Marine sedimentation Chapter 3

choppedspleenMécanique

21 févr. 2014 (il y a 3 années et 6 mois)

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Stratigraphy


The endless rain of sediments
produces layers.



Stratigraphy
is the study of the
thickness and composition of
sedimentary layers.


Such study provides insight into
rates of sedimentation,


sea
-
floor spreading,


and even climate.

Stratigraphy


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


The record is
retained:


climate


environment of
deposition


life forms

Stratigraphy


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 Overview


Here is how these factors work:


1.
Delivery





2.
Dissolution




3.
Dissemination




4.

Dilution



5.
Diagenetic alteration

Atmospheric

Biogeni
c

Rivers

Stratigraphy Analogue


1.
Delivery


2.
Dissolution


3.
Dissemination


4.

Dilution


5.
Diagenetic alteration


Types of Stratigraphy


Lithostratigraphy: sediment chemistry, grain size,
physical characteristics


Biostratigraphy: fossils


magnetostratigraphy: magnetic field direction


stable isotope stratigraphy: ratio of one isotope to
another


chronostratigraphy: absolute age from radiogenic
isotopes



Lithostratigraphy


Distinguish layers basis of lithology or physical
characteristics:


grain size,


mineralogy


mode of deposition etc.


Facies = character of the rock or sediment;


controlled by
environment


A lithologic unit may be continuous over a large
geographic area, but formed at different times in different
areas
-
is diachronous.


diachronous lithologic units = ones which cross time line since
lithologic units are based solely on aspect or facies,


e.g.. Chert beds

Lithostratigraphy


Key correlations between cores from
different areas are by lithologic units
known as marker or key beds:


volcanic ash layers (tephrochronology)


lithologically distinctive,


widespread


dateable


unique chemistry determined by the source


Turbidites


microtektites


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


Biostratigraphy


strata are characterized by their fossil content


organize into units based on fossil characteristics (assemblage,
ranges etc.)


not large fossils (which are rare) but microfossils


Fossils are extremely common in marine sediments; in fact they are
the main constituent in many cases as you'll see in the lab


Organisms are sensitive to their environment
-



live where they are comfortable
-



you don't find polar bears in Florida of alligators in Maine.


We can use these relationships to determine something about the
environment

Biostratigraphy


Biostratigraphy: based on appearances and disappearances
of species


This assumes that EITHER:


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

or


conditions changed locally causing species to migrate


Evolutionary changes:


global


irreversible


allow determination of chronology


Environmental changes:


local or global


reversible


provide environmental information

Chronstratigraphy


Chronstratigraphy: based on age relations


Useful because:


we can determine
rates

(including sediment accumulation rate)


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


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


Based on the predictable decay of radioactive isotopes:


Uranium


Thorium


Carbon


etc..


Chronostratigraphy


Chronostratigraphy


Lots of radioactive isotopes can be used for studies
of the ocean


choice depends on chemistry and half
-
life

Stable 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 '

18
O'




18
O


__


18
O




16
O sample


16
O standard



18
O =
×

1000





18
O





16
O standard


Oxygen Isotope Stratigraphy





18
O


__


18
O




16
O sample


16
O standard



18
O =
×

1000





18
O





16
O standard


standard is PDB (
PeeDee

belemnite (from Cretaceous
belemnite from the
PeeDee

formation of South Carolina)
or SMOW (standard mean ocean water)


A

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


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

Oxygen Isotope Stratigraphy


The
18
O/
16
O ratio of marine carbonate microfossils varies
as you look down a core


Changes are synchronous throughout the world oceans


Both physical and biological processes can lead to fractionation:


Two sources of

's:


1) temperature at which organism grows (biological effect)


2) ratio in seawater (physical effects)


Evaporation of water (any water) preferentially vaporizes
16
O, concentration the
18
O in the remaining water


Freshwater (rain and snow), which is derived from water vapor
which has moved inland from ocean areas is enriched in
16
O.


Saltwater, which is left behind, is enriched in
18
O.


Therefore


18
O is an indicator of salinity at the time the
fossil grew.

Oxygen Isotope Stratigraphy



18
O as an indicator of global climate



in periods of cold climate, ice accumulates on the continents



this ice is derived from snow


During glacial stages,

18
O rises (seawater is enriched in
18
O)


During interglacial stage

18
O lowers (
16
O returns in
meltwater)


The
18
O/
16
O ratio (
18
O) can also be used as a measure of
paleotemperature:



There is a growth temperature effect when organisms take up
molecules containing oxygen:


as seawater temp lowers,
18
O rises in fossil shells


Note that the ice volume and growth temperature effects are
reinforcing


(i.e.. cold temperatures make large ice volumes and both effects
result in increased
18
O ("heavier" = more positive number)


18
O
summary


cold climate


warm climate



more ice



less ice


seawater has more
18
O

seawater has less
18
O




18
O in seawater




18
O in seawater



is less negative


is more negative


(more positive)


(less positive)


seawater is "heavy"

seawater is "light”



Thus
18
O curves should quite clearly indicate glacial
maxima and minimum in sediment cores, particularly in
the Pleistocene.

Stratigraphy and Sea Level History


The rest of the story on continental shelf
sedimentation


Remember we talked about wave energy…….

Continental Shelf Sedimentation


Water depth increases with distance offshore, so:


The energy at the bottom decreases with increasing distance
offshore.


Hence grain size diminishes with increasing distance
offshore.

Continental Shelf Sedimentation (Cont.)


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

Continental Shelf Sedimentation


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


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

Continental Shelf Sedimentation
(Cont.)


Local changes varied quite a bit from global ones


local subsidence or uplift


local sedimentation / accumulation


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

Sea Level History


Radiocarbon dating is almost always involved


These curves are for a variety of samples

Sea Level History


What causes
these changes?


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

Continental Shelf Sedimentation

(Cont.)


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


This map shows the
changes over the last 15,000
years


since the peak of the last ice
age


Will this retreat continue??!!

Sea Level History


Sea Level
variations
control coastal
depositional
patterns

Continental Shelf
Sedimentation

(Cont.)


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


Coarse material
was deposited on
top of fine material.


Deposition and
erosion in the deep
sea increased.


Coral reefs
exposed by
reduced sea level
died.

Continental Shelf Sedimentation
(Cont.)


Actual shelf sediments therefore:


Are coarse
-
grain sediments on the outer shelf.


They were deposited at an earlier time under
different conditions than the surrounding
sediments.


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

Continental Shelf Sedimentation


We call these ancient sediments “Relict
sediments”


because they reflect conditions in a previous
time

Our local environment: Sand


Our local environment: Grain size


Continental Shelf
Sedimentation


During the earth’s history:


the positions of the
continents have changed


(we already covered this)


plate boundaries were
different


climate was different


sea level was often MUCH
higher


much of North America
was under water

Sea Level History


Tectonic changes have
caused even larger
variations


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


rapid fall and slow
recovery


up to 300m higher than
present


up to 120m lower than
present

Summary


Sediment stratigraphy helps unlock the earth’s history


climate


sea level


circulation


evolution


Sea level has changed during earth’s history


substantial fluctuations


caused in part by climate changes


sea level controls


sedimentation patterns


chemistry of seawater