Drilling site locations were selected on the basis of seismic reflection profiles and results from DSDP Site 144. Sites were chosen based on the following criteria: (1) thick sediment packages that include the intervals of interest and (2) a range of paleodepths to provide depth transects for the mid-Cretaceous to Paleogene section. Four sites are primary targets, and six alternate sites are provided for contingency planning.
A 20-nmi-long northwest-southeast seismic reflection line is planned when first arriving on Demerara Rise to provide a crossline over proposed Site DR-3C, intersecting existing line GeoB01-221, and ending at line GeoB01-215. This line is oriented in the dip direction across the proposed site, as none presently exists. It will tie into two existing strike lines to provide closure to the seismic stratigraphic correlations.
The drilling program includes a total of four primary sites (DR-8B, DR-3C, DR-5B, and DR-2) and six alternate sites (DR-1B, DR-3, DR-4B, DR-6B, DR-6C, and DR-7B), spanning water depths from 1895 to 3215 m (for the primary sites). Paleogene to mid-Cretaceous sediments will be sampled at all primary sites. Site DR-2 should include a 300-m-thick Neogene succession. A thin veneer of Quaternary ooze is expected at each site. Site details and drilling order for the primary sites are presented in Table T1. Table T2 contains priorities and details of the alternate sites.
The primary Sites DR-8B, DR-3C, and DR-5B will be multi-advanced piston coring/extended core barrel (APC/XCB) cored to refusal or to the maximum proposed depth of penetration (Table T1). Site DR-2 will be multi-APC/XCB cored and rotary core barrel (RCB) cored to a total depth of 970 mbsf. Depths of maximum penetration of all proposed sites is based on a safe elevation above the estimated depth of Reflector C (see "Seismic Stratigraphy of Demerara Rise"). Interval velocities are not known, so a checkshot experiment is planned at the first site to allow for recalculation of these depth estimates. Multiple coring at each site will ensure 100% recovery of the stratigraphic column. Given the nature of sediments recovered at DSDP Site 144 and interpretation of the seismic reflection profiles, no geologic conditions are expected to provide drilling problems.
Alternate drilling sites have been provided as contingency for unforeseen problems with any of the existing primary sites. The intent is to sample similar sections as proposed for the primary sites but at different locations. They are proposed as alternates because they are viewed as less than ideal from interpretation of the seismic reflection profiles (e.g., the sections of interest are more deeply buried, the target sections are not as thick as at the primary sites, or nearby slumping or faulting draws into question the competency of the stratigraphic section).
Logging operations are scheduled to take place at all of the primary sites (DR-8B, DR-3C, DR-5B and DR-2). The two standard ODP tool string configurations will be deployed. The triple combination (triple combo) tool string logs formation resistivity, density, porosity, natural gamma ray, and borehole diameter, and will be run first, followed by the Formation MicroScanner (FMS)-Sonic tool string, which provides an oriented 360† resistivity image of the borehole wall, logs of formation acoustic velocity, natural gamma ray, and borehole diameter. The Lamont-Doherty Earth Observatory (LDEO) high-resolution Multisensor Gamma Tool (MGT) will be deployed on the top of the triple combo tool string and run on a separate pass at all of the primary sites. In addition, the Well Seismic Tool (WST) is scheduled for use at primary Sites DR-2 and DR-8B. This tool will be used to undertake a checkshot survey, providing accurate traveltime data for calibrating the velocity logs.
The main aims of the leg are the recovery of Paleogene and Cretaceous fossiliferous oozes and chalks and Cretaceous black shales. Borehole logging will provide continuous in situ measured physical properties, which can be used to assess the physical, chemical, and structural characteristics of the formation. Even in the event of complete core recovery, the expansion of sediments resulting from elastic strain recovery requires core data to be depth-shifted (compressed), which can be accomplished by depth matching core multisensor-track (MST) data (density, porosity, and gamma ray) to equivalent log data using the Sagan core-log integration program. The situation is reversed when incomplete core recovery occurs. The recovery of black shale intervals may be problematic, but they should be clearly identifiable using the natural gamma, density, and porosity logs. The continuous logs of density, porosity, gamma ray, and resistivity are readily amenable to cyclostratigraphic analysis, which can identify the influence of orbital forcing on climate cycles evident in sedimentary deposits. Formation density and velocity profiles (derived by splicing core and log data) will be calibrated using the checkshot surveys and used to produce formation acoustic impedance profiles, depth/time models, and synthetic seismograms. The synthetic seismograms provide a direct link between the depth domain core data and the time domain seismic data, allowing the accurate location, dating, and interpretation of the reflectors seen in the regional seismic data. Further details on tools and their application can be found on the LDEO Borehole Research Group (BRG) website: http://www.ldeo.columbia.edu/BRG/.
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