THE ORIGIN AND SIGNIFICANCE OF THE INTERNAL SEDIMENTARY STRUCTURES OF TURBIDITES

Summary The hydrodynamic conditions controlling deposition from turbidity currents are examined and reasons are suggested for the formation or absence of each of Bouma’s five intervals (1962, p. 49). The factors which affect the grain distribution within turbidity currents are discussed and four different types of current are suggested. The nature of the graded division depends upon the type of current from which it formed, but the nature of the other divisions depends mainly on the hydrodynamic conditions existing at the bed-current interface rather than on the current as a whole. The lower laminated division and current rippled division are compared with bed configurations described by Simons et al. (1961) from flume experiments. It is suggested that the upper laminated division was formed by the separation of mixed grains as they settled at different velocities through a laminar boundary layer. The origin of “cut-out” and “truncated” sequences is also discussed and it is tentatively suggested that these may prove to have a palaeogeographical significance.