GEOLOGY
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Volume 43
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Number 8
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www.gsapubs.org 675
Accommodation space, relative sea level, and the archiving of
paleo-earthquakes along subduction zones
Harvey M. Kelsey
1
, Simon E. Engelhart
2
, Jessica E. Pilarczyk
3,4
, Benjamin P. Horton
3,4
, Charles M. Rubin
4
,
Mudrik R. Daryono
5
, Nazli Ismail
6
, Andrea D. Hawkes
7
, Christopher E. Bernhardt
8
, and Niamh Cahill
9
1
Department of Geology, Humboldt State University, Arcata, California 95524, USA
2
Department of Geosciences, University of Rhode Island, Kingston, Rhode Island 02881, USA
3
Department of Marine and Coastal Science, Rutgers University, New Brunswick, New Jersey 08901, USA
4
Earth Observatory of Singapore, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore
5
Research Center for Geotechnology, Indonesian Institute of Sciences, Bandung 40132, Indonesia
6
Department of Physics, Syiah Kuala University, Aceh, Sumatra 23111, Indonesia
7
Department of Geography and Geology, University of North Carolina, Wilmington, North Carolina 28403, USA
8
U.S. Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA
9
School of Mathematical Science, University College Dublin, Dublin 4, Ireland
ABSTRACT
The spatial variability of Holocene relative sea-level (RSL) change influences the capacities
of coastal environments to accommodate a sedimentary record of paleoenvironmental change.
In this study we couch a specific investigation in more general terms in order to demonstrate
the applicability of the relative sea-level history approach to paleoseismic investigations. Using
subsidence stratigraphy, we trace the different modes of coastal sedimentation over the course
of time in the eastern Indian Ocean where RSL change evolved from rapidly rising to static
from 8000 yr ago to present. Initially, the coastal sites from the Aceh, Sumatra, coastal plain,
which are subject to repeated great earthquakes and tsunamis, built up a sedimentary se-
quence in response to a RSL rise of 1.4 mm/yr. The sequence found at 2 sites 8 km apart con-
tained 3 soils of a mangrove origin (Rhizophora, Bruguiera/Ceriops, Avicennia pollen, and/or
intertidal foraminifera) buried by sudden submergence related to coseismic subsidence and 6
tsunami sands that contain pristine subtidal and planktic foraminifera. After 3800 cal yr B.P.
(years before A.D. 1950), sea level stabilized and remained such to the present. The stable rela-
tive sea level reduced accommodation space in the late Holocene, suggesting that the continued
aggradation of the coastal plain was a consequence of periodic coastal inundation by tsunamis.
INTRODUCTION
The ability of a coastal environment to re-
cord successive changes in paleoenvironment
associated with earthquakes and tsunamis re-
quires stratigraphic accommodation space.
Coastal sediment accumulates where accom-
modation space is created by relative sea-level
(RSL) rise. In the Holocene, RSL change is the
result of eustatic, static equilibrium, isostatic,
local, and tectonic processes. The relative im-
portance of these factors varies in space and
time (Clark et al., 1978). The end of significant
eustatic input from Laurentide Ice Sheet melt-
ing 7000 yr ago slowed rates of RSL rise from
~15 mm/yr between ca. 11,400 and 8200 cal yr
B.P. to ~1 mm/yr or less for the remainder of
the Holocene (Lambeck et al., 2014). RSL fell
in near-field areas that were covered by major
ice sheets because of glacio-isostatic rebound
(e.g., Long et al., 2006), while RSL in interme-
diate-field regions reflects the balance among
postglacial isostatic recovery, proglacial fore-
bulge collapse, and hydro-isostatic loading
(Shennan and Horton, 2002). Equatorial and
Southern Hemisphere RSL reconstructions
recorded a mid-Holocene highstand of a few
decimeters to several meters (e.g., Rostami et
al., 2000), but the presence or absence of such
a highstand may be controlled by local tectonic
processes (Briggs et al., 2008).
Coastlines that have submerged during
the Holocene (e.g., Cascadia) are excellent
recorders of paleo-earthquakes and tsunamis
(e.g., Witter et al., 2003) because of the sub-
siding coastline that provides the necessary
accommodation space. In contrast, net emer-
gent coastlines (e.g., Chile) do preserve RSL
changes representative of the earthquake defor-
mation cycle and tsunamis (e.g., Cisternas et
al., 2005; Dura et al., 2015), but the lack of ac-
commodation space makes preservation more
difficult. Records of localized coseismic sub-
sidence and accompanying tsunamis on emer-
gent coastlines are scarce, and where discov-
ered, fragmentary. Here we use a paleoseismic
study from Aceh, Sumatra, a region that was
devastated by the A.D. 2004 Aceh-Andaman
earthquake and tsunami (M
w
~9.2) (Subarya et
al., 2006) (Fig. 1A), as an example to show the
singular role that accommodation space plays
in recording coastal environmental changes
with time. And we show that the lack of ac-
commodation space in a particular setting can
lead to alternative strategies for determining
paleoenvironmental change on tectonically ac-
tive coasts. Deliberate consideration of the role
that accommodation space plays in the preser-
vation of sediments will both better focus re-
search efforts and optimize research outcomes.
GEOLOGY, August 2015; v. 43; no. 8; p. 675–678
|
Data Repository item 2015237
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doi:10.1130/G36706.1
|
Published online 23 June 2015
© 2015 Geological Society of America. For permission to copy, contact editing@geosociety.org.
1000 m
0
0
200 m
50 m
0
4
6
16
11
12
13
Forest
e
d uplan
d
Sunda Megathrust
Sumatra
2004
2005 M
92
o
E
0
o
N
6
o
N
12
o
N
Indian Ocean
Indian
Ocean
Ma
Th
Th
Bu
Banda
Aceh
5
o
30’N
5
o
15’N
95
o
15’E
Seungko
Meulat
1D
1C
Pulot
5 km
96
o
E
North
1B
Upland
50 m
0
3P
4/17P
13P
15P
12P
16P
14P
Coastal
lowland
North
11P
5
o
21.8’N
95
o
15.1’E
95
o
14.1’E
5
o
17.6’N
Indian
Ocean
Coastal
lowland
Indian
Ocean
Indian
Ocean
w
9.2
M
w
8.6
D
C
B
A
D
FE
C
Figure 1. A: Location map of Sumatra, Indo-
nesia, depicting rupture areas for the A.D.
2004 and 2005 subduction zone earthquakes
(Subarya et al., 2006; Briggs et al., 2008).
Bu—Burma; Th—Thailand; Ma—Malaysia;
M
w
—moment magnitude. B: Sites of pa-
leoseismic investigations on coastal reach
south of Banda Aceh. C: Pulot site, Febru-
ary 2005 (Google Earth
®
). D: Seungko Meulat
site, March 2005 (Google Earth
®
). Both C and
D images show area denuded by December
2004 tsunami. E: Core site locations (black
dots) at Pulot site. F: Core site locations
(black dots) at Seungko Meulat site.
676 www.gsapubs.org
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GEOLOGY
ACEH COASTAL PLAIN
STRATIGRAPHY
Based on reconnaissance coring at 17 sites
along the northern Aceh coast (Fig. 1B; Fig.
DR1 in the GSA Data Repository
1
), we selected
2 sites 8 km apart (Fig. 1B; Fig. DR1) where
site stratigraphy defines an early to mid-Holo-
cene record of paleo-earthquakes and tsunamis
(Fig. 2). Stratigraphy is consistent among all
cores at both sites. The lower part of the strati-
graphic section is dominated by three buried
soils abruptly overlain by clastic units. The three
buried soils are organic silts with variable but
minor concentrations of fine sand and/or clay.
