gy 308 strati lecture notes

PRINCIPLES OF PRINCIPLES OF STRATIGRAPHYSTRATIGRAPHY

• The name STRATIGRAPHY is based on

• STRATA = STRATUM = LAYER

• GRAFIA= STUDY/ARRANGEMENT

• IT DEALS WITH THE WAY ROCKS ARE ARRANGED IN SPACE AND TIME IN/ON THE EARTH CRUST

OBJECTIVES OF GY OBJECTIVES OF GY 308308

• The objectives of this course are:

• to establish and understand the way rocks are arranged vertically and laterally in the earths crust

• to establish and understand the processes that led to that arrangement and the was those processes changed spatially and temporally.

HISTORICAL HISTORICAL DEVELOPMENT OF THE DEVELOPMENT OF THE

PRINCIPLES OF PRINCIPLES OF STRATIGRAPHYSTRATIGRAPHY

• Earliest documented records on the knowledge of STRATIGRAPHY is that of the Greeks

Xanthus, Herodotus (400 BC), Aristotle, Pythagoras etc. They observed that rocks have certain arrangement patterns and some rocks had marine and others land fossils. They interpreted that ocean-land configuration has been changing with time. Hence, the present is the key to the past

The Renaissance The Renaissance PeriodPeriod

• The ideas of the Greeks were lost during the early Christian era as the history of the earth was explained by the Creation Story of the Bible.

• Around 1500 however Leonardo da Vinci revived the principles of uniformitarianism and that of ever changing land-sea configuration. No one listened

Steno 1669 Steno 1669

• Original Horizontality and Cross cutting. It says that:

• Sedimentary rocks are deposited horizontally,

• The oldest being at the bottom and become successfully young upwards.

• A younger feature/rock will cut through an older feature/rock

•

Werner, 1700sWerner, 1700s

• Classified the rocks into four groups:• I Alluvium: Formed after receding of

Noah’s flooding• II Secondary: Soft rocks with fossils

also sills dikes, etc formed when

Noah’s flood was receding• III Transition: Limestone,

greywacke, formed during flooding.• IV Basement: Hard rock formed

when God created the earth.

James HuttonJames Hutton

• In the 1700 James Hutton a Scottish,

Advanced the theory of uniformitarianism and that igneous rocks were formed from a melt.

CUVIER 1800

Found that there was a change of fossil types along a lithological succession and proposed that successive lithological units were of successively different ages.

Hence fossils could tell about the age and can be used to establish rocks of the same age.

PRINCIPLES STILL PRINCIPLES STILL VALID TODAYVALID TODAY

The Principles of Stratigraphy are based on:

• Law of Original Horizontality,

• Law of Superpostion and Cross-cutting

• Law of Uniformitarianism

• Law of Inclusion

• Law of Faunal Evolution, Succession and Correlation

• MEASURING GEOLOGIC TIME

• It is now known that rocks and other geological features were formed sequentially (not instantaneously) .

• The question now is WHEN

• The process of assigning an age to a rock or geological feature is known as

GEOLOGICAL AGE DETERMINATION OR GEOLOGICAL DATING

RELATIVE AGE RELATIVE AGE DETERMINATION DETERMINATION

• Here the ages of the rocks are only compared.

• Methods used include:– Superposition– Cross-cutting– Particle inclusion– Fossil types– Degree of

deformation/metamorphism

• Episodes

• 1. deposition of 1,

Fossil datingFossil dating

• It is possible to determine a relative age of rocks on the basis of fossils because;

- Organisms evolve and extinct

- Fossils are incorporated in the cotemporary sediment

Degree of Degree of deformation/metamorphisdeformation/metamorphis

mm• For two rock types A and B

which are nearby; A being more deformed/metamorphosed than the B, then A is normally older. – Reason; deformation occurred before formation of rock B

ABSOLUTE AGE ABSOLUTE AGE DETERMINATIONDETERMINATION

• With this method it is possible to determine the absolute age of a rock/geological feature i.e. the number of years since it was formed

• Methods include: radiometric dating, Fission track, magnetostratigraphy, counting natural rhythmic signatures eg tree rings, glacial rhythimic layers eg varves,

RADIOMETRIC DATING RADIOMETRIC DATING • Some isotopes are radioactive,

their nuclear decay at a specific rate.

