BREAKS

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Chapter 3
UNCONFORMITY
BREAKS IN THE RECORD
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• BREAKS IN THE RECORD
• The stratigraphy is the study of stratified
  rocks.
• So whereever we study these rocks, especially the
  sedimentary rocks we find some gaps.
• These gaps vary or range in magnitude
• from the short span of time, recorded only
  obscure bedding planes,
• to great breaks where a very large gap of time
  is recorded.

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Unconformities

• So far we have discussed vertical relationships
  among conformable strata
• sequences of rocks in which deposition was more
  or less continuous
• Unconformities in sequences of strata represent
  times of nondeposition and/or erosion that
  encompass long periods of geologic time
• millions to hundreds of millions of years
• The rock record is incomplete
• interval of time not represented by strata is a
  hiatus

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Origins of an Unconformity

• Deposition began 12 million years ago (MYA)
• Continuing until 4 MYA

• For 1 million years erosion occurred
• removing 2 MY of rocks

• The last column is the actual stratigraphic
  record with an unconformity

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• BREAKS IN THE RECORD
• Unconformity
• Unconformity is the temporal break in a
  stratigraphic sequence resulting from a change in
  regime that caused deposition to cease for a
  considerable span of time.
• Uplift ? Erosion with the loss of some of the
  previously formed record.
• Unconformity is a HIATUS representing a certain
  span of time.

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• Types of Unconformity
• Nonconformity Stratified rocks rest upon
  non-stratified rocks, either igneous or
  metamorphic rocks.
• Angular Unconformity An angular discordance
  separates two units of stratified rocks.
• Disconformity All strata are parallel but the
  contact between two units is an uneven erosional
  surface. The surface of unconformity is an old
  erosion surface of appreciable relief.
• ParaconformityThe beds are parallel and the
  contact is a simple bedding plane. So the
  evidence of a break is not obvious.

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Figure 50
Types of Unconformity
(Dunbar Rodgers, 1957)
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Nonconformity--Triassic dolostone over
crystalline basement, French Alps
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Nonconformity
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Deeply Eroded Intrusions - NONCONFORMITY
GRANITE INTRUSION
SHALE
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Nonconformity Stratified rocks rest upon
non-stratified rocks, either igneous or
metamorphic rocks.
(Dunbar Rodgers, 1957)
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Angular Unconformity An angular discordance
separates two units of stratified rocks.
(Dunbar Rodgers, 1957)
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Angular Unconformity
Hutton's angular unconformity at Siccar Pt.,
Scotland
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Angular Unconformity
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Angular Unconformity
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Disconformity All strata are parallel but the
contact between two units is an uneven
erosional surface. The surface of unconformity
is an old erosion surface of appreciable relief.
(Dunbar Rodgers, 1957)
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U

• Paraconformity The beds are parallel and the
  contact is a simple bedding plane. So the
  evidence of a break is not obvious.

(Dunbar Rodgers, 1957)
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Late Carboniferous
Early Carboniferous
(Dunbar Rodgers, 1957)
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Types of Unconformities

• Unconformities of regional extent may change from
  one type to another

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CRITERIA for RECOGNITION and EVALUATION

• Three steps
• Recognition of the unconformity
• Determination the magnitude
• Evaluation the importance
• Nonconformity
• No contact metamorphism (Chilled / Baked
  margins)
• Pebbles of the igneous/metamorphic rocks in the
  overlying units.

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NONCONFORMITY ????
(Dunbar Rodgers, 1957)
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ANGULAR UNCONFORMITY
(Dunbar Rodgers, 1957)
The angle of discordance does not indicate the
relative importance of the hiatus. The
discordance may reach any angle up to 900 (a) or
the folds flatten out (f) the discordance
declines even though the hiatus does not decrease.
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ANGULAR UNCONFORMITY ????
Horizontally bedded Carmel formation resting on
cross-bedded Navajo sandstone (both Jurassic)
1.75 miles west of Boulder, Utah.
(Dunbar Rodgers, 1957)
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• DISCONFORMITY
• Erosional relief at the contact
• Where the surface of unconformity cuts across
  beds, it is evident that uplift and erosion have
  occurred during the hiatus.
• Relief makes the unconformity impressive. The
  relief is limited by the amount of the uplift and
  the activity of erosional agent (stream).
• So during a very short time you may have an
  impressive
• relief and this make you think that there is a
  very important unconformity.

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Erosional relief at the contact
(Dunbar Rodgers, 1957)
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• DISCONFORMITY
• Regional beveling of formations or of faunal
  zones
• In conformable rock sequences, an unconformity
  may sometimes be recognized by the systematic
  appearance (or disappearance) of units at a given
  contact over a large region.

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Regional beveling of formations
(Dunbar Rodgers, 1957)
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Regional beveling of faunal zones
(Dunbar Rodgers, 1957)
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CRITERIA for RECOGNITION and EVALUATION

• Abrupt lithologic change
• Evidence of an old erosion surface
• Physical form of the surface
• Irregularities
• Mounds
• Crevices
• Caves
• This surface is overlapped and filled by the
  materials of the overlying unit.
• Evidence of weathering of the rock below before
  deposition of the rock above
• Fire clay
• Old soil profile
• Silicified surfaces
• Calichified surfaces

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Evidence of an old erosion surface
(Dunbar Rodgers, 1957)
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• The abrupt truncation of structural features
• Joints
• Folds
• Faults
• Dykes
• It may be the presence of pebbles from the beds
  below in the basal layers above.
• Lag gravels and sands, concentrates of phosphate,
  manganese, iron or glauconite nodules or grains
  may be suggestive for evidence of unconformity.

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• Paleontological evidence
• The final and the only criterion that gives
  quantitative results for the large
  unconformities.
• The faunal breaks prove a hiatus and at the same
  time indicate its temporal value by placing the
  beds above and below in widely different
  divisions of the geologic column.
• However, an abrupt faunal break is no evidence of
  a hiatus. It may be due to a change in bottom
  ecology without interruption of the deposition.

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• Diastem
• Short breaks due to non-deposition or to
  sublevation of loose sediment on the sea floor
  are called diastems.
• An unconformity records a change in the overall
  conditions of deposition, commonly involving, at
  least regional uplift and erosion.
• Diastems, on the contrary, are smaller breaks
  resulting from the normal changes that occur
  without any basic change in the general regimen.