Abstract

The wealth of knowledge created by recent studies in physical and geological oceanography has brought us answers, as well as many new questions. However, certain basic concepts seem to be emerging, which can be applied to the study of ancient sediments. About 85% of modern shelves are storm-dominated; 10% are tide-dominated, while sedimentation on the rest is controlled by intruding oceanic currents. Shelf storm currents are combined-flow currents in which a wind-driven, mean-flow component and a high-frequency, wave-orbital current component are subequal in value. They are geostrophic currents, which flow predominantly along the shelf. There is frequently an offshore component of flow near the bottom, but flows at high angles to the shelf contours are infrequent and short-lived. Sustained longshore geostrophic currents during storms are much more important for sediment transport than the relaxation currents dubbed “storm-surge ebb”. Hurricanes are not necessarily more effective in sediment transport than mid-latitude cyclones; although they are by definition very intense, many are too localized and rapidly moving to couple efficiently with the shelf water mass.

The origin of shelf stratification is problematic. Ancient shelf sequences frequently contain graded beds with solemarks and a Bouma-like sequence of primary structures, which have been attributed to deposition by density underflows. However, density underflows have not been observed and are considered to be uncommon on theoretical grounds. Geostrophic storm currents, which have been abundantly observed, create beds (tempestites) that are also graded and have Bouma-like primary structures.

Other areas of modern-shelf study that help in our interpretation of ancient deposits are the dynamics of large-scale shelf sand ridges analogous to ancient sand ridge deposits, and studies of Holocene shelf facies patterns analogous to the patterns reported by sequence stratigraphers.