• N = Noe-tλ -------- 1

where N = number of isotope atoms at time t

No = number of isotope atoms at the beginning

e = 2.718 (mathematical constant)

λ = decay constant = 0.693/thl

thl = half life of an isotope

Replacing λ in equation 1 you get

t = thl/0.693*ln N/ No, No = N+ atoms of daughter

isotope

• N = Noe-tλ -------- 1

where N = number of isotope atoms at time t

No = number of isotope atoms at the beginning

e = 2.718 (mathematical constant)

λ = decay constant = 0.693/thl

thl = half life of an isotope

Replacing λ in equation 1 you get

t = thl/0.693*ln N/ No, No = N+ atoms of daughter

isotope

Parent Daughter

Half life

Minerals in which isotopes occur

Dating range

147 Sm 143Nd

106 Bill

Garnets micas All range

87 Rb 87Sr

48.8 Bill

Potassium-bearing minerals (micas, feldspar, hornblende)

All range

238 U 206Pb

4.5 Bill

Uranium-bearing minerals (zircon, uraninite)

All range

40 K 40Ar

1.3 Bill

Potassium-bearing minerals (micas, feldspar, hornblende)

All range

235 U 207Pb

713 Mill

Uranium-bearing minerals (zircon, uraninite)

All range

14 C 14N

5,730 Organic carbon – bearing material

70,000

What is dated in What is dated in radiometric datingradiometric dating

• Igneous rocks, dykes, sills, veins, chemical sediments - date since crystallization

• Metamorphic rocks - time since

cooling or end of metamorphism

• 14C = time since death of organism

Procedure in Procedure in radiometric datingradiometric dating

1. Collect fresh, un-weathered rock sample

(No leakage of parent or daughter isotope)

2. Crash the rock and separate minerals

3. Separate and quantify the isotopes using a mass spectrograph

Assumptions and errorsAssumptions and errors

• That the decay constant of isotopes has been the same.

• That the ratio of parent/daughter at to was the same as today

• No leakage of isotopes in or out.

• No significant analytical errors

COUNTING RYTHMIC COUNTING RYTHMIC SIGNATURESSIGNATURES

• Some natural processes create features which depend on seasonality e.g warm and cold, dry and wet, climatic conditions etc will form dark and bright layers.

• Such rhythmic signatures include: tree growth rings, glacial varves, growth rings in mollusks etc

Tree Growth RingsTree Growth Rings

Glacial varvesGlacial varves

Growth rings in Growth rings in mollusksmollusks

Magneto-stratigraphyMagneto-stratigraphy

• The earth’s magnet POLARITY has been reversing from N-S-N-S with time.

• Iron-bearing minerals formed at a particular polarity will acquire and maintain that polarity.

• A reference polarity column has been constructed and used for comparison

12_21.jpg12_21.jpg

Fission TrackFission Track

• In some minerals like apatite, the ejection of an atom particle during the decay of radioactive isotope damages the nearby crystal lattice, creating a line called FISSION TRACK . The number of the tracks is proportional to the age

12_22.jpg12_22.jpg

GEOLOGICAL TIME GEOLOGICAL TIME SCALESCALE

• The major stratigraphic sections have been measured, named and made reference sections. Example are

• A sequence of rock strata at Cambria, Jura Mts, Ordovis, Devon, Perm etc. These sequences were then known as:

• Cambrian, Jurassic, Ordovician etc

• The exact ages of these sequences were determine after the discovery of radiometric dating.

