Maturity (sedimentology)
Encyclopedia
In sedimentology, maturity describes the composition of grains in sandstones
occurring from various amounts of sediment transportation. It occurs when the grains in a sediment become well-sorted
and well-rounded due to weathering
of the grains. There are two components to maturity: texture
(how rounded and sorted) and composition (how much the composition trends toward stable mineral
s and components). Mature sediment is more uniform in appearance, for the edges of the grains in the sediment are smoother and are of a similar size. Conversely, an immature sediment contains angular grains, diverse grain sizes, and is compositionally diverse.
As the sediment is transported, the unstable minerals react with their surroundings or weather away to leave more stable minerals, such as quartz
. Mature sediments, which contain stable minerals, generally have a smaller variety of minerals than immature sediments, which can contain both stable and unstable minerals. One measure of this maturity is the ZTR index
.
A rock sample from a river downstream is likely to be more mature than one found upstream, since the original sediment has been subject to more weathering conditions as it travels downstream.
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
occurring from various amounts of sediment transportation. It occurs when the grains in a sediment become well-sorted
Sorting (sediment)
Sorting indicates the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Poorly sorted indicates that the sediment sizes are mixed ; whereas well sorted indicates that the sediment sizes are similar .The degree of sorting may also indicate the energy...
and well-rounded due to weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
of the grains. There are two components to maturity: texture
Texture (geology)
Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level. This includes the geometric aspects and relations amongst the component particles or crystals which is called...
(how rounded and sorted) and composition (how much the composition trends toward stable mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s and components). Mature sediment is more uniform in appearance, for the edges of the grains in the sediment are smoother and are of a similar size. Conversely, an immature sediment contains angular grains, diverse grain sizes, and is compositionally diverse.
As the sediment is transported, the unstable minerals react with their surroundings or weather away to leave more stable minerals, such as quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
. Mature sediments, which contain stable minerals, generally have a smaller variety of minerals than immature sediments, which can contain both stable and unstable minerals. One measure of this maturity is the ZTR index
ZTR index
The ZTR index is a method of determining how weathered, both chemically and mechanically, a sediment is. The letters in ZTR stand for 3 common minerals found in ultra-weathered sediments: zircon, tourmaline, and rutile...
.
A rock sample from a river downstream is likely to be more mature than one found upstream, since the original sediment has been subject to more weathering conditions as it travels downstream.
See also
- Rounding
- SortingSorting (sediment)Sorting indicates the distribution of grain size of sediments, either in unconsolidated deposits or in sedimentary rocks. Poorly sorted indicates that the sediment sizes are mixed ; whereas well sorted indicates that the sediment sizes are similar .The degree of sorting may also indicate the energy...
- Sediment transportSediment transportSediment transport is the movement of solid particles , typically due to a combination of the force of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained...