STRATIGRAPHY
Sedimentary Processes &
Tectonics
Stratigraphic Procedures,
Stratigraphic Column,
Sedimentary Paleontology;
Stratigraphic Traps.
Definitions
• Sediments are small pieces of a solid
material derived from pre-existing rock; from
biogenic sources or precipitated by chemical
processes, and deposited at or near the
earth’s surface.
• Sedimentology is the study of sediments,
particularly focusing on sedimentary
processes (how they were generated,
transported and deposited), interpretation and
classification.
Definition Conts.
• Stratigraphy is the study of layered
(stratified) rocks in terms of time and
space. In other words it is the study of
relative spatial arrangement of rock
strata. Thus stratigraphy emphasizes the
analysis of sedimentary strata, the layers
of sedimentary rocks.
Why Study Sediments and
Sedimentary Processes
The application of the knowledge of sediments
and sedimentary processes is to:
• Reconstruct the earth’s history
• Locate economic resources
• Advice on environmental engineering
activities.
Reconstruction of Earth’s
History
• Ocean is an archive of sediment
composition (lithology, fossils, chemistry)
that gives detail information (evidence)
about the earth’s past (climate ~
temperature, ecology and evolution;
ocean chemistry).
• provides an insight into the dynamics of
environmental processes
•Paleogeography gives the configuration
of land masses;
•Paleoclimatology provide information on
greenhouse/ice-house;
•Paleooceanography ........
•Paleoecology and Paleobiology
respectively account for oceanic/terrestrial
environments, evolution and ecosystems.
•Tectonic processes furnish evidence on
mountain building.
Location of Economic
Resources
• Marine sediments deposited millions of years ago
along continental margins (basins, river deltas, etc.)
have resulted in the formation of oil and gas (fossil
fuel).
• Sediments deposited on the continent (coastal
wetlands, swamps, bogs etc.) produced coal. Alluvials
produced mineral resources (gold diamond etc).
• Chemical and Biogenic precipitates have resulted in
producing economic resources that include: limestone,
phosphorites, gypsum, talc, salt, etc.
• Ground water from aquifers etc.
Environmental Engineering
Advice
• Geologic hazards:
–Mass Movement
–Floods
–Earthquakes
–Costal Erosion
–Population Growth and land use
–etc
Weathering of Rocks and
formation of Sediment
• Weathering is a terminology assigned to the group
of processes that result in the transformation of rock
into sediments.
• Weathering includes two general categories of
processes – physical processes collectively
referred to as disintegration and chemical
processes collectively referred to as
decomposition.
• Weathering is important because it is the process
through which rocks are broken down and sediment
is formed.
Formation of Sediments
• Although weathering processes are separated into
physical (mechanical) and chemical, in nature, both
work together to break down rocks and minerals to
sediments or to minerals more stable near the
Earth's surface.
Physical (Mechanical) Weathering:
• Physical weathering breaks intact rock into smaller
grains or chunks collectively called detritus through
the following processes:
Physical Weathering
• Jointing – When overburden
are eroded away from buried
rocks, the release of
pressure on them cause
these rocks to change shape
slightly by naturally cracking
on their surfaces. These
cracks are known as joints
(planar, irregular, curving
etc). Joints can make rocks
break into various sizes.
Joints form free space in
rock by which other agents
of chemical or physical
weathering can take place.
• Frost Wedging - When
water collected in joints
freeze, they expand forcing
the joints to open wider. This
action results in the breaking
of portions of the rock. This
situation occurs mostly in
temperate regions or at high
altitudes areas.
• Root Wedging – Roots of
trees as they grow expand
and applied pressure on their
surrounding. This action can
split open cracks and pores
to pill off rock surfaces.
Physical Weathering
• Salt Wedging – Dissolved
salts in groundwater
crystallized out from solutions
under favourable conditions. As
the crystals grow they weaken
the rocks around them thereby
disintegrating the rocks into
sediments.
• Thermal Expansion –
Temperature is capable of
causing differential expansions
in rock layers. On cooling the
layers contract, creating forces
sufficient enough to make the
outer part of the rock to break
(spall) in sheet-like pieces.
Types of Chemical
Weathering Reactions
• Hydrolysis - H+ or OH- replaces an ion in the mineral. Example:
• Leaching - ions are removed by dissolution into water. In the
example above we say that the K+ ion was leached.
• Oxidation - Oxygen (O2) is more common near the Earth's surface
thus react with minerals to change the oxidation state of an
ion. This is more common in Fe (iron) bearing minerals
Types of Chemical
Weathering Reactions
•Dehydration - removal of H2O or OH- ion from a mineral.
• Complete Dissolution - all of the mineral is completely dissolved
by the water.
Mineral Composition and
Stability in weathering
Environment
• In a weathering environment some minerals weather
more quickly than others.
• Few minerals are readily soluble in slightly acidic water.
• Others are very resistant to weathering thus
persisting for a long time without alteration e.g. quartz.
• Weathering ends eventually with the production of
clay minerals.
• One of the controls on weathering of minerals is
the temperature at which the minerals originally
formed when they crystallized from magma or lava.
Minerals Stability Scheme
Least stable (high temperature minerals)
• Olivine
Ca plagioclase
• Pyroxene
• Amphibole
• Biotite
Na plagioclase
•
Potassium feldspar
•
Muscovite
•
Quartz
Most stable (low temperature minerals)
This is called Goldich Stability Series
Erosion and Sedimentation
Products of weathering may remain in the place
where they have been disintegrated.
• Physical weathering gives rise to eluvial
deposits, whereas in the case of chemical
weathering residual deposits are produced.
• Bulk of sediments are eroded and transported by a
medium (water, air and glacier sometimes by
living organisms).
Erosion and Sedimentation Cont.
• The rate of erosion in the source area determines
the supply of materials for sedimentation.
• Sediments are transported by the medium in the
form of insoluble fragments or in a dissolved
state from source to its final destination.
• Transportation may involve a short violent tumble
down a slope or a long trip with many stopovers
across continent or both.
Erosion and Sedimentation
Cont.
• Deposition is the process by which
sediment settles out of the transporting
medium.
• Sediments are deposited in a certain
sequence (sedimentary differentiation)
mechanically or chemically.
Erosion and Sedimentation
Cont.
• Mechanically, sediments are deposited according to
their size, shape and density.
• Chemical differentiation consists of successive
precipitation of substances depending on a degree
of solubility of the substances and on physicochemical conditions of the solution (concentration,
temperature, pressure, acidity or alkalinity of the
medium etc.).
Sorting
• Very
well
sorted,
• well sorted,
• moderately
sorted
• poorly sorted
Origin of Sediment Colours
• The colour of a deposit may reflect the original
colour of the parent rock from which it was
derived.
• However, a reddish colour may indicate subaerial
accumulation in an environment where ironbearing minerals have been oxidized.
• Sediment colour is not only useful in
determining depositional environments, but also
in geologic mapping and correlation.
Types of Sediment
• Detrital also known as clastic or
terrigenous or silliclastic.
• Chemical
• Biochemical and bioclastic.
Detrital or Clastic
• Dominated by the solid remains of chemical and
physical weathering. Most commonly consists of
quartz, feldspar, and clay minerals.
• Volcanic ash consolidate into volcanic tuff. Ash is
composed of crystalline grains of minerals such as
biotite are used for K/Ar radiometric dating) and
glass shards used for trace element analyses and
volcanic vent sourcing.
• Ash mixed with other sediment are re-deposited (reworked) in topographically low areas.
Detrital or Clastic
• Ash or tuffaceous beds are very useful
in correlation because they record
single or a series of short-lived
events. They represent time horizons.
In some cases, clastic rocks consist of
limestone or even gypsum debris.
Chemical Sediments
• Formed by inorganic precipitation of solids –
gypsum, halite, limestones, and marls.
• The chemical sediments are usually mixed
with lake silts and clays to form marls.
Biochemical and Bioclastic
Sediments
Plants and animals precipitate solids (i.e. shells,
skeletons, coral reefs) as biochemical
sediments. If these are broken up by waves or
other
processes,
become
bioclastic
sediments.
After the rock has been weathered to saprolite;
•The clays will be eroded and transported by running
water to the sea. Clay is fine-grained and remains
suspended in the water column. The clay may
ultimately be deposited in deep quiet water far from
shore.
•As the soft clay is removed, the unweathered,
residual quartz grains will be released from the
saprolite by erosion. The quartz in granite is sandsized, and it becomes quartz sand. The quartz sand
is ultimately transported to the sea, where it
accumulates to form beaches.
•The dissolved ions (sodium and potassium) will be
transported by rivers to the sea, and will become part
of the salts in the sea
.
