Boulder Ring Structures Produced During Jökulhlaup Flows: Origin and hydraulic significance

This paper investigates the origin of a series of ring structures developed on Myrdalssandur, south Iceland, following a major, volcanically - triggered jökulhlaup in 1918. The ring structures are characterized by boulder-rich rims up to 4 m high and 40 m in diameter. The rims were found to be composed of diamicton which dips steeply into the central hollow, which is infilled with laminated fines. The deposit generated by the flood comprises pre-surge, hyper-concentrated flood surge, and post-flood stage sediments. The diamicton deposits indicate that the ring structures were produced by the in situ melting of debris-rich ice blocks, transported on the surface of the hyperconcentrated flood surge, to form 'rimmed' kettles. Laboratory experiments confirmed that ring morphology is dependent on the sediment concentration and depth of submergence of the ice block into the adjacent sediment, and have allowed the interrelationships to be defined by empirical equations. A sequence of four types of kettle holes has been distinguished in response to increasing sediment concentration of the transported ice block: Type 1, a 'normal' kettle hole; Type 2, a 'rimmed' kettle; Type 3, a 'crater' kettle; and Type 4, a 'till-fill kettle' or 'kettle mound'. The results suggest that other recent and Pleistocene kettle sediments, and their host deposits, may also be of jökulhlaup origin.