What are clastic sediments?
Clastic sediments are formed by loose particles of various sizes, which can evolve into sedimentary rocks such as sandstones, shales and conglomerates. They are essentially made up of sand, mud and gravel, formed from the weathering and erosion of pre-existing rocks.
- Weathering comprises physical, chemical and biological processes that alter, disintegrate, or dissolve rocks. Physical weathering occurs when a rock is fragmented through purely mechanical processes, which do not significantly alter its chemical or mineralogical composition. Chemical weathering, in turn, involves chemical and mineralogical changes of the pre-existing rocks, which may result in hydration, oxidation, reduction or dissolution of their mineral components, and formation of new mineral phases stable under surface conditions.
- Erosion is responsible for moving materials derived from weathering, to be carried by the action of water, gravity, wind or ice, and deposited in various marine or continental environments.
It is possible to characterize the sediments that compose a clastic sedimentary rock with the help of a magnifying glass on a hand sample, or through the analysis of a thin section with a polarized microscope. Composition, textural, structural and fabric aspects can be described through the use of Petroledge® software. The program automatically generates textural and compositional classifications, recognizes the tectonic provenience of the sediments, and also interprets diagenetic environments, aspects which will be further addressed in incoming texts.
After erosion, the particles are available for transportation. Water, wind, ice and gravity are the main agents for sediment transport. Gravity can act alone or associated to other agents, such as water, thus constituting the main sediment transport agent in nature. In fact, rivers account for approximately 95% of the sediment flow to the oceans.
The sedimentary particles remain in motion while the flow energy (usually proportional to its velocity) is sufficient to sustain them. This can be represented for the different particle sizes by the Hjulströms Diagram.
Types of sedimentary transport
Sediment transport can be classified according to its competency (related to the transported grain size), its capacity (related to the amount of sediment that the agent can transport) and the load (amount of sediment that the agent effectively carries). A river capable of carrying particles larger than sand sized is a highly competent agent. Also, the greater the volume flow of the river, the greater the load in motion and, therefore, the greater its capacity.
The force that moves the particles during transportation is provided by fluids, and depends mainly on their velocity and viscosity. The flow of fluids can be separated into 2 types: laminar flow and turbulent flow. In laminar flows, the particles immersed in the fluid move parallel to each other, in the direction of transportation. In turbulent flow, however, the particles immersed in the fluid move in all directions, but with a displacement parallel to the transportation direction. This type of flow has a much greater erosion power than laminar flow. As velocity increases, the flow tends to become turbulent.
A dimensionless parameter called Reynolds Number (Re), named after Osborne Reynolds, who studied fluid dynamics during the nineteenth century, indicates the intervals at which a flow of fluid is laminar or turbulent. The Reynolds number is proportional to the velocity of the flow (V) and to the depth of the channel or diameter of a pipe (L), as well as to the ratio between the density (d) and the viscosity of the fluid (u), according to the equation:
For values of Re < 500, the flow is laminar, whereas for values of Re> 2000, it is turbulent. Laminar flows are common in high viscosity or low velocity fluids. Wind flow is, therefore, essentially turbulent due to the low viscosity of air. Water flows can be laminar when their velocity is very low. However, significant volumes of sediment are usually transported by turbulent water flows.
In addition to the effects of fluid viscosity and inertial forces, gravity also influences the way a fluid transfers waves or moves sediment dunes. The Froude Number (Fr), which can be considered the ratio between the average velocity of the flow and the velocity of a wave contained therein, is also a useful dimensionless value for sedimentology studies, expressed as:
where U represents the average velocity of the flow, L the depth of water and g the acceleration due to gravity.
When Fr value is less than 1, the flow is considered subcritical or quiet and a wave can move upstream (against flow). If the value is greater than 1, the waves cannot propagate upstream and the flow is considered supercritical or fast. In addition to being used to define the critical velocity from which a flow is considered subcritical or supercritical at a given depth, the Froude Number is also related to different flow regimes, which are related to characteristic bedforms.
Two flow regimes are recognized: lower and higher. The lower flow regime comprises a subcritical flow in which ripples, dunes and plane-parallel type stratifications are formed. The upper flow regime, in turn, comprises a supercritical flow, in which antidunes and plane-parallel stratifications are stable. This can be understood from a bedform stability diagram (Figure 4). The use of this diagram allows obtaining velocity estimates or identifying changes in velocity or type of flow that resulted in deposition of sediments according to their depositional structures.
Under the action of water flowing over a moving background condition, the solid particles that make up the substrate tend to come into motion. In a simplified way, coarse particles such as sand and gravel will be transported if the energy of the stream overcomes the weight of the particles. In the case of finer particles, such as silt and clay, the cohesive force is the main force of resistance that flow must overcome. The forces acting on a particle submerged in a water stream are: the submerged weight of the particle, the lift force that causes the rise of the particle, and the drag force, which drives the particle in the direction of flow. From the relation between these three forces, we can classify the particles transportation according to four different modalities:
- Suspension: When the lift force balances that of gravity, the particles move floating in the liquid mass.
