Qbb 2023-clastic Sedimentology and Petrography-quiz

The correct answer is "A rock formed from the cementation of transported grains." Clastic rocks are formed from the accumulation and cementation of rock fragments, mineral grains, or organic material that have been transported and deposited by wind, water, or ice. These transported grains are then compacted and cemented together to form clastic rocks.

Sorting refers to the process by which sediments are separated and arranged based on their particle size. As currents vary in velocity, they have the ability to transport and deposit sediments of different sizes. This leads to the segregation of sediments, with larger particles being deposited closer to the source and smaller particles being carried further away. Therefore, the correct answer is sorting.

Chemical weathering is most likely to take place in wet environments because water is a key component in many chemical reactions that break down rocks and minerals. Water can dissolve minerals, react with certain rock types, and facilitate the growth of organisms that contribute to weathering processes. In wet environments, there is a higher availability of water, which increases the likelihood of chemical weathering occurring. Cold and dry environments may still experience some form of weathering, but the presence of water is crucial for the chemical reactions involved in the process.

The statement explains that grain roundness is a characteristic that can be observed easily in hand specimens using a hand lens or under a microscope when a thin-section is cut from an aeolian sandstone. This suggests that the statement is true.

Calcium carbonate (CaCO3) is indeed the principal compound found in limestones. Limestone is a sedimentary rock primarily composed of calcium carbonate, which is formed from the accumulation of shells, coral, and other marine organisms over millions of years. This compound gives limestone its characteristic hardness and is also responsible for its use in various industries, such as construction and agriculture. Therefore, the statement "Calcium carbonate (CaCO3) is the principal compound in limestones" is true.

All three types of currents mentioned in the options - rivers, ocean waves, and wind - have the ability to transport sand grains. Rivers carry sediment, including sand, downstream, and deposit them in other areas. Ocean waves can pick up sand grains from the shore and move them along the coast. Wind can also transport sand grains, causing erosion and deposition in different locations. Therefore, all of the above currents can transport sand grains.

The statement is true because sedimentary rocks are formed from the accumulation of sediments that are deposited in various environments such as rivers, lakes, oceans, and deserts. The fabric or texture of these rocks is influenced by the specific conditions of the depositional environment, including factors like water currents, waves, and wind. By examining the fabric of sedimentary rocks, geologists can gather valuable information about the direction of currents at the time of deposition, which can help in understanding past environmental conditions and processes.

Mudrocks that contain a high proportion of organic material can indeed be driven off as a liquid or gas by heating, and these rocks are commonly known as oil shales. This process, known as pyrolysis, involves heating the organic material to high temperatures, causing it to break down and release hydrocarbons. Oil shales are an important source of unconventional oil and gas resources and can be found in various parts of the world. Therefore, the statement "Mudrocks that contain a high proportion of organic material that can be driven off as a liquid or gas by heating are called oil shales" is true.

Evaporite minerals are indeed formed through a process called precipitation, which occurs when water evaporates and the dissolved ions in the water become more concentrated. As the water evaporates, the ions come together and form solid mineral deposits. Therefore, the statement that evaporite minerals are formed by precipitation out of solution as ions become more concentrated when water evaporates is true.

Cherts are indeed fine-grained siliceous sedimentary rocks composed of silt-sized interlocking quartz crystals and chalcedony. Chalcedony is a type of silica that consists of radiating fibers. Therefore, the statement is true.

If all the clasts in a conglomerate are of the same material, it is considered to be monomict, not polymict. Polymict conglomerates contain clasts of different materials. Therefore, the correct answer is false.

A well sorted deposit contains clasts that are similar in size, rather than a wide range of sizes.

Shale is a type of rock that is formed from the accumulation and compaction of clay minerals. Clay minerals are fine-grained particles that are derived from the weathering and erosion of other rocks. Over time, these clay particles settle and become compacted, forming shale. This process usually takes place in areas with calm water, such as lakes or deep ocean basins, where the clay particles can settle and accumulate. Therefore, the correct answer is clay minerals.

The statement suggests that if over one-thirds of a rock is solid organic matter, it may be called coal. However, this is not true. Coal is a specific type of rock that is formed from the remains of plants that have been compacted and heated over millions of years. While coal does contain organic matter, it is not simply any rock with organic matter. Therefore, the correct answer is false.

Authigenic minerals are indeed minerals that form and grow as crystals in a depositional environment. These minerals are not transported from elsewhere but are instead formed in situ. This process occurs when certain conditions in the environment, such as changes in temperature, pressure, or chemical composition, promote the precipitation and growth of minerals. Therefore, the statement that authigenic minerals are minerals that grow as crystals in a depositional environment is correct.

