Lecture Thirty Four - Biostratigraphy and chronostratigraphy

Question 1

What is biostratigraphy?

Answer 1

The best way to estimate the ages of sedimentary strata.

• Fossils as key to age estimating correlations of sedimentary successions.
• A relative dating method.
E.g.
– ‘absolute dating’ (e.g. radiometric) - provides a number value (e.g. 100 Ma).
– Absolute = preferable, but rare in sed. rocks.

• Ideally: fossils are anchored with absolute dates (e.g. from volcanic ash layers, other
igneous sources) to constrain their age ranges.

• Detecting time-gaps (AKA: unconformities).
– revealed by abrupt ecological overturns.
– biostrat especially useful for ‘paraconformities.’
– can estimate their durations.
– reveals mass extinction events.

• Age-dating:
lithostrat/environmental changes
–> Whilst providing evidence for enviro./ecol. interpretations in the process.

Question 2

What are index fossils?

Answer 2

• Index fossil (or ‘index taxon’) = a fossil used to distinguish
biostratigraphic units. 
• Typically, index fossils
= microfossils:
– Foraminifera
– Algal microplankton
– Ostracods
– Plant spores/pollen
– Conodonts
• Some bigger players (macrofossils):
– Ammonites
– Belemnites
– other molluscs

• What makes a fossil good for
biostratigraphy (i.e. an index fossil)?
E.g. Plant pollen & spores: 
– Extremely widespread
• wind, water & animal dispersal
– Durable (chemically/physically)
• Sporopollenin ~ most stable natural polymer ever discovered. 
– Small & abundant
• Up to x1000s in a single gram of
sediment. 
– Species:
± short stratigraphic ranges
– As a group: long geologic history
• 1st land plants
= Ordovician liverwort
spores (~450-440 Ma)

Question 3

What are biostratigraphic zones?

Answer 3

• Stratigraphy can be subdivided into “biostratigraphic zones” (or “biozones”).
• These often coincide with lithostrat units, but not necessarily… thus treat these
independently.
• There are several different biozone types, each defined in different ways.
– Including (but not limited to):
1) taxon-range zone
2) concurrent-range zone
3) assemblage zone
4) abundance zone

Each zone is named after a distinctive index fossil which characterises that zone.

1, 2 and 3 rely on the evolution or extinction of species.

• Every species has a First Appearance Datum (FAD).
• if extinct: also a Last Appearance Datum (LAD).

Question 4

Explain the differences and similarities between biostratigraphy and lithostratigraphy.

Answer 4

Lithostrat used to infer
environmental changes.
Biostrat intends to provide time
markers… regardless of local
environmental conditions.
But in many cases: fossil content is STRONGLY biased by environment.
That is, fossils may be found in some environments and not in another, even though it is in the same time period.
Therefore you cannot assume that one rock assemblage is from a different time compared to another due to the absence of a certain fossil, as different organisms will exist in one environment and not another in the same time period.

Question 5

What is chronostratigraphy?

Answer 5

Definition:
• Stratigraphic units defined by their time of formation.

Purpose:
• A way for stratigraphic successions to be correlated across the globe.
• Put strata into the context of Earth’s history.

Methods: many different lines of research converge here.
• Lithostrat: especially global environmental changes.
• Biostrat: especially widely distributed (± oceanic) species and global extinction events.
• “Magnetostrat” (palaeomagnetic polarity changes).
• “Cyclostrat” (eustatic SL changes).

Then can be given absolute date values by,
E.g.
1) radiometric dates (“radiochronology”).
2) tree-rings (“dendrochronology”).
3) ice cores (“cryochronology”).

E.g.
The Ediacaran.
The newest (formalised in
2004), yet oldest (~ 635-541 Ma)
chronostratigraphic ‘system’
(= primary chronostrat unit).
–> First new geological
period in 120 years.
Base of the Ediacaran Period correlated to a severe global environmental change curtail to our evolution.
This rock assemblage was found in the Flunders Ranges in South Aus.
Suggests snow ball Earth and its thawing.

Question 6

Outline absolute dating and radiochronology.

Answer 6

Radiometric dating:
♦ All elements have different types of isotopes.
– Variants have slightly different masses.

E.g. three most stable isotopes of carbon:
– 12C & 13C will last for a very long time.
– 14C is radioactive –> will spontaneously decay (into “daughter” isotope: 14N).
• Decay rate is experimentally established.

• Mineral radiometric dating:
– Various radioactive elements are locked away when a mineral crystallises.
E.g. some of the “K” in K-feldspar.

• Trace element impurities (e.g. Uranium).
– Crystals become a closed system.

• No isotopes go in or out.
– So, if we know: rate of decay +
abundance ‘parent’ vs. ‘daughter’ isotopes.
–> work backwards: age of the mineral.

Most minerals form when magma cools or metamorphic recrystallisation.
Radiometric dating of igneous/metamorphic rocks:
–> Constrain ages of sedimentary rocks (and fossils).
–> The fossils (biostratigraphy) takes care of the rest.