This earthcache will teach you about unconformities. An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity is in telling time.
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- [REQUIRED] Please post a photo in your log of yourself or a personal item at the outcrop to prove you visited the site.
- Do you think this is a disconformity, a nonconformity, or an angular unconformity?
- Describe the difference in rock ABOVE and BELOW the unconformity.
- Which layer do you think is the oldest?
There are three kinds of unconformities: disconformities, nonconformities, and angular unconformities.
The rocks above an unconformity are younger than the rocks beneath (unless the sequence has been overturned). An unconformity represents time during which no sediments were preserved in a region. The local record for that time interval is missing and geologists must use other clues to discover that part of the geologic history of that area. The interval of geologic time not represented is called a hiatus.
Disconformities
Disconformities are usually erosional contacts that are parallel to the bedding planes of the upper and lower rock units. Since disconformities are hard to recognize in a layered sedimentary rock sequence, they are often discovered when the fossils in the upper and lower rock units are studied. A gap in the fossil record indicates a gap in the depositional record, and the length of time the disconformity represents can be calculated. Disconformities are usually a result of erosion but can occasionally represent periods of nondeposition.
Angular unconformities
An angular unconformity is the contact that separates a younger, gently dipping rock unit from older underlying rocks that are tilted or deformed layered rock. The contact is more obvious than a disconformity because the rock units are not parallel and at first appear cross‐cutting. Angular unconformities generally represent a longer time hiatus than do disconformities because the underlying rock had usually been metamorphosed, uplifted, and eroded before the upper rock unit was deposited.
Nonconformities
A nonconformity is the contact that separates a younger sedimentary rock unit from an igneous intrusive rock or metamorphic rock unit. A nonconformity suggests that a period of long‐term uplift, weathering, and erosion occurred to expose the older, deeper rock at the surface before it was finally buried by the younger rocks above it. A nonconformity is the old erosional surface on the underlying rock.
The cove near the head of Conception Bay is the site of an unconformity: a gap in the geologic record where older, tilted rocks are overlain by much younger, less deformed ones. The tilting means the older rocks must have ben folded, uplifted, eroded, and submerged before the younger ones were deposited on top.
The surface of the older tilted rocks is uneven, with obvious hollows, hummocks, and cracks. In some parts of the outcrop here, Cambrian sediment includes a lower layer of conglomerate (541 million years ago to 485 million years ago). In other parts, limestone rests directly on the unconformity. There is smoothly worn, red mudstone, one of the rock layers adjacent to the unconformity. There is also a fragmented greenish grey shale layer appearing at the surface. It is the older rock below the unconformity. At the outcrop there are several places where small vertical rock walls provide excellent views across the unconformity.
Below the unconformity the rock layers are not horizontal, but they are so broken up it can be hard to see the tilted layering clearly. Outcrops near where you first entered provide the best example of tilted beds below the unconformity.
The lower, tilted layers are deep-basin Ediacaran sediments (The Ediacaran Period is a geological period that spans 94 million years from the end of the Cryogenian Period 635 million years ago, to the beginning of the Cambrian Period 541 Mya.)
Reference:
- Hickman Hild, M. 2012. Geology of Newfoundland: Touring through time at 48 scenic sites. Boulder Publications, NL, 256p.