Two holes (A and B) were drilled at Site 1080. Hole 1080A was drilled to a maximum depth of 50.8 mbsf before an authigenic dolomite horizon prevented further drilling (see "Operations" section, this chapter). Hole 1080B was also abandoned after intersecting a dolomite horizon at 37.7 mbsf. Moderate core disturbance is common in the top of Section 1 of all cores. Cores 175-180A-4H through 6H are intensely disturbed from the rapid expansion of gas.
Sediments from Site 1080 form one lithostratigraphic unit composed of moderately bioturbated, dark greenish gray (5GY 4/1) to olive-gray (5Y 4/2), diatom-bearing, and diatom-rich silty clays with varying abundances of nannofossils and foraminifers. Diffuse, dark gray color bands, 0.5 cm thick, composed of framboidal pyrite are present in intervals 175-1080A-5H-1, 48–76 cm, and 5H-2, below 100 cm. Rare, whitish gray nodules, 1–2 mm in diameter, are disseminated throughout Cores 175-1080A-4H through 6H and Cores 175-1080B-1H through 3H.
Smear-slide analyses indicate that silty clay is the dominant lithology at Site 1080. The clastic fraction is dominated by coarse silt-sized, angular, mono- and polycrystalline quartz grains with rare subangular feldspar and detrital apatite clasts. Muscovite and biotite also are present in trace amounts. The biogenic component is represented by frequent diatom fragments, foraminifer fragments, and nannofossils. Radiolarians, plant remains, and particulate organic matter are present in trace amounts. Authigenic minerals include rounded glauconitic peloids and framboidal pyrite. Some glauconite grains contain euhedral pyrite grains, indicating secondary consumption of iron to form pyrite. Examination of a smear slide made from a whitish gray nodule from Core 175-1080A-4H reveals that the nodules are composed entirely of sponge spicules and diatom fragments.
X-ray diffraction (XRD) analysis of sediments from Hole 1080A reveals that the clastic fraction is dominated by smectite, kaolinite and perhaps illite, quartz, the feldspar minerals albite and microcline, and muscovite. Pyrite is present as an accessory mineral in all samples. No clear identification could be made for other accessory phases. The smectites are generally poorly crystallized. The XRD patterns do not indicate the presence of dolomite.
Color reflectance was measured, but because of the extensive core disturbance at this site, no discernable stratigraphic pattern was found either in the total reflectance or the red/blue ratio (Fig. 6, Fig. 7). The red/blue ratio, however, follows the general trend of the carbonate concentration (see "Inorganic Geochemistry" section, this chapter). In the top 10 mbsf, there is a high red/blue ratio (average of 1.4) and the carbonate concentration varies between 6 and 26 wt%. Below 10 mbsf, the red/blue ratio is lower (average of 1.15), and the carbonate concentration varies between 1.5 and 6 wt%.