Morphology and internal structure of sandwaves in the Bay of Fundy

Abstract

ABSTRACTIntertidal sandwaves in the Minas Basin and Cobequid Bay, Bay of Fundy, occur under a wide range of conditions (mean grain size 0.274‐1.275 mm; velocity strength index (V1)0.46‐3.34; and velocity symmetry index (V2) 0.011‐0.294), and they vary from symmetrical to strongly asymmetrical in cross‐section. Heights and wavelengths average 0.81 and 37.9 m respectively. They are straight to weakly sinuous and laterally continuous in plan, occasionally show crestal branching reminiscent of wave ripples, and are commonly skewed relative to the strongest currents because of differential migration rates along their length. The average migration rate is 0.11 m/tidal cycle. Megaripples occur on each sandwave crest, at least during spring tides, but the areal extent, sinuosity and size of the megaripples increases as the dominant current speed increases. The megaripples have heights averaging 24% of the sandwave height, are oriented perpendicular to the fastest dominant currents, and have life spans of several tidal cycles. They are believed to be in quasi‐equilibrium with the sandwaves and play a key role in sandwave dynamics and internal structure formation: periods of lee face steepening and rapid forward migration (megaripple crest at sandwave brink) alternate with times of non‐deposition or erosion and slowed or reversed migration (trough at brink).Dominant‐current cross‐bedding predominates in the two intergradational varieties of translation structure observed: Inclined Cross‐Bedding—decimetre‐scale cross‐beds separated by gently inclined (9° average) erosional surfaces; and Large‐Scale Foresets—cross‐beds with thicknesses greater than half the sandwave height, interrupted by weakly erosional to conformable discontinuity surfaces. These are overlain by a vertical growth or repair structure, Complex Cross‐Bedded Cosets, that consists of nearly equal volumes of dominant‐ and subordinate‐current cross‐beds stacked without a preferred set‐boundary dip. The translation structures correspond well to forms predicted by Allen (1980a, fig. 8) but the inclined set boundaries and discontinuity surfaces (master bedding planes) are produced by megaripple troughs rather than by current reversals. Consequently, Allen's regime diagram is unable to predict structure occurrences. The repair structures suggest that ‘curvature‐related mass‐transport’ (Allen, 1980a, b) is important in tidal sandwave maintenance, although it is not necessarily responsible for sandwave initiation.