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Populations, Trends, and Cycles in Combined-Flow Bedforms
There are two size populations of unidirectional-flow bedform spacings, those of ripples and dunes. In oscillatory flow, by contrast, asymmetrical bedforms might have a bimodal population, but symmetrical ripples show a single log-normal population of ripple spacings. In combined flow, data on ripple spacings are rare.
Ripple spacings L and heights H of 182 combined-flow bedforms with a range of L from 0.1 m to 2.8 m were measured in an upward-shallowing, storm-dominated shoreline succession of sandstones of the Fernie Formation (Middle to Upper Jurassic) in Alberta, Canada. Statistical analysis of ripple spacings shows that the population of three-dimensional (3-D) bedforms belongs to a single log-normal population that is distinct from population of two-dimensional (2-D) bedforms. Statistical tests also reveal an upward linear increase and a 16-bed cyclicity in L. The linear trend is interpreted here as a "proximality trend" normally found in such successions and the 16-bed cycle as a shorter-period climatic effect related to a Milankovitch eccentricity cycle. A complex interplay of wave and current parameters (flow depth, orbital diameter, near-bottom unidirectional velocity, mixing length in the turbulence structure, etc.) and climatic cycles of different periodicities (from 0,4 m.y. to 0.1 m.y.) seem to determine the size distribution of combined-flow bedforms in a stratigraphic sequence.
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