118° 119° 120° INDIAN 20° OCEAN
? Phanerozoic cover
M W C
MB
21°
E
Y I O
S MCB
22°
MB WP EP KURRANA MCB 100 km TERRANE FHB KT
SI AHH416 22.11.10
WA Mount Bruce Supergroup East Pilbara Terrane Fortescue and Hamersley Sulphur Springs Group Groups (2780–2450 Ma) (3270–3235 Ma) De Grey Supergroup Kelly Group Croydon Group and Mosquito (3350–3315 Ma) Perth Creek Formation (2970–2930 Ma) Strelley Pool Formation Gorge Creek Group (3426–3350 Ma) 500 km (3050–3020 Ma)
Pilbara Supergroup Pilbara Warrawoona Group (3525–3427 Ma) Rift-related successions Soanesville Group Undivided granitic rocks FHB Fortescue and Hamersley Basins (c.3200–3160 Ma) (various ages) MB Mallina Basin Granitic rocks outside the Faults; MCB Mosquito Creek Basin East Pilbara Terrane exposed, concealed SI Sylvania Inlier KT Kurrana Terrane WP West Pilbara Superterrane EP East Pilbara Terrane
Figure 1. Simplified geological map of the East Pilbara Terrane showing eight granite–greenstone domes. Most of the domes are separated by Paleoarchean–Mesoarchean boundary faults within the greenstones. Dome names: C = Carlindi; M = Muccan; W = Warrawagine; Y = Yule; E = Mount Edgar; S = Shaw; O = Corunna Downs; I = Yilgalong. Modified from Van Kranendonk et al. (2006, fig. 1).
50 GeologicalGeological SurveySurvey of WesternWestern AustraliaAustralia New geoscience
Pilbara Supergroup of the East Pilbara Terrane, Pilbara Craton: updated lithostratigraphy and comments on the influence of vertical tectonics by AH Hickman Abstract Since the entire lithostratigraphy of the northern Pilbara Craton Introduction was formally revised in a Geological Survey of Western Australia 2ECORD IN EVIDENCE FROM NEW MAPPING AND GEOCHRONOLOGICAL )N THE LITHOSTRATIGRAPHY OF THE NORTHERN data has led to various lithostratigraphic revisions, in particular Pilbara Craton was formally revised and redefined FOR THE 'A 0ILBARA 3UPERGROUP 4HIS PAPER PROVIDES A 6AN +RANENDONK ET AL A .EW summary of the changes and briefly introduces the question of the evidence from university researchers, additional relationship between the stratigraphy of the Pilbara Supergroup geochronology, and ongoing evaluation of data and its tectonic environment. from the Pilbara Craton mapping project, have resulted in further revisions for the Pilbara 4HE LITHOSTRATIGRAPHIC CHANGES MADE SINCE THE PUBLICATION Supergroup, which comprises the greenstone are: succession of the Paleoarchean East Pilbara i) the Soanesville Group has been removed from the Pilbara Terrane (Hickman, 2008, 2010; Hickman and Supergroup because it marks the commencement of Van Kranendonk, 2008; Hickman et al., 2010; plate-tectonic processes Van Kranendonk et al., 2010). The accumulated ii) the Budjan Creek Formation, previously assigned to the changes to the lithostratigraphy of the Pilbara 'A 3ULPHUR 3PRINGS 'ROUP IS REASSIGNED TO Supergroup are now causing a problem because the the Soanesville Group based on its sedimentology and most comprehensive lithostratigraphic reference interpreted tectonic setting WORKS 6AN +RANENDONK ET AL A ARE iii) the Strelley Pool Formation has been removed from the now partly out-of-date. This paper summarizes and Kelly Group briefly explains the recent revisions, and presents iv) the Dresser Formation has been correlated with the them in the context of an updated interpretation McPhee Formation, based on reinterpretation of its age of the evolution of the Pilbara Supergroup. The FROM 'A TO 'A revised lithostratigraphy has been incorporated into v) a basaltic succession underlying the Dresser Formation Geological Survey of Western Australia’s (GSWA’s) is correlated with the North Star Basalt instead of the Explanatory Notes database, and is being used in Coonterunah Subgroup. GSWA’s GIS products. The stratigraphy of the Pilbara Supergroup must have been influenced by its depositional setting within an environment of vertical tectonics. This paper reports an example where the Previous interpretation of the Pilbara distribution of one formation — the Apex Basalt — provides Supergroup some support for this concept, and examination of the variable distribution of several other formations, combined with thickness The Paleoarchean East Pilbara Terrane of the and facies data, is expected to provide additional evidence. Pilbara Craton (Fig. 1) is distinguished by its KEYWORDS: Pilbara Supergroup, lithostratigraphy, Soanesville structural style, which has no close analogues Group, Budjan Creek Formation, Strelley Pool Formation, Dresser in terranes formed by plate-tectonic processes. Formation, Apex Basalt, vertical tectonics, East Pilbara Terrane, .UMEROUS PUBLICATIONS OVER THE LAST YEARS HAVE Pilbara Craton, tectonic environment, plate tectonics, depositional explained its characteristic ‘dome-and-syncline’ setting, Sulphur Springs Group. pattern, well displayed on geological maps and satellite imagery, as the product of vertical tectonic processes; earliest accounts were by Hickman AND RECENT CONTRIBUTIONS have included those by Van Kranendonk et al. A B '37! GEOLOGICAL MAPPING BETWEEN AND REVEALED THAT RATHER THAN consisting of alternating domes and synclines, the regional outcrop pattern was produced by a cluster of adjoining granite–greenstone domes (Van Kranendonk et al., 2002). Boundary faults within the greenstones separate these domes (Fig. 1). In total, there are eight granite–greenstone domes, and there are 20 greenstone belts (Fig. 2).
Annual Review 2009–10 51 118° 120° 20° Greenstone belts INDIAN 1 Pilbara Well 2 Cheearra Phanerozoic cover OCEAN 3 Wodgina 4 Abydos 50 km 5 Pincunah 6 East Strelley 20 7 Soanesville 8 Panorama WPS 9 North Shaw MALLINA 10 Emerald Mine greenstone complex BASIN M 11 Tambina greenstone complex C 12 Western Shaw W 19 13 Coongan 14 Kelly 18 15 McPhee 16 Mount Elsie 17 17 17 Marble Bar 18 Doolena Gap 6 8 E 19 Warralong 1 3 20 Goldsworthy 5 7 4 13 17 9 Fortescue Group 2 13 14 Y 10 I De Grey Supergroup 11 O 15 16 12 Greenstone belt/complex HAMERSLEYFORTESCUE BASINS AND S MOSQUITO CREEK Granitic rocks 14 BASIN Geological boundary 13
22°
AHH415 22.11.10 Figure 2. Greenstone belts and complexes of the East Pilbara Terrane. Dome names as in Figure 1. WPS = West Pilbara Superterrane. Modified from Van Kranendonk et al. (2006, fig. 4).
!S DElNED BY 6AN +RANENDONK ET AL presents the revised stratigraphic column for the the Pilbara Supergroup comprises the greenstone Pilbara Supergroup. This lithostratigraphy is succession of the East Pilbara Terrane, and is described as ‘generalized’ because the thicknesses DIVIDED INTO FOUR GROUPS AS SHOWN IN &IGURE and compositions of the formations vary between different greenstone belts. With the exception of s n -A 7ARRAWOONA 'ROUP the change to the Budjan Creek Formation (below) (predominantly volcanic); all of the individual changes summarized in this paper have already been published but with little s n -A +ELLY 'ROUP VOLCANIC or no explanation, except for the changes involving formations above a thin basal sedimentary the Strelley Pool Formation (Hickman, 2008). formation of the group, the Strelley Pool ‘Chert’); s n -A 3ULPHUR 3PRINGS 'ROUP Soanesville Group (volcanic formations overlying a basal sedimentary formation, the Leilira Formation); The Soanesville Group outcrops in the western part of the East Pilbara Terrane where it overlies s n -A 3OANESVILLE 'ROUP THE n 'A 3ULPHUR 3PRINGS 'ROUP 4HE (predominantly sedimentary). lower part of the Soanesville Group is composed OF A M THICK SUCCESSION OF SILICICLASTIC sedimentary rocks containing minor chert and Revisions to the Pilbara Supergroup BIF; in contrast, the upper part of the group is composed of volcanic rocks in the formally &IGURE SHOWS AN OLDER VERSION OF THE defined Honeyeater Basalt (references in Van lithostratigraphy (modified from Van Kranendonk +RANENDONK ET AL AND MAY INCLUDE ET AL WITH lVE LITHOSTRATIGRAPHIC UNITS unnamed volcanic formations in the Wodgina and DISCUSSED IN THIS PAPER HIGHLIGHTED IN RED &IGURE Pilbara Well greenstone belts (Van Kranendonk
52 Geological Survey of Western Australia New geoscience
et al., 2010). Stratigraphic relations between the Schematic stratigraphic Formation Group unnamed formations and the Soanesville Group section DE GREY SUPERGROUP will remain uncertain without further mapping and PYRAMID HILL FM geochronology. HONEYEATER BASALT SOANESVILLE
6AN +RANENDONK ET AL REMOVED THE PADDY MARKET FM Soanesville Group from the Gorge Creek Group ,IPPLE 6AN +RANENDONK ET AL CORBOY FM because recent mapping had indicated an erosional CARDINAL FM
unconformity between the two successions. BUDJAN CREEK FM SULPHUR SPRINGS Previously, the Gorge Creek Group had been KANGAROO CAVES FM 3251–3235 Ma included in the Pilbara Supergroup, and the KUNAGUNARRINA FM REVISION BY 6AN +RANENDONK ET AL RETAINED the Sulphur Springs Group in this supergroup while moving the Gorge Creek Group into the De LEILIRA FM 3273–3255 Ma 'REY 3UPERGROUP 6AN +RANENDONK ET AL CHARTERIS The contact between the Soanesville Group and the BASALT underlying Sulphur Springs Group was described as WYMAN FM A DISCONFORMITY BY 6AN +RANENDONK ET AL 3325–3319 Ma Based on an interpretation that hydrothermal 3346–3335 Ma
alteration at the top of the underlying Sulphur EURO KELLY Springs Group extends into the lower part of the BASALT 3OANESVILLE 'ROUP 6AN +RANENDONK ET AL inferred no significant age difference between the two groups. However, a transition between the Sulphur Springs Group and the Soanesville Group was questioned when Rasmussen et al. 3350 Ma (2007) dated monazite within the Soanesville S.P.C. PANORAMA FM 'ROUP AT C 'A AND INTERPRETED THIS TO BE 3458–3426 Ma the approximate age of deposition. This result APEX INDICATES THAT THE 3OANESVILLE 'ROUP IS MILLION BASALT years younger than the Sulphur Springs Group. Additional geochronology was subsequently M.B.C. DUFFER FM reported by Van Kranendonk et al. (2010) to 3474–3463 Ma support the interpretation that the Soanesville MT ADA 'ROUP WAS DEPOSITED AFTER C 'A BASALT McPHEE FM 3477 Ma WARRAWOONA Banded iron- 4HE CONTACT BETWEEN THE 'A 3ULPHUR NORTH STAR formation 3PRINGS 'ROUP AND THE 'A 3OANESVILLE 'ROUP BASALT Shale marks a major change in the crustal evolution 3498 Ma Coarse clastic rocks OF THE 0ILBARA #RATON &ROM TO 'A Felsic volcani- DOUBLE BAR clastic rocks the Pilbara crust was progressively thickened by FM a succession of plume-related magmatic events Chert that involved ultramafic–mafic–felsic volcanic COUCAL FM Felsic cycles, contemporaneous granitic intrusion, 3515 Ma volcanic rocks vertical deformation, and metamorphism TABLE TOP FM Basalt 6AN +RANENDONK ET AL 4HE TO Ultramafic lavas 'A CRUST FORMED DURING THESE EVENTS IS DRESSER FM Granite now preserved as the East Pilbara Terrane. 3525 Ma Rifting of this terrane probably commenced Unconformity S.P.C. Strelley Pool Chert towards the end of deposition of the Sulphur M.B.C. Marble Bar Chert Mbr 3PRINGS 'ROUP (ICKMAN 0OST 'A deposition and tectonic activity was interpreted by Van Kranendonk et al. (2010) to indicate a AHH417 11.10.10 Figure 3. Summary of the lithostratigraphic column of the Pilbara Mesoarchean Wilson cycle that reached the stage of Supergroup published in the last formal revision (Van Kranendonk ARC ACCRETION AT 'A FOLLOWED BY 'A et al., 2006, fig. 5). Stratigraphic names in red highlight units extension and accretion, with orogenic discussed in this paper.
Annual Review 2009–10 53 Generalized lithostratigraphic section Volcanic Event: deformation, metamorphism, of the Pilbara Supergroup cycle erosion, deposition, and mineralization Pincunah Banded-iron Member Hydrothermal activity, with deep-water chemical KANGAROO CAVES FORMATION sedimentation and VMS mineralization
UR
GS (3252–3235 Ma) KUNAGUNARINNA FORMATION Deepening of depositional basin
GROUP
SPRIN
SULPH LEILIRA FORMATION (c. 3255 Ma) Erosion and siliciclastic sedimentation Major event of vertical deformation (doming) Unconformity (eroded) and regional metamorphism (c. 3290–3270 Ma)
CHARTERIS BASALT Hydrothermal gold mineralization
WYMAN FORMATION Hydrothermal activity, including shallow-water (3325–3315 Ma) environments (stromatolites). Base metal mineralization
EURO BASALT (3350–3335 Ma) Major deepening of depositional basins, with KELLY GROUP KELLY many kilometres of deep-water volcanics overlying the subaerial SPF unconformity
Paraconformity Uplift, crustal extension, local unconformities STRELLEY POOL FORMATION Shallow-water platform deposition (stromatolites) (<3426 Ma) Locally deep (many kilometres) subaerial erosion Unconformity Vertical deformation (doming) and metamorphism in northeast part of the East Pilbara Terrane PANORAMA FORMATION Local uplift; erosion of Apex Basalt (3449–3427 Ma) APEX BASALT Major deepening of depositional basins, with many kilometres of deep-water volcanic rocks
Salgash
Subgroup Subsidence, with influx of clastic material Disconformity Hydrothermal activity in caldera lakes Marble Bar Chert Member Uplift, crustal extension and volcanic collapse
oup
DUFFER FORMATION Explosive felsic volcanic complexes up to (3471–3463 Ma) 8 km thick; debris flows common. Base metal mineralization
MOUNT ADA BASALT (c. 3470 Ma) Deepening of depositional basins
Coongan Subgr Disconformity McPHEE and DRESSER FORMATIONS Crustal extension, with hydrothermal activity (<3481–3477 Ma) in caldera lakes (stromatolites). Base metal mineralization Talga NORTH STAR BASALT (>3490 Ma)
WARRAWOONA GROUP WARRAWOONA Thin ultramafic–mafic–chert cycles suggest either
Subgroup multiple heating events or a distal setting to Talga more complete cyles developed elsewhere No stratigraphic contacts ?
DOUBLE BAR FORMATION
COUCAL FORMATION (3515–3498 Ma)
Subgroup
Coonterunah TABLE TOP FORMATION
Base of succession not preserved (intruded)
AHH419 27.01.11
Banded iron- Chert formation Carbonate rocks Jaspilite Siliciclastic rocks Felsic volcanic rocks
Basalt Komatiitic basalt Komatiite
Granitic rocks Unconformity Intrusive contact
Figure 4. Revised generalized lithostratigraphy of the Pilbara Supergroup (this paper). The succession is interpreted in terms of eight successive volcanic cycles, most of which are separated by unconformities. Events of deformation, metamorphism, erosion, sedimentary deposition, and mineralization are briefly noted.
54 Geological Survey of Western Australia New geoscience
deformation. The earliest stage of this Wilson cycle although in the Doolena Gap greenstone belt it is is represented by thick clastic sedimentation in the almost entirely composed of 1000 m of quartzite. lower part of the Soanesville Group. The regional distribution of the Strelley Pool &ORMATION ACROSS MORE THAN KM2 of the Sedimentological observations made by Eriksson East Pilbara Terrane (Hickman, 2008), together (1981) within the lower Soanesville Group of with sedimentological evidence, indicates that it the Pincunah and Soanesville greenstone belts was deposited across a regionally peneplained land (Fig. 2) are still relevant today, although he surface. MISTAKENLY GROUPED THIS 'A SUCCESSION WITH C 'A FORMATIONS OF THE $E 'REY As recorded by Hickman (2008), the formation Supergroup. Eriksson (1981) interpreted the lower is absent in some greenstone belts where the Soanesville Group as a submarine mid-fan to outer Euro Basalt of the Kelly Group directly overlies fan succession deposited during intracratonic formations of the Warrawoona Group. This feature rifting. could be due to non-deposition of the Strelley Pool Formation across upland areas.
