Sediment Transport on Continental Shelves: Storm Bed Formation and Preservation in Heterogeneous Sediments.

摘要

Many storm beds are constructed of silt/sand layers interbedded with mud. The coarse sediment fraction originates from reworking of marine sands and/or erosion of the coastal active zone, which extends from fair-weather wave base to the beach berm or coastal dune. Observations and modelling studies show that some sand is removed from the active zone to the inner shelf during extratropical and tropical cyclones. On continental shelves that have large wave events superimposed on offshore nearbottom flow, this coarse material is incrementally transported across the shelf. Storm waves and swell sort this sediment during transport and thus produce storm deposits in water depths of 5- 80 m. Observations of storm beds in the Gulf of Mexico indicate initial storm bed thicknesses of millimetres to decimetres. These observations are supported by event-scale numerical models, which also reveal the interaction of oceanographic and geological factors in generating storm beds. Historical records for hurricanes in the Gulf of Mexico suggest recurrence intervals on the order of I0 years for storm-bed deposition. For typical Gulf of Mexico environments, a storm bed must exceed 10cm in initial thickness in order to survive physical and biological reworking. These results are compared to a storm-dominated sequence from the Cretaceous system of Utah for which the preservation interval for storm beds is estimated to be 266 years. By using the recurrence interval for great storms from the Gulf of Mexico, a preservation rate of less than 20% is estimated for storm beds from the past.