GEOLOGIC STRUCTURES

This photo shows deformation of rocks resulting in folded structures--internal Earth forces can cause such geologic structures to form--geologic structures can harbor important energy sources and valuable mineral ore deposits

I. Strike and dip

• are attitudes in the rocks produced by geologic forces and present after the rocks are folded (bent) or faulted (cracked and moved along the crack)

A. Strike

• is an imaginary line with compass direction constructed on the surface of a sedimentary bed or fault in which all points on that line are of equal elevation--the compass direction is usually expressed as a bearing

B. Dip

• is an imaginary line constructed down-slope on a sedimentary bed or fault--the dip direction is perpendicular to the strike direction and usually expressed in bearing and an angle of tilt (dip) measured from the horizontal plane to the top of a bed or fault--a dip angle may not exceed 90 degrees

click here to see strike and dip

II. Folded structures

• are warps in rock layers and occur bended upwards, downwards, or sideways--compressional forces are the prime causes of folds

A. Kinds of folds

1. Anticline

• is a series of up-arched strata with side portions (limbs) dipping in opposite directions away from the central portion of fold split by a plane called the axial plane and observed in the top (plan) view as the fold axis--an eroded surface indicates the rocks become progressively younger away from the fold axis
• click here to see the parts of an anticline
• click here to see a photo of an anticline

2. Syncline

• is a series of down-arched strata with limbs dipping inwards in opposite directions towards the fold axis--an eroded surface indicates the rocks become progressively older away from the fold axis
• click here to see a photo of a syncline

click here to see some photos of folds and anticlines and synclines

3. Dome

• is an up-arched series of strata with beds on all sides dipping away from the center throughout 360 degrees--an eroded surface indicates the rocks become progressively younger away from the center of the structure

4. Basin

• is a down-arched series of strata with beds on all sides dipping in towards the center throughout 360 degrees--an eroded surface indicates the rocks become progressively older away from the center of the structure

click here to see a dome and basin

5. Monocline

• is a bend in the strata resulting in a local steepening in dip of the strata which is almost flat lying on both sides away from the bend-- there is only one direction of dip in the monocline
• click here to see the monocline

B. Specific types of anticlines and synclines

1. Symmetrical fold--is a fold with the sides showing a mirror image with respect to the axial plane

2. Asymmetrical fold--a fold without a mirror image in respect to the axial plane

3. Overturned fold--a fold in which the axial plane is tilted and beds may dip in same direction on both sides of the axial plane

click here to see symmetrical, asymmetrical, and overturned folds

4. Recumbent fold--a fold with the axial plane horizontal

• click here to see a photo of a recumbent fold

5. non-plunging and plunging folds

• plunging is the tilting of the fold towards the front or back end--most anticlines and synclines have some degree of plunge

• in non-plunging folds contact lines separating formations shown in the surface view are parallel and straight---the contact lines in one of the two profile views are horizontal and parallel---the contacts in the other profile view are arched up or down--in plunging folds contact lines in the surface view are curved---contact lines in one of the profile views dip in the direction of plunge---contact lines in the other profile view are arched up or down--the surface contact lines between formations are convexed (closed) in the direction of plunge for the anticline and concave (open) in the direction of plunge for the syncline

• click here to see the formation contacts of plunging anticlines and synclines

III. Fault structures

A. Definition

• is a structure with major displacement of rock material along a crack in a rock

B. Types of faults

• the types of faults are based on relative movement along the cracked rock and include horizontal, vertical, or a combination of movements

1. Vertical or dip slip fault

• is that type with movement along the dip of the fault--the concepts of hanging wall and footwall are important in defining the different types of vertical faults
• if the angle of dip is not 90 degrees the side of the fault with the portion of land which appears to be hanging over a lower portion before movement is called the hanging wall and that on the bottom, the footwall
• click here to see hanging wall and footwall concepts

• Normal fault--hanging wall moves down in respect to footwall

• Reverse fault--hanging wall moves up in respect to footwall--a low angled reverse fault is called a thrust fault

click here to see hanging wall-footwall related to normal and reverse faults

click here to see a photo of a normal fault

click here to see photos of normal and thrust faults

• Horst and graben--a wedge of land which moves up (horst) or down (graben) between 2 normal faults--best example is the Rhine River graben in Europe
• click here to see a horst and graben

2. Horizontal or strike slip fault

• is that type with horizontal movement along the cracked rock--movement can be left lateral or right lateral--best example of right lateral strike slip fault is the San Andreas fault in California--looking down the fault line in the front plan view, the right side of the fault appears to have moved towards you if a right lateral fault or left side moves towards you if a left lateral fault
• click here to see a right lateral strike slip fault

   

3. Oblique fault

• is a fault which has a major dip slip and strike slip component

IV. Joint structures

A. Definition of joints

• fractures in the rocks in which there is no appreciable displacement along the crack--often joints occur in 2 sets of cracks intersecting between 45-90 degrees dividing the rocks into rectangular blocks

B. Causes of joints

1. Unloading or sheeting effects (see Weathering)

2. Stresses in a cooling magma

• can cause columnar jointing in which there are 6-sided fractures comprising the jointing pattern--best example is Devil's Tower, Wyoming
• click here to see a photo of Devil's Tower

V. Unconformity structures

A. Definition

• is a surface of non-deposition or erosion which represents a break in the rock record--is a structure comprised of a sequence of geologic events in which there is often a significant portion of the geologic history lost through massive erosion--unconformities are used to help separate geologic time divisions

B. Kinds of unconformities

1. Disconformity

• a series of sedimentary rocks form, next a non-deposition or massive erosion event occurs, then a series of more sedimentary rocks are deposited
• click here and see a photo of a disconformity--blue line defines the non-deposition or erosion surface

2. Angular unconformity

• a series of sedimentary rocks form, next rocks are folded or tilted, next a non-deposition or massive erosion occurs, then a series of more sedimentary rocks form
• click here to see a photo of an angular unconformity--blue line defines the non-deposition or erosion surface

3. Nonconformity

• igneous or metamorphic rock form, next a non-deposition or massive erosion event occurs, then a series of sedimentary rocks form
• click here to see a photo of a nonconformity
• click here to see figure on the sequential events defining the kinds of unconformities discussed above

VI. Importance of geologic structures

• Oil and natural gas are formed and found trapped in subsurface folds
• Faults, joints, and fractures can act as a passageway for groundwater and a host for valuable mineral deposits as ores of gold, silver, copper etc.
• Unconformities can be used to mark geologic time boundaries for eras, periods, and epochs

page 359 click to return

page 363 click to return

page 362 click to return

page 360 click to return

anticline click to return

syncline click to return

recumbent fold click to return

page 361 click to return

page 365 click to return

page 364 click to return

page 365 click to return

page 368 click to return

page 369 click to return

disconformity click to return

angular unconformity click to return

nonconformity click to return