167 Preliminary Report

Site 1015 (Proposed Site BA-4D)

The primary objective for drilling here was to sample a very high-resolution sediment section for comparison with ODP Site 893 in the Santa Barbara Basin. Hemipelagic sections between turbidites should be sufficiently large to study upper Pleistocene and Holocene paleoceanographic processes. The site should also prove useful for sedimentological study of turbidite deposition and the development of Hueneme Fan. Organic carbon diagenesis will be studied, through detailed pore-water analysis in the upper 100 mbsf geochemical analyses of the solids, in a sedimentary section that has been quickly deposited and that contains a large terrigenous organic component.

The sedimentary sequence recovered from the two holes at Site 1015 consists of a 150-m-thick interval of upper Quaternary (60 ka) sediments. Sediments consist of one lithologic unit, which is dominated by quartz feldspar sand and clayey silt interbedded at decimeter to meter scale. Sand layers with sharp basal contacts, frequently with wood fragments in the upper part, grading upwards into clayey silt are interpreted as turbidite deposits. Thin layers of laminated hemipelagic nannofossil clay and disseminated volcanic glass occur between the turbiditic sediments. Authigenic pyrite is a minor but common constituent. Calcareous nannofossils and, to a lesser extent, foraminifers dominate the calcareous fraction of the sediments. The biosiliceous component is negligible.

Detailed comparisons between the magnetic susceptibility generated using the MST and high-resolution color reflectance measured using the Oregon State University system at the two holes, demonstrated complete recovery of the sedimentary sequence down to 36 mbsf. The existence of gas voids, turbidites, and coring disturbances below that depth precluded an interhole correlation.

Calcareous nannofossil and planktonic foraminifer data indicate that the 150-m sequence of turbidites and hemipelagic sediments are of latest Quaternary age, younger than 60 ka. Sand layers in the sequence are barren of microfossils except for very rare, moderately well preserved, calcareous nannofossils in some intervals. Hemipelagic sediments contain abundant to few, well-preserved planktonic foraminifers, common to abundant, well-preserved benthic foraminifers, and abundant to rare, well-preserved calcareous nannofossils. Radiolarians and diatoms are essentially absent, except for reworked Miocene taxa.

Changes in planktonic foraminifer assemblages in this sequence exhibit evidence of large-scale Quaternary glacial-interglacial oscillations. The Holocene is well marked by interglacial planktonic foraminifer assemblages. Planktonic and benthic foraminifer assemblages suggests that the glacial-interglacial episodes are associated with changes in circulation of upper intermediate waters affecting changes in oxygen levels of the basins of the California Borderland.

Gas voids, turbidites, and coring disturbances precluded the measurement of most physical properties and the determination of a paleomagnetic reversal stratigraphy. Furthermore, the sequence is too young to contain magnetic field reversals.

Carbonate values range from 2 to 7 wt%. Lowest values occur in the turbiditic sequences. The organic carbon record shows hemipelagic background values of about 1 to 1.5 wt%. In the turbidites, organic carbon is below 0.5 wt%. Spikes up to 2.3 wt% result from wood fragments in the sediment. Methane concentrations are high throughout the sediment column, however, no significant amounts of ethane were observed.

Although the chemical composition of the 9 interstitial water samples from this site indicates that organic matter diagenesis, biogenic opal dissolution, and authigenic mineral precipitation and/or ion exchange reactions are significant influences, the nature of the sediments and the effects of drilling and recovery on them make distinguishing primary geochemical signals from contamination of interstitial water samples by seawater drilling fluid problematic.

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