Geochronology

Concept: An unconformity represents a gap in the geological record caused by erosion or non-deposition. Different types of unconformities are identified based on the nature of rock layers:

• Angular unconformity: Tilted or folded older rocks overlain by horizontal younger strata.
• Disconformity: Parallel sedimentary layers with a gap in time.
• Nonconformity: Sedimentary rocks overlying igneous or metamorphic rocks.
• Paraconformity: Layers are parallel with no visible erosion surface but represent a time gap.

Step 1:Understand the rock types involved.
The question specifies sedimentary rocks deposited over eroded igneous or metamorphic rocks.
Step 2:Match with the correct unconformity type.
This situation corresponds to a nonconformity.
Step 3:Differentiate from other types.

• Angular unconformity involves tilted sedimentary layers.
• Disconformity involves parallel sedimentary layers.
• Paraconformity has no visible erosion but represents missing time.

Step 4:Conclude the answer.
Thus, the correct answer is nonconformity.

Concept: A fault is a fracture in the Earth's crust along which movement has occurred. Faults are classified based on the relative movement of the hanging wall and footwall:

• Normal fault: Hanging wall moves downward relative to footwall (tensional forces).
• Reverse fault: Hanging wall moves upward (compressional forces).
• Thrust fault: A low-angle reverse fault.
• Strike-slip fault: Horizontal movement along the fault plane.

Step 1:Understand hanging wall and footwall.
The hanging wall is the block above the fault plane, while the footwall lies below it.
Step 2:Analyze the given movement.
The question states that the hanging wall moves downward relative to the footwall.
Step 3:Match with the correct fault type.
This type of movement is characteristic of a normal fault, which forms under tensional stress when the crust is being stretched.