Abstract

The interpretation of structureless sand units, in terms of flow character and associated depositional mechanism, is notoriously difficult to deduce from localized outcrop, because such units offer no macroscopic features with which to make distinctions of flow parameters. This investigation assesses the significance of directional properties of particle-fabric samples taken both vertically and horizontally within structureless sand units deposited by jökulhlaups (glacier-outburst floods) in southwest Iceland, known to be macroturbulent phenomena based on eyewitness accounts and inferences made from the large-scale sedimentary architectures of the deposits. Given this framework, we use particle fabric to verify flow character and depositional mechanism, highlighting its potential application to deposits where prior knowledge of conditions is unknown. Vertical and horizontal trends in particle fabric indicate a causative mechanism that is manifested both spatially and temporally within visually structureless units. This data indicates sedimentation onto an aggrading bed beneath a turbulent flow, which conforms to the documented turbulent character of the jökulhlaup. We demonstrate that cryptic information related to flow parameters can be gleaned from visually structureless sand units through detailed particle-fabric analysis, and clarify aspects of hyperconcentrated-flow behavior and sedimentation.

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