Search

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.

SUMMARY Many, but not all, current ripples show log-normal grain-size distribution and good sorting (P. D. Trask). A range in medians was measured from fifty to several hundred microns. Turbidite ripples are log-normal, very well-sorted and are restricted to medians of very fine sand to silt (< 130 μ) because the particles have been supplied in suspension at rippling velocity. Experimental suspension current ripples formed at about 50–60 cm/sec are similar in composition and structure to turbidite ripples. The continuous supply results in climbing ripples. The restricted grain size and the internal structure of turbidite ripples both form strong evidence against the action of normal currents. The number of grain-size analyses is still rather small.

SummaryNew analyses are given of the Glen More Ring-dyke. The basic end of the variation diagram is found to differ considerably from that of the Normal Magma Series. The magma variation is explained by gravitative sinking of augite and iron-ore (and plagioclase), the olivine of the Normal Series playing only a subordinate part. Various objections are brought forward to the explanation of the magmas by hybridization. The peculiar microscopic aspect of the intermediate rocks is tentatively attributed to pneumatolytic emanations from the lower reaches of the column. It is shown that a gradual transition from basic to acid already existed before the final consolidation of the magma took place.

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.

I. Introduction The experimental approach to geological problems was introduced by Sir James Hall a century and a half ago at the dawn of Geology as a science, following even earlier tentative beginnings by de Saussure. Since then many have followed his lead. And yet it cannot be said that this type of research is truly flourishing and generally acknowledged as useful. The majority of earth scientists appears to be highly suspicious of, or actually opposed to, experimental attack on their problems. As one who has had recourse to laboratory methods in connection with almost a dozen separate problems in general geology, it behoves me to enquire into the cause of this poor estimate by others. In the present address I shall try to assess the value of past experiments in geology and to present some examples from my own research in an attempt to vindicate this method. However, before an enquiry into the significance of experiments in the science of geology can be started two aspects of this problem must be defined more closely. First it is necessary to state how wide a meaning will be given to the term “Geology” and secondly the term “Experiment” will have to be given a definition. The necessity of first stating what field is to be treated is clearly illustrated by a recent publication (Morey, 1956) bearing the bold title of “Experimental Geology” but limited to a summary of the latest experimental work on the crystallization of silicate melts. In the present This 250-word extract was created in the absence of an abstract

The abrasion of pebbles rolled in a large concrete basin by a revolving current, both on a sandy and on a pebbly floor, was studied-a setup believed to be a substantial improvement on the customary tumbling-mill experiments. One of the most significant results is that, on a sandy floor, abrasion is less than on a pebbly bottom under similar conditions. The difference increases with size of the rolling pebble and can be four or five times less for medium pebble sizes. Hence reports on measurements of roundness in nature should always be accompanied by statements not only of the size of the pebbles, of current velocities, and of type of rock investigated but especially as to the nature of the bottom. On a sandy bottom, weight and velocity have only slight influence on the percentage of abrasion per kilometer. With increasing roundness, there is a small reduction in the rate of abrasion. It was found that abrasion on a pebbly floor is reduced by 10-15 per cent by the introduction of sand. The abrasion on a p...

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...

Pictures in three dimensions have great advantages over flat ones for teaching geology and other sciences. Not only do they give a very much clearer and more detailed view, but the stereoscopic effect is so fascinating that a photograph viewed in this form imprints itself more deeply on the mind. The technique is simple.

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.

The investigation of submarine canyons has been actively pursued in the United States of latter years, principally due to the activities of F. P. Shepard. The Coast and Geodetic Survey has mapped a large number of these interesting chasms, both along the Atlantic and Pacific continental slopes, by sonic soundings; Shepard himself added data by soundings and dredgings along the Californian coast; Stetson performed current measurements, dredgings, and sampling in the canyons off the east coast. The number of data is steadily increasing in the United States, and it is to be hoped other nations will also become active, for much still remains to be learned before a comprehensive view of the many-sided problem can be obtained.

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

SummaryEvidence from Mull and Ardnamurchan is brought forward to show that the injection of cone-sheets is accompanied by a vertical lifting of the upper wall. This confirms Anderson's explanation. It is argued that the fracturing and injection are of a quite sudden character and that it is in the nature of the process that no evidence from recent volcanoes has yet been obtained of contemporary cone-sheet injection.

A definition is suggested for ptygmatic features. Five explanations, with minor variations, have been offered: 1. (1) Injection along meandering cracks (Read, 1931). 2. (2) Viscous flow (Sederholm, 1926). 3. (3) Injection into an incompetent host and crumpling where the ribbon of magma meets a resistance (Wilson, 1952). 4. (4) Anatectic growth in a passive host (Dietrich, 1959). 5. (5) Crumpling of an originally planar vein by subsequent compression (Milch, 1900). 1. (1) is ruled out among others by mis-fit of the walls, 2. (2) is based on unsound flow mechanics, 3. (3) is excluded by variations in thickness of single veins, by structureless hosts, and by the impossibility of hydrostatic overpressure to force the magma into the more mobile host, 4. (4) by its failure to explain uniform wall distance and because of the necessity to assume resorption of vein material concomitant with deposition exactly on opposite sides. (1) and (4) fail to explain the rough relation between thickness and wave length. Analogous structures produced by deformation of sedimentary beds, sand dikes, or quartz veins, and in experiments support Milch's view 5. (5). It is a matter of definition whether these are included under the term ptygmatic. A search for the correct interpretation is important because ptygmatic features may mean either zero reduction in thickness of the host rock, or, conversely, up to six-fold thinning.

