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Lecture

Lecture 3


Department
Earth Sciences
Course Code
ESS102H1
Professor
Christine Burton

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GLG205 t 3
May 18
Geologic time and fossils
How are fossils formed?
- Mineralization: cavities filled with different minerals like silica, calcium carbonate, and iron most
commonly
o Organism dies in/near water and its soft parts are consumed by bacteria
o After being buried, water passes through the soil and minerals precipitate out of the
water into the cavities
o If sediments are fine grained, the fossils are more detailed
o As more sediments pile on, there is more pressure and it lithifies into rock
o Mineral rich water continuously add or modify the shell
o Uplift: the rock that was previously buried is lifted up
o Erosion t ice, wind, water
- Ammonite fossils
o An organic gemstone
o Iridescent aragonite (a mineral) with trace elements (e.g. Fe, Mn, Al, Ba) that causes
coloration
- Trace fossils
o Preserved tracks, trails, burrows, borings; not the organism itself
o Ichnology: the study of trace fossils
o Bioturbation: process of disturbing sediment
By organisms that walk past or burrowing in, or roots
Fossils as clues to ancient environments
- Palaeoecology: the study of ancient organisms and their environments
- Clues from:
o Fossil types
E.g. if coral fossils are found, it used to be a shallow water
o Assemblages
Groups of organisms that live together
o Fossil morphology
The shape of their bodies
o Trace fossils
Shows what kind of organisms were there and how they moved
- E.g. St. Cuthbert rings
o Parts of an ancient sea lily (Crinoid) that were broken up and fossilized
Fossils and stratigraphy
- How do we use fossils in stratigraphy?
o To establish relative age of rock units, correlate units
- What information do we need to do this?
o Relative age of rock units
o Fossil species present in each unit
o Establish time of first and last appearance of each species geologic range
Long-living organisms are not useful compared to short-lived organisms
- William smith
o Was digging in England and found out that the same fossils were found in the same
depths no matter where he dug
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o Came up with the principle of faunal succession
Oldest fossils at the bottom
/(}Pv]uÆ]vUZÇ}v[}ulU}](}Çtheir geologic
range ends there
o What kinds of fossils are most useful for stratigraphic work?
Short-lived, widespread
Assemblages (groups) of species
Index fossil
x E.g. ammonites
o Trilobite growth
They molted regularly so they are very abundant in the fossil record
Geologic time scale
- Most divisions of the geologic time scale mark major environmental changes and associated
biological changes (appearance or extinction of species)
- Example of major environmental changes: changes in temperature and precipitation patterns
atmospheric composition, sea level
o E.g. glaciations can lead to a drop in sea level
- Such changes can be caused by: changes in the amount of solar radiation, changes in the
intensity of plate tectonics, volcanic eruptions, meteorite impacts, and human activities
Geologic theory relating to time and the formation of the earth: A brief history
- 11th century t persion geologists Avicenna
- 11th century t Chinese naturalist Shen Kuo
Western geologic theory pre 1800s
- Heavily based in Christian religion:
o The earth was created in 7 days
o Climate is fixed
o Till, sand, and gravel = dilivium (from the divine/from the biblical floods)
- In 1658, Archbishop ussher of Ireland gave an age of the earth of about 5600 years, based on
the old testament
- Catastrophism
o Georges cuvier
o The idea that earth has been affected in the past by sudden, short-lived, violent events,
possibly world wide
o XPXv}Z[(o}}
- Uniformitarianism
o James Hutton (1788)
Constant cycle of depostition, burial, uplift, erosion, and redeposition
Same processes we observe today must have occurred in the past
o Charles Lyell (1830)
Geologic change = steady accumulation of minute changes over enormously
long spans of time
o Charles Darwin (1830)
Estimated the earth to be ~300,000,000 years
o Christendom (17th and 18th centuries)
Formation of all rocks and other geologic events were placed into a biblical
chronology
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