dissertation d ibba_thurley_ct

The Geology Of the Dibba Zone Durham University

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.”

Callum Thurley 1 of 50


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)


3km150km


Chapter 1 – Sedimentology

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).


10m


Interpretation

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


70cm


rich organisms (Radiolarians and Diatoms). This is likely to have led to there being a

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

water on 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 3 it 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

rich organisms (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 ensuring the 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) which is 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 as well, 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, at least 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 study area 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 slab lead 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 (Figure 19)

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 a shale. The Chert was also

regularly seen folded, sometimes very intricately as seen in

the fourth photo down in Figure 19 where Z folds are

observed. Finally, in the last photo of there 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.


Figure 19 (field photographs of folds seen in the area)

1.5m

1m

5m

3m

3m


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 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 a σ 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

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dissertation d ibba_thurley_ct

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