HR: 0800h
AN: V51A-1475    [Abstracts]
TI: Rejuvenation Stage Volcanics at Laeo Kilauea, Kauai, Hawaii
AU: * Thordarson, T
EM: moinui@soest.hawaii.edu
AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States
AU: Garcia, M
EM: mogarcia@hawaii.edu
AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States
AU: Wanless, D
V51A-1475 AF: Department of Geology and Geophysics, SOEST, University of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States
AU: Tagami, T
EM: tagami@kueps.kyoto-u.ac.jp
AF: Div Earth Planet Sci, Kyoto Univ, Sakyo-ku, Kyoto, 606-8502 Japan
AU: Sano, H
EM: sanoubb@yahoo.co.jp
AF: Div Earth Planet Sci, Kyoto Univ, Sakyo-ku, Kyoto, 606-8502 Japan
AB: The Plio-Pleistocene Koloa volcanic series represents the rejuvenated volcanism on Kauai, one of the oldest main Hawaiian Islands. The Koloa series is made up of highly alkalic basalt and associated sedimentary rocks that rest unconformably on the shield-building Waimea Canyon volcanic series. Koloa vents are dispersed across the eastern two-thirds of the island and typically consist of scoria or lava cones that fed broad lava flow fields blanketing the marginal lowlands on the south, east and north side of the island. The northernmost subaerial Koloa vents are found at Laeo Kilauea on the north shore of the island. At Laeo Kilauea the volcanic succession is unusual in that it contains the only phreatomagmatic vent structures of the Koloa series. Here an ~2-km-long costal cliff face reveals a bedded phreatomagmatic tephra sequence that is >90-m-thick and represents the remnant of an a much large tuff cone (>2-km in diameter). The tuff cone sequence is characterized by decimeter to meters thick layers, where cross-bedded ash beds alternate with massive and poorly sorted lapilli tuff beds. The cross-bedded deposits were produced by dry and wet surges, whereas the poorly sorted beds represent fall deposits produced by sustained eruption column (i.e. continuous up-rush) or tephra jets (i.e. rooster-tail explosions). The juvenile clast population of the tephra consists of olivine-phyric foidite, but it also contains abundant wall-rock lithics, including fragments of reef-limestone. The base of the tuff cone outcrops at Mokolea point on the east side of the outcrop, where phreatomagmatic tephra rests directly on an older Koloa pahoehoe flow, a olivine- and mellelite-phyric foidite lava. The tephra sequence is cut by an ~1-m-thick olivine-bearing basanite dike, which acted as a feeder for the fountain-fed spatter and lava (up to 100-m-thick) that cap the phreatomagmatic tephra sequence. These units are separated by a 2-3 m thick soil horizon formed by weathering of the tuff. These three formations have been dated by Ar-Ar giving 2.65 +/- 0.35 Ma for the age of the basal foidite lava, 1.68 +/- 0.11 Ma for the tuff cone and 0.69 +/- 0.03 Ma for the overlying fountain-fed basanite lava. Important conclusions that can be drawn from the results of this study include: (1) The characteristics of the phreatomagmatic tephra indicate that at times the tuff cone crater was filled with water implying that the eruption site was submarine and most likely located in shallow coastal waters. The presence of reef-limestone fragments in the tephra supports this notion. On the other hand, the underlying and overlying lava flows, which do extend an unknown distance beyond the current shoreline, were clearly deposited on dry land. This implies that Kauai experienced significant changes in sea level in early to mid Pleistocene times. (2) The eruptions that produced the tuff cone and the overlying fountain-fed basanite lava are one million years apart, yet the dikes that fed these eruptions appear to have followed a similar path to the surface. This indicates that the magma is utilizing preexisting structural weaknesses to reach the surface.
DE: 8404 Volcanoclastic deposits
DE: 8425 Effusive volcanism
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005