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Hole 593 includes a spectacular 16.4-m-sequence of thinly bedded, grayish black, lithified volcanogenic sandstones, mudstones, and occasional breccias near its base within chalks. Sandstone-mudstone couplets (up to 50 cm thick) have sharp basal contacts, are normally graded, and include Bouma A, B, occasional C, and E divisions, whereas brecciadominated units are thicker (up to 140 cm), massive or inversely graded, and unsorted. The beds were deposited rapidly at bathyal depths, close to the Eocene/Oligocene boundary, from turbidity currents and/or subaqueous pyroclastic flows originating on the slopes of "Lalitha pinnacle," a now-buried ~ 500-m-high volcanic shield (average 6-8° slopes) located only ~3 km from Site 593. The occurrence in the deposits of rare primary augite and olivine crystals, the abundance of the low Si/Al zeolite phillipsite, and certain chemical considerations support a basaltic parentage. However, as a result of low-temperature oxidative alteration by pore fluids following emplacement, the original basaltic glass fragments that formed the bulk of the deposits have been palagonitized extensively and replaced by phillipsite, smectite, and calcite. "Lalitha pinnacle" is similar to several other (buried) "pinnacles" evident on seismic records from southern Lord Howe Rise, both in its seismic character and structural and age relations to surrounding sediments. This suggests that submarine basaltic volcanism and associated sedimentary redeposition was widespread in the southeast Tasman Sea during the late Eocene to early Oligocene. The activity is viewed as off-rift (western) volcanism that is related to regional extension and the formation of a major western New Zealand rift system associated with eastward propagation of the mid-ocean Southeast Indian Ridge into the south Tasman Sea and through the New Zealand subcontinent at this time. Analogous occurrences of submarine Eocene-Oligocene basalts east of the rift system now crop out on land in eastern South Island, New Zealand.

Nature and Significance of Volcanogenic Deposits at the Eocene/Oligocene Boundary, Hole 593, Challenger Plateau, Tasman Sea