Dott (1964) Classification Scheme
Dott's (1964) sandstone classification scheme is one of many classification scheme used by geologists for classifying sandstones. Dott's scheme is a modification of Gilbert's classification of silicate sandstones, and it incorporates R.L. Folk's dual textural and compositional maturity concepts into one classification system. The philosophy behind combining Gilbert's classification scheme and R. L. Folk's classification scheme is that it is better able to "portray the continuous nature of textural variation from mudstone to arenite and from stable to unstable grain composition". Dott's classification scheme is based on the mineralogy of framework grains, and on the type of matrix present in between the framework grains.
In this specific classification scheme, Dott has set the boundary between arenite and wackes at 15% matrix. In addition to setting a boundary for what the matrix is, Dott also breaks up the different types framework grains that can be present in a sandstone into three major categories: quartz, feldspar, and lithic grains.
Arenites are types of sandstone that have less than 15% clay matrix in between the framework grains.
- Quartz Arenite are sandstones that contain more than 90% of siliceous grains. Grains can include quartz or chert rock fragments. Quartz arenites are texturally mature to supermature sandstones. These pure quartz sands result from extensive weathering that occurred before and during transport. This weathering removed everything but quartz grains, the most stable mineral. They are commonly affiliated with rocks that are deposited in a stable cratonic environment, such as aeolian beaches or shelf environments. Quartz arenites emanate from multiple recycling of quarts grains, generally as sedimentary source rocks and less regularly as first-cycle deposits derived form primary crystalline or metamorphic rocks.
- Feldspathic Arenites are sandstones that contain less than 90% quartz, and more feldspar than unstable lithic fragments, and minor accessory minerals. Feldspathic sandstones are commonly immature or sub-mature. These sandstones occur in association with cratonic or stable shelf settings. Feldspathic sandstones are derived from granitic-type, primary crystalline, rocks. If the sandstone is dominantly plagioclase, then it is igneous in origin.
- Lithic Arenites are characterized by generally high content of unstable lithic fragments. Examples include volcanic and metamorphic clasts, though stable clasts such as chert are common in lithic arenites. This type of rock contains less than 90% quartz grains and more unstable rock fragments than feldspars. They are commonly immature to submature texturally. They are associated with fluvial conglomerates and other fluvial deposits, or in deeper water marine conglomerates. They are emanate under conditions that produce large volumes of unstable material, derived from fine-grained rocks, mostly shales, volcanic rocks, and metamorphic rock.
Wacke are sandstones that contain more than 15% clay matrix in between framework grains.
- Quartz Wacke are uncommon because quartz arenites are texturally mature to supermature.
- Felspathic Wacke are feldspathic sandstone that contain a matrix that is greater than 15%.
- Lithic Wacke is a sandstone that has a matrix greater than 15%.
Arkose sandstones are more than 25 percent feldspar. The grains tend to be poorly rounded and less well sorted than those of pure quartz sandstones. These feldspar-rich sandstones come from rapidly eroding granitic and metamorphic terrains where chemical weathering is subordinate to physical weathering.
Graywacke sandstones are a heterogeneous mixture of lithic fragments and angular grains of quartz and feldspar, and/or grains surrounded by a fine-grained clay matrix. Much of this matrix is formed by relatively soft fragments, such as shale and some volcanic rocks, that are chemically altered and physically compacted after deep burial of the sandstone formation.
Aeolianite is a rock composed of sand grains that show signs of significant transportation by wind. These have usually been deposited in desert environments. They are commonly extremely well sorted and rich in quartz.
Oolite is more a limestone than a sandstone, but is made of sand-sized carbonate ooids, and is common in saline beaches with gentle wave action.
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