What type of contact is found between limestone and cross-bedded sandstone in a uniformly dipping sequence?

• A. Paraconformity
• B. Angular unconformity
• C. Nonconformity
• D. Disconformity

Answer: (A)

The boundary between limestone and cross-bedded sandstone in a uniformly dipping sequence is best described as a paraconformity. This type of unconformity occurs when there is a hiatus in deposition between sedimentary layers that have parallel bedding and show no obvious surface of erosion or angular change. The rocks on either side are both sedimentary and their beds remain parallel, so the time gap isn’t evident in the rock surface, hence the term “hidden” unconformity.

Angular unconformity would require a noticeable tilt between the older and younger beds. A nonconformity involves igneous or metamorphic rocks at the base, which isn’t the case here. A disconformity would show some erosion or nondeposition surface, even though bedding might be parallel; the lack of an erosional surface here points to paraconformity.