The Early Proterozoic Nabberu Basin and associated iron formations of Western Australia

Precambrian Research, I (1978) 129-184 129 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE EARLY PROTEROZOIC NABBERU BASIN AND ASSOCIATED IRON FORMATIONS OF WESTERN AUSTRALIA

W.D.M. HALL and A.D.T. GOODE

Exploration Department, The Broken Hill Proprietary Co. Ltd., Perth, W.A. 6001 (Australia) Exploration Department, The Broken Hill Proprietary Co. Ltd., Melbourne Research Laboratories, Clayton, Vie. 3168 (Australia)

(Received June 20, 1977; revision accepted December 2, 1977)

ABSTRACT

Hall, W.D.M. and Goode, A.D.T., 1978. The Early Proterozoic Nabberu Basin and associated iron formations of Western Australia. Precambrian Res., 7: 129-184.

Rocks of the Early Proterozoic Nabberu Supergroup were deposited in the Nabberu Basin along the northern margin of the Archaean Yilgarn Shield in Western Australia. The Nabberu Basin consists of three tectonic-sedimentary units known as the Earaheedy, Glengarry and Padbury Sub-basins.

The Earaheedy Group within the easternmost sub-basin is divided into a lower Too100 Sub-group and an upper Miningarra Sub-group, representing succeeding sedimentary cycles totalling some 6000 m of shallow water sediments. The Too100 Sub-group comprises thin quartzose to arkosic elastics (Yelma Formation) which rest unconformably on Archaean rocks, and are overlain by chert, shale, iron formations and minor carbonate (Frere Formation), and thinly bedded carbonate, shale and sandstone (Windidda Forma- tion). The overlying Miningarra Sub-group includes sandstone and shale (Wandiwarra Formation), super-mature quartz sandstone and arkosic siltstone (Princess Ranges Quartzite), fine arkosic sandstone, siltstone, shale and carbonate (Wongawol Formation), limestone, shale and sandstone (Kulele Creek Limestone) and sandstone and shale (Mulgarra Sandstone). Distinctive stromatolite assemblages occur in carbonate units throughout the sequence.

Iron formations of the Frere Formation are similar to those of the Lake Superior and Labrador Provinces of North America, and commonly have a distinctive pelletal (intraclastic) texture, but are locally oolitic or laminated. Benthonic microfossils found at one locality are identical to those in the Lake Superior iron formations (Walter et al., 1976).

West into the Glengarry Sub-basin and Peak Hill-Robinson Ranges area the basal elastics become considerably thicker, finer-grained and more varied, and are commonly interbedded with basaltic volcanics and greywackes. The Peak Hill Beds (MacLeod, 1970), Finlayson Sandstone and Maraloou Formation (Bunting et al., 1977) may be lateral equivalents of the Yelma Formation, while the overlying Horseshoe Range Beds, Labouchere Formation and Robinson Range Formation (Barnett, 1975) are possible equivalents of the Frere Formation. The Millidie Creek Formation may be equivalent to the Wandiwarra Formation.

Deformation of the Nabberu Basin has resulted in the development of the Stanley Fold Belt in the north and the Kingston Platform in the south. On the Kingston Platform the rocks dip very gently north, but deformation increases northwards. A slaty cleavage becomes more conspicuous in this direction, while folds become tighter, overturned southwards and cut by north-dipping thrusts. The fold belt trends west-northwest across the eastern

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portion of the basin before swinging to the southwest. Archaean basement is increasingly involved in the deformation and becomes progressively more gneissic as the Early Proterozoic rocks become more strongly schistose. Refolding of early structures is pronounced in the west. Metamorphic grade, based on mineral assemblages in basic, pelitic and carbonate rocks and iron formations, also increases north and northwest, reaching a maximum grade of granulite facies west of the Robinson Ranges. The sediments are essentially unmetamorphosed in the southeastern part of the basin.

INTRODUCTION

The Nabber-u Basin lies along the northern margin of the Archaean Yilgarn Shield in Western Australia (Hall and Goode, 1975; Horwitz, 1975a). The Early Proterozoic rocks of the basin extend at least 700 km in a west- northwest-east-southeast direction between longitudes 117% and 124%, are exposed over a width of 120 km in the vicinity of Lake Carnegie, and cover an area of approximately 60,000 sq km (Figs.1 and 2). The basin has been named from the Nabber-u 1:250,000 sheet, SG.51-5, and is also exposed on parts of the Duketon, Glengarry, Kingston, Peak Hill, Robert, Robinson Range, Stanley, Throssell and Wiluna 1:250,000 sheets.

Geologists of the Broken Hill Proprietary Co. Ltd. began mapping the Nabber-u Basin in 1973 following publication of the aeromagnetic maps of the Nabberu and Stanley sheets by the Bureau of Mineral Resources, Geology and Geophysics. Conspicuous east-west trending magnetic anomalies were interpreted as indicating the presence of banded iron formations, and subsequent field investigations revealed the presence of a large sedimentary basin containing iron formations of the Lake Superior type, the first of their kind discovered in Australia. Further field work during 1974 and 1975 extended the known basin west into the Glengarry, Peak Hill and Robinson Range ~h~-lts and east into the Kingston and Stanley sheets. Sediments correlated Jith those of the Nabberu and Kingston sheets have now been recognised on

the Guketon, Robert and Throssell sheets to the south and east (Jackson et al., 1973; Bunting and Chin, 1975; Bunting et al., 1975; Preiss et al., 1975). West of the Robinson Ranges much of the Early Proterozoic cover has been eroded; here tectonically reworked, strongly metamorphosed Archaean basement rocks are exposed.

The discovery of the Nabberu Basin is significant as two neighbouring and possibly connected Early Proterozoic basins are now known in Western Australia. Banded iron formations are common in both basins but exhibit markedly different features. The well known Hamersley Basin to the north contains very extensive and regularly banded iron formations (Trendall, 1968), whereas the Nabberu Basin contains much more lenticular and reworked iron formations (Goode and Hall, 1976). The facies relationships within and between these basins, when fully elucidated, should give much important data on the origin of iron formations in general. Similar work on Early Proterozoic basins in North America and South Africa (e.g., Chauvel and Dimroth, 1974;

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Eriksson and Truswell, 1974; Anhaeusser and Button, 1974; Button, 1976a) is leading to a much better understanding of the depositional environments of iron formations.

The aim of this paper is to summarise the present knowledge of the stratigraphy, facies relationships and depositional environments, particularly of the eastern Nabberu Basin, as well as the structural configuration and metamorphic zonation. Much further work remains to be done, but it is hoped that these data will establish a basis for future work, as well as provide some constraints on depositional models for Lower Proterozoic banded iron formations.

PREVIOUS INVESTIGATIONS

No recent comprehensive account is available on the geology of the Nabber-u Basin, the extent and existence of which appears to have gone largely un- noticed, although Talbot (1910) recognised the regional structural trend and noted an unconformity between folded slate and gently dipping quartzite in the north of the basin. The gently north dipping sediments in the south of the basin were correlated with the Nullagine Series, while the large belt of slates with “highly inclined” folds in the north were referred to the Mosquito Creek Series. Later, Talbot (1920) described rocks in the eastern portion of the basin in the vicinity of Lake Carnegie.

The unconformity between the Nabberu sediments and Archaean basement in the south of the basin was recognized by Sofoulis and Mabbutt (1963), who also gave a brief outline of the eastern and central portions of the basin. Horwitz and Daniels (1967) mention volcanic rocks in the Nabberu sheet, and also rhyolite bands in sediments in the Kingston sheet, south and west of Lake Carnegie. Sanders and Harley (1971) undertook a regional hydrogeological survey of part of the area, and described the Proterozoic rocks as comprising interbedded quartzite, shale and dolomite, with minor banded iron formation, greywacke, conglomerate and limestone.

AGE RELATIONSHIPS

The sediments from the eastern Nabberu Basin have previously been considered to be equivalent to the lower division of the BangernaB Group (Daniels, 1966) and thus of Middle Proterozoic age (Compston and Arriens, 1968), while those from the Peak Hill sheet in the central, more deformed and metamorphosed part of the basin were mapped as Arehaean by MacLeod (1970). However, MacLeod noted the possibility of the sediments being Early Proterozoic. The authors believe the Lower Proterozoic age to be more likely on the following grounds:

(1) The Nabber-u sediments unconformably overlie the Archaean basement of the Yilgarn Block. This relationship can be directly observed in the field, and is also well illustrated by the magnetic discontinuity between Nabber-u

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and Yilgarn iron formations. Granites in the Yilgarn basement have been dated at 2550-2750 Ma (Compston and Arriens, 1968).

(2) In’ the Carnarvon Range and Mudan Hills folded Nabber-u sediments are unconformably overlain by sandstones of the Bangemall Group which have a minimum age of 1100 Ma (Compston and Arriens, 1968). To the east of Lake Carnegie Nabberu sediments are overlain by Late Proterozoic and Palaeozoic sediments of the Officer Basin (Lowry et al., 1972).

(3) Samples of glauconite from basal Nabberu sediments in the Duketon sheet gave minimum K/Ar ages of around 1,700 Ma and minimum Rb/Sr ages of between 1,590 and 1,710 Ma (Preiss et al., 1975).

(4) The Nabberu succession is intruded by unmetamorphosed dolerite sills and plugs in several areas; one of these in the eastern part of the basin has been dated at 1050 Ma by Preiss et al. (1975).

(5) Abundant, thick iron formations of the Lake Superior type are present in the Nabberu Basin and contain microfossils identical to those from the Early Proterozoic Gunflint and Biwabik Iron Formations of the Lake Superior district (Walter et al., 1976). Such iron formations are considered typical of the Early Proterozoic (Cloud, 1973).

