StratigraphyStratigraphy

The endless rain of sediments The endless rain of sediments produces layers.produces layers.

Stratigraphy Stratigraphy is the study of the is the study of the thickness and composition of thickness and composition of sedimentary layers. sedimentary layers. Such study provides insight into Such study provides insight into

rates of sedimentation,rates of sedimentation, sea-floor spreading, sea-floor spreading, and even climate. and even climate.

StratigraphyStratigraphy

If conditions are If conditions are right, these right, these sediments can sediments can eventually be eventually be converted into converted into sedimentary rocksedimentary rock

The record is The record is retained:retained: climateclimate environment of environment of

depositiondeposition life formslife forms

StratigraphyStratigraphy

Stratigraphy - the process of describing and categorizing marine (or other) sediments - trying to make some sense of them allow comparisons of sediments from different ages,

environments by really understanding any one sequence of

sediments, you can look at others and learn by comparing and contrasting the two.

Sediment OverviewSediment Overview

Here is how these factors work:Here is how these factors work: 1. 1. DeliveryDelivery

2. 2. DissolutionDissolution

3. 3. DisseminationDissemination

4.4. DilutionDilution

5. 5. Diagenetic alterationDiagenetic alteration

AtmosphericAtmospheric

BiogeniBiogeniccRiversRivers

Stratigraphy AnalogueStratigraphy Analogue

1. 1. DeliveryDelivery 2. 2. DissolutionDissolution 3. 3. DisseminationDissemination 4.4. DilutionDilution 5. 5. Diagenetic alterationDiagenetic alteration

Types of StratigraphyTypes of Stratigraphy

Lithostratigraphy: sediment chemistry, grain size, Lithostratigraphy: sediment chemistry, grain size, physical characteristicsphysical characteristics

Biostratigraphy: fossilsBiostratigraphy: fossils magnetostratigraphy: magnetic field directionmagnetostratigraphy: magnetic field direction stable isotope stratigraphy: ratio of one isotope to stable isotope stratigraphy: ratio of one isotope to

anotheranother chronostratigraphy: absolute age from radiogenic chronostratigraphy: absolute age from radiogenic

isotopesisotopes

LithostratigraphyLithostratigraphy

Distinguish layers basis of lithology or physical Distinguish layers basis of lithology or physical characteristics:characteristics: grain size, grain size, mineralogymineralogy mode of deposition etc.mode of deposition etc.

Facies = character of the rock or sediment;Facies = character of the rock or sediment; controlled by controlled by environmentenvironment

A lithologic unit may be continuous over a large A lithologic unit may be continuous over a large geographic area, but formed at different times in geographic area, but formed at different times in different areas -is diachronous.different areas -is diachronous. diachronous lithologic units = ones which cross time line diachronous lithologic units = ones which cross time line

since lithologic units are based solely on aspect or facies, since lithologic units are based solely on aspect or facies, e.g.. Chert bedse.g.. Chert beds

LithostratigraphyLithostratigraphy

Key correlations between cores Key correlations between cores from different areas are by from different areas are by lithologic units known as marker or lithologic units known as marker or key beds:key beds: volcanic ash layers (tephrochronology)volcanic ash layers (tephrochronology)

lithologically distinctive,lithologically distinctive, widespreadwidespread dateabledateable unique chemistry determined by the unique chemistry determined by the

sourcesource TurbiditesTurbidites microtektitesmicrotektites

These units are often bioturbated These units are often bioturbated (organisms living in the mud can (organisms living in the mud can mix lower layers up and vice versa)mix lower layers up and vice versa)

BiostratigraphyBiostratigraphy

strata are characterized by their fossil contentstrata are characterized by their fossil content organize into units based on fossil characteristics organize into units based on fossil characteristics

(assemblage, ranges etc.)(assemblage, ranges etc.) not large fossils (which are rare) but microfossilsnot large fossils (which are rare) but microfossils Fossils are extremely common in marine sediments; in Fossils are extremely common in marine sediments; in

fact they are the main constituent in many cases as fact they are the main constituent in many cases as you'll see in the labyou'll see in the lab

Organisms are sensitive to their environment - Organisms are sensitive to their environment - live where they are comfortable - live where they are comfortable - you don't find polar bears in Florida of alligators in you don't find polar bears in Florida of alligators in

Maine. Maine. We can use these relationships to determine We can use these relationships to determine

something about the environmentsomething about the environment

BiostratigraphyBiostratigraphy Biostratigraphy: based on appearances and Biostratigraphy: based on appearances and

disappearances of species disappearances of species This assumes that EITHER:This assumes that EITHER:

species appeared (evolved) or disappeared (became extinct) species appeared (evolved) or disappeared (became extinct) globallyglobally

oror conditions changed locally causing species to migrateconditions changed locally causing species to migrate

Evolutionary changes: Evolutionary changes: globalglobal irreversibleirreversible allow determination of chronologyallow determination of chronology

Environmental changes:Environmental changes: local or globallocal or global reversiblereversible provide environmental informationprovide environmental information

ChronstratigraphyChronstratigraphy

Chronstratigraphy: based on age relationsChronstratigraphy: based on age relations Useful because:Useful because:

we can determine we can determine ratesrates (including sediment accumulation (including sediment accumulation rate) rate)

another dimension for correlation over long distances helps another dimension for correlation over long distances helps determine sequence of eventsdetermine sequence of events

Compare samples from different areas which don't overlap Compare samples from different areas which don't overlap in timein time

Based on the predictable decay of radioactive Based on the predictable decay of radioactive isotopes:isotopes: UraniumUranium ThoriumThorium CarbonCarbon etc..etc..

ChronostratigraphyChronostratigraphy

ChronostratigraphyChronostratigraphy Lots of radioactive isotopes can be used for studies Lots of radioactive isotopes can be used for studies

of the oceanof the ocean choice depends on chemistry and half-lifechoice depends on chemistry and half-life

Stable Isotope StratigraphyStable Isotope Stratigraphy

Not radioactive! (“Stable”) Based on the relative abundances of isotopes of:

oxygen (18‑16) carbon (13‑12) Hydrogen sulfur

Oxygen isotopes The ratio is expressed (as are others) as a ' 18O'

18O __ 18O 16O sample 16O standard

18O = × 1000 18O 16O standard

Oxygen Isotope StratigraphyOxygen Isotope Stratigraphy

18O __ 18O 16O sample 16O standard

18O = × 1000 18O 16O standard

standard is PDB (PeeDee belemnite (from Cretaceous belemnite from the PeeDee formation of South Carolina) or SMOW (standard mean ocean water)

A 18O value of 2 would mean that the sample was enriched in 18O relative to SMOW by 2 parts per mil

higher values contain more 18O than the standard, so we call them “heavy”

Oxygen Isotope StratigraphyOxygen Isotope Stratigraphy

The The 1818O/O/1616O ratio of marine carbonate microfossils varies O ratio of marine carbonate microfossils varies as you look down a coreas you look down a core Changes are synchronous throughout the world oceansChanges are synchronous throughout the world oceans Both physical and biological processes can lead to fractionation:Both physical and biological processes can lead to fractionation:

Two sources of Two sources of 's:'s: 1) temperature at which organism grows (biological effect)1) temperature at which organism grows (biological effect) 2) ratio in seawater (physical effects)2) ratio in seawater (physical effects)

Evaporation of water (any water) preferentially vaporizes Evaporation of water (any water) preferentially vaporizes 1616O, concentration the O, concentration the 1818O in the remaining waterO in the remaining water Freshwater (rain and snow), which is derived from water vapor Freshwater (rain and snow), which is derived from water vapor

which has moved inland from ocean areas is enriched in which has moved inland from ocean areas is enriched in 1616O.O. Saltwater, which is left behind, is enriched in Saltwater, which is left behind, is enriched in 1818O.O.

Therefore Therefore 1818O is an indicator of salinity at the time the O is an indicator of salinity at the time the fossil grew.fossil grew.

Oxygen Isotope StratigraphyOxygen Isotope Stratigraphy 1818O as an indicator of global climateO as an indicator of global climate

in periods of cold climate, ice accumulates on the continentsin periods of cold climate, ice accumulates on the continents this ice is derived from snowthis ice is derived from snow

During glacial stages, During glacial stages, 1818O rises (seawater is enriched in O rises (seawater is enriched in 1818O)O) During interglacial stage During interglacial stage 1818O lowers (O lowers (1616O returns in O returns in

meltwater)meltwater) The The 1818O/O/1616O ratio ( O ratio ( 1818O) can also be used as a measure of O) can also be used as a measure of

paleotemperature: paleotemperature: There is a growth temperature effect when organisms take up There is a growth temperature effect when organisms take up

molecules containing oxygen:molecules containing oxygen: as seawater temp lowers, as seawater temp lowers, 1818O rises in fossil shellsO rises in fossil shells Note that the ice volume and growth temperature effects are Note that the ice volume and growth temperature effects are

reinforcing reinforcing (i.e.. cold temperatures make large ice volumes and both effects result in (i.e.. cold temperatures make large ice volumes and both effects result in

increased increased 1818O ("heavier" = more positive number)O ("heavier" = more positive number)

1818O O summarysummary

cold climatecold climate warm climatewarm climate more icemore ice less iceless ice seawater has more seawater has more 1818O O seawater has less seawater has less 1818O O 1818O in seawater O in seawater 1818O in seawaterO in seawater

is less negative is less negative is more negativeis more negative (more positive)(more positive) (less positive)(less positive)

seawater is "heavy"seawater is "heavy" seawater is "light”seawater is "light”

Thus Thus 1818O curves should quite clearly indicate O curves should quite clearly indicate glacial maxima and minimum in sediment glacial maxima and minimum in sediment cores, particularly in the Pleistocene.cores, particularly in the Pleistocene.

