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unconformities , introduction , types , conclusion
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- 2. • Unconformities aredepositional contacts that overlie rocks distinctly older than they are. • They are often called “gaps” in the sedimentary record. • The contact represents time when no sediment was deposited. • Usually they also represent time when the underlying rock was being eroded.
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- 4. Sedimentary rocks (which necessarily form atEarth’s surface) Intrusive igneous rocks (which necessarily form deep in Earth’s crust). How can they be directly superposed?
- 5. One way ofcourse is for the sediments to be the country rock or host rock for the intrusion. The other way, and the one that now interests us, is for the original host rock and part of the intrusion to be eroded away, at the surface, before the sediments are laid down above them. The unconformity (symbolized by the wavy line) represents the time of erosion of the host rock and intrusion. This type of unconformity is called a nonconformity. Sediments overlie igneous (or metamorphic) rocks. WHY METAMORPHIC ROCKS?
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- 7. 1-2° Igneous and Metamorphic Rocks (Precambrian and Paleozoic) Sedimentsand sedimentary rocks (Cretaceous and younger) Geologic map of the Fall Line north of Americus, GA The Fall Line is the northern boundary of the Coastal Plain province. At this locality it is also the southern boundary of the Piedmont province.
- 8. Sumner Butte Tapeats Sandstone BrightAngel Shale Muav Limestone Redwall Limestone Supai Formation View NE from Bright Angel Trail Brahma Temple Brahma Schist
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- 10. A second typeof unconformity is shown here. In this case sediments are both above and below the contact, but the ones below meet the contact at an angle. (the ones above are roughly parallel to it). This is called an angular unconformity because the dips of beds above and below it are different.
- 11. In this casethe unconformity represents the time during which the underlying beds were tilted (folded, actually) and eroded. The overlying beds were then deposited on top of that flat erosional surface.
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- 13. Geologic map of theFall Line near Tuscaloosa, AL. The Fall Line is the northern boundary of the Coastal Plain Province. At this location it is also the southern boundary of the Valley and Ridge Province. 1-2° Coastal Plain outliers
- 15. Vishnu Schist Bass Limestone Hakatai Shale YellowLines – Base of Tapeats Sandstone (C) Orange Lines – Bedding in underlying Formation (pC) Tapeats Missing Shinumo Quartzite Dox Sandstone View NNE from Grandview Point
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- 17. In this case,once again, there are sedimentary rocks above and below the unconformity. Unlike the angular unconformity, however, the beds on both sides are essentially parallel to the contact.
- 18. The unconformity in thiscase represents either a simple lapse in the continuity of sedimentation OR that plus erosion of the underlying layer. The first type can be devilishly hard to recognize, that latter type is easier.
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- 20. The Fall Linein Western Alabama and eastern Mississippi. Here the Fall Line swings northward into the Mississippi Embayment. It is still the boundary of the Coastal Plain (now to the west). Here the older sedimentary rocks to the east are not structurally deformed. (They were too far from the collision zone that folded them to the east). Because the Paleozoic rocks are virtually flat and the Cretaceous ones are virtually flat, the Fall Line is a disconformity. Sediments and sedimentary rocks (Cretaceous and younger) Sediments and sedimentary rocks (Pennsylvanian and older) 1-2° Shown on earlier slide 1-2°
- 21. Muav Limestone (Cambrian) BrightAngel Shale (Cambrian) Devonian Silurian MISSING! Ordovician View SW from Bright Angel Trail
- 22. Providence Sand (Cretaceous) highlyweathered Clayton Formation(Paleocene)
- 24. The Fall Lineis an unconformity, with Cretaceous and younger sediments overlying Paleozoic and Younger rocks. Over ~flat sedimentary rocks Over folded sedimentary rocks Over igneous and metamorphic rocks What kind of unconformity is it in each of the three zones indicated?