The analysis and interpretation of stratification cycles

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Abstract
Beds in carbonate successions often form regular patterns. Either there is a cyclic repetition of beds with different thicknesses, or bundles of beds occur separated from each other by some distinct layer. Any such group will be called a stratification cycle. Cycles of this type are from 0.5 m to several meters in thickness, and they seem to be present throughout the geological column. The analysis of sedimentary cycles involves three stages.(1) Regional mapping is necessary to understand the facies of the sediment. Establishing the lateral persistence of bedding eliminates the possibility that bedding is caused by diagenetic processes.(2) Petrographic examination and, in particular, analyses of primary structures are used to reconstruct the sedimentation conditions during cycle formation. Such conditions are extremely important for judging the time content of individual beds.(3) The quantitative analysis of vertical sections using power spectral methods can find superimposed cycles and enables one to judge their degree of regularity. The latter approach is illustrated using lower Carboniferous limestone sections from NW Ireland as an example. Spectral analysis finds a stratification cycle of 1 to 3 m in thickness. Each cycle consists of between five to seven limestone beds. Upper Jurassic limestone marl successions from southern Germany and late Cretaceous and early Tertiary successions from Italy show a very similar cyclic pattern. The scale of the cycles and their regularity suggest that they are ultimately caused by variations in the earth's orbit, possibly by climatic cycles as was postulated by Milankovitch. However, the exact mechanism which links sedimentation with orbital variations is not known.