Preserved pollen in the lower and middle buried
soil at the Pulot site demonstrates a mangrove
environment (e.g., Rhizophora, Bruguiera/
Ceriops, Avicennia pollen) (Table DR1) and
supports an upper tidal environment for soil
deposition (Engelhart et al., 2007). The upper
buried soil was barren of pollen. We sampled
detrital wood fragments for
14
C age determi-
nations from the top of the each soil (Table 1;
Tables DR2 and DR3). The youngest fragment
represents the closest time to the burial of the
soil by overlying inorganic deposits (Cisternas
et al., 2005; Grand Pre et al., 2012). All dates
were calibrated (cal yr B.P.) to a 2s (95% prob-
ability) error range, where zero age is A.D. 1950
(Reimer et al., 2013). The three soils were bur-
ied at ~3800 cal yr B.P., ~5800 cal yr B.P., and
~7000 cal yr B.P. (Fig. 2).
Sand units occur immediately above the
three buried soils, as well as in two layers be-
tween the middle and upper buried soil. How-
ever, in some cases bioturbated muddy sand,
rather than well sorted sand, occurs above buried
soils. We infer the five sand units (Ts1, Ts2, Ts3,
Ts4, and Ts6; Fig. 2; Fig. DR4) to be of local
tsunami origin based upon the similarity of fo-
raminiferal assemblages and grain size with the
2004 tsunami sand (Figs. 2 and 3; Tables DR4
and DR5). Storms are an unlikely source for the
well-sorted sand because the west coast of Aceh
is south of the latitude of cyclonic storms and
resulting storm surges. All five inferred paleo-
tsunami sands contain relatively high abun-
dances of subtidal species (72% ± 9%) such as
Epinoides repandus and Amphistegina spp., and
planktonic species. Two additional sand layers
(Ts5 and Ts7, Fig. 2; Fig. DR5) may also be of
tsunami origin based on their stratigraphic con-
text of well-sorted sand within mud.
The upper part of the stratigraphic section
consists of bioturbated, massive sandy mud to
muddy sand, and lacks buried soils (Fig. 2).
Muddy sand is more common at the Pulot site,
which is proximal to the Indian Ocean, whereas
sandy mud can occur at Seungko Meulat
(Fig. 3), a valley margin protected from open-
ocean exposure (Figs. 1C and 1D). The upper
part of the stratigraphic section is similar to
the bioturbated massive sandy unit that some-
Mangrove soil
Muddy sand to sandy mud, 35% < sand < 90%
Mud to sandy mud, ≤ 35% sand
Mollusks, primarily
Cerithidea cingulata
Detrital wood fragments
6.9, Age, cal yr B.P. x 10
3
2004 tsunami sand
Pre-2004-earthquake soil
Ts1 Tsunami sand
Sand, 90-100 %
Bedrock
Ts6
Ts5
Ts2 Tsunami sand
Ts3 Tsunami sand
Ts4 Tsunami sand
Ts7
Tsunami
sand
Inferred
tsunami
sand
Inferred
tsunami
sand
15P
14P
16P
12P
4/17P
3P
4
6
11
12
13
16
Pulot
Seungko Meulat
Elevation (meters) relative to mean tidal level (MTL)
Upper
buried soil
3800 yr B.P.
Middle
buried soil
5800 yr B.P.
Lower
buried soil
7000 yr B.P.
Deposition
accommodated
by RSL rise
Deposition not
accommodated
by RSL rise
Stacked tsunami deposits;
disturbed by
post-depositional
reworking
and bioturbation
Interbedded mangrove
soil, mud, sandy
mud and
tsunami sand
4.3
6.9
6.9
5.8
5.8
6.5
6.5
4.8
5.3
3.8
4.0
7.5
7.9
7.4
7.4
7.0
5.4
7.0
7.0
Stratal zone that separates
deposition in response to
rising RSL (below) from
deposition in the context
of no rise in RSL (above)
Massive
muddy sand
to sandy mud
1.0 m
0.0 m
-1.0 m
-2.0 m
-3.0 m
-4.0 m
-5.0 m
Figure 2. Correlated stratigraphy for the Seungko Meulat and Pulot sites, Aceh Province, Sumatra, Indonesia. RSL—relative sea level; Ts—
tsunami sand. Core locations are shown in Figures 1E and 1F.
