Unit-3: Structural Geology
Bedding, Dip, and Strike
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. Understanding basic planar features in rocks is essential for all further structural analysis.
Bedding
Bedding is the primary layering or stratification in sedimentary rocks. Each layer represents a distinct period of deposition, varying in composition, color, thickness, and grain size.
Dip
Dip: The maximum angle of inclination of a tilted rock bed measured downward from the horizontal plane.
Dip has two components: the angle of dip (the numerical value in degrees) and the direction of dip (the compass direction toward which the inclined bed slopes steepest).
Strike
Strike: The compass direction of the line formed by the intersection of a tilted rock bed with a horizontal plane.
Important Observation: Strike and dip are always perpendicular to each other in map view and three-dimensional space. If you know the strike, the true dip is measured at a right angle to that strike line.
| Feature | Definition | Measurement / Description |
|---|---|---|
| Bedding | Original layering in sedimentary rocks | Represents sequential depositional events |
| Dip | Maximum inclination angle from horizontal | Measured in degrees and compass direction (perpendicular to strike) |
| Strike | Compass direction of a horizontal line on a tilted bed | Expressed as a bearing (e.g., N 45 E) |
Folds and Types of Folds
When rocks are subjected to compressive tectonic forces over long periods, they behave in a ductile manner and bend rather than break, resulting in folds.
Elements of Fold
To understand fold classification, one must recognize its key geometric components:
- Anticline: An arch-like fold where strata are bent upward.
- Syncline: A trough-like fold where strata are bent downward.
- Axis of Fold: A line connecting the points of maximum curvature along a folded layer.
- Axial Plane: An imaginary plane that divides a fold as symmetrically as possible into two halves, containing the fold axes for all successive layers.
- Limbs: The sides of a fold situated on either side of the axial plane.
- Plunge: The inclination of the fold axis with respect to the horizontal.
Types of Fold
Folds are categorized based on the symmetry of their limbs and the orientation of their axial planes.
- Symmetrical Fold: A fold in which the axial plane is vertical, and the two limbs dip away from (or toward) the axis at equal angles.
- Asymmetrical Fold: A fold in which the axial plane is inclined, and one limb dips more steeply than the other limb.
- Recumbent Fold: An extreme type of fold where the axial plane is nearly horizontal, causing the limbs to be essentially flat-lying.
- Isoclinal Fold: A fold in which the limbs are parallel to one another and generally dip in the same direction at equal angles.
| Fold Type | Axial Plane Orientation | Limb Characteristics |
|---|---|---|
| Symmetrical | Vertical | Equal dip angles on both limbs |
| Asymmetrical | Inclined / Tilted | Unequal dip angles on limbs |
| Recumbent | Horizontal / Near-horizontal | Limbs lie nearly flat |
| Isoclinal | Any orientation | Limbs are parallel to each other |
Faults and Types of Faults
When rocks undergo brittle deformation under tectonic stress, they fracture and move past one another. This fracture along which displacement has occurred is called a fault.
Elements of Fault
The geometry of a fault is described using specific structural terms:
- Fault Plane: The fracture surface along which movement takes place.
- Hanging Wall: The block of rock situated immediately above an inclined fault plane.
- Footwall: The block of rock situated immediately below an inclined fault plane.
- Slip: The relative displacement of formerly adjacent points on opposite sides of the fault plane.
Types of Faults
Faults are classified primarily by the direction of relative movement along the fault plane.
- Normal Fault: A fault in which the hanging wall moves downward relative to the footwall, caused by extensional tectonic forces (pulling apart).
- Reverse Fault: A fault in which the hanging wall moves upward relative to the footwall, caused by compressional tectonic forces (pushing together).
- Thrust Fault: A special type of low-angle reverse fault where the fault plane has a dip of 45 degrees or less, allowing older strata to be pushed far over younger strata.
- Horst: An elevated fault block bounded by normal faults on both sides.
- Graben: A depressed fault block bounded by normal faults on both sides, forming a valley or trough.
| Fault Type | Hanging Wall Movement | Tectonic Regime | Key Feature |
|---|---|---|---|
| Normal Fault | Downward | Extension / Tension | Stretching of the crust |
| Reverse Fault | Upward | Compression | Shortening of the crust (dip greater than 45 degrees) |
| Thrust Fault | Upward | Compression | Low-angle fault plane (dip 45 degrees or less) |
| Horst | Uplifted relative to sides | Extension | Forms elevated fault blocks |
| Graben | Dropped relative to sides | Extension | Forms down-dropped valleys |
Unconformities and Types of Unconformities
Unconformity: A surface of erosion or non-deposition that represents a significant gap in the geological record, separating younger strata from older rocks.
Unconformities indicate intervals of time when deposition ceased, existing rocks were exposed to weathering and erosion, and later subsidence allowed new sediment to accumulate on top.
Types of Unconformities
- Angular Unconformity: An unconformity in which younger, flat-lying sedimentary beds overlie older, tilted or folded sedimentary layers that meet the boundary at an angle.
- Disconformity: An unconformity between parallel sedimentary layers where the boundary surface represents a period of non-deposition or erosion, but the bedding planes above and below are parallel.
- Non-conformity: An unconformity in which younger sedimentary rocks rest directly upon older, eroded igneous or metamorphic rocks.
| Unconformity Type | Underlying Rocks | Overlying Rocks | Relationship at Boundary |
|---|---|---|---|
| Angular Unconformity | Tilted or folded sedimentary rocks | Horizontal sedimentary rocks | Meet at an acute angle |
| Disconformity | Parallel sedimentary rocks | Parallel sedimentary rocks | Parallel, but separated by an erosional contact surface |
| Non-conformity | Igneous or metamorphic rocks | Sedimentary rocks | Sedimentary layers resting on crystalline basement rocks |
Exam-Oriented Note: Always distinguish clearly between disconformities (where strata are parallel above and below) and angular unconformities (where older strata are tilted). For non-conformities, remember that the underlying rock is always non-sedimentary.