Site 1149 was drilled in the northwestern Pacific during Ocean Drilling Program (ODP) Leg 185 and from it was recovered a reference section for the oceanic crust of the Early Cretaceous Pacific Ocean.
The main objectives of Leg 185 were to provide estimates of the sediment and altered basalt inputs (geochemical fluxes) into the Izu-Bonin subduction zone and to contrast crustal inputs to the Izu-Bonin Trench with those for the Mariana Trench (Hole 801C, drilled during Leg 129 and reentered during Leg 185); the sedimentary section recovered also helped to refine the Early Cretaceous paleomagnetic timescale (Plank, Ludden, Escutia, et al., 2000) and better understand fluctuations in the Early Cretaceous carbonate compensation depth (CCD) and equatorial circulation (E. Erba and R.L. Larson, pers. comm., 2001.
Site 1149 is located at 31.3°N, 143.3°E, southeast of Japan on the Pacific plate in the Nadezhda Basin, in a water depth of ~5800 m. The site lies on a slight high ~100 km east of the Izu-Bonin Trench, where the Pacific plate is flexed upward prior to its entry into the subduction zone (Fig. F1). According to Nakanishi et al. (1992) and confirmed by Plank, Ludden, Escutia, et al. (2000), the site lies on the reversed magnetic Anomaly M12.
Four holes were drilled at Site 1149, and a sedimentary section of 408 m total thickness was recovered; carbonate sediments yielding calcareous nannofossil assemblages were recovered only in Holes 1149B, 1149C, and 1149D, providing direct contact with underlying basalts of the Cretaceous oceanic crust. From Hole 1149A, ~200 m of pelagic brown clay, subdivided into two lithologic units (Plank, Ludden, Escutia, 2000), was recovered. Lithologic Unit I consists of noncalcareous clay with common dispersed ash particles, numerous discrete ash layers, and siliceous microfossils (radiolarians, diatoms, silicoflagellates, and ebridians); it reaches 118 m in thickness and has been dated by means of diatoms (Laws, this volume) and silicoflagellates (Lozar and Mussa, this volume) as late Miocene to late Pleistocene in age. Lithologic Unit II consists of 62 m of dark brown pelagic clays with several discrete ash layers in the upper 30 m; Unit II clays are barren of any siliceous or calcareous microfossils but contain ichthyoliths (Plank, Ludden, Escutia, 2000). The sedimentary sections recovered in Holes 1149B and 1149C are very similar and are subdivided into Units III and IV. Lithologic Unit III consists of a 104-m-thick alternation of radiolarian chert with porcellanite and siliceous clay (Plank, Ludden, Escutia, 2000); it has been dated by means of radiolarians and suggests a mid-Cretaceous age (A. Bartolini, pers. comm., 2001). Lithologic Unit IV comprises 125 m of intercalated radiolarian chert, porcellanite, and siliceous chalks or marls. Lithologic Unit V was recovered only as interpillow sediment in fractures in the upper 2 m of basement in Hole 1149B and consists of recrystallized calcareous marlstone, barren of calcareous microfossils.
Units III, IV, and V were only continuously cored in Hole 1149B. Despite the very low recovery rate (between 3% and 11%), the contact between the sedimentary cover and the oceanic basement was recovered in Core 185-1149B-29R at 408.2 meters below seafloor (mbsf). Hole 1149C was only spot-cored at the top of Unit IV (283.6-322 mbsf) and the bottom of this unit, just above the contact with the oceanic crust (388.2-401 mbsf). In this hole the basement lies 7 m shallower than in Hole 1149B. In Hole 1149D, some 5 km to the southeast of Hole 1149C, only the interval just above the contact with basalt was cored (272.2-307 mbsf). The basement in Hole 1149D lies 101 m shallower than in Hole 1149B.
Samples analyzed in this work are from carbonate sediments of Units IV and V (Hole 1149B only) in Holes 1149B, 1149C, and 1149D.