VOLCANISM IN HAWAII Chapter 11 STRATIGRAPHY OF KILAUEA VOLCANO By R. Michael Easton1 ABSTRACT Lockwood, and G.A. Macdonald. As a historical note, it was This paper reviews and revises the lithostratigraphic G. A. Macdonalds intention to revise the stratigraphy of the Island nomenclature of Kilauea Volcano. The major stratigraphic units of Hawaii and Kilauea Volcano in order to make the nomenclature of the volcano are redefined as the Hilina Basalt (oldest), Pahala consistent with the recommendations of the stratigraphic code, but Ash, and Puna Basalt (youngest) in accordance with the strat- igraphic code. Four pyroclastic units within the Hilina Basalt this work was cut short by his untimely death. This paper was can be used as stratigraphic markers; these are formally pro- reviewed by R.L. Christiansen and R.T. Holcomb, and I thank posed and described here and given member rank. These new them for their careful reading and helpful comments and suggestions. stratigraphic units are, from oldest to youngest, the Halape, Kahele, Pohakaa and Moo Ash Members. The Pohakaa has a similar distribution and lithology to the Pahala Ash and repre- PREVIOUS WORK sents a major period of ash deposition on Kilauea at about 40—50 ka. Two pyroclastic units intercalated within the Puna The various lithostratigraphic schemes that have been applied Basalt are the circa 1.5-ka Uwekahuna Ash Member and the to Kilauea Volcano and correlations between them are summarized mostly A.D. 1790 Keanakakoi Ash Member. The origin and in figure 11.1. An essential element of all these schemes is the use of source of these pyroclastic units, particularly the Pahala Ash the Pahala Ash as a marker horizon to divide the lava of Kilauea and the Pohakaa Ash Member, is briefly discussed, in light of new rare-earth-element and petrographic data from these units. Volcano into pre-Pahala and post-Pahala units. This usage arose because the Pahala Ash was considered to be the only stratigraphic unit found on several (four of the five) volcanoes on the Island of INTRODUCTION Hawaii (see Stearns and Macdonald, 1946). It has therefore been A stratigraphic unit, whether it be a group, formation, mem- used as an important stratigraphic marker. As discussed in detail ber, or flow, becomes established through the repeated demonstration later, the Pahala Ash does not always meet the criteria required of a of its utility. Since the Hawaiian Volcano Observatory was estab- marker horizon, and this can present local correlation problems. lished 75 years ago, a number of stratigraphic terms have been Stone (1926) divided Kilauea rocks into the pre-Kilauea introduced to help geologists unravel the stratigraphy of Kilauea Series, comprising all rocks below and including the unit he called Volcano. Some of these terms have become well established through the Pahala Ash, and the Kilauea Series, comprising prehistoric and use; others, although widely used, do not meet all of the criteria historical lavas younger than the Pahala. L.F. Noble and W.O. required of formal stratigraphic units as outlined in Article 3 of the Clark (unpub. data, 1920, in Washington, 1923) made a similar North American stratigraphic code (North American Commission division, but used the terms Pahala Series and post-Pahala Series; on Stratigraphic Nomenclature, 1983). this division was later revised in Stearns and Clark (1930). In both The purpose of this paper is to review the stratigraphic these works, the lavas of Kilauea and Mauna Loa were not nomenclature of Kilauea Volcano and to revise it so as to be differentiated stratigraphically. Stearns and Clark (1930) included consistent with the stratigraphic code and with current needs. Some lava and tuff of both volcanoes in the Pahala Basalt (which included new stratigraphic terms are herein formally proposed so that they will the upper ash member now called the Pahala Ash) and the overlying be available to the geologic community and their utility can be Kamehame Basalt. The Kamehame Basalt was divided into a tested. Stratigraphy is an important tool in aiding our understanding lower prehistoric part, which included all ash units above the Pahala of Kilauea Volcano, and I hope that this revision of stratigraphic Basalt, and an upper part composed essentially of historic flows (fig. nomenclature will make this tool even more effective. 11.1). Wentworth (1938), in his examination of Kilauea pyroclastic rocks, introduced the term Pahala Tuff for what has generally been ACKNOWLEDGMENTS called the Pahala Ash by workers both before and since. He also R.T. Holcomb was in large part responsible for this paper introduced the terms Uwekahuna Formation and Keanakakoi For- through his encouragement, and this paper has benefited from mation for ash units of Kilauea stratigraphically above his Pahala discussion over the years with M.O. Garcia, R.T. Holcomb, J.P. Tuff. 'Ontario Geological Survey, Toronto, Ontario, Canada M53 1B3. 243 U.S. Geological Survey Professional Paper 1350 U1 QUATERNARY O c 0 ~n f1 I CD" > w w ^ O [JT -t O R o 03 QS CU CD 3 7 3 CD CO H m 3- c5_ w' j 0? 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NJ 3 ^ « <c" 9L CO 03 3- CO (O > 5' 3 Ir- oT D ^ a, IIVAXVH NI IAISINVD1OA 1. STRATIGRAPHY OF KILAUEA VOLCANO 245 Modern usage began when Stearns and Macdonald (1946) SOUTH NORTH AND EAST FLANK divided the rocks below the ash member of the Pahala Basalt FLANKS (Stearns and Clark, 1930) into the Kahuku Volcanic Series for lava Keanakakoi Ash Member Uwekahuna Ash Member of Mauna Loa and the Hllina Volcanic Series for lava of Kllauea ^Puna Basalt 50 m (fig. 11.1). In addition, they divided the rocks called the Kamehame -10 ka **&/<&" Ss;*'* Basalt by Stearns and Clark (1930), which overlie the Pahala on -25 ka ^'€li Mauna Loa and Kllauea and include the historical lava of the two Moo Ash Member volcanoes, into the Kau Volcanic Series on Mauna Loa and the Puna Volcanic Series on Kilauea (fig. 11.1), thus abandoning the term Kamehume Basalt. They included the Uwekahuna in the Puna //////////// and called it Uwekahuna Tuff, but treated rocks called the Kea- nakakoi Formation by Wentworth (1938) as an informal member. The Pahala Ash was used to separate these stratigraphic units. Macdonald (1949, p. 65, 67) also included the Uwekahuna in the Puna Volcanic Series but called it the Uwekahuna Ash. Davis and Macdonald (in Avias and others, 1956) included Wentworth's — Pohakaa Ash Member Keanakakoi Formation in the Puna.