1. Introduction Flashcards
What are the two types of geological age?
Relative geological ages: placing rocks and events in their proper relative sequence of formation/occurence
Absolute geological ages: specifying absolute number of years since an event
What are the stratigraphic principles of relative dating?
superposition: each bed is younger than the one below it
cross cutting: a bed (or unit) is younger than the one it cuts through
original horizontality: consolidated strata that are tilted now were originally deposited horizontal
continuity: in a basin, the same bed is of identical age in every part of the basin
inclusion: if an element A is included in an element B, then A is older than B
paleontological identity: all beds including the same fossils are of the same age, unless reworked
Define unconformity
break in the rock record produced by erosion and/or non deposition
Define uniformitarianism
Same processes operating today were active in the past: “the present is the key to the past”
Define angular unconformity
tilted rocks are overlain by flat-lying rocks
define disconformity
strata on either sides are parallel
define nonconformity
metamorphic or igneous rocks in contact with sedimentary strata
what is faunal succession?
observation that assemblages of fossil plants and animals follow or succeed each other in time in a predictable manner, even when found in different places
How are absolute geological ages defined?
Using Marie Curies research into the halflives of radioactive isotopes
What effects our concept of geological time? How can we combat these?
- Gaps in the rock record
- rates of deposition
A chronostratigraphic diagram, using units of time in the y axis, can be shown alongside the stratigraphic log that uses thickness on the y axis
What are the techniques used to relatively date rocks
- what material do you need
1) Lithostratigraphy
- Lithology
2) Biostratigraphy
- Fossils
3) Isotope stratigraphy
- Geochemical data
What are the techniques used to absolutely date rocks
- what material do you need
1) Radiocarbon ( C-14)
- Organic material
2) Uranium-lead and others
- various U,K minerals
3) Magnetostratigraphic
- Volcanic and sedimentary rocks
What is lithostratigraphy?
Subdivision of stratigraphic units on the basis of their lithological character and stratigraphic position
What is the lithostratigraphic hierarchy
1) Supergroup: Assemblage of related groups (regional to provincial scale)
2) Group: relations between formations (small to regional scale)
3) Formation: Fundamental unit
4) Member: Part of formation with distinguishing traits
5) Bed: smallest unit
6) Key or marker bed: thin distinct bed that is widely distributed
how does bio stratigraphy date rocks?
Species evolve into new forms and have a discreet duration before becoming extinct, biozone boundaries characterised by index fossils
- they need to be widespread, can’t be restrictive to certain environments, has a specific morphology.
Need to be combined with absolute ages (of surrounding beds and fossils contained) to be useful and not only give relative ages
periods of geological time may be characterized by:
- presence/absence of a species
- co-existence of several species
- abundance of a species
