Here's why:

* Disconformity: A disconformity is a type of unconformity (a break in the geologic record) where sedimentary rocks are in contact with older sedimentary rocks, but there is a period of erosion and non-deposition represented by the surface.

Essentially, the disconformity represents a period where:

1. Erosion: Existing rock layers are eroded away.

2. Non-deposition: Sedimentation stops for a period of time.

3. Re-deposition: New layers of sediment are deposited on top of the eroded surface.

So, a buried erosional surface is a disconformity because it signifies a significant break in the depositional history of the rocks, marked by erosion and a period of time when no new sediments were being laid down.