The morphology and distribution of submerged reefs in the Maui-Nui Complex, Hawaii; new insights into their evolution since the early Pleistocene


Faichney, I.D.E.; Webster, J.M.; Clague, D.A.; Kelley, C.; Appelgate, B.; Moore, J.G.

Marine Geology 265(3-4): 130-145

2009


Reef drowning and backstepping have long been recognised as reef responses to sea-level rise on subsiding margins. During the Late Pleistocene ( approximately 500-14 ka) Hawaiian reefs grew in response to rapid subsidence and 120 m 100 kyr sea-level cycles, with recent work on the submerged drowned reefs around the big island of Hawaii, and in other locations from the last deglacial, providing insight into reef development under these conditions. In contrast, reefs of the Early Pleistocene ( approximately 1.8-0.8 Ma) remain largely unexplored despite developing in response to significantly different 60-70 m 41 kyr sea-level cycles. The Maui-Nui Complex (MNC--forming the islands of Maui, Molokai, Lanai and Kahoolawe), provides a natural laboratory to study reef evolution throughout this time period as recent data indicate the reefs grew from 1.1 to 0.5 Ma. We use new high resolution bathymetric and backscatter data as well as sub-bottom profiling seismic data and field observations from ROV and submersible dives to make a detailed analysis of reef morphology and structure around the MNC. We focus specifically on the south-central region of the complex that provides the best reef exposure and find that the morphology of the reefs varies both regionally and temporally within this region. Barrier and pinnacle features dominate the steeper margins in the north of the study area whilst broad backstepping of the reefs is observed in the south. Within the Au'au channel in the central region between the islands, closely spaced reef and karst morphology indicates repeated subaerial exposure. We propose that this variation in the morphology and structure of the reefs within the MNC has been controlled by three main factors; the subsidence rate of the complex, the amplitude and period of eustatic sea-level cycles, and the slope and continuity of the basement substrate. We provide a model of reef development within the MNC over the last 1.2 Ma highlighting the effect that the interaction of these factors had on reef morphology.

Marine
Geology
265
(2009)
130-145
Contents
lists
available
at
ScienceDirect
Marine
Geology
journal
homepage:
www.elsevier.com/locate/margeo
*P'
MARINE
GEOLOGY
The
morphology
and
distribution
of
submerged
reefs
in
the
Maui-Nui
Complex,
Hawaii:
New
insights
into
their
evolution
since
the
Early
Pleistocene
lain
D.E.
Faichney
a
,
*,
Jody
M.
Webster
b
'
a
,
David
A.
Clague
C
,
Chris
Kelley
d
,
Bruce
Appelgate
e
,
James
G.
Moore
School
of
Earth
and
Environmental
Sciences,
James
Cook
University,
Townsville,
Qld,
4811
Australia
b
School
of
Geosciences,
University
of
Sydney,
Sydney,
NSW,
2006,
Australia
Monterey
Bay
Aquarium
Research
Institute,
Moss
Landing,
CA,
95039,
USA
d
Hawaii
Undersea
Research
Laboratory,
School
of
Ocean
and
Earth
Science
and
Technology,
University
of
Hawaii,
Honolulu,
HI,
96822,
USA
Scripps
Institution
of
Oceanography,
La
Jolla,
CA
92093,
USA
US.
Geological
Survey,
MS
910,
345
Middlefield
Road,
Menlo
Park,
California
94025,
USA
ARTICLE INFO
ABSTRACT
Article
history:
Received
23
February
2009
Received
in
revised
form
1
July
2009
Accepted
5
July
2009
Available
online
16
July
2009
Communicated
by
J.T.
Wells
Keywords:
platform
morphology
Maui-Nui
carbonate
terraces
bathymetry
Pleistocene
sea-level
change
Reef
drowning
and
backstepping
have
long
been
recognised
as
reef
responses
to
sea-level
rise
on
subsiding
margins.
During
the
Late
Pleistocene
(-500-14
ka)
Hawaiian
reefs
grew
in
response
to
rapid
subsidence
and
120
m
100
kyr
sea-level
cycles,
with
recent
work
on
the
submerged
drowned
reefs
around
the
big
island
of
Hawaii,
and
in
other
locations
from
the
last
deglacial,
providing
insight
into
reef
development
under
these
conditions.
In
contrast,
reefs
of
the
Early
Pleistocene
(-1.8-0.8
Ma)
remain
largely
unexplored
despite
developing
in
response
to
significantly
different
60-70
m
41
kyr
sea-level
cycles.
The
Maui-Nui
Complex
(MNC
forming
the
islands
of
Maui,
Molokai,
Lanai
and
Kahoolawe),
provides
a
natural
laboratory
to
study
reef
evolution
throughout
this
time
period
as
recent
data
indicate
the
reefs
grew
from
1.1
to
0.5
Ma.
We
use
new
high
resolution
bathymetric
and
backscatter
data
as
well
as
sub-bottom
profiling
seismic
data
and
field
observations
from
ROV
and
submersible
dives
to
make
a detailed
analysis
of
reef
morphology
and
structure
around
the
MNC
We
focus
specifically
on
the
south-central
region
of
the
complex
that
provides
the
best
reef
exposure
and
find
that
the
morphology
of
the
reefs
varies
both
regionally
and
temporally
within
this
region.
Barrier
and
pinnacle
features
dominate
the
steeper
margins
in
the
north
of
the
study
area
whilst
broad
backstepping
of
the
reefs
is
observed
in
the
south.
Within
the
Au'au
channel
in
the
central
region
between
the
islands,
closely
spaced
reef
and
karst
morphology
indicates
repeated
subaerial
exposure.
We
propose
that
this
variation
in
the
morphology
and
structure
of
the
reefs
within
the
MNC
has
been
controlled
by
three
main
factors;
the
subsidence
rate
of
the
complex,
the
amplitude
and
period
of
eustatic
sea-level
cycles,
and
the
slope
and
continuity
of
the
basement
substrate.
We
provide
a
model
of
reef
development
within
the
MNC
over
the
last
1.2
Ma
highlighting
the
effect
that
the
interaction
of
these
factors
had
on
reef
morphology.
©
2009
Elsevier
B.V.
All
rights
reserved.
I.
Introduction
In
recent
years,
significant
work
has
been
carried
out
on
Late
Pleistocene
drowned
reefs
on
rapidly
subsiding
margins
such
as
the
Huon
Gulf,
Papua
New
Guinea
(PNG)
and
Hawaii
(e.g.,
Galewsky
et
al.,
1996;
Webster
et
al.,
2004b).
In
Hawaii,
drowned
reefs
have
been
used
to
determine
subsidence
rates
(Ludwig
et
al.,
1991;
Moore
and
Fornari,
1984)
and
investigate
the
timing
of
Meltwater
Pulse
1A
a
catastrophic
sea-level
rise
responsible
for
reef
drowning
during
the
last
deglaciation
(Webster
et
al.,
2004a).
Numerical
modelling
of
Late
*
Corresponding
author.
Tel.:
+61
7
4781
6942;
fax:
+61
7
4781
4020.
E-mail
addresses:
iain.faichney@jcu.edu.au
(I.D.E.
Faichney),
jody.webster@usyd.edu.au
Q.M.
Webster),
clague@mbari.org
(D.A.
Clague),
ckelley@hawaii.edu
(C.
Kelley),
tba@ucsd.edu
(B.
Appelgate),
jmoore@usgs.gov
Q.G.
Moore).
0025-3227/$
-
see
front
matter
C
2009
Elsevier
B.V.
All
rights
reserved.
doi:10.1016/j.margeo.2009.07.002
Pleistocene
reef
growth
on
Hawaii
(Webster
et
al.,
2007b)
also
suggests
that
the
internal
stratigraphy
of
these
drowned
reef
terraces
is
complex,
controlled
by
frequency
and
amplitude
of
eustatic
sea-
level
variations.
Webster
et
al's
(2007b)
numerical
modelling
also
indicate
that
the
terraces'
gross
morphology
was
influenced
by
the
subsidence
and
carbonate
platform
growth
rates.
These
studies
advanced
our
knowledge
of
reef
development
in
response
to
rapid
subsidence
during
the
Late
Pleistocene,
but
little
is
known
about
reef
development
during
the
Early
Pleistocene
in
these
settings.
During
the
Late
Pliocene
and
Pleistocene,
global
climate,
ice
volume
and
eustatic
sea
level
oscillated
at
a
different
period
and
amplitude
compared
with
the
last
0.8
Ma.
From
about
2.5
to
0.8
Ma
(Hays
et
al.,
1976)
this
oscillation
was
dominated
by
a
period
of
41
kyr
and
eustatic
sea-level
fluctuations
with
amplitude
of
60-70
m
(Dwyer
et
al.,
1995).
This
period
was
followed
by
climate
oscillations
with
a
period
of
100
kyr
with
eustatic
sea-level
fluctuations
of
up
to
120
m
RTE-87
D3
T308
91-WA
AP'
RTE-87
D7
P5-218
P5-14
--
-7t.of
t
e
i
(Fig.
2)
(Fig.
1
41111111
Molokai
Lanai
0
0
Cs1
0
O
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
131
sea
level.
The
interval
characterising
the
change
from
one
dominant
climatic
forcing
to
the
next
(i.e.,
40
kyr
to
100
kyr
worlds)
is
known
as
the
Mid-Pleistocene
Transition.
The
Maui-Nui
Complex
(MNC)
(Fig.
1)
has
developed
over
the
last
2
million
years
with
shield
building
volcanic
rocks
from
the
islands
of
the
Complex
(Molokai,
Lanai,
Maui
and
Kahoolawe
in
age
order)
having
been
dated
from
1.90
to
0.75
Ma
(Clague
and
Dalrymple,
1989).
These
ages
indicate
that
the
entire
MNC
evolved
from
the
Late
Pliocene
to
Present
Day.
Further
studies
show
the
MNC's
initial
subsidence
to
be
rapid,
with
a
slowing
as
it
moved
away
from
the
hotspot
(Moore,
1987).
Recent
lithological
and
Sr
isotope
investigations
of
submarine
terraces
in
the
MNC
(Webster
et
al.,
In
review)
confirm
that:
(1)
most
of
these
features
are
coral
reefs;
(2)
the
terraces
get
older
as
they
get
deeper,
but
that;
(3)
the
MNC
terraces
are
significantly
older
than
their
Hawaiian
counter-
parts
at
similar
depths,
initiating
growth
soon
after
the
end
of
major
shield
building
(-13-1.2
Ma).
Webster
et
al.
(In
review)
confirm
that
the
12
reefs
(L1
to
L12,
Table
2)
forming
the
submerged
MNC
terraces
grew
from
the
Early
Pleistocene
to
Present
Day,
i.e.
before
(>_L9),
during
(L8-L5)
and
after
(L4)
the
Mid-Pleistocene
Transition.
As
such,
the
submerged
reefs
around
the
MNC
represent
a
unique
opportunity
to
explore
the
response
of
reefs
to
varying
subsidence
rates
as
well
as
varying
rates
and
amplitudes
of
sea-level
changes
since
the
Early
Pleistocene.
This
study
focuses
on
characterising
the
morphology
of
the
submerged
reefs
around
the
MNC
and
investigating
any
changes
that
would
indicate
variation
in
reef
development.
New
high
resolution
bathymetric
data
around
the
MNC
have
allowed
a
detailed
analysis
of
the
morphology
of
these
reefs.
Field
observations
from
submersible
and
ROV
dives
allow
"ground-truthing"
of
the
reef
growth
structures
associated
with
their
development.
We
document
changes
in
reef
morphology
between
terraces
by:
1)
using
the
new
high
resolution
bathymetric
and
backscatter
to
illustrate
the
structure
and
morphology
of
the
terraces
both
regionally
and
individually,
and
2)
using
observations
from
ROV
and
submersible
dives
to
describe
outcrop
style
and
reef
morphology.
We
then
compare
the
deeper
MNC
reefs
(>800
m)
that
grew
in
response
to
rapid
subsidence
and
41
kyr
global
sea-level
changes
with
the
Hawaiian
reefs
that
grew
in
response
to
rapid
subsidence
in
a
100
kyr
sea-level
cycle.
Finally,
we
use
these
data
to
develop
a
model
to
illustrate
reef
development
around
the
MNC
since
the
Early
Pleistocene.
2.
Location
and
methods
2.1.
Location
and
geological
setting
The
MNC
is
located
toward
the
south-eastern
end
of
the
Hawaiian-
Emperor
Seamount
Chain
and
has
developed
progressively
as
a
series
of
linked
volcanoes
that
grew
and
subsided
due
to
the
passage
of
the
Pacific
plate
over
the
Hawaiian
hotspot
(Fig.
1).
This
study
concen-
trates
on
the
southern
section
of
the
MNC
where
a
series
of
submerged
z
0
0
z
0
O
z
O
0
157°40'0"W
157°30'0'W
157°20'0"W
157°10'0"W
157°0'0'W
156°50'0"W
156°400"W
156°300"W
Legend
*
Dredge
Samples
*
Dive
Samples
*
Dive
Video
Landslide
extent
High
:
-50
I
Terraces
k
)
Low
:
-3500
Maui
T312
Fig.
4)
Noe
Clark
Debris
Avalanche
1531PM
0VV
Oahu
T310
T311
—L1
—L7
L2
LE3
L3
—L9
—L4
—L10
..4.
L5
—L11
2
L6
L12
,
.
i‘
4"
,
..
r
,,,,,,wir•
-...--
-
T295
44ft
P5-217
4,
TU1MV-D9
T309
T294
(Fig.
3)
Kahoolawe
0
Hawaii
5
10
20
Cs1
Km
158',Olny
157°40'0"W
157°30'0"W
157°20'0'W
157°10'0"W
157°0'0"VV
156'50'0"W
15540'0"W
155°30'0"W
Fig.
1.
Introduction
map
of
MNC
showing
dive
locations
and
bathymetry.
Location
and
bathymetric
map
of
the
MNC
within
the
Hawaiian
Islands.
The
locations
of
the
ROV
and
submersible
dives
discussed
in
the
text
are
shown
in
red
stars,
the
dredges
are
shown
in
blue
stars
and
samples
are
shown
as
yellow
stars.
The
locations
of
Figs.
2,
3
and
4
are
also
marked.
(For
interpretation
of
the
references
to
colour
in
this
figure
legend,
the
reader
is
referred
to
the
web
version
of
this
article.)
132
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
terraces
(from
150
m
down
to
1200
m)
has
been
long
recognised
(Campbell,
1986).
Moore
and
Campbell
(1987)
briefly
addressed
their
morphology,
identified
eight
terraces,
correlated
them
by
depth
with
terraces
offshore
Hawaii,
and
proposed
them
to
be
of
similar
age.
Webster
et
al.
(2009-in
review)
confirm
that
these
terraces
are
much
older
than
the
Hawaiian
terraces,
placing
their
earliest
development
from
the
Early
Pleistocene.
Investigations
of
the
shallower
reef
terraces
in
the
Au'au
channel
have
revealed
karst-like
morphologies
(Grigg
et
aL,
2002),
and
coralline
algal
build-up
and
drowning
(Webster
et
al.,
2006).
These
studies
indicate
reef
growth
in
response
to
slow
subsidence,
subaerial
exposure
of
the
terraces
shallower
than
120
m
and
deep-water
algal
re-occupation
of
terraces
1
and
2
during
the
low-stands.
With
the
exception
of
a
few
dredged
samples
from
the
Haleakala
Ridge
(Moore
et
al.,
1990),
Campbell's
(1986)
subsidence
study
and
bathymetric
atlas
(Campbell,
1987)
and
the
recent
Sr
isotope
study
by
Webster
et
al.
(in
review),
the
deeper
terraces
of
the
MNC
have
not
been
investigated.
2.2.
Multibeam
bathymetry
and
backscatter
analysis
All
available
high
resolution
and
regional
bathymetry
and
back-
scatter
data
for
the
MNC
have
been
compiled
from
multiple
sources
including
the
Monterey
Bay
Aquarium
Research
Institute
(MBARI),
the
University
of
Hawaii
(UH),
the
Japan
Agency
for
Marine-Earth
Science
and
Technology
(JAMSTEC),
the
National
Oceanic
and
Atmospheric
Administration
(NOAA),
the
United
States
Geological
Survey
(USGS),
Scripps
Institution
of
Oceanography
(SIO)
and
Woods
Hole
Oceano-
graphic
Institute
(WHOI).
The
data
are
predominantly
from
a
SIMRAD
EM300
(30
kHz)
system
in
the
shallow
regions
(Dartnell
and
Gardner,
1998)
and
co-registered
SIMRAD
EM1002
(100
kHz)
and
EM120
(12
kHz)
systems
in
the
deeper
regions
(UH,
http:
/
/www.soest.
hawaii.edu/HMRG/Multibeam/)
.
The
latter
were
mainly
collected
using
the
UH
vessel
R/
V
Kilo
Moana
in
2005.
Additionally,
coastal
LIDAR
data,
collected
by
the
US
Army
Corp
of
Engineers
(USACE),
was
incorporated
for
the
very
shallow
and
coastal
areas.
Most
of
the
multibeam
data
were
processed
in
MBSystem
(5.1.1beta7)
(Caress
and
Chayes,
2004)
to
remove
artefacts
and
bad
data,
and
gridded
at
30
m
resolution,
with
smaller
regions
around
the
dive
locations
gridded
at
5
m
resolution.
These
grids
were
imported
into
both
ArcGIS
9.2
and
Fledermaus
6.7
to
define
the
morphologic
characteristics
of
the
terraces
as
well
as
map
their
distribution.
Artificially
sun-shaded
images
and
slope
maps
were
created
from
the
grid
files
using
the
3D-
Analyst
tool
in
ArcGIS.
The
slope
maps
show
a
relative
value
in
the
change
in
depth
from
one
pixel
to
the
surrounding
8,
in
a
3
pixel
by
3
pixel
window.
These
were
used
to
locate
the
change
in
gradient
at
the
crest
of
terraces.
In
this
way,
terraces
around
the
MNC
were
mapped
and
correlated
using
the
slope-map
images
within
the
ArcScene
function
of
ArcGIS
and
sun-shaded
bathymetry
models
in
Fledermaus.
Geo-referenced
Chirp
seismic
profiles
collected
by
the
SEA
Education
Association
(SEA)
and
multibeam
backscatter
images
were
also
imported
into
ArcGIS
to
assist
in
the
seafloor
and
sub-
bottom
characterisation
of
the
terraces
and
structures.
Seismic
data
from
the
Benthos
2-7
kHz
Chirp-II
sub-bottom
profiling
system
on-
board
the
SS
Robert
C
Seamans
were
imported
into
SeiSee
2.3-Beta-1.
These
data
were
used
to
measure
sediment
thickness
in
milliseconds
of
two
way
travel
time
which
was
then
converted
to
thickness
in
metres
(m)
assuming
a
sonic
velocity
the
same
as
water
(1500
m/s).
The
spatial
coverage
of
sediment
was
interpreted
from
the
backscatter
images
in
ArcGIS
with
low
backscatter
(dark)
interpreted
as
soft-
sediment
cover
and
high
backscatter
(light)
areas
as
exposed
outcrop
or
steep
terrain.
To
provide
quantifiable
data
on
terrace
morphology,
observations
of
each
terrace
were
made
including
length,
relief,
sinuosity
and
the
calculation
of
a
Rim-Index
defined
as
the
normalised
length
of
raised
rim
divided
by
the
length
of
the
terrace
itself
(i.e.
Rim-Index
=
(R1
+
R2
+
R3
+
Rx)
/
L)
where
R1,
R2,
R3
to
Rx
are
the
lengths
of
sections
of
the
terrace
exhibiting
raised
rims,
and
L
is
the
total
length
of
the
terrace
(Schlager,
2005).
2.3.
Dive
and
dredge
operations
Dive
and
dredge
operations
have
been
carried
out
across
the
MNC
for
the
past
thirty
years,
but
sampling
has
been
concentrated
in
the
south-central
section
of
the
Complex
(Fig.
1).
In
2001,
MBARI
conducted
a
series
of
dives
using
the
ROV
Tiburon
launched
from
the
RV
Western
Flyer.
Dives
used
in
this
study
include
T309,
T294
and
T295
southwest
of
Lanai
at
580
m,
550
m
and
475
m,
respectively,
T310
directly
south
of
Lanai
at
150
m,
and
T311
and
T312
northwest
of
Kahoolawe
at
230
m
and
275
m.
Data
from
these
six
dives
include
139
carbonate
samples
obtained
from
the
slopes
and
the
tops
of
the
submarine
terraces,
and
approximately
15
h
of
video
footage.
Additionally,
the
Hawaiian
Undersea
Research
Laboratory
(HURL)
at
UH
has
conducted
Pisces
submersible
dives
from
the
R/
V
Kaimikai-o-
Kanaloa
across
the
Complex.
Samples
and
video
from
Pisces
dives
(P4-
026,
P4-027,
P5-191,
P5-217,
P5-218,
P5-254)
and
video
from
ROV
dives
(RCV-108,
RCV-109,
RCV-110,
RCV-111,
RCV-115,
RCV-116,
RCV-
117,
and
RCV-118)
have
been
studied
for
patterns
in
outcrop
morphology,
and
to
correlate
morphology
between
sample
sites.
Rock
dredging
operations
(91-WA;
87RTE-D3,
D4,
and
D7;
F2-88-HW-
D32;
and
TUIMOIMV-D9)
by
the
USGS
and
SIO
yielded
a
further
suite
of
samples
that
were
also
analysed
in
hand
sample.
The
F2-88-HW
dredges
were
conducted
over
tentatively
interpreted
coral
reefs
on
the
basis
of
GLORIA
images
and
3.5
kHz
Chirp
profiles.
There
are
a
total
of
234
limestone
samples
collected
from
all
dives
and
dredge
operations
that
have
been
used
in
this
study
to
confirm
their
reefal
composition
and
origin.
The
detailed
lithological
investigation
of
the
samples,
sedimentary
facies
analysis
and
palaeoenvironmental
implications
will
be
presented
separately.
3.
Results
The
region
between
Lanai
and
Kahoolawe,
(south-central
MNC),
Fig.
1,
shows
the
best
development
of
the
submerged
reef
terraces
of
Table
1
Table
of
terrace
morphology
features
and
definitions.
Paper
Modern
Definition
terminology
analogue
Slope
Reef
slope
Distal
side
of
platform
margin
identified
by
seaward
deepening
bathymetric
signature
and
variable
backscatter
values
Crest
Reef
crest
Platform
margin
running
parallel
to
coast
at
the
top
of
the
reef
slope
identified
by
break
in
the
slope
of
bathymetry
sometimes
associated
with
high
backscatter
values.
This
is
the
correlated
feature
and
has
been
defined
by
the
initial
break
in
the
slope
rather
than
main
break
in
the
slope
Flat
Reef
flat
Proximal
side
of
platform
margin
identified
by
expanse
of
flat
bathymetry
landward
of
the
crest,
commonly
with
low
backscatter
and
sediment
present
in
seismic
section
Lagoon Lagoon
Depressed
bathymetry
landward
of
the
crest
commonly
with
low
backscatter
values
and
sediment
present
in
seismic
section.
These
features
are
defined
by
their
enclosed
nature,
either
through
a
circular
depression,
or
separated
from
the
rest
of
the
reef
flat
by
higher
terraces
Patch Patch
reef
Small
scale
(generally
<200
m
across,
<50
m
relief)
flat-
topped
feature
of
raised
bathymetric
signature
correlated
by
depth
with
terraces.
