Geol 135 Sedimentation
J Bret Bennington
Textbook: Nichols, Chapter 4
Sediments are transported from source areas to depositional areas by three main types of processes:
Gravity - mass wasting due to gravity. Fluids may be involved, but are not critical to flow.
Fluid assisted gravity - sediment gravity flows
Fluid flow - transport by air, water, and ice
We will focus on the later two categories in this course.
Sediment gravity flows
Grains are moved by gravity, but are suspended in fluids during movement.
Fluidized sediment flow
Grain flows: cohesionless sediment (e.g. sand) moves downhill due to gravity. Fluid (air or water) acts as a lubricant, but flow is sustained by dispersive pressures caused by grain-grain collisions.
Fluidized sediment flows: flow maintained by pore pressure of fluid supporting grains. Sediment becomes fluidized when something increases pore pressure by disturbing the sediment (for example, seismic shock - liquifaction). Pore water travels upward through the sediment, disturbing the grains and creating mud or sand volcanoes at the sediment surface.
Debris flows and mudflows: larger grains are supported by a slurry-like matrix of water and mud during downhill movement. Debris flows often show reverse grading in which smaller particles are deposited near the base of the flow layer and larger particles toward the top. Slow moving debris flows are capable of carrying very large cobbles and boulders. Automobiles are common clasts when debris flows move through residential areas.
Turbidity currents: gravity flows in which the sediment is kept suspended by the upward turbulence of the fluid trapped within the flow. Turbidity currents are common events on the slopes of lake margins and continental shelves. Storms or earthquakes stir up the bottom sediments and the liquified mass of material moves downslope, often for long distances down very shallow slopes. As the turbidity current slows, it begins to deposit grains. Larger grains fall out first, followed by smaller grains, and finally silt and mud. This produces normal graded bedding in a deposit known as a turbidite
Thick stratigraphic sequences consisting of normally graded beds are common in the rock record, demonstrating that turbidity flows are common features of depositional basins.
In the 1950s, oceanographer Maurice Ewing demonstated that an earthquake that had occurred on November, 18th, 1929 produced a large turbidity current off the Grand Banks of Newfoundland. The turbidity current originated at the top of the continental slope and as it travelled downslope it snapped transcontinental telegraph cables lying on the deep ocean floor. Based on the precise times that telegraph service had been interrupted, Ewing was able to calculate the speed of the turbidity flow - 65 mph along the steepest part of the slope, slowing to 25 mph on the abyssal plain. It is estimated that 280,000 km2 of the seafloor was covered by 100 cm of sediment.
All molecules in the fluid move parallel to the direction of transport with little mixing.
Molecules in the fluid move in all directions, but with a net movement in transport direction and significant mixing of the fluid.
The transition from laminar flow to turbulent flow is governed by several parameters that can be described by a single Reynold's Number.
Re = u (flow velocity) x l (depth of water or diameter of pipe) / v (viscosity of the fluid)
Re > 2000 - turbulent flow
Re < 500 - laminar flow
500 < Re < 2000 - depends on other variables such as bottom roughness and behavior of the fluid.
Inspection of the Reynold's number equation provides some insight into how fluids that transport sediments will behave.
Most flows that are capable of transporting large quantities of sediment are turbulent.
The Froude number is a dimensionless number that relates the flow velocity to the rate at which a wave can be transmitted through the water. In simplified form:
Fr = V (velocity of flow) / sqrt(g [gravitational acceleration] x D [depth])
Flow at Fr < 1 is tranquil or subcritical with a smooth water surface
Flow at Fr > 1 is rapid or supercritical with an uneven surface of wave crests and troughs
The transition from rapid to tranquil flow creates a hydraulic jump, which produces a standing wave that breaks continuously in the upflow direction.
Transport of particles in a turbulent fluid
Particles are moved by a fluid by one of three mechanisms:
Traction is accomplished by frictional drag forces on the particles.
Saltation and suspension require lift forces that can overcome the inertia of the sediment particles to produce entrainment.
Entrainment is caused by the Bernouilli effect, which is also the phenomenon that generates lift across an airplane wing. Basically, in order to conserve the total energy in a moving fluid, an increase in velocity must be compensated for by a decrease in pressure. As flow moves over a sedimentary particle the cross section of the flow is decreases, resulting in an increase in velocity. This creates greater fluid pressure beneath the particle, which generates lift.
In general, as the mass of a particle increases so will the velocity of flow needed to generate drag and lift forces sufficient to move the particle (critical velocity). At any given velocity, particles beyond a critical mass will remain stationary, smaller ones will move by traction, still smaller by saltation, and the smallest by suspension.
Hjulstrom diagram: Shows the relationship between water flow velocity and grain size.
Note that the simple relationship between flow velocity and grain size predicted above does not hold for mud-sized particles. This is because small particles such as silt and clay can be cohesive. Although it takes little energy to keep them entrained, once they are deposited they stick together and are not easily re-entrained. In environments where water energy (flow) various through time (for example, tidal environments), cohesion explains how layers of clay may come to be interbedded with layers of sand.