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COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL • DEVELOPMENT

BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS

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RECORD 1961 No. 56

CARNARVON BASIN AIRBORNE MAGNETIC

AND RADIOMETRIC SURVEY, W.A. 1959

by

A.G. Spence

The information contained in this report has been obtained by theDepartment of National Development, as part of the policy of theCommonwealth Government, to assist in the exploration anddevelopment of mineral resources. It may not be published inany form or used in a company prospectus or statement withoutthe permission in writing of the Director, Bureau of MineralResources, Geology and Geophysics.

BUREAU OF TrIINE.A.L

GEOPHYSICAL UNARYHamm.^

RECORD 1961 No. 56

CARNARVON BASIN AIRBORNE MAGNETIC

AND RADIOM7TRIC SURVT,Y, W.A. 1959

by

A.G. Spence

The information contained in this report has been obtained by theDepartment of National Development, as part of the policy of theCommonwealth Government, to assist in the exploration anddevelopment of mineral resources. It may not be published inany form or used in a company prospectus or statement withoutthe permission in writing of the Director. Bureau of MineralResources, Geology and Geophysics,

4 •

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CONTENTS

Page

ABSTRACT

1. INTRODUCTION^ 1

2. METHOD^ 1

3. EQUIPMENT^ 2

4. GEOLOGY^ 2

5. RESULTS^ 3

6. REFERENCES^ 4

ILLUSTRATIONS

Plate 1^

Locality Map (0246-8) .

Plate 2^

Magnetic and Radiometric Results, Northern Area (0311-17-2)

Plate't^

Magnetic and Radiometric Results, Southern Area (0311-28-2)

ABSTRACT

Airing the period Aw,ust to November 1959, a MC.3 aircraftof the Bureau of Mineral Resources carried out a magnetic and radiometricsurvey of parts of the Carnarvon Basin of Western Australia.

The magnetic results show clearly the boundary between thePrecambrian rocks in the east and the sediments in the west.

Fifty two radiometric anomalies were recorded; all but fourof them were recorded over Precambrian rocks.

1.^INTRODUCTION

The airborne magnetic and radiometric survey describedin this Record was undertaken by the Geophysical Branch of the Bureauof Mineral Resources at the request of the Western Australian MinesDepartment. It forms part of a programme of surveys which willuttimately cover the whole of the Carnarvon Basin and the Perth Basincif Western Australia. Plate 1 shows the location of the CarnarvonBasin surveys and in particular the two areas surveyed in 1959 whichare the subject of this Record,^For convenience these two areas willbe referred to as the Northern Area and the Southern Area. The surveyswere extended an average distance of about 50 miles east of the junctionbetween the sedimentary rocks and the Precambrian.

The survey was carried out during the period August toNovember 1959, using the Bureau's DC.3 aircraft VH-MIN. Bureaupersonnel who took part in the survey were ;- A.G. Spence (Party Leader),R. Wells, Miss N.J. Price, D. Upton, C.J. Braybrook, J.E. Lynne,F.G. Walker, and R. Jones. Personnel from Trans-Australia Airlineswho assisted in the operations were Capt. N.K. Pascoe, First OfficerG. Greene and Plight Engineer W. Briggs.

The Object of the aeromagnetic surveys was to obtainmagnetic data over the entire Basin, in order to obtain informationconcerning its depth and principal structural features.^Conourrentlywith the magnetic survey the area was tested for radioactivity.

2.^METHOD

The variation in total magnetie intensity was systematicallyrecorded throughout the survey by an airborne magnetometer. The areas

. were surveyed by flying a grid of parallel east-west traverses spaced1 mile apart.

A system of tie-lines spaced 15 mile m apart was flown ina north-south direction to provide the- information required to removesystematic errors from the magnetic data and to reduce the profilesrecorded along individual flight lines te a common datum. The gammaradiation from the ground was recorded by scintilloraphs to indicatelocalised areas of above-average radioactivity.

