167 Preliminary Report

Site 1019 (Proposed Site CA-1D)

Site 1019 is located about 60 km west of Crescent City, California, in the Eel River Basin at a water depth of 983 mbsl (Fig. 1). It is the nearshore site of the Gorda Transect. The primary drilling objective was to sample a high-resolution Pleistocene sediment section as part of the coastal transect from 30° to 40°N and as part of the Gorda Transect across the California Current at 40°N. Site 1019 should will provide important information about the development of coastal upwelling as glaciation expanded in the Northern Hemisphere. It will provide important new information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles and through solid-phase analyses. One of the important objectives at this site was to study the formation of the bottom-simulating reflector (BSR) and to sample gas hydrates. A high-resolution pore-water sampling program was carried out to detect evidence of gas hydrate formation. A logging program was added to measure the extent of gas hydrate formation in situ.

A 247-m-thick sequence of upper Quaternary (1.0-0.0 Ma) sediment was recovered at Site 1019. The sediments consist of one single lithological unit that is subdivided into two subunits. Subunit IA (0-30 mbsf) is a mixed siliciclastic and biogenic unit composed mainly of clays and silt with varying amounts of diatoms and nannofossils. Abundant laminae and thin sand beds occur throughout the middle to lower parts of this subunit. Subunit IB (30-246.4 mbsf) is composed predominantly of siliciclastic clays and silts with a minor diatom component and absent or rare nannofossils. Laminae and thin sand beds occur only in the middle part of this subunit. Sedimentation rates are extremely high (400-1000 m/m.y.) until about 800 ka. After this, rates of sedimentation drastically decreased (to about 150 m/m.y.), and further decreased to about 100 m/m.y. following 450 ka. The decreased rate of sedimentation after 800 ka probably resulted from the tectonic development of the bank immediately to the east of Site 1019. Once this bank was of sufficient elevation, it would have blocked the sediment transport from the adjacent continental shelf. Conspicuous glauconite also began to be deposited at 800 ka at the time of reduction in sedimentation rates, and probably reflects its formation on the developing bank. If this is correct, 250 m of bank uplift occurred in 800 k.y., which is 3 cm/100 yr; a very rapid rate of uplift rate associated with the well-known regional neotectonism.

Detailed comparisons between the magnetic susceptibility and the GRAPE density record generated using the MST, and high-resolution color reflectance measured using the Oregon State University system, demonstrated complete recovery of the sedimentary sequence down to 86 mcd.

Biostratigraphy provides limited age control for the sequence at this site. The base of Hole 1019C is dated at about 1 Ma. The middle part of the sequence is dated using the onset of the prominent 100-k.y. paleoclimatic oscillations at 800 ka (at 113 mbsf) and the upper part by a combination of calcareous nannofossil, radiolarian, and diatom datums. Planktonic foraminifer assemblages reflect oscillations between glacial and interglacial episodes. These faunas are marked by cooler elements during both glacial and interglacial episodes, compared to previous Leg 167 sites to the south (Sites 1010 to 1018). Radiolarians are completely dominated by subarctic forms, whereas diatoms are dominated by cool, high-latitude, North Pacific assemblages, with limited subtropical forms. Radiolarian and diatom taxa characteristic of upwelling are inconspicuous at this site, and diatom assemblages contain rare but pervasive coastal planktonic forms. Benthic foraminifer assemblages indicate the presence of relatively low oxygen concentrations in bottom waters throughout the entire sequence. Intervals marked by particularly low oxygen levels occur in the uppermost part of the sequence, younger than 250 ka (above 27.3 mbsf).

Positive paleomagnetic inclinations of the top 75 mbsf most likely represent the Brunhes Chron C1n. Below the normal polarity interval an interpretation of the inclination record was not possible because of core disturbance by XCB coring.

The methane concentration increased in the second core to maximum values and caused frequent gas voids in the recovered cores. Although the gas pressure was very high, no significant amounts of higher hydrocarbons were found. Calcium carbonate concentrations are generally low, and show a fluctuation between 0 and 9 wt%. Total organic carbon varies from 0.5 to 1.5 wt% throughout the sediment section. According to the C to N ratios, the organic fraction is mainly consisting of marine-derived material.

The most striking feature of the chemical gradients in the interstitial waters (Fig. 4), sampled at 1 per core throughout the site, is a pronounced decrease in chlorinity, from 551 mM at 4.45 mbsf to an average of 357 mM at >150 mbsf. The smooth shape of the profile and the lack of dilution in other elemental concentrations are not consistent with the low chlorinity resulting from an artifact of methane hydrate dissociation, instead indicating there must be a source of low-chlorinity fluid in contact with the site. Other chemical gradients reflect organic matter diagenesis, the dissolution of biogenic opal, the influence of ion exchange and authigenic mineral precipitation reactions, and the diffusive influence of reactions in underlying basalt. Alkalinity increases to peak values >95 mM, whereas sulfate concentrations decrease to values below the detection limit (ca. 0.5 mM) by 12.75 mbsf. Phosphate concentrations increase to strikingly high values >340 µM, and dissolved silicate increases to concentrations >1000 µM. Calcium concentrations decrease to a minimum <1 mM at 41 mbsf, then increase with increasing depth to 4.0 mM at 242 mbsf. Magnesium concentrations decrease to a minimum of 25 mM, coincident with the Ca minimum, then increase with increasing depth to 37-39 mM, subsequently decreasing to 31.5 mM at 242 mbsf. There are no apparent changes in the interstitial water profiles at the estimated depth of the BSR.

The 4 Adara temperature measurements yield a thermal gradient of 57°C/km (Fig. 5). Using an average measured thermal conductivity of 0.989 W/(m-K) provides a heat-flow estimate of 57 mW/m2. The index properties data show a sharp decrease in bulk density values down to about 30 mbsf, which coincides with the lithostratigraphic Subunit IA and IB boundary. Below this, index properties values are fairly constant, with occasional fluctuations generally corresponding to lithologic variations.

Color reflectance was generally low, averaging 10% in the visible wavelengths. Downhole variability in the reflectance signal was minimal (standard deviation of 1%). A prediction of opal content was made using a multiple linear regression equation derived from site-survey reflectance and opal measurements. The results were consistent with the lithologic subunits described at Site 1019, showing higher opal content in Subunit IA than in Subunit IB.

Hole 1019C was logged with one full pass of the Triple Combination tool string, two FMS-Sonic passes, and two GHMT passes. Caliper measurements indicated that the borehole was in poor condition, with broad, irregular washouts throughout the logged interval. The hole was highly elliptical in shape with the long axis oriented north-south, consistent with borehole breakout resulting from the regional east-west maximum compressive stress orientation. The poor hole conditions adversely affected those measurements requiring good contact with the borehole wall (e.g., density, porosity, and FMS), although comparision with core-based physical property data demonstrated that the log measurements were valid over most of the hole where good contact was established. The log data were intended to determine the origin of the BSR observed in the seismic profiles of this site. The log data did not indicate any corresponding increases in sonic velocity or density reductions that typically characterize clathrate occurrances. There was, however, an anomalously high-resistivity layer at ca. 100 mbsf (about 5 m thick) that was not associated with corresponding density or velocity increases.

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