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Strata and Land Eras Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Sedimentary Facies

When a sequence of sedimentary rocks is examined from one layer to another, clear differences can be seen between the layers. These differences are based on the environmental conditions at the time they were deposited. For example, some geologists think the dinosaurs were killed off as a result of a huge meteor impact after having discovered a thin layer of dust and ash over much of the Earth. They think the impact caused fires and dust that rose high into the atmosphere and was suspended for years before finally settling back to the Earth as sediment. This ash and dust layer was compressed into sedimentary rock.

Sedimentary facies are a common group of characteristics within a sedimentary layer (unit) that are specialized as a group.

Unique facies are used to interpret the depositional environment. As you move across a continent and then into the ocean, you’ll notice a wide variety of environments with separate characteristics like grain size and shape, color, deposition, stratification, or fossils. Each new facie can be totally different or slowly change to take on a new composition and texture.

Lithofacies or rock facies are the rocks in a specific facies group. For example, one set of strata may be composed of a sandstone facies, a shale facies, and a limestone facies. Where rocks are not exposed to the surface, it is much more difficult to figure out what is going on. If a limestone rock is exposed in one area and a piece of granite in another, it is a lot harder to determine if the two are of the same age, same facies unit, or whether they were deposited at the same time.

Practice problems of this concept can be found at: Strata and Land Era Practice Test

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