by Harold L. Levin
Chapter 5 - page 8
The Sedimentary Archives
Sands and Sandstones
Sandstones are generally classified on the basis of the composition of their grains.
Three components are generally considered:
- Quartz grains.
- Feldspar grains.
- Rock fragment grains.
The particular minerals present provide information on the amount of weathering and transport
experienced by the sand grains.
Intense weathering and long transport tend to destroy the feldspars
and ferromagnesian minerals because they are less stable, and produce a sandstone dominated by
quartz. Such sandstones are referred to as compositionally mature.
Mature quartz sand from the St. Peter Formation, Missouri.
Sandstones with abundant feldspars, and ferromagnesian minerals, on the other hand, indicate relatively little weathering and transport.
These sandstones are compositionally immature.
Major types of sandstone described by Levin:
- Quartz sandstone (also called quartz arenite) - dominated by quartz grains.
- Arkose - contain 25% or more feldspar, with quartz.
- Graywacke - contains about 30% dark fine-grained matrix (clay, silt, chlorite, micas) along with quartz, feldspar, and rock fragments.
- Lithic sandstone (or subgraywacke) - dominated by quartz, muscovite, chert,
and rock fragments with matrix less than 15%. Feldspars scarce.
Four types of sandstone, as seen in thin section under the microscope. Diameter of each field is about 4 mm.
Note: Several other sandstone classification systems exist.
Sandstone Environmental Interpretation
Each type of sandstone implies something about depositional history and environment:
- Quartz sandstone implies a long time in the depositional basin. Deposition typically in tectonically stable, shallow-water environments.
Common sedimentary structures are ripple marks and cross-bedding.
- Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay).
Also implies rapid erosion, arid climate, tectonic activity, steep slopes.
Commonly deposited in fault troughs or low areas along granitic mountains.
Often has a pinkish color due to oxidized iron, suggesting continental deposition.
- Graywacke implies a tectonically active source area and depositional basin, with rapid erosion.
Graded bedding is common. Associated with volcanic rocks, shales, and cherts of deep water origin.
Hand specimen (left) and thin section (right) of graywacke.
- Lithic sandstone is characteristic of deltaic coastal plains, and may be deposited in nearshore marine environments, swamps, or marshes.
Associated with coal and micaceous shales.
Geologic settings where various sandstones are deposited.
A = Quartz sandstone
B = Arkose
C = Graywacke
D = Lithic sandstone
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 17, 2005