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Hons project FINAL DRAFT Nov 21 .pdf


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FELSIC AND PEGMATOIDAL
INCLUSIONS, WITHIN THE
INTRUSIVE MANUBI DOLERITE
SILL AT CEBE, BOWKER’S BAY,
TRANSKEI WILD COAST.

By
Ryan Edwin Blane

Submitted in partial fulfilment of the Bachelor of Science Honours
Degree in Geology. Faculty of Science, Nelson Mandela
Metropolitan University.

Date submitted : November 2012

Declaration of own work
Sign and date

i

Acknowledgments
The undertaking of this project would not have been possible without the input
and support of a number of people.


My family for providing me with the opportunity to study. Their continued
support, encouragement and sacrifice throughout my university career,
both financially and personally has served as motivation for me to perform
as well as possible at all times. Their help with field work and proof
reading is also appreciated.



Ms Donné van Stormbroek, for her endless support, love, patience as
well as her encouragement, motivation and belief in me, without which I
may never have achieved my goals.



Supervisors, Mr Callum Anderson and Mr Gideon Brunsdon for their
academic guidance and assistance.



Dr‟s. Henrik Svensen, Fernando Corfu and Stephane Polteau of the
University of Oslo, for their initial advice on the project.



Mr Willie Deysel for his work in sample and thin section preparation.



Mr Lawrence Blane for giving generously of his time by travelling to the
study site to assist with field work as well as his procurement of
equipment and machinery required for sample collection.



My fellow 2012 Geology Honours classmates for their friendship and a
memorable time. The year was made much easier by your willingness to
assist and offer advice on issues, big or small.



The National Research Foundation for their financial aid.



The Nelson Mandela Metropolitan University for their financial aid.



Mrs Edwina Oates of Department of Economic Development and
Environmental Affairs, East London, for her assistance with permission to
access the 1km protected coastal zone.

ii

Abstract
A study area involving an intrusive dolerite sheet and Beaufort Group sediments,
located in the Transkei Wild Coast region, was analysed with respect to the
general characteristics of the sill and the anomalous felsic and pegmatoidal
features observed in the sill. Sedimentary lithologies show near horizontal
bedding and distinct jointing, while the intrusive sill shows an undulating nature
and a series of closed and open joint systems. The sill contains inclusion
fragments of xenolithic origin which a played role in the generation of
granophyric material. Associated with the granophyre and as a result thereof,
veins of mobilised sediment occur in a variety of forms, namely rheomorphic
vein, schlieren and closed joints. Pegmatite is apparent in the vicinity of the
granophyre and shows large feldspar laths and pyroxene prisms up to 20mm in
length. The pegmatite is in some cases connected to the rheomorphic veins but
in other cases occur independently. Sediment dykes are apparent as sinuous
grey material of a very fine grain and are also in close spatial relation to the
felsic and pegmatoidal material. Thin section petrographic analysis performed
on samples of the various rocks, supports an idea of sediment xenolithic
inclusion into a molten magma, hot enough to extract volatiles and melt the
xenoliths. The subsequent recrystallization resulted in a felsic rock, termed
granophyre. The extracted volatiles reacted with the magma to form sections of
segregated magma, which cooled more slowly than the host rock. The water
extracted from the sediment acted as a flux, improving the mobility of the
magma allowing it to flow along cooling joint cracks, thus permitting the
formation of the variety of veins and joint fillings.
Keywords: Transkei Wild Coast, Karoo Supergroup, dolerite, pegmatite,
granophyre, diorite, sediment dyke, xenolith.

iii

Table of contents
Acknowledgments ............................................................................................... i
Abstract ............................................................................................................... ii
Table of contents ............................................................................................... iii
List of Figures ................................................................................................... vii
List of Tables ................................................................................................... xiv
1.

Introduction .............................................................................................. 1
1.1

Aims...................................................................................................... 2

1.2

Locality ................................................................................................. 3

1.3

Topography .......................................................................................... 4

1.4

Climate ................................................................................................. 5

1.5

Vegetation ............................................................................................ 6

1.6

Study area ............................................................................................ 8

1.7
2.

3.

1.6.1

Site 1 – Muller‟s Point ................................................................ 9

1.6.2

Site 2 – Duck Pond .................................................................. 13

1.6.3

Site 3 – Black Rock .................................................................. 13

1.6.4

Site 4 – The High Rocks .......................................................... 14

1.6.5

Site 5 – The Hump ................................................................... 17

1.6.6

Site 6 – East of The Hump ....................................................... 19

1.6.7

Site 7 – Nxaxo River Cliff ......................................................... 21

Geological setting ............................................................................... 22
Methodology ........................................................................................... 28

2.1

Desktop Study .................................................................................... 28

2.2

Field Study.......................................................................................... 28

2.3

Laboratory Study ................................................................................ 29
Literature review .................................................................................... 30

iv
4.

Structural geology ................................................................................. 33
4.1

4.2

5.

Balfour formation ................................................................................ 33
4.1.1

Sandstone ................................................................................ 33

4.1.2

Mudstone ................................................................................. 36

Dolerite intrusion ................................................................................. 38
4.2.1

Site 1 – Muller‟s Point .............................................................. 39

4.2.2

Site 2 – Duck Pond .................................................................. 41

4.2.3

Site 3 – Black Rock .................................................................. 42

4.2.4

Site 4 – High Rocks ................................................................. 43

4.2.5

Site 5 – The Hump ................................................................... 44

Thin section petrographic analysis ...................................................... 46
5.1

5.2

Site 1 – Muller‟s Point ......................................................................... 46
5.1.1

Host Dolerite: AB – 5; AB – 7 - 1. ............................................ 46

5.1.2

Dioritic Lumps: AB – 1; AB – 9................................................. 48

5.1.3

Pegmatoidal Inclusions: AA – 1; AA – 2 .................................. 52

5.1.4

Granophyric Inclusions: AB – 8; AB – 2 ................................... 55

5.1.5

Rheomorphic Vein Material: AB – 4. ........................................ 60

5.1.6

Closed Joint Filling: AB – 7 – 2 ................................................ 62

Site 2 – Duck Pond ............................................................................. 64
5.2.1

Closed Joint Filling: BA – 2 ...................................................... 64

5.3

Site 3 – Black Rock ............................................................................ 67

5.4

Site 4 – High Rocks ............................................................................ 68

5.5

5.6

5.7

5.4.1

Host Dolerite: DA – 1 ............................................................... 68

5.4.2

Sandstone: DA – 2 ................................................................... 68

Site 5 – The Hump.............................................................................. 71
5.5.1

Horizontal Pegmatoidal Lense: EA – 3 – 2 .............................. 71

5.5.2

Sandstone Dyke: EA – 1 .......................................................... 71

Site 6 – East of The Hump ................................................................. 75
5.6.1

Normally Cooled Dolerite: FA – 1 – A ...................................... 75

5.6.2

Chilled Margins: FA – 1 – B ..................................................... 75

Site 7 – Nxaxo River Cliff .................................................................... 78
5.7.1

Loose Boulder, Cliff Base: GA – 3 ........................................... 78

v
5.7.2
6.

In Situ Upper Contact Rock: GA – 4 ........................................ 80

Discussion .............................................................................................. 87
6.1

6.2

Models ................................................................................................ 87
6.1.1

Rogers & Du Toit (1909) .......................................................... 87

6.1.2

Mountain (1943; 1960) ............................................................. 88

6.1.3

Walker (1953); Walker & Poldervaart (1941; 1949) ................. 89

6.1.4

Svensen et al. .......................................................................... 91

Structural Geology Interpretation ........................................................ 93
6.2.1

6.3

Manubi Dolerite Sill .................................................................. 93

Petrographic Interpretation ................................................................. 95
6.3.1

Host Dolerite ............................................................................ 95

6.3.2

Granophyre Inclusions ............................................................. 96

6.3.3

Dioritic Lumps .......................................................................... 98

6.3.4

Rheomorphic Veins and Schlieren........................................... 99

6.3.5

Pegmatite‟s ............................................................................ 102

6.3.6

Closed Joints / Vein ............................................................... 105

6.3.7

Sediment Dykes ..................................................................... 108

7.

Conclusion ........................................................................................... 110

8.

References............................................................................................ 112

9.

Appendix ............................................................................................... 115
9.1

9.2

Igneous Jointing Readings ............................................................... 115
9.1.1

Site: 1 – Muller‟s Point ........................................................... 115

9.1.2

Site: 2 – Duck Pond ............................................................... 117

9.1.3

Site: 3 – Black Rock ............................................................... 120

9.1.4

Site: 4 – High Rocks .............................................................. 123

9.1.5

Site: 5 – The Hump ................................................................ 125

9.1.6

Site: 6 – East of The Hump .................................................... 127

Balfour Formation Sandstone Bedding and Jointing ........................ 128
9.2.1

Damba Rock (28°33'50.58"E; 32°32'45.69"S) ....................... 128

9.2.2

The High Rocks (28°34'3.29"E; 32°32'38.64"S) .................... 131

vi
9.2.3
9.3

st

1 /2

nd

Beach (28°33'33.72"E; 32°32'55.79"S) ..................... 133

Balfour Formation Mudstone ............................................................ 136
9.3.1