(6) The westernmost part of the Nabber-u Basin is adjacent to the Gascoyne Metamorphic Block, in which anatectic granites intruding Early Proterozoic sediments have been dated at about 1700 Ma (Compston and Arriens, 1968; S. Williams, personal communication, 1977). An undated but probably in- trusive granite occurs west of Mt. Seabrook in psammitic schist derived from Nabberu sediments.

REGIONAL STRUCTURE

The Nabberu Basin is defined as the Early Proterozoic depositional basin which formed along the northern margin of the Yilgarn Block. The basin includes an eastern Earaheedy Sub-basin, a central Glengarry Sub-basin, and a western Padbury Sub-basin (Hall et al., 1977). Probable tectonic control of

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sedimentation led to the development of distinctive facies assemblages of differing thicknesses in each sub-basin.

Deformation of the Nabber-u Basin in the late Early Proterozoic resulted in the formation of the northern Stanley Fold Belt, in which the sediments are strongly deformed and the metamorphic grade locally reaches granulite facies, and the southern Kingston Platform which is virtually undeformed (Fig.3). The boundary between these two tectonic subdivisions extends approximately east-west to west---southwest across the central part of the Earaheedy Sub-basin and southern part of the Glengarry Sub-basin. The Padbury Sub-basin is now entirely within the Stanley Fold Belt.

On the Kingston Platform sub-horizontal to gently-dipping sediments rest unconformably on Archaean granitic rocks with narrow north-northwest trending belts of me&sediments and mafic and ultramafic volcanics and in- trusives. The unconformity follows a slightly irregular line northwest from the western Throssell sheet around a south projecting lobe around the southern boundary of the Lake Teague structure (Butler, 1974) to about 10 km south of Lake Nabber-u. The unconformity then swings sharply south-southwest along the eastern side of the Verscher Range to the northeast corner of the Glengarry sheet, before outlining a broad arc into the west of the Wiluna sheet. It continues westwards across the centre of the Glengarry sheet to the northeast of Meekatharra where it becomes deformed into a series of east-northeast plunging folds. Northwest of the Glengarry sheet the basal unconformity becomes increasingly blurred, and is marked by the contact between granitic gneisses and coarse grained, locally conglomeratic psammatic schist derived from arkose and impure quartzite.

The Earaheedy Sub-basin is deformed into a large synclinorium which opens out to the southeast, and progressively younger stratigraphic units are preserved as the present centre of the structure, immediately north of Lake Carnegie, is approached (Fig.2). In this region the entire stratigraphic sequence, with the exception of the two uppermost units, can be studied in both a deformed and undeformed state, and the progressive development of structural features provides an excellent model to which deformation in the remainder of the basin can be related.

The gentle northerly dips on the Kingston Platform in the south persist almost to the centre of the synclinorium where the first obvious signs of deformation are the regularly curving outlines of the stratigraphic boundaries. In the centre of the sub-basin these are followed by the appearance of saucer- shaped folds with gentle dips of up to 20”. Northwards the folds become oval- shaped, and limb dips increase quite rapidly up to 50” as the folds become more elongate and canoe-shaped. Cleavage, dipping 40-60” northwards, becomes noticeable in the finer-grained rocks, and small north-dipping thrusts appear. Further north the cleavage becomes increasingly more penetrative and the folds tighter and slightly overturned southwards (Fig.4).

In the prominent ridges in the north of the fold belt the resistant iron formation units form elongate, tight to isoclinal, doubly plunging (i.e., closed)

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structures which are commonly asymmetric and overturned southwards, and often dislocated by north-dipping thrusts. The low areas between the ridges are occupied by slates, the north-dipping cleavage planes of which have almost obliterated the strongly folded bedding. Locally within this part of the fold belt the cleavage and the individual folds to which it is related have been refolded into open to close structures, some of which are slightly overturned southwards. Archaean basement rocks reappear in the north of the fold belt in the central Stanley sheet (e.g., Malmac Dome of Horwitz, 1975b), and have a moderately developed planar fabric imparting a slightly gneissic appearance.

The Stanley Fold Belt trends west to west-northwest from north of Lake Carnegie into the west of the Nabber-u sheet, and then swings to a southwest to west-southwest orientation across the north of the Glengarry Sub-basin (Fig.3). In the west of the Nabber-u sheet and westwards across the Peak Hill and Robinson Range sheets, basement rocks become increasingly involved in the deformation, and increasingly gneissic. Sheared granite appears in anticlinal crests in the west of the Nabber-u sheet and east of the Peak Hill Sheet (e.g., Goodin and Marymia Domes of Bunting et al., 1977), and eventually occupies extensive anticlinorial areas on either side of a major synclinorium of schistose Lower Proterozoic rocks. This deep, tight synclinorium closes to the west, and only isolated patches of strongly deformed, intensely metamorphosed Early Proterozoic rocks are preserved within amphibolite and granulite facies gneisses in the central-west Robinson Range sheet.

In the Robinson Range and western Peak Hill sheet, the schistosity of the Early Proterozic rocks is strongly refolded along generally north-northeast and west-northwest trends. A second axial plane fabric locally transposes the first, and numerous steeply plunging second folds occur, as well as large-scale fold interference patterns.

The formation of the Stanley Fold Belt is thus related to two distinct pulses (or phases) of deformation in the east and possibly three in the west. The intensity of each deformation phase increases westwards. For example, second phase folds in the Mudan Hills are only small to medium scale features, while in the Robinson Ranges they are steeply plunging, regionally mapable structures with a well developed axial plane schistosity and abundant minor folds.

One of the most notable features of the Stanley Fold Belt is the abrupt change in trend in the northwest of the Nabber-u sheet. From east to west along this trend change the geology becomes increasingly complicated as the unit containing the iron formation members (Frere Formation) splits into a number of folded and probably thrust-bound blocks. To the southwest the blocks rejoin to form a complex northeast-southwest trending synclinorium cut by a number of strike faults, probably also thrusts.

While changes in strike of the major structures at the western end of the fold belt (Robinson Range and western Peak Hill sheets) can be related to later deformations, no such cause can be attributed to the area in the northwest Nabber-u sheet as no significant second phase structures are evident.

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Two strong north-northwest trending faults cut Archaean rocks south of the Early Proterozoic unconformity in the south and southwest of the Nabberu sheet, and are projected northwards as the boundary between the Earaheedy and Glengarry Sub-basins. The basal Early Proterozoic sediments (Yelma Formation) thicken quite rapidly immediately to the west of each fault, suggesting that the faults were active during sedimentation. The western fault slightly displaces the unconformity near Shepherd Well, while the eastern fault can be projected to a northwest trending anticline-syncline couple at Mt. Lockeridge in the Frere Range, and further northwest bisects the change in trend of the Stanley Fold Belt.

The folds at Mt. Lockeridge are clearly draped over a west-side-down dis- placement in the basement and have been deformed into small but spectacular dome and basin interference structures during the formation of the Stanley Fold Belt.

It is suggested that these basement faults were active over a considerable time span, and not only controlled deposition within the Nabber-u Basin, but also influenced the subsequent deformation of the basin in the same manner as the salients and recesses described by Rankin (1976) from the Appalachians.

The age relationship between the Stanley Fold Belt and the Ophthalmia Fold Belt which deforms the Early Proterozoic sediments of the Hamersley Basin is uncertain. While deformation in the Stanley Fold Belt increases northwards that in the Ophthalmia Fold Belt increases southwards, so that the Yilgarn and Pilbara Shields are flanked by two Early Proterozoic fold belts with opposite symmetry.

REGIONAL METAMORPHISM

Introduction

Regional changes in metamorphic grade accompany the variations in structural deformation in the Nabberu Basin and its Archaean basement (Figs.3,5). The assemblages within various rock suites indicate a general increase in metamorphic grade from essentially unmetamorphosed sediments on the Kingston Platform to a maximum grade of granulite and amphibolite facies in the strongly deformed western parts of the Stanley Fold Belt*. In the high- grade areas basement and cover lithologies are difficult to separate, although characteristic rock types such as serpentinites (Archaean) and bedded carbonates (Early Proterozoic) are useful in a few areas.

Studies of iron oxide mineralogy and quartz grain sizes in the iron formations throughout the Nabber-u Basin have revealed subtle indicators of metamorphic grade which are particularly useful in the low grade areas (Goode and Hall, in preparation). Quartz grain sizes increase regularly from micron size in the east

*The highest grade assemblages could possibly represent an earlier metamorphic episode within the basement only.

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‘Unmetomorphosed’

100 Kilometres

Fig.5. Generalised metamorphic zonation of the Nabberu Basin.

to a maximum of 7 mm in the west. Iron oxides progressively change from fine, red hematite “dust” through platy hematite to magnetite with increasing grade.

The Nabberu Basin has been divided into four major metamorphic areas; an eastern ultra-low grade zone, central lower and upper greenschist facies regions, and a western high grade area.

Kingston Platform (ultra-low grade)

Rocks of the Kingston Platform are essentially unmetamorphosed, and become even less metamorphosed to the southeast, where delicate microfossils are preserved in iron formations (Walter et al., 1976). Slaty cleavage first appears in argillaceous units north of a line extending from the middle reaches of Sholl Creek on the Stanley sheet west-northwest to Mt. Lockeridge on the Nabber-u Sheet, then southwest to Killara Homestead on the Glengarry sheet. This line separates the southern unmetamorphosed area from lower greenschist facies assemblages (Figs.3,5).

The dominant iron oxide on the Kingston Platform is a very fine red hematite dust, which is regarded, along with very fine-grained quartz, aa the usual mineral precipitated from the original chemical gels. Minor quantities of chamosite and the one recorded example of minnesotaite which occurs in the rosette form, are believed to be diagenetic, in agreement with the observations of Dimroth and Chauvel(1973) on the Labrador iron formations.

