African Sedimentary Basins-GEY 357 for Geology Department, University of Ibadan

Sedimentary Basins in Africa

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GEY 357: SEDIMENTARY DEPOSITIONAL ENVIRONMENT AND BASINS OF AFRICA

COURSE DESCRIPTION:
THIS ARE THE PHYSICAL, CHEMICAL AND BIOLOGICAL INFLUENCE ON CONTINENTAL AND MARINE DEPOSITIONAL ENVIRONMENTS
AND THEIR SEDIMENTATION PATTERN; ANALYSIS OF THE AFRICAN SEDIMENTARY BASINS.

COURSE OUTLINE
INTRODUCTION
PRINCIPLES OF UNIFORMITARIANISM
ENVIRONMENTAL FACTORS OF SEDIMENTATION
TECTONIC SEDIMENTARY PROVINCES
CLASTIC SEDIMENTARY ENVIRONMENTS
FACIES AND WALTER’S LAW OF FACIES CHANGE
VERTICAL FACIES CHANGE
DIAGENESIS OF SEDIEMNTARY ROCKS
SEDIMENTARY BASINS OF AFRICA
TEXT BOOKS
Shelf sand and Sandstone Bodies-Geometry Facies and Sequence Stratigraphy Ed- D.J.P. Swift et al., 1991
Deserts Sediemnts; Ancient and Modern Ed- L.E. Frostick & I. Reid by Blackwell Scientific Publications, 1987.
Facies Models. Ed- R.G. Walker. Geoscience Canada
Sand and Sandstone. 2
nd edition Pettijohn, Potter & Siever
Sedimentary Environments;Processes, Facies & Stratigraphy. Ed- H.G Reading 3
rd edition

2. INTRODUCTION
WHAT IS SEDIMENTARY BASIN
A sedimentary basin is an irregular surface which is generally of tectonic
origin and has a mass of rock formed from sediment that were derived
from one or more provenances and are originally deposited in a relatively
uniform environment within a discreet geologic area over a reasonably
long period of geologic time.
Niger Delta is Eocene-Oligocene-Miocene-Pliocene (57.8, 36.6 34.1 ma)
Anambra Basin- Santonia- Miocene
Southern Benue Trough- Aptian-Albian (Lowermost Cretaceous)
Bally (1975) define Sedimentary basin as a realm of subsidence with
thickness of sediment usually exceeding 1km that is today still preserved
in a more or less coherent form.

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However, Sedimentary Basin are related to geodynamics of the earth, they are therefore divided
basically into four major types
Rift type basin- These are restricted to cartonic are which are as a result of faulting.
Stable Area basin- Found in the continental region
Passive Margin basin-Found in the Peri-cratonic area
Mobile basin- These characterised the orogenic belt
All these are related to Plate tectonics, therefore, the mechanism can be found in the theory of
Plate tectonics
ASSIGNMENT- DISCUSS THE CONCEPT OF SEDIMENTARY BASIN: MECHANISMS OF
FORMATION, CLASSIFICATION, TYPES AND IMPLICATION FOR PETROLEUM ACCUMMULATION.
Arising from the initial definition, we can say that;
Basin are;
Repository for sediment
Formed by crustal subsidence relative to surrounding areas
Sometimes surrounding areas are sometimes uplifted.
Because of this, we have different shapes, sizes and mechanisms of formation
In essence, they are related to three basic types of plate boundaries such as Divergent (mid
oceanic ridge), convergent (subduction zones), conservative (strike-slip systems)

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DEPOSITIONAL ENVIRONMENTS
A depositional environment is otherwise known as sedimentary terrain
and they are various type of place in which sediments are being
deposited, which can be a stream, river channel, a lake, or the ocean
deep.

They are also known as a complex of physical, chemical and Biological
factors within which a sediment accumulates (Cumbin & Sloth)
Physical factors- temperature, pressure etc
Chemical factors- Salinity, Eh, pH
Biological Factors- Fossils/organism action

This can also refer to the condition in which sediments are deposited
and examples include beach, glacial, and river environments.

5. DEPOSITIONAL ENVIRONMENT
Depositional
Environments
Exampes
CONTINENAL
ENVIRONMENT
This is also known as Non marine settings or Erosional
Sedimentary environments because the gradient is high
thus erosion is higher. We have; Alluvial Fan, Fluvial
Environments (Rivers), Lacustrine Environments (Lakes),
Aeolian Environments (Deserts), And Paludal
Environments (Swamps)
TRANSITIONAL
ENVIRONMENT
Also known as mixed environments or equilibrium
environments. This is because it is a peneplain. Units here
include Deltas, Beaches and Barrier Islands, Lagoons, Salt
Marshes, And Tidal Flats
MARINE
ENVIRONMENTS
This is the realm of marine activities and it is noted for
deposition. The gradient is very low thus it favours
deposition of of sediments and by extension,
accumulations of petroleum. Units include Reefs, The
Continental Shelf, Slope, Rise, And Abyssal Plain
OTHERS Evaporite and Glacial deposition environments

6. DEPOSITIONAL ENVIRONMENTS
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Continental
Alluvial
Aeolian
Fluvial
Lacustrine

Transitional
Deltaic
Tidal
Lagoonal
Lakes
Lacustrine
Beach
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Marine
Shallow water
Continental shelf
Continental slope Continental Margin
Continental Rise
Inner Neretic
Outer Neretic Neretic
Batyal Deep water
Abyssal
Reef
Others
Evaporite
Glacial

7. DEPOSITIONAL ENVIRONMENT

8. DEPOSITIONAL ENVIRONMENTS

9. Deepwater Environments and Sand pathway
SL10’s to 100’s of km
Blum and Womack, 2009

10. ENVIRONMENTAL FACTORS FOR SEDIMENTATION
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Sedimentation results from the interaction of the following;
Supply of sediments- This allow small proportion to be preserved
Its reworking- storm, wave, tides, instability on slope, chemical
dissolution, channel migration, shoreline advance or retreat
Modification by Physical, Chemical and Biological processes
Accommodation space- sea level changes, climate, tectonic
movement, volcanic activity, subsidence rate etc
So base on the following, we have the following factors;
Sediment supply, climate, Tectonic movement and subsidence
Sea level changes, Physical processes, Biological activity, Water
chemistry and Volcanic activity

11. SEDIMENT SUPPLY
This varies in volume, composition and grain sizes

The variations is as a results of Climate, Basinal water chemistry and
the Tectonics and Bedrock geology.

