The Bouma sequence and turbidite deposits
For a while now, the most popular page on my site has been this one, a photo of a Halloween pumpkin I carved to look like the Bouma sequence. It is the most popular because people are looking for information about the Bouma sequence, so it is time to do a real post on the Bouma sequence, with more detail about turbidite deposits and the turbidity currents that produce them.
Turbidity currents are a type of sediment gravity flow where turbulence is the dominant mechanism for grain support. A turbidity current that is more familiar to most people is a snow avalanche. A turbidity current is structured like the image below, with a head, body, and tail. In (A), grains are represented by the black dots – note that the coarser grains are located near the bed and towards the front of the flow. In (B) is a turbidity current produced in a laboratory experiment that shows the downslope evolution of the flow.
Turbidity currents in the world’s oceans produce spectacular seafloor architectures like canyons, channels, and lobes/fans, depending on the amount of erosion or deposition taking place at a particular location. The sedimentary architecture is influenced by many factors, including grain size and distribution, slope gradient, sediment supply, etc etc etc.
Turbidites are the products of deposition from a turbidity current. The simplest case is a current that is slowing down (waning) and entirely depositional (e.g. on a lobe). , a turbidity current produces the classical turbidite, which was famously described by Arnold Bouma in 1962 and interpreted by Roger Walker in 1965. The Bouma sequence, as it has become known, is the idealized sequence of sedimentary structures that represents the waning of a turbidity current as it passes over a single point. The five Bouma divisions are (in stratigraphic order):
Te – pelagic mud
Td – planar laminated mud produced from suspension settling
Tc – ripple or climbing ripple cross lamination
Tb – high velocity planar lamination
Ta – structureless (aka massive) division
An important concept reflected by these structures is that the energy (bed shear stress) is decreasing upwards as the current passes by, and this is also manifested in the normal grading of the bed – coarser at the base, finer at the top. This photo from the Mt Messenger Formation in New Zealand says a thousand words (this was published in a cool paper in Nature Geoscience):
Note the nice normal grading in the deposit (coarser stuff is slightly tannish in the Ta-Tb, then going to grey in the Tc, and finally to mud in the Td-Te). The squiggly yellow line in the photo is caused by the denser sand loading into the soft mud at the start of deposition. Notice that this loading repeats in the bed above, suggesting fairly high sedimentation rates.
The Bouma sequence can also be expressed in a cartoon fashion:
Many variations on the Bouna sequence are possible: it is common to lose the Ta in distal environments where there is not enough energy in the current, and in proximal settings, amalgamated Ta beds are common, where the rest of the sequence was either never deposited or eroded away.
The next post will focus on the processes of deposition of the Ta division and how the Bouma sequence relates to the ‘Lowe’ sequence, which is typically used to describe much coarser grained turbidites…