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A major unconformity in the Archaean, JonesCreek, Western AustraliaD.W. Durney aa 104 Ward Street, Kalgoorlie, Western Australia, 6430Published online: 16 Jan 2008.

To cite this article: D.W. Durney (1972) A major unconformity in the Archaean, Jones Creek, WesternAustralia, Journal of the Geological Society of Australia, 19:2, 251-259, DOI: 10.1080/14400957208527886

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A MAJOR UNCONFORMITY IN THE ARCHAEAN, JONES CREEK, WESTERN AUSTRALIA

By D. W. D U R N E Y

(With 2 Text-Figures and 2 Plates)

(Received 17 March 1972: read in abstract at Perth, 17 April 1972)

ABSTRACT Evidence is presented to show that a granite conglomerate of the Archaean

Yilgarn succession rests uncomformably on granite in the Jones Creek area of Western Australia, 400 km north of Kalgoorlie. South of Jones Creek the con- glomerate evidently overlies with angular discordance the lower section of Archaean greenstone (mainly metabasalt and layered metagabbro). The relationship between this lower greenstone and granite is obscured by recent cover, hence the granite is either 'middle' Yilgarn in age or represents the basement on which the Yilgarn succession was deposited.

The conglomerate is fairly well stratified, characteristically with rounded granite clasts and an arkosic or marie matrix. Other types of igneous elast are locally present but sediment clasts are absent. Its proposed formation name is Jones Creek Conglomerate.

INTRODUCTION

One of the problems which has long puzzled geologists working on Western Australian Archaean rocks is the apparent absence of a basement beneath the Archaean metasedi- ments and greenstones. In the Yilgarn Block these metasedimentary and metavoleanic rocks (Yilgarn succession or Kalgoorlie succession) occur as NNW-SSE trending belts separated by wide expanses of regionally semi-concordant granite and granite gneiss. It is generally con- sidered that most of the gneiss formed contem- poraneously with the post-Yilgarn metamor- phism and deformation (approximately 2,700 m.y. ago) and that the gr.anite intruded both the gneiss and the Yilgarn succession (up to about 2,550 m.y. ago) (see Maitland, 1919; Wilson, 1958; Prider, 196I, 1965; Horwitz & Sofoulis, 1965; Sofoulis, 1966; Kriewaldt, 1969; Glikson, 1970; age determinations given in Wilson et al., 1960; Turek 1966; Compston & Arriens, 1968; and Arriens, 1971). Although the greenstones and metasediments must have been laid down on a pre-existing basement, no direct evidence of a .basement has hitherto been reported.

Evidence for the existence of old granitic rocks has been inferred from the presence of a granite pebble from the Kurrawang Con- glomerate in the Kalgoorlie region dated at 3,000 to 3,100 m.y. (Turek, 1966). In fact gneisses in the range 2,900-3,I00 m.y. have recently been reported for the Western mar- gin of the Yilgarn Block (Arriens, 1971), but

they evidently postdate the oldest greenstone (Prider, 1944).

However, Arriens (1971, p. 21) pointed out that initial SrSVSr s6 ratios of these old gneisses suggest 'a prior crystal history for the source materials' with a probable minimum limit of duration of 12.0 m.y. i .e , the source materials were possibly pre-Yilgarn.

A partial answer to the basement problem and information bearing on the pre-2,700 m.y. tectonic history of Western Australia, was recently found at Jones Creek, some 100 km south of Wiluna and 400 km north of Kal- goorlie. Here, a metamorphosed Yilgarn granite boulder conglomerate unconformably overlies a basement of granite ,and 'lower Yilgarn' greenstone, indicating (a) a 'middle' Yilgarn or pre-Yilgarn age of the granite, and (b) a 'middle' Yilgarn tectonic disturbance.

