The most extreme climate transitions in Earth history are recorded by Neoproterozoic glacial deposits with overlying "cap" carbonate rocks indicating a potential snowball, or ice-entombed, Earth. Giant symmetrical wave ripples preserved within these rocks are often interpreted to form under extreme wave conditions. Using a new model for ripple formation, we show that the first-order control on forming this type of ripple is sediment size, not wave climate. New measurements of ripple wavelengths and particle sizes from the approx. 635 million-year-old Nuccaleena Formation, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand, indicating that they likely formed under normal wave conditions. Numerical simulations of flow over ripples suggest that the ripples may have formed over long time periods with variable wave climates in conjunction with rapid seabed cementation. Together our analysis indicates that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during this unusual period of Earth history.
Influence of drainage divide structure on the distribution of mountain peaks
James A. Spotila, Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA. Posted online 29 June 2012; doi: 10.1130/G33338.1.
People have long been fascinated by mountainous topography, yet peaks and ridges have been left at the periphery of scientific investigation of landscape evolution, which traditionally has focused on valley-shaping processes. Author James Spotila shows that the distribution of mountain peaks conforms closely with drainage divide structure. Specifically, peaks occur along the intersection of drainage divides, and are not randomly distributed throughout the landscape. This is likely because dra
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Geological Society of America