Print this pageInternal Geometry of Sand Waves: A Comparison Between Modern and Fossil Examples
Recent developments in acquiring and processing very high-resolution geophysical data help us better understand large subtidal sand waves of the French continental shelf. They are compared with ancient analogs, especially from the Miocene Swiss Molasse.
Internal structure, interpreted from seismic sections, vibracorings, and large outcrops, shows a hierarchy comparable to aeolian dunes. (1) Steep (25°-30°) reflectors, dipping leeward, are interpreted as foreset beds. Vibracoring shows that in modern cases they consist of alternating layers of medium- and coarse-grained sand, similar to those produced by sand avalanching. These deposits give the highest porosity values in the central body of the sand wave. They are comparable to the Miocene sand waves of the Swiss Molasse. (2) Erosional reflectors, dipping at lower angles cut across the foresets, are interpreted as reactivation surfaces created by high-energy events (equinox tides, added tidal and wave effects) rather than by the semidiurnal currents occasionally preserved i fossil sand waves. (3) Subhorizontal reflectors were probably created by truncation of sand waves during major storms.
Fossil analogs more like larger present-day sand waves might be difficult to recognize due to the complex internal architecture of the sand body.
AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.