Formation of Precambrian sediment ripples

Abstract

Jerolmack and Mohrig1 suggest that the wave- ripple structure we describe2 from cap carbonates deposited in the aftermath of the Marinoan glaciation was created under storms or hurricanes similar to those experienced today on certain oceanic coasts, citing a documentation of large wave ripples on the seabed off the coast of North Carolina. Side-scan sonar images indicate that such ripples have wavelengths of 0.4−3 m, although the ripple dimensions at sites where samples were obtained for grain-size analysis range from 0.77 to 1.37 m, which is somewhat smaller than the Neoproterozoic examples shown in Table 1 of ref. 2 (1.5−4.5 m). In addition, the side-scan sonar equipment could only be deployed during fair-weather conditions after the passage of several hurricanes, which makes the precise hydraulic conditions responsible for the wave ripples uncertain. Nevertheless, Jerolmack and Mohrig raise an important issue regarding the shear stress (or orbital velocity) required to generate the wave ripples. We argue that, at values of shear stress well above the th