1.6 - Biostratigraphy: A Tool For Reconstructing The Rock Record Flashcards
What is biostratigraphy?
Exploits fossils - use the extinction (LAD - last appearance datum) and origin (FAD - first appearance datum) events
What is a biozone?
Volumes of rock defined on basis of the relative age of the fossils they contain.
Interval of rock between 2 successive biostratigraphic events.
What are biostratigraphic units?
Biozones (volumes (3D) of strata defined on the basis of the contained fossils - exist only where a diagnostic feature is present)
What are the characteristics of biozones?
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
Are biozones chronostratigraphic units?
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
What is the function of biozones?
To determine stratigraphical succession (relative age)
What is a biochronozone? How can it be defined?
A body of rock between 2 isochrons
Can be defined on the basis of a biozone (but the 2 are not synonymous)
What is a biochron?
An equivalent interval of time
Summarise Chronostratigraphic Units characteristics.
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.
The same interval of strata may be zoned differently depending on what?
The diagnostic criteria or fossil group chosen
How many types of biozones are there?
6
Name the 6 types of biozones.
Taxon range biozone Concurrent range biozone Interval biozone Lineage biozone Assemblage biozones Abundance biozone
What is a taxon range biozone?
The known stratigraphic and geographic range of a single taxon
What is a concurrent range biozone?
The overlapping part of ranges of 2 epcified taxa
What is an interval biozone?
The strata between 2 specific biostratigraphic surfaces (can use the lowest or highest occurrences
What is a lineage biozone?
The strata contain species representing a specific segment of an evolutionary lineage
What is an assemblage biozone?
The strata contain a unique association of 3 or more taxa
What is an abundance biozone?
The local interval in which taxon/taxa abundances is ‘significantly greater’ than above and below
What is an abundance biozone also known as?
An ACME biozone
What makes fossil biostratigraphy useful? (2 points)
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.
Name a fossil that is widely used in biostratigraphy.
Graptolites (graptoloids)
What are the characteristics that make for a good zone fossil? (7)
The species is easily recognised High preservation potential High rate of evolution Many different features 'character states' Wide geographic distribution Facies independent Highly abundant
Describe graptolites.
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
Why are graptolites good zone fossils?
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
Why are graptolites geographically widespread?
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
Why are overlapping species important?
In a widespread geographic area this is important because you can find components that are shared and correlate them over long distances
What are the different zones of the water column from shallowest to deepst? (zonation of the water column) (5)
Epipelagic (0-200m) Mesopelagic (200-1000m) Bathypelagic (1000-4000m Abyssopelagic (4000-6000m) Hadopelagic (below 6000m)
What are the characteristics of the epipelagic zone?
Sunlit
Enough light for photosynthesis
Nearly all primary production in the ocean occurs here
What are the characteristics of the mesopelagic zone?
Twilight
Some light penetrates
Insufficient for photosynthesis
At approximately 500m the water becomes oxygen-depleted (more efficient gills needed, minimal movement required)
What are the characteristics of the bathypelagic zone?
Dark
No living plants
Most animals consume marine snow or are predators
What are the characteristics of the abyssopelagic zone?
No light
What are the characteristics of the hadopelagic zone?
Water in ocean trenches or just below 6000m
Which 3 zones are all quite similar and sometimes combined into a single zone?
Bathypelagic, Abyssopelagic, Hadopelagic
What does pelagic mean?
Open water
