GEOLOGIC STRUCTURES

GEOLOGIC STRUCTURES

• Strike and dip

• are attitudes in rocks produced by geologic forces when rocks are folded or faulted

• Strike

• an imaginary line with compass direction (expressed in bearing) constructed on top of a sedimentary bed or fault in which all points on the line are of equal elevation

Characteristics formed in rocks caused by thedisturbance from internal or external forces

GEOLOGIC STRUCTURES

• Dip

• imaginary line constructed on the down slope surface of a sedimentary bed or fault--dip has 2 attributes: 1)bearing of dip is perpendicular to strike direction; 2)angle of dip measured from horizontal plane to top of bed or fault--dip cannot exceed 90 degrees

• In a series of dipping sedimentary rocks, the formations become progressively younger in the direction of their dip

Strike and Dip on a folded structure

GEOLOGIC STRUCTURES• Folded structures

• warps in rock layers and occur folded (bended) upwards, downwards, or sideways—compression forces are the prime cause of folds —important in mountain formation

• Kinds of folds

• anticline

• a series of up-arched strata

• sides (limbs) dip in opposite directions from central fold which is split by axial plane or fold axis

GEOLOGIC STRUCTURES

• an eroded surface indicates a pattern of progressively younger rocks away from the fold axis

GEOLOGIC STRUCTURESTop (map) view of Axial Plane referred to as the Fold Axis

Anticline

GEOLOGIC STRUCTURES• syncline

• series of down-arched strata dipping towards the fold axis on both sides

• formations become progressively older from fold axis on an eroded surface

GEOLOGIC STRUCTURES Anticline and Syncline

Map view

GEOLOGIC STRUCTURES

• Types of anticlines and synclines

• symmetrical fold

• sides between axis or plane are symmetrical--show a mirror image

GEOLOGIC STRUCTURES

• asymmetrical fold

• no mirror image with respect to the axis or plane

GEOLOGIC STRUCTURES

• overturned fold

• axial plane is tilted and beds may dip in same direction on both sides of plane or axis

GEOLOGIC STRUCTURES

• recumbent fold

• axial plane lies essentially horizontal

GEOLOGIC STRUCTURES

• plunging and non-plunging anti-syn(clines)

• plunging is tilting of fold backwards or forwards—all anticlines and synclines have a degree of plunge

• non plunging anticline

syncline

GEOLOG|IC STRUCTURES• plunging folds

Front view reveals type of fold

Top (Map) view reveals type of fold

Curved Outcrop Patterns of Eroded Rocksin Plunging Anticlines and Syncline with

GEOLOGIC STRUCTURES

Plunge and Fold Axes in Red

Anticline and Syncline on a Geologic Map

Aerial Photo of Syncline and Anticline

Anticline fold axis

Syncline fold axis

GEOLOGIC STRUCTURESFormation and Occurrence of Petroleum and Natural Gas in anticlines and synclines

GEOLOGIC STRUCTURES

• monocline

• a bend in strata resulting in a local steepening in dip of strata which is almost flat lying on both sides of bend

• only one direction of dip

GEOLOGIC STRUCTURESMonocline continued:

GEOLOGIC STRUCTURES• dome

• up-arched strata with limbs dipping outwards from center through 360 degrees

• age of rocks become progressively younger away from center on an eroded surface

If circular, no single fold axis—if elongated in shape there can be a fold axis assigned

GEOLOGIC STRUCTURES Salt Dome

Dome in the Sahara Desert

Geologic Map of a Dome in New Mexico

GEOLOGIC STRUCTURES• basin

• down-arched series of strata with all beds dipping in towards center through 360 degrees

• rocks become progressively older away from center on an eroded surface

If circular, no single fold axis—if elongated in shape there can be a fold axis assigned

Geologic Map of the Michigan Basin

GEOLOGIC STRUCTURESDome and Basin

GEOLOGIC STRUCTURES

• Fault structures

• Definition

• major displacement of rock material along a crack in a rock --- important in mountain formation

• Types of faults

• based on relative movement along the cracked rock

• include vertical, horizontal, or a combination of these movements

GEOLOGIC STRUCTURES

• vertical or dip slip faults

• movement along dip of fault• hanging wall and footwall

GEOLOGIC STRUCTURES• normal fault (gravity fault)

• hanging wall moves down in respect to footwall—on a large scale can cause continental lengthening-tensional forces

mountains

GEOLOGIC STRUCTURES• reverse fault

• hanging wall moves up in respect to footwall--low angle crack is called thrust fault—on a large scale, can

mountains

cause continental shortening---compressional forces

Reverse fault

GEOLOGIC STRUCTURES• horst and graben

• wedge of land that moves up (horst) or down (graben) between 2 normal dip slip faults—caused by tensional forces

• best example is along the Rhine River and the Rhine Valley Graben

GEOLOGIC STRUCTURES• horizontal or strike slip fault

• horizontal movement along the strike of the fault—shear forces

• movement can be right or left lateral

right lateral

best example is San Andreas Faultin California--right lateral

GEOLOGIC STRUCTURES

Left Lateral Strike Slip

GEOLOGIC STRUCTURES• oblique fault

• major dip slip and strike slip displacement along the cracked(faulted) rock—tensional and shear forces

Geologic Structures

• Joint structures

• Definition

• cracks in rocks in which there is no appreciable displacement along the cracks

• often joints occur in 2 sets of cracks intersecting between 45-90 degrees dividing rocks into rectangular blocks

GEOLOGIC STRUCTURES• Causes of joints

• unloading or sheeting effects (see weathering)

• compression forces----example of 90 degree jointing

GEOLOGIC STRUCTURES

Photo of 90 degree jointing

GEOLOGIC STRUCTURES• stresses in a cooling magma—hexagonal or

columnar jointing

best example of igneous jointing is Devil’s Tower, Wyoming

GEOLOGIC STRUCTURES

• Unconformity structures

• Definition

• is a surface of non-deposition or erosion which represents a break in the rock record

• includes a sequence of geologic events associated with the massive erosion surface

• Kinds of unconformities

• based on events prior to and after the time of non deposition or erosion

GEOLOGIC STRUCTURES• disconformity

• series of sedimentary rocks appear above and below the non deposition or erosion surface

• contacts of the sedimentary formations and the non deposition or erosion surface are parallel

blue lines represent non deposition or erosion surfaces

GEOLOGIC STRUCTURES• angular unconformity

• folded or tilted series of formations appear below the non deposition or erosion surface and a series of sedimentary beds above--the contacts of the latter are parallel to non deposition or erosion surface

blue line represents the non deposition orerosion surface

GEOLOGIC STRUCTURES• nonconformity

• igneous or metamorphic rock below non deposition or erosion surface and a series of sedimentary beds below--contacts of the latter parallel non deposition or erosion surface

GEOLOGIC STRUCTURES

• 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 pathways for hydrothermal solutions to host valuable mineral deposits as ores of gold, silver and copper, etc.

• Unconformities can be used to mark geologic time boundaries