SITE 1042

We succeeded in reaching and coring the high-amplitude reflection marking the top of the prism. Spot coring every 50 m showed a sedimentary section similar to the deeper parts of Site 1041, although more careful study will be necessary to make any detailed comparisons. We subdivided the sedimentary section into two subunits on the basis of lithological and age differences; Subunits 1A and 1B (Fig. 9). Subunit 1A (48.7-240.1 mbsf) consists mostly of silty claystone including ubiquitous volcanic glass with common to trace amounts of microfossils. Subunit 1A is late Pleistocene to middle Miocene in age on the basis of planktonic foraminifers. Subunit 1B (201.7 240.1 mbsf) is composed dominantly of silty claystone and limestone with matrix supported breccia. Thin layers of glauconitic sandstone were also recognized. The late to middle Miocene age of Subunit 1B was determined from common to well-preserved foraminifers with few to trace amounts of nannofossils and diatoms.

At 316 mbsf, we cored a carbonate-cemented breccia, consisting of angular to subangular sandstone clasts and other clasts that have not yet been analyzed. Between 350 and 360 mbsf we recovered breccia composed of fragments of red chert, doleritic basalt, and altered mafic rock. These rocks are similar to outcrops described onshore. The carbonate-cemented breccia has a density and sound velocity of about 2 g/cm3 and 4.6 km/s, respectively, sufficient to explain both the high-amplitude reflection at the top of the prism and the velocities obtained from ocean-bottom seismometer (OBS) and ocean-bottom hydrophones (OBH) refraction data. Preliminary age determination on the breccia is middle Miocene. Beneath the breccia is a scaly clay of late Miocene age, interpreted to represent a fault zone.

Objectives of the geochemistry program at Site 1042 were similar to those at Site 1041. Additional chemistry at Site 1042 was needed to tie in these observations with those at other sites. In this light, the work was a success. Because coring was done only every 50 m until 200 mbsf, all the pore-fluid samples analyzed were from beneath the sulfate reduction zone; therefore, methane concentrations have high values even in the shallowest sample analyzed at ~50 mbsf. The section drilled lies within the stability field of methane hydrate, and mild gas escape was observed in the few sediment samples obtained from ~100 to 230 mbsf. This effect is most likely responsible for at least some of the Cl freshening also encountered at this site. An abrupt increase in the content of C1 through C3 and traces of higher hydrocarbons through C6 were observed between 150 and 200 mbsf, where a major fault was intersected, suggesting that they formed at >100šC. A change in the pore-fluid chemistry occurs at ~100 mbsf; below 100 mbsf and crossing a major fault to about 230 mbsf, the chemistry is roughly constant with characteristics similar to those observed at Site 1041 beneath the fluid conduit at 280-300 mbsf, and at Site 1040 beneath the conduit at about 180 mbsf. Characteristics include elevated Ca concentration and both Cl concentration and Na/Cl ratios that are lower than seawater.

Downhole logging acquired resistivity and gamma-ray logs with the CDR tool. A major break in resistivity occurs in the interval between 67 and 74 mbsf. This change in resistivity roughly coincides with the location of a normal fault in the seismic reflection records. A large increase in resistivity occurs at 145 mbsf, and below that resistivity values increase gradually with depth. A number of small peaks could represent gas hydrate.

In summary, even though this site was spot-cored for the first 200 mbsf, it was highly successful in contributing to the major objective of determining the physical nature of the top of the prism. Together with Site 1041, the combined sites through the apron have provided a nearly complete picture of mid-slope geology. These sites have clarified the environment and character of the gas hydrate, the pathways of fluid flow, and the sources of the fluids moving through the apron and prism. Physical properties data from cores and from LWD have also contributed in important ways to this knowledge. The primary goal of drilling through the apron, that of understanding the nature of the prism material below, has been met. Thus, Sites 1041/1042 have been fully successful.

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