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1986 Sedimentology and Stratigraphy of the Fig Tree Group, West Limb of the Onverwacht Anticline, Barberton Greenstone Belt, South Africa. Bruce William Nocita Louisiana State University and Agricultural & Mechanical College

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Recommended Citation Nocita, Bruce William, "Sedimentology and Stratigraphy of the Fig Tree Group, West Limb of the Onverwacht Anticline, Barberton Greenstone Belt, South Africa." (1986). LSU Historical Dissertations and Theses. 4315. https://digitalcommons.lsu.edu/gradschool_disstheses/4315

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8710578

Nocita, Bruce William

SEDIMENTOLOGY AND STRATIGRAPHY OF THE FIG TREE GROUP, WEST LIMB OF THE ONVERWACHT ANTICLINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

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

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University Microfilms International

SEDIMENTOLOGY AND STRATIGRAPHY OF THE FIG TREE GROUP, WEST LIMB OF THE ONVERWACHT ANTICLINE, BARBERTON GREENSTONE BELT, SOUTH AFRICA

A Dissertation

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

in

The Department of Geology

by Bruce W. Nocita B.S., Colorado State University, 1974 M.S., San Diego State University, 1977 December, 1986 ACKNOWLEDGEMENTS

I would like to thank Dr. Donald R. Lowe for serving as committee chairman and assisting in the completion of this project. Drs. Gary

Byerly, Jeff Hanor, Dag Nummedal and Jeff Nunn reviewed the entire manuscript and provided many important and useful suggestions for improvement. Barbara Ransom, Michael DiMarco, Patti P hillips, Gail

Fisher, Danny Worrell, Amitava Roy, Maud Walsh, Kathy Duchac and Sam

Upchurch all read portions of the manuscript and made helpful suggestions; to each of them goes my heartfelt appreciation. I would also like to thank Elizabeth Chinn, John Eldridge, and Ezat Heydari for their assistance in the completion of this project.

My deepest appreciation goes to Barbara Ransom who helped me through much of the field work for this project and who provided needed encouragement and support.

The final stages of completion of this project involved many long and sometimes frustrating hours of work. Throughout this time Heather

Lane provided help and encouragement and took care of a lot of l i t t l e things that would otherwise have occupied my time.

This research was supported in part by NSF grants EAR 82-06015 to

Lowe and EAR 79-19907 to Byerly and Lowe, and an ARCO fellowship to the author. TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS i i

LIST OF TABLES ...... v

LIST OF FIG U R E S...... vi

ABSTRACT...... viii

INTRODUCTION ...... 1

REGIONAL GEOLOGY ...... 3

P l a t e ...... 7

STRATIGRAPHY...... 8

STRUCTURE...... * 14

PETROGRAPHY...... 18

LITHOFACIES AND ENVIRONMENTSOF DEPOSITION...... 24

Formations and Lithofacies ...... 24

Lithofacies A j ...... 26

Lithofacies A 2 ...... 30

Depositional Model -Formation A ...... 35

Lithofacies B ^ ...... 3 6

Lithofacies B 2 ...... 41

Lithofacies B ^ ...... 49

Lithofacies ...... 50

Lithofacies C ^ ...... 53

Fan Delta M odel ...... 74

Lithofacies C2 ...... 83

DIAGENESIS AND METAMORPHISM...... 92

GEOCHEMISTRY...... 99

i i i TABLE OF CONTENTS (Continued)

Page

DISCUSSION AND CONCLUSIONS...... I ll

BIBLIOGRAPHY...... 126

APPENDIX...... 137

Stratigraphic Section Localities ...... 138

Stratigraphic Section Descriptions ...... 140

VITA ...... 169

iv LIST OF TABLES

Table Page

1. Major lithostratigraphic units and lithofacies . . . 10

2. Summary of point count data ...... 19

3. Characteristic features of Lithofacies ...... 56

4. Clast composition for pebbles and cobbles of Lithofacies C ^ ...... 59

5. Comparison of alluvial-fan/fan-delta, submarine-fan, and Fig Tree conglom erates ...... 67

6. Characteristics of different fan-delta models. . . . 81

7. Chemical analyses ...... 101

8. X-ray diffraction analyses ...... 102

9. Comparitive abundances of Cr and N i ...... 106

10. Comparitive major element analyses ...... 108

v LIST OF FIGURES

Figure Page

1. Generalized geologic map showing study area .... 4

2. Generalized stratigraphic column of the Swaziland Supergroup ...... 5

3. Generalized stratigraphic column of the Fig Tree Group, this s t u d y ...... 11

4. Comparison of Fig Tree of this study with that of Heinrichs ...... 12

5. QFL and QmFLT plots for s a n d s to n e s ...... 21

6. Photomicrograph of volcanic quartz grain ...... 23

7. Stratigraphic column of basal chert of Fig Tree . . . 25

8. Intraformational, white-chert clast breccia .... 29

9. a. Microphenocrystic ash grain ...... 32

b. Relict volcanic shard ...... 32

10. Stratigraphic column of Lithofacies ...... 33

11. Stratigraphic column of Formation B ...... 37

12. a. Starved ripples in Lithofacies Bj ...... 39

b. Convoluted laminations in Lithofacies .... 39

13. Scour casts on base of sandstone bed ...... 44

14. a. Chert-clast breccia of Lithofacies B £ ...... 45

b. Scour and f il l in Lithofacies ...... 453

15. Stratigraphic column of breccia/sandstone unit of Lithofacies ...... 46

16. Pebbles zone with crude inverse grading ...... 48

17. Barite localities ...... 51

18. Lateral variation of the terrigenous clastic sequence . 54

vi LIST OF FIGURES (c o n t.)

Figure Page

19. Idealized stratigraphic column of the upper structural z o n e ...... 55

20. Typical clast-supported conglomerate ...... 58

21. a. Low angle planar cross-stratification in Lithofacies ...... 60

b. Horizontal stratification in Lithofacies Cj . . . 60

22. Rose diagrams of paleocurrents ...... 62

23. Longitudinal bar deposits ...... 64

24. Partial stratigraphic column of the conglomeratic sequence ...... 65

25. Fining upward cycles ...... 69

26. Distribution of subaerial and subaqueous sequences . . 76

27. Stratigraphic columns representing different portions of the Fig Tree f a n - d e l t a ...... 80

28. Paleogeographic reconstruction of Fig Tree fan-delta . 82

29. Generalized stratigraphic column of Lithofacies C2 . . 85

30. Photomicrograph of pumiceous grain ...... 87

31. Ripple marks ...... 87

32. Partial stratigraphic column of Lithofacies C2 . . . 89

33. Bedding sag in volearn'dastic sequence ...... 90

34. a. Relict sweeping radial extinction in chert cement . 97

b. Multiple generations of silica cement ...... 97

35. Plots of Ba vs. Al^Ogj Ba vs. 1^0; and Ba vs. Sr. . . 104

36. Sedimentological and tectonic evolution ...... 116

37. Thrust sheets showing different internal geometries. . 118

38. Possible style of thrusting ...... 120

v i i ABSTRACT

Sedimentary rocks of the 3.2-3.2 Ga old Fig Tree Group in the southwest part of the Barberton Greenstone Belt in South Africa can be divided into three informal formations. The lowest unit, Formation A, is composed mostly of silicified pyroclastic debris and carbonaceous cherts. Formation B has four lithofacies - siltstone; sandstone; jaspilite; and bedded barite. Two coarse-grained lithofacies make up

Formation C. In the western portion of the study area there is chert-clast conglomerate and in the eastern part of the study area is volcanic-clast conglomerate, breccia, and sandstone.

In the western portion of the study area Formations B and C form a coarsening-upward sequence of siltstone, sandstone, and conglomerate that is interpreted to represent a fan-delta and associated low-energy basin. Three facies define the fan-delta: 1) a subaerial, delta-plain facies consisting of alluvial conglomerate and sandstone in a coarsening-upward sequence; 2) a delta-front facies consisting of subaqueous, sediment-gravity-flow-deposited conglomerate and sandstone derived from an alluvial/fluvial system that prograded into a low-energy body of water, and; 3) a subaqueous-basin facies consisting of laminated siltstone and thin beds of sediment-gravity-flow-deposited sandstone, conglomerate, and breccia. The pyroclastic and volcaniclastic rocks found in the eastern portion of the study area formed in response to dacitic volcanism which occurred during Fig Tree time.

Rocks of the Fig Tree Group have been subjected to diagenetic alteration and low-grade metamorphism which has resulted mainly in

v i i i silicification, along with the production of phyllosilicate. This has produced rocks that are now composed mainly of chert and sericite.

Provenance studies indicate that the source for the Fig Tree fan- delta included uplifted portions of the underlying greenstone belt sequence and penecontemporaneous Fig Tree volcanic rocks.

The tectonic evolution of the Barberton Greenstone Belt from latest Onverwacht through Fig Tree times can be summarized in three stages: 1) a predominantly volcanic, anorogenic stage; 2) a stage of evolving tectonic instability, and; 3) a final orogenic stage in which there was thrust faulting and erosion of both intra- and extra-belt rocks. INTRODUCTION

The Fig Tree Group of the Swaziland Supergroup in the Barberton

Mountain Land, South Africa, represents the first major accumulation of terrigenous debris in the geologic evolution of the Barberton

Greenstone Belt. The Fig Tree rests on a thick volcanic succession, the

Onverwacht Group, made up largely of mafic and ultramafic extrusive rocks with minor accumulations of interbedded felsic volcanic and sedimentary material. Unlike the sedimentary layers in the lower, volcanic sequence, which consist primarily of volcaniclastic debris and which accumulated under anorogenic conditions (Lowe and Knauth,

1977,1978; Lanier and Lowe, 1982), rocks of the Fig Tree Group were derived in part by erosion of orogenic highlands. Locally, however, rocks of the Fig Tree Group are dominantly volcaniclastics or lavas

(Lowe and Byerly, 1986b).

Few sedimentological studies have focused on the upper, predominantly terrigenous-clastic portion of greenstone belts. Studies in Canada (Goodwin, 1973; Henderson, 1975; Walker, 1978; Hyde, 1980),

Western Australia (Eriksson, 1982a,b) and the Barberton Mountain Land of South Africa (Eriksson, 1977,1978,1979,1980a,b; Heinrichs, 1980;

Reimer, 1980) have indicated that sedimentation took place in both deep-sea and alluvial environments. Paralic facies are not common but have been noted by Eriksson (1977,1979) in the uppermost part of the

Barberton succession.

Heinrichs and Reimer (1977) and Reimer (1980,1983) recognized that the Fig Tree Group can be divided into two major facies. In the northern part of the Mountain Land the Fig Tree consists mainly of 2

graywackes and shales and appears to be of deep-water origin (Eriksson,

1980b). The southern facies contains a varied lithology of cherts, tuffs, jaspilites, sandstones, conglomerates, breccias, and sedimentary barites. Although Reimer (1983) suggested that Fig Tree rocks of the southern facies were deposited in shallow water, few details are available on the overall environmental succession. This southern facies is the area of study of this report (Fig. 1).

The objectives of this study are to describe the sedimentologic characteristics of sedimentary rocks in the Fig Tree Group in order to provide an interpretation of the processes and environments of deposition; to discuss their provenance and diagenesis; to examine the stratigraphic succession in the study area and relate it to Fig Tree rocks described elsewhere in the Barberton belt; and to interpret the evolution of this sedimentary sequence in the context of tectonics and sedimentation. Facies identification and interpretation is important because it helps to establish a framework for the understanding of

Archean age rocks, in which virtually no biogenic features are present to aid in interpreting such variables as water depth. These interpretations provide critical information concerning styles of tectonism and related sedimentary basins and will lead to a better understanding of the tectonic evolution of the Barberton Greenstone

Belt. REGIONAL GEOLOGY

The Barberton Greenstone Belt of South Africa is located near the eastern margin of the early Archean Kaapvaal craton of southern Africa

(Fig. 1). The Barberton belt consists of a complexly folded, slightly metamorphosed volcanic and sedimentary succession, the Swaziland

Supergroup, surrounded and intruded by coeval or younger granitoid rocks. In the southern part of the Mountain Land the greenstone belt consists of a series of large northeast-trending anticlines and synclines with almost vertical fold axes (Fig. 1). In the central and northern part of the Mountain Land the major structural features are a series of tight, steeply-plunging synclines.

Three main subdivisions are recognized within the Swaziland

Supergroup (Viljoen and Viljoen, 1969): 1) the Onverwacht Group, consisting mainly of mafic and ultramafic lavas with minor amounts of volcaniclastic and pyroclastic debris and thin sedimentary units; 2) a middle subdivision of non-quartzose clastic rocks and cherts named the

Fig Tree Group, and; 3) the uppermost division, the Moodies Group, consisting largely of more mature, quartzose clastic rocks (Fig. 2).

Extrusive rocks of the Komati Formation in the lower Onverwacht

Group have been dated at approximately 3.5 Ga (Hamilton et a l., 1979;

Jahn et a l ., 1974; Martinez et a l ., 1984). The Middle Marker, which occurs at the base of the Hooggenoeg Formation, has a metamorphic age of approximately 3.3 Ga (Hurley et a l., 1972). The age of the uppermost parts of the Swaziland Supergroup are constrained by 3.2 Ga dates on the Kaap Valley Pluton, which intrudes the Moodies Group (Lowe et a l., 1985). Rocks of the Swaziland Supergroup around the periphery

3 Barberto LEGEND

Greenstone Plutonic and Belt Metamorphic a 3 Moodies Gr.Rocks bO fc.O) ' v. a> : i •/ a Fig Tree Gr. s STUDY co AREA Onverwacht Gr. T3 C A upper Fms. w N 01 Onverwacht Gr. SOUTH 5 Sower Fms. CO AFRICA

Barberton 0 Mountain 10 20 km Land

A frica O 5 0 0 Km

Fig. 1. Generalized geologic map of the southern part of the Barberton greenstone belt showing location of the study area. Modified from Lowe et a l.(1985).

-p» MOODIES •o' Co. Oo\ GROUP - 2 0

FIG TREE — — —

GROUP —------— MSAULI CHERT

Q. UJQi f— M AAAA A 3 " g -1 5 o ac u. cc o ,N <.i - i CO m r - 3 z CO Q. g 3 occ o 3 u. GC o O Ui zo © CO »- o -1 0 X a: o MIDDLE ~ MARKER

5 N CL cc 10 Ui 3 RMAT

> o 0 z CC Ll_ 0 o m y- 3 g CO - 5 o z < 5 2 < ft g H a= Su. CO < £ = < “= 2£, ~3 g I” 1 § - 0 km

Fig. 2. Generalized stratigraphic column of the Swaziland Supergroup on the west limb of the Onverwacht anticline. Symbols: mafic and ultramafic volcanic rocks (unpatterned); mafic and ultramafic pyroclastic and volcaniclastic rocks (open triangles); felsic igneous rocks (random line segments) chert (black); fine-grained clastic sediments including some chert and jasper (horizontal line segments); sandstone (stippled); conglomerate (open circles). Modified from Lowe et al. (1985). of the greenstone belt have been intruded by plutonic rocks having ages ranging from 2.7 to 3.5 Ga (Barton et a l ., 1983).

The present study area is located on the west limb of the

Onverwacht anticline and includes rocks from the uppermost Kromberg

Formation to the mid- to upper Fig Tree Group. Due to structural truncation, the top of the Fig Tree Group is not exposed in the study area. Strata in the study area strike approximately E-W with stratigraphic up to the north (Plate I).

In the present study area the rocks of the uppermost Onverwacht and Fig Tree Groups are stacked and repeated in a series of thrust sheets. Individual structural zones include up to 900 m of allochthonous volcanic and sedimentary rocks and extend as much as 15 km along strike (Lowe et al., 1985). Recognition of repeated structural blocks is based on key marker beds and recognizable repeated stratigraphic sequences.

The entire greenstone belt has undergone lower greenschist grade metamorphism. This metamorphism is responsible primarily for the production of sericite and chlorite. Si 1icification of all lithologies has occurred to some degree. Although this silic ific atio n has altered primary mineralogy, i t has for the most part preserved primary structures and textures. Post-metamorphism alteration includes the production of calcite, pyrite, and hematite. 7

PLATE I (see back pocket) STRATIGRAPHY

The name 'Fig Tree' was first applied by Van Eeden (see Visser,

1956, p. 41) to a sequence of shales, banded cherts and ironstones, graywackes, and minor amounts of conglomerate occurring between the

Kromberg Formation of the Onverwacht Group and the sandstones of the

Moodies Group in the northern part of the Barberton greenstone belt.

Little stratigraphic and sedimentological work was accomplished until

Reimer (1967, formalized by Condie et al., 1970) defined three formations, the Sheba, Belvue Road, and Schoongezicht Formations, within the Fig Tree Group. Type sections were established both to the northeast and west of the present study area. The Sheba Formation

(700-1000 m) consists of graywacke with subordinate shale and banded iron-formation. The overlying Belvue Road Formation (600-100 m) is made up of interbedded graywacke, siltsto n e, chert, banded iron-formation, and local accumulations of pyroclastic rock. The

Schoongezicht Formation (200-600 m) consists mainly of pyroclastic rock, volcaniclastic rock, conglomerate and graywacke.

Sedimentological analysis by Eriksson (1980) has shown these formations to represent mid-fan channel and lower fan/basin plain and basin slope deposits of a submarine fan system.

The formations of Condie et al. (1970) constitute the 'northern facies' of the Fig Tree Group and are distinctly different from those of the 'southern facies' (Heinrichs and Reimer, 1977; Heinrichs, 1980;

Reimer, 1980, 1983). The present study area is located within the southern facies and the formations of the northern facies will not be considered. 9

Heinrichs (1980) worked in the Fig Tree Group in the southern part of the Mountain Land and presented detailed descriptions of the lithologic units. He divided the Fig Tree into four formations. From bottom to top these include: 1) Umsoli Formation, a sequence of carbonaceous chert and silicified volcaniclastic detritus capped by carbonaceous shale and micaceous volcanogenic graywacke; 2) Ngwenya

Formation, consisting of shale, jaspilite, and chert-clast conglomerate; 3) Mapepe Formation, containing micaceous quartzarenite, subarkose, dolostone, bedded barite, feldspathic shale, lithic graywacke, and chert-clast conglomerate, and; 4) Schoongezicht

Formation, in the southern facies consisting of volcanic tuffs and agglomerates.

The Fig Tree Group in the study area can be divided into three main lithostratigraphic units. They include, in ascending stratigraphic order, 1) a basal sequence of cherts; 2) a succession of siltstones and fine-grained sandstones, and; 3) conglomerate and breccia at the top. These three lithostratigraphic units are here informally designated as formations A, B, and C from base to top. Table 1 lists the three informal formations of the Fig Tree Group of the present study along with a subdivision of these formations into nine lithofacies. Figure 3 is an idealized stratigraphic column of the Fig

Tree Group in the southern part of the Barberton belt, in which the formations and lithofacies are shown. More recent field studies

(Byerly and Lowe, 1985,1986; Lowe et a l., 1985; Lowe and Byerly, 1986b) have shown the lower Fig Tree to contain interbedded ultramafic lavas, banded cherts, and carbonaceous cherts. These previously unrecognized

"Fig Tree" ultramafic lavas occur only in higher structural belts, as 10

Table 1. Major lithostratigraphic units and lithofacies in the Fig Tree of this study. See Fig. 4 for stratigraphic relationships.

