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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