THEME 1:
INTRODUCTION TO
STRATIGRAPHY
AND GEOLOGICAL
TIME
When examining the geological history of
any area, we need to bear seven natural
laws in mind:
Principal of Uniformitarianism
Law of Superposition
Law of Evolutionary Succession in the
rock record
Law of Correlation
Law of Original Horizontality
Law of Cross-cutting Relationships
Law of Contained Fragments
Principle of Uniformitarianism
The principle of Uniformitarianism
(Charles Lyell, 1797-1875)
i.e. the present is the key to the
past
(Archibald Geikie 1835-1924)
Fig. 12.04a
Stephen Marshak
Law of Superposition
Fig. 12.04b
Fig. 12.04c
Stephen Marshak
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Law of faunal (evolutionary) sucession
Law of
evolutionary
succession
Fig. 12.07
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Trilobites: Homotelus bromidensis
Law of correlation (original continuity)
Law of original horizontality
Fig. 12.04fg
Fig. 12.04d
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Stephen Marshak
Strata are no longer horizontal
due to tilting or folding
Fig. 12.04e
Stephen Marshak
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Law of cross-cutting relationships
Law of inclusions (contained fragments)
Fig. 12.04h
Fig. 12.04i
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Law of baked contacts
Sort out the relative age-relationships in
this block diagram
Fig. 12.04j
Fig. 12.05
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Fig. 12.05
Erosion of land surface
Intrusion of dyke
Faulting
Intrusion of granite
Folding
Intrusion of sill
Bed 7
Bed 1
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Young
Old
Relative timescale based on
principles of cross-cutting
relationships, inclusions and
baked contacts
In order to study the changes
in the Earth, we need to have
an appreciation of Geological
Time. The study of stratified
rocks (i.e. those with layers) is
known as ‘stratigraphy’
So just how old is the Earth?
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Geological Time
Humans tend to think of time in terms of
human lifetime. e.g. recorded human
history began 200-400 generations ago,
i.e. c.4000BC
Many cultures have viewed geological
time within the same timeframe. e.g.
Earth formed October 23rd 4004 B.C.,
based upon the bible (Archbishop James
Ussher, 1654).
The Pyramids of Giza: 2650 BC
If the Earth formed 6000 years
ago, the pyramids are two
thirds of the age of the Earth.
So how come they are so
pristine when there is so much
evidence for large-scale
erosion?
These rocks formed 40km underground.
Charles Lyell (1797-1875):
British geologist was an early
worker who found it difficult to
accept diluvial models of
stratigraphy (i.e. associated with
the biblical flood).
Lyell thought it might be possible
to work out the age of stratified
rocks by measuring the rate of
sedimentation
By measuring the thickness of strata
it would then be possible to know
how long they took to accumulate.
He assumed that sedimentation in
the sea occurred at a constant rate,
and that there were only conformable
relationships in strata.
Ages of 100s of millions of years for
all strata ever deposited were
proposed
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But both Lyell’s assumptions were
incorrect!
sedimentation rates vary from
place to place, and through time
there are breaks in the record
(sea level changes/ tectonic
movement)…..periods of
nondeposition or erosion.
There are therefore gaps in the
sedimentary record
=unconformities
Siccar Point, Scotland
(The world’s most
important geological
exposure?)
Red Sandstone and
conglomerate
(Continental sediment)
Greywacke
(Marine sediment)
Hutton recognised that surface processes he saw
in Scotland (erosion and deposition) had been
responsible for formation of sedimentary rock. So
slow processes today had occurred in the past
and would occur in the future
James Hutton (1726-1797)
“The father of geology”
Unconformites….gaps in the record
James Hutton’s appreciation of the
magnitude of geological time was
based upon outcrop at Siccar Point,
where he envisaged a gap in the
geological record where tilting and
erosion of sedimentary strata had
taken place, before the record of
deposition resumed
“The mind seemed to grow giddy
staring back into the abyss of time”
John Playfair
1785 book “The theory of the Earth with Proofs
and Illustrations”
“no vestige of a beginning, no prospect of an end”
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There are three types of
unconformity:
An angular unconformity
A disconformity
Fig. 12.09a
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A nonconformity
Fig. 12.09b
Fig. 12.09c
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What about considering the geological
principles together with the fossil record?
Fig. 12.09d
This idea came to William Smith (17691839), an English canal engineer
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William Smith noted
that using
lithological and
fossil correlation
together, he could
follow formations
over an entire
region. He drew
where each
formation
outcropped on the
surface, and so
produced the first
geological map and
cross-section
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