169 Prospectus


The relative order of operations and the exact placement of some drill holes in the massive sulfide deposits will need to be determined at sea based on weather conditions and results of completed drill holes. Successful CORK emplacements and packer deployments require relatively quiet seas. The first operation site proposed for this leg is a return to Site 858 from ODP Leg 139. During that leg, Hole 858G (48°27.360'N, 128°42.531'W, Figs. 1, 5) was instrumented with a CORK and thermistor string. Submersible observations of Hole 858G show that the CORK has failed and this hole is now exuding hydrothermal fluid (Davis and Becker, 1994b). We plan to extract the thermistor and data logger from the CORK through the drill string before recovering the CORK. The kevlar cable supporting the thermistor string will be sampled for thermophilic bacteria which may have colonized the string during the 5 years it has been bathed in hydrothermal fluid. In order to ensure minimal disturbance of bore fluids, we intend to acquire a temperature log and a water sample prior to recovering the CORK. Minimizing the thermal disturbance of the fluid in the borehole is intended to limit the possibility of reestablishing the strong down-hole fluid flow initiated when the hole was first drilled. The slender diameter of the Becker temperature tool should allow a sandline deployment through the hole in the CORK head left by removal of the thermistor and data logger. Since water temperatures are expected to be in excess of 200°C which is beyond the operational limits of the WSTP or Fisseler water samplers, we propose deployment of the Los Alamos water sampler. This tool also has a slimline configuration, and can be deployed by sandline prior to removal of the existing CORK. The water sample should be taken from the interval below the borehole casing string (260 mbsf). In the event that the thermistor and data logger cannot be removed without pulling the CORK, both will be recovered and the temperature log and water sample will be recovered on a second pipe trip. Once the CORK is recovered, the seals will be inspected to determine if seal failure is responsible for the fluid leakage recognized by submersible observations. If the seals are determined to have failed, a new CORK instrumented with a thermistor string and data logger will be installed. One ultimate benefit of successful deployment of an instrumented CORK will be acquisition of temperature and pressure data that may detect the transient overpressure that will be induced when we unseal and deepen Hole 857D, which is located 1.6 km to the south. Alternatively, if seal failure cannot be demonstrated as the cause of fluid leakage from the CORK, an open pipe artificial chimney will be installed in the borehole to allow future deployment of sandline/wireline tools. During the transit to the next site, we propose to offset to a position halfway between Sites 858 and 857 (~800 m SSW) and deploy the first of three Pop-Up Pore Pressure Instruments (PUPPI's). The experiment designed to use these instruments will be described below.

In order to maximize the time available for thermal recovery of Hole 858G prior to deepening Hole 857D, our second operation will be at ODP Site 856 (48°26.020'N, 128°40.859'W, Figs. 1, 5), the Bent Hill massive sulfide deposit (Fig. 7, 8). The highest priority for drilling at the Bent Hill deposit will be to complete a hole through the massive sulfide deposit and into the underlying alteration zone. Hole 856H was drilled to a depth of 94 mbsf prior to abandonment on Leg 139 (Fig. 7, 8). This hole is equipped with a reentry funnel and 12 m of 11-3/4 in drill-in casing. The hole was left open and unobstructed. The introduction of oxygenated seawater into the massive sulfide deposit will likely have altered the pyrrhotite/pyrite massive sulfide deposit to iron oxide/oxyhydroxide. Since degradation of the borehole walls during operations on Leg 139 was the cause for abandonment of this hole, we suspect that alteration may have cemented and stabilized the borehole. Taking advantage of the 94 m of penetration already achieved, we intend to reenter Hole 856H and drill through the massive sulfide deposit and into the alteration zone that is anticipated to underlie this deposit. Total depth of this hole will be dependent on drilling conditions and nature of the recovered core. If we are unable to successfully deepen this hole, we will offset a few meters and attempt to drill in a casing to isolate the upper part of the formation. We intend to deepen this hole (BH1) to at least 250 m, through the massive sulfide deposit and into the underlying alteration zone, possibly taking advantage of the MDCB to enhance recovery in the massive sulfide deposit. Drilling operations will be followed by a full logging suite.

Completion of this hole (BH1) through the massive sulfide deposit and alteration zone will accomplish one of the primary objectives of drilling in Middle Valley. However, we also wish to characterize the lateral variability and timing of hydrothermal activity recorded in sediments adjacent to the sulfide deposit. The exact placement of holes to accomplish this will be determined by the size of the massive sulfide deposit and underlying alteration zone as well as by the time available for drilling operations. Due to the large uncertainty in the amount of time necessary to deepen and CORK Hole 857D, we deem it necessary to proceed with this operation after completing our deep penetration hole. With this strategy we will maintain the flexibility to complete as much as possible of the other drilling planned for Middle Valley prior to departing this site for Escanaba Trough.

After completing operations at BH1, we will transit to Hole 857D (48°26.517'N, 128°42.651'W, Figs. 1, 5), where a second instrumented CORK was installed during Leg 139. During the transit, we intend to deploy a second PUPPI at a location along a line between Sites 858 and 857 about 400 m north of Site 857. Upon arrival at Site 857, a third PUPPI will be deployed. At Hole 857D, the CORK, data logger and thermistor string will be recovered, and a temperature log and down-hole fluid samples will be collected. The high temperatures expected in the hole will be lowered by circulation while drilling for approximately 36 hours with the RCB/MDCB, allowing us to deepen the hole by as much as 200 m. Circulation of cold seawater in the borehole will induce a transient overpressure of ~1MPa relative to in situ pore pressure. If the rocks on the ridge axis are in a tensional state near failure by normal faulting, then the increased pore pressure should induce localized failure detectable by an OBS array scheduled for deployment prior to drilling. Calculations based on measured pore pressure and permeability indicate that the overpressure induced by introduction of cold seawater into Hole 857D during drilling may be detectable in Hole 858G. This represents the first active hole-to-hole hydrologic experiment conducted on the scale of a seafloor hydrothermal system. We will also attempt to record this pressure pulse with the three PUPPI's deployed in an array between holes 857D and 858G. Since an agitated sea state diminished the quality of the logging data collected from Hole 858G during Leg 139, we will run a full suite of logs in the deepened hole. We expect to run an FMS log as well to help define the structural complexity at this site. Finally, the hole will be CORKed with a new, longer thermistor string, thereby completing an important unrealized objective of Leg 139.

Our plan for the operational time remaining after completion of this portion of the drilling program will need to be highly flexible and will be determined by 1) the amount of time remaining before we must move to Escanaba Trough and 2) the results of drilling at BH1. The investigations we wish to conduct relate to the formation of hydrothermal mounds in the Dead Dog vent field and the lateral variations in the composition and form of the Bent Hill massive sulfide deposit. The following is an ideal scenario assuming that all preceding operations are concluded in a timely fashion.

Following a transit to Site 858 (48°27.336'N, 128°42.600'W, Figs. 1, 5), we will begin a short camera survey to locate Site DD1. Our strategy for investigating the formation of the hydrothermal mound within the Dead Dog vent field is to drill 3 to 4 holes separated by ~25 m each along a transect down the flank of one of the larger mounds. Each of these APC/XCB/MDCB holes will penetrate ~50 m, with particular attention to recovering the hard layer encountered at 30 mbsf at this site during Leg 139. An extensive data set including ~200 heat flow measurements and 20 sediment cores sampled for pore water was obtained within the immediate vicinity of the Dead Dog vent field in the summer of 1995. Evaluation of this data prior to the cruise may argue that the transect of holes should be moved from Dead Dog Hill to either Chowder Hill mound or the Ropos vent field (Fig. 4) if it is determined that they represent better targets for accomplishing the objectives of drilling in this area. Regardless of which mound is chosen for operations, the scientific objectives and drilling strategy will remain the same. All of the mounds are within a few hundred meters of each other, and are considered to be a single drilling site. The proposed transect will include drilling in the top, flank, and periphery of the mound in order to test the subseafloor inflation model for the formation of these mounds by evaluating the composition of the sediment that occurs directly beneath and on the flanks of an active hydrothermal mound. Pore-fluid and temperature gradients will also be measured to evaluate the direction and magnitude of fluid flow in the shallow subsurface. If time permits, the transect will be continued beyond the edge of the acoustic side scan sonar reflector that defines the presently active vent field (Fig. 4).

