EMPLACEMENT OF FLYSCH-TYPE SAND BEDS

Recently several attempts have been made to explain deep-sea sands or flysch-type sandstone beds by normal currents, instead of by turbidity currents. The arguments that are offered against turbidity currents and those in favour of normal currents are inconclusive. Current measurements and calculations indicate 1 m from the bottom on abyssal plains velocities are less than 30 cm/sec. The ubiquitous structures: sole markings, graded bedding, fine-grained ripple mark between a lower and a covering set of horizontal laminae, and convolution, are shown each in turn to be inexplicable on the basis of normal traction currents and the same holds for the uniform bed thickness. On the other hand these features develop readily in a circular flume from overloaded suspension currents. These experiments show that to support a heavy charge of fine sand in a clay suspension a current must exceed 100 cm/sec, and in clear water double that amount is needed. The inadequacy of normal currents both in velocity and kind is thus established. This lends powerful support to the case for turbidity currents. Many authors claim to have found evidence for the deflection of turbidity currents or for currents flowing across the paleo-slope. Explanations offered include the Coriolis force, normal currents, multiple turbidity currents, or surge waves. Analysis shows that all are open to serious doubts. The author suggests, quite tentatively, that the deflections may be only simulated by the development of lamination and grain orientation oblique and perpendicular to the current direction. Sagging of the trough floor may also play a part by confusing the determination of paleo-slope. Another possibility is that the turbidity current deviated from its original direction by “internal slope”, by momentum, by centrifugal force, or by lack of space. Admittedly, a problem remains, for the swift deposition deduced from the climbing ripples is in contradiction with the supposed stretching of the turbidity current inferred from grading.