Budjan Creek Formation Van Kranendonk et al. (2002) interpreted the Strelley Pool Formation (then ‘Strelley Pool Chert’) 4HE "UDJAN #REEK &ORMATION &IG IS A to conformably overlie the Panorama Formation succession of clastic sedimentary rocks that and to be genetically related to it. Later, Van UNCONFORMABLY OVERLIES THE n 'A +ELLY +RANENDONK ET AL RE ASSIGNED THE Group north and west from Nullagine. U–Pb Strelley Pool Formation to the Kelly Group on zircon data indicate a maximum depositional age the basis that it overlies a regional unconformity OF C 'A 6AN +RANENDONK ET AL and represents the first stage of the mantle plume Eriksson (1981) described the composition and event that was responsible for the Kelly Group. sedimentary structures of the formation, and Hickman (2008) reviewed more-recent evidence interpreted the depositional environment as one indicating that only the upper volcaniclastic part of of intracratonic rifting progressing to deposition the Strelley Pool Formation is genetically related to on a rifted continental margin. Numerous stacked the Euro Basalt. In the East Strelley and Panorama progradational cycles were interpreted as evidence greenstone belts (Fig. 2), detailed stratigraphic for sinking graben and deepening lakes. Van studies have found this upper part of the formation +RANENDONK ET AL USED THE MAXIMUM to consist of a boulder conglomerate containing DEPOSITIONAL AGE OF 'A TO TENTATIVELY ASSIGN lithified clasts derived from the underlying part of the Budjan Creek Formation to the top of the the Strelley Pool Formation (Allwood et al., 2007; Sulphur Springs Group, but the sedimentology of Wacey et al., 2010). Above the conglomerate is the formation and its rift-related deposition now a thin volcaniclastic unit that includes silicified strongly support its inclusion in the Soanesville VOLCANIC ASH ,OWE RECORDED ASHFALL TUFF AND Group. volcaniclastic units at the top of the formation that he interpreted to represent distal volcanic deposits. Mantle plume events are commonly preceded by Strelley Pool Formation uplift and erosion, so the relatively thin topmost section of the Strelley Pool Formation, above the 4HE 'A 3TRELLEY 0OOL &ORMATION IS A conglomerate, may be of similar age to the Euro succession of clastic sedimentary rocks, carbonate Basalt. However, the main part of the Strelley Pool rocks, and chert that separates the volcanic Formation contains no volcaniclastic material, successions of the Warrawoona and Kelly Groups and is composed of sedimentary facies indicating in eleven greenstone belts of the East Pilbara depositional environments unrelated to volcanism. Terrane (Hickman, 2008). The formation was deposited in shallow-water shelf, stromatolite reef, Other lines of evidence that argue against the estuarine, beach, sabkha, lacustrine, and fluviatile main part of the formation being deposited as the environments. Extensive silicification of primary first stage of the Kelly Group mantle plume event carbonate and fine-grained clastic rocks has include: locally transformed much of the formation into s Previous plume events in the Warrawoona secondary ‘chert’, explaining why it was initially Group, as represented by five volcanic cycles NAMED ,OWE AND FOR MANY YEARS REFERRED &IG INVOLVED VOLCANIC ERUPTION LESS THAN to, as the ‘Strelley Pool Chert’. The Strelley Pool about 10 million years after deposition of the &ORMATION IS ONLY TO M THICK IN MOST AREAS last volcanic unit of the preceding cycle. Unless
