Turbidite

turbidite, a type of sedimentary rock composed of layered particles that grade upward from coarser to finer sizes and are thought to have originated from ancient turbidity currents in the oceans. They are integral components of sedimentary deep-sea fans adjacent to the base of continental slopes, and they are also found below the major river deltas of the world where they build features called abyssal cones. In addition, abyssal plains are formed by the accumulation of turbidites beyond the limits of deep-sea fans and abyssal cones in locations where there is a very large sediment supply.

Turbidites were first described by Arnold H. Bouma (1962), who studied deepwater sediments and recognized particular fining-up intervals within deep water, fine grained shales, which were anomalous because they started at pebble conglomerates and terminated in shales.

The Bouma sequence is the idealized sequence of sedimentary structures that represents the waning of a turbidity current as it passes over a single point. Each layer described by Bouma has a specific set of sedimentary structures and a specific lithology (see below), with the layers overall getting finer-grained from bottom to top. Most turbidites found in nature have incomplete sequences. The five Bouma divisions are (in stratigraphic order):

E: Massive, ungraded mudstone, sometimes with evidence of trace fossils (i.e., bioturbation). The Bouma E layer is often missing, or difficult to differentiate from the Bouma D layer below.
D: Parallel-laminated siltstone.
C: Ripple-laminated fine-grained sandstone. Often the ripple laminations are deformed into convolute laminations and flame structures.
B: Planar-laminated fine- to medium-grained sandstone. The base of Bouma B often has features known as sole markings, such as flute casts, groove casts and parting lineation.
A: Massive to normally graded, fine- to coarse-grained sandstone, often with pebbles and/or rip-up clasts of shale near the base. Dish structures may be present. The base of the sandstone, below A, is sometimes eroded into underlying strata.

As flows move downslope the following processes take place to create the layers of the Bouma sequence.

Bouma E is the last layer deposited. It results from suspension settling where essentially no current exists. Clays generally remain suspended until the water chemistry changes and allows the clays to flocculate and settle out. Because the Bouma E layer, if deposited at all, is easily eroded by subsequent turbidity currents, it is often not present.

Bouma D is deposited by suspension settling where a slight current exists. Subtle changes in current energy causes alternating laminations of coarser and finer grains of silt to settle out.

Bouma C is deposited under lower flow regime conditions where there is enough energy for the flow to carry fine sand by saltation, wherein grains hop and bounce across the surface beneath the flow. As grains settle out, current ripples develop, with climbing ripples developing if sedimentation rates are high enough.

Bouma B is deposited under upper flow regime conditions where energy is high enough to carry sand grains by traction, wherein they slide and roll across the surface beneath the flow. The current energy is such that sole marks such as groove casts, flute casts and parting lineation can form on top the bed beneath the flow, and be preserved as molds and casts on the underside of the Bouma B layer.

Bouma A is the first layer deposited by a flow, provided the flow has sufficient energy. Otherwise Bouma B, C or D will be the first layer deposited. Bouma A is deposited when the flow energy is high enough that fluid turbulence is able to keep the coarsest grains in suspension. When energy drops below a critical level, the grains tend to settle out all at once to create a massive bed.

If flow energy drops more slowly, then the coarse grains may settle out first, leaving the fine grains still in suspension. This results in coarse-tail graded bedding, which means that there is a bimodal distribution of grain sizes with the coarse grains becoming progressively smaller towards the top of the bed, and the finer grains being randomly distributed between the coarse grains (i.e., the finer grain sizes are ungraded). As grains settle out, water displaced by grain compaction can move upward to create dish structures. Also, erosion can take place at the base of the flow and tear up shale from an underlying bed so that shale-rip clasts are incorporated into the base of the Bouma A layer.

The Bouma sequence.

In (A), grains are represented by the black dots – note that the coarser grains are located near the bed and towards the front of the flow. In (B) is a turbidity current produced in a laboratory experiment. From Zane Jobe.

New Zealand turbidite. From Zane Jobe.

Upper Devonian Turbidite from Rheinisches Schiefergebirge with gradded and convolute bedding. Complete Bouma sequence (Bouma-Sequenz (de.)), each part of the sequence is labeled. Nehden formation, former sandstone quarry, Becke-Oese..

Foto
Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 2x (Field of view = 7mm)	Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 2x (Field of view = 7mm)	Laminated turbidite deposit. (resulting from a pool experiment). XPL image, 2x (Field of view = 7mm)
Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 10x (Field of view = 2mm)	Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 10x (Field of view = 2mm)	Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 10x (Field of view = 2mm)
Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 10x (Field of view = 2mm)	Laminated turbidite deposit. (resulting from a pool experiment). PPL image, 10x (Field of view = 2mm)	Laminated turbidite deposit. (resulting from a pool experiment). XPL image, 10x (Field of view = 2mm)