Structural Geology

Front 1

What is a geologic structure?

Back 1

A geologic structure is a geometric feature in rock whose shape, form, and distribution can be described.

Front 2

How can be described a particular shape of a structure?

Back 2

I. Classification based on geometry, that is, on the

shape and form of a particular structure

• Planar (or subplanar) surface

• Curviplanar surface

• Linear feature

Front 3

How can the structures be classified based on geologic significance?

Back 3

II. Classification based on geologic significance

• Primary: formed as a consequence of the formation

process of the rock itself

• Local gravity-driven: formed due to slip down

an inclined surface; slumping at any scale driven

by local excess gravitational potential

• Local density-inversion driven: formed due to

local lateral variations in rock density, causing a

local buoyancy force

• Fluid-pressure driven: formed by injection of

unconsolidated material due to sudden release

of pressure

• Tectonic: formed due to lithospheric plate interactions,

due to regional interaction between the

asthenosphere and the lithosphere, due to

crustal-scale or lithosphere-scale gravitational

potential energy and the tendency of crust to

achieve isostatic compensation

Front 4

Based on timing of formation, how can a geologic structure be classified ?

Back 4

• Syn-formational: formed at the same time as the

material that will ultimately form the rock

• Penecontemporaneous: formed before full lithification,

but after initial deposition

• Post-formational: formed after the rock has

fully formed, as a consequence of phenomena

not related to the immediate environment of

rock formation

Front 5

Based on the process of formation (the deformation mechanism), how can the geologic structures be classified?

Back 5

• Fracturing: related to development or coalescence

of cracks in rock

• Frictional sliding: related to the slip of one

body of rock past another, or of grains past one

another, both of which are resisted by friction

• Plasticity: resulting from deformation by the

internal flow of crystals without loss of cohesion,

or by non-frictional sliding of crystals past

one another

• Diffusion: resulting from material transport

either solid-state or assisted by a fluid

(dissolution)

Front 6

Based on the mesoscopic cohesiveness

during deformation, how can geologic structures be classified?

Back 6

• Brittle: formed by loss of cohesion across a

mesoscopic discrete surface

• Ductile: formed without loss of cohesion across

a mesoscopic discrete surface

• Brittle/ductile: involving both brittle and ductile

aspects

Front 7

Based on the strain significance,usually the earth structures are defined as

Back 7

• Contractional: resulting in shortening of a

region

• Extensional: resulting in extension of a region

• Strike-slip: resulting from movement without

either shortening or extension

Front 8

Based on the distribution of deformation

in a volume of rock, the structures can be classified as

Back 8

• Continuous: occurs through the rock body at all

scales

• Penetrative: occurs throughout the rock body, at

the scale of observation; up close, there may be

spaces between the structures

• Localized: continuous or penetrative structure

occurs only within a definable region

• Discrete: structure occurs as an isolated feature

Front 9

What is the stress?

Back 9

The stress (σ) acting on a plane is the force per unit area of the plane (σ = F/area).

Front 10

What is deformation?

Back 10

Deformation refers to changes in shape, position, or

orientation of a body resulting from the application of

a differential stress (i.e., a state in which the magnitude

of stress is not the same in all directions). More specifically,

deformation consists of three components: (1) a rotation, which is the pivoting of a body around a fixed axis, (2) a translation, which is a change in the position of a body, and (3) a strain, which is a distortion or change in shape of a body.

Front 11

In order to describe deformation, it is necessary to define a reference frame. What is it?

Back 11

The reference frame used in

structural geology is loosely called the undeformed state. We can’t know whether a rock body has been moved or distorted unless we know where it originally was and what its original shape was.

Front 12

What is the categories of structural analysis?

Back 12

Front 13

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the apparent dip?

Back 13

Dip of a plane in an imaginary vertical plane that is not perpendicular to the strike. The apparent dip is less than or equal to the true dip.

Front 14

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Attitude

Back 14

Orientation of a geometric element in space

Front 15

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Cross section?

Back 15

Plane perpendicular to the Earth’s surface

Front 16

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is true dip?

Back 16

The slope of a surface; formally, the angle of a plane with the horizontal measured in an imaginary vertical

plane that is perpendicular to the strike (Figure 1.10 a)

Front 17

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Dip direction?

Back 17

Azimuth of the horizontal line that is perpendicular to the strike

Front 18

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Foliation?

Back 18

General term for a surface that occurs repeatedly in a body of rock (e.g., bedding, cleavage)

Front 19

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Lineation?

Back 19

General term for a penetrative linear element, such as the intersection between bedding and cleavage or alignment of elongate grains

Front 20

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the Pitch?

Back 20

Angle between a linear element that lies in a given plane and the strike of that plane (also rake) (Figure 1.10b)

Front 21

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Plunge?