FORMATION LITHOFACIES

A - Basal cherty unit A. - Black carbonaceous and banded chert

A? - Silicified volcaniclastic ash and la p illi

B - Fine grained siltstone and B. - Thinly bedded, laminated sandstone units ferruginous siltstone and very fine sandstone with locally thicker and coarser sandstone beds

Bp - Structureless, graded and flat laminated sandstone with locally inter­ bedded breccia

Bg - Jaspilite

B^ - Bedded barite

C - Conglomerate and breccia Chert clast conglomerate and sandstone

Volcaniclastic breccia, conglomerate, and sandstone meters 700, A A - Plagioclase porphyry breccia/conglomerate C 2 a

600- H i - Conglomerate and sandstone Cj

500. - Sandstone with interbedded breccia B 2

Siltstone and fine-grained sandstone 400 . Jaspilite, and Bedded Barite B 3 ,B4

Banded black chert and volcaniclastic . 300 - sandstone 1,rt2

* •* - Msauli Chert • •

Fig. 3. Generalized stratigraphic column of the Fig Tree Group of this study. Lithofacies designations at right. meters 700

-.0. > O Q chert clast cong­ Schoonge- zicht Fm. lomerate and volcanic agglomerates and tuffs clast conglomerate and breccia o-

conglomerate . O' o . > P / graywacke

C sandstone/conglomerate Mapepe Fm. shales and slates bari te quartz arenite

siltstone and chert grit and conglomerate fine-grained sandstone Ngwena Fm. shale and fine sandstone jaspilite shale jaspilite graywacke shale ~ black and white Umsoli Fm. black and white banded chert A banded chert _ silicified accretionary siliceous “oolite" lapilli

4 - Comparison of (A) Fig Tree stratigraphy of the western portion of this study with (B) Fig Tree stratigraphy of Heinrichs (1980). No vertical scale available for (B). discussed in the following section, and indicate that the contact between the Onverwacht and Fig Tree is diachronous (Lowe et a l., 1985).

Figure 4 compares the lithologic units of Heinrichs (1980) with those of the present study, and it can be seen that significant lithologic differences exist between the two areas. While there is obvious correlation between certain units, e.g. the basal chert

(formation A) of this study with the lower portion of the Umsoli

Formation of Heinrichs (1980), there are several problems with utilizing the formations of Heinrichs (1980): 1) a variety of lithologic units including quartz arenite, shale, slate, subarkose, and dolostone do not occur in the present study area but are included in the Fig Tree of Heinrichs (1980); the formational boundaries of

Heinrichs (1980) are defined by the appearance or disappearance of units which do not crop out in the present study area. These differences render correlation between the two strati graphic sequences of no practical use. It should be pointed out that the tuffs and agglomerates of the Schoongezicht Fm. (Fig. 4) may correlate with the volcaniclastic sequence (lithofacies 0.^) of the present study.

However, for the sake of consistency within the present report,

Schoongezicht Fm. will not be used. STRUCTURE

Most rocks in the Barberton Mountain Land dip steeply, usually with angles of 70 degrees or greater. Large, northeast-trending anticlines and synclines, with near-vertical plunges, are the major structural features in the southern part of the Mountain Land (Fig. 1).

Smaller anticlines and synclines are present on a meter scale. Ramsay

(1963) first documented the polyphase nature of the deformation in the northern Mountain Land. More recent work (Dokka and Lowe, 1984; Lowe et a l., 1985; de Wit et a l ., 1983; de Wit, 1982; and Fripp et a !.,

1980) has shown the existence of thrust- and fold- nappes (Plate I).

In the study area the Fig Tree Group occurs as several major, and some minor, structurally repeated packages. The general stratigraphic sequence within these structural packages includes a basal unit of silicified black carbonaceous chert and volcaniclastic debris overlain by a coarsening upward sequence of siltstone, sandstone, and conglomerate. The basal chert is underlain by silicified, altered ultramafic volcanic rocks which are commonly sheared. These volcanic rocks represent the uppermost Kromberg Formation of the Onverwacht

Group (Fig. 2). Correlation between structural belts is based on the presence of distinctive lithologies and on the repetition of stratigraphic sequences in the structural packages. For the purpose of discussion of correlation between structural zones, only the major structural belts are considered. These belts are here termed structural zone-1 (SZ-1) and the overlying structural zone-2 (SZ-2).

Lithofacies A2 and the Msauli Chert serve as key marker beds between structural zones (Table 1 and Fig. 2). These units are 14 15

composed of s ilic ifie d volcaniclastic debris having distinctive sedimentological and mineralogical characteristics which yield a high confidence level for correlation. The Msauli Chert and Lithofacies occupy the same relative stratigraphic position in the two structural zones, and in both zones the basal chert is overlain by a

1ithologically identical terrigenous clastic sequence. Lowe and Byerly

(1986a) have recently documented another distinctive bed which can be found at the same stratigraphic horizon in repeated structural blocks.

It consists of thin layers containing silica spherules which are interpreted as originating from melting due to impact of meteorites or comets.

The structural blocks are separated by stratiform faults which developed within the serpentinized, ultramafic rocks in the upper

Kromberg Formation. Sense of shear analysis indicates that the northern blocks have moved east relative to the southern blocks (D.R.

Lowe, pers. comm., 1986). These faults can be traced regionally on the basis of truncation of folds, repetition of distinctive lithologic units and stratigraphic sequences, and by the presence of narrow zones of deformation (Lowe et al., 1985). Locally there is evidence that the fault planes were originally in a near-horizontal position. This is shown by the presence of quartz-filled cavities that occur along stratiform fault zones (Lowe et a l., 1985). The quartz displays pendant structures which indicates the cavity was approximately horizontal during infilling.

Field mapping of these structural zones has shown that within each block there is stratigraphic younging to the north. There is also an overall younging trend from southerly to northerly blocks (D.R. Lowe, 16

Pers. Comm., 1986). This is shown by the loss of Kromberg Formation in the lower portion of northerly blocks, such that the basal rocks are

Fig Tree; and the concomitant appearance of Moodies Group rocks in the uppermost structural zone. Harris and Milici (1977) documented a similar structural/stratigraphic relationship in the Appalachians and attributed it to an upward migration of the basal detachment zone.

This relationship will be discussed in more detail in the Discussion section of this report.

Field evidence suggests that stacking of the structural sheets of

Onverwacht and Fig Tree rocks occurred during deposition of the upper part of the Fig Tree Group. Conglomerate in the upper Fig Tree contains distinctive clasts that can be identified as being derived from lower Fig Tree rocks. Also, many stratiform faults in the Fig

Tree have served as loci for the intrusion of hypabyssal dacitic rocks

(Lowe et a l., 1985). These intrusive rocks appear to be co-magmatic with extrusive dacitic rocks in the upper Fig Tree Group.

Conglomerates at the top of the Fig Tree Group contain clasts of this dacite, which suggests that faulting and uplift accompanied volcanism and intrusion during late Fig Tree time (Lowe et a l ., 1985). The structural evolution of the study area will be discussed in more detail in the final section of this report.

More modern thrust sheets fall into one of two categories (Hatcher and Williams, 1986): crystalline thrust sheets, which transport metamorphic and/or igneous rocks and which have relatively deep basal detachments; and thin-skinned foreland thrusts, which possess properties similar to those of crystalline thrust sheets but are proportionately thinner and smaller. Thin-skinned foreland thrust sheets more closely resemble the thrust sheets seen in the Barberton

Greenstone Belt. One major difference is the very thin nature of the thrust of the Barberton Greenstone Belt. Thickness is controlled by depth to basal detachment which depends in part upon lithology and geothermal gradient (Hatcher and Williams, 1986). The uniqueness of komatiites and of the Archean geothermal gradient may be factors in this difference between styles of thrusting. PETROGRAPHY

Petrographic study of sandstones from the Fig Tree Group shows that all framework grains except megaquartz have been altered by early si 1icification and low-grade metamorphism. Volcanic-rock fragments, sedimentary-rock fragments, and non-quartzose monomineralic grains are now composed of a mosaic of granular microcrystalline quartz and phyllosilicate minerals.

Detrital framework modes of sandstone suites have had wide application in interpreting provenance and tectonic setting (Dickinson,

1970; Graham et a l ., 1976; Dickinson and Suczek, 1979; Dickinson and

Valloni, 1980). These sandstone suites are quartzose and contain measurable amounts of feldspar. The QFL plots used by Dickinson and

Suczek (1979) and in other similar studies do not lend themselves to interpretation of sandstones of the Fig Tree Group. Table 2 shows modal analysis data for 12 samples of Fig Tree sandstones and it can be seen that chert grains comprise an average of over 80% of the framework-grain population. Quartz is the other dominant grain type, with polycrystal 1ine and monocrystalline types together making up an average of 13% of the framework-grain population.

The difficulties of utilizing the detrital framework modes to interpret Fig Tree sandstones are severalfold: 1) the combination of d etrital chert grains undergoing both silica cementation and further silicification results in sandstones composed almost entirely of chert.

Grain boundaries are sometimes totally obscured, making point count data unreliable; 2) chert grains commonly contain varying amounts of phyllosilicate minerals, usually sericite, in abundances ranging from

18 Table 2. Summary of point count data for selected silicified sandstones. Qm = monocrystalline quartz; Qp = polycrystalline quartz; GMC a = chert grains with less than 10% intergrown phyllosilicate; GMC b = chert grains with greater than 10% intergrown phyllosilicate.

Sample Black No. Qm QP GMC

37-1 4.9 3.0 30.7 61.4 - - -

37-2 7.8 2.6 48.4 40.3 - 1.0 -

37-3 5.5 1.3 32.0 50.5 - 10.7 -

37-5 10.0 9.7 52.0 28.0 * - -

37-9 9.3 6.6 58.7 22.7 2.3 - -

37-10 8.7 8.7 44.7 35.3 2.7 - -

37-12 7.3 8.0 41.0 37.7 4.0 - -

37-14 8.0 4.2 48.2 39.3 -- -

36-14 8.2 6.8 46.5 3.1 2.3 33.1§ -

2-1A 5.7 2.7 33.6 57.9 - - -

2-4 6.7 5.8 34.8 52.7 - - -

24-2 7.2 3.8 64.7 17.1 6.5 --

X 7.5 5.4 44.6 37.2 1.5 3.7 -

* _ trace amount, less than 1%

§ - altered ultramafic grains trace amounts to over 50% of the grain. Cherts of known volcaniclastic origin usually contain sericite, so sericite-rich chert grains are interpreted as representing eroded, silicified fine-grained volcaniclastic debris. An arbitrary cut-off of 10% sericite was chosen to divide the chert grains for modal analysis, with those having >10% sericite being classified as volcaniclastic lithic fragments. The accuracy of this is questionable because observation of cherts of known volcaniclastic origin show some to contain just trace amounts of sericite. It is also questionable because some sericite-rich grains may be altered, silicified feldspars. However, feldspars are ruled out on the basis of the greenstone belt provenance as shown geochemically and by conglomerate clast studies; 3) plots of QFL and QmFLt (Fig. 5) for the data of Table 2 fall along the line joining quartz and lithic fragments. These plots fall into the "recycled orogen" provenance

(Dickinson and Suczek, 1979), which is defined as deformed and uplifted stratal sequences in subduction zones, along collision orogens, or within foreland fold-thrust belts. It is not known however if these specific plate-tectonic settings existed in the Archean. The unique tectonic-sedimentary nature of greenstone belts, the high degree of silicification, and the uncertainty that Phanerozoic-style plate tectonics can be applied to Precambrian terranes, render detrital framework modes unreliable.

While grain composition has been obscured by alteration, grain textures are remarkably preserved. This preservation allows for the identification of microphenocrysts based on crystal morphology and the recognition of important grain-types such as volcanic shards and pumiceous grains. These features are present mostly in Lithofacies A^. Qm

F

Fig. 5. QFL and QmFLt plots for medium grain-sized sandstones of the Fig Tree Group. Q = monocrystalline and polycrystalline quartz grains; Qm= monocrystalline quartz; F= total feldspar; L= volcanic and aphanitic sedimentary rock fragments; Lt= polycrystalline quartzose grains + Lt. See Graham et a l . (1976) for detailed discussion of these grain parameters. The bulk of the sandstones (Formation B) are made up of grains of fine­ grained chert and chert-sericite mosaics. These grains were apparently already chert when they were deposited, and contain no remnant textural features that are useful in determining the original mineralogy.

Point counts were done on selected medium grain-sized sandstones in order to establish quantitatively the distribution of framework grains and also to document abundances of grain types which do provide specific information on source area but which are not grain types which fall into the categories of Dickinson and Suczek (1979)(Table 2).

Detrital megaquartz grains make up an average of 13% of the framework-grain population. Chert is the most abundant grain type, and has been subdivided into two groups based on the estimated amount of intergrown phyllosilicate mineral. This parameter was chosen to possibly identify grains of volcanic origin, but as previously discussed, the accuracy of this is uncertain. Three different grain types provide useful provenance information. Volcanic quartz grains are identified by the presence of straight edges and resorption embayments (Fig. 6). Black-chert grains are easily identified, and indicate a source from uplifted greenstone-belt terrane. Chert grains with high amounts of intergrown chlorite probably represent altered, silicified mafic or ultramafic rock, and also are derived from uplifted portions of the greenstone belt. While modal analysis of silicified sandstones of the Fig Tree Group of this study cannot provide information with regard to plate-tectonic setting, it does indicate that these sandstones appear to be derived from underlying greenstone-belt rocks rather than extrabasinal terranes. Fig. 6. Photomicrograph of volcanic quartz in sandstone of Lithofacies B2. Polarized light. Sample 36-7. LITHOFACIES AND ENVIRONMENTS OF DEPOSITION

The Fig Tree Group in the study area has been divided into three informal 1ithostratigraphic units or formations (A,B,C) and nine lithofacies C ) (Table 1). The formations include a lower section of cherty sedimentary rocks (A), a middle division of silt- to sand-sized detrital units including a thin bedded barite zone

(B), and an upper sequence of coarse conglomerate and breccia (C). Two types of intrusive rock occur within the Fig Tree in the study area: diabase dikes having a NW-SE orientation, and dacite (Plate I).

Formation A

Two lithofacies make up Formation A, which has a thickness of up to 40 m (Fig. 7). These are: A^ - black, carbonaceous chert and banded black and white chert, and; k^ - discontinuous pods of s ilic ifie d , volcaniclastic ash and la p illi. Immediately beneath the black chert of Lithofacies A^ is a unit of silicified volcanic dust, ash and accretionary lapilli named the Msauli Chert by Stanistreet et a l . (1981) (Fig. 7). This unit has been described in detail by Lowe and Knauth (1978), Stanistreet et al. (1981), and Heinrichs (1984).

There remains some controversy as to the depositional environment of the Msauli Chert. Stanistreet et a l . (1981) and Heinrichs (1984) interpreted the normally graded beds of accretionary lapilli to represent turbidites deposited in deep water. Lowe and Knauth (1978) suggested that these deposits represent airfall pyroclastic debris which accumulated in a shallow subaqueous environment characterized by weak to moderate, discontinuous current activity.

24 25

meters

Ferruginous siltstones and fine­ grained sandstones. Lithofacies

Green chert (A 2 ). Pods of silicified volcaniclastic debris,

Black/banded chert ( )

m

Msauli Chert

upper Kromberg Fm.

Fig. 7. Basal chert (A ) of the Fig Tree Group. All sedimentary units and beds are idealized and diagrammatic. Lithofacies designations in parentheses. 26

Lithofacies - Black and Banded Chert

This lithofacies varies from 20 to 40 m in thickness. Lithofacies

Aj is a complex association of microfacies including layers of structureless, carbonaceous chert; laminated carbonaceous chert; silicified, current-deposited detritus; translucent chert; and ferruginous chert.

Description and Composition. Black carbonaceous chert forms beds ranging from 2 cm to 2 m in thickness and is the most abundant rock type in Lithofacies A^, comprising an average of between 40 and 60% of most stratigraphic sections. Thinner layers (<10 cm) are commonly interbedded with other chert types in this lithofacies.

Carbonaceous matter occurs as irregular and rounded, sand-sized clumps; fine, disseminated grains; irregular streaks and patches; and less commonly, fine laminations. Throughout the black and dark-gray chert units the quantity, shape, and structure of carbonaceous matter changes vertically on a scale of millimeters. Cross lamination occurs locally but is subordinate to flat lamination. Rhombic carbonate, probably dolomite or ankerite, pyrite, hematite, and silt-sized grains of quartz and chert are present in trace amounts, and the rock is cemented with microquartz. Silica also occurs as stratiform and cross­ cutting veins of quartz which locally attain a width of up to 10 centimeters.

Current-deposited detrital layers make up less than 15% of

Lithofacies A1 and range from 5 to 50 cm in thickness. They consist of moderately sorted, fine- to medium-grained sandstone composed of grains of relatively pure white and gray chert, grains of chert containing 27

sericite and chlorite, volcaniclastic grains, and fragments of black chert. Detrital monocrystaline quartz makes up less than 10 percent of these units. Most of these detrital layers are either structureless or normally graded. Flat lamination, when present, usually overlies a normally graded unit, This facies also includes at least two thin (<50 cm), discontinuous beds of accretionary lapilli. Two types of chert- clast breccias, in units 5 to 40 cm thick, occur in Lithofacies A^: one is an intraformational breccia composed of plates of white chert in a black-chert matrix, and the other is made up of white-chert clasts in a sandy matrix. Each of these breccia-types makes up no more than 10% of Lithofacies A^. The black-chert-matrix breccias usually contain white chert plates of subequal size, averaging about 1 cm in thickness and up to 15 cm in length. The sand-matrix breccias are less well sorted regarding chert-plate size, with maximum thicknesses and lengths of 2 cm and 30 cm respectively. The smallest chert-plate clasts have dimensions on a millimeter scale. There is a complete spectrum of clast sizes present between these two extremes.

Translucent chert occurs as bands 1 to 5 cm thick and makes up between 10% and 20 % of Lithofacies A^. White-chert bands are separated by very thin black-chert laminations. No primary sedimentary structures are present, but some layers show thin, discontinuous streaks. No admixed sediment is present and petrographically the translucent chert bands are composed of fine-grained microquartz.

Layers of ferruginous chert occur as thin bands averaging 2-3 cm in thickness and make up 10% to 20% of Lithofacies A^. Many layers contain fine, even laminations. Obvious detrital textures are obscured by silicification and production of hematite. In thin section this 28

rock is predominantly a mozaic of microquartz and hematite. Carbonate rhombohedra of secondary origin are locally abundant but usually occur

in minor amounts.

Sedimentation. The rocks of Lithofacies Aj represent three main primary sediment types: 1) carbonaceous matter, now altered to chert;

2) non-terrigenous sediment of probable orthochemical origin, and; 3) terrigenous and volcaniclastic silt-, sand-, and granule-sized clastic detritus.

Carbonaceous chert shows no evidence of high-energy deposition.

Flat- and slightly wavy-lamination is present in 30-60% of the black chert in a given stratigraphic section, the remainder being structureless. Cross lamination and scour are rare, suggesting that the laminations are not of high-velocity origin. Low-energy sedimentation is also suggested by the draping of laminations over irregular surfaces. The precursor to the carbonaceous chert appears to have behaved as soft, deformable sediment after deposition, as shown by dikes of black chert that disrupt and brecciate surrounding layers and by the common presence of load-features.

The white-weathering, translucent chert, because of its compositional purity and lack of sedimentary structures and detrital textures, probably originated as either primary precipitated silica or as fine-grained chemical sediment, such as carbonate, that has since been silicified. Thin, white-chert layers show evidence of brittle behavior in the form of intraformational breccias, suggesting that lith ificatio n and sedimentation were penecontemporaneous (Fig. 8). Fig. 8. Intraformational, white-chert-clast breccia. Lithofacies Ai. The white-chert precursor appears to have behaved in a b rittle manner relative to the black-chert precursor. 30

Clastic detrital beds represent periods of locally higher energy conditions when debris was transported and deposited by currents, and also periods of lower-energy supension sedimentation. Deposition by sediment gravity flow is suggested by the presence of normally graded or massive sandstone beds which locally have flat lamination at the top, a sequence which resembles the Ta and Tb divisions of the Bouma turbidite sequence. These beds are thin (5 to 20 cm) and sandwiched between black or banded chert layers, and thus represent a brief influx of sediment which interrupted the otherwise quiet conditions of deposition that prevailed. The laminated ferruginous chert shows no evidence of current deposition. Original grain size is difficult to determine because of secondary alteration but the presence only of fine, even laminations suggests that deposition was from suspension-sedimentation of silt-sized and finer sediment.

Rocks of Lithofacies A^ formed in a low-energy environment subject to discontinuous current activity, with periods of high-energy reflected by the presence of coarser detrital units. The overall low energy is reflected by the abundance of fine-grained sediment, the evenness and fineness of laminations, the presence of drapes of fine-grained sediment over coarser grained beds, and the paucity of current-generated structures and scour.

