Question

In the given schematic diagram, cross beds are exposed on a vertical rock face. The feature XY (bold line) represents a/an: 

• A. reactivation surface.
• B. foreset of cross bed.
• C. scoured channel base.
• D. angular unconformity.

Hint:

Inside cross-bedded sets, a thin erosional line that truncates older foresets and is overlain by new ones with a slightly different dip is a reactivation surface; major concave-up erosions at bedset bases indicate channel scours.

Answer 1

The Correct Option is A

Step 1: What is a reactivation surface?
Within a set of cross beds produced by migrating dunes, episodes of brief flow-strength changes or minor scour create a gently erosional surface that truncates older foresets and is then overlain by a new set of foresets with a slightly different dip—this is a reactivation surface.

Step 2: Read the diagram features.
The bold XY line: 

•  cuts across and truncates underlying cross-laminae, 
•  is overlain by new cross-laminae with a slightly different orientation,
•  is internal to a cross-bed set (not at the base of a thick erosional channel). 

These are hallmark traits of a reactivation surface.

Step 3: Eliminate incorrect options.
- (B) Foreset: a foreset is a bed inclined in the flow direction, not a truncation surface; XY clearly truncates foresets.
- (C) Scoured channel base: would display a major concave-up erosional surface separating distinct lithofacies; XY is a thin internal surface within cross-beds.
- (D) Angular unconformity: separates older tilted strata from younger overlying beds at a large angular discordance; XY is much smaller-scale within a single dune set.

Final Answer:
\[ \boxed{\text{Reactivation surface}} \]