Back 21

Angle of linear element with earth’s surface in imaginary vertical plane

Front 22

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Plunge direction?

Back 22

Azimuth of the plunge direction

Front 23

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the Position?

Back 23

The geographic location of a geometric element (e.g., an outcrop).

Front 24

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is Profile plane?

Back 24

Plane perpendicular to a given geometric element; for example, the plane perpendicular to the hinge line

of a fold

Front 25

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the rake?

Back 25

Angle between a linear element that lies in a given plane and the strike of that plane (also pitch)

Front 26

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the strike?

Back 26

Azimuth of the horizontal line in a dipping plane or the intersection between a given plane and the horizontal surface (also trend) (Figure 1.10a)

Front 27

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is trace?

Back 27

The line of intersection between two nonparallel surfaces

Front 28

TERMINOLOGY RELATED TO GEOMETRY AND REPRESENTATION OF GEOLOGIC STRUCTURES_What is the Trend?

Back 28

Azimuth of any feature in map view; sometimes used as synonym for strike

Front 29

WHAT ARE SOME GUIDELINES FOR THE INTERPRETATION OF DEFORMED AREAS?

Back 29

Front 30

SOME TERMINOLOGY OF STRATIFICATION_What is Bedding?

Back 30

Primary layering in a sedimentary rock, formed during deposition, manifested by changes in texture, color, and/or composition; may be emphasized in outcrop by the presence of parting

Front 31

SOME TERMINOLOGY OF STRATIFICATION_What is Compaction?

Back 31

Squeezing unlithified sediment in response to pressure exerted by the weight of overlying layers

Front 32

SOME TERMINOLOGY OF STRATIFICATION_what is Overturned beds?

Back 32

Beds that have been rotated past vertical in an Earth–surface frame of reference; as a consequence, facing is down

Front 33

SOME TERMINOLOGY OF STRATIFICATION_what is Parting?

Back 33

The tendency of sedimentary layers to split or fracture along planes parallel to bedding; parting may be due to weak bonds between beds of different composition, or may be due to a preference for bedparallel

orientation of clay.

Front 34

SOME TERMINOLOGY OF STRATIFICATION_What is Strata?

Back 34

A sequence composed of layers of sedimentary rock

Front 35

SOME TERMINOLOGY OF STRATIFICATION_Stratigraphic facing?

Back 35

The direction to younger strata, or, in other words, the direction to the depositional top of beds

Front 36

SOME TERMINOLOGY OF STRATIFICATION_Younging direction?

Back 36

Same as stratigraphic facing

Front 37

What does define a bedding?

Back 37

It has a definable top or bottom and can be distinguished from adjacent beds by differences in grain size, composition, color, sorting, and/or by a physical parting surface.

Front 38

What does the parting form?

Back 38

Parting forms when beds are unroofed (i.e., overlying strata are eroded away) and uplifted to shallower depths in the crust. As a consequence, the load pushing down on

the strata decreases and the strata expand slightly.

During this expansion, fractures form along weak bedding planes and define the parting. This fracturing reflects the weaker bonds between contrasting lithologies

of adjacent beds, or the occurrence of a preferred orientation of sedimentary grains (e.g., mica)

Front 39

What is fissility?

Back 39

It is the tendency of a sedimentary rock to have closely spaced partings. Shale, which typically

has a weak bedding-parallel fabric due to the

alignment of constituent clay or mica flakes, is commonly

fissile.

Front 40

What are the three reasons why platy grains like mica have a preferred orientation in a sedimentary rocks?

Back 40

First, the alignment of grains can reflect settling of asymmetric bodies in Earth’s gravity field. Platy grains tend to lie down flat. Second, the alignment of grains can reflect flow of the fluid in which the grains were

deposited. In a moving fluid, grains are reoriented so that they are hydrodynamically stable, meaning that the traction caused by the moving fluid is minimized (as is the case if the broad face of the grain parallels the flow direction). Third, the alignment of grains can form as a consequence of compaction subsequent to deposition. As younger sediment is piled on top, water is progressively squeezed out of the older sediment below and the grains mechanically rotate into an orientation with their flat surfaces roughly perpendicular

to the applied load.

Front 41

Why is important the recognition of bedding in structural analysis?

Back 41

Because bedding provides a reference frame for describing deformation of sedimentary rocks, because when sediments are initially deposited, they form horizontal or nearly horizontal layers, a concept referred to as the Law of Original Horizontality. Thus, if we look at an outcrop and see tilting or folding, what we are noticing are deviations in bedding attitude from original horizontality.

Front 42

Why the study of certain depositional structures within beds and on bedding surfaces is useful in tectonic analysis?

Back 42

Because they may provide important information on depositional environment (the setting in which the sediment was originally deposited), on stratigraphic facing or younging direction (the direction in which strata in a sequence are progressively younger), and on current direction (the direction in which fluid was flowing during deposition).

