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  • This article was downloaded by: [University of Western Ontario]On: 17 November 2014, At: 07:46Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

    Journal of the Geological Society ofAustraliaPublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/taje19

    Revolution in earth historyA. F. Trendall aa Geological Survey of Western Australia , Mineral House, 66Adelaide Terrace, Perth, Western Australia, 6000Published online: 03 Aug 2007.

    To cite this article: A. F. Trendall (1972) Revolution in earth history, Journal of the GeologicalSociety of Australia, 19:3, 287-311, DOI: 10.1080/00167617208728798

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  • REVOLUTION IN EARTH HISTORYPresidential Address delivered in Brisbane on 25 May 1971

    By A. F. TRENDALL

    (With 2 Tables, 9 Text-Figures and 6 Plates)

    (Received 9 July 1972)ABSTRACT

    The 2,400-metre thick Hamersley Group is characterised by its 900 m of bandediron formation, and is the middle of three constituent groups of the Mt BruceSupergroup, which forms the contents of the 2,300-1,800 m.y. old Hamersley Basin.The Hamersley Basin initially covered about 150,000 km2 of northwestern Australia,and in its present widespread outcrop, the Mt Bruce Supergroup is mostly littledisturbed or metamorphosed. Within one iron formation unit 142 m thick, the DalesGorge Member of the Brockman Iron Formation, within the Hamersley Group, thereare three scales of stratification, termed macrobanding, mesobanding (the normal'banding' of banded iron formation) and microbanding. Microbands are thin (0.2-2.0 mm) regular laminae, alternately rich and poor in iron, within chert mesobands.Microbands, mesobands and macrobands may all be correlated over the whole of thepresent outcrop. Microbands are believed to result from annual seasonal control ofthe primary precipitation in the basin, while alternations between microbanded chertmesobands and the adjacent non-microbanded chert-matrix are thought to reflect a25-year environment cyclicity. There is also a higher-order cyclicity. The microbandsare chemical evaporitic varves. There are many published accounts of modern non-glacial varves of a similar order of thickness whose identity as varves is establishedby direct evidence. Many of these are couplets of laminae, one half being largelyorganogenic, and similarly structured couplets have been widely accepted as varvesin descriptions of Phanerozoic rocks. Evaporitic laminae have also been so accepted,although there are no exact modern analogues. Microbands are closely similar tothese in geometry, and all varves are characteristically regular.

    Secular variations in all depositional environments are likely to be related, inthe absence of major tectonic changes, to variations in the total annual receipt ofsolar radiation. The controls of insolation are well known, but their past variationis only usefully calculable back to 1 m.y. Variations of these controls, and especiallyof rotational obliquity, outside the Pleistocene limits, seems to be the most likelysource of an explanation for the various depositional cyclicities of the Dales GorgeMember. The history of Earth's revolution around the Sun (Revolution in EarthHistory) is a topic concerning which much evidence still remains to be read directlyfrom the stratigraphic record.

    INTRODUCTION stantially abbreviated, and its style has beenThis Address begins with an introductory modified into a form closer to that of a con-

    summary of the geological development of the ventional research paper. However, the originalHamersley area of Western Australia. It goes sequence of themes is retained, and the de-on to describe the progress and results of liberate ambiguity of the title is resolved inresearch on the Hamersley Group iron for- the final sentence,mations carried out over several years. It isthen shown how one specific problem arising GEOLOGICAL DEVELOPMENT OF THEfrom that workthe interpretation of banding HAMERSLEY AREAin banded iron formationled into an ex- The location and regional setting of theamination of aspects of geology which would Hamersley area are shown in Figure 1. Thenot initially have been thought to be remotely oldest rocks are those of the Pilbara Block,relevant. From this examination came specu- They comprise sinuously curved synclinallations on a possible significance of the total strips of metasedimentary and metavolcanicresult for the general interpretation of Earth rocks, usually termed 'greenstone belts', whichhistory from the geological record. For its separate, and are intruded by, irregularly ovoidpresent publication this Address has been sub- domes of gneiss and granite. The granites have

    Journal of the Geological Society of Australia, Vol. 19, Pt. 3, pp. 287-311, Pis. 13-18, November, 1972.