1
GSA Data Repository item 2015237, Tables
DR1–DR5 and Figures DR1–DR6, is available online
at www.geosociety.org/pubs/ft2015.htm, or on request
from editing@geosociety.org or Documents Secre-
tary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
TABLE 1. ELEVATION AND AGE OF BURIED
MANGROVE SOILS, ACEH, SUMATRA
Core site* Elevation
(m)
†
Sample age
§
Upper buried mangrove soil
SM 11 13A–0.44 3717–3902
SM 11 13B–0.48 3844–4080
SM 11 04 –0.464248–4421
PU 11 16 –0.754654–4960
PU 11 16 –0.865066–5446
PU 11 15 –0.995301–5463
Middle buried mangrove soil
PU 07 04 –1.355743–5917
PU 11 15 –1.695745–5904
PU 11 14 –1.846402–6537
PU 11 16 –2.076409–6626
SM 11 04 –1.596748–6958
SM 11 04 –1.446789–6951
Lower buried mangrove soil
PU 07 03 –2.966791–7142
PU 11 16 –2.706912–7160
PU 07 03 –3.076949–7166
PU 11 11 –3.326966–7167
PU 11 19 –2.336965–7173
SM 11 16 –3.447320–7482
SM 11 16 –2.977324–7475
SM 11 13 –3.527424–7558
PF 07 12 –4.877474–7656
SM 11 13 –4.587837–7965
*PU—Pulot; SM—Seungko Meulat; PF—Seudu.
All core logs except PU 11, PU 13, PU 19, and PF
12 are depicted in Figure 2.
†
Elevation relative to mean tide level. Estab-
lished by survey over ≥2 tidal cycles.
§
Calibrated radiocarbon age at 2σ range
(Calib 7.0; Reimer et al., 2013), in cal yr B.P. (yr be-
fore A.D. 1950).
GEOLOGY
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times is present above buried soils. The biotur-
bated massive sandy unit is distinctive from the
standpoint of both foraminiferal taxonomy and
taphonomy (i.e., test condition) (Fig. 3). The
unit consists of an intertidal assemblage (72%
of species were intertidal) that showed evidence
of subaerial exposure through a high degree of
abrasion (65% of individuals), which is charac-
teristic of beach and dune deposits (e.g., Berke-
ley et al., 2009; Pilarczyk et al., 2012). But the
unit also has a relatively high abundance (25%)
of deeper dwelling subtidal (e.g., Operculina
ammonoides) and planktic species that are not
consistent with beach and/or dune as the sole
source. Rather, we infer that the original source
area of the sand is wholly or in part offshore.
Given that the Aceh coast is immediately on-
shore of the Sunda megathrust, the most likely
transport mechanism bringing subtidal foramin-
ifera–bearing sand from offshore is a tsunami.
For example, the 2004 tsunami sand, which is
the uppermost stratigraphic unit, is a 15–55-cm-
thick medium-fine sand dominated by subtidal
(75%) foraminifera (Figs. 2 and 3).
