FORMATIONAL CONTACTS
DESCRIPTIONS: Formational contacts represent shifts in ancient environments that are displayed in rapid changes in rock color, composition, texture of sediments and bedding thicknesses.
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Figure 1 - Contact between Ankareh (Triassic), red and maroon shales and purple limestone and the overlying Nugget (Jurassic), buff to pink Quartz Sandstone. Sage Junction, WY. (Photo 1983)
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Figure 2 - Contact between Woodside Fm. (red) (Triassic) and Thaynes (Fm) (Triassic) at Wolfe Mountain, WY exposed on a steep cliff. (Photo 1983)
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Figure 3 - Contact between Woodside Fm. (red) (Triassic) and Thaynes (Fm) (gray) (Triassic) at Wolf Mountain, WY. Beds are inclined. Woodside sediments are composed largely of shale which weathers easily into a reddish soil which supports pant life. (Photo 1983)
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Figure 4 - Some contacts are subtle as is seen here between the Ordovician Carters Fm. below the hammer (relatively thick limestone beds) and the Hermitage Fm above the hammer (argillaceous thin-bedded limestone).
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Figure 5- Subtle contact between the Ordovician Hermitage argillaceous limestone below the hammer and the thicker bedded limestone beds of the Cannon Fm. above the hammer.
INCLINED TO VERTICAL STRATA
DESCRIPTION - During intense tectonic deformation, sedimentary strata can be tilted to the vertical. In some cases, beds can be completely overturned so that the top of a bed faces directly downward.
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Figure 1 - Vertical beds near Periodic Springs, Swift Creek Canyon, WY. (Photo 1982)
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Figure 2 - Don't be fooled. These are perfectly vertical beds found in the northern Rockies. Rotate the photo 90 degrees counterclockwise for the true photo angle. (Photo 1982)
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Figure 3 - Near-Vertical Beds of limestone and siltstone. Differential weathering gives greater relief to the limestone beds. Triassic Thaynes Fm. Little Muddy Creek, WY. (Photo 1983)
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Figure 4 - Inclined strata, Nova Scotia. (Photo 1974)
UNCONFORMITIES
DESCRIPTION: An Unconformity is a plane representing a large gap in geologic time between the rocks above and below.the plane.
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MILE 49.5. Unconformity
REDWALL LS (Miss)
MUAV LS (Camb)
Figure 1 - Grand Canyon - Unconformity (Disconformity). A Disconformity is a gap in time between parallel sedimentary strata. Notice that the Cambrian Muav Limestone is directly adjacent to the Mississippian Redwall Limestone. That is to say, there are no Ordovician, Silurian, or Devonian rocks here. In places within the Grand Canyon there are Devonian rocks (Temple Butte Formation). Here, the unconformity represents about 145 million years of missing time. The upper limit of the Redwall is marked by the beginning of the Supai red shales. (B13)
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TAPEATS (570 MYA)
VISHNU SCHIST (1.7 BYA)
Figure 2 - Grand Canyon . Unconformity (Nonconformity). Outcropping of the Great Unconformity (1.1 billion years missing) A Nonconformity is a gap in time between sedimentary rock and crystalline rock.( D11)
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Figure 3 - Grand Canyon. Angular Unconformity. Viewed from Navajo Point at the east end of the park. Note the distinctive Angular Unconformity between the PreCambrian Grand Canyon Series below and the Cambrian Tapeats above ("The Great Unconformity"). An Angular Unconformity is one in which the beds below the gap in time are at an angle to the beds above and parallel to the the gap.