GEOLOGIC
STRUCTURES
GEOLOGIC STRUCTURES
Characteristics formed in rocks caused by the
disturbance from internal or external forces
• 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
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 STRUCTURES
Top (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
Top (Map)
view
reveals type
of fold
Front view
reveals type
of fold
GEOLOGIC STRUCTURES
Curved Outcrop Patterns of Eroded Rocks
in Plunging Anticlines and Syncline with
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 STRUCTURES
Formation 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 STRUCTURES
Monocline 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 STRUCTURES
Dome 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
cause
continental
shortening--compressional
mountains
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
best example is
San Andreas Fault
in California--right
lateral
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 or
erosion 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