Sedimentary Rocks For this lab, you will be making observations and interpretations about sedimentary rocks. I recommend that you print the data table provided and fill it in as you analyze each sample. Then, use your table to answer the questions at the end of the lab. Also answer these questions on our online class site so that you can get credit for the laboratory exercise. Sediment and Sedimentary Rocks Sedimentary Rocks formed by the consolidation of sediment. The word sediment comes from the Latin word sedere, meaning “to settle.” Sediments are solid materials that settle from a state of suspension or solution in a fluid. For example, take sediment in a lake. Sediment can be an accumulation of sand that has settled to the bottom of the lake. However, sediment can also be salt that was at first dissolved in the water but is now precipitating out of that solution and is settling to the bottom of the lake. These two types of sediment can be lithified (converted into sedimentary rock) as different types of sedimentary rocks. Therefore, there are two main types of sedimentary rock, clastic sedimentary rocks and chemical sedimentary rocks. Clastic Sedimentary Rocks Clastic sedimentary rocks are composed of fragments of older rocks. Therefore, their formation begins with weathering. Mechanical weathering breaks down rock into smaller particles of sediment. Chemical weathering can decompose minerals (example: feldspars to clays). Now that the original rock was weathered, erosion removes the grains from that location to a new location. The grains of sediment get transported and eventually deposited, or set down. Over time, they get buried by younger sediment. The pressure of overlying material causes compaction of the sediment. Fluids passing through the buried sediment may precipitate minerals in the pore spaces between grains leading to cementation. Therefore, lithification has begun, which is the process of converting sediments into sedimentary rock, which involves compaction and cementation. The texture and composition of clastic sedimentary rocks help us to make interpretations about the formation of these rocks as well as the environment in which they formed. Textural details that help us to interpret clastic sedimentary rocks are grain size, sorting, and grain shape. Grain size is simply a measurement of how large the grains are in a sedimentary rock. In order, from largest to smallest, grain sizes include: boulders, cobbles, pebbles, granules, sand, silt, and clay. Each of these terms has a specific size requirement, noted below. Boulders Cobbles Pebbles Granules Sand Silt Clay greater than 256 mm 64 mm to 256 mm 4 mm 64 mm 2 mm to 4mm 1/16 mm to 2 mm 1/256 mm to 1/16 mm less than 1/256 mm Analyzing the grain sizes of the sediment in a rock is important because grain size reflects the strength of the process that transported the grains. For example, boulders can only be transported by rivers during flood stages, mudflows, mass wasting (landslides), or some other high-energy process. Therefore, if a layer of sedimentary rock has boulders in it, you know that it was deposited during a time of flooding, or some other high energy environment. Another example is a layer of sedimentary rock that has a lot of silt and clay in it. Silt and clay will not be deposited in swift flowing rivers, since. Instead, they are deposited in low-energy environments where water is rather still, allowing the grains to settle out of the water. Sorting describes the distribution of grain sizes, meaning how similar the grains are in size to each other. Sorting is described as being either very well sorted, well sorted, poorly sorted or very poorly sorted. If all of the grains have the same size, it is very well-sorted. If most of the grains are similar in size, it is well sorted. If grains are a variety of grain sizes, it is poorly-sorted. If grains vary greatly in size, it is very poorly sorted. Sorting is important because it reflects if the transport process was able to sort the sediment according to its grain size. It tells you whether or not the sediment has been reworked after deposition. For example, waves rework sediments, concentrating grains of the same size at the beach, producing well-sorted sediment. Another example would be a debris flow, which deposits sediment rapidly with little to no reworking, producing poorly-sorted sediment. Grain shape refers to how smooth the grains of sediment are. It is described as being angular, subangular, subrounded, and rounded. Grains that have sharp or rough corners are angular. Grains that have smooth surfaces are rounded. Shape is important because it is used to determine the distance of transport. Rough edges get ground down and smoothed as they bump into other grains during transport or reworking. Therefore, the more smooth and rounded the grains, the further the grains have been transported from where they started. For this lab, if a rock is made of fine-grained sediment such as silt or clay, we will assume that the grains are rounded. Mineralogy is also important in determining the source of the sediment, as well as the environment and the distance of transport. Quartz is the most common mineral in sedimentary rocks because it is hard, has no cleavage, and has a low temperature of formation. Therefore, quartz survives long, turbulent trips better than most minerals. If a rock contains an abundance of