Unconformities
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Unconformities represent gaps or missing time in the geologic record due to non-deposition or erosion. There are several types of unconformities that can form, such as angular unconformities, disconformities, and nonconformities. Unconformities are important as they provide information about periods of geologic activity, like folding or erosion of the land, and help place boundaries on geologic timescales. They can be identified in the field based on features like a lack of parallel bedding above and below the contact, presence of erosion surfaces, and fossils of widely different ages across the boundary.
Unconformities
- 2. Introduction Rocks involved in unconformities Types of unconformity Recognition of unconformities Significance of unconformities Conclusions References
- 3. It is one of the most common geological feature found in rocks or in succession. It is different then all other geological structures viz. the fold, joints and faults. Unconformities are resulted due to tectonic activity in form of uplift or subsidence of land. It is referred to a period of non-deposition. UNCONFORMITIES = gaps in rock layers ◦ Form when layers of rock are removed by erosion ◦ Form when a period of time passes without any new deposition to produce new layers of rock
- 4. A) Sedimentary rocks are both above and below the unconformity B) Volcanic rocks above sedimentary rocks below C) Volcanic rocks above and below D) Sedimentary rocks above, plutonic rocks below E) Volcanic rocks above and plutonic rocks below F) Sedimentary rocks above, volcanic rocks below
- 5. Angular unconformity Disconformity Local unconformity Non conformity Blended unconformity Para conformity Buttress unconformity
- 6. •An angular unconformity forms when rock deposited in horizontal layers is folded or tilted and then eroded. When erosion stops, a new horizontal layer is deposited on top of a tilted layer. • When the bedding planes of the older rock layers are not parallel to those of the younger rock layers deposited above them, an angular unconformity results.
- 7. layers of sediments are uplifted without folding or tilting and are eroded. Eventually, the area subsides and deposition resumes. The layers on either side of the boundary are nearly horizontal. Although the rock layers look as if they were deposited continuously, a large time gap exists where the upper and lower layers meet. This gap is known as a disconformity.
- 8. Local unconformity is a similar to disconformities, but as the name implies, it is distinctly local in extent; the time involved is short . In the deposition of continental sediments such as gravels, sands and clays the streams may wander back and forth across the basin of deposition.
- 9. Nonconformities must be distinguished from intrusive igneous contacts. The rocks above a nonconformity may contain fragments of the older igneous rock , either as readily recognized pebbles and boulders or as small fragments recognized only under the microscope.
- 10. a surface of erosion may be covered by a thick residual soil that grades into the underlying bed rock. Younger sediments deposited above this erosion surface may incorporate some of the residual soil , and a sharp contact may be lacking . such a contact is called a blended unconformity.
- 11. •A paraconformity is a type of unconformity (gap in the geologic system) in which there is no evidence of a gap in time, because the planes above and below the gap are parallel and there is no evidence of erosion •Para conformities challenge the geologic timescale. The lack of evidence of time at the surface of the underlying layers of a paraconformity, especially the lack of erosion, suggest that the long ages never occurred.
- 12. A buttress unconformity is when younger bedding is deposited against older strata thus influencing its bedding structure.
- 13. Unconformities may be recognized in single outcrop is most satisfactory. If the unconformity is on angular one, the lake of parallelism of the beds on opposite sides of the contact will be readily apparent . this may be observed in vertical section. Under favorable conditions, disconformities may be readily recognized in outcrops , road cuts , and quarries. The most important criteria for recognizing unconformities in the field are usually sedimentary but unconformities can also be recognized from gaps in the paleontological record. Recognition of unconformity can be presence of conglomeratic bed at the interface. It is fossils of wildly different ages which mark the variation of attitude.
- 14. An angular unconformity, as its name suggests, is an unconformity between layers that are not parallel but instead are at an angle. The upper, horizontal, layers were deposited a top the erosion surface, so we can step back in time by removing them. Steno's principle of lateral continuity tells us that the tillted layers were originally continuous before being eroded. Steno's principle of original hozontality tells us that these tilted layers were deposited as horizontal layers and later tilted.
- 15. A disconformities is an ancient surface of erosion between layers that are parallel. The upper layers were deposited a top the erosion surface, so we can step back in time by removing them. Prior to the erosion, there may have been layers of sediment that were eroded, and of which no trace remains.
- 16. A nonconformity is an ancient surface of erosion between overlying sedimentary rock layers and underlying unlayered igneous or metamorphic rocks. The layers were deposited a top the erosion surface, so we can step back in time by removing them. Prior to the erosion, there has to have been rock into which magma was intruded or beneath which rocks were metamorphosed. Erosion of all that rock would require much time.
- 17. Unconformities are erosional or non-depositional breaks in a sequence of rocks. Unconformities are structural features and they play an important role in investigation of deformed rocks and represents break in stratigraphic sequence. Rocks of various origin may participate in unconformities like sedimentary rocks, volcanic igneous rocks, plutonic rocks or metamorphic rocks may be involved. Formation of an unconformity may be attributed to three main processes like erosion, deposition and tectonic activity.
- 18. STRUCTURAL GEOLOGY- Marland P. Billings- Prentice Hall of india Pvt Ltd-3rd edition- pp no- 278-292 1976, Bruce E.Hobbs, Winthrop D.Means, Paul F.Williams, AN OUTLINEOF STRUCTURAL GEOLOGY, John Wiley and Sons,Pp:154-155. www.google.com www.wikipedia.com