1 minute read
Detrital Sedimentary Rocks
by AudioLearn
• Coal —this is a biological rock made from decaying plants that build up faster than they can be completely decayed. As the layers get compacted, the partially decayed stuff becomes coal.
• Halite, gypsum, anhydrite, and certain limestones are all precipitate types of sedimentary rock. Layers of these are known as evaporites. Even though some are mostly dissolvable in water, they become crystallized as rock and are layered to become sedimentary rock. The different chemical sedimentary rocks are travertine, which is just precipitated limestone. Dolostone is the same but has much more magnesium in it.
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DETRITAL SEDIMENTARY ROCKS
When you look at a clastic rock, you can say a lot about its deposition. If the particles are large, the transportation was largely very short. The opposite is true of fine-grained rocks. You also need to look at the difference in sizes. Is it chaotic or uniform? This tells you how the particles were sorted and the energy needed to do the sorting. High energy leads to larger fragments; low energy leads to sorting due to the density of the sediment. This means that both size and density affect the sorting.
Some examples involve water and wind sorting. Wind sorting is usually even and wellsorted; beach-related sorting is also even because of the high energy involved. Streams are not well-sorted because the energy is so variable.
Rounding happens due to abrasion. Look at the degree of rounding. If you see a lot of rounding, you can assume that a rock has undergone a lot of transportation cycles, or that there was a lot of abrasion involved.
Sediment maturity also matters. This means how long a piece of sediment has been in the sedimentary cycle overall. Mature sediment is much more well-rounded than less mature sediment. Sorting that is even also indicates a greater maturity of the sediment. As sediment is transported further and further, those with unstable minerals like olivine and plagioclase become less and less. More stability in mineral content means a more mature sediment. Beaches have mature sediment mostly made of quartz. This quartz has been around for a while to get to that point.
