Bouma sequence
A Bouma (pronounced: Bauma) sequence (according to Arnold H. Bouma, 1962) is the typical sediment sequence (typical sediments and their structures) of a turbidite . It is caused by the continuous decrease in the speed of a submarine turbidity flow , which leads to the deposition of the sediment particles carried along. The coarse-grained particles are deposited first. A complete Bouma sequence consists of five units (A to E) from the bottom (coarse-grained layers) to the top (fine-grained layers):
E Pelagic position (floating load of the background sedimentation; clays and silts )
D Upper laminated compartment (sedimentation after the current has ended; silt and fine sands)
C Department with convolute bedding , plate structures and flow ripples (sedimentation after the flow velocity slows down; cross-layered medium and fine sands)
B Lower laminated compartment (sedimentation in fast-moving currents; shallow stratification, laminated sands )
A Graded section (rapid deposition due to alluvium , often erosion of older layers; coarser sand and gravel below ; finer towards the top)
The pelagic layer (E) represents a separating layer between two turbidites deposited on top of one another (cyclic deposition). Units A to D are deposited in a relatively short period of time (a few hours to days), while unit E usually takes several tens of thousands of years to do so. Units A to D are mainly composed of shelf material and fossils , while unit E consists of particles of pelagic and plankton . As a rule, not all five units are delivered, as the following turbidity flows often erode the already deposited, finer units C, D and E before solidification . Units A and B, on the other hand, rarely extend into the distal area and are therefore more often retained. The thickness of a Bouma sequence varies from a few centimeters to several meters (megaturbidite).
literature
- Füchtbauer, H. (1988): Sediments and Sedimentary Rocks. pp.821. Schweizerbart'sche Verlagbuchhandlung, Stuttgart
- Bouma, AH, Normark, WR & Barnes, NE (eds.) (1986): Submarine Fans and Related Turbidite Systems. pp. 351. Springer Verlag, New York.
- Tucker, ME, Wright, VP: Carbonate Sedimentology. pp. 482. Blackwell Science