Submarine Fans

A submarine fan is a body of sediment on the sea floor deposited by mass-flow processes that may be fan-shaped, but more elongate, lobate geometries are also common. They vary in size from a few kilometres radius to depositional systems covering over a million square kilometres and forming some of the largest geomorphological features on Earth. The morphology and depositional character of submarine fan systems are strongly controlled by the composition of the material supplied, particularly the proportions of gravel, sand and mud present. In this sense submarine fans are very much like other depositional systems such as deltas which also show considerable variability depending on the grainsize distribution in the material supplied. Note that although coarse-grained deltas are sometimes referred to as fan deltas and are largely submarine, the term submarine fan is restricted to fan-shaped bodies that are deposited by mass-flow, mainly turbidity current, processes. A submarine fan could form of any clastic material, but the larger fans are all composed of terrigenous clastic material supplied by large river systems. Carbonate shelves can be important sources of sediment redeposited in the ocean basins by turbidites, but the supply of carbonate sediment is rarely focused at discrete points along the continental slope: submarine fans composed of carbonate material are therefore rarely formed, and most carbonate turbidites are associated with slope-apron systems.

Architectural elements of submarine fan systems

A submarine fan can be divided into a number of ‘architectural elements’, components of the depositional system that are the products of different processes and subenvironments of deposition. Submarine fan channels form distinct elements on the fan surface and may have levees associated with them: these channels may incise into, or pass distally into, depositional lobes, which are broad, slightly convex bodies of sediment.

Submarine fan channels and levees

The canyons that incise into the shelf edge funnel sediment and water to discrete points at the edge of the ocean basin where turbidity currents flowing down the canyons pass into channels. Unlike the canyons, the channels are not incised into bedrock, but may scour into underlying submarine fan deposits. Submarine fan channels are variable in size: some of the larger modern examples are several tens of kilometres wide and over a thousand metres deep, and in the stratigraphic record there are submarine fan channels with thicknesses of up to 170 m and 20 km across. The deposits in the channel are typically coarse sands and gravel that form thick, structureless or crudely graded beds characterised by Tab of the Bouma sequence and S1–3 of the ‘Lowe-type’ high-density turbidite model. The lateral extent of these turbidite beds is limited by the width of the channel, which, when it is filled, forms a lenticular body made up of stacked coarse-grained turbidites.

Most of an individual turbidity flow is confined to the channel but the upper, more dilute part of the flow may spill out of the channel laterally. This is analogous to the channel and overbank setting familiar from fluvial environments. The overbank flow from the channel contains fine sand, silt and mud and this spreads out as a finegrained turbidity current away from the channel to form a submarine channel levee. The levee turbidites consist of the upper parts of Bouma sequences and they thin away from the channel margin with a low-angle, wedge-shaped geometry. Levee successions can build up to form units hundreds of metres thick, especially if the channel is aggrading, that is, filling up with sediment and building up its banks at the same time. Channel and levee complexes are also preserved when the channel migrates laterally or avulses, to leave its former position abandoned.

Depositional lobes

At the distal ends of channels the turbidity currents spread out to form a lobe of turbidite deposits that occupies a portion of the fan surface. An individual lobe is constructed by a succession of turbidity currents that tend to deposit further and further out on the lobe through time. A simple progradational geometry results if fan deposition is very ordered, with each turbidity current event of approximately the same magnitude and each depositing progressively further from the mouth of the channel. However, turbidity currents are of varying magnitude and so the pattern tends to be more complex. As the lobe builds out the flow in the more proximal part tends to become channelised. Lobe progradation continues until the channel avulses to another part of the fan. Avulsion occurs because an individual lobe will start to build up above the surrounding fan surface and eventually flows start to follow the slightly steeper gradient on to a lower area of the fan. The succession built up by depositional lobe progradation is ideally a coarsening-up succession capped by a channelised unit. Individual turbidites will show normal grading but as the lobe progrades currents will carry coarser sediment further out on the fan surface. Successive deposits therefore should contain coarser sediment and hence generate an overall coarsening-up pattern. A thickening-up of the beds should accompany the coarsening-up pattern. Commonly this overall coarsening-up and thickening-up is not seen because of the complex, often random pattern of deposition on depositional lobes. Therefore there may not be any consistent vertical pattern of beds deposited on a submarine fan lobe. Depositional lobe deposits often contain the most complete Bouma sequences. The whole lobe succession may be tens to hundreds of metres thick and an individual lobe may be kilometres or tens of kilometres across. Lobes will be stacked both vertically and laterally against each other, although the lateral limits of an individual lobe may be difficult to identify.

Turbidite sheets

Turbidite sheets are deposits of turbidity currents that are not restricted to deposition on a lobe but have spread out over a larger area of the fan. They are thin, fine-grained turbidites characterised by Bouma divisions Tc–e and Tde with little or no organisation into patterns or trends in grain size and bed thickness. Interbedding with hemipelagic mudstones is common.

1 comment: