Leg 166 Transect Summary
Leg 166 will drill two main transects to answer the sea-level and fluid-flow objectives. The first transect will be along an extension of the Western Geophysical line on which the holes Unda and Clino were drilled. The holes follow a transect from shallow to deeper water (BT-1 through BT-4) (Figs. 2, 6, 7, 9). These holes primarily will address the sea-level objectives. One additional hole (BT-20) will be drilled along strike to examine lateral variation in facies. Also drilled along this first transect will be F-3, a hole designed to address the fluid-flow objectives. As a result of time constraints, Site F-3 will be a single advanced hydraulic piston corer (APC) hole and Sites BT-2, BT-1 and BT-4 will be APC and extended core barrel (XCB) holes. The second transect, located to the south of the first transect, consists of three holes, F-4, F-5, and F-6, extending from shallow to deep water (Figs. 1, 8). These three sites will be single APC holes. Due to shallow-water drilling-safety considerations, the drilling strategy for the two shallowest sites (BT-1, 290 m; and F-4, 245 m) may have to be modified.
Schedule and Sequence of Drilling
Leg 166 is scheduled to proceed from San Juan to Panama. The transit time from San Juan to the first site is projected at 3.2 days (Table 2). The first site will be BT-2 because of few expected drilling difficulties and a procedure of drilling from deeper to shallow water on parts of this transect for safety precautions. In addition, the projected drilling time of 7.4 days should allow the science party to become fully acquainted with shipboard routine and laboratories. BT-2 will be followed by Sites F-3, BT-1/F-1, BT-4, BT-20 and BT-3. Site BT-3 will be left as the last site of the first transect because it will be the deepest objective, thus allowing us to ascertain the possibility of deepening the hole to achieve the middle Cretaceous objective at this stage. The final three sites in the southern transect should require 3.5 days. If time savings are attained, we have contingency plans that will enable us to deepen sites (Table 3) or drill alternate sites (Table 3).
The drilling at each site will follow a strategy in which the sites initially are cored using the APC/XCB until refusal or target depth is met. If deeper objectives are required, the strategy will be to wash down using the rotary core barrel (RCB) and to begin coring slightly above the interval last cored with XCB. If time permits we will in some instance triple APC the upper interval. Cores will be orientated during the APC interval of the first hole at Sites BT-2, BT-3, and BT-4.
Logging Plan
Three standard logging runs (Quad-combination, Formation MicroScanner, and geochemical tool strings) are planned for all sites except the shallow penetration fluid-flow sites (all sites labeled F). The geological high-sensitivity magnetic tool (GHMT) is planned at two sites, and a vertical seismic profile (VSP) experiment will be conducted at three sites.
The Quad-combination string provides measurements of sonic velocity, porosity, density and electrical resistivity that will aid in characterizing lithology, sediment fabric, degree of lithification, and diagenetic alteration. The Formation MicroScanner (FMS) produces electrical-resistivity images that can be used to distinguish thin beds, sedimentary structures, diagenetic features, and fractures. Once calibrated to cores and to other logs, FMS images can help characterize facies variations within the different sequences. The geochemical logging tool (GLT) will be useful to delineate mineralogical changes (e.g., dolomite/calcite) downhole. The standard suite of logging measurements will utilize site-to-site correlation along the transect.
Continuous susceptibility measurements by the GHMT can be used to detect changes in magnetic-grain concentration. Provided that the magnetization of the sediments is not too weak, the total field data in conjunction with the magnetic susceptibility can be used to derive a magnetic-reversal sequence for the logged interval.
The integration of core and logging data to the high-resolution multi-channel seismic-reflection data will be done via synthetic seismograms generated from the sonic velocity and density logs together with core measurements. At three holes, a VSP "check shot" survey will be conducted to calibrate the time-depth relationships and sonic velocity logs. (Further information on logging tools can be obtained from the Borehole Research Group home page at http://www.ldeo.columbia.edu).