Transcript Document

GEOLOGICAL TIME
GL1 III KI 2 d

The study of geological time is known as
STRATIGRAPHY

It leads to the understanding of
– The materials of the crust
– Their structure
– And past life
Geological study is an attempt to unravel
the history of the Planet Earth piece by
piece.
 And different people can have different
interpretations as to what and how the
events have happened.


Layers (stratum) of SEDIMENTARY ROCKS
are
– Deposited with the oldest rocks at the base
– And youngest rocks at the top of the
sequence
Youngest rocks deposited later
Oldest rocks deposited first

This is known as the
–LAW OF SUPERPOSITION
oldest at the base
youngest at the top

Rocks are originally laid down horizontally

This is known as the
LAW OF ORIGINAL
HORIZONTALITY
Rocks originally deposited in horizontal
layers

Rocks are linked
through the ROCK
CYCLE
And rocks are often
DEFORMED
 By SQUEEZING rocks
into FOLDS (the rocks
needs to be flexible
like plasticine)
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Or by cracking and
BREAKING into
FAULTS
DIVISIONS OF GEOLOGICAL TIME
Our studies focus on the last 600,000,000
years of Earth history
 The are three main divisions:

 CAINOZOIC
younger
 MESOZOIC

 PALAEOZOIC
older
We can use basic words to subdivide these
further
 Eg. Using the words LOWER (older) and
UPPER (younger)
 So the Palaeozoic can be divided into 2
parts, the Lower Palaeozoic (the older
portion) and the Upper Palaeozoic (the
younger portion)

BEFORE THE PALAEOZOIC
Time called PRE-CAMBRIAN
 4,600 million years (straight after the Big
Bang) through to 570 million years ago
 Earliest known rocks 3,700 my old
 Earliest life – BACTERIA (3,000 mya)
 Many impression and trace fossils (jellyfish
like creatures)
 Primitive life forms evolved 1,000 mya

PRE-CAMBRIAN
Atmosphere had very little OXYGEN 1,000
mya
 marine bacteria respired and produced O2
as a by-product
 Excess O2 escaped from the water to build
up in the atmosphere and form the ozone
layer

PRE-CAMBRIAN
Rocks from this time have not remained
unaltered, they have been deformed many
times
 Deformed by: folding, faulting,
metamorphism, weathering, erosion
 Trying to unravel their origins is difficult

ERA’S

Time is conveniently grouped into ERA’s
CAINOZOIC
(most recent time)
MESOZOIC
PALAEOZOIC
(older time)
Era’s are divided into
GEOLOGICAL PERIODS
LOWER PALAEOZOIC

“Ancient Life”
mya
395
SILURIAN
435
ORDOVICIAN
500
CAMBRIAN
570
OUTCROPS
CAMBRIAN
570-500 mya
 Cambrian life explosion (“sudden
appearance” of life with hard parts –
preservable)
 Cambria – ancient name for Wales
 Outcrops cover much of Wales
 Fossils - TRILOBITES
 Rocks – mainly deepwater shales

CAMBRIAN
FOSSILS
ORDOVICIAN
500-435 mya
 Named after the Ordovices (ancient British tribe of
the Welsh Borderlands)
 6 series, type localities (Tremadoc, Arenig, Llanvirn,
Llandeilo, Ashgill – Much of Wales and some of the Lake District)
 Rock types – dark shales (deep oceans floor
sediments)
 Fossils – mainly TRILOBITES and
GRAPTOLITES

ORDOVICIAN
FOSSILS
SILURIAN
435-395 mya
 Named after the Silures (an ancient British tribe from
Shropshire area)
 Fossils – mainly BRACHIOPODS and
TRILOBITES also GRAPTOLITES and
CORALS
 Mainly thick mudstones and shales with
massive limestones (Wenlock Edge)
 Rocks show the seas getting shallower
(shelf seas)