A second objective of drilling within the Dead Dog vent field is to establish the presence, continuity, and nature of a suspected "caprock" horizon underlying the vent field. Drilling on Leg 139 encountered, but did not recover, a lithified layer at approximately 30 m depth throughout the present extent of the vent field. This layer may be controlling lateral gradients in pore-fluid composition that appear to be supported by lateral fluid flow at approximately this depth. The absence of, or permeable path ways through this layer may also control the location of the individual vent sites. Authogenic cementation of the sediment by either carbonate or silica are though to be the likely cause of this apparent hydrothermal caprock. Hydrothermal caprocks appear to be an important component of near surface fluid flow and localization of ore deposition in ancient hydrothermal deposits, but it is difficult to unambiguously establish the time of formation or genetic importance of such features in on land analogs. Recognition of this horizon (based on Leg 139 rate of penetration) allows us to target recovery of the rocks and to evaluate gradients in the pore-fluid composition at this potential barrier to hydrological flow.

While completion of BH1 will accomplish our highest priority goals for investigating the Bent Hill massive sulfide deposit, other important objectives at the Bent Hill site include examining the lateral extent of sulfide mineralization, the compositional variations in the sulfide, the timing of hydrothermal activity recorded in the sediments, and the tectonic controls on fluid flow. These objectives will be addressed by drilling a transect of holes across the western edge of the Bent Hill deposit starting from BH1 (Fig. 7). The total number, spacing and depth of the holes in this transect will need to be determined by the results of drilling at BH1 and the amount of time left for drilling operations at the Middle Valley site. The first site (BH3) will be located ~50-100 m west of BH1 and will be drilled by APC/XCB to a depth of 50 to 200 m, dependent on recovery. Each of these holes will require a short camera survey for precise site location. The nature of the recovered material and the drilling conditions will determine if we should step inward toward the center of the deposit or move to more distal sites. It will also determine whether the APC/XCB system is the appropriate drilling technology for sampling the sediment buried fringe of the deposit, or whether we should use the RCB to core through the buried sulfide mound. The most westerly hole in this transect (BH6) is targeted at a buried fault (Site 856 fault) that may have served as a conduit for hydrothermal flow, but this hole, which is approximately midway between Sites 856 and 857 will also serve as a reference hole for evaluating the physical properties of the Middle Valley sediments away from the thermal anomalies of hydrothermal upflow zones. If time permits, we will drill an APC/XCB hole (BH8) on the north flank of a second mound of massive sulfide that occurs approximately 300 m south of BH1 (Sunnyside Up deposit, Fig. 7). Pore-water profiles and fluid inclusions in hydrothermal minerals formed in the zone of hydrothermal fluid upflow that formed this massive sulfide mound may help constrain the stability of vent fluid compositions with time and may give important clues to the origin of salinity variations in hydrothermal vents, which remains one of the important unanswered questions in the geochemistry of these systems.

An alternate site to be drilled, as time and conditions warrant, aims to extend the N-S transect begun on Leg 139 across the Bent Hill deposit. Proposed Hole BH7 is located approximately half way between the Bent Hill and Sunnyside Up deposits (~100m south of BH1) and completes the transect BH1, BH7, BH8 (north to south). This Hole is intended to provide constraints on the geometry, extent, and timing of the sulfide mineralization and alteration. It should also provide the least hydrothermally disturbed section of the sediment column overlying the footwall of the Site 856 fault.

In order to complete the drilling objectives at Escanaba Trough we will need to leave the Middle Valley drill sites on or about 28 days into the cruise. After recovering the PUPPI's, a two day transit from Middle Valley to Escanaba Trough (40°57.50'N, 127°30.50'W) will put us on location for Site ET7 (Fig. 10). This hole will be drilled to establish the sedimentary sequence away from the NESCA volcanic/hydrothermal center. A reference hole is needed to provide the necessary background information to evaluate the sedimentary and thermal history in an area away from a hydrothermal upflow zone. Determination of the nature of the igneous basement at a site away from the igneous intrusions that define the volcanic/hydrothermal centers is important to our understanding of the larger scale thermal and hydrologic structure of sediment-covered spreading centers. The reference hole will be sited along seismic line 89 05 (Fig. 11), which shows relatively little disturbance of the sediment and along which a heat flow profile was collected.

