Geological Setting of the Celebes and Sulu Seas

Abstract

The Sulu and Celebes Seas lie to the southeast of the South China Sea, within a complex geodynamic environment marked by active arc-arc or arc-continent collision zones, subduction zones, and long strike-slip fault zones. Prior to Leg 124, neither the ages nor the mode of formation of these basins were clear. The basins are located near the zone of complex junction between the Pacific, Indian Ocean, and Philippine Sea plates (Fig. 1). They may have been related to the history of one of these plates prior to collision or be the remnants of a subducted ocean that previously extended between these major plates (the Molucca Sea or neo-Tethys Ocean). Alternatively, they may have formed by back-arc spreading processes or, as in the case of the South China Sea, as a result of continental rifting unrelated to arc activity. Numerous models have been proposed for the opening of small ocean basins in a position marginal to a continent or a piece of continent by convergent boundaries. In the west Pacific region (Fig. 1), a widely accepted model is that of extension in the back-arc area (Chase, 1978; Molnar and Atwater, 1978; Uyeda and Kanamori, 1979). This model might be applied to the Sulu Sea (Mitchell et al., 1986; Rangin, 1989) but not obviously to the Celebes Sea. The South China Sea has been interpreted as an Atlantic type (Taylor and Hayes, 1980) or a pull-apart basin, possibly related to transcurrent shear zones controlled by intracontinental deformation processes (lapponnier et al., 1982; Lallemand and Jolivet, 1985). The Atlantic-type model could be extended to other marginal or intracontinental basins of the Eurasian margin, such as the Japan Sea or the Yellow Sea (Taylor and Rangin, 1988). Paleogene rifting and Oligocene-Miocene spreading from Borneo to Japan and the suspected presence of dominantly north-south right slip faults along the east Asian margin suggests that an extensional stress regime was present along the Eurasian margin during this period. These marginal basins could also be interpreted as trapped fragments from a larger ocean, as a consequence of subduction zone flipping or jumping along the volcanic arcs bordering these basins. If these basins have had an exotic origin relative to Asia, their stratigraphic histories will be unrelated to the Eurasian continental margin. The Celebes and Sulu seas are deep (4000-5000 m) restricted basins located between the island of Borneo to the west and the north-south elongated Philippine Archipelago to the east. They are part of a succession of subparallel oceanic basins trending roughly northeast. Separating the basins from South China to Banda are narrow, elongated ridges such as the Cagayan Ridge,

Calcareous Nannofossil Biostratigraphy of the Celebes and Sulu Seas

“…This is in good agreement with the age determined from planktonic foraminifers and radiolarians at this site, and with radiolarians at Site 767. Since the late Oligocene, Site 770 has subsided below the CCD and sedimentation has changed to pelagic brown claystone with extremely low carbonate content (Rangin et al, 1990). From the middle Miocene to the Holocene this site was not influenced by turbidites because of its elevation, and almost no calcareous nannofossils are found in the hemipelagic clay.…”

“…Carbonate content is low throughout the sediment section, indicating that deposition at this site was below the regional CCD. Four distinct lithologic units are recognized (Rangin, Silver, von Breymann, et al, 1990 Figure 2 and the nannofossil check list is shown in Table 2 (back pocket). The youngest biohorizon recognized is the lowest occurrence of Emiliania huxleyi, which marks the beginning of Zone NN21 in Sample 124-767B-2H, CC.…”

“…1, Table 1). The recovered sediment section is divided into five distinct lithologic units (Rangin, Silver, von Breymann, et al, 1990). Unit I is late Pliocene to Holocene in age and consists of interbedded pelagic carbonate marl, sparse thin volcanic ash layers, and some turbidite beds.…”

“…One hole was drilled at this site to complement Site 769. The recovered sediment column can be divided into two units: an lower upper Pliocene to lower middle Miocene nannofossil clay and marl, and a massive pre-lower middle Miocene lapillistone, tuff, and basalt flow (Rangin, Silver, von Breymann, et al, 1990). This site is only 2856 m below sea level, well above the local CCD.…”

“…3, Table 9). Taking into account the high Pleistocene deposition rate at all three sites (Rangin, Silver, von Breymann, et al, 1990), the lowest occurrence of Gephyrocapsa oceanica s.l. must be very close to the Pliocene/ Pleistocene Boundary.…”