Paraconformity in geology refers to a type of unconformity where the upper young and old lower rock layers or masses have the same orientation, i.e., parallel and with little or no noticeable erosional features. Therefore, it is hard to recognize or discern as rock beds appear as conformable bedding plane contact.
The only way to recognize a paraconformity is by considering the age of the upper and lower rock strata. One such method is using fossil records.
Today, we will discuss paraconformity, giving you an elaborate meaning and some examples. We will also discuss the formation and how it differs from disconformity.
Overview
An unconformity is a buried erosional and/or non-depositional surface between older lower and younger upper rock layers. It represents a time gap in the rock record or hiatus. During this time, erosion removed already deposited rock, and/or no new sediments were deposited. Consequently, there was no rock record.
James Hutton (1787-1788) postulated unconformity while referring to angular unconformity. However, many years later, the term unconformity was coined, and various types have been recognized.
These unconformities are angular, blended, buttress, paraconformity, and disconformity. None of these types represent a certain time gap length or hiatus but rather its characteristics.
What is paraconformity?
A paraconformity forms when the rock beds below (older) and above (younger) unconformity are parallel and don’t show any discernable erosion features to identify it. It has a rock record gap (hiatus) with a bedding plane with no or little erosional features and the same spacing. Therefore, you will only recognize it using the relative age of the strata between the break, like the fossil records.
Sometimes, it is known as pseudoconformity or non-depositional unconformity, with some calling it obscure unconformity. And in geological maps, you will represent a paraconformity with a missing rock unit. However, it may appear as a rock unit thinning out in some areas.
Besides the absence of erosional features, paraconformities are hard to notice if they have rocks of the same composition above and below. Also, subsequent diagenesis may obscure the unique diagenetic texture.
Therefore, identifying paraconformities or such rock record gaps, lack of faunal zones, or abrupt changes in fauna will help.
For instance, Microcodium formed by calcification of roots of terrestrial (not apparent) helped confirm the paraconformity in the Paleogene sequence of Malatya Basin in eastern Turkey.
Studying this “Eocene-Oligocene boundary west of Malatya in eastern Turkey, located between the Asartepe Member of the Darende Formation (Eocene) and the overlying Muratlı Formation (Rupellian-early Chattian)” (Kayğılı et al., 2019), reveals missing the Priabonian stage in the strata below the boundary.
That is not all. A wrong assumption you shouldn’t make is that paraconformity represents a short duration than other types. That is not true. For instance, the Soft Quaternary clays are paraconformably to the soft Cambrian clays and have an age gap of 400 million years.
Lastly, some argue that paraconformities are just a case of disconformity if you laterally trace them. What you see as parallel with a normal contact bedding place represents a small area. Therefore, erosion may be apparent if you follow it over a wider area.
How do paraconformities form?
Paraconformities form due to nondeposition and erosion, resulting in a rock record gap or hiatus. However, the lack of prominent erosional features suggests nondeposition played a major role in hiatus formation. Also, very little deposition to make rocks that keep a meaningful record is a possible cause.
Possible events that result in the formation of paraconformities include eustatic sea level rise and fall. The fall resulted in no deposition of sediments, perhaps with little erosion. Since no new rock layers formed without sediment deposition, no rock record was created, resulting in a hiatus.
On the other hand, rising sea levels will result in an episode of sediment deposition that forms the younger strata above the unconformity.
Another possible way is where an uplift happens without any tilting. It will result in erosion or cessation of deposition. And in case it later subsides, new, younger deposits will accumulate. These events will result in this unconformity provided no apparent erosional features.
Significance
The significance of paraconformity is the same as other unconformities. They help understand past events like the rise and fall of sea levels often associated with their formation.
Since they represent rock record gaps, stratigraphers and paleontologists must properly identify these unconformities. Otherwise, they will not accurately compute relative fossil ages without considering the time gap in geological records that unconformities represent. Remember, even with radiometric dating, you cannot conduct tests if no deposition happened or erosion removed deposited strata.
Lastly, some unconformities, including paraconformities, are associated with water aquifers, oil, and mineral deposits. Therefore, understanding them can help explore and exploit these resources. Why? Because they localize porosity and may seal over reservoirs of water, oil, etc.
Examples of paraconformity
There are many examples of paraconformities. Common ones include:
- Marshall Paraconformity
- Eocene-Oligocene boundary west of Malatya in eastern Turkey
- Paraconformity between carbonate grainstones at Isla Cancun, Mexico.
Also, possible ones include one at Dead Horse Point, Utah, USA, with an age gap of 10-20Ma, Pennsylvanian and Permian contact, etc.
What is the difference between paraconformity and disconformity?
Both these two unconformities have strata parallel to each other. However, disconformity has an apparent erosional surface to help identify it. Therefore, you will have an irregular surface between older and younger strata. In contrast, paraconformity will not have discernable erosional features, leaving fossil study the only way to identify it. You will see a regular bedding plane contact representing the age gap.
References
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Kayğılı, S., Aksoy, E., Jones, B., & Acar, Ş. (2019). The use of microcodium to identify a paraconformity: An example from the Paleogene sequence of Malatya Basin (eastern Turkey). Sedimentary Geology, 380, 83–93. https://doi.org/10.1016/j.sedgeo.2018.11.015
- Prothero, D. R., & Schwab, F. (2014). Sedimentary geology: An introduction to sedimentary rocks and Stratigraphy (3rd ed.). W.H. Freeman and Company.
- Spencer, E. W. (2018). Geologic maps: A practical guide to preparation and interpretation (3rd ed.). Waveland Press.
- Brookfield, M. E. (2004). Principles of stratigraphy. Blackwell Pub.