A Bouma (read: Bauma ) sequence (after Arnold H. Bouma, 1962) is the typical sedimentary sequence ( typical sediments and their structures ) of a Turbidits. It is caused by the continuous deceleration of a sub - marine turbidity current, which leads to the deposition of entrained sediment particles. The coarse particles are deposited here first. A complete Bouma sequence is from the bottom ( coarse-grained layers ) to top ( fine-grained layers ) of five units (A to E):
D Upper laminated division ( deposit after the end of the flow; silts and fine sands)
C Division with convolute bedding, dishes structures and current ripples (deposition by attenuating the flow rate, cross- stratified medium and fine sands)
B Lower laminated Division (deposition in fast -shooting flow, shallow stratification, laminated sands)
A Graded Division (rapid deposition by alluvial, often erosion of older strata; below coarser sands and gravels; upwards fine )
The pelagic layer ( E ) shows a separation layer between two successive turbidites deposited is (cyclic deposition ). The units A to D are (up to a few hours days ) deposited in a relatively short time, while unit E usually takes several ten thousand years to do so. The units A to D are composed mainly of shelf material and fossils, the unit e other hand, consists of pelagic particles and plankton. In general, not all five units are delivered because the following turbidity currents often erode the already deposited, finer units C, D, and E before solidification. The units A and B, however, rarely reach up into the distal region and remain therefore be given more often. The cardinality of a Bouma sequence varies from a few centimeters to several meters ( Megaturbidite ).