Which sediment grains sink

Another name for a lithic wacke is greywacke. Some examples of sandstones, magnified in thin section are shown in Figure 6. A thin section is rock sliced thin enough so that light can shine through. Clastic sedimentary rocks in which a significant proportion of the clasts are larger than 2 mm are known as conglomerate if the clasts are well rounded, and breccia if they are angular.

Conglomerates form in high-energy environments where the particles can become rounded, such as fast-flowing rivers. Breccias typically form where the particles are not transported a significant distance in water, such as alluvial fans and talus slopes.

Some examples of clastic sedimentary rocks are shown on Figure 6. The table below shows magnified thin sections of three sandstones, along with descriptions of their compositions. Using Table 6. Skip to content Chapter 6 Sediments and Sedimentary Rocks. Exercise 6. Transportation One of the key principles of sedimentary geology is that the ability of a moving medium air or water to move sedimentary particles, and keep them moving, is dependent on the velocity of flow.

Figure 6. When the photo was taken the river was not flowing fast enough anywhere to move the boulders and cobbles visible here, but it is fast enough when the discharge is higher. The larger clasts, resting on the bottom bedload , are moved by traction sliding or by saltation bouncing. Smaller clasts are kept in suspension by turbulence in the flow.

Table 6. The quartz arenite and arkose have relatively little silt-clay matrix, while the lithic wacke has abundant matrix. Magnified Thin Section Description Rock name? A micron is a millionth of a metre. There are 1, microns in a millimetre. Previous: Chapter 6 Sediments and Sedimentary Rocks.

Next: 6. Share This Book Share on Twitter. Porosity is the void fraction of the sediments or rock. Over time compaction will remove most of the voids, and the density and sound velocity will both increase.

Permeability in the interconnectedness of the voids. It is actually more important than porosity for oil and gas production, and fracking or more technically, hydraulic fracturing increases the rock's permeability.

Runoff can carry away top soils, pushing the sediment into nearby streams and rivers. In addition to the influence of wind and rain, sediment transport is also affected by the local topography The amount of sediment that enters the water and the distance that it travels is due to the terrain that a waterway runs through Bedrock streams are less likely to contribute to the sediment load, as the channel is resistant to quick erosion These rivers, as well as man-made channels with no sediment, are considered non-alluvial channels.

The majority of rivers however, are alluvial, or self-formed Alluvial rivers and streams create their own path by carrying sediment away. In an alluvial stream, the depth and breadth of the waterway will depend on the strength of the water flow and the material that makes-up the channel boundaries Rivers that run through soft soil typically have a higher sediment transport load than rivers exposed to bedrock, as much of the sediment load is taken from the sides and bottom of the channel.

In addition to non-erodible bedrock terrains, highly vegetated areas are less subject to runoff erosion during flood events, as the roots of the plants hold the soil in place In addition to the effects that geomorphology has on sediment transport rates, the process itself plays a part in creating the terrain. In addition to the mineral-based aspect, sediment can be organic in source. Organic sediment comes from decaying algae, plants, and other organic material that falls in the water such as leaves 4.

Bacteria attached to this detritus or other inorganic matter are also categorized as organic Organic sediment transport is will vary by location and season. Some phytoplankton can play a unique role in their contribution to sediment loads. In addition to the organic factor they provide, specific phytoplankton such as diatoms can contribute an inorganic component as well 1. This inorganic material comes from diatom frustules and calcium carbonate detritus. While this material is not specifically organic, it is organic in origin 1.

Sediment transport is not constant. In fact, it is constantly subject to change. In addition to the changes in sediment load due to geology, geomorphology and organic elements, sediment transport can be altered by other external factors.

The alteration to sediment transport can come from changes in water flow, water level, weather events and human influence. Water flow, also called water discharge, is the single most important element of sediment transport. The flow of water is responsible for picking up, moving and depositing sediment in a waterway Without flow, sediment might remain suspended or settle out — but it will not move downstream.

Flow is required to initiate the transport There are two basic ways to calculate flow. Water discharge can be simplified as area a cross-section of the waterway multiplied by velocity, or as a volume of water moved over time The equations describing the relationship of water flow and sediment transport are a bit more complex. The complexity of sediment transport rates are due to a large number of unknowns e. The sediment transport rate in particular is difficult to measure, as any measurement method will disturb the flow and thus alter the reading.