• The age of the earth was subdivided into smaller time intervals corresponding to these rock sequences. The subdivided total age is known as GEOLOGICAL TIME SCALE

Geo-chronological UnitGeo-chronological Unit• Time interval during which a

specific rock sequence was formed. E.g. Jurassic Period is the time between 200 Ma - 145 Ma ago which was originally defined by a rock sequence at Jura Mts in France (Type section).

• Any rock anywhere in the world formed during this time will also belong to this Geo-chronological Unit.

Chronostratigraphic Chronostratigraphic UnitUnit

• This is a rock sequence formed during a specific geochronlogic unit.

Relationship: Chronologic Relationship: Chronologic and Chronostratigraphic and Chronostratigraphic

unitsunits

Chronologic units (Time)

Chronostratigraphic units (Rock)

Eon (e.g.) Phanerozoic

Eonothem

Era (e.g) Mesozoic

Erathem

Period (e.g) Permian

System

Epoch (e.g) Miocene

Series

Age (e.g) Early

Pleistocene

Stage

Stratigraphic Stratigraphic CategoriesCategories

• Rocks sequence (Strata) can be classified under the following categories:

• LITHOLOGY = LITHOSTRATIGRAPHY

• FOSSIL CONTENT= BIOSTRATIGRAPHY

• AGE = CHRONOSTRATIGRAPHY

LITHOSTRATIGRAPHIC LITHOSTRATIGRAPHIC CLASSIFICATIONCLASSIFICATION

Classification of rocks on the basis of their lithological characteristics: such as rock type, mineralogy, colour, chemistry, grain size, sorting, geological structures, fosils etc.

LITHOSTRATIGRAPHIC UNIT

It is a body of a rock unified by having a certain lithological characters which are distinctively different from the surrounding ones. Example igneous, metamorphic, sandstone

LITHOSTRATIGRAPHILITHOSTRATIGRAPHIC UNITC UNIT

• Lithostratigraphic unit are sub-divided into:

• Complex

• Group Super Group

Sub-group

• Formation

• Member

• Bed

FormationFormation

• It is a formal lithostratigraphic unit• It should be mappable and clearly

identified over a large area with clear upper and lower boundaries

• Its thickness may vary from few m to 1000s of m

• Composed of igneous, or sedimentary or metamorphic or a combination

• It should have a Geographical name after a river, town, village, valley mountain e.g. Mchuchuma Formation

Mbuyura Mbuyura FormationFormation

MBUYURA FORMATIONMBUYURA FORMATION

MemberMember

• This is part of a Formation

• It posses lithologic characteristics distinguishing it from the rest of the Formation

• Not all Formations are subdivided into Members

Bed Bed

• Layer(s) characterized by certain lithological characteristics eg. Green Beds or Red Beds

• Thickness is > 2 cm to a few meters

GroupGroup

• It is formed by two or more contiguous Formations with unifying lithologic features which are different from the surrounding rocks, eg Karoo Group

• The name of a Group should come from a geographical feature where or near the location of the type section.

ComplexComplex

• It is a lithostratigraphic unit composed of any class of rocks and characterized by highly complicated structure to the extent that the original sequence of the component rocks may be obscured.

• Example: Usagaran Complex or Basement Complex

Procedure of forming Procedure of forming lithostratigraphic unitslithostratigraphic units

1. Map the area and draw lithostratigraphic column

2. Subdivide the column into possible units starting from FORMATION then Member, Beds, Group etc

3. Select best exposures, establish grid ref. and describe the section in detail –These sections will form the TYPE SECTION of each unit at corresponding TYPE LOCALITY.

4. Give the units Geographical names

5. Determine the age

6. Extend the unit to the other areas by establishing equivalent units

BIOSTRATIGRAPHIC BIOSTRATIGRAPHIC CLASSIFICATIONCLASSIFICATION

Is the classification or subdivision of rock strata/ bodies into units on the basis of their fossil contents.

Biostratigraphic units are bodies of rock strata unified by having similar type(s) of fossils which are distinctively different from those around that rock body.