Note:
Where the supply of land derived (terrigenous) sediment is abundant,
siliciclastic sediment is abundant.

Where it is low, evaporates, carbonates, diatomites, cherts, Ironstones,
phosphorites and carbonaceous sediments predominates.

The above scenario leads to extra basinal and intra basinal particles
except for erupting volcanoes.

12. CLIMATE
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Two main aspects of climate are Temperature and Precipitation
Also local wind regimes may be significant
The common mean annual temperature and Precipitation important
Also, their fluctuations both seasonal and non-seasonal
Finally, the magnitude and frequency of extreme events
Climate governs sediment yields e.g
equable climate as in the present day humid tropical belt and temperate
humid mid-latitude regions have promoted a good cover of vegetation and
erosion is kept to minimum with clay being the dominant sediment
Inequable climate tend to yield greater volumes of sediments especially
sands and gravels

13. TECTONIC MOVEMENT AND SUBSIDENCE
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This affects sedimentation in a number of ways and on many different scales
One is through Isostasy (Isostatic movements)
On a global scale, the distribution and movements of lithospheric plates lead to
the changing pattern of oceans and continents which in turn controls the size and
nature of the larger source areas, sediment transport paths and sedimentary
depocentres.

Others are Continental collision zones such as the Himalayans which produces
the largest volumes of sediments

On a smaller scale, movements along faults, growth of folds, block tilting,
differential subsidence and uplift on a scale of 1-100s meters provide a critical and
delicate control on the type, thickness and distribution of sedimentary facies.

14. SEA LEVEL CHANGES
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This is of two types; Eustatic Sea level change and Relative sea level change
Eustatic Sea level change: This is a function of sea-surface movement relative to
some fixed point such as the centre of the earth.
Relative Sea level change: This is measured relative to some moving point in the
underlying subsiding crust or near the sea floor, thus, it is a function of both sea
surface movement and sea floor movement in the fixed frame.
Longer term variations in relative sea level arise through the interplay of changes
in global sea level and basin floor, subsidence and uplift.
The most important effects on regional scale are mountain building , volcanism,
sediment compaction and isostatic movements.
When we have sediment loading and unloading we have Isostatic movement
When it is ice, we have Glacioisostasy
When it is water, we have Hydroisostasy
The effects of Relative sea level change are felt primarily along;
Shorelines which is reflected in transgressions (landward retreat of the shoreline)
and Regression (seaward advnce of the shoreline)

15. PHYSICAL PROCESSES
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These are the main means by which terrigenous clastic
sediments are eroded, transported and deposited.
They are also important in the transport and deposition of many
biochemical and chemical sediments.
Agents are wind, wave, tide and storm induced currents, gravity
induced mass flow especially turbidity currents, slow moving
oceanic and lacustrine currents and glacier
Features include
Their ability and competence to transport various grain sizes
Whether they are more or less steady or fluctuate in strength
and if so over what range and at what periodicity or whether
they are catastopic

16. BIOLOGICAL ACTIVITY
Animals and plants not only produce large quantities of potential sediments.
They trap, stabilize, mix, erode and govern a whole range of chemical and
biological processes of sedimentation and diagenesis.
Their first role is as precipitators of mineral matter ,ainly calcium carbonate-
construction of major reefs and platforms (coral, Bryozoanz, Sponges,
Stromatoporoids, algae and Rudists)
Main mixers of sediments are
Plant roots
Burrowing animals- destroys sedimentary laminations, homogenize and sort
We also have Bio-erosion by micro borers, raspers, crushers and burrowers
which can remove carbonate sediments as fast as they are produced and
can degrade lithified limestones to fine particles leading to mud grade
carbonate sediments

17. WATER CHEMISTRY
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Ionic concentrations and composition of sea and Lake water are prime
controls for the formation of evaporates and carbonate and other
biochemical sediemnts.
They are govern by
Temperature,
Organic activity,
Nature of inflowing waters and
The degree of isolation from or communication with open oceans that restricted basins experienced
Sediments are formed when water composition or Salinity changes due to
Movement of water from one place to another
Changing conditions of the basin
Chemical variations in sedimentation most times reflects climatic changes

18. VOLCANIC ACTIVITY
This involves catastrophic processes.
It can also upset completely all geological rules of sedimentation
Ash falls are precise chronostratigraphic markers in geology.
They also preserve fossils including footprints, soft-bodies in places where fossils are not
normally preserved e.g Savannah of East Africa.
Volcanic activities can provide coarse clastic sediment from within the basin as well from
outside the basin
The enormous nature of the sediments over a short distance and short space of time and
of unusual composition is the trade mark.
The coarse clastic sediment is texturally and compositionally immature and often localised
in distribution.
In deep ocean basins, leaching of hot pillow lavas by sea water, formation of clay minerals
through chemical exchange with sea water and associated hydrothermal discharge of
metal rich fluids have an important effect upon sedimentation

19. TECTONIC SEDIMENTARY PROVINCES
Tectonics not only deals with larger features of the earth but with forces and
movements that produced them.

Tectonics is the process of mountain building which involves the conversion
of marine sediments and volcanics into upstanding elongate ridges.

The concept of Tectonism can be viewed from two aspects
Orogeny and Epeirogenesis

Orogeny are spatial, episodic, irreversible tectonic process which also deform
rock structures

Epeirogenesis is the opposite of orogeny, it involves the uplift and subsidence
of large portion of the earth crust that have produces broader topographic
features such as basins and plateaux.