These
features
are
also
seen
in
seismic
section
buried
under
sediment
Pinnacle Pinnacle
Large-scale
(generally
>200
m
across,
>100
m
relief)
feature
of
raised
bathymetric
signature
correlated
by
depth
with
terraces.
These
features
commonly
rise
multiple
terraces
in
vertical
relief
Barrier
Barrier
reef
large-scale
(generally
>1
km
length,
>100
m
relief)
feature
of
raised
bathymetric
signature
correlated
by
depth
with
terraces.
These
features
commonly
rise
multiple
terraces
in
vertical
relief
Fringe
Fringing
Type
of
reef
morphology
that
forms
abutting
the
coastline.
reef
Distinct
from
patch,
pinnacle
and
barrier
morphologies
which
are
reef
types
separated
from
the
coastline
60-75
0.00
n/a
40-60
0.00
n/a
50
0.00
n/a
Low
as
fringing,
Up
to
9
moderate
between
the
islands
Low
Nil
on
open
terrace.
6-8,
with
11
max
in
lagoon
Low
to
moderate
Nil
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
133
the
MNC.
We
have
mapped
the
reefs
throughout
the
entire
MNC,
but
this
paper
concentrates
on
the
most
well
developed
terraces
in
this
south-central
section.
Twelve
separate
fringing-reef
terraces
(L1
-L12)
have
been
identified
and
mapped
in
this
region
(Fig.
1)
and
Table
1
defines
the
morphologic
terminology
used
to
describe
the
features
and
their
likely
modern
reef
analogs.
3.1.
Structure
and
morphology
of
the
MNC
On
a
regional
scale,
the
terraces
of
the
MNC
generally
follow
the
flanks
of
the
volcanoes
of
the
Complex;
however,
they
are
also
present
where
volcanic
rifts
extend
from
the
summits
i.e.
Penguin
Bank,
west
of
Kahoolawe,
and
north
and
east
of
Maui
(Fig.
1).
The
seaward
margin
of
the
Complex
is
marked
in
most
places
by
a
sharp
steep
scarp
(Fig.
1),
that
Price
and
Elliot-Fisk
(2004)
associated
with
the
end
of
shield
building.
This
scarp
rises
between
1100
and
1800
m
above
the
sea
floor,
but
towards
the
far
eastern
and
far
western
ends
of
the
Complex
it
is
much
less
pronounced.
Large-scale
submarine
landslides
are
prominent
around
the
Main
Hawaiian
Islands
with
six
separate
events
identified
as
originating
from
the
MNC
(Moore
et
al.,
1989).
The
extent
and
boundaries
of
the
slide
features
are
visible
in
the
bathymetry
data
(Fig.
1).
The
Clark
Debris
Avalanche
southwest
of
Lanai
(Fig.
1)
is
the
only
landslide
feature
relevant
to
this
paper
as
it
falls
within
the
designated
geographical
boundaries
of
this
study.
Examination
of
new
high
resolution
bathymetric
data
reveals
large
blocks
(<2
km
in
size)
on
the
abyssal
plain
southwest
of
the
MNC,
with
large-scale
(3-7
km
across)
hummocky
terrain
at
the
foot
of
the
slope
(Fig.
1).
Proximal
to
where
Moore
et
al.
(1989)
defined
the
avalanche
head
on
Lanai,
the
major
platform
margin
is
broken
by
a
fault
trending
northeast.
Moore
et
al.
(1989)
identify
the
series
of
steplike
reefs
(L5-12,
Fig.
1)
as
growing
on
subaerial
post-avalanche
volcanic
flows.
Immediately
offshore
the
coastlines
on
the
north
of
Molokai,
the
northeast
and
southwest
of
Oahu
and
south
of
Kahoolawe,
submarine
canyons
incise
deeply
(200
m-600
mat
the
foot
of
the
slope)
into
the
flanks
of
the
volcanic
islands
(Fig.
1).
These
canyon
incisions
obscure
terrace
identification
and
correlation
in
these
areas
of
the
MNC,
with
widely
spaced
cuts
in
the
breaks
in
the
slope
of
the
terraces.
These
incisions
widen
further
at
greater
distance
from
the
coast
so
the
deeper
reef
terraces
are
more
affected.
Additionally,
common
erosive
slump
features
on
the
steep
margins
of
the
canyons
also
make
identification
of
terrace
breaks
in
the
slope
more
difficult
to
differentiate
from
erosive
scarps.
The
deeper
reef
terraces,
where
identifiable,
generally
have
much
higher
vertical
relief
than
the
shallower
terraces
(Table
2).
The
shallower
slopes
of
these
canyon
regions
are
clear
of
the
slump
features
however
the
crests
of
the
terraces
are
much
smoother
and
less
distinct
(south
of
Kahoolawe
in
Fig.
1).
The
south-central
MNC
can
be
divided
into
three
regions
based
on
the
presence
of
reef
terraces
and
their
differing
morphology:
immedi-
ately
south
of
Molokai
and
Penguin
Bank
the
northern
region
(Fig.
2),
the
south
of
Lanai
the
southern
region
(Fig.
3)
and
between
the
islands
of
Lanai,
Maui
and
Kahoolawe
the
central
region
(Fig.
4).
The
pinnacle
and
barrier
structures,
clearly
observed
in
the
bathymetry
data
of
the
northern
section,
do
not
appear
on
the
broader
flats
of
the
southern
section
or
elsewhere
within
the
MNC.
Additionally,
the
deeper
reef
terraces,
L10
to
L12,
do
not
appear
in
the
southern
region
except
for
L10
Table
2
Summary
table
of
terrace
morphology
and
structure.
Terrace
Depth
to
Mapped
Relief
Rim-
Rim
height
Shape
Sediment
Badcscatter
Structure
Age(Ma)a
top
of
length
(m)
Index
and
width
(sinuosity)
cover(m)
terrace(m)
(km)
(m)
Ll
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
L12
Variable
across
fringing
terrace
in
the
north.
No
data
in
the
south
No
data
No
data
No
data
No
data
No
data
40
0.00
n/a
Low
as
fringing,
2
high
between
the
islands
80
0.45
H:
10-12
Very
low
in
the
Nil
W:
30-35
south
and
moderate
to
high
in
the
north
55
0.13
H:
8-10
Low
2-11,
also
W:
35-45
exhibits
buried
patch
reefs
60
0.41
H:
15-25
Low
in
the
south
2-13,
from
crest
W:
40-100
and
moderate
to
of
L7
to
foot
of
high
in
the
north
L6,
numerous
reflectors
35-90
0.29
H:
20-40
Low
in
the
south
2-4
on
reef
flat
W:
50-100
and
moderate
to
high
in
the
north
100-600
0.78
H:
40-80
Low
to
moderate
1-6
on
reef
flat,
W
100-400
max
13
100-400
0.78
H:
10-25
Low
to
moderate
Nil
W:
60-100
910-1040
21
40-60
1.0
H:
10-12
Low
Nil
W:
35-45
1170-1270
47
300-450
1.0
H:
15-30
Low
Nil
W:
40-60
Low
angle
of
the
slope,
patches
and
lagoons.
Karst
formations
including
small
scale
depressed
sinkholes
and
raised
patches.
Low
promontory
terrace.
Variable
slope
of
terrace
from
gently
to
moderately
sloping.
Fringing,
fairly
narrow
terrace.
0.533
Fringing
terrace
with
patches
evident
Winnowed
dip
at
the
foot
of
the
0.708
slope.
Fringing,
very
wide
terrace.
Winnowed
dip
at
the
foot
of
the
0.643-0.715
slope.
Hummocky
structures
on
reef
flats.
Cut
by
drainage
ravines
to
the
south
of
Kahoolawe.
Northern
section
shows
barrier
and
0.643-0.788
pinnacle
features.
Fringing
terrace
in
southern
section.
Northern
section
shows
barrier
and
pinnacle
features.
Fringing
terrace
in
southern
section.
Raised
rim
along
terrace
crest.
0.509-1178
Very
steep
high
high-relief
scarp.
Depressed
sinkholes
on
flat.
Barrier
and
pinnacle
features
1.094
in
northern
section
Fringing
terrace
along
northern
section
Fringing
terrace
in
northern
section.
Dramatic
vertical
relief
scarp.
Moderate
across
most
of
the
terrace
Very
high
across
terrace
crest
and
flat
High
along
the
terrace
crest
High
across
entire
slope,
and
on
crest
High
across
entire
barrier
(North).
High
across
slope
of
fringing
(South)
High
along
terrace
crest
in
the
north,
no
data
in
the
south
100-150
281
230-270
35
285-330
110
355-380
28
365-555
105
334-780
129
520-790
110
605-1050
210
605-1140
152
700-835
120
a
Ages
are
averaged
according
to
depth,
after
Webster
et
al.,
2009
(In
review).
\`'
RT
E
87-D4lb
P5-191
1)
i.-
A"
0
ACP
0
1.5
3
Km
6
r
l
m
a
L12
157°10'0"W
CV
to
to
.LN
z
I
157°20
.
0"W
157°15'0"W
F
T308
0
RCV-201
,11
.
I
/
4
.
RTE87-07
a
91-WA
Legend
*
Dredge
Samples
*
Dive
Samples
*
Dive
Video
Bathymetry
High:
-50
mm
Low
:
-2000
Backscatter
Lanai
High
:
255
III
Low
:
0
-
Backscatter
extent
C-
(
1
L11
L10
L9
c
L8
157°5'0"W
-11
91
Terraces
Li
—L7
L2
L8
L3
—L9
L4
—L10
L5
—L11
L6
L12
31
6.'L5
L3
Ll
157°0'0"W
z
LU
LU
134
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
157°20
.
0"W
157°15'0"W 157°10'0"W
157°5'0"W 157°0'0"W
0
Lc,
Fig.
2.
Expanded
section
of
Fig.1
-
the
Northern
region.
An
expanded
section
of
Fig.1
exhibiting
pinnacle
and
barrier
structures.
Available
backscatter
data
is
displayed,
and
dives
and
samples
are
displayed
as
in
Fig.
1.
occurring
as
small
ledges
protruding
from
the
scarp
that
marks
the
seaward
margin
of
the
Complex,
(Fig.
1).
The
central
region
is
characterised
by
lagoonal
and
patch
morphologies
and
also
illustrates
karst
features
shallower
than
120
m
(LO
and
Ll
in
Fig.
4).
3.2.
Structure
and
morphology
of
the
reefs
To
quantify
the
morphological
variation
between
the
reef
terraces,
different
measures
of
morphology
such
as
Rim-Index,
sinuosity,
vertical
relief,
sediment
cover
and
shape
are
summarised
below
(see
also
Table
2).
These
regional
observations
are
enhanced
wherever
possible
by
a
detailed
analysis
of
individual
reefs
using
dive
observations
from
video
footage
of
the
RCV
and
Tiburon
ROVs
and
Pisces
submersible
dives
across
the
MNC.
Basic
lithological
summaries
of
samples
taken
from
these
terraces
are
also
included
to
support
the
observations
made.
3.2.1.
Terrace
Ll
The
shallowest
terrace
identified
within
the
complex,
Ll,
is
of
low
relief,
with
an
average
vertical
rise
of
40
m
(Fig.
5a)
at
an
average
incline
of
12°.
It
ranges
in
depth
from
100
to
150
m
depending
on
location
within
the
MNC
and
exhibits
no
Rim-Index
value.
This
is
the
best
defined
terrace
within
the
MNC
with
a
total
mapped
length
of
281
km.
Ll,
displaying
a
moderate
degree
of
sinuosity
(Fig.
4),
follows
the
coasts
of
the
islands
relatively
closely
except
for
Penguin
Bank
and
the
shallow
areas
between
the
south-eastern
four
islands.
In
these
shallowest
regions
between
the
islands,
Ll
shows
lagoon
and
patch
morphologies.
