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1986 Stratigraphy, Sedimentology, and Sedimentary Petrology of the Early Archean Coongan Formation, Warrawoona Group, Eastern Pilbara Block, Western Australia (Sandstones, Precambrian). Michael J. Dimarco Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Dimarco, Michael J., "Stratigraphy, Sedimentology, and Sedimentary Petrology of the Early Archean Coongan Formation, Warrawoona Group, Eastern Pilbara Block, Western Australia (Sandstones, Precambrian)." (1986). LSU Historical Dissertations and Theses. 4296. https://digitalcommons.lsu.edu/gradschool_disstheses/4296

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8710559

DiMarco, Michael J.

STRATIGRAPHY, SEDIMENTOLOGY, AND SEDIMENTARY PETROLOGY OF THE EARLY ARCHEAN COONGAN FORMATION, WARRAWOONA GROUP, EASTERN PILBARA BLOCK, WESTERN AUSTRALIA

The Louisiana State University and Agricultural and Mechanical Col. Ph.D. 1986

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STRATIGRAPHY, SEDIMENTOLOGY, AND SEDIMENTARY PETROLOGY OF THE EARLY ARCHEAN COONGAN FORMATION, WARRAWOONA GROUP, EASTERN PILBARA BLOCK, WESTERN AUSTRALIA

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

in

Department of Geology

by Michael J. DiMarco B.A., State University of New York at Buffalo, 1979 M.S., Kansas State University, 1983 December, 1986 ACKNOWLEDGEMENTS

Numerous people contributed to the success of this dissertation project. The members of my advisory committee, Drs. Donald Lowe, Gary Byerly, Ray Ferrell, and

Dag Nummedal of the Department of Geology at L.S.U. and

Dr. Kenneth Eriksson of the Department of Geological

Sciences at Virginia Polytechnic Institute, provided guidance and discussion throughout all phases of this work. In particular, I thank Dr. Lowe for suggesting this project to me and encouraging my interest in Archean studies. Drs. Eriksson and Byerly are especially thanked for the time spent in the field and for discussions on the outcrop. Many of the sedimentological conclusions were refined through discussions with Drs. Lowe, Eriksson, and

Nummedal. Drs. Ferrell and Byerly contributed greatly to aspects of petrology, alteration, and geochemistry discussed in the text.

In addition to the members of my advisory committee, other members of the faculty of the Department of Geology at L.S.U. gave of their time and expertise. Dr. Jeffery

Hanor shared many of his ideas and the results of studies on alteration of Archean terranes. Dr. Ajoy Baksi discussed some of the peculiarities of geochronology and the difficulty of applying isotopic techniques to Archean terranes in general.

Two able-bodied field assistants, Edmund Burhans and

ii Randolph McBride, provided invaluable help under sometimes harsh field conditions. They were worth their weight in gold. For additional logistical help in the field, I thank the French Family of Panorama Station, Western Australia, and the Bradfield Family of Marble Bar, Western Australia.

Several individuals from the Department of Geology at the University of Western Australia, including Drs. David

Groves, Mark Barley, and Rodger Buick, shared many ideas and provided helpful tips for planning several of the field seasons. Dr. Alec Trendell, Director of the

Geological Survey of Western Australia spent several sessions with the author discussing geology of the Pilbara

Block.

I thank Maud Walsh, Bruce Nocita, Amitava Roy, Louise

Hose, Gail Fisher, Phil Lowry, and other fellow students at L.S.U. for listening to many of the ideas presented here, especially when they were in their formative stages.

Wanda LeBlanc, Research Associate at L.S.U., helped with the polytype analysis presented in paper four.

Finally, I thank my wife, Ramona B. DiMarco, for continued support and understanding throughout the duration of this project. Her contribution to this project cannot be understated.

This research was funded through a National Science

Foundation Grant, No. EAR-81-10863, to Dr. Donald Lowe.

iii Contributions from the Department of Geology at L.S.U., the O.R.F. Fund at L.S.U., and the Society of Economic

Paleontologists and Mineralogists partially defrayed the costs of presenting some of the results of this dissertation at professional meetings. The author received an Alumni Federation Fellowship from L.S.U. during his course of studies for the Ph.D. degree. CONTENTS

Acknowledgements...... ii

Abstract...... x

Dissertation Introduction...... 1

Stratigraphy and Sedimentology of the early Archean

Coongan Formation, Eastern Pilbara Block, Western

Australia, with Special Reference to the Duffer Member and Implications for Crustal Evolution...... 3

Abstract...... 3

Introduction...... 5

Geologic Setting...... 6

Stratigraphy...... 9

Facies of the Duffer Member...... 12

Facies A: Structureless and graded breccia.... 13

Facies B: Complexly organized conglomerate.... 16

Facies C: Turbiditic tuff and tuff-breccia....19

Facies D: Lavas...... 22

Sedimentology of Associated Units...... 23

Panorama Member...... 23

Strelley Pool Chert...... 24

Sequences and Environments of Deposition...... 25

Coongan belt ...... 26

Kelly belt...... 28

Marble Bar and North Shaw belts...... 30

Discussion...... 32

v Controls on sedimentation of the Duffer

Member...... 32

De ve 1 opment a 1 mode 1...... 33

Relationship of Plutonism toFelsic Volcanism...36

Early Archean Crustal Evolution in the

Eastern Pilbara Block...... 39

Comparison to modern settings...... 41

Conclusions...... 44

References...... 46

Figure Captions and Figures...... 60

Shallow-water Volcaniclastic Deposition in the Early

Archean Panorama Member of the Coongan Formation, Eastern

Pilbara Block, Western Australia...... 89

Abstract...... 89

Introduction...... 91

Geologic Setting...... 92

Stratigraphy...... 94

Facies of the Panorama Member...... 95

Facies A: Volcaniclastic breccia...... 96

Facies B: Chert-clast conglomerate...... 97

Facies C: Turbiditic tuff and tuff-breccia.... 99

Facies D: Cross-stratified volcaniclastic

sandstone...... 102

Facies E: Laminated and cross-laminated

siltstone...... 105 Facies F: Banded chert...... 107

Facies G: Silicified evaporite...... 109

Facies H: Lava...... 112

Facies Associations.,...... 112

Facies association 1...... 112

Facies association 2...... 113

Facies association 3...... 114

Facies association 4...... 116

Sequence Evolution...... 117

Western occurrences...... 117

North Pole Dome...... 117

Pilgangoora belt...... 120

Eastern occurrences...... 121

Coongan belt...... 121

Marble Bar belt...... 122

Kelly belt...... 123

Depositional Model...... 124

Summary and Conclusions...... 126

References...... 129

Figure Captions and Figures...... 141

Petrographic Characteristics and Provenance of Silicified

Early Archean Sandstones, Eastern Pilbara Block, Western

Australia...... 175

Abstract...... 175

Introduction...... 177

vii Geologic Setting...... 178

General Character of the Sandstones...... 180

Original Framework Components...... 183

Megaquartz...... 183

Rock fragments...... 184

Feldspar...... 187

Ferromagnesian minerals...... 188

Pumice and shards...... 188

Estimate of framework-grain proportions...... 189

Modal Results...... 191

Provenance...... 193

Alteration...... 197

Comparison to Other Archean Sandstones...... 200

Conclusions...... 202

References...... 204

Figure Captions and Figures...... 216

Tables...... 233

Polytypes of 2:1 Dioctahedral Micas in Silicified Meta- volcaniclastic Sandstones, Warrawoona Group, Pilbara

Block, Western Australia...... 236

Abstract...... 236

Introduction...... 237

Geologic and Petrographic Background...... 238

Analytical Methods...... 241

viii Results...... 243

Discussion...... 244

Summary...... 248

References...... 249

Figure Captions and Figures...... 253

Table...... 260

Appendix 1...... 261

Appendix 2 ...... 263

Appendix 3...... 314

Vita...... 324

ix ABSTRACT

The Coongan Formation, defined and described here, is an approximately 3.5 Ga-old unit of felsic volcanic and volcaniclastic rocks, representing a major episode of early Archean felsic volcanism, in the predominantly basaltic Warrawoona Group, eastern Pilbara Block, Western

Australia. The Coongan Formation is divided into three members: the Duffer and Panorama Members and the Stre1ley

Pool Chert.

The oldest and volumetrically greatest part of the

Coongan Formation is the Duffer Member, which consists largely of volcaniclastic breccia, conglomerate, tuff, and felsic lava. Vertical sequences, up to 5 km thick, indicate that the Duffer Member represents coarse-grained, shoaling-upward debris-aprons chat flanked felsic volcanic centers. The Panorama Member, consisting predominantly of silicified volcaniclastic sandstone, tuff, siltstone, chert-clast conglomerate and banded chert, caps or interfingers with the upper part of the Duffer Member in many sections, or, in other localities, is developed to the exclusion of the Duffer Member. Sequences of the

Panorama Member, generally less than 300 m thick, represent a spectrum of environments, including fan(?)- delta and laterally flanking shoreface environments as well as subaqueous settings under the direct influence of pyroclastic volcanism. After a period of erosion, ortho-

x and biochemical sediments represented by the Strelley Pool

Chert accumulated on a broad, post-volcanic platform.

Petrographic techniques, developed here for analysis of highly silicified sandstones, indicate that felsic volcanic sources provided most of the detritus to the

Panorama Member. No evidence for granitic detritus was found in either the Duffer or Panorama Members, indicating that granitoid plutons in the eastern Pilbara Block were not exposed during the felsic volcanic episode. The maximum temperature attained during alteration of Panorama sandstones was 200° to 300° C, based on the present mineralogy of these rocks.

Paleocurrent evidence suggests that sediment was dispersed from felsic centers, now mostly eroded, that coincided with the present positions of parts of Archean granitoid batholiths in the eastern Pilbara Block. These data suggest that the oldest granitoid rocks in the batholiths are genetically related to felsic volcanic rocks of the Coonagn Formation, and support similar hypotheses made by others based on geochemical data.

xi DISSERTATION INTRODUCTION

This dissertation embodies four papers dealing primarily with the stratigraphy, sedimentology, and sedimentary petrology of the Coongan Formation, a newly identified formation defined and described here. This unit, consisting of felsic volcaniclastic and volcanic rocks, represents the surficial expression of the earliest major period of felsic magmatism in the early

Archean Pilbara Block of Western Australia.

The first paper deals with the stratigraphy of the

Coongan Formation and emphasizes the sedimentology of coarse-grained deposits of the Duffer Member, the oldest and thickest member of the Coongan Formation. The relationship of the Coongan Formation to felsic plutonism in the Pilbara Block is explored along with the resulting implications of this magmatic episode for early Archean crustal evolution. The second paper documents the sedimentology of the Panorama Member of the Coongan

Formation, a silicified, predominantly sandy unit representing the dispersal of late-stage felsic debris within shallow-water and adjacent environments. The third paper focuses on the petrography and provenance of sandstones of the Panorama Member, emphasizing their derivation from felsic-volcanic sources and the difficulty in applying conventional petrographic techniques to these highly altered rocks. The final paper addresses the polytype distribution of fine-grained dioctahedral micas, common as replacement minerals in altered Panorama sandstones. The polytype characteristics are used to interpret the thermal history of the rocks. STRATIGRAPHY AND SEDIMENTOLOGY OF THE EARLY ARCHEAN

COONGAN FORMATION, EASTERN PILBARA BLOCK, WESTERN

AUSTRALIA, WITH SPECIAL REFERENCE TO THE DUFFER MEMBER

AND IMPLICATIONS FOR CRUSTAL EVOLUTION

ABSTRACT

Sequences of metamorphosed felsic volcaniclastic

breccia, felsic lava, and lesser amounts of tuffaceous

rocks and mafic lava, up to 5 km thick, form the Duffer

Member of the Coongan Formation in the predominantly basaltic Warrawoona Group, eastern Pilbara Block, Western

Australia. Together with the sandy Panorama Member and the

Strelley Pool Chert, the Duffer Member forms the oldest and volumetrically greatest part of a newly defined

lithostratigraphic unit, the Coongan Formation, traceable through at least six greenstone belts in the eastern

Pilbara Block.

The Duffer Member is divided into four facies: (A) structureless and graded volcaniclastic breccia, (B) complexly organized conglomerate, (C) turbiditic tuff- breccia and tuff, and (D) lavas. These facies represent deposition on predominantly subaqueous volcaniclastic aprons flanking felsic volcanic centers. A three-stage model of the evolution of these aprons includes (A) an early stage of subaqueous felsic and mafic eruptions and subaqueous deposition, (B) a middle stage, during which the bulk of the sequence was deposited, of shallow-water to subaerial felsic eruptions and subaqueous deposition, and (C) a terminal stage of subaerial felsic eruptions and subaerial deposition. Post-Duffer felsic volcanism was marked by explosive pyroclastic activity resulting in the widespread deposition of ash in surrounding shallow-water and subaerial environments. This phase of activity is represented by the Panorama Member of the Coongan

Formation. Following cessation of volcanism, orthochemical and biochemical sedimentation, represented by the Strelley

Pool Chert, took place on a broad platform eroded on the underlying volcanic sequence.

Paleocurrent evidence from the Duffer and Panorama

Members confirms earlier proposals that the intrusion of granitoid rocks in the eastern Pilbara Block was coeval with felsic volcanism. It seems likely that felsic volcanism represented by the Coongan Formation was the surficial expression of plutonism and represented the initial phase of cratonization in the eastern Pilbara

Block.

The evolutionary spectrum of clastic environments represented by the Coongan Formation is similar to that of many Phanerozoic arcs and marginal basins. Shoaling- upward felsic volcaniclastic sequences would be expected, however, in any setting where felsic volcanoes are active in water. The abundance of shallow-water facies, non­ linear and closely spaced distribution of felsic centers, association with thick sequences of basalt erupted at or near sea level, sparse syn-volcanic deformation, and total

lack of granitic detritus suggest that the felsic volcanic province represented by the Coongan Formation was unlike

those of modern arcs but was associated with a shallow,

simatic platform in an anorogenic setting.

INTRODUCTION

Felsic volcanic units, including lavas and fragmental

rocks, are important components of virtually all Archean greenstone belts (Viljoen and Viljoen, 1969; Goodwin and

Ridler, 1970; Hickman and Lipple, 1978; Dimroth, 1977;

Goodwin, 1977; Lambert, 1978; Hickman, 1983; Barley et al, 1984 and many others). These units form broadly

lenticular deposits that show major thickness and facies changes (Goodwin and Ridler, 1970; Barley et al, 1979;

Lowe, 1980, 1982a; Barley et al, 1984). The thickest parts of felsic volcanic sequences, commonly interpreted as proximal or vent complexes (e.g. Goodwin and Ridler, 1970;

Lowe, 1980, 1982a), can reach several km in thickness and characteristically contain coarse fragmental rocks and lavas. Although numerous studies have focused on flow and proximal pyroclastic facies (e.g. Dimroth and Demarke,

1978; Tasse et al, 1978; Ayres, 1982), few details are available on the spectrum of facies that form broad flanking aprons around early Archean felsic eruptive centers.

This paper focuses on the early Archean Duffer Member, the oldest member of the Coongan Formation, a new unit 6

defined here, in the Warrawoona Group of the eastern

Pilbara Block, Western Australia. The Duffer Member,

previously termed the Duffer Formation (Hickman and

Lipple, 1978; Hickman, 1977, 1981, 1983), consists of

coarse volcanogenic breccia and smaller amounts of felsic

lava and tuffaceous rocks. The rocks have calc-alkaline affinities and were largely dacitic in composition (Barley

et al 1984). The sedimentology of the Duffer Member was briefly addressed by Barley and co-workers (Barley, 1978,

1980, 1981; Barley et al, 1979, 1984) in the course of petrologic studies. This paper provides a systematic account of facies, sequences, and depositional environments of the Duffer Member and an outline of the stratigraphic and sedimentologic relationship of the

Duffer Member to other members of the Coongan Formation.

Paleocurrent data from the Coongan Formation are integrated with previously published evidence to constrain the relationship between early Archean felsic volcanic rocks and plutonic rocks in the eastern Pilbara Block.

These results are applied to interpreting early Archean crustal evolution in the Pilbara Block.

GEOLOGIC SETTING

The Pilbara Block (Fig.l), with an area of about 56,000 2 km , is the smaller and older of the two Archean granite- greenstone terranes in Western Australia (Hallberg and

Glickson, 1981) and includes one of the oldest well- preserved supracrustal sequences onearth. Greenstone

sequences in the eastern Pilbara Block are composed of

metamorphosed lavas and sedimentary rocks and form steeply

infolded, keel-like synforms and, locally, more gently

arched domes, separated by ovoid, granitoid batholiths.

The batholiths are complex bodies consisting of schists,

paragneisses, and amphibolites and are intruded by

granitic rocks ranging in composition from tonalite to

granite (Bettenay et al, 1981; Bickle et al, 1983; Barley

et al, 1984). Rocks studied here crop out within an area 2 of about 5000 km in the eastern Pilbara Block where

greenstone sequences are exceptionally well preserved.

Deformation has produced steeply dipping to overturned

strata in most belts, although gentler dips, ranging from

30° to 70°, are locally present, such as around the North

Pole Dome. In many areas, the rocks show little evidence

of strain, but in others, cleavage is well developed and

the rocks have been altered to greenschist grade or higher. Structural histories of the Pilbara Block proposed by Hickman (1981) and Bickle et al (1983) involve several

stages of deformation. Despite structural complexity, recent studies by Lipple (1975), Hickman and Lipple

(1978), and Hickman (1981, 1983) have suggested that the supracrustal sequence in the Pilbara Block formed a tabular, regionally continuous sheet prior to deformation.

Metamorphism within the Warrawoona Group is generally at greenschist grade but ranges from prehnite-pumpellyite to amphibolite grades (Barley, 1980). The highest metamorphic

grades are restricted to zones adjacent to gneissic

portions of the batholiths (Hickman and Lipple, 1978).

Although pervasive metasomatism has destroyed most primary

minerals (Barley et a l , 1979; Barley, 1980; Lowe, 1983;

Barley et al, 1984; DiMarco and Lowe, 1984, and this

volume, chapter 3), relatively unstrained sedimentary

rocks in the Warrawoona Group generally contain well-

preserved textures and structures. Thus, the prefix "meta"

will be dropped from lithologic nomenclature in this paper

and rocks will be described in terms of their protoliths.

Maximum isotopic ages of about 3.5 Ga, determined by Pb-

Pb, U-Pb, and Sm-Nd methods (Richards, 1977; Sangster and

Brook, 1977; Pidgeon, 1978a,b; Hamilton et al 1981; Jahn

et al, 1981), are reported for basaltic and felsic rocks

of the Warrawoona Group, including the Duffer Member, and are generally interpreted as primary. Ages obtained by the

Rb-Sr method for Warrawoona rocks (e.g. Oversby, 1976;

Cooper et al, 1982) show a range from <3.0 to 3.5 Ga. Most

Rb-Sr ages reflect Rb and Sr mobility during alteration episodes, although Rb-Sr ages obtained from least-altered rocks appear to be primary (Barley and deLaeter, 1984).

Similar 3.5 Ga ages were obtained from the oldest granitoid rocks in the batholiths using U-Pb, Pb-Pb, and

Sm-Nd methods (Pidgeon, 1978c; Bickle et al, 1983;

Collerson and McCulloch, 1983; Williams et al, 1983). STRATIGRAPHY

Early Archean supracrustal rocks in the eastern Pilbara

Block have been assigned to the Pilbara Supergroup by

Hickman (1981, 1983). This succession is divided into the

lower, predominantly basaltic Warrawoona Group and the

overlying, mainly sedimentary Gorge Creek and Whim Creek

Groups. The Warrawoona Group consists of 5 to 20 km

(Lipple, 1975) of mostly mafic lavas that include thinner

units of felsic volcanic and fragmental rocks and minor

layers of cherty sedimentary rocks. Hickman (1981, 1983)

further subdivided the Warrawoona Group into nine

formations, which he correlated throughout the greenstone

belts of the Pilbara Block. His correlation is based

largely on the inferred recognition in these belts of the

Towers Formation, which includes the distinctive black-,

red-, and white-banded Marble Bar Chert (MBC) as a member

(Hickman, 1977, 1981, 1983).

Regional correlation of felsic volcanic and

volcaniclastic strata, outlined here, indicates departures at the formation level from Hipkman's correlation of units and has resulted in the establishment of a new

lithostratigraphic unit, the Coongan Formation, named after the Coongan River, an important drainage in the eastern Pilbara Block. This formation, found in at least six greenstone belts (Fig. 2), consists of felsic volcanic breccia and sandstone, felsic tuff, lava, and chert, in sequences up to 5 km thick in the upper half of the Warrawoona Group. It is overlain and underlain by thick sequences of basalt, although differential preservation of lower parts of the Warrawoona Group in some greenstone belts has resulted in underlying basaltic sequences ranging from 0 to over 6 km thick. In some belts, the lower part of the Coongan Formation is intruded by or in tectonic contact with granitoid rocks.

The Coongan Formation is divided into the Duffer,

Panorama, and Strelley Pool Chert (SPC) Members (Fig. 3).

The type section, (Loc. 30, Fig. 1) which contains all three members, is near the north end of the Coongan greenstone belt, at 21° 22'S, 119° 37' E, about 30 km southwest of Marble Bar (Fig. 2). The two lower members, the Duffer and Panorama Members, formerly known as tna

Duffer and Panorama Formations (Hickman and Lipple, 1978;

Hickman, 1977, 1981, 1983), make up the bulk of the formation. The Duffer Member consists largely of felsic volcaniclastic breccia and felsic lava. Its type section

(Loc. 63, Fig. 1) is at 21°03'S, 119°4l'E, in the Marble

Bar belt, about 14 km north of Marble Bar. A reference section (Loc. 35, Fig. 1) is designated at 21*31' S, 120° 01*

E, in the Kelly belt, in the vicinity of Spinaway Creek.

The Duffer Member is well developed in the Coongan, Marble

Bar, Kelly, and North Shaw belts but is absent in the

North Pole Dome (NPD) and the Pilgangoora belt. The

Panorama Member is made up primarily of felsic volcaniclastic sandstone and felsic tuff. Its type section (Loc. 34, Fig. 1) is at 21° 16'S , 119° 28'E, at the eastern

end of Panorama Ridge on the southeast flank of the NPD.

The Panorama Member is developed extensively along the

northern and eastern flanks of the NPD. It also occurs

locally above or interbedded with the upper part of the

Duffer Member. The Duffer and Panorama Members are

overlain by the SPC in the NPD, Kelly, and west flank of the Coongan belts; and by the MBC, or a closely spaced

sequence of cherts including the MBC, considered here a

lateral correlative of the SPC, in the Marble Bar, North

Shaw, and east flank of the Coongan belts. The SPC is present without the underlying clastic members on the north flank of the Pilgangoora belt.

The stratigraphic interpretetion presented here differs from earlier syntheses (Lipple, 1975; Hickman and Lipple,

1978; Hickman, 1977, 1981, 1983) largely because of differences in the regional correlation of thick chert marker-beds. The establishment of the Coongan Formation is based largely on (1) the recognition, in several belts, of the SPC as a cap on the lower felsic members of the

Coongan Formation, and (2) the validity, in other belts, of correlating tha SPC to cherts of Hickman's Towers

Formation, particularly the MBC. In belts where it is recognized, the SPC is the only thick chert cap over felsic volcanic and volcaniclastic units. It shows a regionally uniform sequence of facies, including laminite, stromatolite, and evaporite, representing shallow-water, evaporative sedimentation (Lowe, 1983). The SPC is

overlain by a sequence of basalts, generally 2 to 3 km

thick, that includes from two to five thin, green to gray

cherts (Fig. 4). These cherts are made up of silicified

massive, laminated, and, locally, cross-laminated ash and

dust, representing air-fall pyroclastic material, and are

uncommon or absent in basalt sequences below the Coongan

Formation. In belts where the SPC is not observed, the

MBC, or a closely spaced sequence of cherts including the

MBC, is correlated with the SPC. Both the SPC and the MBC occupy a similar position in the upper part of the

Warrawoona Group, overlie a thick felsic succession, and are never found in the same greenstone-belt sequence. The basaltic sequence above the MBC also contains several thin gray to green cherts, again interpreted as ash deposits, which, as in belts containing the SPC, are rare or absent in the lower basaltic succession (Fig. 4). The MBC and the

SPC appear to represent a major hiatus in volcanic activity.

FACIES OF THE DUFFER MEMBER

The Duffer Member includes a restricted range of rock types that are here divided into four facies: (A) structureless and graded breccia, (B) complexly organized conglomerate, (C) turbiditic tuff-breccia and tuff, and

(D) lavas. Facies A: Structureless and graded breccia

Description

The bulk of the Duffer Member consists of poorly bedded volcaniclastic breccia (Fig. 5) interbedded with sparse beds of turbiditic tuff of facies C. Typically, the breccia has a disorganized, bimodal fabric consisting of clasts of light-colored felsic volcanic rock in a sand­ sized volcaniclastic matrix. Clasts make up 30 percent or less of most units but a small precentage of units, less than 20 percent, have 40 to 50 percent clasts and are clast supported. The clasts are generally equant to elongate, angular, and pebble- to cobble-sized, although, locally, they range up to 1 m in diameter. Units with a maximum grain size of less than 8 cm are rare. Most units contain from 5 to 10 percent rounded clasts, although in some units, over 50 percent of the clasts are rounded

(Fig. 5). The clasts are composed of a variety of aphanitic to porphyritic lavas, containing abundant phenocrysts of plagioclase, a few pseudomorphs, now composed of micromosaics of chert, sericite, and black oxides, after pyroxene and amphibole, and rare primary quartz. A few breccia layers contain sparse tabular clasts of white, red, or banded chert, and, in several sections, one or two layers of chert-clast breccia are present. The matrix typically consists of poorly to moderately sorted, medium- to very coarse-grained sand composed of blocky, microporphyritic to aphanitic volcanic rock fragments. Locally, the clasts are surrounded by lapilli-sized

pumice(?) fragments and chloritic aphanitic debris.

Bedding is commonly weakly developed or absent, and its

recognition is hindered by poor exposure and preservation.

Sections with preserved bedding (Fig. 6), amounting to

about 10 percent of observed sections, contain breccia

beds ranging from 20 to 240 cm thick, with most being 50

to 90 cm thick. They are structured, in order of

decreasing abundance, into ungraded, normally graded,

inverse-to-normally graded, and inversely graded units.

Grading is defined by size changes in the gravel size

fraction and in most graded units is only weakly

developed. The thickest units tend to be ungraded or

normally graded and generally have sand-matrix support.

Inversely and inverse-to-normally graded units are

generally thinner, averaging about 50 cm thick, and can have clast or sand-matrix support. None of the breccia

types have wel1-developed imbrication. A few normally graded and ungraded units have finer grained, inversely graded bases less than 20 cm thick.