Lithofacies A 2 - S ilicified Volcaniclastic Sandstone

Description and Composition. This facies occurs as 2 to 4 m thick discontinuous lenses of silicified volcaniclastic debris. These lenses are 20 to 100 m in length and are interbedded with Lithofacies A^ (Fig. 31

7). The base of Lithofacies locally shows small-scale scour into underlying, laminated black chert, but more commonly the basal contact

is sharp and shows no evidence of erosion.

Siltstone to medium-grained sandstone forms 50 to 60% of

Lithofacies the remainder being very coarse-grained sandstone to granule conglomerate, with local occurrences of pebble conglomerate.

Petrographically, rocks of this lithofacies are composed of microquartz with small amounts (<5%) of sericite, chlorite, and iron oxide. The volcaniclastic nature of the sediment is shown by the following features: 1) volcanic grain textures are preserved as silica pseudomorphs after feldspar (Fig. 9a); 2) chert pseudomorphs of volcanic shards are present (Fig. 9b) and; 3) volcanic accretionary lapilli are present.

Cross lamination, flaser bedding, and wavy bedding all occur, but are greatly subordinate amounts to flat-1 ami nation in the finer-grained portions of Lithofacies A^. These finer-grained units generally occur on top of coarser-grained beds which are either massive, or more commonly, cross stra tifie d , thus forming fining-upward cycles ranging from 20 to 80 cm in thickness (Fig. 10).

An intraformational conglomerate commonly caps Lithofacies Ag.

Units of this conglomerate rarely exceed 1 m in thickness and 50 m in strike width. Clasts are composed of silt- and sand-sized sediment and there is a complete spectrum of clast sizes from lengths of less than 1 cm to over 20 cm. The clasts are plate-shaped and commonly are imbricated. Pore spaces are filled with clear silica or less commonly black chert. Geopetal features are common and are recognized by either Fig. 9a. Microphenocrystic ash grain in rock of Lithofacies A 2 showing pseudomorphs after a silicate microphenocryst, possibly feldspar. Sample 8-3. Plane light.

Fig. 9b. Relict volcanic shard in Lithofacies A 2 . Plane light. Sample SAF 142-25 (from collection of D.R. Lowe). meters

Intraformational conglomerate

Flaser and wavy bedding Discontinuous zone of climbing ripples Form sets

Single layer of accretionary lapilli Cross-laminated ash Cross-stratified lapilli

Flat-laminated ash containing sparsely dispersed larger grains

Discontinuous bed of graded accretionary pebble 1a?111i- sand

Fig. 10. Stratigraphic column of the green, volcaniclastic chert (Lithofacies Ap). Note coarse-to-fine cycles (1-3) of ash and lapilli, and overall fining-upward trend. Top of unit is locally reworked to form an intraformational conglomerate. Measured Section S-18. 34 fine-grained sediment trapped above large clasts or silica-filled cavities below large clasts.

Sedimentation. Three cycles of high- to low-energy deposition are shown in Fig. 10. These cycles begin with cro ss-stratified or graded lapilli and ash overlain by cross-laminated and flat-laminated ash. An overall fining-upward trend is shown by the presence of both flaser bedding and fewer cro ss-stratified , coarse-grained layers in the upper portion of the sequence. The intraformational conglomerate that caps portions of Lithofacies A2 represents an abrupt increase in energy, perhaps related to storms.

Sediments of Lithofacies A2 were composed largely of locally reworked pyroclastic debris. They were deposited during an interval of current activity after which Lithofacies Al deposition resumed. The thickness of Lithofacies A2 suggests that a single, or several, closely-spaced eruptive events introduced the sediment into the low-energy environment in which Lithofocies A^ was forming. The graded bed of accretionary lapilli shown at the base of Lithofacies A2 in Fig.

10 is an unreworked a irfall deposit. The rest of the sequence shows evidence of current activity. The cycles are representative of several periods of current activity, each showing a decline in energy.

A shallow site of deposition for Lithofacies A2 is suggested by local reworking of the top of the unit to form an intraformational conglomerate, by the abundance of current-generated sedimentary structures, including large-scale cross stratification, climbing ripples, cross lamination, and flaser bedding, and by the presence of accretionary lapilli. While accretionary lapilli may possibly survive 35 settling through deep water, they are more likely to be preserved under conditions of rapid burial in a shallow, subaqueous environment (Moore and Peck, 1962; Sisson, 1982).

Depositional Model - Basal Chert

The cherts of Formation A formed under essentially anorogenic conditions. Lithofacies is composed of siightly-reworked, pyroclastic debris which accumulated during a period of nearby volcanic activity. The banded and black chert of Lithofacies formed during periods of volcanic quiescence and overall low energy conditions.

The Msauli Chert was deposited on the volcanic edifice of the

Kromberg Formation. Evidence indicates that water depth was shallow and current activity intermittent. The onset of A^ deposition shows no evidence for dramatic deepening of the basin. Stromatolites have been found within the basal chert in structural zone 2 (Byerly et al.,

1986). If the microorganisms that formed the stromatolites were photosynthetic this would limit the depth of stromatolite formation to the photic zone (Walter, 1976). The black carbonaceous cherts of Aj are similar to black cherts from the Hooggenoeg and Kromberg Formations which contain filamentous micro-fossils (Walsh and Lowe, 1985), and may be a product of biogenic processes.

Lithofacies k ^ contains features indicative of relatively shallow water deposition and lends support to an overall shallow water site for the basal chert. A gradual deepening and generation of a slope is suggested by the following criteria: 1) cessation of black chert deposition, which may be the result of water depths becoming too great 36

for the microorganisms to survive; 2) sediment gravity flow deposits are interbedded with the black chert. The generation of such flows is enhanced by slopes, and similar deposits continue to appear and become thicker and more abundant in the overlying lithofacies of Formation B.

Formation B

Four lithofacies make up Formation B, which attains a maximun thickness of approximately 500 m. Lithofacies B^ is made up predominantly of laminated siltstone and fine-grained sandstone with locally thicker and coarser sandstone beds. This lithofacies is the most abundant in Formation B, making up an average of approximately 80% of the stratigraphic thickness (Fig. 11), although the complex folding that has occurred throughout this lithofacies renders thicknesses somewhat uncertain. Lithofacies B ^ is made up of structureless, graded, and flat-laminated sandstone with interbedded breccia. A persistent unit of jaspilite forms Lithofacies B^, and several thin units of bedded barite make up Lithofacies B^. The jaspilite of

Lithofacies B^ occurs approximately 50 m above the top of Formation A.

Either Lithofacies B^ or may directly overly Formation A, but more commonly Lithofacies B^ is the lowermost facies. The barite usually occurs a few meters above the top of the basal chert, but in some localities i t , too, rests directly on Formation A.

Lithofacies B^ - siltstone and fine-grained sandstone

Description and Composition. Lithofacies B^ is composed of thinly laminated to very-thinly-bedded ferruginous siltstone and very fine- to meters 500 i

LEGEND

» Breccia. Angular clasts of chert B2 in a sandstone matrix 400 B2 ii Sandstone

Abundant thin chert layers inter­ bedded with ferruginous siltsto n e

J a sp ilite - banded white/ 300- Be jasper chert

B - Bedded barite SSL

3 I I _ Ferruginous siltstone and very 1 I___ I ” fine grained sandstone

2 0 0 - = r - Flat lamination -=ST

=V ,-rfA - Cross lamination

_ = = ; A - Normal grading f W T - J

1 0 0 - \ y - Reverse grading

- Convoluted lamination ZJ Ss S P C

Fig. 11. Stratigraphic column of Formation B in structural zone 1. Lithofacies designations at left of symbols. Ss, S, P, and C on scale bar indicate siltstone, sandstone, and pebble and cobble conglomerate. Section S-34. 38

medium-grained sandstone. Sandstone occurs in beds averaging 1 to 2 cm thick and is less abundant than siltstone. Sandstone abundance and overall average grain size increase up-section (Fig.11).

Lithofacies Bj is composed predominantly of hematite and a microcrystalline mosaic of quartz, sericite, and chlorite. Hematite was identified by x-ray diffraction. Detrital quartz makes up an average of 2.7% of these rocks. The detrital quartz grains are non-undulose, monocrystal 1ine, and angular to very angular.

Flat lamination is the most common sedimentary structure in

Lithofacies B^. Cross lamination occurs throughout this lithofacies,

(Fig. 12a) but forms no apparent cycles. Its overall abundance appears to increase slightly toward the top of the section. Low-angle cross stratification with average foreset angles of 15 degrees occurs in this lithofacies, and like cross-lamination, is more common in the higher parts of the section. It occurs in tabular sets with angular to slightly tangential bases, and, locally in concave foreset. Convoluted lamination in layers 3 to 5 cm thick is present in a few layers (Fig.

12b). Some laminae show normal grading. Normally graded layers may be as thin as 2 mm but more commonly are 1-2 cm thick. The proportions and sequences of sedimentary structures present in Lithofacies B^ show no apparent cyclicity other than a slight increase in abundance of cross-bedding up-section

Several observations suggest that the original sediment of

Lithofacies B^^ was silt-sized and finer: 1) there is a preponderence of fine, even laminations; 2) the present texture is homogenously fine­ grained, and; 3) there is a paucity of scour features, and virtually no large-scale scour. 39

Fig. 12a. Starved ripples in rock of Lithofacies Bj. Note draping of sediment over ripple form.

Fig. 12b. Convoluted laminations in s ilic ifie d siltstone of Lithofacies Bj. Note upper layer of undeformed siltstone overlain by laminated white-weathering chert. 40

Thin (5-10 cm), continuous layers of white-weathering, laminated chert occur throughout Lithofacies Bj. Locally, these chert layers form up to 50% of the lithofacies in zones 1 to 2 m thick, where they alternate with siltstone layers. Overall, however, these chert layers form a minor portion of Lithofacies B^. White-weathering chert also occurs as nodules. Finely-laminated sediment bends around these nodules, which themselves show internal laminations, suggesting that the nodules formed very early and later compaction has caused deformation of surrounding sediment around the rigid nodules.

A lens of breccia composed of clasts of both chert and siltstone in a siltstone matrix crops out in the eastern portion of the study area (see siltstone breccia in Plate I and Measured Section S-33 in

Appendix). This breccia has a maximum thickness of about 55 m, a lateral extent of about 500 meters, and the basal contact with the underlying siltstone is irregular. The breccia unit is matrix supported and is unstratified. Chert clasts are angular to subangular, average 2 cm in diameter, and include white chert, black chert, banded chert, and jasper. Siltstone clasts are angular to subrounded, average about 4 cm in diameter, and appear to be of the same composition as the siltstones of Lithofacies Bj. Two megaclasts of siltstone were observed in this breccia, the larger having dimensions of approximately

4 x 20 meters.

Sedimentation. Deposition of the siltstones and sandstones of

Lithofacies B^ took place under predominantly low energy conditions.

This is shown by the abundance of flat-lam inated, fine-grained sediment, by the lack of scour and other high-energy features, by the 41

paucity of current structures, and by the lack of wave-induced structures.

Thin (2-20 mm), normally graded layers are best explained as resulting from suspension sedimentation. Many of the laminae, both graded and ungraded, have a very uniform thickness, and may represent accumulations of volcanic ash. Petrographically these layers provide no unequivocal evidence for a volcanic ash origin, such as the presence of shards, accretionary lapilli, or or other recognizable volcanic fragments. The quartz is non-undulose and very clear in plane light, but cannot conclusively be said to be of volcanic origin.

Deposition from tractive currents is indicated by the presence of cross-bedded layers. Cross-lamination formed from migrating ripples, which are now preserved as lenticular bedding. Harms et al. (1982) interpret low-angle cross-sets which are similar to those seen in

Lithofacies as forming from migrating sand waves.

The siltstone-clast breccia displays characteristics suggestive of debris-flow deposition, most notably the matrix-supported texture, the lack of current structures or other bedding features, and the presence of two very large clasts which would require a flow with matrix strength in order to be transported. This breccia is interpreted as a slump deposit in which already lithified siltstone and chert were ripped up to form clasts in a matrix of unlithified silt and clay.

Lithofacies Bg - Sandstone and Breccia

Description and Composition. Rock of Lithofacies B 2 occurs as thin, lenticular units of sandstone and breccia. Lithofacies Bg sandstone may occur as a single bed 50 cm thick or may form a sequence 42

of sandstone and breccia up to 40 m thick. Beds range in thickness from 3 cm to 1.5 m and are locally discontinuous. Laterally, this lithofacies pinches out and interfingers with siltstones of Lithofacies

Bj. Basal contacts of B2 sandstones or breccias are generally sharp and show no evidence of erosion, except for the local occurrence of scour casts.

Most sandstones in this lithofacies are medium- to coarse-grained, but some beds are very coarse sand- to granule-sized. Sorting varies; the finer grained beds are moderately- to well-sorted and the very coarse sand to granule beds are moderately- to poorly-sorted.

The sandstones of Lithofacies B2 are composed of grains of gray chert, black carbonaceous chert, white chert, and detrital quartz, all cemented by microquartz. Most grains are subrounded to subangular but si 1icification has commonly obscured grain boundaries, especially for chert grains. Trace amounts (<«!%) of barite, plagioclase, and zircon have been identified. Pyrite, hematite and rhombohedral carbonate grains are common authigenic constituents of Lithofacies B2 sandstones.

Megaquartz grains constitute <10% of the framework grains of all samples examined in Lithofacies B2 (Table 2). Monocrystalline and polycrystalline quartz occur in subequal amounts. Identifiable volcanic quartz makes up <10% of the quartz grains in sandstones of B2

(Fig. 6).

Normal grading and flat-lamination are the most abundant sedimentary structures in B2 and where these structures occur in the same sedimentation unit, flat-1aminations overly the graded bed.

Cross-lamination occurs but is not common, usually overlying flat-lamination. Many beds appear to be structureless. Sole marks 43

resembling filled -in scours were observed on the base of one bed (Fig.

13).

Associated with the sandstones of Lithofacies are chert-clast breccia beds. Pebble- and cobble-sized clasts occur as plates of white chert, banded chert, jasper, black chert, and rarely, siltstone, chao­ tically arranged in a sandstone matrix (Figs. 14a and 15). Some breccia beds contain only white chert plates (Fig. 14a). Breccias are both clast-supported and sand-matrix-supported, with clast-supported textures being more common. Inverse and normal grading are present in many of these breccia beds (Fig. 15). Bases of beds are generally sharp and show no evidence of erosion although small-scale scour occurs locally (Fig. 14b).

Sedimentation. Sedimentological evidence in the form of structureless and normally graded sandstones, sole marks, and inversely graded breccia suggests deposition by sediment gravity flow rather than traction flow. A structureless sand bed may reflect depositional conditions or it may be the result of destruction of original lamination. Biogenic disruption is ruled out on the basis of the age of the sandstones. Physical disruption due to liquefaction and subsequent water escape can destroy lamination but should produce features such as dish and pillar structures, none of which were seen.

The absence of lamination may also result from rapid deposition from suspension, most probably by the deceleration of a heavily sediment­ laden current.

In the lower structural zone (SZ-1) many thin sandstone beds occur as single sedimentation units. These units commonly have a graded Fig. 13. Scour casts on base of sandstone bed, Lithofacies Bg. Measured section S-2. Book is 19 cm long. 45

Fig. 14a. Chert-clast breccia of Lithofacies B 2 . Plates of white chert in a sandy matrix. Bending and deformation of some of the white chert clasts indicates that these clasts were still somewhat plastic during deposition. Sample 2-10

Fig. 14b. Small-scale scour and fill in Lithofacies B 2 . Measured partial section S-9. mete

2r^ LEGEND

n ,_^d - Breccia ya*

Sandstone

Siltstone

O C~S --OVQ fl ’Pjsr'afK. Flat lamination 1 -

v, —Jafe $'.“^ od 4s p.cjta. _ . - Inverse grading [fi)

&■ ■ * —1--' -.° »°.*.6 o^

Fig, 15. Stratigraphic column of breccia/sandstone unit of Litho­ facies Bp- Note inversely.graded debris flow deposits and overall fining and thining upward trend. S, P, C, and B on scale bar indicate sand, pebble, cobble, and boulder average size. Section S-9. 47

basal portion which may be overlain by flat-lamination. If present, cross-laminations overly the flat-laminations and a thin chert layer caps all of these sandstone beds. This sequence resembles the Ta, Tb» and Tc subdivisions of the Bouma turbidite sequence. The chert cap may represent the Td division.

The breccia beds show both normal and inverse grading. The inversely graded beds have pebbly sandstone at the bottom and clast- supported pebble and cobble conglomerate at the top (Fig.15). These beds are interpreted as sediment gravity flow deposits in which the clasts were largely supported by dispersive pressure. The normally graded beds are interpreted as being deposited by fully turbulent, high-density turbidity currents. Several breccia beds in Figure 15 show an increase in matrix content near the top. This texture has been noted by Nemec and Steel (1984) and Nemec et al. (1984) and is considered to be typical of subaqueous debris-flow deposits.

Figure 16 shows a portion of a breccia bed that has a non-erosional basal contact and is characterized by near-horizontal stratification. Clast layers are both sand-matrix supported and framework supported. Three crudely inversely graded pebble zones can be seen in Fig. 16. Inverse grading may be produced by shearing of a high concentration of clasts. The base of these inversely graded beds is non-erosive. It has been suggested (Walker, 1966) that basal inertia-flow carpets may form a buffer between the current and the substrate, which would inhibit its erosive capacity. Stratification bands such as these may be due to a surging character of the flow in which periodic 'freezing' took place (Hiscott and Middleton, 1980; Fig. 16. Breccia bed of Lithofacies Bp showing three crudely inversely graded pebble zones. See text for discussion. 49

Lowe, 1982a). Massari (1984) described very similar resedimented deposits from a Miocene fan-delta in Italy.

The sandstones and breccias of Lithofacies formed during periods of relatively higher-energy deposition in which sand and gravel were deposited by sediment gravity flow into a predominantly low-energy environment. A sediment gravity flow origin is suggested by the massive nature of many beds, the abundance of normally graded and amalgamated beds, and by the presence of partial Bouma sequences and sole marks. Lack of reworking of the tops of beds as well as the inferred low-energy environment of the underlying and overlying laminated ferruginous siltstones suggest that the overall energy of the environment was low.

Lithofacies B^ - Jaspilite

Description and Composition. A prominant, jasper-rich horizon has been correlated approximately 8 km along strike (Plate I). This horizon varies from 3 to 8 meters in thickness. The jasper is thinly bedded and thinly laminated. Grain size of sediment in Lithofacies B^ is mostly in the silt- and clay-sized range. Thin layers of fine sand-sized sediment which generally show less jasperization occur locally.

Jasper-rich layers can be seen to grade laterally into ferruginous siltstones of Lithofacies B£, suggesting that the jasper is a diagenetic alteration of the ferruginous sediments. Jasper layers locally may be interbedded with white chert layers in which the white chert is commonly brecciated. 50

Sedimentation. Rock of Lithofacies Bg does not display significantly different sedimentological characteristics from the fine­ grained siltstones of Lithofacies Br>. Both are predominantly fine­ grained and flat laminated. Locally the two lithofacies are gradational, both laterally and vertically, into one another, further suggesting similar processes of sedimentation. The jaspilite does exhibit evidence of early 1ithification in the form of brecciated white-chert layers within intact jasper layers. The overall energy of the depositional environment of Lithofacies Bg was low as shown by the absence of scour, coarse-grained sediment, or other high energy indicators.

Lithofacies B^ - Bedded Barite

Description and Composition. Thin barite deposits have been found throughout the Onverwacht and Fig Tree Groups in the Barberton Mountain

Land (Heinrichs and Reimer, 1977; Reimer, 1980). Two outcrops of bedded barite are located within the study area (D.R. Lowe, per. comm.,

Fig. 17), each occurring as a thin (20-30 cm) bed with a lateral extent of no more than 100 m. Each of these barite beds is located just above the top of the basal chert, and may correlate with the "lower Fig Tree baryte" of Reimer (1980, p. 402).

