Abstract

Primary sedimentary structures can be used to infer conditions of flow which prevailed within fluid currents which deposited the sediment. Three particularly important conclusions about fluid flow conditions can be drawn from primary sedimentary structures: 1) The mechanisms of suspension and traction, as defined by G. K. Gilbert in 1914, are fundamentally different, and can be recognized by the different gross structures which they produce in sediments, and by the different effects which are brought about by the distinctive paths followed by particles which move under the influence of each mechanism; 2) The effects of grain-to-grain encounters in cohesionless sediment, which arise in response to current flow, can also be recognized in primary sedimentary structures, as can analogous effects in cohesive sediments; and 3) Mass shearing effects at the fluid-sediment interface may produce deformation in cohesive sediments. The interpretations of primary sedimentary structures summarized herein suggest new lines of experimental investigation of fluids and sediments, in which attempts should be made to duplicate some of the mass shearing effects which have been recognized in cohesive sediments found in nature.

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