A R C H A E A N ROCKS OF THE JONES CREEK REGION

The Tectonic Map of Australia assigns all the greenstones and metasediments of the Jones Creek region (which includes the con- glomerate whose relative age is discussed on p. 256) to the Yilgarn succession. They form a NNW-SSE trending belt which extends from Agnew to Wiluna. The strata are generally steeply dipping, affected by greenschist and in places amphibolite-facies regional metamor- phism, varying degrees of tectonic deformation, and are transected by numerous quartz veins and a NW-SE set of post-tectonic pegmatite dykes.

Journal of the Geological Society of Australia, Vol. 19, Pt. 2, pp. 251-259, Pls. 9-10, October, 1972.

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The Yilgarn succession rocks and the nearby granites of the Jones Creek region may be grouped into five different divisions. These are (1) the conglomerate, (2) the 'Western granite', (3) the 'Western greenstone' (2 and 3 both west of the conglomerate), (4) the 'Eastern greenstone and metasediment', and (5) the 'Eastern granite' (east of 4). In addi- tion there is (6) 'Southern granite' which is indirectly related to the 'Western greenstone' but cannot yet be correlated with either of the other granites. Divisions 2 and 3 and the northern part of division 1 have undergone the least deformation and possibly the least metamorphism; division 4 has the most com- plex structure.

(1) JONES CREEK CONGLOMERATE

The granite conglomerate extends over a known strike of 90 km (Agnew to Mt Keith). In view of its stratigraphical significance and distinctive lithological character it will be called the Jones Creek Conglomerate (or simply the Conglomerate in the following text) after /ones Creek where its relations with adjacent rocks are probably clearest.

A complete section, arbitrarily chosen as the type section, is exposed immediately north of Jones Creek (Fig. 2). The terrain here is fairly rugged but readily accessible from the ~outhwest or northeast.

The total thickness (I,000 m) and the thick- nesses of the constituent conglomeratic and arkosic beds of the formation can be measured directly from the map in Figure 2. This is because the bedding is vertical and hence the map shows a sectional view of the Con- glomerate.

The degree of tectonic deformation in the Conglomerate of the type area is generally slight owing to its high competency. Generally the granite boulders .are rather round and there is only a weak cleavage (strike 030 °) in the arkose (bedding strike 175°). This informa- tion together with the absence of fold closures or offsetting of strata, and the asymmetric sequence of beds, indicates that the succession within the Conglomerate of the type area has not been repeated .by faulting or isoclinal fold- ing.

Current-bedding structures such as the one in Plate 9, Figure 1 face consistently east. Thus the western contact, against 'Western granite', marks the base of the Conglomerate. This is a very irregular surface (Fig. 2 and Plate 9, Fig. 2). In contrast, the top of the Conglome- rate (the eastern contact) is concordantly

D. W. DURNEY

overlain by well-laminated feldspathic para- amphibolite of the 'Eastern greenstone and metasediment'.

The rhythmic succession of arkose and con- glomerate is shown in Figure 2. Arkose beds remain uniform in texture and composition throughout the formation. Conglomerate beds are all characterized by the presence of round granite clasts but there is a significant change in their facies up the succession. Briefly, the vertical facies variation consists of a change from poorly sorted basal conglomerate to moderately sorted conglomerate (where there is a preponderance of clasts of a certain size, usually cobbles), then a change from granitic arkose matrix to schistose greenstone matrix accompanied by a progressive decrease in the 'pebble' matrix ratio. Up to the point where the conglomerate matrix changes composition, the formation is entirely of granitic or adamel- litic composition.

Arkose beds The arkose beds are all composed of fairly

well-sorted medium-grained detrital feldspar and quartz with subordinate ferromagnesian minerals (quartz and feldspar 0.05-2.0 rnm, sub- angular to subrounded, shapes and sizes modi- fied to a certain extent by compaction welding, deformation and polygonization). A charac- teristic bedding is imparted by thin persistent and remarkably parallel micaceous laminae. The arkose could easily be mistaken for a sheared granite, but its sedimentary origin is shown by the presence of current bedding and rare granite cobbles (round and virtually un- deformed, e.g. Plate 10, Fig. 1), and by the fact that the micaceeus l.aminae are not parallel to the tectonic cleavage but parallel to the bedding as outlined by adjacent con- glomerate beds. Type A conglomerate beds

In each of the three depressions at the base of the Jones Creek Conglomerate is situated a poorly sorted basal conglomerate interpreted as being a valley-infilling talus. In addition to rounded granite clasts of various sizes there are angular granite blocks and aplite pebbles all tightly packed in an arkose matrix. At the base of each depression is a transitional zone of ,broken granite (passing down into normal massive granite) which makes it difficult to pin-point the unconformity on outcrop scale, although over the intervening elevations the unconformity is quite sharp. Type B conglomerate beds

The succeeding seven to eleven conglomerate

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UNCONFORMITY IN ARCHAEAN, WESTERN AUSTRALIA

beds consist of tightly packed, well-rounded, high (0.7-0.9) sphericity, massive granite elasts, set in an arkose matrix of slightly higher marie content than the elasts but other- wise petrographically similar to the arkose beds described above (e.g. Plate 10, Fig. 2). The clasts are predominantly 8-20 cm in dia- meter (cobbles) 'but do range from 1 or 2 cm up to approximately 1 metre in some horizons. There generally appears to be a scarcity of particles in the range 2 mm-1 cm, so the overall size distribution is possibly hi- modal. In addition to massive granite, clasts of faintly ,banded granite (possibly granitic gneiss) and of saccharoidal vein quartz are also locally present.

Owing to its compact granite-like nature type B conglomerate is typically exposed in the form of smooth convex exfoliation sur- faces. Type C conglomerate beds

The uppermost conglomerate beds possess well-rounded granite clasts very similar to those of type B, but they are often loosely packed owing to a higher proportion of matrix, and the matrix is composed of para-amphibo- lite (e.g. Plate 10, Fig. 3). Small rhyolite cobbles are also locally abundant. Lateral [acies variation

Superimposed on the vertical conglomerate facies variation described above is a pro- nounced lateral facies and thickness variation. Five km north of the type area most of the conglomerate beds wedge out leaving the for- mation reduced in total thickness and com- posed mainly of arkose.

Southwards from the type area (1-5 km south of Jones Creek) arkose beds almost disappear from the lowest third of the suc- cession and at the top of the succession the type C conglomerate increases in thickness and becomes diversified into: (a) granite- pebble conglomerate with predominant amphi- bolite matrix, (b) granite- and granite-rhyolite- cobble-boulder conglomerate with subordinate amphibolite matrix and associated thin impure sandstone beds, (e) para-amphibolite with sparse granite boulders and cobbles, and (d) para-tremolite-chlorite or chlorite-tremolite schist with sparse granite boulders.

Ten km south of Jones Creek a wedge of gabbro-bas.alt-sparse granite-pebble conglome- rate with pseudo-dolerite matrix appears at the base of the succession.

The rhyolite and basalt pebbles could have been derived from penecontemporaneous vol-

253

canic centres, but at least the granite and gab- bro pebbles must have been significantly older than the Conglomerate.

No pebbles of sedimentary quartzite, jaspi- lite, chert, banded ironstone, slate, dolomite, other sedimentary rock, serpentinite or original metamorphic schist have been found anywhere in the Conglomerate. (2) 'WESTERN GRANITE'

The granite west of the Conglomerate and the greenstone belt is massive, internally un- deformed, medium- to coarse-grained and apparently rich in plagioclase. The coarse- grained variety usually contains stubby pheno- crysts of potash-feldspar. (3) 'WESTERN GREENSTONE'

A suite of layered metagabhros and a mono- tonous metabasalt sequence situated west of the Conglomerate south of Jones Creek (Fig. 1) known 1ocally as the 'Kathleen gabbro' and 'Mt Goode basalt' respectively, are grouped together as the 'Western greenstone'.

