Dissertation d ibba_thurley_ct

The Geology Of the Dibba Zone Durham University
Callum Thurley 1 of 50
The Geology of the Dibba Zone, Hajar Mountains, United Arab Emirates.
Callum Thurley
Department of Earth Sciences Durham University
2012/13
This dissertation is submitted in partial fulfilment of the requirements of the
degree “Geology.”

Abstract
The Dibba Zone studied shows a North East to South West trending section consisting of
deep-water facies sediments ranging in age from the late paleozoic and Mesozoic. These
units are comprised of a carbonate black shale (Mayah Formation), inter-bedded Chert and
Carbonate Smarls (Nayid Formation), bedded fine-grained Chert and Glauconitic Chert
(Sid‟r Formation). The depositional environment for these rocks is likely to be in deep
water, at a very similar depth to the lysocline, in order to produce carbonate and chert
interchanges at a high frequency. These lithologies have undergone at least three stages of
structural deformation, firstly by extension from a spreading ocean during the evolution of
the Tethys in the Paleozoic and early Mesozoic. Secondly compressional deformation from
the emplacement of the Hajar Ophiolite in the late Cretaceous, which lead to thrusting with
easterly dipping faults, and tectonic transport of the sediments from East to the West.
Thirdly, more recently and even current day strike slip deformation from the Dibba Fault,
which shows a sinistral offset trending approximately North East to South West through the
centre of the map.
The emplacement of the Hajar Ophiolite occurred 65-70Ma by marginal basin spreading,
and has been interpreted as an oblique obduction, due to lithological heterogeneities in the
Green Glauconitic Chert (Sid‟r Formation).Partial serpentinization of the ophiolitic
harzburgite, around its perimeter suggests that the hydration of the harzburgite and the two
main thrust faults are closely related.

Contents
Abstract – P.1
Acknowledgments – P.4
Introduction – P.5
Chapter 1 – Sedimentology - P. 6-20
i) Lithology 1 – Mayah Formation P. 6-7
ii) Lithology 2 – Sidr Formation P 7-9
iii) Lithology 3 - / Nayid Formation P. 9 -17
i) Sub-lithology 1 – P. 10
ii) Sub-lithology 2 – P. 10-2
i. Likely cause of Turbidity Currents – P. 12
iii) Sub-lithology 3 – P. 13
iv) Sub-lithology 4 – P..13-4
v) Sub- lithology 5 – P.15
vi) Sub-lithology 6 – P. 15-6
vii) Overall Interpretation Nayid Formation – P.16
i. Sedimentary Log of Nayid Formation – P17
iv) Lithology 4 - Brown Chert –P. 17-8
i. Possible formation hypotheses – P.17-18
v) Lithology 5 – Dendritic Chert – P. 18-9
vi) Lithology 6 – Wadi Conglomerate – P.19-20
vii) Chapter Conclusion – P.20
Chapter 2 – Igneous & Metamorphic Geology – P. 21-8
i) Lithology 1 – Harzburgite – P.21-2
i) Mineralogy from field observation – P.21
ii) Mineralogy under thin section – Appendix i

ii) Lithology 2 – Serpentinite – P.23-4
i) Mineralogy – P.23-4
iii) The Contact between Serpentinite and Harzburgite – P. 25-6
i) Mineralogy from thin section – Appendix ii
iv) Lithology 3 – Mélange – P.27-8
v) Lithology 4 - Serpentinized intrusion – P.28
Chapter 3 – Structural Geology P. 29-45
i) Normal Faults – P.29-31
ii) Thrust Faults – P.32-3
i) Emplacement of the Samail Ophiolite P. 33-4
iii) Folding - P. 36-38
a. Stereonets, Figure 20 – P. 38
iv) Deformation of Sediments – P. 39-41
i) Possible interpretations – P.39-40.
ii) Further research – P. 41
v) Strike Slip Faults – P.41- 43
vi) Oblique Slip Faults – P. 44
vii) Mélange and shear sense indicators – P. 45
Chapter 4 – Economic Potential – P.46
i) Hydrocarbon Potential – P.46
ii) Mining Industry – P. 46
Chapter 5 – Geological History – P.47-48
Bibliography – P 49-50

Acknowledgements
This project could not have been carried out without the generous help given by various
individuals. Thanks must go to Ali Mohammed Ahmed Qasim, head of Fujairah
Department of Natural Resources. The whole mapping project would not have happened
without the permission he granted. The hard work put in by Vanessa Jackson in organising
meetings with the government in order to obtain permission forms, which was greatly
appreciated. Paul Oliver must also be thanked for providing us with accommodation and
relief from the heat. Geological advice from two geologists (Sheikh Ali and Edgar
Akobyan) from Crescent Petroleum was also extremely helpful. Whilst out in the field, the
general welcoming attitude and kindness of farmers in the mountains by allowing us to
park on their land and offers of water was very much appreciated. Gulf Rock must also be
thanked for their support, help in obtaining permission forms, and advice on the geology of
the region. Dr Mark Allen who agreed to be supervise the project, supported us and allowed
us to undertake this mapping project and has given his advice on any geological questions.
Joao Trabucho-Alexandre has also aided this project by giving his opinion on
Sedimentological issues that were encountered. Ben Jackson must finally be thanked for
being my field partner for the duration of the mapping project.

Introduction
This study was carried out in July - August 2012 and focused upon the Dibba Zone in the
Hajar Mountains located in the Eastern United Arab Emirates. The Dibba Zone is to the
west of Ghub and Dahir, and approximately 15km away from the town Dibba (see Figure 1
below). This region has not been extensively studied, apart from one major study by the
British Geological Survey in 2003-5 and another by Mike Searle in the 1970‟s.
An integral geological feature of the Dibba Zone is the Semail Ophiolite, which was
obducted 65-70Ma and stretches for approximately 600km along the east coast of the
United Arab Emirates and Oman. It is the largest and best exposed ophiolite in the world
(M.P.Searl et al 1990). Beneath and in front of the Semail Ophiolite are thrust sheets
composed of distal deep sea sediments, the Hayibi and Hawasina Complexes. During the
course of this study, the Hawasina Complex was observed, which is composed of distal
Limestones, Calciturbidites and Cherts, all of which have been faulted and folded during
the ophiolite emplacement. The specific mapping area is highlighted in red below (Figure
1).
Figure 1 (Location Map, Google Earth Copywrite)
Chapter 1 – Sedimentology
3km150km

This chapter will describe all of the sediments in the area stratigraphically, starting from the
basal units and working upwards.
Lithology 1 - Mayah Formation
Observations
This is the lowest stratigraphic unit in the area and is an organic rich carbonate (Figure 2).
This rock is well sorted, grey in colour, on a weathered surface, dark in colour on a fresh
surface and very fine grained. There is a distinct lack of evidence of past life, with no
fossils present. This limestone is calcareous, evidenced by the classic honeycomb
weathering patterns and pits seen on the surface, as well as the reaction documented upon
the addition of hydrochloric acid. This lithology was also in beds that ranged from
approximately 10cm to 1m in thickness. After observation under thin section, it was still
apparent that this was very fine grained, with little other than a single spongue spicule
(exoskeleton) being seen.
Figure 2. (Locality 1, showing the carbonate black shale, North facing).
Interpretation
10m

The dark colour of the rock on a fresh surface is an indicator that the rock has a high
organic content. It is likely that it was deposited in a deep water environment under an area
of high productivity in order to yield sufficient organic matter to produce such a dark
colour, therefore it can be classified as a pelagic deposit. The organic matter is likely to be
comprised of Coccolithopores and Foraminfera, which fell through the water column as
“marine snow” (Mike Leeder,2011) or as faecal pellets from animals at higher trophic
level. This paleoenvironment is also likely to be anoxic, as hinted by the presence of
organic material, the lack of oxygen means that there are no scavengers, therefore
preserving the organic material. Therefore this may be described as a Sapropel, which is a
term commonly associated with organic rich lithologies. Due to the distinct lack of fossil
assemblages (apart from a singular Spongue Spicule) it can be interpreted that this is a
distal Carbonate, and may even be described as Carbonate Black Shale. The fact that this
rock contains carbonate, and fizzes under the application of hydrochloric acid indicates that
this rock was deposited above the Calcite Compensation Depth and is therefore likely to be
a Lime Mudstone. This has been deposited on the outer carbonate ramp, as a calcareous
ooze, which was then buried and lithified. Searle et al 1990 suggests that the area
underwent high amounts of slumping due to steep slopes, however no evidence of this was
seen in the mapping area. The Mayah formation is part of the Sumeini Duplex, which is
found stratigraphically below the Hawasin Complex.
Lithology 2- Sid‟r Formation
Observations
The second lithology, was very fine grained, friable, green in colour, did not contain any
fossils and did not react upon the application of hydrochloric acid. This lithology was found
running along the base of one wadi sampled within the study. Angular unconformities

between this lithology and wadi conglomerates were scattered all over the mapping area
(Figure3).
Figure 3(Field photograph showing an angular unconformity between folliated Green
Chert and Wadi Conglomerate, South Facing).
Interpretation
This lithology was deposited in a low energy, deep-water environment, with a low rate of
sedimentation. The green colour could be attributed to the mineral Glauconite, which also
supports the hypothesis that there was a slow rate of sedimentation (H.S Chafetz & A Reid,
Oct 2000). Due to the fineness of the grains and the fact that there was no reaction with
hydrochloric acid, this is a distal silica rich lithology. This lithology is quite similar to the
lithology found at the bottom of the Turbidite Smarl / Nayid Formation. Due to the lack of
any Carbonate, the depositional environment was below the calcite compensation depth.
Therefore the depositional environment was underneath an area of an upwelling of silica
rich organisms (Radiolarians and Diatoms). This is likely to have led to there being a
70cm

reduction in the oxygen minimum zone, a rise in the calcite compensation depth and all
Carbonate tests being dissolved prior to deposition. The upwelling of silicate organisms led
to the deposition of silicate biogenic ooze, which underwent diagenesis and was lithified
into a “Green Glauconitic Chert.”
Lithology 3 - Calciturbidite Smarl Formation/ Nayid Formation
The third lithology studied was a Calciturbidite sequence, consisting of a sequence of inter-
bedded limestones, smarls, and thinly bedded cherts (see Figure 4). Each lithology within
the sequence has been separated into sub-lithologies and observed and interpreted. It has
been described chronologically from the bottom to top of the sequence.
Figure 4. (Field photograph, showing the inter-bedded Chert and Carbonate, SW Facing).
15cm

Sub-lithology (i)
Observations:
Starting from the bottom of the sequence, there is a finely bedded (1/2cm thick) smooth
green rock type and this contained approximately 10% muscovite crystals. Abundant
dendrites were observed along the bedding planes of this lithology. This rock is also dark
on a fresh surface, indicating that there is a high organic content. This rock did not react
upon the application of hydrochloric acid.There is a distinct lack of any fossils within the
rock and the only sedimentary structure shown is planar bedding.
Interpretation:
Due to the fact that there was no reaction upon the application of hydrochloric acid, and is
very finely grained, this rock does not contain any carbonate and is a silicate, clay rich
rock. The green colour of this lithology may be caused by the presence of the mineral
Glauconite. The planar bedding, fineness of the grains and the potential presence of
Glauconite indicate that this was deposited in a low energy marine environment, in deep
wateron the continental shelf, with a low rate of sedimentation (H.S Chafetz & A Reid, Oct
2000). Dendrites are the result of a chemical change during diagenesis,, the dendrites that
have been studied look as if they are composed from Manganese. It can also be
hypothesised that this rock was deposited below the calcite compensation depth due to the
lack of carbonate in the rock. This rock is Glauconitic Shale and it looks as if it could be
similar to the Green Glauconitic Shale.