Types of Types of Biostratigraphic UnitsBiostratigraphic Units

• Biostratigraphic units = Biozones

• 1. Assemblage Zone

• 2. Range Zone

• 3. Acme Zone

• 4. Interval Zone

Assemblage ZoneAssemblage Zone

• Assemblage Zone is a body of rock strata unified by having certain kinds of naturally occurring fossils which are distinctively different from those in the adjacent rock body.

• Boundaries of A.Z. are the outer limits of occurrence of such assemblage.

• A.Z is named after two or more prominent members, eg Eponides-Planorbulinella Assemblage. Zone

Assemblage ZoneAssemblage Zone

Taxon Range ZoneTaxon Range Zone

• Is a body of rock strata representing a total range of occurrence (vertical and horizontal) of a particular Taxon (species, genus, family etc)

Concurrent Range Concurrent Range ZoneZone

• It is a body of rock strata defined by having concurrent or coincident parts of the range zone of two or more specified taxons selected from the total forms contained in a sequence of strata.

• It is named after at least two concurrent members defining the zone.

CONCURRENT CONCURRENT RANGE ZONERANGE ZONE

Acme ZoneAcme Zone

• Is a body of rock strata representing the interval where there was maximum development of a specific Taxon.

• It may be either abundance of specimen of a taxon or a number of species in a genus.

• It is sometimes known as a peak zone or flood zone.

• It is named after the taxon eg. Didymograptus Acme-zone

Interval ZonesInterval Zones

• Is an interval between two distinct biostratigraphic horizons (surfaces, boundaries)

• Such a zone is NOT a range zone or concurrent zone of any Taxon.

• It is used for correlation purpose and

• It is named after the names of the boundaries eg Globigerinoides sicanus/Orbulina saturalis

Problems in Problems in biostratigraphybiostratigraphy

• Due to poor fossil preservation in some cases, fossil identification may be erroneous.

• Fossils may not be regularly distributed in a stratum and hence samples taken from a barren point will not be a representative sample for the whole stratum

• There may be fossil infiltration hence younger fossils in older formation

• There may be fossil reworking, hence older fossils in younger formation

CHRONOSTRATIGRACHRONOSTRATIGRAPHYPHY

• Def.: It is the subdivision of the stratigraphic sequences into units formed at the same time.

• Chronostratigraphic Unit is all bodies of rock strata formed at the same time. They maybe continuous or isolated. Hence Permian System are all rocks in the world formed between 300 – 251 Ma

Relationship: Chronologic Relationship: Chronologic and Chronostratigraphic and Chronostratigraphic

unitsunits

Chronologic units

Chronostratigraphic units

Eon (e.g.) Phanerozoic

Eonothem

Era Mesozoic

Erathem

Period Permian

System

Epoch Miocene

Series

Age Early Pleistocene

Stage

Chron Chronozone

Subdivision of Subdivision of Chronologic and Chronologic and

Chronostratigraphic unitsChronostratigraphic units

Chronologic units

Chronostratigraphic units

Late Permian

Middle Permian

Early Permian

Upper Permian

Middle Permian

Lower Permian

Relationship between Relationship between Chronostratigraphy and Chronostratigraphy and BiostratigraphyBiostratigraphy

• Hence all rocks can be classified Lithostratigraphically and chronostratigraphically but not all rocks can be classified biostratigraphically

Stratigraphic boundariesStratigraphic boundaries

• Lithostratigraphic Boundary

It is a surface separating two types of lithologies. It can be abrupt or gradational.

• Biostratigraphic Boundary

It is a surface, sometimes not visible, separating rocks with different bio characteristics. The boundary is abrupt.

Stratigraphic Stratigraphic boundariesboundaries

• Chronostratigraphic Boundary

It is a surface, sometimes not visible, separating rocks with different ages. The boundary is abrupt. Chrostratigraphic boundary is said to be isochronous.

• Two or all these three boundaries may sometimes coincide. Examples:

Multi-type BoundariesMulti-type Boundaries

• The lower boundary of a volcanic ash layer is isochronous hence it is both a litho- and chronostratigraphic boundary.