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  • 288 A. F. TRENDALLan approximate age of about 3,000 m.y.(Compston & Arriens, 1968), although thereare significant local variations from this (deLaeter & Blockley, 1972).

    After the igneous and metamorphic activityrepresented by these rocks there was upliftand erosion to a surface of fairly low relief.On this surface, as it later sank, there beganto be laid down, about 2,300 m.y. ago, asequence of lavas, tuffs, and subordinateclastic sediments, which together constitute thepresent Fortescue Group. Along the southernedge of the Pilbara Block the basal uncon-formity of the Fortescue Group now dipsgently south, and reappears farther south in anumber of inlying domes (Fig. 1). The de-position of the Fortescue Group, probablyover an ovoid area at least 550 km long (east-west) and about 400 km across (north-south),marked the beginning of a regional crustal sagwhich probably persisted for some 500 m.y.(roughly equivalent to the whole of the Phane-rozoic) which we now know as the HamersleyBasin. The outcrops of the two groups thatsucceeded the Fortescue Group in the Hamers-ley Basinthe Hamersley Group and theWyloo Groupalso appear in Figure 1.

    The Fortescue Group attained its greatestthickness (about 4,250 m) in the central partof its total outcrop area. The details of itsstratigraphic nomenclature are confusing, andare not important in the present context. Thesignificant things are that much of it is directlyor indirectly volcanic and was laid down sub-aerially or in shallow water; it can be assumedthat the sinking associated with its depositionwas steady, that it kept pace with the infillingof the basin, and that it was centred on thearea which now has the thickest section. In thelower part of the Fortescue Group there is agood deal of minor lateral stratigraphic varia-tion, but the uppermost Jeerinah Formation, ablack shale, is laterally persistent over virtuallythe whole outcrop area, which defines theminimum extent of a quiet and extensive seawhich existed at the end of Fortescue Grouptime. The deposition of the Fortescue Groupprobably occupied about 250 m.y.

    The Fortescue Group is succeeded upwardswith perfect conformity by the HamersleyGroup, which has a thickness of about 2,400m. It is characterised by the presence ofbanded iron formation, and is not further de-scribed at this point since it is the main topicof the immediately following part of thisAddress. The uppermost group of the Hamers-ley Basin, the Wyloo Group, is locally con-

    formable and locally disconformable over theHamersley Group, and contains a mixture ofclastic sediments, thick and partly algal dolo-mite, and locally important basalt. It wasdeposited in a terminal phase of HamersleyBasin development in which the centre ofdeposition moved south, and we will not befurther concerned with it.

    Figure 2 is a more detailed map of part ofthe area of Figure 1, and illustrates thestructure more clearly. In the north the For-tescue Group has a southerly dip of only afew degrees, and it is virtually undisturbed.This is so also for the overlying HamersleyGroup along the north face of the HamersleyRange, which is defined by a broad open syn-clinorium of the Hamersley Group in whichdips on either side rarely exceed 10. To thesouth of the Hamersley Range, however, thesituation is different. The folding becomessteadily more intense, and in the south-eastern part there is even overturning. Ingeneral, along their southern margin, the rocksof the Hamersley Basin are either faultedagainst, or overlain unconformably by, theyounger (1,000 m.y. old) clastic sediments ofthe Bangemall Basin, but in the southwest theWyloo Group grades into metamorphic equiva-lents and is intruded by diapiric granites whoseage (1,600 m.y.) gives an upper limit to thatof the sediments which they intrude. It hasalready been noted that the lowest FortescueGroup rocks are about 2,300 m.y. old; withinthe Hamersley Group the Woongarra Vol-canics give a rather reliable age of 2,000 m.y.(Compston & Arriens, 1968).

    STRATIGRAPHY OF THE HAMERSLEYGROUP

    The Hamersley Group has an average thick-ness of about 2,400 m, of which some 900 mconsists of banded iron formation (BIF),750 m of acid volcanic rocks, 300 m ofdolerite sills, and the remaining 450 m ofa mixture of shale, tuff, and carbonate. Thedistribution of these lithologies, and the formalstratigraphic names, appear in Figure 3. Thedifferent formations are easily distinguishedand mapped by their contrasting topographicexpressions. The iron formations are oftenwell exposed on steep cliffy hillsides and indeeply incised gorges, and contribute to thegreat natural beauty of the area.