These
depressions
are
up
to
2.5
km
long
and
1
km
wide,
and
contain
thin
(2
m)
sediment
packages
along
the
top
of
the
terrace.
The
central
region
between
the
islands
displays
an
upper
terrace
development,
that
we
designate
LO,
which
shows
common
karst
features
such
as
solution
basins
and
solution
ridges,
as
described
by
Grigg
et
al.
(2002).
3.2.1.1.
T310
and
T311
dive
observations.
Dive
T310
exhibits
a
stepped
profile
in
Ll
with
a
prominent
ledge
halfway
up
the
slope
at
155
m
that
is
well
lithified
and
at
least
4-5
m
thick
and
is
directly
overlying
a
sandy
bottom.
The
crest
reveals
a
small
rubble
field
that
continues
landward
across
the
flat
before
giving
way
to
a
sand
sheet
with
low-
relief
outcrops
running
perpendicular
to
the
terrace
crest
protruding
through
the
sediment.
The
dive
T311
site
of
Ll
shows
a
slightly
different
outcrop
style,
with
nodule
fields
being
the
only
outcrop
style
right
up
to
the
crest
of
the
terrace
(163
m)
where
the
nodules
are
cemented
into
a
pavement-style
outcrop
that
stretches
landward
across
the
flat.
Coralline
algal
limestones
dominate
the
samples
from
these
dives
confirming
coralgal
deposits
on
Ll.
3.2.2.
Terrace
12
L2
is
a
35
km
long
terrace
and
was
observed
during
two
ROV
dives.
It
has
vertical
relief
of
75
mat
T311's
location
and
60
mat
T312
(Fig.
5b
and
c)
at
a
depth
of
220
m
and
280
m
respectively.
L2
is
characterised
by
low
sinuosity,
has
no
Rim-Index
and
forms
a
lagoon
between
Lanai
and
Kahoolawe
as
it
creates
a
bridge
between
the
islands
(Fig.
4).
The
Chirp
data
across
this
region
reveal
no
sediment
on
L2
and
backscatter
data
show
mottled
high
values
across
this
promontory
with
high
values
being
characteristic
of
all
of
L2.
Between
the
series
of
patch
reefs
of
Ll,
landward
of
the
main
lagoon,
Chirp
data
indicate
sediment
up
to
9
m
thick
(Fig.
6a)
however
the
limited
mappable
extent
of
L2
makes
it
unclear
whether
this
sediment
is
on
L2
or
13.
L12
L1
a
T310
T295
T309
-_-
..
L
P5-217
T294
(Fig.
6b)
L10
L9
Legend
Bathymetry
Backscatter
High
:
-50
High
:
255
Low
:
-800
1
Low
:
0
*
Dredge
Samples
-
-
-
Extent
Dive
Samples
Sediment
*
Dive
Video
-
Chirp
Line
air
\.•
Terraces
L1
—L7
L2
L8
L3
—L9
L4
L10
L5
—L11
L6
L12
t
L5
.;
0
1.5
3
6
L7
L6
i
Km
/
z
O
CO
z
cn
157°10'0"W
157
°
5'0"W
157
°
0'0"W
156°55'0"W
0
0
M
0
156°50'0"W
z
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
135
157°10'0"W
157°5'0"W
157°0'0"W
156°55'0"W
156°50'0"W
Fig.
3.
Expanded
section
of
Fig.1
-
the
Southern
region.
An
expanded
section
of
Fig.1
exhibiting
Chirp
seismic
navigation
line
displayed
as
a
black
line
with
the
location
of
sediment
packages
on
the
reef-flat
identified
by
the
grey
colouring
overlying
the
black
navigational
line.
Available
backscatter
data
is
displayed,
and
dives
and
samples
are
displayed
as
in
Fig.
1.
3.2.2.1.
7311
and
7312
dive
observations.
Outcrop
begins
on
dive
311
at
the
foot
of
the
slope
as
nodules
cemented
into
a
pavement-style
outcrop
that,
along
with
minor
hummocky
outcrop,
composes
the
face
of
the
slope
(Fig.
5b).
The
pavement
outcrop
gives
way
to
a
rhodolith
field
at
the
crest
at
a
depth
of
163
m
that
stretches
across
the
flat
to
the
foot
of
the
slope
of
Ll.
This
rhodolith
field
has
previously
been
correlated
with
the
mottled
backscatter
of
the
L2
flat
(Webster
et
al.,
2006).
T312
video
reveals
a
very
similar
pattern
with
nodules
at
the
foot
of
the
slope
that
are
cemented
into
a
pavement
outcrop.
At
this
location
though,
the
pavement
provides
the
only
outcrop
style
for
the
entire
terrace
face.
At
the
top
of
12
this
pavement
gives
way
to
the
same
style
of
rhodolith
fields
that
are
seen
at
the
T311
site
(Fig.
5c),
which
continue
across
the
flat.
Samples
from
these
dives
confirm
coralline
algal
nodules
to
be
the
main
limestone
across
these
reef
terraces
with
a
minor
occurrence
of
a
Halimeda
facies.
3.2.3.
Terrace
13
13
is
characterised
by
high
backscatter
values
along
the
crest
of
the
terrace
and
is
best
developed
around
and
between
the
islands
of
Lanai
and
Kahoolawe.
It
lies
at
a
depth
of
between
285
m
and
330
m
depending
on
location.
It
shows
no
elevated
rim,
moderate
relief
of
60
m,
and
a
low
degree
of
sinuosity
for
most
of
its
110
km
length.
In
contrast
to
the
rest
of
13,
the
section
between
Lanai
and
Kahoolawe
shows
moderate
sinuosity
and
lower
vertical
rise
of
40
m.
Chirp
data
indicate
little
or
no
sediment
was
deposited
seaward
of
the
12
promontory
in
contrast
with
the
lagoon
behind
12,
where
the
data
indicate
both
thick
sediment
deposits
(averaging
6-8
m
with
maximum
of
11
m)
and
buried
patch
structures
(Figs.
4
and
6a).
Sediment
is
visible
in
dive
video
(RCV-108)
with
low
backscatter
values
in
the
same
region
that
cover
most
of
the
floor
of
the
lagoon.
There
are
no
ROV
or
submersible
dives
over
the
slope
or
crest
of
13
seaward
of
the
12
promontory,
so
no
other
observations
have
been
made
for
this
part
of
the
terrace.
3.2.4.
Terrace
L4
L4
is
the
least
developed
terrace
with
a
total
length
of
28
km
and
ranges
from
360
m
to
380
m
depending
on
location.
It
exhibits
low
sinuosity
crossing
the
Kealaikahiki
Channel
between
Lanai
and
Kahoolawe,
before
wrapping
around
the
western
rift
zone
of
Kahoolawe.
A
large
patch
located
centrally
on
the
flat
of
L5
is
correlated
with
L4.
Its
50
m
vertical
rise
is
larger
than
the
fringing
sections
of
the
terrace
that
show
a
more
modest
vertical
relief
of
35
m.
Chirp
data
across
this
terrace
show
no
sediment
deposits
associated
with
L4.
Backscatter
data
exhibit
low
values
over
the
central
patch
reef
indicating
sediment
coverage
but
there
is
no
corroborating
Chirp
data.
High
backscatter
values
are
shown
along
the
slope
of
L4
and
across
the
flat
at
the
crest
of
the
terrace
which
is
confirmed
by
a
lack
of
sediment
in
the
Chirp
data
in
these
areas.
This
terrace
displays
no
raised
rim,
and
thus
no
Rim-Index
can
be
calculated,
and
due
to
its
limited
extent
there
are
no
ROV
or
submersible
dives
across
L4
and
so
no
direct
observations
were
made.
3.2.5.
Terrace
L5
L5
exhibits
very
low
sinuosity
along
56
km
south
of
Lanai
and
it
is
the
shallowest
terrace
not
to
trend
shoreward
toward
the
Kealaikahiki
-
o
z
0
b
z
-
co
136
I.D.E
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
156°45'0"W
J
156°40'0"VV
156°35'0"W
-,4
40
-
C.L
RCV-118
RCV-116
T311
P4-027
RCV-108
14
7
RCV-111
(Fig.
6a
T312
RCV-115
Terraces
—Li
—L3
P5-2
54
-.A.
Legend
—L2
—L4
*
Dive
Samples
—Chirp
Line
0
1
2
4
*
Dive
Video
`Sediment
Km
P4-026
Bathymetry
Backscatter
*-
-RCV-109
High
:
-50
High
:
255
RCV-11
L4
L3
* 7
Li
Kahoolawe
=Low
:
-400
Low
:
13
O
z
0
Cn
156°45'0"W
156°40'0"W
156°35'0"W
Fig.
4.
Expanded
section
of
Fig.
1
-
the
Central
region.
An
expanded
section
of
Fig.
1,
displaying
available
backscatter
data,
with
the
dives
and
samples
displayed
as
in
Fig.
1
and
the
Chirp
data
is
displayed
as
in
Fig.
3.
Channel
between
Lanai
and
Kahoolawe.
The
80
m
vertical
rise
of
L5
is
marked
by
a
winnowed
depression
at
the
foot
of
its
slope.
The
depth
of
this
terrace
ranges
from
360
m
to
550
m
depending
on
location.
Chirp
data
show
no
sediment
either
on
the
terrace
flat
or
within
the
dip
at
the
foot
of
the
slope.
This
is
confirmed
by
video
footage
of
P5-217
at
the
bottom
of
the
terrace
but
the
dive
did
not
reach
the
crest
so
there
is
no
ground-truthing
at
the
top
of
the
terrace.
High
backscatter
values
are
observed
along
the
slope
of
the
terrace
in
the
southern
region,
and
across
the
entire
barrier
structure
in
the
northern
region;
however
both
these
areas
are
steep
terrain,
and
thus
this
is
not
interpreted
as
a
hard
substrate.
The
northernmost
49
km
of
L5
shows
a
moderate
to
high
sinuosity
where
the
terrace
consists
of
pinnacles
and
barriers
separated
from
the
island's
coastline.
These
barrier
and
pinnacle
features
account
for
the
moderately
high
Rim-Index
of
0.45.
3.2.5.1.
P5-217
dive
observations.
Outcrop
begins
on
L5
with
a
low-
relief
(-1
m)
rocky
scarp
rising
from
a
sediment
covered
plain
(Fig.
7a),
with
numerous
corals
visible
within
the
face
of
the
outcrop
and
small
loose
limestone
blocks
sitting
at
its
base.
This
scarp
is
the
first
in
a
series
of
stepped
ledges
at
the
foot
of
the
main
slope
that
are
also
visible
in
the
high
resolution
bathymetry
(Fig.
3).
These
scarps
commonly
show
1-
2
m
of
vertical
relief,
with
large
blocks
(>4-5
m)
immediately
down
slope
of
the
face.
The
flat
of
the
lowest
of
these
scarps
contains
individual
coral
colonies
in
growth
position,
steadily
increasing
in
number
toward
the
second
ledge
until
there
are
continuous
coral-rich
outcrops
at
the
base
of
scarp
face.
From
this
point
(430
m)
up,
the
outcrop
on
the
flatter
sections
of
the
ledges
appears
smoother
and
more
weathered
often
in
layers
lying
in-dip
with
the
slope,
with
common
large
blocks
and
rubble
fields
at
the
base
of
the
scarp
faces.
Outcrop
in
the
faces
of
the
scarps
has
common
vertical
elements
of
corals
in
growth
position
with
layering
that
lies
across
the
direction
of
the
slope.
The
main
slope
of
L5
consists
of
similar
form
with
vertical
elements
that
look
like
in-situ
coral
in
horizontal
bedding.
This
reef
face
is
of
an
order
of
magnitude
bigger
(approximately
25
m)
than
the
stepped
scarps
below,
but
also
has
a
relatively
large
rubble
field
and
larger
blocks
at
its
base.