Siliciclastic Sediment
Maturity
• Depending on the degree of weathering and the
constituents of the sediments, a silicclastic
material(s) may be described as:
–Mature Sandstones
–Immature Sandstones
•Immature Sandstones: Retain lots of unstable
igneous/mm minerals from source area (top of Bowen
series). Sediments mineralogically look much like source
rocks. Often fragments of source rock preserved whole,
even in sandstones. One can easily infer if sediment
source was granite or andesite. It requires lots of
mechanical, not much chemical weathering.
•Mature Sandstones: Sand grains dominated by quartz
with lesser amounts of K-feldspar ± muscovite. Lots of
chemical weathering unstable minerals. Clays washed
away, only quartz and feldspar left.
Controls on Sediment
Maturity
These factors control the maturity of sediments:
–Soil development
–Global controls on soil development and
sediment production
–Topographic Relief
Soil development
 For a stable land surface, there is lots of time for rainwater to
seep down and chemically alter all of the less stable
minerals.
Trees and other plants force their roots down into the
relatively fresh bedrock, thus opening more pathways
for
alteration.
Fire lay bare this patch of ground; rain erode away clay
minerals and unweathered mineral grains.
The duration of chemical weathering associated with soil
development result in production of mature sediments.
 On a mountain, it is tough to make a good soil. As soon as
the rock is physically broken up by some chemical and
physical weathering, it is easy for the loose particles to
fall away. There is not the time for extensive chemical
weathering, hence relatively immature sediments are
produced.
Climate
Weathering intensity depends most strongly on rainfall .
Good vegetation → plants leading to mechanical breakdown;
production of corrosive organic acids.
Lots of water to carry dissolved solids away to prevent chemical
equilibrium with unweathered minerals.
Highest rainfall found in tropics: rocks rapidly weathered to clay
minerals stripped off everything except Al, Al and Si, and Fe oxides.
Sands tend to be mostly quartz. Very thick but nutrient-poor soils.
Subtropics dominated by deserts: low rain, low chemical
weathering and thin soils results in fairly immature sediments, but
not much export by rivers.
Temperate region: more rain, thicker soils, a lot more
nutrients retained in dirt. Best farmland. Moderately mature
sands.
Arctic: low rain, too cold for many plants. Low chemical weathering,
thin soils results in fairly immature sediments.
Topographic Relief
The steeper the slopes, the faster any loosened
minerals are eroded away.
Mechanical erosion is very fast on
mountains. It is, however, tough to predict
sediment maturity due to relief alone.
Sediments are transported in major
flood events. From the sources the streams
carry the immature sediments over the
flood plains and their banks at the lowlands.
These sediments have ample opportunity to
weather on the river flood plains before they
eventually are eroded and taken to the sea.
Sediments Characteristics
• Sediments are derived from the weathering of
pre-existing rocks. The grain sizes of
sediments depend on:
the types of rocks in the source area
the textures and mineralogy of the
rocks and
composition of the resulting
sediment.
Texture Interpretation
• Texture is an indicator of energy levels in the
environment of deposition.
 Moving water (such as waves or currents) is considered
to be a high energy environment.
Quiet water or still water (water without waves or
currents) is considered to be a low energy environment.
Deep water environments commonly have quiet water,
because wave motion is restricted to the upper part of
the water column.
Determination of Energy Levels in
Depositional Environment from
looking at sediment
The place where sediments accumulate, perhaps a
beach, a riverbed, a lake, or a delta is referred to as
depositional environment.
Factors for determination of energy levels in
depositional environment include:
Grain size
Sorting
Grain shape
Grain size
Coarse-grained sediments (sand, gravel) indicate high
energy environments.
Fine-grained sediments (clay or silt) indicate low energy
environments.
Sorting
Well-sorted grains indicate that the sediment was probably
transported for a long time in a fairly high energy
environment (waves or currents). The finer grains were
probably washed or winnowed away.
Poorly sorted grains indicate that the sediment has not been
transported very far from the source area. It also suggests
fluctuating energy levels, and a fairly short time in the
depositional environment.
Good sorting implies consistent energy
(washing)
Poor sorting implies inconsistent energy
(dumping)
Grain shape:
A well rounded sand grain indicates that the sediment has
been transported far from the original source area,
and that it has been in the depositional environment
for a long time. (High energy environment)
Angular sand grains have probably only been transported
for a short distance from the source area, or they have
been in the depositional environment for a short time.
(low energy environment)
Textural Maturity
Three steps are involved:
•Winnowing or washing out of fines - makes
an immature sediment become submature
•Sorting of grain sizes - makes a submature
sediment become mature
•Rounding - makes a mature sediment
become supermature
Mineralogical Composition
of Sediments
• Identifying the minerals present is important
because sandstones are classified based on the
composition of their grains.
• Three components are considered when naming
sandstones:
• Quartz grains
• Feldspar grains
• Fine-grained rock fragments. Possibilities include
shale, slate, phyllite, basalt, rhyolite, andesite,
chert, and possibly schist.
 The three major types of sandstone are:
1. Quartz sandstone (also called quartz arenite) - which
is dominated by quartz
2. Arkose - which is dominated by feldspar
3. Litharenite or lithic sandstone (commonly but
imprecisely called greywacke) - which is dominated by
rock fragment grains.
 Other minerals may also be present in sands and
sandstones: gypsum grains, olivine grains, shells and
shell fragments of marine organisms (made of calcium
carbonate - calcite or aragonite), rutile, tourmaline,
zircon, garnet, kyanite, staurolite, apatite, olivine,
pyroxene, amphibole, magnetite, ilmenite, hematite,
 Heavy minerals are important indicators
which can tell us the type of rocks that
existed in the sediment source area.
Rocks are Archives of Historic Records;
Sandstone Interpretation Guide
 Sandstone textures and compositions may be
used to interpret many things about the history of
the sand, including source area lithology,
paleoclimate, and tectonic activity, processes
acting in the depositional basin, and time
duration in the basin.
Source Area Lithology
Composition of sediments gives the key information
(minerals or rock fragments present).
• Sand-sized quartz grains could come from the
weathering of source area rocks such as granite,
gneiss, or other sandstones which contain quartz
(recycled sandstones).
• Sand-sized feldspar grains could come from the
weathering of source area rocks such as granite or
gneiss.
• Sand-sized rock fragment grains come from the
weathering of fine-grained source rocks. Possibilities
include shale, slate, phyllite, basalt, rhyolite,
andesite, chert, and possibly schist.
Paleoclimate
Paleoclimate refers to the climate which existed in
the source area.
Weathering rates are of particular concern here.
• If feldspar is present in sand, it indicates that the
climate was probably arid. (Or that erosion rates
were very rapid, and that tectonic activity was
extremely high - lots of uplift, and steep slopes.)
• If quartz is the dominant mineral in the sand, the
climate was probably humid (all of the feldspars
weathered away to clay).
 If rock fragments are present in sand, it helps to
know what lithology they are.
 If they are rock types which would weather rapidly
(such as basalt or limestone fragments), the climate
was probably arid.
 If they are rock types which would be relatively
stable (shale, slate, or chert), the climate may have
been temperate to humid.
 If rock fragments are present and no rock types are
given, a good compromise answer would be
temperate climate.
Tectonic Activity in the
Source Area
Basically tectonic activity can be classified as
"active" or "passive".
Tectonically active areas include: steep slopes,
mountains close to the sea, lots of earthquakes,
tectonic uplift, and volcanic activity are taking
place.
Tectonically passive areas include: broad, flat
coastal plain, few or no earthquakes, no uplift,
and no volcanic activity.
 If sand has a lot of feldspar or rock fragments, it
probably indicates high tectonic activity.
 If sand has a lot of quartz, it probably indicates low
tectonic activity - a passive setting.
 Tectonic activity also influences sorting, time duration
in the depositional environment (and to some
extent, compositional maturity).
 High tectonic activity might produce rapid dumping of
sediments into the basin with little or no time for
sorting.
 Low tectonic activity means little uplift, low erosion
rates, and therefore little sediment supplied to the
basin.
 what sediment that is there is likely to wash around for
a long time and become well sorted and rounded,
and grains other than quartz are likely to be
destroyed (by abrasion or chemical weathering).
Processes Acting in Depositional Basin
This refers to energy levels ("high" vs. "low") and
consistency of energy. Texture gives the key information.
 Grain size: Coarse sediments generally indicate high
energy, and fine sediments indicate low energy.
 Sorting: Well sorted sediments indicate consistent,
fairly high energy levels. (Winnowing and washing.)
