Stratigraphic context- dating methods Flashcards
What are the 5 main methods of correlation and dating?
Biostratigraphy
Geochronology
Cyclostratigraphy
Magnetostratigraphy
Chemostratigraphy
What is biostratigraphy?
correlating and assigning relative ages of rock strata using the fossil assemblages
contained within them
What is geochronology?
determining absolute numerical age of rocks, fossils, sediments by measuring the amount of decay of a radioactive isotope found within rocks with known half-life
What is cyclostratigraphy?
absolute ages from astronomically forced climate cycles within sedimentary successions due to the gravitational interaction of Earth’s orbit with other masses in solar system
What is magnetostratigraphy?
correlating and assigning relative ages of rock strata by using sequence
of normal and reverse polarity reversals of the Earthʼs magnetic field registered within magnetic minerals
What is chemostratigraphy?
relative ages from changes in relative proportions of trace elements and
isotopes (mainly carbon and oxygen) within and between lithological units
Who formulated the principle of faunal succession?
William smith
Who introduced the concept of biozones?
1856 Albert Oppel
What is the biozone concept?
a way to describe strata characterised by overlapping range of fossils
What does a biozone represent?
interval between the appearance of species at the base of the zone and the appearance of another species at the base of the next zone
What does FAD and LAD mean on biozone graph?
FAD- First Appearance Datum
LAD- Last Appearance Datum
How are Oppels zones named?
After a distinctive fossil species (index fossil)
What type of method is biostratigraphy?
Relative - not absolute years
What issues comes about with using different fossils for biostratigraphy?
Not all fossil groups suitable
What fossil groups are most suited to biostratigraphy?
rapidly evolving lineages with wide
dispersal strategies, high organismal abundance and small body sizes (i.e. microfossils)
How does taphonomy affect biostratigraphy?
Fossil content is subject to physical, biological and chemical processes (fossil might have been present in past)
How can absence of fossils be explained ecological factors affect biostratigraphy?
species distributions are spatially heterogenous across different habitats and regions
Where is biostratigraphy most effective?
one physical domain doesn’t suit wider application as readily
What substance works for both marine and terrestrial ecosystems with biostratigraphy?
Pollen
What makes biostratigraphy cheap and easy in the field?
presence of macro fossils
What are the two parts of a decay of unstable nuclei called?
Parent- original
Daughter- what is produced after decay
What is an isotope?
An element with the same number of protons and electrons but a change in number of neutrons
What the only application of the decay of Rb?
due to 48 bn yr half life cant be applied to earth only out of planet systems
How can the distribution of Zircon be described?
Ubiquitous
What does zircon contain?
Uranium
Thorium
What can the uranium and Thorium in zircon be used for?
radiometric dating
How will zircon form in magma chambers?
crystal grow over long period
old crystals at core young at exterior
How can zircon be used?
Individual crystals can be isolated and used to provide very accurate measurements
What do you need to be careful of when using zircon that has undergone metamorphism?
aggrading (get larger)
Metamorphism age crystal exterior
Crystal age interior
What can mean that zircon can provide incorrect ages for sedimentary rocks?
Naturally resistant and prone to recycling through rock cycle
What are tuffs?
lithified ash deposits
What is zircon preferentially used to date tuffs?
contains abundant zircons formed in eruption and deposition of ash
Where are tuffs common?
Terrestrial deposits (can be difficult to recognise)
Is radiometric dating absolute?
no
What is error range/ standard error? (radiometric dating)
+/- to represent variation from different samples, method or location
How can absolute dates help constrain other unknown units?
If you know the age of the rocks surrounding deposits then you know it falls in that range
Does geochronometry work on all rock types?
No
What is the practicality issue of radiometric dating with zircon?
Has to be done in the lab which can cost £10,000 for 100 samples
How are climate cycles often directly reflected?
In cyclical changes in sedimentation
What are astronomical cycles?
variations of the Earth’s orbit around
the sun through gravitational interaction between planets
What scale do the astronomical cycles act on?
Ka (thousand years)
What are the main orbital cycles?
Eccentricity - 100Ka
Obliquity- 41Ka
Procession- 19-23Ka
How many eccentricity cycles have been recognised?
95Ka
100Ka (most dominant)
125Ka
400Ka
What is the range for obliquity?
+/- 1.5* from present 23.5
What are varves?
Winter/summer seasonal sedimentary variation
What can and cant Cyclostratigraphy determine?
Can- duration of cyclic deposition
Cant- when it commenced or ended
What does Cyclostratigraphy work on?
geological phenomenon with cyclicity related to orbital forcing – bed thickness, elemental compositions, bed compositions
What is magnetic inclination?
The angle between the Earth’s surface and the magnetic field at that point
What will the dip of the magnetic field be like at the poles and equator?
Max dip at poles
No dip at equator
What will the inclination value be like in the northern and southern hemisphere?
NH-positive
SH- negative
How can magnetic fields be preserved in rock?
Iron bearing minerals when in liquid form will align to polarity which will then be locked into rock record
Magnetic minerals in magma chambers can be aligned to Earth’s magnetic field
What is the curie point?
temperature in which igneous minerals take on Earth’s magnetic field
What is the typical value of the curie point?
Can vary with minerals - typically 585*c
What is remnant magnetism?
magnetic alignment of magnetic minerals locked into crystals below Curie Point. Preserves magnetic orientation of crystals with respect to Earths magnetic field when formed
How can magnetism occur in marine sediments?
Magnetic minerals become aligned during settling through the water column.
What does paleo magnetism refer to?
past magnetic inclination when rocks formed
What information can be acquired from palaeomagnetism?
Palaeo-latitude but not palaeo-longitude
What are palaeomagnetic reversals?
represent period reversed polarity linked to earths core
What can palaeomagnetism be key to understanding?
How continental plates have moved over geological time
How has the UK’s location changed through geological time?
moved from mid southern latitudes to mid-northern latitudes over the last 500 million years
What are the problems with palaeomagnetism?
No absolute age
Not all units have significant magnetic properties
Broad window
Why is palaeomagnetism difficult with terrestrial settings?
erosion and unconformities represent breaks of unknown duration
