1. 1
King Saud University
College of Science
Department of Geology and Geophysics
SEDIMENTOLOGICAL CHARACTERISTICS
OF LOWER HANIFA FORMATION,
CENTRAL SAUDI ARABIA
By:
AHMED MEDHAT RASHAD
Under The Supervision
Dr. Osama E.A. Attia
Submitted For The Requirement of GEO 498
To the Department of Geology and Geophysics
King Saud University
1435H - 2014 AD
2. 2
LIST OF CONTENTS
SUBJECT PAGE#
CHAPTER I: ---------------------------------------------- 4
INTRODUCTION--------------------------------------- 4
- Introduction ----------------------------------------- 5
- Aims of study --------------------------------------- 6
- Geologic setting ------------------------------------ 6
- Previous work--------------------------------------- 13
CHAPTER II: -------------------------------------------- 16
METHODS OF STUDY ------------------------------ 16
- Field works-- --------------------------------------- 17
- How to make a thin section--------------- -------- 17
CHAPTTER III: ----------------------------------------- 27
FIELD DESCRIPTION AND PETROGRAPHY - 27
- Petrography characters of the studied samples 33
- Diagenesis------------------------------------------- 34
CHAPTER IV: ------------------------------------------ 42
SUMMARY, CONCLUSIONS AND RECOMMENDATION 42
REFERENCES ------------------------------------------ 46
3. 3
ABSTRACT
The Hanifa Formation derives its name from Wadi Hanifa of Riyadh at the
coordinates N24o
57′ 38.7" E46o
12′ 56.4". Diriyah super-group consists of
Buryadha Group, Shaqra Group, and Thamama Group, which that Hanifa
Formation regards one of the formations that consist in Shaqra Group. The Hanifa
Formation consisting of alternating aphanitic and calcarenitic limestone, much of it
oolitic. The representative carbonate lithofacies from base to top consisting of
alternating layering of wackestone, mudstone, grainstone, and boundstone
following the classification of Dunham (1962). Petrographic investigations
revealed the occurrence of different diagenetic processes including cementation,
porosity and dolomitization. Dolomitization occurred due to the influx of the
brines from the associated lagoonal environment as detected by the occurrence of
bottom nucleated gypsum crystals. These brines have high Mg/Ca ration causing
the dolometization of the pre-existing carbonate sediments
5. 5
LEGEND:
Study Location
240
200
400
440
160
480
520
320
280
LEGEND:
Study Location
Introduction:
The Hanifa Formation derives its name from Wadi Hanifa of Riyadh at the
coordinates N24o
57′ 38.7" E46o
12′ 56.4" (Fig.1). Moreover, Wadi Hanifa takes its
name from the ancient Arab tribe of Banu Hanifa, who were the principal tribe in
the area at the dawn of Islam, though in those days the valley was better known as
Al-Irdhsrtogn a r lgs fgl iooofnss oos fo gn rts valley, including Uyaynah,
Jubaila, Irqah, Diriyah, and Ha'ir.
Fig. 1: Location map of the studied section.
6. 6
The importance of Upper Jurassic carbonate rocks of Saudi Arabia lies in its
considered to be the most prolific oil producing carbonate rocks reservoirs in the
world. They have been subjected to geological investigations since the discovery
of oil in Saudi Arabia in 1938 (Steineke et al., 1958).
Aims of Study:
- How to collect samples from the field and write field observations.
- Training how to make a scientific research.
- Making petrographic thin sections and describe some petrographic
characteristics of Lower Hanifa Formation of Late Jurassic age.
- Deduce the different porosity types observed in the rock samples.
- Trying to elucidate the paleo-environments of Lower Hanifa Formation.
Geological Settings:
Riyadh area is found between latitudes 24° N and 25° N and longitudes
46°30′ and 48° E. It is located in the eastern part of the Najd province in east-
central Saudi Arabia. Generally, Riyadh area is underlain by (Phanerozoic)
Mesozoic to Cenozoic sedimentary rocks of the Arabian shelf and covered to a
large extent by Quaternary deposits. According to Vaslet et al. (1991), Jurassic to
7. 7
Early Cretaceous rocks (Fig. 2) cropping out in Riyadh area and assigned to the
newly defined Diriyah supergroup consist of (Fig. 3):
1. The informal Byradah group, comprising Late Permian to Triassic
deposits, mostly crops out to the west of the quadrangle boundaries.
