Various types of geological unconformities.

Read this page. There is an interactive table of examples at the bottom for you to practice recognizing the various types of geological features.
This is the last of four "interactive" pages.

f. Unconformities

Unconformities represent periods of non-deposition of sediment or active erosion of strata. They help us appreciate that the geological record in any one location is NOT complete but contains gaps. Unconformities may represent important periods of activity in Earth history such as mountain building events where strata are being actively uplifted and eroded.
Figure 1.22 Schematic of selected types of unconformity.

Several types of unconformity are recognized (refer to figure above):
  • Disconformity: exists where the layers above and below an erosional boundary have the same orientation
  • Nonconformity: develops where sediments are deposited on top of an eroded surface of igneous or metamorphic rocks
  • Paraconformity: strata on either side of the unconformity are parallel, there is little apparent erosion
  • Angular unconformity: strata is deposited on tilted and eroded layers (such as at Siccar Point)
These six photos are examples of different types of unconformities from around the world. Use dropdown boxes to label these correctly and click the "Check" button to see if you are correct. Remember the discussion board for exchanging questions, ideas and answers with fellow students or a TA. To see full size images right-click (alt-click) and pick "view image".

You saw this view of Siccar point earlier.
Figure 1.26 A band of igneous rock within sedimentary rocks at the base of the Grand Canyon, Arizona. The diagonal band is roughly 1-2 metres thick.
Figure 1.27 from Guatemala, photo at the Geology and Geophysics Science Centre, US Geological Survey.
This Precambrian - Cambrian interface indicates a 1 Ga gap in the geological record. Image from Wikipedia, annotations by S. Sutherland.
ho7fk6.jpgThis interface is between folded Middle Jurassic and overlying Late Jurassic strata, in Utah. The bigger trees are probably about 2m high. Image copyright © by Thomas McGuire, made available via the Earth Science World Image Bank.
From <a href="". The little green bushes are roughly 20-30 cm high.

Creative Commons License EOSC 326 Active Reading by F.Jones, L.Longridge, 2014-2016.
Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.