Structural evolution of the Outalpa Inliers, Olary Domain ... - PIRSA

Geological Note 

Structural evolution of the 

Outalpa Inliers, Olary Domain, 

Curnamona Province 

Richard Flint (Geosurveys Australia Pty Ltd) 

Introduction 

PIRSA, as part of the BHEI program, 

contracted Geosurveys Australia Pty 

Ltd to reappraise the geology of the 

Outalpa Inliers in the central portion of 

the Olary Domain, southern Curnamona 

Province (Fig. 1), and produce a series 

of new digital and hardcopy lithological, 

lithostratigraphic, structural and metallogenic 

maps for the region (Flint, 

2001). Surface lithological mapping, 

and structural and airborne geophysical 

data were integrated into a new 

coherent 1:25 000 lithostratigraphic 

map incorporating both exposed and 

concealed portions of the inliers, and 

are the basis of a companion paper 

(Flint, in prep.). The present paper 

concentrates on the characteristics and 

nature of deformational episodes that 

have affected the Willyama Supergroup 

within the Olary Domain. 

A great challenge to a better 

understanding of the detailed strati- 

Fig. 1 Location plan of the Outalpa Inliers within the Olary Domain, Curnamona Province (after Flint and Parker, 1993; Preiss and Conor, 

2001). 

34 

MESA Journal 26 July 2002


graphy for the Willyama Supergroup 

within the domain has been recognition 

of structural elements for the major 

deformational phases within the 

Mesoproterozoic Olarian Orogeny. At 

least three main phases were identified 

more than 20 years ago, but since 

then relatively few workers within the 

region have successfully applied this 

classification scheme. This is due, in 

part, to a poor appreciation of the subtle 

differences between structural elements, 

and axial planes for various phases 

having a similar orientation in some 

areas. Orientation alone is not a sufficient 

distinguishing criteria — style of folding 

is also important, and the combination of 

style, orientation and local overprinting 

criteria are all important to successfully 

delineate the various phases of the 

Olarian Orogeny. 

No detailed regional study on 

structural evolution of the Olary 

Domain has been undertaken, though 

syntheses by Laing (1995a,b), primarily 

on lithology and lithostratigraphy, partly 

addressed structural considerations; 

Clarke et al. (1986) produced a regional 

structural synthesis. Considerable 

structural data now exist for the Outalpa 

Inliers, with University Honours 

mapping projects by Beckton (1993), 

Buckley (1993), Eykamp (1993), 

Middleton (1993), Peterson (1993), 

Benton (1994), Baulch (1995), Freeman 

(1995), Morton (1995), Fraser (1997), 

Pelling (1997) and Zdziarski (1997), 

an incompleted Ph.D project by Flint 

(1981), and mapping by Archibald 

(1980). All their point structural 

observations were digitally captured 

and, along with additional data collected 

during a 30-day field mapping program, 

now form a database containing >5450 

structural readings, permitting a detailed 

reappraisal of the tectonic evolution of 

the Outalpa Inliers. 

Regional orogenesis 

Within the Olary Domain, deformation 

of the Olarian Orogeny occurred at 

~1600–1580 Ma during three main 

phases (OD 1 

to OD 3 

) that have produced 

a complex distribution of units (Glen et 

al., 1977; Berry et al., 1978; Grady et 

al., 1989; Clarke et al., 1986; Flint and 

Parker, 1993). The effects of refolding 

during two further deformational phases 

(of the Palaeozoic Delamerian Orogeny) 

are relatively minimal. 

During OD 1 

, a layer-parallel 

schistosity and gneissosity developed 

during peak metamorphism at 

middle amphibolite facies, with local 

development of migmatite. However, 

the degree of folding during OD 1 

is 

enigmatic. Mesoscopic OD 1 

folds 

have only been observed rarely, but 

macroscopic recumbent folds with 

downward-facing stratigraphy have been 

proposed (Pointon, 1980; Clarke et al., 

1986; Grady et al., 1989). Recognition 

of other OD 1 

folds is hampered by 

the intensity of refolding during later 

deformational phases. 