 Poorly sorted sediments indicate inconsistent energy
levels - rapid dumping (which might involve short
episodes of high energy), followed by low energy
conditions.
Time Duration in Depositional Environment
 Both mineralogy and texture can be used to determine
time in the depositional environment.
 Sand with abundant quartz grains suggests a long
time in the depositional environment.
 Quartz is more resistant to abrasion than feldspar or rock
fragments.
 Sand with abundant feldspar or rock fragment grains
suggests a short time in the depositional
environment.
 Textural maturity is also useful in interpreting time
in the depositional environment.
 Immature or sub-mature sediments probably
spent only a short time in the basin before
burial.
 Mature or super-mature sediments were
probably rolling around in the basin for a long
time before burial.
 Roundness is a good clue to a long time in the
depositional environment.
 Rounding of grains takes a long time; it is
more likely in a tectonically passive situation.
 Desert sands are often well rounded because of the
"sandblasting" process of wind transport. Hence, in
an arid desert, it is possible to get a well-rounded
(super-mature) arkose.
SEDIMENTARY ENVIRONMENT
 A sedimentary environment is an area on the earth's
surface where sediment is deposited as shown below. It
can be distinguished from other areas on the basis of its
physical, chemical, and biological characteristics.
 Types of sedimentary environments present on the
earth today: Continental, Transitional and Marine.
Continental Environments
Continental environments are those
environments which are present on the
continents including:
Alluvial fans are fan-shaped deposits formed
at the base of mountains. Alluvial fans are
most common in arid and semi-arid regions
where rainfall is infrequent but torrential, and
erosion is rapid. Alluvial fan sediment is
typically coarse, poorly- sorted gravel and
sand.
Fluvial environments consisting of braided and
meandering river and stream systems. River channels,
bars, levees, and floodplains are parts (or subenvironments) of the fluvial environment. Channel
deposits consist of coarse, rounded gravel, and sand.
Bars are made of sand or gravel. Levees are made of
fine sand or silt. Floodplains are covered by silt and clay.
Lacustrine environments (or lakes) are diverse; they
may be large or small, shallow or deep, and filled with
terrigenous, carbonate, or evaporitic sediments. Fine
sediment and organic matter settling in some lakes
produced laminated oil shales.
Deserts (Aeolian or aolian environments) usually
contain vast areas where sand is deposited in dunes.
Dune sands are cross-bedded, well sorted, and well
rounded, without associated gravel or clay.
Swamps (Paludal environments) Standing water
with trees. Coal is deposited.
Transitional Environments
 Transitional environments are those environments at
or near the transition between the land and the sea.
These include:
 Deltas are fan-shaped deposits formed where a river
flows into a standing body of water, such as a lake or
sea. Coarser sediment (sand) tends to be deposited
near the mouth of the river; finer sediment is carried
seaward and deposited in deeper water.
 Beaches and Barrier Islands are shoreline deposits
exposed to wave energy and dominated by sand with
a marine fauna. Barrier islands are separated from
the mainland by a lagoon. They are commonly
associated with tidal flat deposits.
 Lagoons are bodies of water on the landward side of
barrier islands. They are protected from the pounding
of the ocean waves by the barrier islands, and contain
finer sediment than the beaches (usually silt and mud).
Lagoons are also present behind reefs, or in the center
of atolls.
 Tidal flats border lagoons. They are periodically
flooded and drained by tides (usually twice each day).
Tidal flats are areas of low relief, cut by meandering
tidal channels. Laminated or rippled clay, silt, and fine
sand (either terrigenous or carbonate) may be
deposited. Intense burrowing is common. Stromatolites
may be present if conditions are appropriate.
Marine Environments
Marine environments are those environments in the
seas or oceans.
 Reefs are wave-resistant, mound-like structures
made of the calcareous skeletons of organisms
such as corals and certain types of algae. Most
modern reefs are in warm, clear, shallow, tropical
seas, between the latitudes of 30oN and 30oS of the
equator. Sunlight is required for reef growth
because of the presence of symbiotic algae called
zooxanthellae which live in the tissues of corals.
Atolls are ring-like reefs surrounding a central
lagoon
 The continental shelf is the flooded edge of the
continent. The continental shelf is relatively flat (slope <
0.1o), shallow (less than 200 m or 600 ft deep), and may
be up to hundreds of miles wide.
 Continental shelves are exposed to waves, tides, and
currents, and are covered by sand, silt, and mud.
 The continental slope and continental rise are located
seaward of the continental shelf. The continental slope is
the steep (5- 250) "dropoff" at the edge of the continent.
The continental slope passes seaward into the
continental rise, which has a more gradual slope. The
continental rise is the site of deposition of thick
accumulations of sediment, much of which is in
submarine fans, deposited by turbidity currents.
 The abyssal plain is the deep ocean floor. It is basically
flat, and is covered by very fine-grained sediment,
consisting primarily of clay and the shells of microscopic
organisms
IDENTIFY ANCIENT SEDIMENTARY
ENVIRONMENTS
Sedimentary rocks contain clues that help us to
determine the sedimentary environment in which
they were deposited millions of years ago.
 By an examination of the physical, chemical, and
biological characteristics of the rock, we can
determine the environment of deposition.
Examine hand specimens of sedimentary rocks and
describe their physical, chemical, and biological
features, and then, interpreting their possible
sedimentary environments of deposition.
Sedimentary Tectonics
• Tectonics is
• Two kinds of information can be used for
interpretations.
1. The first is the actual mineral and rock fragment
content of the rock.
2. The second is empirical (actually observed) evidence
of the QFL (Quartz, Feldspar and Lithic) composition
of sandstones derived from specifically known tectonic
regimes
1. Because many minerals form only under specific conditions it
is possible to identify source-land composition with some
confidence. Specific rock fragments, of course, tell directly
what the source-land is made of.
2. Using sediments samples from four tectonic regimes , a QFL
ternary diagram (see illustration below) shows that:
 stable craton,
 block faulted continent (including continental rifts)
 volcanic arcs and
 recycled orogens (arc-continent and continentcontinent collision mountains).
tend to be distinct, indicating that sandstone
Composition can be used, judiciously, to make tectonic
interpretations.
STRATIGRAPHY
It is primarily used in the study of sedimentary
and layered volcanic rocks.
Few basic principles have been developed for
the study of layered (and other) rocks to
determine the sequence of events recorded by
them.
By a sequence we mean placing events in
order from oldest to youngest without
knowing the exact duration of the events in
years - (Relative Age).
STRATIGRAPHY
Since sequences of such beds provide evidence of
sequences of geological events, from the strata
(their nature, distribution and structures) the
geological history of an area can be deduced.
Hence stratigraphy is also referred to as “Historical
Geology” because it deals with the history of the
rocks of the earth’s crust with special emphasis on
their approximate time of formation and the
changes they have undergone since their formation.
Information are available in the form of structural
deformations and physical
dislocations caused by
the forces from within as well as morphological
changes brought about by external forces (wind,
water, glacier etc.).
Arranging the available information revealed by
layered rocks, the geological history – sequence of
geological events in chronological order- of that span
of time might be constructed.
STRATIGRAPHY
Stratigraphy includes two related subfields:
–lithologic or lithostratigraphy and
–biologic stratigraphy or biostratigraphy.
Lithostratigraphy
• Lithostratigraphy, or lithologic stratigraphy, is the most
obvious. It deals with the physical lithologic, or rock
type, change both vertically in layering or bedding of
varying rock type and laterally reflecting changing
environments of deposition, known as facies change.
Chemostratigraphy is based on the changes in the
relative proportions of trace elements and isotopes
within and between lithologic units. Carbon and oxygen
isotope ratios vary with time and are used to map
subtle changes in the paleo-environment. This has led
to the specialized field of isotopic stratigraphy.
• Cyclostratigraphy documents the often cyclic changes
in the relative proportions of minerals, particularly
carbonates, and fossil diversity with time, related to
changes in palaeo-climates.
Biostratigraphy
Biostratigraphy or paleontologic stratigraphy is
based on fossil evidence in the rock layers.
Strata from widespread locations containing the
same fossil fauna and flora are correlatable in time.
Biologic Stratigraphy was based on William Smith's
principle of faunal succession once most powerful
lines of evidence for, biological evolution.
Biologic Stratigraphy provides strong evidence for
formation (speciation) of and the extinction of
species.
The geologic time scale was developed
during the 1800s based on the evidence of
biologic stratigraphy and faunal succession.