Sedimentation

The overall texture, structuring, and lithologic association of facies A suggest that most breccia units were deposited subaqueously by flows ranging from debris flows to high-density turbidity currents. The lack of imbrication, poor sorting, and presence of a sand-sized matrix indicate that most flows were relatively cohesionless and that the clasts were supported by the

combined effects of dispersive pressure, buoyancy,

turbulence, and hindered settling, similar to coarse,

dense, gravity flows discussed by Lowe (1979, 1982b),

Nemec and Steel (1984) and Surlyk (1984). Ungraded and

normally graded units with sand-matrix support appear to

represent sandy, relatively cohesionless debris flows

whereas inverse-to-normally graded and inversely graded

units that tend to have clast support probably represent

high-density turbidity currents. Breccia units of facies A

are similar in grain-size, structure, and fabric to coarse

sand-matrix-supported deposits discussed by Hendry (1978),

Nemec et al (1980), and Surlyk (1984). The clasts in breccia of facies A were derived from volcanic centers close to the depositional sites. The presence of sub- to well-rounded clasts in most units suggests high-energy modification of some of the gravel-sized components in alluvial and energetic shallow-water environments. The association of debris-flow deposits and high-density turbidites with thin turbiditic tuffs of facies C and the

lack of traction deposits characteristic of shallow-water and terrestrial settings suggest deposition below wave base. The absence of lutitic interbeds or partings, considered characteristic of coarse subaqueous deposits

(Nemec and Steel, 1984), suggests that the thick breccia sequences were deposited rapidly with little time between successive flows and that little clay-sized detritus was being supplied. Similar subaqueous volcaniclastic breccia/

containing clasts modified in high-energy zones, are

important components of sequences developed around

Phanerozoic island volcanoes (Jones, 1967; Mitchell,

1970) .

Facies B : Complexly organized conglomerate

Description

Complexly organized conglomerate and sandy conglomerate

(Fig. 7), containing thin interbeds of stratified tuff, were observed only in the type section of the Coongan

Formation in the northern part of the Coongan greenstone belt. They form a well-bedded sequence approximately 250 m thick that appears to conformably overlie up to 2.7 km of structureless and graded breccia of facies A. Both sand- matrix-supported and clast-supported conglomerate occur in beds from a few cm to 50 cm thick. The clasts are typically subangular to rounded, although they range from angular to well-rounded. They are composed exclusively of felsic volcanic rock containing abundant plagioclase phenocrysts or sericite pseudomorphs after feldspar, smaller amounts of relict amphibole phenocrysts, now composed of sericite, chert, and black oxides, and sparse primary quartz. The sandy matrix is composed largely of compositionally similar, sand-sized volcanic rock fragments, sericite and chert pseudomorphs after feldspar, and a small percentage of red and gray chert fragments (Chapter 2, this volume). The conglomerate has a wide

range of fabric types but can be divided into two

subfacies (Fig. 7): subfacies B , sand-matrix-supported 1 conglomerate, and subfacies B , clast-supported 2 conglomerate.

Subfacies B , which makes up the bulk of facies B, 1 consists of matrix-supported, ungraded, normally graded,

or rarely, inversely graded pebble- to cobble

conglomerate. Poorly sorted units, 10 to 50 cm thick, are made up of clasts, generally 10 to 20 cm in diameter, but ranging up to 60 cm long, supported by a sandstone matrix.

Clast concentration ranges from as low as 10 percent, resulting in a widely dispersed clast fabric, to up to about 40 percent, resulting in "tight" matrix support.

Although most units have a disorganized fabric, some contain isolated zones or lenses of clast-supported conglomerate in which imbrication is weakly developed. A few beds contain matrix-supported, imbricate "megaclasts", up to 60 cm long, which protrude from the tops of the beds.

Conglomerates of subfacies B are characterized by an 2 imbricate, clast-supported fabric. Units are poorly sorted but the gravel size fraction is generally moderately to well sorted. Maximum clast size, ranging up to 8 cm, and bed thickness, generally less than 30 cm, are smaller than that of subfacies B . Complex interbedding of subfacies 1 B and B on a very small scale is characteristic. Thin 1 2 lenticular units of subfacies B commonly overlie with 2 scoured contact or interfinger laterally over a few

decimeters with units of subfacies B . 1 Stratified tuffaceous sandstone is a minor component of

facies B, forming interbedded units up to 1 m thick within

the conglomerate, but generally making up less than 5

percent of individual sections. Units are lenticular over

a few tens of m. Their bases are commonly scoured as much

as 50 cm into underlying conglomerate units, although

pronounced channels are absent. The rock is well sorted

and predominantly flat laminated, but low-angle cross­

laminated zones are present locally.

Sedimentation

Matrix-supported, poorly sorted units of subfacies B , 1 characterized largely by disorganized fabrics and locally showing protruding clasts, were deposited by relatively cohesionless debris flows and hyperconcentrated flows, in which concentration-related effects, such as buoyancy, hindered settling, and dispersive pressure were important.

Clast-supported conglomerate lenses within some of these units suggests that traction movement was locally important. The description and mechanics of similar deposits were described by Beverage and Culbertson (1964),

Lowe (1979, 1982b), Smith (1986) and many others.

Associated units of stratified tuff and conglomerate units of subfacies B , with imbricate, clast-supported fabrics, 2 represent traction-dominated current deposits that, in 19

some cases, were erosive into underlying units.

A number of features of facies B, including the wide

range in conglomerate fabrics, abundant evidence for

scour, complex small-scale interbedding characterized by

lateral subfacies interfingering over a few decimeters,

and lack of turbiditic units are characteristic of

subaerial conglomeratic sequences (e.g. Heward, 1978;

Nemec and Steel, 1984, Smith, 1986) where reworking and

current-dominated processes are ubiquitous. These deposits

are similar to coarse alluvial sequences deposited around

Phanerozoic subaerial stratovolcanoes (e.g. Mulleneaux- and

Crandell, 1962; Swanson, 1966; Crandell, 1971; Mills,

1984; Smith, 1986).

Facies C ; Turbiditic tuff-breccia and tuff

Description

Tuff-breccia and tuff, conspicuous because of their fine grain size and well-developed cyclicity, are associated with breccia of facies A. Tuff-breccia and tuff occur as tabular, laterally continuous units, 20 cm to about 3 m thick. At least two subfacies of facies C can be distinguished: subfacies C , thin- to thick-bedded tuff, 1 forming isolated turbidites that are sparsely interbedded with breccia of facies A, and subfacies C , thick- to very 2 thick-bedded tuff-breccia, forming sequences from 30 m thick to as much as several hundred m thick, made up of numerous individual turbidites. Subfacies C occurs widely 1 in the Duffer Member, but subfacies C was observed only 2 in the Coongan belt.

Thin- to thick-bedded tuff of subfacies C (Fig. 8) 1 occurs as cyclic units, 20 to over 100 cm thick, consisting, from base upward, of (1) structureless or normally graded tuff, (2) flat- to subtly cross-laminated tuff, and (3) cross-laminated tuff. The cycles are normally graded from coarse- to fine-grained tuff. Stoss- side laminae are preserved in some cross-laminated beds and others show convolute laminations. This ideal cycle is not always developed. In many places, only the structureless or the structureless plus flat-laminated divisions are present. The tuff is made up of angular, aphanitic to sparsely porphyritic volcanic rock fragments.

Thick- to very thick-bedded tuff-breccia of subfacies C 2 (Fig. 9) occurs as cyclic units averaging over 1 m thick.

They consist of structureless or graded tuff-breccia that is capped by flat-laminated tuff. Many beds contain two types of grading: (1) normal or inverse-to-normal coarse- tail grading of equant clasts, 1 to 10 cm in diameter, and

(2) inverse coarse-tail grading of flat, horizontally elongate, easily eroded clasts, 2 to 5 cm long. Both clast types are dispersed in a matrix of coarse ash. Megaclasts of rounded volcanic rock fragments up to 40 cm in diameter are present in several beds. The coarse equant clasts are composed of aphanitic to porphyritic volcanic rock.

Laminated tuff caps consist of coarse-grained, moderate- 21

to well-sorted, flat-laminated ash.

Sedimentation

The lateral continuity and sedimentary-structure

sequences of beds of subfacies C and C resemble those of 1 2 turbidites. Volcaniclastic turbidites can be deposited

from several types of flows including (1) turbidity

currents resulting from subaqueous transformations of

subaerial pyroclastic flows that entered water, (2)

pyroclastic flows originating under water, or (3)

turbidity currents resulting from mass movement on unstable subaqueous slopes (Fisher, 1984). Isolated but widely occurring turbidites of subfacies C , interbedded 1 within thick breccia sequences, suggest deposition from

low-density sandy turbidity currents of the third type, perhaps spawned from high-density turbidites or debris flows, similar to those in the model of Lowe (1982b).

Younger sandy turbidites that occur within predominantly conglomeratic sequences were discussed by Walker (1970) and were believed to represent low-density end-member flows in an environment in which a spectrum of sediment- gravity-flow types were active. Units of subfacies C , 2 forming thick turbidite sequences and containing rounded clasts, appear to represent resedimented deposits, similar in structure and sequence to thick turbidite deposits elsewhere in the geologic record (e.g. Mutti and Ricci

Lucchi, 1978). Flattened clasts in subfacies C are 2 interpreted as resedimented pumice, based on their compressed habit, ease of erosion, and inverse grading,

which is characteristic of pumice in many volcaniclastic

deposits (e.g. Sparks, 1976).

Facies D ; Lavas

The Duffer Member contains felsic and smaller amounts of

mafic lavas that are volumetrically subordinate to

structureless and graded breccia of facies A but are more

abundant in most sections than rocks of facies B and C.

The lavas form poorly exposed sequences up to several km

thick and are especially prominent in the Marble Bar belt

at Marble Bar and in the Kelly belt in the vicinity of

Spinaway Creek (Loc. 35, Fig. 1). In many areas the lavas

are highly sericitized and cut by numerous chert dikes.

Felsic lava characteristically forms massive units in

which bedding cannot be distinguished. Basalt, interbedded

with the felsic lavas, occurs as massive, pillowed, and

hyaloclastite beds.

Most lavas contain abundant albitized plagioclase or

sericite pseudomorphs after feldspar phenocrysts, which made up 15 to 26 percent of observed rocks. Sericite,

chlorite, or silica pseudomorphs after amphibole and

biotite phenocrysts form 0 to 2 and 0 to 5 percent of the

rock, respectively. Quartz phenocrysts are generally

absent and never constitute more than 1 percent of the

rock. The phenocrysts are set in a recrystallized felty, pilotaxitic, or trachytic groundmass now composed largely of a micromosaic of sericite and chert. Most lavas were

originally dacitic to andesitic in composition (Barley,

1981; Hickman, 1983; Barley et al, 1984). Basalt contains

intersertal or subophitic lath-like plagioclase and

chlorite pseudomorphs after granular pyroxene and probable

glass. Further petrographic details and geochemical data

of Duffer lavas are given in Barley (1981), Hickman

(1983), and Barley et al (1984).

SEDIMENTOLOGY OF ASSOCIATED UNITS

The Panorama Member and the Strelley Pool Chert (SPC)

form the upper parts of the Coongan Formation. Details of their sedimentology are given in chapter 2 of this volume and in Lowe (1983) and are only summarized below.

Panorama Member

The Panorama Member consists of silicified volcaniclastic sandstone and tuff with smaller amounts of silicified breccia, conglomerate, siltstone, felsic lava, and chert (Chapter 2, this volume). Clastic rocks of the Panorama Member are composed of felsic debris, originally dacitic to rhyolitic in composition, with smaller amounts of basaltic and cherty detritus (DiMarco and Lowe, 1984 and chapter 3, this volume). There is no evidence for detrital contributions from plutonic or metamorphic sources. These rocks represent mainly pyroclastic with lesser epiclastic debris reworked in a spectrum of shallow-water and subaerial settings including delta, shoreface, and braided-stream environments as well as quiet subaqueous environments where sheets of ash-fall and turbiditic tuff accumulated (DiMarco and Lowe, 1986).

The upward increase of ash-sized detritus in the Coongan

Formation suggests that there was a progressive increase in explosive pyroclastic activity and correlates with the eruption of increasingly evolved, and presumably more hydrous and viscous, magmas with time (Hickman, 1983, p.

154; Barley et a l , 1984).

Strelley Pool Chert

The SPC (Lowe, 1983) consists of silicified laminite, stromatolite, evaporite, and arenite that represent a post-volcanic period of orthochemical and biogenic sedimentation. Deposition occurred under low-energy, evaporative conditions in a restricted, shallow-water to intermittently subaerial setting. The Marble Bar Chert, considered to be correlative with the SPC, is a red-, white-, and black-banded chert containing only local evidence for evaporative conditions. It appears to represent a deeper water facies of the SPC (Lowe and Yuan, in prep.).

A sharp depositional contact separates the Duffer and

Panorama Members from the SPC. Although there is no observable angular discordance, a number of sedimentologic factors indicate collectively that this contact is unconformable. The Duffer and Panorama Members have a diverse spectrum of facies, in places showing lateral 25 facies transitions over relatively short distances, and represent a genetically related spectrum of subaqueous to subaerial clastic environments. The SPC, however, has a restricted range of facies representing- a regionally uniform pattern of evaporative and biogenic sedimentation.

Environments represented by the Duffer and Panorama

Members are unrelated to and show no upward transition into environments represented by the SPC. In most areas, this contact appears to represent a paraconformity, in the sense of Dunbar and Rodgers (1957), defined only by the sharp break that separates the contrasting rock types.

Locally, on the south flank of the North Pole Dome, a chert conglomerate, apparently representing a lag of units eroded prior to the deposition of the SPC, occurs just above the contact. In the Pilgangoora belt and locally in the Kelly belt, a thin sandstone sequence with well-sorted and rounded grains, approaching the composition of a quartz arenite in the Pilgangoora belt, is present at the base of the SPC and appears to represent a transgressive deposit above an erosional unconformity (Chapter 2, this volume).

SEQUENCES AND ENVIRONMENTS OF DEPOSITION

The Duffer Member of the Coongan Formation was examined in the Coongan, Kelly, Marble Bar and North Shaw belts. In these areas, the Duffer Member is represented by sections in which structureless and graded breccia of facies A and lava of facies D predominate.

Coongan belt

Sequences

In the Coongan belt, the Duffer Member consists of

coarse felsic breccia, felsic lava, tuff-breccia, and tuff

(Fig. 10). On the west limb of the Coongan belt, where the rocks are relatively unstrained, the Duffer Member is underlain by basalt and capped sequentially by the

Panorama and SPC Members of the Coongan Formation. Rocks of the Duffer Member are also present on the east flank of the Coongan belt, but there the felsic sequence is intruded by granitic rock and overlain by banded chert of the Marble Bar facies, and, in general, the rocks are highly strained.

Two stratigraphic sections, sections 52 and 30, separated by a strike distance of 10 km, illustrate the characteristic vertical and lateral relations in the western Coongan belt (Fig. 10). Section 52, about 10 km south of the northwest end of the Coongan belt, is 2.0 km thick and includes, from base upward, structureless and graded breccia, tuff-breccia and tuff, and dacitic lava.

The sequence is capped by 18 m of current-deposited tuff of the Panorama Member and then by the SPC. At section 30, at the northwest end of the Coongan belt, 2.5 km of structureless and graded breccia containing intercalated units of felsic lava are capped by 250 m of complexly organized conglomerate of facies B, which is absent in 27

section 52. In section 30, the Duffer Member is capped by

200 m of subaqueous ash-fall tuff of the Panorama Member.

Although the Duffer-Panorama sequence is characterized

by a general upward decrease in grain size, the Duffer

Member in the Coongan belt shows little systematic

vertical variation in lithology. The thick sequence of

structureless and graded breccia, forming the bulk of

these sections, shows no systematic trends in clast size

or shape. Felsic detritus predominates throughout the

sections. Within the complexly structured conglomerate at

locality 30, however, there is an upward decrease in clast

size and bed thickness and an increase in clast roundness.

Environment of Deposition

The Duffer Member in the Coongan belt represents coarse, shoaling-upward volcaniclastic aprons flanking one or more

felsic volcanic centers. The coarse grain size of the breccia and conglomerate and the abundance of lava flows and tuff-breccia suggest a proximal setting close to the felsic sources. Clear indications of a vent area, such as intrusive stocks or dike complexes, were not seen; however, the greater thickness of the Duffer Member, higher proportion of coarse breccia, larger number of felsic interbeds, and the subaerial cap of facies B suggest that section 30 lay closer to a volcanic center than section 52. In both sections, thick sequences of structureless and graded breccia with interbedded turbiditic tuff and tuff-breccia indicate that deposition

was largely subaqueous and that the felsic center(s) had

appreciable relief. Alluvial conglomerate forms the upper

part of the Duffer Member in section 30 and indicates the

local volcanic build-up into the subaerial environment.

The thinning- and fining-upward trend in these alluvial

deposits suggests, by comparison to alluvial-fan

environments (Schluger, 1973; Heward, 1978), a progressive

denudation of the highland volcanic source and probable

waning of volcanic activity. Subaerial vents were

apparently established early, however, because rounded

clasts, probably formed in shallow-water or alluvial

environments, occur in the breccia throughout the section.

Kelly belt

Sequences

In the Kelly belt, the Duffer Member includes a thick

sequence of structureless and graded breccia and felsic

and mafic lava with minor units of tuff. The supracrustal rocks are intruded on the west by granitic rocks and a quartz-plagioclase porphyry. The porphyry was termed the

Boobina Porphyry by Lipple (1975) and was thought to be genetically related to the felsic rocks by Barley et al

(1984). Although the Duffer Member extends continuously along strike for 25 km in the Kelly belt, numerous faults cut the sequence. In places, anomalously thin sections of

Coongan Formation that lack the capping SPC, which is well developed in nearby thicker sequences, suggest that the upper part of the formation has been removed by faulting.

Three sections, sections 35, 56, and 70, separated by a

cumulative strike distance of 22 km, illustrate facies of

the Duffer Member in the Kelly belt (Fig. 11). The

thickest and lithologically most diverse is section 35,

the reference section of the Duffer Member. This sequence

is 2.8 km thick and consists of (1) a lower division, 800 m thick, of interbedded basaltic flows and pillow breccias

and hyaloclastites, felsic tuff, and structureless and

graded breccia, (2) a middle division, 700 m thick, of

structureless and graded breccia made up of very angular

to angular volcanic clasts, and (3) an upper division,

1300 m thick, of massive, highly altered felsic lavas. At

locality 35, the sequence is capped by the SPC. Sections

70 and 56, 17 km north and 5 km south of locality 35, respectively, are made up largely of structureless and graded breccia and felsic lava. In both sections, rounded clasts in the breccia become more abundant upward. In section 56, the breccia forms sequences up to 250 m thick in which the maximum clast size increases upward.

Additionally, volcaniclastic sandstones of the Panorama

Member, containing evidence suggestive of deposition in a tide-dominated setting (Chapter 2, this volume), are interbedded with the Duffer Member near the top of the sequence at locality 56. Environment of Deposition

A proximal volcanic environment for the Duffer Member in

the Kelly belt is indicated by (1) thick sequences of lava

and coarse volcanic breccia, and (2) the possible presence

of subvolcanic porphyries. At locality 35, the thickest

and most complete section examined, early eruptions produced both mafic and felsic products. The presence of pillow breccias and hyaloclastite indicates that early deposition was subaqueous. The lack of rounding of clasts

in the breccia suggests that most of this facies originated from subaqueous eruptions. Late-stage activity was entirely felsic and culminated with the emplacement of a thick sequence of felsic flows.

The lower percentage of lavas at localities 56 and 70

suggests that these sites may represent more distal environments than locality 35. The upward increase in clast roundness in the breccia and development of shallow- water facies indicate progressive shoaling of the volcaniclastic aprons. The upward increase in clast size in breccia sequences several hundred m thick suggest that these units represent progradational, coarse-grained debris-aprons.

Marble Bar and North Shaw belts

Description

In the Marble Bar and North Shaw belts, the Duffer

Member is represented by thick sequences of structureless and graded breccia and felsic lava capped by the banded Marble Bar facies or banded cherts interbedded with basaltic units. In the Marble Bar belt at Marble Bar, the

Duffer Member consists of 2.5 km of felsic lavas capped by

300 m of structureless and graded breccia containing sparse intercalations of thin-bedded turbiditic tuff. At the type section of the Duffer Member, 14 km north of

Marble Bar, a sequence, 5 km thick, of structureless and graded breccia and felsic lava is overlain by 1000 m of subaqueous resedimented tuff and felsic lava of the

Panorama Member (Chapter 2, this volume). A progressive upsection change from andesite to dacite lavas has been suggested for felsic rocks in this section based on geochemical data (Hickman, 1983). Although details of bedding and sedimentation units are poorly preserved, subrounded to rounded volcanic clasts were observed in the breccia as in other belts.

In the North Shaw belt, moderately to highly strained sequences of structureless and graded breccia again predominate in the Duffer Member. Locally, up to 100 m of sheared tuff and sericite schist, probably correlative to the Panorama Member, cap the breccia, similar to sequences in the Coongan and Marble Bar belts.

Environment of Deposition

Thick sections of coarse breccia and felsic lavas, in this and other areas, were apparently deposited close to the volcanic centers. Depositional characteristics of the breccia and the presence of turbiditic tuff attest to

largely subaqueous deposition. Although subaerial facies were not recognized in these belts, the presence of

rounded clasts in the breccia suggests that, as in other

areas, the vents had become shallow to subaerial and

coarse volcaniclastic aprons were constructed primarily by

sediment-gravity flows around these felsic volcanic centers.

DISCUSSION

Controls on Sedimentation of the Duffer Member

Volcanism controlled virtually all aspects of Duffer

sedimentation. Felsic volcanism provided detritus of a restricted compositional range; with the exception of a small percentage of chert clasts, debris of non-volcanic origin is absent. Continued felsic activity ensured a

large supply of volcanic debris. Because of the frequency of explosive eruptions and high viscosities of silicic magmas, felsic eruptions commonly construct steep, elevated volcanic centers. Steep slopes flanking felsic centers and the abundance of volcanic debris promoted the development of sediment-gravity-flow deposits in the

Duffer Member. Many of these flows were subaqueous lahars made up of debris that originated on upper parts of the volcanic edifice. Although volcanism was entirely responsible for sediment composition and supply, epiclastic modification of volcanic sediment in energetic subaerial and shallow-water environments is indicated by the structure and fabric of complexly organized

conglomerate of facies B and the presence of rounded

clasts in structureless and graded breccia of facies A and

tuff-breccia of facies C.

Deve1opmenta1 Model

Stratigraphic and sedimentologic data indicate that the

Duffer Member represents shoaling-upward sedimentation developed primarily as coarse volcaniclastic debris-aprons

flanking felsic volcanic centers. Steeply dipping attitudes in greenstone belts of the eastern Pilbara Block provide cross-sectional views through these proximal volcanic piles. A generalized developmental model for the

Duffer Member based on the four belts studied includes three stages (Fig. 12). During the first stage, the vents were largely subaqueous and both felsic and basaltic eruptions contributed to the construction of the volcanic edifices. Basaltic eruptions produced flows and hyaloclastites whereas felsic activity provided an abundance of fragmental debris and widely distributed flows. Many of the early felsic fragmental units contain only angular debris, suggesting an origin from (1) subaqueous block-and-ash flows, (2) subaqueous lahars, or

(3) hyaloclastic or autoclastic processes (e.g. Pichler,

1965). The second stage was characterized by deposition around volcanic vents that lay near or above water-level and volcanism that was entirely felsic. Although preserved units were deposited largely under subaqueous conditions,

shallow-water and subaerial reworking of volcanic material

is indicated by the presence of rounded clasts in breccia

and tuff-breccia. Sequences deposited during this stage

can be several km thick but show no convincing evidence

for deep-water deposition. The great thickness could be

accommodated easily by subsidence rather than infilling of

deep-water basins. During the last stage, emergent volcanic islands were well developed and flanking alluvial

facies were deposited. The depositional model for Duffer

sedimentation is similar to the evolutionary model for greenstone-belt volcanoes of Ayres (1982) and the general model for island-volcano growth of Fisher (1984). Thick

felsic sequences such as these are commonly found in or near Phanerozoic volcanic arcs, especially oceanic-arc types (Jones, 1967; Mitchell, 1970; Karig and Moore, 1975;

Carey and Sigurdsson, 1984). Characteristics of the Duffer volcaniclastic aprons, including thick sequences of volcaniclastic breccia, lack of well-defined facies transitions, absence of large-scale feeder channels, and inferred multiple source areas, suggest th-'t early Archean felsic volcaniclastic aprons are directly r parable to their Phanerozoic counterparts and bear little resemblance to submarine-fan systems. Processes responsible for the construction of these aprons, however, are entirely volcanologic and sedimentologic, and thick felsic sequences such as these would not be restricted to specific tectonic settings but could be found in any

setting where felsic volcanoes build up in water.

The terminal stage of felsic volcanism during the

deposition of the Coongan Formation was marked by ash-

producing pyroclastic eruptions, which were probably

related to increasingly differentiated magmas. This ash

was dispersed in a spectrum of predominantly shallow-water

environments represented by the Panorama Member. Following

the end of volcanism, a period of erosion reduced the

volcanic edifices to a relatively flat surface and

subsequent orthochemical and biogenic sedimentation on

this platform is represented by the SPC.

Despite the abundance in the Duffer Member of thick

sequences of coarse breccia and felsic lava interpreted as

proximal volcaniclastic deposits, unequivocal evidence for

vent accumulations is sparse. Criteria that are

collectively used for identification of vent facies

(Smedes and Protska, 1972; Dickinson, 1974; Vessel and

Davies, 1981; Fisher and Schminke, 1984) include: (1)

poorly bedded or unbedded pyroclastic breccias with

intrusive relations to surrounding rocks, (2) abundant

lava flows, (3) abundant fallout deposits, (4) evidence

for fumarolic activity, and, in volcanic terranes that

have undergone erosion or deformation, (5) exposed stocks,

sills, and dikes. Intrusive breccias, however, might be difficult to distinguish in thick sequences of massive breccia such as that of the Duffer Member. Although lava flows and possible subvolcanic porphyries are present in

many sections of the Duffer Member, virtually all sections

appear to represent proximal, but not vent facies.

Although ash was abundant, thick volumes of ash-flow

tuff and associated calderas, common in many Phanerozoic

continental felsic terranes, were also not observed in the

Coongan Formation. However, a sequence, about 1000 m

thick, of turbiditic tuff with minor amounts of debris-

flow deposits and felsic lava (Chapter 2, this volume),

crops out in the Marble Bar belt and is assigned to the

Panorama Member. This sequence is similar to subaqueous

felsic volcaniclastic deposits developed in association with a caldera complex in the Mesozoic of California

(Busby-Spera, 1985), and may represent a similar caldera- related facies.

RELATIONSHIP OF PLUTONISM TO FELSIC VOLCANISM

Several lines of evidence suggest that felsic volcanic and volcaniclastic rocks of the Coongan Formation are related genetically to emplacement of the some of the oldest granitoid rocks exposed in the batholiths of the eastern Pilbara Block, as originally postulated by

Hallberg and Glickson (1981), Jahn et al (1981), Hickman

(1981) and others. The plutonic rocks are closely associated spatially with the Coongan Formation and locally intrude it or the underlying basalts. A zircon U-

Pb age of 3453 ± 16 Ma (Pidgeon, 1978a) and primary Pb- model ages of between 3400 and 3500 Ma (Sangster and Brook, 1977; Richards et a l , 1981) were reported for the

Duffer Member. Rb-Sr ages of 3506 ± 62 Ma (Cooper et al,

1982) and 3471 ± 125 Ma (Barley and deLaeter, 1984) were

also reported for the Duffer Member, but most Rb-Sr ages

were apparently reset during alteration episodes (Barley

and deLaeter, 1984). U-Pb, Pb-Pb, and Sm-Nd ages for the

oldest granitoid rocks include 3417 ± 40 Ma (Pidgeon,

1978c), 3499 ± 22 Ma (Bickle et al, 1983), 3485 ± 30 Ma

(Williams et a l , 1983), and 3260 to 3460 Ma (Collerson and

McCulloch, 1983). Pidgeon (1984) dated the Boobina

Porphyry, which intrudes the Coongan Formation in the

Kelly belt, at 3307 ± 19 Ma, using the U-Pb method. These data suggest that the felsic volcanic rocks of the Coongan

Formation are coeval with the oldest felsic plutonic rocks and that plutonic activity continued for a few hundred m.y., similar to magmatic relationships in many

Phanerozoic felsic provinces (e.g. Pitcher, 1975, 1978).