The second deformational episode 

(OD 2 

) varied in intensity across the 

Olary Domain. Mesoscopic folding 

ranges from intense development of 

tight to isoclinal folds with an inclined 

axial planar schistosity or crenulation 

cleavage, to open folds with minor 

crenulations and crenulation cleavage. 

Refolding during OD 2 

was significant 

(Parker, 1972; Berry et al., 1978; 

Archibald, 1980; Flint, 1981), but 

has been poorly recognised or totally 

overlooked by some subsequent workers. 

Elucidation of OD 2 

structures is critical 

in establishing a detailed stratigraphic 

sequence for the Willyama Supergroup 

within the domain. 

The OD 2 

schistosity and crenulation 

cleavage regionally trend approximately 

northeasterly, are defined by sillimanite, 

biotite and/or muscovite, and clearly 

predate the strong and pervasive folding 

by OD 3 

(Flint and Parker, 1993). 

Mesoscopic to macroscopic OD 3 

folds are common, are open to close, 

and have steeply inclined to upright 

axial planes (trending east and 

northeast). Axial planar crenulations and 

crenulation cleavage are well developed, 

as are discrete retrograde shear zones 

characterised by assemblages of quartz, 

sericite and chlorite. Metamorphic grade 

during OD 3 

was greenschist facies and 

significantly lower grade than during 

earlier phases of the Olarian Orogeny. 

The fourth and fifth deformational 

phases within the Willyama Supergroup 

are assigned to the Delamerian Orogeny, 

though deformation is largely restricted 

to reactivation along major faults, broad 

warping of earlier structures and strata, 

and regional uplift by west-directed 

thrusts (Berry et al., 1978; Preiss, 1995; 

Paul et al., 2000). 

Outalpa orogenesis 

Of mapping projects undertaken within 

the Outalpa Inliers since 1980, only 

those by Archibald (1980), Flint (1981), 

Baulch (1995) and Morton (1995) 

clearly and successfully delineated 

structural elements for the three phases 

of the Olarian Orogeny (Table 1). 

Mapping by Buckley (1993), Beckton 

(1993), Eykamp (1993), Middleton 

(1993), Peterson (1993), Benton (1994) 

and Freeman (1995) did not distinguish 

younger deformational phases (post-D 1 

), 

whereas Fraser (1997), Pelling (1997) 

and Zdziarski (1997) grouped all post- 

D 1 

structural features into a single event. 

Clarke et al. (1986) adopted a different 

nomenclature for the deformational 

episodes — their D 2 

is equivalent to 

D 3 

of most other studies whilst younger 

development of retrograde shear zones 

was regarded as a separate phase (their 

D 3 

; Table 1). 

During this project, all the point 

structural data from previous work 

were reassessed and, where possible, 

reclassified into a common, regional 

classification scheme (Table 1, Fig. 2). 

A series of plans showing interpreted 

trend lines for bedding and axial planar 

surfaces have been compiled (Fig. 

3). Trends were simply derived from 

plotting all appropriate field structural 

observations onto a base map and, where 

data were sufficiently concentrated, 

extrapolated to achieve regional trends. 

Trend lines do not indicate fold hinges, 

they simply reflect the orientation of 

the structural element whether primary 

or modified by later deformational 

episodes. The trends are generalised 

and, though not all local variations in 

orientations are shown, are extremely 

useful in defining broad patterns, 

extent and coherency for individual 

deformations. Further discussion on 

delineation of significant macroscopic 

fold hinges and lithostratigraphic 

considerations are contained in Flint 

(2001, in prep.). 

Bedding 

A layer-parallel schistosity and 

gneissosity, partly or totally overprinting 

MESA Journal 26 July 2002 35


Fig. 2 Stereographic diagrams for principal structural elements, Outalpa Inliers. 

36 

MESA Journal 26 July 2002


Table 1 Comparative deformational classifi cation schemes for the Willyama Supergroup of the Outalpa Inliers, Olary Domain. 