Platy hematite also occurs throughout the Kingston Platform, but its abundance and grain size decrease eastwards.

Stanley Fold Belt - eastern low grade areas

Lower greenschist facies assemblages occur in the Stanley Fold Belt from the Kingston Platform in the east to the eastern Robinson Range sheet in the west. Argillaceous to arenaceous rocks typically contain fine-grained metam-

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orphic sericite, resulting in the formation of fine-grained schists and slates in which sedimentary structures are often preserved. Basic rocks in the west also contain low-grade assemblages, viz., plagioclase-tremolite-epidote- calcite-chlorite-quartz. Iron formations typically contain quartz (generally 5-100 pm in size), magnetite, platy hematite and minor red hematite dust.

Stanley Fold Belt -Mt. Seabrook area

Upper greenschist facies assemblages are observed in the Mt. Seabrook area, central Robinson Range sheet. Psammitic to semi-pelitic rocks consist of medium to coarse-grained quartz-muscovite schists with accessory chlorite, fuchsite and magnetite. Talc schists, mined by Westside Mines Limited near Mt. Seabrook, occur within dolomitic units (Lipple, 1975). Basic rocks variably consist of tremolite-chlorite-magnetite + carbonate, and blue/green hornblende-epidote-quartz-sphene.

Iron formations contain minor metamorphic muscovite, chlorite and biotite, but consist essentially of quartz (generally 50 to 500 pm in size), magnetite and lesser platy hematite.

Stanley Fold Belt - western high-grade areas

Amphibolite and granulite facies assemblages, some partly retrograded, occur in the vicinity of Errabiddy and Moorarie Homesteads in the western Robinson Range sheet, and at Conical Hill in the eastern Glenburgh Sheet. Observed assemblages include: (a) basic rocks

(i) plagioclase-clinopyroxene-orthopyroxene-hornblende-opaques f garnet

(ii) brown/green hornblende-quartz-garnet (iii) quartz-b1 ue / green hornblendegarnet-plagioclase-clinopyroxene-

epidote-opaques-muscovite-chlorite-sphene (b) pelitic rocks

(i) quartz-anthophyllite-cordierite-r-utile (ii) quartz-anthophyllite-cordierite-orthopyroxene (-garnet-i-utile) (iii) quartz-garnet-anthophyllite-rutile (* plagioclase)

Assemblage (i) is probably derived from lower grade greenschist facies assemblages by the dehydration reaction chlorite + quartz -+ anthophyllite + cordierite + HzO.

The orthopyroxene in assemblage (ii) may be a further dehydration reaction (at about 800°C - Turner, 1968) anthophyllite -+ orthopyroxene + quartz + H,O. Assemblage (iii), and transitional assemblage (ii) are probably derived by a further dehydration reaction cordierite + orthopyroxene (or anthophyllite?) + garnet + HzO. This reaction is a “sliding” one i.e., reactants and products coexist over a finite P-T interval.

These assemblages are indicative of upper amphibolite to granulite facies

metamorphism (Hensen and Green, 1973), but no alumino-silicates have yet been found in the pelitic rocks. (c) talc-silicates

(i) dolomite-tremolite-olivine-calcite (ii) dolomite-diopside-calcite t- olivine (iii) serpentine-tremolite-diopside-chlorite-spine1 f plagioclase

(d) banded iron formations (i) quartz-magnetite (or martite) + clinopyroxene t orthopyroxene

+ tremolite The pyroxenes contain exsolution lamellae in some instances, a most un-

usual occurrence as these features are normally found only in igneous pyroxenes. Exsolution lamellae also occur in pyroxenes in high grade iron formation in the Lake Superior district, U.S.A. (Simmons et al., 1974; Bonnichsen,.1975). Quartz grain sizes are always greater than 100 pm, and usually greater than 500 pm. A maximum grain size of 7 mm has been observed near Errabiddy. (e) quartzo-feldspathic gneisses-granitic gneisses dominate the terrain around Errabiddy and in some cases contain garnet or orthopyroxene.

STRATIGRAPHY

Introduction

The term Nabber-u Supergroup is introduced to encompass the Early Proterozoic rocks deposited in the Nabber-u Basin. The most complete stratigraphic succession within the Nabber-u Basin is preserved on the Kingston Platform portion of the Earaheedy Sub-basin in the Kingston and Stanley sheets. Eight formations have been recognized, and comprise a total thickness of nearly 6,000 m of largely shallow-water marine sediments, named the Earaheedy Group (Fig.6). To the west and north in the Nabberu sheet slightly deeper water facies rocks appear, some of the units recognised to the southeast die out, and thicknesses of the lower units in particular show marked changes.

The type sections of all but one of the formations have been taken in the Kingston-Stanley area, because of the relatively easy access, the moderately good exposure and the lack of structural or metamorphic complications. Much of the stratigraphic description that follows is therefore valid only for the Earaheedy Sub-basin but, where possible, correlations are made with the stratigraphy developed in the Glengarry and Padbury Sub-basins.

The division of the Earaheedy Group into a lower Too100 Sub-group and an upper Miningarra Sub-group is based on its deposition in two distinct cycles of sedimentation. The stratigraphic units were defined by Hall et al. (1977) and type sections were described for each formation.

FORMATION

Mulgarra Sandstone

Kulele Creek LlmeStOtle

APPROX. THICKNESS LITHOLOGIES

Sandstone, rmnor shale and limestone

Limestone, minor sandstone and shale

Wongawol Formation 2000m Sandstone, rmnor shale and hmestone

Princess Ranges Quartzite zoom puartzlte, sandstone and minor slItstone

Wandiwarra Formation 350m Sandstone, quartzite and shale

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Windidda Formation 12mm Limestone. lImestone conglomerate, shale, minor sandstone and chert

; i Frere Formation Iron formation, chert. SlItstone.

shale and rare Ilmestone, dolomite

Yelma Formatton pSondstone, rmnor conglomerate, 130m

- siltstone and iron formation

Fig 6. Generalised composite stratigraphic section of the Earaheedy Group.

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Too100 Sub-group

The Too100 Sub-group comprises the Yelma Formation at the base, the Frere Formation, and the Windidda Formation at the top.

Yelma Formation The Yelma Formation, oldest unit in the Earaheedy Sub-basin, is named

from exposures six kilometres northwest of Yelma outstation in the west central portion of the Kingston sheet. The formation was defined by Hall et al. (1977) as the medium to coarse-grained, quartz-rich elastic rocks which lie unconformably on the Archaean basement. It varies from 10 m to about 150 m in thickness along the southern edge of the Earaheedy Sub-basin.

The type Yelma Formation is composed of buff weathering white to cream, medium to very coarse-grained clean quartz sandstones, sometimes arkosic, with minor bands of quartz pebble or quartz cobble conglomerates. The sandstones are flat bedded or laminated,’ with occasional cross-bedding and ripple marks, and rare mud pellets.

Petrographically the sandstones are supermature and composed of well- rounded, well-sorted quartz grains, sometimes with minor rounded chert fragments, microcline and plagioclase, and accessory tourmaline, zircon and limonite grains. They are typically quartz cemented. Rounded glauconite pellets are present in the Yelma Formation in the Duketon and Throssell sheets (Bunting and Chin, 1975; Bunting et al., 1975), but have not as yet been observed further west except for one possible occurrence in the Glengarry sheet. Pebbles and cobbles of vein-quartz in the conglomerates are usually rounded but may be angular, especially where quartz reefs occur in the immediately underlying basement.

Three kilometres west of the type section the Yelma Formation includes finely laminated, grey to white siltstone with a 5-cm thick band of shaly, microbanded iron formation, while a kilometre east of the type section a thin band of red-grey oolitic/pelletal iron formation within shales lenses out towards the type section as it approaches a small topographic high of about 30 m relief along the unconformity surface.

Probable equivalents of the Yelma Formation occur in the deformed northern portion of the Earaheedy Sub-basin in the Stanley and Nabberu sheets. The stratigraphy and thickness of these rocks have not been determined with certainty, but they appear to include a lower medium-grained, grey-brown impure quartzite unit, with scattered rounded quartz pebbles and thin grit bands, now psammitic quartz mica schists, and an upper phyllite and slate unit containing thin bands of schistose quartzite and conglomerate. These rocks are equivalent to the Malmac Formation of Horwitz (1975b).

Due to increasing metamorphism and deformation which leads to conver- gence of basement and cover lithologies, the Yelma Formation is less well known to the west. It appears to be much thicker and finer-grained in the Glengarry Sub-basin and includes a lower quartzite with minor conglomerates

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(Finlayson Sandstone of Bunting et al., 1977) and an upper siltstone with minor chert, sandstone, conglomerate and dolomite (Maraloou Formation of Bunting et al., 1977) in the south, and interbedded slate and psammitic schist in the north. Possible equivalents of the Yelma Formation in the Peak Hill area (Peak Hill Beds of MacLeod, 1970) are composed of fine to coarse g-rained elastic sediments, now quartz-mica and chlorite schists and quartzites with occasional conglomeratic beds. Thin beds of iron formation are also reported by MacLeod who suggests a minimum thickness of 1,700 m for the Peak Hill Beds.

In the western Robinson Range sheet possible Yelma Formation equivalents are similar to the sediments in the northern Nabber-u sheet, but of higher metamorphic grade. Thin meta-conglomerate and meta-arkose bands and rare banded iron formations are contained in psammitic quartz-muscovite schists in the Mt. Seabrook area. Fuchsite-bearing schists occur locally. Similar rocks are included in the Mt. James Beds further to the northwest, and occur as outliers in the Gascoyne Block (S. Williams, personal communication, 1977).