Bickle et al (1983) and Barley et al (1984) documented the geochemical and isotopic similarity of these plutonic and felsic volcanic rocks, suggesting that they collectively represent a 3.5 Ga calc-alkaline province in the eastern Pilbara Block. Variably foliated granodiorites, subvolcanic porphyries, and gray gneisses in the batholiths and felsic volcanic rocks of the Duffer

Member have overlapping major- and trace-element contents including similar HREE-depleted REE signatures. These abundances are similar, except in Ni and Cr, to those in corresponding rocks of many Phanerozoic Andean-arc

assemblages (Bickle et al, 1983; Barley et al, 1984).

Regional paleocurrent patterns in the Coongan Formation

(Fig. 13) strongly suggest that the felsic vent areas, now

mostly removed by erosion, coincided generally with many

of the the major batholithic domes in the eastern Pilbara

Block. Paleocurrent data from the flanks of the North Pole

Dome (NPD) indicate sediment dispersal from a source near

the center of the dome. A bimodal paleocurrent pattern in

alluvial deposits in the northwestern part of the Coongan

belt suggests sediment derivation from felsic sources over

the present outcrop areas of the Shaw and Corunna Downs

Batholiths. Sparse paleocurrent data from the Marble Bar

belt indicate that deposition there was unrelated to debris dispersal from vent areas associated with the NPD.

Sequences of breccia, up to 5 km thick, in the Coongan

Formation in the Marble Bar belt, however, suggest proximity to a volcanic source, possibly related to the

Mount Edgar Batholith. Locally, small granitic bodies, possibly representing subvolcanic intrusions, intrude the

Coongan Formation along the edge of the Mount Edgar

Batholith near Marble Bar. Further east, Barley (1981) proposed that thick sequences of felsic volcaniclastic breccia correlative with the Duffer Member and flanking the McPhee Dome represent another felsic vent centered over this dome. Collectively, these patterns indicate the presence of numerous centers of felsic volcanism in the eastern Pilbara Block during the deposition of the Coongan

Formation. The arrangement of centers shows no linear

trend but a striking coincidence with exposed domal

granitoid batholiths.

EARLY ARCHEAN CRUSTAL EVOLUTION IN THE EASTERN PILBARA

BLOCK

The oldest rocks in the Warrawoona Group are basalts, dated at 3560 ± 32 Ma by Hamilton et al (1981). Despite

suggestions for a primordial, pre-greenstone granitic basement (e.g. Hickman, 1981), no direct evidence for

sialic rocks older than the Warrawoona Group has been reported. The basalts that make up the bulk of the

Warrawoona Group were apparently erupted in relatively

shallow water, based on evidence from thin intercalated units of silicified sedimentary rocks (Barley et al, 1979;

Dunlop and Buick, 1981). Evidence for deformation during the deposition of the Warrawoona Group is sparse (Hickman,

1981). This thick sequence of basalts apparently represents an anorogenic simatic platform, as described by

Lowe (1980, 1982a), Rutland (1982), and Groves and Batt

(1984).

The Coongan Formation is the oldest major felsic volcanic unit in the Warrawoona Group. The lack of granitic clasts in the Duffer Member and the lack of petrographic evidence for a sialic provenance for the

Duffer, Panorama, and SPC Members indicate that plutonic rocks were not exposed during the deposition of the

Coonagn Formation. Paleocurrent evidence presented here

and geochemical and isotopic evidence presented by Bickle

et al (1983) and Barley et al (1984) suggest that the

felsic volcanism was the surficial expression of

predominantly granodioritic intrusive events. It seems

likely that the Coongan Formation represents the earliest

supracrustal reflection of cratonization in the eastern

Pilbara Block. During this felsic magmatic event, the

Warrawoona Group was underplated by felsic plutons and

overplated, in the sense of Elston (1969), by felsic

volcanic and volcaniclastic strata. A complete transition

from a simatic volcanic terrane to a stable sialic block

was not accomplished, however, because after the

deposition of the SPC, there was renewed eruption of

basalt and a restoration of the simatic platform. A less

extensive felsic episode, possibly related to continuing

plutonic activity (e.g. Pidgeon, 1984), is represented by

the Wyman Formation, at the top of the Warrawoona Group.

Following the deposition of the Warrawoona Group, large-

scale orogenesis and unroofing of plutonic rocks occurred,

resulting in the deposition of siliciclastic sediments

containing granitic, sedimentary, and metamorphic detritus, represented by the Gorge Creek Group (Hickman,

1981, 1983; Krapez, 1984). This deformation apparently

culminated in a pre-2.7 Ga period of cratonization and

stabilization in the Pilbara Block (Blake, 1984). 41

Comparison to Modern Settings

Several workers (e.g. Tarney et al, 1976; Tarney and

Windley, 1981; Windley, 1984, p. 61-65) have suggested that volcanic arcs and arc-related marginal basins represent the best modern analogues for greenstone belts.

Bickle et al (1983) and Barley et al (1984) suggested that the calc-alkaline sequences in the Pilbara Block are similar to those of Phanerozoic Andean-type arcs based on major- and trace-element data, the relative abundance and relationships among rock types, and limited sedimentologic data.

The abundance of shallow-water facies, distribution of felsic centers, and evidence for little syn-volcanic deformation suggest, however, that the felsic volcanic province represented by the Coonagn Formation was unlike those of most Phanerozoic arcs. Most Phanerozoic arc and marginal-basin assemblages with thick, shoaling-upward felsic sequences similar to that of the Coongan Formation are made up predominantly of deep-water facies (e.g.

Mitchell, 1970, Karig and Moore, 1975; Carey and

Sigurdsson, 1984). Even in those arc assemblages that contain a significant shallow-water component (e.g.

Dickinson, 1974), the distal facies are generally represented by deep-water deposits. There is no direct evidence for deep-water deposition, however, in the

Coongan Formation and shallow-water facies predominate, especially in the upper part of the Duffer to Panorama

succession. Numerous felsic centers active during the deposition of the Coongan Formation were apparently

related to subvolcanic plutons that were distributed

uniformly throughout the eastern Pilbara Block. The

apparently non-linear distribution of felsic centers

contrasts with the curvilinear trend of volcanoes in most arcs. The distance between volcanoes in many modern arcs is about 70 km, which is greater than the approximately 30 km distance between granitoid domes in the eastern Pilbara

Block. Only local syn-volcanic deformation affected the

Warrawoona Group (Hickman, 1981), and felsic strata of the

Coongan Formation are underlain by thick basaltic sequences representing an anorogenic, simatic platform. In most Phanerozoic arcs, including oceanic and Andean types,

large-scale deformation is an important part of arc evolution (e.g. Windley, 1984). The lack of granitic detritus in the Coongan Formation indicates that, unlike many Phanerozoic orogens, the plutonic roots were never exposed to erosion during the evolution of the volcanic episode. The wide areal extent of granite-greenstone terrane and the overall pattern of deformation of the

Warrawoona Group in the eastern Pilbara Block are also not compatible with the expected late closure patterns of marginal basins (Groves et al, 1978; Rutland, 1982). Rocks of the Coongan Formation and related plutonic rocks have lithologic and geochemical similarities to rocks of 43

environments. Caldera complexes and thick ash-flow tuff

units, common in many Phanerozoic continental felsic

provinces, appear to be absent or poorly developed in the

Duffer and Panorama Members. After a period of erosion,

sediments represented by the Strelley Pool Chert and its

probable correlative, the Marble Bar Chert, were deposited

on a broad, post-volcanic platform.

Paleocurrent evidence from the Duffer and Panorama

Members indicates sediment dispersal from felsic source

areas close to the centers of presently exposed granitoid plutons in the eastern Pilbara Block, supporting earlier proposals that early granitoid rocks and felsic volcanic rocks are genetically related. Apparently, felsic volcanic and volcaniclastic rocks of the Coongan Formation, which contain no evidence for detrital contributions from granitic sources, represent the supracrustal expression of the now-exposed, deeper level plutons. This episode of magmatism marks the initiation of sialic cratonization in the eastern Pilbara Block.

Rocks of the Coongan Formation represent sediments deposited in a spectrum of environments similar to those developed around volcanoes in oceanic-arc and marginal- basin settings. A number of features, however, such as the apparently non-linear distribution of felsic centers, widespread development of shallow-water environments, the lack of evidence for widespread syn-volcanic deformation, and association with thick sequences of basalt erupted Andean-arc assemblages but were associated with a shallow

anorogenic basaltic platform. They appear to represent a

tectonic setting without a good Phanerozoic analogue.

CONCLUSIONS

The 3.5 Ga Coongan Formation is a newly defined

lithostratigraphic unit in the Warrawoona Group, eastern

Pilbara Block, Western Australia. It includes three members: the Duffer and Panorama Members and the Strelley

Pool Chert. The lowermost and thickest of these, the

Duffer Member, consists of coarse volcaniclastic breccia and felsic lava with minor amounts of turbiditic tuff, tuff-breccia, and mafic lava. The Duffer Member is overlain by or interfingers in its upper part with the

Panorama Member, consisting predominantly of silicified volcaniclastic sandstone and tuff. The Duffer and Panorama

Members are overlain by the Strelley Pool Chert, consisting of silicified orthochemical and biogenic deposits.

Rocks of the Duffer Member represent largely subaqueous volcaniclastic debris-aprons deposited around felsic volcanic centers. With time, the debris-aprons were built up into shallow water and were capped, locally, by sequences of alluvial conglomerate. Rocks of the Panorama

Member represent tephra and erosional debris produced during the late eruptive stages characterized by pyroclastic activity. This debris was dispersed to a spectrum of predominantly shallow-water and subaerial 45 into shallow water indicate that the Coongan volcanic province was unlike those of Phanerozoic arcs. Felsic flows and volcaniclastic sediment represented by the

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Viljoen, M.J., and Viljoen, R.P., 1969, The geological and geochemical significance of the upper formations of the Onverwacht Group. Spec. Publ. Geol. Soc. South

Africa, v. 2, p. 113-152.

Walker, R.G., 1970, Review of the geometry and facies organization of turbidites and turbidite-bearing basins, in Lajoie, J., ed., Flysch sedimentology in North

America. Geol. Assn. Canada Spec. Pap. 7, p. 219-251.

Williams, I.S., Page, R.W., Froude, D., Foster, J.J., and

Compston, W., 1983, Early crustal components in the

Western Australian Archaean: Zircon U-Pb ages by ion microprobe analysis from the Shaw Batholith and Narryer

Metamorphic Belt. Geol. Soc. Aust. Abstracts, v. 9, p.

169.

Windley, B.F., 1984, The evolving continents, 2nd edition. Wiley and Sons, New York, 399p. 60

FIGURE CAPTIONS

1. Generalized geologic map of the eastern Pilbara Block

showing the outcrop extent of the Coongan Formation in

black and locations of sections mentioned in text.

2. Proposed correlation of the Coongan Formation among

greenstone belts of the eastern Pilbara Block.

3. a) Previous stratigraphic interpretation of the

Warrawoona Group, after Hickman and Lipple (1978). b)

Present interpretation, showing position of the Coongan

Formation in the Warrawoona Group, based on thickness of

the greenstone sequence in the North Pole Dome.

4. Correlation between greenstone sequences in the North

Pole Dome and the Marble Bar belt. Basalts shown in

white, felsic volcanic and volcaniclastic breccia in

hachured pattern, felsic tuff and sandstone in stippled

pattern, and cherts in black. Chert types are: SPC,

Strelley Pool Chert; MBC, Marble Bar Chert; SP,

spherule-bearing chert; and G, green chert representing

silicified ash- and dust-fall deposits. Note

stratigraphic equivalence of the SPC and the MBC and the presence of green cherts in the upper basaltic

sequences.

5. Fabrics of structureless and graded volcaniclastic breccia drawn from outcrop photos. a) Sand-matrix- supported breccia with mostly angular clasts; lens cap for scale. b) Probable clast-supported breccia, made up of angular to rounded clasts and showing a single megaclast; 15-cm rule for scale.

6. Two sequences with well-preserved details of sedimentation units of facies A. Note presence of rounded megaclasts and several types of grading as mentioned in text. Mps = maximum particle size.

7. Typical layering in sequences of complexly organized conglomerate showing small-scale interfingering of units with imbricate clast support and matrix support. Note protruding clasts in one matrix- supported bed.

8. Turbiditic tuff of facies C showing single high- density turbidite with capping surge deposit.

9. Outcrop photo of turbiditic tuff-breccia of facies C showing large rounded clast in the T division. a

10. Representative sections of the Duffer Member in the

Coongan belt.

11. Representative sections of the Duffer Member in the

Kelly belt.

12. Developmental model of Duffer volcaniclastic debris- aprons: a) Eruption of basaltic and felsic lavas was wholly subaqueous. b) Entirely felsic eruptions at or near water level, some reworking of clasts in high-

energy environments but deposition was below wave-base,

c) Subaerial eruption of felsic material with both

subaerial and subaqueous deposition.

13. Paleocurrent and dispersal patterns of

volcaniclastic debris in the Coongan Formation: dip direction of cross-strata in fluvial cross-bedded

sandstone of the Panorama Member, Locs. 33, 15, 7, and

66; Imbrication of clasts in alluvial conglomerate of

Panorama and Duffer Members, Locs. 25 and 30; dip direction of cross laminae in Tc division of turbiditic tuff, Loc. 50; dip direction of cross-laminae in fine­ grained strata in loc. 63. Figure vO Younger Sedimentary Rocks

Archean Granitoid Marble Rocks Marble Beit Gorge Creek Group Warrawoona Gp + + + + + + + with outcrop + + + + + + + + + + + + ♦ + + + of Coongan Fm. + + + + + + + + + T + + + + + ♦ + + + + + + + + + + + + + + + + + + + + + + *k*t

Pilgangoora

0

North Corunna Downs Shaw Shaw

Coongan Kelly Belt Belt I Figure vD v£>

NORTH POLE DOME COONGAN BELT KELLY BELT 20 km 16 km 32 km 5 km ^ \ \ ^ \ \ / ^ / ^ - \ ✓ 16 mj * * • • • • * 98 m • ■ t* V ^ - • • • • N\ — Loc. 61 500 m Loc. 34 900 m

2.7 km 2.8 km Strelley Pool Chert

Panorama Member Loc. 65

Duffer Member Loc. 56

+ + Granitoid Rock

' '/I Mafic Rock

+ + +

Loc. 35 Loc. 30 Figure 68

Wyman Fm.

Salgash Subgp.

Panorama SPC Fm. Panorama Mbr. J Salgash t>0 Subgp. Duffer Towers Mbr. Fm Duffer Fm Taiga Taiga Subgp. Taiga Taiga Subgp.

A B

LEGEND

Felsic tuff and sandstone

Felsic lava and breccia

Chert units □ Mafic and ultramafic units Figure Gorge Creek Fortescue■ Warrawoona Gp. Gp. G p .

to ^ u

C / 1 O O

(Dtd o'I—' M (0 rt W fu A s Coongan Fm. W o o n

'-I o Figure 72

r ^ ^ ' . K -

< 0 , '^:r7-7-\.Vv

$ 3 $

;sn AS=>-S. •. ^ .£ E r ^ r • V O •’ ; r ^ v ' ("□ U u Li L i u ill [> — L / • * • /— '

r & l Figure 6 74

Coongan Belt Marble Bar Belt Loc. 52 Loc. 63 82 oilSoi. ■::S o o"'3= ' , « C 32

■:&&!• ° 22 ■ <5?.f.o«, 13

18

$.

5 20 >Q — • * . &• * <«=>£ i 20 a Vo . *

\A.'.A>. utfc >•* o''-‘, oD 17 I & - o r a Z 1 m s £ h * z & 27 f . Q y V 24 15 8 25 M p s , 52

36

M p s •(cm) Figure 76 Figure 8 78

Surge Figure 9 15 cm 81

Figure 10 CM 00

N <-

CCu _iO UJi coE - Opl, =u

o < > ■ O'. z ° o c 57 o« Strelley Pool Chert Q O p e bble Panorama Member imbrication ?'<*4 n = 40 Structureless and Graded

0.0. Breccia: Facies A Complexly Organized ib'ci**? Conglomerate: Facies B

o'o- Turbiditic Tuff and Tuff- z o Breccia: Facies C u o O' Felsic Lava: Facies D

o. <0. Mafic Lava Not Part of

■ • o. < 0 <=> Coongan Formation

9 <0 ?** ° A ■

17 km 5 km rS

'O <: V y —* □ Strelley Pool Chert *o-o:o y f s Black Chert

- 'x Intrusive Ultramafic Rock

g sa - V Panorama Member S 'db 'O as Structureless and Graded U. U-. O o Breccia: Facies A £3 Loc. 56

C00NGAN Q • < 6 . Fig. 1 Strained Breccia: Facies A 0 ° '•« Turbiditic Tuff and Tuff- ■•'o-o' - • Breccia: Facies C

Felsic Lava: Facies D ■<3 •

Mafic Lava: Facies D + + Granitoid Rock Locally Showing Intrusive Contact Loc. 70 Fig. 1

km

L o c . 35 Fig. 1 Figure 86

n v6. <3 0 ^ OD

60

0 [ p / O C ? j Figure 13 88

n = 54 n = 33 n = 4 n = ~ 4 Loc. 15 Loc. 25 Loc. 50 Loc. 7

1

n = 19 n = 40 n = 17 n = 7 Loc. 33 Loc. 30 Loc. 63 Loc. 66

o Mou gar Btlh. North Pole Dome Marble

Pilgangoora Belt

North

Shaw Shaw Btlh. Belt

KM

70 Coongar Kelly Belt till Dell r e = i SHALLOW-WATER VOLCANICLASTIC DEPOSITION IN THE EARLY

ARCHEAN PANORAMA MEMBER OF THE COONGAN FORMATION, EASTERN

PILBARA BLOCK, WESTERN AUSTRALIA

ABSTRACT

The 3.5 Ga Panorama Member of the Coongan Formation in the eastern Pilbara Block, Western Australia, consists predominantly of silicified volcaniclastic sandstone and tuff deposited at the close of a major period of early

Archean felsic volcanism. The bulk of the Coongan

Formation consists of sequences, up to 5 km thick, of felsic volcaniclastic breccia and lava of the Duffer

Member deposited on volcaniclastic debris-aprons around felsic centers. Deposition of the overlying Panorama

Member was associated with pyroclastic eruptions, which provided abundant sand-sized detritus. The lower clastic members are overlain unconformably by the Strelley Pool

Chert, which represents a period of evaporitic and biogenic sedimentation on a broad, erosional, post- volcanic platform.

The Panorama Member is divided into eight facies: (A) volcaniclastic breccia, (B) chert-clast conglomerate, (C) well-bedded tuff and tuff-breccia, (D) cross-stratified volcaniclastic sandstone, (E) laminated and cross­ laminated siltstone, (F) banded chert, (G) silicified evaporite, and (H) felsic lava. Wave-formed structures are

89 found in facies B, C, D, and E. The eight facies occur in

four facies associations (F.A.): (1) sequences of banded chert with lesser amounts of tuff, representing elastic-

starved basinal environments, (2) sequences of siltstone with variable amounts of interbedded turbiditic tuff, representing lower shoreface and prodelta environments,

(3) sequences of cross-stratified sandstone with one or more other facies, representing upper shoreface, fan-delta front, and braided-stream environments, and (4) sequences of turbiditic and ash-fall tuff, representing subaqueous settings under the direct influence of pyroclastic volcanism. In the North Pole Dome, the Panorama Member is developed directly over basalt and consists of shoaling- upward sequences of F.A. 1, 2, and 3, indicating sedimentation in prograding fan-delta complexes flanked laterally by shoreface environments. In other areas, units of F.A. 3 and 4 are developed over thick, locally subsided sections of the Duffer Member. Felsic volcanism controlled sediment composition, sediment size, and the rate of sediment supply to surrounding epiclastic systems.

Deposition took place regionally in shallow-water to subaerial environments in which deep-water distal facies were lacking. Despite the abundance of wave-worked shallow-water environments, evidence for tides is rare, probably because deposition took place under regionally microtidal marine conditions. 91

INTRODUCTION

The 3.5 Ga Warrawoona Group of the Pilbara Block, Western

Australia, contains thick sequences of basaltic flows interbedded with felsic calcalkaline volcanic and volcaniclastic strata and thin units of cherty meta- sedimentary rocks (Lipple, 1975; Hickman & Lipple, 1978;

Hickman, 1981, 1983; Barley et al., 1984). The meta- sedimentary units represent evaporitic and biogenic sediments and dispersed volcaniclastic debris deposited primarily in shallow-water environments (Barley, 1978;

Dunlop, 1978; Barley et al., 1979; Walter, Buick & Dunlop,

1980; Lowe, 1980, 1983; Dunlop & Buick, 1981; Buick &

Barnes, 1984). These units of shallow-water origin have a wide stratigraphic distribution among the basalts and indicate that the depositional surface lay at or near water level throughout the accumulation of most of the

Warrawoona Group (Rutland, 1982; Lowe, 1982; Groves &

Batt, 1984).

A widespread assemblage of silicified, predominantly sandy strata, named the Panorama Member, occurs near the top of the newly defined Coongan Formation, a major felsic unit in the upper part of the Warrawoona Group (Chapter 1, this volume). Similar to the cherty units within the basalts, the Panorama Member shows abundant evidence of shallow-water deposition. This report documents the regional spectrum of facies and environments represented by the Panorama Member and details volcaniclastic

sedimentation patterns during the terminal stage of a major episode of early Archean felsic magmatism.

GEOLOGIC SETTING

The Pilbara Block of Western Australia (Fig. 1) is a 2 56,000 km granite-greenstone terrane that contains some of the oldest, well-preserved supracrustal rocks on earth.

The greenstone sequences, made up of low-grade meta- volcanic and sedimentary strata, crop out in steeply dipping synforms or belts and, locally, more gently folded domes. The greenstone belts are separated by ovoid, granitoid batholiths composed of schists, amphibolites, gneisses, and variably foliated granitic rocks (Hickman,

1983; Bickle et a l ., 1983; Barley et a l ., 1984).

Early Archean meta-volcanic and sedimentary rocks in the eastern Pilbara comprise the Pilbara Supergroup

(Hickman, 1981, 1983). This sequence is divided into the lower, predominantly volcanic Warrawoona Group and the upper, mainly sedimentary Gorge Creek and Whim Creek

Groups. The Warrawoona Group consists of 5 to 20 km

(Lipple, 1975) of low-grade metamorphosed mafic volcanic rocks plus minor ultramafic and felsic units and thin sedimentary strata made up largely of silicified volcaniclastic debris and chert. Maximum ages of 3.5 Ga are reported for the Warrawoona Group using U-Pb, Pb-Pb, 93 and Sm-Nd methods (Richards, 1977; Sangster & Brook, 1977;

Pidgeon, 1978a, b; Hamilton et al., 1981; Jahn et a l .,

1981) and similar 3.5 Ga ages have been obtained from the oldest granitoid rocks (Pidgeon, 1978c; Bickle et al.,

1983; Collerson & McCulloch, 1983; Williams et al., 1983).

Polyphase deformation of the supracrustal sequence, most of which post-dates the deposition of the Warrawoona

Group, has resulted in steeply dipping to overturned strata in most areas, although gentler dips predominate locally (Hickman & Lipple, 1978; Hickman, 1981, 1983).

Despite structural complexity, the rocks in many sections show little evidence of strain, although sheared rocks are common adjacent to some batholiths.

The Warrawoona Group has been regionally metamorphosed to the greenschist facies but, locally, the grade ranges from prehnite-pumpellyite to amphibolite near some batholith contacts (Barley, 1980; Hickman, 1983). The rocks have undergone a complex alteration history, resulting in intense silicification, carbonation, or sericitization of most primary constituents (Barley, 1980;

1984; Barley et a l ., 1984; Hickman, 1983; Buick & Barnes,

1984; DiMarco & Lowe, 1984). Although pervasive metasomatism has destroyed most primary minerals, low- strain meta-sedimentary rocks of greenschist grade or lower generally show good preservation of primary depositional textures and structures. 94

STRATIGRAPHY

Regional correlation of felsic meta-volanic and meta-

sedimentary strata of the Warrawoona Group of the eastern

Pilbara Block has resulted in the establishment of a new

formation, the Coongan Formation (Fig. 1, 2) in the upper part of the Warrawoona Group (Chapter 1, this volume). The

Coongan Formation has been divided into three members

(Chapter 1, this volume): (1) the basal Duffer Member, (2) the Panorama Member, and (3) the capping Strelley Pool

Chert (SPC) (Fig. 2). The Duffer Member, which makes up the bulk of the formation, consists of poorly bedded volcaniclastic breccia and felsic lava, in sequences up to

5 km thick, representing coarse, shoaling-upward debris- aprons developed around insular felsic centers (Chapter 1, this volume). These rocks were previously assigned to the

Duffer Formation (Lipple, 1975; Hickman & Lipple, 1978;

Hickman, 1981, 1983).

Finer grained tuffaceous rocks of the Panorama Member locally cap or interfinger with the upper part of the

Duffer Member in the eastern part of the study area and are developed to the exclusion of the Duffer Member in western areas such as the North Pole Dome (NPD) (Figs. 1,

2, and 3). The Panorama Member comprises well-bedded sequences, generally 20 to 200 m thick, but locally ranging up to 1000 m thick. These sequences consist 95

predominantly of felsic volcaniclastic sandstone and tuff

with lesser amounts of chert, chert-clast conglomerate,

volcaniclastic breccia, siltstone, and felsic lava

(DiMarco & Lowe, 1986). These rocks were previously

assigned to the Panorama Formation (Hickman & Lipple,

1978; Hickman, 1981, 1983).

In all greenstone belts studied, the lower clastic

members of the Coongan Formation are overlain, generally

unconformably (Chapter 1, this volume), by the SPC (Lowe,

1983) or its inferred lateral equivalent, the Marble Bar

Chert. The Strelley Pool-Marble Bar Chert represents a

period of orthochemical and biogenic sedimentation that

accompanied a magmatic hiatus following cessation of

felsic volcanism.

FACIES OF THE PANORAMA MEMBER

The Panorama Member can be divided into eight facies. In order of decreasing grain size, these are (A) volcaniclastic breccia, (B) chert-clast conglomerate, (C) well-bedded tuff and tuff-breccia, (D) cross-stratified

sandstone, (E) laminated and cross-laminated siltstone,

(F) banded chert, (G) silicified evaporite, and (H) felsic

lava. The characteristics, physical interpretation, and preliminary environmental interpretation of each facies are given below. Facies interpretation and detailed environmental interpretations are presented in the next section. Because of the excellent preservation of primary 96 depositional features in many sections, the prefix "meta-" will henceforth be omitted and rocks referred to in terms of their protoliths.

Facies A: Volcaniclastic breccia

Description

Poorly bedded volcaniclastic breccia occurs as massive or poorly structured units, 30 cm to several m thick. The breccia is poorly sorted and composed of equant, angular to rounded clasts of felsic volcanic rock, 5 to 40 cm long, in a poorly sorted sand-sized volcaniclastic matrix.