    Two general points are worth making con-cerning the stratigraphy of the HamersleyGroup. Firstly there is a massive concentrationof igneous rocks in the upper part of the

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  • REVOLUTION IN EARTH HISTORY 289

    I N D I A N O C E A N

    REFERENCE

    Phanerozoic

    Precambrian rock younger thanMount Bruce Supergroup

    mWyloo Group

    Hamersley Group

    Fortescue Group

    Mount BruceSupergroup

    f Archaean

    50 o 50 100 150 200

    KILOMETRES

    LOCATION MAP

    Fig. 1. Generalised geological map of the northwestern part of Western Australia, showing the out-crop areas of the three component groups of the Mount Bruce Supergroup.

    Journal of the Geological Society of Australia, Vol. 19, Pt. 3, pp. 287-311, Pis. 13-18, November, 1972.

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  • 290 A. F. TRENDALL

    V V V V

    v v v v

    REFERENCE

    Wyloo Group

    Boolgeeda Iron Formation

    Woongarra Volcanics

    Brockman Iron Form.; Weeli Wolli Form.Mt McRae Shale; Mt Sylvia Formation;Wittenoom DolomiteMarra Mamba Iron FormationFortescue Group

    Archaean

    10 0 10 20 30

    KILOMETRES

    Southern edge of Mount Bruce Supergroup outcrop

    Fig. 2. Generalised geological map of part of the Hamersley Range area (outlined inFig. 1) showing the regional structure.

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  • REVOLUTION IN EARTH HISTORY 291

    group. The extrusive rhyolites and dacites ofthe Woongarra Volcanics form over a quarterof the thickness of the group, while thedolerites of the Weeli Wolli Formation, whichwere later intruded below these, constituteanother eighth. Secondly, if these igneousrocks are disregarded, and only the sedimen-tary rocks taken into account, there are fivemain units of banded iron formation, eachunderlain by a thinner unit consisting mainlyof shale. These iron formation units are, frombelow upwards:

    5. Boolgeeda Iron Formation4. Iron formation of Weeli Wolli Formation3. Joffre Member of Brockman Iron For-

    mation2. Dales Gorge Member of Brockman Iron

    Formation1. Marra Mamba Iron Formation.

    A remarkable feature about each of thesefive units of iron formation is that each ischaracteristically different from the others, ina number of subtle but quite consistent ways.This feature of the five banded iron formationunits is a major puzzle of the basin, which Ionly mention here in passing.

    BLUE ASBESTOS: A STIMULUS FORRESEARCH

    The brief outline that has so far been givenof the stratigraphy and regional setting of theHamersley Group is based on regional map-ping at a scale of 1:250,000 by parties of theGeological Survey of Western Australia dur-ing the years 1961-1963, especially of thefollowing Sheets:Dampier and BarrowIsland (Kriewaldt, 1964); Roebourne (Ryan,1966); Yarraloola (Williams, 1968); Pyramid(Kriewaldt & Ryan, 1967); Wyloo (Daniels,1970); Mount Bruce (de la Hunty, 1965);Roy Hill (MacLeod & de la Hunty, 1966);and Newman (Daniels & MacLeod, 1965).The regional results, with particular referenceto iron ore, have been synthesised by MacLeod(1966). The mapping was done at that timebecause in December 1960 the CommonwealthGovernment had lifted an embargo, which hadbeen imposed in 1939, on the export of ironore; and it is interesting to reflect on what anabundance of fascinating geology had, as anindirect result, gone undiscovered for so longup to that time. Naturally, other factors wereinvolved, but there is no doubt that the em-bargo was one important reason why little wasknown about Hamersley area geology before1961.

    In 1963 it was decided that the GeologicalSurvey of Western Australia should conductan investigation of blue asbestos in the Hamers-ley Range area. Blue asbestos, or crocidolite,occurs in clusters of thin confor...