The
other
major
feature
of
the
L5
is
the
crest
at
-390
m
that
is
composed
of
a
massive
unit
lying
stratigraphically
above
the
lattice-work
of
the
main
slope.
Samples
from
P5-217
consist
of
shallow
reef
building
corals,
which
when
taken
with
the
dive
observations,
confirm
coral
reef
development
on
L5.
3.2.6.
Terrace
L6
Terrace
L6
is
a
steep
terrace,
averaging
18°
along
a
total
length
of
129
km
from
west
of
Lanai
around
the
canyons
south
of
Kahoolawe
and
ranges
in
depth
from
330
m
to
780
m.
For
much
of
this
length
L6
has
a
depression
at
the
foot
of
the
slope
(Fig.
3),
similar
to
L5.
However
L6
displays
none
of
the
barriers
or
pinnacles
prevalent
in
northern
L5,
and
subsequently
has
a
much
lower
Rim-Index
(0.13).
Whilst
Chirp
data
indicate
sediment
on
the
flat
of
L6,
that
thickens
from
2
m
near
the
hummocky
structures
in
the
centre
of
the
flat
to
11
m
approaching
the
base
of
L5
(Fig.
2b),
this
sediment
is
not
observed
in
the
dive
video
over
most
of
the
terrace
slope.
Backscatter
data
define
the
crest
of
L6
in
the
northern
region
with
high
values
along
its
length.
This
pattern
is
not
visible
in
the
limited
coverage
of
data
in
the
southern
region
with
low
values
displayed
across
the
whole
terrace.
L1
QC
Dive
T310
Legend
z)
250
200
150 100
50
0
a)
130
140
170
Coral
stand
160
c=2:.<
INEbb.
In-situ
coral
framework
200
L1
Hummocky
outcrop
E
240
L2
Yti
Outcrop
through
sediment
layer
Broken
block
sitting
on
sediment
0
280
11
11ii
Cliff-face
320
Dive
T311
tfta
Lava
flow
cliff-lace
Sheet
pavement
outcrop
1200
1000
800
600
400
200
Unit
layering of
Coral
reef-face
846
Nodule
field
Rubble
field
280
290
E
300
L2
a.
310
320
330
340
Dive
T312
(0.295
Ma
+/-
0.37
Mal
400
350
300
250
200
150 100
50
0
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
137
Distance
along
traverse
(m)
Fig.
5.
Outcrop
style
and
terrace
schematics.
The
different
outcrop
styles
represented
are
taken
from
ROV
dive
observations.
(a)
Terrace
Ll
from
dive
video
of
T310.
(b)
Terraces
Ll
and
12
from
dive
video
of
T311.
(c)
Terrace
12
from
dive
video
of
T312.
Ages
are
averaged
ages
relative
to
depth,
taken
from
Webster
et
al.
(In
review).
3.2.6.1.
T295
dive
observations.
Outcrop
starts
at
the
foot
of
the
slope
with
large
(2
m
across)
blocks
protruding
through
the
sediment
(Fig.
7b).
The
first
of
these
outcrops
are
loose,
likely
derived
from
upslope.
Further
along
the
dive
track,
sitting
in
the
dip
at
the
foot
of
the
slope,
the
blocks
give
way
to
similar
outcrops
that
appear
as
a
broken
unit,
with
piles
of
coral
appearing
between
blocks.
At
the
foot
of
the
slope
proper,
the
sedimentary
blocks
are
overlain
by
pavement-
style
outcrop.
This
pavement
continues
into
the
slope
and
is
overlain
firstly
by
unconsolidated
sediment,
and
then
the
same
blocky
outcrop
appears
down
slope.
These
hummocky
blocks
are
broken
into
smaller
pieces
in
parts.
Further
upslope
these
are
overlain
by
a
smooth
pavement
drape
in
a
small
scarp,
and
this
pavement
continues
upslope
to
the
crest
of
the
terrace
at
-490
m.
The
top
of
the
terrace
is
marked
by
a
small
rubble
sheet
with
a
low
linear
outcrop
of
pavement
running
parallel
with
the
crest,
set
back
about
30
m.
Thin
sediment
covers
the
flat
landward
of
the
crest.
Samples
from
T295
confirm
coral
reef
growth
on
L6.
3.2.7.
Terrace
L7
This
terrace
rises
vertically
over
60
m
with
an
average
incline
of
11°
in
dive
294
(Fig.
7c)
where
the
crest
lies
at
550
m.
At
the
northern
end,
dive
T308
is
characterised
by
a
steep
slope
of
31°
(Fig.
8a)
at
520
m
depth
however
this
dive
was
on
a
pinnacle
rather
than
a
terrace,
which
could
account
for
the
difference
in
the
slope.
L7
shows
a
marked
difference
in
morphology
over
part
of
its
mapped
110
km
length.
In
the
northern
region,
west
of
Lanai,
L7
shows
a
fringing
morphology
proximal
to
the
landmass,
and
a
series
of
barrier
and
pinnacle
structures
seaward
of
this.
In
the
southern
region,
south
of
Lanai,
the
fringing
terrace
continues
but
there
are
no
pinnacle
or
barrier
structures
associated
with
it.
L7's
moderately
high
Rim-Index
is
associated
with
the
occurrence
of
the
barrier
and
pinnacle
features
in
the
northern
region.
The
fringing
terrace
illustrates
a
very
low
sinuosity
along
its
entire
length
and
contrasts
with
the
high
sinuosity
of
the
pinnacle
and
barrier
features
of
L7.
Backscatter
data
for
L7
only
cover
the
fringing
terrace
section
in
the
northern
region
and
show
low
values
for
this
section.
Chirp
data
cross
the
southern
end
of
L7's
mapped
extent
revealing
several
prominent
sub-bottom
reflectors
across
the
flat.
These
reflectors
represent
multiple
sediment
packages
that
thicken
from
2
m
close
to
L7's
crest,
to
13
m
close
to
the
dip
at
the
foot
of
the
slope
of
L6
(Fig.
6b).
3.2.7.1.
T294
and
T308
dive
observations.
At
the
T294
location
(Fig.
7c),
outcrop
begins
at
the
foot
of
the
slope
as
solitary
blocks
protruding
through
unconsolidated
sediment.
Further
up
the
slope
at
-590
m,
these
Uninterpreted
500
m
Interpretation
L
Sediment
onlap
Buried
patch
reefs
500
m
a)
r
-
n
-
200
E
240
I=
›.
280
320
150
165
Ff
180
195
Uninterpreted
1
000
m
Interpretation
L6
L7
Multiple
sub-bottom
reflectors
1
000
m
b)
660
g
700
740
n.
780
(N
820
495
525
Q.
555
585
615
138
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
Fig.
6.
Chirp
seismic
data
showing
reef
structure
development.
(a)
Interpretation
of
Chirp
data
from
Ll,
as
marked
in
Fig.1,
showing
patch
reefs
buried
under
the
sediment
pile
and
sediment
on-lap
at
the
foot
of
the
terrace
within
the
lagoon
environment.
(b)
Interpretation
of
Chirp
data
from
L7,
as
marked
in
Fig.
1,
showing
several
reflectors
indicating
multiple
sediment
packages,
and
sediment
pinch-out
at
winnowed
foot
of
L6.
individual
blocks
merge
to
appear
as
near-continuous
low-lying
outcrop
that
is
bedded
at
the
same
angle
as
the
slope.
Up
the
slope,
the
pavement-
style
outcrop
is
divided
by
small
rubble
fields
in
hollows
with
small
low-
relief
(<1
m)
cliffs
of
exposed
outcrop
at
the
edges
of
these
hollows.
The
pavement-style
outcrop
is
unbroken
at
the
crest
of
L7
at
-550
m
however
here
L7
exhibits
a
hummocky
outcrop
style.
The
only
outcrop
style
visible
in
the
T308
dive
location
(Fig.
8a),
is
a
similar
near-continuous
outcrop
that
forms
a
smooth
pavement.
This
pavement
is
also
at
the
same
angle
as
the
slope,
and
there
are
no
rubble
fields
evident
at
the
T308
location.
Samples
from
both
these
dives
confirm
limestones
across
the
terrace,
and
samples
from
dive
T294
indicate
coral
reef
growth
for
at
least
part
of
L7's
extent.
3.2.8.
Terrace
L8
L8
is
of
low
relief,
with
a
vertical
rise
of
35
m
along
its
continuous
margin.
It
shows
a
similar
morphology
to
L7
in
the
northern
section
of
its
210
km
mapped
length,
with
barrier
structures
rising
up
to
90
m.
The
terrace
crest
lies
at
between
605
m
and
1050
m
depending
on
location.
The
sinuosity
of
L8
is
moderate
to
high
in
both
the
northern
and
southern
sections
however
it
lowers
at
the
most
southern
end,
near
the
canyons
south
of
Kahoolawe.
L8
is
characterised
by
a
relatively
low
Rim-Index
of
0.29.
Hummocky
structures,
similar
but
larger
in
scale
to
those
of
L6,
are
visible
in
the
bathymetric
data
on
the
southern
flat,
and
Chirp
data
also
indicate
3-5
m
of
sediment
across
the
flat.
Due
to
its
more
distal
location,
there
are
no
ROV
or
submersible
dives
across
L8.
3.2.9.
Terrace
L9
Terrace
L9
is
one
of
the
most
prominent
terraces
of
the
MNC
lying
at
the
top
of
a
high-relief
escarpment
(100-600
m)
(Figs.
1,
2
and
3)
that
marks
the
seaward
edge
of
the
MNC.
This
terrace
also
exhibits
a
very
steep
slope
averaging
53°,
(Fig.
8b),
with
its
crest
exhibiting
low
to
moderate
sinuosity
along
its
152
km
length
that
ranges
from
605
m
to
1140
m
in
depth.
The
terrace
has
a
high
Rim-Index
(0.78)
with
a
distinctive
raised
rim
that
is
up
to
45
m
above
the
flat
of
the
terrace.
Spatially,
L9
extends
well
to
the
west
of
Kahoolawe,
creating
an
extensive
flat
up
to
18
km
wide
in
parts.
Lagoons
are
observed
on
this
terrace
flat,
clearly
visible
in
the
bathymetric
data,
and
are
up
to
40
m
deep
and
7.5
km
long.
These
areas
are
similar
to
those
seen
on
Ll
but
are
an
order
of
magnitude
larger.
The
flat
around
the
lagoons
reveals
patchy
sediment
cover,
visible
in
the
Chirp
data,
generally
1.5-6
m
thick
with
a
maximum
of
13
m.
3.2.9.1.
T309
dive
observations.
Basalt
flows
overly
the
first
carbonate
outcrops
present
in
the
T309
location.
The
carbonate
outcrop
is
a
lithified
pavement
that
dips
in
the
same
direction
as
the
slope
and
contains
many
basalt
cobbles
that
appear
to
be
cemented
into
the
carbonate
pavement
(Fig.
8b).
The
pavement
itself
is
weathered
and
winnowed,
giving
it
a
reef-like
appearance,
but
without
obvious
in-
situ
corals
present.
This
outcrop
style
continues
all
the
way
up
the
reef
slope
until
approximately
90
m
below
the
crest
where
branching
in-
situ
reef
framework
was
observed
for
the
first
time
this
outcrop
was
also
described
by
Webster
et
al.
(in
review).
This
steep
face
continues
all
the
way
up
the
slope
to
the
crest
of
the
terrace
exhibiting
at
least
five
apparent
stepped
layers
within
the
reef
face,
defined
by
flat
joint-
planes
and
either
uniform
recesses
or
protrusions.
A
distinct
lip
in
the
outcrop
at
602
m
signifies
the
end
of
this
reef
face
and
the
crest
of
the
terrace.
Behind
the
crest
are
small
rubble
fields
and
hummocky
outcrop
that
continue
out
onto
the
flat.
Samples
from
this
dive
confirm
coral
reef
limestones
across
the
terrace
and
the
detailed
composition
and
chronology
of
these
samples
are
presented
in
Webster
et
al.