2. The informal Shaqra group, of Jurassic age, the Middle to Late Jurassic
part of which is represented in the quadrangle.
3. The Thamama group comprising Early Cretaceous rocks.
Fig. 2: Simplified geologic map of Saudi Arabia.
8. 8
Fig. 3: Jurassic to Early Cretaceous rocks cropping out in Riyadh area (after
Vaslet et al., 1991)
The type locality of Hanifa Formation (Early Kimmeridgian) lies in Wadi
Hanifa near Lat. 24°57' 38.7'' N and Long. 46° 12' 56.4''E (Fig. 4) was regarded
originally as a member of the Tuwaiq Mountain Formation by Steineke (1937,
cited in Powers et al., 1966), but was raised to formational rank within the Tuwaiq
Mountain Group by Bramkamp (1945, cited in Powers et al.. 1966). This
Formation was described in detail by Steineke and Bramkamp (1952) and Steineke
et al. (1958).
9. 9
Fig. 4: Satellite image of the studied location.
The thickness of the Hanifa Formation remains remarkably constant
throughout and is without doubt the most consistent of all Jurassic units. In the 400
km from Wadi al 'Atk (Lat 25°25' N.) to Sha'lb al Haddar (Lat 21°55' N.), the
Formation shows only 10 m variation in thickness from 106 to 116 m. Some
thinning is evident to the north and south but even this is limited to a very few
meters. In the vicinity of Al-Ghat the upper Hanifa limit cannot be definitely
placed; however, at least 60 m of section can be assigned to the formation with
certainty. South of Sha'ib Al-Haddar, thickness is gradually reduced to 75 m at
240
57′ 47.90″
460
13′ 19.67″
240
57′ 08.55″ 240
57′ 37.5″
460
12′ 10.06″
240
58′ 14.52″
460
12′ 29.34″ 460
13′38.65″
10. 10
Wadi Al-Dawasir. Beyond, there is apparently no appreciable change as
measurements vary only from 75 to 82 m. The lithology of Hanifa Formation is
somewhat more variable than the underlying Tuwaiq Mountain Limestone
basically; it is another nearly pure carbonate unit (Fig. 5). The Hanifa Formation is
made up of alternating aphanitic and calcarenitic limestone, much of it oolitic.
Beds of golden-brown, oolitic calcarenite occur at several levels and become
increasingly prominent in the upper part of the Formation (Fig. 5). In fact, the top
of the Hanifa Formation is marked by a massive bed of oolitic-pelletic calcarenite
that can be traced without interruption almost the entire length of the outcrop. In
some sections in the north and south calcarenite increases in amount until it
becomes the dominant type present. Argillaceous limestone units and some shale
are present, particularly in the northern area; dolomite is common in the south. A
few coral-bearing beds occur in the central area and become increasingly
prominent southward. The golden-brown calcarenite beds, although thin, are
ordinarily resistant and form obvious benches or weather to slabby debris.
Generally the Hanifa Formation is sufficiently covered with fragments of this
calcarenite so that the entire interval appears brown from a distance, in strong
contrast to the lighter colored formations above and below.
11. 11
Fig. 5: Generalized Stratigraphic section of Late Jurassic in central Saudi Arabia
JubailaFormationHanifaFormationTuwaiqFormation
OxfordianKimmeridgianTime
Rockunit Lithology Rock type according to Dunham’s classification (1962)
Lime-mudstone and dolomite with sub-ordinate lime-
grainstone/packstone
Lime-mudstone/wackestone and grainstoneLpackstone with
corals and stromatoporoids
Lime-mudstone with alternation of lime-grainstone/packstone.
Corals and stromatoporoids are in the top part
Dolomite
Lime-mudstone/wackestone
Lime-grainstone/packstone
Corals and stromatoproids
Legend
12. 12
The late Oxfordian to early Kimmeridgian Hanifa considered to be source
rock deposited in the intrashelf basin of the differentiated Arabian basin. These
deposits are mostly low-energy, laminated, dark, organic-rich lime muds deposited
under anoxic bottom-water conditions. To the north of this intrashelf basin, high-
energy, shallow-water grainstones and evaporitic peritidal sediments accumulated
across the Rimthan arch (McGuire et al., 1993). To the east, the continental margin
separated the platform from the open ocean in which little or no deposition took
place (Murris, 1980).
