ASBOG Practice Exam

Which unconformity is characterized by layers of sedimentary rock that are parallel but separated by an erosion surface?

1. Paraconformity

2. Angular unconformity

3. Disconformity

4. Nonconformity

The correct answer is: Disconformity

The correct answer is disconformity, which is a specific type of unconformity that occurs when layers of sedimentary rock are deposited in parallel but are later interrupted by an erosion surface. This erosion surface signifies a period of non-deposition or erosion that removed some of the material between the layers. The key feature of a disconformity is that the sedimentary layers remain parallel to each other above and below the unconformity, which distinguishes it from other types of unconformities. In contrast, paraconformity lacks a visible erosion surface; instead, it represents a gap in the geological record where rock layers are assumed to be deposited without interruption but may not show physical evidence of the gap. Angular unconformity represents a scenario where older layers have been tilted or folded and then eroded before younger layers are deposited on top at an angle. Nonconformity occurs between sedimentary rocks and older igneous or metamorphic rocks, highlighting a significant distinction where sedimentary layers are deposited over eroded or exposed rocks of different origins. The clear identification of parallel sedimentary layers separated by an erosion surface is the hallmark of a disconformity, making it the correct choice for this question.