Dynamic Submarine Flanks of Hualalai Volcano, Hawaii

Hammer, JE

Shamberger, PJ

Four remote and manned submersible dives examined the Hualalai midslope bench scarps, NW rift zone, and an elongate ridge cresting at 3900 mbsl during 2001 and 2002 JAMSTEC cruises. Here we report the results of stratigraphic, petrographic, and geochemical studies of the latter feature (dive S692). The ridge is 2x8 km, 300-700 m above datum, oriented parallel to the midslope bench, and 1 km east of the Mauna Loa Alika 2 landslide chute and levee deposit. Although the vast majority of the ridge is sediment covered, dive videos and sampling of the steep seaward side of the ridge revealed the following in-place lithologies, from base to crest: (1) bedded, landward dipping glass sandstones consisting of arcuate, angular to subangular glasses, moderately vesicular, tholeiitic and fairly uniform in composition, dominantly low in S, and intensely chemically altered toward the base, (2) olivine basalt breccia with fresh tholeiite glassy matrix, including S-rich grains, (3) dense olivine basalt lava blocks, (4) coarsely crystalline, vesicular, and oxidized lava, and (5) a capping unit of layered, volcaniclastic siltstone beds rich in radiolarians. Finally, an apron of talus and a superficial coating of muddy clastic materials drape the base of the ridge. Samples of this material are compositionally distinct from the in-place samples: they include transitional basalt and S-rich hawaiite.

Key inferences about the ridge deposits are: (1) glass sands were produced as shield stage Hualalai lava erupted subaerially or in shallow water, (2) sands were cemented and overlain by breccia and lava blocks following minimal transport as grain flows to a depth that allowed incorporation of high-S grains; the entire sequence was transported to

its current deep water location as a coherent package, (3) the zone of intense hydrothermal alteration and mineralization at the base is consistent with fluid flow in a region of distributed strain, possibly associated with gravitational spreading of Hualalai volcano, (4) the alkalic and transitional materials may represent pre-shield Hualalai volcanism. Alternatively, they could represent pre-shield Mauna Loa lavas excavated and transported to their present location by the Alika 2 landslide, which truncated the package, exposed the observed outcrop, and capped the sequence with fossiliferous glassy silt.