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Megatsunami Deposits Vs. High-Stand Deposits in Hawai'i

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Megatsunami Deposits Vs. High-Stand Deposits in Hawai'i NSF Tsunami Deposits Workshop, Seattle, WA Megatsunami Deposits vs. High-stand Deposits in Hawai‘i June 12–15, 2005 GERARD J. FRYER and GARY M. McMURTRY School of Ocean & Earth Science & Technology, University of Hawai‘i at Manoa, Honolulu, HI 96822, USA. email: [email protected] Summary Subsidence and uplift. As Two hypotheses have been proposed to explain fossiliferous marine the Big Island sinks, ‘Oahu conglomerates on the Hawaiian islands of Lana‘i and Moloka‘i. Either rises. But what about the is- lands in between? The low- they were left by prodigious tsunamis generated by giant submarine Mahukona landslides, or they are the result of prodigious uplift. Flexural models er figure shows the static Harbor t 2 in 0 Po 0 flexure computation of a require subsidence rather than uplift, but these models assume the m o a along-chain flexural properties are the same as the across-chain prop- Watts & ten Brink (1989). K Tsunami deposits in Kaluakapo Crater, Lana‘i. erties. Since that assumption may not be valid, the arguments have The red line marks the zero Above: at an elevation of 180 m a deposit raged. With the discovery of matching conglomerates on Kohala, the crossing, which runs along N 20º10' chokes a gully. Note that it overlies a soil which northernmost volcano of the Island of Hawai‘i, it is now clear that the the Ka‘iwi Channel between has been undercut. This deposit is also 120 ka. conglomerates are indeed from tsunamis: Kohala is known to be sub- Moloka‘i and ‘Oahu. North- Top right: A distinctly different unit, with much siding at at least 2.5 mm/yr, which puts the 110 ka Kohala conglomer- west of that line land is ris- larger clasts, overlies the smaller-clast 120 ka ates at an elevation of at least 275 m at the time of their deposition. ing; to the southeast it is conglomerate. This large coral boulder is 80 ka. sinking. P The date of the Kohala deposit actually matches that of a 400-m-deep o 2 0 l 0 Middle right: Two tsunamis? Higher up the o terrace offshore, implying that the incoming tsunami reached runups l u same stream bed, Gary McMurtry stands on the s of 400 m. c V i o n upper of two thin units (120 ka and 80 ka?). l N 20º08' a c c a Comparing the megatsunami deposits with reefs and other shoreline l n o Bottom right: A clastic dike at 190 m elevation i V structures exposed by the uplift of ‘Oahu (lithospheric flexure lifts c s laden with marine fossils. ‘Oahu at about 0.08 mm/yr), we can begin to work out the differences i K eawe w 'ula B ay a upraised shorelines and tsunami deposits. Details of the tsunami deposit at H Kea Megatsunami deposits on Lana‘i wanui 326m Kohala, Unit 2. Left: the deposit Bay Our conclusions are tabulated lower right. Highstand deposits (Hulopo‘e Gravel) 300 overlying a truncated soil with Unit 2 as originally mapped by Stearns (1938) y Unit 3 Ba Wawaionu Stearn's Mahana Stand root casts. Because corraline 1 km nt Tsunami deposits we have sampled Mala'e Poi ssand in the tsunami deposit has W 155º54‘ 200 200 Ka luak ter dissolved and reprecipitated, the 20°45´ apo Cra unit is well cemented. The soft- The “Stearns Site” at Keawe‘ula Bay, Kohala, Big Island. The upper photograph, of a 100 er underlying soil is preferen- supposed high stand, is from Stearns & Macdonald (1946). Inconsistency in geographic tially eroded. names left the location uncertain. The lower image is from our visit in May 2002. The 100 A approximate framing of the original photo is shown in yellow. The tsunami deposit, rc he olo Above right: a rare coral clast at g Manele y which we call Unit 2, is about 0.5 m thick. It is laden with back-reef fossils (quite un- G Beach u l the Stearns Site. l y like the fauna od the present shoreline), and is about 120 ka. It overlies another carbo- K Manele Bay a la e Below right: rip up clasts of an nate conglomerate, Unit 3, which we have been unable to date. The maximum elevation o k y a a Hulopo‘e h oxisol incorporated into the unit. of Unit 2 is 61 m, about 750 m inland (map). B a 'a Hulopo'e Beach n a o ih p Bay a K Kaluako'i Pu'upehe 0 1 20°44´ Point km 156°54´ 156°53´ 156°52´ 4 3 2 3 4 0 4 0 5 k k a k a Uplift. Wai‘anae Coast of ‘Oahu. The bluff below the cars with a white (coral) boulders at its base is an upraised reef. A higher, larger, older (but less photogenic) terrace exists off to the right. Tsunami Deposit High Stand Deposit 1 8 Appearance Random mix of materials of offshore Massive reef off paleo-shoreline, sand 1 k 3 a 0 k Subsidence. By comparing the depth and spacing of drowned reef terraces off a and onshore provenance or beach rock onshore; distinct facies 6 0 0 k Lana‘i (right) and Kohala (below right) with global sea level curves, Cambell a Corals Randomly oriented In growth position (1987) worked out average subsidence rates of 1.9 mm/yr for Lana‘i and 2.4 mm/yr for Kohala (below center). His Kohala computation was subsequent- Grading Both normal and reversed grading None ly verified by sampling and dating (Ludwig, et al., 1991). The red line on the present, never more than poorly de- Kohala map is the location of the coastline at the time of the Alika slide (loca- veloped tion map, below left). 21ºN Hana Fining Continuous landward fining Absent or discontinuous (coral clasts Maui 0 on back reef, sand on beach) Hana I k Lana'i r Ridge a l C Pololu Elevation Smeared upslope like plaster, variable Terraces of uniform elevation oe II oeh Laupah 400 Ko III elevation 20ºN PC01 MK North Puna ) B13 s r IV Ridge e t Thickness 0-2 meters 0.5-10 meters A West of Lanai li e k Hu m 1.91 mm/yr ( 800 a North 2 Hawai'i h t Kona p V ML e Soil Often overlies friable soil Never overlies friable soil D Megatsunami. Maximum wave heights if the Alika-2 landslide Ki Alika 1 West of Hawaii a VI were to occur today. The tsunami easily accomplishes the run- ilin 2.43 mm/yr Fossils Need not match paleoenvironment Must match paleoenvironment H 1200 u lu ups we infer from tsunami deposits (McMurtry, et al., 2004). 19ºN a n South Kona u P Best preserved in valley bottoms on promontories and ridges Loihi Slides 1600 600 400 200 0 Age relationships Younger deposits reach to higher Because of progressive uplift, the Time (kiloyears) West East Ka Lae Ka Lae among deposits elevations because of progressive highest deposits are the oldest 157ºW 156ºW 155ºW subsidence.
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