During deposition of the Hanifa Formation, the Arabian basin transition
from the northern shelf-margin into the southern intrashelf sag was gradual. The
northern margin of the basin by the Rimthan arch was characterized by shallow-
marine oolitic and peloidal grainstones. There were indications of peritidal
conditions and islands developing along the northern transition between the
Rimthan arch and the Gotnia basin to the north (Koepnick and Waite, 1991). The
basin gradually and gently dips to the southwest (slope < 0.5 degrees) forming a
ramp-like depositional surface with a deeper gentle intrashelf depression to the
southwest. This is indicated by a gradual southwestward thinning of the carbonate
deposits and thickening of the evaporite deposits (salinas) in later regressive and
13. 13
restrictive stages. The Hanifa formation is overlain by the Lower Kimmeridgian
shallow-water Jubaila Formation.
There are many explanations for the formation of intrashelf basins, but most
of these basins appear to form on the shallow passive stable margins of plates.
These shallow margins are extremely susceptible to the effects of marine
transgression, particularly since in such settings tectonic subsidence tends to be
minimal and the resulting sedimentary fill geometries are largely the products of
sea-level variations (Aigner et al., 1989). Often these intrashelf basins develop as a
result of a rapid eustatic sea level rise in which carbonate margins build up around
an isostatically sagged deeper basin floor while the sedimentary fill lags at a slower
rate of sedimentation (Read, 1985). As indicated above most intrashelf basins are
short-lived and may be filled during the succeeding transgression event.
Previous Work:
Many works were published dealing with the Upper Jurassic of Saudi
Arabia, especially Hanifa Formation. Among these, Murris, (1980) recognized that
the source rocks for the upper Jurassic Arabian reservoirs were deposited in an
intrashelf basin on the Arabian platform that was flooded by a major transgression
initiated in the upper Middle Jurassic (late Oxfordian to early Kimmeridgian).
14. 14
These high stand carbonate deposits "kept up" with the rising sea level, finally
surpassing the rate of rise and prograded seaward during the late stages of a sea
level high stand. The later stages of the high stand (of the 2nd order eustatic cycle)
were characterized by increasingly more regressive deposits (the Arab formation)
and were finally capped by extensive evaporites (the Hith formation) that
accumulated during the arid climate of the next sea level lowstand.
Ayres et al. (1982) recognized that the upper Middle Jurassic source rocks
were deposited in an anoxic intrashelf basin separated by grainstone and
dolomitized facies from the open-marine environment of the Neo-Tethys Sea to the
east. This intrashelf basin most probably resulted from the differential build-up of
grainstones that kept up with a eustatic rise in sea level at the margin of the
isostically sagged region. The high salinity conditions on the floor of this intrashelf
basin aided the preservation of the organic matter.
Aigner et al. (1989) concluded that the intrashelf basin in which the Hanifa
Formation was deposited was the result of a rapid eustatic rise in the sea level
across a stable tectonic region (the passive margin of the Neo-Tethys Sea). They
contended that the role of tectonic subsidence in the development of the cratonic
basin was minimal and the role of sea-level rise in the deposition of carbonates on
15. 15
the margins of an isostatically sagged platform interior was large. These intrashelf
basins are different from the intra-cratonic basins and tend to be short-lived and
may have been filled during the subsequent seal-level cycle.
Droste (1990) identified two transgressive-regressive sub-cycles within the
2nd
order transgressive event that constituted the Hanifa Formation. Each of these
shoaling up-ward sub-cycles contains three recognizable lithofacies: a lower
lithofacies which is composed of grain-rich lime-mud/wackestones with a TOC of
1-6% wt. (with a high gamma ray response) deposited in an anoxic environment;
this is the primary source rocks for the Upper Jurassic reservoirs. The middle
facies is a bioturbated lime-mud to peloidal packstone with a TOC of < 1% wt.
deposited in an oxygenated environment. The upper lithofacies is formed by
anhydrite (massive and vertically elongated nodular) and it was deposited in sub-
aqueous conditions and forms the capping horizons over the underlying rocks.
Droste (1990) also concluded that most of the intrashelf deeper sediments were
storm-generated deposits derived from the surrounding platform margins.
17. 17
II.1. Field work:
Field trip is carried out to Sadus area in order to study the type section of
Hanifa Formation (Hanifa Formation) (Fig. 6). Field geologic tools were used such
as compass to detect the orientation, GPS to know the coordinates, camera,
hammers, markers and sample bags, to collect selective samples for macro- and
microscopic studies. Samples were marked and described in the field.
