Gravel Ripples on the Inner Scotian Shelf

Abstract

ABSTRACT Sidescan sonar and photo/video surveys have revealed the presence of symmetrical gravel bedforms, interpreted as wave-generated ripples, with wavelengths of 1.3 to 3.0 m, occurring in patches and narrow ribbons in depths of 15 to 65 m on the inner Scotian Shelf. Direct observations and sediment sampling from the Pisces IV submersible have been used to ground-truth surface-based survey dam and to quantify relationships between the bedforms and the processes responsible for their formation. The ripples are formed in polymodal sandy gravels with modal sizes ranging from 0.1 to 45 mm. Evaluation of threshold criteria for coarse sediments together with an 11-year record from a nearby wave-rider buoy suggests that existing models for wave entrainment of gravel on rippled surfaces ove estimate the critical velocity by a factor of approximately 2 and that conditions suitable for transport of the coarsest (45-mm) size mode sampled in the ripple crests occur, on average, 43 hr/yr. Analysis of gravel ripples on the inner Scotian Shelf and of other examples noted in the recent literature extends the range of empirical data for oscillation ripple wavelengths by a factor of 3 and for ripple grain size by a factor of 5. The gravel ripples display a linear scaling to wave-orbital diameter similar to that reported for orbital ripples in sand. Use of such relations for paleoenvironmental interpretation of gravel ripples preserved in the rock record is complicated by the difficulty of selecting appropriate grain size and wave parameters. Conditions suitable for potential formatio of large-scale, symmetrical ripples in coarse sediments extend from the outer continental shelf to shallow shore face settings with possible fetch lengths as short as 20 km under intense storms.