The barite occurs as gray to light green blades which are oriented at high angles to bedding, and as barite sand. Locally, bundles of blades diverge upward, forming what Heinrichs and Reimer (1977) term

"cauliflowers". At locality 1 (Fig. 17) the barite is underlain by three meters of bluish-gray chert and overlain by a sequence of fine­ grained, cross-laminated cherts and pea-conglomerate. 0 500 m

Basal Chert | |- Siltstone and sandstone

Conglomerate I^fl- Diabase dikes

Fig. 17. Barite localities for the Fig Tree Group of this study. A and B: localities 1 and 2 in text. C: general geologic map of study area showing localities of A and B. See Plate I for detailed geologic map. Scale bar is for A and B.

cn 52

Sedimentation - A sedimentary origin for the barite is suggested by its occurrence at the same stratigraphic horizon at different localities and by its inclusion in structural disturbances that affected the surrounding sedimentary rocks. Reimer (1980) determined that all barite deposits he studied in the Fig Tree Group were of sedimentary origin based on sim ilar c rite ria , as well as on the presence of primary sedimentary structures.

The blades in the barite lithofacies are a diagenetic growth phenomenon, growth taking place during or shortly after deposition

(Reimer, 1980). According to Reimer (1980) the lower Fig Tree barites are of detrital origin, forming either from erosion of uplifted

Onverwacht barite or reworked from contemporaneous hydrothermal exhalative deposits. The close similarities of the barite deposits of this study with those of Reimer (1980) suggest similar depositional histories. Although no primary sedimentary structures were seen in sedimentary barites in the present study area, all other aspects are analogous to the deposits of Reimer (1980).

Formation C

The upper portion of the Fig Tree Group of the present study is made up of two lithofacies: 1) Lithofacies - a chert-clast conglomeratic sequence which is most abundant in the western portion of the study area, and; 2) Lithofacies C ^ -a volcaniclastic sequence composed of breccia, conglomerate, and sandstone which occurs in the eastern map area. Petrologic and field relationships indicate that these two lithofacies occupy similar stratigraphic levels. 53

Lithofacies

The lateral relationships of the chert-clast conglomerate (here referred to as the conglomeratic sequence) and clastic rocks of formation B are shown in Fig. 18. Lithofacies Cj can be subdivided into three main subfacies on the basis of grain size and sedimentary structures. Two subfacies predominate. These are 1) unstratified to crudely stratified coarse, clast-supported conglomerate, locally sand- matrix supported, and 2) crudely horizontally stratified sandy conglomerate. The third subfacies, cross-stratified conglomerate, is less abundant. All three may contain interbedded sandstone. These subfacies are interbedded with one another both laterally and vertically, and locally grade into one another. The overall character of Lithofacies is that of a coarsening and thicknening upwards conglomeratic sequence (Fig. 19). The general characteristics of

Lithofacies are summarized in Table 3.

Description and Composition. Unstratified to crudely stratified conglomerate of subfacies 1 is mostly clast-supported with well-rounded pebbles and cobbles up to 30+ cm in diameter (Fig. 20). The average size is in the pebble to small cobble range. This subfacies makes up about 40% of Lithofacies C^. The matrix is sand-sized sediment with a composition similar to the sandstones of Lithofacies B£ and also to the interbedded sandstones of Lithofacies C^. Clasts are composed of varieties of chert, including black chert, banded chert, and white chert (Table 4). Volcanic-porphyry clasts occur and may be locally abundant. A-A' C-C D-D' meters O ’q-.'q -.0 B-B1 ° h -16 it>‘* 600 "•'°Vo “ i.'/.V.o? '.M’.o-.®; 500 0 . 0* 0 ?0 .*-8 T^<> •?< 0 •*« * V. 400 f.Oo •• >!•. ».•* <*• _p.ro < Location Map 300 P <1 :®ro. ■Vli 500 m

200 - W

100 - -n — i r r *—»

conglomerate

sandstone f*-{ - Diabase dikes

siltstone S a n d s to n e pTs| - Landslide Qeposlt

S i l t s t o n e xf-' - Fault

Fig/ 18. Stratigraphic columns showing lateral variation of the terrigenous clastic sequence. Inset shows location of each column (see Plate I). Column A-A' is a complete section, all others have the lower and part of the upper portion of siltstones omitted due to structural thickening.

Cl -P* meters 600_ i ' N / N / s / V ^ ,Oqo00 0OaO

500-

d)O d0oQ° O d ^ 3 ° A r > < J ,-tfi j LEGEND

- Clast supported conglomerate M 8 8 t ^ 400- ««*>o Clast supported conglomerate with interbedded sandstone and stratified conglomerate

VoS=k'" 0o».a.^V. ?:v.;r7) - Sandstone 300- Vi*'

»Oc& - B reccia

Ferruginous siltstone and □ fine-grained sandstone

- Flat lamination

-rt'. - Cross-lamination A - Normal grading

Fig. 19. Idealized stratigraphic column of the upper structural zone (SZ-2) of the terrigenous clastic sequence in the western portion of the study area. Formational divisions to le ft of symbols. Table 3. Characteristic features of Lithofacies Cj - conglomeratic sequence

Depositional unit Bed geometry Sedimentary features Grain size Interpretation (subfacies)

Clast-supported Maximum 2 m th ick . Massive, crudely bedded, and Pebble and Longitudinal bars and conglomerate. normally graded. Local cobble. channel lag. Locally basal inverse grading. debris flow. Imbrication common. Erosive base. Interbeds of massive and planar s t r a t if ie d sandstone.

Horizontally .2 to 1 m thick. Horizontal stratification. Predominantly Bar tops and upper stratified sandy Stringers of pebbles common. sand and pebble flow regime channel conglomerate and Some normal grading. Cross­ sized grains. d e p o sits. coarse sandstone. laminations in sandstones. Local over­ sized clasts.

Planar cross- 20-40 cm thick Planar tabular cross­ Granules and Transverse bars. stratified conglom- tabular sets. stratified granules and pebbles. erate. pebbles. 57

Conglomerate beds in the unstratified subfacies of Lithofacies range in thickness from 0.5 to 1.5 meters. Amalgamated units may reach

6 m or more in thickness, producing essentially a massive conglomerate with no d istin ct bedding.

Crude stratification is defined by variation in clast size and concentration. Bedding planes are rare. When beds are distinct, they are laterally discontinuous and erosional bases are common.

Imbrication of clasts is also common. Normal and reverse grading are both present in the coarse conglomerates, with normal grading being the more prevalent. Most clasts are discoid to spheroidal in shape, preventing long axis orientation measurements.

The second subfacies in Lithofacies C^ consists of horizontally stratified sandy conglomerate and coarse sandstone, and makes up approximately 30-35% of Lithofacies C^. Beds are 0.2 to 1 m thick.

Clast composition is the same as that of subfacies 1.

Horizontal stratification is shown by stringers of small cobbles or pebbles in a sandy matrix or by more subtle alternations of grain size in coarse sand- to small pebble-sized sediment (Fig. 21b). Normal grading is present within many horizontally stratified layers.

Sandstone layers locally display trough cross-lamination and cross-strati fi cati on.

Subfacies 3 of Lithofacies C^ is planar cross-stratified granule/ pebble conglomerate. Tabular cross-sets are 20 to 40 cm thick (Fig.

21a). Clasts are subangular and composition is the same as for the rest of the conglomeratic sequence. The cross-stratified conglomerate has sharp contacts with underlying beds. This subfacies makes up approximately 15% of Lithofacies C^. Fig. 20. Typical clast-supported pebble conglomerate o f Lithofacies C^. 59

Table 4. Clast composition for pebbles and cobbles of Lithofacies C.. Total sample of 198 clasts from different beds in Fig Tree conglomerates.

black chert 25% white chert 18% banded chert 15% gray chert 10% plagioclase porphyry 10%* ferruginous chert 7% chert arenite 5% unknown 5% quartz porphyry 3% cavity-fill quartz 1% volcaniclastic sandstone <1% fuchsitic rock <1% siltstone <1% accretionary lapilli <1%

* in some units, plagioclase porphyry clasts may make up as much as 40% of the conglomerate. 60

Fig. 21a. Low-anale, planar cro ss-stratification in Lithofacies C^. This feature is interpreted to be the product of migrating transverse bars.

Fig. 21b. Horizontal stratification in fine conglomerate of Lithofacies C^. This type of stratification forms as a result of sheetflow on bar ty p s or on the unconfined surface of an alluvial fan. 61

Paleocurrent directions were measured from imbricated pebbles and cobbles in the conglomeratic sequence. These paleocurrent measurements are dispersed, with paleoflows generally oriented west-northwest and southwest (Fig. 22). Complications in paleoflow patterns in the Fig

Tree conglomerates arise in part from the complex structural deformation of the area. Measurements were corrected for dip, but the entire Fig Tree sequence in the study area is part of a major structural feature, the Onverwacht Anticline (Fig. 1). The regional strike of the west limb of the Onverwacht Anticline, roughly east-west, was used as a reference orientation for paleocurrent reconstruction.

Sedimentation. Lithofacies Cj rocks show abundant evidence of forming in a high energy, current-dominated depositional system.

Widespread imbrication was produced by unhindered clast interaction.

Scour and fill structures are common and represent the infilling of channels. These channel features may have a basal layer of lag gravel covered by massive or stra tifie d sandstone or they may be normally graded conglomerate and sandstone. Lateral migration of these channels produced complex erosional surfaces and internal lithofacies associations. Numerous erosion surfaces and poorly developed strati­ fication produce a disorganized appearance that reflects complex history of erosion and deposition.

Massive and crudely stra tifie d conglomerate forms a major portion of Lithofacies (Fig. 23). These conglomeratic units resemble those formed from the building of longitudinal bars in fluvial systems, and correspond to facies Gm of Miall (1977). Formation of longitudinal bars is by clast-by-clast accretion over an obstacle or a channel lag 62

N l 6 .4 ■ 4 •2

n=30 J f S - 17 V S -3 2

N

16 ■8 12 ■8 n=30 •4 ■4 •2

s * s c

S-10A S-lO BS-lOB

500 m 01?

10, 10A lOB^f;

■ : ' r ■

|^ ; ’[ - Conglomerate |:-[ - Oiabase dikes

|;=..-W| - Sandstone ftls| - landslide Deposit

Si U s tone -f" - Fault

Fig. 22. Rose diagrams of paleocurrents for Lithofacies All readings are from imbricate clasts. 63

deposit (Mial1 , 1977). Interbedded sand lenticles are probably formed from low water accretion processes as described by Mi al1 (1977). Some of the conglomerates are sand-matrix supported and some have inversely graded basal layers (Fig. 24). These conglomerate beds are debris flow deposits. The inverse grading is due to dispersive pressure set up as a result of grain-to-grain collisions during transport. Debris-flow deposited conglomerate is not common in this subfacies as shown by the paucity of both matrix-supported conglomerate and inverse grading, and by the lack of any relationship between bed thickness and maximum particle size. A positive relation between these parameters is considered to be a good indicator of mass-flow deposition (Nemec and

Steel, 1984).

Horizontally stra tifie d conglomerate and sandy conglomerate commonly cap more massive conglomerate beds in the upper portion of the conglomeratic sequence. Beds up to a meter thick of horizontally stratified coarse sand- and granule-sized sediment occur more commonly in the lower portion of the sequence. These horizontally stra tifie d layers are interpreted as bar-top deposits that formed under waning but s till high flow regime conditions. They may also form as high flow regime channel deposits when the channel floor sediment undergoes virtually continuous particle movement (Mial1 , 1977). The horizontally stratified coarse sand- and fine gravel-sized sediment corresponds to the Sh facies of Mi al1 (1977). Small-scale cross- laminations are reported by Mi al1 (1977) to be a minor component of the Sh facies and are a minor component in the predominantly horizontally stratified sandstone (subfacies 2) of Lithofacies Z y The paucity of 64 meters

• a* ooC?^ Sfc

LEGEND

IK Q » fe - Clast-supported conglomerate w m M |3%ggfe & & sg ? \? 'g ■2c?r,S'0<’% °., Horizontally stratified o* ® t*Q J fine conglomerate *o^ Sc>» S -o. * Ao%**0‘ “.

Sandstone

p p ft:^ 0 o^5 lI£ - ' A£\ 4*\ XT'*OOo'O 'c° ■ ° Normal grading “oV^.oVO-'5 ■ • o .»1=1. .0' /I

.0 .U,“„?u-. Q. 0 «’/) o •• j u ^7* o.v: q 0.0.-°'

“•in . “o '. J^£“o_l\11 . O Ol* OoO^olggnt?^] 0 S P c B

Fig. 23. Partial stratigraphic column of Lithofacies C, showing predominantly unstratified conglomerate interpreted as superimposed longitudinal bars on the proximal portion of an alluvial fan. S, P, C, and B on scale bar indicate sand, pebble, cobble, and boulder size. Section S-20. meters

P&-m LEGEND

- Clast-supported m conglomerate

2 ° J & g fA V g { Matrix-supported 6»o 5roac>S.| conglomerate *2

SfsSSI4* - Normal grading

- Inverse grading

1 m bPS X - Imbrication

POfiVA.*'*

. O . >»).» /v* *. ’

*• ^ •'C n"5 '

•.-.■^v.o-.T...IJ-IMMI | I, B

Fig. 24. Partial strati graphic column of the conglomeratic sequence. Note inverse-to-normal grading in upper conglomerate bed. S, P, C, and B on scale bar indicate sand-, pebble-, cobble-, and boulder­ sized clasts. Section S-17. 66

cross-bedding indicates that persistent flow, resulting in the formation of ripples and dunes, did not take place.

The final subfacies of Lithofacies is planar cross-stratified conglomerate. Low-angle bedding inclination may arise from the migration of bars with low channelward-dipping slopes or may reflect local scouring and infilling of shallow channels. Cross-stratified conglom-merate occurs most commonly as tabular sets 20 to 40 cm thick.

This internal geometry conforms more to that produced by migrating bars rather than channel in f i ll, and corresponds to facies Gp of Mi a11

(1977).

Detailed studies of thick sequences of Archean conglomerates

(Walker and Pettijohn, 1971; Turner and Walker, 1973; Henderson, 1975;

Goodwin, 1977; Walker, 1978; Hyde, 1980; Eriksson, 1978, 1982) have shown the common existence of two major depositional systems: alluvial fan-braided stream and submarine fan. The distinction between conglomerates deposited in deep water and alluvial/fluvial conglomerates is based on descriptive features and facies associations.

An alluvial/fluvial depositional system for Lithofacies Cj is suggested by both positive and negative evidence. Table 5 lists differences which may help to distinguish between alluvial/fluvial and submarine fan deposits along with crite ria for Lithofacies Cp It can be seen from these data that conglomerates of Lithofacies resemble deposits of alluvial/fluvial systems more closely than those of submarine-fan systems.

In addition to physical characteristics of Lithofacies Cp the facies associations are not those typically found in submarine-fan systems. Repetitive sand-shale units with great lateral continuity, Table 5. Comparison of conglomerate characteristics of alluvial/fluvial fan-delta, submarine fan, and lithofacies C, of this study. Data for alluvial/fluvial fan- delta and submarine fan from Hein (1984).

Criteria Alluvial/Fluvial- Submarine Fan Lithofacies C. Fan Delta

Grading in Avg. 74% ungraded, 10% normally Avg. 29% ungraded, 61% normally 67% ungraded, 30% normally Conglomerates graded, 16% inversely graded graded, 4% inversely graded graded, 3% inversely graded

Texture of Mostly clast supported (some Matrix supported in slope and Mostly clast supported Conglomerates matrix supported in arid-region very proximal submarine feeder fans) channels, otherwise clast supported

Sedimentary Mostly massive to crude hori­ Slump and fold strata in Coarse conglomerate mostly Structures zontal bedding in coarse proximal slope and feeder massive. Planar tabular conglomerate. Planar tabular channel deposits with massive cross-bedding common. Hori­ cross-bedding common on gravel to graded conglomerate. Planar zontal stratification mostly bars. Horizontal stratification tabular cross-bedding very layers of clast-supported, shown by layers of different rare. Horizontal str a tific a ­ m atrix-filled conglomerate clast sizes and/or layer with tion shown by layers of diff­ of differing grain size clast-supported, matrix-filled erent clast sizes and/or layers texture alternating with open with clast-supported texture work texture alternating with clast-dispersed texture 6 8

common in the mid- and outer-fan regions of submarine fans, are not present. Recognizable levee and overbank deposits are not associated with the conglomerates of Lithofacies C^. No slump structures were seen. Interchannel areas on submarine fans are characterized by very thin and evenly bedded sandstones alternating with greater amounts of interbedded mudstone (Nelson and Nilsen, 1974) and this association is not present in the Fig Tree conglomerates. Sedimentary features common to sandstones of submarine fans but absent in the sandstones associated with the conglomerates of Lithofacies include complete Bouma sequences, de-watering structures, abundant substrata! markings, and highly contorted beds. The coarsening-upward nature of the sequence along with its extremely coarse grain-size indicate deposition in a proximal, prograding alluvial-fluvial system.

Figure 25 shows a portion of the conglomeratic sequence that exhibits three upward-fining sequences. Large-scale abandonment of a braided alluvial tract will form sequences such as these (Nemec and

Steel, 1984). However, very large flood sequences, especially in pre-Devonian settings, because of the absence of vegetation, may also form fining-upward facies sequences on a scale of several to tens of meters (Nemec and Steele, 1984). Similar sequences have been described by Heward (1978) and were interpreted as mid-fan lobe deposits. The facies sequence of Heward (1978) begins with coarse unstratified or crudely stratified conglomerate resting usually with an erosional contact on underlying finer-grained sediments. Overlying the coarse conglomerate is a sequence of finer grained, horizontally stratified or locally planar tabular cross-stratified conglomerate, with thin sandstone beds occurring throughout. meters

crudely horizontally stratified conglo- (2) merate with in te r­ bedded sandstone

cross-strati fied conglomerate

crudely horizontally 40 - (2) s tra tifie d conglo­ merate with inter­ bedded sandstone

■ ■’■ P* vqd unstratified coarse conglomerate

otobo.~*o -oo.y

sand 4 8cm

Fig. 25. A - Three fining upward cycles representing channel fill sequences. B - Detailed section of a portion of A showing complex vertical and lateral lithologic associations. Facies 1 - low relief longitudinal bar; Facies 2 - tops of longitudinal bars and interbar channels; Facies 3 - transverse bars. Measured partial section 31. 70

Not all facies sequences in the Fig Tree conglomerates are as well developed as those in Fig. 25. Much of the upper portion of the conglomeratic sequence is massive or crudely stratified conglomerate that displays poorly developed bedding. Debris-flow deposits are recognized by the presence of inverse grading and make up a small proportion of Lithofacies C1 (Fig. 24). Most of the Fig Tree conglomerates are clast supported, and in the absence of inverse grading the distinction between clast-supported debris-flow deposits and "flashy", ephemeral stream flow deposits is difficult to make.

Nemec and Steel (1984) recognized this problem and lis t a number of criteria to help make this distinction. These include widespread clast imbrication, evidence of scouring or channelized flow, vague to distinct stratification, and lack of well-defined bedding. It should be noted, however, that "flashy" deposits may closely resemble those of fluidal sediment-gravity flows or turbulent and/or surging debris flows

(Nemec and Steel, 1984). Another criterion that can be used to distinguish these two types of deposits is orientation of the long axis of clasts, but the nature of most of the outcrops in the Fig Tree conglomerates, combined with a predominance of discoid shaped clasts prevented collection of such data.

