Carbonate Bedding Cycles in Cretaceous Pelagic and Hemipelagic Sequences

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Abstract
Many shales, marls and carbonates show a rhythmic oscillation in carbonate content, resulting in alternate carbonate-richer and carbonate-poorer beds (bedding couplets). Spacing of couplets may be even, irregular, or patterned (bundled), and bundles may be grouped into superbundles. Along with spacing, couplet sequences may also be modulated by changes in composition. These cycles record depositional response to cyclic, climatically induced changes in the rate of carbonate supply (productivity cycles), in rate of detrital supply (dilution cycles), and in carbonate removal (dissolution cycles). Systems vary in sensitivity, some recording every beat of the cycles, others only the strongest ones. The question of diagenetically introduced cycles remains in debate. Nearly a century ago Gilbert suggested that such oscillations in the Cretaceous of Colorado reflect climatic oscillations caused by the Earth’s orbital variations. Varve data suggest that these variations have influenced depositional systems for at least 250 ma, and that they have maintained much the same periods over that time span. Segments of a Cretaceous pelagic sequence in Italy show this hierarchy of cycles. Here the couplets reflect the precession, the bundles represent the short cycle of eccentricity, and the superbundles correspond to the long cycle of eccentricity. Couplets and bundles in the Niobrara Formation probably are of this origin. In other sequences couplets, generally unbundled, seem to represent the 41 Ka obliquity cycle. The Greenhorn cycles are of this type. Most long pelagic sequences are complex: some segments show the simple oscillations suggestive of the obliquity signal, others show the precession-eccentricity hierarchy, and yet others appear chaotic. There are indications that high latitudes such as northern Germany may yield long and relatively uncomplicated record of the obliquity cycle. Hope for sorting out complex sequences lies largely in instrumental scans of cores and in time-series analysis. The cycle geochronometry envisioned by Gilbert is not yet practicable but may come into reach. Cycle studies raise important questions for sedimentology and paleoceanography. For stratigraphy, they provide ultra-fine correlation within the cyclic facies. Furthermore, they imply that in some facies complexes the more limestone-rich parts of the shale facies are age equivalent to the more shaly segments of the limestone facies.

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