Pumice is absent. In some units, tabular chert clasts comprise up to a few percent of the gravel size fraction.

Where sedimentation units can be distinguished, they consist of massive or normally graded, matrix-supported beds, 30 to 130 cm thick, that show flat, non-erosive bases, no internal stratification, and disorganized clast fabrics, similar to volcaniclastic breccia that makes up the bulk of the Duffer Member of the Coongan Formation

(Chapter 1, this volume).

Sedimentation

The structuring, geometry, and sorting of this facies suggest that most units were deposited by cohesion-poor debris flows. The presence of rounded volcanic clasts and chert clasts and the absence of pumice suggest that, in most cases, these units probably represent lahars.

Facies B: Chert-clast conglomerate

Description

Chert-clast conglomerate, interbedded with cross­

stratified sandstone, is divided into two subfacies, B 1 and B (Fig. 4) . 2 Subfacies B consists of chert-pebble conglomerate, 1 with maximum grain size of 10 cm but generally about 3 cm, that fills very broad but shallow channels with a maximum depth of 3 m (Fig. 4a). In their thickest parts, the channels are filled by clast-supported conglomeratic units, 20 to 50 cm thick, that are sheet like and whose contacts show evidence of scour. The conglomerate units grade laterally over 100 to 150 m into cross-bedded chert- granule sandstone at the channel edges. Imbrication in many clast-supported conglomerate units shows polymodal paleocurrent patterns (Fig. 4a). The gravel size fraction consists of well-sorted and rounded red, white, and gray chert fragments with no volcanic rock clasts, although some consist of breccia made up of silicified siltstone clasts. Felsic volcaniclastic sand fills the framework intersticies. These units are commonly interbedded with sandstone showing evidence of deposition by waves.

Subfacies B consists of chert-cobble conglomerate 2 occuring as channel-fill units up to 25 m deep and over

100 m wide (Fig. 4b). Most channels are filled by fining- upward sequences that grade from framework-supported

cobble conglomerate at the base, having clasts up to about

40 cm in diameter, to sandy pebble conglomerate at the

top. Other channels contain complex fills made up of

alternating clast-supported and disorganized sand-matrix-

supported beds. Imbrication is well developed in clast-

supported conglomerate and shows unimodal paleocurrent

patterns (Fig. 4b). Seventy to eighty percent of the

clasts are chert fragments. The remainder of the coarse

fraction consists of felsic volcanic rock fragments. The

gravel size fraction is moderately sorted and subangular

to well rounded, with most clasts being rounded. Felsic

volcaniclastic sand fills the intersticies. This subfacies

is restricted to the northern flank of the NPD, where it

is enveloped by thick sequences of planar cross-stratified

sandstone that also shows unimodal paleocurrent patterns.

Sedimentation

Subfacies B is interpreted as wave-worked gravel 1 deposits, similar to shallow-marine units described by

Clifton (1973) and Bourgeois & Leithold (1984). It shows

sheet-like geometry, imbricate clast-supported fabric, a

polymodal paleocurrent pattern, textural and compositional maturity, and association with wave-formed sandstone.

Subfacies B , with larger clast size and less mature 2 compositions, suggests deposition in a more proximal 99

setting than subfacies B . Units with well-developed 1 imbrication, moderate sorting, and good rounding indicate

deposition from traction currents, although the presence

of disorganized, matrix-supported beds in some sequences

suggests that some units were deposited from debris flows.

Facies C: Well-bedded tuff and tuff-breccia

Description

Well-bedded tuff and tuff-breccia is divided into two subfacies, C and C (Fig. 5). 1 2 Subfacies C occurs as cyclic, fining-upward units, 1 <1 to 5+ m thick, consisting from base to top of (1) a structureless or normally graded division, 20 cm to 5 m thick, composed of moderately to poorly sorted tuff- breccia or coarse tuff, (2) a stratified middle division, a few cm to 2 m thick, composed of beds, 10 to 30 cm thick, of flat-laminated, or, less commonly, cross­ laminated tuff, and (3) an upper chert cap, generally less than 20 cm thick (Fig. 5a). The lower division is sharp based and, locally, fills shallow scours. Some lower divisions contain inversely graded intraclasts, 3 to 50 cm long, of green chert and flat-laminated tuff. The stratified division generally fines upward from coarse­ grained tuff near the base to fine-grained tuff at the top. Where present, cross-laminated beds in the middle division are randomly interstratified with flat-laminated 100

tuff. In cross-stratified units, low-angle planar and

trough cross-stratification forms sets 10 cm or less

thick. The chert cap is structureless to finely laminated.

Locally, the chert caps are absent and C cycles are 1 capped by tuff having large-scale, low-angle cross­

bedding, showing convex-upward bounding surfaces and

wavelengths of 30 to 90 cm. In some sections, cycles are

incompletely developed; parallel-sided units of massive to

weakly graded tuff in which the upper divisions are absent

are interbedded with cross-bedded sandstone of facies D.

Subfacies C forms cyclic units, similar to those of 2 subfacies C , except that traction structures are poorly 1 developed or absent. Ideal C cycles (Fig. 5b) consist, 2 from base to top, of (1) a massive to graded lower

division, 25 cm to 4 m thick, consisting of poorly sorted

tuff-breccia or coarse tuff, (2) a middle division, made

up of beds, 10 to 30 cm thick, of poorly to moderately

sorted, crudely layered or unstructured tuff that commonly

shows normal grading, and (3) a chert cap, 10 to 20 cm

thick. Some chert caps contain silty laminae, which can be molded into symmetrical ripple-form sets a few cm thick.

This facies is composed primarily of equant, aphanitic

to microporphyritic, very angular to subrounded felsic

VRF's with lesser amounts of volcanic quartz and sericite pseudomorphs after feldspar. In subfacies C , jasper and 1 black-chert fragments form a small percentage of grains, felsic VRF's have a moderate diversity of texture, and pumice was never observed. Subfacies C consists 2 predominantly of angular volcaniclastic material that

shows blocky, shard-like shapes and sparse relict pumice grains.

Sedimentation

The succession of structure intervals resembling the Bouma sequence (Bouma, 1962) and presence of eroded, admixed intraclastic debris suggest that subfacies C was 1 deposited subaqueously from turbidity currents. The lower division is T , the middle division is T and T , and the a b e chert cap is T . Some lower division beds containing de inversely graded clasts may have been deposited from concentrated flows in which turbulence was suppressed and the large clasts supported by buoyant lift, perhaps more like debris flows than turbidity currents. Low-angle cross-bedding with convex-upward bounding surfaces that locally caps C cycles appears to be hummocky cross 1 stratification (HCS) (Dott & Bourgeois, 1982; Swift et al. , 1983; and many others), although the 30 to 90 cm wavelengths of these structures are smaller than the 1 to several m wavelengths that characterize most HCS. The presence of HCS suggests, by analogy to Phanerozoic deposits (e.g. Swift et al., 1983; Klein & Marsaglia, in prep.), that some C units were deposited in environments 1 subject to storm wave activity or possibly seismic shocks.

Elsewhere, C units with poorly developed cyclicity, 1 commonly preserving only massive tuff and invariably

associated with cross-stratified sandstone of facies D,

were apparently deposited in shallow, periodically

agitated water. Although not normally considered typical

of shallow-water sedimentation, gravity-flow deposits are

commonly interbedded with traction deposits in delta and

fan-delta settings (Kleinspehn et a l ., 1984).

The presence of well-graded sedimentation units that generally lack current structures and are composed largely of poorly sorted, angular, felsic VRF's with shard-like

shapes indicates that subfacies C was deposited as 2 subaqueous ash falls. Scarce ripple form-sets with

symmetrical geometries in the chert caps suggest that some ash-fall units were reworked by weak, post-depositional wave activity, implying deposition in a shallow but rather quiet water body.

Facies D: Cross-stratified volcaniclastic sandstone

Description

Silicified volcaniclastic sandstone occurs as well-bedded sequences, a few to 100 m thick, made up of units <10 to

80 cm thick. Locally, the sandstone is cut by barite veins. Most units consist of moderate to well-sorted, medium- to very coarse-grained sandstone and pebbly 103 sandstone. The bulk of the grains are angular to rounded felsic volcanic-rock fragments, sericite pseudomorphs after feldspar, and volcanic quartz. Chert fragments, however, may constitute up to 20 percent of the grains and are preferentially concentrated in the gravel size fraction (DiMarco & Lowe, 1984; Chapter 3, this volume).

In most sections, the rocks consist of trough cross­ stratified units, 1 to 50 m thick, made up of individual sets, 8 to 35 cm thick (Fig. 6a). Locally, tabular cross­ beds, 20 to 80 cm thick, form cosets 50+ m thick.

Paleocurrent data from tabular sets and from some trough cross-sets show unimodal patterns (Fig. 6b). Flat- laminated sandstone and pebbly sandstone, forming units up to 1 m thick, can be interbedded with cross-stratified sandstone. Collectively, these structure types form subfacies D . 1 In some sections, units with structuring, grain size range, and paleocurrent patterns distinct from subfacies

D are present and define subfacies D and D . Subfacies 1 2 3 D consists of cross-laminated, very fine- to medium- 2 grained sandstone in which low-angle cross-laminations, <1 to 8 cm thick, are interlaminated with flasers or drapes of silicified silt-sized debris (Fig. 6c, 7). Where preserved by overlying drapes of fine-grained sediment, ripple form-sets have symmetrical or chevron-shaped profiles with form-discordant cross-stratification (Fig. 104

7). Top and foreset laminae from one set commonly extend

to adjacent sets where they interfinger or drape over

other laminations (Fig. 7), identical to the draping and

"offshooting" laminae of de Raaf, Boersma & vanGelder

(1977). Paleocurrent patterns are polymodal (Fig. 6c). In

some sections, subfacies D makes up the upper part of 2 crude cycles, overlying 2 to 6 m of typical cross-bedded

sandstone of subfacies D (Fig. 6c), and is associated 1 with wave-influenced conglomerate units of subfacies B . 1 Subfacies D is restricted to the Kelly belt, where it 3 occurs locally with massive tuff of subfacies C , in 1 sequences up to 40 m thick (Fig. 6d). It consists of

interbedded units of (1) large-scale, flat- and low-angle

cross-bedded sets, up to 50 cm thick, of medium- to

coarse-grained, well-sorted sandstone, (2) small-scale

cross-stratified sets, 5 to 15 cm thick, of medium- grained, well-sorted sandstone, and (3) thin drapes of highly silicified structureless to finely laminated silt- and possibly clay-sized sediment. Low-angle scoured

surfaces, filled by drapes of sandstone and silicified mud-sized debris, and rip-up clasts of fine-grained sediment are common (Fig. 6d). The small-scale cross­ stratification shows bipolar paleocurrent patterns (Fig.

6d ) . 105

Sedimentation

Good sorting and we11-developed trough and planar cross­

stratification indicate that typical units of D sandstone 1 were deposited by traction currents, mainly under lower

flow-regime conditions. Subfacies D , with symmetrical and 2 chevron-shaped ripple cross-sections, polymodal

paleocurrent patterns, and draping and offshooting cross­

laminae, was deposited under the influence of waves.

Subfacies D was deposited by traction currents showing 3 varying intensity that had periodically reversing

directions. Several characteristics of these units,

including closely associated large- and small-scale cross­

stratification, bipolar paleocurrent patterns, sediment-

draped scours, and abundant fine-grained rip-ups,

collectively suggest deposition from tidal currents,

similar to sequences described by deRaaf & Boersma (1971) and Johnson (1978). Features such as tidal cyclicity,

tidal bundles or sigmoidal cross-bedding, however, which would strengthen the tidal interpretation (e.g. Kreisa &

Moiola, 1986), were not observed, and similar sequences are known from storm-dominated environments.

Facies E: Laminated and cross-laminated siltstone

Description

Highly silicified siltstone forms well-bedded sequences up to 40 m thick. It is composed of laminated and cross­ laminated layers, <1 to 4 cm thick, of coarse silt- and

very fine sand-sized debris intercalated with laminae of

fine silt- and probable clay-sized sediment (Fig. 8).

Coarse silt- and sand-sized debris is made up of well-

sorted, angular to rounded, equant grains that are

structureless to sparsely microporphyritic and appear to

represent volcaniclastic sediment. Finer laminae are

intensely silicified and much textural detail, including

very fine silt and smaller grain sizes, has been obscured.

They were probably composed of volcaniclastic dust and

detrital material.

In most sections, the coarser layers occur as

lenticular laminae that swell laterally over a few cm into

isolated, starved, ripple-form sets <1 to 5 mm thick (Fig.

8). In many cases, these structures have symmetrical

cross-sectional shapes with form-discordant stratification

(Fig. 8). Paleocurrent patterns are polymodal, unimodal or bipolar. The finer layers are very finely laminated and drape over coarser cross-laminae. In other sections, the coarse siltstone occurs predominantly as even, laterally

continuous, normally graded laminae, with little evidence

for scour at the base of each layer.

Sedimentation

The fine grain size and presence of cross-lamination

indicate that many of these units were deposited in quiet, subaqueous settings swept periodically by weak traction currents. The symmetrical cross-sectional shapes,

form-discordant stratification, and polymodal paleocurrent

patterns of some cross-laminae suggest deposition from waves. Wave-formed structures of similar shape and scale were described from younger mudstone sequences by de Raaf

et al. (1977). The laterally continuous, normally graded

layering of other coarse siltstone units suggest deposition from suspension. Slow sedimentation from

suspension is also indicated for the finer layers by their very fine laminations and draping geometries.

Facies F: Banded chert

Description

Chert, a fine-grained rock composed predominantly of granular microcrystalline quartz (GMC), forms sequences up to 200 m thick. It consists of alternating bands, generally 1 to 10 cm thick, of flat-laminated jasper, flat-laminated black and gray chert, and structureless white, gray, or black chert. Laminated cherts are opaque but structureless cherts are translucent. Bands are laterally continuous although contacts are irregular and show features resembling load structures. Translucent chert also forms elongate pods up to about 10 cm long and

7 cm thick. Elongate cavities, filled with centripetal drusy megaquartz, are present in all types of opaque chert. Jasper commonly replaces black laminated chert. In some cases, banded-chert sequences up to several m thick 108 are brecciated and clasts of white, red and gray chert are deformed plastically and set in a homogeneous, black-chert groundmass. Internally, jasper is composed of tightly packed peloids, 0.02 to 0.1 mm in diameter, with concavo- convex boundaries. Locally, jasper contains GMC pseudomorphs, generally about 0.05 mm long, after minerals shaped like displacive lenticular gypsum (Chapter

3, this volume), similar to that described by Schreiber et al. (1982). Black laminated chert is composed of discrete silt-sized grains. Locally, yellow-brown euhedra of ferroan dolomite replace the chert.

Locally, unbanded black chert, in units 5 cm to 10 m thick, is interbedded with sandstone of facies D. This chert variant is structureless or very finely laminated and is composed of discrete, silt-sized, probable relict carbonaceous grains.

Sedimentation

Banded, jasper-bearing chert represents orthochemical and possibly biochemical subaqueous sedimentation in volcanically quiescent environments relatively free from clastic influx. Its structuring is similar to that in many lutitic banded iron formations (e.g. Gole & Klein, 1981;

Beukes, 1984). The homogeneity and podiform habit of the translucent chert indicate deposition as an orthochemical precipitate. Minute grain size and fine laminations of the 109

other chert types suggest slow deposition of the chert

precursor from suspension. Irregular, probable loaded

contacts and soft-sediment deformation features suggest

that the sediment was soft or plastic prior to

lithification. The structure of the brecciated zones, in which the black chert always occurs as the matrix,

indicates that the gray- and red-chert precursors

lithified early and underwent brittle deformation prior to the solidification of the black-chert precursor.

Facies G: Silicified evaporite

Description

Silicified evaporite was identified at only at one locality on the northeast flank of the NPD, where it is interbedded with sandstone of facies D. It consists largely of gray translucent chert containing up to 30 percent isolated and coalescing nodules of white chert or cavity-filling megaquartz (Fig. 9). Also present in the bottom half of the unit are (1) a 10 cm-thick bed of flat to undulose, light and dark chert laminae, and (2) a 10 cm-thick unit of vertical to sub-vertical white chert pseudomorphs after a coarsely crystalline precursor. The gray chert is megascopically structureless except for isolated laminae and cross laminae of volcaniclastic and black-chert sand. Microscopically, it shows poorly preserved, pseudomorphed lenticular grains, 0.2 to 1.0 mm long, oriented subparallel to bedding. The nodular structures, 2 to 5 cm long and <2 cm high, are

concentrated in crude layers parallel to bedding, in

places forming coalescing masses with intersitial gray

chert.

The thin laminated unit consists of alternating light

and dark chert laminae, 0.1 to 1.0 mm thick, that contain

sparse grains of silt-sized debris. They are flat-

laminated or define low-relief domal structures approximately 1 cm across and 0.5 cm high. Some of the dark laminae are crinkly. Both laminae types are locally replaced by elongate, cavity-filling drusy quartz.

White, vertically oriented coarse crystal pseudomorphs, arranged in rows parallel to bedding, are 5 to 10 cm long and 0.3 to 0.5 cm wide, becoming thicker toward crystal tops. Gray translucent chert fills spaces between the pseudomorphs. The tops of some crystal rows are truncated by horizontal discontinuities, marked by thin layers of silicified dark gray sediment. Irregular discordant patches, up to 5 cm across, of gray chert and broken crystal pseudomorphs, occur interstitially between groups of well-preserved pseudomorphs.

Sedimentation

The coarsely crystalline pseudomorphs, with vertical crystal habits, discordant patches suggestive of dissolution, and horizontal truncation of crystal tops, are morphologically similar to Miocene gypsum evaporites of the Mediterranean region (Schrieber et al., 1976,

Shearman, 1978) and other shallow-water evaporite deposits

(Schreiber & Kinsman, 1975). Smaller lenticular pseudomorphs, making up the bulk of the gray chert, are similar in habit to gypsum crystals developed in modern evaporative supratidal environments (Schreiber et al.,

1982). Isolated sand laminae in the gray chert appear to represent current-deposited debris. The size, shape, structure, and crude bedding of the white chert nodules are similar to those of modern evaporite nodules. The tight packing of the nodules resembles chickenwire structure of many modern sulfate nodules in supratidal deposits (e.g. Shearman, 1978). The flat and domal structuring, crinkly dark laminae, and elongate concordant cavities of the laminated chert unit are similar to features of other Archean cherts interpreted as stromatolites (Walter, Buick & Dunlop, 1980; Lowe, 1983;

Byerly, Lowe & Walsh, 1986). Sparse silt-sized grains may represent trapped, wind-blown detritus. The vertical arrangement of the components of this lithofacies, with layers of stromatolite and coarsely crystalline pseudomorphs after gypsum at the base overlain by gray nodular chert, resembles Phanerozoic shoaling-upward, shallow-water to subaerial evaporitic sequences (e.g.

Kendall & Skipwith, 1969). 112

Facies G: Lava

Felsic lava is distinctly subordinate to clastic rocks in

the Panorama Member. The lavas contain 7 to 20 percent

tabular sericite pseudomorphs after feldspar phenocrysts,

0 to 2 percent sericite or GMC pseudomorphs after mica and

amphibole phenocrysts, and 0 to 2 percent resorbed primary

quartz set in a highly altered, fine-grained groundmass

composed of a micromosaic of sericite and chert. They

apparently had a dacitic to rhyolitic composition. Sheared

and altered lavas on the southeast flank of the NPD,

considered felsic lavas belonging to the Panorama

Formation by Hickman & Lipple (1978) and Hickman (1983), are silicified mafic lavas underlying the Coongan

Formation.

FACIES ASSOCIATIONS

The Panorama lithofacies can be grouped into four facies associations (F.A.) representing four major environmental settings: F.A.l, basinal areas relatively free from clastic influx; F.A.2, lower shoreface, prodelta, and shelf environments; F.A.3, alluvial, fan-delta, and upper shoreface environments; and F.A.4, subaqueous settings under the direct influence of pyroclastic sedimentation.

Facies association 1

F.A.l occurs as sequences, 40 to 200+ m thick, made up of banded chert of facies F and smaller amounts of

interbedded tuff of facies C (Fig. 10). It is restricted to the NPD, where it occurs locally at the base of the

Panorama Member. The tuff occurs as thin sequences, up to

20 m thick, of thin-bedded distal ash-fall units and T ab and T turbidites. The tops of some tuff beds show wave- abc formed structures.

The predominance of banded chert and presence of turbiditic tuff indicates that F.A.l represents subaqueous basinal settings largely free from clastic influx. These evironments lay adjacent to the advancing margins of marginal, progradational sequences, as evidenced by their interfingering upward with F.A.2 and 3. Basin-plain cherts are similar sedimentologically and in facies association to lutitic banded-iron formation units that occur at the base of prograding shallow-marine sequences in some younger Archean sequences (Eriksson, 1983).

Facies Association 2

F.A.2 includes sequences, 10 to 100 m thick, made up of interbedded siltstone of facies E, turbiditic tuff of facies C (Fig. 10, 11), and, in some sections, smaller amounts of breccia of facies A. The relative proportion of siltstone to tuff can vary between a fine-grained endmember (Fig. 10), made up exclusively of siltstone, to a thick, coarse-grained endmember consisting largely of turbiditic tuff. The turbiditic tuff shows more proximal characteristics than that of F.A.l, with a predominance of thick T and T units. In all sections, F.A.2 grades up a ab into sequences of cross-stratified sandstone of F.A.3, which commonly include units with wave-formed structures.

The position of F.A.2 below F.A.3, its local interfingering with F.A.l, and the presence of turbidites indicates that F.A.2 represents subaqueous deposition on the outer fringes of progradational basin-margin volcaniclastic sequences.

Facies association 3

F.A.3 is characterizied by cross-stratified sandstone of facies D interbedded with rocks of other facies and forming sequences, 10 to 100+ m thick, that cap shoaling- upward sequences (Figs. 10, 11, 12) or are locally interbedded with the upper part of the Duffer Member.

Thick and thin sequences of F.A.3- are recognized. Thin units of F.A.3, up to 30 m thick (Fig. 10), commonly overlie fine-grained sequences of F.A.2. They consist of simple coarsening-upward sequences of well-sorted sandstone that becomes compositionally more mature upward in showing increasing concentrations of quartz and chert.

Cross-bedding in the sandstones commonly shows polymodal paleocurrent patterns, even in upper parts of the sequences. Thick units of F.A.3 (Figs. 11, 12) cap coarse­ grained sequences of F.A.2, and often include lower subdivisions dominated by texturally mature, chert-bearing

sandstone commonly showing wave-formed structures, and

upper subdivisions characterized by less mature sandstone

showing unimodal paleocurrent patterns. Stacked F.A.2-

F.A.3 sequences 100 to 190 m thick (e.g. Fig. 11, 12),

composed of coarse, turbidite-bearing F.A.2 units and

thick F.A.3 units, thin laterally over several km and are

replaced by thinner F.A.2-F.A.3 sequences, similar to that

of Figure 10.

In stacked F.A.2-F.A.3 sequences, the overall

coarsening- and maturing-upward trend and upward

development of current- and wave-formed structures suggest

that F.A. 3 represents the tops of shoaling-upward, progradational, basin-margin environments. Thick F.A.3

sequences show (1) lower subdivisions, with mature compositions and wave-generated structures indicating high-energy, shallow-subaqueous deposition, and (2) upper subdivisions, with less mature compositions, unimodal paleocurrent patterns, predominance of planar- and trough cross-stratifications without wave-generated structures, and position in the vertical sequence strongly suggesting a braided-fluvial origin. The lateral thickening of F.A.2-

F.A.3 sequences to those localities where a progressively shoaling subaqueous subdivision is capped by a braided- fluvial sequence suggests that thick F.A.2-F.A.3 sequences represent deltaic complexes (see also Fig. 11), in which F.A.2 represents the prodelta and F.A.3 represents the

delta front and upper delta plain. The abundance of wave-

formed structures in some lower F.A.3 subdivisions and the

predominance of sandy, cross-stratified units without

shale in upper subdivisions indicates that the Panorama

deltas were wave-dominated, sedimentologically similar to

some Phanerozoic fan-deltas (e.g. Ethridge and Wescott,

1984), and only grossly comparable to large modern river

deltas (e.g. Coleman and Prior, 1982). Thinner, F.A.2-

F.A.3 sequences appear to represent shoreface and shelf-

shoreface sequences based on their laterally flanking

interdeltaic positions, simple coarsening- and maturing- upward trends, polymodal paleocurrent patterns, and lack of prodelta turbidites and fluvial caps.

Facies association 4

F.A. 4 consists of sequences of turbiditic and air-fall tuff of facies C, 200 to 1000 m thick, that include smaller amounts of siltstone, breccia, and felsic lava of facies E, A, and H (Fig. 13). Some F.A.4 sequences are dominated by proximal volcaniclastic turbidites (Fig. 12) whereas others are made up predominantly of ash-fall units. Wave-formed cross-laminae are locally present in some air-fall tuffs and hummocky cross stratification

(HCS) occurs in the upper parts of some turbiditic units, especially toward the tops of thick turbidite successions. Thick sequences of F.A.4, showing abundant turbiditic tuff, ash-fall units with wave-formed features, and local

HCS, indicate deposition in subaqueous settings under the direct influence of pyroclastic volcanism. The presence of wave-formed structures indicates that water depths were, at least locally, shallow.

SEQUENCE EVOLUTION

Comparison across the eastern Pilbara Block suggests that the Panorama Member includes two main types of sections:

(1) western occurrences, not associated with the Duffer

Member and developed over a basaltic substrate, typified by deltaic and shoreface sequences in the NPD, and (2) eastern occurrences, consisting primarily of F.A. 4 deposited over thick sequences of the Duffer Member (Fig.

13) .

Western occurrences

North Pole Dome

Complex deltaic sequences (Fig. 14 a,b,c) crop out on the north, northeast, and southeast flanks of the NPD.

Paleocurrent evidence suggests that the sediment was fed to these marginal environments from volcanic centers within the NPD (Chapter 1, this volume).

The sequence on the north flank of the NPD (Fig. 14a) interfingers eastward over 2 km with volcaniclastic breccia and with felsic lava and appears to represent a proximal, volcanically controlled fan(?)-delta complex.

Basal cherts of F.A.l are overlain by "proximal"

turbidites of subfacies C , interpreted as prodelta 1 deposits and similar to deposits in other fan-deltas, the

subaqueous parts of which were dominated by gravity-flow

processes (Nemec et a l ., 1984; Postma, 1984; Massari,

19 84). The prodelta turbidites grade upward into wave- and

current-formed sandstone intercalated with beds of massive tuff interpreted as fan-delta front deposits. The massive tuff beds, with constituent framework grains that are compositionally and texturally similar to those of the interbedded sandstones, appear to represent high-density turbidites or liquefied-flow deposits generated from delta-front or fluvial debris. Interbedded gravity-flow and current-lain deposits are common in many fan delta- front sequences (e.g. Wiley & Moore, 1983; Nemec & Steel,

1984; Kleinspehn et al., 1984) and reflect the interplay among complex hydraulic conditions, slope characteristics, and rates of sediment accumulation at the delta front. The delta-front deposits are capped by a thick sequence of unchannelized planar cross-bedded sandstone with interbedded channelized conglomerate (Fig. 4b) that is interpreted to represent two styles of fluvial deposition: unchannelized fluvial deposition is characterized by sandstone that forms sequences similar to Platte-type braided streams (Miall, 1977) and represents an aggrading sheet-flow-dominated braid plain, similar to modern

braided outwash plains (Boothroyd & Nummedal, 1978). The

Panorama braid plain probably formed during periods of

pyroclastic activity. During times of volcanic quiescence,

the supply of loose ash was quickly eroded and streams

downcut into their braid plains. Epiclastic material,

represented by chert-cobble conglomerate, formed

channelized deposits within the incised braid plains

during volcanically quiescent periods. Similar

sedimentation patterns are present in the modern of

central America (Kuenzi, Horst & McGehee, 1979; Vessel &

Davies, 1981), where periodic pyroclastic eruptions result

in aggradation of fluvial systems and progradation of

deltas. Between eruptive phases, dissection of stream

deposits takes place.