• A boundary of a coral limestone is both a litho- and biostratigraphic boundary

• The lowers side of deep sea ooze is a ltho-, chrono- and bio-stratigraphic boundary

STRATIGRAPHIC STRATIGRAPHIC CORRELATIONCORRELATION

• Stratigraphic correlation is establishing the time equivalence of rock sequences at different localities.

• Thus, it is establishing which rock strata at different areas were formed at the same time.

Ways of correlating Ways of correlating rock stratarock strata

1. LITHOLOGICAL CORRELATIONa) Here a bed or layer of the same

lithology is considered to be formed at the same time . In some cases it may not be true.

b) Lateral continuity of rock strata The continuity of a rock strata can

be established by i) walking along the strike

ii) use air photo or satellite

imagery

A typical outcrop A typical outcrop sectionsection

Walk along the outcropWalk along the outcrop

Use of aerial Use of aerial photographsphotographs

Satellite imagerySatellite imagery

Ways of correlating rock Ways of correlating rock stratastrata

c) Use the similarity in lithostratigraphic succession pattern in the outcrops at different localities

d) Using the principle of lateral facies changes. For example coastal sand will interfinger laterally with reef limestone and then with deep water silt/clay

Ways of correlating rock Ways of correlating rock stratastrata

2. PALEONTOLOGICAL METHOD

Fossils are useful tools in age determination hence they are widely used in correlation sometimes in conjunction with lithology and sometimes alone

Ways of correlating Ways of correlating rock stratarock strata

3. RADIOMETRIC AGES 3. RADIOMETRIC AGES METHODMETHOD

• Absolute ages are of course the most accurate in correlating. This is useful in rock sequences with igneous and metamorphic rocks. The Chimala Rocks sequence is correlateble with the Uha Formation which are middle part of the Bukoban System. Corelation was based on Radiometric dates from the Andesites

4. SEISIMIC METHOD4. SEISIMIC METHOD

• This method is used to get detailed subsurface stratigraphic continuity.

• Seisimic survey is normally complemented with borehole drilling

Stratigraphic configuration Stratigraphic configuration from seismic studiesfrom seismic studies

Scales of correlationScales of correlation

• Correlation within a short distance will give arrangement of rock strata as they were deposited in a depositional basin plus post-depositional modifications. Hence a basin configuration is obtained.

• Long distant Correlation normally give contemporary systems formed in different basins unless the basin was very extensive eg. Oceans.

Reasons for Stratigraphic Reasons for Stratigraphic CorrelationCorrelation

• Establish the patterns of geological processes operating on the earth’s surface/crust spatially and temporally

• Determine the paleogeographic configuration of a certain place in a certain time.

FACIES ANALYSISFACIES ANALYSIS• Facies is a term used to define

or name a rock with respect to its lithology or/and the process of its formation eg sandy facies, shallow water facies, alluvial facies etc.

• Facies Analysis is studying the rocks from limited exposures with the aim of establishing lateral and vertical relationship between them.

Facies changesFacies changes• Lateral variation of rock types is a

reflection of lateral variation of the processes that were responsible for the formation of those rocks.

• At any given time sediments are being deposited in various sub environments within a depositional basin.

• With time, depositional environment shifts laterally leading into vertical changes of rock type

Lateral Variation of Lateral Variation of FaciesFacies

Meandering river depositsMeandering river deposits

DeltaDelta

Stratigraphic Stratigraphic RelationshipRelationship

• Because of changes in depositional environment both laterally and temporally, one rock type will pass laterally and also vertically into another rock type.

• A body of rock with essentially uniform lithology is called a LITHOSOME.

Lithosome, Lithotope & Lithosome, Lithotope & LithofaciesLithofacies

• Lithosome = A body of rock with essentially uniform lithology

• Lithotope = sediment types formed by a specific depositional environment at a given time e.g mud lithotope and sand lithotope.

• Litho facies is a general term for any lithological unit serving a particularly definition e.g sandy facies >50% sand < 50% silt

Boundaries between Boundaries between LithosomeLithosome

• Horizontal Boundaries• a) Abrupt boundaries

(contacts) such as:• Erosional• Conformable primary contacts

are formed in areas with very slow

rates of deposition• Those formed by secondary

post- depositional processes eg

dolomitization

BoundariesBoundaries• b) Gradational contacts

- Continuous mixed gradational

- Intercalated gradational

Vertical Boundaries

Abrupt

a) Fault

b) Erosional

c) Pinch out and intertonguing

d) Gradational

Facies Changes Facies Changes (Lateral)(Lateral)

DeltaDelta

Facies RelationshipFacies Relationship

Lateral + vertical facies Lateral + vertical facies build upbuild up

Facies RelationshipFacies Relationship

Presentation of Facies Presentation of Facies ModelsModels

• Cross-sections Models

• Geological Litho-Maps (isolith Maps)

• Isopach and Isobath Maps

• 3 Dimension Model

Cross-sectionsCross-sections

MapsMaps

• Maps are 2D presentation geological features when projected on a horizontal plane. There are 3 types of maps used in presenting facies

• i) Geological maps

• Ii) Isolith maps

• Iii) Isopach maps

• Iv) Isobathy maps

Geological MapsGeological Maps

• 2D qualitative presentation of rock types (lithologically) as they intersect the earth’s surface

Geological mapGeological map

Geological MapGeological Map

Isolith MapIsolith Map

• 2D quantitative presentation of rock types (lithologically) on a horizontal plane.

• Here the lithological characteristics are quantified (e.g. sandstone/mudstone ratios) and plotted as contours.

• Contours are lines joining points with the same values.

Isolith mapIsolith map

Isopach and Isobath Isopach and Isobath mapmap

• Isopach map is a map with contours joining points with same thickness of a specific rock stratum

• Isobath map is a map with contours joining points with same depth from the earth’s surface to a specific surface at depth e.g. from the surface to the basement

Isobath MapsIsobath Maps

3D - BLOCK 3D - BLOCK DIAGRAMSDIAGRAMS

• This is a kind of facies presentation in three dimension.

• In 3D presentation it is possible to see how rock types vary laterally and vertical

3D - BLOCK DIAGRAM3D - BLOCK DIAGRAM

3D - BLOCK 3D - BLOCK DIAGRAMDIAGRAM

Coastal Margin Facies Coastal Margin Facies DevelopmentDevelopment

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

++++++++++++++++++++

Rift Basin Facies Rift Basin Facies DevelopmentDevelopment

Steps in Developing a Steps in Developing a facies Modelfacies Model

• Collect stratigraphic succession data from outcrops and boreholes

• Place the data in form of stratigraphic columns at their corresponding locations

• Select a Datum e.g. Present day ground surface, or an isochronous marker horizon or basement surface

• Join the columns using the principles of correlation and facies relationship

BREAKS IN GEOLOGICAL BREAKS IN GEOLOGICAL RECORDSRECORDS

• GEOLOGICAL RECORDS

• Rocks are holders of records of what has been taking place throughout the geological time.

Example a pillow lava on top of a mudstone with ooze show that volcanic eruption occurred that spread lava on deep sea sediments.

BREAKS IN GEOLOGICAL BREAKS IN GEOLOGICAL RECORDSRECORDS

• In some periods of the earth’s history there are places where rocks did not form i.e. no sedimentation or igneous activity. Such periods are said to represent breaks in geological records

UnconformitiesUnconformities

• Breaks in geological records are revealed by unconformities.

• Unconformities are surfaces in rocks sequences indicating non-deposition which may be followed by erosion.

Types of Types of UnconformitiesUnconformities

• i) Disconformity:

Erosional surface separating stratified rocks of the same attitude

• ii) Angular unconformity

Erosional surface separating stratified rocks of the different attitude

• iii) Nonconformity

Erosional surface separating stratified rocks overlying non-stratified rocks

• iv) Paraconformity

Straight erosional surface separating rocks of the same attitude

DisconformityDisconformity

Angular unconformityAngular unconformity

Angular unconformity Angular unconformity

NonconformityNonconformity

HiatusHiatus

• The time interval between the rocks over and under the unconformity is called hiatus. It is the time interval without the geological record.

