The Influence of Grain Size from the Nature of Oscillatory-Flow Bedforms: Experimental Results Using Very Fine and Coarse Sand
Workers have long believed, based on outcrop studies, that the bed configurations created by purely oscillatory flow are different in fine and coarse sand, but few experimental studies have been undertaken to determine the influence of grain size. Our experiments using very fine sand and coarse sand in a wave tunnel show that large, oceanic-scale waves (orbital diameters 1-4.5 m) produce very different bed configurations. In very fine sand, a greater diversity of bed forms is possible. Small, generally straight-crested, size-invariant (anorbital) ripples (wavelengths ~ 10 cm, heights < 1 cm) occur exclusively in very fine sand at low oscillatory velocities. Large, 3D orbital ripples with wavelengths (λ) that scale with orbital diameter (do) according to the relationship λ ~ 0.6 do appear as oscillatory velocities increase and coexist with anorbital ripples over a range of intermediate velocities. The large orbital ripples become more subdued and rounded as oscillatory velocity increases, with flank slopes of 5-15 degrees. Aggradation of these bedforms would generate structures resembling hummocky cross stratification. Small anorbital ripples and round-crested large orbital ripples are absent from coarse sand. The only bed forms consist of large, straight-crested, orbital ripples that scale with orbital diameter according to the relationship λ ~ 0.4 do. These large coarse-grained ripples are distinctly steeper than their counterparts in very fine sand, with flank slopes of 15-22 degrees. If such ripples were formed in the presence of even a weak unidirectional current, they would generate cross stratification that could be mistaken easily for the deposit of a current-generated dune, leading to the potential for significant misinterpretation of the environmental setting.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009