--> Abstract: The Influence of Orbital Climatic Cycles on Organic Carbon Distribution in Marine Sediments, by F. S. P. Van Buchem, P. L. De Boer, and I. N. McCave; #90987 (1993).

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VAN BUCHEM, FRANZ S. P., Institut Francais du Petrole, Rueil-Malmaison, France; P. L. De BOER, University of Utrecht, NL; and I. N. McCAVE, University of Cambridge, UK

ABSTRACT: The Influence of Orbital Climatic Cycles on Organic Carbon Distribution in Marine Sediments

Many good (potential) source rocks show a typical rhythmic bedding pattern of alternating organic rich and organic poor layers (e.g. Kimmeridge Clay in UK and North Sea, Pliensbachian Belemnite Marls in the Wessex basin, Oligocene evaporitic series in the Mulhouse basin, Cenomanian/Turonian of the North Atlantic, Mediterranean sapropels).

An elegant explanation for this type of cyclicity may be offered by the theory of astronomical forcing of high frequency climatic changes, which has now been recognized in sediments throughout the Phanerozoic. The direct coupling of astronomical, atmospheric, oceanographic, biological and geological systems has important consequences for the understanding of fine scale sedimentation patterns in time and space. In particular the production and distribution of sediment of biologic origin, like marine and terrestrial organic matter, can be expected to be directly influenced by orbitally induced changes in a sufficiently sensitive sedimentary environment.

An essential point however is to separate:

1. The larger scale trends (millions of years) caused by plate tectonics affecting palaeogeography and topography, long term eustatic sea level and climatic changes `icehouse' and `green-house'), the interplay of which allowed organic matter to be stored and Milankovitch cyclicity to be expressed; and

2. On a smaller scale, once conditions are suitable for organic matter preservation, the high frequency (<500 ka) climatic changes which may modulate the degree to which organic matter is produced and preserved. And since each sedimentary environment responds differently to climatic changes, it seems possible to predict timing and direction of shifts in the organic matter depocentres. In this respect it is important to realize that orbital influences do not produce a regular, sinusoidal-like cyclic sedimentation pattern. The orbital signal is the sum of numerous different frequencies and amplitudes producing a complicated rhythmic pattern, which is found back in the sedimentary record and very suitable for correlations over great distances.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.