RELATIVE SEA LEVELS OF ACEH,
SUMATRA
We reconstruct a 8000 yr RSL record for
the coast of Aceh (Fig. 4) using sea-level index
points (n = 12) and limiting data (n = 11) from
3 buried soils (Fig. 4; Table 1; Tables DR2 and
DR3). In employing index points, we assume
that coseismic subsidence is balanced out by
interseismic uplift. Where foraminifera were
present within the organic soils or pollen dem-
onstrated a mangrove source, the sample was
classified an index point and assigned a refer-
ence water level (RWL
i
), which is a level half-
way between highest astronomical tide (HAT)
and mean tide level (MTL) (Horton et al., 2005;
Engelhart et al., 2007). Where organic content
supported formation above MTL but foramin-
ifera were absent and/or pollen did not provide
unequivocal support for a mangrove origin, we
classify the sample as an upper limiting data
point (Fig. 4; Table DR3) with a reference water
level of MTL, indicating that the sample must
have formed above this elevation. The age of
a gastropod (Cerithidia cingulata) in one core
(Grand Pre et al., 2012) provided a lower limit-
ing data point (Fig. 4; Table DR3). A sea-level
index point estimates the unique position of
RSL in space and time. For each index point
and limiting data point, we reconstructed RSL
using the equation
=RSLA–RWL
ii i
, (1)
where A
i
is the elevation of the sample measured
relative to local MTL (Table 1). Each index point
and limiting data point has a unique vertical er-
ror estimated from the indicative range of man-
grove [(HAT–MTL)/2] (Engelhart et al., 2007)
and seven other factors inherent in the collection
and processing of samples for sea-level research
(Shennan and Horton, 2002) (Table DR3).
The index points and limiting data points
demonstrate that Holocene RSL rise in Aceh
was not linear (Fig. 4). Using index points for
the period 5800–7600 cal yr B.P., the overall
mean rate of sea-level rise is 1.4 m/k.y. with
95% credible interval of 2.5–0.4 m/k.y. (Fig. 4;
Fig. DR6). The age of the youngest index point
precludes this rate continuing post–5800 cal yr
B.P., when RSL was ~–2 m. Terrestrial limiting
data points suggest that RSL was below present
until at least 3800 cal yr B.P. (Fig. 4). The high
degree of abrasion of the intertidal foraminifera
in the post–3800 yr sandy mud to muddy sand
unit (Fig. 3), characteristic of postdepositional
reworking, and the absence of coastal geomor-
phic features associated with a highstand suggest
that RSL did not rise above present. Therefore,
we infer that RSL rose slowly to present level
since 3.8 ka or stabilized within 0.4 m of modern
RSL at 3800 cal yr B.P. (Fig. 4). Either scenario
eliminates accommodation space as a driver for
coastal sediment deposition after 3.8 ka.
ACEH COASTAL PLAIN:
PALEOSEISMIC ARCHIVES UNDER
CONDITIONS OF SHRINKING
ACCOMMODATION SPACE
The beginning of the Holocene coincides
with a millennial-scale period of high (to ~15
mm/yr) rates of global mean sea-level rise (e.g.,
Lambeck et al., 2014). However, by 8000 cal yr
B.P., RSL slowed sufficiently for the establish-
ment of widespread mangrove swamps in Aceh
and elsewhere in southeast Asia (e.g., Horton et
al., 2005). From 8000 to 3800 cal yr B.P., the
Aceh mangrove-vegetated coastal plain built
upward at an average rate of 0.9 m/k.y. (3.8 m
sediment thickness; Fig. 2) in response to the
accommodation space provided by rising RSL.
Aggradation was interrupted at least three times
when RSL rose instantaneously during coseismic
subsidence, burying the mangrove soils with in-
organic deposits (Fig. 2). Well-sorted sand pre-
served above buried soils is of tsunami origin,
and two additional tsunamigenic sand layers oc-
cur in mud between the middle and upper buried
soils (Figs. 2 and 3). The lower part of the strati-
graphic section therefore records 5 subduction
Seungko Meulat core 4
Pulot core 16P
fragmented
pristine
abraded
subtidal
intertidal
planktic
BSM
(2004)
Ts
Species
ecology
Test
condition
0
0
1
1
2
2
3
3
4
4
Lower buried
soil
Middle buried
soil
Upper buried
soil
Test
condition
Species
ecology
Ts1
Ts2
Ts3
Ts4
BSM
BSM
Ts6
S
Ts
... tsunami sand
S... mangrove soil
BSM... bioturbated
sandy mud to
muddy sand
Depth (m)
Relative Sea Level (m)
Age (cal yr B.P.)
