law of superposition- in any undisturbed sedimentary...

Biostratigraphy

Law of superposition- in any undisturbedsedimentary sequence the oldest bed is at thebase and the youngest is at the top.

Faunal succession- biotas have followed oneanother in an orderly succession throughgeologic time.

Principle of Faunal Succession

• Fossils have proved to be the only widelyapplicable tool of time correlation insedimentary rocks.

• The use of fossil for stratigraphiccorrelations is called biostratigraphy

Correlation with fossils

• It should be evident that biostratigraphicunits can only be used in an approximateway to determine time relationships.

• Establishing the time equivalence of twospatially separate stratigraphic units isknown as “correlation”.

Alcide d’Orbigny

• He showed the importance of assemblagesas a key for correlations.

• All of the strata defined by one fossilassemblage is a stage.

• Originally proposed ten.

Friedrich Quenstedt and AlbertOppel

• Orbygny’s stages too broad and vague.• He began to divided the sections according to

stratigraphic ranges of individual fossil taxa.• He realized that the pattern was similar all over

Europe.• These patterns can be divided into aggregates

based on the first occurrence of taxas oroverlapping range zones.

Controls: Evolution andpaleoecology

EVOLUTION• Biogeography and species changing from

one to another.• This is because evolution: organism as they

colonized new areas evolve also.

Paleoecology• The limiting factor on biostratigraphic

distributions of organism.• This is because no organism has ever

inhabited every environment on Earth.• They are facies-controlled.

Biostratigraphic Zonation

• The key to use fossils for telling time is thecarefully plotting of vertical stratigrpaphicdistribution, or Range, of every fossil in a localsection. Accurate stratigraphic data should becollected at the time of collection of eachspecimens.

• Zonation immediately divides the time in to threedifferent parts: the time before the fossil appeared,the time during which it existed, and the timesince its disappearance.

• All the strata that actually contain a givenfossil species are said to be in its rangezone.

• In a local section, the observed range of afossil is its partial range zone, or teilzone.

• The various classes of zones are calledbiozones.

Biostratigraphic Zonation

The absence of a fossil in an area can result from a number offactors. The fossil first evolve in province - 1, but the barrier toprovince 2 is presence, so its migrational first appearance inprovince 2 is later than its true first appearance. Likewise, it dies offfirst in province 1but lasts longer inprovince 2, where iteventuallydisappears byevolving intoanother species.Also, range zonescan be shortened bynonpreservation orby erosion ormetamorphism.Fig. 16.4, p. 372

• The first appearance of a fossil is called firstappearance datum (FAD), or first occurrencedatum (FOD).

• The last appearance whether by extinction orby emigration, is called last appearancedatum(LAD) or last occurrence datum(LOD).

• If the fossil record were perfect everywhere,biostratigraphic ranges would be simple functionsof four factors:– Evolution– Extinction– Immigration, and– Emigration.

• However other factors involve, facies,preservation, erosion etc.

Zones according to the most recentstratigraphic code.

• Classes of interval zones: bounded by twospecific first or last occurrence of taxa.

Classes of Interval Zones

The most important isThe most important isthe the concurrent rangeconcurrent rangezonezone, defined by the, defined by theoverlap of two or moreoverlap of two or morespecific first or lastspecific first or lastoccurrence of taxa.(Fig.occurrence of taxa.(Fig.16.5 (A), p. 373)16.5 (A), p. 373)

• (B) Taxon range zone,defined by the first andlast appearance of a singletaxon

• ( C) Lineage zones, orsuccessive taxa in alineage. It is based on asuccessive evolutionaryfirst and last occurrencewithin a single lineage.

• (D) Interval zonedefined by twosuccessive first or lastoccurrence ofunrelated taxa.

Assemblage zones• The second class of zones is the assemblagezones, which are based on the association ofthree or more taxa.

Oppel zones

• Overlap of severaltaxa

Abundance zone• Peak or acme zone• Fig. 16.7 Correlation based

on climatically controlledchanges in coiling directionin the foraminiferGlobotruncanatruncatulinoides in the SouthAtlantic. Prodominantlyright -coiling

Rock-stratigraphic unit

• Stratigraphy deals with units, primarily units ofrocks and of time that are used to dividestratigraphic sequences and geologic history.

• The stratigraphic record is divided into three-dimensional rock bodies on the basis of lithology.

• Rock units or “rock-stratigraphic units, arerecognized without regard to time or fossilcontent.

• The basic rock-stratigraphic unit is then the“Formation” that can be divided into smaller unitscalled “Members” or lumped together into higherunits called “Groups”.

• A single bed or lamina of sedimentrepresents a single event.

• Surfaces between beds or laminae thenindicates gaps in the rock record.

• Discontinuousness of deposition formsgaps (due to short time periods ofnondeposition or erosion) are term“Diastems”.