Introduction and stratigraphic methods Flashcards
What can stratigraphy be used for?
Determining tectonic, biological, and climatic events
What are the two ways to determine geological age?
Relatively or numerically
Define relative geological aging
Placing rocks and events in their proper sequence of formation/occurence
Lithostratigraphy
Define numerical (absolute) geological aging
Specifying absolute number of years since an event
Geochronology
Describe the principle of superposition
Relative aging
Each bed is younger than the one below it
Thrust faulting can invert the order
Describe the principle of cross-cutting
Relative aging
A bed or unit is younger than the one it cuts across
Describe the principle of original horizontality
Relative aging
Consolidated strata that are now tilted were originally deposited horizontally
Describe the principle of continuity
Relative aging
In a basin, the same bed is of identical age in every part, beds terminate were deposition was not possible or where they have been eroded or deformed after deposition
Describe the principle of inclusion
Relative aging
If an element A is included in an element B, A is older than B
Describe palaeontological identity
Relative aging
All beds including the same fossils are of the same age
Relies of the principle of faunal succession
Not applicable where there has been reworking
Describe an unconformity
Relative aging
A break in the geological record produced by erosion and/or non-deposition
Describe an angular unconformity
Relative aging
Tilted rocks are overlain by flat-lying rocks
Describe a disconformity
Relative aging
Strata on either side a parallel, there have been a period of erosion
Describe a non-conformity
Relative aging
Metamorphic or ignous rocks in contact with sedimentary strata
Describe isotopic ratios
Relative aging
Applicable when changes are globally synchronous
O2 linked to temp
C linked to biological activity
S linked to igneous processes
Correlates within basin
Supported by absolute age dates
Define uniformitarianism
The same processes operating today were active in the past
Define numerical (absolute) geological aging
Specifying the absolute number of years since an event
Describe radioactive decay
Absolute aging
Time that it takes U to decay to Pb (parent to daughter isotope)
What does the use of radioactive decay for absolute dating require?
A closed system that contains no original daughter nuclides
Describe mangetostratigraphy
Absolute aging
Magnetic minerals orient to the pole
Pole changed from S to N ~700,000 ya
Minerals formed before this will orient to old pole
When do magnetic minerals orient to the pole?
Below the Curie point, ~570C, above this they orientate in random directions
Describe radiocarbon (C-14) dating
Absolute aging of organic material
Upper limit of ~70,000 years
Define chronostratigraphic units
The relative relations and ages of rock bodies
Define geochronological units
Dating and determining the time sequnce of depositional/deformity events
Provides absolute ages
Give an example of how a bed can be destroyed
By compaction during lithification
This compaction can make it so the bed’s thickness no longer indicates geological time
What is the subdivision of stratigraphic units based on?
Their lithological character and stratigraphy
Boundaries are placed at lithological changes (can be sharp/gradational)
Give the subdivision of stratigraphic units, in order from smallest to largest
Bed, member, formation, group, supergroup
Describe a bed subdivision
Smallest unit, can be a key/marker bed, a thin distinctive bed that is widely distributed
Describe a member subdivision
Part of formation with distinguishing traits
Describe a formation subdivision
Fundamental unit
Describe a group subdivision
Expresses natural relations beween formations
Small to regional scale
Describe supergroup subdivsion
Assemblage of related or superposed groups
Regional to provincial scale
What does biostratigraphy depend on?
Species evolve into new forms and have a discreet duration before becoming extinct
What defines a biozone?
The assemblage of index fossils
What is a limitation of biostratigraphy?
The biases of the rock record and taphonomy
Organisms can evolve at different rates around the world
Describe radiometric dating
Absolute aging
Radioactive clock measures time of crystallisation
Give four requirement of geochronology that can limit its efficacy
Requires a closed system with no original daughter nuclides (Pb), fresh samples, and a variety of minerals
