souths Range

Aralkd

Formdtlon i

Areyonga

FormatIon

--

Areyonga

Mt Hams Basalt Bloods Range beds

Fig. 2. Stratigraphic

terminology

in the Amadeus

sediments. Classical Jura (foreland) folds (e.g. Laubscher, 1978) may develop above a dtcollement between the crystalline basement and the

Stuart

Dyke

Swarm

Basin (After Wells et al., 1970 and Kennard

et al., 1986)

sedimentary sequence. Thus structures associated with this type of basin are major basement-related thrusts that probably extend to deep in the crust,

8

LITHOSPHEAIC

and decollement-related in the basin sequence.

STRETCHING

thrusts and foreland folds

The two types of basins conveniently basins

can be referred

to

as stretching basins and shortening

respectively

(Dewey,

1982).

Other

subsi-

dence mechanisms such as meteorite impact, salt solution and salt migration (Dewey, 1982) have all

(b) Thermal relaxation stage

operated in the Amadeus Basin but are of local significance Fig. 3. Development of a sedimentary basin due to lithospheric stretching, showing results of the rift phase followed by the thermal recovery phase. On the left is the McKenzie (1978) model which involves symmetrical extension, occurring presumably on opposed bounding faults in the upper crust. However, movement on these faults cannot occur simultaneously (Gibbs, 1984) and most rift basins are in fact asymmetric half-grabens (Bally, 1982) with a single bounding fault at depth. On the right is the model for asymmetrical extension based on Wernicke (1981) and Wernicke and Burchfiel (1982). Note in this case the locus of subsidence for the thermal relaxation phase is markedly offset from that of the rift phase. As with thrust fault geometry, a wide variety in a geometry of extensional faults and adjacent blocks can occur leading to wide variations in basin morphology (see examples in Wernicke, 1981; Wernicke and Burchfiel, 1982; Gibbs, 1984; Etheridge et al., 1984; Lister et al., 1986). Load refers to sediment loading of the crust during deposition.

ADVANCING THRUST SHEETS

_

SHORTENING BASIN

PERIPHERAL BULGE

only and will not be discussed

in

detail. Thermomechanical models have been developed for rifted, Atlantic-type passive margins and foreland basins and in simple terms can adequately account for the evolution of these types of basins (e.g. Beaumont et al., 1982; Quinlan and Beaumont, 1984; Chadwick, 1985). However, broad, shallow intracratonic basins have not been as amenable to modelling, and in some aspects, still remain a problem. Ollier (1985) showed that most continents topographically have mid-continental sags, and these are the sites of some present-day intracontinental basins. Dewey and Pitman (1982) considered that intracratonic basins were complex with neither a simple shortening

nor stretching

origin.

Dewey

(1982) proposed a mechanism of multiple stepping-out of rifts that, when followed by thermal recovery, would produce a large intracratonic sag. In a model developed specifically for the intracratonic basins of central Australia, Lambeck (1983, 1984) proposed that the basins formed by buckling of the crust by long lasting horizontal compression. His model predicts that sedimentation rates (that is, subsidence) must increase with time. This is not consistent with the patterns of

LOiD

the raw and tectonic subsidence curves (see below, and Lindsay and Korsch, in press) and thus we consider Fig. 4. Formation of a sedimentary basin due to shortening of the crust (after Beaumont, 1981; Quinlan and Beaumont, 1984). Advancing thrust sheets form a load which causes downward

that

the

mechanism

proposed

by

Lambeck is unlikely to have produced the intracratonic basins of central Australia (including the Amadeus Basin).

flexuring on the crust to form the basin; compensating flexuring forms the peripheral bulge that also acts as a barrier to sedimentation. The thrust sheets, by thickening the crust and

Tectonic

elevating the topography, provide an adjacent source of debris.

The subsidence history of sedimentary basins can be examined by constructing raw subsidence curves, that is, by graphically comparing the cumulative thickness of sediment against time.

As the thrust sheets advance, proximal in character,

the sediments become more

both vertically and also laterally to-

wards the sheets, producing coarsening-upwards sequences that often end in very coarse boulder conglomerates.

subsidence

curves

and basin type

9

processes such as crustal thinning, thermal cooling

deformational events that should be associated with basins that developed by a particular thermo-

and loading.

of

mechanical mechanism.

of

For stretching basins, McKenzie (1978) introduced the concept of p as a measure of the

Subsidence patterns result from a combination

sediment

In order to remove the effects

loading

to determine

the amount

of

tectonic subsidence the raw subsidence curves can be backstripped. This technique was developed by

amount of extension that a basin has undergone.

Sleep (1971)

When p = 1 there is no extension, and when p =

and Watts

and Ryan

(1976)

and

described in detail by Steckler and Watts (1978),

infinity the crust has been thinned to zero. The

Sclater

and Christie

and Ungerer et al.,

normal ranges are p = 1 to about 6. For a crustal

(1984).