Front 43

Why is the facing indicators important in structural analysis?

Back 43

Because Facing indicators allow you to determine whether a bed is right-side-up (facing up) or overturned (facing down), with respect to the Earth’s surface. Recognition of facing is powerful both for stratigraphic studies and for structural studies.

For example, the structural interpretation of a series of

parallel beds in two adjacent outcrops depends on the facing—if the facing is the same in both outcrops, then the strata are probably homoclinal, meaning that they have a uniform dip. However, if the facing is opposite,Because Facing indicators allow you to determine whether a bed is right-side-up (facing up) or overturned (facing down), with respect to the Earth’s surface. Recognition of facing is powerful both for stratigraphic studies and for structural studies.

For example, the structural interpretation of a series of parallel beds in two adjacent outcrops depends on the facing—if the facing is the same in both outcrops, then the strata are probably homoclinal, meaning that they

have a uniform dip. However, if the facing is opposite, then the two outcrops are likely on different limbs of a fold whose hinge area is not exposed. then the two outcrops are likely on different limbs of a fold whose hinge area is not exposed.

Front 44

What can inform the patterns within beds?

Back 44

Patterns within beds may contain information about stratigraphic facing and current directions that are often critical for tectonic interpretations.

Front 45

How is the graded beds?

Back 45

Graded beds display progressive fining of clast/grain size from the base to the top (Figure 2.2), and are a consequence of deposition

from turbidity flows.

Front 46

What is a tubidity flow?

Back 46

A turbidity flow is a cloud of sediment

that moves down a slope under water because the density of the sediment-water mixture is greater than that of clean water, and denser liquids sink through less dense

liquids. Turbidite flows are triggered by major storms or earthquakes

Front 47

How does the turbidity flow work?

Back 47

Typically, a flow is confined to a submarine channel or canyon; when a broadening of the channel or a decrease in slope slows the speed of a turbidity current, the sediment cloud settles. During settling, the largest grains fall first, and the finest grains last. Each turbidity flow produces a separate graded sequence or a turbidite, which is often capped by pelagic sediment, meaning deep-marine sediments like clay and plankton shells. Turbidites display an internal order, called a Bouma sequence2 (Figure 2.2), which reflects changing hydrodynamic conditions as the turbidity current slows down.

Front 48

What is a flysch?

Back 48

In pre–plate tectonics geological literature (i.e., pre- Beatles), thick sequences of turbidites were referred

to as flysch, a term originating from Alpine geology. Flysch was considered to be an orogenic deposit, meaning a sequence of strata that was deposited just prior to and during the formation of a mountain range. Exactly why such strata were deposited, however, was not understood. Modern geologists now realize that the classical flysch sequences are actually turbidites laid down in a deep trench marking an active plate boundary (like a subduction zone).

Front 49

What are cross beds?

Back 49

Cross beds are surfaces within a bed that are

oblique to the overall bounding surfaces of the bed

Front 50

How does the cross beds form?

Back 50

Cross beds, which are defined by subtle

partings or concentrations of grains, form when sediment moves from the windward or upstream side of a dune, ripple, or delta to a face on the leeward or downstream

side, where the current velocity is lower and the sediment settles out

Front 51

What are the three parts of a cross bed?

Back 51

Thin beds parallel to the upper

bounding surface are called topset beds, the inclined layers deposited parallel to the slip face are called foreset beds, and the thin beds parallel to the lower bounding surface are called bottomset beds.

Front 52

Why do the topset beds tend to be truncated at the upper bedding surface, whereas they are asymptotic to the lower

bedding surface?

Back 52

The foreset beds, which typically are curved (concave up) and merge with the topset and bottomset beds, are the cross beds. If the topset beds and the upper part of the foreset beds are removed by local erosion, the bottomset beds of the next higher layer of sediment are juxtaposed against the foreset beds of the layer below.

Thus, cross beds tend to be truncated at the upper bedding surface, whereas they are asymptotic to the lower bedding surface. This geometry provides a clear stratigraphic facing indicator. The current direction

in a cross-bedded layer is taken to be approximately perpendicular to the intersection between the truncated foresets and overlying bed.

Front 53

COMMON SURFACE MARKINGS - What is animal tracks?

Back 53

Patterns formed when critters like trilobites, worms, and lizards tromp over and indent the surface (the characteristic trails of these organisms are a type of trace fossil).

Front 54

COMMON SURFACE MARKINGS-What is Clast imbrication?

Back 54

The shingle-like overlapping arrangement of tabular clasts on the surface of a bed in response to a current. Imbrication develops because tabular clasts tend to become oriented so that the pressure exerted on them by the moving fluid is minimized.

Front 55

COMMON SURFACE MARKINGS-What is Flute casts?