Pests Rock (28°34'1.24"E; 32°32'40.12"S) ............................ 136

vii

List of Figures
Figure 1-1: Location of Cebe, Bowker‟s Bay in South Africa ............................... 3
Figure 1-2 : Location of Cebe in relation to Centane and Kei Mouth ................... 4
Figure 1-3 : Avicennia marina (White Mangrove) on the Eastern bank of the
Nxaxo River ............................................................................................... 7
Figure 1-4 : Study sites 1-7 in relation to the Nqgusi and Nxaxo Rivers as well
as Cebe Cottages. ..................................................................................... 9
Figure 1-5 : Site 1: Muller‟s Point displaying a pegmatite pond along with
rheomorphic veins and associated schlieren striking North-South in host
dolerite. .................................................................................................... 10
Figure 1-6 : Site 1: Muller's Point sediment dyke in host dolerite ...................... 10
Figure 1-7 : Dolerite pegmatite inclusion in a boulder at Site 1: Muller's Point. . 11
Figure 1-8 : Dioritic protrusions showing weathering resistance and gradational
boundaries with the host dolerite ............................................................. 11
Figure 1-9 : Dioritic protrusion at Site 1: Muller's Point displaying a more felsic
appearance than the host rock. ............................................................... 12
Figure 1-10 : Granophyre inclusion giving rise to a rheomorphic vein and
schlieren with sharp contacts at Site 1: Muller's Point. ............................ 12
Figure 1-11 : Dioritic patches at Site 3: Black Rock resembling those from Site 1
without the protrusive nature. ................................................................... 13
Figure 1-12 : Site 4: High Rocks displaying a contact zone between Balfour
Formation sandstone and the Manubi dolerite sill ................................... 15
Figure 1-13 : Contact zone of the Manubi sill with Balfour Formation sandstone
displaying accentuated bedding planes. .................................................. 15
Figure 1-14 : Contact zone displaying a hornfels resulting from the
metamorphism of Balfour Formation sandstone by the intrusive dolerite 16
Figure 1-15 : Sinuous sediment dykes striking East to West in dolerite from
Site 5: The Hump ..................................................................................... 18
Figure 1-16 : Horizontal pegmatoidal lense in close proximity to the sediment
dyke at Site 5: The Hump ........................................................................ 18
Figure 1-17 : 4mm wide fibrous, green translucent closed joint striking North South ....................................................................................................... 19

viii
Figure 1-18 : Dolerite dyke displaying sediment dykes and a contact with the
Balfour Formation sandstone. .................................................................. 20
Figure 1-19 : Nxaxo River cliff exhibiting a columnar jointing pattern of dolerite.21
Figure 1-20 : Spatial locality of Karoo Groups, Subgroups and Formations.
Study area, Cebe, indicated by red dot. (After: Johnson et al., 2006) ..... 23
Figure 1-21 : Extent of the Karoo Igneous Province in the Karoo Supergroup.
Cebe indicated by red dot. (After Svensen et al., 2010) .......................... 24
Figure 1-22 : Geological map of the study area displaying the Beaufort Group,
alluvial cover and the intrusive Qolora Sheet and the Manubi Sill. The
Transkei Gap Dyke and the Nxaxo lineation are also indicated. Study
area indicated by red polygon. (After: OneGeology 2012) ....................... 25
Figure 1-23 : Large scale geological map with study area, Cebe, indicated along
with Nxaxo River mouth and Qora River mouth. The Qolora Sheet,
Manubi Sill and the Gap Dyke are also specified..................................... 26
Figure 1-24 : Small scale geological map with study area, Cebe indicated as
well as the Manubi Sill and the existing undulation. The dyke at Site 6
cross cutting the sill is marked. ................................................................ 26
Figure 2-1 : Sample collecting by means of a paving breaker. .......................... 29
Figure 3-1 : Location of study area in respect to sites studied by Mountain
(1960). ..................................................................................................... 31
Figure 4-1 : Balfour Formation sandstone dominant jointing directions S1, S2
and S3 . .................................................................................................... 34
Figure 4-2 : Stereogram depicting the poles to planes of the Balfour Formation
sandstone bedding................................................................................... 34
Figure 4-3 : Stereogram depicting a contoured projection of the Balfour
Formation sandstone bedding. ................................................................ 35
Figure 4-4 : Rose diagram indicating Balfour Formation sandstone jointing
directions.................................................................................................. 35
Figure 4-5 : Balfour Formation mudstone dominant jointing directions S1; S2. .. 36
Figure 4-6 : Stereogram depicting the poles to planes of the Balfour Formation
Mudstone bedding. .................................................................................. 37
Figure 4-7 : Stereogram depicting a contoured projection of the Balfour
Formation mudstone bedding. ................................................................. 37


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