Uncomformities
Ancient erosional surface that represents a time gap or a hiatus. There are 3 types:
- Angular uncomformity
- Discomformity
- Noncomformity
Angular discomformity
Different orientations above and below
Angle->erosion->deposition of rock
Discomformity
Strata have same orientation above and below
No obvious visual quo that there is a gap in time
Nondiscomformity
Unit below is not sedimentary
2 different rock types associated with each other where we know one is older than the other (e.g. igneous and then sedimentary)
Development of geologic time scale
- methods of relative dating of strata were used to first develop a geological time scale
- numerical ages were then added subsequently by using isotopic methods
- to this day, many boundaries are defined by fossils
K-T boundary
Croatian timescale
Review chart in lecture slides
What lead Hutton to think of the Earth as older than it is
It’s stratigraphy
How the GTS was built
- Lord Kelvin: magma cooling (20-100 million years)
- John Joly: estimated how long it would have taken for the oceans to accumulate salt from erosion processes. 80-100 million years old. Used radioactivity too
- Samuel Haughton: used stratigraphic thickness and deposition rates to come up with the age of 200 million years
- Arthur Holmes: in The Age of The Earth, reviewed and added to all available radiometric ages and came up with a rough sketch of a Geologic Time Scale
The original GTS had
Four time division:
Primary, secondary, tertiary, quaternary (only one left)
ASIC structure of GTS
- Eons (4)
- Eras (10)
- Periods (22-10)
- Epochs (lots)
- Age (even more)
Hadean Eon
- 4.5-4.0 GA
- period before earliest known rock
- heating from radioactive decay melted most of early Earth
- intense meteorite bombardment forming moon (between 4-4.2 GA)
Archean Eon
- starts with earliest known rocks-4.0 GA
- interior of earth was hotter than now (more radioactive isotopes)
- atmosphere mainly CO2, nitrogen, methane
- first signs of life around 3.5 GA (stromatolites)
Proterozoic Eon
- start defined at 2.5 GA
- tectonic system probably similar to today’s
- single called organisms became abundant
- “Great Oxygenation Event”
- Birth of multicellular organisms
Phanerozoic Eon: Paleozoic Era
- starts with first Shelly fossils (545 MA) known as Cambrian explosions
- things start creeping out of the ocean
- “the Great Dying” (end of permian extinction)
- defined by 6 systems that saw major leaps in the evolution of modern Earth systems and biology
Paleozoic Era summary
-review chart
Phanerozoic Eon: Mesozoic era
- starts with “The Great Dying” and ends with another ~65 MA (K-T boundary)
- age of dinosaurs on land
- giants ruled the sea, land and sky
- three famous periods
Mesozoic era
Review summary chart
Phanerozoic Erin: Cenozoic Era
- starts at a major extinction event ~65 MA
- continues to today
Cenozoic Era summary
- the age of mammals
- general cooling to the “ice Age” defined by the quaternary
- Holocene is today but debate about Anthropocene
