Clastic Sedimentology And Petrography-jan 2018-quiz II-

The statement suggests that when grains are transported by wind (aeolian transport), they tend to be effectively and selectively separated, resulting in well-sorted deposits. This means that the grains are sorted according to their size, with larger grains settling closer to the source and smaller grains being transported further away. This sorting process occurs due to the differential settling velocities of different grain sizes in the wind. Therefore, the statement is true.

The statement is true because quartz arenites are types of sandstones that are primarily composed of quartz grains. Wind-deposited sands, both in modern and ancient times, often contain a high percentage of quartz grains due to their durability and resistance to weathering. Therefore, it can be concluded that most wind-deposited sands are quartz arenites.

The explanation for the given correct answer is that oxidizing conditions, which involve the reaction of oxygen with other substances, in deserts prevent the preservation of a significant amount of fossil material. These conditions cause the breakdown and degradation of organic material, making it difficult for fossils to form and be preserved. Additionally, the sediments found in deserts are often red or yellow in color, which further supports the idea that oxidizing conditions are present.

The statement accurately describes a desert as a continental area that receives little precipitation. Deserts are characterized by their arid climate, with very little rainfall throughout the year. This lack of precipitation results in a scarcity of water and vegetation, making deserts harsh and inhospitable environments. Therefore, the correct answer is true.

Load casts are geological structures that are formed when dense sand sinks into a layer of fluid mud beneath it. This process occurs when the weight of the sand causes it to penetrate the less dense mud, creating distinctive structures in the sedimentary layers. Therefore, the statement "Load casts are formed by the sinking of dense sand into an underlying fluid mud" is true.

Sand dunes can indeed form adjacent to beaches in any climatic setting. Aeolian action refers to the process of wind transporting and depositing sand particles. In coastal areas, winds blowing from the sea towards the land can carry sand particles from the beach and deposit them further inland, creating sand dunes. This process can occur in various climatic settings, including both arid and humid environments. Therefore, the statement is true.

At high wind velocities, silt- and clay-sized particles are not carried as bed load. Instead, they are carried as suspended load in the air. Bed load refers to larger particles, such as sand and gravel, that are transported by rolling or bouncing along the surface. Silt and clay particles are much smaller and lighter, so they can be easily picked up and transported by the wind as suspended load. Therefore, the statement is false.

The last material to be deposited from a turbidity current will not necessarily be the coarsest. While it is true that coarse particles are more likely to settle out of suspension first, the order of deposition can also be influenced by other factors such as the velocity of the current and the size and density of the particles. Therefore, it is not always the case that the coarsest material will be the last to be deposited.

High-density turbidity currents are characterized by their high concentration of sediment particles, which causes them to have a higher density than the surrounding water. This higher density allows them to flow rapidly and transport large amounts of sediment. Therefore, it is true that high-density turbidity currents have a bulk density of at least 1.1 g cm-3.

Debris flow material can vary in size, ranging from clay particles to large boulders. This means that it is possible for debris flows to contain a wide range of materials, from small particles to larger rocks. Therefore, the statement that debris flow material can form any size from clay to large boulders is true.

Turbidity currents are underwater currents that carry sediments, and turbidites are sediment deposits formed by these currents. They can occur in any body of water where there is an abundant supply of sediment and a slope. This is because the sediment particles become suspended in the water and flow downhill due to gravity, creating turbidity currents. Therefore, it is true that turbidity currents and turbidites can occur in water wherever there is sediment and a slope.

The statement suggests that sand grains become angular or even very angular after a sustained period of aeolian transport. However, this is not true. In reality, sand grains that undergo aeolian transport become rounded due to the constant abrasion and collision with other grains. Therefore, the correct answer is false.

This statement is true because avalanches are indeed mechanisms of mass transport down a steep slope. They involve the movement of a large amount of snow, ice, and debris down a mountain or slope, often with destructive force. The term "grain flows" is also used to describe avalanches, referring to the granular nature of the snow and ice particles that make up the avalanche.

Small-scale erosional features on a bed surface are referred to as sole marks. This statement is true because sole marks are commonly formed by the action of flowing water or ice on the bed surface. These marks can include grooves, scratches, or small depressions that are left behind as the sediment or rock is eroded. Sole marks are important in understanding the past flow direction and intensity of currents or glaciers.

Rill marks are indeed small-scale dendritic channels formed by the erosion of non-cohesive sand. This erosion occurs when water flows over the sand, creating these distinctive patterns. Therefore, the statement is true.

The statement in the question is false. Turbidity currents are actually less dense mixtures compared to debris flows. Turbidity currents consist of sediment suspended in water, while debris flows are composed of a mixture of water and a high concentration of sediment. Additionally, turbidity currents typically have a higher Reynolds number, indicating a more turbulent flow, compared to debris flows.

The explanation for the given answer "False" is that the statement is stating that the winnowing effect, which is the selective removal of finer grains from sediment in a flow, is more effective in water flows due to the higher density and viscosity of water. However, this statement is incorrect. The winnowing effect is actually more effective in air flows, not water flows. In water flows, the density and viscosity of water make it more difficult for the finer grains to be selectively removed.

The statement claims that ripple heights range from less than a meter to more than ten meters. However, this is not true. Ripple heights typically range from a few millimeters to a few centimeters, not reaching heights of more than ten meters. Therefore, the correct answer is false.

The statement in the question suggests that well-developed grains may avalanche down into the adjacent trough, forming planar-lamination. However, this is not true. Planar-lamination is formed when fine-grained sediment settles out of suspension and forms thin, parallel layers. It is not formed by the avalanching of well-developed grains. Therefore, the correct answer is False.

Turbidity currents can flow down slopes or over a horizontal surface regardless of the thickness of the flow. The statement in the question is incorrect.

Aeolian carbonate deposits are actually more stable features compared to dunes made of quartz sand. This is because carbonate deposits are composed of more cohesive materials, such as limestone or calcite, which are less prone to erosion and movement. In contrast, dunes made of quartz sand are more easily shifted by wind and water, making them less stable over time. Therefore, the correct answer is False.

Raindrop impressions are not formed by rain falling onto a non-cohesive sediment surface. Rather, raindrop impressions are typically formed on cohesive surfaces such as clay or mud, where the raindrops can cause the surface to become temporarily indented or marked. Therefore, the correct answer is False.

The statement suggests that flute casts are symmetric in cross-section with one steep edge opposite a tapered edge. However, this is not true. Flute casts are actually symmetric in cross-section with two tapered edges. This means that both edges of the flute cast taper towards the center, rather than having one steep edge and one tapered edge.

The grain flow mechanism is not most effective in poorly sorted material falling under gravity down a steep slope such as the slip face of an aeolian dune. Instead, it is more effective in well-sorted material where the grains can easily slide past each other.

The given statement is false. Flow in a turbidity current is not maintained by the density contrast between the sediment-water mix and the water. Instead, it is maintained by the gravitational force acting on the denser sediment-water mixture. If the density contrast is reduced, the flow will actually decrease, as there will be less gravitational force acting on the mixture.

Desiccation cracks are not often filled with finer sediment. When an area dries out and the soil shrinks, cracks can form. These cracks are typically wider at the surface and narrower at depth. They are not typically filled with finer sediment, but rather they are empty spaces or gaps in the soil.

The statement is false because Draa is not an area where sand has accumulated due to aeolian processes. Draa is actually a river valley located in southern Morocco, known for its lush vegetation and palm groves. The sand dunes and desert areas in Morocco are typically found in regions such as the Sahara Desert, not in Draa.