What is the Bouma sequence?

The Bouma sequence is used to identify turbidite (underwater debris flows) deposits within the rock record. It has five subclassifications:T_e - massive mudstoneT_d - planar laminated siltsT_c - rippled or cross bedded sands fining to siltsT_b - laminated planar sandsT_a - sandstones which are normally graded or massive with an erosive base
These classifications are rarely found in perfect sequence due to the destructive nature of turbidites. If there are multiple events, the most recent turbidite can destroy previous deposits, removing them from the lithological record. Bouma sequences can also be used to identify if the deposit is more proximal (closer) or distal (further away) to the source of the tsunami. For example, if whilst in the field you find a deposit that looks like T_a-c , this is more than likely a proximal deposit due to coarser grains settling first in the water column.