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== == Clastic Sedimentary Rocks ==

Clastic (or detrital) sedimentary rocks are made up of pieces of pre-existing rocks/minerals, called “clasts”, loosened and eroded by weathering, then transported to some accumulation zone where they are trapped. Clast is a general term referring to sediments that range in size from less than a micron to as big as a whole apartment block. Clastic rocks are the most common ones among the sedimentary rocks. The “birth process” of a clastic rockis composed by many steps: ♦Erosion: it’s the process by which clasts are created from pre-existing rocks/minerals. ♦Transportation: it’s the movement of the clasts away from the original source, usually due to water, wind or ice action. Leads to rounding of the sediments in a directly proportional way to the abrosion processes during transportation. ♦Lithification: can be divided in three interconnected steps: 1.Deposition: happens when friction and gravity overcome the forces driving sediment transportation, allowing them to accumulate. 2. Preservation and Compaction: occur when material continues to accumulate on top of the sediment layer, squeezing the grains together and driving out water while also being preserved. The mechanical compaction is aided by weak attractive forces between the smaller grains of sediment. 3.Cementation: is the process of cementing minerals coating the sediment grains and gluing them together into a fused rock, groundwater typically carries cementing agents into the sediment. Clastic rocks are classified based on their grain shape, grain size, and sorting. Grain size: there are six main grain-size categories in the Udden-Wentworth size scale, five of which are broken into subcategories as the chart on the side shows: ·Boulder and cobbles ·Pebbles ·Sand ·Silt ·Clay Grain shape: Three different but related properties determine particle shape: particle form, roundness, and surface texture. ♦Particle form is the overall shape of particles, typically defined in terms of the relative lengths of the longest, shortest, and intermediate axes. Particles can be spherical, prismatic, or bladelike.

♦Roundness (or angularity) is a measure of the smoothness of particles.

♦Surface texture refers to the presence or absence of small, variously shaped markings (pits, polish, scratches) that may occur on grain surfaces.

Sorting: it’s the range of sediment sizes within a sedimentary rock. ♦A “well sorted” rock contains clasts that share almost the same size. ♦A “poorly sorted” rock contains many different sizes of clasts in a medium size range. ♦An “unsorted” rock contains different sizes of clasts in a wide size range. ♦A “graded” rock contains clasts of different size organized in a progressive distribution. Sorting can be a measure of the erosion and transportation processes that happened in the rock formation process.

The classification of clastic sedimentary rocks is shown the following chart: