geologic structures strike and dip are attitudes in rocks produced by geologic forces when rocks are...
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GEOLOGIC STRUCTURES
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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
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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
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Strike and Dip on a folded structure
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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
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GEOLOGIC STRUCTURES
• an eroded surface indicates a pattern of progressively younger rocks away from the fold axis
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GEOLOGIC STRUCTURESTop (map) view of Axial Plane referred to as the Fold Axis
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Anticline
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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
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GEOLOGIC STRUCTURES Anticline and Syncline
Map view
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GEOLOGIC STRUCTURES
• Types of anticlines and synclines
• symmetrical fold
• sides between axis or plane are symmetrical--show a mirror image
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GEOLOGIC STRUCTURES
• asymmetrical fold
• no mirror image with respect to the axis or plane
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GEOLOGIC STRUCTURES
• overturned fold
• axial plane is tilted and beds may dip in same direction on both sides of plane or axis
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GEOLOGIC STRUCTURES
• recumbent fold
• axial plane lies essentially horizontal
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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
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GEOLOG|IC STRUCTURES• plunging folds
Front view reveals type of fold
Top (Map) view reveals type of fold
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Curved Outcrop Patterns of Eroded Rocksin Plunging Anticlines and Syncline with
GEOLOGIC STRUCTURES
Plunge and Fold Axes in Red
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Anticline and Syncline on a Geologic Map
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Aerial Photo of Syncline and Anticline
Anticline fold axis
Syncline fold axis
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GEOLOGIC STRUCTURESFormation and Occurrence of Petroleum and Natural Gas in anticlines and synclines
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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
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GEOLOGIC STRUCTURESMonocline continued:
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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
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GEOLOGIC STRUCTURES Salt Dome
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Dome in the Sahara Desert
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Geologic Map of a Dome in New Mexico
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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
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Geologic Map of the Michigan Basin
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GEOLOGIC STRUCTURESDome and Basin
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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
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GEOLOGIC STRUCTURES
• vertical or dip slip faults
• movement along dip of fault• hanging wall and footwall
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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
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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
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Reverse fault
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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
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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
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GEOLOGIC STRUCTURES
Left Lateral Strike Slip
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GEOLOGIC STRUCTURES• oblique fault
• major dip slip and strike slip displacement along the cracked(faulted) rock—tensional and shear forces
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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
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GEOLOGIC STRUCTURES• Causes of joints
• unloading or sheeting effects (see weathering)
• compression forces----example of 90 degree jointing
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GEOLOGIC STRUCTURES
Photo of 90 degree jointing
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GEOLOGIC STRUCTURES• stresses in a cooling magma—hexagonal or
columnar jointing
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best example of igneous jointing is Devil’s Tower, Wyoming
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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
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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
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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
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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
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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