Stratigraphy and Sea Level HistoryStratigraphy and Sea Level History

The rest of the story on continental shelf The rest of the story on continental shelf sedimentationsedimentation

Remember we talked about wave energy…….Remember we talked about wave energy…….

Continental Shelf SedimentationContinental Shelf Sedimentation

Water depth increases with distance offshore, so:Water depth increases with distance offshore, so: The energy at the bottom decreases with increasing The energy at the bottom decreases with increasing

distance offshore.distance offshore. Hence grain size diminishes with increasing distance Hence grain size diminishes with increasing distance

offshore.offshore.

Continental Shelf Sedimentation (Cont.)Continental Shelf Sedimentation (Cont.)

In the ideal case, grain size would look like this:In the ideal case, grain size would look like this:

Continental Shelf SedimentationContinental Shelf Sedimentation

All of this assumes a “steady state” condition, but this All of this assumes a “steady state” condition, but this isn’t true!isn’t true!

Sea level has been rising so rapidly over the last Sea level has been rising so rapidly over the last 10,000 years that conditions have not had a chance 10,000 years that conditions have not had a chance to equilibrateto equilibrate

Continental Shelf Sedimentation Continental Shelf Sedimentation (Cont.)(Cont.)

Local changes varied quite a bit from global onesLocal changes varied quite a bit from global ones local subsidence or upliftlocal subsidence or uplift local sedimentation / accumulationlocal sedimentation / accumulation

Even so, global changes can be determined if you’re diligentEven so, global changes can be determined if you’re diligent

Sea Level HistorySea Level History

Radiocarbon dating is almost always involvedRadiocarbon dating is almost always involved These curves are for a variety of samplesThese curves are for a variety of samples

Sea Level HistorySea Level History

What causes What causes these changes?these changes?

We think it has to We think it has to do has to do with do has to do with the earth’s orbital the earth’s orbital parameters:parameters:

Continental Shelf SedimentationContinental Shelf Sedimentation(Cont.)(Cont.)

As sea level rises, the As sea level rises, the coastline (where water and coastline (where water and land meet) retreatsland meet) retreats

This map shows the This map shows the changes over the last 15,000 changes over the last 15,000 yearsyears since the peak of the last ice since the peak of the last ice

ageage Will this retreat continue??!!Will this retreat continue??!!

Sea Level HistorySea Level History

Sea Level Sea Level variations variations control coastal control coastal depositional depositional patternspatterns

Continental Shelf Continental Shelf SedimentationSedimentation

(Cont.)(Cont.)

River channels River channels extended onto the extended onto the shelf during periods of shelf during periods of lowered sea level. lowered sea level.

Coarse material was Coarse material was deposited on top of deposited on top of fine material.fine material.

Deposition and Deposition and erosion in the deep erosion in the deep sea increased.sea increased.

Coral reefs exposed Coral reefs exposed by reduced sea level by reduced sea level died.died.

Continental Shelf Sedimentation Continental Shelf Sedimentation (Cont.)(Cont.)

Actual shelf sediments therefore:Actual shelf sediments therefore: Are coarse-grain sediments on the outer shelf.Are coarse-grain sediments on the outer shelf. They were deposited at an earlier time under They were deposited at an earlier time under

different conditions than the surrounding different conditions than the surrounding sediments.sediments.

They provide evidence of a previous time of They provide evidence of a previous time of lowered sea level.lowered sea level.

Continental Shelf SedimentationContinental Shelf Sedimentation We call these ancient sediments “Relict sediments”We call these ancient sediments “Relict sediments” because they reflect conditions in a previous timebecause they reflect conditions in a previous time

Our local environment: SandOur local environment: Sand

Our local environment: Grain sizeOur local environment: Grain size

Continental Shelf Continental Shelf SedimentationSedimentation

During the earth’s history:During the earth’s history: the positions of the the positions of the

continents have changedcontinents have changed (we already covered this)(we already covered this)

plate boundaries were plate boundaries were differentdifferent

climate was differentclimate was different sea level was often MUCH sea level was often MUCH

higherhigher

much of North America much of North America was under waterwas under water

Sea Level HistorySea Level History Tectonic changes have Tectonic changes have

caused even larger caused even larger variationsvariations

Over the millennia, sea Over the millennia, sea level fluctuations have level fluctuations have been the rule rather been the rule rather than the exceptionthan the exception

rapid fall and slow rapid fall and slow recoveryrecovery

up to 300m higher than up to 300m higher than presentpresent

up to 120m lower than up to 120m lower than present present

SummarySummary

Sediment stratigraphy helps unlock the earth’s historySediment stratigraphy helps unlock the earth’s history climateclimate sea levelsea level circulationcirculation evolutionevolution

Sea level has changed during earth’s historySea level has changed during earth’s history substantial fluctuationssubstantial fluctuations caused in part by climate changescaused in part by climate changes sea level controls sea level controls

sedimentation patternssedimentation patterns chemistry of seawaterchemistry of seawater