Essentially

it involves an iterative process

thickness of 31.2 km and lithosphere thickness of

(1980)

of progressively removing the effects of sediment

125 km (McKenzie,

compaction,

advanced when p = 4.43 that oceanic crust forms.

palaeobathymetry

and eustatic

sea

1978)

rifting is sufficiently

level changes (see Ungerer et al., 1984, fig. 2j. A

Values of p can be determined

similar technique, termed geohistory analysis, was

techniques

described

applicable here is to compare tectonic subsidence

by Van Hinte (1978)

and Falvey and

(see Dewey,

1982)

by a variety of but one method

De&ton (1982). One benefit of tectonic subsidence curves is that they provide an indication of the origin of the basin even if the shape of the basin has changed due to subsequent deformation, and therefore knowledge of the original shape of the basin is not needed to interpret basin evolu-

curves generated for stratigraphic successions with theoretical curves. Using this approach, we find that for stage 1 in the Amadeus Basin p is less than 2 and for stage 2, /3= 1.2 to 1.5.

tion through subsidence curves. Stretching basins and shortening

(Fig. l), namely sub-basins where the sediment pile is thickest, interconnecting troughs between

basins have

The Amadeus Basin has been subdivided into three main morphological depositional features

distinctive subsidence curves (Fig. 5) that are use-

the sub-basins, and platform areas where the sedi-

ful in understanding

ment pile is thinnest

the evolution

of complex

(Lindsay

and Korsch,

press). To illustrate

as a predictive

representative of the three morphological regions have been selected for detailed study (Fig. 6a): (1)

tool to infer the nature of the

basin evolution

in

sedimentary basins. These curves can thus be used

three areas

the Carmichael sub-basin as typical of the subbasins, (2) the Missionary Plains Trough as representative of the interconnecting trough areas between sub-basins, and (3) the Erldunda #l well (25.3” S, 133.2” E) as typical of the platform areas where basin fill is exceptionally thin. In the case of the Carmichael sub-basin and Missionary Plains Trough the curves are based on seismic data (Lindsay and Korsch, in prep.). The evaporites of the Bitter Springs Formation have been deforming since shortly after their deposition thus affecting Fig. 5. Theoretical subsidence curves for a stretching basin (a) and a shortening basin (b). In a stretching basin, the initial rift (stretching)

phase

results

in rapid subsidence

and is then

followed by subsidence due to thermal recovery which decays following an exponential

curve over a period of the order of

150 Ma. For the shortening basin, subsidence would increase as the thrust sheets approach. After cessation of thrusting, the sediment loading effect would produce further subsidence, but this would not be sustained for a substantial period because of the absence of any thermal recovery.

the deposition of subsequent formations (Lindsay, 1987a). Consequently it is difficult to select a single stratigraphic succession that can be regarded as representative. It is possible to overcome some of these problems by using seismic data to derive average sediment thicknesses across the area of the whole morphological feature. In the case of the platform areas where seismic data are sparse and difficult to interpret, well data

10

lb)

(a) Time(m.y.) 800

.

ri

0.

stage

0

(a) and tectonic

Korsch,

in press).

A-platforms,

whereas

stage 3 is a shortening a subsidence

evaporites

(squares)

300

900

I

Jl2

.

subsidence

turbidites

(T).

The

times

of the abscissa

(Fig. 2) with the numerical

were used to determine the thickness of deeper units. Raw subsidence curves (Fig. 6a) and tectonic subsidence curves (Fig. 6b) imply that the Amadeus Basin developed in three distinct stages, Stage 1 being a stretching basin, Stage 2 also being basin (but with a shortening

basin

developing simultaneously in the southwestern part of the basin-see below), and Stage 3 being a shortening basin. The stratigraphic distributions of these stages are shown in Fig. 2. Crustal stage

deformation

curves

in the Amadeus

B and

Basin

during

1

The base of the Amadeus Basin is traditionally taken as the base of the Heavitree Quartzite (e.g. Wells et al., (1970) but for the purposes of their basin model Lindsay and Korsch (in prep.) extended the basin to include the Mount Harris Basalt, Bloods Range beds and Dixon Range beds in the southwest (Wells et al., 1964; Forman, 1966) and an unnamed succession of sediments and bimodal felsic and mafic volcanics in the

were

Tlme(m.y.) BOO

IT

500

basin, sediments by a shortening determined

300

I

units of the Amadeus C, stages

time scale of Harland

400

1

area. Below each graph the symbols indicate

(Erldunda #l) were used to determine sediment thickness to the top of the Gillen Member of the Bitter Springs Formation whereas seismic data

a stretching

For

part of the basin, stage 2 is represented

formations

0.

(b) curves for the morphological

C-sub-basins.