A feature of the Peak Hill-Robinson Range area is the occurrence of large piles of metabasaltic rocks (tremolite schists) interbedded with the basal sedimentary unit. A volcanic origin for the basalts is supported by the occurrence of abundant volcano-elastic greywackes in the east-central Peak Hill sheet, and basic tuffs in the northwest Glengarry Sheet. The greywackes are locally cross-bedded, poorly sorted and immature. They are composed of abundant rounded to angular clasts of quartz, chert, sandstone, plagioclase (trachytic texture), meta-dolerite, chlorite, epidote and feathery feldspar (devitrified glass?) in a chloritic matrix. The igneous rock and mineral fragments are angular, suggesting a local source, while the quartz grains are typically well rounded, indicating an admixed, second more mature provenance (see also Trendall, 1967, and Blockley, 1968). The fluviatile Kaluweerie Con- glomerate (Allchurch and Bunting, 1976) and other outliers up to 120 km south of the Glengarry Sub-basin may also be equivalent to the Yelma Formation.

Frere Formation The Frere Formation is named from the Frere Range along the northern

side of Lake Nabber-u on the Nabberu sheet, and consists dominantly of ferruginous chemical sediments with interbedded fine-grained elastics and minor carbonates. It lies between the Yelma Formation and the Windidda Formation east of Lake Teague, but to the west it is overlain by the Wandiwarra Formation, lowest unit of the Miningarra Sub-group.

The formation thickness of between 1,300 m and 2,000 m appears to remain fairly constant along the southern margin of the Earaheedy Sub-basin, but within this interval the formation exhibits some quite strong facies changes. An iron formation/shale ratio of about 80:20 in the west of the Nabber-u sheet changes to a ratio of less than lo:90 in the southeast of the Kingston sheet, as individual bands of iron formation thin from a maximum of 200 m thick in

Fig.7. Interbedded lenticular iron formations and shales of the Frere Formation, Frew Range. Hammer for scale.

the west to generally less than a metre in the southeast. The iron formations also tend to become more siliceous eastwards, and the top of the Frere Forma- tion in the Kingston 1: 250,000 Sheet is marked by a distinctive chert unit (Hall et al., 1977). The iron formations are of several different types. (1) Intraclastic (pelletal)

The most common iron formation is a steel grey to maroon, often mottled and internally unbanded ferruginous chert. This usually occurs in evenly spaced, slightly podded bands 15 cm thick, interbedded with bands up to 10 cm thick of microbanded hematitic ironstone, non-ferruginous and ferruginous chert, siltstone, shale and rare fine conglomerate and grit. Lensing in the iron formations is relatively common (Fig.7).

The ferruginous cherts have a distinctive elastic (pelletal) appearance, comprising rounded to rarely angular intraclasts of ferruginous chert (hematite- quartz + magnetite) and/or chert up to 2 mm in diameter, in a chert or chalcedonic matrix (Figs.8a, b). Detrital phases such as muscovite and tourmaline are occasionally present. The pellets are generally sub-spherical in shape, although in some units they may be irregularly tabular and curved (Figs.&, 11). The latter texture, especially as illustrated in Fig.ll, is considered to result from soft-sediment compaction as very similar textures are observed in the younger Wandiwarra Formation where irregular glauconite pellets show differ- ential compaction features into shale (mud) lenses (Fig.8d).

Fig.8. a. Rounded to subangular intraclasts (pellets) of ferruginous chert in chert matrix. b. Angular intraclasts of ferruginous chert in chert matrix. c. Elongate intraclasts of ferruginous chert in chert matrix. d. Elongate to wispy pellets of glauconite and minor rounded quartz grains. Note indentation of glauconite pellets into shale (mud) layer, indicative of soft sediment deformation.

Shrinkage or syneresis cracks filled with clear, non-dusty quartz are common in individual intraclasts (Fig.Sa); authigenic quartz crystals up to 0.5 mm in size also rarely contain texturally preserved shrinkage cracks (Fig.Sb). In some instances intraclasts are inhomogeneous, comprising iron-rich rims (usually magnetite) and chert cores (Fig.Sa-c). In view of the metamorphic nature of magnetite in these iron formations (Goode and Hall, in preparation) these inhomogeneous pellets are regarded as being produced by low-grade metamorphic segregation from the normal homogeneous pellets.

The cement or matrix to the hematite or chert intraclasts usually comprises either fine-grained chert (Fig.Sc) or chalcedonic quartz. These components are regarded as the equivalents of micritic matrix and sparry cement respectively in carbonate rocks (cf., Dimroth and Chauvel, 1973; Dimroth, 1975). In some cases two stages of chalcedonic cement can be recognised; an older thinner, regular rimming of the pellets and younger coarser fan-aggregates filling the remaining interstices (Fig.Sd). In other cases coarse equigranular aggregates of quartz may irregularly replace both pellets and matrix (Figs. lOa, b); these textures are regarded as diagenetic in origin.

Locally the pelletal ferruginous chert is prominently mesobanded, and consists of alternating layers of pelletal ferruginous chert and microbanded non-pelletal chert (Fig. 11). Individual bands, usually of microbanded chert, are commonly disrupted or fragmented especially in the upper part of the Frere Formation. Large, distinctive tabular chert clasts up to 4 cm long are incorporated in the pelletal units (Fig.12a) in which thinning (podding) and small scale scouring are common. These features are interpreted as indicating later or contemporaneous shallow water reworking of originally banded iron formations of Hamersley type, and are very similar to textures and structures in the granular iron formations of the Lake Superior region and Labrador Trough of North America (Gross, 1972; Mengel, 1973; Dimroth and Chauvel, 1973; Floran and Papike, 1975; Klein and Fink, 1976). Although such features may not necessarily form in high energy environments (see Adeleye, 1975), it is considered from the bulk of evidence that most of the Frere iron formations resulted from deposition during storm and wave action of varying intensity.

The alternation of pelletal ferruginous cherts and micro-banded non-pellet& chert described above is of some interest. An example of this combined textural and chemical cyclicity led Goode and Hall (1976) to originally suggest possible varve-like deposition of the layers, i.e., alternating quiet depositional periods with associated silica precipitation, and turbulent periods with com-

Fig.9. a. Shrinkage (syneresis) cracks in rounded chert intraclast. Note rim of euhedral magnetites on intraclast. b. Textural relic of shrinkage crack in single authigenic quartz crystal within magnetite- rimmed intraclast. c. Rounded intraclasts of chert, and chert rimmed by magnetite, in chert matrix. Crossed polar-s. d. Two stage chalcedonic cement between rounded ferruginous chert intraclasts; an older stage of regular thickness, and a younger stage of coarse fan-aggregates. Crossed polars.

I

Fig.lO. a, b. Rounded chert and ferruginous chert intraclasts with shrinkage cracks. Nott! irregular presence of coarse-grained quartz in both intraclasts and matrix (11 in (srossetl polars).

bined iron and silica deposition. Such a periodicity could have been seasonal and may even have had some relationship to glacial conditions as suggested for other iron formations by Williams (1972) and Young (1976). However, zrrnther simpler explanation is also available.

If, during periodic storms on the shallow water Kingston Platform, areas of ferruginous chert deposition (e.g., lagoonal flats?) were swept and eroded by storm waves, the resultant detrital load could be carried elsewhere and deposited in areas of quiet-water deposition where only silica was being precipitated. Cessation of storm activity could then allow more quiet-water deposition of silica, and thus a cyclic (or apparently varved) sequence could be built up. In view of the occurrence of such rocks at the top of the Frere Formation at a time of regression (see later), this latter explanation is now preferred. (2) Oolitic

Thin units of oolitic iron formations occur within the pelletal ferruginous cherts, especially in the southeast of the Earaheedy Sub-basin. The ooliths are usually elliptical in shape, and rarely contain multiple nuclei of ferruginous chert or chert (Fig.13a). The banding is caused by thin chert layers alternating with thicker ferruginous chert bands. Syneresis cracks are common, and the matrix is always silica.

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(3) Oncolitic, stromatolitic About 2.5 km northeast of Camel Well on the Wiluna sheet abundant algal

filaments and microfossils occur in a thin oncolitic and stromatolitic iron formation (Figs.l3b, c) interbedded with pelletal and oolitic ferruginous cherts (Walter et al., 1976). The micro-fossils show strong similarities to benthonic micro-organisms from the Early Proterozoic Gunflint and Biwabik Iron Formations of the Lake Superior region, although their preservation is not as good (cf., Barghoorn and Tyler, 1965). Minor rosettes of minnesotaite(?) occur with the microfossils. Large syneresis cracks in the oncolite cores (Fig. 13b) are filled with small rounded intraclasts indicating gel dewatering and recrystal- lization at an early stage prior to final sedimentation. (4) Laminated

In the west of the Nabberu sheet a 200 m thick banded iron formation, the Illagie Iron-Formation Member, is developed at the top of the Frere Formation.

Fig.11. Alternating mesobands of intraclastic ferruginous chertlchert (with rare large chert intraclasts) and microbanded chert.

Fig. 12. Banded iron formation and chert, with large, tabular chert clasts included in the intraclastic iron formation, from the top of the Frere Formation, Frere Range. Length of specimen 17 cm.

This is a dark purple to steel grey and black, microbanded hematite-rich ironstone with minor chert (Fig.l3d), and is believed to be a deeper water, non-reworked equivalent of the pelletal ferruginous chert. The basal 2-3 m are very siliceous, and include rare mesobands of pelletal ferruginous chert. This member thins to the east, and is absent east of Canning Gap near Mt. Lockeridge.

The elastic units separating individual bands of iron formation in the eastern Earaheedy Sub-basin are generally grey, yellow, and maroon siltstones, shales and sandy shales up to 100 m thick (e.g., Lockeridge Shale, Wy-Uda Shale, and Murgurra Shale Members). The siltstones are finely laminated and often have small scale cross-bedding, and frequently include bands of chert less than 3 m thick. Climbing ripples are observed in some siltstone units.

