time introduction to earth science

Lecture 10 Stratigraphy and Geologic
Time
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Stratigraphy
Basic principles of relativ e age dating
Unconformities: Markers of missing time
Correlation of rock units
A bsolute dating
Geologic Time
How old is the Earth? W hen did v arious geologic ev ents
occur? Interpreting Earth history is a prime goal of geology .
Some know ledge of Earth history and geologic time is also
required for engineers in order to understand relationships
betw een geologic units and their impact on engineering
construction.


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Stratigraphy :

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Stratigraphy is the study of rock lay ers (strata)

and their relationship w ith each other.

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Stratigraphy prov ides simple principles used to
interpret geologic ev ents.


Two rock units at a cliff in Missouri. (US Geological Survey)


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Basic principles of relativ e age dating
Relativ e dating means that rocks are placed in
their proper sequence of formation. A formation is a
basic unit of rocks. Below are some basic principles
for establishing relativ e age betw een formations.

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Principle of original horiz ontality

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Principle of superposition

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Principle of faunal succession

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Principle of cross-cutting relationships


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Principle of original horiz ontality :

Lay ers of sediment are generally
deposited in a horiz ontal position.
Thus if w e observ ed rock lay ers that are
folded or inclined, they must, w ith
exceptions, hav e been mov ed into that
position by crustal disturbances sometime
after their deposition.


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Most lay ers of sediment are deposited in a nearly horiz ontal position.
Thus, w hen w e see inclined rock lay ers as show n, w e can assume that
they must hav e been mov ed into that position after deposition.
Hartland Quay , Dev on, England by Tom Bean/DRK Photo.


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Principle of superposition:

In an undeformed sequence of
sedimentary rocks, each bed is older
than the one abov e and y ounger than the
one below .
The rule also applies to other surfacedeposited materials such as lav a flow s
and v olcanic ashes.


Principle of superposition. (W.W. Norton)


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A pply ing the law of superposition to the lay ers at the upper
portion of the Grand Cany on, the Supai Group is the oldest and
the K aibab Limestone is the y oungest. (photo by Tarbuck).


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Principle of cross-cutting relationships:

W hen a fault cuts through rocks, or w hen
magma intrudes and cry stalliz es, w e can
assume that the fault or intrusion is
y ounger than the rocks affected.


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Cross-cutting relationships: A n intrusiv e rock body is
y ounger than the rocks it intrudes. A fault is y ounger
than the rock lay ers it cuts. (Tarbuck and Lutgens)


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Unconformities: Markers of missing time
W hen lay ers of rock formed w ithout interruption, w e call
them conformable.
A n unconformity represents a long period during w hich
deposition ceased and erosion remov ed prev iously formed
rocks before deposition resumed.

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A ngular unconformities

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Disconformity

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Nonconformity


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A ngular unconformities:

A n angular unconformity consists of tilted
or folded sedimentary rocks that are
ov erlain by y ounger, more flat-ly ing
strata.
It indicates a long period of rock
deformation and erosion.


Formation of an angular unconformity. An angular unconformity
represents an extended period during which deformation and erosion
occurred. (Tarbuck and Lutgents)


Angular unconformity at Siccar
Point, southern Scotland, that
was first described by James
Hutton more than 200 years ago.
(Hamblin and Christiansen and
W.W. Norton)


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Disconformity :

A disconformity is a minor irregular
surface separating parallel strata on
opposite sides of the surface.

It indicates a history of uplifting abov e
sea (w ater) lev el, undergoing erosion, and
low ering below the sea lev el again.


Formation of disconformity. (W.W. Norton)


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Disconformities do not show angular discordance, but an erosion
surface separates the tw o rock bodies. The channel in the central part of
this outcrop rev eals that the low er shale units w ere deposited and then
eroded before the upper units w ere deposited. (Hamblin and
Christiansen)


Nonconformity

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A nonconformity is a break
surface that dev eloped w hen
igneous or metamorphic rocks
w ere exposed to erosion, and
y ounger sedimentary rocks w ere
subsequently deposited abov e
the erosion surface. (Tarbuck and
Lutgens)

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A nonconformity at the Grand Cany on. The
metamorphic rocks and the igneous dikes of the inner
gorge w ere formed at great depths and subsequently
uplifted and eroded. Y ounger sedimentary lay ers w ere then
deposited on the eroded surface of the igneous and
metamorphic terrain. (Hamblin and Christiansen)


Types of Unconformity
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This animation show s the stages in the
dev elopment of three main ty pes of unconformity
in cross-section, and explains how an incomplete
succession of strata prov ides a record of Earth
history . V iew 1 show s a disconformity , V iew 2
show s a nonconformity and V iew 3 show s an
angular unconformity . [by Stephen Marshak]

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Play A nimation W indow s v ersion >>

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Play A nimation Macintosh v ersion >>

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Distinguishing nonconformity and intrusiv e
contact

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Nonconformity :
The sedimentary rock is y ounger. The erosion surface is
generally smooth. Dikes may cut through the igneous body
but stop at the nonconformity .

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Intrusiv e contact:
Intrusion is y ounger than the surrounding sedimentary
rocks. The contact surface may be quite irregular. A z one of
contact metamorphism may form surrounding the igneous
body . Cross-cutting dikes may penetrate both the igneous
body and the sedimentary rocks.


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Contrasting field conditions for (a) a nonconformity
and (b) an igneous intrusion. (W est, Fig 9.4)


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The three basic ty pes of unconformities illustrated by
this cross-section of the Grand Cany on. (Tarbuck and
Lutgents)