700

stage

basin. For curve A, stage 1 is a stretching

curve for this geographic and

800

stage 3

B-trough,

basin. In the southwestern

construct

400

stage 2

1

Fig. 6. Raw subsidence

shortening

500

Basin (after

1 and 2 represent of stage 2 are absent,

Lindsay

stretching

and stage 3 is a

basin, but there are insufficient the timing of volcanic

by correlating

the ages

and

basins, data to

rocks (triangles), of the geological

et al. (1982).

northwest (Wells et al., 1965; Mutton et al., 1983). For the purposes of this paper, we refer to the base of the basin in the sense of Lindsay and Korsch (in press) Several lines of evidence suggest that the basin was initiated in the Late Proterozoic by a phase of crustal extension. In the southwest the association of fluvial elastic sediments (mainly cross-bedded sandstones and conglomerates) and basalt of the Dixon Range beds and Mount Harris Basalt suggests comparisons with modern rift settings. In the northwest, the Heavitree Quartzite is unconformable on an unnamed succession of basalt and felsic volcanics. Immediately to the north of the Amadeus Basin, from 134.5’ in the east to 131.3” in the west, a distance of over 300 km, the Stuart Dyke Swarm (Black et al., 1980) intrudes metamorphic rocks of the Arunta Block. These fine to medium-grained dolerite dykes are generally up to 10 m wide and, except in the west where they are E-W, generally have a N-S orientation (Fig. 7). The marked changes in orientation of the dyke swarm suggest possible later rotation of blocks or thrust sheets and hence the extension direction is unknown. The dykes are the last recognised event along the northern margin prior to the deposition of the Heavitree Quartzite, have a Rb-Sr isotopic

11

IO

132”OO’

130-30

129~00’

135”OO’ I

133”30’

1: 16”:30’ !3900’

1'

24”OO’

25~00’

I

-I

_-___~__.~.__________~___________~__________*__________

lfmw5

SA

j{(f

26”OO’

Dolerite dykes

Fig. 7. Distribution of sediments in the Amadeus Basin related toyhe rifting and thermal subsidence phases of Stage 1. Also shown are dolerite dykes, which to the north of the basin form the Stuart Dyke Swarm, and have an isotopic age of 897 k 9 Ma (Black et al.. 1980).

age of 897 + Ma (Black et al., 1980) and are correlated with the Mount Harris Basalt and the basalt in the northwest

by the present authors.

Officer Basin to the southwest, the Townsend Quartzite (Jackson and Van der Graaff, 1981) is also a direct equivalent. The nature of lithospheric

The above evidence suggests that there was a period of crustal extension immediately prior to deposition of the Heavitree Quartzite and subsequent formations. The rift sequence on the southwestern margin of the basin (Fig. 7) would be the site where syn-sedimentary extensional faults would be expected, but this is the area that has

flexuring associated with the thermal recovery phase indicates it is unlikely that major crustal deformation would occur during this relatively

been extensively

One mechanism

deformed

by the nappes

developed in the Petermann Ranges (Forman, 1966) during stage 2. Following

that

Orogeny

the rifting phase, a long period (c.

200 m.y.) of subsidence took place due to thermal and mechanical recovery of the crust and lithosphere. This resulted in widespread sedimentation (Fig. 7) the first unit of which was the Heavitree Quartzite. The isopach pattern of the Proterozoic succession is asymmetric with the greatest thickness of sediments occurring along the southwest margin (Wells et al., 1970) adjacent to the site of the inferred rift. Sedimentation during the thermal recovery phase possibly extended continuously over much of central Australia. In the Georgina and Ngalia basins to the north, the Yackah beds (Walter, 1980) and the Vaughan Springs Quartzite (Wells and Moss, 1983) respectively are direct correlatives of the Heavitree Quartzite; in the

long period of time. The recognition that several interior basins in Australia were initiated by extension at approximately the same time (Lindsay et al., 1987) implies the presence of several rifts. to explain this was provided by

Dewey (1982). If the strain rates are low, stretching will occur over a longer period of time, and as such thermal recovery will commence

during the

stretching phase. This will lead to the rocks becoming harder and stronger in the stretched zone than those outside the stretched zone. Hence the stretching would stop or move to adjacent areas where the rocks are weaker. Thus as stretching spreads it would lead to sagging on a widespread, even sub-continental scale. This would produce an overall subsidence for the basin, but beneath which there could be a series of rifts over a wide area. Crustal

deformation

during stage 2

The shapes of the tectonic subsidence curves (curves B and C, Fig. 6b) indicate that during stage 2 (from about 620 Ma to about 410 Ma) the

25"OO'

:,

_-_i--_---_-.-I-_-.------

26~00'

SA

UL%w

,...... .z:
Rift phase

Fig. 8. Distribution interval

620-410

The shortening

I

:

