An angular unconformity occurs when rock beds above and below the unconformity lie at an angle. The younger rock beds lie flat on tilted, folded, or deformed older rock strata dipping at a different angle.
The formation of angular unconformity indicates tectonic activities that resulted in the older rock beds’ uplift, tilting, folding, and deformation. Also, erosion truncated old beds before new sediments created the younger rock strata.
Learn more about angular unconformities, including historical background, how they form, and their significance. We will also give popular examples, including Grand Canyon and Siccar Point.
- 1 Historical background
- 2 What is angular unconformity in geology?
- 3 Examples of angular unconformities with images
- 4 How does angular unconformity form?
- 5 Significance
An unconformity is a buried erosional or non-depositional surface between rock strata or masses of different ages. It represents a hiatus or a gap in the geologic record. During this time, no strata were formed, erosion removed some beds, or both resulted in a rock record gap.
Nicolaus Steno (1638-1687) and then John Strachey (1671-1743) had angular unconformities sketches but only very vaguely understood the idea. After that, Jean Etienne Guettard (1715-1786) published a geological map showing this unconformity. However, they didn’t explain and only vaguely understood it.
In 1785, James Hutton (1726-1797) presented the idea of an angular unconformity. Two years later, he found an example at the Isle of Arran and shortly after found others at Jedburgh and Siccar Point.
The presence of sedimentary features like ripple marks of tilted strata made Hutton conclude that the vertical layers were originally horizontal when they formed. However, he didn’t use the term unconformity. It was Robert Jameson who first applied it.
Today, there are many types of unconformities. Common ones are angular unconformity, disconformity, and nonconformity. Others are paraconformity, buttress and blended unconformities.
What is angular unconformity in geology?
An angular unconformity is an unconformity type where the older, lower rock beds dip at different angles from the younger, upper rock strata. Usually, the younger rock sequence rests flat on an eroded surface of tilted, folded, or deformed old rock beds with a different dip, strikes, or both.
This unconformity represents the discordance between old and younger rock strata. A discordance in geology means two rock strata dip at different angles, i.e., strikes or dips have different angles or are misoriented. Where the rock beds are parallel and separated by an unconformity, you will have disconformity if you see erosional features. If you have no visible erosional features, you have a paraconformity.
Since the younger and older strata have different dip angles, an angular unconformity is easy to notice, and the place of contact will be obvious. Also, you may see irregular surfaces and some rock beds. Even fossils and strata age will further reveal a geologic age gap.
Like other unconformities, angular unconformity can be local or regional. Some will appear on cliffs, outcrops, cut roads, etc. However, you may not easily notice those low dips over a small area and will. Therefore, you must map a large area to identify those with low slopes.
Lastly, on geologic maps, angular unconformity appears like contact lines with strata having different strikes and dips. These dips and strikes occur on the older strata. However, in case of a subsequent deformation after the unconformity, you can also have them on the younger beds.
Examples of angular unconformities with images
There are numerous examples in various parts of the world. Common ones include the following:
1. Siccar Point unconformity or Hutton’s Unconformity
Siccar Point in Scotland is one of the important geological sites where James Hutton observed and understood the significance and consequently explained his angular unconformity idea. It has younger red sandstone and breccia (65 million years younger) resting above nearly vertical tilted Silurian-age sandstones, mudstones, and graywacke.
In 2022, IUGS listed Siccar Point as the first of the 100 geological heritage sites in the world. Sometimes, the term Great Unconformity refers to this formation.
2. The Grand Canyon, Arizona, USA
The Grand Canyon is another important site with not just angular unconformity but also disconformity and nonconformity. At this site, the younger horizontal beds of the Tonto Group overlie the older tilted and faulted Grand Canyon Supergroup (GCS), forming the Great Unconformity.
The Grand Canyon Supergroup is Mesoproterozoic to Neoproterozoic, while Tonto is Cambrian. The Great Unconformity represents an age gap of 175 Ma to 725 Ma from the youngest in Tonto Group and GSC.
3. Angular unconformity at Praia do Telheiro
Praia do Telheiro in Algarve, Portugal, has a spectacular example of this unconformity. At this site, parallel, nearly horizontal Late Triassic (237 Ma and 201.4 Ma) red and yellow rock beds overlie tilted greywackes and shales of Carboniferous age (359.2 to 299 Ma). The deformation of the carboniferous rocks happened at about 290 Ma during the Variscan orogeny.
The unconformity at Salina Canyon, Central Utah, USA
This Flagstaff Formation over Twist Gulch Formation is a great example of an angular unconformity. It has vertically oriented reddish siltstones, mud shales, and sandstone overlaid by horizontal grayish sandstone.
The vertical rocks are part of the Middle Jurassic Twist Gulch Formation and the horizontal mid-Paleocene Flagstaff Formation. The tilting happened due to salt dome tectonics.
5. Altiplano angular unconformity in Chile
As shown by an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) image, the Altiplano unconformity is another example. It has tilted and eroded Cretaceous sediments (146-65 Ma) to the right side. On top of it lies younger loose flat rock made of pyroclastic material from volcanic activities.
How does angular unconformity form?
Angular unconformity indicates sedimentation and lithification formed old rock beds. Then tectonic movements tilted, faulted, folded, or deformed these strata. Afterward, erosion truncated the old strata before sedimentation and consequent lithification of the younger rock beds above the old.
Step 01: Formation of old rock beds
Sedimentation happens in the sea floor, followed by lithification to form the old rock beds or sequence. These rock beds were nearly horizontal, considering the law of original horizontality. Any deformation happened afterward.
Step 2: Plate tectonic movements
Tectonic plate movements, including mountain building, uplifted, tilted, folded, faulted, or deformed the originally horizontal old rock layers. The uplifting exposed the deformed strata to erosion and stopped deposition.
Step 03: Erosion and weathering
Weathering and erosion removed or truncated part of the deformed rock beds removing things like edges of tilted or folded (anticline) beds. Therefore, you ended up with a flat surface or a peneplain with rock beds at an angle to the surface. This process can take millions of years and will plane down an uplifted area.
Step 04: Land subsided, or sea level rose
Land subsided, or sea level rose, covering the eroded, truncated old strata. These events resulted in sedimentation and consequent lithification of the younger near-horizontal rock beds.
The younger strata will be almost horizontal without major tectonic movements. However, should there be some movements, it may tilt or deform, but the discordance will remain where there was angular unconformity.
A good example is Siccar Point, where the younger rock sequence gently slopes while the older is nearly vertical. It means some Earth movement happened. However, you can still see where the angular unconformity was.
Like other unconformities, they represent a major gap in the rock record. Therefore, correctly identifying them and understanding formation can help in the following ways:
- Piece together past events, including sea level changes, plate tectonic movements like mountain building, etc.
- Paleontologists and stratigraphers can correctly get relative age since any unconformity represents a missing rock record.
- Oil, mineral, and underground water exploration since unconformity formation may be associated with these important economic resources.
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