Bouma sequence

The Bouma Sequence (after Arnold H. Bouma, 1932–2011) describes a classic set of sedimentary structures in turbidite beds deposited by turbidity currents at the bottoms of lakes, oceans and rivers.

The Bouma sequence specifically describes the ideal vertical succession of structures deposited by low-density (i.e., low sand concentration, fine-grained) turbidity currents. An alternate classification scheme that is generally called the Lowe sequence exists for the ideal vertical sequence of structures deposited by high-density flows.

The Bouma sequence is divided into 5 distinct layers labelled A through E, with A being at the bottom and E being at the top. 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 - Bouma describes the ideal sequence where all layers are present.

The layers are as follows.

The Bouma sequence is deposited during waning flow as turbidity currents move downslope. In other words, flows steadily lose energy as they react to changes in the slope of the surface over which they travel, and/or as flows move from being confined within a channel to unconfined when they exit the channel and spread out. Surges and/or hydraulic jumps caused by changes in slope can reinvigorate flows briefly to increase flow energy, but ultimately energy decreases as flows move away from their points of origin.

When energy within a flow is highest, it can carry the maximum amount of sediment and the largest grain sizes, but as energy decreases, the carrying capacity reduces, and the coarsest grains quickly settle out, sometimes almost instantaneously. High-energy flows may also erode into underlying beds, thereby incorporating new material into the flow, which will tend to decrease flow energy. Flows in channels can also undergo flow stripping, in which the upper part of the flow, where the finer grains tend to concentrate, separates and travels out over the top of the channel, leaving the lower part of the flow, where the coarser grains accumulate, within the channel. Ultimately, only clay particles remain, suspended in a stagnant water column with essentially no current movement.

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