Thermal subsidence phase

of sediments

in the Amadeus

Ma (Stage 2). The direction basin is related

to the thrusts

o>zz E3

Basin related

Shortening basin

to the stretching

of onlap shows the progressive and nappes

of the Petermann

the area up to, and immediately

northern sub-basins and troughs were subsiding due to thermal recovery after a short period of stretching. Stratigraphically, stage 2 represents the formations from the Arumbera Sandstone to the Mereenie Sandstone (Fig. 2). The tectonic subsidence curve for the southern platform area (curve A, Fig. 6b) indicates that there was no (or very little) subsidence in this area during stage 2. In the southwest, a limited shortening basin developed in association with the Petermann Ranges Orogeny. The distributions of the stretching and thermal subsidence phases and the shortening basin are

growth Ranges

1-------_3

Direction of onlap

and shortening

basins

of the basin during Orogeny.

south of, the shortening

that

formed

the thermal

The Petermann

Ranges

in the time

recovery Nappe

phase. occupies

basin.

an asymmetric half-graben (e.g. Bally, 1982; Gibbs, 1984; Etheridge et al., 1984) and then thins abruptly at the present northern margin implying that there could be a major bounding fault on the northern margin of the basin. The thick salt horizon within the Bitter Springs Formation (Lindsay, 1987a) severely hinders collection of seismic reflections beneath it making direct observation of the fault difficult. However, isopachs of the Arumbera Sandstone show that a deep E-W trending trench appeared along the northern margin of the Missionary Plains Trough during

shown on Fig. 8.

deposition of the Arumbera Sandstone (Lindsay, 1987b). The deep trench is clearly visible on the

Stretching

N-S section of the Missionary Plains Trough (Fig. 9B). North of this trench the Arumbera Sandstone

basin

During stage 2, the deep sub-basins, which were the depocentres for the Arumbera Sandstone (Lindsay, 1987b) were aligned approximately E-W along the present northern margin of the basin (Fig. 1). Although the depositional margin was destroyed during the later Alice Springs Orogeny, we consider that during Arumbera Sandstone time, it was located just to the north, very close to the present, structural margin. As shown by a detailed seismic stratigraphic study (Lindsay, 1987b), the Arumbera Sandstone thickens dramatically towards the north (Fig. 9) as would be expected in

thins abruptly whereas south of the trench it thins gradually as it onlaps the Central Ridge. There is no evidence of the existence of the trench during deposition of earlier formations suggesting that it appeared immediately prior to or during deposition of the Arumbera Sandstone. For stage 2, we have determined a fl value from the tectonic subsidence curves (Fig. 6b) of 1.2 to 1.5 which is significantly less than that required for oceanic crust to form (normally about /I = 4.43) and also much less than that required for the extrusion of basaltic rocks (see Dewey,

13

tion of the crust occurred, except for flexuring associated with the thermal subsidence.

Shortening basin Along the southwestern margin of the Amadeus Basin, major thrusting and nappe formation took place in the latest Proterozoic and is referred to as the Petermann

Ranges Orogeny (Forman,

1966).

Limited isotopic work by P.J. Leggo (in Forman, 1966) suggested that Rb-Sr mineral ages on biotite and microcline were reset at about 600 Ma, and this age is generally interpreted as the approximate age of the Petermann Ranges Orogeny (e.g. Wells et al., 1970; Forman and Shaw, 1973). Recent work by Webb (1985) on the geochronology of the Musgrave Block has shown considerable variability of K-Ar mineral ages; they tend to cluster at greater than 920 Ma, although five analyses cluster between 581 Ma and 598 Ma. We interpret

these

younger

ages to represent

the

Petermann Ranges Orogeny. These ages imply that orogeny and intense shortening was occurring along the southwestern margin of the basin at or close to the same time as extension and deposition Fig. 9. North-south

cross sections of the Arumbera Sandstone

(after Lindsay, 1987b) showing a typical half-graben fill morphology with thickening towards the north. Datum is top of the Arumbera Sandstone. The blank area is the late Proterozoic sediments between the Bitter Springs Formation

and

Arumbera Sandstone. Section A is from the Carmichael subbasin, B is from the Missionary Plains Trough, and C is from

of the Arumbera Sandstone were occurring along the northern margin (Fig. 8). The Petermann Ranges Orogeny produced the Petermann Ranges Nappe (Fig. 11) which extends at least 320 km in an east-west direction and has its middle limb overturned and overthrust for at

the Orraminna sub-basin.

least 50 km towards the north (Forman and Shaw,

1982). Nevertheless, basaltic extrusives occur at this period in several adjacent sedimentary basins, namely the Officer, Warburton, Ord and Georgina

tions of the thermal subsidence phase of stage 1 (Heavitree Quartzite and Bitter Springs Formation equivalents). At the same time, the younger sedi-

basins (see references in Lindsay et al., 1987) indicating that for these basins the amount of

ments of the thermal subsidence phase of stage 1

extension must have been greater than in the Amadeus Basin. During the thermal subsidence phase, the sediments onlapped progressively towards the south and west (Fig. 10) and eventually, by late Stairway Sandstone time, occupied a similar area to that of stage 1. However, during this phase, it is unlikely that any significant deforma-

1973). The nappe was basement-cored, but also involved the rift sequence (Mount Harris Basalt and Blood Range beds) and the lower two forma-

became detached and slid northwards on a decollement surface in the Bitter Springs Formation (Forman, 1966) which was probably the thick salt horizon (Lindsay, 1987a). It is interesting to note that the decollement surface is within the sedimentary succession and not at the basement-cover interface, as for example in the Jura foreland folds (e.g. Laubscher, 1972, 1974).