quartz, and not many other minerals, this means that it has probably survived multiple weathering events and trips, when all other minerals have not. Clastic sedimentary rocks are classified primarily by grain size. Figure 1 contains common sedimentary rocks. Notice that they are named due to the size of the sediment. For example, if it is made of sand, it is called sandstone. If it is made of silt, it is a siltstone. If it is made of clay, it is a claystone. However, if it is made of silt or clay and is fissile (breaks in flat planes), it is called shale. If it is made of grains larger than sand, there are only two options; it is either a conglomerate or a breccia, depending on if the grains are round or angular. If they are more rounded, the rock is a conglomerate. If the grains are more angular, it is a breccia. Figure 1. Clastic Sedimentary Rocks Grain Size Grain Name Distinctive Features Greater than 4mm Boulders, cobbles, Angular grains in finer sediment that pebbles, and/or surround them (poorly sorted) granules Greater than 2 mm Boulders, cobbles, Rounded grains in finer sediment that pebbles, and/or surrounds them (poorly sorted) granules 1/16 mm to 2 mm Sand Can be well-sorted to poorly-sorted; well-rounded to angular; composition often includes various amounts of quartz, feldspars, biotite, hornblende, etc. 1/256 mm to 1/16 mm Silt Can’t see grains with naked eye. Not layered. Does not stick to tongue, but instead feels gritty when chewed. less than 1/256 mm Clay Can’t see grains with naked eye. Not layered. Sticks to tongue and feels sticky when chewed. Up to 1/16 mm Silt or clay Can’t see grains with naked eye. Has a finely laminated/layered structure that allows it to split along planes (fissile). Depositional Environment High energy environments. Product of mass wasting processes such as rockslides, mud and debris flows, rockfalls, etc. High energy environment, such as rivers, ocean shorelines, and alluvial fans. Well-sorted sandstones form on beaches, windblown sand dunes, continental shelves. Moderatelysorted sandstones form in rivers, deltas, alluvial fans. Very poorlysorted sandstones form from debris flows or underwater slumps. Form in low energy environments such as lakes, deeper parts of ocean, floodplains of river channels Form in very low energy environments such as outer parts of deltas or floodplains, deep lake, deep ocean floor, etc. Low to very low energy environments. Silt: lakes, deeper parts of ocean, floodplains of river channels. Clay: outer parts of deltas or floodplains, deep lake, deep ocean floor, etc. Rock Name Breccia Conglomerate Sandstone Siltstone Claystone Shale Chemical Sedimentary Rocks Chemical sedimentary rocks are formed when minerals precipitate from ions dissolved in water. Biochemical rocks are subgroup of chemical sedimentary rocks that are an accumulation of organic debris. Naming chemical sedimentary rocks is done mostly on the basis of the mineral that was being precipitated out of the water. For example, if gypsum is precipitated out, settling to the bottom of the water, a sedimentary rock called rock gypsum will form. You can test for gypsum because the rock will scratch easily with a fingernail, since gypsum only has a hardness of two. If halite is precipitated out of the water, rock halite or rock salt will form. You can test for halite by licking the rock. If it tastes salty, it is halite. Rocks that form from calcium carbonate (calcite) precipitating out of the water are collectively called limestone. You can test for limestone by dropping hydrochloric acid (HCl) on it. If it fizzes in acid, it contains calcite, and is limestone. There are many types of limestones, depending on the different forms the calcite takes as it is coming out of solution. Many organisms use calcium (Ca) carbonate (CO32-) dissolved in sea or lake water to make their shells. When they die, their shells accumulate to make limestone. For example, if the limestone is made entirely of coarse shell fragments that have accumulated, it is called coquina. If the limestone has an abundance of fossils surrounded in fine-grained limestone mud, it is called fossiliferous limestone. If the limestone is fine-grained and is either made of almost microscopic organisms with calcite shells and skeletons, or just precipitated out as soft (powdery), fine-grained calcite, it is called chalk. If the limestone is made of ooids (spherical or “egg shaped” grains composed of concentric layers of calcium carbonate), it is called oolitic limestone. If the limestone lacks all of these above distinctive features, it is micrite, or is often just called limestone. Occasionally, some of the calcium in limestone can be replaced by magnesium as magnesium-rich water flows through an area. If this happens, the calcite in the limestone changes to dolomite, a calciummagnesium-carbonate mineral. A rock made of the mineral dolomite is called dolostone. You can tell if the rock is dolostone if it no longer fizzes in acid, unless it is scratched first, producing a powder that will fizz in acid. As a reminder, if life is involved with helping to produce a chemical sedimentary rock, it can also be called a biochemical sedimentary rock. For example, any of the limestones that contained shells or skeletons would be biochemical. Another biochemical sedimentary rock is coal. Coal forms by accumulati

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