SILURIAN
FOSSILS
End of the Lower Palaeozoic large changes
 Plate tectonics (destructive boundaries)
brought the northern and southern parts
of the British Isles together
 Forming the Caledonian mountain range
 And a change from marine strata to
terrestrial ones
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UPPER PALAEOZOIC
mya
225
Permian
280
Carboniferous
345
Devonian
395
OUTCROPS
DEVONIAN
 395-345
mya
 Named after Devon (area of the UK where the
majority of rocks are of this age)
 Fossils – BRACHIOPODS and
GONIATITE AMMONOIDS, AND
GASTROPODS
 ROCKS – Old Red Sandstone
 ENVIRONMENT – shallow seas and
land
DEVONIAN FOSSILS
CARBONIFEROUS
345 -280 mya
 Named after carbon bearing rocks (coal)
 Fossils – PLANTS, BRACHIOPODS and
GONIATITE AMMONOIDS, AND
GASTROPODS
 ROCKS – Coal, Limestone, Sandstone
 ENVIRONMENT – cycle of swamp toppeddeltas building up and subsiding into
shallow seas
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CARBONIFEROUS FOSSILS 1
CARBONIFEROUS FOSSILS 2
END OF CARBONIFEROUS
Extensive earth movements buckled rocks
into troughs and basins
 Led to large areas of land with terrestrial
deposition in arid and semi arid conditions
(British Isles was situated in a mid continental position centred around the
Equator at this stage)

PERMIAN
280 -225 mya
 Named after Perm, an area in Russia
 Fossils – very few, mainly Brachiopods
 ROCKS – New Red Sandstone, some
evaporites, Magnesian Limestones (in upper
part)
 ENVIRONMENT – terrestrial beds with
isolated lakes, some reptile tracks
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PERMIAN FOSSILS
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Permian marks the
extinction of a number of
fossils groups (trilobites,
and some corals)
New group (reptiles)
became widely
established and severed
association with water!
Big change in plant fossils
too (from primitive forms
to more advanced
conifers).
Permian conditions continued into the next
era.
 Onto the next major era

 CAINOZOIC
younger
 MESOZOIC

 PALAEOZOIC
older
MESOZOIC
The so-called “age of the reptiles”
 225 – 70 mya
 However, very few reptiles to be found in
the British Isles, better described as “age
of the ammonites”

OUTCROPS
TRIASSIC
225 – 195 mya
 Named from German localities which show
a three-fold division
 Often use the term Permo-Trias since
conditions continued
 Fossils – very few, mainly Brachiopods
 ROCKS – Conglomerates, Sandstones and
marls
 ENVIRONMENT – terrestrial land surface,
some lakes, shallow sea towards end

TRIASSIC FOSSILS
JURASSIC
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195 - 135 mya
Named from Jura mountains of France
Fossils – Ammonoids (showing rapid evolutionary change), UK
lacks the classic dinosaur fossils, although some
Ichthyosaus and Plesiosaurs from Lyme Regis!
ROCKS – Shales, Limestones, Oolites
ENVIRONMENT – deepening of sea and
connections established with distant regions
JURASSIC FOSSILS 1
JURASSIC FOSSILS 2
JURASSIC
FOSSILS 3
CRETACEOUS
135 – 65 mya
 Greek – creta - chalk
 Fossils – Bivalves, Brachiopods,
Ammonites, Plants (fossil forest near Lulworth)
 ROCKS – Clay, Greensands, Chalk
 ENVIRONMENT – intermittent seas in
south to start with freshwater and land,
developing into wide Chalk seas

CRETACEOUS FOSSILS
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And finally ….
 CAINOZOIC
 MESOZOIC
 PALAEOZOIC
younger

older
CAINOZOIC
Alternate spellings: Caenozoic, Cenozoic
 Last 65 million years of geological time

OUTCROPS
TERTIARY
65 – 2 mya
 London and Hampshire basins – Clays with
fossils similar to modern species, plants
etc. of marine and land origins
 Some warm temperate flora found in Mull

TERTIARY DIVISIONS
PLIOCENE
 MIOCENE
 OLIGOCENE
 EOCENE

TERTIARY FOSSILS
QUATERNARY
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Last 2 million years
PLEISTOCENE – Ice Ages with Interglacials
(warmer)
HOLOCENE – “Human History”, Bones, Teeth,
Pollen and fossils similar to modern fauna
ROCKS: Glacial sands, gravels and clays
ENVIRONMENT – generally colder globally with
glacial erosion, marine and land deposition
QUATERNARY FOSSILS
Selected references
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Fossil diagrams from British Museum texts
(British Palaeozoic Fossils, British Mesozoic
Fossils, British Cainozoic Fossils).

JLC  1/03 updated 2/06