Evidence indicates that ephemeral, flash-flood deposition took place on the Fig Tree alluvial fan. The coarseness of the unit, general absence of mud, widespread imbrication, and abundance of channelization features suggest vigorous bedload transport. The sandy portions of Lithofacies Cp when not associated with a gravel bed, are almost exclusively flat laminated or massive, with locally developed cross-laminated tops. These sandstones are interpreted as 71

hyperconcentrated flood-flow, or HFF, deposits (Smith, 1986). This

term is preferred over the older term, sheetflood deposits, because

sheetfloods are defined as essentially unchanneled thin flows of

sediment-laden water. It is not always possible to determine how

"channelized" a deposit was, but more importantly, hyperconcentrated flood-flow (HFF) is a process term, referring to high-discharge flows

in which neither turbulence is the lone sediment support agent nor in which deposition occurs en masse. This term emphasizes rapid transport and deposition of sediment during high sediment and water discharge events. Smith (1986) points out that this type of deposition is probably important in areas where high sediment loads are episodically

introduced into fluvial systems, such as alluvial fans and the proximal portions of braided streams. These ephemeral flows usually develop under upper-flow regime conditions and may cover a few to 10 km (Rahn,

1967; Bull, 1972). Sand-dominated hyperconcentrated flood-flow deposits are characterized by the dominance of horizontal stratification, with the rare presence of cross-bedding. Parallel lamination is the major stratification type in many deposits described as being of "sheetflood" origin (McKee et al., 1967; Scott and Gravlee,

1968; Frostick and Reid, 1977; Schramm, 1981; Tunbridge, 1981;

Ballance, 1984; Wells, 1984; van Der Muelen, 1986). Galloway and

Hobday (1983) suggest that the sedimentary record of a "flashy" depositional system may be dominated by deposits formed during high sediment and water discharge events. Ballance (1984) documents

"sheetflood-dominated" alluvial fans of the Miocene of central

California. 72

Gravel may be moved and deposited by HFF processes but recognition of such deposits is difficult as they closely resemble the deposits of longitudinal bars (Ballance, 1984). Smith (1986) documents HFF gravel deposits with long axes of clasts oriented parallel to flow, unlike flow-perpendicular orientation typical of longitudinal bars. As previously mentioned, long-axis orientation measurements were not able to be taken. HFF-deposited gravel is not necessarily well stra tifie d , but shows some evidence of gradual accretion, such as internal partings and lateral and vertical variations in grain size and sorting (Wells,

1984). The matrix is mostly sand, cross-bedding is uncommon, but some normal grading may occur (Wells, 1984; Smith,1986). Wells (1984) has also demonstrated that there is no correlation between bed thickness and maximum particle size in gravelly sheetflood and sheet debris-flow deposits. Abundant HFF deposition may be related to the absence of vegetation, which in turn means a lack of water and sediment-storage capacity (Schumm, 1968). This would lead to an immediate response to storms in the form of flash floods. In the Archean, sediment transportation on alluvial fans would probably have been characterized by strongly fluctuating discharges because of the absence of vegetation

(Schumm, 1968).

Alluvial-fan deposits are ideally divisible into proximal, mid-fan, and distal associations (McGowan and Groat, 1971). In an ideal sequence coarse conglomerate of the proximal fan is deposited by both debris-flow and streamflow processes. The mid-fan area consists of extensive water-laid conglomerate and sandstone. The distal fan usually is composed of sand- and silt-sized sediments showing parallel bedding, planar cross-stratification, and trough cross-bedding. This 73

ideal sequence may show strong variations, depending on proportions of water-laid and debris-flow deposits which in turn are dependent upon climate, nature of source terrane, and proximity of the fan to a nearby body of water. The picture now emerging of alluvial fans is that of a diverse and complex depositional system that may be predominantly coarse- or fine-grained, and may be dominated by a particular style of sedimentation (e.g. Ballance, 1984). Only recently have models been proposed for alluvial fan sedimentation (Mial1 , 1977, 1978), and i t is becoming clear that many ancient fans are not easily pigeonholed into one of these models.

The inferred alluvial sequence of the Fig Tree Group does not display a wel1-developed distal facies with abundant cross-bedded sandstone. The alluvial portion of the Fig Tree is composed predominantly of conglomerate which has characteristics similar to those seen in the proximal and mid-fan regions of an alluvial fan

(Table 5) and that have been reported from both modern (Hooke, 1967;

Bull, 1972; Schramm, 1981) and ancient (Heward, 1978; Daily et a l.,

1980; Sailer and Dickinson, 1982; Ballance, 1984; Kerr, 1984; and many others) alluvial fan settings.

On the basis of a poorly developed recognizable distal alluvial fan facies, and on the basis of the subaqueous nature of the underlying and interbedded rocks of formation B, i t is proposed that the alluvial fan was prograding into a nearby body of water. This type of alluvial fan is termed a fan-delta (Holmes, 1965; McGowan, 1970). 74

Fan Delta Model

Ideally, a fan-delta deposit will contain either fossil evidence indicating that subaqueous sediments intertongue with alluvial sediments, or signs of wave or tidal action indicating a transition from subaerial to subaqueous deposition. However, neither of these c rite ria need be mandatory. Fluvially dominated fan-deltas may lack well-developed transitional facies, and Archean-age rocks will contain no useful fo ssils. In the former case, channel-mouth and fan-surface processes will dominate the sedimentary record. According to Rust and

Koster (1984) the deltaic connotation is misleading because fans are dominated by terrestrial processes. Rust and Koster (1984) further state that coarse grained fan-deltas are not significantly different from completely subaerial fans. Nevertheless, the term fan-delta will be used in this discussion.

A sedimentary accumulation of inferred fan-delta origin has alluvial fan deposits in its upper portion and delta front/delta slope deposits in its lower parts. Although the deltaic nature of a fan-delta may not be clearly demonstrable, however, one should be able to show either progradation of alluvial fan sediments into a nearby body of water, or show interfingering of alluvial with subaqueous sediments.

Lithofacies Cj, the conglomeratic sequence, has been interpreted as forming in an alluvial fan setting. The coarseness of the conglomerate increases up-section, which suggests the system was progradational. The occurrence of conglomerate interbedded with fine-grained rocks of Lithofacies indicates that these two 75

lithofacies were side-by-side, and most likely that the alluvial fan was building out into a nearby body of water. This interbedding could be due to either intrabasinal or extra- basirial processes such as water level changes, tectonism, or avulsion of fluvial channels. The relative contribution of any or all of these processes cannot be evaluated at the present time.

Figure 26 shows two stratigraphic columns of formations B and C along with the proposed division into different fan-delta environments.

The proximal subaerial fan is easily identified by the coarseness of the clasts, the abundance of channelization features, the overall high energy of the system and the general sim ilarities of the sequence with coarse alluvium found in the proximal regions of alluvial fan systems

(column B, Fig. 26).

The lower subaerial fan has finer grained conglomerate than the proximal fan and contains a higher percentage of sandstone beds. Lower fan deposits show planar tabular cross-stratification and horizontal layering of pebbles.

Beneath the inferred lower-fan facies are the subaqueous deposits of formation B (Fig. 26, column B). There is not a well-developed subaerial-to-subaqueous transition facies such as identifiable beach deposits or channel mouth bar complexes. This subaqueous zone is inferred to be delta front and possibly distal fan (for the upper portion) deposits. The reasons for assigning a possible distal fan facies to the upper portion of this zone is as follows.

Sandstone of Lithofacies B£ commonly underlies conglomerate sequences (Fig. 18). These sandstones show evidence of deposition from a current of waning energy, with massive or normally graded layers B meters meters 600., 500 t '" '•f' OpO «Q.Q

I A 500 Proximal Subaerial Fan 400- = a Delta Front/Prodelta

400. OJ-qU a> r2 o 9?.o 9, a o« joq / 300- i m M M $ $ 1

300 .•p.Vcfrv* v .v .v .r • O/x*]?!? p cP.JO t^ a«=*» oofl ... Lower Subaerial Fan OOO Q « 0 . , S-7V . ' ° ’; v :^ vhy » coV.' °0 a •••••••pCbV/ 200 - 0««»» Pop 2^/ , * Prodelta/Basin 200 -

D istal Fan (?) /

100 - Delta Front

Ss S P C

Fig. 26. Generalized stratigraphic profiles of the two main structural zones of the Fig Tree Group showing organization into subaerial and subaqueous sequences. See Fig.19 for symbol explanation. Column A is SZ-1 and column B is SZ-2.

CT) 77 overlain by flat-laminated layers and locally cross-laminated zones.

This sequence of sedimentary structures can be produced from turbidity current deposition but can also be formed from sheetflood processes.

Some workers (Long, 1981; Ballance, 1984) have suggested that sheetfloods may evolve into turbidity currents when the flows enter the water and become turbulent. The sediment-gravity-flow deposits of

Lithofacies are inferred to represent subaqueous continuations of subaerial sheetfloods. Thus the proposed distal fan/delta front sandstones may represent very near shore continuations of sheetflood deposits.

Conglomerate beds are interbedded with sandstone and siltstone.

These conglomerates are separated from the proximal-fan conglomerates by a thick sequence of siltsto n es. These lower conglomerates are interpreted as a fan lobe building into the inferred delta-front zone and possibly forming subaerial or subaqueous mouth bars in the prograding fan. Kleinspehn et a l . (1984) described a similar sequence in late Paleozoic fan-deltas of western Spitzbergen.

Present within the inferred delta-front zone are fine-grained sandstones with large-scale low angle cross-stratification, which are interpreted as forming from migrating low amplitude sand waves. The currents which generated these sand waves may have been related to prolonged river flood events. Interfingering of conglomerate and siltstone implies side-by-side depositional environments and it seems likely that the fluvial processes operating on the fan would influence nearshore subaqueous processes in a quiet body of water. Torrential floods would bring large quantities of sediment into the basin and generate moderately strong nearshore currents as well as turbidity 78

currents, which could then distribute fine-grained sediment to the

deeper portions of the basin.

Column A in Fig. 26 does not have any proximal subaerial-fan

deposits. The divisions between delta front, prodelta, and basin are

not easily dilineated. There are more numerous and coarser sandstone and breccia beds interbedded with the siltstones in the upper portion of the sequence, indicating more proximal deposits in the upper

portion. There is no clear indicator of a recognizable slope, such as numerous slump folds. The large slump deposit of siltstone-clast breccias in the eastern portion of the study area (see Lithofacies description) may be evidence for a slope.

Prodelta sediments are found in the lower portion of the sequence

in column A of Fig. 26. This portion of the sequence contains abundant thin, white-weathering chert layers, fewer of the thin sandstone beds than the rocks above it, and has an overall finer-grained character than the inferred delta-front/delta-slope sediments in the overlying rocks.

Transport of sediment to the prodelta region of the fan-delta system was by suspension sedimentation and possibly by dilute turbidity currents. Weak bottom currents generated from circulation patterns may have caused slight reworking of the fine-grained sediment but the overal; energy of the environment was low. Very dilute, thin turbidity currents and low-velocity tractive currents have been cited as the primary depositional agents for interbedded siltstone and fine sandstone in ancient fan-delta front and prodelta deposits in the North

Sea (Stow, 1982; Kessler and Moorhouse, 1984). The prodelta region of the Fig Tree fan-delta behaved essentially the same as any low-energy 79

basin with water depths below wave base. Figure 27 shows representative stratigraphic columns from the inferred fan-delta sub-envi ronments.

Holocene fan-deltas and fan-delta deposits have been studied in order to establish the data base necessary for erecting models for recognition of ancient fan-delta deposits and for inferring depositional processes (McGowan, 1970; Galloway, 1976; Wescott and

Ethridge, 1980; Ethridge and Wescott, 1984). Presently there are three basic models for fan-deltas (Wescott and Ethridge, 1980; Ethridge and

Wescott, 1984): 1) shelf-type model; 2) slope-type model, and; 3)

Gilbert-type model. Postma and Roep (1985) have recently added to this with a modification of the Gilbert-type. All three models are coarsening upward sequences. The slope- and shelf-types build into a marine environment and have well-developed transition zones consisting of beach, tidal lagoon, and tidal channel deposits. The Gilbert-type fan-delta is found only in lacustrine and intracratonic basins, and is not nearly as well documented as the other two models. Postma and Roep

(1985) point out that the recognition of a Gilbert-type fan-delta depends on the presence of a unit of steeply inclined foresets, although its absence does not exclude mass-flow-dominated fan-deltaic sedimentation.

Table 6 lists the characteristics of the three fan-delta models of

Ethridge and Wescott (1984) along with those of the Fig Tree fan-delta.

It can be seen that the Fig Tree fan-delta does not conform well with any of these models. The least amount of correlation is with the

Gilbert-type model and the best correlation is with the shelf-type model. The Fig Tree fan-delta may be termed a 'hybrid1 shelf-type 80

m eters LEGEND 700- - clast supported conglomerate - matrix supported conglomerate

- sandstone -1 s i lt s t o n e 600-» - black chert

- Msauli Chert

:= - horizontal lamination 5 0 0 -S ^ - cross bedding

400-^2

300- -si

200 - =1 m cobble pebble 100-

lm

Fig. 27. Representative partial stratigraphic sections from different portions of the Fig Tree fan-delta. A - predominantly sandstone with interbedded siltstone in prodelta zone (section S-14); B - transition zone with debris-flow conglomerate and fluvially deposited conglomerate (section S-17); C and D - predominantly coarse, fluvial, clast-supported conglomerate from subaerial portions of the fan (sections S-10,S-21). Table 6. Comparison of fan-delta model characteristics from Ethridge and Wescott (1984) with the Fig Tree fan-delta of this study.

Slope-type Shelf-type Gilbert-type Fig Tree

Overall coarsening upward Coarsening upward sequence Overall sequence is thin Coarsening upward sequence sequence may not be well well developed compared to o th er two is well developed developed models Proximal subaerial fan - Proximal subaerial fan - Proximal subaerial fan - poorly bedded, well-imbrica­ Large-scale gravelly poorly bedded, abundant im­ poorly sorted, massive ted poorly sorted coarse fo resets brication, mostly ungraded conglomerate interbedded conglomerate coarse conglomerate with cr-dely horizontally Reported only from lacus­ bedded conglomerate and Mid-fan - imbricated fine­ trine and intracratonic Mid-fan - horizontally conglomeratic sandstone. grained conglomerate and basins bedded and planar tabular Landward dipping imbrica­ planar cross-bedded and cross-bedded fine conglomer­ tio n horizontally stratified ate and conglomeratic sandstone sandstone Distal subaerial fan - crudely horizontally Distal subaerial fan - Distal subaerial fan - bedded, landward-imbrica interbedded planar cross­ sheetflood deposited sand­ ted conglomerate and lam inated and rip p le d r i f t stone conglomeratic sandstone. sandstone. Distal sand may Rare trough cross-bedded be reworked into spits Transition zone character­ conglomeratic sandstone ized by tu rb id ite s which Coastal zone may be charac­ represent subaqueous T ransition zone - w ell- te riz e d by broad lagoon w ith continuations of subaerial sorted conglomerate and active tidal channels sheetfloods sandstone with horizontal bedding, swash laminations, Slope and basin deposits and seaward-dipping im bri­ are very fine-grained cation sandstone and s ilts to n e

Coarse-grained slump deposits Slump deposits rare - indi­ just seaward of transition cating little development zone of a slope

Slope deposits are muds and matrix supported conglomerate with abundant slumps Fig. 28. Paleogeographic reconstruction of the Fig Tree fan-delta system.

CO ro 83

fan-delta: it has characteristics common to the shelf-type but also has characteristics of the slope-type and characteristics that are not part of any of the current models. Similarities to slope- and shelf-types can be seen in Table 6: The coarsening upward sequence is well developed; proximal and mid-fan regions display features similar to those of the shelf-type; an inferred slump deposit may be indicative of the presence of a slope as might the presence of turbidites interbedded with prodelta and basin deposits. Differences between the Fig Tree fan- delta and the three models include: no well-developed transition zone with such features as beach or tidal channel deposits and inferred distal fan deposits which are sheetflood-dominated. A paleoreconstruction of the Fig Tree fan-delta featuring the inferred relationship of the different eubenvironments is shown in Fig. 28.

This system is one in which a streamflow- and sheetflood-dominated alluvial fan or fans prograded into a nearby body of quiet, relatively shallow water.

Lithofacies C£ - Volcaniclastic sequence

General Characteristics. The eastern portion of the study area is marked by the appearance of a distinctive sequence of volcaniclastic plagioclase-porphyry breccias and conglomerates with interbedded volcaniclastic sandstone (Plate I). The volcaniclastic sequence attains a maximum thickness of approximately 400 meters. The volcanic nature of the sediments is shown by the presence of pumiceous grains, microphenocrystic grains, volcanic quartz, and abundant euhedral 84 plagioclase grains. A schematic stratigraphic column of the volcaniclastic sequence is shown in Fig. 29.

Description and Composition. Sandstones in Lithofacies C 2 occur as several thick units and as numerous thinner beds interbedded with conglomerate and breccia (Fig. 29). The thinner beds may be laterally continuous for only a few or a few tens of meters and even the thicker beds grade laterally into either plagioclase-porphyry conglomerate or breccia. Weathering of Lithofacies C2 sandstone is locally severe, producing a crumbly, poorly indurated rock.

Volcaniclastic sandstones of Lithofacies C2 have a range of colors including grayish yellow (5YR 8/4), yellowish gray (5Y7/2), greenish gray (5GY 6/1), grayish yellow green (5GY 7/2), and pale olive (10Y

6/2). Grain size ranges from medium-grained sandstone to fine conglomeratic sandstone, with most beds being coarse- to very coarse­ grained sandstones. Grains are irregular in shape, showing little effects of rounding. Sandstones locally have a streaky, smeared out appearance, with grains that appear to have been flattened, and also may have flecks of small white grains evenly distributed through the rock.

In hand sample grains of quartz and euhedral plagioclase can be recognized. Quartz never comprises more than 10% of the sandstone, whereas plagioclase may locally make up as much as 25%.

Petrographically, the sandstones of Lithofacies C2 are composed of grains of chert, quartz, and plagioclase, with minor amounts of iron oxide and calcite or dolomite. The chert grains have intergrown sericite in amounts ranging from <10% to >70% with an average of about Meters

intrusive

400

Porphyry-clast breccia intruded by plagioclase porphyry

300 Covered - probably sandstone

Volcaniclastic sandstone 200-■ Volcanic-porphyry-clast conglomerate ‘ f f - n * 6 V a .ft.■®'2 • * a»Q'*.e y-o'- '■£*

Covered - probably sandstone. Laterally breccia is exposed

,V'-; f/. ; • A 100 Volcaniclastic sandstone

• .*.* . o'*► *O, . °• V #0 •' - +■'* i '*• V •■ °_ v *—g •.''. •-•' Volcani c-porphyry-clast conglomerate with sandstone interbeds Sedimentary-chert-clast conglomerate

cong1omerate/brecci a

sandstone

Fig. 29. Generalized stratigraphic column of the volcaniclastic sequence (Lithofacies C 2 ) in the Fig Tree Group. Section S-33. 8 6

30%, and locally contain small amounts of chlorite. The matrix of the sandstones is chert and intergrown chert/sericite. The chert/sericite matrix may be deformed framework grains, and thus be more properly termed pseudomatrix (Dickinson, 1970). Volcanic quartz is recognized by the presence of resorption embayments and planar edges intersecting at 90 and 120 degrees. Plagioclase, when it can be identified, is heavily altered to sericite or less commonly to calcium carbonate.

Pumiceous grains are present, showing a flattened, vesicular texture

(Fig. 30).

Silicification and sericitization have obscured many textural and compositional features in Lithofacies C ^ sandstones. Areas of cherty sericite that resemble squashed and elongate grains may represent altered and deformed aphanitic volcanic rock fragments. Rectangular shaped areas of sericite and cherty sericite may represent altered plagioclase grains.

Sedimentary structures present in Lithofacies C ^ include flat- lamination, cross-lamination, low-angle cross-stratification, and festoon cross-stratification. These structures form no recognizable cycles or sequences. Small cut and fill structures are common, producing fining upward sequences. Sinuous-crested ripple marks were seen at one locality (Fig. 31).

Beds of volcaniclastic breccia and conglomerate occur as predominantly stuctureless layers ranging in thickness from <1 m to units over 8 meters. These very thick units may be an amalgamation of several beds but display no discernable bedding. Breccias are more 87

Fig. 30. Photomicrograph of pumiceous grain from sandstone of Lithofacies C 2 . Plane lig h t, 4x. Sample 60-4.

Fig. 31. Ripple marks in sandstone of Lithofacies C 2 . 8 8

common than conglomerate, comprising about 60-65% of the coarse grained portion of Lithofacies C2.

Breccia and conglomerate beds are poorly to very poorly sorted.

Clasts range in size up to a maximum of approximately 20 cm in diameter, averaging approximately 3 to 4 cm in diameter. The majority of clasts in the breccias and conglomerates of Lithofacies C2 are composed of plagioclase porphyry. These porphyry clasts are identical in appearance to the massive intrusive plagioclase porphyry which occurs throughout the study area. Other clast types present include black chert, gray chert, translucent chert, jasper, fuchsitic rock, and clasts of very dark green aphanitic rock. The matrix of these breccias is typically a light green fine-grained ashy looking sediment, and commonly contains dispersed euhedral plagioclase grains.

