Cordilleran Section - 113th Annual Meeting - 2017

Paper No. 22-7
Presentation Time: 11:05 AM


THORDARSON, Thorvaldur, Faculty of Earth Sciences, University of Iceland, Sæmundargötu 2, Reykjavik, 101, Iceland and GARCIA, Michael O., Department of Geology and Geophysics, University of Hawaiʻi at Mānoa, Honolulu, HI 96822,

The Plio-Pleistocene Koloa volcanic series on the islands of Kauai represents the most voluminous (58 km3) rejuvenation-stage volcanism and is made up of highly alkalic basalt and associated sedimentary rocks. At Laeo Kilauea the succession features the only phreatomagmatic vent structures of the Koloa series. Here an ~ 2-km-long costal outcrop reveals a >90-m-thick, bedded phreatomagmatic tephra sequence; a remnant of a much large tuff cone (>2-km in diameter). This sequence is characterized by decimeter to meters thick layers, where massive to diffuse-bedded lapilli tuff beds are commonly punctuated by cross-bedded ash beds. The cross-bedded deposits were produced by dry and wet surges, whereas the lapilli beds represent fall deposits. The tephra is olivine-phyric foidite and also contains abundant wall-rock lithics, including fragments of reef-limestone. The base of the tuff cone outcrops at Mokolea point, where phreatomagmatic tephra rests directly on an older Koloa lava; an olivine- and mellelite-phyric foidite pahoehoe. The tuff cone is cut by an ~1-m-thick basanite dike, which acted as a feeder for the fountain-fed basanite spatter and lava (up to 100-m-thick) that rests on top of the sequence. The tuff cone and the basanite lava are separated by a 2-3 m of lateritic soil formed by chemical weathering of the tuff. The Ar-Ar ages for these three Koloa formations are 2.65±0.35 Ma for the basal foidite lava, 1.68±0.11 Ma for the tuff cone and 0.69±0.03 Ma for the fountain-fed basanite lava (1).

Important deductions drawn from this study include: (1) The facies associations in the tuff cone indicate that at times the crater was filled with water, implying that the eruption site was submarine and the presence of reef-limestone fragments in the tephra implies shallow coastal waters. The underlying and overlying lava flows, on the other hand, imply subaerial emplacement. This suggests that Kauai experienced significant changes in relative sea level in early- to mid-Pleistocene times. (2) The tuff cone event and the following basanite eruptions took place one million years apart. Yet the conduits that fed these eruptions followed similar paths to the surface suggesting that the magma is utilizing preexisting structural weakness to reach the surface.

(1) Garcia et al 2010. J. Pet. 51, 7: 1507-40.