This timescale remained a relative scale until
the development of radiometric dating, which
gave it and the stratigraphy it was based on an
absolute time framework, leading to the
development of chronostratigraphy
Physical Principles for
defining Relative Age
Six basic principles developed by some famous
scientists are very important in stratigraphic studies
and used for defining Relative Age include:
1. Original Horizontality
2. Superposition
3. Crosscutting Relationships
4. Inclusions (Included Fragments)
5. Fossil Succession
6. Uniformitarianism
Original Horizontality
 First developed by Nicholas
Steno in the 17th century
(1600s).
 Steno determined that sand
and mud settle out of water in
relatively horizontal layers.
 Where layers of sandstone
and mudstone (and other
sedimentary rocks) are found
tilted at a large angle to the
horizontal, some force must
have acted on them after
they were deposited.
Superposition
This principle was also
proposed by N. Steno.
This principle states that
where sedimentary
layers (or volcanic ones,
for that matter) are
stacked one on top of
another, the oldest
layers are found at the
bottom of the pile.
Crosscutting Relationships
This principle was first described by
James Hutton in the late 18th century, and
is applied to rock units and geological
structures that are not necessarily
horizontal to start with.
A fault that cuts a rock layer, for example,
must be younger than that layer.
Crosscutting Relationships
Inclusions (Included
Fragments)
Another James Hutton principle, which states
that a fragment of one rock included within
another must be older than the rock that
contains it.
 Also organic remains preserved in sediments
may be older or about the same age as the
sediments within which they are found.
Inclusions (Included
Fragments)
Conglomerate
• A pebble in a
conglomerate must be
older than the
conglomerate (if you
think about this carefully
it can’t work the other
way around – a
conglomerate can’t be
made of pebbles that
didn’t exist when the
conglomerate was
deposited).
Fossil Succession
• The British engineer
William Smith
(around the year
1800) discovered
that in thick
sequences of
sedimentary rock,
the fossils they
contain change in a
systematic fashion.
Fossil Succession
These patterns is
repeated throughout
the world, and thus
by finding distinctive
fossils geologists
can correlate rocks
in widely separated
areas and determine
that they have the
same relative age
Uniformitarianism
 The assumption that sedimentary rocks were formerly
laid down by the deposition of sediments, formed in just
the same way as sediments are being formed at the
present day, are part of the philosophical approach to
geology known as uniformitarianism. This observation
is credited to James Hutton.
 The principle of uniformitarianism is best
summarized in the phase that “the present is the
key to the past”.
 It postulates that the geological processes which can
be observed today (weathering and erosion, deposition
and accumulation of sediments as well as outpouring
and solidification of lavas) have taken place in an
analogous manner in the past geological ages.
Uniformitarianism
Two geological processes:
1. Regional metamorphism or
emplacement of plutons
2. Continental drifts
are known to have taken place but cannot
however be directly observed to be
happening, are exceptions to the principle of
uniformitarianism.
Original Continuity
This principle postulates that sediments
generally accumulate in continuous
sheets. If today we find a sedimentary
layer cut by a canyon, then we can
assume that the layer once spanned the
canyon but was later eroded by a river
that formed the canyon.
Stratigraphic Classification
Stratigraphic classification encompasses all rocks of
the earth’s crust based on their physical properties
or time of their origin. Classification include:
Lithostratigraphic Units are based on the
lithographic properties or material properties of the
rock bodies.
Chronostratigraphic Units based on the time of
formation of the rock bodies.
Biostratigraphic Units
Unconformity-bound Units
Stratigraphic Classification
Two types of stratigraphic classification are used:
1. Rock Stratigraphic Units (Lithostratigraphic Units)
2. Time stratigraphic units (Chronostratigraphic
Units).
 Rock Stratigraphic Units employ distinctive
bodies of rocks that differ from the rocks above
and below in the general characteristics i.e. marker
beds. The basic unit is a formation.
Stratigraphic Classification
Time Stratigraphic Units refer to bodies of
rocks that were deposited during the same
geologic time interval. The basic unit is a
period.
Outcrop consists of various varieties of rock
Stratigraphic observation involves with the
description and nomenclature of rock bodies
based on their lithology and their relations with
each other.
Key elements of stratigraphy involve
1. understanding how certain geometric
relationships between rock layers arise.
2. what these geometries mean in terms of
depositional environment.
So the observed rock bodies are organized and
classified into lithostratigraphic units that form the
fundamentals of geologic mapping classification.
Stratification
This is the building up of layers or arranging
into layers of separate units or rocks with
fabrics and textures distinct from the directly
overlying and underlying rock.
In rocks of igneous origin, particularly volcanic
rocks, the layering may be due to flow-structure
initiated in the molten state or- on a large scaleit may be due to the accumulation of successive
lava flows.
In metamorphic processes, layering may be present due to
the re-orientation of platy minerals example being the
development of cleavages in many slates or indeed to
the creation of new minerals ‘adapted’ to the stress
conditions e.g. schistosity.
In the vast majority of rocks, conspicuous layering is
called bedding or stratification and owes its origin
to the manner in which the rocks are accumulated,
successive layers of strata –
or
beds
–
being
deposited one on top of another.
A relatively uniform and lithologically distinct layer is called a
bed; this is the fundamental sedimentary unit.
Bedding planes or bounding planes are the upper and lower
planer surfaces surrounding beds and the stratal
geometry
is overall shape of a bed or series of beds.
Thickness of Beds
Thickness Units
Lamina
Very thin bed
Thin bed
Medium bed
Thick bed
Very thick beds
< 1 cm
1 - 3 cm
3 - 10 cm
10 - 30 cm
30 cm - 1 m
>1m
Events
This refers to accumulation of sediments in one
specific episode.
An event could be a:
large storm,
landslide,
volcanic eruption, or
flood.
Events Cont.
 The uniform (or often the erratic) nature of the
sediments in relation to the surrounding sediments is
the only clue that a particular bed might have been
deposited in a single event.
 Sandstone, for instance, that is well-sorted, contains
erratic fossils (like brachiopods) and is wedged
between sandstones that are generally poorly-sorted
and contain minor siltstone layers and contains no
fossils, can be interpreted as "tempestite".
Events Cont.
Other event indicators could be lava flows, lahars,
and glacial ice-dam breaks- all of which have been
identified in the rock record. Volcanic rocks, both
lava flow and bedded volcanic tuffs, can be dated
relatively by the position in a stratigraphical
sequence.
Care must be taken to distinguish between
volcanic rocks, contemporaneous with the
deposition of the strata, from intrusive igneous
rocks, particularly sills which have been
intruded at a later date
Igneous intrusions are necessarily later than any of the
strata which they can be seen to intrude, or to
thermally metamorphose.
. Note that in some instances parts of the crustal strata
have been thrusted or have slid over the top of more
recently deposited, younger, rocks. In this situation the
structural position of the strata is no longer a guide to
their relative age.
Discuss how to determine the event(s) in a situation
like this.
Principal lithostratigraphic
units
Hierarchical order:
Supergroup
Group
Formation
Member
Bed
Formation it generally consists of a mappable unit of a
single lithology or of two or more lithologies that
is/are intimately associated, differ from rocks that
occur above or below, has/have distinctive
characteristics and fossils such that it/they can be
recognized over wide area(s).
In other words boundaries of a formation with the
overlying and underlying units may be well-defined
along clearly distinguishable contacts, or drawn
arbitrarily within a zone of gradation.
Formations are given a formal name, normally a
geographic name after the type area followed by a
simple lithologic term after the dominant lithology.
If it consists of two or more lithologies, for
example, interbedded mudstones and siltstones,
then it might be called Bimbila Formation.
If it is a single rock type, then only the rock
name is specified in the formation name, for
example the Otekpelu Limestone.
Member is a subdivision of a formation; it
possesses lithologic properties distinguishing it
from the adjacent parts of the formation such as
a variation in lithology within the unit.
A formation may or may not consist of
members, which need not be named separately
unless particularly useful for stratigraphic
purposes such as a marker bed. If requiring a
distinctive identity, it may be given a distinctive
name followed simply by the rank term. (e.g.
Basal Tillite/Conglomerate of the Buipe Member
in the Oti-Pendjari Group).
Bed is the smallest unit of a stratified sequence
and consists of a single stratum distinguishing it
from other layers above and below. A bed is
generally not given a formal name unless it is of
a distinctive name, such as a coal bed, or a
marker bed of distinctive lithology that
characterizes a stratigraphic unit of higher rank.
Group is a formal unit comprising a succession of
two or more contiguous or associated
formations with significant and diagnostic
properties in common. For example the KwahuMorago Group which is made up of Kwahu,
Yabraso, Damango, Mpraeso, Abetifi, Obocha,
and Anyaboni Formations.
Supergroup is a formal unit composed of two or
more associated groups and/or formations of
the same stratigraphic sequence, and bears a
regional geographic name. Example is the
Voltaian Supergroup that comprise of KwahuMorago, Oti-Pendjari and Obosum groups.
Voltaian Basin
Supergroup
Groups:
Voltaian
Kwahu-Morago Oti-Pendjari Obosum
Formations: Anyboni Obocha Abetifi Mpreaso Damongo, Yabraso Undifferentiated
Members: Sandstones (Arkosic, medium grained, etc.) Mudstones
Beds:
Thin, thick, laminated, fossiferous, etc.
etc.
Correlation
Rock formation and its attendant deformation and
destruction followed by re-formation are still on-going
everywhere on the earth.
To demonstrate the correspondence in stratigraphic
position between geographically separated geologic
sections or rocks bodies, difficult though it might be,
yet possible, is to link or connect or relate rocks
formed at different places during the same geological
time, as well as establishing relative age of rocks
formed in the same region at different times.
This process is called correlation and it is a
fundamental concept of great importance in
stratigraphy.
Correlation Principles Cont.
The most important kinds of evidence used for
correlation are:
 physical and
 paleontological (fossils).
The physical evidence consists of lithological
similarity and continuity of strata and its position in
the stratigraphic succession, unconformities and
metamorphism.
The paleontological evidence is based on similarity
of contained fossils – early method of
stratigraphic classification.
The latest physical methods (modern methods) are
based on radiometric age determinations and
determination of residual magnetism preserved
in rocks.
The occurrence of rocks occupying the same
position in stratigraphic or fossil sequences in
separate or different areas is known as
Homotaxis.
Divisions of rock-stratigraphic units or
biostratigraphic units are said to be
homotaxial.
Strata of the same age are called coeval.
Diachronism occurs when a lithological units that
appears to be continuous, actually represents
development of the same
facies at different times
and different
places.
Facies is the sum total of features (lithological,
sedimentological or faunal) that reflect the
specific environmental conditions under which a
given rock was formed or deposited
Elements of Correlation
These are:
Units
Lithology
Fossils
Structural Features
All the Physical Principles for
defining Relative Age
Units
Used for correlating rocks formed at
different places during the same time
and at different times in the same place.
 Certain geological events results in the
formation of rocks. Since these events
occur within certain times in geological
history, it implies that rocks are formed
at different times therefore a set of rocks
can be assigned to a particular time on
the geological time scale.
Hence there is a correspondence between
time-frame and a set of rocks formed within
the span of time on the frame.
The aspect of stratigraphy that deals with the
age of strata and their time relation is
known as Chronostratigraphy.
Categories of chronostratigraphic units and
their nomenclature are given below.
Chronostratigraphic
Units
Geochronologic Units
Eonothem
Eon
Erathem
Era
System
Period
Series
Epoch
Stage
Age
Substage
Substage
Chronozone
Zone
Simplified Time-units and Rockunits
Time-units
Era
Period
Epoch
Age
Rock-units
Group
System
Series
Stage
On the basis of time-units and rock-unit, the
geological time scale was established. The geologic
time scale is a chronologic scheme relating
stratigraphy to time, to describe the timing and
relationships between events that have occurred
during the history of Earth.
Lithology
It signifies the nature, composition and type of rock
as found in a particular formation. Thus sandstone,
limestone and shale are board rock types, which
may further be distinguished into ferruginuous
sandstone, siliceous limestone and carbonaceous
shales. Lithological similarities in formations of
rocks developed in different areas in a region form
very important elements of correlation.
Fossils
These are remains or traces of organisms that have
lived on this earth from time to time in the past.
 That is fossils are indications of former existence of
living organisms, both plants and animals. They range
from preservation of complete animal to indirect
evidence of animals.
Usually only the hard skeletal parts or shell of the
animal are preserved as fossil, although chitinous
(a hydrocarbon related to cellulose) skeletons and
carapaces (hard shell that covers or protects
animals) are commonly preserved as carbonaceous
films as also are plants. Animals lacking a skeleton
or shell are infrequently preserved as fossil.
Fossils Cont.
Another condition which must generally be fulfilled,
if an organism is to be preserved as a fossil is that:
quite soon after death it must be covered by
sediment to reduce the chance of bacterial
attack and decay. Thus organisms in water
are much more likely to become fossilized
than terrestrial creatures.
A rock may be devoid of fossil (barren) for three
reasons:
it may have been deposited in conditions
which were unable to support life
life may have existed but failed to be
preserved as fossils
life may have existed and become
fossilized yet subsequently destroyed as a
result of major changes of texture and
composition of the parent rock by
metamorphic processes.
Strata could be identified by the study of the
contained fossils supplementing the lithological
characteristics. The recognition of strata by their
fossil content is dependent on two factors:
throughout geological time there has been
repeatedly extinction of species and genera
with new ones arising to replace them. This
provided constantly changing patterns of life.
A single species (or a genus) which dies
out does not recur; extinction is complete
and final.
Structural Features
Of these, unconformities are the most important.
Unconformities indicate breaks, gaps or
“interruptions” in periods of deposition. These are of
great significance in correlating the formations.
Features such as folding, faulting and
metamorphism are also helpful in correlating rock
formations if clearly understood.
Physical Principles for
defining Relative Age
These simple but vital fundamental principles of
stratigraphy applies to all layered rocks except
those which have been subjected to tectonic
upheaval of such magnitude such that:
 the strata have been intensely folded that the
succession is in places inverted (upside
down) when the strata will still be arranged in
the original sequence, although now the
highest stratum is the oldest and
successively younger beds occur at lower
levels.
Correlation Principles
• In order for rock units to be
correlated over wide areas,
they must be determined to
be equivalent.
• Determination of equivalence
is based first on lithologic
similarity. If the rock units
have the same type of rocks
and look similar then they
may correlate.
Sometime very distinctive rocks that do not
change over large distances can be identified.
These are referred to as key beds or marker
beds.
Relative age must also be taken into account.
If rocks are equivalent they must have the same
relative age relationships to surrounding rocks in
all areas.
Finally, fossils, since they are key indicators of
relative age as well as depositional environment,
can be used to determine equivalence.
In simple terms determination of equivalence is
based on:
Relative Age - if two rock units are equivalent
they must have the same age relative to rocks
that occur below and above. The laws of
stratigraphy enable this determination.
Physical Criteria - similarity in rock type, but
only if relative age is equivalent. Key beds, like
widespread volcanic ash layers that are the
same over wide areas are often used to
establish equivalence.
Fossils present - fossils are key indicators of
relative age (life has evolved through time) and
environments of deposition.
Correlation Procedure
The following methods either in isolation or in
combination have been used widely for rock
correlation:
 Lithological and Structural Controls
Biostratigraphic Controls
Radioactive Dating Control
Lithological and Structural
Controls
In these methods attempts are
made to establish lithological
nature of the formations and their
lateral continuity, also
development of characteristic
structures such as unconformities,
folding, faulting and jointing.
This is achieved by detailed geological mapping
using conventional and where available latest tools
and techniques for surveying including geophysical
methods.
It may come out that some contacts are continuous
over considerable areas whereas others are
obliterated only locally due to changes that have
taken place subsequent to the deposition of these
formations.
Similarly, presence of unconformities demarcating
sequences with different lithological and/or structural
features is specially searched for.
Biostratigraphic Controls
• Every variety of life from singlecell organisms to highly evolved
animals and plants require a
definite set of environmental
conditions to grow, propagate and
evolve.
An environmental relationship is thus established
on the basis of fossil content of the rocks.
Fossils have been used as the greatest
biostratigraphic controls for correlating rock
formations lying at present adjacently or widely
separated from each other.
The presence of fossils of a particular type of
animal or plant kingdom in a number of rock
formations now separated is clearly indicative of the
fact that those rock formations were formed
contemporaneously.
Radioactive Dating Control
Most rocks which are unfossiliferous and also those
which are un-stratified present a lot of difficulties in
correlation.
 Under such difficult situations rocks can be
correlated on time-scale only through their
relationship with the overlying or surrounding
sedimentary formations.
Radioactive dating has been used for fixing
geological age of very old and very young rocks as
well unfossiliferous and un-stratified rocks.
Correlation Procedure
Problems of Correlation
Lithological:
 The relative ages of beds in a succession
can be determined in a limited area
according to the Law of Superposition by
the relative position of those beds.
 While it is possible by geological mapping,
to trace some beds across country and so
build up more complete successions, others
can only be traced for relatively short
distances.
Majority of beds, members and formations cannot
be traced directly along their outcrop for great
distances.
The whole geological succession cannot be seen
at any one locality or in any one section.
However, some beds known as marker horizons
or index horizons which are both of wide extent
and readily identifiable character can be
recognized at widely separated localities. If beds
were constant character, difficulties would still
arise.
Due to post-depositional tectonic structures, such as folding
and faulting of the strata , coupled with subsequent erosion,
create discontinuous outcrops.
Geological strata are generally not of a constant character
laterally, especially in areas of complex geology which had
tectonic history including several periods of orogenesis.
Area of deposition which is either shrinking or expanding, there
will be changes with time, in the distribution of the deposition of
the different lithological types.
In expanding basin of marine sediments the lowest bed will not
be of as great extent as the succeeding one, which will spread
beyond it and is said to overlap it.
Shrinking basin of deposition will result in successive beds being
of lesser extent and this is known as off-lap.
Problems of Correlation Cont.
Fossils:
 Complicated by several
factors, the most
important of which is
the lateral (or
geographical) variation
in a faunal assemblage,
which may be relatable
to observed lithological
differences only in
some cases.
Geographical factors, principally climatic, will affect
the composition of faunal.
In deposits of past geological ages are found
lateral faunal variations resulting from differences in
physical conditions.
The sum total of the physical conditions under
which a deposit is formed is known as the facies.
Facies is the sum total of features (lithological,
sedimentological or faunal) that reflect the
specific environmental conditions under which a
given rock was formed or deposited
Thus we have littoral facies, arenaceous facies, rudaceous facies
etc.
Strata of the same geological age, if of similar facies, will naturally
have very similar faunal assemblages if not the same.
Strata in geographically distant areas, the funal differences may be
conspicuous.
Rocks formed contemporaneously but of different facies will have
very different faunas.
Due to non-recurrence of fossil species, rocks formed under
closely similar conditions but at different geological periods will not
contain the same fossil assemblages.
 To be useful for stratigraphical purposes, fossils must be of
animals and plants which were readily distributed and which became
geographically widespread.
Stratigraphic Column
A succession of rocks
laid down during a
specified interval of
geologic time.
A simplified columnar
diagram relating a
succession of named
lithostratigraphic units
from a particular area
to the sub-divisions of
geologic time.
A typical stratigraphic column shows a sequence of
Formations or rocks with the oldest at the base and
the youngest on top,
the relative thicknesses of each Formation or
Group,
the Formation Name, and gives an approximate
idea of whether the rocks are hard-cliff forming units
or softer more easily erodable units.
In areas that are more geologically complex, such as
those that contain intrusive rocks, faults, and/or
metamorphism, stratigraphic columns can still indicate
the relative locations of these units with respect to one
another.
However, in these cases, the stratigraphic
column must either be:
 a structural column, in which the units are
stacked with respect to how they are observed
in the field to have been moved by the faults
 or a time column, in which the units are
stacked in the order in which they were formed.
 In the past, some areas were above sea
level and being eroded and other areas were
below sea level where deposition was
occurring. Thus, in order to develop a
complete record, correlations must be
undertaken in order to see how everything fits
together.
Breaks in the Stratigraphic Record
Earth's crust is continually changing due to uplift,
subsidence, and deformation, erosion and
deposition.
When sediment is not being deposited, or when
erosion is removing previously deposited sediment,
there will not be a continuous record of
sedimentation preserved in the rocks.
Such a break in the stratigraphic record is known as
hiatus.
Evidence of a hiatus in the stratigraphic record is
called an unconformity.
Unconformities
An unconformity is a surface of erosion or nondeposition. Three types of unconformities are
recognized:
Angular Unconformity
Disconformity
Nonconformity
Angular Unconformity
• Angular
unconformities are
easy to recognize in
the field because of
the angular
relationship of layers
that were originally
deposited
horizontally
Disconformity
• Disconformities
(sometimes called parallel
unconformities) are much
harder to recognize in the
field, because often there
is no angular relationship
between sets of
layers. Disconformities
are usually recognized by
correlating from one area
to another and finding that
some strata is missing in
one of the areas.
Nonconformity
• Nonconformities occur where
rocks that formed deep in the
Earth, such as intrusive
igneous rocks or
metamorphic rocks, are
overlain by sedimentary
rocks formed at the Earth's
surface. The nonconformity
can only occur if all of the
rocks overlying the
metamorphic or intrusive
igneous rocks have been
removed by erosion.
Variation of Unconformities
The nature of an
unconformity can change
with distance. If we are
only examining a small
area as shown, we would
determine a different type
of unconformity at each
location, yet the
unconformity itself was
caused by the same
erosional event
Divisions of the
Stratigraphical Column
Naturally occurring breaks, unconformities, had
been used to divide up the stratigraphical
column.
 Within a limited area this is a readily usable
method and hence its frequent employment.
However, since any unconformity represents a
period of time, which in the area under
consideration is not represented by strata, the
geological record is necessarily incomplete.
Thus if series of strata defined by natural breaks above
and below are equated the correlation may be erroneous.
This is because deposition may commence in one area
sooner than in another and sedimentation may be
terminated earlier in one area than the other.
Clearly it is desirable that stratigraphical divisions should
be contemporaneous, i.e. that the boundaries between such
divisions should be time-planes.
Although this may be, in fact an unattainable ideal, the
use of fossils in correlation brings a certainty to correlation
that lithological correlations generally lack.
Breaks in the Geological
Record
Every bedding plane of a sedimentary rock represents
a plane which was at one point in time the interface
between sediment and water (in the case of water-laid
sediments).
 Normally if the sediment settles in relatively quiet,
undisturbed water, these bedding will be parallel and
the rock is said to be even-bedded.
 It may be true that the bedding planes do in fact
represent very temporary pauses in sedimentation.
 Clearly much of geological time is not represented by
sediments.
Gaps in the geological succession proved by
incompleteness of the sequence are called diastems.
Whether a rock possesses few bedding planes and is
massive, or is well bedded, probably depends less on
whether sedimentation was continuous or interrupted than
on the nature of the material.
Where the particles are rounded, e.g. sand grains of
ooliths, massive sandstones and limestones occur, but
argillites composed of ultra microscopic platelets of
materials are commonly very well bedded.
Intervals in sedimentation, perhaps quite brief but long
enough to permit organisms to bore and burrow into the
semi-consolidated substrate, or to encrust its surface, are a
common occurrence in fossiliferous strata
A break in sedimentation provable by erosion surface is called an
unconformity and the younger bed is said to be unconformable on the
older.
The post-unconformity sediments frequently commence with
conglomeratic deposit. Frequently the pebbles in a conglomerate at
or near the base of a sedimentary series are fragments derived from
an older series of rocks below the plane of unconformity.
The nature of the pebbles in a conglomerate can indicate
something of the nature of the land surface supplying the material –
provides information of the paleogeography.
In the case of any angular unconformity the lowest bed of a series
of strata is seen to rest on beds of differing ages. A geological
succession which includes unconformities is an incomplete record.
The Geologic Column
No one locality on earth provides a complete record of
the entire planet’s history, because stratigraphic
columns do contain unconformities.
Over the past 150 years or more, detailed studies of
rocks throughout the world based on stratigraphic,
paleontologic, and correlation studies have allowed
geologists to correlate rock units throughout the world
and composite them into time stratigraphic unit called
the geologic column, which breaks relative geologic
time into units of known relative age as shown below.
The Geologic Time Scale
The geologic time scale is the calendar that geologists
have constructed to communicate information about the
history of the earth.
 Like a calendar it is divided into large units, which are
subdivided into smaller units, which are subdivided into
still smaller units.
 The units are not of equal length, but they do
completely fill the time in the next larger time unit.
The geologic time of the earth's past has been
organized into various units according to events which
took place in each period. Different spans of time on
the time scale are usually delimited by major geological
or paleontological events, such as mass extinctions.
For example the Cretaceous period and the Paleogene
period are defined by the extinction event, known as the
Cretaceous–Tertiary extinction event that marked the
demise of the dinosaurs and of many marine species.
Older periods which predate the reliable fossil record are
defined by absolute age. The largest defined unit of time is
the supereon, composed of eons. Eons are divided into
eras, which are in turn divided into periods, epochs and
ages. The terms eonothem, erathem, system, series, and
stage are used to refer to the layers of rock that
correspond to these periods of geologic time
In geology, we use similar principles to
determine relative ages, correlations
(connection between two or more items),
and absolute ages.
Relative ages - Principles of
Stratigraphy
Correlations – Fossils, key beds,
physical criteria
Numerical (Absolute) ages Radiometric dating
Because of the mode of formation the
deposits the oldest layers occur
below younger layers.
Thus the relative age of the layers in
each of the deposits can be
determined in order from youngest
to oldest.
Note that at this point we do not
know exactly how old any layer really
is. Thus we do not know the numerical
age of any given layer.
Given the geologic environment of deposits,
othe geologic activities in operations at the time of
deposition,
o availability of chemical elements for minerals
formation,
othe rate of mineral formation and deposition,
o pressure variation, similar features will exist in
the two deposits
if they form at the same time.
 Thus based on same or similar features
the
connection (linkage) between two or more
layers in the
two deposits can be
established by reasoning that they were
deposited during the same time period.
If any layer in one of the deposits is not found or
not seen in the other deposit, it could be that
conditions responsible for the formation of
that particular layer perhaps never prevailed
in the environment in the deposit in which it
was not found, because we assumed that no
deformation had taken place.
 The surface marking in the break in deposition
is called an unconformity in geologic terms,
and represents time missing from the
depositional record.
If there are some inclusions of other foreign
materials in the deposits they can give
clues to numerical age.
Age of earth estimated on the basis of how long it would
take the oceans to obtain their present salt content. Assumes
that we know the rate at which the salts (Na, Cl, Ca, and CO3
ions) are input into the oceans by rivers, and assumes that
we know the rate at which these salts are removed by
chemical precipitation. Calculations in 1889 gave estimate
for the age of the Earth of 90 million years.
Another attempt is to estimate the age from time required
to cool from an initially molten state. Assumptions include,
the initial temperature of the Earth when it formed, the
present temperature throughout the interior of the Earth, and
that there are no internal sources of heat. Calculations gave
estimate of 100 million years for the age of the Earth
Relative and Numerical (Absolute) Age
• Relative age - Relative means that we can determine if
something is younger than or older than something
else. Relative time does not tell how old something is,
all it tells us is the sequence of events. For
example: the sandstone in this area is older than the
limestone.
• Numerical (Absolute) age - Numerical age means that
we can more or less precisely assign a number (in
years, minutes, seconds, or some other units of time) to
the amount of time that has passed. Thus we can say
how old something is. For example: The sandstone is
300 million years old.
Numerical (Absolute)
Geologic Time
Although geologists can easily establish relative
ages of rocks based on the principles of
stratigraphy, knowing how much time a geologic
Eon, Era, Period, or Epoch represents is a more
difficult problem without having knowledge of
numerical (absolute) ages of rocks. In the early
years of geology, many attempts were made to
establish some measure of absolute geologic time.
Relative and
Numerical Age
Assume two undeformed sedimentary
chemical deposits
with sequence of layers,
though quite variable,
each layer has a
distinctive kind of
features that
distinguishes it from
other layers in the pits.
In 1896 radioactivity was discovered, and it was
soon learned that radioactive decay occurs at a
constant rate throughout time.
 With this discovery, Radiometric dating
techniques became possible, and gave us a
means of measuring absolute geologic time.
Radiometric Dating
This involves the use of radioactive decay as a
clock to determine the age in years of events in
earth history. It relies on the fact that there are
different types of isotopes.
An isotope consists of all atoms that have both the
same atomic number (number of protons) and
atomic mass (protons plus neutrons). An element
may thus have more than one isotope.
Stable and Unstable Isotopes: Some isotopes are
stable, and the nucleus of each atom will remain
unchanged unless powerful outside forces are applied to
it. In contrast, some combinations of protons and
neutrons are inherently unstable, and will break down
spontaneously. This is called radioactive decay.
Several types of radioactive decay will cause an
isotope to break down into a completely new element.
For example, radioactive carbon 14 will break down into
stable nitrogen 14.
Radioactive Decay: Process by which a radioactive
parent element loses elementary particles from its
nucleus and in doing so becomes a stable daughter
element. The rate of decay is constant for a given
element and is a very precise and accurate device for
the measurement of geologic time.
The key to understanding radioactive decay is to recognize that it
is a statistical process, and large numbers of atoms must be studied
to obtain meaningful results.
Radioactive Isotopes are isotopes (parent isotopes) that
spontaneously decay at a constant rate to another isotope.
Radiogenic Isotopes - isotopes that are formed by radioactive
decay (daughter isotopes).
The rate at which radioactive isotopes decay is often stated as the
half-life of the isotope (t1/2). The half-life is the amount of time it
takes for one half of the initial amount of the parent, radioactive
isotope, to decay to the daughter isotope.
Numerical (Absolute) Dating
and Geologic Time Scale
 Radiometric dating can be used to date the time when
igneous rocks formed and when metamorphic rocks
metamorphosed, but not when sedimentary rocks were
deposited. So how do we determine the numerical age of
sedimentary rocks? It is achievable by adding numerical age
to the geologic column. Hence geologists obtain ages of
sedimentary rocks by using the methods of numerical dating
of metamorphic or igneous rocks or both, and their crosscutting relationships with the sedimentary rocks, eventually
being able to establish the numerical (absolute) times for the
geologic column. For example, imagine some cross section
such as that shown below. From the cross-cutting
relationships and stratigraphy we can determine that:
The Oligocene rocks are younger than the 30 m.y old
lava flow and older than the 20 m.y. old lava flow.
The Eocene rocks are older than the 57 m.y. old dike and
younger than the 36 m.y. old dyke that cuts through them.
The Paleocene rocks are older than both the 36 m.y. old
dike and the 57 m.y. old dike (thus the Paleocene is
younger than 57 m.y.
By examining relationships like these all over the world, the
Geologic Time scale has been very precisely correlated with the
Geologic Column. But, because the geologic column was
established before radiometric dating techniques were available, note
that the lengths of the different Periods and Epochs are variable.
The dated geologic column is called the geologic time table an
example given below:
The Age of the Earth
Theoretically by dating the oldest rock that occur, we
should be able to determine the age of the earth.
The oldest rock found and dated has an age of 3.96
billion years.
So is this the age of the Earth? Probably not,
because rocks exposed at the earth's surface are
continually being eroded, and thus, it is unlikely that the
oldest rock will ever be found.
But, we do have clues about the age of the earth from
other sources:
Meteorites - These are pieces of planetary material that
fall from outer space to the surface of the earth. Most of
these meteorites appear to have come from within our
solar system and either represents material that never
condensed to form a planet or was once in a planet that
has since disintegrated. The ages of the most primitive
meteorites all cluster around 4.6 billion years.
Moon Rocks - The only other planetary body in our solar
system that we have samples of are moon rocks (samples
of Mars rocks have never been returned to earth). The
ages obtained on Moon rocks are all within the range
between 4.0 and 4.6 billion years. Thus the solar system
and the earth must be at least 4.6 billion years old.
 Thus evidence from radiometric dating of other sources
materials indicate that the Earth is about 4.570 billion
years old. However, we do not find 4.6 billion-year-old
rocks in the crust because during the first half-billion
years of earth history, rocks in the crust remained too hot
for radiometric clock to start (their temperature stayed
above the blocking temperature). Geologists have named
the time interval between the birth of the earth and the
formation of the oldest dated rock the Hedean Eon.
Petroleum Reservoir
Stratigraphy is also commonly used to delineate the
nature and extent of hydrocarbon-bearing reservoir
rocks, seals and traps in petroleum geology.
A petroleum reservoir or an oil and gas reservoir
is a subsurface pool of hydrocarbons contained in
porous or fractured rock formations. The naturally
occurring hydrocarbons are trapped by overlying
rock formations with lower permeability.
 Traps: The traps required in the last step of the reservoir
formation process have been classified by petroleum geologists
into two types: structural and stratigraphic. A reservoir can be
formed by one kind of trap or a combination of both.
 Structural Traps (Fold and Fault): Structural traps are formed by
a deformation in the rock layer that contains the hydrocarbons.
Domes, anticlines, and folds are common structures. Faultrelated features also may be classified as structural traps if
closure is present. Structural traps are the easiest to locate by
surface and subsurface geological and geophysical studies. They
are the most numerous among traps and have received a greater
amount of attention in the search for oil than all other types of
traps.
An example of this kind of trap starts when salt is
deposited by shallow seas. Later, a sinking seafloor
deposits organic-rich shale over the salt, which is in turn
covered with layers of sandstone and shale. Deeply buried
salt tends to rise unevenly in swells or salt domes, and
any oil generated within the sediments is trapped where
the sandstones are pushed up over or adjacent to the salt
dome.
Stratigraphic Traps: Stratigraphic traps are formed when
other beds seal a reservoir bed or when the permeability
changes (facies change) within the reservoir bed itself.
Stratigraphic traps can form against either younger or
older time surfaces.