(in
review).
3.2.10.
Terrace
L10
L10
marks
the
edge
of
the
MNC
to
the
southern
end
of
its
120
km
length,
protrudes
from
the
face
of
L9's
slope
in
the
central
section
of
its
mapped
extent,
and
displays
the
same
barrier
structures
as
L8
and
L6
in
the
northern
section.
Depths
for
this
terrace
range
from
700
m
to
835
m.
All
three
regions
of
L10
record
low
sinuosity,
with
the
entire
length
of
the
crest
of
the
terrace
displaying
a
raised
rim
yielding
a
Rim-Index
of
1.0.
There
are
no
ROV,
backscatter
or
Chirp
data
in
the
central
or
southern
section
that
provide
any
more
detail
for
these
regions.
3.2.10.1.
P5-191
dive
observations.
The
first
outcrops
near
the
foot
of
the
slope
of
L10
(Fig.
8c)
are
large
limestone
blocks
composed
of
corals
embedded
within
the
matrix,
and
are
typical
of
the
common
outcrop
style
over
the
rest
of
the
terrace.
One
of
these
blocks
displays
in-situ
coral
framework,
however,
it
is
impossible
to
determine
whether
the
block
itself
is
actually
in-situ.
The
foot
of
the
slope
proper
at
975
m,
exhibits
a
shift
to
pavement-style
outcrop
on
a
much
steeper
slope.
This
outcrop
style
characterises
most
of
the
rest
of
the
slope
of
the
pinnacle
and
varies
primarily
in
the
angle
of
the
slope.
However,
at
a
depth
of
approximately
850
m,
there
is
a
25
m
vertical
reef
face
with
in-situ
corals
clearly
visible
and
near
the
top
of
the
pinnacle
is
a
sandy
flat
area
with
scattered,
thinly
sediment-veiled,
hummocky
boulders.
These
two
regions
are
the
only
other
types
of
outcrop
style
seen
on
the
pinnacle.
The
fringing-reef
section
of
the
terrace
exhibits
similar
outcrop
style
to
the
pinnacle
terrace,
with
a
continuous
outcrop
of
in-
a)
E
O
390
400
410
420
430
440
450
b)
490
500
L6
E
510
520
0
a
,
530
540
(0.689
Ma
+1-
0.141
Ma)
(0.643
Ma
-14-
0.664
Ma)
Dive
T295
200
150 100
50
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
139
L5
Dive
P5-217
1000
800
600
400
200
0
c)
550
L7
560
(0.643
Ma
-al-
6.047
Ma)
E
570
Legend
(if
Coral
stand
A.
In-situ
coral
framework
Hummocky
outcrop
Outcrop
through
sediment
layer
Broken
block
sitting
on
sediment
Cliff-face
Lava
flow
cliff-face
Sheet
pavement
outcrop
Unit
layering
of
Coral
reel-face
8
Nodule
field
4
14,
Rubble
field
(0.788
Mar/.
0.130
Ma)
Dive
T294
r
a
580
590
600
400
300
200
100
0
Distance
along
traverse
(m)
Fig.
7.
Outcrop
style
and
terrace
schematics.
The
different
outcrop
styles
represented
are
taken
from
ROV
dive
observations.
Part
(a)
is
not
to
scale
as
there
was
no
navigational
data
to
construct
transects
from
this
figure
is
purely
observational.
(a)
L5
from
dive
video
of
P5-217.
(b)
Terrace
L6
from
dive
video
of
T295.
(c)
Terrace
L7
from
dive
video
of
T294.
Ages
are
averaged
ages
relative
to
depth,
taken
from
Webster
et
al.
(In
review).
situ
coral
framework
sloping
at
the
same
angle
as
the
terrace
slope.
This
pavement
is
thinly
covered
by
sediment,
with
the
corals
evident
as
blocks
protrude
through
the
mud.
Samples
of
shallow
water
corals
and
associated
coralline
algae
confirm
this
terrace
to
be
coral
reefal
in
origin.
3.2.11.
Terrace
L11
L11
is
the
second
least
developed
terrace
identified
in
the
MNC
with
its
21
km
mapped
extent
all
occurring
as
fringing
reef
on
the
substrate
of
the
Clarke
debris
avalanche
to
the
west
of
Lanai.
Its
crest
lies
at
between
930
m
and
1040
m
depth
and
exhibits
relatively
low
relief
of
40-60
m.
L11
has
a
raised
crest
(1.0
Rim-Index)
of
low
sinuosity,
and
no
distinctive
features,
with
no
backscatter,
Chirp,
or
submersible/
ROV
data
to
confirm
the
details
of
this
terrace.
3.2.12.
Terrace
L12
L12
is
also
a
fairly
poorly
developed
terrace
with
only
47
km
mapped.
Due
to
its
distal
location
on
the
edge
of
the
MNC,
L12's
vertical
relief
is
one
of
the
largest,
ranging
between
300
and
450
m.
The
crest
lies
at
1170
m
to
1270
m
depth
and
shows
low
sinuosity
except
for
one
small
section
directly
west
of
the
northern
tip
of
Lanai
and
has
a
Rim-
Index
of
1.00.
There
is
no
evidence
on
the
nature
and
composition
of
this
terrace
as
no
dives
or
dredges
have
been
conducted
over
it.
Additionally,
the
quality
of
Chirp
data
recovered
over
L12
was
poor
due
to
the
terraces
depth
and
its
location
on
a
steep
incline.
4.
Discussion
4.1.
Regional
and
temporal
differences
in
reef
development
The
large-scale
morphology
of
the
submerged
reefs
of
the
MNC
shows
major
regional
variation
with
the
presence
of
pinnacles
and
barrier
structures
west
and
south
of
Lanai
and
the
absence
of
these
features
elsewhere.
Additionally,
the
southern
region
of
the
south-
central
MNC
(Fig.
3)
shows
greater
terrace
width
than
the
northern
region
(Fig.
2),
and
the
central
region
(Fig.
4)
displays
lagoon
and
patch
morphologies
that
are
absent
elsewhere
within
the
MNC.
These
variations
in
the
distribution
and
morphology
of
the
reefs
indicate
regional
change
in
factors
such
as
accommodation
space
and
substrate
across
the
MNC.
More
importantly
the
reefs
were
dominated
by
a
140
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
a)
480
520
560
600
640
E
0
a)
O
L7
(0.811
Ma
+/-
0.24
Ma)
Dive
T308
400
300
200
b)
100
0
600
650
L9
(0.509
Ma
+/-
0.338
Ma)
(1.178
Ma
+1-
0.077
Ma)
Legend
Coral
stand
E
700
41:k
-
(0.802
Ma
+1-
0.012
Ma)
Oar
In-situ
coral
framework
0
a)
750
Hummocky
outcrop
800
Outcrop
through
sediment
layer
850
(1-002
Ma
+1-
0-314
Ma)
Broken
block
sitting
on
sediment
900
Dive
T309
'
4
1.1
gum
Cliff-face
Lava
flow
cliff-face
300
300
200
100
c)
Sheet
pavement
outcrop
Unit
layering
of
Coral
reef-lace
700
CZX
Nodule
field
800
Rubble
field
L10
(1.036
Ma
+/-
0.19
Ma)
c.
a)
900
10
(1.110
Ma
+/-
0.043
Ma)
O
1000
1100
Dive
P5-191
400
350 300
250
200
150 100
50
0
Distance
along
traverse
(m)
Fig.
8.
Outcrop
style
and
terrace
schematics.
The
different
outcrop
styles
represented
are
taken
from
ROV
dive
observations.
Part
(c)
is
not
to
scale
as
there
was
no
navigational
data
to
construct
transects
from
this
figure
is
purely
observational.
(a)
Terrace
L7
from dive
video
of
T308.
(b)
Terrace
L9
from
dive
video
of
T309.
(c)
Terrace
L10
from
dive
video
of
P5-191.
Ages
are
averaged
ages
relative
to
depth,
taken
from
Webster
et
al.
(In
review).
temporal
change
in
sea-level
cyclicity
the
Mid-Pleistocene
Transi-
tion
(MPT).
In
the
following
sections
by
looking
at
the
pre-MPT,
the
MPT,
and
the
post-MPT
reefs
independently,
we
show
how
this
temporal
change
is
the
primary
factor
influencing
the
observed
variations
in
the
reef
morphology.
4.2.
Pre-MPT
Maui-Nui
Complex
vs
post-MPT
Hawaiian
Reef
growth
prior
to
the
MPT
was
restricted
to
the
outer
margin
of
the
Complex
to
the
south
and
west
of
Kahoolawe
and
the
southwest
and
west
of
Lanai
(Fig.
9).
Additionally,
the
complex
was
subjected
to
eustatic
sea-level
cycles
of
60-70
m
amplitude
over
a
41
kyr
period
during
this
time.
Using
the
Sr
age
data
Webster
et
al.
(2009
(in
review))
calculated
an
average
subsidence
rate
of
0.85
m/kyr
for
the
deepest
dated
terrace,
L10.
Given
the
last
250
kyr
has
likely
been
nearly
stable
(Webster
et
al.,
2007a),
it
is
probable
that
this
0.81
m/
kyr
linear
rate
is
a
poor
approximation,
with
the
MNC
undergoing
initially
rapid
subsidence
that
slowed
as
the
Pacific
plate
moved
the
complex
beyond
the
hot
spot.
Thus
with
the
growth
of
Lanai,
West
Maui
and
Kahoolawe
all
occurring
during
the
time
of
L12-L9
(-1.3-
0.9
Ma),
the
MNC
would
have
been
experiencing
rapid
subsidence,
approaching
the
modern
subsidence
rate
of
2.2
m/kyr
of
the
big
island
of
Hawaii
(Moore
et
al.,
1996).
Given
the
subsidence
rates
for
the
pre-MPT
MNC
and
Hawaii,
it
is
reasonable
to
look
to
the
submerged
reefs
offshore
of
the
Big
Island
of
Hawaii
for
comparison
with
pre-MPT
MNC
in
respect
to
morphology
and
development
patterns.
Campbell
(1986)
was
the
first
to
identify
the
Lanai
terraces
and
correlate
them
with
the
terraces
offshore
Hawaii
based
on
depth
and
gross
morphology.
Campbell
(1986)
also
identified
barrier
reef,
lagoon
and
patch
reef
features
on
the
Hawaiian
425
m
terrace
and
presented
two
bathymetric
profiles
comparing
the
MNC
terraces
in
the
northern
region
with
the
series
of
terraces
offshore
Hawaii.
Campbell's
figure
shows
that
the
Hawaiian
reefs
are
more
broadly
backstepping
than
the
terraces
on
the
MNC,
and
display
none
of
the
offshore
barrier
and
pinnacle
features
present
within
the
MNC.
Campbell's
(1986)
barrier
and
lagoon
system
on
the
425
m
terrace
was
also
highlighted
by
Jupiter
et
al.
(2002)
who
conducted
a
detailed
morphologic
study
of
this
terrace.
These
features
are
similar
to
those
found
on
the
upper
terraces
of
the
MNC
(Fig.
3)
that
also
grew
in
response
to
100
kyr
sea-level
cycles.
Legend
Location
of
volcanoes
Active
terraces
Submerged
terraces
Centre
of
subsidence
Exposed
at
Sea-Level
Max.
El
Exposed
at
Sea-Level
Min.
Maui-Nui
Complex
Islands
ie:
ob
ib
ibo
Clark
Landslide)
-1.3
MA
\
/
1
Kahoolawe
Rift
After
Price
&
Elliot
Fisk
(2004)
1.6
1.4
12
1
A
0.8
0.6
0.4
02
0
\)\
,41
1
A
66-70
m
N
Sea-level
oscillation
Faulting
&
Clarke
debris
avalanche
(-1.3
Ma)
Rapid
Subsidence
(-2.0
mm/yr)
B
B'
66-70
m
Sea-level
oscillation
Multipile
Packages
of
Vertical
Reef
Development
Rapid
Subsidence
(-2.0
mm/yr)
Faichney
et
al.