02000400060008000
-6
-5
-4
-3
-2
-1
0
1
Upper limiting data point, RSL
must be below this limit
Lower limiting data point, RSL
must be above this limit
Index point, estimate
of RSL
Figure 3. Foraminifera
data; small circles de-
pict sampling intervals.
Black circles—intervals
containing foraminifera.
White circles—intervals
where foraminifera are
absent. Pie diagrams,
percentages for spe-
cies ecology (subtidal,
planktic, intertidal) and
test condition (abraded,
fragmented, pristine) for
bioturbated sandy mud
(BSM), tsunami sand (Ts),
and soil (S) are shown.
Colors for stratigraphic
units are as in Figure 2.
Figure 4. Aceh (Sumatra, Indonesia) relative
sea-level curve. Solid line spanning 7600–
5800 cal yr B.P. defines an overall mean rate
of sea-level rise, defined by index points, of
1.4 m/k.y. (Fig. DR6; see footnote 1). Dashed
line delineates the period from 5800 cal yr
B.P. to present when relative sea-level (RSL)
trend is constrained, to a lesser extent, by
limiting data.
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zone earthquakes between 7000 and 3800 cal yr
B.P., based on 5 tsunami sands, 3 of which over-
lie the buried soils. And a sixth and seventh ad-
ditional candidate tsunami sand occurs between
the lower and middle buried soils and below the
lower buried soil, respectively (Fig. 2).
The 1.3–2.0-m-thick bioturbated massive
muddy sand unit, which overlies the sequence
of buried soils, contains mostly abraded fora-
minifera that are 28% subtidal (open-ocean
sourced) and the rest intertidal (Fig. 3). The unit
reflects aggradation of the coastal plain over the
past 3800 yr due to on-land transport of tsunami
sand and subsequent reworking of both sand
grains and foraminifera by tidal processes. Each
subsequent tsunami further built up the coastal
plain, and the 2004 tsunami is the most recent
of such events.
We therefore infer that the post–3800 yr
Aceh coastal plain consists of a sequence of
stacked tsunami deposits (Fig. 2), the number
of which depends on the recurrence of tsuna-
mis that deposit sand. Tsunami recurrence es-
timates, using the underlying buried soil unit
(Fig. 2) where 5–7 tsunami sands were depos-
ited over a 3600 yr interval from to 7400 to 3800
cal yr B.P., are 600–900 yr. If the past 3800 yr
represents the period of deposition of stacked
tsunami deposits (Fig. 2), then this bioturbated
massive muddy sand unit can account for 4–6
tsunamis (3800/900–3800/600). Given the 1.3–
2.0 m thickness of the unit (Fig. 2), individual
tsunami deposits would be 0.2–0.5 m thick if
4–6 tsunamis are represented. The 2004 tsunami
deposit at the 2 study sites ranges from 0.15 to
0.55 m thick, which is not inconsistent with the
inference that the aggraded unit represents as
few as 4 or as many as 6 tsunami deposits.
EXPLOITING THE FORM OF THE RSL
CURVE
Knowing the RSL record for a coastal region
on a subduction zone margin is the initial step
in investigating paleoseismic history. For mid-
latitude coasts that border subduction zones,
sequences of buried soils may provide a long-
duration, subsidence stratigraphic paleoseismic
record that spans to the present (e.g., Witter et
al., 2003); but in other settings such as the Aceh
coastal plain, joint research approaches, for ex-
ample targeted foraminiferal analyses and paly-
nology, are required to both exploit the changing
form of the RSL curve and characterize coastal
evolution in the context of the diminishing im-
portance of accommodation space.
ACKNOWLEDGMENTS
This study received support from National Science
Foundation grants EAR-0809392, EAR-0809417, and
EAR-0809625. We thank E. Yulianto and D. Natawi-
djaja, and students and staff at Syiah Kuala University,
Aceh, for field assistance. The paper was improved by
comments from three anonymous reviewers and edi-
tor R. Cox.
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Manuscript received 12 February 2015
Revised manuscript received 21 May 2015
Manuscript accepted 24 May 2015
Printed in USA