Annual Review 2009–10 55 the Kelly Group plume event was very different explained by the interpretation that the Dresser from its predecessors, this suggests that it Formation was deposited in a caldera setting (Van COMMENCED NO EARLIER THAN C 'A %URO +RANENDONK ET AL "ASALT VOLCANISM COMMENCED AT 'A s Previous plume events in the Pilbara Supergroup did not produce volcanic cycles Basalts underlying the Dresser Formation commencing with widespread deposition Following revisions to the interpreted age and of conglomerate, sandstone, and carbonate stratigraphic position of the Dresser Formation, rocks. It could be argued that the Sulphur the underlying basaltic succession in the Panorama Springs Group, which unconformably overlies greenstone belt has been correlated with the North the Kelly Group, does contain a basal clastic Star Basalt of the Talga Talga Subgroup (Hickman, succession, the Leilira Formation. However, the 2010). Leilira Formation differs from the Strelley Pool Formation in that it (a) contains felsic volcanic and volcaniclastic components and wacke (Van Apex Basalt Kranendonk, 2000); and (b) is of similar age to the overlying volcanic formations of the group Throughout the Marble Bar greenstone belt, the (Buick et al., 2002). !PEX "ASALT IS AN APPROXIMATELY M THICK s Geochronology on the clastic sedimentary formation of komatiite, komatiitic basalt, tholeiitic rocks of the Strelley Pool Formation in four basalt, and thin units of chert. The formation greenstone belts (reviewed by Hickman, 2008) DISCONFORMABLY OVERLIES THE 'A $UFFER reveals detrital zircon populations ranging Formation and conformably underlies felsic FROM TO 'A AGES CONSISTENT WITH VOLCANIC ROCKS OF THE TO 'A 0ANORAMA detritus derived by erosion of the underlying &ORMATION 4HE !PEX "ASALT IS M THICK IN Warrawoona Group. Of the 129 zircon grains the eastern part of the Warralong greenstone belt, analysed, 12 of the youngest had U–Pb model but is absent in the western Warralong, Panorama, AGES OF BETWEEN AND 'A AND TWO and Coongan greenstone belts where the Panorama ZIRCONS HAD AGES CLOSE TO 'A 4HESE DATA Formation directly overlies the Duffer Formation. do not support deposition of the main part These stratigraphic differences can be explained of the Strelley Pool Formation shortly before by differential uplift of the granite–greenstone eruption of the volcanic rocks of the Kelly domes (Fig. 1) across the boundary faults prior 'ROUP AT 'A TO 'A )F THE BOUNDARY FAULTS WERE ACTIVE FOLLOWING TO 'A DEPOSITION OF THE $UFFER Formation they could have controlled the extent Dresser Formation of the depositional basins of the Apex Basalt. 6AN +RANENDONK ET AL PLACED THE $RESSER Alternatively, if the boundary faults became active Formation in the Coonterunah Subgroup of shortly after deposition of the Apex Basalt, this the Warrawoona Group based on a preliminary could have led to it being eroded across those 3(2)-0 5n0B ZIRCON DATE OF Ò -A FOR A domes that were uplifted. Hickman and Van felsic volcaniclastic unit. However, the maximum +RANENDONK INTERPRETED THE BOUNDARY age of deposition of this rock was later revised to FAULTS AS TO 'A STRUCTURES AND THE PRESENT Ò -A 6AN +RANENDONK ET AL SUGGESTION THAT PRE 'A 7ARRAWOONA 'ROUP when it was recognized that a younger concordant stratigraphy was influenced by these boundaries zircon population was also present. Hickman is a new concept. Evidence that the Apex Basalt and Van Kranendonk (2008) therefore supported was eroded from the Coongan greenstone belt a correlation between the Dresser Formation may lie in the existence of numerous dolerite AND THE 'A -C0HEE &ORMATION OF THE DYKES THAT INTRUDE THE 'A $UFFER &ORMATION Talga Talga Subgroup (Van Kranendonk et al., in this greenstone belt; a similar situation exists 2007a,b). The Dresser and McPhee Formations in the Marble Bar greenstone belt where dolerite are each composed of basalt, chert, carbonate dykes in the Duffer Formation were feeders to rocks, and minor felsic volcaniclastic rocks, and the overlying Apex Basalt (Hickman and Van BOTH FORMATIONS UNDERLIE THE 'A -OUNT Kranendonk, 2008). More-detailed mapping is Ada Basalt. The Dresser Formation also contains required to establish if any of the dolerite dykes of hydrothermal barite, evaporites, and stromatolites the Coongan greenstone belt are truncated at the not present in the McPhee Formation. This is base of the Panorama Formation.
56 Geological Survey of Western Australia New geoscience
Discussion the Sulphur Springs Group. The Strelley Pool Formation is no longer assigned to the Kelly Group &IGURE PRESENTS A REVISED LITHOSTRATIGRAPHIC for reasons previously published (Hickman, 2008), succession for the Pilbara Supergroup, and and is interpreted to be a sedimentary formation interprets the main events that were responsible that separates the Warrawoona Group from the for the deposition of the succession. A key feature +ELLY 'ROUP 4HE 'A $RESSER &ORMATION HAS of the Pilbara Supergroup is that it is composed of been moved from the Coonterunah Subgroup VERTICALLY REPEATED ULTRAMAlCMAlCFELSIC VOLCANIC to the Talga Talga Subgroup following revised cycles, each of which is attributed to plume-related geochronology and a new correlation with the partial melting of the upper mantle and lower crust McPhee Formation; accordingly, basalts underlying (Van Kranendonk et al., 2002; Smithies et al., the Dresser Formation in the Panorama greenstone !NOTHER IMPORTANT FEATURE IS THE PRESENCE belt (Fig. 2) have been assigned to the North Star OF SEVERAL UNCONFORMITIES IN THE SUCCESSION &IG Basalt (Hickman, 2010). All these revisions are Two major unconformities separate the three SHOWN ON &IGURE groups, and disconformities or paraconformities separate the volcanic cycles within the groups. The restriction of the Apex Basalt to particular Some of the disconformities are marked by laterally greenstone belts is now thought to be the result of discontinuous clastic sedimentary lenses formed differential vertical movement (several kilometres) by erosion of small-scale surface structures at the OF THE EAST 0ILBARA DOMES BETWEEN AND tops of the cycles. These structures were most 'A 4HE DOMES DIFFER NOT ONLY IN SIZE BUT likely formed either by crustal extension (related also in variable amounts of uplift. This raises the to regional doming or local subvolcanic intrusion) possibility that regional thickness variations in or by volcanic collapse that produced calderas. other formations are also at least partly due to Alternating stages of uplift and subsidence are variations in the amplitude and timing of domal evident from the repeated examples of subaerial uplift, and to varying subsidence in the different erosion and/or shallow-water deposition followed greenstone basins. Differences in uplift may be BY PILLOW BASALT SUCCESSIONS MORE THAN M explained by varying extents of intrusion by the thick (Mount Ada, Apex, Euro, and Charteris four granitic supersuites of the East Pilbara Terrane. Basalts). Such rapid and major variations in basin These supersuites were largely responsible for depth are best explained by periods of domal INmATION OF THE DOMES BETWEEN AND 'A uplift and basin subsidence. Structural evidence (ICKMAN AND 6AN +RANENDONK 6AN (ICKMAN #OLLINS 6AN +RANENDONK +RANENDONK ET AL !VAILABLE GEOCHRONOLOGY ET AL A B (ICKMAN AND 6AN indicates that the volumes of individual supersuites +RANENDONK DEMONSTRATES THAT THE DOMES vary considerably between the domes, and some were formed by vertical uplift. Geochemical domes lack any intrusions of some supersuites (Van EVIDENCE 3MITHIES ET AL 6AN +RANENDONK ET AL &IGS Kranendonk et al., 2007a,b) supports a relationship to mantle plumes. Stratigraphic evidence, such as that discussed here for the Apex Basalt, could be investigated to provide more information on differences in the timing and amounts of uplift and SUBSIDENCE IN THE EIGHT GRANITEGREENSTONE DOMES
Conclusions Further work since a formal lithostratigraphic definition of the Pilbara Supergroup (Van +RANENDONK ET AL NECESSITATES REVISION OF its composition and succession. The Soanesville Group is now formally excluded from the Pilbara Supergroup because its deposition marked a major change in tectonic processes operating in the Pilbara Craton (Van Kranendonk et al., 2010). The age, sedimentology, and interpreted tectonic setting of the Budjan Creek Formation support assignment to the Soanesville Group instead of
Annual Review 2009–10 57 References
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