Stratigraphic Classification Help

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

Stratigraphic Classification

The Earth’s crust is constantly changing inside and out. Sedimentation and erosion processes are high for long periods (millions of years) and then something changes and they slow or stop. As we saw with plate tectonics, continental landmasses are shoving and compressing at their margins, while ever-widening ocean ridges spread apart. The crust is always on the move and brings about everything from nearly bottomless trenches and impossibly high mountains to crystalline beaches and black lava fields. Our “blue planet” is anything but boring.

Unlike newly erupted igneous surface rock, sedimentary rock strata give us snapshots of individual climates and geological events throughout history. When geologists put all these snapshots together, it creates a “family album” of all the species, habitats, landscapes, and temperatures of the Earth.

By studying layers of different kinds of rock, geologists get not only knowledge of strata composition, but also a window into the experiences and influences that were in place during a specific time period. Geologists pull all this mixed information together in a system of stratigraphic classification that can be used by scientists all over the world.

Rock Stratigraphy

The study of rock stratigraphy is basically a grouping exercise. It reminds me a lot of the sorting exercises we did as children. What belongs with what? Which of these things goes together and in what order? At first glance, the many layers of a sedimentary rock structure look like a crayon box full of different colors or an artist’s box of paints. Differences are easily spotted between natural tones and earthen hues. But in addition to that, geologists have the added benefit of texture. Figure 5-2 shows a cross-section of the ancient (Precambrian) and more recent (Paleozoic) sedimentary rock layers that make up Arizona’s Grand Canyon in the United States. Some layers are thin, some thick, some rocky, and some smooth, but all have a place in the geological stack. Each particular band added together makes up the total vertical picture. Some layers are separated by unconformities. The Grand Canyon is a well-known example of stratigraphic sedimentary rock layers that lie above metamorphic and original plutonic rock.

Strata and Land Eras


Fig. 5-2. The Grand Canyon is a colorful stratigraphic record of sedimentary rock.

An individual band in vertical strata, with its own specific characteristics and position, is called a rock-stratigraphic unit or rock unit.

When several rock-stratigraphic units are stacked vertically, they add up to a formation which geologists can then describe and map as part of the geological record.

Formations are collections, then, of many rock-stratigraphic units grouped together into a section with the same physical properties. Formations are commonly thick enough to be seen in a lot of different places where various strata layers are exposed. Igneous and metamorphic rock layers also have specific formations. Two or more formations can also be bunched together into groups .

When drawing geological maps, different formations are called by name like the Green River formations. When naming a formation, geologists usually use the name of a surrounding area or the formation’s major stone type, like the red sandstone formations in Red Rock Canyon, Nevada.

For even more detail, geologists subdivide the physical characteristics of formations into smaller rock-stratigraphic units called members and even smaller divisions, called beds .

When studying sedimentary rock strata, even more than igneous or metamorphic rock, it is important to remember the huge stretches of time that have led to the layer upon layer of solidified rock. Thousands and millions of years have added atom upon atom, crystal upon crystal to slowly build each layer. It is a lot like watching paint dry multiplied a million times slower. In order to better understand the super slow deposition of sedimentary rock, geologists divide strata by periods of time called time-stratigraphic units . Then, when they are discussing a certain formation, they can further divide it into sandstone formed at one time, compared to nearly identical sandstone, formed much later.

Time-stratigraphic units are the rock layers with known characteristics that formed during a specific period in geologic time.

Time-stratigraphic units are commonly based on the fossil groups they contain and are sorted to represent progressively shorter time periods. These major groupings are combined into systems and systems are combined into erathems . Smaller geologic time units are further divided into eras , periods , epochs , and ages . These help to further track changes in stratigraphic rock layers over time.

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