Abstract:
A study of sedimentary organic matter (palynofacies) was carried out for paleoenvironmental purposes in the siliciclastic, Miocene upper-slope sediments of Ocean Drilling Project (ODP) Site 903, Leg 150. Two groups of organic constituents were distinguished: a fraction derived from the continent, made of phytoclasts, pollen, and spores, and a marine fraction consisting of amorphous organic matter (AOM), dinoflagellate cysts, and foraminiferal linings. Palynofacies data reflect the overall trend of the Haq et al. (1987) eustatic curve. The rise in eustatic sea level during the early Miocene is associated with high levels of AOM and total organic carbon, indicative of low-energy, starved, deeper-water conditions. In contrast, during a fall in sea level from the early middle Miocene to the late Miocene, AOM became diluted by continental organic matter associated with prograding clinoforms. Site 903 penetrated the distal part of progradational, well-developed clinoforms of Miocene age. Most of the seismic reflections bounding these clinoforms are the pelagic correlatives to sequence boundaries observed beneath the adjacent shelf, and they correlate well with the sea-level records of Haq et al. (1987). Within packages of clinoforms, palynofacies show important percentage variations. In most cases, seismic reflectors (i.e., sequence boundary correlatives) seem to be associated with an increase in AOM indicative of a condensed section. Seismic stratigraphic interpretation landward of Site 903 indicates that these condensed sections may correspond to the downlap surface of prograding lowstand or highstand wedges. Above the condensed section, AOM becomes progressively diluted by the renewed progradation of terrigenous sediments. The concentration of marine AOM near sequence boundaries, the ages of which correlate with the Haq et al. (1987) eustatic curve, tends to demonstrate that the distribution of AOM in the middle and upper Miocene slope sediments at Site 903 may be to a large extent related to relative sea-level variations.