Internal Structures of Shallow-Marine Tidal Sand Waves: ABSTRACT

Abstract

Asymmetric sand waves (average height 0.86 m, and average wavelength 46.6 m) with superimposed megaripples 0.05 to 0.2 m high, occur commonly in medium to coarse sand on tidal sandbars in the Bay of Fundy. Their internal structures are complex, but three main types can be distinguished: (1) inclined sets of descending and ascending cross-bedding (0.1 to 0.3 m thick) that have set boundaries dipping at an average angle of 9° in the dominant transport direction; (2) large-scale foresets having set thicknesses comparable to the sand wave lee-face height and average inclinations of only 20°; and (3) complex cosets up to 0.5 m thick of thin (0.05 to 0.15 m thick) cross-bedded sets with abundant herringbone cross-stratification. Types 1 and 2 are formed during sand wa e lee-face migration, whereas type 3, which overlies the lee-face structures, is produced by the superimposed megaripples during vertical growth of the sand waves following degradation by storms or winter ice. Inclined sets are the most common lee-face deposit in the Bay of Fundy. Their formation is favored by the high current speeds, low to intermediate sediment transport, and the migration of large megaripples (relative to the sand waves) which characterize this area. Large foresets are relatively rare, and extensive development of large-scale, angle-of-repose cross-bedding has never been observed. Large-scale foresets may be more abundant in other areas where there are larger sand waves, lower current speeds, and higher sediment transport, but they should contain numerous reactivation surfaces, and be overlain by vertically accreted complex cosets. The internal structures of tidal sand waves should differ significantly from those in aeolian dunes. End_of_Article - Last_Page 696------------