- Sliding: occurs when particles slide on the bed.
- Rolling: occurs with coarser particles, which shapes allow them to rotate.
- Saltation: occurs through the interaction between the lift and drag forces, a situation in which particles move through jumps (such as a suspension transport in which the particle sometimes comes into direct contact with the bottom).
The viscosity of ice is so high that it is not directly recognized as a fluid. However, ice may transport large amounts of sediment. The ice flow is very slow and laminar, and carries suspended sediments of all sizes.
Air is a fluid of very low density and viscosity. The principles involved in wind transport are similar to those present in water transport; however, the low density and viscosity of air reflect at different thresholds for particle transportation. Usually, air is capable of transporting in suspension only particles below the fine sand size, while coarser sediments are transported by rolling and saltation. Transport occurs at relatively high speeds, in a turbulent way. Like water, wind transport results in the deposition of strata that occur in ripple size up to dunes many meters high.
The action of gravity can transport sediments in subaerial or underwater conditions. In gravitational flows, it is common that 20 to 70 percent of the sediment is transported in suspension.
Through gravity, grain flows can occur, in which part of the grains are suspended by contact interactions between the grains. It is common for grain flows to occur on the sliding face of wind dunes. Highly concentrated sediment and water mixtures can generate mud flows or debris flows. In this case, coarse particles are supported by a mud matrix, which has cohesive strength. This characteristic makes the flow less predictable, with non-Newtonian characteristics.
Liquefied or fluidization flows are a type of gravitational flow in which the grains are held in suspension by the upward movement of the interstitial fluid that escapes with the gravitational settlement of the grains, or by forced ejection of the fluid filling the pores. This occurs in shallow-buried sediments, which can behave as fluids after a sudden shock, which causes an instantaneous loss of contact between the grains thereafter suspended in the fluid.
Turbidity currents are a type of density current that occurs at the bottom of a sea or lake, consisting of turbid mixtures of variable density of sediments temporarily suspended in water. They are less dense than the debris flows, with a relatively high Reynolds number. The result of the deposition of sediments transported by this type of current are the turbidites. Turbidity currents slow down over time and as they move away from their source, resulting in deposition. Since the coarser suspended materials are the first to be deposited, the turbidites have a coarsening upward character.
Turbidity currents from low to medium density ideally form a sequence known as the Bouma Sequence, consisting of 5 divisions. This sequence is represented by poorly selected and massive sands at base (a), overlain by laminated sands of upper flow regime (b), sands with cross lamination (c), laminated silt (d) and hemipelagic mud at the top (e).
High-density turbidity currents, in which the density is greater than 1.1g cm-3, form thick deposits of coarse texture and massive structure at their base, as a result of the interaction between particles. The top of the deposit is characterized by fluid escape structures and is more similar to the Bouma Sequence, representing deposition from lower density flows.
Deposition of clastic sediments
Sedimentary particles are deposited when the transportation agent loses competence to carry them or when the force that causes the movement is cancelled. Loss of competence for water or air transportation may be related to decreased flow velocity. In the glaciers, the deposition occurs with the stagnation or retention of the glacier, which may occur due to increase in the melting rate, or decrease of snow accumulation rate. Deposition rates in the different environments are very variable, and usually present values from 3 millimeters per year, in abyssal seabeds, to tens of meters per year in deltaic regions, in which the sediment supply is very high.
The deposition of sediments transported by gravity, water, wind or ice can occur along several transportation cycles. Deposition may be temporary (when the sediment moves repeatedly) or permanent (when the deposited sediments remain immobilized and are buried).
Sedimentation may be episodic or continuous. Episodic sedimentation is characterized by processes of great magnitude separated by long periods of non-deposition. It occurs when an instantaneous depositional process is triggered. This makes estimating the deposition rate quite complex, so that an annual estimate cannot be used. Continuous sedimentation is associated with virtually constant depositional conditions over long periods of time. In the abyssal sea floor, the continuous sedimentation results in the slow accumulation of fine particles. However, in this environment, turbidites deposits may accumulate during events of episodic deposition. It is common, therefore, that events of episodic deposition represent an important part of deep marine sequences.
The superposition of depositional events results in the vertical stacking of layers that record the processes involved in sediment deposition. These sequences can be seen in outcrops or cores, and are of extreme importance for the characterization of depositional environments.
The description of sedimentary layers recorded in cores and outcrops can be performed efficiently through Strataledge®, a software that allows their complete and systematic description, comprising the types of rock, minerals and other constituents, textures, structures, thickness and types of contact between beds. With this tool, all the data is acquired in a digital and organized way. In addition, the system allows an easy integration with petrographic data, geophysical logs, photos and other media.
- Nichols, Gary. Sedimentology and stratigraphy. John Wiley & Sons, 2009.
- Elias Cembrani da Rocha – Endeeper