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Based on stratigraphic data accumulated over the years, it is clear that the majority of preserved
sedimentary rocks are marine (proportion not estimated)
There are several reasons for the dominance of marine sediments
The light SIALIC materials that forms a large proportion of Continental masses is limited in volume,
about 30% of the earth’s surface whereas the marine sediment account for 70% of the earth’s surface.
Because of the movement and subduction of oceanic crust and mantle at the edges of some
continental margins adjacent to raised orogenic areas.
There has been pronounced tendency through time for broad areas of the continental blocks to be
depressed, resulting in widespread incursions of the ocean into the interiors of the continents. Causes;
a. Eustatic Sea level changes
b.Phase changes caused by variations in heat flow in Asthenosphere
c.Melting of polar ice caps due to climate change


IV. Continental deposits are by definition formed above sea level and hence are subject to removal
should the rate of accumulation fall behind the erosional rate.

Note that the stratigraphic section in the continental blocks contain more unconformities while deeper marine or oceanic sections
are more complete

21. EPICONTINETAL SEDIMENTS

These are sediments formed as a result of depression of the
continental blocks resulting in widespread incursion of the
Ocean into the interiors of the continents.

The resulting shallow water sediments are laterally extensive
on the craton, although they are usually very thin

22. GEOSYNCLINE
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A Geosyncline (originally called a geosynclinal) is an obsolete geological concept to explain
orogens, which was developed in the late 19th and early 20th centuries, before the theory of
plate tectonics was envisaged.
In geology, geosyncline is a term still occasionally used for a subsiding linear trough that
was caused by the accumulation of sedimentary rock strata deposited in a basin and
subsequently compressed, deformed, and uplifted into a mountain range, with attendant
volcanism and plutonism.
It is also considered to be large-scale depression in the earth's crust containing very thick
deposits. An elongated downwarp in the Earth's crust in which a great volume of sediments
accumulated.’
They are usually elongate, basinlike depression along the edge of a continent, in which a
thick sequence of sediments and volcanic deposits has accumulated.
The definition of a geosyncline is a long trough in the surface of the earth where sediments
and deposits collect, thought of in the late 19th century and early 20th century as the origin
of most mountains.
An example of a geosyncline is the Adelaide Rift Complex in South Australia.
The term is divided into two parallel segments
Miogeosyncline and Eugeosyncline

23. MIOGEOSYNCLINE
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Characterised by clear cut depositional contacts with underlying
continental basement and a stratigraphic section whose marine
sediments were all deposited in shallow water.
They lack interstratified volcanic rocks or volcanic detritus.
The sediments grade landward into continental, cratonic
sediments rich in quartz.
On a first look , it can be interpreted as a thick accumulation of
strata on a margin of the continents
They are found in Appalachian region and are exposed in the
valley and ridge province.

24. EUGEOSYNCLINE
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These are formed in the deeper part of an Ocean basin adjacent to the
shallow water miogeosynclinal strata.
They are characterised by equivocal contact relationships with continental
basement, an abundance of volcanic rocks and detritus within
sedimentary sequence
There is a conspicuous absence of evidence of shallow water deposition.
The boundary between Miogeosyncline and Eugeosyncline is characterised
by intensive disturbed sedimentary rock unit called Melange
Melange is a tectonically chaotic mixture of very large fragments of older
sedimentary and crystalline rocks, some, several kilometres in length set in
muddy matrix
Another definition put it as chaotic mixture of blocks and fragments of rock
of a variety of composition both local and exotic. They are formed via the
process called Olistostrome

25. CLASTIC SEDIMENTARY ENVIRONMENTS
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This is an environment composed of clastic materials only i.e sediments derived from the continent exclusive
of the marine regime. We shall examine each depositional environment under the following headings;
Definition
Geological facies model
Lithology (Composition and Texture)
Structure
Boundaries
Sequences
Geometry of Bodies and Directional flow model
Surrounding facies
3. Well log responses and characteristics
Electro- Lithofacies
Dipmeter, Response and Dip patterns
Boundaries
Electro-sequences
Confusion with other environments
The environments are Glacial, Alluvial Fan, Aeolian, Braided, Meandeing, Deltaic Environment

26. GLACIAL ENVIRONMENT
DEFINITION OF TERMS:
GLACIAL:
This can be define as a period during an ice age in which there is a considerable increase in the total
area covered by Glaciers and ice sheets. During the last Glacial, ice covered about 32% of the land. It
can also be viewed as pertaining to the products or processes of glaciers and ice sheets eg Jurassic,
Pleistocene, Holocene.

GLACIER:
This is a mass of ice which deforms and moves due to the force of its own weight. Mass (ice and water)
and heat are constantly exchanges between the glacier and both the atmosphere above and the bed
or water body below. It occurs where winter snow fall exceeds summer snow melt.

GLACIAL ENVIRONMENT:
It is an environment characterised by deposits on the continents, in lakes or seas resulting from the
melting of ice masses which carried along detrital materials, landslide, avalanched or pulled out from
bedrocks by erosion and abrasion of the glacier floor during their movement. The present day glacial
deposition is naturally largely impossible to observe, since it is taken place beneath the major ice
sheets and only the marginal areas are visible

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The movements of glaciers consists principally of two components

Internal Plastic flow (Creep)
Basal Sliding
Creep involves rearrangement of the ice crystals into layers of atoms
parallel to the glacier surface under the great pressure exerted by the
weight of the ice.

The gliding of these layers over one another causes ice to creep.

Basal sliding refers to the movement of the glacier over bedrock under
the pull of gravity

28. Glacier Budget:
This is the balance between the rate of addition of
snow (accumulation) to a glacier and the loss of
ice (Ablation) by melting, calving etc

The equilibrium line is the boundary between the
accumulation zone where there is net addition and
the ablation zone where there is net loss.

29.  Glacial Landforms

30. LITHOLOGY- Two parameters are considered
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COMPOSITION:
Show wide type of lithology from rock fragments (Igneous, Metamorphic or
Sedimentary) even clay minerals.
This varieties of lithology is controlled by several factors.
The character of bedrock on which the glacier moves
The morphological characteristics and flow velocity of the glaciers
Position of the transported material in relation to the glacier and flow
velocity of the glacier
Mode of deposition
Subsequent reworking by melt water
Glacial deposits are commonly known as drift which can be divided into
stratified and Un-stratified

31. The Un-stratified glacial drift:
This is laid directly by glacier ice (called Till or mixtite, commonly found in Moraines and
Drumlins- They are generally characterised by the constant presence of certain amount
of gravel fraction and the equilibrium among the sand, silt and clay fractions).
The important feature is the presence of numerous labile minerals e.g feldspars,
ferromagnesian, minerals as unaltered.