Fig. 6: Field works along Hanifa Formation
II.2. How to make a thin section
Thin section are made from small slabs of a rock sample glued to a glass slide
(~ 1 inch by 2 inches, Fig. 7), and then ground to specific thickness of 0.03 mm (30
18. 18
microns). At this thickness most minerals become more or less transparent and can
then be studied by polarized microscope using transmitted light.
Fig. 7: Schematic diagram for thin section preparation.
Thin section making equipment
There are five main tools used to prepare the thin section: The slab saw, the
trim saw, the grinder, the cut-off saw, and the lap wheels.
1-Finding the rock sample:
The first step to make thin section is how to choice the rock sample from the
collected samples from field (Fig. 8). For example, check the matrix around some
of your fossils and you will likely find pill-shaped, torpedo-shaped and round
inclusions. If you are lucky and the formation is rich, you may find identifiable
fossils that are smaller (juvenile) versions of the mega-fossils. In the field you
might find a rock that looks for the entire world-like a collection of spherical or
Cigar-shaped grains. If you know the matrix to contain abundant mega-fossils then
it is likely full of micro-fossils and you would do well to bring back a rock of the
material.
Sla
b
Glass
slide
Canada balsam or
epoxy
Roc
k
Thin slab of rock on slide
19. 19
Fig. 8: Collecting samples from the field.
2- Cutting the hand specimen out:
Back in the lab cut off a piece of one end of
the rock. The harder the rock is the better. You
don't want it crumbling. (We will deal with the
crumbly ones later.) You do not need to cut off
more than one inch square piece and would be
better off if it were less for a first effort. If you do
not have a motorized saw, a simple hack saw
should do well. Cut another piece off the main rock
as thin as you can, 2-4 mm will do but 1-2 mm is
better (Fig. 9). If it breaks then save the pieces. The
important part is that you have a little slab with
roughly flat, parallel sides under 1 square inch area.
Should you have a nice sized sample with rough sides that are anything but
Fig. 9: Samples ready for grinding
on the first surface
20. 20
parallel, you can make it a little more regular by rubbing it against a circular
sanding pad of about 40-60 grit. At this point it only has to be approximately
parallel.
3- Materials for grinding
Grinding powders is used in this stage. For the next steps a selection of the thin
6-8 inch squar lapping disks are handy. I use 100,
500 and 1200 almost exclusively. I have used the
thick lapping wheels (Fig. 10) but find the thin
ones are much cheaper and if set on a good flat
surface (like a piece of glass) will work
excellently well.
Other items will be needed:
Cerium polishing powder.
A piece of twill or denim.
One can of spray contact cement.
Polarized petrographic microscope.
Epoxy (60 min. or longer)
Some pieces of brass shim stock or equivalent, preferably in several thicknesses
and at least twice as long as the narrowest dimension of the microscope slides
being used. Cerium offers a good compromise and is a very fast polisher
especially if attention is paid to the last grinding step to make sure it is "fined"
thoroughly.
Fig. 10: Examples of diamond
grinding disks, thin and
thick.
21. 21
4- Grinding of the slab:
Take the little slab, place it on the coarsest grinding disk with some water
from a rinse water bowl or pan that you should have nearby (Fig. 11). Use only one
finger on the back of the piece and grind
with a medium to light pressure and move
in a zig-zag or circular or epi-cycular
motion around the tool. Epi-cycular
motion consists of moving the piece
around in circles near the edge of the disk.
It's a good idea to move about the disk as
best you can so you don't wear a low spot
in one area. Grind it until the entire saw or
roughing marks are gone. Use a 10x hand
lens to inspect the piece as you grind it. If
you had a steady hand on the saw and the
cut marks have little relief, you could start
with the 400/600 abrasive grade. After
each abrasive be sure to inspect the
surface to see if the pits left by the
abrasive are uniform and that all the bigger pits and scratches from the previous
abrasive have been removed. Especially check the edges for residual imperfections
in the saw cut. Wash up the work between disks. One grain of rock in the finer
grades will ruin your hard work. All this is particularly important if you are only
using the three minimum grades of abrasive for grinding. Once all the saw marks
have been ground off the specimen and you are sure on inspection that the piece is
still flat and the grind pattern on the plate is even, you can go on to the next grade.
