ASBOG Practice Exam

Which term describes an unconformity where the older surface remains horizontal during non-deposition?

1. Disconformity

2. Angular unconformity

3. Nonconformity

4. Paraconformity

The correct answer is: Disconformity

The term that describes an unconformity where the older surface remains horizontal during non-deposition is disconformity. A disconformity occurs when there is a break in the sedimentary record between two sets of sedimentary layers, but the layers above and below the unconformity remain parallel. The older layers are not tilted or folded; instead, they stay horizontal during the period of non-deposition, which can indicate a significant amount of time during which no sediment was deposited over the older strata. In contrast, an angular unconformity involves tilted or folded layers that contrast with the overlying horizontal layers, showing that the older rocks have undergone tectonic forces before deposition resumed. Nonconformity refers to the relationship between sedimentary rocks and older igneous or metamorphic rocks where the older rock type is exposed and then covered by younger sedimentary layers. Paraconformity is characterized by a gap in the geologic record without any noticeable angular discordance, but it typically involves sediment layers that appear to be conformable upon initial observation. The key distinction for disconformity is the horizontal nature of both the older and the younger sedimentary layers, which is what makes it the correct term in this context.