麻豆传媒

Lower Cretaceous pelagic cyclic sequences from the Atlantic Ocean, Vocontian Basin (Southeast France) and an Upper Cretaceous pelagic cyclic sequence from the Indian Ocean (Exmouth Plateau) were quantitatively studied to test the theory that orbital periodicity has induced cyclic sedimentation.

These cyclic sequences have more similarities than differences. The cycles generally are composed of a more calcareous portion and a more marly portion, and are on a decimeter scale. Clay minerals, silt and microfossils vary with calcium carbonate content. In the Upper Cretaceous cyclic sequences from the Indian Ocean, systematic differences in K, Na, Si, Al, Ti, Fe and Mn between the more calcareous portion and the more marly portion, reflecting alternation of sedimentary conditions, were detected. In sequences from the Atlantic Ocean and the Indian Ocean, burrows which introduce light-colored sediments into the underlying dark-colored sediments are observed. All these lines of evidence indicate that these sequences are depositional in nature rather than diagenetic products. Discrete time series were built on the basis of lithological features (color, sedimentary structure, weathering appearance) of the cyclic sequences and were analyzed with the Walsh spectral method. Examination of the Walsh power spectra, using both wavelength ratios of the important frequency peaks and their duration from estimated average pelagic sedimentation rates, reveals that the periodicities in the cyclic sequences are correlative to the periodicities of orbital elements (eccentricity, obliquity and precession). The results support an orbital-related climatic origin for these cyclic sequences. In these Cretaceous cyclic sequences, the obliquity signal is generally stronger than the other two orbital elements.

A general model regarding cyclic sedimentation under the control of orbitally induced climatic variation proposes that the more calcareous portions are deposited during cold and dry climatic phases when the ocean is better convected with high bio-productivity but less terrestrial input. The more marly portions represent warm and humid phases when the ocean is less well convected with decreased bioproductivity but more terrestrial input as a result of enhanced weathering on land. This study also found that the dominant orbital signal in sediments varies with time, implying changes in the controlling role of the three orbital elements. The variation in cyclic sequences (i.e., cycle pattern changes and hue periodicity to refine time scales is tested in the Vocontian Basin. The duration of the Valanginian and Hauterivian estimated with the orbitally tuned pelagic sedimentation rate is 6.11 m.y. and 5.36 m.y.respectively.

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Supervisor: Felix Gradstein