Sub-lithology (ii)
Observation:
Above this lithology there is a carbonate, which shows a slight coarsening upward
sequence from fine silt to very fine sand. This lithology showed planar bedding, with the
beds varying in thickness between 20-45cm. Once again this rock was black/dark grey on a
fresh surface and therefore also has a high organic content. Chert nodules were observed
within the beds of this carbonate, and thin beds of chert were present, which were not
consistent in their thickness (Figure 4). Flutecasts petruded from the base of the beds as
seen in Figure 5, with a rough orientation of 040°. (Figure 5).Some quartz overgrowths
were also seen.
Figure 5(A field photograph illustrating Flute Casts on the base of a carbonate bed.)
Interpretation:
The presence of Chert nodules in the carbonate may suggest that the deposition of this
Carbonate was in a shallower water paleoenvironment. The reason for the Chert nodules
may be diagenetic unmixing or segregation of originally mixed biogenic ooze. (João
Trabucho Alexandre et al Nov 2011). From looking at Figure 3it can be seen that the Chert
30cm

beds are not consistent with their thicknesses, these look like pinch and swell structures
(boudinage), which formed by extension, where the more competent rock (Chert) breaks up
and is stretched into a long thin shape (Hans Ramberg,1955). Flute casts form from erosion
by turbidity currents, which are vortexes of water during submarine avalanches which
produce triangular shaped structures that open out in the direction of the
paleocurrent(Figure 6). The presence of flute casts indicates that there were submarine
avalanches, and the fact that this is a carbonate means that it may have been deposited on a
Carbonate Turbiditic Apron. The presence of quarts overgrowths, means that diagenesis
occurred in a warm wet, oxic environment. This can be called a fine grained Cherty
Carbonate.
Figure 6(Rose Diagram Showing flute cast orientations).
Likely Cause of Turbidity Currents
The likely cause of turbidity currents may be attributed to various processes.
Build of sediment in one area can cause overloading and slope instability, which
then leads onto submarine avalanches.
It is quite likely that earthquakes in the region caused slope failure, and turbidity
currents.
Decay of organic material within these organic rich lithologies leads to the release
of methane gas into the pore spaces in the rocks. This exerts a pressure, and
eventually, this pressure may exceed the internal strength of the rock. This leads to

slope instability and can cause turbidity currents when the slope fails and the
material fall downs as a submarine avalanche.
Sub-lithology (iii)
Observations:
The next lithology in this sequence of beds is beige in colour and is extremely finely
bedded on a millimetre scale, powdery and clay rich. This rock fizzed under the application
of hydrochloric acid, has a dark grey colour on a fresh surface indicating once again a high
organic content, and shows planar bedding. Suggesting that the environment of deposition
was one of low energy. The entire bed of this marl is only approximately a metre in width.
Interpretation:
Due to the fineness of the grains, the presence of clay and the small scale of the bedding,
this may have been deposited in a deep water environment with a low rate of
sedimentation. It can also be reasonably determined that is rock is a carbonate, due to the
reaction witnessed after the addition of hydrochloric acid. Due to the high percentage of
silicate minerals within the carbonate, determines this rock as a Smarl(João Trabucho
Alexandre et al Nov 2011.
Sub-lithology (iv)
Observation:
Above the bed described in sub-lithology iii there is another fine grained Siltstone which
contains Chert nodules (Figure 7) in the lower section of the bed with a small band of Chert
in the upper part of the bed, which also showed planar bedding. Flute casts were observed,
with an average orientation of 030° and this therefore gives us evidence for submarine
avalanches. These chert nodules in a smarl carbonate are typically named flint, which show

conchoidal fracture and are composed of quartz. The veins that run through the chert here
are comprised of calcite as they fizzed under the application of hydrochloric acid and were
easily scratched with a fingernail.
Figure 6(Field photograph showing a Chert/flint Nodule)
Interpretation:
The fact that both beds and nodules of Chert/Flint are present, within the same lithology,
suggests that the deposition may have occurred at or around the Lysocline. This means this
was deposited during a period of high productivity, when there was an upwelling of silica
richorganisms (Radiolarians).Therefore there was an increase in the oxygen minimum zone
and a decrease in the Calcite Compensation Depth. This means that all calcite-containing
tests, such as Foramonifera and Coccolithopores would be dissolved at this depth, and none
would be preserved. This would lead to the deposition of silica rich rocks and most namely
chert. However during the periods where there was not high productivity, the calcite
compensation depth would be deeper, and therefore some of the calcium carbonate tests
would be preserved, and ensuringthe preservation of a carbonate rock with some clay
minerals rather than a pure silicate, this can also be described as Smarl.
5cm

Sub-lithology (v)
Observation
The following lithology in the sequence represents a fining upward sequence from coarse to
medium sand. This is matrix supported and has lithic fragments of size ranging from 1-
3mm. It is black on a fresh surface and it fizzed under the application of hydrochloric acid.
Under thin section we could see Ooids, circular structures and a cubic mineral which
showed some clear fabric. Some smaller circular structures that were darker in colour than
the ooids were also observed.
Interpretation
This is a carbonate mudstone, more specifically an Oolitic Micrite. Micrite has formed as
diagenetic cement and therefore forms the matrix. Some of the Ooids are difficult to
distinguish because they are also composed of micrite. The presence of Ooids indicates that
this was originally a higher energy environment on the carbonate ramp, towards the
offshore during periods of lower energy. This can be interpreted due to its occurrence with
other ramp carbonate facies. Other inclusions of dolomite show that dolomitisation has
occurred, which is a diagenetic change typical of dry arid environments. The small pellets
are a form of carbonate material, maybe faecal pellets from organisms higher up in the
water column. The pyrite seen indicates that during diagenesis this was an iron rich anoxic
environment.

Sub-lithology (vi)
Observations
The top of the sequence is capped with a green coloured rock that fractured in a conchoidal
manner, sugary texture and showed planar bedding. It is monomineralic microcrystalline
quartz.
Interpretation
The likely depositional environment for this was probably around the lysocline, when there
was an upwelling of silica producing organisms such as Radiolarians and Diatoms. This led
to an increase in the oxygen minimum zone, therefore raising the Calcite Compensation
Depth, resulting in the deposition of a silicate ooze which was lithified during diagenesis to
form green Chert/Jasperite. The Green colouration may also be due to the presence of
Glauconite.
Overall interpretation of the depositional environment of this inter-bedded Turbidite Smarl
Chert sequence
This sequence represents a series of distal facies carbonates, shale, and cherts. They are
deep-water facies and due to the Carbonate/Chert cyclicity they were deposited at or around
the lysocline. Carbonate rocks were deposited during times of calm and chert or shale was
deposited when there was an upwelling and high productivity. The presence of flute casts in
some of the carbonate beds means that these were deposited on the continental slope where
slope instability was prominent. This may have occurred during the closing of the Tethys
Ocean 75-65 Ma (M.P.Searle et al 1990), leading to submarine avalanches on a turbiditic
apron (reasons for submarine avalanches can be seen on P 13). The sedimentary Log of
the Turbidite Smarl/Nayid formation can be seen below (Figure 8). These can be classified
a Smarl due to that fact that they are a Silica rich Marl (J.Trabucho Alexandre et al 2011).

Figure 8 (Sedimentary Log of the Turbidite Smarl / Nayif Formation)
Lithology 4 - Brown Chert
Observations
This lithology formed in uniform beds, which were approximately 10cm in width. The rock
was brown in colour, did not react under the application of hydrochloric acid, showed
conchoidal fracture and was microcrystalline. (A field photograph of this lithology can be
seen in the structural section on page 30, Figure 15).
Interpretation
Due to the fact that the rock here is formed in beds, completely silica rich with carbonate
present, we can conclude that this is a Chert deep-water facies.
Possible Reasons for Formation
The formation of chert, is one of sedimentologys greatest questions, however below here
are some possible interpretations.

There may have been a sustained period of high productivity and upwelling of silica
containing tests (Radiolarians & Diatioms). Although this is a feasible explanation,
it is unlikely due to the extremely low sedimentation rates of these rocks. Also the
upwelling would have to last for an unrealistic amount of time for this thickness of
Pelagic Chert to be deposited.
A marine transgression could have produced such a thick formation of Chert. A
marine transgression would lead to the starvation of carbonate from the
environment, due to the Calcite Compensation Depth being now at a shallower
depth than the floor of the paleoenvironment.
The most likely reason for the formation of this Chert however is Diagenetic
Secondary Replacement. This has been localised to a narrow band of Turbidite
Smarl and two other spots on the map, therefore leading to the formation of a band
silicified Turbidite Smarl. During burial, there may have been a change in both
temperature and pressure and maybe even the addition of some fluid that dissolved
the calcite and left silica remaining. This silica may then have formed biogenic opal,
which was then changed into microcrystalline Quartz. This localised diagenesis,
explains the fact that there are two smaller groups of this lithology in the mapping
area. This also explains why there is a small slice of Turbidite Smarl on the top of
this mountain; primarily due to the fact that the diagensis / replacement has been
localised and therefore hasn‟t fully silicified the whole formation.
Lithology 5 – Dendritic Chert
Observations
This rock was dark red, orange and green in colour on a fresh surface, contained both thin
(5cm) and thick (30cm) beds and shows conchoidal fracture. Dendrites were also identified
and it was microcrystalline.

Interpretation
This is another deep-water facies Chert lithology. The predominant mineral is probably
quartz due to the hardnessand conchoidal fracture. The depositional environment once
again is one of deep water with a low rate of sedimentation and below the Calcite
Compensation Depth. This has been called Dendritic Chert on the Geological Map.
Lithology 6 – Wadi Conglomerate
Observations
This lithology contains poorly sorted angular clasts, between 3-8mm and 20cm, cemented
by finer grained but unstable sediment due to a lack of diagenesis and compaction, meaning
that this rock is matrix supported. Horizontal bedding with zero dip is observed at many
localities, and fining upward sequences are seen. This lithology is only found at the base of
wadis, throughout the mapping area, laid down uncomformably above the green chert, and
with the metamorphic rocks in the area Serpentinite and Harzburgite. The clasts within the
rock are composed of rocks in the local area, for example there are large clasts of the
metamorphic Serpentinite, Harzburgite, and many inclusions of the surrounding carbonates
and silicates. Imbricate structures are present here, where the long axis of each clast has
aligned to the direction of the paleo current, which was at a bearing of 244 degrees or in a
West, South-West Direction. (See Figure 3 for a field photograph)
Interpretation
It can be hypothesised from the lack of structural deformation seen in the form of folds and
faults throughout the mapping area, that this is a much more recent deposit and is unrelated
to the deposition of the Carbonates and Cherts. This is a Wadi Conglomerate, deposited
during a very high-energy event and is likely to have been deposited over a period of a few
hours. This often occurs when there is heavy rainfall in an arid environment that has

impermeable rocks due to intense heating and therefore drying out or crystalline lithologies.
This leads to an extremely rapid rise in the water level and results in a flash flood. These
can be extremely powerful and can transport boulder-sized clasts, which are deposited first
and then the finer grains are deposited, leading to a fining upwards sequence. The
paleocurrent simply just runs in a direction down and out of the wadi.
Chapter conclusion
It can be concluded that all of the carbonates in the area are sapropellic, due to their black
colouration on a fresh surface. All of the sediments except for the Wadi Conglomerate were
deposited on the continental shelf of the Arabian platform or Musandam Shelf edge, some
of which were above the Calcite Compensation Depth producing carbonate grainstones,
and some below forming microcrystalline Chert lithologies. Most recently flash flood
deposits have led to the deposition of Wadi Conglomerates, that outcrop uncomformably in
the Dibba Zone.

Chapter 2 - Igneous and Metamorphic Geology
Some key features of this mapping dissertation were defined by the presence of igneous
and metamorphic lithologies. All of these lithologies have got an ophiolitic origin.
Lithology 1- Harzburgite
Observations
This lithology was black in colour, crystalline with Interlocking crystals, and very dense.
Figure 9 shows a thin section.
i) Mineralogy from field observation.
10% - Dark green, circular mineral, poor cleavage. Circular Pits
with rust coloured residue in them. This mineral is Olivine, and the
residue in the pits is known as Iddingsite, which is weathered olivine.
10% - Black elongate mineral, vitreous lustre, cleavage at
approximately 120°. This mineral is amphibole.
10% - Bronze coloured mineral, platey with a metallic lustre, at least
2 directional cleavage. This mineral is called Bronzite, which is an
Orthopyroxene.
70% - The finer grained material is all a dark mineral and in the
field it is hard to identify, but maybe Augite.
See appendix i for full mineralogical description of thin section.
Interpretation
Due to the very high density of this rock and the presence of these orthopyroxene
mafic/ultramafic minerals such as Bronzite, this rock can be classified as Harzburgite.
Bronzite is an iton rich weathered form of Enstatite (Encyclopedia Britannica (1911)
Enstatite) whichis indicitive of this rock type. Harzburgite is found at the ophiolite sole.
This is a cumulate rock, meaning that it forms by the accumulation of minerals.