• A time interval representing short break in deposition e.g. that of a season is called DIASTEM

HiatusHiatus

Introduction to PaleontologyIntroduction to Paleontology Paleo = Old

Ntology = life

Paleontology is the study of ancient life shown to have existed by the presence of fossils in the rocks.

Fossils: are remains of any ancient organisms

HistoryHistory• Fossil (Latin) originally meant

any object dug from the ground including minerals, fossils themselves or archeological artefact.

• Fossils were considered to represent failed attempt to become live but never became organisms.

• The present meaning for “Fossil” started from late 18th Century when Paleontology became a scientific subject.

HistoryHistory

• It was Conrad Gesner (1516-1565) and Niels Stensen (Steno) who showed that fossils were once living organisms.

• Georges Cuvier (1769-1832) was able to show that mammoths existed but now were extinct

FossilsFossils

• There are two types of fossils:• 1. Body Fossil • 2. Trace Fossil• Body Fossils are preserved

elements of original body of an organism, and have undergone the process of fossilisation.

• In practice recently dead and buried organisms are not considered as fossils. The cut-off point is placed at the base of Holocene.

Trace FossilsTrace Fossils

• Trace fossils are the physical evidence of the existence of organism through their traces; such as burrows, tracks (foot print and trails) and boring which disturb the bedding surface and fabric of sedimentary rocks.

Coprolites (fossil faeces) are also considered to be trace fossils

FossilizationFossilization• Fossilization is transformation

of living of organisms into fossils i.e. preservation of the physical aspects of the organisms. The science is called TAPHONOMY

• There are 3 stages involved:

• - death

• - pre-burial

• - post-burial

DeathDeath• Can be caused by being

attacked by predators, infection, parasites, or environmental changes (draught, flood, CO2, Temperature) old age, accident etc.

• In some fossils it is possible to establish the causes of death eg under volcanic ash, mass accumulation, broken or fractured bones etc

Pre-Burial StagePre-Burial Stage• Breakdown and Decay

On the surface there is immediate commencement of breakdown and decay of body parts due to presence of O2, and scavengers and also H2O, high temperature.

• In almost all cases the soft parts of the organisms are destroyed or the the organism are destroyed completely.

•

Pre-Burial StagePre-Burial Stage• Transportation

On the surface body remnants may be subjected to transportation by water/air

Forming Allchthonous fossils

• Preservation

In some rare cases, preservation may take place during Pre-burial Stage eg. mummified bodies in dry hot or cold environments

Post-BurialPost-Burial

• After burial the dead organism can be affected by a variety of physical-chemical factors.

• Most important effect is that the supply of oxygen is cut off or reduced hence slowing or stopping decay

• The soft part are replaced by fine sediments.

Post-BurialPost-Burial• Some time the soft parts are

replaced by minerals like pyrite, carbonate, phosphate and silicate which preserve the structure of the organism.

• As burial progress loading and hence compaction will take palce.

• The two above are known as DIAGENESIS

Diagenesis dafter BurialDiagenesis dafter Burial

The following can occur:

•Preservation of the original material.

Here no effective flow of pore fluids. This is common in younger rocks.

•Recrystallization

Here no change of chemistry but xtal may change in habit or size eg Aragonite to Calcite,.

Replacement may take place by eg SiO2 or FeS2

Diagenesis dafter BurialDiagenesis dafter Burial

• Impregnation (Petrification)

For porous material like wood or bone, the pore space may be filled will minerals like calcite silica,

• Solution.

This occurs by complete removal of the original material forming a natural cavity(external mould)

Internal mould occurs if the inner space of a shell was first filled by sediments before desolution

Post-BurialPost-Burial

• .