HR: 1330h
AN: T12B-0451 INVITED     [PDF]
TI: Petrogenesis of the Crystal Vitric Tuffs recovered 300 km Northeast of Oahu, from ODP Leg 200: Pyroclastic or Landslide Deposit?
AU: * Sherman, S B
EM: bean@higp.hawaii.edu
AF: SOEST, Dept. Geology & Geophysics, 1680 East West Rd, Honolulu, HI 96822 United States
AU: Garcia, M O
EM: garcia@soest.hawaii.edu
AF: SOEST, Dept. Geology & Geophysics, 1680 East West Rd, Honolulu, HI 96822 United States
AB: Hawaiian volcanoes frequently experience flank failure that generate landslides, debris flows, and turbidity currents. The largest landslide known to have occurred on the Hawaiian islands is the Nuuanu landslide. The Nuuanu landslide resulted from the collapse of Koolau volcano on the island of Oahu, Hawaii approximately 2 million years ago where 40% of the Koolau volcano was removed. An ancillary objective of ODP Leg 200 was to drill the distal facies of the deposits emplaced during the Nuuanu landslide. The actual date and the mechanics of this landslide had not been well constrained. It is not known whether the landslide occurred as a single catastrophic event or as a series of collapses. One of the justifications for drilling the Nuuanu landslide deposit was thus to better understand how this and other similar landslides occur. Because they are relatively common events on volcanic islands such an understanding is highly desirable. In addition, there are major potential hazards associated with landslides, e.g., tsunami generation and as in the case of Mt. St. Helens, explosive eruptions. The location of site 1223a is 300 km northeast of Oahu on the crest of the Hawaiian arch. The depth of the hole reaches 41 meters below the seafloor (mbsf) and 23.5 m of core was recovered (61% recovery). Three main types of lithologies were recovered: unconsolidated clays and volcaniclastic turbidites, consolidated volcaniclastic siltstones and claystones, and lithified crystal vitric tuffs. Only 12.7 mbsf was drilled before encountering an $\sim$ 3 m thick bed of lithified crystal vitric tuff. A second 4 m thick crystal vitric tuff was recovered at 33 mbsf. The main components of both of the crystal vitric tuffs, in order of abundance, are volcanic glass, mineral fragments (the majority are olivine and plagioclase with minor amounts of pyroxene) clay and zeolites, predominantly phillopsite. The volcaniclastic siltstones and claystones contain the same components. Chlorite is found in all lithologies in minor amounts. Both of the tuffs have an average particle size of 2 mm, the upper tuff is normally graded. The major element chemistry of individual glass fragments from the upper crystal vitric tuff is heterogeneous. All of the glass fragments are tholeiitic basalts and have sulfur abdunances below 0.04 wt% indicating they were erupted subaerially or under very shallow marine conditions. The majority of the samples have MgO concentrations between 6 and 8 wt%. The major element chemistry of the crystal vitric tuffs have affinities with Koolau and have SiO$_{2}$ abduances up to 55 wt%. The two crystal vitric tuffs may be related to the giant Nuuanu landslide deposit. However, it is not known if the tuffs are the result of a pyroclastic eruption or if they are simply a product of the landslide.
DE: 1020 Composition of the crust
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3665 Mineral occurrences and deposits
SC: Tectonophysics [T]
MN: 2003 Fall Meeting