Site CR-1 (Middle America Trench Axis)
Proposed Site CR-1 is located on Swath Line 20, seaward of the Middle America Trench, and acts as a reference for the age, thickness, lithology, physical properties, and fluid composition of the incoming sedimentary section. The planned depth of penetration is 500 m (sub-bottom), including 50 to 100 m into oceanic basement. This site has taken on greater significance as a result of the discovery of regional, extremely low heat flow on the lower plate during the February 1994 cruise of the Alvin. Deeper drilling would certainly be helpful but would seriously impact other major objectives. We plan to take heat-flow measurements with the Adara temperature tool on every other core recovered with the advanced hydraulic piston core (APC) technique, as well as five heat flow measurements with the downhole water sampler, temperature, and pressure probe (WSTP). The downhole logging program consists of wireline logging (triple-combo and Formation MicroScanner [FMS] and sonic), as well as logging-while-drilling (LWD). Porosity data from the LWD Compensated Density Neutron (CDN) tool and velocity data from the standard logging suite (C1 and C2, see Table 1) will be used to empirically correlate velocity to seismic data at the other holes.
Proposed Site CR-2 is located on Swath Line 20, 1.5 km from the toe of the accretionary wedge, near the base of the slope. Drilling targets include drilling through the accretionary wedge, the décollement, and an additional 150 m beneath the décollement to a total depth of 750 mbsf. The objectives for this site include understanding the rate of dewatering of wedge sediments, the hydrogeology of the décollement, and constraining the amount of dewatering occurring in the underthrust strata. We plan to take heat-flow measurements with the Adara tool on every other APC core, as well as five heat-flow measurements with the WSTP tool. The logging program consists of LWD; however, if time permits, standard wireline logging tools may be utilized. Deployment of the Lamont shear sonic tool is subject to availability of the tool onboard, as well as time available.
Site CR-3 (Lower Trench Slope)
Proposed Site CR-3 is located on Line CR3D-177, 14 km landward of the trench, where the hole can penetrate the slope apron, the high-amplitude reflector at the base of the apron, and one or more of the interpreted fault zone reflectors. This key site will constrain the age, lithology (hemipelagic or carbonate), structure, and physical properties of the slope apron, the prism, and important boundaries. It will allow us to utilize geochemical markers to determine the significance of fluid flow along the décollement, as well as through other fault zones within the wedge. This site also has potential for providing some input on the concept of a flow channel, proposed by Cloos and Shreve (1988a, 1988b), although the drilling depths will be too shallow to fully test the idea. Target depths are 500 m of sedimentary apron material and a total depth of 1400 m to sample out-of-sequence thrusts within the prism. We plan to take heat-flow measurements with the Adara tool on every other APC core, as well as five heat-flow measurements with the WSTP tool . The logging program consists of LWD; however, if time permits, standard wireline logging tools may be utilized. Deployment of the Lamont shear sonic tool is subject to availability of the tool onboard, as well as time available.
Site CR-4 (Middle Trench Slope)
Proposed Site CR-4 is located on 3D line 132, 23 km landward of the trench. This site will sample a portion of the slope apron that is thicker and less deformed than the portion at proposed Sites CR-2 or CR-3 and also will penetrate the high-amplitude reflector, to a total depth of 950 mbsf. The purpose of sampling the slope apron is to date regionally correlatable seismic sequences and thereby constrain a large portion of the upper slope. Additional penetration of the apron-prism boundary is important to confirm the nature of this seismic impedance boundary and to calculate the growth rate of a larger portion of the accretionary complex. This site is upslope from a major structural disturbance and change in thickness of the slope apron. Proposed Site CR-4 will allow us to calibrate the seismic stratigraphy, which is well developed in the middle and upper slope regions of the margin, but more poorly developed on the lower slope. Penetration of the apron prism boundary will provide the second datum for determining an age gradient for the rate of underplating (if that process is documented) and information on variation in material properties of the subapron prism. It will allow us to determine whether the underlying rocks are recently underplated or if they are older basement rocks such as those found off Guatemala. Standard wireline logging is planned at this site.
Site CR-5 (Lower Trench Slope)
Proposed Site CR-5 is located on 3D seismic line 172, 5.5 km landward of the base of the trench slope, and is designed to intersect a major out-of-sequence thrust that has abundant fluid vents where it crops out at the surface. Because of time constraints, proposed Site CR-5 is considered an alternate site to be drilled if one of the other sites (CR-1 or CR-2) cannot be drilled. We plan to drill to a depth of 1150 m, which would cross the OOST and penetrate into the décollement. If this site is drilled in place of either Site CR-2 or CR-3, then we plan to carry out LWD, as well as Adara and 5 WSTP heat-flow measurements at this site.
Additional Operational Notes
Should additional time become available as a result of more favorable drilling conditions than anticipated at this time, we would add the following, unranked list of operations. The ranking will depend on the amount of additional time and the results of the program at the time of the decision.
Add 2 days to proposed Site CR-2 to carry out shear sonic logging, which will allow measurement of both P-waves and shear waves.
Drill deeper into basement at proposed Site CR-1.
Drill proposed Site CR-4.
Because of the focus on structural fabrics, mechanical properties, and fluid flow within the wedge, sampling of whole rounds for structural and geotechnical analyses and interstitial pore fluid is needed. We anticipate needing whole-round sampling in excess of the specimen number and specimen length outlined in the general ODP curation policy.
In Situ Fluid Sampling
It is planned to attempt sampling of formation fluids with the WSTP and if feasible with the developmental "Fissler Water Sampling Tool." Alternatively, the Pressure Core Sampler (PCS) could be deployed for pore-water sampling. If gas hydrate is encountered, hydrate sampling may be attempted with the PCS as well. The anticipated total number of fluid and gas hydrate sampling runs will be limited by the operational time available.
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