Berry et al. 

(1978) 

Archibald 

(1980) 

Flint 

(1981) 

Clarke et al. 

(1986) 

Flint and 

Parker (1993) 

Baulch, Morton 

(1995) 

Fraser, Pelling, 

Zdziarski (1997) 

This study Conor (2001) 

deformational 

phases 

OLARIAN OROGENY 

D 1 

D 1 

D 1 

OD 1 

D 1 

D 1 

OD 1 

Faugh-a-Ballagh 

D 2 

D 2 

D 2 

D 1 

OD 2 

D 2 

} 

OD 2 

Mount Mulga 

D 3 

D 3 

D 3 

D 2 

OD 3 

D 3 

}D 2 

} 

OD 3 

Waterfall 

RSZ* RSZ D 3 

RSZ* RSZ* } 

RSZ* Walter–Outalpa 

} 

DELAMERIAN OROGENY 

D 4 

D 4 

DD 1 

D 4 

D 5 

D 5 

DD 2 

D 5 

*Retrograde schist zones (RSZ) regarded as syn- to late-D 3 

features. 

Fig. 3 Trends for S 0 

to S 5 

axial surfaces, Outalpa Inliers. 



original bedding, developed in many 

regions of the Outalpa Inliers during 

OD 1 

. This has resulted in a complex 

compositional layering, certainly in 

part tectonic, but also reflecting the 

orientation of original bedding in some 

circumstances. 

Observed sedimentary structures 

include cross-bedding, graded bedding, 

trough scours and syneresis features. 

Previously, younging criteria had 

been recorded at 36 localities within 

the Outalpa Inliers, but some are of 

doubtful value, based on either assumed 

reverse graded bedding (coarsening 

upwards) or tectonically induced 

pseudo-cross-bedding. An additional 

37 younging observations were noted 

during fieldwork; observed graded 

bedding was assumed to be normal 

(fining upwards). Only 17 of the total 73 

facing observations indicate overturned 

strata, and of these only three have dips 


Fig. 4 Superimposed structural trends within exposures near Ameroo Hill (left) and Outalpa Springs Bore (right). 

psammitic, psammopelitic and albitic 

horizons, and crenulations in pelitic 

schists. Axial planar crenulations, kinks 

and crenulation cleavage are typical OD 3 

elements. 

Macroscopic F 3 

folds have wavelengths 

and amplitudes of a few hundred 

metres to several kilometres, 

with examples in the Doughboy Inlier, 

northwest of Tommie Wattie Bore and 

northeast of Sylvester Bore. Trends of S 2 

(and S 1 

) axial surfaces in the Doughboy 

Inlier show a marked regional variation, 

indicating significant refolding about 

a northeast-plunging OD 3 

antiformal 

Tight to isoclinal OD 2 

folds with curved axial traces due to 

refolding during OD 3 

, north of Outalpa Springs Bore. (Photo 

48854) 

axis that is truncated by Neoproterozoic 

sediments (Fig. 4). 

Open warping of bedding and S 1 

and 

S 2 

axial surfaces is evident northeast 

of Sylvester Bore. Trends of S 3 

axial 

surfaces are at a very high angle to all 

earlier fabrics and mesoscopic folds 

are common. Similar warping of strata 

occurs south of Sylvester Bore. 

Discrete ductile–brittle retrograde 

shear zones, up to 50 m wide, are 

characterised by assemblages of quartz, 

sericite and chlorite (±Fe oxides) and 

extend across the inliers. They trend 

predominantly ~090–105°. Dips are 

subvertical to steeply 

south-dipping, with 

lineations plunging to 

the south and southeast 

with south block up 

and oblique reverse 

sense of displacement. 

These retrograde shear 

zones are considered 

to be late OD 3 

features 

rather than a discrete 

younger event as 

proposed by Clarke et 

al. (1986), as S 3 

axial 

planar crenulations also 

locally trend ~090– 

105° in the vicinity of 

the retrograde shear 

zones. Uplift along them represents the 

final deformational effect of the Olarian 

Orogeny (Flint, 2001). 

