Organic sedimentation of ODP Sites 124-767 and 124-768

This study relates the organic sedimentation characteristics to the lithostratigraphic successionsthat were observed at Site 767 (Celebes Sea) and Site 768 (Sulu Sea) during ODP Leg 124. It is based on the total organic carbon content (TOC) of the sediments, on the petrographictype and maturity of the organic matter, and on the TOC accumulation rates calculated for the lithostratigraphic units. In the Celebes and Sulu Seas sediments, the organic matter is mainly of terrestrial origin with the highest concentrations and TOC accumulation rates occurring in the middle Miocene turbiditic sequences that correspond to a major compressive event between the Philippine Mobile Belt and the Palawan, Cagayan, and Sulu Ridges. Petrographic analysis of the Eocene and lower Miocene organic matter in the Celebes Sea shows that it consists only of highly degraded terrestrial particles. This observation and the very low TOC accumulation rates indicate poor conditions for organic carbon preservation during this open-ocean phase of the Celebes Basin formation. The organic matter, either of marine or terrestrial origin, is much better preserved in the younger sediments, suggesting physico-chemical changes in the depositional environment. Because of the dilution phenomena by turbidites, it is difficult to observe the progressive improvement of the organic matter preservation throughout the turbiditic series. The same change in preservation is broadly observed in the Sulu Sea from the early Miocene (rapid opening phase of the basin with massive pyroclastic deposits) to the present.