Deltaic deposits (Fig. 14 b,c) on the northeast and

southeast flanks of the NPD represent depositional sites

further removed from felsic volcanic centers. This is

shown by a lack of closely associated felsic lava and the

smaller proportion and more distal character of prodelta

turbidites. On the northeast flank of the NPD, deltaic deposits (Fig. 12, 14b) include a sequence of cross-bedded distributary deposits overlain successively by parellel-

stratified bar-top or beach deposits, lower delta-plain evaporite-pond deposits, and fluvial cross-bedded sandstones, similar to many fan-delta sequences constructed on shallow shelves (e.g. Ethridge & Wescott,

1984). Evaporative deposits on the lower delta plain,

represented by silicified evaporite of facies G, are

similar to those on the modern Colorado River Delta

reported by Thompson (1968).

Interdeltaic sites are represented by thinner stacked

F.A.2-F.A.3 sequences. An example of these is the sequence

at Loc. 61 (Fig. 14d), consisting of a simple coarsening-

and maturing-upward sequence only 15 m thick.

Pilganqoora Belt

Although now eroded, the former presence of the Panorama

Member in the Pilgangoora belt is indicated by a laterally discontinuous, quartz-rich sandstone that is the basal member of the SPC (Lowe, 1983). The highly silicified

sandstone, containing medium to coarse, well-sorted grains, shows large-scale, low-amplitude trough cross­ bedding. Framework quartz, ranging from 63 to 75 percent of the framework-grain population, is exclusively volcanic in origin (Lowe, 1983; DiMarco & Lowe, 1984, and chapter

3, this volume). The remainder of the framework grains consists of silicified felsic VRF's.

The thinness, discontinuity, and textural and compositional maturity of this unit suggest deposition under high-energy, shallow-water conditions (Lowe, 1983).

These rocks apparently represent transgressive deposits resulting from migration of a shallow-water system over underlying basalt and patchy felsic sediments. The deposit

itself represents a winnowed, concentrated residue of now

eroded and probably originally thin quartzose tuffaceous

sediments of the Panorama Member. The poor preservation of

the underlying units and the maturity of the sandstone

indicate slow transgression in the Pilgangoora belt.

Eastern occurrences

Coonqan belt

The Panorama Member is present on the west limb of the

Coongan belt. In the type section of the Coongan Formation

(Chapter 1, this volume) at the northwest end of the

Coongan belt, the Panorama Member consists of about 100 m

of ash-fall tuff of F.A.4 (Fig. 14e) that rest on alluvial

fanglomerate of the uppermost part of a 3 km-thick section

of the Duffer Member (Chapter 1, this volume). Locally, a

thin sequence of cross-stratified sandstone of F.A.3,

generally less than 10 m thick, caps the ash-fall deposits.

In the Coongan belt, quiet subaqueous deposition was directly associated with pyroclastic volcanism. The preservation of underlying fluvial deposits suggests concurrent subsidence. Deposition of the ash took place mainly below wave base. Local capping of tuff by F.A.3

indicates that late stages of tuff sedimentation took place under gradually shoaling conditions. Marble Bar belt

The Panorama Member is locally developed in the Marble Bar belt, particularly in the vicinity of Bowl's Gorge (Loc.

63, Fig. 1), where it consists of about 1000 m of F.A.4 in which turbidites predominate (Fig. 14f). The sequence is traceable over a strike distance of about 10 km, thinning and partially truncated by faults at either end, and caps over 5 km of volcaniclastic breccia and lava of the Duffer

Member. It consists predominantely of turbidites interbedded with debris-flow deposits and rare beds of siltstone (Fig. 13). Toward the top of the sequence, beds with HCS cap some T and T cycles. a ab Subaqueous deposition is indicated by the (1) presence of turbidites, (2) interbedding of siltstone, representing weak current reworking and fine-grained suspension deposition during periods between generation of turbidity currents and (3) presence of HCS, suggesting the possible role of wave- or seismic activity (Swift et a l ., 1983;

Klein & Marsaglia, in prep.). Shoaling of the sequence is suggested by the progressive upward development of HCS

Although no direct evidence for associated calderas was found, the great thickness of this sequence, predominance of felsic turbiditic tuff, and association with and felsic lava are similar to subaqueous felsic volcaniclastic deposits developed in association with a caldera complex in the Mesozoic of California (Busby-Spera, 1984), and may represent a similar, caldera-related facies.

Kelly belt

F.A.3 of the Panorama Member is locally developed in the

Kelly belt. In outcrops between Copper Hills and Spinaway

Creek (Locs. 56 and 66, Fig. 1) it includes about 50 m of sandstone of subfacies D and massive tuff that 3 interfinger with matrix- and clast-supported volcaniclastic rudite of the Duffer Member (Fig. 14g).

The unit is traceable along strike for about 2 km and is terminated at either end by faults. This is the only locality where the Panorama Member has evidence suggestive of tides.

Although not associated with any clearly littoral or alluvial deposits, the sequence shows a number of features like those of the subaqueous parts of some fan-delta deposits: (1) The interbedding of stratified and unstratified arenites is characteristic of many fan-delta fronts (e.g. Kleinspehn et: a l ., 1984) where current, wave, and tidal processes alternate with gravity-flow processes.

Massive tuffs probably represent proximal turbidites or liquefied-flow deposits, produced by sediment overloading and failure. Reworking of the delta-front deposits during abandonment or slack-water probably produced tide(?)- formed features. (2) Interbedded clast- and matrix- supported rudite of the Duffer Member represents debris- flow and density-modified grain-flow deposits (Chapter 1, this volume), which are common in many fan-delta environments (Nemec et a l ., 1984; Postma, 1984). These

units indicate that, at least locally, the top of the

Duffer Member formed in shallow-water environments.

Subaerially deposited tuff and conglomerate cap the Duffer

Member about 10 km south of the sequence described here

(Barley, 1978).

DEPOSITIONAL MODEL

Sedimentation within the Coongan Formation was characterized by a progressive change from coarse to fine material and a progressive shoaling with time. Early volcaniclastic breccia and lava deposits of the Duffer

Member constitute the bulk of the Coongan Formation and represent predominantly subaqueous, shoaling-upward deposition on coarse debris-aprons developed around the flanks of volcanic centers (Chapter 1, this volume). The termination of felsic volcanism was marked by the deposition of sandy debris, represented by the Panorama

Member, in predominantly shallow-water environments.

Volcaniclastic deposition was succeeded by the deposition of evaporitic and biogenic sediments on a broad, post- volcanic platform (Lowe, 1983). Stages in the evolution of the Coongan Formation, in which the Panorama Member represents a transition period during the return to regional platform conditions that prevailed during the deposition of most of the Warrawoona Group (Chapter 1,

this volume), are outlined in Figure 15.

Petrographic evidence indicates that most of the sandy

debris in the Panorama Member was tephra (Chapter 3, this

volume). The upward development of sand-sized volcanogenic

detritus in the Coongan Formation suggests that explosive

pyroclastic eruptions became more important and correlates

with the eruption of increasingly more evolved magmas

through time in Pilbara calcalkaline sequences (Barley et

a l ., 1984; Hickman, 1983, p. 154). Thus, tephra production

is probably related to an increase in viscosity and

volatile content of the late magmas. The scarcity of clay

suggests that chemical weathering was unimportant compared

to volcanism and the production and accumulation of ash

and gravel-sized volcaniclastic detritus.

Tephra originated from several centers during Panorama times (Chapter 1, this volume) and was dispersed by reworking within fringing epiclastic systems and by direct pyroclastic sedimentation. In eastern areas, the Panorama

Member caps volcaniclastic breccia of the Duffer Member mostly as well-bedded, laterally persistent subaqueous ash-fall deposits and turbidites, but locally occurs as probable fan-delta units interbedded with the upper part of the Duffer Member. The wide preservation of subaqueous deposits, local preservation of probable fan-delta deposits, and the evidence for erosion at the SPC contact (Chapter 1, this volume) suggest that much of the Panorama

Member was removed by erosion prior to the deposition of

the SPC.

Western sections of the Panorama Member are developed

directly over Warrawoona basalts. In the NPD, tephra

reworked from proximal volcanic environments was deposited

in fluvial and coastal systems including a diverse

assemblage of shallow-water facies. Proximal-to-distal

sedimentation patterns are represented by eruption-tied

deltaic packages, in which banded chert, the most distal

facies, locally shows evidence for shallow-water

deposition.

This study provides the first evidence suggestive of

tidal deposition in the volcanic portions of early Archean

greenstone belts, although well-developed tidal facies have been described in the upper sedimentary portion of early Archean greenstone belts in South Africa (Eriksson,

1977, 1979). The abundance of wave-formed structures in the Panorama Member suggests that most deposition took place in low-energy shallow-water environments.

SUMMARY AND CONCLUSIONS

Silicified, predominantly sandy meta-volcaniclastic rocks of the Panorama Member represent predominantly shallow- water, volcaniclastic deposits in the 3.5 Ga Warrawoona

Group, Western Australia. Together with the underlying Duffer Member and the capping Strelley Pool Chert, the

Panorama Member occurs in the upper part of the Coongan

Formation, a widespread assemblage of volcaniclastic breccia, sandstone, tuff, felsic lava, and chert developed primarily in response to felsic volcanism.

The Panorama Member is divided into eight facies of which felsic volcaniclastic sandstone and felsic tuff are the most important. Wave-formed features are widespread but evidence suggestive of tides was found only locally.

The eight facies occur in four facies associations (F.A.):

F.A.l, dominated by banded chert with lesser turbiditic and air-fall tuff, representing basinal environments relatively free from clastic influx; F.A. 2, made up of siltstone with varying proportions of turbiditic tuff, representing prodelta, lower shoreface, and shelf deposits; F.A. 3, composed mainly of cross-atratified sandstone with one or more other facies, representing delta-front, alluvial, and upper shoreface deposits, and

F.A.4, made up of turbiditic tuff, felsic lava, and volcaniclastic breccia, representing subaqueous environments under the direct influence of pyroclastic sedimentation.

Deposition of the Panorama Member was influenced greatly by volcanism, which provided an abundant supply of ash and freshly eroded volcaniclastic debris. In western parts of the study area, sediment was reworked in fringing epiclastic systems and deposited directly over basalt mainly in fan(?)-delta and flanking shoreface systems.

Associated braided-stream and proximal volcanic environments are poorly preserved. In the eastern part of the study area, units of F.A. 4 were deposited over thick,

locally subsided sequences of the Duffer Member. The

local evidence suggestive of tides and the abundance of wave-formed features suggests that subaqueous deposition took place mainly under microtidal marine conditions. The lack of rocks containing detrital clay minerals is notable and is probably .caused by the predominance of volcanic effusion and rapid transportation of loose ash in comparison to chemical weathering. 129

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FIGURE CAPTIONS

1. Generalized geologic map of the eastern Pilbara

Block showing outcrop extent of the Coongan Formation in

black and locations of sections mentioned in text.

2. a) Previous stratigraphic interpretation of the

Warrawoona Group, after Hickman and Lipple (1978). b)

Present stratigraphic interpretation showing inferred

position of the Coongan Formation.

3. Correlation of members of the Coongan Formation

through several greenstone belts in the eastern Pilbara

Block.

4. Subfacies of chert-cobble conglomerate of facies B: a) Sheet-like geometry and polymodal paleocurrent pattern of subfacies B . b) Deeply channelized geometry 1 and unimodal paleocurrent pattern of subfacies B . 2 Additional criteria described in text.

5. Subfacies of tuff and tuff-breccia of facies C: a)

Turbiditic units of subfacies C with well-developed T 1 a and T divisions. b) Air-fall units of subfacies C b 2 characterized by normal grading, crude layering, and, except for sparse development of wave-formed structures in chert cap, lack of traction-produced structures.

6. Stratification in cross-stratified sandstone of facies D: a) Typical sandstone units showing trough cross-stratification. b) Planar cross-bedded sandstone showing unimodal paleocurrent patterns. c) Trough- cross-stratified sandstone sequence capped by a sequence of wave-worked sandstone of subfacies D . 2 Note polymodal paleocurrent pattern of wave-worked strata. d) Interbedding of probable tide-influenced sandstone of subfacies D and massive tuff. Note bimodal 3 paleocurrent pattern of small-scale sets.

7. Slab photo of wave-worked sandstone of subfacies D 2 showing symmetrical and chevron-shaped cross-sectional outlines of sets and drapes and flasers of silicified finer grained sediment.

8. Slab photo of facies E showing laminated and cross­ laminated fine sandstone, siltstone and probable silicified mudstone. Note symmetrical cross-sectional geometries of ripple form-sets.

9. Diagram of silicified evaporite of facies G showing chert with nodular structures, vertically oriented pseudomorphs after probable gypsum, and possible stromatolite.

10. Facies association 1 and its position in basal part of a shoaling-upward clastic sequence.

11. Facies association 2 and its position below traction-dominated part (F.A.3) of a thick shoaling- upward clastic sequence.

12. Facies association 3 showing the details of traction-dominated units at the top of a deltaic assemblage. Lower part of figure schematically illustrates the depositional model for deltaic deposition at Loc. 7. Contrast with deltaic center on north flank of the North Pole Dome (Loc. 15) in text and

Fig. 14a.

13. Facies association 4 illustrating the predominance of gravity-flow deposits.

14. Sequence evolution in the Panorama Member. Western occurrences, typified by Locs. 15, 7, 34, and 61, consist of shoaling-upward sequences made up of facies associations 1,2, and 3, representing deltaic and shoreface packages. In the eastern part of the study area, facies association 4 and, locally, facies association 3 are developed over thick sequences of the Duffer Member, as typified by Locs 30, 63, and 56.

Location of sections shown in Fig. 1. N.P.D. = North

Pole Dome; C.B. = Coongan belt; M.B.B. = Marble Bar belt; K.B. = Kelly Belt.

15. Depositional model for the Panorama Member of the

Coongan Formation (no scale implied). I. Deposition of coarse volcaniclastic debris-aprons of the Duffer Member, mainly as sediment-gravity-flow deposits flanking volcanoes that were constructed on a basaltic platform and built up in water. II.Deposition of the

Panorama Member during the terminal stage of felsic volcanism. In eastern areas, such as the Kelly, Marble

Bar, and Coongan belts, ash-fall deposits, turbidite sequences, and, locally, probable fan-delta deposits are preserved over locally subsided sections of the

Duffer Member. In western areas, such as the North

Pole Dome, debris was dispersed from volcanic centers, now mostly eroded, and depositied in flanking alluvial and shallow-water deposystems. III. Subsequent erosion resulted in the development of a peneplain at or near water level and the deposition of the Strelley Pool

Chert upon this platform. Figure Younger Sedimentary Rocks

Archean Granitoid Marble Bar H Rocks Marble Bar Belt y + + + + + + [jjlllj Gorge Creek Group + + + + +„ Warrawoona Gp. with outcrop of Coongan Fm. r

Mount Edgar Btlh.*■

North Dome

Marble 9 Pilgangoora

'0

North Corunna Downs Shaw Shaw Btlh.

m

Coongan Figure 148

Wyman Fm. Wyman Fm.

Salgash Subgp.

Panorama SPC Fm. a. 3 Panorama o M M b r . 1 e> Salgash n) Subgp. a Duffer o Mbr. o Towers Fm 3 Duffer * — M Taiga tflM Fm. Es _l 1x Taiga Subgp. Taiga Taiga Subgp.

A

LEGEND

Felsic tuff and sandstone

^ ✓» A Felsic lava and breccia

Chert units □ Mafic and ultramafic units Figure o m

NORTH POLE DOME COONGAN BELT KELLY BELT 14 km 20 km 16 km 32 km 5 km ^ N t

16 mj,

Loc. 61 500 m Loc. 34 900 m

2.7 km 2.8 km Strelley Pool Chert

Panorama Member Loc. 65

Duffer Member Loc. 56 + + Granitoid Rock

V| Mafic Rock \ + ♦ + Loc. 35 Loc. 30 Figure 152 Figure Chert cap Chert cap

Crudely layered tuff and tuff-breccia

A: Subfacies B: Subfacies Figure A

n = 54

Planar and trough Massive sandstone cross-stratified representing high- I sandstone density turbidites or n = liquefied-flow deposits

Sub facies D2 : Subfacies D3 : Large- wave-worked and small-scale cross­ sandstone with stratified sandstone with lutite flasers green chert drapes representing probabale tidal deposits Figure

Figure 160 Figure 9 162

LEGEND

Silicified gypsum

Silicified stromatolite

Nodules of white chert in gray chert matrix with isolated lenses of sand-sized grains Figure 164

FACIES ASSOCIATION 3 FACIES ASSOCIATION 2

FACIES ASSOCIATION 1

FACIES DESCRIPTION INTERPRETATION

Silicified Basinal suspen­ banded chert sion deposits without current and chemical structures. precipitates F fringing clastic progradational sequences.

Thin-bedded Distal clastic turbidites and deposits in, at air-fall tuff. least locally, Tops of some shallow water. units are wave worked. C

F As above. As above. Figure 166 100 - o. 7 Loc. N.P.D.

/ — D D & E & D FACIES & E & C bedded with bedded and Laminated siltstone inter­ siltstone ittn inter­ siltstone and Laminated sandstone. cross-stratified with calated mostly showing mostly cross-laminated Ta* cross-laminated ubdtc tuff turbiditic FACIES ASSOCIATION 2 ASSOCIATION FACIES DESCRIPTION

aa delta-front Basal deposits. INTERPRETATION Prodelta deposits.

Figure 168

FACIES ASSOCIATION 3

FACIES DESCRIPTION INTERPRETATION

Trough cross-stratified Alluvial deposits. and flat-laminated D & A sandstone interbedded with volcaniclastic breccia whose top is reworked.

Silicified evaporite Local evaporative G and probable stromat­ environment on olite . upper fan-delta plain. Compositionally Well-winnowed fan- D & B mature sandstone and delta front chert-granule con­ deposits. glomerate.

D Flat-laminated sand­ Beach deposits. stone and chert-pebble conglomerate.

Trough cross-stratified Fan-delta front D sandstone. deposits. 0 -

FACIES ASSOCIATION 2

A

ALLUVIAL FAN-DELTA FRONT PRODELTA

FACIES ASSOCIATION 3

basaltic platform B Figure 13 170

Loc. 63 Marble Bar Belt Facies FACIES ASSOCIATION 4 FACIES DESCRIPTION INTERPRETATION

Turbiditic and Subaqueous air-fall tuff. pyroclastic C and resedi- mented depsoits.

Volcaniclastic Subaqueous breccia. debris-flow deposits. A

Laminated and Background cross-laminated deposition of siltstone commonly silt- and clay­ showing lenticular sized debris. bedding. E Figure CM

Loc. 15 Loc. 7 Loc. 34 Loc. 61 Loc. 30 Loc. 63 Loc. 56 N.P.D. N.P.D N.P.D N.P.D. C.B. M.B.B. K.B. 290 100 - 100 100l 900. 140.

3 3 7

F. A. S 3 3 s F.A.

F.A. D 2 « / v \ V E G

FACIES C 0 F. A. F.A. 1 B F. A F FACIES E m 13 A F. A. FACIES D c> ° FACIES F DUFFER MBR. FACIES A E

FACIES G 1 MAFIC UNITS ffigg FACIES B Figure 15 M M w

N. PETROGRAPHIC CHARACTERISTICS AND PROVENANCE

OF SILICIFIED EARLY ARCHEAN SANDSTONES,

EASTERN PILBARA BLOCK, WESTERN AUSTRALIA

ABSTRACT

Intense post-depositional alteration has affected most

sandstones in the lower, principally volcanic portions of

early Archean (3.5 to 3.3 Ga) greenstone belts. In these

rocks, most primary minerals have been completely destroyed and compositions altered by pervasive metasomatism, but internal framework-grain textures are commonly well preserved. Consequently, preserved microtextural information coupled with present alteration compositions as determined petrographically can be used to reconstruct original framework modes.

Pervasively silicified early Archean volcanic arenites of the Panorama Member of the Coongan Formation in the eastern Pilbara Block, Western Australia, were originally composed of (1) a variety of volcanic- (VRF) and sedimentary- (SRF) rock fragments, (2) quartz, feldspar, and traces of ferromagnesian minerals, and (3) generally negligible amounts of pumice. Only volcanic megaquartz framework grains, with common corrosion embayments and beta-quartz habits, remained stable during alteration. All other primary components were replaced by granular microcrystalline quartz (GMC) or fine-grained sericite. In

175 176

most areas, relict dacitic to rhyolitic VRF's, now totally

replaced by sericite-poor GMC, predominate in the

framework population and are recognized by preserved

microporphyritic textures. In other areas, relicts of

quartz-poor dacitic to andesitic(?) VRF's dominate the

detrital mode. Minor amounts of altered SRF's and mafic

to ultramafic VRF's, now replaced by GMC, are recognized

on the basis of color and internal textures, and preserved

microquench textures, respectively. Detrital feldspar is

represented by blocky, sericite-rich domains.

A semi-quantatative point-count scheme was devised and

indicates the following modal assemblage for Panorama

arenites: quartz, 0-28%; feldspar, 0-28%; VRF's, 58-86%;

and SRF's, 0-25%. In about half the point-counted samples,

feldspar could not be distinguished from rock fragments.

Both were counted as one grain type, Lv', which ranges

from 84 to 100 percent of the framework mode of these rocks.

This assemblage was derived from a terrane dominated by felsic volcanic rocks with minor mafic to ultramafic and local sedimentary rocks. Much, but not all, of the felsic volcaniclastic sand represents reworked pyroclastic debris. There is no evidence for contributions from granitic sources. This modal assemblage, derived from an anorogenic felsic volcanic setting, represents a distinctly different provenance than typical Archean 177

graywackes and other siliciclastic rocks, common in the

upper sedimentary portions of early Archean (3.5 to 3.3

Ga) greenstone belts and in younger greenstone belts (3.0

to 2.7 Ga), that were derived from the erosion of complex

orogenic terranes.

INTRODUCTION

Petrographic study of detrital modes of Phanerozoic

sandstones has proven useful in inferring sand provenance

and tectonic setting of ancient basins (e.g. Dickinson,

1970; Folk, 1974; Dickinson and Suczek, 1979). This

approach, however, has been of limited use in the analysis

of early Archean sandstones in basal volcanic sequences

of older (3.3 to 3.5 G a ), well-preserved greenstone belts.

In these rocks, alteration minerals, particularly microquartz and fine-grained phyllosilicate minerals, partially to completely replace original constituents

although the primary internal textures of framework grains are commonly well preserved (Barley et al., 1979; Dunlop and Buick, 1981; Heinrichs, 1984; DiMarco and Lowe, 1984;

Buick and Barnes, 1984). These highly altered arenites contrast markedly with better preserved, younger Archean sandstones, including greywackes, that comprise the major

lithology of the predominantly sedimentary parts of both older (3.5 to 3.3 G a ) and younger (3.0 to 2.7 G a ) greenstone belts (e.g. Condie et al., 1970; Walker and

Pettijohn, 1971; Turner and Walker, 1973; Ayres, 1983; 178

McLennan, 1984). In this paper, it is demonstrated that,

despite pervasive post-depositional changes in composition

and texture, it is possible in many cases to see through

the screen of alteration of early Archean sandstones and

study these rocks using standard petrographic methods.

GEOLOGIC SETTING

The 3.5 Ga Warrawoona Group (Pidgeon, 1978; Jahn et

al., 1981; Hamilton et a l ., 1980), forming the lower two-

thirds of the supracrustal sequence in greenstone belts of

the eastern Pilbara Block of Western Australia (Fig. 1),

contains some of the earth's oldest well-preserved

sedimentary and volcanic rocks. These rocks occur as

tightly folded synforms and broadly arched domes separated

by large, ovoid, granitoid batholiths. Altered tholeiitic

and lesser amounts of komatiitic lavas comprise the bulk

of the Warrawoona Group, but felsic meta-volcanic and

silicified meta-sedimentary rocks occur throughout the

succession (Barley, 1978; Barley et al., 1979; Dunlop and

Buick, 1981; Lowe, 1982; Buick and Barnes, 1984; Barley et al., 1984)). The Warrawoona Group is overlain, with local unconformity (Hickman, 1983), by the predominantly

sedimentary Gorge Creek Group.

Although rocks of the Warrawoona Group have generally undergone only low-grade metamorphism (Barley, 1980), most have been affected by pervasive metasomatic alteration. This alteration probably occurred over an extended period,

beginning shortly after deposition and continuing through

later low-grade, early Archean metamorphism. Clastic meta-

sedimentary rocks in particular have commonly been extensively silicified and recrystallized, although some

show nearly complete carbonation.

A regionally identifiable unit of felsic volcaniclastic sandstone, volcaniclastic breccia, and lava is interbedded with basalts in the upper part of the

Warrawoona Group in almost all greenstone belts in the eastern Pilbara Block (Fig. 1). Regional correlation of felsic volcanic units and cherty volcaniclastic strata at this stratigraphic position (Chapters 1 and 2, this volume) has resulted in the establishment of a new lithostratigraphic unit, the Coongan Formation (Fig. 1).

This unit consists of three members: the Duffer and

Panorama Members, made up primarily of volcaniclastic and volcanic rocks, and the overlying Strelle'y Pool Chert, which includes silicified evaporitic, stromatolitic, and carbonaceous sediments (Lowe, 1983). Sequences of volcaniclastic rudite and lava, up to 5 km thick, form the

Duffer Member, formerly termed the Duffer Formation

(Lipple, 1975; Hickman and Lipple, 1978; Hickman, 1983), and make up the bulk of the Coongan Formation. Finer grained felsic volcaniclastic sandstone and tuff of the

Panorama Member, formerly termed the Panorama Formation (Lipple, 1975; Hickman and Lipple, 1978; Hickman, 1983),

cap or interfinger with the upper part of the Duffer

Member in greenstone belts in the eastern part of the

study area and are developed to the exclusion of the

Duffer Member in the North Pole Dome. Regional geochemical

studies (Hickman, 1983, Barley et a l ., 1984) of felsic units of the Warrawoona Group indicate that these rocks were derived from calcalkaline magmas.

The petrology of sandy rocks of the Panorama Member, which accumulated in predominantly shallow-water depositional environments developed around felsic volcanic centers (Chapter 1, this volume), is the focus of this report. Several quartz-rich sandstones from the base of the Strelley Pool Chert are also included. Rocks described here are from the flanks of the North Pole Dome, western part of the Coongan Belt, and north limb of the

Pilgangoora Syncline (Fig. 1). Because this report emphasizes the sedimentary aspects of these rocks, the prefix "meta-" will be dropped and rocks referred to in terms of their protoliths with the understanding that the sandstones are highly altered and that the Warrawoona

Group is regionally metamorphosed to the prehnite- pumpellyite or greenschist grades.

