GEOLOGIC STRUCTURES

1. GEOLOGICSTRUCTURES

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 —importantin 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 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 • 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 45-90 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