14.4: Unconformities

Unconformities

The history of the Earth is written into strata in much the same way a book is written; occasionally, paragraphs are erased or pages are torn out. In order to fully understand the relative order in which a sequence of rocks formed, we need to know what is missing. Layers of rock or sediments can be described as having a conformable contact or an unconformable contact (unconformity) between them. A conformable contact Links to an external site. represents unbroken deposition, with no hiatus (break or interruption in the continuity of the geologic record). The surface strata resulting are called a conformity (Figure 14.9).

An unconformable contact is a period of erosion or nondeposition. The resulting surface is called an unconformity Links to an external site. (Figure 14.9), surfaces between superjacent Links to an external site. bodies of rock that reflect missing pages or chapters of Earth history. A sea level fall (regression) may cause sedimentation to cease for a period, or uplift and erosion may remove large volumes of rock from a given region. The amount of time missing can be relatively short or may represent billions of years.

Left, layers (strata) with a conformable contact. Right, layers (strata) with an unconformable contact. The squiggly, dark line represents an unconformity.

Figure 14.9: Left, layers (strata) with a conformable contact. Right, layers (strata) with an unconformable contact. The squiggly, dark line represents an unconformity. (CC-BY 4.0, Chloe Branciforte, own work)

There are three types of unconformities:

1. An angular unconformity Links to an external site. (Figure 14.10) is perhaps the easiest to recognize of the three types of unconformities. Angular unconformities occurs when there is a degree of angular discordance Links to an external site. between the layered rocks located above and below the plane of the unconformity. These angles form from tilting, folding, or other forms of deformation related to tectonic processes.

Sedimentary rock are deposited, then uplifted, tilted and eroded. Lastly the area subsides, and new deposition of sedimentary layers occur.

Figure 14.10: Block diagrams illustrating the formation of an angular unconformity. (CC-BY-SA 3.0, Bradley Deline Links to an external site.; modified by Chloe Branciforte, own work)

2. A nonconformity Links to an external site. (Figure 14.11) is where layered sedimentary rocks overlie an erosion surface developed on metamorphic or igneous rocks. Because the crystalline rocks that underlie nonconformities form deep in the crust were magmatism and regional metamorphism occur, the nonconformity reflects a period of tectonic mountain building followed by a prolonged period of regional erosion. You can remember nonconformities because they form on top of nonlayered rocks.

Intrusive igneous rock intrude into sedimentary rocks. Uplift and erosion occur removing the sedimentary layers and uncover the igneous rock. Lastly the area subsides, and new deposition of sedimentary layers occur.

Figure 14.11: Block diagrams illustrating the formation of a nonconformity. (CC-BY-SA 3.0, Bradley Deline Links to an external site.; modified by Chloe Branciforte, own work)

3. A disconformity Links to an external site. (Figure 14.12) is a surface of buried erosional relief between parallel layers of sedimentary rock. This makes disconformities more difficult to recognize in the rock record; frequently fossils are needed to determine the exact amount of geologic time missing between layers. Unfortunately, disconformities are fairly common in the rock record, as they frequently form as a result of the natural fluctuations in sea level.

Sedimentary rocks are deposited. Uplift and erosion occurs removing some of the sedimentary layers. Lastly the area subsides, and new deposition of sedimentary layers occur.

Figure 14.12: Block diagrams illustrating the formation of a disconformity. (CC-BY-SA 3.0, Bradley Deline Links to an external site.; modified by Chloe Branciforte, own work)

Use this flow chart (Figure 14.13) to determine the unconformity types:

Flow chart says, Step 1: Are the rock below the unconformity igneous or metamorphic? If yes, the unconformity is a nonconformity. If no, move to Step 2: Are the rock layers directly above and below the unconformity parallel? If yes, the unconformity is a disconformity. If no, the unconformity is an angular unconformity.

Figure 14.13: Flow chart illustrating how to distinguish between the three major unconformity types. (CC-BY 4.0, Chloe Branciforte and Kelli Wakefield)