Ch. 6: Correlation & Dating of the rock recordfaculty.chas.uni.edu/~groves/EHCh06lecture.pdf · Ch....

Earth History, Ch. 6 1

Ch. 6: Correlation & Dating of

the rock record

• The geologic time scale

• Geologic systems

• Stratigraphic units and correlation

• Radioactivity and absolute ages

• Dating by other stratigraphic features


Geologic time scale

• The geologic time scale was first established using the relative ages of rocks

– William Smith conceived the Law of Faunal Succession

• Fossil content of a rock unit can be used to distinguish that unit from units above and below

– Georges Cuvier established that entire biotas have become extinct and then replaced by entirely new biotas

• 7 distinct biotas recognized in the Cenozoic rocks of the Paris Basin


1830’s: Earliest definition of

geologic systems

Crystalline rocks

Adam Sedgewick:

Cambrian System

Sedimentary rocks with

few fossils, resting directly

on crystalline rocks

Roderick Murchison:

Silurian System

Sedimentary rocks

containing distinctive

fossils, resting on

other sedimentary rocks


1870’s: Revision of geologic

systems

Crystalline rocks

Ordovician System

(second fauna)

Cambrian System

(first fauna)

Silurian System

(third fauna)

Charles Lapworth:

Recognized that the original

Cambrian and Silurian systems

actually contain three distinct

faunas, with the lower Silurian

overlapping the upper Cambrian.

He proposed Ordovician System

for the intermediate (second) fauna.


1870’s through present:

Continued revision of systems

• New systems have been recognized

• Boundaries between systems have been revised repeatedly (as recently as 2001!!!)

• Phanerozoic time scale is now complete, so that all boundaries are formally defined and there are no gaps and no overlaps

– Any Phanerozoic rock can be assigned to one and only one system


Permian-Triassic system

boundary

“Golden spike” was driven inAugust, 2001 in bed 27 atMeishan, China, coincident with evolutionaryappearance of a conodontspecies (Hindeodus parvus)


Geologic time scale

Phan

erozo

ic E

on

Archaean Eon

Proterozoic Eon

Paleozoic Era

Mesozoic Era

Cenozoic EraNeogene System

Paleogene System

Cretaceous System

Jurassic System

Triassic System

Permian System

Carboniferous System

Devonian System

Silurian System

Ordovician System

Cambrian System


Stratigraphic units

• Stratigraphy = the study of layered rocks,

especially their geometry, composition, modes of

origin, and age relations

• Correlation = the procedure of demonstrating

correspondence between geographically separated

units

– Lithologic correlation = correspondence of rock types

– Age (temporal) correlation = correspondence in age

between separate rock units


Stratigraphic units

• Kinds of stratigraphic units:

– Rock units

– Time-rock units

– Time units

– Biostratigraphic units


Rock units

• Rock units (aka “lithostratigraphic units”) are

three-dimensional bodies of rock characterized by

a particular physical attribute (rock type, color,

etc.)

– Group (two or more formations)

– Formation (mappable rock unit)

– Member (subdivision of a formation)

• Rock units at a given locality can be portrayed

graphically by a stratigraphic section


Lithologic (rock) correlation

Sandstone

Shale

Limestone

correlation

correlation

10 milesSandstone

Shale

Limestone


Time-rock units vs. time units

• Time-rock unit = all the rocks in the world that

were depositied during a given interval of time

• Time unit = the interval of time from the base of a

time-rock unit to its top

• For example:

– Cambrian System (time-rock) = all rocks deposited

between 543 and 510 million years ago

– Cambrian Period (time) = the 33 million years of time

between 543 and 510 million years ago


Time-rock units vs. time units

Time-rock Unit Time Unit

Erathem……………………..Era

System………………………..Period

Series…………………………Epoch

Stage………………………….Age

There is a one-to-one correspondence between time-rock

units and time units. Thus we say that the rocks of the

Cambrian System were deposited during the Cambrian Period.


Biostratigraphic units

• Biostratigraphic units = bodies of rock that are

characterized by their fossil content

– usually called zones or biozones

– usually defined by the stratigraphic ranges of one or

more key fossils

• Stratigraphic range = the total vertical interval

through which a fossil species occurs in rocks

(point of evolutionary origin through point of

extinction)


Age correlation

Sandstone

Shale

Limestone

Sandstone

Shale

Limestone

Extinction of

trilobite species

Origin of

urchin species


Biostratigraphic units (cont.)

