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ABSTRACT: Organic Sedimentation Response to Tectonic and Paleogeographic Influences in Celebes and Sulu Basins, West Pacific (ODP, Leg 124)

Philippe Bertrand, Manuel Pubelier, Ulrich Berner, Marta Von Breymann, Elisabeth Lallier-Verges, Alain Desprairies

The objectives of Leg 124 of the Ocean Drilling Program were to determine the age and stratigraphy of the Celebes and Sulu seas, which are thought to record the history of complex tectonic and oceanographic events that have battered this Western Pacific region throughout the Cenozoic. The present study attempts to relate the organic sedimentation characteristics to the general history of the basins based on the organic carbon content of the sediments, the organic accumulation rates, and the type and state of preservation of the organic matter.

Both geochemical and petrographic methods were used for the organic matter characterization: organic carbon concentration, Rock Eval pyrolysis, and petrographic analysis of chemically isolated organic matter. Information on the diagenetic processes was obtained from the molecular and isotopic composition of the hydrocarbon gases released from the sediments, interstitial water chemistry, and elemental composition of the whole sediment.

In the Celebes (Site 767) and in the Sulu (Site 768) sediments, the organic matter is mainly of continental origin with the highest concentrations and accumulation rates occurring in the middle Miocene turbiditic sequences. The autochthonous organic material is relatively higher, and sometimes dominant in sediments younger than middle to late Miocene. In sediments older than middle Miocene, the relative proportion of marine organic matter is thought to be mainly controlled by dilution with massive turbiditic (Celebes and Sulu) or pyroclastic (Sulu) deposits.

Petrographic analysis of the Eocene and lower Miocene organic matter in the Celebes Sea shows that it consists only of highly degraded terrestrial particles, indicating poor conditions for organic matter preservation during this period. By comparing the organic matter in the upper and lower parts of the Celebes series, it is obvious that the organic matter is much better preserved in the younger sediments, suggesting changes in the paleoenvironmental conditions of the basin which correspond to its tectonic evolution. However, because of the dilution phenomena, it is difficult to observe the progressive improvement of the conditions for organic matter preservation throughout the series.

The thermal maturity of the organic matter indicates a much higher paleogeothermal gradient in the Sulu than in the Celebes Sea. Indeed, in the Celebes Sea, the organic matter is thermally immature independent of its age, while in the Sulu Sea the organic matter older than late Miocene has reached maturity and is associated with thermally generated gases.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990