Harzburgite is a type of peridotite that forms when Lherzolite is melted and the
Clinopyroxene is lost, leaving a rock with Orthopyroxene and Olivine and a Harzburgitic
content. In ophiolites, Harzburgite is the most common type of peridotite found.
Figure 9(Thin section of True Harzburgite away from the apparently serpentinized zone in
cross polarised light, see appendix i)
4mm

Lithology 2 - Serpentinite
Observations
This lithology is green in colour and has dark patches. It is medium grained, has a fibrous
texture with interconnecting calcite veins. Dark inclusions can be up to 30 cm wide, and
some darker minerals have a fabric and are aligned to a bearing of 113/293 degrees. A
photograph of this lithology can be seen below (Figure 10).
Figure 10 (A field photograph of Serpentinite, South East Facing)
i) Mineralogy
 10% - Dark elongate minerals with cleavage at approximately 120°,
this is amphibole.
 10% - Dark green, circular mineral, poor cleavage. Circular Pits
with rust coloured residue in them. This mineral is Olivine, and the
residue in the pits is know as Iddingsite, which is weathered olivine.
 10% - Lighter green plated mineral, this mineral was also rounded
and had a vitreous lustre, this mineral is Chlorite.
 5% - A very small amount of a blue elongate mineral, which has 2
directional cleavage, this minteral is Glaucophane.
50cm

 10% - Dark red/brown elongate mineral, with a waxy lustre. This
may be Jasper.
 10% - Crysotile Aspestos, although this varied from locality to
locality.
 Veins scratch easily with a steel knife, and weather like a limestone,
therefore these are calcite veins.
Interpretation
This lithology is a metamorphic Serpentinite, probably with a Harzburgite protolith. It is
formed by the low-grade metamorphism and hydration of the Harzburgite ophiolite sole, by
the addition of water (Patricia Fryer,2002). Harzburgite is unstable at the earth‟s surface
due to the fact that is formed in the mantle at vastly higher temperature and pressures.
Therefore when these rocks were exhumed by thrusting, the addition of seawater and the
change in physical conditions leads to metamorphosis. On a mineral scale, serpentinization
occurred, this led to the hydration of ultramafic minerals such as Olivine and
Orthopyroxene first and the formation of Serpentine.
During metamorphosis there is only a relatively small change in temperature and pressure,
and the minerals metamorphose in order to reach equilbrium with their environmental
conditions. This may have occurred when subduction began. When the rocks were
exhumed by thusting, the reduction in both temperature and pressure, along with the
addition of sea water led to the serpentinization of the ultramafic Harzburgite, forming the
serpentine minerals and the rock Serpentinite. The formation of this Serpentinite has a
structural origin. (See next observation & Interpretation).

The Contact between Serpentinite and Harzburgite.
Observations
The contact between these two lithologies is not a distinct change, but a gradual one. There
is a transition zone between these two major lithologies, where the rock seems to be of a
higher density than the pure Serpentinite but less dense than the true ophiolite
(Harzburgite). It seems to contain more serpentine minerals aswell, giving it more of a
green colouration, but also contains some Bronzite minerals that are exclusively found in
the Harzburgite. Some of the rocks found here are extremely rich in Bronzite minerals.(See
below for a thin section Figure 11). Detailed mineralogical descriptions can be seen in
appendix ii. (disc in at the back of the book).
Figure 11(Thin section from the Harzburgite/Serpentinite transition zone in cross polarised
light)
Interpretation
This is most likely to be some form of metamorphic contact and the serpentinization,
doesn‟t extend very far into the Harzburgite. This may be due to the fact that hydration of
4mm

the anhydrous Harzburgite by seawater depends on percolation along a conduit such as a
thrust/fault plane. (Patricia Fryer, 2002).Therefore seawater may not have been able to
penetrate very far into the Harzburgite, and thus partial serpentinization has occurred
(Figure 12). Serpentinite is weaker than the surrounding Harzburgite and so allows shearing
to be focussed at the hydrous/anhydrous boundary. This hypothesis is supported by the fact
that Sepentinite is found at lower altitudes, in wadis below and surrounding the
Harzburgite. This has led to the separation of the two lithologies by a second thrust
fault.The thrusting and tectonic activity may have stopped before the serpentinization was
complete, therefore leaving some of the Harzburgite in its „pure‟ form whilst some of it was
metamorphosed into Serpentinite.
Figure 12(schematic diagram showing the segregation of hydrated Serpentinite and the
anhydrous “pure” Harzburgite).
Pure Harzburgite, is
hasn’t been affected
by the
serpentinization
process.
The Serpentinite is
hydrated Harzburgite.
Sea water percolated
along the floor thrust
fault to create a narrow
shear zone, and that is
where the Serpentinite
formed.

Lithology 3 - Mélange
Observations
This lithology was too small to map on a 1:10,000 scale map. It has the same fibrous
texture as the Serpentinite and is brown in colour. Some very large inclusions that can be
up to 50cm in size are present. It looks like a fragmental rock and the matrix looks as if it
has flowed in a ductile manner, around the larger clasts. (see structural section for more on
this). Some of the inclusions seem to consist of the underlying carbonates (Nayid
Formation) whilst others seem to be composed of Serpentinite. This rock looks as if it is a
large Breccia but on a larger scale (Figure 13), and looks as if it has also been
serpentinized.
Figure 13(Field photograph of Locality 41, showing the Mélange, North facing.)
Interpretation
This is a mega-breccia, in other words Mélange. Where the ophiolite has been thrusted over
the Carbonates beneath and the lithologies have been tectonically sheared together into a
40cm

mega breccia. This lithology may also have been serpentinized during the exhumation of
these rocks, just like the Harzburgite has been.
Lithology 4 – Ultramafic Dyke
Observations
This lithology was also of a scale too small to map, however it should be noted as part of
this study. It was a linear outcrop of emerald green and white rock on a weathered surface.
The outcrop stands alone with scree on either side of it. It is a much darker green than the
surrounding Serpentinite. In some places on a fresh surface this appeared to be white.
Mineralogy
o 20% - Dark green, circular mineral, poor cleavage, this mineral is Olivine.
o 80% - Black Groundmass, it is hard classify in the field, however it may be
some form of Augite or Amphibole.
Interpretation
This is an ultramafic dyke that has been serpentinized. It still looks like Serpentinite,
however it is a different kind of Serpentinite to that of the majority. This may have come
from a dyke that was more ultramafic or more Olivine rich than Harzburgite. It was found
very near to an area that has been have interpreted as a thrust plane. Therefore this may
have been where the hydrating fluid has been concentrated. This along with the fact that
this is an ultramafic dyke could have led to this exceptional serpentinization. This may even
be a Carbonatite, see Economic Potential, Metaliferous Mining on page 46 for more detail.