A till is un-lithified sediment that has been transported and deposited primarily by
glacier ice with subordinate modification by other processes



Stratified glacial Drift:
These are ice transported sediment that has been washed and sorted by glacial melt
waters according to particle size. They are laid down in recognizable layers eg Eskers,
Kames, Varves

32. TEXTURE:
Sorting is very poor in un-stratified deposits (range from Boulders to silt/clay or colloids).
The shape is angular especially in sand fraction. Pebbles and boulders are better
rounded and show striations on their surface

STRUCTURE:
The un-stratified sediments lack sedimentary structures with no bedding planes except
when pebbles are imbricated.

In stratified sediments, we have cross bedding, fore-sets and Scour and fill structures
and laminations in Verves.

A fining upward sequence in outwash deposits and in Varves (Coarse grains deposited
during the summer period, finer materials during the winter

Generally, glacial deposits grade into braided fluvial deposits and it terminates in a
water body, delta like features are produced and they grade to lake or marine sediments

33. Well Log Responses:
The natural radioactivity, the thorium and Potassium content can vary
significantly, depending on the source rock type and chemical maturity

Dip Metres Response and Dip Patterns:
Generally, tills and Outwash deposits are characterised by very noisy dip-
meter resistivity curves.
Varve deposits are characterised by numerous dips with some azimuth
and magnitude ( this is because of the contrast in resistivity between
summer and winter deposit)
Note:
Tills and Outwash deposits can be confused with Alluvial fan and debris
flow turbidite deposits.

34. ALLUVIAL FAN ENVIRONMENT
Alluvial fan:
These are lobate deposits of alluvium on land. They form best on
lowlands adjacent to highlands in arid regions where little or no
vegetation exists to stabilize surface materials.
Alluvial fan Environment:
This is define as the continental environment characterised by coarse
sediments shaped like an open fan deposited by an emerging
mountain stream at the outlet of a narrow valley upon a plain or a
broad valley

The abrupt change of gradient eventually reduces the velocity of the
transport of sediment by the issuing stream.

35. LITHOLOGY
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COMPOSITION:
They are composed of rock fragments, the mineralogy of which depend on the geological source.
The matrix of fan gravels is sand or mud (which can be Primary or secondary in origin- deposited by
filling of intergranular porosity by finer materials deposited by an overlying braided river on the
previous fan)
Ranges from arkoses to greywackes plus plant fragments at times
TEXTURES:
Particle size range from boulder to clay size
Abrupt change in maximum or mean particle size and roundness are characteristics.
The particle shapes vary from angular to very well rounded
Particle size generally decreases from fan head(proximal fan) to fan base (distal fan) but down the
base, coarser particle accumulate at the base and act as strainers or sieves holding back finer
materials
Grain supported Conglomerates are the predominant facies of Stream flow/water laid deposits.
Matrix supported Conglomerates are more characteristic of debris flow/mud flow deposits.

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STRUCTURE:
Trough cross-bedded grain supported gravel is the major structure.
Planar cross-bedded gravel, trough and planar cross stratified sands
are the minor structures present in the environment
Graded bedding is also frequent, mostly fining upward but may be
coarsening upward at times if tectonic uplifts occur.

SURROUNDING FACIES:
Fans may inter-finger with Talus deposits towards the source area
and its margin may inter-finger with deposits of playa. Flood plain,
Aeolian dunes, fluvial sands.
Playa lake deposits are easily distinguished by their dominant clay
size and chemical deposits (Carbonates, evaporates).
Flood plain deposits have better stratification than alluvial fans.
While Aeolian sand are recognised by their dip patterns

37. i.
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WELL LOG RESPONSES:
The radioactivity, the K and Th content are generally medium to high, reflecting
chemical immaturity and presence of Feldspars, micas and rock fragments
from Igneous, Metamorphic or even Sedimentary Origin
Because of oxidising conditions, Uranium is present only if the source rock
contains Uranium bearing minerals (granite for example).
DIP PATTERNS:
Grain Supported Conglomerates:
High resistivity, low amplitude activity on the curves, scattered dips
Matrix /Mud Supported Conglomerates:
High amplitude, isolated resistivity peaks corresponding to compact indurated
pebbles embedded in a more conductive matrix

Generally, Alluvial fan deposits can be confused with glacial deposits or with Debris
flow turbidite deposits.
And the direction of flow is not easily extracted from the dip meter data

38. AEOLIAN ENVIRONMENT
Definition:
This is a continental environment
characterised by deposits
resulting from wind action often
mixed with Sabkha facies(Fluvial)

Three sub units have been
distinguished within Aeolian
Environment;
Dune

Inter-dune

Sand sheet
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Aeolian Transport
Suspension: grains held over substrate- clay and
silt (Loess)
Saltation: grains bounce off each other- fine and
medium sand
Surface creep: short steps- coarse sand and
granules

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Aeolian Deposition
Heavier material drops out of the wind before lighter materials. Sand gets deposited
before silt particles.
Deposition of sand forms sand dunes.
Deposition of clay and silt forms loess.

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Dunes are mounds of loose sand and the most well-known aeolian
feature. Size and shape of a dune depend on the wind direction and
amount of sand available.
Ripple marks result from high velocity winds which form fine, well
sorted, long, flat ripples.
Loess is a clastic, silt sized sediment that is formed by accumulation
of wind-blown dust.
Desert pavement is a desert surface covered with closely packed,
interlocking angular or rounded rock fragments of pebbles and
cobble size.
Desert vanish are coatings found on the exposed surfaces of desert
rocks due to the presence of clay particles in deposited materials.
MAJOR AEOLIAN DEPOSITIONAL FEATURES

41. MAJOR AEOLIAN EROSIONAL FEATURES
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Deflation basin is a depression in the land formed due to the
removal of particles by wind.
Ventifacts are stones that have one or more highly polished,
flattened facets as a result of erosion by windblown sand.
Mushroom rocks form when the top of an outcrop is eroded by
wind at a slower rate than the base of the outcrop.
Lag deposits are angular rocks and sand grains left behind after
the finer particles have been removed by Aeolian processes.
Yardangs are protruding rocks formed by wind erosion through
abrasion process.

