STRESS - direct pressure on the rock. Compressive = shorten Compressive = shorten Tensional = extension Tensional = extension STRAIN – when rock is stressed to the point of failure = permanent deformation. Deformation
Beds are said to be dipping if they are oriented more than 0 o from horizontal. (A cross-section sort of thing.) Dip direction is the compass direction a marble would roll down the top of the individual bed. (Not the topography…) (A map view sort of thing.) Which compass direction do these beds appear to be dipping? (view looking approximately south) W
Dip angle is the angle between horizontal and the flat face of the bed. (A cross-section sort of thing.) How much do these beds appear to be dipping? ~35 o W (view looking approximately south)
Strike is the compass direction 90 o from the dip direction, and is one direction in which beds are laterally continuous. (A map view sort of thing.) Approximately which compass direction do these beds appear to be striking? ~35 o W (view looking approximately south)
What does this look like in map view? (view looking southwest) Topo map review – find the valley and the ridges Geol map stuff – find the strike and dip symbols What are the map units relative to the picture? T S2 Tertiary seds T B1 T B2 Tertiary basalt 1 Tertiary basalt 2 20 N * * A A’
What does this look like in cross-section? A A’ (view looking southwest) 20 N Sketch only – NOT TO SCALE. T S2 Tertiary seds T B1 T B2 Tertiary basalt 1 Tertiary basalt 2 * * A A’
Quick Review: Identify the approximate dip direction, dip angle, and strike in the close outcrop. ~?? o
The name of a landform is determined by how the beds dip… A side effect is the relations of older and younger beds…
Folds and Block Diagrams Anticline = oldest rocks exposed in the center. Syncline = youngest rocks exposed in the center
Which directions are these beds dipping? What are the relations of older and younger beds? What are the names of the landforms?
Which way are the beds dipping? What are the landforms? CFW ‘07
web.uct.ac.za/depts/geolsci/dlr/laingsburg Which feature has on overturned limb? Which way is up on each limb?
Economic Look Folds in alternating permeable and impermeable layers = traps. Can trap water, oil, etc… Can trap water, oil, etc…
Mt. Withrow As the Rocky Mountains formed, the strata (rock layers) of the Earth's crust were subjected to stress which resulted in strain (folding). Ridges = resistant sandstone Valley = much less resistant shale. Which way are the beds dipping? Where is the nose? What is the name of the structure? Which way is it plunging?
Stress Types and Faults Compressive Stress: causes shortening of the body on which it acts Reverse or Thrust Fault: one rock gets pushed up over another. Reverse or Thrust Fault: one rock gets pushed up over another.
Tensional Stress: Causes lengthening of the body on which it acts Normal Fault: One rock slides down and away from another Normal Fault: One rock slides down and away from another Stress Types and Faults
Shear Stress: Causes shearing/tearing of the body on which it acts. Strike-Slip Fault: One rock slides past another horizontally. Strike-Slip Fault: One rock slides past another horizontally. Stress Types and Faults
Quick Review of Map Views – In each picture: Which units are oldest/youngest? Which way are the beds dipping? What structures are present? Quick Review of Map Views – In each picture: Which units are oldest/youngest? Which way are the beds dipping? What structures are present?
Seismic Interpretation Offshore Netherlands “F3” dataset courtesy of Paul deGroot of dGB Open-source seismic software from OpendTect.org
Seismic Interpretation Lab 1) Strike and Dip Put at least 10 strike-and-dip symbols on the horizontal section, consulting the vertical section as needed. Identify and label a dome. Identify and label a basin. For the dome and basin you interpreted, explain the observations that support your interpretations. Point out the observations noted on your sections: Offshore Netherlands “F3” dataset courtesy of Paul deGroot of dGB Open-source seismic software from OpendTect.org
2) Plunging Fold On the east side of the horizontal section is a plunging fold. Mark some strikes and dips around it, referring to the vertical sections. Interpret whether the fold is an anticline or syncline: Interpret which direction the fold is plunging: Label the fold on the horizontal section with the proper symbol at its axis. Describe how your observations support your interpretations:
3) Fault Tracing On the east side of the horizontal section is a prominent fault. Mark the fault on the horizontal section. Mark the same fault on each of the vertical sections, if it appears on that section. What is the average strike direction of the fault? Average dip and direction? What is the sense of offset on the fault: horizontal; or vertical? What type of fault is it: reverse; thrust; normal; or strike-slip? Complete the labeling of the fault on the horizontal section with the proper symbol. Mark where the fault terminates on the horizontal section. Discuss whether you can rule out that this fault has oblique slip. Could it be a mixture of types?
4) Unconformities On the east side near the middle of each vertical section there are several unconformities (originating in a hiatus of sedimentation between deposition of delta lobes). These are evident as terminations of one bed (reflection) against another. Go through the 5 vertical sections and mark at least 5 terminations on each, with a short arrow pointing along a bed to its termination. Mark one unconformity across all 5 vertical sections with the wiggly line symbol. 5) Reservoirs Reservoirs of fluid (oil, gas, water, steam) trapped in the rock pore spaces tend to make seismic bright spots. Circle a few bright spots on the Inline vertical sections 102 and 190. Discuss whether these potential oil reservoirs may be associated with any of the other features you have interpreted.