14

L

._____NT__l.

4

__-.____-_i___-.-._--_A-_-_-_-_-_-l-_-_-_-___

26”OO 12,NT,67

SA 1

Arumbera

Sandstone

5

Lower Staway

2

CambrIan

clast~c umts

6

Mlddle

3

Pacooia

7

Upper

4

Horn Valley

Sandstone SlItstone

Starway Staway

8

Stokes

9

Carmichael

Sandstone Sandstone Sandstone

Formation Sandstone

Fig. 10. Map of the Amadeus Basin showing distribution of formations deposited during the stage 2 stretching basin event. The rift phase (Arumbera Sandstone) is limited in extent to the northern margin, and then during the thermal subsidence phase, succeeding units progressively onlap (towards the south and west) onto sediments deposited during stage 1. Based on data in Wells et al. (1970).

Immediately

north of the northern limit of the

Petermann Ranges Nappe, a thick molasse wedge developed (Fig. 8). These rocks are the Mount Currie Conglomerate, best seen at Katatjuta (Mount Olga) and the granule-size arkose at Uluru (Ayers Rock), and represent continental or fluvial sediments deposited in a shortening basin that developed in front of an advancing thrust nappe. The sediments are unconformable on older Proterozoic sediments of stage 1. Following Forman (1966) and Wells et al. (1970), we interpret the uplifted Petermann Ranges also to have been the source of detritus for the

immature Arumbera Sandstone (Lindsay, 1987b), which was probably being deposited at the same time as the Mount Currie Conglomerate. Apart from the shortening basin, which is relatively localised in extent (Fig. 8), the southern platform was an area of sediment bypass (curve A, Fig. 6) and finer-grained detritus was transported towards the north and northeast to be deposited in the developing rift as the Arumbera Sandstone (Lindsay, 1987b; Lindsay and Korsch, in press). The available evidence indicates that a shortening basin was forming in the southern part at the same time as a stretching basin was forming in the northern part of the Amadeus Basin. The problem is to reconcile the apparent conflicting stress regimes associated with the compressional and extensional events. This situation is not unique either in space or time, and several examples were cited by Gordon and Hempton (1986). In the Amadeus Basin the elongations of the shortening and

I

I

,

Fig. 11. Cartoon cross section of the Petermann Ranges Nappe (after Forman, 1966). The middle limb of the nappe is overturned and has been transported at least 50 km towards the north. The Late Proterozoic succession slid northwards above a dt?collement that developed in the salt horizon in the Bitter Springs Formation.

stretching basins are at an angle of about 35 “, which is very similar to the angles between the Keeweenawan rift and the Grenville deformation front in North America (30 “) and the Upper Rhine Graben and Alpine erogenic front (50 “) (Gordon and Hempton, 1986). Thus these regions

15

are analogies

for the development

and shortening

basins

ing stage 2. A similar

in the Amadeus conflict

the shortening

Amadeus

Basin dur-

still awaiting

tion also exists for stage 3 between Canning

the Mereenie

of the stretching resolu-

elongations

for

(Smith,

along

Amadeus Group

1984).

deformation

during stage 3

associated

opment

of the basin

of the

Pertnjara

is that of molasse

Group

(Fig.

(1967) and Jones (1972) identified unit

of the

Pertnjara

Group,

glomerate,

as a synorogenic

the Alice

Springs

Orogeny

sediments

12). Wells

et al.

the uppermost

the

Brewer

deposit associated which

with

isotopically

is

dated at 400-300 Ma (Shaw et al., 1984), although major fault movements probably culminated at

thrust results

about 350 Ma (Shaw, 1987). The structures associated with this stage are the most impressive in the basin and tend to dominate the structural pattern.

followed

The

Sandstone

erosion

locally

from

Pertnjara

to the Brewer laterally

towards wedge

from the advancing,

the increasing

and

produces

An inverted

by felsic

and

then

upwards

the Brewer Conglomerate 1962; Wells et al., 1970). Within the Pertnjara

se-

proximity

immature

to

lithic-

stratigraphy,

of older Amadeus

Block, occurs

the

from the Parke Siltstone,

sheets. The coarsening-upwards

sheets,

sediments.