Each is about 3,500 m thick and dips steeply NW and strikes NE-SW (40 ° to 50 ° oblique to the main trend of the greenstone-metasedi- ment belt). Despite regional metamorphism to actinolite-plagioclase and green hornblende- labradorite assemblages, igneous textures are well preserved, e.g. pseudomorphs after pyrox- ene crystals in the gabbro and vesicles in the basalt.

Compositional variations in the 'Kathleen g~bbro' due to crystal settling indicate a SE facing (I. Gemuts, pers. comm., 1971), i.e. the gabbro underlies the ,basalt.

Unfortunately the contact between 'Western greenstone' and 'Western granite' is not ex- posed so at present it is not possible to say which is the older. (4) 'EASTERN GREENSTONE AND

METASEDIMENT' The 'Eastern greenstone and metasediment'

comprise phyllite, black slate, jaspilite, sedi- mentary quartzite, quartz grit, serpentinite, metagabbro, metabasalt, para-amphibolite and magnesian amphibolite. Tigh,t folds and at least one large strike fault are known ~o ,be present. (5) 'EASTERN GRANITE'

Granite along the eastern margin of the greenstone-metasediment belt contains amphi- bolite rafts and possesses dykes and apophyses which intrude amphibolite of group (4). Both the granite .and amphibolite have a strong ver- tical tectonic schistosity with a steep south- plunging mineral lineation. The minor intru-

Journal of the Geological Society of Australia, Vol. 19, Pt. 2, pp. 251-259, Pls. 9-10, October, 1972.

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254 D . W .

sions have been deformed such that most of them are now only slightly discordant to the schistosity. This granite is consequently younger than the amphibolite but older than or coeval with the post-Yilgarn orogenic activity. (6) 'SOUTHERN GRANITE'

The 'Southern granite' (Fig. 1) really lies outside the Jones Creek region but is relevant to the following discussions.

It is intrusive into the base of the 'Agnew greenstone' (Fig. 1) and like the 'Eastern granite' appears to have ,been de{ormed to- gether with the greenstone after intrusion (simi- lar orientations of schistosity across intrusive contacts). The intrusions take the form of innumerable granite sills and dykes in the base of the greenstone, and stoping of large seg- ments of greenstone by rather homogeneous deformed granite.

The relation between this 'Southern granite' and the Conglomerate is not yet known.

EVIDENCE OF T H E U N C O N F O R M I T Y BETWEEN C O N G L O M E R A T E A N D 'WESTERN G R A N I T E '

1. Current bedding such as the example in Plate 9, Figure 1 indicates that the Conglome- rate faces away from the granite.

2. The granite detritus must have been derived from a granite basement and the size of the boulders implies that the source could not have been far away. Hence the adjacent granite was probably the source.

3. Granite clasts in the Conglomerate are identical in lithological appearance to the 'Wes- tern granite'.

4. The contact is too irregular to be a fault and there is no shearing, fault breccia, slicken- sides or other structure along the contact which could be interpreted as implying fault move- ment.

5. There are no signs of granitization, thermal metamorphism or appreciable ductile deformation at or near the Conglomerate.

6. There are no dykes or sills of granite passing into the Conglomerate and there are no xenolitbs and no platy flow texture, chilled margin or autobreeciation in the granite along the contact.

7. The granite has undergone a complex history of joinfing, whereas joints in the Con- glomerate usually consist only of a few sets at high angles to bedding.

8. Shear joints and certain aplite veins in the granite terminate abruptly at the contact.

9. The shape of the contact resembles an incised land surface profile (Fig. 2).

DURNEY

REGIONAL GEOLOGICAL SETTING :

Agnew - Jones Creek q . . . . . . . . . ~. 2o kitor~etres

b . . . . ~ tb mites

i t~n~e*

r~tone timent "

. ,- ~,°,. r- ~~.~: Fig. 1.

10. Successive arkose and conglomerate beds overlap onto the granite with discor4ant sedi- mentary contact (Fig. 2, and Plate 9, Fig. 2).

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LrNCONFORMITY IN ARCHAEAN, WESTERN AUSTRALIA 255

)0

Oieo,|

// //

//

/ / //

/ / / /

// / /

p 2 / II

~V~

v A

Fig. 2,

Geological Map of Unconformity at

Jones Creek (controlled photo - compilation )

I 1 kilometer 1 2000 feet

Archean

/ pegmatite dyke qt quarfzfte g metagabbra ml magnesian amphibolffe

(tpemo/itic) ol ortho-amphiboUte (mefabasaft) t talc schist tr tcemolite schist cu coarse-grained ultrabaslc fu fine-grained ultrobasic

m b black slate pl pana-amphibolite

! I ~ cl amphibolite with gr boulders a ar~ose

i l ~ c conglomerate (amphit~ matrix) c conglomerate (arkose matrix)

. ~ L ~ c conglomer~ (basal) gr granite (Western]

Symbols

geological contact ,.-'~, outct~o

drainage fence

=~=~e tPock j "breakoway"

access point bedding

"~r~so cleavage 4o=~ lineation

• -> current bedding facing

Journal of the Geological Society of Australia, VoL 19, Pt. 2, pp. 251-259, Pls. 9-I0, October, 1972.

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256

11. At map or outcrop scale (Fig. 2, and Plate 9, Fig. 2) the bedding drapes over pro- tuberances and sags into depressions of the granite contact as one would expect as a result of sedimentary compaction. Related bedding compaction structure can be seen in arkose surrounding isolated granite cobbles (e.g. Plate 10, Fig. 1).

12. The lack of stratigraphical correlation between sediments in adjacent depressions (Fig. 2) can only be satisfactorily explained by independent sedimentation in valleys separated by intervening ridges.

AGE OF THE CONGLOMERATE 1. The Conglomerate is cut by post-tectonic

pegmatite dykes. (Pegmatite within the Yilgarn Block so far dated all belongs to the Arch- aean).

2. It was deposited prior to (or in other words has been affected by) the post-Yilgarn deformation and metamorphism which is gen- erally believed to have taken place about 2,700 m.y. ago. This is shown ,by the schistose and metamorphic nature of the amphibolitic matrices, deformation of small pebbles, and the near vertical attitude of bedding.

3. It is older than the close of ultrabasic in- trusive activity (small serpentinite intrusion in the Conglomerate 8 km south of Jones Creek).

4. The Conglomerate appears to be the basal unit of the 'Eastern greeustone and metasedi- ment'. This conclusion is based on (a) the east facing of the Conglomerate, (b) the gradual upward transition from Conglomerate to amphibolite south of Jones Creek, (c) the apparent conformity of stratification between Conglomerate and 'Eastern greenstone and metasediment', and (d) the absence of sedi- ment pebbles, particularly of resistant types such as quartzite and jaspilite, from the con- glomerate.

5. It is younger than the 'Western granite' and the 'Western greenstone'. (The latter point is discussed below).

EVIDENCE OF UNCONFORMITY BETWEEN CONGLOMERATE AND 'WESTERN GREENSTONE'

Figure 1 shows that there is a pronounced angular discordance between the 'Western greenstone' and the Conglomerate. It could be argued that this discordance is a major fault, and indeed relict igneous textures in the 'Wes- tern greenstone' are notably deformed or 'sheared' at some places along the contact.

D. W. DURNEY

But the following points indicate that it is more likely to be an unconformity.

1. Bedding in the Conglomerate near the 'Western greenstone' is sub-parallel to, and evidently continuous with, the bedding at Jones Creek; in particular there is a good correla- tion between type B and type C conglomerates of the two areas. Thus, although no facing structures were found in the Conglomerate here, there are reasonable grounds for suppos- ing that it faces east and could therefore overly the 'Western greenstone'.