Montmorillonite minerals can absorb water within their structure. This is because they have a layered structure with a high surface area and a negative charge that attracts water molecules. This property allows them to swell and retain water, making them useful in various applications such as in drilling fluids, soil amendments, and as a component in cosmetics and pharmaceuticals.

Sedimentary rocks are formed from the accumulation and solidification of sediment, which can contain various features. Bedding is a characteristic feature of sedimentary rocks, as it refers to the distinct layers that form as sediments settle over time. Fossils are also commonly found in sedimentary rocks, as they are the preserved remains or traces of ancient organisms. However, foliation is not associated with sedimentary rocks. Foliation is a feature commonly found in metamorphic rocks, where minerals are aligned in parallel layers or bands due to intense pressure and heat.

Silt is not defined as the finest grade of clastic sedimentary particles less than 4 microns in diameter. Silt is actually defined as particles ranging from 0.002 to 0.06 mm in diameter. Therefore, the statement is false.

The statement that feldspar is the most common mineral seen in silt deposits is false. While feldspar is a common mineral, it is not the most common one found in silt deposits. Silt deposits typically contain a mixture of minerals, including quartz, clay minerals, and various other minerals. Therefore, the correct answer is false.

The given statement is false. The subsoil is not the region of most intense biological activity. It is the topsoil that supports the majority of biological activity, including insects, worms, and plant roots. The subsoil, located beneath the topsoil, is generally less fertile and contains fewer organic materials, limiting its ability to support extensive biological activity.

The statement is false because arenite refers to a sandstone with less than 15% matrix, not more than.

At high wind velocities, silt- and clay-sized particles are not carried as bed load. Instead, they are carried as suspended load. Bed load refers to larger particles that are transported by rolling, sliding, or bouncing along the bottom of a river or stream. Silt and clay particles, being smaller and lighter, can be easily lifted and transported in the air as suspended load by strong winds. Therefore, the statement is false.

The explanation for the given correct answer is that fabric in a rock refers to the arrangement and alignment of its constituent particles or minerals. If a rock has a tendency to break in a specific direction or shows a strong alignment of elongate clasts (fragments), it indicates a distinct fabric. Therefore, the statement that describes this characteristic of rocks as fabric is true.

The statement is false because sediments and sedimentary rocks with a high proportion of organic matter are actually termed as carbonaceous rocks, not carbonates. Carbonates are sedimentary rocks that are primarily composed of carbonate minerals such as calcite or dolomite.

The statement is false because when clay- and silt-sized particles are mixed in unknown proportions as the main constituents in unconsolidated sediment, the material formed is not shale. Shale is a type of sedimentary rock that is formed from the compaction and cementation of fine-grained clay and silt particles over time. In this case, the material is still in its unconsolidated state and has not undergone the processes necessary to become shale.

Fine clastics are the most abundant type of sediments. This is because they are composed of smaller particles, such as silt and clay, which are easily transported by wind or water and can be deposited over large areas. In contrast, coarse clastics, which include larger particles like sand and gravel, are less abundant because they require higher energy environments to be transported and deposited. Chemical and biochemical sediments, which are formed through precipitation or the accumulation of organic remains, respectively, are also less abundant compared to fine clastics.

The statement "Cement - Fine-grained material occurring between the sand grains" is incorrect. Cement is actually a binder material used in construction that hardens and binds other materials together. It is not a fine-grained material occurring between sand grains.

The statement suggests that clast sphericity increases with prolonged agitation of the sediment, which is false. In reality, clast sphericity is a function of the transport history of the material, not prolonged agitation. The roundness of the clasts is determined by the processes they have undergone during transportation, not how long they have been agitated.

Erosion of sediments is not an important cause of subsidence during the development of a sedimentary basin. Subsidence occurs when the Earth's crust sinks or settles, and it is primarily caused by processes such as deposition of sediments, cooling and contraction of the crust, and tectonic down faulting. These processes can lead to the accumulation of sediments, the shrinking of the crust due to cooling, and the movement of rocks along fault lines, which result in subsidence. However, erosion of sediments involves the removal or wearing away of sedimentary material, which does not contribute to the sinking or settling of the crust.

The explanation for the answer being False is that the presence of oil in the pores of the sediment does not prevent the formation of cement. In fact, the oil can act as a cementing agent itself, helping to hold the sediment together. Therefore, tar sands can be lithified not just by bitumen, but also by the cementation of the sediment.