Geophysical Studies in Southern and Central Rockall Trough, Northeast Atlantic Philip AntOny ...

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Geophysical Studies in Southern and Central
Rockall Trough, Northeast Atlantic
Philip AntOny Dicker Bentley
Ph.D.
University of Edinburgh
1986
DEDICATION
To my father
Cohn Dicker (1926-1963)
For whom I have achieved all this.
EPIGRAM
In a word, new matter offers to new observation, and they who
write next, may perhaps find as much room for enlarging upon us, as
we do upon those that have gone before ".
Daniel Defoe, 1724
ABSTRACT
This study is based on the interpretation of approximately 8600 line km of
underway marine geophysical data over southern and central, Rockall Trough. The
data consist of seismic reflection profiles, and gravity and magnetic anomaly
observations. Some of this data was newly acquired for this work; the
remainder was made available from other sources. This work represents the
first time that most of the geophysical information from this area has been
inspected and integrated as a whole.
The bathyal bathymetric deep that is Rockall Trough is bordered by rifted
passive continental margins. These are backed by continental crust of fairly
typical thicknesses - about 30 km. Rapid attenuation of this crust occurs
across the steep continental margins over a distance of 50-60 km. BeyOnd the
margins previous gravity and seismic refraction studies, and the free-air
gravity modelling presented here, define a 200-250 km wide zone of thin crust
flooring Rockall Trough. This thin crustal layer varies in thickness from 5 km
(locally) to 8 km but it is generally fairly uniform at about 7 km. The Moho
typically models at 14-15 km depth.
Conflicting interpretations have been proffered for the thin crustal layer.
This research supports the balance of recent opinion which favours an oceanic
origin and evolution for Rockall Trough,.though perhaps under less than normal
conditions of accretion. But the possibility of highly stretched continental
crust being present cannot be excluded. The size and geometry of the Trough,
the oceanic layer 2 appearance on many seismic profiles, and the structures
defined by the gravity models are most readily interpreted in terms of an
oceanic origin. The absence of a coherent oceanic magnetic anomaly pattern is
accounted for by advocating the lack of an ordered spreading ridge - transform.
system and a diffuse style of accretion, perhaps under high sediment
accumulation rates.
Two previously unreported features are identified, mainly from the seismic
reflection data. Firstly, the Barra volcanic ridge system is a large zone of
arcuate volcanic ridges in southern Rockall Trough. The ridges attain heights
of 3-4 km and widths of up to 40 km. This volcanic province is related to the
last pulses of spreading in Rockall Trough, possibly in the E.Cretaceous. It
also seems to be associated with a broad upwarp (c.2 km) in the underlying
mantle, a feature defined by a conspicuous 60 mgal free-air anomaly. The Barra
volcanic ridges are associated with curved magnetic anomalies having
amplitudes of up to 1000 nT. Secondly, a number of sill and ?lava complexes
are mapped out within the thick sedimentary fill to the Trough. These igneous
events are dated as L.Palaeocene - E.Eocene from a consideration of the
regional seismic and volcanic stratigraphy. It is likely that they are coeval
with the Thulean igneous episode and the onset of spreading between Greenland
and Rockall Plateau.
The continuation of the Gibbs F.Z. eastward into the Clare Lineament and
thence to the base of the continental margin is proven for the first time.
Detailed mapping of the seismic basement here has clarified the nature and
significance of the Clare Lineament as an ocean-ocean or ocean-continent
fracture zone. This combined fundamental discontinuity marks the southern
tectonic limit of Rockall Trough and separates it from the mature Late
Cretaceous ocean crust to the, south in Porcupine Abyssal Plain.
The seismic stratigraphy in the Rockall area is reviewed and a strat-
igraphic/reflector framework developed for the Trough based on slight
modifications of previous schemes. The R4 reflector frequently discussed in
the literature concerning this area is reassigned a R2 label and given a new
E.-M. Miocene age. There does not appear to be a well developed L.Eocene
-E.Oligocene reflector (i.e. R4) in Rockall Trough. A distinct regional
reflector R5, dated roughly as earliest Eocene, marks the onset of differ-
ential deposition in Rockall Trough and the construction of Feni Ridge drift.
A conspicuous. reflector R7 (? M.Albian) defines the top of a deep, seismically
well layered sedimentary sequence that is present in the Trough but absent
over oceanic crust south of the Gibbs F.Z. - Clare Lineament.
