1. GEOLOGIC STRUCTURES

2. 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

3. 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

4. Strike and Dip on a folded structure

5. 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

6. GEOLOGIC STRUCTURES
an eroded surface indicates a pattern of progressively younger rocks away from the fold axis

7. GEOLOGIC STRUCTURES
Top (map) view of Axial Plane referred to as the Fold Axis

8. Anticline

9. 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

10. GEOLOGIC STRUCTURES
Anticline and Syncline
Map view

11. GEOLOGIC STRUCTURES Types of anticlines and synclines symmetrical fold
sides between axis or plane are symmetrical--show a mirror image

12. GEOLOGIC STRUCTURES asymmetrical fold
no mirror image with respect to the axis or plane

13. GEOLOGIC STRUCTURES overturned fold
axial plane is tilted and beds may dip in same direction on both sides of plane or axis

14. GEOLOGIC STRUCTURES recumbent fold
axial plane lies essentially horizontal

15. 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

16. GEOLOG|IC STRUCTURES plunging folds
Top (Map) view reveals type of fold
Front view reveals type of fold

17. GEOLOGIC STRUCTURES in Plunging Anticlines and Syncline with
Curved Outcrop Patterns of Eroded Rocks
in Plunging Anticlines and Syncline with
Plunge and Fold Axes in Red

18. Anticline and Syncline on a Geologic Map

19. Aerial Photo of Syncline and Anticline
Anticline fold axis
Syncline fold axis

20. GEOLOGIC STRUCTURES
Formation and Occurrence of Petroleum and Natural Gas in anticlines and synclines

21. 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

22. GEOLOGIC STRUCTURES
Monocline continued:

23. 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

24. GEOLOGIC STRUCTURES
Salt Dome

25. Dome in the Sahara Desert

26. Geologic Map of a Dome in New Mexico

27. 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

28. Geologic Map of the Michigan Basin

29. GEOLOGIC STRUCTURES
Dome and Basin

30. 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

31. GEOLOGIC STRUCTURES movement along dip of fault
vertical or dip slip faults
movement along dip of fault
hanging wall and footwall

32. 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

33. 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 forces
mountains

34. Reverse fault

35. 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

36. 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

37. GEOLOGIC STRUCTURES
Left Lateral Strike Slip

38. GEOLOGIC STRUCTURES oblique fault
major dip slip and strike slip displacement along the cracked(faulted) rock—tensional and shear forces

39. 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 degrees dividing rocks into rectangular blocks

40. GEOLOGIC STRUCTURES Causes of joints
unloading or sheeting effects (see weathering)
compression forces----example of 90 degree jointing

41. GEOLOGIC STRUCTURES
Photo of 90 degree jointing

42. GEOLOGIC STRUCTURES
stresses in a cooling magma—hexagonal or columnar jointing

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

44. 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

45. 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

46. 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

47. 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

48. 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