Arunta

of

significant

These rocks form a sedimentary

quence

senting

loading.

and

asymmetric

margin

and also coarsens

elevated

rich

Group

13), reflecting

upwards

that has been derived

the thrust

Con-

(Fig.

Hermannsburg

Conglomerate

with stage 3 of devel-

northern

due to thrust

coarsens

the north. Sedimentation

of a major

the

Basin

through Crustal

and Pertnjara

the development

depression subsidence

Basin and the stretching

Basin to the northwest

heralds

Sandstone

repre-

Basin sediments

mafic

units

of the

in the boulders

(P&hard Group,

in

and Quinlan, several

uncon-

The basin can be divided into at least three main structural styles (Fig. 13).

formities have been recognised (Wells et al., 1970) and are also visible on seismic sections (Lindsay, 1987b). We interpret them to represent phases in

Sediment fill

the growth of the shortening basin due to significant southward movements of the advancing thrust

A major sequence boundary (the Pertnjara Movement of Wells et al., 1970) occurs between

sheets. These thrust sheets induced loads necessitated reorganisation of the depocentres hence sedimentation

that and

patterns.

24"OO'

PERTNJARA

’

_-_-_---_~_-------_-_~~--

GRO

-_-.-___I__

________

I___

____

____l2~YOO’

SA

Fig. 12. Map of the Amadeus Basin showing present distribution of formations deposited during stage 3 (Pertnjara and Finke groups). These units are interpreted as deposits filling a shortening basin that developed mainly along the northern margin of the Amadeus Basin in the Devonian, related to major thrust faults in the Arunta Block. Circles represent the Brewer Conglomerate in the north and the Polly Conglomerate in the south. The easternmost, isolated unit was originally deposited further to the north, and has been transported to the south as part of a nappe complex recognised in the northeastern part of the basin by Wells et al. (1970). Shaw et al. (1983) and Stewart and Oaks (1987).

2%00’

Zd”O0

2 -

Major

---

Anhchne

-

--

25”OO’

Thrust

fault

II

sheets

CENTRAL

I

2

Santa

Teresa

3 Olympic

Syncline

L

4 Camel

NT __-_-_____~.-_-.-_-___~____~-~-_-_~_-___-_-_-~-_-________ SA

Flat 2O”OO’

Fig. 13. Sketch map of the Amadeus Basin showing location of the major structures (anticlines, synclines and faults) that developed during stage 3 (after Lindsay, 1987a). The basin can be divided into three zones, according to the dominant structural style present: (I) a series of stacked nappe and thrust sheets in the east, (II) (III)

thrusts and folds above a major decollement in the central part, and

a complex fold pattern reminiscent of interference patterns in the west. Boundaries between the zones are only approximate. For details of the structural elements in the eastern zone see Stewart and Oaks (1987).

Deformation

in the eastern Amadeus

Basin

Several allochthonous sheets (nappes) were recognised in the northeast by Wells et al. (1970) and Shaw et al. (1983). More recently, work by Stewart and Oaks (1987) has led to the identification of at least five allochthonous sheets with movements of 30 km to 60 km. The sheets display classic nappe structures such as frontal thrusts, rear normal faults (lags), and lateral ramps and strike-slip (transfer) faults. Later folding and erosion have produced klippen and back thrusts (Stewart and Oaks, 1987). Deformation

Within the basin, the central part is represented by a series of macroscopic synclines and anticlines with axial surface traces of a sinuous nature that often can be mapped for over 100 km (Fig. 13). The wavelengths of the folds are much greater north of the Central Ridge which is a major structural high (McNaughton and Huckaba, 1978) separating the extremely thick pile of sediments in the sub-basins of the north from the considerably thinner platform sediments of the south (Lindsay

S

ARUNTA AMADEUS

in the Central Amadeus

BLOCK

NGALIA

N

BASIN

Basin

Along the present northern margin of the Amadeus Basin, a series of discrete nappes involving both basement and sedimentary cover below the Bitter Springs decollement developed during the Alice Springs Orogeny. The nappes involved complex folding and thrusting and are best developed in Ormiston nappe complex (Marjoribanks, 1976), Blatherskite nappe near Alice Springs (Stewart, 1967) and the Arltunga nappe in the northeast (Forman, 1971). An overall model for the nappe development (Fig. 14) was proposed by Teyssier (1985).

Fig. 14. Schematic cross section, after Teyssier (1985) the central part of the Anmta

through

Block and Amadeus Basin

showing the relationship between major thrust zones in the Arunta Block and folds and thrusts in the Amadeus Basin. Note that the decollement in the basin is positioned within the sedimentary succession at the salt horizon in the Bitter Springs Formation, and not at the basement-cover interface. Thus the sedimentary units below the salt horizon are intimately involved in the basement nappes.

