Unconformity in geology & different types of unconformity

Definition

An unconformity is a surface of erosion or non-deposition indicating a gap in geological record. It signifies a time break between two rock formations, indicating past geological events such as uplift, erosion, or changes in sedimentation rates. The strata lying above the unconformity are always younger than the strata lying below.

The younger rocks are always of sedimentary origin and deposited on the surface of the older rock, which is a surface of erosion or non-deposition. Unconformity is developed in three stages;

Types of unconformity

Unconformity are classified into different types based on the relationship existing between the underlying and overlying rock beds. Broadly unconformity is classified into 5 categories as follows;

Angular unconformity

An angular unconformity is developed when younger sedimentary strata are deposited over an eroded surface of tilted strata of different strike and dip. In this case, the older strata are tilted first and exposed to prolonged erosion and subsequent deposition of younger sedimentary strata takes place.

The tilting of strata is mostly caused by tectonic uplift or folding of the older rock. Therefore, angular unconformities are indicative of a period of tectonism

Nonconformity

This type of unconformity is commonly applied to the structure where older rock is of igneous origin and is overlain by a sedimentary rock layer. Nonconformities are typically marked by a surface of erosion, as indicated by scour features, or by a paleosol, which is a soil horizon formed from weathering prior to deposition of the overlying sequence. It is also called Heterolithic Unconformity by some geologists.

Nonconformity is originated by upliftment and exhumation of plutonic igneous or metamorphic rock that subsequently undergoes weathering and erosion before the formation of an overlying sedimentary layer.

Disconformity

Disconformity is a type of unconformity refers to a surface of erosion representing a period of erosion and strata lying above and below the erosional surface have similar attitude (means similar strike and dip). It is also marked by the presence of paleosol indicating little apparent erosion. This type of unconformity is developed where rocks are not affected by diastrophism.

It is also called Parallel unconformity since both overlying and underlying beddings are parallel.

Paraconformity

Paraconformity refers to a surface of little erosion of older sedimentary unit before deposition of younger sedimentary unit. In this case underlying and overlying strata are parallel in nature similar to Disconformity representing the time gap in deposition in geological record. Paraconformity can be distinguished from Disconformity based on degree of erosion of older strata.

Buttress Unconformity

This type of unconformity occurs where younger sediments deposited above the surface of residual soil which developed over older rock in absence of sharp contact. This may occur in environments with slow and continuous erosion and deposition.

This is characterized by gradual changes from older to younger without development of sharp contact.

Why unconformity is important?

Unconformities are very important for geologists because they influence the formation and accumulation of valuable natural resources, including hydrocarbons, mineral deposits, and groundwater. The economic importance of unconformity is described bellow.

Petroleum and Natural Gas Exploration

• Trap Formation: Unconformities act as structural or stratigraphic traps for hydrocarbons by sealing porous reservoir rocks with impermeable layers.
• Reservoir Development: Eroded unconformities may create high-porosity and high-permeability zones that serve as excellent reservoirs for oil and gas.
• Source Rock and Migration Pathways: Unconformities can provide migration pathways for hydrocarbons, aiding their accumulation in reservoirs.
• Example: The Great Unconformity in North America plays a role in hydrocarbon accumulation in the sedimentary basins.

Mineral Deposits

• Unconformity-Related Uranium Deposits:
  • Some of the world’s largest uranium deposits, such as those in Canada (Athabasca Basin) and Australia (Northern Territory), are found near Proterozoic unconformities.
  • The basement rocks provide reducing conditions that precipitate uranium from oxidized fluids moving along the unconformity.
• Gold and Base Metals:
  • Erosional unconformities can concentrate placer gold deposits in ancient riverbeds.
  • Unconformities can be associated with lead-zinc deposits in carbonate sequences, such as those in Mississippi Valley-type (MVT) deposits.

3. Groundwater Reservoirs

• Aquifer Formation:
  • Unconformities can separate porous and permeable aquifers from impermeable layers, forming important groundwater reservoirs.
  • Example: The Ogallala Aquifer in the United States is influenced by an unconformity that controls water storage and movement.
• Karst Development:
  • Unconformities in carbonate rocks (limestone and dolomite) can lead to the formation of karst systems, which serve as major groundwater sources.

Basic questions on unconformity

How can geologists identify an angular unconformity in the field?

Answer:
An angular unconformity is identified by an obvious difference in the orientation of older, tilted layers beneath a younger, horizontal sedimentary sequence. It may also show an erosional surface between the two rock sets.

Why does a nonconformity suggest a significant geological event?

Answer:
A nonconformity suggests a major geological event because it indicates that older igneous or metamorphic rocks were uplifted, exposed to erosion for a long period, and then buried by younger sedimentary rocks.

What is the difference between an angular unconformity and a disconformity?

Answer:

• Angular unconformity – The older rock layers are tilted or folded before deposition of newer layers.
• Disconformity – The layers on either side of the unconformity are parallel, but there is a missing time gap due to erosion.

What does a paraconformity indicate about deposition conditions?

Answer:
A paraconformity suggests a period of non-deposition rather than significant erosion, meaning that sedimentation stopped for a time before resuming under similar conditions.

How do unconformities help in relative dating?

Answer:
Unconformities help geologists establish relative ages by marking significant time gaps. The principle of superposition allows geologists to determine that the rocks below the unconformity are older than those above.

What role do unconformities play in petroleum exploration?

Answer:
Unconformities can act as traps for hydrocarbons by forming sealing surfaces that prevent oil and gas from escaping. They also indicate potential reservoir rocks buried beneath erosional surfaces.

How can radiometric dating be used to determine the age of an unconformity?

Answer:
Radiometric dating can be applied to the igneous or metamorphic rocks below a nonconformity or to the youngest layers beneath an angular unconformity. This provides constraints on the minimum and maximum ages of the unconformity.