Week 1 - Stratigraphy Flashcards
What is stratigraphy?
The branch of geology concerned with the order and relative position of strata and their relationship to the geological timescale.
What is geochronology?
Defines intervals of geological time
What is chronostratigraphy?
Defines units of rock formed during intervals of geological time (sometimes referred to as time-rock units)
What are golden spikes?
-One way to define objectively the divisions of geological time is to assign Global Standard Section and Points (GSSP) (AKA shows the start of a geological period)
-Otherwise known as golden spikes defined by figuratively hammering in a spike to a type section to define a boundary
-More than half of all Phanerozoic Stages are assigned GSSPs
What are the key stratigraphic methods used to place rocks in order?
*Lithostratigraphy
* Biostratigraphy
* Magnetostratigraphy
* Astrochronology
* Radioisotopic dating
* Isotope stratigraphy
* Event stratigraphy
What is lithostratigraphy?
-Lithostratigraphic units are bodies of rock defined on the basis of lithological features that make them distinct from other bodies of rock.
-Key to this is recognizing the relative stratigraphic relationship of rock units to each other to determine which are older and which are younger.
-This can only be done by examining rocks in the field and/or a geological map of these rock units.
- Even where one rock horizon can be traced over a long distance, it may not represent the same time everywhere.
What are diachronous rocks?
- During sea-level rise a sandstone characteristic of the beach environment will move across the landscape. Such a rock layer is said to be diachronous or time-transgressive
What are isochronous rocks?
- Certain rock horizons are isochronous (same time everywhere) and very widespread. —The best are volcanic ashes, which fall across a wide region in a (geologic) instant. They are also radiometrically datable.
What is biostratigraphy?
- Use of fossils to date age of rocks
- Life evolves fast so creates distinct features in each time period so can show the age of rocks.
- Are usually fossil remains of animals and plants (actual preserved body parts, casts, or impressions of body parts, or traces left by the passage of an organism such as burrows or footprints)
What are fossils that show the age of rocks called?
Index fossils
What makes a good index fossil?
- Limited age
- Wide geographic location
- Fast evolving organisms
-Preserved well
-Spatial distribution
-Mobility of organism
What is radiometric dating?
- Relative order of the geological timescale is defined mainly by litho/biostratigraphy… but numbers assigned to boundaries are determined by radioisotopic dating
- Excellent for igneous (volcanic ash) and metamorphic rocks, but not so good for sedimentary rocks.
What is the equation for the radioactive decay of a substance?
N0: The initial number of atoms in the substance
N: The number of atoms remaining after time
lambda : The decay constant for the substance
t: The time elapsed (usually rearrange to find this)
What are the three methods used to date using radiometric dating?
Carbon 14 –> Carbon 12 (used for <50Ka) - Wood/peat (Common dated minerals)
Potassium 40 —-> Argon 40 (Used for > 100Ka) - Mica, hornblende and volcanic rocks.
Uranium 238 —> Lead 206 (Used for >5Ma) - Zircon
Uranium 235 —-> Lead 207 (Used for >250 Ma) - Zircon
