Resumo

VILLAMIL, T.. C.T.F Cienc. Tecnol. Futuro [online]. 1996, vol.1, n.2, pp.05-23. ISSN 0122-5383.

Spectral analyses of depositional and geochemical time series were conducted on three stratigraphic sections of the Cretaceous Vil I eta Group, Colombia. Results show statistically-valid cyclicity in distal parasequence stacking patterns and in geochemical variations. Cycles are in the low to high frequency spectra (500 ky - 20 ky). Timing of cyclicity suggests climatic controls on deposition, probably caused by Milankovitch orbital oscillations. Stratigraphic intervals enriched in total organic matter or organic matter indicators such as barium coincide with condensed intervals that are generally associated with observable high-frequency cyclicity. Very thin stratigraphic cycles (< 0,4 m) contain moderate amounts of organic matter because condensation allows organisms to recycle organic carbon. Condensation also allows time for gradual oxidation of organic matter. Thin to moderately thick (0,6-5,0 m) cycles show the highest quantity of organic matter. Sedimentation rates are high enough to inhibit biological destruction and low enough not to dilute organic carbon within the sediments. Thick cycles are characterized by low organic matter because sedimentation rate dilutes organic material, organic matter content not only depends on sedimentation rate but also on paleoproductivity and on grain size and shape. The model presented shows how cycle thickness trends may be used as a very simple approach to predict the amount of total organic carbon.

Palavras-chave : Depositional and geochemical ciclicity; statigraphy; cretaceous Villeta group.

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