The breccias and conglomerates of Lithofacies C2 are predominantly matrix supported. There are local occurrences of clast-supported beds that display cut and fill features. Crude inverse grading is developed in some of the breccias and conglomerates (Fig. 32), but most are massive and structureless and have a chaotic appearance. Bedding sags occur at one locality (Fig. 33) with pebble-sized volcanic clasts sinking into and deforming underlying laminated sediment.

Sedimentation. Sedimentation in Lithofacies C2 was dominated by debris flow processes. This is suggested by the presence of inversely graded breccia/conglomerate beds, by the dominance of matrix-supported units over clast-supported units, and by the massive character of many of the beds and paucity of tractive-current structures. Figure 32 is a partial stratigraphic column of a portion of the volcaniclastic Meters

.■•o'.* 0 o'.O ’ • • , t0 * ’ l b 0 Matrix-supported conglomerate t ■ 0 ° ‘' ■«. .V • c 0 No-' s / covered

5 Fine conglomerate and very coarse-grained sandstone. Channels into underlying beds.

Interbedded sandstone and chert

4 Coarse- to very coarse-grained inversely graded sandstone

3 Poorly exposed. Appears to be fla t- laminated sandstone.

2 Festoon cross-stratified sandstone

Flat-laminated sandstone

1 Irregular contact 0 '.CD. Matrix-supported conglomerate with crude o inverse grading

0 conglomerate/brecci a sandstone

Fig. 32. Partial strati graphic column of conglomerate and sandstone in Lithofacies C 2 . Measured section S-24. Fig. 33. Bedding sag in volcaniclastic sequence of Lithofacies C 2 . Laminated sediment at arrows is deformed by overlying pebbles. 91

sequence. A crude fining upward trend is shown with a debris-flow breccia at the bottom, overlain by flat-laminated, massive, and cross­ stratified sandstone. The top of the trend is marked by a half-meter thick sequence of interlayered chert and fine-grained sandstone, which in turn is channeled by the overlying conglomerate unit. This type of sequence is not not well developed throughout the volcaniclastic sequence. More commonly, there may be many meters of unstratified, matrix-supported breccia/conglomerate.

The plagioclase-porphyry composition of the volcaniclastic sequence suggests a genetic link between the volcaniclastic sequence and the intrusive plagioclase porphyry. Possibly the volcaniclastic sediments are a surface expression of the igneous activity responsible for the generation of the intrusive porphyry.

The dominance of debris flow processes indicates either unstable slopes or very rapid sedimentation, or both. It cannot be said whether or not the debris flows of the volcaniclastic sequence were generated on the slopes of a volcanic edifice, but clearly the debris was not transported great distances. Most of the clasts are very angular and obviously have not traveled far. The inclusion of chert clasts suggests that uplift either exposed bedded cherts which were then eroded, or that older coarse sedimentary beds were reworked. The fuchsitic clasts may have a xenolithic origin. They are very angular and possibly were incorporated into the volcaniclastic debris during eruption.

The volcaniclastic rocks of Lithofacies of the Fig Tree Group record a major magmatic episode, possibly the last such activity in the evolution of the Barberton greenstone belt. DIAGENESIS AND METAMORPHISM

Much of the original mineralogy of the Fig Tree Group sedimentary rocks has been altered or obliterated by silic ific a tio n , metasomatism, and low-grade metamorphism. Alteration has in many cases preserved original grain texture and sedimentary structures, while generating an entirely new mineralogy. This discussion will outline the major alteration effects, especially silicification.

Alteration. The type of alteration and alteration products varies with grain types. Detrital megaquartz grains remain essentially unaltered. Quartz grains do not show strain effects such as undulose extinction or grain suturing and crushing.

Recognizable detrital feldspar grains are extremely rare in Fig

Tree sedimentary rocks of this study. Only a few grains of plagioclase and orthoclase were seen in examination of over 40 thin sections. Most sandstones are now composed of chert and microcrystalline phyllosilicate, mainly sericite. The possibility exists that some of these grains were originally feldspar. Reliable indicators such as euhedral or lath-like morphologies were not noted, with most grains having irregular, subrounded to subangular shapes. Volcaniclastic grains in Lithofacies A2 do, however, contain recognizable subhedral and euhedral chert pseudomorphs which can be identified as feldspar by grain morphology (Fig. 9a). It is apparent that feldspar grains may be completely replaced by chert-sericite mosaic and that such grains may be unrecognizable, but present, in Fig Tree sandstones. The paucity of subhedral and euhedral grains suggests that feldspar is an insignificant component of Fig Tree sedimentary rocks.

92 93

Grains of known volcaniclastic origin (e.g. Lithofacies Ag) undergo several types of alteration. Most commonly the altered grains are a micromosaic of chert and sericite or chert and chlorite.

Volcaniclastic grains may be essentially pure chert. In some cases, grains have a brown oxidized rim. The presence of phyllosilicate alteration product is ubiquitous in volcaniclastic units and is suggestive of the original presence of aluminosilicates.

Fig Tree sandstones and conglomerates are composed predominantly of chert grains, and evidence suggests that these grains were deposited as already silicified grains, although further silicification has occurred after deposition. This evidence is threefold: 1) The sandstone grains contain veins of both megaquartz and chert that do not extend beyond the grain boundary. In the source area, therefore, silica veins cross-cut already lithified, and probably silicified, rock. 2) The chert grains have textures suggesting that the source rock was fine-grained. Silicification produces remarkable preservation of coarse-grained and porphyritic textures (Fig. 9a) and presumably one would see such textural features if the original rock contained them.

The chert grains in most Fig Tree sandstones are rather homogeneous, containing varying amounts of phyllosilicates intergrown with chert.

This texture suggests that the original rock was a fine-grained sedimentary or volcanic rock. The angular shape of many of the sandstone grains supports the idea of lithification (possibly silicification) prior to transport and deposition. 3) The pebbles and cobbles of the Fig Tree Group are composed mostly of varieties of chert

(Table 4). These chert clasts almost certainly were chert prior to deposition as shown by a wide variety of chert types occurring as clasts in a sandy matrix which is not commonly silicified (although the sand grains in the matrix are commonly chert). Silicification after deposition would presumably affect the whole rock and not just the pebbles and cobbles. Simonson (1985) reports the common occurrence of sand and gravel-sized sediment that was chert prior to detrital reworking in the Early Proterozoic Wishart Formation in Newfoundland.

The evidence suggests then that silica was both common and abundant, and that sand and gravel was already silicified prior to deposition and further silica cementation and replacement. Recent work by de Wit et al. (1982) in the Barberton greenstone belt and Simonson (1985) in

Proterozoic rocks of the Labrador Trough, as well as earlier work by

Lowe and Knauth (1977) in the Barberton Mountain Land, documents early silicification of sediments before incorporation into younger sedimentary rocks.

Silicification of rocks of the Barberton Mountain Land has been attributed to hydrothermal alteration that occurred prior to any tectonic deformation (deWit et a l., 1982, Hanor and Duchac, 1985, Paris et a l., 1985). Simonson (1985) suggests that chert of the Proterozoic

Wishart Formation of the Labrador Trough formed from ascending thermal waters. Hanor and Duchac (1985) suggest that silicification took place in conjunction with the ascension of hydrothermal solutions. As the fluids rise, the decrease in temperature favors precipitation of silica and muscovite. This mineral association is common in grains in Fig

Tree sandstones. The work of Hanor and Duchac (1985) is applied to the silicification of komatiites and komatiitic basalts. It has been suggested in the present report that many of the sandstone grains were chert prior to deposition. That some of these chert grains may be 95 silic ifie d ultramafic rock is suggested by the high values seen for both Cr and Ni in bulk chemical analysis of Fig Tree sandstones.

The multiple episodes of silicification make it difficult to state with certainty the sequence of alteration events in Fig Tree sandstones. Low-grade metamorphism most likely resulted in sericite and chlorite production after Fig Tree deposition, although it cannot be ruled out that some detrital grains had been altered prior to transport and deposition (deWit et a l., 1983). S ilicification has been severe enough in some cases to totally obscure grain boundaries, making paragenetic reconstruction difficult.

A common detrital framework grain-type is carbonaceous chert.

These grains are mixtures of microquartz intergrown with clumps and wisps of carbonaceous matter, and they are also interpreted to have been silic ifie d when deposited. Carbonaceous chert grains appear to have undergone l i t t l e to no post-Fig Tree alteration. They are composed of microquartz and carbonaceous matter along with trace amounts of diagenetic pyrite, and dolomite (see below). The black chert grains appear to be identical to bedded black cherts found toward the base of the Fig Tree Group and throughout the Kromberg Formation.

Cementation. The primary cementing agent in Fig Tree sedimentary rocks is microquartz. Early cementation is shown by the cherts of

Lithofacies A^. L ittle compaction took place as evidenced by the presence of uncrushed delicate volcaniclastic grains and by the loose, grain-supported texture of A^ and other sandstones. A locally occurring intraformational conglomerate (Fig. 10) contains clasts made up of coarse sand-sized grains. Original deposition of the sediment now composing these clasts was as loose, cohesionless detritus. Some degree of lithification (cementation) must have occurred for this sediment to be ripped up and form clasts.

Fracturing closely followed cementation in many rocks, producing silica-cemented brecciated layers. In some cases the direction of fracture-filling can clearly be determined by the inclusion of fragments of distinct layers found in close proximity. This type of fracture filling has also been noted by Paris et al. (1985). Lowe and

Knauth (1977) documented black chert veins which behaved in a similar manner, brecciating surrounding rock.

Figure 34a shows re lic t sweeping radial extinction in chert cement. The sweeping extinction indicates that before recrystallization to chert this cement was fibrous. This once-fibrous pore-filling resembles chalcedony and locally gives way to megaquartz in the center of pores. The overwhelming abundance of silica throughout the Fig Tree Group and the lack of impurities in the cement suggest that original cementation was siliceous rather than replacement of some other mineral by silic a . In some cases the cement may have originally been chert, but in other cases chalcedony formed and has since been recrystallized to chert. Several generations of cement can be seen in Figure 34b.

Numerous post-cementation quartz veins occur in the study area.

These veins range in size from m icrofracture-fil1ings less than a millimeter in width to large cavities 20 cm in thickness (Lowe et al.,

1985). Cross-cutting relationships of the smaller veins (.5 to 2 mm thick) indicate at least three generations of silica fracture-filling. Fig. 34a. Relict sweeping radial extinction in chert cement. Lithofacies Ao.

Fig. 34b. Two, possibly three generations of cement in rock of Lithofacies A 2 . A thin radial fringe possibly followed by chert, with megaquartz in the center. Some larger veins are quite disruptive and can be seen to have incorporated and moved small pieces of the surrounding rock.

Late Stage Alteration. Other authigenic minerals present include dolomite and pyrite. Partial replacement by carbonate is locally intense and tends to obscure the textural features of the rock. The dolomite occurs as well-developed rhombohedra within chert, indicating growth after s ilic ific a tio n . Hematite occurs as rhombohedral replacement of carbonate suggesting that the carbonate was iron rich, possibly siderite or ankerite. Authigenic pyrite occurs as cubes and pyritohedrons. Commonly the distribution of pyrite is controlled by bedding planes. GEOCHEMISTRY

Analytical Methods

Twenty six samples of Fig Tree rocks were selected for bulk chemical analysis. The samples were prepared using the metaborate method of Ingamells (1970), and analysis was carried out using an ARL

3400 ICP spectrometer. Sixteen elements were analyzed by fixed detector system. SPEX solutions were used as standards for trace elements. The U.S.G.S. standard AGV-1 was run along with the samples as an unknown to determine the accuracy of the runs, and the following data were obtained:

ICP values Quoted U.S.G.S. values (wt.%) (wt.%)

Si02 59.94 ± 1.64 59.64

Ti02 1.05 ± 0.014 1.06

A1203 17.23 ± 0.68 17.19 FeO 6.22 + 0.68 6.9 Na20 3.54 ± 0.16 4.32

K2 0 2 . 8 6 ± 0.06 2.92 MgO 1.53 ± 0.10 1.52 CaO 4.82 ± 0.11 4.94 MnO 0.11 + 0.02 0.10

P2 05 0.52 ± 0.16 0.51

Ba 1293 ± 6 6 1200* Sr 669 ± 46 660*

*ppm

99 100

In addition to ICP elemental analysis, all samples were examined by x-ray diffraction using a Philips XRG-3000 diffractometer at 40 KeV with copper radiation. Results of the ICP and XRD analyses are shown in Tables 7 and 8 respectively. Totals for weight percents in Table 8 are in most cases less than 100. Water and other volatiles make up the remaining mass, most of which probably reside in the phyllosilicates

(G. Byerly, pers. comm., 1985).

Sandstones and si Itstones were chosen for analysis. Two groups of sandstones (locality 36 and 37, Table 7) are from two measured sections, with locality 37 being stratigraphically above locality 36.

Three samples of volcaniclastic sandstone were analyzed. The si Itstones were collected throughout the study area.

Geochemical Relations

Major elements. The major element composition of sandstones from the Fig Tree Group are characterized by high to very high Si (69 to 92 wt.%), and very low Na (less than .05 wt.%) and Ca (not present in measurable quantities) contents. Siltstones and shaley siltstones have lower Si values than the sandstones (Table 7) and this is reflected in the overall lower amount of silicification of the siltstones over the sandstones. Average Fe contents of Fig Tree Siltstones is higher than that of sandstones (Table 7).

Abundances of many of the elements appear to be correlated with proportions of muscovite or chlorite. A positive correlation was found between the concentration of I^O and MgO and corresponding XRD peaks of Table 7. Chemical composition of rocks of the Fig Tree Group, this study. Major elements in wt.%; trace elements in ppm. Localities 36 and 37 are measured partial sections S-14 and S-15 respectively. Other samples are from various sites within the study area.

Volcaniclastic ------locality 36------locality 37- . ------Siltstones ------S a n d s to n e ------

S i0 2 85.01 79.94 6 8 .9 79.76 83.1 82.1 8 5 .3 7 6 .5 8 9 .8 8 8 .9 92.7 82.7 91.9 79.24 7 6 .6 6 5 .8 6 1 .1 3 5 8 .8 66 7 2 .5 6 6 .3 89 9 7 .8 6 7 .6 74.7

T i0 2 .297 .23 .432 .32 .13 .2 6 .21 .2 9 .1 9 .21 .116 .6 5 .2 .47 .5 .67 .66 .73 .74 .8 .7 .09 .3 3 .69 .3 6 a i 2o 3 4 .4 6 7 .4 9 9.89 5.95 5.5 5.87 3 .6 8 5 .7 7 2.97 3 .3 9 1 .9 9 12.6 3.21 10.7 10.1 14.1 15.7 18.13 14.62 1 7 .6 15.52 2 .4 .9 16.7 16.03

FeO 5 .4 8 8.07 11.32 8.49 6.15 5.85 5.8 13.74 4 .9 3 1 .0 8 1 .9 6 .25 1.67 2.93 3.28 8 .6 11.78 12.77 7.77 4 .2 3 10.16 7.29 .16 6.68 2.84

HnO .0 2 .03 .038 .024 .0 2 .02 .009 .02 .02 .002 .08 - .008 .007 .01 .02 .23 .035 .003 .07 .078 .01 .002 .04 .072

MgO 1 .7 6 3.27 4 .5 2 2 .9 9 2.21 2 .2 9 .05 .078 .04 .08 .49 .18 .02 .17 .15 .24 1.84 1.07 .19 .80 2.52 .043 .12 .344 1.29

C a O ------.73 -- - - .002 - - .03 - .027 -- .23 na 2o .0 0 6 - .016 --- .02 .05 - .004 - .04 - .12 .055 .11 -- .12 .24 -- .03 .35 .044 k 2o -• .036 - - - .856 1 .0 6 .5 9 .5 8 .342 2 .0 4 .54 2 .2 2 .0 3 .7 3 .2 4.01 3.02 2.72 3.6 .41 .64 3.4 3.89

£ 97.03 99.03 96.15 9 7 .5 3 97.11 9 6 .4 9 5 .9 9 7 .5 98.54 94.34 98.4 9 8 .4 97.55 95.84 92.7 93.24 94.54 9 5.55 92.4 99.04 98.9 99.24 100.12 9 5 .8

Ba 132 54 261 131 105 334 4463 9504 4946 6127 3803 26459 4494 13968 8992 7512 2509 5013 2607 7497 2129 1187 121 1027 706

Sr 11 5 8 6 - 5 12 U 7 5 8 21 - 24 19 26 5 8 3 1 61 5 12 5 124 54

Zr * * - 6 8 - 14 - - 17 30 54 -' 50 - 66 66 - 79 - 12 3 - 124

Cr 230 291 65 376 123 399 322 365 217 321 186 1012 247 1190 1277 1130 1079 1626 1349 1308 1255 138 537 486 18

Hi 58 180 111 85 - 102 - 29 -- 73 ----- 308 112 - 236 2 6 8 - 186 91 -

V 98 136 153 81 61 81 64 68 34 52 3 1 1 179 12 190 143 213 205 247 251 2 6 5 2 2 0 47 62 127 23

Co 32

O 102

Table 8 . Results of XRD analysis of southern Fig Tree siltstones and sandstones.

QUARTZ SERICITE CHLORITE HEMATITE

Sample

36-14 X X 36-13 X X 36-10 X X 36-9 X X 36-8 X X 36-7 X X 37-13 X X 37-12 X X 37-11 X X

37-10 X X X X X X X X X X 37-9 X X 37-7 X X X 37-6 X X 37-5 X X X 37-3 X X X 37-2 X X

9-12 X X X 55-1 X X X 48-4 X X X 13-1 X X X 33-6 X X X 6-4 X X X 12-1 X X X 4-5 X X 52-2 X X 18-2A X X X 65-2 X X 103 muscovite and chlorite respectively. Petrographic examination corroborates this correlation. While XRD analysis shows the presence of muscovite, petrographically this mineral is more properly termed sericite.

SiOg values range from 58.8 wt.% to 97.8 wt.%. The average Si0 2 value for the sandstones at localities 36 and 37 (measured sections) is

82.8 wt.% (Table 7). This high value is a reflection of the abundance of original chert framework grains which are now cemented by GMC (see

Diagenesis section). Lowest silica values are seen in the ferruginous siltstones of Lithofacies-Bl while the highest silica value is from

Lithofacies-Ag (Table 7).

Absolute abundances of Fe and Mn show a crude correlation with the concentration of Mg. This relation is probably due to incorporation of these elements into chlorite, or possibly into carbonate which is present in small amounts in some samples. This carbonate occurs as small, euhedral rhombohedra and is interpreted to be dolomite.

McLennan, Taylor and Eriksson (1983) note a similar relationship in

Archean shales from the Pilbara Supergroup of Western Australia.

Trace elements. There is a direct correlation between Ba and (K

+ Al) abundances, with high Ba values corresponding to high K and Al values (Table 7 and Fig. 35). XRD analysis shows samples with high Ba values to contain muscovite. The high Ba values are probably a reflection of incorporation of Ba into the muscovite structure (J.