Other important questions regarding the sedimentary history of Escanaba Trough will also be addressed at this site. The only previous drilling in Escanaba Trough was DSDP Site 35 approximately 35 km south of the NESCA area, but this site was located east of the spreading axis. Site 35 was drilled to 390 mbsf without reaching basement, but only 95 m of the interval was cored. The turbidite sequence records the sedimentary record of glacial activity and sea level variations during the Pleistocene. Leg 5 scientists recognized a major change in provenance between the sediments recovered from the lower and upper parts of this hole, defining a change in source from sediments derived from the Klamath Mountains to Columbia River drainage, respectively (Vallier et al., 1973). A second area of investigation into the sedimentary history of Escanaba Trough involves the controversial "transparent" layer present in all seismic profiles across the Trough (Morton and Fox, 1994; Davis and Becker, 1994a). This layer occurs at approximately 100 ms depth and is about 50 ms thick. Davis and Becker (1994a) have interpreted this zone as a homogeneous sandy layer related to the Bretz floods caused by the catastrophic draining of ice-dammed Lake Missoula at approximately 13,000 ka. Normark et al. (1994), however, interpret this zone as a muddy interval formed during an interglacial high-stand of sea level. Either interpretation has ramifications for the large scale hydrology of the Escanaba Trough and can be tested by subseafloor sampling. The former scenario would imply that this unit could be an aquifer for transport of hydrothermal fluid to sites of cross-stratal permeability, such as faults or volcanic highs. The second scenario would suggest that this unit may be an aquiclude that seals the hydrothermal system. Hole ET7 will be logged with the standard suite of logging tools, including FMS.

We will then proceed to the ET1-4 (Figs. 10, 12) transect across the Central Hill massive sulfide deposit with the highest priority again to drill through the massive sulfide deposit and into the alteration zone near the center of the hydrothermal upflow zone. Drilling objectives at this site are similar to those at the Bent Hill deposit and we hope to establish the causes of the major compositional differences between the deposits at Middle Valley and Escanaba Trough. Our drilling strategy will be to APC/XCB a series of shallow exploratory holes and will, therefore, be targeted primarily at the sediment-covered areas of the seafloor between exposed mounds of massive sulfide. We wish to establish the extent, composition, and drillability of the massive sulfide in this area prior to starting a deeper drill hole. Drilling on the exposed massive sulfide outcrops will not be possible using the APC/XCB system and we may be forced to switch to the RCB system during the exploratory phase of drilling to enable us to drill on the outcropping massive sulfide in order to determine the best site to initiate a hole designed for deeper penetration.

The exploratory drilling will provide important information on the mineralogy, composition, and extent of massive sulfide mineralization in the shallow subsurface at this site, and will contribute to some of our high priority goals. In order to achieve all the goals for this site we will need to establish a reenterable drill hole that penetrates through the massive sulfide deposit and recovers altered rock from the hydrothermal upflow zone. This hole (ET5) will most likely be drilled with the RCB system and initiated by installing a length of drill-in casing. In order to address the questions regarding the compositional differences between the Escanaba Trough and Middle Valley deposits it will be necessary to examine the nature and composition of the rocks underlying the deposit. Our contingency plan, in case establishing a deep hole in this environment proves difficult, is to move to the sediment-covered edges of the deposit and attempt to intersect the flanks of the alteration zone. The complex geology at this site precludes accurate estimation of the required depth of penetration at this site. It is our intent to drill deeply enough to warrant running a full suite of logs in this hole, including the GMT, for comparison to the Bent Hill deposit, and drilling will be terminated at the appropriate stage to enable logging of the hole prior to departing the site for the San Diego port call.

In the event that time is available after the conclusion of ET5 operations, we propose to offset to a location over the flat top of the recently uplifted SW Hill (ET6). This hill is similar to Bent Hill in Middle Valley, but is larger and rises more than 120m over the surrounding turbidite plain. Massive sulfides have been observed on every camera and submersible track that have crossed the basal scarp that defines the hill. Drilling at SW Hill could test the hypothesis that laccolithic intrusion is responsible for uplift of these hills, which are common in Escanaba Trough.

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