Back 55

Asymmetric troughs formed by vortices (mini tornadoes) within the fluid that dig into the unconsolidated substrate. The troughs are deeper at the upstream end, where the vortex was stronger. They get shallower and wider at the downstream end, because the vortex dies out. Flute casts can be used as facing indicators.

Front 56

COMMON SURFACE MARKINGS-What are Mudcracks?

Back 56

Desiccation of mud causes the mud to crack into an array of polygons and intervening mudcracks.

Each polygon curls upwards along its margins, so that the mudcracks taper downwards and the polygons resemble shallow bowls. Mudcracks can be used as facing indicators, because an individual crack tends to taper downwards

Front 57

COMMON SURFACE MARKINGS-What are Raindrop impressions?

Back 57

Circular indentations on the bed-surface of mudstone, formed by raindrops striking the surface while it was still soft.

Front 58

COMMON SURFACE MARKINGS-What are Ripple marks?

Back 58

Ridges and valleys on the surface of a bed formed as a consequence of fluid flow. If the current flows back and forth, as along a beach, the ripples are symmetric, but if they form in a uniformly flowing current, they are asymmetric (Figure 2.5). The crests of symmetric ripples tend to be pointed, whereas the troughs tend to be smooth curves. Thus, symmetric ripples are good facing indicators. Asymmetric ripples are not good facing indicators, but do provide current directions.

Front 59

COMMON SURFACE MARKINGS-What is Traction lineation?

Back 59

Subtle lines on the surface of a bed formed either by trails of sediment that collect in the lee of larger grains, or by alignment of inequant grains in the direction of the current to diminish hydraulic drag.

Front 60

COMMON SURFACE MARKINGS-What are Worm burrows?

Back 60

The traces of worms or other burrowing organisms that live in unconsolidated sediment. They stand out because of slight textural and color contrasts with the unburrowed rock.

Front 61

Disrupted Bedding - What are load casts?

Back 61

Load casts, which are also called ball-and-pillow structures, are bulbous protrusions extending downward from a sand layer into an underlying mud or very fine sand layer (Figure 2.6). They form prior to lithification where a denser sand lies on top of less dense mud and a disturbance by a storm or an earthquake causes blobs of sand to sink into the underlying mud. Load casts are useful stratigraphic facing indicators when they retain some connection to the host layer.

Front 62

What are sand volcanoes?

Back 62

Where sand and mud layers are progressively

buried, it is typical for the mud layers to compact and consolidate before the sand layers do. As a consequence, the water in the sand layer is under pressure. If an earthquake, storm, or slump suddenly cracks the permeability barrier surrounding the sand, water and sand are released and forced into the mud along cracks. When this happens near the Earth’s surface, little mounds of sand, called sand volcanoes, erupt at the ground surface.

Front 63

What are clastic dikes?

Back 63

Where sand and mud layers are progressively

buried, it is typical for the mud layers to compact and consolidate before the sand layers do. As a consequence, the water in the sand layer is under pressure. If an earthquake, storm, or slump suddenly cracks the permeability barrier surrounding the sand, water and sand are released and forced into the mud along cracks.The resulting wall-like intrusions of sand (or in some localities, even conglomerate) are called clastic dikes.

Front 64

What is disrupted bedding?

Back 64

Where sand and mud layers are progressively

buried, it is typical for the mud layers to compact and consolidate before the sand layers do. As a consequence, the water in the sand layer is under pressure. If an earthquake, storm, or slump suddenly cracks the permeability barrier surrounding the sand, water and sand are released and forced into the mud along cracks.At depth, partially consolidated beds of sand and mud break into pieces, resulting in a chaotic layering that is known, simply, as disrupted bedding (Figure 2.8).

Front 65

What do the studies of disrupted bedding, sedimentary dikes, and sand volcanoes in lake and marsh deposits inform?

Back 65

Studies of disrupted bedding, sedimentary dikes, and sand volcanoes in lake and marsh deposits provide an important basis for determining the recurrence interval of large earthquakes. In these studies,

investigators dig a trench across the deposit and then look for disrupted intervals within the sequence. Radiocarbon dating of organic matter in the disrupted layers defines the absolute age of disruption events and allows us to estimate the recurrence of earthquakes.

Front 66

What are the three contacts types in rocks?

Back 66

There are three basic types of contacts:

(1) depositional contacts, where a sediment

layer is deposited over preexisting rock; (2) fault contacts, where two units are juxtaposed by a fracture on which sliding has occurred; and (3) intrusive contacts, where one rock body cuts across another rock body.

Front 67

What is a conformable contact?

Back 67

Relatively continuous sedimentation in a region

leads to the deposition of a sequence of roughly parallel sedimentary units in which the contacts between adjacent beds do not represent substantial gaps in time.

Front 68

How can sedimentation gaps be identified in a parallel contact between sedimentary units?

Back 68

Gaps in this context can be identified from gaps in the fossil succession

Front 69

what is an unconformable contact?

Back 69

If there is an interruption in sedimentation, such that there is a measurable gap in time between the base of the sedimentary unit and what lies beneath it, then we say that the contact is unconformable.Unconformable

contacts are generally referred to as unconformities, and the gap in time

represented by the unconformity

(that is, the difference in age between the

base of the strata above the unconformity and the top of the unit below the

unconformity) is called a hiatus.

Front 70

What are the four types of unconformities?

Back 70

The principal types of unconformities: (a) disconformity, (b) angular unconformity, (c) nonconformity, (d) buttress unconformity.

Front 71

TYPES OF UNCONFORMITIES - What is Disconformity?

Back 71

At a disconformity, beds of the rock sequence above and below the unconformity are parallel to one another, but there is a measurable age difference between the two sequences. The disconformity surface represents a period of nondeposition and/or erosion (Figure 2.9a).

Front 72

TYPES OF UNCONFORMITIES - What is Angular unconformity?

Back 72

At an angular unconformity, strata below the unconformity have a different attitude than strata above the unconformity. Beds below the unconformity are truncated at the unconformity, while beds above the unconformity roughly parallel the unconformity surface. Therefore, if the

unconformity is tilted, the overlying strata are tilted by the same amount. Because of the angular discordance at angular unconformities, they are quite easy to recognize in the field. Their occurrence means that the sub-unconformity strata were deformed (tilted or folded) and then were truncated by erosion prior to deposition of the rocks above the unconformity. Therefore, angular unconformities are indicative of a period of active tectonism. If the beds below the unconformity are

folded, then the angle of discordance between the super- and sub-unconformity strata will change with location, and there may be outcrops at which the two sequences are coincidentally parallel (Figure 2.9b).

Front 73

TYPES OF UNCONFORMITIES - What is Nonconformity?

Back 73

Nonconformity is used for unconformities at which strata were deposited on a basement of older crystalline rocks. The crystalline rocks may be either plutonic or metamorphic. For example, the unconformity between Cambrian strata and Precambrian basement in the Grand Canyon is a nonconformity (Figure 2.9c).

Front 74

TYPES OF UNCONFORMITIES - what is Buttress unconformity?

Back 74

A buttress unconformity (also called onlap unconformity) occurs where beds of the younger sequence were deposited in a region of significant predepositional topography. Imagine a shallow sea in which there are islands composed of older bedrock. When sedimentation occurs in this sea,

the new horizontal layers of strata terminate at the margins of the island. Eventually, as the sea rises, the islands are buried by sediment. But along the margins of the island, the sedimentary layers appear to be truncated by the unconformity. Rocks below the unconformity may or may not parallel the unconformity, depending on the pre-unconformity structure. Note that a buttress

unconformity differs from an angular unconformity in that the younger layers are truncated at the unconformity surface (Figure 2.9d).

Front 75

How do you recognize an unconformity (Figure

2.12) in the field today?

Back 75

Well, if it is an angular unconformity or a buttress unconformity, there is an

angular discordance between bedding above and below the unconformity. A nonconformity is obvious, because crystalline rocks occur below the contact. Disconformities, however, can be more of a challenge to recognize. If strata in the sequence are fossiliferous,and you can recognize the fossil species and know their age, then you can recognize a gap in the fossil succession.Commonly, an unconformity may be marked by a surface of erosion, as indicated by scour features, or by a paleosol, which is a soil horizon that formed from weathering prior to deposition of the overlying sequence. Some unconformities are marked by the occurrence of a basal conglomerate, which contains

clasts of the rocks under the unconformity. Recognition of a basal conglomerate is also helpful in determining whether the contact between strata and a plutonic rock

is intrusive or whether it represents a nonconformity.

Front 76

Compaction and Diagenetic Structures - What structures differential compaction can form?

Back 76

Differential compaction within a

layer can lead to lateral variation in thickness that is called pinch-and-swell structure. Pinch-and-swell structure can also form as a consequence of tectonic stretching, so again, you must be careful when you see

the structure to determine whether it is a depositional structure or a tectonic structure.

Front 77

what does the compaction of the clay lead to?

Back 77

The compaction of mud leads to development of a preferred orientation of clay in the resulting mudstone. The preferred orientation of clay flakes, as we have seen, leads to bedding plane fissility that produces a shale.

Front 78

What does Deeper compaction lead to?

Back 78

Deeper compaction can cause pressure solution, a process by which soluble grains preferentially dissolve along the faces at which stress is the greatest.

In pure limestones or sandstones, this

process causes grains to suture together,

meaning that grain surfaces interlock with one another like jigsaw puzzle pieces. In conglomerates, the squeezing together of pebbles results in the formation of indentations on the pebble surfaces creating pitted pebbles (Figure 2.13). In limestones and sandstones that contain some clay, the clay enhances the pressure solution process. Specifically, pressure

solution occurs faster where the initial clay concentration is higher. As a result, distinct seams of clay residue develop in the rock. These seams are called stylolites

Front 79

What is the difference in styllolites rich and poor in clay?

Back 79

In rocks with little clay (<10%), stylolites

are jagged and tooth-like in cross section, like the sutures in your skull. The teeth are caused by the distribution of grains of different solubility along the stylolite. In rocks with more clay, the stylolites are wavy and the teeth are less pronounced, because the clay seams become thicker than tooth amplitude.

Front 80

What are pitted pebbles?

Back 80

Deeper compaction can cause pressure solution, a process by which soluble grains preferentially dissolve along the faces at which stress is the greatest. In pure limestones or sandstones, this process causes grains to suture together, meaning that grain surfaces interlock with one another like jigsaw puzzle pieces. In conglomerates, the squeezing together of pebbles results in the formation of indentations on the pebble surfaces creating pitted pebbles.

Front 81

what happens with the dissolved ions removed at pressure in some rocks (stylolites)?

Back 81

Some of the dissolved ions that are removed at pressure-solved surfaces precipitate locally in the rock in veins or as cement in pore spaces, whereas some get transported out of the rock by moving groundwater. The proportion of reprecipitated to transported ions is highly variable, but as much as 40% of the rock can be dissolved and removed during formation of stylolites.

Front 82

What is Liesegang banding?

Back 82

Some sedimentary rocks exhibit color banding that cuts across bedding. This color banding, which is called Liesegang banding, is the result of diffusion of impurities, or of reactions leading to alternating bands of oxidized

and reduced iron. Because it can be mistaken for bedding or cross bedding, it is mentioned in the context of primary sedimentary structures. To avoid mistaken

identity, search the outcrop to determine whether sets of bands cross each other (possible for Liesegang bands, but impossible for bedding), and whether the bands are

disrupted at fractures or true bedding planes, because these are places where the diffusion rate changes.

Front 83

what is debris flows?

Back 83

When sediments move down the slope. If the flowing mixture of sediment and water is dominantly sediment, it churns into a slurry containing chunks and clasts that are

suspended in a matrix. Such slurries are called debris flows, and where preserved in a stratigraphic sequence, they become matrix-supported, poorly sorted conglomerates

containing a range of clast sizes and shapes.

Front 84

what is the difference between debris flow and turbiditic flow ?

Back 84

It the sediment completely mixes with water in deeper waters, it will become a turbidite. If the flowing mixture of sediment and water is dominantly sediment,it will become debris flow. (not depeer waters).

Front 85

What is slumping?

Back 85

It is the movement down the slope of beds which were lithified sufficiently prior to movement, so that they maintain cohesion. The folds and faults formed during

this slumping are called penecontemporaneous structures,

because they formed almost (Greek prefix pene) at the same time as the original deposition of the layers.

Front 86

What is the characteristic of Penecontemporaneous folds and faults?

Back 86

Penecontemporaneous folds and faults are characteristically chaotic. The folds display little symmetry, and folds in one layer are of a different size and orientation than the structures in adjacent layers. Penecontemporaneous faults are not associated with pronounced zones of brittle fracturing.

Front 87

How can we recognize slump strcutures?

Back 87

One key to the recognition of slump structures in a sedimentary sequence is that the deformed interval is intraformational, meaning that it is bounded both above and below by relatively undeformed strata. Commonly, intervals of penecomtemporaneous structures occur in a sequence that also includes debris flows and turbidites, all indicative of an unstable

depositional environment. While slump structures can be mistaken for local folding adjacent to a tectonic detachment fault, the opposite, tectonic folds mistakenly

interpreted as slump structures, may also occur.

Front 88

Are landslides relatively small structures?

Back 88

The geologic record shows that catastrophic landslides of enormous dimension have occurred on occasion. In northern Wyoming, for example, a giant Eocene slide in association with volcanic eruption displaced dozens of mountain-sized blocks and hundreds of smaller blocks.

One such large block, Heart Mountain, moved intact for several tens of kilometers, apparently riding on a cushion

of compressed air above a nearly planar subhorizontal (detachment) fault.

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How is the environment of the deposition of salt?

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Salt deposits accumulate in any sedimentary

basin, meaning a low region that is the site of deposition. Particularly thick salt deposits lie at the base of passive-margin basins.

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What is difference between the salt and other sedimentary rocks? (in the point of view of the deformation)

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Salt differs from other sedimentary rocks in that it is much weaker and, as a consequence, is able to flow like a viscous fluid under conditions in which other sedimentary rocks behave in a brittle fashion. In some

cases, deformation of salt is due to tectonic faulting or folding, but because salt is so weak, it may deform solely in response to gravity, and thereby cause deformation

of surrounding sedimentary rock.

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what is the name of the salt movement in which only the gravity is responsible for deformation (not tectonics)?

Back 91

halokinesis

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What are the factors which lead to halokinesis?

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Halokinesis begins in response to three factors: (1) the development of a density inversion, (2) differential loading, and (3) the existence of a slope at the base of a salt layer. All three of these factors occur in a passive-margin basin setting.

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What is density inversion (halokinesis)?

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Salt is a nonporous and essentially incompressible material. So when it gets

buried deeply in a sedimentary pile, it does not become denser. In fact, salt actually gets less dense with depth, because at greater depths it becomes warmer and expands. Other sedimentary rocks (like sandstone and

shale), in contrast, form from sediments that originally had high porosity and thus become denser with depth because the pressure caused by overburden makes them compact. This contrast in behavior, in which the

density of other sedimentary rocks exceeds the density of salt at depths greater than about 6 km, results in a density inversion, meaning a situation where denser rock lies over less dense rock.

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What is positive buoyancy? (halokinesis)?

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A density inversion is an unstable condition because the salt has positive buoyancy.

Positive buoyancy means that forces in a gravity field cause lower density materials to try to rise above higher density materials, thereby decreasing the overall gravitational potential energy of the system.

Negative buoyancy, the reverse, is a force that causes a denser material to sink through a less dense material

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What is differential loading? (halokinesis)

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Differential loading of a salt layer takes place

when the downward force on the salt layer caused by the weight of overlying strata varies laterally. This may occur where there are primary variations in the thickness or composition of the overlying strata, primary

variations in the original surface topography of the salt layer, or changes in the thickness of the overlying strata due to faulting. Regardless of its cause, differential loading creates a situation in which some parts of the

salt layer are subjected to a greater vertical load than other parts, and the salt is squeezed from areas of higher pressure to areas of lower pressure.

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What is neutral buoyancy? (halokinesis) ?

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The combination of differential loading and buoyancy force drives salt upward through the overlying strata until it reaches a level of neutral buoyancy, meaning the depth at which it is no longer buoyant. At this level, salt has the same density as surrounding strata.This process, which is also driven by gravity (above the level of neutral buoyancy, the salt is subjected to a negative buoyancy force), is known as gravity spreading.

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What are the two types of salt structure growth?

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If the salt rises after the overlying strata have already been deposited, then the rising salt will warp and eventually break through the overlying strata. This process is called upbuilding. If, however, the rise of the salt relative to the source layer occurs coevally with further deposition, the distance between the source layer and the surface of the basin also increases. This process is called downbuilding. As salt moves, it deforms adjacent strata and creates complex folds and local faults. When salt diapirs approach the surface, the overlying strata are arched up and therefore are locally stretched, resulting in the development of normal faults in a complex array over the crest of the salt structure

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Gravity-Driven Faulting and Folding - How the faults and folds are developed in halokinesis?

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Salt is so weak that it makes a good glide horizon on which detachment and movement of overlying strata occurs. In fact, on many passive margins, a thick package of sedimentary rocks tends to detach and

slump seaward, gliding on a detachment fault in the layer of salt at its base.As slumping occurs, the landward portion of the basin is stretched and is therefore broken by a series of normal faults whose dip tends to decrease with depth. This change in dip with depth makes the faults concave up, which are called

listric faults. As movement occurs on a listric fault, the strata above the fault arch into rollover folds.

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How the magma intrusions are created?

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Once formed, magma is less dense than the surrounding rock, and buoyancy forces cause it to rise. Thedensity decrease is a consequence of the expansion that accompanies heating and melting, the formation of gas bubbles within the magma, and the difference in composition

between magma and surrounding rock.

Magma moves by oozing up through a network of cracks and creeping along grain surfaces. The difference between the pressure within the magma and the pressure in the surrounding rock is so substantial that,

as magma enters the brittle crust, it can force open new cracks.Magma continues to rise until it reaches a level of neutral buoyancy, defined as the depth where pressure in the magma equals lithostatic pressure in the surrounding rock, meaning that the buoyancy force is zero. At the level of neutral buoyancy, the magma may form a sheet intrusion, or may pool in a large magma chamber that solidifies into a bloblike intrusion called a pluton. If the magma pressure is sufficiently high, the magma rises all the way to the surface of the Earth, like water in an artesian well, and is extruded at a volcano.

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What is intrusion foliation?

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This foliation is defined by alignment of inequant crystals and by elongation of xenoliths. Such fabric is a consequence of shear of the magma against the walls of the magma chamber and of the flattening

of partially solidified magma along the chamber walls in response to pressure exerted as new magma pushes

into the interior of the chamber. Similarly, intrusion foliation can be developed along the margins of dikes.