Chapter 3 – Structural Geology
The Dibba Zone has undergone, atleast 3 phases of structural deformation, and there is still
some strike slip deformation occurring today. The following structural features have been
explained in chronological order, in terms of the apparent geological history.
Normal Faults
Normal faults were observed in all of the sedimentary sequences that were mapped (Figure
13/14/15). At locality 1, in the carbonate black shale, all of the normal faults observed had
the southerly side downthrown. All of the faults observed showed fault drag and some
smaller degrees of fault bend. In some areas the faults did seem to show an en-echelon
pattern. Normal dip slip faults were seen on scales with offsets of up to 6.5m down to 3cm,
see Figure 14 for schematic diagrams of the faults, see figures below. Other faults were
seen to form in conjugate sets and some smaller faults showed a listric habit. (Figure 13).
Figure 13(A normal fault, seen in the carbonate black shale, North East Facing).
10cm

Figure 14(sketches to show the types of normal faults seen)
Figure 15(outcrop showing normal fault system in fine grained Chert, South Facing)
2m

Interpretation
The fact that normal faults were only present in the sedimentary rocks in the area, suggests
that the tensional deformation occurred prior to the ophiolite emplacement and the
compressional deformation. This would have occurred when the Tethys Ocean was still
spreading and the Carbonates and Cherts were still being deposited in the ocean basin
during the Permain period (M P. Searle et al 1990). Due to the fact that some of the normal
faults in the area show an en echelon pattern, this may suggest that this was a transtensional
margin, or an “oblique rift” margin. (M P. Searle et al 1990).
It has also been suggested that the normal faulting may have been caused by the
subduction, a process known as “Marginal Basin Spreading.” This has been described and
explained fully in the Obduction of the Semail Ophiolite section (Gary Feulner 2005).
Due to the fact that the ophiolite and serprentinite were not visibly affected by normal
faulting, it could be argued that this occurred before the ophiolite was obducted onto the
Arabian continental margin. However, it may have happened in the early stages of
subduction due to marginal basin spreading. Alternatively, spreading of a conventional
manner in the Tethys Ocean could have been the cause of these tensional faults.

Thrust Faults
Observations
The main type of structural deformation in the Dibba Zone has been compression; all of the
rocks in the area have been affected by the thrusting and therefore are all allochotonous
units. The vast number of folds and tilted sedimentary sequences in the area reflects this.
There are two main thrust faults in the area, one runs along the boundary between the
Harzburgite, and the Serpentinite, the other between the Serpentinite and the Turbidite
Limestone contact. Furthermore, the faults generally follow the contours of the mountains.
Slicken lines and slickenfibre steps (Figure 16) were seen all along these contacts, and
throughout the Serpentinite. Riedel shears were seen throughout the Serpentinite, and
occasionally in the Green Chert.
Figure 16(field photograph, showing a slickenside, and slickenfibre steps).
Interpretation
As we can see from the fair copy map the general direction of tectonic transport is from
East to West and due to the fact that the faults generally follow the contours this means that
we probably have a shallowly dipping thrust. If this was a steeply dipping thrust plane then
10cm

the relationship with contours would be a cross cutting one. Riedel shears are features that
are associated with Strike or Oblique-Slip Faults. (Twiss Robert J,Moore Eldrige M, 2006)
and thus this may indicate the presence of a transpressional deformation zone. The
slickenfibre steps form from the reactivation of faults along the same plane and the
direction of stepping up indicates the direction of tectonic transport. As demonstrated in
Figure 16 the direction of transport is up the rock face.
Finally, thrusting from the ophiolite emplacement has led to the tectonic transport of all
sedimentary lithologies in the area. Therefore they are all allochtonous units, as they are not
currently sat in the position that they were originally deposited. All of the pelagic and
turbidic sediments in the area are part of an accretionary prism (Figure 17) where the
sediments have been scraped off from the oceanic plate. This was formed during the
ophiolite obduction, these are the deep water facies that immediately over-lie the oceanic
plate.
http://www.classroomatsea.net/general_science/images/acc_prism.jpg
Figure 17 (a diagram to show the formation of an accretionary prism)

Emplacement of the Samail Ophiolite
The Samail Ophiolite represents a section of the Tethyan Oceanic crust, formed by the
spreading in the Cenomanian/Mid Cretaceous (Robert G Coleman 1981). The ophiolite
exposure extends for approximately 600km and is made up of 12 blocks, separated by
major faults (Lippard et al 1986). During the dispersal of Gondwanaland, the Arabian Plate
drifted north and Eurasia and Africa rotated in an opposite direction. This led to the closing
of the Tethyan Ocean during the Turonian and Campanian or end cretaceous period 65-70
Ma (M P. Searle et al 1990).
The Ophiolite in the Mapping Area
The ophiolite the studyarea is called the „Hajar Ophiolite‟ and it has been suggested by
Gary Feulner that it was not obducted in the conventional manner. Due to the fact that the
ophiolite has been aged between 90Ma and 100Ma, it would suggest that these rocks were
obducted soon after their genesis. Gary Feulner has also implied that the ophiolite was not
formed from a conventional spreading ridge, but from a process called „Marginal Basin
Spreading‟ which takes place on an overriding plate close to the subduction zone. This is
caused by physical tension in the overriding plate caused by rapid decent of the subducting
plate. During the process frictional heating and the release of water vapor and other
volatiles from the subducted slablead to the melting of the overriding plate. Marginal basin
crust is thinner than true oceanic crust (due to a smaller source), this crust was young,
buoyant and hot. Collectively these features facilitated the obduction of the Hajar Ophiolite
(Gary Feulner 2005)It has been proven that this is method of obduction is correct due to
the fact that the protoliths in the metamorphic sole are not the same age as the ophiolitic
rocks above (Mike Searle & Jon Cox, 1999). Therefore the subduction can not have been
initatied from a mid-ocean ridge.