GENERAL CHARACTER OF THE SANDSTONES

Rocks of the Panorama Member have been divided into eight facies, of which cross-bedded volcaniclastic 181

sandstone, turbiditic and air-fall tuff-breccia and tuff, and chert-clast conglomerate are the most common (DiMarco and Lowe, 1986; Chapter 2, this volume). These rocks are best developed in the North Pole Dome, where the Panorama

Member consists of we11-bedded coarsening-upward sequences representing prograding deltaic and laterally flanking shoreface environments developed in response to felsic volcanism (DiMarco and Lowe, 1986; Chapter 2, this volume). Elsewhere in the eastern Pilbara Block, the

Panorama Member consists predominantly of sheets of air- fall and turbiditic tuff deposited in shallow subaqueous settings. Thin layers of highly reworked sandstone from the Strelley Pool Chert represent transgressive deposits.

In the outcrop, the highly silicified sandstones and tuffs of the Panorama Member are typically light gray to tan. Locally, the rocks are carbonated and weather dark brown. Most framework grains appear as light gray to white particles, which will be shown to be relict volcanic rock fragments. Most rocks also contain less than ten percent vitreous quartz and red, black, gray, and green chert fragments.

Cross-bedded sandstones are generally well sorted and consist of subangular to well-rounded, medium to very coarse, sand-sized grains. Turbiditic and air-fall tuffs contain angular to subrounded particles and, in general, are more poorly sorted than the cross-bedded rocks because 182

of the presence of silt- and fine sand-sized grains. Most

samples show little evidence for compaction or framework-

grain crushing. Microtextures of framework grains are

commonly well preserved and point contacts among framework

grains are common. Cementation apparently began early, based on the loose packing and the preservation of primary textures and grain microtextures.

The present mineralogic composition of the intensely silicified sandstones bears little resemblance to original rock compositions. All framework grains except megaquartz and some detrital micas, as well as any original cements and matrix materials, have been replaced by micromosaics of granular microcrystalline quartz (GMC) with lesser amounts, generally less than ten percent, of fine-grained sericite. Traces of black oxides and rutile are also present, especially in grains representing altered ferromagnesian minerals. The GMC consists of equant optical domains of microquartz, <35//m in diameter, with ill-defined boundaries that change shape as the microscope stage is rotated under crossed nicols. X-ray analysis of the sericite indicates that it is a mixture of 2M and 1M polytypes of dioctahedral mica (Chapter 4, this volume).

Expandable clays are absent. X-ray analysis of the local carbonate phase indicates that it is ferroan dolomite or ankerite. In many cases, silicification was so intense that original framework-grain boundaries, except those of 183

megaquartz, have been erased and the distinction between

framework grains and interstitial material has been lost

(Fig. 2) .

Evidence for dissolution of framework grains (e.g.

McBride, 1985) is rare. Good preservation of primary

textures of labile framework grains and the relatively

loose packing suggests that dissolution was unimportant.

Complete dissolution of rounded framework grains was

inferred for rare grains in a few thin-sections where the grain site is now occupied by meta-chalcedony. Long and

sutured contacts among quartz grains in a few quartz-rich

samples also suggest partial dissolution of megaquartz.

ORIGINAL FRAMEWORK COMPONENTS

In this section criteria are presented for recognition of the following primary framework components in Panorama sandstones: (1) megaquartz, (2) volcanic and sedimentary rock fragments, (3) feldspar, (4) ferromagnesian minerals, and (5) pumice and shards. The first three components form the basis of many commonly used sandstone classifications

(e.g. Dott, 1964; Folk et al., 1970; Folk, 1974) and their relative proportions have important genetic implications in Phanerozoic systems (e.g. Dickinson and Suczek, 1979).

Megaquartz.-Because of its stability during alteration and metamorphism, megaquartz is the only remaining primary detrital component of the Panorama sandstones (Fig. 3).

Framework quartz occurs mainly as monocrystalline sand­

sized grains. These are largely strain-free and have few

inclusions. In many cases, megaquartz has a bipyramidal

habit and well-developed corrosion embayments, both of

which indicate a volcanic source. Megaquartz overgrowths

rimming detrital quartz grains, recognized on the basis of

fine-grained inclusions that trace original boundaries,

are present in some of the most quartz-rich samples. In

most samples, however, detrital-megaquartz grain

boundaries are sharply defined and lack syntaxial

overgrowths; hence megaquartz is amenable to conventional

modal point-count techniques.

Rock Fragments.-Relict rock fragments are the most

abundant framework constituents of the Panorama

sandstones. All rock fragments were unstable during

alteration and are presently composed largely of GMC (Fig.

4). Primary volcanic and sedimentary textures, however,

are commonly well-preserved. Three main types of relict

rock fragments can be recognized on the basis of preserved

internal textures: (1) felsic volcanic rock fragments

(FVRF), (2) sedimentary rock fragments (SRF) and (3) mafic

to ultramafic volcanic rock fragments (MVRF).

Altered FVRF's, recognized by the presence of relict microphenocrysts surrounded by a recrystallized, GMC-rich groundmass, constitute the bulk of the rock-fragment and 185 total framework population (Fig. 4). The groundmass of most relict rock fragments is composed of essentially pure

GMC, although in some samples, the recrystallized groundmass contains a small amount, generally less than ten percent, of fine sericite. The most common microphenocryst relicts in FVRF's are pseudomorphs after feldspar. These can be recognized by (1) their tabular or

lath shapes, and (2) their high contents, generally approaching 100 percent, of fine-grained sericite in comparison to surrounding sericite-poor groundmass sites.

Some FVRF's also contain bipyramidal microphenocrysts of megaquartz (Fig. 4) and rare GMC pseudomorphs after mica and amphibole. In many samples, a high proportion of framework grains occurs only as uniformly GMC-replaced particles with no primary microtextural detail. These grains appear to represent microphenocryst-poor FVRF's based on observations that (1) there is a continuous spectrum in many thin-sections from VRF's with common, through sparse, to those lacking microphenocrysts, (2) most SRF's are chert having distinctive colors and textures, and (3) relict SRF's and MVRF's rarely contain fine sericite.

Although altered FVRF's are the most abundant rock- fragments, a small proportion, generally less than twenty percent, of altered SRF's are present in some sandstones.

Different types of SRF's are distinguished on the basis of 186 color and preserved internal textures. Jasper rock fragments are bright red in slabbed samples and commonly have a we11-developed peloidal texture that shows diagenetic layering (Fig. 5). The well-sorted, equant peloids, generally 0.05 to 0.1 mm in diameter, have a clotted texture and mutually concavo-convex boundaries, similar to the diagenetic microquartz in Precambrian banded iron formation described by Dimroth and Chauvel

(1973) and Dimroth (1979). Some jasper SRF's contain (1) silica pseudomorphs, averaging about 0.05 mm long, after apparent displacive lenticular gypsum, similar to that reported by Cody (1979) and Schreiber et al. (1982) from younger rocks, and (2) silt-sized pyrite euhedra. Other varieties of SRF's include pale-green chert, representing silicified volcanic dust, and black carbonaceous chert, gray translucent chert, and white opaque chert, probably representing ortho- and biochemical sediments (e.g. Lowe,

1982). The SRF types in Panorama sandstones represent much of the spectrum of cherts found as thin interflow sedimentary units within the Warrawoona Group. The

SRF/VRF ratio is significantly higher in many conglomerate units that are interbedded with the sandstones, suggesting that volcanic and pyroclastic source rocks and sediments eroded more readily to yield sand-sized debris than more resistant chert units.

Another type of altered rock fragment that occurs only rarely in a few Panorama sandstones consists of GMC pseudomorphs after subparallel to radiating sheaves of prismatic to bladed crystals surrounded by a recrystallized matrix consisting of GMC plus black oxides

(Fig. 6). This appears to represent a microquench texture, suggesting that the original rock fragments were mafic to ultramafic in composition.

Feldspar.-The former existence of feldspars as framework grains is indicated by the presence of abundant lath­ shaped and tabular pseudomorphs. Pseudomorphs after feldspar consist of blocky sand-sized domains now composed of fine sericite intergrown with lesser amounts, generally less than ten percent, of GMC. The sericite imparts a distinct first-order yellow interference color to these pseudomorphs that contrasts with the first-order gray of sericite-free GMC after other primary phases. Many of the blocky domains clearly represent feldspar euhedra (Fig.

7). It is not possible to determine exactly the original feldspar composition or internal structuring of the original feldspar grains. However, the lath-like shape is similar to that of plagioclase in intermediate and felsic lavas. Petrologic studies (e.g. Barley et al., 1984) indicate that felsic volcanic rocks in the Warrawoona

Group are altered calc-alkaline lavas. The replacement sericite may represent (1) recrystallized clay minerals inherited from feldspar weathering in the source area, (2) clay produced during feldspar degradation prior to

silicification, or (3) clay formed wholly during

alteration, similar to that in many altered Phanerozoic

deposits (e.g. Beaufort and Meunier, 1983).

Ferromagnesian Minerals.-Pseudomorphs after ferromagnesian minerals occur in some samples (Fig. 8). These grains usually have euhedral or subhedral outlines. The most common and easily recognized appear to represent biotite, commonly showing weak brown pleochroism. Some biotite grains show high-order interference colors where not completely replaced by GMC. In rare cases, silicified detrital amphibole can be recognized on the basis of its prismatic habit and characteristic rhombic cross-sectional shape with interfacial angles of 120° and 60°. Fine-grained rutile or black oxides or both commonly form thin rims along the pseudomorph boundaries or occur as disseminated grains within relict ferromagnesian minerals.

Pumice and Shards.- Pseudomorphs after pumice and lunate to crescentic shards are rare in most Panorama sandstones. Lunate-shaped grains, now composed of monocrystalline quartz, were observed in a single volcanic grain. This rock fragment probably represents a detrital tuff fragment or an accretionary lapillus. Similar megaquartz crystals after original vitric shards were reported in early Archean sandstones by Condie et al.

(1970). Other more poorly sorted tuffs, forming graded units in the Coongan Belt and in other areas, are composed

mostly of angular, phenocryst-poor, equant ash, which may

represent blocky pyroclastic material.

Pumiceous-textured grains were observed only rarely in

thin-section. Pumice fragments or their pseudomorphs,

however, may be difficult to detect in highly altered

rocks (Fiske, 1969). Some of the more poorly sorted

sandstones and tuffs from the north flank of the North

Pole Dome and from the top of the section at the Coongan

Belt include sparse undeformed to slightly flattened

silicified grains with ragged edges, in some cases still

preserving cellular texture. These grains, which contrast

markedly with more equant, smaller, and better preserved

lithic fragments, probably represent pumiceous clasts.

Because of the early cementation of Panorama arenites, it

is unlikely that pumice would have been destroyed by

crushing during post-depositional compaction.

Estimate of framework-qrain proportions

Because post-depositional alteration has obliterated many framework-grain boundaries and replaced most primary minerals and glass with GMC and sericite, it was not

possible to apply conventional point-counting techniques, using direct mineral-identification procedures, to modal analyses of these rocks. Nevertheless, a quantitative estimate of framework modes would provide a means by which

Panorama sandstones could be compared to other Archean 190

sandstones. A semi-quantitative procedure for estimating

framework modes was devised based on criteria presented in

the previous section. The relative percentage of quartz,

feldspar, and lithic fragments could be determined only

for those samples, about 50 percent of those examined, in which virtually all microscopic sericite was confined to blocky, sand-sized domains. In these samples, an estimate of the original QFL modal assemblage was determined as

follows:

(1) The relative proportions of quartz (Q), feldspar

(F), and lithic fragments (L) were determined from point- counts of 300 framework grains assuming the following:

(a) meqaquartz remained stable during alteration,

(b) feldspar is represented by blocky, sericite

domains, and

(c) rock fragments are replaced by GMC and generally

preserve some primary textural features.

(2) The relative percentage of SRF's (Ls) in the whole rock, which are represented by distinctive red, black, green, and gray chert fragments that stand out boldly in polished slabs, was visually estimated in slabs using standard comparitor charts. In thin-section, the distinctive color of SRF's is subdued. Light green and gray SRF's in particular can be easily mistaken for aphanitic VRF's in thin-section but are readily distinguishable in slab. 191

(3) The relative percentage of VRF's (Lv), which comprise the bulk of the framework population, was determined as the difference

L - Ls = Lv.

In about half of the samples examined F could not be determined. In these rocks the suspect grains were counted as L, and

L - Ls = L v ' where L v ' equals the mode of VRF's plus any feldspar that was present.

Only those thin-sections in which grain boundaries of rock fragments were discernable, 30 of the 50 thin- sections examined here, were amenable to this semi- quantitative technique. For the remaining twenty, however,

Ls could be determined by visual estimate and a close approximation of Q was determined from 300 point counts of the whole rock per thin section. These rocks show Ls and Q contents similar to those in the better preserved samples.

MODAL RESULTS

Modal results for Panorama arenites amenable to the semi-quantitative technique described above are given in

Table 1 and plotted on a QFL diagram (Fig. 9) Rocks from all localities contain abundant VRF's, with Lv and

Lv' ranging from 58 to 86 and 74 to 100 percent, respectively, and can be classified as volcanic arenites 192

(Folk, 1974). Framework modes of Panorama sandstones and tuffs are virtually identical to Phanerozoic sandstones derived from undissected volcanic arcs (Dickinson et al.,

1983).

Sandstones from the base of the Strelley Pool Chert in the Pilgangoora Syncline are quartz-rich, with Q ranging from 64 to 75 percent of the mode. Quartz enrichment in these units was caused by epiclastic concentration in high-energy environments based on sedimentologic criteria

(Lowe, 1983; Chapter 2, this volume). Similarly, in some sequences of the Panorama Member in the North Pole Dome, quartz content and roundness increase upward and again can be related to sedimentologic control. Tuffs of the

Panorama Member from the Coongan Belt are distinctly depleted in modal quartz, with Q amounting to 1 percent or less, compared to arenites from other localities that have

Q ranging from 1 to 19 percent. Variations in abundance and type of SRF are also observed. SRF's are virtually absent from tuffs from the Coongan belt, but are common, ranging from 0 to 25 percent of the mode, in sandstones from other localities. Jasper SRF's are the dominant SRF at the north flank of the North Pole Dome but black and white SRF's are more abundant at other SRF-bearing localities. 193

PROVENANCE

Based on observed and inferred detrital modes, at

least three source-rock types provided sediment to the

Panorama deposystem. The predominance of FVRF's, presence

of volcanic quartz, and presence of interbedded or

subjacent felsic lavas indicate that felsic volcanic

sources provided the bulk of the framework grains to the

Panorama sandstones. In some areas, such as the North Pole

Dome, abundant volcanic quartz and sparse detrital mica

indicate dacitic to rhyolitic sources. In contrast, rocks

in the Coongan Belt are impoverished in volcanic quartz,

have quartz-free VRF's, and contain some FVRF's with

trachytic textures, suggesting dacitic to andesitic(?)

sources in this area. Many modern intermediate to felsic

volcanic terranes also show variations in compositions of

eruptive rocks (Gill, 1981). These variations do not

require detrital contributions from separate, genetically

unrelated volcanic sources.

The presence of SRF's indicates local sedimentary

sources. The chert types that predominate in the SRF population of individual sections appear to represent

immediately underlying or stratigraphically older cherts

found in the same general area. Cherty metasedimentary rocks in the Warrawoona Group represent air-fall ash and dust, orthochemical precipitates, and biogenic sediments

(Lowe, 1982). Apparently, local uplifts, probably .194

associated with the felsic centers, exposed these sedimentary rocks to erosion.

Although rare, the presence of microquench-textured

MVRF's in a few samples indicates a contribution from mafic to ultramafic sources. This component was probably minor but in light of the overall abundance of mafic rocks in the Warrawoona Group and the extreme chemical instability of these rocks during weathering (Ingersoll,

1984), MVRF's are probably under-represented in Panorama sandstones. These grains also appear again to represent detrius derived from local, volcanic-related uplifts.

Age relationships between granitoid batholiths and greenstone belts are a fundamental and yet unresolved problem in the Pilbara Block and in many other Archean terranes (e.g. Hickman, 1981). Bickle et al. (1983) and

Barley et al. (1984) suggested that there is a petrogenetic relationship between felsic volcanic rocks and some of the granitoid plutons in the eastern Pilbara

Block. There is no evidence in Panorama sandstones, however, for any detrital contributions from granitic sources. Monocrystalline, bipyramidal quartz with characteristic corrosion embayments in the Panorama suite clearly indicates derivation from dacitic to rhyolitic sources. Although original compositions and internal features of detrital feldspar could not be determined, their euhedral shapes are strong evidence for a volcanic 195

source. Additionally, granitic clasts were never observed

in associated conglomerates. This evidence is compatible with the suggestion of Bickle et al. (1983) and Barley et

al. (1984) that some of the Pilbara granitoid units represent magma sources for the felsic volcanic rocks and that, at the time of volcanism, the plutonic rocks were not yet exposed.

The abundance of VRF's suggests that Panorama sandstones represent a first-cycle sand whose origin was strongly controlled by volcanogenic processes. Inferred modal abundances combined with sedimentologic criteria

(DiMarco and Lowe, 1986; Chapters 1 and 2, this volume) indicate that these arenites represent reworked volcaniclastic sediments and air-fall and turbiditic tuffs. Several lines of evidence suggest that much of the felsic detritus, especially in the North Pole Dome, represents reworked volcaniclastic detritus, much of which was probably unconsolidated pyroclastic debris: (1) Ayres

(1982) outlined four criteria that he suggests can be used to identify reworked pyroclastic debris: (a) an heterolithic grain population, (b) moderate rounding of quartz grains, (c) a dominance of lithic over vitric grains, and (d) an increase in quartz in comparison to associated flows. All of these features characterize

Panorama sandstones. (2) Some associated conglomerates in the Panorama Formation consist predominantly of well- rounded pebble- to cobble-sized SRF clasts in a sand-sized

VRF matrix. This association indicates that although

sufficient energy was available to transport gravel-sized

clasts, much of the felsic volcanic detritus was sand

sized when transportation began, suggesting that it

represents volcanic ash. (3) Microphenocrystic to aphanitic textures of FVRF's are similar to those of many modern felsic volcanic ashes (Heiken, 1974). (4) The presence of euhedral grains, including megaquartz and pseudomorphs after feldspar, mica, and amphibole, is commonly taken as evidence for pyroclastic detritus in an otherwise non-pyroclastic sediment (Ross et al., 1928;

Pettijohn et al., 1972). In other areas, such as the

Coongan Belt, Panorama tuffs represent air-fall pyroclastic debris (Chapter 2, this volume). The abundance of blocky, angular to subangular, glassy VRF's and scarcity of pumice in these deposits suggests that they are similar to those produced by hydrovolcanic eruptions

(Heinken, 1974 ) .

Not all the volcaniclastic detritus, however, represents pyroclastic material. In addition to chert clasts representing sedimentary sources, associated conglomerates also include clasts of felsic to intermediate flow rock and intraclasts of silicified volcanic arenite.

The above petrographic results indicate that the source terrane for sandstones of the Panorama Member was lithologically diverse but dominated by felsic to intermediate volcanic rocks. Eroded pyroclastic material predominated in the sedimentary system, but previously lithified volcanic, volcaniclastic, and sedimentary rocks were also exposed to erosion and yielded detrital sediment.

During the accumulation of the Warrawoona Group, the eastern Pilbara Block was largely a low-relief, shallow- water platform upon which mafic volcanic lavas plus minor amounts of felsic volcanic lavas and sediments were deposited (Barley et a l ., 1979; Lowe, 1983; Chapters 1 and

2, this volume). Felsic volcanic complexes provided the only appreciable relief (Barley et al., 1979). Inferred framework modes of Panorama sandstones are clearly compatible with this scenario, but the rare presence of

MVRF's and SRF's suggests that local uplift around felsic volcanic centers was also important.

ALTERATION

Pervasive silicification is an important characteristic of Panorama sandstones and appears to characterize clastic sedimentary rocks in the lower volcanic parts of early Archean greenstone belts as a whole (e.g. Lowe and Knauth, 1977; Lowe, 1982; de Wit et al., 1982; Heinrichs, 1984; Paris et a l ., 1985). Much 198

silicification probably represents an early diagenetic

episode based on a variety of factors including: (1) loose

packing of framework grains, (2) a notable lack within the

silicified sandstones of other minerals characteristic of

low-grade metamorphism, (3) preserved pseudomorphs after

early diagenetic and primary evaporite minerals, both in

the sandstones and associated cherts, and (4) presence of

chert and silicified sandstone clasts in less silicified

interbedded conglomerates. In a few samples, particularly

on the north flank of the North Pole Dome, additional

silicification due to late Cenozoic silcretization is

indicated (e.g. Williamson, 1957; Watts, 1978; Soegaard

and Eriksson, 1985) by (1) scalloped megaquartz grain

boundaries and (2) GMC that overprints an earlier

interstitial chalcedonic phase. Early silicification of

Panorama sandstones, in which originally labile VRF's and

feldspar were replaced by silica and sericite, prevented

the formation of paragenetic assemblages that typically

characterize the alteration of volcaniclastic sandstones

at higher grades of diagenesis and low-grade metamorphism

(e.g. Boles and Coombs, 1977; Iijima, 1978).

Major-element geochemistry of Panorama sandstones is

shown in Table 2. Even least silicified rocks have greater

than 80 percent Si02. Table 3 contains recalculated primary compositions of a typical Panorama framework assemblage based on modal assemblages estimated earlier and reasonable chemical compositions for the unaltered

framework components. These recalculated compositions

compare quite well with modern volcaniclastic sands (Table

3). As shown in Figure 10, the present alteration

assemblage of Panorama sandstones and tuffs is extremely depleted in most major elements and enriched in Si and K on a weight-percent basis relative to the inferred pre­ alteration framework composition.

In many early Archean terranes, widespread regional

silicification and its close association with volcanism strongly suggest that alteration resulted from rock-fluid interaction, possibly related to heat sources associated with magmatic activity (e.g. deWit et al., 1982; Gibson et a l ., 1983; Duchac and Hanor, 1985). Barley (1984) and

Buick and Barnes (1984) have also suggested that shallow hydrothermal systems, similar to those described by

Spooner and Fyfe (1973) and Henley and Ellis (1983), were responsible for most early silicification in the

Warrawoona Group. Common features of younger hydrothermal systems, however, such as hydrothermal breccias and localized alteration aureoles marking fluid ascent paths

(Nairn and Wiradiradja, 1980), have not been identified within early Archean terranes. Other workers have suggested that much silicification occurred in response to low-temperature rock-water interaction (Knauth and Lowe,

1978) . 200

The maximum temperature of alteration in sandstones of the Panorama Member can be constrained by mineralogic evidence. Inclusion-poor quartz framework grains with straight, well-defined boundaries indicate stability during alteration. Quartz is inert in known hydrothermal systems up to about 300°C (Hoagland and Elders, 1978).

Replacement sericite in Panorama sandstones is well- ordered 2:1 dioctahedral mica that contains no expandable fraction (Chapter 4, this volume). In modern hydrothermal systems, illite is present over the range 150°to about 300°

C (Henley and Ellis, 1983). Hower et a l . (1976) indicate that the conversion of smectite to illite is essentially complete at 175°C. These data suggest that the quartz- sericite alteration assemblage in the Panorama sandstones equilibrated with hydrothermal fluids within the range 175° to 300°C. This temperature range probably represents the last stage of equilibration. Although silicification began early, it is not clear whether it persisted through the final stages of alteration.

COMPARISON TO OTHER ARCHEAN SANDSTONES

Silicified volcaniclastic sandstones of early Archean age are known from other parts of the Warrawoona Group

(Barley et al., 1978; Dunlop and Buick, 1981; Buick and

Barnes, 1984) as well as from the Onverwacht Group of

South Africa (Lowe and Knauth, 1977; Lanier and Lowe,

1982; Heinrich, 1983). Greywackes and shales, however, are 201

notably absent in these sequences. Like Panorama

sandstones, other examples from the Warrawoona and

Onverwacht Groups have (1) predominant volcanic lithic modes, reflecting a volcanic provenance, (2) quartz, if present, of volcanic origin, and (3) an absence of debris

representing plutonic and metamorphic source terranes. As

in the case of the Panorama Member, these early Archean

sandstones are intensely silicified and represent predominantly shallow-water deposition.

In contrast, many younger Archean sandstones, including detrital units in the upper, predominantly sedimentary parts of older (3.5 to 3.3 Ga) greenstone belts in the

Pilbara Block (Hickman, 1983) and Barberton greenstone belt (Condie et al., 1970, McClennan, 1984) and in the volcanic portions of the younger, pre-3.0 Ga belts in the

Superior Province of Canada (see Ojakangas, 1985 and

Ayres, 1983 for reviews), differ significantly in modal composition from the older Panorama-type sandstones. Many of these, especially those from the Fig Tree Group and from the Canadian belts, are greywackes, which tend to be richer in quartz and impoverished in lithic fragments compared to the Panorama-type volcaniclastic sandstones

(Fig. 9). The framework modes of Archean greywackes are interpreted to represent a diverse provenance that included volcanic, plutonic, and supracrustal sources

(e.g. McLennan, 1984). The modal differences between these 202

two Archean sandstone types clearly represent differences

in tectonic setting of the source areas. Early Archean

volcaniclastic sandstones, such as those of the Warrawoona

and Onverwacht Groups, were derived from a setting that was volcanically active but otherwise anorogenic, in which

local uplifts, probably related to felsic centers, provided a small admixture of sedimentary and mafic volcanic detritus. In these settings, felsic volcanic activity controlled almost completely the generation of siliciclastic sediment. Without volcanism there would have been little or no deposition of sand. In contrast, the diverse framework modes of Archean greywackes indicate sediment derivation from orogenic source regions, in which large-scale deformation and intrusion resulted in the uplift, exposure, and erosion of a heterogeneous assemblage of source rocks.

CONCLUSIONS

This study has shown that despite pervasive, post- depositional silicification, the modal composition of early Archean sandstones can be effectively interpreted using standard petrographic techniques. Even in highly silicified sandstones, in which most framework grains are overprinted by replacement GMC and sericite, modified point-counting methods can provide useful, semi- quantitative estimates of framework modes. In Panorama sandstones, detrital volcanic quartz remained stable during alteration and is easily recognized. All other

framework constituents were replaced by GMC or sericite or

both, although internal framework-grain textures are generally well preserved. Volcanic rock fragments are recognized and subdivided on the basis of their preserved volcanic textures. Sedimentary rock fragments are recognized and subdivided by their internal texture, color, and presence of pseudomorphs after evaporite minerals. Detrital feldspar is recognized as blocky, sericite-rich domains. Panorama volcanic arenites were derived from an anorogenic, predominantly felsic to intermediate volcanic terrane, in which underlying mafic volcanic and sedimentary source rocks were only locally exposed. Much of the volcaniclastic sand represents reworked pyroclastic debris. No compelling evidence for exposed granitic source rocks was found. Early Archean volcaniclastic sandstones, such as those of the Panorama

Member, have distinctly different modal assemblages than younger Archean sandstones, which were derived from a heterogeneous assemblage of source rocks exposed in orogenic terranes. 204

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FIGURE CAPTIONS

1. Generalized geologic map of the eastern Pilbara

Block. Outcrops of the Coongan Formation shown in black.