I go straight from #100 or #200 to 500 and then 5 micron or #1200. This is the
Fig. 11: Grinding wheel
22. 22
minimum amount of steps you should use but you can omit the #100/200 if the
original saw cut is smooth enough. Again with the finer grades, push the piece
around in the abrasive with a medium to light pressure. Don't rush the grinding or
you will break the specimen especially if it is thin. If in the finest stage the piece
kicks and sticks, you need to go back to the previous disk and grind more. You
probably have a hollow in the center of the piece. Some fossils, especially bone,
can absorb water very rapidly and stick badly on the final grind. Keep the sample
and tool good and wet.
5- Polishing of the slab:
We now need that very flat yielding
medium to hold our cerium polishing
compound. It also needs to be firm enough to
not change the shape of the ground surface of
the specimen. Find a very flat piece of metal
or glass about 70-100mm diameter to act as
the substrate in your polishing tool (Fig. 12A).
If the edges are sharp make sure to bevel them
off with a grinding stone. Polish with a back
and forth motion and good (but not hard)
pressure. If the specimen is ground off well on
the finest abrasive, it should polish up in just a
few minutes. If not, or if the surface is
scratched, go back to the finest abrasive tool
and charge it with a little of the same grade
loose abrasive (aluminum oxide is
Fig. 12: A- Glass plate used to polish
the grinding sample. B-
Well-polished sample.
A
B
23. 23
recommended). Use light pressure and grind until the scratches are gone (Fig.
12B).
6- Slide Mounting:
All slides must be cleaned thoroughly. This is a most important step in
making a successful slide. No slides are clean as delivered by the factory, in fact,
they can't be. If the glass slides were perfectly clean cohesion of the glass surfaces
would make it impossible for you to pry them apart. To avoid this manufacturers
use a light dusting of powders like talc or thin films of silicone oils on the glass to
keep them from sticking. This must be completely cleaned off for bonding to be
strong enough to resist the tremendous shearing forces of polishing. Lay the slides
on a layer of paper several sheets thick on a solid flat surface. Be sure the surfaces
are clean. Use slow set epoxy. The faster set epoxies can yellow in just a couple
years or with exposure to sun. Additionally, you should avoid the fastest setting
glues (5 min) and go for the longer (20 min. or longer) as these have different
chemical formulae and bond to the glass better. With most epoxies the rock will
grind much faster than the glue because of the glue’s slightly plastic texture. It also
slows the polishing. If epoxy is used mix enough to do four or five slides at a time.
Take a Popsicle stick and wipe the glue on the specimen first to fill any pores.
Then put a drop or two on the slide and push the sample into it working it back and
forth a bit to seat it and get a good bead of glue around the outside (Fig. 13). Do
not use too much glue. It will become a messy job and a wide glue margin around
the work will slow grinding and polishing. These will need 24 hours to bond
completely before any grinding. If you need to clean off any epoxy use a liquid
(not gel) paint stripper and a Q-Tip. Don’t let any of that get near the sample or it
may get sucked in (by capillary action) and loosen the bond. In both cases, after the
glue has set but not bonded you can look at the backside to inspect for bubbles. It
24. 24
is unlikely that there will be any but wherever there are will be a weak spot in the
slide that may cause problems later. Be aware of this as you grind and polish the
specimen. There is virtually nothing that can be done to fix this so you will have to
treat this slide with greater care.
Fig. 13: Mounting the slab on the glass slide.
7- Thinning the mounted slab:
Now that the little rock slab is glued to the glass microscope slide on its
polished side it is necessary to grind it down to the required thickness for
transparency so you can literally see through rock without x-ray vision. The most
important thing now is to make sure the front surface being ground becomes
parallel to the glued down polished side, or the surface of the microscope slide
itself. The latter is much easier and, it turns out, is easy to do. "Wedge" is when
one side of the specimen is thicker than the other. As you hold it up to the light
usually one corner or side will be darker than the other. The cause can be poor
grinding, a defective clip, a bit of grit or dust under one end of the specimen when
25. 25
it was glued down or a combination of these or it can be that the rock is just more
opaque at that point. You will have to determine this. When grinding in these final
stages put the fingertip pressure over the darkest area when grinding (Fig. 14). The
slide and specimen are fairly flexible now and this will put a little more pressure on
that spot causing it to grind a little more
in that spot. Don't overdo it as the other
side needs to stay co-planar with the
corner you're grinding. Go slow with
the grinding in these stages, don't rush.