The area is near the most northerly part where the ophiolite has been obducted and we have
the rocks from the very base of the oceanic crust. Harzburgite, is found in the metamorphic
sole and is a form of Peridotite. The other layers of Tethyan oceanic crust are found further
south and east along the coast for example. The Aswad Block shows a complete ophiolite
section from mantle harzburgites to the upper crustal Pillow lavas to the East(Goodenbough
et al 2009). The ophiolitic rocks in the area of study are actually part of a klippe, which
have been detached from the main ophiolite by erosion. (Figure 18).There are two smaller
scale klippes in my area, both are in the North East section of the map, one of Harzburgite
in the Serpentinite, and one of Calciturbidite Smarl in Serpentinite.
Figure 18(schematic diagram showing the formation of a klippe in the Dibba Zone)

Folding – Ductile Deformation
Observations
The study area was dominated by folds,
ranging from a few centimetres to tens of
metres in scale. Many of the folds were
open or closed, upright folds
(Figure19)and were more recumbent,
whilst a few cylindrical folds were
spotted(Figure 19). Most of the fold
hinges measured in the area measured
have azimuths between 90 and 160
degrees or south-eastern direction and
had an eastward vergence. From the
photos to the left (Figure 19) at the top a
“S” fold is observed, with low inter-limb
angles, this is a closed fold. Below this a
recumbent fold has formed with Cherty
Siliceous rocks in the middle of the fold,
with the carbonate Smarl Formation on
the outside of the fold. Finally a
cylindrical fold in the Mayah Formtion,
this unit was heavily folded, possibly
due to its low competency, due to being
1.5m
1m
5m
3m

a shale. The Chert was also
regularly seen folded, sometimes
very intricately as seen in the fourth
photo down in Figure 19where Z
folds are observed. Finally, in the
last photo ofthere are another set of
open folds that are fairly
symmetrical, forming multiple
anticline syncline folds. Recumbent
folds (100m across) are found
connected to eachother (See Figure 20 for detailed analysis PTO) that had an eastward
vergence.
Interpretation
Due to the fact that the folds don’t seem to be cross cut by any of the faults, it can be
concluded that they were produced at the same time as the ophiolite obduction.Folding is
an example of ductile deformation, as opposed to faulting which is brittle deformation.
These rocks have been deformed at depth and pressure, which allowed for them to behave
in a ductile manner to form folds. Stereonets from the entire mapping area (Figure 20) give
us evidence that the in general, the hinges of folds in the area are dipping in a southerly
direction, and predominantly South East. This means that the deformation has originally
come from the South West and North East producing a hinge dipping South East. The
deformation in the form of folding seems to be fairly homogenous, as many folds are
upright, with near verticle axial planes and are nearly symmetrical. Both S and Z folds have
formed on either side of the same mountain in both Turbidite Smarl and Chert formations.
Figure 19 (field photographs of folds seen in
the area)
3m

Figure 20 of STERONETS

Deformation of sediments
Observations
The Green Chert lithology is not laterally homogenous, for example at the South Western
tip of this unit qualities such as, high friability, foliaton and cleavage are seen. These are
usually features that associated with metamorphic Schist. Elongate minerals were also seen
in the South Western tip where the rock was heavily foliated and powdery and could be
classified as chlorite. However, the North Eastern section of this unit is dominated by a
greater number of finer grained, less folliated, and chlorite minerals than are seen to the
South West. Generally most of the beddings seem to dipping in a Southeasterly direction.
Interpretation
The lack of consistency in cleavage and mineralogy throughougt this unit may suggest that
there has not been a homogenous amount of deformation throughout the area of study. The
rocks to the south west have experienced a higher degree of shearing than the rocks to the
north east and have therefore led to heavy foliation of the rocks to the southwest.The
bedding dipping to the South East suggests that the compression has come from that
direction also. Robertson, A.H.F et al 1990 has suggested that there is a thurst contact
beween the Mayah Formation and Hawasina complex. On the fair copy map map this runs
along the boundary between the Mayah Formation and the Sid‟r formation. There was not
time time to properly research this whilst out in Dibba, however below are a few potential
causes for the intriguing heterogeneity in this lithology.
Possible interpretations
i) This could have been caused by an oblique convergence when the ophiolite was
obducted. There may have been rotation hinged in the North Eastern part of section

of the mapping area, (Figure 21), which would mean that the rocks in the nothern
area of the map have been translated much less than the rocks in the South West.
Therefore less deformation has occurred here and so they show less evidence for
shearing.
Figure 21(schematic sketch to show an oblique convergence and possible rotation of
the ophiolite)
ii)The heterogeneity may have been caused by a small lithological or mineralogical change,
allowing shearing to to be fully distributed throughout the entirety of the lithology to the
south. Whereas in the North Western section of the map, the shearing has been
concentrated in the narrow fault planes, and so the rocks arent as foliated. This
heterogeneity may have been due to the fact that the rocks to the north had more clear cut

horizons/planes.Therefore allowing faulting and deformation to use these pre existing lines
of weakness as a slip surface.
Further research
If the study was to continue in the Dibba Zone, a couple of tests could be run, in order to
resolve this issue.
i) Study the lithology more closely in both areas, and obtain more data. Take
thin sections of the rocks from both localities and study them under the
microscope to look for mineralogical differences.
ii) Measure slickenline plunge and azimuths more in the areas to obtain see if
there are any preferred orientations, this may give more information about
how the ophiolite was obducted and direction of thrusting.
Strike Slip Faults.
Observations
Many of the contacts in my area have been offset by faults, most of which showed no
vertical offset whilst and others did. Eight major strike slip faults were seen in varying
locations, most created offsets in the Serpentinite and Harzburgite thrust contacts.
Indicators of shear sense and shear band fabrics were observed along fault planes (Figure
22). All of the faults apart from one seem to show an orientation between 090° and 170°.

Figure 22, (A Field photograph, showing the sense of a shear band fabric, where we
can see the minerals aligning to the fault plane, the diagram to the right, shows the strain
ellipsoids of the minerals across the fault plane).
Interpretation
Using the law of cross cutting relationships it can be said that, this is the most recent form
of structural deformation based on the fact that many of the contacts in my area were offset
by strike slip faults. This particular fault shows a dextral sense of shear and we can see that
the strain ellipsoids are stretched to a prolate form along the fault plane and then plain
strain on either side of the fault. On a Flinn Plot, the strain here would plot somewhere in
the zone of apparent constriction. The minerals are rotated into parallelism with the shear
plane, suggesting that the strain increases towards the centre of the shear zone. The strike
slip faults that cross cut the thrust faults produce structures called “non-coplanar imbricate
thrusts” (Robert J. Twiss, Eldridge M. Moore, 2006)(Figure 23.) Due to the South Easterly
orientation of the faults in my area, this may suggest that there is a major strike slip fault
running underground in the same direction through the centre of my study area. This may
be the Dibba fault, which separates the ophiolites to the South and the mesozoic carbonates
of the Musandam peninsula (P.D.Clift, D.Kroon, and J Craig, 2002) and it has been

speculated that this was once an ocean continent tranform fault between the Arabian plate
and Tethys Ocean (A.S.Alsharam, A.E.M. Nairn, 1997). However, this may even be linked
to the escape tectonics occurring in the Zagros region in Iran. This fault renders many of
the potential oil and gas reserves in the region useless by providing a conduit for the
petroleum to escape, leaving dry wells.
Figure 23(Diagram of Non-Coplanar Imbricate Thrusts)

Oblique Slip faults
Observations
A few strike slip faults observed in my area showed both some horizontal and vertical
offset. The main example of this was at locality 36, I found a contact that was offset
horizontally by 30m and vertically by approximately 10m up-hill. (Figure 24)
Figure 24, (Diagram of an oblique slip fault)
Interpretation
As we can see from Figure 24, this is an oblique slip fault, due to the fact that both
horizontal and vertical displacement is shown. This fault that was observed showed some
dextral sense of shear, which is anomalous for the area. In general the strike slip faults have
a sinistral sense which may be due to the fact that it happened at the same time as the
obduction of the Samail Ophiolite, rather than from the more recent strike slip faulting and
as such this may have been an oblique convergent boundary.

Mélange, and shear sense indicators
Observations
Carbonate and Serpentinite inclusions were found in the Mélange, with some finer grained
material that looked as if it had flowed around it in a ductile manner. (Figure 25)
Figure 25, (A field photograph of a σ shear sense indicator)
Interpretation
Figure 25 shows aσ shear sense indicator(Robert J. Twiss & Eldridge M.Moores,2006)
formed in a ductile shear zone during the thrusting of the ophiolite. It has formed due to
ductile flow in a deformable matrix, and we can see this porphoryclast of a local carbonate
carbonate. (Robert J. Twiss & Eldridge M.Moores,2006) Here we can see that the
asymetric „tails‟ have been recrystallised from the edges of the porphoryclast itself. This
particular example (Figure 25) shows a right lateral shear sense and this gives us supporting
information that the ophiolite obduction did in fact have some component of dextral
shearing as well as thrusting. This supports the hypothesis discussed in the interpretation of
oblique slip faults.

Chapter 4 – Economic Potential
Hydrocarbon Potential
The carbonates in the area are perfect resevoir lithologies, due to their high porosity,
permeability, high organic content, and presence of impermeable cap rocks Unfortunately
the structural geology extinguishes their economic potential, due to the fact that this is a
complicated fault zone and ensures that there are far too many routes of escape for potential
hydrocarbons to be retained in traps.
Mining Industry
Principally, much of the limestone in the area is mined, where the lime is removed and used
within the production of cement. Chippings of both Limestone and the ophiolite are also
used for aggregate.
i) Metaliferous mining
Chromite is the main economic income in the metaliferous mining of Dibba, from the
chrome-spinel in Harzburgite and other mafic/ultramafic lithologies. It is then used to
induce hardness and chemical resistance in steel. Large bodies of Carbonatites are found
within the metamorphic rocks beneath the Semail Ophiolite near to Dibba, which are
associated with meta-volcanics and Radiolarian Cherts. (Alleman, F., and Peters, T. 1972.)
Carbonatites contain the highest percentage of Rare Earth Elements (REE) of any other
igneous rock type. The abundance of Niobium bearing minerals such as Phyrochlore make
the Carbonatite economically viable for mining. It is then used in Iron and other alloys and
occasionally jewelry. Tantalium is another REE that is mined from Carbonatites, both of
these elements are used extensively within the high-tech industry. There was a small dyke
observed, which is described in more detail on page 27 and this could have potentially been
an intrusive Carbonatite.

Chapter 5 - Geological History
The geological history of the Dibba Zone has been written in chronological order, from past
to present.
Paleozoic
Deposition of the distal carbonate Mayah Formation in the spreading Neo-Tethys Ocean,
this was a transtensional oblique rift margin in the Permian period.
Mesozoic
After this there was an upwelling of silica rich organisms, leading an increase in the oxygen
minimum zone and lysocline and resulting in the deposition of a Glauconitic Chert.
Tectonic activity and gas pressure caused submarine avalanches leading to density flows
producing a Calciturbidite sequence (Nayid Formation). Fluctuations in the lysocline due to
periods of high productivity caused the Nayid formation to be a section of inter-bedded
Carbonates and Cherts. Normal faults from the spreading ocean penetrate sediments from
the north western section of the map.
In the late cretaceous 65-70Ma the Semail Ophiolite was obducted when the Neo-Tethys
spreading rate slowed and the rate of subduction overcame this. This was an oblique
obduction suggested by the presence of riedel shears, and the heterogeneity in the green
chert. This compression principally came from the South West and North East, as the
Arabian plate collided with the Eurasion plate. Compressional forces caused thrusting and
folding of sediments during the ophiolite emplacement. During the obduction, interactions
between seawater and Harzburgite, lead to the partial serpentinization of the ultramafic
ophiolite sole. Most folds in the area have azimuths facing South East, therefore suggesting
once again that the deformation was orientated South West / North East.

Cenozoic
The most recent form of deformation is strike slip, offsetting the thrusts forming non-
coplanar imbricate thrusts. These strike slips faults all seem to show a sinistral shear sense,
hinting that they may have been caused by the Dibba Fault which runs underground in
close proximity to the mapping area and is known to be a sinistral strike slip fault.
The most recent sedimentology in the area is the deposition of Wadi conglomerated at the
base of the Wadis forming unconformities with all lithologies. These have been deposited
during flash floods, which occur when there is a heavy rainfall in an arid environment

Bibliography
1. Alleman, F., and Peters, T. 1972, The ophiolite-radiolaite belt of the northern oman
mountains,Eclogolgy geology Helvitica 65, pp. 657-697,
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