I. Continental shelf: avg. width = 75 km; avg. depth = 130 m; avg slope = 10^-3 (0.1°)

II. Types of sediment:

A. Pelagic sediment: i.e. sediment NOT derived from land, but produced at sea in the water column. This could be carbonate material or siliceous material. Also hemipelagic muds are clay deposits that might have come from land, but might have formed at sea (e.g. volcanic ash settling out at sea).

• Sedimentation rates are slow: 1 - 6 cm/ka (thousand years).

B. Carbonate Compensation Depth (CCD): defines as where carbonate falling through water column becomes completely dissolved, so no limestone deposited on seafloor below this depth.

• Since depth of CCD is a function of such things as carbonate production rate by biota near surface waters and temperature of water, the CCD depth reflects climate.

C. Turbidity Currents- dense, sediment laden (i.e. turbid) water gets carried downslope as a mass flow. How do we get sediment into the water column to become a turbidity current? Catastrophic events such as mudflows, debris flows, underwater landslides can introduce sediment into water quickly. Also dense river flows, during times of flooding, can carry dense sediment rich water into ocean where it sinks downhill. These are called hyperpycnal flows (hyper = over; pycnal = dense). Click here for movie of turbidity current.

• As flow moves over shelf/slope break it picks up speed which means it can entrain more sediment to become denser and so flow moves even faster. Process is called ignition. (Click here for a movie showing ignition).
• As material falls out of suspension, for example where slope runs out, or flow becomes more dilute, get sedimentation of a turbidite.
• Bouma Sequence (below) is classic turbidite deposit as finer and finer grained sediment is deposited at lower and lower shear stresses.
• Over distance, farther down flow, get less of the Bouma a and b beds deposited, because shear stresses are too low.

Below left: Ideal Bouma Sequence. On right: Illustration of concept that with increasing distance downflow, the lower parts of the Bouma Sequence are not found.

III. Submarine Canyon - Where erosion, primarily due to turbidity current ignition, dominates along the continental slopes you can develop a channel or a canyon feeding these flows down into deeper water. The largest submarine canyons tend to form offfshore of river mouths. These are sites where lots of sediment that eventually become turbidity currents, are delivered to the shelf. The inner walls of canyons can also collapse generating large slump structures and subaqueous debris flows. These canyons and/or channels are erosional cutting down into underlying slope deposits.

Click here for movie showing submarine canyons off of southern California.

IV. Submarine Fans - These are essentially turbidite dumps most typically at the mouths of the submarine canyons that feed them. Much, much, much has been written about the types of depositional facies seen in fans. This falls into various schools of thought. The key for this class is to remember that modern submarine fans are not described from eye observations. Instead the topography of the fan, from sonar and/or the internal seismic structure of the fan is observed. It is unclear is a simple facies model describes how fans look. Clearly many fans have channels, mostly aggrading features where the channel levees build above the channels. The channels may persist across the entire fan and some are sinuous. Other channels seem to split apart in a distributary pattern into smaller and smaller channels. (See Amazon fan below). These small channels may die out completely down the fan. At the mouths of these small channels, sheets of turbidites are deposited that may build up into fan lobes. Lobe deposits will thicken and coarsen up section. Lobes may or may not have channels build out across them.

V. Deep geostrophic currents - Once sediment is deposited on the abyssal plains (or basinal plains), sluggish bottom hugging currents, caused mostly by dense cold water moving along parallel to the continental slopes. These geostrophic currents can rework the sediment into bedforms, such as sand waves and ripples, with current directions oriented parallel to the continental slope.