■ Summary of Discussion 



Dr. Tulley inquired as to the origin in nature of the material postu- 

 lated in the flows in Dr. Kuenen's experiments. It was answered that 

 the cause might be found in a slump, with the material taking up water 

 to reach a stage where flow is possible. Dr. Shepard stated that this 

 had been proved to occur in the case of mud at the head of a canyon. 

 A process of filling up took place until a state of instability was reached 

 and the semi-fluid material commenced to slide down. Dr. Kuenen 

 further instanced mud-flows on land after floods, or the action of rivers 

 containing sufficient silt to provide a specific gravity approximating 

 to 2. Or, again, sediment might be churned up by waves to produce 

 a mixture of high specific gravity. 



Mr. Banwell mentioned the examples of coastal drift on west coast 

 of South Island of New Zealand, causing harbour silting. It had been 

 estimated that about 5,000,000 cubic yards of material were so shifted 

 annually along 150 miles of coast-line. Mr. Gage commented on the 

 absence of erosion of previous sedimentations which was regarded by 

 Dr. Kuenen as characteristic of this mode of deposition. He pointed 

 out to the contrary that erosive effects are frequently encountered in 

 sedimentary sequences of New Zealand Tertiary and pre-Tertiary. Had 

 Dr. Kuenen alwa3^s found an absence of erosion in graded bedding. 

 x\nd, referring to Mr. Banwell's remarks, why was it found that at times 

 sorting took place, and that at other times characteristically ungraded 

 beds resulted from slumping ? 



Dr. Kuenen attributed this variation to differences in the type of mud, 

 slope, particle size, and water admixture, through all gradations from a 

 sliding slump to a true turbidity current. Erosion may in part occur, 

 but is often far from complete. Mr. Gage agreed that in cases where 

 chips have been torn up by erosion the phenomenon was very noticeable, 

 but that the converse case of lack of erosion tended to be less readily 

 noticed. 



Dr. Lillie referred to Bailey's views on submarine landslipping. Were 

 turbidity currents regarded as an explanation in these cases ? It was 

 pointed out that Bailey is not clear on this point. Dr. Lillie referred to 

 variations in graded bedding in quarries, often with peculiar effects. 

 There may be coarse to fine grading, interrupted however by lenses 

 of thin bands often less than half a millimetre thick of carbonaceous 

 material. Dr. Kuenen thought these might be attributable to remains 

 of seaweeds and marine plants. 



Dr. Shepard spoke of cores in submarine canyons, as in Monterey 

 Ba}', showing well graded pebble sequences, with sharp breaks in the 

 cores between the coarse stratum below and the fine above. He 

 instanced many cases where fine material of turbibity currents has been 

 carried considerable distances. Was it possible that the fine material 

 might be carried on when the coarser grades had been precipitated ? 

 Dr. Kuenen inclined to the view that muddy water might get caught 

 between the coarse particles and silt filtered off. 



Dr. van Baren objected that the total amount of mud in relation 

 to water in Dr. Kuenen's experimental buckets where S.G. 2 is attained, 

 might far exceed anything occurring in Nature. Could we expect such 

 a density of mud to flow in Nature without dispersion ? Dr. Kuenen 

 held that this depended on the shape of the bottom. Flows emerging 

 from a canyon will disperse, but momentum will carry material a great 

 distance. 



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