You've put a lot of work into this and
you don't want to wreck it now. Inspect
the work frequently. When you can
clearly see details in the fossils through
the sample, go to the last grinding grit.
Better to have to spend more time on
the last stage than over shoot with the
coarser stage. If a fossil pops out of the
sample or disintegrates in grinding, or the specimen grinds through above a trapped
bubble in the glue underneath, then stop. You can fill the hole in with a little glue,
let it dry for a day and then briefly go to the previous stage of grinding to level out
the dried glue and then go to the final grind stage. This will not produce a high
quality slide but it will save the slide to that point. To go on further with the grind
would result in the disintegration of all or most of the slide. Better to have
something than nothing! Use the last grinding grit until the work has a shine to it
when a light is reflected off of it at a low angle. Experience will eventually be your
guide. The last wet with the finest grinding grade should be with very light
Fig. 14: Final grinding of the mounted
rock slab.
26. 26
pressure and should just skid about on the tool. This will smooth out the pits of this
final grade and make the polish go even easier.
8- Final Polish
Once the specimen passes the above test and there are no scratches and the
specimen which should be quite transparent when wet, then it is time to polish.
Again, using polish until the epoxy and specimen is as clear as possible. The
details in the fossils will become more and clearer as the specimen takes on a
polish. It will be necessary to put a cover glass over the specimen after the
polishing.
9- Cover Glass
If it is necessary to put a cover glass on then wash the specimen and slide
well making sure the entire polishing residue is gone. Then let it dry for a day.
Obtain a set of microscope cover glasses or cover-slips and also try to get low
viscosity cyano-acrylic glue and its solvent. The latter is most important. The
larger cover glasses are best to ensure the entire sample is covered. Apply the drop
of the glue and gently push the cover into it softly applying pressure. Watch out
that the glue does not get on the fingers. Wipe the excess with tissue or paper towel
and don't worry about smears at this stage. You will have about 30 seconds to
place the cover glass correctly so waste no time. After a few minutes, when the
cover glass is well affixed, get a tissue or Q-Tip and put several drops of the
solvent on it. Use this to clean up the excess glue and smears. You'll be pleasantly
surprised how readily these clean up. Stubborn drops and glops of glue can be
coaxed free with a single-edged razor blade.
28. 28
In this study, the classification
of Dunham for carbonate rocks (1962)
is followed (Fig. 15). This
classification defines carbonate rocks
depending on whether they are grain-
supported or matrix supported, on the
dominant type of grain (allochem), and
whether their matrix is dominated by
micrite or sparry calcite. Types include
mudstone wackestone, packstone,
grainstone, rudstone, baffelstone,
bindsone and framestone. The
Dunham’s classification system is most
useful for micro-facies interpretation of carbonates depositional environments.
As mentioned above, Hanifa Formation composed of alternating aphanitic
(any rock with fine grain that cannot see it with naked eye) and calcarenitic
limestone (sedimentary rock formed of calcareous particles ranging in diameter
from 0.06 to 2 mm that have been deposited mechanically rather than from
solution), where much of the calcarenite limestone is oolitic limestone (spherical
nodules with concentric structure). Beds of golden-brown, oolitic calcarenite occur
at several levels and become increasingly prominent in the upper part of the Hanifa
Formation is marked by a massive bed of oolitic-pelletic calcarenite.
The following is a brief field description of the sequence observed at Lower
Hanifa Formation at Jabal Al-Abakkayn, central Saudi Arabia (Fig. 16). The
Formation, generally, composed of various carbonate lithofacies intercalated with
shale beds at several levels (Fig. 16). A single lithofacies can repeat several times
along the section, consequently a representative lithofacies will describe.
Matrix-support
Grain-support
Fig.15: Dunham's Classification
Mudstone (‹ 10% grains)
Wackestone (› 10% grains)
Packstone
Grainstone
Boundnstone
Grains ›2 mm
Grains 0.25 – 2 mm
Micrite
30. 30
These representative carbonate lithofacies, from base to top, are as follows (Fig.
16):
- Wackestone: This is mud-supported carbonate rocks containing more than
10 percent grains (Dunham, 1962). Lower Hanifa Formation characterized
by wackestone rich in sponge spicuales and molluscan grains (Fig. 17).
Fig.17: Photomicrograph of wackestone with sponge spiculas and molluscan
grains. Plane polarized light.
