Volcaniclastic sediments of ODP Leg 136 sites

Sediment cores recovered from three holes drilled during Ocean Drilling Program Leg 136 include volcaniclastics probably derived from the Hawaiian islands. The volcaniclastics shallower than 10 meters below seafloor are fresh and are composed of basaltic glass (sideromelane), basaltic fragments (mainly tachylite), plagioclase, olivine, pyroxene, and opaque minerals. Most of these glasses are probably products of hydrovolcanism. Visibly, some of these volcaniclastics are recognized as bedded ash layers having thicknesses that range from 5 to 10 cm. However, many volcaniclastics are disrupted by bioturbation to some degree, and are sometimes totally mixed with ambient brown clays. No visible correlative ash layer among these holes was found. It seems that many ash layers thinner than the bedded layers were disrupted by bioturbation because of the low sedimentation rate of volcaniclastics. The volcaniclastics were probably transported one of two ways: through air fall and pelagic settling or through turbidity-current transport. Other archipelagic apron volcaniclastic sediments of volcanic seamounts suggest that turbidite transport is the favored explanation of origin.

Geochemistry of Pliocene-Pleistocene volcanic sands of ODP Site 136-842

Several distinct, thin (2-7 cm), volcanic sand layers ("ashes") were recovered in the upper portions of Holes 842A and 842B. These holes were drilled 320 km west of the island of Hawaii on the outer side of the arch that surrounds the southern end of the Hawaiian chain. These layers are Pliocene to Pleistocene in age, graded, and contain fresh glass and mineral fragments (mainly olivine, plagioclase, and clinopyroxene) and tests of Pleistocene to Eocene radiolarians. The glass fragments are weakly vesicular and blocky to platy in shape. The glass and olivine fragments from individual layers have large ranges in composition (i.e, larger than expected for a single eruption). These features are inconsistent with an explosive eruption origin for the sands. The only other viable mechanism for transporting these sands hundreds of kilometers from their probable source, the Hawaiian Islands, is turbidity currents. These currents were probably related to several of the giant debris slides that were identified from Gloria sidescan images around the islands. These currents would have run over the ~500-m-high Hawaiian Arch on their way to Site 842. This indicates that the turbidity currents were at least 325 m thick. Paleomagnetic and biostratigraphic data allow the ages of the sands to be constrained and, thus, related to particular Hawaiian debris flows. These correlations were checked by comparing the compositions of the glasses from the sands with those of glasses and rocks from islands with debris flows directed toward Site 842. Good correlations were found for the 110-ka slide from Mauna Loa and the ~1.4-Ma slide from Lanai. The correlation with Kauai is poor, probably because the data base for that volcano is small. The low to moderate sulfur content of the sand glasses indicates that they were derived from moderately to strongly degassed lavas (shallow marine or subaerially erupted), which correlates well with the location of the landslide scars on the flanks of the Hawaiian volcanoes. The glass sands may have been formed by brecciation during the landslide events or spallation and granulation as lava erupted into shallow water.

(Table 1) Compressional- and share-wave velocieties at elevated confining pressure of ODP Holes 136-842C and 136-843A

Identification of a sediment/basement contact using seismic reflection recordings has proven to be extremely difficult in wide areas of the North Pacific Ocean owing to the presence of massive, highly reflective chert layers within the sediment column. Leg 136 of the Ocean Drilling Program recovered coherent pieces of chert of sufficient size for the first comprehensive laboratory measurements of the seismic properties of this material. Compressional-wave velocities of six samples at 40-MPa confining pressure averaged 5.33 km/s, whereas shear-wave velocities at the same pressure averaged 3.48 km/s. Velocities were independent of porosity, which ranged from 5% to 13%, suggesting that pores within the samples were mostly high aspect ratio vugs as opposed to low aspect ratio cracks. Back-scattered electron images made with a scanning electron microscope confirmed this observation. Acoustic impedances were calculated for the chert samples and from shipboard measurements of the red clay sediment overlying the chert layers. An extremely large compressional-wave reflection coefficient (0.73) characterized the interface between the two lithologies. A synthetic seismogram was calculated using chert and typical pelagic carbonate properties to illustrate the influence of chert layers on a marine seismic-reflection section. Compressional-wave to shear-wave velocity ratios of the chert samples (Vp/Vs =1.53) are close to that of single-crystal quartz in spite of variable porosity. Shear-wave reflection coefficients are estimated to be approximately 0.94. A compressional-wave reflection coefficient for a basement/sediment (carbonate) interface is estimated to be approximately 0.50, significantly less than that of sediment/chert.