Clastic Tidalites—A Partial Retrospective View
Research on tidalites, sediments deposited by tidal currents, evolved through four phases during the last half century:
PHASE I, Facies mapping of Holocene tidalites in Germany, Holland, the United Kingdom, and Canada identified the seaward-coarsening pattern of sediment distribution, a distinct zonation of sedimentary structures, and provided a fining-upward facies model used to recognize ancient counterparts. Mapping in subtidal areas showed that extensive sheets of tidally molded-and-deposited sand accumulations characterized continental shelves that were both wide, and funnel-shaped in plain view. Similarly, extensive work was completed on carbonate tidalites, although it is not discussed herein.
PHASE II. Study of sedimentary structures was followed by a detailed analysis of sediment transport dynamics on intertidal sand bodies in Canada, where time-velocity asymmetry is the major factor controlling sand body geometry, orientation of bedforms, grain size distribution, sediment dispersal, and sand body orientation. Parallel work in tide-dominated continental shelves of the Yellow Sea of Korea and the southern North Sea showed similar patterns. These studies confirmed that tidal sand bodies are likely to be preserved in the rock record and provide a counterpart facies that is likely to dominate ancient cratonic seas.
PHASE III. In tide-dominated estuaries of The Netherlands, cross-bedded units were observed to be organized into discrete bundles that were correlated to neap and spring tides. These observations were replicated in ancient counterparts.
PHASE IV. Detailed analysis of the Schelde Estuary, The Netherlands, demonstrated that parallel-bedded couplets of sand and mud (tidal bedding) could be correlated directly to neap-spring tidal cycles. Recognition of such couplets, particularly in Mississippian and Pennsylvanian sediments of the midcontinent of North America, can be correlated to lunar dynamics and tidal patterns.
ALL of these studies demonstrated that tidalites accumulated rapidly and were preserved widely. Where preserved in the stratigraphic record, tidalites represent accumulation during very short time intervals. Consequently, in many sequences where such facies are preserved, the time gaps in the stratigraphic record were far longer than previously interpreted.