Compare and contrast: a. textural immaturity and maturity of sediments, describi
ID: 231972 • Letter: C
Question
Compare and contrast:
a. textural immaturity and maturity of sediments, describing how sediments become more textually mature.
b. compositional immaturity and maturity of sediments, describing how and why mineral compositions of sediment change as sediment becomes increasingly more compositionally mature.
c. the oldest known assemblages of crustal rock, the greenstone belts and gneiss belts. Discuss how they formed, and what they tell us about the conditions that existed at and near the surface of the Earth during the Archean.
d. orthodox and alternative views for the oxygenation of the ocean and atmosphere during the Precambrian.
Explanation / Answer
a) A texturually immature sediment is one with poor sorting, with a large proportion of matrix and angular particles, while a texturally mature sediment composes of little or no matrix, and is well sorted with well rounded grains. The progression of sediments is to go from larger, angular rock fragments to well rounded fine single-mineral grains. At first, the sediment is a mix of the parent material, but as erosion and transport occur, the sediments segregate into sands, silt, clay and deposit at different locations. An immature sediment is one that has seen little migration and erosion, so it consists of a mix of minerals and rock fragments, all of which are generally poorly rounded (angular).
b) Compositionally mature sediments consists entirely of minerals that are stable near the surface and compositionally immature sediments consist of a high proportion of unstable minerals. In a sediment one can find 2 types of minerals : authigenic (minerals formed at the site of deposition either by chemical precipitation or by later diagenetic processes) and allogenic(minerals formed elsewhere and transported into the area of deposition). The minerals that are seen in mature sediments are those that are stable at conditions near Earth's surface and do not readily dissolve or chemically alter.
The longer a mineral is in the weathering and transportation cycles of sedimentary rock forming processes, the more likely it is to break down to a more stable mineral or disappear altogether and on the basis of this, compositional maturity can be measured.
The longer a mineral is in the weathering and transportation cycles of sedimentary rock forming processes, the more likely it is to break down to a more stable mineral or disappear altogether and on the basis of this, compositional maturity can be measured.
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