2. The 'Western greenstone' discordance lies in a direct line with the mean strike of the known unconformity at Jones Creek just to the north.

3. Conversely, if the .discordance were a major fault it would pass N-NNW-wards through the 'Western granite' or perhaps through the Conglomerate, but no fault has been found in either of these units.

4. There is a large smoothly curved embay- ment of type B granite conglomerate into the 'Western greenstone' near its northern end (just south of Jones Creek). With a depth/ breadth ratio of 1:2 (depth 380 m and breadth 750 m) and inflexion angles of approximately 45 ° to the mean strike, the shape of this em- bayment could hardly .be ascribed ~to fault curvature. Rather, it is homologous to the type of embayment seen in the 'Western granite'. Two other embayments and one broad protuberance with amplitudes between 100 m and 200 m are also known further south along the 'Western greenstone' discordance.

5. As mentioned previously there is a basic conglomerate, composed mainly of metagabbro and metabasalt pebbles, at the base of the Jones Creek Conglomerate some 10 km south of Jones Creek. These pebbles indicate a sub- stratum, such as the 'Western greens tone', composed of these rock types.

6. The relationship between Conglomerate and greenstone at Agnew is relevant. The 'Agnew greenstone' is remarkably similar to the 'Wes- tern greenstone' in that it consists of a lower, layered metagabbro and an upper metabasalt (facing determined from crystal settling in one of the metagabbros). Jones Creek Conglome- rate lies in discordant contact against the western margin of this greenstone (the meta- basalt sequence is completely truncated) and it faces west in the following stratigraphical order:

(a) arkose, intermediate composition, fac- ing from ripple bedding,

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UNCONFORMITY IN ARCHAEAN,

(b) granitic arkose with scattered granite (iv) pebbles, lithologically equivalent to type B conglomerate of Jones Creek, facing from current bedding and graded bed- ding,

(c) para-amphibolite, (d) granitic arkose, (e) granite boulder conglomerate with para-

amphibolite matrix, equivalent to type C of Jones Creek, contains some cob- bles and pebbles of tremolite schist and metagabbro, some granite boulders attain 1.2 m diameter, and

(f) granitic arkose. Not only .does this sequence face in the

correct direction to overly the 'Agnew green- stone', but also, some of the metagabbro peb- bles in it can be matched with a distinctive and rare type of metagabbro which is known onIy near the top of the layered metagabbro. This rock has a spotty appearance due to scattered ovoid pseudomorphs after pyroxene a few mm in diameter in a feldspar ground- mass.

Thus the 'Agnew greenstone' and 'Western greenstone' are two parts of a single green- stone sequence, and on independent evidence both appear to underlie the Conglomerate.

DISCUSSION

History The unconformity is evidently a major ero-

sion surface because it has cut down into plutonic igneous rocks and truncated a thick greenstone sequence which had already been affected by moderate folding. The palaeotopo- graphical relief of the unconformity and the petrology of the Conglomerate suggest that the erosion was terrestrial and fairly rapid, possibly on the flank of an uplifted land mass. The chief diastrophic event was to come after a period of subsidence during which there was deposition of argillites and quartzites accom- panied by further basic volcanicity and fol- lowed by ultrabasic intrusion. These events may be summarized as set out below.

(i) Widespread granite intrusion, granitiza- tion or differentiation of primordial crust (?).

(if) Erosion, then extrusion of basic volcanics followed by intrusion of layered gabbro.

(iii) Broad flexuring along a NE-SW axis (cal- culated from the 'Western greenstone' discordance after removing vertical tilt of Conglomerate). Some granite intrusion could also have occurred at this time.

WESTERN AUSTRALIA 257

Erosion, then deposition of Jones Creek Conglomerate and 'Eastern greenstone and metasediment', with basic and ultra- basic intrusion.

