Abstract
Synopsis This paper provides new data on Torridonian alluvium on Raasay, confirming its braided origin and refining its environmental interpretation. Fining-upwards cycles appear to be absent. This may be an important criterion in distinguishing deposits of braided rivers from those due to meandering rivers. A quantitative study reveals a close correlation between grain size and sedimentary structures. With decreasing grain size, the sedimentary structures are broadly arranged in the following order: trough cross-bedding, planar cross-bedding, flat bedding, massive bedding, micro-cross-lamination, lamination. Two types of siltstone sequences are distinguished. The more common type lies between two erosional surfaces and is attributed to deposition in abandoned channels. Its thickness suggests that streams seldom attained a depth of more than one metre in this environment. The second type is attributable to overbank deposition, having an erosional top and a transitional base. Finally, the origin of the ubiquitous Torridonian quicksand structures is considered in the light of experimental work. Previous writers have put forward three possible triggering mechanisms for these bedding disturbances, viz.: springs, earthquakes and current turbulence. Simulation of all three processes produced quicksands in the laboratory but the weight of evidence suggests current turbulence to be the most probable triggering mechanism.
Published Version
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