During Ocean Drilling Program (ODP) Leg 186, two sites (Sites 1150 and 1151) were drilled in the deep-sea terrace west of the Japan Trench (Fig. F1) (Sacks, Suyehiro, Acton, et al., 2000). At each site, middle Miocene to Holocene sedimentary sections were recovered that were over a kilometer thick. Sedimentation rates vary between 20 and 248 m/m.y. over the entire section, but average ~119 m/m.y. at Site 1150 and ~103 m/m.y. at Site 1151 over the Brunhes normal polarity interval (0-0.78 Ma). A sedimentary record spanning the past 1 m.y. is thus preserved in the upper 100 m of the section, in which multiple holes have been cored. These multiply cored intervals, particularly those at Site 1151, are the primary focus of this study.
The sediments are very homogeneous, consisting mostly of diatomaceous clays and oozes interbedded with thin volcanic ash layers. Although the sites are part of an accretionary wedge along an active subduction zone, the upper 300 m of sediment sustained little or no visual deformation. The upper portions of the sedimentary sections at these sites thus have the potential to provide a wealth of information about the paleoceanographic, geologic, and volcanic history of the region.
As is typical for a single drill hole, coring gaps and intervals affected by coring disturbances result in incomplete recovery of the sedimentary section. Thus, ODP has adopted a strategy of coring multiple holes at a site to ensure complete or nearly complete recovery of a continuous sedimentary record. During Leg 186, this strategy was employed at Site 1151 but only after the primary objective of installing a geophysical observatory had been successfully completed. Because of time constraints, Hole 1151A was cored with the rotary core barrel (RCB) system to a depth of 1113.6 meters below seafloor (mbsf) to determine lithologic changes that are important for establishing casing depths (Table T1). As part of this, the upper 78 m of the section were washed away during a jet-in test (Shipboard Scientific Party, 2000c). The geophysical instrument package was then installed in Hole 1151B, where no cores were recovered. With time remaining, the upper portion of the sedimentary section at Holes 1151C and 1151D were cored with the advanced piston corer (APC) to 97.2 and 93.0 mbsf, respectively. In addition, to ensure stratigraphic overlap and optimize recovery between Holes 1150A and 1150B, a short interval at ~720 mbsf was double cored. In this study, we place the multiply cored intervals at each site into a common stratigraphic framework by conducting between-hole correlation and establishing a common or composite depth scale.
A composite depth scale (measured in meters composite depth, or mcd) places coeval, laterally continuous stratigraphic features into a common frame of reference by shifting the mbsf depth scales of individual cores to maximize correlation between holes (e.g., Hagelberg et al., 1995; Acton et al., 2001). By definition, the individual cores are shifted vertically, without permitting expansion or contraction of the relative depth scale within any core. In essence, the composite depth scale overcomes many of the inadequacies of the mbsf depth scale, which is based on drill pipe measurements and is unique to each hole. By ODP convention, the drill pipe measurement defines the depth to the top of each core.
Deviations of mbsf depths from true depths arise from errors and uncertainties in depth measurements, most of which can be attributed to ship motion, in addition to core expansion, incomplete recovery, and tides. Therefore a horizontal feature present in recovered material from several holes, which in the absence of local bathymetric variations should have the same true depth, will likely have different mbsf depths (Fig. F2). Errors in the mbsf depth scale range from a few centimeters to several meters, though rarely more than ~10 m (e.g., Acton et al., 2001, and references therein). After establishing an mcd scale, more complete stratigraphic records are spliced from the data from multiple holes.
The primary goals of this study are to establish common depth scales and spliced stratigraphic records for (1) the three holes at Site 1151 and (2) the two holes at Site 1150. Commonly, this type of work is completed during an ODP cruise. This was not possible during Leg 186 because a stratigraphic correlator had not been staffed for the leg and insufficient time was available between the completion of coring in Holes 1151C and 1151D and the end of Leg 186. Below, we present the data used to construct the mcd scales and outline the methods to achieve accurate between-hole correlations.