(v) Tight folding and faulting on NNW-SSE axes, regional metamorphism, graniti- zation and granite intrusion.

(vi) Intrusion of NW-SE pegmatite dykes. Thus on the basis of the unconformity the

Archaean of this region is divided into at least two tectonic cycles; an early Yilgarn cycle (if and iii) and a late Yilgarn cycle (iv, v and vi). I t would therefore be of interest to establish the ages of these major events, and to resolve the age of the 'Western granite' with respect to the early Yilgarn cycle, by radiometric methods. Work on these lines is being carried out by staff of the Australian National University.

Seen in a wider context, the two cycles may correspond to what some observers con- sider to be two different Yilgarn provinces on either side of the belt shown in Figure 1. These are respectively a province with extensive basic intrusions to the west (? lower Yilgarn) and a more sedimentary province to the east (? upper Yilgarn). Granite--greenstone relations in the Yilgarn Block

This study has given an indication of the type of 'basement-cover' relationship which can exist within the Archaean (specifically, within the Yilgarn succession). Future re- search and exploration might well disclose a similar relationship for the base of the lower Yilgarn succession, but because of the probable high crustal geothermal gradients and heat flow early in the earth's history (Sutton, 1971) the older the basement the less chance it had of surviving in an u~disturbed state.

Another question stemming from the pre- Conglomerate age of the 'Western granite' concerns the age of granites, such as the 'Southern granite', which intrude only the lower Yilgarn succession. Clearly, if pre-tec- tonic (affected by post-Yilgarrt deformation and metamorphism), such granites need not neces- sarily be post-Yilgarn but could be 'middle' Yilgarn (end of lower Yilgarn cycle) in age, or indeed any age between the post-Yilgarn deformation and the youngest intruded host rock. Granitized Conglomerate

Since granite has a lower melting point than other common rocks, rock with the com- position of a granite would be expected to be

Journal of the Geological Society of Australia, Vol. 19, Pt. 2, pp. 251-259, Pls. 9-10, October, 1972.

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most susceptible to granitization or remobiliza- tion. Arkose and type A and B beds of the Jones Creek Conglomerate belong to this category. In fact in the vicinity of the Kathleen Valley Hotel these beds gradually loose their sedimentary character southwards along strike and take on the appearance of a deformed granite, probably as a result of locally strong regional metamorphic recrystallization, or thermal metamorphism due to a hidden in- trusion.

This susceptibility of granite-derived sedi- ments to granitization may account for their rarity in the Yilgarn Block. Unconformable relationships between granitic sediments and granite basement would also tend to be selec- tively b~urred in regions of strong regional metamorphism or where later intrusive granites are widespread.

It may also be worth mentioning that the old gold-quartz vein workings of the Kathleen Valley are concentrated in granitized type B conglomerate. These veins may have been a product of remobilization of material originally dispersed in the Conglomerate.

Comment on Archaean sedimentation The Jones Creek Conglomerate sediments

D. W. DURNEY

are somewhat peculiar by Phanerozoic or Pro- terozoic standards in that (a) the detritus was derived entirely from igneous rocks and (b) although mineraloglcally immature the sedi- ments are generally moderately well sorted and well rounded, especially arkose and type B conglomerate (pp. 252-253). The presence of labile minerals was possibly due to their pre- servation in an inert Archaean atmosphere. These minerals help to account for the igneous appearance of some of the sediments, for example the arkose and the basic and even ultramafic sediments near the top of the Con- glomerate.

A C K N O W L E D G M E N T S Permission to publish was kindly given by

L. C. Ranford, Chief Geologist of Anaconda Australia Inc. (Jones Creek data) and B. P. Webb, Chief Geologist of Newmont Pty. Ltd. (Agnew data) . Encouragement f rom former colleagues was greatly appreciated, in particu- lar I. Gemuts who first drew attention to the conglomerate problem and critically examined the manuscript. Dr P. Coleman of the Univer- sity of Western Austral ia also made useful suggestions for improving the text.