CONTENTS
Page
Number
ACKNOWLEDGEMENTS(cid:9) 1
INTRODUCTION: LAYING THE FOUNDATIONS.(cid:9) 3
1.1 Scientific curiosity or future harvest?(cid:9) 3
1.2 Geography and bathymetry of Rockall Trough.(cid:9) 4
1.3 History of research activity.(cid:9) 9
1.4 Rockall Trough: Its setting in the evolution of the
Worth Atlantic.(cid:9) 20
The Crustal Foundations.(cid:9) 20
The Consolidation of Pangaea.(cid:9) 22
The Opening of the North Atlantic Ocean.(cid:9) 25
1.5 Layout of the thesis.(cid:9) 31
DATA ACQUISITION, REDUCTION AND INTERPRETATION.(cid:9) 32
2.1 Geophysical surveying at sea.(cid:9) 32
2.2 The R.R.S. Challenger 1/84 cruise.(cid:9) 34
Cruise logistics and track policy.(cid:9) 34
Data collection and reduction.(cid:9) 37
2.3 Previous geophysical surveys.(cid:9) 40
2.4 Interpretation and modelling of gravity and magnetic
anomalies.(cid:9) 41
2.5 Interpretation of seismic reflection profiles.(cid:9) 46
SEISMIC STR.ATIGRAPHY IN THE ROCKALL AREA. 49
3.1(cid:9) Introduction. 49
3.2(cid:9) Previous stratigraphic work. 50
3.3(cid:9) A seismic stratigraphy for the Rockall Trough:
correlation with DSDP results. 68
Seismic stratigraphy at Site 550. 70
Seismic stratigraphy at Site 610. 75
Seismic continuity between Sites 550 and 610. 82
MARGIN AND THE CHARLIE-GIBBS FRACTURE
4. THE ROCKALL OFFSET
87
ZONE.(cid:9)
4.1 Introduction: bathymetry and early studies.(cid:9) 87
4.2 Geophysical investigations across the Rockall offset
91
margin.(cid:9)
4.2.1 Basement structure and sediment geometry as
shown by seismic reflection profiles.(cid:9) 91
Basement structure and distribution.(cid:9) 93
The sedimentary cover to the Charlie-Gibbs
109
Fracture Zone.(cid:9)
4.2.2 Evidence from gravity observations.(cid:9) 114
4.2.3 Evidence from magnetic observations.(cid:9) 126
135
ROCKALL TROUGH: SEISMIC REFLECTION PROFILING.
135
5.1(cid:9) Seismic basement.
Continental basement beneath the margins of
135
Rockall(cid:9) Trough.
145
The Barra Volcanic Ridge System.
153
Deep layered basement in the Rockall Trough.
5.2(cid:9) The sedimentary infill(cid:9) to Rockall Trough. 159
162
Pre-R7 seismic sequence.
167
R5 - R7 seismic sequence.
169
R2 - R5 seismic sequence.
172
Post-R2 seismic sequence.
174
5.3(cid:9) Igneous sills in Rockall Trough.
ROCKALL TROUGH: GRAVITY ANOMALY DATA.(cid:9) 181
181
6.1 Free-air anomaly chart.(cid:9)
6.2 Gravity modelling in two and three dimensions.(cid:9) 187
Challenger 80-1 and 84-4 gravity model.(cid:9) 188
Challenger 80-4 gravity model.(cid:9) 196
Challenger 84-7 gravity model.(cid:9) 202
210
Charcot 1969 gravity model.(cid:9)
ROCKALL TROUGH: TOTAL INTENSITY MAGNETIC ANOMALY DATA.(cid:9) 213
7.1 Regional magnetic anomaly charts.(cid:9) 213
7.2 Forward and inverse two-dimensional magnetic
modelling.(cid:9) 220
Forward magnetic anomaly modelling.(cid:9) 220
Inverse magnetic anomaly modelling.(cid:9) 225
DISCUSSION: THE NATURE, ORIGIN AND EVOLUTION OF ROCKALL
TROUGH.(cid:9) 236
8.1 Prologue.(cid:9) 236
8.2 Continent or Ocean?(cid:9) 237
8.3 Palaeozoic or Mesozoic?(cid:9) 251
8.4 Rockall Trough: a three-fold geological evolution.(cid:9) 267
Early continental rifting.(cid:9) 267
Mesozoic development of a small ocean basin.(cid:9) 270
Late Mesozoic - Cenozoic sea floor spreading in
the Worth Atlantic.(cid:9) 279
8.5 Summary.(cid:9) 283
8.6 Epilogue.(cid:9) 287
REFERENCES.(cid:9) 290
ENCLOSURES IN BACK POCKET FOLDER.
FIGURES
Figure 3.16 Seismic correlation between DSDP sites 550 and 610.
Figure 3.17 Seismic correlation from DSDP site 550 to central
Rockall Trough.
Figure 4.5(cid:9) Interpretation of seismic profile CM-04, sps.2850-4700
Figure 5.3(cid:9) Interpretation of seismic profile Shackleton 79-14.
Figure 5.4(cid:9) Interpretation of seismic profile Challenger 80-1.
Figure 5.5(cid:9) Interpretation of seismic profile GSI-1.
Figure 5.6(cid:9) Interpretation of seismic profiles NA-1 and NA-1 Ext.
Figure 5.19 R2 - R5 reflector isochron chart.
Figure 5.20 Post-R2 reflector isochron chart.
CHARTS
CHART 1 Geophysical data across southern and central Rockall
Trough.
CHART 2 Free-air gravity anomaly chart.
CHART 3 Total intensity magnetic anomaly map around southern and
central Rockall Trough.
CHART 4 Acoustic basement isochror, chart.
CHART 5 Total sediment thickness isochron chart.
CHART 6 Distribution of major sills in sediments of Rockall Trough
CHART 7 Free-air gravity anomaly chart and selected structural
elements in Rockall Trough.
ACKNOWLEDGEMENTS
My warmest thanks. are extended to my supervisor, Or Roger Scrutton,
who for over three years maintained a high level of interest,
commitment and encouragement toward this research project. In a
working environment where the eccentricities and idiosyncracies of a
scientific department seemed only too prevalent it has been a great
pleasure to work alongside someone who still appreciates the
personal and leisure aspects, of academic life, and who values the
art of everyday communication. In addition, his astute and thought-
provoking conversations concerning matters geological were a
constant source of direction and are, I hope, reflected in this
piece of research.
lam also greatly indebted to Doug Masson, Lindsay Parson and Pete
Miles at the Institute of Oceanographic Sciences, Surrey for their
interest and enthusiasm for this project. Their combined knowledge
and expertise on passive continental margins, especially those
bordering the North Atlantic, has been of enormous benefit to me. I
eir:
am particularly grateful to Doug and Hilary Masson for th
marvellous open hospitality on two occasions when it was necessary
for me to work at I.O.S. for extended periods.
I thank the Master, Officers, technicians and crew of R.R.S.
"Challenger" for their professionalism at sea from 6 to 21 June
1984, during which scientific cruise data was collected for this
study. Or Martin Sinha and his colleagues from the Bullard Laborat-
ories, Cambridge University uncomplainingly provided scientific
assistance throughout the cruise. And Mick Geoghegan, acting as
Irish observer, entertained all aboard with his contagious lepre-
chaun humour.
Or Ian Hill and Rob Young of Leicester University kindly invited me
on their scientific cruise around the Cape Verde Archipelago, off
West Africa during December 1982 and January 1983. I heartily thank'
them and the Master, Officers, scientists, technicians and crew of
R.R.S. "Shackleton" for making the cruise so enjoyable and. educat-
ional. As a first-hand introduction to the methods, collection and
processing of underway geophysical information it enabled me to
prepare better for my own data collecting cruise in June 1984.
I am grateful to the staff and technicians at N.E.R.C. Research
,Vessel Services base in Barry, S.Wales, in particular Doriel Jones,
for their able and concerned assistance both at sea during the two
aforementioned cruises and also at later times on dry land.
I gratefully acknowledge the staff of the British Geological Survey,
Murchison House, Edinburgh who unhesitatingly provided their
services for, or access to, special reproduction and automatic
digitising facilities, and also unusual items of literature in their
excellent library. Special thanks are due to Mr Dave Smythe of
B.G.S., Hydrocarbons Unit for his interest in this research, his
numerous valuable comments and suggestions, and for allowing me both
to look at the product of his own investigations in the Rockall area
and to inspect a number of important multichannel seismic profiles
in my own study area.