Palaeontology
Palaeontology is the science concerned with fossil
animals and plants and includes the reconstruction
of organisms from their fossil remains and the study
of the processes of fossilization.
The entire universe is divided into kingdoms:
 Plant
Kingdom,
 Animal Kingdom and
Mineral Kingdom.
 The first two come in the ambit of paleontology. Paleontology is a rich field, filled with a
long and interesting past and an even more hidden and hopeful future. It is not just the
study of fossils but much more. Paleontology is traditionally divided into various
subdisciplines:
 Micropaleontology: Study of generally microscopic fossils, regardless of the group
to which they belong.
 Paleobotany: Study of fossil plants; traditionally includes the study of fossil algae
and fungi in addition to land plants.
 Palynology: Study of pollen and spores, both living and fossil, produced by land
plants and protists.
 Invertebrate Paleontology: Study of invertebrate animal fossils, such as mollusks,
echinoderms, and others.
 Vertebrate Paleontology: Study of vertebrate fossils, from primitive fishes to
mammals.
 Human Paleontology (Paleoanthropology): The study of prehistoric human and
proto-human fossils.
 Taphonomy: Study of the processes of decay, preservation, and the formation of
fossils in general.
 Ichnology: Study of fossil tracks, trails, and footprints.
 Paleoecology: Study of the ecology and climate of the past, as revealed both by
fossils and by other methods.
 Hence, paleontology is the study of what fossils tell us about the ecologies of
the past, about evolution, and about our place, as humans, in the world.
Paleontology incorporates knowledge from biology, geology, ecology,
anthropology, archaeology, and etc. to understand the processes that have led
to the origination and eventual destruction of the different types of organisms
since life arose.
 The animal kingdom is divided into two:
 The Protozoa ( single-celled animals) and
 The Metazoa (multi-celled animals).
 Though sponges are multicellular they are not usually included in the Metazoa.
 Though fossils are the remains of once-living organisms, animal and plant, which
typically comprise the bodies or parts of the organisms themselves, it also
include any traces of their activities and their faeces. The term is usually
restricted to the remains of organisms that lived prior to the Ice Age.
 Fauna is the entire animal population of a particular geological time, formation,
region or habitat. Flora is the entire plant population of a particular geological
time, formation, region or habitat. Biota refers to the entire animal and plant life of
any unit
Processes of Fossilisation
As the organism dies and is buried in sediments, it generally
gets destroyed. But its more durable parts are liable to get
preserved. The shells of molluscs and brachiopods, the
exoskeletons of crustaceans and trilobites, the skeletons of
echinoderms, the structures of corals and bryozoa, the
bones and teeth of vertebrates, and the woody parts of
plants have the chance to get preserved as fossils.
Soft parts are rarely preserved, when even the most
delicate structures are preserved as in case of some
feathers of birds and worms, inserts trapped inside fossil
resins called amber, and the entire bodies of mamnoths
buried in snow. Fossils are mostly found in sedimentary
rocks but in rare cases remains have been preserved in
metamorphosed sedimentary rocks and entrapped in lavas.
 The main processes of fossilisation are:
1. A mineral substance such as silica and carbonates present in
saturated form in the sediment water, completely replaces the hard
parts, a process known as petrification. Calcification occurs when
organic material, generally hard parts are replaced by calcite (calcium
carbonate).
2. A shell gets completely filled with sediment and then gets dissolved,
leaving a mould.
3. The original shell is replaced by a different mineral substance that
produces a pseudomorph.
4. A shell or plant leaves its exterior shape impressed on the sediment.
5. Plant materials are replaced by a film of carbon preserving the
structure.
The study of process by buried, decay and preservation that affect animal
and plant remains as they become fossilized is called taphonomy.
Classification and
Nomenclature of Fossils
In order to make sense of the diversity of
organisms, it is necessary to group similar
organisms together and organize these groups in a
non-overlapping hierarchical arrangement.
 Classification is therefore, identification, naming,
and grouping of organisms into a formal system
based on similarities such as internal and external
anatomy, physiological functions, genetic makeup,
or evolutionary history, making things easier to find,
identify, and study.
Scientific classification groups all plants and animals on the basis of
certain characteristics they have in common. Scientific classification uses
Latin and Greek words to give each animal and plant two names (similar
to a first and last name) that identify the animal or plant.
The science of naming and classifying animals and plants, both living and
fossil is called Taxonomy [Greek taxis, arrangement or order, and nomos,
law, or nemein, to distribute or govern]. In a broader sense it consists of
three separate but interrelated parts: classification, nomenclature, and
identification.
Probably the first scientific study of plants and animals was the attempt to
classify them. Because of the limited knowledge at first artificial
classifications begun. These simple categories merely catalogued in a
tentative way, the rapidly accumulating material, in preparation for a
classification based on natural relationships. Modern taxonomic
classification, based on the natural concepts and system of the Swedish
botanist Carolus Linnaeus, has progressed steadily since the 18th
century, modified by advances in knowledge of morphology, evolution, and
genetics.
Scientists classify organisms using a series of hierarchical
categories called taxa (taxon, singular).
This hierarchical system moves upward from a base containing a
large number of organisms with very specific characteristics. This
base taxon is part of a larger taxon, which in turn becomes part of
an even larger taxon. Each successive taxon is distinguished by a
broader set of characteristics.
The base level in the taxonomic hierarchy is the species. Broadly
speaking, a species is a group of closely related organisms that are
able to interbreed and produce fertile offspring . On the next tier of
the hierarchy, similar species are grouped into a broader taxon
called a genus (genera, plural). The remaining tiers within the
hierarchy are formed by grouping genera into families, then families
into orders, and orders into classes. This conforms with the
Linnaean system of classification widely accepted and is still the
basic framework for all taxonomy in the biological sciences today.
The Linnaean system uses two Latin name categories,
genus and species , to designate each type of
organism.
A genus is a higher level category that includes one or
more species under it. Such a dual level designation is
referred to as a binomial nomenclature or binomen.
genus
species
genus
species
species
species
Linnaeus also created higher, more inclusive classification
categories.
order
family
genus
family
genus
genus
genus
species species species species species species species species
Hierarchical Order
The hierarchical order is:
• kingdom
• phylum (division for plants)
• class
• order
• family
• genus
• species
All living things are organized into a family tree starting with five "Kingdoms".
1. Bacteria (Monera)
2. Protista
3. Fungi
4. Plants and
5. Animals
The next level of the family tree under each of these kingdoms is
called "Phyla". The plural of phylum is phyla.
The animal (animalia) kingdom for example is divided into
approximately 38 smaller phyla branches of the tree.
The next levels down the tree are "Class", Subclass", "Order",
"Suborder", "Family", "Genus", and "Species".
For the "Paramecium" which is a common pond microscopic
animal.
Kingdom: Animalia (Protista)
Phylum: Ciliophora
Class: Ciliatea
Subclass: Rhabdophorina
Order: Hymenostomatida
Suborder: Peniculina
Family: Parameciidae
Genus: Paramecium
Species: aurelia, bursaria, or caudatum .............
 Phylum is a major group or category or taxon, of organisms with a common design
or organization. This design is shared by all members of the phylum, even though
structural details may differ greatly because of evolution. The assumption is made by
biologists that all members of a phylum have a common ancestry. It is an arbitrary
grouping; that is, it is developed from a combination of scientific observation,
theorizing, and guesswork in an attempt to find order in the complexity of living and
extinct life forms. The same is true of all classification levels above and below it
except for species, which consist of organisms known to be capable, at least
potentially, of interbreeding (Species).
 Genus, is a group of species closely related in structure and evolutionary origin. The
position of a genus, in classification of the kingdoms of living forms, is below family
or subfamily, and above species.
 A genus name always differs from the name used for any other genus of living
forms. An organism is named by assigning it a binomial, consisting of a genus name
followed by a species name. In zoological nomenclature, the genus and species
names may be identical; the gorilla, for example, is Gorilla gorilla. In botanical
nomenclature, the genus name may never be assigned as a species name. The
scientific name applied to a family is always a modification of the name of one of the
genera; the genus involved is termed the type genus.
Nomenclature is "the branch of taxonomy concerned with
the assignment of names to taxonomic groups in
agreement with published rules.“
The method of naming organism, living or fossil, is
binomial, the first is the name of the genus (that is
capitalized), followed by the generic name (which is
always in lower case even if named after a person or
locality).
The names are in Latin form. Both are printed in italics
(or written underlined). The other hierarchical rank terms
i.e. family and above, are also capitalized but informal
reference is in lower case. Examples:
Brachiopoda/brachiopod; Gastropoda/gastropod;
Gymnospermae/gymnosperm.