17

24"00'

5Okm

0

I

I

24'30'

Contours in metres Ix 10) Contour interval=

I

500 m

Fig. 15. Structural

12#?rm

I

I

contour map of the base of the salt horizon in the Bitter Springs Formation, mapped at a local seismic datum 550 m above sea level. The horizon is a near-planar surface dipping towards the north.

and Korsch, in press). This ridge began to form early in the history of the basin, and much of its later growth is due to Bitter Springs salt flowage

of

that dips towards the north at dips of up to 16” (Lindsay, 1987a). Probably

the most impressive structure in the

into the ridge (Lindsay, 1987a). It is a significant

central zone is the Gardiner Range Anticline (Fig.

feature that has had a prolonged influence on the sedimentological and structural development of the basin. To the north of the Central Ridge, the

in its core, sitting on and just to the north of the

anticlines collement

and synclines developed above a deat the base of the Bitter Springs salt

horizon that still has a near planar base (Fig. 15)

16), a large anticlinal structure, with a thrust fault Central Ridge. Its axial trace extends for over 120 km. In the central part of the anticline, the Gardiner Fault shows over 5 km vertical displacement and at least 20 km horizontal displacement

Stage 3

Sediments

K-l

Thermal subsidence phase

eismic line 83-A3

Bitter Springs Formation

-

Fadt

-

Formational boundew

__+-

Anticlinel

axis

Fig. 16. Sketch map of the Gardiner Range stages of basin evolution,

Anticline (after Wells et al., 1970) showing sedimentary units associated the Gardiner Fault and location of the seismic line shown in Fig. 17.

with the three

18

3

4

0

Fig. 17. North-south

5 km

seismic line 83-A3 across the central part of the Gardiner Range Anticline, showing the position of the

Gardiner Fault, a major thrust with over 5 km of vertical displacement and at least 20 km of horizontal displacement. Although shown as a relatively simple thrust fault, the presence of several splays cannot be discounted. Further to the south off the section from where the thrust flattens out it must ramp again, down to the Bitter Springs decollement. To the south of the thrust the Arumbera Sandstone was traced from surface outcrops and to the north of the thrust it was tied to well data.

(Fig. 17). However, at the surface the thrust is hinged at either end and only the middle section has moved upwards. The anticline is still situated

and steep to overturned beds in the foreward (northern) limb, suggests that they are fault-propagation folds (Suppe, 1985) that developed at the

above the thrust ramp. On the Gardiner Fault overthrusting has been from the south, whereas overthrusting in the Arunta Block and along the northern margin of the basin has been from the north (Stewart, 1967;

tips of the propagating dominant deformational

Deformation

Forman, 1971; Marjoribanks, 1976; Teyssier, 1985). This led Teyssier to interpret the Gardiner Fault as a minor backthrust associated with the main thrusting events in the Arunta Block. The

The outcrop pattern of beds in the western part of the Amadeus Basin, particularly in the Western Australia sector is indicative of a complex fold

large amount of displacement on the Gardiner Fault argues against a backthrust hypothesis and the fault is one of several south-dipping thrusts that developed in the sedimentary pile above and to the south of the Central Ridge. The geometry of anticlines in the southern part of the basin with their strongly asymmetric shape

interference pattern (e.g. Ramsay, 1967; Thiessen, 1986). One of the fold generations in the western sector produced folds that had N-S axial surfaces, in contrast to the rest of the basin, where the major structures are aligned E-W, or approximately so (Fig. 13). The interference pattern suggests that the N-S deformation was later than the

thrust faults. Thus the mechanism is one of

shortening by thrust fault mechanisms. in Western Amadeus

Basin

19

E-W

one, but its timing can only be constrained

as post-Proterozoic

and pre-Permian.

During all deformational

stages of the Amadeus

Basin, no internal defo~ation

of the sediments

took place. That is, the fossils and other marker

boundaries within the succession. During stage 1, the sequence boundaries are represented by the Areyonga and Souths Range unconformities

(Fig.

2). During stage 2, the sequence boundaries are represented by several unnamed unconfo~ties,

structures still retain their original shape, except

many of which have been detected by breaks in

for some flattening

the fossil record

attributable

in the bedding plane that is

1986).

During

this

stage the basin grew southwards and westwards

to flattening during compaction.