42. MAJOR AEOLIAN LANDFORMS

43. Desert most times have basin of internal
drainage which allows for intermittent rivers/
streams. But because the rate of evaporation
exceed the available precipitation, fine silts
and clay that are laid down gives rise to
playa lakes which on evaporation form salt
flat (Sabkha).

44. GENERAL CHARACTERISTICS OF AEOLIAN DEPOSITS
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Lithology: sand and silt.
Minerology: mostly quartz, few cases of carbonates and other grains.
Bed geometry: sheets or sand dunes.
Texture: well to very well sorted fine silt to medium sand.
Fossils: rare in desert dune deposits, vertebrate bones are found
sometimes.
Bed geometry: tabular or large scale lenses of sand
Sedimentary structures: large scale dune cross-bedding and parallel
stratification in sands.
Colour: yellow to red due to presence of iron hydroxides and oxides.
Palaeocurrents: dune orientations reconstructed from cross bedding
indicate wind direction.
Facies associations: alluvial fans, ephemeral river, lake facies in deserts,
also with beach deposits or glacial outwash facies.

45. LITHOLOGY
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COMPOSITION:
Typical dunes are made up of Proto quarzites sands (>85% Quartz
grains).
But this is largely depended on clastic source and transport distance
Based on composition sand dunes are considered chemically
mature.
Clays are minor components which are found in Wadi, interdune,
Coastal sabkha and Playa Lakes or as post depositional authigenic
materials.
Cement may be calcitic or dolomitic due to rise of the water table

46. TEXTURE:
Aeolian consists of fine to medium sand grains (0.2 to 0.5mm)

They are well rounded

They are well sorted

They are positively skewed

The absence of fines(clays) and micaceous minerals is due to Aeolian
winnowing. This give rise to high grain/matrix ratio

47. Cross bedding formed as a result of Aeolian Processes (Navago Sandstones in Zion National park, Utah) Ken
Hamblin

48. KEY CRITERIA FOR RECOGNIZING
AEOLIAN DEPOSITS
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well sorted grains
pitted, frosted grains
thick cross bed sets
high angle foresets
high degree of oxidation
which gives red coloration.

49. 1.
2.
3.
4.
5.
6.
STRUCTURE:
Medium to large scale cross bed sets facing downward with foreset
dips at angle of repose (up to 34⁰).
Successive boundaries separating individual cross sets, horizontal or
downward at low angle dip.
Cross sets become progressively thinner towards the top of dunes, as
the dunes grow upward, winds frequently truncate the upper laminae,
producing the thinning upward pattern of sets.
Dipping foresets are progressively larger in a downward direction.
Contorted bedding
Rare ripple laminae.

50. SURROUNDING FACIES
Aeolian dunes are found in dry deserts, along rivers and along
coastlines.

Consequently, Aeolian sands may grade laterally with nearshore
marine sands, silts or clays, fluvial overbank silt and clays or
Alluvial fan, playa and evaporite deposits of the desert setting.

51. WELL LOG RESPONSES
The radioactivity of dune sands is low.

Cross plot representative points of massive dunes fall very close to sandstone line (if there is no gas influence)
indicating quartz as the main component.

The increase of radioactivity corresponds to interdune deposits (Wadi or Sabkha) which are also reflected by density
and Neutron responses.

Dolomite and anhydrite (or gypsum) are common in interdune sabkhas or bottom sets.

Sabkha deposits can be recognised, they correspond to interdune dolomitic or anhydritic shaly deposits.

Wadi deposits are characterised by coarser, poorly sorted sediments richer in clay material and consequently a little
more radioactive.

They show scattered dips (trough cross bedding) or even no dip (conglomeratic level or mud cracked clays)

Generally, the dipmeter are so characteristics that confusion with other environments is unlikely

52. FLUVIAL ENVIRONMENT: BRAIDED AND MEANDERING
SYSTEMS
•
•
•
•
The river is the most important transporting agent over much of the world’s
land surface,
Apart from localised accumulations of weathered material on hillslopes,
the main fluvial sedimentary environment is the river floodplain.
This usually consists of a long but narrow accumulation of layered
sediments
The sediment erosion, transport and deposition by running water are related
to particle size and flow velocity

53. BRAIDED SYSTEM
This is a continental environment characterised by deposits resulting from
river system of an interlaced network of low sinuosity channels.

It possess an intricate network of dividing and rejoining channels.

Braided develops when a stream is supplied with excessive sediment which
over time is deposited as sand, gravel bars within its channel.

During high water stages, these bars are submerged but during low water
stages, they are exposed and divide a single channel into multiple channels.

They have broad and shallow channel.

54. At-Bashi River, Tien Shan Mtns., Kyrgyzstan
Braided stream.  Tien
Shan Mtns, Kyrgyzstan
Son-Kul River, Tien Shan Mtns., Kyrgyzstan

55. Resurrection River, Kenai Peninsula,
Alaska
Resurrection River, Kenai Peninsula,
Alaska
Kicking Horse River, Yoho National
Park, British Columbia

56. COMPOSITION
•
•
•
•
They are typically composed of texturally and chemically immature gravels
and sands.
The sand-shale ratio is greater than 1
They can be classified as lithic arenites to lithic wackes
We also have minor amount of silts approximately 10% (which is
correspond to abandoned channel deposit.
COMMON MINERALS
Quartz, feldspars, mica, glauconite is absent (non marine deposit).
Carbonaceous organic matter is very rare due to the oxidizing nature of the
environment.
Gravels and pebbles are rock fragments (composition of which depends on
the source areas.
Shale, pebbles and reworked clay-ironstones concretions may be present.

57. TEXTURE:
Poor to moderate sorting (gravel to sand ) with low sphericity is observed.
They have low grain-matrix ratio.
They have abundant silt in fine end tail

STRUCTURE
There is an asymmetrical small scale ripples.
Also have abundant well oriented cross bedding, commonly unimodal are
observed as well as small scale cross-laminations.
The bedding may be either massive or graded.
Depositional bars are abundant

58. Braided River core examples from the Cretaceous of British Columbia (from University of British Columbia, Canada)

59. a.
C.
Both sandy and gravelly braided rivers migrate laterally leaving sheet-like or wedge shaped deposits of channel and
bar complexes preserving only minor amounts of floodplain materials.
Three main geomorphological bodies are recognised;
CHANNELS
-Vary considerably in size
- Can be composite stream channel, stream channel, channel bars
- The basic sedimentary fill succession is fining upward .
- In cross section, the channels are erosional occurring in a very high frequency association.
- In channel bars, they are composed of coarse grained lag deposits of the stream (gravels)
b. BARS
- Three types exist; Longitudinal (most abundant=95%), Transverse and point bars.
- The surface is never smooth, including a very wide range of small and large scale structures
- Bars tend to be built up by accretion down stream lateral, the upstream end is partly eroded.
ISLANDS
- These are the most permanent features on the valley floor within a braided system
- They are formed when channel bars formed are stabilised by the deposition of fine grained sediment on its
top during high flows and later colonised by vegetation.

60. WELL LOG RESPONSES AND CHARACTERISTICS
The SP shape is typically smooth cylinder.

The potassium/Thorium cross-plot confirms the presence of feldspar and also highlights
the high thorium concentration from heavy radioactive minerals such as zircon.

The sands are massive apparently homogenous with a few randomly distributed highly
resistive peaks. This also show low GR and hydrogen content and high density.

The mineralogy associated with such level is not obvious but hydrated silica (chert) may
be present OR could be limonite (hydrated FeO) with concentration of Thorium bearing
minerals (Zircon).

The general tendency within the sands appears to be fining upward sequence.

The cylinder shape of braided channels can be confused with the same features found in
Chute bars (Meandering system)

61. MEANDERING SYSTEM
a)
b)
c)
d)
e)
f)
Meandering depositional environment is a sub division of the fluvial
environment which is classified under the continental sedimentary environment.
A continental environment characterised by deposits resulting from a river
system of single high sinuosity channel generated by a mature stream swinging
from side to side across its flood plain on gentle slope.
In meanders, a spiral or helicoidal flow is created by heaping up of water against
outside shores of the bends. This mechanisms is responsible for the lateral
accretion of point bars.
The outer bank is called cut bank because flow velocity and turbulence are
greatest on that side of the channel where it is eroded.
In contrast, flow velocity is at minimum near the inner bank which slope gently
into the channels.
The valley floor of meandering stream are commonly marked by crescent
shaped ox bow lakes which are actually cut off meanders.

62. DEPOSITIONAL ENVIRONMENT

63. MEANDERING DEPOSITIONAL ENVIRONMENT
Oxbow lake

Cutoff

Point bar

Abandoned
channel

Alluvial
deposits

64. LITHOLOGY







The main minerals are quartz, altered feldspars and micas.
The sandstones range from quartzitic to lithic arenites due to
their low to high chemical maturity.
In the bedload, clay pebbles can occur (from levee slumping).
Glauconite is absent.
Peat and coals are present as beds (flood plains and fragments
(channels).
Carbonate and Iron concretions may be formed in areas with
high rate of evaporation (flood plain).
Clays are generally kaolinitic but other types may be present
depending on the climatic conditions and the distance from the
source area

65. TEXTURE





They show normal grading.
The grading are typically composed of sands, silts and
shales with sand-shale ratio lower than 1.
The basal zone is poorly sorted and grain size ranges
from conglomerates to coarse grained sands.
It grades upward to a well sorted medium to fine sands.
The upper zone is generally composed of very fine sands,
silts and some clays and may be poorly to fairly sorted.

66. STRUCTURE



The sedimentary structures are related to the flow regime
and consequently organised in sequences.
The sequence starts with an erosional surface with scour
troughs followed by medium scale cross bedding; parallel
laminations which are related to upper flow regime, forest
bedding in Point bar or chute bar with small trough sets.
Flood plain deposits show horizontal or convolute bedding,
generally destroyed by bioturbation and rootlets may be
present

67. SEQUENCES
o
o
o
o
o
Fundamentally, the sequence is fining upward.
It consists of in channel deposits (lateral accretion).
The lag deposits cover a near-horizontal erosional surface
and are capped by trough cross bedding (sands) which in
turn is overlain by small scale trough cross lamination (silts).
After the lateral channel migration, the sequence continues
with vertical accretion deposits (silts and muds) introduced
at flood stage.
Root traces and desiccation cracks can be observed and in
the humid climates, vegetation may grow sufficiently to
form coal seams.

68. WELL LOG RESPONSES
•
•
•
•
•
•
•
•
•
•
•
•
The radioactivity is medium to high depending on the importance of chemical maturity.
Potassium % is generally less than 1% if the rock is relatively mature or between 1 and
2% if immature (with feldspar or mica)
Th/K ratio tends to be higher than 10.
Uranium is often very low (oxidising conditions) except in flood plain deposits rich in
organic matter
On the desity/neutron crossplot, representative points fall between sandstone and shale
region.
Few points may indicate carbonates (caliche-like)
Few points with high density may correspond to laterite or limonite presence
Sand/Shale ratio is lower than 1.
Most times, environment show no dip or scattered dips(trough cross bedding)
Blue and red pattern may be present, sometimes with the same azimuth indicating
foreset beds.
For the electro sequence, bell shape often serrated, for each individual sedimentary
sequence is observed which reflects a general fining upward sequence.
A succession of bell shape features with variable thickness is the rule
NOTE:Insomecasesfluvialdepositscanbeconfusedwithturbiditedepositsformedby

69. Deltaic Environments
a)
b)
c)
d)
e)
A transitional environment characterised by sediments that have been
transported to the end of a channel by current of continental water.
They are deposited mostly sub-aqueously but partly sub-aerially at the
margin of the standing body of water (Sea, Lake, Ocean) into which the
channel discharges or still discharging.
A delta may forms where river systems brings more sediment into the
sea than can be reworked by marine currents and waves.
When a stream flows into another body of water, its flow velocity
decreases rapidly and it deposits its sediment, as a result, delta is form
causing a local shoreline to build out (prograde).
Deltas in lakes are common but marine delta are larger and important
as areas of natural resources

71. 