Hanor, pers. comm., 1984). Sr and Ba have a positive correlation with one another (Fig. 35), and most likely the Sr is also residing in the 104

zo - D □ □B □ Al 203 0 (wt%) 0 0 10 O 0 sandstones, ©0 ° locality 36 0 <*0 0

□ □ - siltstones

10 20 30 Ba, 1000 ppm

k2o (wt%) B

1000 ppm

20 '

Ba lOOOPpm

10 -

Sr ppm

Fig. 35. (A) Plot of Ba vs. AI 2 O3 . (B) Plot of Ba vs. K2 O. (C) Plot of Ba vs. Sr. Note positive correlation. It is suggested that the mineral muscovite controls the distribution. Samples are from locality 37, Section S-15 (see -Table 7). In (A) sandstones from locality 36, and siltstones from various localities are shown for comparison. 105

muscovite. An interesting segregation of Ba concentrations can be seen between samples of lo calities 36 and 37 in Table 7. Samples from locality 36 have low Ba values and correspondingly low K and Al values compared to samples from of locality 37. XRD results of locality-36 samples show them to contain no muscovite (Table 8 ), thus the low Ba concentrations. The reason for this difference in Ba and muscovite abundance between the two stra ti graphic localities is not entirely clear. The two stratigraphic sections that show this large difference in barium abundance do not display any sedimentologic or petrographic evidence of having different provenances. Barite deposits occur in the

Barberton Mountain Land (Reimer, 1980; Heinrichs and Reimer, 1977; this report) and possibly one of these areas was weathered and eroded, providing barium in aqueous solution. Hanor and Duchac (1985) suggest that Ba enrichment of silicified komatiitic lavas took place by reaction with ascending hydrothermal solutions. This alteration was also responsible for the silicification of the volcanic rocks and possibly Ba enrichment accompanied the silic ific atio n of the Fig Tree sandstones. Another possibility is that conditions similar to those when barite deposits formed, that is, locally higher barium concentrations, existed during deposition or diagenesis of the sandstones of locality 37.

The abundances of Cr and Ni in Fig Tree sedimentary rocks are high compared to abundances seen in younger sedimentary rocks. Table 9 compares Cr and Ni abundances from rocks of this study with Cr and Ni values from other Fig Tree sediments, other Archean terranes, and from younger sedimentary rocks. 106

Table 9. Comparative abundances of Cr and Ni for Fig Tree rocks and other clastic sedimentary rocks. All values are ppm.

1 2 3 4 5 6 7

445 1116 860 * 641 90 50 43 144 495 261 342 6 8 23

1: Average for sandstones, this study. 2: Average for siltstones, this study. 3: Average for Fig Tree shales (Danchin, 1967). 4: Average for Fig Tree graywackes (Condie et a l., 1970). * - no measurement made 5: Average for Gorge Creek Shales, Pilbara Block, Australia (McLennan, Taylor, and Eriksson, 1983). 6 : Average shale (Danchin, 1967). 7: Average fresh water sedments (Danchin, 1967). 107

The anomalously high Cr and Ni are similar to those seen in other

Archean terranes (Collerson et a l., 1976; Naqvi, 1978; McLennan,

Taylor, and Kroner, 1983; and Table 9). Laskowski and Kroner (1985) have shown a secular decrease in Cr/Ni and Cr/TiOg ratios for shales through geologic time, and the Cr/Ni and Cr/TiOg for siltstones of the present study fall within their Archean fields. Danchin (1967) first noted the unusually high Cr and Ni values for shales of the Fig Tree, and attributed these abundances to an ultramafic source. Mclennan,

Taylor, and Eriksson (1983) show that the anomalously high values for

Archean shales of Western Australia are not explained by provenance alone, and suggested that enrichment through weathering may have affected the Cr and Ni abundances. It cannot be said if some sort of secondary enrichment process such as weathering has resulted in the observed values of Cr and Ni for Fig Tree sedimentary rocks of this study, but it seems clear that the source, in part, was considerably more mafic than for post-Archean sedimentary rocks (Table 9).

Geochemical comparisons and provenance

Table 10 shows the major element abundances of Fig Tree sedimentary rocks along with average Archean, Proterozoic, and

Phanerozoic clastic sedimentary rocks. The most notable differences are the very high silica content of sandstones of this study, and the low Mg and Na. Sandstones of this study (column 1, Table 10) average almost 83 wt.% Si0 2 with some samples containing over 90 wt.% SiO^.

These remarkably high Si0 2 values are a reflection of the severe 108

Table 10. Major element analysis of Fig Tree sedimentary rocks and average Archean, Proterozoic, and Phanerozoic clastic sedimentary rocks.

1 2 3 4 5 6 7

Si02 82.8 6 8 . 0 63.04 68.74 64.37 65.9 70.0 70.6

Ti02 0.3 0.63 0.53 0.27 0.52 0 . 6 0 . 6 0.7

a i 2 o 3 6.03 14.32 13.85 13.86 1 1 . 1 1 14.9 14.5 14.2

FeO 5.4 8.84 4.88 3.09 6 . 8 8 6.4 5.5 5.2

MgO 1 . 2 1 . 0 2 3.6 2.08 4.47 3.6 2 . 1 2.4

CaO 0.05 0.007 2.62 1 . 0 0 2.37 3.3 1.7 2 . 2

Na2 0 0 . 0 2 0.06 2.29 0.36 2.13 2.9 1 . 8 1 . 8

k2o 0 . 6 8 3.3 2.54 5.58 1.72 2 . 2 3.2 2 . 8

1: Lithic arenites of Lithofacies EL, this study. 2: Siltstones of Lithofacies B., this study. 3: Graywackes from Mapepe Fm., Fig Tree Group (Heinrichs, 1980). 4: Loenen micaceous graywacke Member, Umsoli Fm., Fig Tree Group (Heinrichs, 1980). 5: Average for Sheba Fm. and Belvue Rd. Fm., Fig Tree Group (Condie et a l., 1970). 6 : Average Archean clastic sedimentary rocks (McLennan, 1982). 7: Average Proterozoic clastic sedimentary rocks (McLennan, 1982). 8 : Average Phanerozoic clastic sedimentary rocks (McLennan, 1982). 109

diagenetic alteration that has affected rocks of the Barberton belt on a regional scale. The original rock may be almost completely replaced by SiOg (e.g. sample 4-5, Table 7). Weathering and metamorphism can also affect the chemistry, and it is therefore somewhat hazardous to try to relate the composition of Fig Tree sedimentary rocks to that of a source area. Heinrichs (1980) has compared the major element chemistry of Fig Tree sandstones with the chemistry of the Hooggenoeg

Formation of the Upper Onverwacht Group (Fig. 2) and concluded that the

Hooggenoeg was the source area for these Fig Tree sandstones.

Comparisons of this type are invalid due to effects of alteration as shown in the present study.

The geochemical and mineralogical analyses of southern Fig Tree rocks have resulted in four conclusions. Most importantly, it has been shown that the silicification of sedimentary rocks can be virtually complete, including removal of relatively immobile elements such as Al.

The precise nature of the silicification process is not yet clearly understood, but is thought to be related to hydrothermal alteration processes (de Wit et a l., 1983; Hanor and Duchac, 1985). Rare earth element patterns, due to the immobility of REE during sedimentary and metamorphic processes, may yield useful information regarding provenance and crustal evolution (McLennan, Taylor, and Eriksson, 1983;

McLennan, Taylor, and Kroner, 1983). Due to limitations of the ICP, however, rare earth elements were not analyzed. Secondly, high Cr and

Ni values reflect a mafic or ultramafic source. High Cr and Ni values are common in other Archean sedimentary rocks as well (McLennan,

Taylor, and Eriksson, 1983; Collerson et al., 1976). Thirdly, unusually high Ba values in one suite of samples (locality 37, Table 7) indicate locally higher Ba concentrations, possibly due to erosion of an older, barite-bearing terrane, or the presence of muscovite. The variable enrichments of K and Ba, which tend to vary with the presence of sericite, are not easily associated with any greenstone belt source and may be produced by some unspecified alteration process. Lastly, all sedimentary rocks analyzed have extremely low Ca values (Table 7).

McLennan, Taylor, and Eriksson, (1983) note similar Ca abundances for

Gorge Creek sediments of the Pilbara Block in Western Australia. The most resonable explanation seems to be that the source of the sediment was "calcium-free" before erosion, possibly as a result of silic ific a tio n . Hanor and Duchac (1985) document the strong depletion of Ca during silicification of komatiites. DISCUSSION

Sedimentary Evolution

Sedimentary rocks of the Barberton Mountain Land display definite trends, most notably differences in both composition of sediment and in styles of sedimentation, between the Onverwacht Group and the overlying

Fig Tree and Moodies Groups. Sedimentary rocks of the Onverwacht Group are predominantly silicified volcaniclastic rocks, carbonaceous cherts, and banded cherts (Lowe and Knauth, 1977, 1978; Lowe, 1980; Lowe 1982b;

Lanier and Lowe, 1982). Significant amounts of terrigenous debris derived by weathering and erosion did not accumulate until the onset of

Fig Tree sedimentation. The Moodies Group represents further sedimentary evolution of the greenstone belt, being composed in part of quartzose sandstones. Sedimentation of the Moodies Group has been described in detail by Eriksson (1977, 1978, 1979).

Fig Tree sedimentation commenced with the deposition of the Msauli

Chert, which is a slightly reworked, silicified pyroclastic deposit.

The Msauli Chert is overlain by a sequence of carbonaceous black cherts and black and white banded cherts. The deposits of the Fig Tree basal chert units do not differ significantly from silicified volcaniclastic sediments and banded cherts described from the underlying Onverwacht

Group (Lowe and Knauth, 1977, 1978; Lanier and Lowe, 1982) and formed under similar, essentially anorogenic conditions. However, within the banded cherts of Lithofacies are local occurrences of thin sandstone beds that record the initial stages of significant input of terrigenous debris. On the basis of similar composition, texture, and overall

111 112

appearance, these sandstones are interpreted as being from the same source as sandstones in the overlying formation B.

Point count data from Fig Tree sandstones reveal general information regarding provenance. The low percentages of monocrystalline quartz (maximum of 1 0 %) and the virtual absence of recognizable feldspar argue against a continental, sialic source area.

Rather, the source appears to be uplifted portions of the greenstone belt. This is supported by the high Cr and Ni values seen in Table 9, which suggest at least in part a mafic or ultramafic source, and by grain types such as black chert which are recognizable as being derived from underlying rocks. Carbonaceous black chert grains occur in many of the Fig Tree sandstones and are probably derived from the immediately underlying Fig Tree cherts of Formation A or from black chert units in the upper Kromberg Formation (Fig. 2).

Pebbles and cobbles from the conglomeratic sequence of the Fig

Tree provide unambiguous evidence of specific source rocks. Table 4 lis ts relative abundances of different clast types found in the Fig

Tree conglomerates. Several distinctive clast types pinpoint provenance and have important implications regarding tectonics of the source area. These clast types include chert-grain sandstone, jasper, silicified accretionary lapilli-bearing volcaniclastic rock, and plagioclase porphyry.

The clasts composed of chert-grain sandstone are identical to the sandstones of Lithofacies Both binocular and petrographic microscopic examination shows these clasts to have textural and compositional characteristics that leaves little doubt that sandstone of Lithofacies B 9 was their source. In addition, no other similar 113

sandstones occur in any of the underlying sedimentary rocks in the

Barberton Mountain Land.

Jasper clasts occur in small quantities in Lithofacies (Table

4), but are relatively more abundant in the breccias of Lithofacies B^.

The source for these jasper clasts almost certainly was the jaspilite of Lithofacies B^.

The clasts of silicified accretionary lapilli closely resemble rock of the Msauli Chert. There are, however, other silic ifie d accretionary lapilli-bearing units in the Hooggenoeg and Kromberg

Formations (Lowe and Knauth, 1977), making it d ifficu lt to pinpoint the exact source. Clearly, though, these clasts were derived from uplifted greenstone belt rocks.

Clasts of a hypabyssal, dacitic rock occur in Fig Tree conglomerates, usually in relatively low abundances, but locally comprise as much as 40% of the clast population. These porphyry clasts are similar in composition and textural features to the hypabyssal intrusive that occurs throughout the lower Fig Tree and upper

Onverwacht Groups (Lowe et a l., 1985). When considered along with the other clast types found in Fig Tree conglomerates, it is likely that the source for these porphyry clasts was the intrusive porphyry.

In addition to the clast types just described, there are also clasts of black chert, banded chert, and altered ultramafic rock containing the green mica mariposite; lithologies which are common in the greenstone belt. Evidence from the conglomerates indicates then, that the source was uplifted portions of the greenstone belt. While many of the clast types could be derived from either uplifted Fig Tree or from underlying rocks (e.g. black chert, banded chert, accretionary 114

1apiHi), two distinctive types - chert-grain sandstone and jaspilite - indicate that Fig Tree rocks were being uplifted and eroded to provide detritus for younger Fig Tree rocks. This evidence would support a Fig

Tree and upper Kromberg source for virtually all conglomerate clasts because all clast lithologies can be found within either the Fig Tree or Kromberg.

Tectonic Evolution

The occurrence of clasts of Fig Tree rocks in the coarse clastic rocks of the Fig Tree Group, combined with the occurrence of structurally repeated strati graphic packages, has important implications regarding the tectonic evolution of the Barberton greenstone belt. Prior to the tectonism which initiated Fig Tree sedimentation, the Barberton greenstone belt evolved under essentially anorogenic conditions. Sedimentation was directly related to volcanic activity (Lowe, 1982b). Toward the top of the Hooggenoeg Formation

(Fig. 2) is a 'disturbed zone' up to 2 km thick in which large blocks of volcanic rock have been rotated and deformed (Lowe et a l., 1985).

The exact age of this deformation is not known but field relations indicate a pre-Moodies age (Lowe, et a l ., 1985). The relation of this upper Hooggenoeg disturbance, if any, to the deformation affecting the uppermost Kromberg and lower Fig Tree rocks of the present study is unknown. Sustained orogenic activity which resulted in prolonged sedimentation began with the onset of Fig Tree sedimentation and continued through Moodies time. Fig Tree terrigenous clastic sedimentation in the present study area can be directly related to the deformation which produced the repeated stratigraphic packages. 115

Detailed field work has shown that the uppermost Kromberg

Formation and Fig Tree Group of this study have been broken into a series of imbricate sheets, emplaced along low-angle thrust faults

(Dokka and Lowe, 1984; Lowe et a l ., 1985; this study). Each imbricate sheet detached along the altered ultramafic rock zone of the uppermost

Kromberg Formation. Apparently these ultramafic rocks provided a relatively weak zone along which thrusting could take place. These structural belts repeat the uppermost portion of the Kromberg

Formation, the Fig Tree Group, and, as shown by more recent work (D.R.

Lowe, Pers. Comm., 1986), the lower part of the Moodies Group. Within each belt the younging trend is to the north and there is an overall younging trend between structural belts.

Figure 36 graphically summarizes the sequence of tectonic and sedimentation events leading up to and including the imbricate thrust faulting. Stage 1 is a volcanic platform or anorogenic period, characterized by upper Onverwacht and lower Fig Tree rocks. This stage displays little tectonic activity (Fig. 36A). The second stage begins the development of tectonic in stab ility , as shown by the appearance of second cycle clastic detritus. Local uplifts supplied shallow-level intraformational debris to small sedimentary systems that were characterized by widespread lateral facies changes (Fig. 36B,C). The final stage is that of imbricate thrusting and orogenesis (Fig 36D,E).

Sediments of the Fig Tree Group were derived from various levels within the greenstone belt. Overlying Moodies Group sediment had source terrains which were rich in quartz and k-spar, and which were found outside the greenstone belt. 116

S e a L e v e l

T rm rm n'lTll n in I n hititi itimi-i n iiTiiiTiinm fi ri n m rriT

Vv/VVvV'/VVVV'/vvV'/ U U U V V * SI S£ Si V Si V \/ V V V

im-mi nrnim m m Tnrnrrnn uu in u n rm n irmm iriTrr VV'/V'VV’ VVV'VV'/V'/V/v' V SI V V v V. . ,V y SC \ / \ S \ / V v N/

c

ulTi i n n~i11 i m i nrii iin n in n i i m m i innrn |ii iittldihin

-V V V V V V V v V - V V V W V V/ vy

E

V

0 2 km

Fig. 36. Sedimentological and tectonic evolution of the Barberton Greenstone Belt from latest Onverwacht to earliest Moodies times. (A) - Anorogenic stage, predominantly a period of intermittent lava extrusion with interbedded carbonaceous cherts and volcaniclastic units. This stage includes lowermost Fig Tree rocks. (B,C) - Stage of evolving tectonic instability characterized by extensive volcaniclastic and pyroclastic vol- canism and associated sedimentation. At this stage, local uplift and shallow erosion of the greenstone belt supplied terrigenous debris. (D,E) - A final, orogenic stage in which there was thrust faulting and erosion of both intra- and extra­ belt rocks, resulting in widespread clastic sedimentation. ® = point of absolute reference. 117

The precise nature and timing of the thrusting is difficult to discern. Complications arise from: post-thrusting deformation; unknown magnitudes of movement of individual thrust sheets; and possible differential levels of erosion related to post-thrusting deformation. Although it is not known how far individual sheets travelled, it appears that movement was not great because the juxtaposition of facies that may have formed in widely separated parts of the basin is not seen. Evidence suggests that thrusting was initiated during late Fig Tree time. Many of the thrust faults in the

Fig Tree serve as loci for the intrusion of dacitic hypabyssal rocks, which appear to be co-magmatic with dacitic extrusive rocks in the upper Fig Tree (Lithofacies C 2 ). Conglomerates at the top of the Fig

Tree Group contain clasts of upper Fig Tree dacitic hypabyssal rocks as well as clasts of chert derived from lower stratigraphic levels. This relationship suggests that faulting and u p lift accompanied volcanism and intrusion in late Fig Tree time.

The type of thrusting that occurred in the Barberton Greenstone

Belt is thin-skinned (Harrison and Milici, 1977; Boyer and Elliot,

1982; Coward, 1983; Hatcher and Williams, 1986) rather than crystalline

(Hatcher and Williams, 1986), in which the faults are deep-crustal and move high-grade metamorphic rocks. Typically, thrust faults propogate upward and outward from an in itial fau lt, usually along a decollenient zone. The presence of a decollement zone is not prerequisite, and it cannot be said at this time whether or not one exists in the Barberton

Greenstone Belt. In addition to producing repeated lithologic packages, thrusting may produce different internal geometries (Fig.

37). This, combined with different levels of erosion may produce SOUTH NORTH

Fig. 37. Diagramatic sketch showing possible thrusting mechanism. In this example thrust sheets propagate out, to the south. Note how individual thrust sheets have different internal geometries, which would result in different surface exposures depending on the level of erosion. Lithologic units do not represent those of this study. It is not known if a true decollement zone exists in the Barberton Greenstone Belt.. Modified from Harris and Milici (1977). 118 119

significant variations in lithology between structural belts. It is possible that important, diagnostic facies of the Fig Tree Group have been structurally removed - absent due to the vagaries of deformation and erosion.

One aspect of the style of thrusting seen in the Barberton

Greenstone Belt is different from most other thin-skinned thrust-faulted areas. There is an overall younging trend (northward) between the structural belts. This is seen in the appearance of

Moodies Group rocks in uppermost structural zones with the concomitant disappearance of Kromberg Formation rocks. The appearance of Moodies

Group rocks could be explained by the thrust system being active over a geographically wide area, so as to include rocks of diverse origin.

The disappearance of Kromberg Formation rocks, however, implies that the thrust faults are migrating stratigraphically upward. This observation is not easily explained by standard thrusting models.

Harris and Milici (1977) proposed a style of thin-skinned thrusting for the Appalachians that would explain the structural and stratigraphic relationships seen in the Barberton Greenstone Belt. In their model, an abandoning process becomes operative during movement, so that the stratigraphic position of the detachment faults migrates vertically.

This style of deformation is diagramatically shown in Figure 38 for rocks of the Barberton Greenstone Belt. The observations just discussed may be better explained by noting that upper-structural-zone faults are s till located in ultramafic rock - in this case lower Fig

Tree rather than upper Kromberg. The upward migration of the faults may be minimal, or the possibility exists that the volcanic rocks are diachronous. vv | - Kromberg Fm

- Lithofacies A 1 N | | - Lithofacies

- Lithofacies B£ -> -Lithofacies

-Moodies Group

/ - Younging Direction

V VVVV V V V V V V y v w V V V V V V V V V VV VVV/V/VV

Fig. 38. Idealized and diagramatic representation of style of thrusting seen in study area. Basal detachment fault migrates upward and propagates to north in this example. It is not known if there is a true decollement zone in the Barberton Greenstone Belt. Not to scale and exaggerated thrust angles. Modified from Harris and Mi 1ici (1977). Compare with Fig. 37.

rvj o 121

When interpreting Phanerozoic sedimentary basins, tectonics and sedimentation is regarded in the context of plate interactions. The question of whether or not plate tectonics operated in some fashion in the Archean is impossible to resolve at this time. It has been popular to try to extend modern plate tectonic processes to the Archean and to use these processes to explain the sedimentation volcanism, plutonism, and deformation of greenstone belts (Burke et al., 1976; Tarney et al.,

1976; Windley, 1984; and many others). Another view, which takes into account major differences in the geology of Archean terranes as compared with Phanerozoic settings, incorporates the assumption that global tectonics has not been stric tly uniformitarian. This assumption is based on the changing physical conditions of the lithosphere and underlying mantle. Proponents of this view speculate on "primitive" plate tectonic processes including accelerated spreading and subduction, the presence of many micro-plates, and the limited opening and closing of ocean basins (Burke et a l ., 1976; Baer et a l., 1981;

Fyfe, 1981; Lambert, 1981). About the only aspect of the early history of the earth agreed upon by those working in Precambrian terranes is that the earth's geothermal gradient was probably higher in the

Archean.

