ABSTRACT

Wave-formed, coarse-grained ripples (CGR) form in water depths ranging from 3 to 160 m on transgressive surfaces, in upper and lower shoreface sediments and in finer-grained offshore-transition deposits. CGR have crest spacings of 0.25-3.0 m with amplitudes of 5-35 cm. Internal stratification is rarely preserved, but where present, crossbeds dip en echelon away from the ripple crest and individual laminae feather out away from the crest. Waves that form CGR commonly have wavelengths on the order of 2-5 m with periods of 8 to 14 sec. These are not large storm events, especially on a geological scale. CGR commonly occur in close association, laterally and vertically with hummocky cross stratification. It is postulated that if sediment is medium to very coarse-grained sand,or pebbly sand then two-dimensional CGR will be formed from oscillatory or oscillatory-dominant flow. If, however, sediment is very fine to fine-grained sand, then three-dimensional hummocky cross-stratification will be formed. The wave conditions under which CGR form are similar to those which would form hummocky cross-stratification; it is only the grain size that is different.Reported and calculated wave conditions for hummocky cross-stratification are almost identical to those for CGR. Modern CGR crests trend parallel to the local shoreline and bathymetry; as such, ancient ripple crests are a useful tool for basin reconstruction.