I wish to thank Dr Alan G Nunns and Dr Graham Westbrook, formerly of
Durham University, for their assistance with and permission to use
the FORTRAN magnetic anomaly inversion program (Chapter 7) written
by the first-named person.
Edinburgh Regional Computing Centre, Edinburgh University provided
computing facilities and frequent related advice, without which it
would have been impossible to perform a large proportion of the
manipulation and interpretation of the gravity and magnetic anomaly
data (Chapters 4,6 and 7).
In addition to the underway geophysical data collected during the
"Challenger 1/1984 cruise in Rockall Trough I was fortunate indeed
to be able to inspect or utilise a considerable amount of multi-
channel seismic reflection, gravity anomaly and magnetic anomaly
information held by the Institute of Oceanographic Sciences, Surrey
and also the Irish Department of Industry and Energy, Dublin.
Without the incorporation into this thesis of the good quality
material from the former source, and the accompanying discussions
with and foresighted advice of Doug Masson, this study would have
been a far sadder and simpler affair.
During my short residence at Edinburgh University I have benefited:
from conversations with many people: notably, Professor M.H.P.Bott
of Durham University; Dr Roger Hipkin of the Geophysics Department
here for valuable advice and access to computer programs regarding
matters gravitational; and the teaching staff and postgraduates at
the Grant Institute of Geology.
I owe a large vote of thanks to Cohn Chaplin and Diana Baty for
their superb conscientious work in the photographic and drawing
departments, especially when often faced with large strangely-shaped
material! Mrs Thea Grieve provided assistance on countless occasions
when trying to locate difficult references in the library. And Peder
Aspen kindly helped me to locate a variety of bathymetric charts in
the map collection.
A special thank you is. in order for the secretaries at the Grant
Institute, Marcia Wright, Patricia Stewart, Denise Wilson and
Heather Hooker, who have always dealt with my enquiries, difficult-
ies and complaints with a smile. Their constant light-heartedness
and jocularity have undoubtedly served to create a more pleasurable
working atmosphere. I am particularly in the debt of Denise Wilson
for her rapid and accurate typing of this thesis, a task she
performed with splendid flexibility.
This research was funded for three years by the Natural Environment
Research Council under award number GT4/82/GSI3O. Nonetheless I
would not have been able to maintain this study were it not for the
considerable additional financial assistance and understanding from
my bank in Birmingham, the Law Courts branch of the National
Westminster Bank.
Finally I would like to express my gratitude to the residents of the
City of Edinburgh, a remarkale place in so many respects, for making
my three years and more stay there such an enjoyable, fulfilling and
learning time ... thank you.
3
1. INTRODUCTION: LAYING THE FOUNDATIONS
1.1 Scientific curiosity or future harvest?
Why study the geology of Rockall Trough? In reply to this
prompt one could argue simply that the acquisition of further
scientific knowledge is in itself grounds for justification. However,
an investigation of this small ocean basin - ocean being used in the
broadest sense - would seem to hold more import than just academic
curiosity. The main objective of this work has been to understand
the general, large-scale geological evolution of the southern and
central Rockall Trough (some people prefer to call it Rockall
Channel) and its adjacent continental margins and shelf during much
of the long Phanerozoic period. The nature of the data used in this
research and their interpretation do not allow the finer details of
the area's history to be resolved with confidence. It has been a
high priority to step back and view the Rockall Trough in its broader
regional context; namely to consider its significance in the
development of the North and, less obviously, the Central Atlantic
Ocean since Late Palaeozoic times. This is important when one
recognises that the true origin and composition of Rockall Trough,
and its evolutionary position within the framework of North Atlantic
continental drift, are as yet poorly understood.
In addition, the importance of the considerable thicknesses of
sediment in certain areas of the trough as potential sources of
hydrocarbons cannot be overlooked, despite the large technological
difficulties which would have to be surmounted in order to produce
oil and gas from a sedimentary basin where water depths are an order
of magnitude greater than the hydrocarbon-producing areas of the
North Sea. In fact there has been much interest shown recently in
the hydrocarbon prospectivity of the Rockall Trough, particularly to
the north-east, and with the arrival of the tension-leg drilling
platform some exploratory work may be witnessed in the near future.
However, it seems that development and production in such a deep-
water area is a long way off yet.