Stratigraphic context- dating methods Flashcards by Andrew Jeavons

What are the 5 main methods of correlation and dating?

Biostratigraphy
Geochronology
Cyclostratigraphy
Magnetostratigraphy
Chemostratigraphy

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What is biostratigraphy?

correlating and assigning relative ages of rock strata using the fossil assemblages
contained within them

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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

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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

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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

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What is chemostratigraphy?

relative ages from changes in relative proportions of trace elements and
isotopes (mainly carbon and oxygen) within and between lithological units

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Who formulated the principle of faunal succession?

William smith

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Who introduced the concept of biozones?

1856 Albert Oppel

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What is the biozone concept?

a way to describe strata characterised by overlapping range of fossils

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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

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What does FAD and LAD mean on biozone graph?

FAD- First Appearance Datum
LAD- Last Appearance Datum

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How are Oppels zones named?

After a distinctive fossil species (index fossil)

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What type of method is biostratigraphy?

Relative - not absolute years

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What issues comes about with using different fossils for biostratigraphy?

Not all fossil groups suitable

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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)

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How does taphonomy affect biostratigraphy?

Fossil content is subject to physical, biological and chemical processes (fossil might have been present in past)

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How can absence of fossils be explained ecological factors affect biostratigraphy?

species distributions are spatially heterogenous across different habitats and regions

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Where is biostratigraphy most effective?

one physical domain doesn’t suit wider application as readily

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What substance works for both marine and terrestrial ecosystems with biostratigraphy?

Pollen

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What makes biostratigraphy cheap and easy in the field?

presence of macro fossils

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What are the two parts of a decay of unstable nuclei called?

Parent- original
Daughter- what is produced after decay

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What is an isotope?

An element with the same number of protons and electrons but a change in number of neutrons

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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

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How can the distribution of Zircon be described?

Ubiquitous

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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?

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?

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