(Fig. Significance of unconformities

(Shergold,

in the basin succes-

9) with the limits to sedimentation

constrained

being

by the peripheral bulge (in part the

Central Ridge) which also migrated in those direc-

sion

tions. It was not until middle Stairway Sandstone In the Amadeus Basin, several unconformities have been recognised and interpreted as resulting

time that the bulge had sufficiently

diminished in

size to enable the sediments to break out across it

from erogenic events or to tectonic movements (e.g. Wells et al., 1970). Note that Wells et al. use

and spread over the rern~~ng

the term orogeny for only those events that caused

In shortening basins, unconformities are produced by subsidence caused by loading of successive thrust sheets. In the northern Amadeus Basin

basement reactivation and use the term movement for events with no evidence of basement reactivation that produced unconformities or disconformities in the sedimentary succession. Some of these are shown under the term “event” in Fig. 2. Here we refer to the unconformity by the formal name proposed for that event. Evidence for the Petermann

Ranges

been discussed

and Alice

Springs orogenies

above, and both are related

major shortening events involving well as the basin succession. In the stretching

basement

has

sediments of stage

1.

the first significant influence of the thrust sheets as an elevated source area is heralded by the formation of the Pertnjara unconformity (Wells et al., 1970) and above it occur the immature sediments of the Pertnjara Group which increase in

to

coarseness upwards as the thrust sheets approach. Several unnamed ~confor~ties within the Pert-

as

njara Group are related to the loading effect of the thrust sheets as the shortening basin develops.

basin model, unconformities

develop at several stages in the evolution of the basin. During initial stretching, an unconformity will form between the basement and the first sediments of the rift phase. During stages 1 and 2 in the Amadeus Basin this would equate with the Ormiston unconfor~ty (Shaw et al., 1984) and the Petermann Ranges unconfor~ty (between the Julie and Arumbera units) respectively. Following the rifting phase, a major unconformity develops below sediments of the thermal

Discussion An intimate relationship exists between basin dynamics and sediment deformation of the Amadeus Basin. There is also a clear relationship between the location of the rift phases of the stretching basins and the location of the following thrusts and shortening basins. Stretching in stage 1 is largely concentrated on the present southwestem margin of the basin, and this is the site of the

subsidence phase, and is represented in the Amadeus Basin by the unconformity beneath the Heavitree Quartzite (stage 1) and an unnamed

Petermann Ranges Orogeny during stage 2. Stretching in stage 2 is concentrated along the present northern margin of the basin, and this

unconformity that occurs above the Arumbera Sandstone and Todd River Dolomite (stage 2). During the thermal subsidence phase several unconformities of regional extent develop on the basin margins but pass laterally basinward into conformities. These represent major sequence

area is also the focus of the Alice Springs Orogeny during stage 3. A possible explanation is that during stretching phases, the crust is sufficiently thinned to enable crustal scale thrust faults, that develop during the shortening stages, to more easily cut through this thinner crust than adjacent

20

a zone of

can be regarded as a series of three stacked basins,

structural weakness that nucleates inversion zones

the first being a stretching basin, the second being

thicker

crust.

The

stretching

forms

Chad-

a stretching basin in the north and a small shor-

wick, 1985; Etheridge, 1986; Stockmal et al., 1986).

tening basin in the southwest, and the third being

Coupled with this there would be a significant

a shortening basin.

(Dewey, 1982;

Dewey and Pitman, 1982;

contrast in the thickness of sediments in the rift and adjacent to it, leading to a heterogeneity

that

During these stages, the evolution of the basin is intimately related to deformational

processes in

could be exploited during thrusting. During stage 3 the deformation is intracratonic in nature and located at a considerable distance

the crust, whether they be stretching accompanied

from the nearest plate boundaries which were located in easternmost Australia and in the Tethys

shortening due to thrust faulting.

region (e.g. Veevers and McElhinny,

Acknowledgements

was a time when the continents

1976). This

by extensional

faulting, aseismic flexuring of the

crust during thermal subsidence,

or loading and

showed strong

convergence and Pangaea was being assembled (Scotese et al., 1979) leading to deviatoric com-

We wish to thank Magellan Petroleum Australia Limited on behalf of the Amadeus Joint Venture

pression in the crust. Gordon and Hempton (1986) consider that intracontinental shortening and thickening begins near the suture that developed during continental collision after the oceanic crust has been eliminated. But in central Australia, evi-

Partners for access to unpublished data and for permission to publish Fig. 17, and the following for fruitful discussions on the geology of the

dence of angular integrity is inferred by palaeomagnetism (e.g. Veevers and McElhinny, 1976) and by essentially identical sedimentary facies in the Georgina, Amadeus and Officer basins, indicating that the intracratonic shortening must have been located at a considerable distance

Amadeus Basin: M.R. Walter, J.D. Gorter, M. Owen, J.M. Kennard, R.S. Nicoll, P.N. Southgate and J. Bradshaw. Comments on the manuscript were provided by M.R. Walter and A.T. Wells. J. Tipper assisted with the backstripping and M. MacDonald and J. Wilford assisted in analysing the seismic data. The figures were drafted by C. Knight. Published with permission of the Direc-

from any plate margin. Hence stresses must be transmitted away from the suture, through a rela-

tor, Bureau of Mineral Resources.

tively homogeneous crust into the interior of the supercontinent to reactivate old fault systems such as the Redbank, Harry Creek and Oolera deformed zones in the Arunta Block.

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