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