Disconformity

Disconformity is a type of unconformity that occurs between parallel sedimentary beds, representing a period of non-deposition or erosion that separates two layers whose bedding planes are approximately parallel. The surface separating the beds marks a gap in the geologic record, with no tilting of the beds on either side.

Formation and characteristics: A disconformity forms when deposition of sediment pauses or when exposed surfaces are eroded during a time of tectonic uplift or drop in sea level, followed by renewed deposition that continues to lie parallel to the older beds. The contact surface is typically planar or gently curved and can be difficult to detect because the adjacent layers share similar lithology and orientation.

Identification and interpretation: Because the beds above and below a disconformity are parallel, recognition relies on evidence of a time gap rather than structural discordance. Common indicators include fossil assemblages that differ across the boundary, geochemical or paleomagnetic signals, radiometric dating that shows a hiatus, or an erosional surface with signs of weathering. The absence of tilting distinguishes disconformities from angular unconformities.

Examples and occurrence: Disconformities are widespread in sedimentary sequences and can occur in both marine and nonmarine settings. They often separate packages of the same or similar lithology, such as sandstone overlying sandstone, with a measurable interval of missing deposition.

Relation to other unconformities: A disconformity is one of several types of unconformities. It is distinguished from angular unconformities by parallel bedding on both sides and from nonconformities by the lack of exposed igneous or metamorphic basement beneath the younger strata.