UNCONFORMITY
Unconformities are gaps in the geologic record that may indicate episodes of crustal
deformation, erosion, and sea level variations. They are a feature of stratified rocks, and are
therefore usually found in sediments (but may also occur in stratified volcanics). They are
surfaces between two rock bodies that constitute a substantial break (hiatus) in the geologic
record (sometimes people say inaccurately that "time" is missing). Unconformities represent
times when deposition stopped, an interval of erosion removed some of the previously
deposited rock, and finally deposition was resumed.
Unconformities are the record of major episodes of uplift, erosion and subsidence during the
growth of the continents as earth history progressed. They are therefore important evidence
for crustal mobility throughout earth history.
Commonly four types of unconformities are distinguished by geologists:
a) Disconformity (Parallel Unconformity)
Disconformities are usually recognized by
correlating from one area to another and
finding that some strata are missing in one
of the areas.
35 b) Angular Unconformity
Angular unconformities are easy to
recognize in the field because of the
angular relationship of layers that were
originally deposited horizontally.
c) Nonconformity
Nonconformities occur where rocks that
formed deep in the Earth, such as intrusive
igneous rocks or metamorphic rocks, are
overlain by sedimentary rocks formed at the
Earth's surface. The nonconformity can
only occur if all of the rocks overlying the
metamorphic or intrusive igneous rocks
have been removed by erosion.
36 d) Paraconformity
Paraconformities are widespread gaps in the
geologic layers where there is essentially no
irregular erosion of the surface at the gaps,
hence the sedimentary layers below and
above the gaps are parallel. As you consider
paraconformities keep in mind two
characteristics: a gap and parallel layers.
Para means “near”, as in nearly conformable. An unconformity with no obvious erosion
surface. There is a distinct gap in the fossil record.
RELATIVE DATING
Relative dating utilizes six fundamental principles to determine the relative age of a formation
or event.

Principle of Original Horizontality
sedimentary beds deposited near horizontal

Principle of Superposition
layers young from bottom to top

Principles of Lateral Continuity
tapering, thinning near edges

Principle of Cross-Cutting Relationships
disrupted pattern is older than disruption
truncation of older rock bodies

Principles of Intrusive Relationships

Principle of Fossil Succession
principles of faunal succession
concept of index fossil
fossil assemblage
37 In the following figures, ages of units are A, B, C, D, E, F, G from oldest to youngest.
38 39 APPLICATIONS
1. Determine the relative sequence of events in the diagram below. Enter the letter of the
rock unit or geologic structure in the proper Event sequence. The geologic events of tilting,
folding and erosion do not have single letter labels. Type the word "tilting", "folding" or
"erosion" in the proper position. Event 1 is the oldest event. Event 20 is the most recent or
youngest event.
40 2. According to the geological map;
a) Show the inclination of the valley bottom lines (IVBL) on the map.
directions of the beds by writing the appropriate V-rules.
Find the dip
b) Draw the map symbols of the all beds on the map.
c) Put all the beds in vertical order, from youngest to oldest.
d) Draw the geological cross-section A-B and C-D (Vertical Scale: 1/10000).
Dip values of the older series are 40°, younger series are 20° for A-B section,
Dip values of the younger series are 15°, find the dip values of the older series for C-D
section.
41 UNCONFORMITIES
Representing times of
nondeposition, erosion
or both
Paraconformity
Disconformity
Angular unconformity
Nonconformity
1
Prof.Dr.Kadir Dirik Lecture Notes
A simplified diagram showing the development of an unconformity and a hiatus.
(a) Deposition began 12 million years ago (MYA) and continued more or less uninterrupted
until 4 MYA. A 1-million-year episode of erosion occured, and during that time strata
representing 2 million years of geologic time were eroded.
Time of
erosion
Amount of
rock removed
by erosion
2
Prof.Dr.Kadir Dirik Ders Notları
Prof.Dr.Kadir Dirik Lecture Notes
(Monroe&Wicander, 2005)
(c) A hiatus of 3
billion years exists
between the the
older strata and the
starata that formed
during arenewed
episode of
deposition that
began 3 MYA.
(d) The actual
stratigraphic record.
The unconformity is
the surface
separating the
strata and
represents a major
break in our record
of geologic time.
DISCONFORMITY
Uplift
Deposition
Disconcormity
(Monroe&Wicander, 2005)
Uplift and erosion
Deposition
3
Prof.Dr.Kadir Dirik Lecture Notes
ANGULAR UNCONFORMITY
Deposition
Angular
unconformity
(Monroe&Wicander, 2005)
Uplift
Erosion
Uplift and tilting
Deposition
4
Prof.Dr.Kadir Dirik Lecture Notes
NONCONFORMITY
Uplift
Deposition
Nonconformity
(Monroe&Wicander, 2005)
Uplift and erosion of overlying sediments
Intrusion of magna
5
Prof.Dr.Kadir Dirik Lecture Notes
(Monroe&Wicander, 2005)
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Prof.Dr.Kadir Dirik Lecture Notes
Deposition
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Uplift, tilting, and faulting
Prof.Dr.Kadir Dirik Lecture Notes
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Erosion
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Prof.Dr.Kadir Dirik Lecture Notes
Deposition
Prof.Dr.Kadir Dirik Lecture Notes
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Intrusion
Prof.Dr.Kadir Dirik Lecture Notes
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Uplift and erosion
Prof.Dr.Kadir Dirik Lecture Notes
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Deposition
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Intrusion
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Lava flow
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Deposition
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Prof.Dr.Kadir Dirik Lecture Notes