An Introduction to Geological Structures and Maps - BENNISON
Geological structures- التراكيب الجيولوجيه
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Transcript of Geological structures- التراكيب الجيولوجيه
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Geological Structures What are Geologic Structures?
Division of Structures
A- Primary structures Ripple marks Mud cracks Cross bedding Graded bedding Burrows
B- Secondary Structures Folds Faults Joints Unconformities
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What are Geologic Structures?
Geologic structure is any feature in rocks
that results from deformation, such as
folds, joints, and faults.
Geologic structures are usually the result of
the powerful that occur
within the earth. These forces fold and
break rocks, form deep faults, and build
mountains .
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Division of Structures Primary (or sedimentary)
structures: such as ripple marks,
cross-bedding, and mud cracks form
in sediments during or shortly after
deposition.
Secondary structures:
is that structures formed after the
formations of any kind of rocks, such
as folds, faults, or unconformities.
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Primary structures
They are any structures in sedimentary rock
formed at or shortly after the time of
deposition: such as:
Ripple marks
They are wavelike (undulating) structures produced in
granular sediment such as sand by unidirectional wind
and water currents or by oscillating wave currents.
Wind and current ripples. (Asymmetric
Wave ripples. (Symmetric
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Mud cracks
Mud crack is a crack in clay-rich sediment
that has dried out.
Cross bedding
• Cross-bedding takes place when individual
beds or strata are deposited at an
angle to the surface upon which they
accumulate.
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• Origin of cross-bedding by deposition on
the sloping surface of a desert dune.
• Cross-bedding is also common in dune like
structures in stream and river channels.
Graded bedding
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Origin of Graded Bedding in Which Particle
Size decreases Upward Within a Single
Layer (a) A turbidity current flows
downslope along the seafloor or a lake
bottom. (b) The flow slows and deposits
progressively smaller particles, thus
forming a graded bed.
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Burrows
is a hole or tunnel excavated into the ground by an
animal to create a space suitable for habitation
Burrows are also commonly preserved in the fossil record as a
type of trace fossil.
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Secondary Structures Folds
Fold is a type of geologic structure in which
planar features in rock layers such as bedding
and foliation have been bent.
Anticline is a convex upward fold in which
the oldest exposed rocks coincide with the
fold axis and all strata dip away from the
axis.
Syncline is a down-arched fold in which the
youngest exposed rocks coincide with the
fold axis and all strata dip toward the axis.
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Faults
Fault is a fracture along which movement
has occurred parallel to the fracture
surface.
Normal fault is a dip-slip fault on which the
hanging wall block has moved downward
relative to the footwall block.
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Reverse fault is a dip-slip fault in which the
hanging wall block has moved upward
relative to the footwall block.
Thrust fault is a type of reverse fault with a
fault plane that dips less than 45 degrees.
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Origins of Horsts and Grabens:
Many of the mountain ranges in the Basin
and Range Province of the western United
States and northern Mexico formed in this
manner.
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Joint
: is a fracture along which no movement
has occurred or where movement has been
perpendicular to the fracture surface.
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Unconformities
Unconformity is a break in the geologic
record represented by an erosion surface
separating younger strata from older rocks.
An unconformity is a contact between two
rock units in which the upper unit is usually
much younger than the lower unit.
Unconformities are typically buried
erosional surfaces that can represent a break
in the geologic record of hundreds of
millions of years or more.
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Types of Unconformaties
1. Angular Unconformity
An unconformity below which older
rocks dip at a different angle (usually
steeper) than the overlying strata
2. Disconformity
An unconformity above and below
which the strata are parallel.
3. Nonconformity
An unconformity in which stratified
sedimentary rocks overlie an erosion
surface cut into igneous or
metamorphic rocks.
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Nonconformity