/
Marine
Geology
265
(2009)
130-145
141
Fig.
9.
Model
of
MNC
reef
terrace
development
(pre-MPT).
Map
showing
the
aerial
extent
of
the
MNC
during
1.2-1.0
Ma.
Two
profiles
(A-A'
on
the
Clarke
debris
avalanche
terrain
and
B-B'
in
the
southern
region
of
the
complex)
are
shown
displaying
reef
morphologies
in
response
to
rapid
subsidence
and
small
amplitude,
short
period
sea-level
oscillations.
Note
the
proximal
centre
of
subsidence
causing
the
rapid
subsidence
rate.
Data
from
Shackleton
et
al.
(1995).
Given
the
likely
similar
subsidence
rates
of
the
pre-MPT
MNC
and
the
current
rates
observed
on
Hawaii,
we
propose
that
it
is
the
difference
in
eustatic
sea-level
cyclicity
either
side
of
the
MPT
that
is
the
major
driving
factor
in
the
variation
in
terrace
morphology
observed
between
the
MNC
and
offshore
Hawaii.
We
propose
that
the
conditions
of
rapid
subsidence
and
short,
low-amplitude
sea-level
oscillations
dominant
for
the
MNC
prior
to
the
MPT
produced
a
different
mode
of
reef
morphology.
During
regression,
flank
sub-
sidence
allowed
reef
growth
on
these
platforms;
however
instead
of
drowning
and
backstepping
during
sea-level
rise,
the
smaller
amplitude
of
the
sea-level
oscillations
and
shorter
time-scale
for
subsidence
to
take
place
allowed
the
reefs
to
re-occupy
and
resume
growth
upon
the
next
regression.
This
process
allowed
numerous
cycles
of
reef
growth
in
the
same
location
before
island
subsidence
submerged
a
terrace
out
of
the
growth
zone.
Importantly,
in
the
northern
region,
extensive
backstepping
did
not
occur
due
to
the
steep
substrate
and
rapid
sea-level
cycles,
with
the
re-occupation
and
re-growth
producing
offshore
barriers
and
pinnacles.
Additionally,
in
the
absence
of
raised
and
isolated
substrate
in
the
southern
region,
(Le.
no
complex
topography
associated
with
the
Clarke
debris
avalanche)
re-occupation
episodes
were
expressed
as
a
thicker
fringing-reef
face.
This
formation
of
pinnacles
and
barrier
structures
in
the
northern
region
and
thick
reef
face
to
the
south
results
in
higher
Rim-Index
values
for
these
terraces.
The
proposed
model
is
supported
by
both
the
pinnacle
and
barrier
features
themselves,
and
thick
in-situ
reef
face
observed
during
dive
T309
(Fig.
8b).
In
Hawaii,
where
submerged
reef
terraces
have
also
developed
in
response
to
rapid
subsidence,
terrace
backstepping
is
the
dominant
terrace
morphology.
In
the
Hawaii
case
however,
eustatic
sea-level
cycles
were
of
much
larger
amplitude
and
longer
period,
hence
terrace
re-occupation
could
not
take
place
to
the
same
extent
as
in
the
MNC.
4.3.
MPT
terraces
The
time
frame
between
900
and
800
ka
represents
the
Mid-
Pleistocene
Transition,
and
as
such,
the
period
of
transition
from
41
kyr
cycles
to
100
kyr
sea-level
cycles.
It
also
represents
a
time
when
the
centre
of
subsidence
was
moving
from
fairly
central
MNC
(west
Maui)
to
a
more
marginal
location
on
the
complex
(south-eastern
Legend
Location
of
volcanic
cones
Active
terraces
Submerged
terraces
Centre
of
subsidence
Exposed
at
Sea-Level
Max.
Exposed
at
Sea-Level
Min.
Maui-Nui
Complex
Islands
B'
After
Price
&
Elliot
Fisk
(2004)
1,6
1A
1.2
1
0
0.8 0.6
0.4
0.2
0
kt1
0-
o
0
A
A'
70
-
100
m
Sea-level
oscillation
Offshore
Pinnacle
Development
On
Steepened
Substrate
Near
Stability
(+0.1
to
-0.8
mm/yr)
B
B'
70
-
100
m
Sea-level
oscillation
Beginning
of
Terrace
Backstepping
Near
Stability
(+0.1
to
-0.4
mm/yr)
142
I.D.E
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
Fig.
10.
Model
of
MNC
reef
terrace
development
(during
MPT).
Map
of
the
aerial
extent
of
the
MNC
at
-0.8
Ma
using
the
same
shading
as
Fig.
9.
Two
profiles
are
shown;
A-A'
in
the
Northern
region
and
B-B'
across
the
southern
region.
Note
the
migrating
centre
of
subsidence
(near
Haleakala)
causing
a
slowing
rate
of
subsidence,
and
the
change
in
amplitude
and
period
of
the
sea-level
oscillations.
Data
from
Shackleton
et
al.
(1995).
Haleakala)
as
Haleakala
became
the
dominantly
growing
volcano
and
Kohala
started
forming
(Fig.
10).
This
movement
of
the
centre
of
subsidence
is
reflected
in
a
slowing
subsidence
rate
across
the
complex.
Thus
the
L8
to
L5
terraces
developed
in
a
response
to
a
situation
of
slowing
subsidence
and
changing
eustatic
sea-level
cyclicity.
Reef
growth
in
this
period
was
located
west
and
south
of
Lanai
and
to
the
west
and
south
of
Kahoolawe
(Fig.10).
The
terraces
of
the
MPT,
L8
to
L5,
exhibit
variable
but
relatively
uniform
morpholo-
gical
characteristics
(Table
2).
There
is
not
any
identifiable
pattern
or
general
trend
within
this
group.
When
compared
to
terraces
both
before
and
after
the
MPT,
however,
trends
are
apparent
in
both
vertical
relief
and
Rim-Index.
The
pre-MPT
terraces
of
the
MNC
(L12-I9)
have
more
continuous
raised
platform
margins,
i.e.
Rim-Index
values
significantly
higher
(averaging
0.89)
than
that
of
terraces
developed
within
the
MPT
(L8-L5)
which
average
0.256.
The
MPT
terraces
in
turn
have
a
higher
Rim-Index
than
post-MPT
terraces
(L4-L1)
for
which
it
is
zero
in
each
case.
The
vertical
relief
of
the
terraces
also
diminishes
from
pre-MPT,
to
MPT,
to
post-MPT
time
periods.
Schlager
(2005)
defined
a
Rim-Index
as
a
measure
of
the
continuity
of
a
platform
rim
and
used
this
measure
to
comment
on
wave
energy
entering
the
lagoon.
Alternatively,
we
suggest
that
the
Rim-Index
could
be
interpreted
as
a
proxy
for
the
terrace
edge
as
being
the
preferred
location
of
frame-building
corals.
Kennedy
and
Woodroffe
(2002)
highlight
six
models
of
fringing-reef
development
on
various
substrate
types
and
tectonic
settings,
and
show
that
with
ample
accommodation
space
vertical
growth
is
faster
on
the
reef
crests.
As
such,
we
apply
this
index
to
comment
on
the
accommoda-
tion
space
of
a
terrace
as
it
develops;
i.e.
a
high
Rim-Index
indicates
that
the
terrace
was
created
by
an
active
coral
reef
crest
with
ample
accommodation
space.
We
propose
that
this
temporal
variation
in
Rim-Index
is
a
direct
reflection
of
a
change
in
accommodation
space
caused
by
variation
in
subsidence
rate,
eustatic
sea-level
cycles
and
the
slope
of
the
substrate.
The
more
rapid
subsidence
experienced
by
the
MNC
pre-MPT
coupled
with
the
smaller
and
more
rapid
sea-level
oscillations
resulted
in
continuous
formation
of
accommodation
space
that
allowed
rapid
reef
growth.
With
either
a
slowing
in
subsidence,
or
a
shift
in
sea-level
cyclicity,
(both
are
seen
with
the
onset
of
the
MPT),
this
creation
of
accommodation
space
is
reduced
and
rapid
reef
growth
at
the
terrace
rim
will
slow
down,
lowering
the
Rim-Index.
It
is
important
to
note
that
solutional
erosion
of
reefs
when
subaerially
exposed
can
amplify
the
elevated
rims
of
fossil
reefs,
however
the
data
we
present
on
the
morphology
of
the
reefs
indicate
the
terraces
more
Legend
Location
of
volcanic
cones
Active
terraces
---
Submerged
terraces
Centre
of
subsidence
Exposed
at
Sea-Level
Max.
CI
Exposed
at
Sea-Level
Min.
Maui-Nui
Complex
Islands
e„.
A
B'
After
Price
&
Elliot
Fisk
(2004)
1.6
1.4
1.2
1.0
0.8 0.6
OA
0.2
0
#)11
A
120
m
Sea-level
oscillation
•••
Offshore
Pinnacle
Development
On
Steepened
Substrate
Near
Stability
(+0.1
to
-0.8
mmlyr)
A'
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
143
likely
to
be
exposed
and
subject
to
extensive
subaerial/solution
erosion
(ID,
Ll,
L2
and
13)
show
little
to
no
Rim-Index,
and
we
argue
that
our
interpretation
of
the
Rim-Index
is
the
reason
why
this
is
the
case.
The
changing
conditions
within
the
MPT
gave
rise
to
variation
within
these
terraces
(Table
2),
however
no
overall
temporal
pattern
for
this
period
can
be
identified.
Even
within
the
time
frame
of
the
MPT,
the
L8
to
L5
reefs
show
significant
variation
in
the
nature
of
backstepping
evident
between
the
terraces
(Figs.
2
and
3).
Generally
speaking,
the
slope
of
the
underlying
substrate
is
critical
to
regional
terrace
morphology
with
respect
to
the
amplitude
of
any
terrace
backstepping.
Within
the
southern
region
of
the
MNC,
the
transition
from
the
steeper
distal
flanks
of
the
volcanos
(broadly
evident
in
the
pre-MPT
terrace
L9),
to
the
more
gently
sloping
upper
substrate
(Fig.
3)
allowed
larger
backstepping
to
occur.
This
change
in
substrate
slope
coincided
with
the
start
of
the
increase
in
amplitude
of
eustatic
sea-level
cycles
(Marine
Isotope
Stage
19,
21).
In
contrast,
the
steeper
substrate
of
the
northern
region
(Fig.
2)
produced
closer
spacing
of
terraces
under
the
same
conditions,
similar
to
the
spatial
distribution
of
the
deeper
terraces.
We
propose
that
two
factors
contribute
to
the
pinnacles
and
barriers
continuing
to
form
on
MPT
terraces
(L8,
L7
and
L5).
These
two
factors
are
1)
the
reduced
nature
of
the
backstepping
in
the
northern
region
due
to
steeper
substrate,
and
2)
the
persistence,
albeit
reduced
in
strength,
of
the
41
kyr
periods
of
interglacial/glacial
cyclicity
throughout
the
MPT.
4.4.
Post-MPT
terraces
These
reefs
(L4-L1)
grew
in
response
to
the
conditions
since
the
Mid-
Pleistocene
Transition.
For
this
period,
the
interglacial/glacial
cycles
are
dominated
by
a
100
kyr
period,
and
the
centre
of
subsidence
was
further
from
the
MNC,
near
Kohala
(Fig.11).
Reef
growth
in
this
period
was
located
primarily
between
the
islands
of
Molokai
and
Lanai,
and
Lanai
and
Kahoolawe
(Fig.11).
Terraces
developing
at
this
time
exhibit
no
raised
rim
and
are
of
lower
vertical
relief
than
the
earlier
terraces.
Webster
et
al.
