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Antidunes in the Mount Toby Conglomerate (Triassic), Massachusetts
Bryce M. Hand (2), James M. Wessel, Miles O. Hayes
Nineteen consecutive sand waves, interpreted as antidunes, occur on an exposed bedding surface of granule conglomerate within the Mount Toby Conglomerate (Triassic) near Sunderland, Massachusetts. The waves are nearly symmetrical, with smoothly rounded crests somewhat sharper than the intervening troughs. Slip faces are not developed. Average wave length is 63.3 cm, average height 6.5 cm. Cross laminae within the waves are parallel to the upstream wave fronts (maximum steepness about 22°), indicating that the antidunes were migrating in the upstream direction. Current direction is known independently from cobble imbrication in adjacent beds, regional grain size trends, oriented structures, and known source areas.
Hydrodynamic calculations indicate that the preserved antidunes formed in water about 1.6 cm deep, flowing at 100 cm/sec down a paleoslope of at least 2.7°. These conditions would be compatible with sheetwash on the surface of an alluvial fan.
Upstream-dipping cross-laminae are not limited to the unit displaying sand waves, but occur abundantly in many other beds at the Sunderland exposure and elsewhere within the Mount Toby Conglomerate. Thus, antidune structures may be far more abundant in the geologic record than has been assumed.
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