The shapes of deltas varies considerably;
Where reworking by the sea is minimal, individual distributary channels
push sediment lobes out to form the classic bird’s foot shape e.g
Mississippi delta.
However an arcuate plan are formed by creating a coastal barrier beach
e.g Nile delta.
While Ganges delta has a coastline of mangrove swamp encouraged by
high tidal range.
PROGRADING DELTA
Bottom set beds------------ Fore set beds----------------------Top set beds
The finest sedt beyond the stream Sand/silt are deposited in gentleCoarse grained sediments in a
mouth where they settle down from inclined layer network of distributary channels
suspension traversing the top of the delta

72. DELTA SUBENVIRONMENTS
I.

i.
ii.

a)
b)
Delta can be subdivided into several sub environments
The Delta Plain: This is subaerial lowland part of the delta consisting
of active and abandoned channels separated by shallow water
environments and emergent or near emergent surfaces. This includes;
Upper Delta Plain Deposits:
Migratory distributary channel deposit (braided, meandering, natural
levee and point bar deposits).
Lacustrine delta fill, inter-distributary flood plain deposits
Lower Delta Plain Deposits:
Bay fill deposits (inter-distributary bay, natural levee, marsh, crevasse
splay).
Abandoned distributary fill deposits.

73. 2. The Delta Front OR Sub-aqueous Delta
Plain
•
•
•
•
This is high energy environments where the sediments are
constantly reworked by tidal currents, wave action, marine
longshore currents.

It includes delta front sheet sands, distributary mouth bar
deposits.

It is represented by a relatively large scale coarsening upward
sequence which records the vertical upward facies change
from fine grained offshore.

These sequence result from progradation of the delta front
and they usually sandstone dominated

74. PRODELTA
•
•
This is the transitional sub-environment
between delta front and normal shelf
deposits.


It is the part of the delta that is below the
effective depth of wave erosion

75. COMPOSITION
Siliciclastic deposits with chemical and textural maturity relatively good are the
dominant lithofacies.

Mica, lenses of heavy minerals, coal beds and fragments are common.
Glauconite and phosphates may be present depending on the marine influence

Shell debris, thin limestones are present

Siderite, pyrite, limonite and other iron rich compounds are presents in small
amounts.

Evaporites would be present if climatic conditions favour them

76. TEXTURE
Grain size is mainly medium sand to clay.

Sorting is medium to well developed

Grain size tends to alternate in cyclic sequences

Roundness is moderate to good

But each sub-environments models will leave their imprints over textural
parameters

77. CLASSIFICATION
1.
2.
Note that the distribution, orientation and internal geometry of
deltaic deposits are controlled by a variety of factors. These
include climate, morphology, vegetation, water discharge,
sediment load, river mouth processes (waves, tides, winds and
currents), tectonics and geometry of the receiving basin.
Based on these complex sets of variables, the deltas is classified
into two or three
River dominated deltas= High constructive
Wave and Tide dominated deltas= High destructive

78. RIVER DOMINATED DELTAS (RDD)
•
•






This happens when wave, tide and long shore currents are weak and the
volume of sediments carried by the river is high, leading to rapid seaward
progradation.
We have the following events;
As progradation proceeds, the river slope is flattened.
Flow becomes less competent
A breach in the subaerial levee occur upstream during high discharge
termed crevasse
The breach offer a shorter route to the sea via an interdistributary bay
causing a major flow diversion
A sub delta crevasse splay deposit may dvelop rapidly
Eventually the crevasse become a major distributary and the process is
repeated.

79. RIVER DOMINATED DELTAS SUB
ENVIRONMENTS
1.
2.
There are two main types;
Bird foot, elongate- Under this type, river discharge is steady with
high suspension load. Have distributaries eg Mississippi river (Gulf
coast.

Lobate Shape- Here, the river discharge fluctuate with high
proportion of bed load in the transported sediments, greater
number of distributaries eg Orinoco, Ebro, Niger

Generally, River dominated deltas generate a large cyclical deposition.
The rapid seaward progradation of these deltas gives rise to the most
characteristics feature of deltaic sediments; the coarsening upward
sequence/cycle

80. WAVE DOMINATED DELTAS (WDD)
This take place in an environments of strong wave activity in which mouth bar deposits are continuously reworked
into series of superimposed coastal barriers.

Sand bodies tend to parallel the coastline in contrast to those of RDD which are nearly perpendicular to the coast.

The surface facies distribution shows a strong reworking of the sand, creating more cuspate subaerial forms.

Shale, coal, evaporate, shell beds and heavy minerals may be present.

Sands and Shales tend to be burrowed and rooted

Pro delta clays are rich in mud pellets.

Due to wave action, the flow pattern tend to be bi modal with large scattering.

The sequence revealed coarsening upward

81. TIDE DOMINATED DELTAS (TDD)
When tidal range is high, the reversing flow that occurs in the distributary channels during flood and ebb may
become the principal source of sediment dispersal energy.

Within and seaward of the distributary mouths, the sediments can be reworked into series of parallel linear or
digitate ridges parallel to the direction of tidal current.

Deltas of this type may be difficult to recognise in ancient sedimentary deposits.

The modern example is the Klang- Langat River in Malaysia

Common structures include thin bedding with parallel to oblique laminations

Small and large bi-directional and uni-directional trough cross bedding, flase structure, mudcracks in shales,
algal structures and intense bioturbation of sand tops and shales.

