Bouma Sequence

The Bouma sequence is an idealised description of the vertical succession of sedimentary structures preserved in a single turbidite bed - a bed deposited by one episode of turbidity current flow. ^[1] The model describes five structural units, labelled A through E from base to top, each reflecting a different stage in the waning of turbulence and current strength as the flow decelerates from a high-energy upper-flow regime down to a gentle, almost still condition. ^[1] The sequence is ideal in the sense that any individual turbidite bed rarely preserves all five units - which units are present and which are missing tells a geologist a great deal about the energy, grain size, and position of the deposit relative to the source.

The Five Structural Units

The five units of the Bouma sequence each record a distinct phase in the life of the depositing current. The lower units, A and B, reflect the most energetic phase of deposition, when the flow is still fast and sediment concentrations are high. The upper units, C through E, record progressively quieter conditions as turbulence decays and the tail of the flow takes over.

The complete sequence, from base to top, consists of a massive or graded sand unit at the base (A), followed by a planar laminated sand unit (B), then a ripple cross-laminated or convolute sand unit (C), overlain by an upper laminated or structureless silt and mud unit (D), and capped by a pelitic (mud) unit (E). ^[1] The scour marks - flutes, tool marks, grooves - that often accompany a turbidite are found not within the sequence itself but on the sole of the bed, cut into whatever substrate underlay the flow before deposition began.

What Turbidites Actually Look Like

Because the ideal five-unit Bouma sequence is rare in practice, understanding which units tend to be present in which types of turbidites is essential for field identification.

Thick, coarse-grained turbidites - the kind deposited close to the source from high-density, high-concentration flows - tend to show well-developed A and B units, while units C through E are commonly poorly developed or absent altogether. ^[1] The reverse is true for thin, finer grained turbidites: these typically display good C through E units and may have poorly developed or absent A and B units. ^[1] This complementary pattern makes intuitive sense: the lower units require a high-energy flow to deposit massive or planar-laminated coarse sand, while the upper units require a slow, waning flow capable of producing ripples and finally draping the bed with mud.

The Hsü Simplification

A more streamlined interpretation of the Bouma sequence was proposed as an alternative to the original five-unit framework. This view holds that Bouma’s D unit rarely occurs in practice and that most turbidites can be reduced to just two units: a lower horizontally laminated unit combining what Bouma called A and B, and an upper cross-laminated unit corresponding to Bouma’s C. ^[1] The E unit presents a separate interpretive challenge, because the fine material at the top of many turbidite beds may have settled slowly from the overlying water column after the turbidity current itself had fully dissipated, making it potentially a background pelagic or hemipelagic deposit rather than a true part of the turbidite flow unit. ^[1] The question of whether E is part of the turbidite or simply the ambient background drape is, in many cases, genuinely difficult to resolve in the field.

Proximal to Distal Variation

The character of turbidite beds changes systematically with distance from the source, and this variation maps broadly onto which Bouma units are preserved. Near the source, particularly within the main transport channel where suspended sediment concentrations remain high, turbidites tend to be coarse-grained, massive, or poorly laminated. ^[1] Some very high-concentration flows can produce coarse-grained turbidites even far down the channel, well away from the source area. ^[1] Thin, fine-grained turbidites are deposited where the current has spilled over the channel banks and spread across the seafloor, thinning and diluting as it goes, as well as in genuinely distal locations far from the source. ^[1]

This produces not only vertical grading within individual beds but also horizontal grading across the turbidite system: thick, coarse-grained deposits near the source grade laterally into thinner and finer grained sediments with increasing distance. ^[1] When a turbidity current system is active over a long period, repeated flows build up a rhythmic succession of turbidite beds stacked one atop another, forming sequences that may reach hundreds of metres in thickness. ^[1]

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