Revolution in earth history
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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
To link to this article: http://dx.doi.org/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.
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, w