Deformation 4 

D 4 

is manifest within the Willyama 

Supergroup as rare crenulations and 

kinks that have subvertical axial planes 

striking ~010–020° (Fig. 3). These 

structural elements are best observed 

within very pelite-rich schists in the 

Doughboy Inlier and northern Bimbowrie 

Inlier. No major D 4 

macroscopic fold has 

yet been identified within the Willyama 

Supergroup. 

Deformation 5 

D 5 

corresponds to the major fold-forming 

event of the Delamerian Orogeny within 

the Adelaide Geosyncline (Berry et al., 

1978; Preiss, 1995). Crenulations and 

kinks within the Willyama Supergroup 

of the Outalpa Inliers have subvertical 

axial planes striking ~100–110°, though 

the latter may vary locally in close 

proximity to major pre-existing thrusts, 

shears and faults (Fig. 3). Distinguishing 

these crenulations from often similarly 

orientated OD 3 

features is difficult, 

and can only be successfully done in 

basement pelitic schists within close 

proximity to Neoproterozoic sediments. 

No major, macroscopic D 5 

fold has 



yet been identified within strata of the 

Outalpa Inliers. 

Conclusions 

A detailed, universally accepted 

lithostratigraphy for the Willyama 

Supergroup within the Olary Domain 

of the Curnamona Province has 

proven elusive over the years despite 

considerable regional mapping and an 

established stratigraphic succession for 

the Broken Hill Domain. Ambiguities 

exist principally due to structural 

complexities, particularly fold repetition 

of gneissic sequences containing few 

marker horizons, similarly orientated 

but discretely different deformational 

episodes, and misinterpretation of 

structural overprinting criteria. Characterising 

the number and nature of the 

major deformational episodes that have 

affected the Willyama Supergroup in the 

Olary Domain is a prerequisite before 

local lithostratigraphic considerations 

and regional structural correlations. 

Original bedding and sedimentary 

structures within lithologies of the 

Willyama Supergroup are best preserved 

in northern portions of the Outalpa 

Inliers. Only 23% of facing observations 

indicate overturned strata, and only 4% 

have dips


Resources. Open fi le Envelope, 9788 

(unpublished). 

Flint, R.B. (in prep.). Lithostratigraphy of the 

Willyama Supergroup within the Outalpa 

Inliers, Olary Domain, Curnamona 

Province. MESA Journal. 

Fraser, G., 1997. The geology of the Outalpa 

Springs area, Olary, South Australia, 

with special reference to metasomatic 

and mineralisation processes. University 

of Melbourne. B.Sc. (Hons) thesis 


Freeman, H.S.R., 1995. A geochemical and 

isotopic study of mafic and intermediate 

rocks in the Olary Province, South 

Australia — magma series discrimination 

and geological framework. University 

of Adelaide. B.Sc. (Hons) thesis 


Glen, R.A., Laing, W.P., Parker, A.J. 

and Rutland, R.W.R., 1977. Tectonic 

relationships between the Proterozoic 

Gawler and Willyama Orogenic Domains, 

Australia. Geological Society of Australia. 

Journal, 24:125-150. 

Grady, A.E., Flint, D.J. and Wiltshire, R.J., 

1989. Excursion guide for Willyama 

Supergroup and related rocks, Olary 

district, SA. South Australia. Department 

of Mines and Energy. Report Book, 89/23. 

Laing, W.P., 1995a. Lithological map 

sheets: Billeroo North, Billeroo South, 

Boolcoomata South, Bulloo North, 

Bulloo South, Kalabity North, Kalabity 

South, Koolka North, Koolka South, 

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North, Plumbago South. South Australia. 

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stratigraphy of the Willyama Supergroup, 

west of Bimbowrie, Olary, South 

Australia. University of South Australia. 

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structural study of portion of the Olary 

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South Australia. University of Adelaide. 

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Structural evolution of the Outalpa Inliers, Olary Domain ... - PIRSA

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