Hummocky cross-stratification and post-vortex ripples: length scales and hydraulic analysis

Duke (1987) argues that the structure reported by Greenwood & Sherman (1986) as hummocky cross-stratification from a shallow lacustrine environment is, instead, bedding formed by simple post-vortex oscillation ripples. His primary concern is with the length scale of the structures identified in box cores. However, using measured flow data, it is shown here that existing models of bedform generation based upon wave orbital velocity, orbital diameter and grain diameter do not support a simple post-vortex ripple interpretation. In particular, the relative steepness of the structure refutes such an origin. The structural characteristics of the lacustrine cores most closely match those associated with classic hummocky cross-stratification and were produced under a combined oscillatory/quasi-steady flow field.