2. Photomicrograph of typical Panorama sandstone,

showing replacement of most framework grains and original

cements by GMC (chert). Note preserved megaquartz

framework grains and the difficulty in discerning grain

boundaries of rock fragments. GMC replacing framework

grains tends to have smaller sized domains than that

replacing intergranular minerals. Also present are

preserved megaquartz framework grains (arrows). Crossed

polars. Field of view is 12.8 mm wide.

3. Photomicrograph of sand-sized detrital megaquartz.

Several megaquartz grains are visible showing varying

stages of extinction. Large grain at center shows

bipyramidal habit, crystal faces, and corrosion embayments characteristic of volcanic quartz. Remainder of view is GMC. Crossed polars. Field of view is 5 mm wide.

4. Photomicrograph of a volcanic rock fragment showing characteristic sericite replacement of feldspar microphenocrysts, preserved quartz microphenocryst, and

sericite-poor, GMC-replaced groundmass. Also present in

lower right corner of photo are a few grains of black 217

oxide. Remainder of view is GMC. Crossed polars. Field of

view is 5 mm wide.

5. Photomicrograph of two sand-sized jasper SRF's. Both

are bright red in hand specimen. The opaque grain

contains quartz pseudomorphs after displacive lenticular

gypsum. The lighter grain shows typical peloidal texture

of jasper SRF's. Remainder of photo is GMC plus a few

irregular patches of sericite plus hematite. Plane-

polarized light. Field of view is 12.8 mm wide.

6. Photomicrograph of mafic or ultramafic VRF showing

GMC pseudomorphs after a bladed or lath-like precursor

surrounded by a matrix composed of GMC and black oxides.

Plane-polarized light. Field of view is 5 mm wide.

7. Photomicrograph of sericite pseudomorphs after

feldspar. Note presence of euhedral shapes. Remainder of

photo is predominantly GMC. Crossed polars. Field of view

is 12.8 mm wide.

8. Photomicrograph of detrital mica, probably biotite,

that preserves high-order interference colors (arrows).

Also note detrital quartz (almost extinct) and relict

VRF's. Crossed polars. Field of view is 12.8 mm wide.

9. QFL plot for Panorama sandstones. Filled circles represent data from the North Pole Dome and Coongan Belt

(n = 8). These rocks plot in the field representing 218

Phanerozoic sands derived from undissected arcs

(Dickinson et al.f 1983) Open circles represent

typical Archean graywackes (McLennan, 1984), which plot

in the field of Phanerozoic sands derived from recycled

orogenic provinces (Dickinson et al., 1983). See text

for further details comparing Panorama-type sandstones to

Archean graywackes.

10. Relative comparison of weight percentages of major

oxides of Panorama sandstones using present abundances

and inferred pre-alteration composition calculated from

inferred modal abundances. Figure 220 Pilgangoora Marble Bar Uljjf] m Gre re Group Creek Gorge arwoa Gp Warrawoona Younger eietr Rocks Sedimentary rha Granitoid Archean ih outcrop with f ona Fm. Coongan of Rocks

North Shaw Belt ot Pl Dome Pole North O Shaw PI17&3G Coongan Belt

+ + oun Downs Corunna abe a + + + + + . + + + + Bar Marble + + + + + + + + + + + + + + + + + + + + + ♦♦^▼444444 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 Marble ++ + + + + + + + + + + + + + + + + + + Btlh.1- + Edgar Mount + + + + + + 444444444 444444444 + + + + + + + + + + + + + + + + + + + + + + +**r^-r-r-r-r-++ 444444444 + + + + + + + + + + + + + + + + + + + 44444444 + + + + + + + + + + + 44444444 + + + + + + + + 4

4

4 4 4

4

4 4 4

4 4

4 4 4 4 el Belt Kelly

4 4 4

4 4 4 4 4

4

4 4

4 4 4 4 4

4 4 4 4

4

4 4 4 4 4 4 4 4 221

Figure 2

Figure 3

223

Figure 4

Figure 5 224 Figure 6

Figure 7

Figure 8

Figure 230 70 80 90 RECYCLED 0R0GEN UNDISSECTED ARC Figure Present composition (measured) Log Original framework composition (calculated) I N> 1 1 1 1 ■ 1 C/1 H- NJ >(—■ N> CO •n u> H S OQ

o

z o ■■■■to o 232 233

Table 1: Modal results for Panorama sandstones. Symbols explained in text.

Q F Ls Lv Lv

N.E. Flank NPD

PI -7 -2 28 0 10 62 _ -7 -4 8 3 3 86 - -7-13 7 0 7 86 - -7-14 9 4 3 84 - -7-15 3 - 1 - 96

N. Flank NPD

PI -8 -3 17 0 25 58 _ -8 -7 8 - 1 - 91 -8 -9 6 15 0 79 - -8-14 12 20 2 66 - -8-16 9 28 1 62 - -8-24 13 2 7 78 - -8-26 13 - 3 - 84 -15 -6 3 - 1 - 96 -15 -9 2 - 0 - 98 -15-22 16 - 10 - 74

-26 -2 3 16 1 80 -

S.E. Flank NPD

PI-33 -1 13 7 2 78 — -33 -3 10 - 1 - 89

N.W. End Coongan Belt

PI-17 -4 <1 _ 0 — 99 -17 -5 <1 - 1 - 99 -17 -6 <1 - 0 - 99 -17 -7 <1 - 0 - 99 -30 -3 0 - 0 - 100 -30 -4 <1 - 0 - 99 -30 -5 <1 - 0 - 99 -30 -6 0 - 0 - 100 -30 -7 0 - 0 - 100

Pilgangoora Belt

PI-20 -1 75 _ 2 _ 23 -20 -8 64 - 1 - 34 -20 -9 70 — 5 - 25 Table 2: Major-element contents* of silicified Panorama samples. Those with <90 percent Si02 represent least-altered sandstones.

Sample PI-15-16 PI-16-5 PI-8-7 PI-8-14 Average S. African ^ 2 Archean grawacke Location CB CB NPD NPD BML

Si02 92.25 82.27 90.04 83.35 70.7 a i 2o 3 4.57 11.69 6.59 9.52 10.9

0.32 0.32 0.12 0.25 6.7 Fe2°3T MgO 0.00 0.02 0.07 0.13 4.8

CaO 0.03 0.08 0.02 0.03 2.1

Na20 0.02 0.01 0.04 0.06 1.9 k 2o 1.19 3.23 1.79 2.62 1.7

Ti02 0.13 0.42 0.10 0.16 0.6

0.03 0.28 0.02 0.03 - P2°5

MnO 0.01 0.01 0.03 0.20 -

S 0.04 0.01 0.03 0.20 -

Notes: 1. Data for Panorama sandstones by X-ray fluorescence. 2. Average S. African Archean graywacke from Condie et al. (1970). 3. Locations as follows: CB “ Coongan Belt, NPD = North Pole Dome, BML = Barberton Mountain Land. 235 Table 3: Estimated composition of Panorama sand prior to alteration assuming Q:F:Lv of 10:10:80 and 10:10:80 of Q:F:Lv assuming alteration to prior sand Panorama of composition Estimated 3: Table A12°3 MgO Si02 Fe2°3 FeO k Na20 CaO 2 o i 20:20:60. Fo etjh (1975). Pettijohn ^From ^Recalculated assuming F * oligoclase composition reported in Cox et al. (1979, Appendix 5), Lv ■ Lv 5), (1979, Appendix al.et Cox in reported composition oligoclase * F assuming ^Recalculated dacitic composition reported in Cox et al. (1979, Appendix 2), and Q ■ 100 percent SiO^. 100percent ■ Q and 2), (1979, Appendix al.et inCox reported composition dacitic eacltdcmoiino aoaa ^ Panorama of composition Recalculated sand prior to cementation and alteration and cementation to prior sand 10 F v F Lv F Q Lv F Q 0 0 0 0 0ryltc mixed rhyolitic 60 20 20 80 10 847. 73.50 71.8 68.4 4313.6 14.3 . 4.4 4.2 3.8 1.9 1.8 1.5 . 1.6 1.8 2.6 1.4 1.4 1.1

Modern volcaniclastic sand volcaniclastic Modern 3313.89 13.3 0.56 0.56 4.01 2.34 1.55 .33.10 1.13 61.69 3.89 2.20 2.20 2.20 1.88 2

Polytypes of 2:1 dioctahedral micas in silicified

meta-volcaniclastic sandstones, Warrawoona Group,

Pilbara Block, Western Australia

ABSTRACT

Analysis of polytypism in dioctahedral 2:1 micas in

pervasively silicified meta-volcaniclastic sandstones of

the early Archean Panorama Member of the Coongan

Formation in the Warrawoona Group, Western Australia was undertaken to provide evidence bearing on the post- depositional history of these rocks. Fine-grained

"sericite" aggregates occur as (1) pseudomorphs after

feldspar framework grains and feldspar microphenocrysts

in dacitic rock fragments, and (2) irregular masses within the now silicified sandstone. Height ratios of the

2.80A and 2.58A illite-muscovite peaks for 13 samples

from two widely spaced (35 km) localities were obtained by X-ray diffraction of random powder mounts after the method of Maxwell and Hower (1967). Relative percentage of 2M polytype was determined by comparison to the

2.80A/2.58Aratio of pegmatitic muscovite, assumed to be

100 percent 2M.

Abundance of 2M polytype (2M/2M + 1M) for the 13 samples ranges from 44 to 95 percent. There exists a positive correlation between percentage 2M and degree of

236 silicification: Better silicified samples from the northern locality have higher 2M percentages, ranging from 68 to 95 percent, than those of the less-silicified southern locality, which range from 44 to 80 percent. The wide range in 2M percentages, both regionally and at the outcrop scale, coupled with other geologic and petrographic evidence suggests that the polytypism is a function of hydrothermal alteration and not regionally uniform low-grade metamorphism. The maximum temperature of the alteration fluids at the time of polytype equilibration is estimated to have been 175°to 300°C.

INTRODUCTION

The fundamental structural motif of illite-muscovite crystal structures is the 2:1 dioctahedral layer.

Stacking of the 2:1 layers parallel to the c- crystallographic axis results in the growth of mica crystals. Theoretically, there are six simple ways in which 2:1 layers can be stacked, giving rise to the 1M,

2M , 2M , 20r, 3T, and 6H polytypes (Yoder and Eugster, 1 2 1955). In nature, however, only the 1M (1M and lMd) and

2M polytypes are common among dioctahedral micas (Yoder and Eugster, 1955).

Following the work of Yoder and Eugster (1955), many authors have shown that the 1M polytype prevails at low temperature, progressively reacting to form the 2M polytype at higher temperatures. Hence, dioctahedral mica polytypes are correlated with geologic occurrence: 1M varieties dominate in sedimentary and weathering environments (Velde and Hower, 1963; Bailey et al. 1962) whereas 2M polytypes become dominant in higher temperature magmatic and metamorphic mica associations

(Reynolds, 1963; Maxwell and Hower, 1967; Karpova, 1969).

Smith and Yoder (1956) reported that all true muscovites are of the 2M type.

This report presents results of polytype determinations of 2:1 K-bearing dioctahedral micas from silicified meta-volcaniclastic sandstones of early

Archean age. These rocks are now composed of replacement silica with subordinate dioctahedral mica. The 2:1 polytype distribution present in these rocks can provide constraints for interpreting the post-depositional history of these meta-sandstones.

GEOLOGIC AND PETROGRAPHIC BACKGROUND

Micas analyzed in this study occur in silicified meta-volcaniclastic sandstones of the Panorama Member

(DiMarco and Lowe, 1984, 1986; Chapters 2 and 3, this volume), a regionally widespread unit of silicified volcaniclastic rocks and dacitic lavas near the top of the 3.3 to 3.5 Ga Warrawoona Group (Pidgeon, 1978; Jahn et al., 1981) in the eastern Pilbara Block of Western

Australia. Felsic volcanic and volcaniclastic rocks such as those of this study form volumetrically minor but important stratigraphic units in the predominantly basaltic, 15+km-thick Warrawoona Group.

Several episodes of alteration have affected these rocks since their deposition. Regionally, the rocks have undergone low-grade burial metamorphism, generally of prehnite-pumpellyite to greenschist grade (Hickman and

Lipple, 1978; Barley, 1980). Additionally, much of the

Warrawoona sequence shows the effects of early metasomatic alteration, possibly due to a shallow hydrothermal system developed in conjunction with volcanism (Barley, 1984). Originally porous units such as volcaniclastic sandstones were in particular pervasively altered by hydrothermal fluids. Hence, all fragmental rocks in the Warrawoona Group are silicified or locally carbonated.

The present mineralogic composition of the meta­ sandstones, consisting of replacement granular microcrystalline quartz (GMC) plus subordinate amounts of finely divided, K-bearing, dioctahedral mica, bears little resemblance to original compositions, which can be inferred on the basis of well-preserved internal features of sandstone framework grains (DiMarco and Lowe, 1984;

Chapter 3, this volume). The sandstones were composed of felsic volcanic rock fragments, volcanic quartz, feldspar, sedimentary rock fragments, and mafic- to ultramafic rock fragments. All rock fragment types as well as any original cement and matrix were replaced by

GMC. Strain-free volcanic quartz with sharp grain boundaries remained stable during alteration. Fine­ grained mica, often termed sericite by petrographers, occurs as (1) pseudomorphs after feldspar framework grains and feldspar microphenocrysts in felsic volcanic rock fragments, and (2) irregularly shaped aggregates without pseudomorphous relations within replacement GMC.

The alteration process apparently began early, prior to significant burial and compaction, as evidenced by the lack of tightly packed or deformed framework grains as well as the good preservation of internal framework-grain textures.

Samples analyzed were collected from two localities separated by about 3 5 km. The northern location, on the north flank of the North Pole Dome (Fig. 1), represented by samples prefixed by PI-8, is in general more highly silicified than the more poorly silicified southern locality, on the northwest part of the Coongan Syncline, represented by samples prefixed by PI-30, PI-17, and PI-

18. Samples at each locality represent no more than 100m of stratigraphic height and were collected along traverses no more than 0.5 km wide. 241

ANALYTICAL METHODS

Unweathered rock chips were spalled off from hand samples with a geologic hammer. Chip size was reduced to a maximum size of approximately 0.5 to 1.0 mm with a mechanical pulverizer. Several runs of several seconds duration through the pulverizer were required for complete sample disaggregation. The fine fraction, which contained some rock flour, was removed after each run to minimize reduction of particles to ultrafine (< sizes, which can adversely affect polytype determination

(Maxwell and Hower, 1967). Following pulverization, mica flakes were liberated from crushed rock using an ultrasonic generator. The <5 micrometer (equivalent spherical diameter) size fraction was then obtained by sedimentation through 5 cm for 22 minutes in a 0.1 percent solution of Na3PO^. The resulting suspensions were concentrated centrifugally, and then dried overnight in a 100°C oven.

X-ray diffractograms were obtained for randomly oriented powdered samples using a Phillips APD-3500 diffractometer with Cu K* radiation. Data were collected by step counting (0.02 2 ©interval) for one second with the chart full scale = 16 00 counts. Random powder mounts were prepared using a side-loading Al sample holder. 242

The relative abundances of 2M and 1M mica were determined using the method of Maxwell and Hower (1967) by comparing the ratio of the height of the 2.80A peak, which is unique to 2M polytypes, to that of the 2.58A peak, which is of equal intensity for both polytypes. The

3.74A peak, which is also unique to the 2M polytype and was used by Velde and Hower (1963) for mica polytype determination, could not be used here because of additive interference from a probable kaolinite in a few samples.

It was assumed that pegmatitic muscovite, used as a calibration standard, represents 100 percent 2M polytype as indicated by Smith and Yoder (1956). Reproducibility results based on five replicate analyses of sample PI-17-

9 suggest a maximum standard deviation of ±9 percent of the measured 2.80A peak and a maximum experimental uncertainty in 2.80A/2.58Aratios of ±10 percent.

In order to check that the mechanical pulverizer did not destroy any 2M polytype that may have been present, the diffractogram for pulverized rock flour of sample PI-

30-18 was compared to that of micas released during ultrasonic treatment of physically separated sand-sized fragments of the same sample. The 2.80A/2.58Aratio for both fractions are identical within the estimated experimental error. Hence, there is no evidence that 2M polytypes were destroyed during sample preparation. 243

RESULTS

Heights of the mica 2.58A and 2.80A peaks and the

2.80A/2.58A peak ratios for analyzed samples are listed in Table 1. Note that there is a significant range in the

2.80A/2.58A ratios for the group as a whole (0.09 to

0.21) as well as for subset samples from each locality.

Also, the muscovite control sample, assumed to be 100 percent 2M, has the highest 2.80A/2.58Aratio. In the absence of evidence for destruction of 2M polytypes during sample preparation, these data indicate that a mixture of 1M and 2M polytypes is present in the

Warrawoona samples. Figure 2 shows a comparison of a diffractogram with a high proportion of 2M mica to that with a low 2.80A/2.58Aratio.

Precise quantification of the percentage of 2M polytype present in each sample is difficult to make.

Maxwell and Hower (1967) demonstrated empirically that curves of percentage 2M polytype versus 2.80A/2.58Aratio approach linearity. Figure 3 shows a linear plot relating the 2.80A/2.58A ratios to percentage 2M polytype for micas in this study, assuming that the muscovite control is 100 percent 2M. As noted by Reynolds (1963), such polytype percentage values obtained in this way have little absolute significance because of the uncertainty of the 2M content of the calibration material and the unverified assumption of a linear relation between 2.80A 244 peak height and actual percentage of 2M. Thus, values of percentage 2M polytype obtained in Figure 3 are correctly termed "peak-height ratio percentages." However, for the sake of brevity, they will be termed "percentage 2M polytype" in this report.

Samples from the northern locality are richer in 2M

(68 < percent 2M < 95) than those from the southern location (44 < percent 2M < 80). There is an apparent positive correlation between percentage 2M polytype and degree of silicification as estimated both at the outcrop and in thin-section.

DISCUSSION

Any model for the origin of mica polytypes in these meta-volcaniclastic sandstones must be consistent with

(1) characteristics of the polytype population present,

(2) considerations inferred from the petrographic assemblage of the sandstones, and (3) known or inferred post-depositional events that could account for the presence of the mica. For meta-sandstones in the

Warrawoona Group, the most likely origins for replacement mica are from (1) low-grade burial metamorphism, and (2) hydrothermal alteration.

Because of the wide range in percentage 2M polytype, even as observed at the scale of a single outcrop, an origin during low-grade burial metamorphism is unlikely. Regional prehnite-pumpellyite (200” to 300" C) to greenschist-grade (300° to 400° C) metamorphism would provide a percentage of 2M greater than 80 percent as observed by Maxwell and Hower (1967) in the Belt

Supergroup of Idaho. Theoretically, the great lenghts of time available for mineralogic equilibration during burial metamorphism should also provide a more restricted range of 2M percentages, especially at the outcrop scale.

In addition, the assemblage quartz plus K-bearing dioctahedral mica without zeolites is not compatible with low-grade burial metamorphism of felsic volcanic compositions (Iijima, 1978; Boles and Coombs, 1977).

As suggested by the positive correlation between the percentage 2M polytype and the degree of silicification, it is more likely that the dioctahedral polytype distribution present in these samples was caused by hydrothermal alteration. An origin from weathering is ruled out by the lack of expandable clays in any sample and because of the correlation between the percentage of

2M and the degree of silicification. Unlike regional burial metamorphism, which would be expected to produce uniform mineralogic trends in geologic space, hydrothermal fluids follow fracture systems, lithologic inhomogeneiti’es, and stratigraphic paths of least resistance, often producing complex and tortuous alteration networks (Henley and Ellis, 1983). Geologic 246 evidence for hydrothermal alteration at the Warrawoona outcrops where the samples were collected include the presence of silica- and barite-filled vein networks and higher degrees of silicification in originally more porous units such as sandstones in comparison to more massive lava flows. The pervasive silicification in these rocks is quite similar to silica "self-sealing" of primary porosity observed in many modern hydrothermal systems (Keith et al., 1978). The wide range in 2M percentages observed in the Warrawoona samples, even over a few tens of meters, is compatible with highly complex alteration patterns associated with hydrothermal systems.

A similar range in 2M percentages (38 < 2M percentage <

79) was noted by Reynolds (1963, p. 1105) for Proterozoic rocks from Idaho that were collected from outcrops that are "abundantly silicified and mineralized."

Precise temperature data on the transformation of 1M to 2M dioctahedral micas are unknown because of the sluggish nature of the reaction (Yoder and Eugster,

1955). However, temperature estimates of the 1M-2M transition, combined with thermal information inferred from the observed petrographic assemblage, can constrain the temperature of hydrothermal fluids presumed to have produced the polytype population present in these samples. X-ray diffractograms indicate that no expandable clays are present. This places a lower limit of 247 alteration at 175°C, the approximate point at which the conversion of smectite to illite is complete (Hower et al., 1976). In modern hydrothermal systems, illite is present over the range 150*to >300°C (Henley and Ellis,

1983). Rocks of the Belt Supergroup, which equilibrated at 200* to 300° C according to oxygen-isotope data

(Eslinger and Savin, 1973), contain 80 to 100 percent 2M mica. Weaver et al. (1984, p. 86) suggested that the 1M-

2M trasformation in the southern Appalachians occured between 280° and 360° C. Quartz framework grains in the

Warrawoona sandstones remained physically and chemically stable during the alteration event. Hoagland and Elders

(1978) found that quartz is inert in hydrothermal systems up to about 300*C. A synthesis of these data suggests that hydrothermal alteration and polytype generation in the sandstones of the Panorama Member probably took place at approximately 200* to 300' C.

These temperature estimates probably represent the highest temperature that affected these micas during the alteration episode. Clearly, alteration began early and continued up to the time of low-grade metamorphism. Thus, the data presented here might not reflect the temperature at which the mica began to form but rather the highest tmperature of mica equilibration. 248

SUMMARY

A wide range in percentage of 2M polytype in dioctahedral 2:1 micas is present in pervasively silicified meta-volcaniclastic sandstones of the early

Archean Warrawoona Group, Western Australia. This wide range persists even at the scale of a single outcrop.

These data, in conjunction with geologic and petrographic information, suggest that hydrothermal alteration, not regionally uniform low-grade burial metamorphism, generated thepolytype population of K-bearing dioctahedral micas in these altered rocks. Based on temperature estimates for the 1M-2M transformation and thermal implications of the petrographic assemblage present in these samples, the mica-altering hydrothermal fluids probably attained a temperature of 200°to 300°C. 249

REFERENCES

Bailey, S.W., Hurley, P.M., Fairbairn, H.W., and Pinson,

W.H., 1962, K-Ar dating of sedimentary illite polytypes. Geol. Soc. Amer. Bull., v. 73, p. 1167-1170.

Barley, M.E., 1980, The evolution of Archean calc- alkaline volcanics: a study of the Kelly Greenstone

Belt and McPhee Dome, eastern Pilbara Block, Western

Australia, unpub. Ph.D. thesis, Univ. West. Australia.

Barley, M.E., 1984, Volcanism and hydrothermal alteration, Warrawoona Group, East Pilbara, _in Muhling,

J.R., Groves, D.I., and Blake, T.S., eds., Archaean and

Proterozoic basins of the Pilbara, Western Australia:

Evolution and mineralization potential, p. 23-36.

Boles, J.R. and Coombs, D.S., 1977, Zeolite facies alteration of sandstones in the Southland syncline, New

Zealand. Geol. Soc. Amer. Bull., v. 86, p. 982-1012.

DiMarco, M.J. and Lowe, D.R., 1984, Problems in interpreting framework modes of altered early Archean sandstones: an example from the Pilbara of Western

Australia. Geol. Soc. Amer. Abst. with Prog., v. 16, p.

489.

DiMarco, M.J. and Lowe, D.R., 1986, Widespread shallow- water and subaerial facies of the Panorama Member in the 250

3.3 to 3.5 Ga, predominantly basaltic Warrawoona Group,

Western Australia. Abstracts of the 12th International

Sedimentological Congress, Canberra, Australia, p. 83-84.

Eslinger, E.V. and Savin S.M., 1973, Oxygen isotope geothermometry of the burial metamorphic rocks of the

Precambrian Belt Supergroup, Glacier National Park,

Montana. Geol. Soc. Amer. Bull., v. 84, p. 2549-2560.

Henley, R.W. and Ellis, A.J., 1983, Geothermal systems ancient and modern: a geochemical review. Earth-

Science Rev., v. 19, p. 1-50.

Hickman, A.H. and Lipple, S.L., 1978, Explanatory Notes:

Marble Bar 1:250000 geologic sheet SF50-8, Geol. Surv.

West. Aust.

Hoagland, J.R. and Elders, W.A., 1978, Hydrothermal mineralogy and isotopic geochemistry of Cierro Prieto geothermal field, Mexico, 1. Hydrothermal minaral zonation. Geotherm. Resour. Counc. Trans., v. 2, p.

283-286.

Hower, J., Eslinger, E.V., Hower, M.E., and Perry, E.A. ,

1976, Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence. Geol.

Soc. Amer. Bull., v. 87, p. 725-737.

Iijima, A., 1978, Geological occurrences of zeolites in 251

marine environments, in Sand, L.B., and Mumpton, F.A.,

eds., Natural zeolites: occurrence, properties, use.

Pergammon, Oxford, p. 175-198.

Jahn, B.-M., Glickson, A.Y., Peucat, J.J., and Hickman,

A.H., 1981, REE geochemistry and isotopic data of

Archean silicic volcanics and granitoids from the

Pilbara Block, Western Australia: implications for the

early crustal evolution. Geochim. et Cosmochim. Acta,

v. 45, p. 1633-1652.

Karpova, G.V., 1969, Clay mineral post-sedimentary ranks

in terriginous rocks. Sedimentology, v. 13, p. 5-20.

Keith, T.E.C., White, D.E., and Beeson, M.H., 1978,

Hydrothermal alteration and self-sealing in Y-7 and Y-8

drillholes in northern Geyser Basin, Yellowstone

National Park, Wyoming. U.S. Geol. Surv. Prof. Pap.

1054-A, 26p.

Maxwell, D.T. and Hower, J., 1967, High-grade diagenesis and low-grade metamorphism of illite in the Precambrian

Belt series. Am. Minaral., v. 52, p. 843-857.

Pidgeon, R.T., 1978, 3450-m.y.-old volcanics in the

Archean layered greenstone succession of the Pilbara

Block, Western Australia. Earth and Planet. Sci. Let., v. 37, p. 421-428. 252

Reynolds, R.C., Jr., 1963, Potassium-rubidium ratios and

polymorphism in illites and microclines from the clay

size fraction of Proterozoic carbonate rocks. Geochim.

et Cosmochim. Acta, v. 27, p. 1097-1112.

Smith, J.V. and Yoder, H.S., 1956, Experimental and

theoretical studies of the mica polymorphs. Mineral.

Mag., v. 31, p. 209.

Velde, B. and Hower, J., 1963, Petrologic significance of

illite polymorphism in Paleozoic sedimentary rocks. Am.

Mineral., v. 48, p. 1239-1254.

Weaver and associates, 1984, Shale-slate metamorphism in the southern Appalachians. Elsivier, Amsterdam. 239p.

Yoder, H.S. and Eugster, H.P., 1955, Synthetic and natural muscovites. Geochim. et Cosmochim. Acta, v. 8, p. 225-280. 253

FIGURE CAPTIONS

1. Generalized geologic map of the eastern Pilbara Block showing outcrop extent of the Coongan Formation and locations of samples analyzed.

2. Comparison of sample PI-8-49, with 2.80A/2.58Aratio of 0.19, to sample PI-30-18, with 2.80A/2.58Aratio of

0.09. These ratios correspond to 2M percentages of 85 and

44 percent, respectively.