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How can we distinguish intrusion foliation from schistosity?

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It may be difficult to distinguish intrusion foliation from schistosity resulting from tectonic forces. Plutons tend to act as mechanically strong blocks, so that regional deformation is deflected and concentrated along the margins of the pluton. Interpretation of a particular foliation therefore depends on regional analysis and the study of

deformation microstructures. For example,

if the fabric remains parallel to the boundary of the intrusion, even when the boundary changes and individual grains show no evidence for solid-state deformation, the foliation is likely a primary igneous structure.

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How the pillow lava are created?

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If basaltic lava is extruded beneath seawater, the surface of the flow cools quickly, and a glassy skin coats the surface of the flow. Eventually, the pressure in the glass-encased flow becomes so great that the skin punctures, and a squirt of lava pushes through the hole and then quickly freezes. The process repeats frequently, resulting in a flow composed of blobs (centimeters to meters in diameter) of lava. Each blob, which is called a pillow, is coated by a rind of finegrained to glassy material. .As a result, pillows commonly have a rounded top and a pointed bottom (the “apex”) in cross section, and this shape can be used as a stratigraphic facing criterion.

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what is an ignimbrite?

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1.

An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, which is a hot suspension of particles and gases flowing rapidly from a volcano driven by having a greater density than the surrounding atmosphere. (When the ash stopped moving, it settled into a hot layer that welded together. Such a layer of welded tuff is called an ignimbrite, often displaying a foliation).

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How is the foliation formed in ignimbrites?

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Volcanic ash is composed of tiny glass shards with jagged spinelike forms that are a consequence of very rapid cooling. When the ash settles, the glass shards are still hot and soft, so the compaction pressure exerted by the weight of overlying ash causes the shards to flatten, thereby creating a primary foliation in ignimbrite that is comparable to bedding.

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How is the flow foliation forrmed in rhyolites?

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Rhyolitic lavas commonly display subtle color

banding, called flow foliation, that has been attributed to flow of the lava before complete solidification. The banding forms because lavas are not perfectly homogeneous

materials. Since the temperature is not perfectly uniform, there may be zones in which crystals have formed, while adjacent regions are still molten.Shear resulting from movement of the lava smears out these initial inhomogeneities into subparallel bands.

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Cooling Fractures - What is columnar jointing?

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As shallow intrusions and extrusive flows cool, they contract. Because of their fine grain size, these bodies are susceptible to forming natural cracks, or joints, in response to the thermal stress associated with cooling. When such joints are typically arranged in roughly hexagonal arrays that isolate columns of

rock, the pattern is called columnar jointing. The long axes of columns are perpendicular to isotherms (surfaces of constant temperature) and thus they are typically perpendicular to the boundaries of the shallow intrusion or flow. If you look closely at unweathered columnar joints, the surfaces of individual joints are ribbed

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What are cryptovolcanic structures?

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Ancient impact sites that are no longer

associated with a surficial crater dot the Midcontinent region of the United States. These sites are defined by relatively small (less than a few kilometers across) semicircular disruption zones, in which the generally

flat-lying Paleozoic strata of the region are fractured, faulted, and tilted. They were originally called cryptovolcanic structures (from the Greek crypto, meaning

“hidden”), because it was assumed that they were the result of underlying explosive volcanism. Typically, steeply dipping normal faults, whose map traces are roughly circular, define the outer limit of these structures.

These faults are cross cut by other steep faults that radiate from the center of the structure like spokes of a wagon wheel.that radiate from the center of the structure like spokes

of a wagon wheel. This fracture geometry is similar to that around volcanoes, which appeared to support the volcanic interpretation. Near the center of

the structure, bedding is steeply dipping, and faulting juxtaposes units of many different ages. Locally, the strata are broken into huge blocks that jumbled together to create an impact breccia. Throughout this

region, rocks are broken into distinctive shatter cones, which are conelike arrays of fractures similar to those found next to a blast hole in rock (Figure 2.26). The apex of the cone points in the direction from which the

impacting object came. In impact structures, shatter cones point up, confirming that they were caused by impact from above, as would be the case if the structure was due to an incoming meteor.

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Why do impact structures have the geometry that they do?

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To see why, think of what happens when you

drop a pebble into water. Initially, the pebble pushes down the surface of the water and creates a depression, but an instant later, the water rushes in to fill the depression, and the place that had been the center of the depression rises into a dome. In the case of meteor impact against rock, the same process takes place. The initial impact gouges out a huge crater and elastically compresses the rock around the crater. But an instant

later, the rock rebounds. At the margins of the affected zone it pulls away from the walls, creating normal faults, and in the center of the zone, it flows upward, creating the steeply tilted beds.