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Geol 307: Sedimentation andStratigraphySemester 071

2008

Lecture 20Lecture 20BiostratigraphyBiostratigraphy

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INTRODUCTION Remains of past animals and plants occur

mainly in sedimentary rocks as fossils Fossils with no living counterparts indicate

that life on Earth has evolved throughgeological time

Different rock units contain differentassemblages of fossils, using stratigraphicprinciples they can be either younger orolder

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INTRODUCTION Thus fossils are essential to stratigraphy

and provide high resolution technique instratigraphic analysis

The divisions and sub-divisions of thegeological time scale are based oncharacteristic fossil assemblages

Time in Stratigraphy: Lithostratigraphy -Biostratigraphy - Chronostratigraphy

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BASIS FOR BIOSTRATIGRAPHY

Evolution of life General trend of increasing complexity

and sophistication of life forms fromprimitive to advanced Emergence and diversification Extinction

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SPECIES CONCEPT INPALEONTOLOGY

Species is the basic unit of the Linnaean System ofbiological classification, i.e. organisms can berelated to each other in a hierarchical manner. It isalso used to classify fossils

Kingdom Phylum Class Order

Family Genus Species

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SPECIES CONCEPT INPALEONTOLOGY

A biological species is an interbreeding populationthat share genetic characteristics

A paleontological species is a morphospeciesbased on the morphological characters of thefossils because the biological species concept can

not be applied to fossils Closely related species are considered to belongto the same genus

Binomial nomenclature means that each individualorganism has two names, generic followed byspecies name. e.g. Homo sapiens is the biologicalname for human species

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PATTERNS OF EVOLUTION Phylogeny: An evolutionary lineage is a line of

descent of organisms. Its pattern will show twocomponents; 1. it may show numerousbranches from one lineage as new speciesform thus number of species increase, and 2. asingle species may change through time intoanother species i.e. Phyletic evolution hencedoes not increase number of species

Both of these patterns of evolution are usefulin stratigraphy as they involve appearance ofnew taxonomic groups through time

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PATTERNS OF EVOLUTION Phyletic gradualism: evolution is a

gradual process Punctuated equilibria: since missing

links between two species are not foundin the fossil record, this theory says thatevolution occurs in episodic way. Forlong stasis no change occurs followedby short periods of rapid change inresponse to environmental stresses

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FOSSILS IN STRATIGRAPHY Depositional Environments: Environmental

control on the distribution of taxa Problem of correlation between continental

and marine environments because few

animals or plants found in both settings Provincialism due to geographical isolation; it

necessitates development of differentbiostratigraphic schemes for different parts ofthe world

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FOSSILS IN STRATIGRAPHY Abundance and size: smaller fossils are more

abundant than larger ones, thus are moreuseful in biostratigraphy Preservation Potential: Fossil record

represents only a minor portion of the past life

because (a) only hard parts are fossilized withfew exceptions (b) different environments havedifferent fossilizable potential (c) stratigraphicrecord is incomplete, only few environmentalniches are preserved in sedimentary rocks

Rate of Speciation: Differing rates ofspeciation observed in fossil record

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Taxa used in BiostratigraphyMarine Macrofossils: Trilobites: mainly in Cambrian Graptolites: Ordovician-Silurian

Brachiopods: Silurian-Permian Ammonoids: Mesozoic

Echinoderms: Cretaceous Corals: Devonian-Lower Carboniferous

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Taxa used in BiostratigraphyMarine Microfossils:

Foraminifera: Phanerozoic, widely used inoil industry

Radiolaria: Phanerozoic, widely used inpaleoceanographyNannofossils: Mesozoic-CenozoicMicroplankton: Phanerozoic

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Zonation Schemes Total range biozone: FAD and LAD of a

single taxon Consecutive range biozone: a part phyletic

lineage Partial range biozone : partial range of asingle taxon

Assemblage biozone: many taxa Acme biozone: abundance of a taxon

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Biozones and DepositionalEnvironments

Reasons for presence/absence of fossils fromsedimentary rocks

Under the same depositional environmentappearance of a taxon may be due to speciationevent thus of regional stratigraphic significance

Migration of already existing species could also takeplace thus have only local stratigraphic significance

Disappearance of taxon from the stratigraphicsuccession is likely an extinction event under thesame depositional environment regional

significance Disappearance can also occur due to change indepositional environment local significance

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Graphical correlation schemes Thickness of a biostratigraphic unit at any

place is determined by the rate ofsedimentation during the time periodrepresented by the biozone

A reference section is defined for a givenbiozone which can be used for correlation

Graphical correlation methods are used toidentify changes in rates of sedimentationor a hiatus in deposition

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What is high resolutionstratigraphy?

It depends on where you are in the geological time scale. Ageneral consensus is following achievable resolution forintegrated stratigraphy in marine sediment sequences

Quaternary < 1 to 3 ka (1980:c. 20 ka)

Late Cenozoic 5 to 10 ka (1980:c. 100 ka) Early Cenozoic 10ka to 1Ma Late Cretaceous 100 ka to 1 Ma

Early cretaceous ca. 10 Ma Jurassic 50 ka to 150 ka Triassic 225 ka to 2 Ma