(2007a)
propose
that
the
MNC
is
currently
nearly
stable
with
calculations
indicating
between
0.1
m/kyr
uplift
and
0.4
m/
kyr
subsidence
over
the
last
30
kyr
and
perhaps
the
last
250
kyr.
Similarly,
Webster
et
al's
(in
review)
Sr
dating
indicates
that
since
13's
formation
at
0.533
Ma,
there
has
been
an
average
of
0.58
m/kyr
subsidence.
These
data
indicate
a
slowing
in
subsidence
within
the
MNC,
that
correlates
with
the
movement
of
the
MNC
away
from
the
Hawaiian
hotspot
as
the
Pacific
plate
migrated
to
the
northwest.
This
B
120
m
B'
Sea-level
oscillation
t
I
Terraces
potentially
exposed
on
sea-level
fall
Near
Stability
(+0.1
to
-0.4
mm/yr)
Fig.
11.
Model
of
MNC
reef
terrace
development
(post-MPT).
Map
of
the
aerial
extent
of
the
MNC
at
-0.4
Ma
using
the
same
shading
as
Fig.
9.
Two
profiles
are
shown;
A-A'
in
the
Northern
region
and
B-B'
across
the
southern
region.
Note
the
distal
centre
of
subsidence
(near
Kohala)
causing
a
slower
rate
of
subsidence,
and
the
larger
amplitude,
longer
period
sea-level
oscillations.
Data
from
Shackleton
et
al.
(1995).
144
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
Y-axis:
Angle
of
Substrate
10
12
Steep
L9
—4
1
1(
LB
L11
I
I I
I
I
I I
I I
I
I
I I
/ /
I
I
./
.1
(
L7
I
I
Gently
(6
I
I
Dipping
Stable
/
I
I
Upid
Su
s
dence
X-axis:
41
Kyr
// //
Subsidence
I
Rate
/// /1
-74
/
/—
I
/—
Mid
Z-axis:
Climate
&
/// 1(
1_4
Pleistocene
Transition
Sea-level
//
Oscillations
//
L3
Fossil
reefs
100
Kyr
L1
2
Ce
around&
Big
Island
of
Hawaii
Fig.
12.
Conceptual
diagram
showing
main
factors
influencing
reef
development.
This
figure
shows
the
three
main
factors
controlling
reef
development
within
the
MNC
and
plots
each
of
these
within
three
dimensional
space.
Each
terrace
is
plotted
with
trace
lines
for
where
on
each
axis
it
plots.
The
Mid-Pleistocene
Transition
is
represented
by
the
red
band
with
41
kyr
terraces
grouped
in
green,
and
100
kyr
terraces
grouped
in
blue.
The
region
that
the
Hawaiian
reefs
would
plot
within
this
conceptual
3D
space
is
also
marked.
(For
interpretation
of
the
references
to
colour
in
this
figure
legend,
the
reader
is
referred
to
the
web
version
of
this
article.)
time
frame
also
coincides
with
the
onset
of
the
domination
of
the
100
kyr
oscillation
in
eustatic
sea-level
cyclicity.
Both
the
amplitude
and
style
of
sea-level
oscillation
during
the
100
kyr
cycles
changed
from
pre-MPT
sinusoidal
60-70
m
cycles
to
a
more
saw-tooth
pattern
and
a
greater
amplitude
of
up
to
120
m.
Large
amplitude
of
sea-level
variation,
slow
subsidence
(or
stability)
of
the
complex,
and
limiting
reef
growth
to
the
central
region
of
the
complex
have
created
reef
growth
conditions
where
reef
terraces
stack
on
each
other,
with
little
or
no
lateral
movement
or
backstepping
possible.
The
promontory
relationship
of
12
and
Ll
and
the
lagoonal
feature
of
L3
(Fig.
3)
suggest
stacking
of
the
terraces
with
little
or
no
backstepping
with
reef
stacking
evident
in
the
antecedent
topography
Grigg
et
al.
(2002)
highlighted
in
the
Au'au
channel.
These
reefs
forming
the
post-MPT
terraces
developed
during
sea-level
high-
stands,
similar
to
the
modern
environment,
with
the
successive
terraces
developing
on
the
successive
sea-level
high-stands.
Age
control
(Webster
et
al.,
in
review)
indicates
that
13
developed
during
Marine
Isotope
Stage
(MIS)
11
with
12
then
developing
during
the
next
cycle
(MIS
9),
Ll
during
MIS
7
and
LO
during
MIS
5.
This
would
suggest
that
these
terraces
have
also
undergone
some
subsidence
to
be
at
their
current
depths,
implying
that
Webster
et
al's
(2007a)
potential
subsidence
rate
of
0.4
m/kyr
is
more
likely
than
their
potential
0.1
m/kyr
uplift
rate.
Additionally,
during
the
sea-level
low-
stands,
at
least
Ll,
L2
and
13,
and
possibly
L4
were
likely
reoccupied
by
intermediate
to
deep-water
coralline
algal
nodule
and
coralline
crust
development
(Webster
et
al.,
2006;
Webster
et
al.,
2009).
Lithologic,
chronologic
and
morphologic
data
(Webster
et
al.,
2006)
confirm
that
this
low-stand
re-occupation
scenario
has
taken
place
on
Ll
and
12
during
the
LGM
(MIS
2).
With
such
large
amplitudes
and
long
periods
of
sea-level
oscillations,
the
upper
part
of
Ll
(Le.
LO)
would
have
been
subjected
to
significant
subaerial
exposure
during
successive
low-stands.
This
conclusion
is
supported
by
terrace
morphology,
(Fig.
4)
and
the
presence
of
karst-like
features
identified
by
Grigg
et
aL
(2002).
Grigg
et
al.
(2002)
suggest
that
around
14
ka
sea
level
was
-
82
m
in
the
Au'au
Channel
-
the
upper
parts
of
LO
would
have
been
exposed.
Assuming
recent
stability
and
using
a
60
18
sea-level
proxy
from
ODP
site
677
and
current
depth,
over
the
past
500
kyr,
LO
would
have
been
repeatedly
subaerially
exposed
for
a
total
of
at
least
145
kyr.
This
sort
of
time
frame
would
have
been
ample
to
produce
the
well
developed
and
defined
karst
morphologies
evident
in
the
multibeam
bathymetry.
Repeated
subaerial
exposure
of
this
mag-
nitude
and
over
this
time
frame
would
lead
to
higher
erosion
rates
than
experienced
by
deeper
terraces
that
remained
submerged.
The
total
exposure
of
145
kyr
would
have
occurred
over
5
distinct
periods,
each
greater
than
10
kyr,
with
the
longest
period
being
just
over
50
kyr.
These
sorts
of
periods
and
lengths
of
subaerial
exposure
would
produce
significant
amounts
of
mud
and
silt
by
mechanical
erosion
in
addition
to
solution
weathering
of
the
karst
landscapes.
5.
Conclusions
1.
We
have
identified
12
reefs
offshore
Lanai
in
the
Maui-Nui
Complex
that
line
the
volcanic
flanks
of
the
islands,
and
range
in
depth
from
150
m
to
1200
m.
Modern
reef
features
such
as
slopes,
crests,
lagoons
I.D.E.
Faichney
et
at
/
Marine
Geology
265
(2009)
130-145
145
and
patch
reefs
have
been
identified
on
the
submerged
terraces,
and
video
observations
and
samples
confirm
the
terraces
to
be
reefs.
2.
The
morphology
and
structure
of
the
reef
terraces
vary
spatially
within
the
study
area
displaying
three
distinct
morphologies
in
three
different
regions.
The
northern
region
is
dominated
by
offshore
pinnacle
and
barrier
structures,
the
southern
region
is
characterised
by
large-scale
terrace
backstepping,
and
the
central,
most
recent,
region
shows
lagoonal,
patch
and
karst
development.
3.
The
morphology
of
the
reefs
displays
distinct
temporal
variation
correlating
with
ages
before,
during
and
after
the
Mid-Pleistocene
Transition.
Thick
reef
faces
and
offshore
pinnacle
and
barrier
structures
characterise
the
Pre-MPT
reefs.
There
is
a
shift
to
broad
backstepping
in
the
south
and
a
reduction
in
the
offshore
pinnacle
and
barrier
structures
in
the
northern
region
with
the
onset
of
the
MPT.
The
Post-MPT
reefs
show
evidence
of
reef
stacking
with
karst
development
features
evident
on
the
shallowest
reef
terrace.
4.
Over
the
past
1.2
Ma,
the
northwest
migration
of
the
Pacific
Plate
has
carried
the
Maui-Nui
Complex
away
from
the
Hawaiian
hotspot.
Coupled
with
the
development
of
the
big
island
of
Hawaii,
this
migration
has
resulted
in
the
movement
of
the
centre
of
subsidence
away
from
MNC
towards
Hawaii
and
a
slowing
in
the
subsidence
rate
experienced
within
the
complex.
5.
We
argue
the
observed
variation
in
morphology
in
MNC
reef
development
was
controlled
by
three
main
factors
(Fig.
12);
(a)
the
subsidence
rate
of
the
MNC.
Subsidence
varied
from
rapid,
to
slow,
to
finally
nearly
stable
with
the
migration
of
the
Pacific
plate
across
the
Hawaiian
hotspot
and
affected
the
style
of
reef
terrace
backstepping
in
addition
to
inducing
subaerial
exposure,
erosion
and
karst
dissolution
of
the
shallowest
reef
terraces.
(b)
the
amplitude
and
period
of
eustatic
sea-level
cycles.
This
forcing
changed
in
both
amplitude
and
frequency
during
Mid-
Pleistocene
Transition
and
affected
reef
growth
morphologies
with
multiple
re-occupations
of
reef
terraces
under
41
kyr
cycles
producing
large
vertical
relief
structures
(e.g.
pinnacles,
barriers
and
thick
fringe
reef
faces)
not
replicated
under
100
kyr
cycles.
(c)
the
slope
and
continuity
of
the
substrate.
This
aspect
varies
at
the
location
of
the
Clarke
debris
avalanche
which
steepened
the
slope
such
that
pinnacle
and
barrier
structures
could
be
formed
and
closely
spaced
reef
terraces
could
dominate.
6.
We
present
a
model
of
terrace
and
reef
development
within
the
MNC
(Figs.
9,
10
and
11)
that
is
consistent
with
the
available
data
and
could
be
tested
with
scientific
drilling
of
the
pinnacles
or
barrier
structures
to
reveal
the
internal
structure
of
these
features
and
further
test
our
model.
Alternatively,
deep
seismic
profiling
across
the
northern
and
southern
regions
could
image
the
subsur-
face
of
the
reef
terrain
and
potentially
reveal
the
reef
thickness
overlying
the
volcanic
flanks
on
the
broad
flats
in
the
southern
region.
Acknowledgements
Bathymetry
data
used
in
this
study
were
collected
by
MBARI,
UH,
JAMSTEC,
NOAA,
USGS,
SIO,
WHOI
and
coastal
LIDAR
data
were
collected
by
the
USACE.
Pisces
submersible
dives
were
conducted
by
HURL,
UH
under
a
grant
from
NOAA,
and
Tiburon
ROV
dives
were
conducted
by
MBARI
with
support
from
the
David
and
Lucile
Packard
Foundation
through
a
grant
to
MBARI.
Subsequent
processing
and
laboratory
study
by
IDEF
was
supported
both
by
an
internship
at
MBARI,
and
an
APA
at
James
Cook
University
(JCU).
We
thank
Jenny
Paduan
and
David
Caress
from
MBARI
for
their
help
with
the
bathymetric
data.
We
thank
Barbara
Block
and
Rob
Dunbar,
the
chief
scientists
during
the
collection
of
the
Chirp
data.
We
also
thank
Gabi
Laske
for
making
dredge
time
available
during
the
TUIM
cruise
to
deploy
seismometers
for
the
PLUME
experiment,
which
was
funded
under
NSF
grant
OCE-00-02470.
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