REFERENCES AR~ENS, P. A., 1971: The Archaean geochrono-

logy of Australia. Spec. Pubis geol. Soc. Aust., 3, pp. 11-23.

BUREAU OF MINERAL RESOURCES, 1960: Tectonic Map of Australia, 1:2,534,000. Bur. Miner. Resour. Geol. Geophys. Aust.

COMPSTON, W., & AV~tmNS, P. A., 1968: The Pro- cambrian geochronology of Australia. Can. I. Earth. ScL, 5, pp. 561-583.

GLICKSON, A. Y., 1970: Geosynclinal evolution and geochemical affinities of early Precam- brian Systems. Tectonophys., 9, pp. 397-433.

HORWITZ, R. C., & SOFOULIS, J., 1965: Igneous activity and sedimentation in the Precam- brian between Kalgoorlie and Norseman, Western Australia. Proc. Australas. Inst. Min. Metall., 214, pp. 45-59.

KILIEWALDT, M. I. B., 1969: Explanatory Notes: Kalgoorlie 1: 250,000 geological sheet, SH/51-9. Geol. Surv. West. Aust.

MAITLAND, A. G., 1919: A summary of the geo- logy of Western Australia. Mere. geol. Surv. West. Aust., 1, pp. 3-35.

PRIDEI~, R. T., 1944: The geology and petrology of part of the Toodyay District, Western Aus-

D. W. Durney, 104 Ward Street, KaIgoorlie, Western Australh~ 6430

tralia. 1. Proc. R. Soc. West. Aust., 28, pp. 83-137.

, 1961: The 'greenstones' of southwestern Australia. J. Proc. R. Soc. West. Aust., 44, pp. 1-9.

, 1965: Geology and mineralization of the Western Australian Shield. Proe. 8th Commonw. Min. metall. Congr., 1, pp. 56-65.

SoFouLIS, J., 1966: Explanatory Notes: Widgie- mooltha 1:250,000 gelogical sheet, SH/51-14. Geol. Surv. West Aust.

SUTTON, J., 1971: Some developments in the crust. Spec. Publs geol. Soc. Aust., 3, pp. 1-10.

TUREK, A., 1966: Geochronology of the Kalgoor- lie area. Ph.D. Thesis, Aust. Nat. Univ. [unpublished].

WILSOr~, A. F., 1958: Advances in the know- ledge of the structure and petrology of the Precambrian rocks of south-western Australia. J. Proc. R. Soc. West. Aust. 41, pp. 57-83.

, COMPSTON, W., JEFFERY, P. M., & RILEY, G. H., 1960: Radioactive ages from the Pre- cambrian rocks in Australia. J. geol. Soc. Aust., 6, pp. 179-196.

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UNCONFORMITY IN ARCHAEAN, WESTERN AUSTRALIA 259

EXPLANATION OF PLATES

PLATE 9

Fig. 1. Current bedding in granitic arkose, facing east. Foreset beds belong to the southern flank of a broad festoon bedding structure. NNE-SSW cleavage just visible in foreset beds.

Fig. 2. The unconformity. Laminated granitic arkose overlying an angular protuberance of 'Western granite' (hammer head at contact).

PLATE 10

Fig. 1. Granite cobble in granitic arkose (next to hammer head). Fig. 2. Type B conglomerate; granite boulders and cobbles in granitic arkose matrix. Fig. 3. Type C conglomerate; granite boulders and pebbles in para-amphibolite matrix.

Journal of the Geological Society of Australia, Vol. 19, Pt. 2, pp. 251-259, Pls. 9-10, October, 1972.

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D. W. DURNEY PLATE 9

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Journal of the Geological Society of Australia, Vol. 19, Pt. 2, Pls. 9-10, October, 1972.

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