Formation of Precambrian sediment ripples

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Arising from: P. A. Allen & P. F. Hoffman Nature 433, 123–127 (2005); Allen and Hoffman reply.  Quantitative estimation of environmental properties using sedimentary structures preserved in rocks is complicated by the fact that some relationships between the fluid flow, sediment transport and bed topography are not unique. Allen and Hoffman1 propose that large, wave-generated sand ripples (orbital ripples) in Precambrian rocks were generated by sustained, extreme winds driven by rapid climate change after termination of the Marinoan glaciation. We show here that these features could equally well have formed under normal storm conditions in tens of metres of water. We therefore contend that the ripples do not provide direct evidence for a climatic transit after the break-up of a snowball-Earth's global ice cover.

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Figure 1: Height (dashed lines) and length (full lines) of possible ripple-forming surface waves calculated for a range of water depths.


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    Allen, P. A. & Hoffman, P. F. Nature 433, 123–127 (2005).

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    Ardhuin, F. et al. J. Geophys. Res. 107, 3143 (2002).

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    Komar, P. D. & Miller, M. C. J. Sedim. Petrol. 43, 1101–1110 (1973).

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    Hoffman, P. F. et al. Science 281, 1342–1346 (1998).

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Correspondence to Douglas J. Jerolmack.

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Jerolmack, D., Mohrig, D. Formation of Precambrian sediment ripples. Nature 436, E1 (2005) doi:10.1038/nature04025

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