The elastics vary in grain size from medium silt to very fine sand, and are composed of detrital quartz with minor muscovite, clay, plagioclase, limonite, chlorite and tourmaline. In some units, unusual thin, angular “broken glass” slivers of quartz are common. The origin of these grains is unknown, but recent investigations by us in the Hamersley Basin have shown a possible association between “sliver” quartz in siltstones and thin tuffaceous units

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Fig.13. a. Ferruginous chert oo]iths, including rare mult ip le oo]iths, in chert matrix. b. Oneolite wi th algal layers coating intraclastic iron format ion core. Note large shrinkage crack f i l led with intraelast~. c. Algal f i l amen t s (Gunflintia minuta) in fe r ruginous che r t onco l i t e near Camel Well. d. L a m i n a t e d fe r ruginous cher t , Illagie I r o n - F o r m a t i o n Member .

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containing broken (?) quartz crystals of similar morphology. Thus the "sliver" quartz in the Nabberu sediments could indicate either some previously un- recognised tuffaceous units or a provenance from Archaean crystal tuffs.

An alternative explanation relates to the suggested peneplained and weathered nature of the Archaean basement at the time of the Early Proterozoic transgression. The "sliver" quartz is similar in shape to some angular, "shard-like" quartz grains in Tertiary silcretes (e.g., Milnes and Hutton, 1974), thought to result from prolonged pedogenic processes in stable, low relief environments (Hut ton et al., in press). Consequently, the "sliver" quartz in the Early Proterozoic sediments could also be interpreted as erosion products of late Archaean or Early Proterozoic silcrete.

Carbonate units occur near both the base and top of t he Frere Formation. At the base of the formation 7 km southeast of Mt. Deverell, light grey, welt bedded, locally oolitic dolomite 20 m thick contains an assemblage of three distinctive stromatolites (Fig. 14; M.R. Walter, pets. comm. ):

(a) small columnar stromatolites with common slightly divergent branch- ing and gently convex laminae (up to 8 cm high, 0.5--2 cm wide, with minimum growth relief of 5 cm);

(b) large, subcylindrical, non-parallel columnar stromatolites with gently convex laminae, locally sharply deflexed at column margins (20 cm high, 15 cm wide, with minimum growth relief of 2 cm);

(c) pseudocolumnar and linked bulbous stromatolites with included tabular and subspherical oncolites up to 2--3 cm long. Gently convex laminae with a distinctive, regular, finely banded microstructure. Partially replaced by chert. Pseudocolumnar units 0.5--2 cm wide and up to 5 cm high; bulbous structures 10--15 cm wide, 10--15 cm high (maximum growth relief of about 2 cm).

The stromatolites usually contain relatively abundant carbonaceous material

u

4crn I I

Fig. 14. Small columnar (a), large columnar (b) and linked bulbous (c) stromatolites from basal Frere Formation, southeast of Mt. Deverell.

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along the algal laminae, especially in (b), and also contain a minor terrigenous silt fraction.

Stromatoli tes have also been found in isolated blocks of carbonate about 13 km southeast of this locality, and above the Yelma Formation in the southeastern Kingston sheet (J. Bunting, pers. comm., 1976), and the northeastern Duketon sheet (Preiss, 1976).

A fine-grained limestone of unknown thickness and extent occurs just above the lowest iron formation, 8.5 km west-southwest of old Windidda Homestead on the Kingston sheet.

Near the top of the Frere Format ion 4 km northeast of Simpson Well in the west of the Nabberu sheet, a 40-m thick banded marble with chert concretions grades down into banded calcareous chert and pale pelletal chert, and is overlain by ferruginous chert. At about the same horizon in the southeast of the Nabberu sheet, near Oxby 's Well, pink quartzose limestone with poorly developed stromatolites underlies the uppermost pelletal ferruginous chert unit.

A completely silicified, crossbedded, sandy oolitic rock interbedded with claystone and gritty sandstone on the southern side of the von Treuer Table- land (eastern Kingston sheet) may also represent an oolitic carbonate near the top of the Frere Formation. Large rounded, oblate concret ionary pisoliths with asymmetrical layering are also present in the unit (long axis vertical; banding eccentricity consistently on lower side of the pisoliths). These features indicate the pisoliths are vadose in origin (Dunham, 1969; Assereto and Folk, 1976), which in turn indicates a dry climate at the time of formation, and repeated subaerial exposure.

The Frere Format ion is tentatively correlated with the Horseshoe Range Beds, Labouchere Format ion and Robinson Range Formation (MacLeod, 1970; Barnett, 1975) of the Peak Hill and Robinson Range sheets. These units total about 3,000 m thick, and consist dominant ly of medium to fine grained clastics including flysch-like units, with rare pelletal iron formation, rare thin dolomite, and banded iron formation at the top (Robinson Range Formation) and b o t t o m (Horseshoe Range Beds) of the succession. The Horseshoe Range Beds in particular have many sedimentary and textural features in common with the iron formations of the Frere Formation. The occurrence of large blocks (olistostromes) of banded iron formation within the Peak Hill Beds (Horwitz, 1976) may indicate contemporaneous slumping of the Frere Forma- tion or its equivalents into the Peak Hill Beds.

Windidda Formation The Windidda Formation, named from Windidda Homestead in the central

part of the Kingston sheet, consists of carbonates and fine-grained clastic sediments lying between the Frere Format ion and Wandiwarra Formation.

In the type area the basal few metres of the Windidda Formation consists of brown ankeritic carbonates with interbedded pelletal ferruginous cherts, pelletal chlorites, black sulphidic cherts and stromatolitic horizons. There is

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textural evidence that at least some of these basal carbonate units have been formed by replacement of pre-existing cherts. Sedimentary chlorite with a chamositic composition occurs as very fine-grained material on bedding planes and algal laminations, and as reworked pellets within carbonates and cherts (Fig. 15). Rare detrital chlorite is also associated, and some laminated chamosite has been found finely intergrown with chert. Chamosite and also glauconite, which is found elsewhere in the Nabberu Basin, have rarely been observed in Precambrian rocks (James, 1966). Apatite, ankerite, chalcopyrite and sphalerite also occur in some of the thin, dark chert bands. The apatite occurs as small stubby hexagonal prisms (Fig.16a).

At about the same stratigraphic level just west of the Wiluna--Carnegie road, finely laminated, pinkish fine sandstone bands 2--3 cm thick are interbedded with finely laminated pinkish to maroon shale bands 5--10 cm thick. Thin lenses of impure limestone occur in both the sandstone and shale bands.

The basal units in the type area are overlain by grey to pink locally conglomeratic laminated limestones containing abundant stromatolitic beds, and interbedded with calcareous shales which become more common higher in

Fig. 15. Chamosite pellets in ankeritic carbonate, basal Windidda Formation near Tooloo Bluff. Scale 2.8 cm.

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Fig. 16. a. Apatite prisms in chert interclast, basal Windidda Formation. b. Size-grading in fitted calcite polygons, upper Windidda Formation. c. Rounded glauconite pellets and quartz grains in ferruginous sandstone, Wandiwarra Formation. d. Radial calcite pisoliths with wavy concentric banding in ferruginous gtauconitic sandstone. Note rare inclusions of glauconite pellets within the pisoliths.

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the sequence. Finely laminated red chert and rare pelletal chert are also found near the base, and ferruginous concretions and manganese bloom are frequently observed.

Bands of sandstone and intraformational carbonate conglomerate become common higher in the sequence, so that the formation consists of limestone, carbonate conglomerate, pink to grey-green, fine-grained, finely laminated and cross-bedded sandstone and minor maroon shale in bands up to 2 m thick, but generally less than 50 cm thick. The carbonate conglomerates are both evenly and irregularly banded, and composed of angular fragments of green, pink and grey limestone up to 15 cm long in a carbonate mud matrix. Towards the top of the Windidda Formation the conglomerate becomes a mixed car- bona te -che r t conglomerate, and is interbedded with pink to grey-green, fine micaceous sandstone with abundant mud pellets.

The top of the Windidda Formation is marked by a prominent 1--2 m thick band of intraformational limestone conglomerate which forms a small but conspicuous strike ridge for about 8 km west of the Wiluna--Carnegie road (Fig.17). The conglomerate contains large (5 cm) flat clasts of laminated limestone in a quartz--glauconite sand matrix. Irregularly layered calcite of variable grain size and with both normal and reverse size grading occurs in the conglomerate (Fig.16b). Microtextural studies show that this calcite has a polygonal fitted texture with some polygons preserving a fibrous radiating morphology with rare concentric banding. Large, wavy concentric pisolites

Fig. 17. Intraformational limestone conglomerate, upper Windidda Formation. Scale 2 cm.

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Fig. 18. a. Vertical sec t ion view of large bulbous s t romato l i t e , basal Windidda F o r m a t i o n near Too loo Bluff. Scale 2.8 cm. b. Plan view of co lumnar s t romato l i t e s in Windidda F o r m a t i o n near Moonl igh t Bore. Scale 2.8 cm.

are also found in the overlying Wandiwarra Formation, and are very similar to those described by Dunham (1969) from pisolitic carbonates of vadose origin in the Capitan Reef. This implies subaerial exposure of the uppermost Windidda carbonate sediments in a dry climate.

The Windidda Formation is confined to the southeast of the Earaheedy Sub-basin. Its apparent absence to the north and northwest is probably due to the transition in sedimentary environment from a southern carbonate shelf to a deeper water area to the north. However, local areas of thin carbonate deposition near the suggested top of the Frere Formation in the Nabberu sheet may represent the equivalent stratigraphic interval.

The Windidda Formation has been traced eastwards into the Robert sheet south of Moonnight Bore where ankeritic stromatolitic carbonates and intraformational limestone conglomerates are intruded by porphyritic dolerite. Fine-grained scapolite occurs in the limestone above the dolerite contact. Isolated outcrops of the Windidda carbonates have been traced as far east as the central Robert sheet (Jackson et al., 1973).

