1.6 - Biostratigraphy: A Tool For Reconstructing The Rock Record

Question 1

What is biostratigraphy?

Answer 1

Exploits fossils - use the extinction (LAD - last appearance datum) and origin (FAD - first appearance datum) events

Question 2

What is a biozone?

Answer 2

Volumes of rock defined on basis of the relative age of the fossils they contain.
Interval of rock between 2 successive biostratigraphic events.

Question 3

What are biostratigraphic units?

Answer 3

Biozones (volumes (3D) of strata defined on the basis of the contained fossils - exist only where a diagnostic feature is present)

Question 4

What are the characteristics of biozones?

Answer 4

Dynamic (spatially and temporally eg if new fossil found)
They do NOT need to coincide with lithostratigraphic units (relative age)
Biozones defined on one criterion can be different shapes from those defined on other paleontological criteria

Question 5

Are biozones chronostratigraphic units?

Answer 5

Ideally they would be but strictly no.
The lower surface of a biozone cannot be an isochron (and practical limitations) eg biozones end because of a change in the depositional facies/variations in conditions for fossilisation and preservation of fossils etc).
If there is rapid disperal (eg. globally near instantaneous extinction event) it can approximate a chronostratigraphic unit but does not strictly occur

Question 6

What is the function of biozones?

Answer 6

To determine stratigraphical succession (relative age)

Question 7

What is a biochronozone? How can it be defined?

Answer 7

A body of rock between 2 isochrons

Can be defined on the basis of a biozone (but the 2 are not synonymous)

Question 8

What is a biochron?

Answer 8

An equivalent interval of time

Question 9

Summarise Chronostratigraphic Units characteristics.

Answer 9

Encompass all rocks formed within certain time spans of Earth history regardless of compositions or properties.
Include rocks of only a certain age (basis of their time of formation).
Their boundaries are synchronous.

Question 10

The same interval of strata may be zoned differently depending on what?

Answer 10

The diagnostic criteria or fossil group chosen

Question 11

How many types of biozones are there?

Answer 11

6

Question 12

Name the 6 types of biozones.

Answer 12

Taxon range biozone
Concurrent range biozone
Interval biozone
Lineage biozone
Assemblage biozones
Abundance biozone

Question 13

What is a taxon range biozone?

Answer 13

The known stratigraphic and geographic range of a single taxon

Question 14

What is a concurrent range biozone?

Answer 14

The overlapping part of ranges of 2 epcified taxa

Question 15

What is an interval biozone?

Answer 15

The strata between 2 specific biostratigraphic surfaces (can use the lowest or highest occurrences

Question 16

What is a lineage biozone?

Answer 16

The strata contain species representing a specific segment of an evolutionary lineage

Question 17

What is an assemblage biozone?

Answer 17

The strata contain a unique association of 3 or more taxa

Question 18

What is an abundance biozone?

Answer 18

The local interval in which taxon/taxa abundances is ‘significantly greater’ than above and below

Question 19

What is an abundance biozone also known as?

Answer 19

An ACME biozone

Question 20

What makes fossil biostratigraphy useful? (2 points)

Answer 20

All species originate and go extinct (restricted to a particular interval of geological time.
The restriction of a particular species of fossil to a narrow unit of time is therefore important, but not the only attribute that would favour its use as a zone fossil.

Question 21

Name a fossil that is widely used in biostratigraphy.

Answer 21

Graptolites (graptoloids)

Question 22

What are the characteristics that make for a good zone fossil? (7)

Answer 22

The species is easily recognised
High preservation potential
High rate of evolution
Many different features 'character states'
Wide geographic distribution
Facies independent
Highly abundant

Question 23

Describe graptolites.

Answer 23

Cambrian to Carboniferous in age
Hemichordates
Colonial
Sessile attached benthos (some dendroids)
Planktonic - free living in the water column (some dendroids and all graptoloids)
Graptolites important re use as zone fossils

Question 24

Why are graptolites good zone fossils?

Answer 24

Easily recognisable
High preservation potential (decay resistant - recalcitrant periderm)
Evolution: high rate of species turnover, short species duration, allows fine-scale division of geological time-scale, fast rate of evolution
Geographic distribution: widespread (planktonic)
Facies distribution: typically widely distributed within a basin (dependent on water depth as closer to surface = more widely distributed)
Abundant: often abundant in fine-grained lithologies that accumulated slowly (pelagic/hemipelagic) - high ratio of fossil:sediment

Question 25

Why are graptolites geographically widespread?

Answer 25

Water currents circulating control where they are distributed.
East to West orientation, North to South changes in fauna
Ordovician graptolites: define 2 faunal provinces: high latitude in warm Atlantic province and low latitude in cold Pacific province
Closer they were to water surface the more widely distributed they are

Question 26

Why are overlapping species important?

Answer 26

In a widespread geographic area this is important because you can find components that are shared and correlate them over long distances

Question 27

What are the different zones of the water column from shallowest to deepst? (zonation of the water column) (5)

Answer 27

Epipelagic (0-200m)
Mesopelagic (200-1000m)
Bathypelagic (1000-4000m
Abyssopelagic (4000-6000m)
Hadopelagic (below 6000m)

Question 28

What are the characteristics of the epipelagic zone?

Answer 28

Sunlit
Enough light for photosynthesis
Nearly all primary production in the ocean occurs here

Question 29

What are the characteristics of the mesopelagic zone?

Answer 29

Twilight
Some light penetrates
Insufficient for photosynthesis
At approximately 500m the water becomes oxygen-depleted (more efficient gills needed, minimal movement required)

Question 30

What are the characteristics of the bathypelagic zone?

Answer 30

Dark
No living plants
Most animals consume marine snow or are predators

Question 31

What are the characteristics of the abyssopelagic zone?

Answer 31

No light

Question 32

What are the characteristics of the hadopelagic zone?

Answer 32

Water in ocean trenches or just below 6000m

Question 33

Which 3 zones are all quite similar and sometimes combined into a single zone?

Answer 33

Bathypelagic, Abyssopelagic, Hadopelagic

Question 34

What does pelagic mean?

Answer 34

Open water