Week 7,8 - Deep water sedimentary systems: process to product

Define deep water systems

Below the storm-wave base

Topographically low → particles moved there under gravity and find it difficult to get out

The Grand Banks disaster

1929 earthquake

Upper continental slope with a steep margin

20km deep strike slip fault caused a 50m/s landslide that broke seafloor cable

First direct evidence of natural turbidity currents

Features of turbidity currents

A flow driven by the action of gravity upon the density excess of a sediment laden fluid within an ambient fluid. Sediment is maintained in suspension by the upwards (and chaotically) directed turbulent flow cells

Flows have a dominantly Newtonian rheology

In the absence of turbulence, particles will settle from suspension

In the absence of sediment, the flow will stop (unlike a river)

Deposit character of turbidites is directly linked to the rate of deposition, which, in turn is linked to the steadiness of the flow

Hypopycnal flow

Inflow water density < basin water density

Less dense fluid enters the more dense fluid and floats across the surface

Hyperpycnal flow

Inflow water density > basin water density

More dense fluid enters a less dense fluid and travels along the bottom

Density of freshwater and seawater

Freshwater = 1g/cm³

Seawater = 1.02g/cm³

Deposit of turbidity currents

Turbidites

Deposit of debris flows

Debrites

Deposit of hyperpycnal flow

Hyperpycnite

Deposit of pelagic sediments

Pelagite

Deposit of hemipelagic sediment

Hemipelagite

Deposit of channels

Channel fills

The Bouma sequence

Low Density Turbidites

High Density Turbidites

Distribution of LDTs

Billow out in clouds of sediments forming a lobe shape thinning outwards

Distribution of HDTs

Branching of channels

Combination of turbidites

Classic fan model

Thick bedded turbidites

Structureless

Often deposited by high density turbidity currents in channels

Thin bedded turbidites

Often deposited in levees

Normally graded often with ripples

Debris flows

A debris flow is a high concentration (typically >40%) mixture of fluid and sediment which flows downslope due to gravity. Particles are supported in the flow by the sediment and pore pressure in the flow

Flows have a dominantly non-Newtonian rheology

Particles are supported by the flow matrix strength and pore fluid pressure which creates a buoyancy force

Deposit character is directly linked to the flow content. Flows freeze ‘en-masse’ and the deposit represents what the flow was at that time

Slides and slumps

Basal detachment

Dominated by intact, but rotated bedding

Occur mainly on slopes

AKA mass transport deposits

Thermohaline currents

Flows driven by temperature and salinity