In the Kingston and Robert sheets the lower part of the Windidda Forma- tion in particular contains two common, distinctive and separate stromatolite assemblages (see also Preiss, 1976). Large, partially linked bulbous stromatolites up to about 15 cm in diameter (Fig.18a) are probably associated with a very high energy environment (marine intertidal?), as the stromatolite growth is sometimes interrupted by erosion, and coarse carbonate clasts up to 5 cm across are occasionally found in the inter-space sediments (M.R. Walter, personal communication). Water depth was at least 15 cm. The other common stromatolite consists of sub-circular, sub-parallel columns 2--5 cm wide and at least 10--12 cm high (Fig.18b). The stromatolites had at least several centimetres of growth relief. The interspace sediments are generally coarse sand size.

Metamorphosed often talcose dolomites are found above thin iron forma-

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tion in the core of the Mt. Seabrook synclinorium in the central Robinson Range sheet, and possibly represent Windidda Formation equivalents. Other- wise no equivalents of the Windidda Formation are known in the central and western Nabberu Basin.

Miningarra Su b-gro up

The Miningarra Sub.group comprises the Wandiwarra Formation at the base, the Princess Ranges Quartzite, Wongawol Formation, Kulele Creek Limestone, and Mulgarra Sandstone at the top.

Wandiwarra Formation The Wandiwarra Formation is named from Wandiwarra Well 12 km east

of Windidda Homestead on the Kingston sheet, and lies between the Windidda Formation and the Princess Ranges Quartzite.

At Wandiwarra Well the formation is approximately 350 m thick, and consists largely of pink to grey, finely laminated, cross,bedded micaceous fine sandstone with occasional ripple marks and small mud pellets. Interbedded with the fine sandstone are one to two metre thick bands of grey,brown, impure micaceous quartzite and finely laminated maroon shale. At the top of the formation is a siliceous, micaceous shale.

About 15 km west of the type section, finely laminated fine sandstone with thin chert bands at the base of the Wandiwarra Formation is overlain by finely laminated yellow, brown to maroon, micaceous sandy shale and a 3--4 m thick, cross-bedded, finely banded chert and fine sandstone which forms a prominent scarp at Jackie Lookout and the adjacent isolated hills. This is followed by about 500 m of pink to grey, fine micaceous sandstone with thin intercalations of medium to fine grained, impure quartzite and pink, siliceous micaceous shale.

In the vicinity of the old Wiluna-Carnegie road, the base of the Wandiwarra Formation is marked by a thin but prominent medium-grained, ferruginous, mature quartzose glauconitic sandstone (Fig.16c), which is overlain by a similar ferruginous glauconitic sandstone containing fiat shale pebbles and distinctive calcite pisoliths of possible vadose origin. These pisoliths exhibit a radial structure and an unusual wavy concentric banding unlike that observed in normal sedimentary pisoliths (Fig. 16d). Scattered glauconite pellets within the pisoliths also suggest in situ replacement of the host arenite. The rounded glauconite pellets in the upper unit are largely replaced by calcite. Further to the west, the basal Wandiwarra unit is a ferruginous, poorly sorted muddy glauconitic litharenite containing large fiat shale clasts up to 8 cm long, and some chert pellets. Detrital quartz sand grains in these units are typically well rounded.

In the southeast of the Nabberu sheet the Wandiwarra Formation consists of finely laminated, locally cross-bedded sandstone with thin bands of micaceous shale. The finer grained rocks become more dominant to the north

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and northwest, where the Frere Formation is overlain by maroon, grey and yellow-brown, finely banded siltstone with occasional thin bands of sandstone, impure quartzite, and banded iron formation.

In the east of the Earaheedy Sub-basin in the western Rober t sheet the Wandiwarra Formation appears to be absent. Medium-grained glauconitic quartz sandstone typical of the overlying Princess Ranges Quartzite rests directly on conglomeratic limestones of the Windidda Formation. The sandstones contain mud pellets and are interbedded with micaceous siltstones and fine sandstones.

In the type area the Wandiwarra Format ion appears to be conformably overlain by the Princess Ranges Quartzite representing the incoming of extensive mature quartz sand deposition. The nature of the lower contact with the Windidda Formation is less certain. Where best exposed, at Wandiwarra Well, Jackie Lookou t and on the old Wiluna--Carnegie road south of Wongawol Homestead, the uppermost Windidda Formation member is a reworked carbonate, suggesting that the Wandiwarra Format ion is disconformable above. Glauconite is a common const i tuent in the Wandiwarra Formation and the Princess Ranges Quartzite bu t is not normally observed in the Windidda Formation.

To the northwest the ferruginous conglomerate at the base of the Wandiwarra Formation also suggests a locally disconformable relationship. Further north and northwest, however, the Wandiwarra Format ion is finer grained and appears to be conformable with the Frere Formation.

The glauconite in the Wandiwarra Format ion is very aluminous, and has a skolitic composition. X-ray studies show that it is well ordered. The glauconite usually forms well rounded, spheroidal to tabular pellets 0.2--0.4 mm long, with an internal random microcrystalline fabric (terminology of McRae, 1972). The aluminous nature of the glauconite may have genetic implications as Keller (1958) has reported glauconitic mica (17.9% A1203) from the non-marine Morrison Format ion in Colorado, and similar material has been found in non- marine sediments near Lake Eyre in South Australia (K. Norrish, pers. comm., 1975).

The Wandiwarra Formation is probably stratigraphically equivalent to the Millidie Creek Formation (Barnett, 1975) of the Robinson Ranges area. The Millidie Creek Formation includes slates, chert, quartz schist and conglomerate, and is the youngest formation preserved in the Robinson Range--Peak Hill area.

Princess Ranges Quartzite The Princess Ranges Quartzite is named from the Princess Ranges west of

Wongawol Homestead on the Kingston sheet and lies conformably between the Wandiwarra Format ion and Wongawol Formation.

The formation is dominated by three main units of massive quartzite interbedded with units of fine sandstone and sandy siltstone. The total thickness of the formation in the type area is about 200 m, with individual beds varying from 2 to 20 m in thickness. The quartzite units are medium to coarse grained,

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supermature quartz-cemented quartz sandstones, with rare grit bands. The quartz grains are typically well sorted and well rounded. In some units, particularly in the Kingston, Rober t and eastern Nabberu sheets, rounded glauconite clasts are present in variable amounts, while rounded green tourmaline grains are a rare accessory.

The quartzites are commonly finely laminated and frequently cross-bedded, and ripple marked. Mud pellets up to 1--5 cm in size are common in the quartzite units throughout the Earaheedy Sub-basin, and are usually con- centrated in specific horizons (Fig.19). Some units are ferruginous, possibly due to weathering of the glauconite, and in many cases the weathered surface of the quartzite has a distinctive "Dalmatian" texture. Ferruginous holes up to several centimetres across and sometimes partly filled with detrital quartz grains give the quartzite a speckled or "b lo t chy" appearance. Their origin is not understood, but they are quite distinct from weathered out mud pellets.

Interbedded with the quartzite units are bands of pale grey-brown and pink fine-grained sandstone with common mud pellets, and finely laminated, cross-bedded khaki and brown micaceous fine sandstone to sandy siltstone which are quartzose to subarkosic in composition. Ripple marks are common in some units and rare overturned limbs were observed in some crossbeds near Lake Teague. (These latter indicate liquefaction of the sand coupled with current drag, Lowe, 1975.) The siltstones are composed of medium to coarse

Fig. 19. Rounded mud pellets in bedding plane of quartz sandstone, Princess Ranges Quartzite. Scale 2.3 cm.

167

silt-sized angular quartz with lesser mica (muscovite, biotite, chlorite) and plagioclase. The angular nature of the clastic grains, together with the relatively abundant presence of mica and feldspar, indicates a different source of sediment to the supermature quartzite units. The siltstones are similar to those within the overlying Wongawol Sandstone. A thin biotitic quartz siltstone with a manganiferous cement was observed in the central part of the formation near Lake Teague, and some units of ferruginous glauconitic quartz sandstones with flat shale pebbles occur in the Princess Ranges.

The Princess Ranges Quartzite is generally quite similar to the Yelma Forma- tion at the base of the Earaheedy Group. However, it can usually be empirically distinguished by the general absence of conglomeratic bands, and by the occurrence of mud pellets. "Dalmat ian" texture is usually more common in the Princess Ranges Quartzite.

Wongawol Formation The Wongawol Formation is named from Wongawol Homestead on the

Kingston sheet, and lies conformably between the Princess Ranges Quartzite and the Kulele Creek Limestone.

The Wongawol Formation consists of a rather monotonous sequence of grey to pinkish-brown, finely laminated, very fine-grained micaceous sandstone and minor siltstone units about 2,000 m thick. Strongly developed sedimentary features include small scale festoon cross-bedding, ripple marks, small scour channels and rounded mud pellets up to 2 cm across, and are indicative of shallow water sedimentation. Rare mud cracks indicate local subaerial exposure of the formation. A few thin bands of shale and small pebbles also occur in the sandstone as well as a number of 50 cm thick bands in which the bedding has been contorted into small slump rolls. These latter are particularly abundant in the upper part of the formation (Fig.20).

The fine sandstones are mature, well sorted and subarkosic to arkosic in composition, consisting of angular to subangular coarse silt to very fine sand- sized quartz with lesser plagioclase, as well as muscovite and minor tourmaline and limonite. Dolomitic or calcitic cement occurs in some samples.

West of the type area the sandstone is typically pink to grey and exhibits the same well-developed sedimentary features, but differs in the greater development of shale, commonly maroon to yellow-brown and micaceous, so that locally the formation consists of alternations of 10 cm of fine sandstone and 70 cm to 1 m of slightly sandy shale.

In the upper part of the formation, previously referred to as the Sholl Creek Formation (Hall and Goode, 1975) but now given only member status, the sandstones are interbedded with bands of up to two metres thick of micaceous maroon and chocolate shale, fine calcareous sandstone, grey oolitic limestone, pink to grey oolitic limestone conglomerates, and occasional one metre thick stromatolitic limestones. The stromatolites are domal, similar to those in the overlying Kulele Creek Limestone.

The lowest carbonate unit in the formation comprises a thin (1--2 m), lam-

168

Fig.20. Slump roll in fine-grained arkosic sandstone, Sholl Creek Member of Wongawol Formation.

inated, silty oolitic limestone and conglomeratic limestone containing 1--2 cm micritic carbonate intraclasts. This unit is interbedded with cross-bedded sandy shales and fine-grained micaceous sandstone containing minor 1 cm mud pellets.

3he carbonates usually contain minor quantities of coarse terrigenous silt, g~ading up ra~eiy to a medium sand: fraction. The limestones and associated silt are usually mature and well winnowed sediments with little if any micritic matrix, and are typically shallow water sediments. The ubiquitous presence of ooliths indicates strong tidal current action. Carbonate intraclasts in the conglomerates are frequently micritic, and indicate quieter depositional environments in the vicinity of the obser~ ed conglomerates.

In the central Nabberu sheet ~he Wongawol Format ion forms quite spectacular breakaway country. Excellent exposed sections show that in the fold belt the formation is strongly cleaved and cut by low angle, north-dipping thrusts (Fig.21). Quartz and dark green chlorite (iron-bearing clinochlore) occur within these thrust zones.

Kulele Creek Limestone The Kulele Creek Limestone, named from Kulele Creek at the northwest

corner of Lake Carnegie, lies conformably between the Wongawol Formation and Mulgarra Sandstone, and is about 300 m thick.

169

The Kulele Creek Limestone comprises repeated (cyclic) sequences 5--10 m thick (Fig.22). At the base of the formation the cycles consist of purple shales and carbonate conglomerates which give way to white, massive to cross-bedded oolitic limestones, which axe in turn overlain by pink stromatolitic limestones with interdomal conglomeratic and pisolitic carbonate. At the top of the formation shales are absent, although minor carbonate conglomerates are present. This upper part of the formation is dominated by pink stromatolitic limestones with a white pisolitic limestone matrix.

Within each cycle, several generalized gradations exist: (a) the terrigenous content and grain size decrease upwards both within

and between cycles. Within the basal cycle terrigenous grain sizes decrease from medium sand to coarse silt;

(b) the carbonates show a gradation from conglomeratic (intertidal beach environment) to oolitic to stromatolitic facies (subtidal reef environment). The shales at the base of the sequence may represent supra-tidal to lagoonal mud flats;

(c) the orthochemical consti tuents of the carbonates show an increase in micritic content upwards. The pisoliths deposited between the stromatolitic mounds also frequently have micritic nuclei, sometimes up to pebble size.

These features are interpreted as indicating a general deepening of water within and between cycles, i.e., a rhythmical deepening and progressively

Fig.21. Folds and incipient north-dipping thrusts (T) in Wongawol Format ion west of Windich Spring.

170

• o o o o ' o0tm', o .~_. ~ . ' . : - ; I oo~# %.o PINK STROMATOLITIC LIMESTONE WITH INTERDOMAL

[ l l J [ I II I_l ! l

F~ '~ -~ - - ~ . PURPLE 5HALES. CARBONATE CONGLOMERA TE.

Fig.22. Schematic representation of sedimentary cycle in basal Kulele Creek Limestone.

transgressive sequence from supratidal mud-flats to shallow sub-tidal reefal conditions.

The stromatoli tes are large domal structures, up to 1--2 m across and a metre high (Figs.23a, b). The domes may be simple or consist of a broad internal dome or column structures. The fine algal laminae are characterised by an upward cuspate shape, and have incorporated sand to pebble-sized carbonate and terrigenous grains during growth. The interdomal cavities are filled with cobble-sized carbonate clasts, pisoliths and ooliths with mixed coarse terrigenous silt, micrite and sparry calcite matrix. Stromatoli tes of this general size and shape are indicative of shallow sub-tidal conditions (Serebryakov and Semikhatov, 1974). The trapping of sand-sized material as well as the association of pisoliths, ooliths and coarse carbonate clasts in interdomal hollows probably indicates strong current activity in the vicinity of the stromatolite colonies.

Carbonate ooliths and pisoliths of two distinct types coexist in the interdomal cavities of the Kulele Creek Limestone stromatolites. The coarser pisoliths contain large tabular to rounded nuclei of generally micritic calcite, or rarely multiple oolites of the second type (Fig.24). These coarser pisolith

Fig.23. Large domal stromatolites in Kulele Creek Limestone north of Lake Carnegie. Note coarse-grained interdomal limestone detritus in b.

171

Fig. 24. Pisol i ths and spheru l i t i c oo l i ths o f calci te in Kulele Creek L imes tone . No te assoc ia t ion of micr i t i c m a t r i x w i th pisol i ths and spar ry calci te c e m e n t wi th the smal ler spheru l i t i c ool i ths . - :

cores are surrounded by well banded rinds of calcite. The smaller ooliths contain no obvious nuclei, and possess a well-developed radial structure ("spherulites" of Friedman et al., 1973).

Davies and Martin (1976) and Davies et al. (1976) have suggested that radial ooliths may form in non-agitated environments (not necessarily under hypersaline conditions as suggested by several other authors). The pisoliths on the other hand may represent more agitated environments of deposition, as evidenced by their reworked micritic cores and non-radial carbonate mor- phologies.

It is interesting to note that the ooliths are generally associated with a matrix of sparry calcite and terrigenous silt (even when the ooliths are incorporated as multiple nuclei in the pisoliths), whereas the pisoliths are usually associated with a micritic matrix. This further indicates their separate development prior to deposition in Weir present position.

Several prominent bands of sandstone occur in the upper part of the formation.

Mulgarra Sandstone The Mulgarra Sandstone is named from Mulgarra Pool in the northeast of

the Kingston sheet. It is at least 100 m thick, and appears to rest disconformably on the underlying Kulele Creek Limestone.

172

The formation consists dominantly of medium-grained, grey to brown ferruginous quartz sandstone with minor glauconite, and exhibits small-scale cross bedding, slump rolls, and rare load casts, and contains mud pellets. Thin shale and carbonate bands occur in the middle part of the sequence.

The Mulgarra Sandstone is the youngest known unit of the Earaheedy Group, and is exposed only in the extreme east of the Earaheedy Sub-basin. It possibly represents the beginning of a third transgressive phase of sedimentation, or alternatively represents a regression at the end of the second sedimentary cycle.

HISTORY OF SEDIMENTATION

Initial sedimentation and basin development

Sedimentation in the Nabberu Basin began during a marine transgression over Archaean basement rocks. The presence of glauconite in the basal clastics, the upward lithological variation in the Tooloo Sub-group of the Eara- heedy Subbasin from sand to silt to chemical sediments, and the occurrence of probable deeper water iron formations at the top of the Frere Formation indicate progressively deeper water conditions associated with transgression. East of the Glengarry Sub-basin the outcrop trace of the basal unconformity is essentially linear for some 300 km despite the very low dip of the sediments. This suggests that the dominantly granitic basement was essentially a peneplain at the time of the Early Proterozoic transgression as also suggested by Allchurch and Bunting (1976), although the basal Yelma Formation drapes over low hills in the western Kingston sheet, and small outliers of the Yelma Formation occasionally occur south of the main unconfirmity.

The peneplain concept is supported by the general lack of thick, coarse- grained clastic sediments in the basal Tooloo Sub-group and in the Glengarry Sub-basin. Higher in the Tooloo Sub-group abundance of silt is further evidence of the lack of relief in the Archaean hinterland. Although some feldspar occurs in the Yelma Formation, the Tooloo Sub-group is generally poor in feldspar detritus despite the granitic nature of the basement. This is considered to indicate that the basement was deeply weathered*, thus con- forming with the peneplain concept. During Miningarra Sub-group deposition, however, this weathered mantle seems to have been largely stripped, as the sandstones of the Wongawol Formation are noticeably arkosic in composition. The deeply weathered basement concept may also be supported by the earlier discussed possibility of a Late Archaean silcrete being developed on the basement rocks.

In the Miningarra Sub-group the feldspar is plagioclase with virtually no K-feldspar, despite the dominantly quartz monzonitic (adamellitic) composition of the basement rocks to the north and south. If the depositional order

*Precambrian weathering to produce aluminous regoliths has also been suggested in North American and South African sequences (e.g., Vogel, 1975).

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177

CLQstic components of sedimentary sequence

(observed) Weathering Profile on basement

(inferred) Mineral Stabi l i t ies

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Fig. 25. Earaheedy Group elastic sedimentary sequence and inferred Late Archaean weathering profile on granitic basement.

of the sediments reflects the reverse of the weathering profile in the basement, then the profile may be represented as in Fig.25 (showing plagioclase as more stable than K-feldspar). It is generally considered that K-feldspars are more stable than plagioclases in modern weathering environments (see Loughnan, 1969), although breakdown processes are imperfectly understood. If early Precambrian atmospheres were poor in oxygen and rich in carbon dioxide and other gases, as suggested by a number of authors (e.g., Cloud, 1972, 1974; Carver, 1974), the resulting effect on critical weathering factors such as pH and Eh could be important. For example, hydrogen ions produced by the hydrolyzing action of mineral surfaces play an important part in modern-day breakdown of silicate structures (Loughnan, 1969, p. 31), and their activity could be significantly altered in differing Eh--pH conditions. This may in turn alter the types of breakdown processes operating.

Volcanics which occur in the basal sediments in the Glengarry and Padbury Sub-basins may be associated with faulting and tectonic downwarping which led to the development of the basin (Fig.26). A similar association is recorded in the Hamersley Basin where large volumes of basaltic volcanics dominate early sedimentation (Trendall, 1968). Mafic dyke swarms have been dated at about 2400 Ma within the Yilgarn Block (Compston and Arriens, 1968) and at 2329 Ma -+ 98 Ma in the Pilbara Block (Lewis et al., 1975), and are con- ceivably feeders for the volcanics, although the altered basaltic sequence in the Hamersley Basin has been dated as slightly younger at 2190 -+ 100 Ma (Compston and Arriens, 1968).

Pre-existing Archaean structures also appear to have had an important control on sedimentation in the Nabberu Basin. The Glengarry Sub-basin, which

178

contains a much greater thickness of basal sediments (of probably deeper- water origin than those in the Earaheedy Sub-basin), is bounded in the east by the extension of a number of major north-northwest trending faults parallel to and within the Wiluna greenstone belt. Other "o ld" structures trending northeast parallel to and within the western part of the Stanley Fold Belt may also have been important in basin development (J. Bunting and D. Gee, pets. comm., 1977). Similar effects of older Archaean structures on early sedimentation have been observed in the Early Proterozoic Transvaal Basin of South Africa (Anhaeusser and Button, 1974).

Deposition of the Tooloo Sub-group

The earliest Tooloo Sub-group sediments were deposited during a regional transgression. However, features at the top of the Windidda Formation indicate sub-aerial exposure, which in turn indicates that deposition of the Tooloo Sub-group ended in a regressive phase of sedimentation. This is partly supported by the presence of reworked aluminous glauconite in the overlying Miningarra Sub-group, possibly indicating non-marine deposition following the regression but prior to the Miningarra transgression.

The occurrence of the Frere Formation above transgressive carbonates and below regressive carbonates suggests that the banded iron formations were deposited during the t ime of maximum transgression, i.e., they represent a facies of chemical deposition further off-shore than carbonate precipitation and occupy a specific stratigraphic niche in the succession. The presence or absence of reworked textures in the chemical sediments depends on basinal water depth, as well as distance off-shore (i.e., it will depend to some extent on basin floor slopes). The compression of the transgressive facies, and at tenuation of the regressive facies (in terms of relative thicknesses) may result from either a relatively slower regression, or, alternatively, slower sedimen tation rates during the transgression.

An idealised sedimentary facies model for deposition in the Earaheedy Sub- basin is shown in Fig.27. This is based on the observed lateral and vertical facies relationships described above. In reality, of course, deposition is much more complicated than this, being especially dependent on local environmental/ palaeogeographic factors and on the interplay between clastic supply and chemical precipitation. The transition from carbonates to iron formations or vice versa, however, is becoming increasingly well known, particularly in the classic Early Proterozoic sequences e.g., Transvaal Basin (Button, 1976b), Hamersley Basin (Davy, 1975). In this regard the occurrence of interbedded ferruginous cherts, chlorites and carbonates at the contact between the Frere and Windidda Formations provides an interesting contrast where closely associated, shallow-water depositional environments have given rise to variably oxide (hematite-quartz), carbonate (ankerite) and silicate (chamosite) facies iron formations. Fur thermore associated black laminated cherts contain abundant apatite, rare sulphides and micro-fossils. The chamositic pellets are

179

f

/'/" ff~/~~ 100-200 m (maximum depth of wave action and light penetration)

d) @ @ ® @ low retief hinterland sandstones siltstones limestones oxide facies iron formation

shares (dolomi~) t (more cherty shorewards)

transition zone including iron silicates

iron carbonates

Fig.27. S c h e m a t i c facies mode l showing re la t ions b e t w e e n near shore elastics, c a r b o n a t e s and of f -shore i ron f o r m a t i o n s dur ing s e d i m e n t a t i o n o f the T o o l o o Sub-group. T h e inter- sec t ion of the wave base -pho t ic zone l imi t w i th the basin f loor will d e t e r m i n e presence or absence of s e d i m e n t a r y r ework ing and p h o t o s y n t h e t i c b e n t h o n i c organisms in the sediments .

considered to be of reworked diagenetic origin (James, 1966; Leone et al., 1975), and the association with organic black cherts could be important in the formation of the chamosite (Leone et al., 1975).

The increasing silica content and decreasing thickness of the iron formations eastward in the Earaheedy Sub-basin, together with the occurrence of a thick, distinctive chert member at the top of the Frere Formation (Hall et al., 1977) are also thought to be related to the overall facies model, i.e., iron formations become more cherty shorewards.

The bulk of the sediments of the Tooloo Sub-group were deposited in shallow water conditions, and the presence of algal stromatolites throughout the sequence indicates water depths of less than 100 m (i.e., within the photic zone). In fact, depths were probably much less than this along the southern margin of the Kingston Platform. Decreasing water depths and increasing nearness to the basin edge south and southeast are suggested by a large number of observations in the Earaheedy Sub-basin:

(a) The Yelma Formation thins eastwards along the southern margin of the Earaheedy Sub-basin and becomes thicker and more argillaceous on the northern margin.

(b) Banded iron formations of the Frere Formation thin and become more siliceous eastwards, where shales dominate over chemical sediments. Deeper water (unreworked) iron formations are only found near the Glengarry Sub-basin at the western end of the Earaheedy Sub-basin.

(c) Thick shallow-water carbonates of the Windidda Formation are confined to the southeast of the Earaheedy Sub-basin.

The dominance of shallow-water iron formations, and the minor occurrence of "deep-water" iron formations to the northwest is of great interest when comparing the Nabberu and Hamersley Basins of Western Australia. The banded iron formations of the Hamersley Basin are characterised by their continui ty over large distances, and Trendall and Blockley (1970) have been able to match microbanded sequences over 300 km apart. Both the Hamersley iron

180

formations and associated carbonates are devoid of shallow-water reworking features or benthonic algal structures, and consequently are best interpreted as of deeper water shelf origin, i.e., forming at more than 100 to 200 m water depths below storm wave base and the photic zone. Since it seems possible on regional grounds that the Hamersley and Nabberu Basins may originally have been part of one basin, the Nabberu Basin and particularly the Kingston platform can be considered to represent the shallow shelf equivalent of the Hamersley Basin.

Deposition of the Miningarra Sub-group

Although more detailed work is needed, it appears that the Miningarra Sub-group was deposited disconformably on the Tootoo Sub-group in the southeast of the Earaheedy Sub-basin following subaerial exposure of the Windidda Formation. However, the two sub-groups appear to be conformable in the northwest of the sub-basin.

The presence of glauconite in the basal parts of the Miningarra Sub-group, and the general transition from coarse to fine clastics to carbonates indicate that the group was deposited in a second major transgression. Detailed analysis of cyclic units in the Kulele Creek Limestone near the top of the Miningarra Sub-group shows that the transgression was still continuing at that time, although the Mulgarra Sandstone may represent a regressive phase of sedimentation.

As in the Tooloo Sub-group all units exhibit abundant shallow-water sedimentary structures, and rare features indicating local subaerial exposure. Increasing distance offshore to the northwest is supported by:

(a) The Wandiwarra Formation appears to be absent in the extreme southeast of the Sub-basin, and becomes finer-grained to the northwest. It rests disconformably on the Tooloo Sub-group in the southeast, but appears to be conformable in the northwest.

(b) The Princess Ranges Quartzite thins towards the northwest. (c) The Wongawol Formation becomes finer grained to the northwest. The abundance of detrital plagioclase in the Miningarra Sub-group*, as

well as providing information on the weathering profiles on the Archaean basement, also indicates fairly rapid erosion of the basement. This may in turn be related to tectonic reactivation of the basement, in contrast to the generally stable relations existing during deposition of the Tooloo Sub-group. Similar features have been noted in the development of the Hamersley Basin following the deposition of the banded iron formations, and indeed the regression ending deposit ion of the Tooloo Sub-group may have been caused by this renewed tectonic activity.

*The abundance of feldspar in the finer clastics of the Sub-group (compared to the coarser arenites of the Princess Ranges Quartzite) may support the results of Odom (1975) who noted a relationship between grain size and feldspar abundance in Cambrian sediments in the U.S.A.

181

ACKNOWLEDGEMENTS

An investigation of this nature would be impossible to complete without the help and co-operation of a large number of individuals and organisations. The authors are particularly grateful to the following officers of The Broken Hill Proprietary Company Limited for assistance in fieldwork and discussion:

The late D.G. Moye who authorised and encouraged our early work on the basin, J. Daly who recognised the significance of the aeromagnetic anomalies, J.E. Harms, I.W. Reid, S.V. Bell, C. Adsett, C. Arndt, G. Beavis, G. Davis, M. Hanlon, P. Haslett, A. Heath, B. Hewitt, P. Hillsdon, L. Hollingworth, J. Macfarlane, G. Maguire, H. Meyer, M. Rose, L. Smith, G. Torr, R. Turner, G. Williams and V. Zani. The authors are also very appreciative of discussions with geologists of several other organisations, viz., Geological Survey of Western Australia (D. Gee, J. Bunting, P. Commander, S. Williams, M. Elias), Bureau of Mineral Resources (M. Walter, J. Jackson), CSIRO (R. Horwitz, A. Milnes, K. Norrish), South Australian Geological Survey (W. Preiss). The assistance of local station personnel and of Jayrow Helicopters is also warmly acknowledged.

This paper is published with the permission of the General Superintendent Exploration of the Broken Hill Proprietary Company Limited.

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The Early Proterozoic Nabberu Basin and associated iron formations of Western Australia