A

B

C D

Zone 1 = the total

stratigraphic range of “A”

Zone 2 = the overlapping

stratigraphic ranges of “B”

and “C”



• Index fossils (aka “guide fossils”) are fossils that are

particularly useful in biostratigraphic correlation.

Characteristics of index fossils are:

– Abundant

– Easily identified

– Geographically widespread

– Occurs in a variety of rock types

– Short stratigraphic range (rapid rates of evolution)



• Examples of good index fossils

– Planktonic organisms, especially microfossils

• Planktonic forams, dinoflagellates, calcareous nannoplankton, radiolarians, graptolites

– Nektonic or mobile organisms (swimmers, crawlers)

• Cephalopods, trilobites, conodonts

• Examples of poor index fossils

– Sessile, benthonic organisms

• Corals, stromatoporoids, sponges, echinoderms

– Organisms adapted to narrow ecologic niches

• Most terrestrial organisms, diatoms


Other types of correlation and

age dating

• Magnetostratigraphy

• Radiometric dating

• Key beds


Magnetic stratigraphy

• Motion of magnetic material (iron) in the liquid outer core is responsible for Earth’s magnetic field

• The magnetic field is polarized—north and south poles

• Polarity has reverseditself repeatedly over the course of Earth’s history


Magnetic “striping” of oceanic crust


Magnetic “striping”

of oceanic crust


Magnetic stratigraphy (cont.)

• Magnetic sediment grains typically align themselves according to Earth’s polarity at the time of deposition

• Reversals of polarity can be detected by magnetic sampling of sedimentary rocks in a vertical sequence—magnetic stratigraphy

• Polarity time-rock units are correlatable packages of rock deposited during a particular normal or reversed polarity time interval


Radiometric dating

• Some elements have two or more isotopes:

i.e., varieties with the same number of

protons, but different numbers of neutrons

• Some isotopes are radioactive: i.e., they

decay into other isotopes by losing or

gaining protons


Radiometric dating (cont.)

• Radioactive decay of

each radioactive

isotope occurs at a

constant rate

• Half life = the time

(in years) required

for the decay of one

half of the original

amount of the parent

isotope

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

half-lives

% o

f p

are

nt

rem

ain

ing


Radiometric dating (cont.)radioactive

isotope

half life

(years)

daughter

product

Rubidium 87 48.6 billion Strontium 87

Thorium 232 14 billion Lead 208

Potassium 40 1.3 billion Argon 40

Uranium 238 4.5 billion Lead 206

Uranium 235 0.7 billion Lead 207

Carbon 14 5730 Nitrogen 14

Note: Carbon 14 has a very short half life, so it is useful in

dating materials only as old as 50,000 to 70,000 years


Radiometric dating (cont.)

• Some caveats:

– Radiometric “clock” starts ticking when a

mineral grain crystallizes, and the clock can be

reset by subsequent metamorphism

– Must deal with a closed system, i.e. no loss or

gain of either parent or daughter isotopes


Accuracy of radiometric dates vs.

fossil correlation• Radiometric dates usually have an analytical error

of ± 1%

– 50,000 years ± 500 years

– 500,000,000 years ± 5,000,000 years

• Fossil zones usually are ± 1,000,000 years in duration, depending on rates of evolution within a given taxon

• When dating sedimentary rocks, it may be difficult to find unaltered, datable minerals (usually found in interbedded volcanic ash layers)


Accuracy of radiometric dates vs.

fossil correlation

0

2

4

6

8

10

12

10

million

50

million

100

million

200

million

500

million

1 billion

age of dated material

dati

ng

err

or

(millio

ns o

f years

)

fossil error

radiometric error


Correlation of key beds

• Key beds (= “marker beds”)

– A thin layer of rock deposited over a broad region, and known to be of the same age throughout its areal distribution

– Such a layer represents a “geologic time line”

– Example: volcanic ash beds(deposited instantaneously in geologic terms)

Ch. 6: Correlation & Dating of the rock recordfaculty.chas.uni.edu/~groves/EHCh06lecture.pdf · Ch....

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