Site 1149, drilled during Ocean Drilling Program (ODP) Leg 185, is located in the Nadezhda Basin southeast of Japan and ~100 km east of the Izu-Bonin Trench axis (Fig. F1). Site 1149 is located on magnetic Anomaly M11 (Nakanishi et al., 1992) with an assigned age of ~132 Ma (Gradstein et al., 1994, 1995; Plank, Ludden, Escutia, et al., 2000). The principal objective of Leg 185 was to determine the inputs into the Izu-Bonin Trench for "subduction factory" studies. Site 1179, on the abyssal seafloor ~1650 km east of Japan (Fig. F1), was drilled during ODP Leg 191. Site 1179 is located on magnetic Anomaly M8 (Nakanishi, et al., 1992) with an assigned age of 129 Ma (Gradstein et al., 1994, 1995). The main objectives of Leg 191 were to install a seismic monitoring station and to test the drilling and casing capabilities of the hard-rock reentry system ("hammer drill") (Kanazawa, Sager, Escutia, et al., 2001).
Despite the different scientific objectives of Legs 185 and 191, the sedimentary sections recovered from Sites 1149 and 1179 are the two most complete sections recovered from the northwestern Pacific Basin by either the Deep Sea Drilling Project (DSDP) (i.e., Legs 6, 20, 32, and 86) or ODP (i.e., Legs 185 and 191) (see Fig. F4 in Shipboard Scientific Party [2001]). During Leg 185, a complete sedimentary section (410 m) and an additional 133 m of highly altered volcanic basement were recovered. The Miocene to Pleistocene section (i.e., upper ~150 m) recovered from Site 1149 includes lithostratigraphic Unit I (0–118.2 meters below sea floor [mbsf]) and Subunit IIA (118.2–149.5 mbsf) of Plank, Ludden, Escutia, et al. (2000) and consists of ash- and biogenic silica- bearing clay, radiolarian-bearing clay, silt-bearing clay, ash-bearing siliceous ooze, and diatomaceous clay, with numerous discrete volcanic ash layers (Plank, Ludden, Escutia, et al., 2000). During Leg 191, a near-continuous 375-m-thick sedimentary section was recovered in addition to 100 m of basaltic basement. The upper 221.5 m of the sedimentary section at Site 1179 (i.e., within lithostratigraphic Unit I of Kanazawa, Sager, Escutia et al. [2001]) consists of upper Miocene to Pleistocene clay- and radiolarian-bearing diatom ooze containing numerous discrete ash layers. The presence of discrete ash layers within the Miocene to Pleistocene sedimentary section at both Site 1149 and 1179 provides a unique opportunity to conduct 40Ar/39Ar ash chronology to refine the excellent magnetostratigraphic records (based on the scale of Berggren et al., 1995) obtained shipboard from both sites (Plank, Ludden, Escutia, et al., 2000; Kanazawa, Sager, Escutia, et al., 2001).
In this data report we present the analytical results from the 40Ar/39Ar incrementally heated analyses and provide a new combined late Miocene to Pleistocene 40Ar/39Ar and magnetostratigraphic chronology for the northwestern Pacific.
1Escutia, C., Canon, M., and Gutierrez-Pastor, J., 2006. Data report: 40Ar/39Ar chronology of discrete ash layers in the northwestern Pacific: ODP Sites 1149 and 1179. In Ludden, J.N., Plank, T., and Escutia, C. (Eds.), Proc. ODP, Sci. Results, 185 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/185_SR/015/015.htm>. [Cited YYYY-MM-DD]
2C.S.I.C.-Universidad de Granada. Instituto Andalúz de Ciencias de la Tierra, Campus de Fuentenueva, 18002 Granada, Spain. Correspondence author: cescutia@ugr.es
3Scandpower Petroleum Technology, 11490 Westheimer Road, Suite 500, Houston Texas 77082, USA.
Initial receipt: 8 September 2004
Acceptance: 28 November 2005
Web publication: 3 March 2006
Ms 185SR-015