Hole 1223A, the Nuuanu site (22°58.4095´N, 155°39.2590´W, water depth = 4235.1 m), was drilled to 41 mbsf. The advanced piston corer (APC) was used for depths shallower than 12.7 mbsf. At this depth, indurated sediments were encountered and the drilling system was switched to the extended core barrel (XCB). Figure F20 shows the core description (Garcia et al., 2006). Two types of major lithological layers are found: fine-grained clay and silt layers and volcanic sand layers. Sediments shallower than 12.7 mbsf are unconsolidated. Deeper in the hole there are weakly consolidated fine-grained sediments and indurated sands that are estimated to be volcaniclastic sandstones. Above 12.7 mbsf, six unconsolidated sand layers (sand Layers 1–6) were identified. Below 12.7 mbsf two volcanic sandstones and several volcaniclastic claystone units were identified (Stephen, Kasahara, Acton, et al., 2003). A part of the upper sandstone was lost during APC drilling (sand Layer 7 in Fig. F20). The thickness is unknown. The sandstone is moderately indurated and normally size graded with angular to subrounded fragments of fresh glass, minerals, and vitric and lithic clasts in a radiolarian-bearing clayey matrix.
At 32 mbsf, lower sandstone (sand Layer 8) was encountered. The upper part of sand Layer 8 is poorly consolidated and highly disturbed. The lower part is well lithified. Characteristics of the lower sandstone (sand Layer 8) are similar to those of the upper (sand Layer 7) except for the more advanced stage of glass alteration (Garcia et al., 2006).
Results of physical property measurements are presented in Figures F21 and F22. Units are as described in the Leg 200 Initial Results volume (Stephen, Kasahara, Acton, et al., 2003). Sand layers shallower than 12.7 mbsf are unconsolidated and have correspondingly low VP (~1.7 km/s). Among the sand layers, VP and density oscillate, indicating graded bedding in the sand layers. Deeper than 12.7 mbsf, VP drastically increases to >3.3 km/s, suggesting induration of the sand layers. Some specimens, however, show VP between 1.8 and 2.4 km/s, which corresponds to clayey layers. The deepest sand layer (sand Layer 8 in Fig. F20) at ~35 mbsf shows VP of ~4 km/s, indicating induration. These physical property measurements are consistent with lithological studies.
Radiolarians were found in layers at 3.02–3.9 and 5.10–6.70 mbsf. The shallower layer suggests that sand Layers 1–3 were emplaced in the Neogene. Radiolarians in the deeper sand Layer 6 give an Eocene age, suggesting collapse and reworking of the older sediments. Radiolarian studies from several intervals in the cores yield ages ranging from Quaternary to early Eocene, indicating that the turbidity currents that deposited the sand eroded older sediments around the Hawaiian Islands (Garcia et al., 2006; Popova-Goll and Goll, this volume).
A magnetostratigraphic study shows that Site 1223 cores are dated from Chron C1n (0–0.78 Ma) to Chron 2r (1.95–2.581 Ma). Chron 1r.1n (0.99–1.07 Ma) appears from 0.80 to 1.23 mbsf, and Chron C2n extends from 2.02 to 7.19 mbsf. Below 7.19 mbsf, the section is interpreted to be entirely reversed polarity and is estimated to have been deposited within Chron C2r (1.95–2.58 Ma) (Garcia et al., 2006).
Sand layers contain abundant basalt detritus including fresh glass, olivine, and plagioclase fragments. Sand Layer 1 is distinct from the other sand layers, with abundant rock fragments and sparse glass. The other sand layers have abundant glass. The indurated volcaniclastic sandstones are petrographically similar to unconsolidated sand layers except for their more advanced stages of alteration and lithification and the presence of zeolites in voids (Garcia et al., 2006).
The composition of the glass sands was determined by microprobe. More than 800 samples were analyzed; they are all tholeiitic and typical of Hawaiian shield volcanoes (Fig. F23) (Garcia et al., 2006). MgO contents are distributed at 6–8 wt%, the common range for Hawaiian tholeiitic glasses (e.g., Clague et al., 1995; Davies et al., 2003). Three glasses in sand Layer 2 show unusually high MgO (up to 12.3 wt%), suggesting relatively primitive magma compositions and high quenching temperature (~1290°C). Sulfur contents are low (>0.03 wt%), suggesting subaereal eruption.