- Mudstone: This is mud-supported carbonate rocks containing less than 10
percent grains (Dunham, 1962). Lower Hanifa Formation characterized by
mudstone with sponge spicules (Fig. 18).
Sponge
spicules
Molluscan
grains
253µm
31. 31
Fig. 18: Photomicrograph of carbonate mudstone with sponge spicules.
Plane polarized light.
- Grainstone: Mud-free carbonate rocks, which are grain supported
(Dunham, 1962). Lower Hanifa
Formation characterized by pelletal
grainstone with pellets, molluscan
grains intraclasts, molluscan fragment
(Figs.19, 20, 21 and 22, respectively)
and brecciated intraclasts (Fig. 23).
Also, molluscan grainstone is recorded.
Sponge
spicules
253µm
Pellets
253µm
Fig.19: Photomicrograph of pelletal
grainstones. Plane polarized
light.
32. 32
Fig. 20: Photomicrograph of pelletal grainstones with pellets and molluscan grains.
Plane polarized light.
Fig. 21: Photomicrograph of pelletal grainstones with pellets and intraclasts. Plane
polarized light.
250µm
Pellets
Intraclasts
250µm
Molluscan
grains
Pellets
33. 33
Fig. 22: Photomicrograph of pelletal grainstones with molluscan fragment. Plane
polarized light.
Fig. 23: Photomicrograph of grainstone with brecciated intraclasts and molluscan
grains. Plane polarized light.
Brecciated
intraclasts
Molluscan
grains
253µm
250µm
Pellets
Molluscan
fragment
34. 34
- Boundstone: Carbonate rocks showing signs of being bound during
deposition (Dunham 1962). Embry and Klovan (1972) further expanded
the boundstone classification on the basis of the fabric of the boundstone.
From the types of boundstone is framestone which the organisms build
framework (Fig. 24). There are two zones of bounstone observed in the
sequence; one in the lower part and the other at the top part (Fig. 16).
Fig. 24: Noticed coral reef in lower zone of Lower Hanifa Formation.
Cementation:
In the Hanifa Formation, cement of calcite occurs within chambers and
hollows of many skeletal grains (Fig. 25). This took place on shallow sea floor
(Alexanderson, 1972). The cement, which occupies the majority of the original
pore spaces, is a clear equant calcite (Fig. 26). This diagenetic process of cavity or
35. 35
open-space filling through chemical precipitation of material from solution on a
free surface (substrate) is indicated by the presence of partial or complete calcite
spar. The Hanifa carbonate rocks are characterized by their high induration. This is
apparent in the grainstone facies where the pore system is completely filled by
calcite spar cement (Fig. 26 and 27).
Fig. 25: Calcite filling champers of the reefs and occupy the open space pores>
Notice the relics of chambers (arrows) that completely replace with
calcite.
Calcite
37. 37
Porosity:
Primary porosity, either between the depositional grains (intergranular) or
within the skeletal framework of bioclasts (intragranular), has been completely
destroyed by the deposition of both early cement coats and most importantly the
late sparry calcite cement (Fig. 27). These diagenetic cements have effectively
blocked the primary pores in the Hanifa grainstones. The only type of pore space
which can be identified microscopically in the Hanifa carbonates is always of
secondary origin. These secondary pores have apparently resulted from dissolution,
which have affected the earlier fabric of the rocks at different times during
diagenesis. The voids created by the dissolution of carbonate constituents may
remain open, but they are often found to have been filled by the subsequent
precipitation of sparry calcite cement. Four types of dissolution void porosity are
recognized in the limestones of the Hanifa rocks. These are the moldic, vuggy,
intercrystalline, and rhombohedral porosity.
1. Moldic porosity:
In the limestones, moldic pores were selectively created by the local
dissolution of some carbonate grains in the boundstone and grainstone fabrics (Fig.
25). The dissolution of grains is, in many instances, incomplete, and irregular
intragranular voids are rarely observed in Hanifa rocks (Fig. 25). These skeletal
molds are now filled with sparry calcite cement. Figure 28 shows the intraparticle
cementation with slight moldic porosity in the bounstone facies at Lower reefal
facies of Lower Hanifa Formation.
38. 38
Fig. 28: Molding porosity filled with sparry calcite. Nicols Crossed.
2. Vuggy porosity:
Dissolution vugs irregular voids and in many instances resulted from the
indiscriminate dissolution of the original limestone elements such as allochems and
intergranular sparry calcite cement. Most of this secondary porosity is filled by
younger generation of late diagenetic calcite cement (Fig. 29). These dissolution
vugs are filled with micrite and/or sparry calcite cement.
