Model for the combined-flow origin of hummocky cross-stratification

Abstract

Many shallow marine sandstones are characterized by gently dipping trough cross-stratification. There appear to be significant similarities between this low-angle cross-stratification and typical hummocky cross-stratification, and it is proposed that much of what is currently recognized as hummocky cross-stratification is related to low-relief megaripple bed forms that are equilibrium structures for fine-grained sand under combined wave surge and unidirectional currents. The geometry that obtains when these bed forms migrate is low-angle trough cross-stratification. High rates of sedimentation from incipient suspension and limited lateral migration of individual bed forms promote preservation of convex-upward depositional surfaces (hummocks) between troughs. Highly unsteady flow leads to abundant internal scour and drape surfaces. Neither random scour and drape nor oscillatory currents seem sufficient to generate hummocky bed forms.