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A brief review is offered of the many problems where knowledge of the ocean floors and of marine processes in shallow water is indispensable for the further advancement of geology. The subject of turbidity currents is treated in greater detail, to demonstrate the interrelation of several aspects of marine geology with sedimentologic and paleogeographic investigations.

Recent improvements in the methods of raising cores from the deep ocean, of studying them and of measuring the total thickness of sediment are described. The possibility of turbidity currents carrying coarse material far into the deep ocean is discussed.

Nesteroff discovered, among other interesting facts, that many oceanic turbidites are covered by a thin film of pelagic shells, and he attributed this to deposition from the tail of the turbidity current. Any later pelagic deposit is supposed to have been washed away by the following current, and is therefore missing. Nesteroff's finding is important in itself and throws new light on ancient turbidites. However, the present author favours an explanation through winnowing of the shells from a pelagic stratum by the current that deposited the covering turbidite. This would mean that the shell pavement does not mark the end but the beginning of a turbidite, and that the upper part of the pelitic interval with pteropods and planktonic Foraminifera is a pelagic deposit. Several arguments are presented in favour of the latter view. As Nesteroff and Heezen are led by their interpretation to doubt similarly the presence of pelagic strata between flysch turbidites, a brief survey is made of this subject. It shows that most of the ancient turbidite sequences also contain many beds with a pelagic part towards the top of the pelitic interval. Up to the present, however, no shell pavement has been reported, but careful search will probably reveal examples of such fossil pavements.

SUMMARY The matrix (< 40 μ) of turbidites forms a possible clue to the density of turbidity currents and the origin of the graywacke matrix. Experiments in a circular flume provide a mechanism to study the relation between composition of suspensions at various speeds and their deposits. There is a close analogy to the lower part of turbidity currents. The lutum content of samples with median diameters greater than 400 or 500 μ is found to correspond to the suspended load of the pore water. The higher value for finer deposits can be recalculated to suspension concentration by use of the “sedimentation factor”. Hence, each turbidite carries, as it were, a sample of its depositing current. The lutum content depends not on the ratio of sand to lutum in the current, as tacitly assumed by many authors, but mainly on the ratio lutum to water, although also influenced by velocity. The average lutum density of coarser recent deep-sea sands is 1-2%. This indicates turbidity currents with 5-10% lutum by weight (density 1.03–1.07). The sand must be added to ascertain the current density. In first approximation turbidity currents tend to have densities at their nose of 1.1–1.2, but higher and much lower values also occur. The maximum original lutum percentage of coarse turbidites is below 10%. Higher values are very scarce and are due to post-depositional mixing, or we are dealing with slides. However, in fine-grained turbidites there is more matrix up to 20% for a median of 100 p. Hence, coarse graded marine graywackes with 20 or more per cent matrix are presumably weakly metamorphic turbidites, that originally held the same modest amount of lutum as recent turbidites of the same grain size. The Trask sorting of the experimental deposits is very good, like the average of natural turbidites. Most cumulative curves of turbidite grain-size analyses on arithmetic probability paper show a characteristic bend in fine sand or silt sizes.

Experiments on currents in which processes of long duration are involved, e.g., on slowly depositing, eroding, or decelerating suspensions, cannot be carried out in normal, straight flumes. Einstein and Krone therefore used a channel with a narrower and faster return flow. The disadvantage is the turbulent mixing during this return, but they nevertheless obtained noteworthy results. I employ a circular flume with vertical paddles moving horizontally in a circle along the axis. Centrifugal force and differences in velocity along the inner and outer wall are troublesome, but turbulence due to the drive is uniform and slight. The experiments described here are concerned with the origin of horizontal lamination as shown by nearly all turbidites of finer grain than coarse sand. The origin has been attributed to various causes, but usually to current pulsations of different kinds. Lamination is quoted as evidence for, but also against, deposition by a turbidity current. However, current pulsations are so numero...

SUMMARY 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.

(1951). Mechanics of varve formation and the action of turbidity currents. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 73, No. 1, pp. 69-84.

SUMMARY The sedimentological characteristics of typical flysch sediments are confirmed for the Roumanian flysch. The emplacement by turbidity currents of the coarse beds alternating with the shales and marls is again abundantly confirmed by the studies in the area under discussion.

Submarine canyons have generally been lumped together and a common explanation of their origin has been sought. The author suggests that some, named submarine ravines, are drowned river valleys (Corsica). The other extreme, the New England type, is the result of glacial turbidity currents. Daly9s explanation of the California type as drowned subaerial valleys, smothered by sediment, and excavated by glacial and recent turbidity currents is accepted with some change of emphasis. This type is thus tentatively ascribed to the same turbidity-current mechanism, but erosion has exhumed some buried valleys, cleaned out parts of others, laid bare parts of old mountain slopes, and has even developed new channels. Shepard9s new hypothesis of drowned river valleys kept permanently open by submarine processes, comes near to this picture and is accepted for the submarine ravines, but it cannot account for the features that indicate submarine erosion in the recent past off New England and California. His main argument against erosion by turbidity currents is the absence of scour on lake-delta fronts, but conditions in lakes must be highly adverse to erosion and hence this objection is eliminated. On the other hand Ewing and his associates give convincing evidence for the great importance of turbidity flow in the oceans.