3. Determination of percentage 2M dioctahedral mica in the mica fraction as a linear function of 2.80A/2.58A ratio for meta-volcanic sandstones of the Warrawoona

Group. Muscovite calibration was assumed equal to 100 percent 2M. Figure 1 255 Pilgangoora Marble & m m og Cek Group pmn Creek Gorge u rha Granitoid Archean arwoa Gp. Warrawoona Younger eietr Rocks Sedimentary ih outcrop with f ona Fm. Coongan of Rocks

North Shaw

ot Pl Dome Pole North o Shaw NORTHERN NORTHERN IOC. Coongan Beit OTEN +4 4 + + 4 4 SOUTHERN

oun Downs Corunna Marble + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + 4 + + + + + + + Mount 4 + + 4 + 4 + 4 4 4 + + + + + + + + + + + 4 4 4 4 4 4 4 4 el Belt Kelly 4 Figure 2 H o o X 2M + 1M 100 0 0.1 .0 / 2.582.80A / ekhih Ratio Peak-height A 0.2 Envelope for Southern Locality Southern for Envelope + Samples from Northern Loc. Northern from Samples + 0 Samples from Southern Loc. Southern from Samples 0 ucvt calibration Muscovite Envelope for Northern Locality Northern for Envelope 0.3

Figure 3 259

4> O

Qtz 110

U> Mica 2.58 A TJ Ui M I ro 00 © I Mica 2.80 A v£> Ot o

tn

O

Qtz 110

Mica 2.58 A

, ^-Mica 2.80 A

ro Ln 260

Table 1: Raw peak-heights for 2.80A and 2.58A peaks, meta-volcaniclastic sandstones, Warrawoona Group. Chart full scale = 1600 cts, peak heigt of 10 approx. = 150 c t s .

Sample 2.80 A 2.58 A 2.80A/2.58

Muscovite control 12.4 60.2 0.21

PI-8-26 not detected 7.7 — 1 -30-18> 1.8 18.7 0.10 -8 -49 4.9 24.7 0.20 -30-13 2.7 30.7 0.09 -18-7 2.0 13.5 0.15 2 -30-18< 1.6 13.6 0.12 -8-4 2.0 12.7 0.16 -8-9 interference 14.0 - 3 -30-18 2.2 24.0 0.09 -8-49 4.8 24.7 0.19 -17-9 8.3 47.5 0.17 -8-15 4.0 22.0 0.18 -8-35 1.8 12.0 0.15 -8-36 2.0 13.9 0.14 -8-59 2.4 12.0 0.20

Notes: 1. Fraction of micas liberated ultrasonically from

sand-sized rock fragments.

2. Fraction of micas present in crushed rock

flour.

3. Mixture of micas released from sand-sized

fragments and from rock flour. APPENDIX 1: Locations of Sections

Section Structural Belt Location

7 N. Pole Dome 21’ 04' 52' S 119' 31' 25' E 8a N. Pole Dome 21 00 51 S 119 28 51 E 8c N. Pole Dome 21 00 42 S 119 28 59 E 15 N. Pole Dome 21 00 41 S 119 28 14 E 17 Coongan Belt 21 22 36 S 119 36 59 E 20 Pilgangoora Belt 21 06 49 S 119 08 18 E 22 N. Pole Dome 21 11 31 S 119 18 13 E 24 N. Pole Dome 21 00 49 S 119 28 43 E 25 N. Pole Dome 21 01 10 S 119 28 42 E 26 N. Pole Dome 21 01 05 S 119 29 16 E 28 N. Pole Dome 21 04 50 S 119 31 41 E 29 N. Pole Dome 21 03 46 S 119 30 03 E 30 Coongan Belt 21 23 12 S 119 35 22 E base 21 22 09 S 119 37 26 E top 31c N. Pole Dome 21 11 37 S 119 18 15 E 32 N. Pole Dome 21 15 25 S 119 29 18 E 33 N. Pole Dome 21 14 33 S 119 30 09 E 34 N. Pole Dome 21 15 53 S 119 28 16 E 35 Kelly Belt 21 33 40 S 119 59 35 E base 21 34 50 S 120 01 20 E top 37 N. Pole Dome 21 00 43 S 119 28 23 E 50 N. Pole Dome 21 01 23 S 119 29 56 E 51 N. Pole Dome 21 02 10 S 119 31 36 E 52 Coongan Belt 21 25 50 S 119 37 52 E base 21 26 16 s 119 39 05 E top 53 N. Pole Dome 21 16 00 s 119 27 37 E 54 N. Pole Dome 21 16 12 s 119 26 18 E 55 N. Pole Dome 21 16 04 s 119 24 53 E 56 Kelly Belt 21 37 24 s 119 59 00 E base 21 37 30 s 120 00 56 E top 58 N. Shaw Belt 21 19 34 s 119 21 02 E 60 N. Shaw Belt 21 22 45 s 119 17 47 E 61 N. Pole Dome 21 16 23 s 119 19 20 E 62 Marble Bar Belt 21 12 21 s 119 45 56 E base 21 11 06 s 119 42 13 E top 63 Marble Bar Belt 21 03 22 s 119 44 53 E base 21 03 51 s 119 41 13 E top 63a Marble Bar Belt 21 04 07 s 119 41 15 E 64 Coongan Belt 21 26 17 s 119 43 17 E 66 Kelly Belt 21 37 52 s 119 59 39 E 70 Kelly Belt 21 28 00 s 120 12 36 E base 21 27 09 s 120 10 39 E top 71 N. Pole Dome 21 21 05 s 119 15 59 E 72 N. Pole Dome 21 21 14 s 119 16 29 E

261 Younger

Archean Granitoid Marble Bar Rocks Marble m Gorge Creek Group Warrawoona Gp. 4 4 4 4 4 4 4 [jX with outcrop 4 4 4 4 4 4 4 4 aof( Pnnnnan Coongan Fm 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 + «*■-*■-*••*■•*■ + + + O ♦ + Mount Edgar Btlh.* North Pole Dome 4 4 4 4 4 4 4 4 4 4444444444 4 4 4 4 4 4 tttTTMarble Bar + + + ++ + + + + + ++ + Pilgangoora 44444444444 4444444444 4444444444 4 4 4 4 4 4 4 4 4 444444444 44444444 4 4 4 4 4 4 4 4 + + + + + + 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 S 4 4'4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 444 4 4 44 4 4 4 4 44 4 4 4 4 4 4 4 4 4 4 4 4 4 44 j North 4 4 4 4 4 4 4 4 4 4 Corunna Downs 4 4 4 4 I Shaw + + Shaw Btlh. + + 4 4 4 + + + + + + + + + 4 4 4 4 l + + + + + + + + 4 4 44 * + + + + + + + + 4 44 4 W j 4 4 4 4 4 4 4 4 + 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 + “ 4 4 4 44 4 + KM + 4 44 4 4 + + I------1------1 + + 4 4 4 4 + + 0 10 20 + 4 44 4 + + + Coongan 4 4W94 + + + + + + + + + + 44 4 4 i*- + + + + + + + + + + APPENDIX 2: Detailed Measured Sections

A total of thirty-seven sections, representing an approximate aggregate thickness of 23,500 m, of those parts of the Coongan Formation that include the Duffer or

Panorama Members or both was measured during this study.

Locations of sections are given in Appendix 1. For those sections in which the Panorama and Duffer Members are both present,' the contact between the two is indicated.

Individual sections are presented in numerical order and are preceeded by a legend to the sections.

263 Thickness in meters unless indicated otherwise Panorama Mbr. Panorama Other symbols used in section graphics section in used symbols Other ‘ c v r \ : r vf sand or less or sand vf m sand m sand f sand c Barite vein Barite hae rock Sheared ais H Facies Facies G Facies F Facies ais E Facies Facies D Facies ais C Facies Facies B Facies A Facies or bed or sand

EEDT ESRD SECTIONS MEASURED TO LEGEND

• Mxmmpril ie (cm) size =• particle Maximum 5 45 e m co H •H H CD•H ■u CJ*H »j ^ ai T C co co cfl C o 0 o; u (U (U B S £j telyPo Chert Pool Strelley P£ j CO r T > 43 43 •H —1 G g 0 o h C

xfi-i-l sample of = Location Duffer or Panorama Mbrs. Panorama or Duffer te nt o pr o the of part not units Other Duffer Mbr. Duffer • lrmfc nrsv rock intrusive Ultramafic xSZSfcS? rntc rock Granitic Chert Bar Marble — • • ■ . / V. T ’> • • •■. - / ■ & s e a r- — " n ‘. ■ <•

Facies D Facies Facies D Facies Sheared rock Sheared ais C Facies mfc lava) (mafic Facies C Facies fli lava) (felsic ais B Facies Facies A Facies Facies A Facies (Tuff) (Tuff-breccia) (sheared)

264 265

Sec. 7

b rT •gg •<*. ’fv_

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[fls, x r»-7-io

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6 PC Sec. 8c

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Sec. 15

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Pi- I6-S7 l7o 2)0 -

P i - I S - 6 Z . Pv-ic-ii j• i_*oO** ^ .*3

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\gt> tfo 270

Sec. 15 27o 351 »■

*■ PI-IS--JI

* Pi- iS-41

x f i -\S- 4 0

„ Pi-iS-45

Zfco

0 < ? 5 H

K pl-lS-lto

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Pl-is- U.

2

4 4 0

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V Pl-iS- 14 271

Sec. 17

ts

to

* pi-n-fe

x Pt-n-s

r n-n-j

x P'-n-z

» x Pl-I7-'C>

ypi-n-i x p\-r?-n

o 272

Sec. 20

Laminite of SPC, member 2 (Lowe, 1983) l o - y Pl-Zo-7

Quartz-rich sandstone of SPC, * P l - Z o - 8 member 1 (Lowe, 1983) y „ ?i-!o -u> A o o Q n 1 1 n I I I I

cn (U O

N>

M 'O CO 274 275

Sec. 24

A

Highly silicified

zn and sheared rock

I sz

70 276

Sec. 25

*c Pi-Z5-U

< Pl-rS-3

* Pi-JS - S' .) ^ Cj d Zo

163

ys x P 1 - 2 S - Z

170

x PI-2S-I

Mostly covered but showing a few exposed units similar to those in basal third of secs. 15 and 24.

o 277

Sec. 26

6c>

3 O

To 278

Sec. 28 4o

20

-» r I -A

A <

to -

< T A <

A

I I I" I •fc. CP > tillM 280

Sec. 30 SPC.

see next page Panorama Mbr.

Duffer Mbr. x Pi - 3o-*i x Pi -3o-« *7* P’l-'ao-ie Pi-3o-i)

.oTo<

0 0 9

Typical layering 3o2>.

3 h> ^ <

, Pi-3o--M „ Pi-3o - Z1

Pi-J ©-«* 281

Sec. 30 Upper 100 m

xp/- io -/f

y p/-t o -/I x Pi-io-tj

5

x p /-3 0 -/7

So x PhSo - 31 X rt-3o-3* X Pl-fOSV

10

x-pi-io-nf

XU-3 •-/*>

2 5 > fi-So-t5

5

X ?/-tc> -J b

o 282

Sec. 30 Upper 100 m (cont'd)

SPCj 93

8S ■

75 -

* p \-y o -v z-

*. VI- So - U

11 i i I 283

Sec. 31c

Panorama and Duffer Mbrs, not present

SfZ log x Pl--Me - >

* PI-3U- Z

x Pi-3lc-5

36 -

37

35 -

33 - ^ W PI-JlC-S" 3> -

2

■Z7 284

Sec. 32 ■20% -i

-

IfeS

jasper HS * pi-j-z--*-

light-colored translucent and finely laminated chert

T - n 285

207 Sec. 33

\o

t /5 i*> I Z97 Debris flotf-roll of laminated siltstone at channel edge. Channel filled A P l - > S '7 by debris-flow deposit. jePi - -t M

X p.- H k x Pl-S'J-fc

> PI-3J-Z2.

X Pl-JS-ZI

--- 1 ,I V\I I' I x pi-*3-z«

2 9 7 numerous faults in this chert sequence 286

Sec. 33

rPI-3> :?i -yi-J

xPl-J?-"

xP*-3V-yo

r Pl-73-1

x P1-53-S

. pv- 3J-2-S

X Pi-73-I < ?i-3»-2fc

X P>-73-7.

Typical cycles above 337 m.

\ nn

2-t, x Pl-37‘27 6 ->0

• . -j'J; :.‘ir \ * * .* *• 11 1 i - n f I t « i * "h' ii ' i jw 3 V

X

N5 00 VJ N3 00 00 289

Sec. 35

A A PI-55-12.

Pi-i s -2.4 PI-3S- >3

A.

a.

< ? <7 <7

o : ^

Pi-iS-Zi x ft-JS- Ji x />/-35-32. x Pi-^-io X Pl'3S-Z. o * ft e. 37 Sec.

\oro o 291

Sec. 37

!»(•

3

-

i>2- .

i^O ys

i3! -3 7 t 292

S PC ( 120 Sec. 50

7 — 7=7 « f \ . S O ~ e *.*. ‘.'/irrj

See next page 293

Sec. 50 "t\ (basal part)

89

x Pi-50-4,

jasper band

pod of trans^ lucent chert'

I o - finely laminated light gray chert

< Pi-50-8 294

Sec. 51

Base of Pan. Mbr.

/

AD 295

Sec. 52 m At Pf9Z-/» Pan. Mbr. ■f Duffer Mbr SZ

I

S 3©

30

y P t - S Z - Z . x pi-sz -i ' t ~■ ‘0 V > r PI-S2-3

xPh91-3

* ' *£>•-° A ‘ ■» x»,- » „» P / - * 2 - 7 " / ■

xfl'-SZ.-fc See next page 296

Sec. 52 Sec.52 about 500 m directly under SPC

SPC I

Inverse grading of probable relict pumice io 297

Sec. 53

Bands of jasper and S P C - 1 X PI S i - 2. white chert. *.

A o

Tabular white chert clasts in black chert matrix.

<23

2 b

5 298

Sec. 54

l?o s ?c

K pi-94-4

Pods of translucent chert.

9 o - Finely laminated black and gray chert. Banded jasper and white chert. * 4 / 60 s i / S t / S t / w s i S 4o s t S v Pi-94-1

S'

t o ' 299

Sec. 55

Pi- 55 -s"

f-

pi-e^-4

Base of Panorama Mbr.

Pl-Sff-Z

z'. 300

Sec. 56

7C

4Z

•93 ZO iZ > P l - f f b ' b 10

/ 7

Z1

Panorama Mbr . <> ‘ Duffer Mbr

Panorama Mbr.Mbr (see next page)

Duffer Mbr

r a o

• •< 25

o 301

Sec. 56 at about 800 m

J 5

x P |-5 6 - 7

:.v j

~ T i ~ M

g s ^

i fri > Jfe 302

Sec. 58

<,£ “I Mftc

C-S ■ />hse~ t

c-j. f>hS9> *4 Panorama Mbr. <— Duffer Mbr.

O l ­

a-2

0 1-

o - Kn. 303

Sec. 60

me>c.

xfl-fcO-'

Panorama Mbr. ^Duffer Mbr.

go

o CM

M m . Sec. 61

g e loo dp

H E K 5 2 i S

k f\-<>\-y

K f l- 61 - b

„ Pl-fcl-ff 1°

bo -

-22^ IS v n- fci--* „ Pl-bi-3 31

« PI-W-l 305

Sec. 62

mac

A fit-bt-r- pt-ei- s x k fl-bt- 'S

> Pt-tz-j

X Pt-bZ-G

A Pl-bZ-b

* Pl-bZ - 7

X Phbt-8 x i7/-^ - 7

x Pl-tZ-'O 306

Sec. 63

reen chert «• pl-63-ifc

K Cl'H

0 , (& • measured section of next few pages

K Pt-4»-'"7 >< Pi-C.-3-lfc

0-1 ir

* Pl-t.-5-tO Panorama Mbr. <------Duffer Mbr.

About 4 to 5 km of felsic lava, vol- caniclastic breccia, and Pi-c 7 • tuff-breccia. 4 - S’ Kir) 307 308

Sec. 63

■>

/so probable lO HCS

c m

a o

0 :

8 0 309

Sec. 63a 4o 62

6c x — welded tuff

gas- escape/"?) cavities ",

t o

so

flattened''^ pumice ? 310

Sec. 64

o\e£^ti

3< U ' 0 . ; */7-fcV-/ A PI-47<-2 311

Sec. 66 A sequence of Panorama Mbr. in upper part of Duffer Mbr. Correlative to similar sequence within Sec. 56. I-OL-IJ *

•V>'o

:0 * f,-OL-U < c> ^ .«=> ( - OL-M X ' o > > V s , 0 7-Oi.-lc> x

V > -* v > v > 1/

S-ol-td X 3-^7- - Oi - Id * - / - •' \ “T = ■>

0L ’33S

Zl£ 313

Sec. 71 Sec. 72

>2 -\ SPC ) I * rc- 1

o ~_____ / t r i ■ i

I I I I I APPENDIX 3: Paleocurrent Data

Paleocurrent information was measured on a variety of structure and texture elements in order to help define facies and environments of deposition as well as determine paleo-sediment-dispersal paths in the Coongan Formation.

Gathering of this data was hindered because (1) the bulk of the formation comprises the Duffer Member, which is composed predominantly of debris-flow deposits with random clast orientation and (2) alteration, especially silicification, made paleocurrent measurement difficult, especially in the Panorama Member. Additionally, much of the current information in the Panorama Member is the result of shallow subaqueous processes, hence could not be used to infer sediment-dispersal routes. Raw paleocurrent data are given in tabular format on the following pages.

314 Section and Data element and Attitude 315 section attitude location in section element

7: N30W, 42NE Dip of trough N, 51E

cross bedding in N2W, 53E

upper 20 m of N15W, 65E

section in N6E, 59E

fluvial deposits N58W, 38N

N9E, 79E

N42W, 66E N36W, 36N N20W, 59E

N22W, 52E N22E, 81E N25W, 55E

N20W, 57E N6E, 71E

15: W, 59N Dip of low angle E, 72N

cross bedding

in turbidites

above basal

chert

15: N87E, 47N Dip of trough and N80W, 65N

planar cross bedding N65W, 7 ON

in upper parts of N70E, 65N

section N84E, 80N

N57W, 75N

S71W, 72N E, 81N N86E, 74N

S78W, 64N N87E, 80N N82E, 81N

N82E, 62N N70E, 78N N84W, 75N

N89E, 75N N89E, 79N N70E, 60N

N89E, 65N N83W, 75N N80Wr 69N

N81W, 64N N60W, 75N N84W, 70N Section and Data element and Attitude of section attitude location in section element

N70W, 71N S82W, 70N N64W, 44N

N72E, 64N N88W, 71N N75E, 68N

N82E, 62N N85W, 66N N81W, 7 ON

N80W, 83N N84W, 74N N85W, 77N

N88W, 76N N75Wr 71N N81W, 82N

N78W, 74N N65W, 74N N85E, 66N

N76W, 67N N80E, 72N N81E, 71N

N87E, 80N N80E, 65N N85W, 75N

N89E, 74N

tf, 67N Imbrication of N46W, 55E

clasts in clast- N22W, 85E

supported conglom­ N32W, 50E

erate near top of N15W, 60E

section N5E, 52E

N23E, 65S

N44W, 46N

N80W, 46N N35W, 79E N54W, 6 ON

N18E, 42SE N24W, 69F, NllW, 23E

N5Wf 47E N70W, 80N N32W, 56E

N34W, 57E N20W, 40E N21W, 46E

25: N67W, 67N Imbrication of N30W, 46E

clasts N18E, 63W

N63W, 58E 317 Section and Data element and Attitude of section attitude location in section element

N40W, 40E N12W, 68E N20W, 48E

N34W, 41E N50E, 35S N52W, 52E

N36W, 39E N49W, 73N N79W, 53N

N45E, 68SE N36E, 38NW

30: N30W, 76SW Imbrication of N50W, 75SW

overturned clasts in alluvial N39W, 72SWS

conglomerate at N44W, 84NE

top of Duffer Member N60W, 69SW

N50W, 85SW

N79W, 64NE N75W, 78NE N38W, 66SW

N79W, 64NE N29W, 87SW N52W, 77SW

N38W, 66SW N57W, 66SW N41W, 87NE

N46W, 70NE N82W, 67NE N48W, 89NE

N40W, 90 N34W, 87SW N42W, 82SW

N46W, 66NE N26W, 75SW Nl9Wr 64SW

N46W, 75NE N39W, 78NE N35W, 78SW

N26W, 82SW N45W, 85SW N49W, 66SW

N50W, 64SW N34W, 63SW N34W, 85SW

N40W, 85SW N49W, 82SW N50W, 78SW

N35W, 62SW N15W, 71NE N50W, 90

N60W, 75SW N44W, 50SW N44W, 75NE

N42W, 54SW Section and Data element and Attitude of section attitude location in section element

, 45S Dip of cross N65W, 54S

laminations in N72W, 60S

turbidites and N56E, 56S

siltstones in lower N71W, 55S

part of the clastic N65W, 50S

sequence N78E, 55S

N78E, 51S

N80W, 55S

N64W, 40S

N78W, 48S

N75W, 53S

, 45S Dip of cross N65E, 50SE

laminations in N75E, 65SE

sandstones at top N65E, 61SE

of section N48E, 56S

N58E, 54S

N87W, 55S

N78E, 50SE N58E, 58E N72E, 60S

N84E, 51SE N50E, 56SE N35E, 42SE

N79W, 48S N80W, 45S N72W, 42S

N55E, 58S N45W, 50S N49E, 55S

N50E, 50S Section and Data element and Attitude of section attitude location in section element

34: N80E, 42S Imbrication of clasts N52W, 45S

in conglomerate at N45W, 53SW

68m mark of section N45W, 65SW

N86E, 64S

N87E, 45S

N73W, 38S N89E, 60S N72E, 26S

N51W, 21SW N24E, 51W W, 74S

N57E, 56S N52E, 58S N65W, 65S

N19W, 53E N27W, 40E N84E, 86N

N76E, 52S N89E, 59S N87E, 38S

N51W, 51SW

34S Imbrication

conglomerate near 88 N65W, 62S

m mark of section N76E, 55S

N80W, 59S

W, 50S

N10W, 72E N78E, 55N N69W, 60S

W, 78S NlOE, 70W N75E, 46S

N22W, 56E N16E, 57E N44E, 52S

N68E, 51S N60W, 60S N89E, 87N

N17W, 89E N24E, 54S N19E, 56S

N87W, 75S N89E, 59S N44W, 66S

N15W, 90 N56W, 60S N55W, 36N

N84W, 60S N70W, 66S N1E, 66E

N89E, 42S N72W, 66S N65W, 43S 320 Section and Data element and Attitude of section attitude location in section element

N64W, 50S N75W, 86S N89W, 74S

N72W, 54S

34: N80E, 42S Dip of cross N76W, 42S

lamination in wave- N61W, 45S

molded sandstone N86W, 55S

near top of section N56E, 29S

N65E, 13N

W, 57S

N85E, 35S N65W, 7W N76W, 42S

N70E, 54S N39W, 30S N80W, 50S

N7 5E, 15S N68W, 40W

34: N80E, 42S Dip of micro N54E, 30N

cross lamination N72W, 58SW

in siltstones N55E, 52S

throughout the N81E, 55S

section N75E, 50S

N89W, 55S

N55W, 54S

N50W, 35SW N76W, 50S N70E, 55S

N52W, 45S N52W, 45S N74E, 49S

N80W, 50S N81E, 56S Section and Data element and Attitude < section attitude location in section element

37: N80E, 58N Dip of planar N80E, 65N

cross beds near N70E, 62N

top of section N82E, 72N

N82E, 62N

N80E, 66N

N67E, 65N N81E, 64N N76W, 63N

N78W, 54N N82E, 64N W, 58N

50: N79W, 60N Dip of ripple N68W, 68N

cross laminations N65W, 66N

of Tc division of N65W, 68N

turbidites inter­ N62W, 72N

bedded with cherts

at base of section

52: N13W, 80S Dip of cross N35W, 75W

overturned stratifications N40W, 78W

in sandstones at • N24W, 87W

top of section N50W, 90

53: N10E, 34E Dip of cross N21E, 66E

stratifications N6E, 56E

in sandstones Nl3E, 60E

at top of section N5E, 60E

N10W, 56E

N19E, 56E Section and Data element and Attitude of section attitude location in section element

N10E, 52E N5E, 46E N12W, 61E

N48E, 54S N15W, 55E N24W, 57E

N58E, 48S N44E, 57S N54E, 45S

53: N89E f 52S Dip of cross N74E, 65S

laminations in N72E, 66S

siltstones near N36E, 65S

base of section N65E, 60S

N72W, 45SW

N50W, 51SW

N68W, 59S N61W, 49S N4 5E, 63S

N87E, 46S N54E, 34S

55: N66E , 45SE Dip of cross N48E, 56SE

stratification N51E, 61SE

in sandstones N38E, 42SE

near top of section N24E, 64E

56: N9E, 89W Dip of large- and N39E, 79W

small-scale cross N32E, 82W

stratifications in N22E, 80W

sandstone units at N, 84W

about the 800m mark N18E, 90

of section. Data N30E, 76W

with an asterisk N1W, 84W*

taken from bipolar N20E, 90* 323

Section and Data element and Attitude of section attitude location in section element

oriented small-scale N26E, 77W

sets N24E, 89W

N14E, 87E

N20E, 87W

N22E, 90 N2E, 80W* N15E, 78W*

N21E, 72W* N7W, 70W* N34E, 90*

N5W, 76W* N5E, 69W NlW, 70W

N24E, 78N N25E, 77W N32E, 75W

N4E, 76E N20E, 87W N3 6E, 90

N42E, 72W N42E, 89E N13E, 82E

N, 73E N, 73E N36E, 90

N30E, 75W N2E, 80W

61: N88E, 65S Dip of cross N59E, 65S

stratification in N67W, 65S

sandstones and pebbly N60E, 80S

sandstones at top N60E, 78S

of section N62E, 75S

N81E, 90

62: N32W, 75NE Attitude of tabular N50W, 90

overturned rip-up clasts N42W, 89S

in Ta division of N54W, 69NE

turbidites near top N62W, 82NE

of section N32W, 65NE

N35W, 86NE VITA

Michael Joseph DiMarco was born on April 26, 1957, to

Vincent and June DiMarco, in Buffalo, N.Y. He attended

Canisius High School, where he graduated in 1975. He graduated with a B.A. in Geology from the State University of New York at Buffalo in 1979. He earned a M.S. in

Geology from Kansas State University in 1983, where his thesis topic was the petrology of some Oligocene volcanic rocks, in the Wet Mountains, Colorado. At L.S.U., Michael

DiMarco undertook a dissertation project on the sedimentology of early Archean felsic volcaniclastic rocks in Western Australia. In addition to his Archean research, he published on sedimentology and mineralogy of several

Holocene deposits of the Mississippi Delta while at L.S.U.

Upon the completion of his Ph.D., Mr. DiMarco will be employed by Shell Oil Company.

324 DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Michael J. DiMarco

Major Field: Geology

Title of Dissertation: Stratigraphy, sedimentology, and sedimentary petrology of the early Archean Coongan Formation, Warrawoona Group, eastern Pilbara Block, Western Australia

Approved:

Chairman

D e a n of the Graduate

EXAMINING COMMITTEE:

Date of Examination:

September 17, 1986