The aircraft was flown at an average height of 500 ftabove ground level, and was navigated by comparing ground featureswith details on mosaics of vertical aerial photographs. The trackflown was recorded by a vertical camera. An air position indicatorsystem provided information on the aircraft's air position in two forms ;as a continuous plot on a "Recti-riter" recorder, and in the form ofco-ordinates on a mechanical register which was photographed every 12seconds.

All records were correlated by a system of fiducialmarks and numbers.

2.

3.^EVIPILEIIT

The magnetometer used was a modified AN/ASQ-6 fluxgatemagnetometer with the head mounted in a boom on the tail of theaircraft. The output of the magnetometer, which measured totalfield intensity, was recorded on a "Speedomax" recorder.

The radiometric equipment measured the intensity ofgamma radiation at two heights, nominally 500 ft and 200 ft aboveground level. The inboard equipment consisted of two Chalk Riverscintillation detector heads feeding into an.M.E.L. ratemeter.The outboard equipment consisted of a scintillation detector mountedin a "bird" and towed below and behind the aircraft on 500 ft of cable.The output of the detector was connected via the cable to an M.E.L.ratemeter in the aircraft. The output of. both systems was recordedon a dual-channel "Recti-riter" recorder.

An STR-30B radio altimeter was used for altitude control;the altitude was recorded on a single-channel "Recti-riter" recorder.

An "Aeropath" strip camera was used together with a Mk 1Aair position indicator, to provide the information required to plotthe aircraft's track.

4. 'GEOLOGY

The following brief geological notes are based primarilyon the "Strntigraphy of Western Australia" (Meilia;e et al. , 1956).

The continental area of the Carnarvon'Basin occupiesabout 4500 square miles of the central coastal region of WesternAustralia. Its eastern boundary is delineated by the junction of thesediments with the Precambrian. Its north-eastern boundary is not SOclearly defined but its northern limit is tentatively accepted to liein the coastal region north of Onolow adjacent to Barrow Island. Theline formed by the Madeline, Yandi o and Hardabut faults is regarded asthe boundary between the Carnarvon and Perth basins. The Perth Basinoccupies the southern part of the Western Australian coastal region westof the Darling scarp.

Silurian, Devonian, Carboniferous, Permian, Jurassic,Crotaceousl and Tertiary sediments occur in varying thickness in thesebasins. The maximum thickness of sediments in the Carnarvon Basin.occurs east of Wandagee Hill, where the exposed sedimentary sequenceexceeds 21,000 feet.

The eastern portions of the surveyed areas form part ofthe Precambrian Yilgarn-Pilbara Block. In general, the rocks aremetamorphosed and highly folded and include granitic rocks, schists,slate, sandstone, volcanic sediments, pegmatites,and basic dykes.The less disturbed Nullagine Series is present in the Northern Areaand consists of thicl: beds of conglomerate with interbedded basicand acid lava flows and occasional beds of limestone and dolomite.

'RESULTS

The results of this survey will be finally presentedin a series of maps on a scale of four miles to one inch which willshow contours of total magnetic intensity and positions of radiometricanomalies. The observations that follow wore made from a preliminaryexamination of the profiles during the course of the survey; theydeal only with the more obvious features recorded. A detailed inter-pretation cannot be made until the final maps are produced.

The preliminary examination of the scintillograph recordsresulted in the plotting of approximately 80 radiometric anomalies,which were numbered in a separate series for each of the two areassurveyed. In the final interpretation a total of 52 anomalies wereselected as warranting Eround follow-up. The positions of thoseanomalies, which arc all interpreted as due to sources of small arealextent, are shown in Plates 2 and 3. The original numbering has boonretained in these plates.

(1) NORTHERN AREA (Plate 2)

Magnetic Results

The outstanding feature of the magnetometer profiles istho contrast between the smoothness over the sediments and tho largovariations over the Precambrian region. The points whore this transitionin the character of the profiles occurs follow approximately the boundaryof the Precambrian outcrop. In this respect the area follows the generalpattern revealed in the other areas of the Carnarvon and Perth Basinssurveyed to date (Parkinson 1957, Newman 1959 and Forsyth 1960).

Although the profiles over the Precambrian region are generallyvery disturbed there is a pronounced reduction in the magnetic disturbancein the oast of the Telfor River, Maroonah, and Carrina one-mile areas.This area of moderato magnetic activity corresponds approximately to theregion whore undifferentiated Proterozoic sediments overlie Archaeansediments and mota-sediments.

Tho main feature of the profiles over the sediments is abroad anomaly of low intensity which trends slightly cast of north andextends from the southern to the northern boundary of the area. Itappears to terminate, just beyond tho southern boundary, in the WandagoeHill anomaly. It can be traced beyond the northern boundary, to the westof Giralia, then on to the northern limits of the areas so far surveyed(Parkinson, 1957). This anomaly is complex and has been interpreted asa basement ridge which divides the basin into two troughs. The westerntrough is thought to have at least 12,000 feet of sediments in the north;it is broader than the eastern trough and appears to be deeper. Thethickness of sediments in both troughs is considered to increase to thesouth. This is shown more clearly in the eastern trough which appearsto deepen from about 5000 feet in the north to over 10,000 feet in thesouth. The basement ridge is probably within 2000 feet of the surfacein the north, and increases in depth in the south to about '6000 foot.

A fault exposed in the southern part of the area and postulatedby Condon to extend as far north as Exmouth Gulf largely coincides with themagnetic disturbance marking the basement ridge. It appears that theridge may be an expression of such a fault.

4.

Radiometric•Results

, Ton anomalies wore detected, and their positions aro shownin Plato 2:of them are in the Precambrian region;

84§tr.

(2) SOUTHERN AREA (Plate 3)

Magnetic Results

Here also the main feature of the magnetometer profilesis the contrast between 'the smoothness over the sediments and thelarge variations over the Precambrian region; the contact between thetwo regions coincides approximately with the points of transition onthe profiles. This does not appear to be the case in the Woodrarrungand 1 239t one-mile areas, whore a faulted block of Precambrian rooksexhibits only an isolated area of Magnetic disturbance;

Tho Ajana Inner of Precambrian rocks (part of the GreenoughBlock) is indicated on the profiles as a magnetically disturbed area.However, the variations in the profiles over the Ajana Inner are not aslargo as those over the Precambrian shield. It has boon suggested as aresult of gravity surveys in this area (Thyor and li*oringham, 1956) thata ridge of Precambrian rocks with a relatively thin sedimentary coverextends between the Greenough Block and the Carrandibby Inlior (bothPrecambrian outcrop areas). -Thoro appears to be no magnetic evidenceto support this suggestion.

Radiometric Results

In general the Precambrian regions show a higher radiationlevel than the sedimentary regions. - Many of the changes in radiationlevel coincide with known geological boundaries; for instance the AjanaInlior is clearly indicated on the scintillograph profiles.

Forty-two radiometric anomalies wore detected, and theirpositions arc shown in Plato 3. All except one are in the Precambrianregion.

6.^REFERENCES

CONDON, M.A.^ 1954^Condensed progress report on thegeology of the Carnarvon (N eW„)Basin, Western Australia,Bur. Min. Resour k Aust. Roc,1954/36.

. FORSYTH, W.A.L.^1960^Carnarvon Basin airborne nagncticand radiometric survey, W.A. 1957.Bur. Min. Rosour. Aust. Roc. 1960/132.

McWHAE, J.R.H., PLAYFORD, P.E. 1 1956^Tho stratigraphy of WesternLINER, A.W. 9 GLENISTER, B.F.,^Australia.and BALME, B.E.,^ J. geol. Soc. :Lust. 4 (2).

5.

PARKINSON, W.D.^1957^Preliminary report on airbornesurveys (scintillograph andmagnetometer) of Geraldton-Onslowregion, W.A. 1956.Bur. Min. Resour, Aust. Rec.1957/9.

THYER, R.F. and EVERINGHAM,I.B. 1956^Gravity survey of the Perth Basin,W.A.Bur. Min. Resour. Aust. Bull. 33.

NEWMAN, J.K.^1959^Preliminary report of anaeromagnetic survey of thePerth Basin 1957. Bur. Min.Resour. Aust, Rec. 1959/64.

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