Site U1336

Abstract

Integrated Ocean Drilling Program Site U1336 (site survey PEAT-5C; 7°42.067?N, 128°15.253?W; 4286 meters below sea level [mbsl]) (Fig. F1; Table T1) is in the central area drilled during the Pacific Equatorial Age Transect (PEAT) program (Expedition 320/321). Site U1336 (~32 Ma crust) is between Site U1334 ~410 km to the west and Site U1335 ~330 km to the southeast and ~100 km to the west of Deep Sea Drilling Project (DSDP) Site 78. Site U1336 is also ~30 km north of the center of the Clipperton Fracture Zone. The site survey data (Lyle et al., 2006; Pälike et al., 2008) show that Site U1336 is on abyssal hill topography draped with thick sediment (Fig. F2). The fabric of the abyssal hills is oriented slightly west of due north.

Water depth in the vicinity of Site U1336 is relatively shallow for the age of the crust, between 4200 and 4400 m. Surprisingly, the crust south of the Clipperton Fracture Zone is only 100 m deeper than the crust north of the zone, despite a water depth of nearly 5 km in the middle of the fracture zone trace. A few oblique ridges and depressed topography occur in the south of the survey area, showing some interaction between the fracture zone and the Site U1336 region. The site is thickly covered with sediment (300–400 ms two-way traveltime [TWT]) (Fig. F2) and has a very thin layer of recent to middle Miocene sediment, with frequent sediment erosion on flanks of abyssal hills. Based on correlation to the Neogene central equatorial Pacific seismic stratigraphy of Mayer et al. (1985) and the Paleogene equatorial Pacific stratigraphy of Lyle et al. (2002), we estimated ~120 m of Oligocene sediment for an average Oligocene sedimentation rate of 13 m/m.y., assuming a crustal age of 32 Ma. The total sediment thickness, using the velocity-depth conversion for DSDP Site 574 (Mayer et al., 1985) was estimated as 253 m prior to drilling.

Based on stage-pole reconstructions of Pacific plate motion, observations of basement age from previous drilling sites, and magnetic anomaly maps (Cande et al., 1989), we estimated that Site U1336 is on 32 Ma crust. The best control on age is information from Site 78, ~100 km east of Site U1336. The base of Site 78 reaches the lower Oligocene.

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