Lecture 2 Flashcards
Who first observed unconformity and when
Hutton and playfair in 1788
Siccar point unconformity
a surface with horizontal Devonian sandstones on top of almost vertical silurian slates
Geological cycle
- Sedimentation
- Burial
- Folding, faulting, mountain-forming and igneous intrusion
- Erosion
Back to sedimentation as eroded rocks transported by rivers
What is lithophagia
when organisms gain nutrients from rocks—>form of erosion
What did Ussher work on
He was an archbishop and in 1654 estimated the creation of the earth at 4004 BC based on the bible
Lord kelvin’s contribution
Using loss of heat from a sphere he estimated 40-100 million years old—>however made many invalid assumptions
Stratigraphy
The study of rock layers in order to determine relative ages and therefore form a relative geological timescale
4 laws of stratigraphy
- Superposition, so rocks on top are newer
- Lateral continuity, so if rocks eroded away then lateral construction can be formed as originally continuous
- Initial horizontality, assumption that strata were formed relatively horizontally
- Cross-cutting relations
Biostratigraphy
Stratigraphy with fossils and fossil zones as these are generally more uniform than the differing conditions across the surface of earth which leads to different rock types
Facies
‘fash-ees’
environment of deposition. SO different environments can exist in different places at the same time, so mud and sand on river estuary and beach.
Problems with biostratigraphy
x5
- facies
- Many rocks lack fossils, especially continental ones
- Fossil zones may not extend beyond a single continent, if that far
- Few useful fossils prior to Cambrian and evolution of hard parts and almost 90% history before that
- Gaps are common—>called an unconformity
Unit Ma
Means million years before present
What is chalk formed of
Mainly coccolithophores, which are formed of CaCO3
Break between proterozoic and cambrian
Evolution of ‘hard’ parts, bone, shells and other less fleshy parts
Paleozoic to Mesozoic
Volcanic eruption in siberia
Mesozoic to Cenozoic
Asteroid impact removed charismatic megafauna
Pliocene to Pleistocene
Development of ice sheets in the northern hemisphere
The decay of parent, P, isotopes to stable daughter, D, isotopes gives us a clock, what equations do we use.
GRAPH 5
What are the most common systems used in radioactive dating
Rb—>Sr
Thorium and uranium to lead
Potassium to argon
Carbon to nitrogen
Best crystal to test
Zircons in Jack Hills
4.4B
No lead when formed but some uranium so any lead present was due to decay
Only closed systems are dated. Why?
IF the daughter atoms are lost we need to reset the clock as it makes it look younger than it is.
Closure occurs when below closure temp which changes due to isotope system and mineral involved.
Why is zircon a good closed system
Closes at >750 degrees so very close to actual formation time
What do we actually mean by date?
—why do we have to ask that question?
Age of deposition of the sediment may be much younger than the formation of the crystals within it
Estimate of the earths age using U-Pb system
~4560Ma
How good is stratigraphic correlation?
Using the example between deep-sea boreholes in the Southern Ocean
-GRAPH
- Magnetic polarity, rock types and fossil zone shown
- Rock type does not correlate at all (variability of facies)
- Magnetic polarity reversals are effectively synchronous everywhere
- Sedimentation rates very variable
- Fossil zone correlate quite well with the magnetic reversals
