Megatsunami Deposits vs. High

Megatsunami Deposits vs. High-stand Deposits in Hawai‘i
NSF Tsunami Deposits Workshop, Seattle, WA
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]
Subsidence and uplift. As
the Big Island sinks, ‘Oahu
rises. But what about the islands in between? The lower figure shows the static
flexure computation of
Watts & ten Brink (1989).
The red line marks the zero
crossing, which runs along
the Ka‘iwi Channel between
Moloka‘i and ‘Oahu. Northwest of that line land is rising; to the southeast it is
sinking.
m
oint
aP
200
Ka
o
Mahukona
Harbor
Tsunami deposits in Kaluakapo Crater, Lana‘i.
Above: at an elevation of 180 m a deposit
chokes a gully. Note that it overlies a soil which
has been undercut. This deposit is also 120 ka.
N 20º10'
Top right: A distinctly different unit, with much
larger clasts, overlies the smaller-clast 120 ka
conglomerate. This large coral boulder is 80 ka.
Unit 2
Unit 3
1 km
W 155º54‘
B
Wawaionu
t
Tsunami deposits we have sampled
n
Mala'e Poi
200
200
he
olo
Ka
pi h
MK
Clark
Al
a2
ik
North
Kona
Hu
Hawai'i
ML
Alika 1
Ki
ina
Hil
South Kona
Puna
luu
19ºN
North Puna
Ridge
Depth (meters)
PC01
B13
Elevation
II
400
III
West of Lanai
1.91 mm/yr
800
IV
V
VI
1200
Megatsunami. Maximum wave heights if the Alika-2 landslide
were to occur today. The tsunami easily accomplishes the runups we infer from tsunami deposits (McMurtry, et al., 2004).
West of Hawaii
2.43 mm/yr
Loihi Slides
1600
West
Ka Lae
157ºW
East
Ka Lae
156ºW
155ºW
600
400
Time (kiloyears)
200
0
no
a
Fining
Tsunami Deposit
Random mix of materials of offshore
and onshore provenance
Randomly oriented
Both normal and reversed grading
present, never more than poorly developed
Continuous landward fining
I
oe
Hulopo‘e
Beach
ah
Lau
Ko
eh
paho
Hulopo'e
Bay
ok
Pololu
ae
ka
ka
Lana'i
y
Ba
Manele Bay
Kaluako'i
Point
0
Hana
Ridge
Manele
Beach
K al
18
130
a
0k
20ºN
Corals
Grading
Hana
Maui
r
Pu'upehe
0
1
km
156°53´
156°52´
245 ka
a
0k
a
340 k
43
Appearance
60
21ºN
apo Crate
100
156°54´
Subsidence. By comparing the depth and spacing of drowned reef terraces off
Lana‘i (right) and Kohala (below right) with global sea level curves, Cambell
(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 subsequently verified by sampling and dating (Ludwig, et al., 1991). The red line on the
Kohala map is the location of the coastline at the time of the Alika slide (location map, below left).
luak
100
20°44´
Uplift. Wai‘anae Coast of ‘Oahu. The bluff below the cars with
white (coral) boulders at its base is an upraised reef. A higher,
larger, older (but less photogenic) terrace exists off to the right.
Ka
20°45´
ul l y
gy G
The “Stearns Site” at Keawe‘ula Bay, Kohala, Big Island. The upper photograph, of a
supposed high stand, is from Stearns & Macdonald (1946). Inconsistency in geographic
names left the location uncertain. The lower image is from our visit in May 2002. The
approximate framing of the original photo is shown in yellow. The tsunami deposit,
which we call Unit 2, is about 0.5 m thick. It is laden with back-reef fossils (quite unlike the fauna od the present shoreline), and is about 120 ka. It overlies another carbonate conglomerate, Unit 3, which we have been unable to date. The maximum elevation
of Unit 2 is 61 m, about 750 m inland (map).
326m
0
Volcanics
a
Highstand deposits (Hulopo‘e Gravel)
as originally mapped by Stearns (1938)
Stearn's Mahana Stand
c
Below right: rip up clasts of an
oxisol incorporated into the unit.
Megatsunami deposits on Lana‘i
Keawanu
i Bay
Ar
Above right: a rare coral clast at
the Stearns Site.
Keawe'
ula B
ay
30
Our conclusions are tabulated lower right.
Bottom right: A clastic dike at 190 m elevation
laden with marine fossils.
a'a
Details of the tsunami deposit at
Kohala, Unit 2. Left: the deposit
overlying a truncated soil with
root casts. Because corraline
ssand in the tsunami deposit has
dissolved and reprecipitated, the
unit is well cemented. The softer underlying soil is preferentially eroded.
Middle right: Two tsunamis? Higher up the
same stream bed, Gary McMurtry stands on the
upper of two thin units (120 ka and 80 ka?).
Hawi
ics
can
Vol
N 20º08'
Comparing the megatsunami deposits with reefs and other shoreline
structures exposed by the uplift of ‘Oahu (lithospheric flexure lifts
‘Oahu at about 0.08 mm/yr), we can begin to work out the differences
upraised shorelines and tsunami deposits.
200
lu
Polo
Two hypotheses have been proposed to explain fossiliferous marine
conglomerates on the Hawaiian islands of Lana‘i and Moloka‘i. Either
they were left by prodigious tsunamis generated by giant submarine
landslides, or they are the result of prodigious uplift. Flexural models
require subsidence rather than uplift, but these models assume the
along-chain flexural properties are the same as the across-chain properties. Since that assumption may not be valid, the arguments have
raged. With the discovery of matching conglomerates on Kohala, the
northernmost volcano of the Island of Hawai‘i, it is now clear that the
conglomerates are indeed from tsunamis: Kohala is known to be subsiding at at least 2.5 mm/yr, which puts the 110 ka Kohala conglomerates at an elevation of at least 275 m at the time of their deposition.
The date of the Kohala deposit actually matches that of a 400-m-deep
terrace offshore, implying that the incoming tsunami reached runups
of 400 m.
y
Summary
Thickness
Soil
Fossils
Best preserved
Age relationships
among deposits
Smeared upslope like plaster, variable
elevation
0-2 meters
Often overlies friable soil
Need not match paleoenvironment
in valley bottoms
Younger deposits reach to higher
elevations because of progressive
subsidence
High Stand Deposit
Massive reef off paleo-shoreline, sand
or beach rock onshore; distinct facies
In growth position
None
Absent or discontinuous (coral clasts
on back reef, sand on beach)
Terraces of uniform elevation
0.5-10 meters
Never overlies friable soil
Must match paleoenvironment
on promontories and ridges
Because of progressive uplift, the
highest deposits are the oldest