Most flow rate and sediment transport rate equations attempt to simplify the scenario by ignoring the effects of channel width, shape and curvature of a channel, sediment cohesion and non-uniform flows The two main flow factors in sediment transport are the settling rate and the boundary layer shear stress The settling rate also called Stokes settling is the rate at which sediment falls through a liquid and it is controlled by the drag force keeping a particle suspended and the gravitational force a function of the particle size Understanding this relationship helps to define some of the forces that sediment transport has to overcome relative to particle size.

Shear stresses in the boundary layer of a sediment bed explain how much force is required for water flow to overcome relative inertia and begin sediment transport through bedload or suspended load In the ocean and in other more complex water systems, this equation is inadequate. Instead, the Von Karman-Prandlt equation should be used. The shear stress is influenced not only by the viscosity of the liquid, but the roughness of the sediment The turbulent eddies created at the bottom by water flow must also be accounted for.

This is also known as the Law of the Wall The above equations help to give a basic understanding of some of the forces acting on sediment in the water. To further understand the conditions required for sediment transport, the Shields stress equation can be used. Shields stress, along with the particle Reynolds number, can be used to predict how much flow is required for substantial sediment transport In other words, the Reynolds number demonstrates whether or not a flow is viscous enough to overcome the relative inertia of sediment.

For sediment transport, the Reynolds number for flow through a sediment bed can be calculated from the boundary layer shear stress equation:. The point at which water flow begins to transport sediment is called the critical Shields stress This creates an empirical curve to approximate at what flow rate a sediment particle will move based on particle size While these equations help define minimum flow rates for sediment transportation, they do not determine sediment load and sediment transport rates themselves.

One sediment transport rate equation was developed by van Rijn, for the bedload transport of particles between 0. The suspended load transport rate still assuming cohesionless sediment and a sediment size of 0. Other sediment rating curves have been developed, but they cannot be equally applied to all water bodies This is because in any application, there are seven main variables that have an effect on sediment transport rates 11, The sediment transport rate is a function of these seven variables, as well as the size-shape-density distribution often assumed as a standard deviation of the particle diameter of the suspended particles In addition, the largest river discharge does not automatically mean that a river will have the largest sediment load.

The quantity and material of the sediment particles, as well as the geography of the local terrain will still play a contributing role in the sediment load The sediment load itself is calculated as a depth-integrated sediment mass above a unit area It is variable for multiple reasons, but can be estimated with a time-average collected sediment concentration While it is dependent on flow to initiate and continue transport, it is not calculated from flow rates, as the main variables in sediment load come from environment factors.

Sediment transport relies on water flow to move a load downstream. Water flow is variable, affected not only by the local terrain e. Most changes in water level are due to weather events such as rainfall Precipitation causes water levels to initially rise, and then return to previous levels base flow over the course of hours or days.

Rainfall, whether slight or heavy can affect water flow and sediment transport. The extent to which a weather event will influence sediment transport is dependent on the amount of sediment available.

Snowmelt in a glaciated area will result in a high sediment load due to glacial silt Heavy rainfall over an area of loose soil and minimal vegetation will create runoff, carrying loose particles into the waterway.

Likewise, flooding will also pick up sediment from the local area. Increased water level creates additional volume in a channel, and increases the hydraulic radius cross-sectional area of a waterway. The increased hydraulic radius increases the discharge rate, regardless of whether or not flow is uniform or non-uniform Increased flow will increase the stress on the bed, making it more likely for water flow to initiate sediment transport.

The higher velocity also increases erosion rates as flow overcomes the shear stress of sediment Seasonal effects are also responsible for changes in water level and flow Most seasonal changes are due to precipitation levels and events such as snowmelt. During low precipitation and low flow periods, sediment transport falls.

During the peak of snowmelt, the sediment load can increase by a factor of 15 or more Climate change can also play a role in sediment transport, as it affects both the timing and magnitude of floods and other weather events Anthropogenic factors, such as dams and altered land use will affect both the sediment load and sediment transport rate