The most popular plate tectonic model for greenstone belt evolution is that of the the marginal back-arc setting (Tarney et a l.,

1976; Groves et a l ., 1978; Windley, 1984). After final welding to a craton, these basins have synclinal form, greenschist metamorphic grade, and a volcano-sedimentary rock association, all of which are similar to greenstone belts. However, there are several problems with a back-arc basin model for Archean greenstone belts. First, back-arc 122

basins form with simatic crust at the base, and known marginal back-arc basins always contain ophiolite sequences (Windley, 1984). No modern-type ocean-floor ophiolite has been reported from an Archean greenstone belt (Fyfe, 1981). Until a good example of an ophiolite is found in an Archean complex i t is premature to discuss modern sea-floor spreading processes occurring in the Archean. Secondly, the volcanic arcs that are associated with marginal basins would provide detritus to the volcanic portion of greenstone belts. Lowe (1980) points out that there is a lack of arc-derived sediment in Archean greenstone belts. A final discrepency between Archean greenstone belts and marginal back-arc basins has been outlined by Platt (1980). In his study of the

Agnew greenstone belt of Western Australia, Platt (1980) notes that the marginal basin model cannot explain the voluminous pre-tectonic tonalitic magmatism of this belt. Recent back-arc basins show no evidence for this type of magmatism. This discussion deomonstrates that the style and extent of Archean tectonics is as yet unknown.

Although there exist rocks which appear to occur exclusively in

Archean greenstone belts (e.g. komatiites), others occur that are essentially the same as those generated in Phanerozoic plate-tectonic settings. An example would be calc-alkaline suites. In the

Phanerozoic, calc-alkaline rocks are erupted mostly in volcanic arcs presumably in genetic relation to subduction. The hypabyssal dacitic porphyry which is so abundant in the study area is of calc-alkaline affin ity (Byerly and Lowe, 1985). Field evidence suggests that this rock is Fig Tree in age, with faulting, uplift, and erosion taking place at the time of this igneous activity (Lowe and Byerly, 1986b;

Lowe et a l., 1985). It seems likely that the processes responsible for 123

the generation of the calc-alkaline rocks is related to the drastic change in tectonic style we see between Fig Tree rocks and those of the

Onverwacht Group. Bickle et a l . (1983) have noted similar occurrences

in the Pilbara Block of Western Australia. Based on the work of Byerly and Lowe (1985, 1986) and others (Barley et al., 1984; Bickle et al.,

1983) these late-stage volcanic rocks may be magmatic-arc derived. The style of deformation within the uppermost Onverwacht and Fig Tree

Groups is similar to that of fold and thrust belts associated with

Phanerozoic subduction systems. The presence in the Barberton

Greenstone Belt of both structural style and igneous rock suites that are similar to those seen in Phanerozoic plate tectonic settings provides a forceful argument for a setting comparable to a modern, arc-type environment. This setting was no doubt some modified form of today's subduction/arc complex. Evidence suggests that plate-tectonic processes as we under- stand them today seem to have been operating at least as far back as 2.1 b.y. (Kerr, 1986), and the simplest solution to explain the occurrence of many of the features of greenstone belts is to utilize processes we understand rather than invoke unknown settings produced by unprovable dynamics constrained by unmeasurable parameters.

The sedimentary-volcanic record preserved in the Onverwacht Group is not easily reconciled with an Andean-type tectonic setting. Both mafic and felsic volcanism occurred in a relatively quiet, shallow-water environment. This volcanic platform stage was mostly an interval of lava extrusion with little tectonic activity (Lowe and

Byerly, 1986). The Fig Tree represents f ir s t a transition, with a continuation of mafic and ultramafic volcanism, then widespread felsic 124

volcanism, and finally a period of folding and thrusting (Lowe and

Byerly, 1986).

The preceding discussion points out some of the difficulties of trying to describe the tectonic evolution of Archean greenstone belts utilizing modern plate tectonic processes. Currently, marginal basins are the most popular plate tectonic model for greenstone belt development, and they do satisfy many of the known structural, stratigraphic, geochemical and field relationships of greenstone belts.

However, there are fundamental differences, some mentioned above, that indicate a marginal basin (as forming today) does not provide a satisfactory model for greenstone belt development. The same types of problems arise with other plate tectonic settings such as rift basins

(Condie and Hunter, 1976; Groves et a l., 1978); and hot spots (Fyfe,

1978; Lambert, 1981) and i t is becoming apparent that Archean greenstone belts may have formed in unique and diverse tectonic settings.

It seems likely that if plate tectonics operated in the Archean it was in some modified form from that of today, and that this difference in process is reflected in the unique features of Archean terranes. A comprehensive tectonic model of greenstone belt evolution is beyond the scope of this paper. Whether or not some form of plate tectonics operated in the Archean is still an unanswered question, but recent work has shown that Archean plates tectonics was indeed possible (Sleep and Windley, 1982; Arndt, 1983; Nisbet and Fowler, 1983).

The results of this study lead to several conclusions regarding tectonics and sedimentation in the southwestern portion of the

Barberton greenstone belt. Sediment supply was in itia lly from both 125

volcanic eruptions and uplifted portions of the greenstone belt itself.

There is no evidence for a nearby continental block or volcanic arc in the rocks of the study area, however a contintental-type source area is

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Walker, R.G. and Pettijohn, F .J., 1971. Archaean sedimentation; analysis of the Minnitaki basin, northwestern Ontario, Canada. Geol. Soc. America B ull., v. 82, p. 2099-2199.

Walsh, M.M. and Lowe, D.R., 1985. Filamentous microfossils from the 3,500-Myr-old Onverwacht Group, Barberton Mountain Land, South Africa. Nature, v. 314, p. 530-532.

Walter, M.R., 1976. Stromatolites. Elsevier, Amsterdam, 790 p.

Wells, N.A., 1984. Sheet debris flow and sheetflood conglomerates in Cretaceous cool-maritime alluvial fans, South Orkney Islands, Antarctica, in_: Koster, E.H. and Steel, R.J. (eds.), Sediment­ ology of Gravels and Conglomerates. Canadian. Soc. Pet. Geol. Mem. 10, p. 133-145. Wescott, W.A. and Ethridge, F.G., 1980. Fan-delta sedimentology and tectonic setting - Yallahs Fan Delta, Southeast Jamaica. Amer. Assoc. Petroleum Geologists Bull. v. 64, p. 374-399.

Windley, B.F., 1984. The Evolving Continents, 2nd. ed.. Wiley, London, 385p. APPENDIX

STRATIGRAPHIC COLUMN LOCALITIES AND DESCRIPTIONS

137 138

Measured Section Localities

Measured Section S-l 25°54 '24" S 30°55 ■25" E

Measured Section S-2 25°53 ■26" S 30°59 ■03" E

Measured Section S-3 25°54 '03" S 30°59'40" E

Measured Section S-4 25°54 •57" S 30°57 '56" E

Measured Section S-5 25°54‘19" S 30°57 • 04" E

Measured Section S-6 25°53,56" S 30°56 ■34" E

Measured Section S-7 25°53155" S 30°56 '15" E

Measured Section S-8 25°53 ’57" S 30°56 '25" E

Measured Section S-9 25°54 ■30" S 30°57 ■04" E

Measured Section S-10 25°531'48" S 30°551'57" E S-10A 25°53''48" S 30°551'57" E S-10B 25°53,47" S 30°55'57" E

Measured Section S-ll 25o55'05" S 30°00' 33" E

Measured Section S-12* 25°54' 47' S 31°01108" E

Measured Section S-13* 25°54' 46" S 31°01107" E

Measured Section S-14 25°54‘16" S 30°55155" E 25°54‘13" S 30°55155" E

Measured Section S-15 25°53* 43" S 30°57'00" E

Measured Section S-16 25°53142" S 30°56' 59" E

Measured Section S-17 25°53' 53" S 30°56* 13" E

Measured Section S-18 25°54' 56" S 30°56' 26" E

Measured Section S-19 25°53' 45" S 30°56135" E

Measured Section S-20 25°53' 52" S 30°55153" E

Measured Section S—21 25°53147" S 30°56'25" E

Measured Section S-22 25°53' 55" s 30o56‘32" E

Measured Section S-23 25°53152" s 30°59157" E Measured Section L o c a litie s (c o n t.)

Measured Section S-24 25°53'41" S 31°00'■02" E

Measured Section S-25 25°53 ■28" S 31°00''36" E

Measured Section S-26 25°54 ■28" S 30°58''54" E

Measured Section S-27 25°54 '32" S 30°59''15" E

Measured Section S-28 25°54 '10" S 30°591'10" E

Measured Section S-29 25°531'35" S 30°57' 00" E

Measured Section S-30 25°531153" S 30°59158" E

Measured Section S-31 25°531'55" S 30°56' 30" E

*These sections located outside study area.

Measured sections 32, 33 and 34 are many hundreds of meters thick. The locations given are for the base of the section, which is the southern-most portion of the section, with thickness increasing to the north.

Measured Section S-32 25°54'12" S 30°56'15" E

Measured Section S-33 25°53'58" S 30o59'55" E

Measured Section S-34 25°54,50" S 30o57‘49" E 140

EXPLANATION FOR MEASURED SECTIONS

black chert 3 normally graded bed

massive bed inversely graded bed

crudely stratified conglomerate & cobble pebble sand

==■ - flat stratification - scour cast

- cross lamination - fla se r bedding

V S- - cross stratification •A - accretionary lapilli

W - planar cross stratification - breccia

- horizontal stratification "white chert bands

- scoured base and lag •=asS" - intraformational conglomerate

X ~ imbrication ® - sample

L-Bg - lithofacies designation Measured Section S-l

2 -5 CO

amalgamated 40 90 beds co

2-1

covered

80

CM CO co 2-2 70

L-B

amalgamated beds

60

2-6 CM GO 2-4

amalgamated beds

2-7 Om 50 142

Measured Section S-2

30

covered

25 - ® J 11-5

2 0 -

15 - , banded, silty, ferruginous chert

10 - a '11-4

5 -

covered | ^ | H-3 11-2 0 m a 143

Measured Section S-3

35

I 19-11

30 -

25 - 60-

19-10

20 55- 19-9 covered «=cI 19-8 15 - 50-

« I 19-4 10 - 45-

19-3 5 - ^ banded Fe-rich Green chert 1 layers and white chert

/ 0 nr 7 ~ l I ‘l 144

Measured Section S-5

Measured Section S-4 4 I® 27-7,27-7A

aiitlaa® 27-6

massive

CM «=c 2 - I

0 massive

, ,, basal contact not exposed 145

Measured Section S-7

10

Measured Section S-6

9- 4 • a

3 ® 28-5A.B

i-

7-

28-4 siltstone rip-ups S'-z. ss:3

u covered

24-4

0 ® -= 28-2

' *•!<• is 28-1 K 0m- - r t f 146

Measured Section S-9

Measured Section S-8 147

Measured Section 10-A

^ 44-2

Measured Section 1Q-B

Measured Section 10

30-3 30-2

44-6

covered

0 148

Measured Section S -ll

31-5 ■=cCM I

31-4

0 149

Measured Section S-14

54-,

same as below

same as below 50

36-5 ®?f l'Ai 36-12

same as below

36-11

36-10 44-

36-9

36-4 repetitious alternation of thin (l-5cm) bands of fine to coarse sandstone and flat- lam inated v.f. sandstone to siltstone. Local thicker beds of coarse to v. coarse san d ston e.

3£t8__

36-7 36-6

covered

36-3 22 36-2 36-1

20 Measured Section S-14(cont.)

62

£i 36-14

60 - covered

36-13 58 -

J&- covered 56

54 % 151

Measured Section S-15

16 T

14 - 37-7

37-6

10

CM COI 8 * 37-8

CM CO

37-3

4* ■ covered

0 m. 16 Measured Section S-16

Measured Section S-17 153

Measured Section S-18

L-A

42-4A,B

42-3 42-2

42-5

42-1B 0 42-1A Measured Section S-19

2Ch

16-

covered

10

covered

matrix supported

43-1 155

Measured Section S-20

Measured Section S-21

45-4

ow

47-1

45-3

45-2

0 156

Measured Section S-24

Measured Section S-22

®-=5 48-4

4 58-5

2 58-4 58-3 58-2 58-1 0 Om Measured Section S-23

5 7 -5

57-4 4- matrix supported

i 57-3

0 ITT -i“V- 157

Measured Section S-25

matrix supported

covered

CSJ COI

61-1 158

Measured Section S-26

20

18 contorted bedding •^i J1 16 ’a siltstone rip-ups

14

53-3

12

® - 3& 63-2

10

8 1 covered

63-1

6

cvj CD

4

s 2 !s 3 >* 4*. * s 0m -r*P 20 159

Measured Section S-27

6 12

5 11 - 64-3

64-1 4 IQ

o 3 */ I %

2 8 -

covered 1

matrix supported

64-2 Om 160

Measured Section S-28

28

COI

24- CM 0 3 65-4 >--A2 65-2 I

22 ‘

65-1

20 -

«=ci

I 18-

Mixture of black and green chert 16- ferruginous chert y 65-3 l- b2

14- 161

Measured Section Measured Section S-29 S-30

3 8 67-2 67-3

67-1 2 6

1 4

OCM

0 2

Om 162

Measured Section S-31

60

covered

50

oI

40 90-

O I 30 80-

co t

o Jt3 *' , 20 70-

*5? •: o I COI

covered

Om 60 ?SL 163

Measured Section S-32

200 400i

matrix supported 180'

matrix supported O

160 360-

140- 340-

120 - 320'

CO

100- 300

80 ■ 280

60 ' 260

40 - 240

20 • 220 (SJ

ri*** l-/ 0 n r , 200 164

600

Measured Section S-32 (cont.) 580 -

matrix supported 560

540 680

matrix supported 520 660-

500 640 covered

480 620- - s i

460 L-B 600

covered

440

420

400 165

Measured Section S-33

200 400 53-12

55-3

180

55-23 55-11

160 55-10

140 s-- 55-2 340-

120 320- 55-9

B r- 55-1 100

covered 55-8

55-7 80 • 280 S1ltstone-clast breccia. This deposit is a chaotic arrangement of siltstone and chert clasts in an unstratified siltstone 55-6 matrix

60 - 260

L-B,

40 • 240

55-5

20 ‘ 220 55-4

L-A, 0 m 200 166

600

55-16,16A,17, 18,19 580-

Measured Section S-33 (cont.)

560-

very poorly exposed - i probably sandstone

540-

520 720 -

500- 700 - ® $ 55-15

3®

480 ® 55-14 680 - % 55-13

% 55-12

460- 660

55-20

440 640 -

covered 420' 620 -

400 600 167

Measured Section S-34

200-1 400

L-B

I f i L -B ,

180- 380

160- 360 i) L" 2

140-- 340

L-B,

120 - 320

100- 300

L-B,

80 - 280

L-B, 60 260

40 - 240

20 220 L-B,

TT-I 200 168

Measured Section S-34 (cont.)

-f

540

520

500 •

480 -.

460

440 •

420

400 VITA

Bruce William Nocita, son of Paula J. and John W. Nocita, was born

May 19, 1952 in Paso Robles, California. He graduated from McLean High

School, in McLean Virginia, in 1970. His first two years of college were spent at Randolph-Macon in Ashland, Virginia. In 1972 he

transferred to Colorado State University in Fort Collins, Colorado and graduated with a B.S. in geology in 1974. After working for the U.S.

Forest Service for six months he began graduate work at San Diego State

University where he graduated with an M.S. in geology in 1977. Mr.

Nocita enrolled in the Ph.D program in geology at Louisiana State

University in the fall of 1980 after teaching at several colleges and universities for three years. He currently is an Assistant Professor in the Department of Geology, University of South Florida.

169 DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Bruce W. Nocita

Major Field: Geology

Title of Dissertation: Sedimentology and Stratigraphy of the Fig Tree Group, West Limb of the Onverwacht Anticline, Barberton Greenstone Belt, South Africa

Approved:

•hairmanMajor Professor •hairmanMajor

EXAMINING COMMITTEE:

/

Date of Examination:

October 6.1986

LIBRARY g* WEST LIME

GEOLOGIC MAI IT LIMB OF THE ONVERWACHT ANTld

ST. *• * ***» V'( ><> •- “► * • * • * * « » » * «w-f„ .* » » ISAcj;/*V>-y

11 w —y ^ f * 1 V jl" flu 4 4 *_ * * - ^ Shu/tzenhnrxt jjq

H9 •'w>

^V-'» ,*, 4 ,* t > 4»•— »i ^ H H n S 2 r T ’ ^ \y*-\ * * * f r ► »J«L*Sj^VQfti H / * 4*<■*** * * * * »* ^R S > 4 r\« » « ft «« ..► ,► k * 4 •^ * 4 * * *; a * vvSB w *JV *'***' ► <* A , « » A fc ■«» . ™ £i* ’■*.« ».**»“ « * » » * o ,” * ," »«* » ► . k*■. ►* . ,-•'*» *}5£ss«3rf£

4A?- 2^*3*=^

*/

IAP OF THE FIG TREE GROUP, ICLINE, BARBERTON GREENSTONE el

SJ* * * * . *********** tk.* * * k * ’ fckA* * « ** .

BELT, SOUTH AFRICA

EXPLANATION

; - Ma Kromberg Formation

- serpentinized komatlite ban

-silicified and b la carbonated komatiite

grei

Fig Tree Group

Maeuli Chart sandstone Recent Cover

landslide depoa bandad chart - chert-claat conglomerate and recent collu

n n ft ft plagioclaae-porphyry black chert Y i « • * . conglomerate Igneous Rocks p green volcanlclaatic chert ptagiocftaae por plagioclase-porphyry breccia ■■ \ •Aifm2...... ?ra 3....>,# \

measured section 0 S - 7

strike and dip

c o n t a c t

contact.located approximately

contact, inferred

1:12300

0 1/2 1km

Namibia Zim b ab w e

Botswana -o

South B ar A fric a G reenst

1km

Namibia Zimbabwe

South B arberton Greenstone Beit Africa

\

EXPLANATION

Ms Kromberg Formation

serpontlnized komatilte b an '///A

-silicified and - bli carbonated komatlite

or

s ilt

Fig Tree Group

s a n d s t o n e Msauli Chert Recent Cover

(wj,-*. landslide deposi m banded chert chert-clast conglomerate i w § l - m 'sa and recent collu

■war1 ".tfirjr plagioclase-porphyry '•® efy e or 9 . black chert ,® o O- l « o e o o ^ o ' conglomerate Igneous Rocks

green volcaniclastic chert -plagioclase-porphyry breccia plagiocla.se p n, - « m il \ \ ferruginous siltstone Bffii volcaniclastic sandstone diabase dikes locally jasper-bearing lm 2 a k s£ siltstone-clast breccia

ijofluni

riO uiT ii

measured section 8-7

ver % > — strike and dip de deposits contact sent coliuvium

>cks contact,located approximately

slase porphyry contact, inferred \ \ ' is dikes fault

stratiform fault

5 - 2 7 : pr. ■ ^

'^^'uJffrrnjLrjiu iurmmu

1:12300

Namibia Zimbabwe

Botswana

South Bor Africa Greens

Geology by Bruce W. No< 3I°G0'

1km

Namibia Zimbabwe o- Botswana

South B arberton Africa Greenstone Belt

_ . O _ _ 4 Geology by Bruce W. Nocita 1981,1982 31 00 i