This figure shows a disconformity between an upper dark layer (being deposited by the sea)and a lower light-colored layer.What happened to form this disconformity?
A)The lower layers were tilted and untilted before deposition of the upper layers.
B)The boundary between the two units indicates that for a time there was either erosion or no deposition.
C)The lower limestones were converted into another rock type by seawater.
D)The upper layer must have been deposited by volcanic eruptions.

Question

This figure shows a disconformity between an upper dark layer (being deposited by the sea)and a lower light-colored layer.What happened to form this disconformity? 
A)The lower layers were tilted and untilted before deposition of the upper layers. 
B)The boundary between the two units indicates that for a time there was either erosion or no deposition. 
C)The lower limestones were converted into another rock type by seawater. 
D)The upper layer must have been deposited by volcanic eruptions.

Quizplus · Accepted Answer

The boundary between the upper dark layer and lower light-colored layer indicates a period of either erosion or no deposition. This suggests that there was a gap in time between the deposition of the two layers, and that the lower layer was exposed to the elements (wind, water, etc.) for some period of time before the upper layer was deposited. This type of gap in sedimentary layers is known as a disconformity. None of the other answer choices adequately explain the formation of this disconformity.

This Figure Shows a Disconformity Between an Upper Dark Layer