The sequence show coarsening upward followed by eventual fining upward without a well define boundary

82. WELL LOG RESPONSES AND CHARACTERISTICS
a.
b.
c.
The SP and Resistivity log shapes of Funnel, Bell and Cylinder are in used to recognise sedimentary facies.
The mineralogical composition is obtained by log and cross plot analysis.
A very low radioactive interval can correspond to
Coarse grained sands/well washed sands
Coal (peat or lignite)
Limestone beds (accumulation of shell fragments)
While low to moderate radioactive intervals are representative of medium to fine sands
The thorium and Potassium help to define the predominant clay mineral types and Uranium correlate with the
percentage of organic material.
Also the textural information can be extracted from porosity level and its evolution in combination with gamma rays and
SP analyses
High porosity (30-40%) = well sorted sand
Medium porosity (15-25%)= poorly to moderately sorted sand or to quartz cemented sand if the representative points
fall on the sandstone or siltstone line
None or scarce dips reflect homogenous , heterogenous or bioturbated shales
Numerous dips with same azimuth and small changes in magnitude indicate laminated silty shales without strong
bioturbation.
For sandy intervals, None or scattered dips reflect bioturbation or cross bedding whereas numerous dips correspond to
ripple marks

83. DIAGENESIS OF SEDIMENTARY ROCKS








The concept of sedimentary rocks do not stop with environmental
interpretations, there is still much to be told of events after deposition,
that is during diagenesis.
During diagenesis, indurated rocks are produced
The process begins immediately after deposition and continue until the
onset of metamorphism.
Therefore diagenesis can be define as the sum total of the physical and chemical changes taking
place in a sediment before the onset of metamorphism.
It can also be seen as the process reaction that occurred in a rock from the time the crystal or grains
are deposited until the time the texture or mineralogy of the resulting sedimentary rock is markedly
changed. The range of temperature and pressure etc is low.
It can be divided into two; Early diagenetic events and Late diagenetic events
EARLY DIAGENETIC EVENTs- These are events taking place from sedimentation until shallow burial
LATE DIAGENETIC EVENTs- These are events occurring during deep burial and subsequent uplift

84. i.
ii.
The diagenetic processes include all or any of the following;
CompactionDissolutionAuthigenesis
RecrystallisationReplacementCementation

The processes are important for several reasons
Considerably modify a sediment (composition, texture and
sometimes original structure are completely destroyed)
It also affect both porosity and permeability of the sediment
(these are the two controlling properties of potential reservoir)
Based on these, we are going briefly examined the diagenetic
processes of three sedimentary rocks of important to us.
Sandstones, Mudrock & clay minerals and Carbonate

85. DIAGENESIS OF SANDSTONE
Physical Processes-
Compaction and Pressure solution (both depends on depth of burial).
Chemical processes includes-
Precipitation of minerals
Dissolution of the unstable grains
Replacement of grains by other minerals
Note that chemical process take place in the medium of water therefore salinity, Eh, pH (redox reaction)
, porosity and permeability are of critical importance to diagenesis.
Factors affecting Sandstone Diagenesis:
1. The depositional environment 2. Composition and texture of sediment which are primary factors
3. Pore fluid migrations4. The burial history
PRINCIPAL PROCESSES IN SANDSTONE DIAGENESIS: 1. Compaction and Pressure solution
2. Silica and Calcite Cementation3. Clay mineral Authigenesis
4. Others such as Hematite cementation and Pigmentation

86. DIAGENESIS OF CLAY MINERALS AND MUDROCKS
Clay minerals can be altered during early and late
diagenesis even unto the onset of metamorphism.

The physical post depositional process affecting the
mud rock as a whole is compaction.

Compaction expels water and reduces the thickness
of the deposited sediment by a factor of up to 10.

Further compaction leads to more water being expel
and at greater depth.

87. DIAGENESIS OF CARBONATE ROCKS (LIMESTONE)
•
•
•
•
•
•
•
•
•
•
•
The onset of carbonate is difficult to define in carbonate sediment.
This is because sedimentation processes such as algal micritisation may be contemporaneous with
diagenetic processes such as cementation
The diagenetic processes in Carbonate is referred to as ISOCHEMICAL ( ie no major change in
chemistry of the sediment in its transformation into a rock)
They are referred to as Allochemical when chemical changes are involved (dolomitisation and
silicification)
Principal isochemical diagenetic processes taking place are cementation and neomorphism
Neomorphism involved changes in the mineralogy or fabric of the sediment. It is a process of
replacement which include all transformation between one mineral and itself OR a polymorph.
There are two aspects to this neomorphism
The WET POLYMORPHIC TRANSFORMATION of Aragonite to Calcite
The WET RECRYSTALLISATION of Calcite to Calcite
Both are wet because they take place in the presence of water through solution- re-precipitation
Note that most neomorphism in Limestone is of the aggrading type leading to coarser mosaic of
crystals.

88. THE BASICS OF LOG EVALUATION
•
•
1.
2.
Need for method whereby fundamentals properties of rocks and fluid
content could be quickly and reliably determine within the subsurface
necessitated the introduction of electric logs and its evaluation.
Logs are powered by electric currents.

Functions of Electric logs:
This is divided into two;
Continuous reading with depth of all measurable physical
properties of the rocks penetrated by the bit and fluid content
(downhole geology)
the facilitation of correlation between wells and even between
drilled areas.

89. •
I.
II.
III.
LOG TYPES
Logs used to to quantify hydrocarbon in place can be classified into
three families
RESERVOIR THICKNESS LOGS- (These include Gamma Ray,
Spontaneous Potential) It discriminates reservoir from non
reservoir.

POROSITY LOGS- (Density, Neutron, Sonic) These logs are used to
calculate porosity, identify lithologies and differentiate oil from gas.

RESISTIVITY LOGS- (Laterog, Induction , micro-resistivity). These
logs together with porosity logs are used to calculate hydrocarbon
saturations.

90. 



OTHERS ARE;

Side Wall Sampler- Takes small rock samples which
are used for lithology and fluid types confirmation.

Formation Tester- This measures formation
pressures and can retrieve fluid samples.

Dipmeters and Formation Micro Scanner(FMS)-
These measures dip and azimuth of the layers

Well Shoot and Vertical Seismic Profiling (VSP)- this
is used in calibrating seismic.