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.

Abrasion by eolian transport was studied in three different types of wind tunnel. Limestone, feldspar, and quartz were tested in the shape of cubes, crushed crystals, or natural grains. Abrasion increases with size, wind velocity, angularity and roughness of surface. Polished medium-sized quartz grains sustain no loss. Wind abrasion of quartz is 100 to 1,000 times more effective over the same distance than the mechanical action of a river. But eolian abrasion is slight for fine quartz sand and drops to zero at 0.05 mm. in diameter. No quartz particles are produced in the size range dominating in loess. Experimental abrasion of ventifacts indicates that these are cut slowly in scores of years by medium strong winds, but in a matter of days by storm winds, and in a matter of hours if the cutting sand is coarse and angular. These results must be looked upon as first approximations, for many factors in nature are either unknown or not reproduced in the experiments. Experiments on frosting and defrosting will ...

Eolian action does not produce quartz particles between 20 and 50 microns in mass, either by abrasion of larger grains or by spalling of chips in that size range. Aqueous spalling is insignificant compared to disintegration of phyllite or similar material and of coarser crystalline rocks. Glacial action is particularly important.

The term 9turbidite structures9 recently used by Sen (1967) is better avoided, because only a combination of features together with the absence of others can be diagnostic. The distinctions in meaning between the terms 9turbidites9 and 9fluviatile deposits9 should not be confused. The explanation of convolute lamination by exaggeration of ripple mark is supported by experimental results.

The sediments of the East Indian Archipelago, owing to the complicated topography of the land and sea bottom, vary greatly in character in different areas. In the shallow Java Sea they are relatively fine-grained and rich in volcanic debris on the south side near Java, and relatively coarse-grained and rich in quartz on the north side near Borneo. The sediments of deep water are similar to sediments in deep water in other parts of the ocean, though in general they contain less calcium carbonate, owing in part to the relatively large amount of terrigenous debris they contain. The calcium carbonate content of the sediments decreases with depth of water from an average of about 50 per cent for sediments in water less than 500 meters deep to less than 5 per cent for water mor than 5000 meters deep. In areas in which the sea floor slopes more than 5°, the calcium carbonate content is greater at depths of 2500 to 3500 meters than at depths of 1500 to 2500 meters, perhaps owing to slumping of the sediments. The sediments of Kaoe Bay contain hydrogen sulphide and iron sulphide. They also are rich in organic matter and black in color. Evidently, like in the Black sea and some of the Norwegian fiords, a condition of stagnation prevails in this bay, which is 500 meters deep and whose connection with the sea is only 50 meters deep.

The tidal flats on the southeastern border of the North Sea are developed behind a chain of barrier islands. In this area the grain size of the sand decreases from the inlets towards the inner shores and the tidal watersheds behind the islands. Likewise the clay content increases, so that muddy sands and even sandy muds are formed. This condition is explained by the action of tidal currents combining with other, mainly biological phenomena. Pellets and flakes are formed and these are carried inwards in consequence of the "settling lag effect" and the "scour lag effect." In estuaries and tidal lagoons similar conditions may be responsible for lateral changes in clay content. In ancient basins such lateral increase in fine grained material may indicate approach to the mainland shore and increasing distance from passes between sandy barrier islands. This study is an elaboration of suggestions offered by Postma (1954). A more detailed account with extended bibliography and frill acknowledgements has been given in an earlier paper (van Straaten and Kuenen, 1957).

Publisher Summary This chapter describes the pivotability of sands with a shape-sorter, each size grade between 0.15 and 1 mm being tested separately. Pivotability plays a modest part in transportation causing sorting. The influence is opposite in traction and in suspension transportation. Source materials (weathered granite) show low pivotability. It increases by abrasion, especially in the desert. Coastal dunes are more pivotable than the feeding beaches in the fine and coarse sand grades and less pivotable in medium sizes. The pivotability decreases inland. Minimum pivotability is found at different localities along the south Californian beach for different grain-sizes. In sorting processes the role of repositories is important.

(1929). Experiments on the formation of windworn pebbles (pl. 3–7) Geologiska Foreningen i Stockholm Forhandlingar: Vol. 51, No. 1, pp. 129-130.

The Polish Carpathian Mountains are occupied for the greater part by rocks of the flysch type; in which graded bedding with the usual accompanying sole markings and other features of turbidites are ubiquitous. For several years Polish geologists have successfully applied the concept of turbidity flow to explain features formerly thought to be conflicting. This paper deals with paleogeographic problems and summarizes results so far attained by numerous measurements of current direction. A few separated troughs persisted throughout the Cretaceous and Paleogene sedimentary history. Each of these oblong basins had its separate history in which location and direction of supply varied greatly with time. Most units show a consistent system of transport. Dominantly lateral supply is obvious in some cases. In others the measurements indicate a distant supply and from thence longitudinal transport in the flysch trough with or without local coarse supply from the sides. However, evidence, mainly paleogeographic, strongly suggests that local cordilleras between or flanking these troughs acted as the main source. Continued field work may throw light on this controversial topic which occurs in many other geosynclines that contain turbidite formations. A number of sedimentological problems are examined on the basis of evidence mainly from Poland: upper margins of graded beds, suspended currents, convolute lamination, doubtful turbidites, glauconite in turbidites. A few new sole markings are named and described.

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.