Fig. 28: Vuggy porosity filled
with micrite and
recrystallized sparry
calcite. Nicols Crossed
63 µm
123µm
39. 39
Dolomite
The dolomite in the Hanifa rocks has originated through the replacement of
original calcium carbonate sediments (Fig. 29). The replacement is determined by
the growth of dolomite crystals within and across the mutual boundary between
sparry calcite cement. This piece of evidence, beside other criteria, clearly
indicates that dolomitization took place during or after the introduction of late,
post-compaction cement, and consequently, the replacement process must be of
burial diagenetic origin
Fig. 29: Dolomitization of sparry calcite cement (arrows). Nicols Crossed.
Evidence of lagoonal environment:
Bottom nucleated gypsum crystals with its twin character is dectected and
replace by microcrystalline aggregates of secondary gypsum (Fig. 30). This
indicate the restriction of sea water in the back reef environment where the rate
of evaporation exceed the rate of influx. This also support dolomitization of the
carbonate cement either by the brine or by the dissolution of evaporite minerals.
60 µm60 µm
43. 43
By applying the famous geologic rule which is “Present is the key to the
past”, we can deduce the depositional environment of the studied section. The
modern northeastern coast of Saudi Arabia is an example of a modern carbonate
ramp. In the deeper water of the Gulf, below about 30 m, lime mud is being
deposited. As water depth gradually shallows toward the Arabian Shield, skeletal
wackestones pass shoreward, via skeletal packstones, into shallow water oolite
grainstones and reefs that accrete around Pleistocene limestone islands (Purser,
1973). Carbonate muds, algal stromatolites, and evaporites form in sabkhas
(Arabic-salt marsh) in sheltered coastal lagoons and embayments (Evans et al.,
1969). These form upward-fining sequences analogous to those of temperate
terrigenous intertidal flats.
Consequently, the studied section consists of alternating bedding of
grainstone, shale, wackestone, mudstone, and boundstone (Fig. 16).
Both wackestone and mudstone indicate depositional environments
characterized by calm water and restriction of grain-producing organisms (low-
energy depositional setting) (Dunham, 1962). Moreover, grainstone deposited in
moderate- to high-energy environments, but their hydraulic significance can vary.
Dunham (1962) mentioned that grainstone can be formed in: (1) high-energy
44. 44
environment where grain-productive environment exist and mud cannot
accumulate, (2) environment where currents, which drop out the grains, bypass
mud to another area, and/or (3) environments of winnowing of previously
deposited muddy sediments. Additionally, boundstone is generally deposited in
high energy environments, where currents can provide nutrients to the organisms
which that form the boundstone as seen in Figure 25. Al-Dhubaib (2010)
mentioned that depositional cyclicity of the Jurassic carbonates in Saudi Arabia
have revealed the stratigraphically and biostratigraphically complexity, that mainly
result from relative sea-level changes. Late Jurassic carbonates of Saudi Arabia
were deposited on a very extensive shallow submarine platform that extended over
most of the Arabian Plate (Al-Dhubaib, 2010).
Generally, Dolomitization is a widespread phenomenon in the Hanifa
carbonates. Dolomite shows a preferential replacement of lime mud matrix over
associated lime sand, thus being common in lime mudstone and wackestone and
relatively rare in grainstone and packstone. Petrographic observations suggest that
the dolomite has been formed while original lime mud matrix was still aragonitic,
and the replacement of the latter into calcite took place only during the final stages
of the replacement process. Consequently, the majority of dolomite is of post-
depositional origin. Also, dolomitization was caused by Mg-rich brines from the
45. 45
associated lagoonal environment which predominated during the deposition of the
Lower Hanifa Formation. The existence of lagoonal environment confirmed by the
occurrence of gypsum crystals of bottom nucleation (Figs. 30 A and B). An
upwards transition from inner lagoon, back reef to reef palaeoenvironmental trend
is suggested by Enay et al. (1987). This is consistent with the present observations
Recommendation
Further studies needed to determine the environmental sensitivity and sea-
level fluctuation during the deposition of Lower Hanifa Formation. This is can be
done by using benthic foraminifera and, associated microfossils where
macrofossils considered to be a potentially valuable technique for determining sea-
level fluctuations.
46. 46
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