The Western Whakatane Graben: What’s Up?
Steven Hochman
Frontiers Abroad Program: University of Auckland, Auckland, New Zealand
Pomona College, Claremont, California
Abstract
The oblique subduction of the Pacific plate beneath the Australian plate in the Bay of Plenty (BOP)
region, New Zealand, has generated a complicated magmatic and structural setting whose history is
currently only poorly constrained. A closer inspection of the stratigraphy and faulting on the Western flank
of the Whakatane Graben (WWG) near Matata, an extensional feature associated with magmatic activity in
the Taupo Volcanic Zone (TVZ), has yielded data allowing for a more complete understanding of the
history of sedimentation, volcanic activity, local and regional faulting, and graben formation in this region.
Fitting the stratigraphic observations and dated volcanic units with world sea level curves has allowed for
the observation that New Zealand sea level has not varied significantly from world curves in the last 700 ka.
The stratigraphy and sea level curves are also employed to illustrate an uplift rate for the western margin of
the Whakatane Graben of 0.4 – 1.0 m/ka between 700 ka and 322 ka.
1. Introduction
The Whakatane Graben (Fig. 1) is found at
the north most region of the Taupo Volcanic
Zone (TVZ) where dextral strike-slip faulting of
the North Island Shear Zone (NISZ) is truncated
by the normal faulting found within the TVZ
(Mouslopoulou et al., 2007).
The TVZ represents the focus of young
volcanism, within c. 2 Ma, in New Zealand
(Wilson et al., 1995). An in depth review of the
TVZ performed by Wilson (1995) carefully
characterized the properties of the TVZ. The
total length of this volcanic zone is
approximately 300 km, 200 km from the center
of the North Island to the Bay of Plenty (BOP)
coast, and 100 km offshore. Consideration of the
volume of magma extruded in this zone is
estimated between 15-20,000 km3 without
considering older erupted material that more
recent eruptions may have covered. The magma
composition is dominantly rhyolite in the central
portion of the zone, with lesser amounts of
andesite focused around the northern and
southern ends, and only minimal basalt and
dacite found throughout.
Normal faulting trending NNE has also been
categorized throughout the TVZ, which is
currently rifting at a rate between 7 and 18
mm/yr (Wilson et el., 1995). This rifting may be
Fig. 1. Overview map of the TVZ in the Bay of Plenty region,
New Zealand. Calderas active between 340 and 240 ka
outlined in black, and the extent of ignimbrite emplaced in
light gray. The Whakatane Graben is located in the northeast
most portion of the on land TVZ.
related to the rise of magma from depth,
although this is still a topic of intense debate.
In the last 5500 years there have been three
fault zones active within the Whakatane Graben,
the Rotoitipaku, Matata, and the Onepu-
Edgecumbe, the latter responsible for the March
1987 Edgecumbe Earthquake along the
Edgecumbe Fault (Nairn and Beanland, 1989).
The center of the graben is subsiding at a rate
greater than 0.8 m/ka, and igninbrites once
deposited at sea level are now 2-4 km below sea
level and overlain by fluvial sediments from
inland and coastal deposits near the shoreline
(Nairn and Beanland, 1989). Recent uplift along
the margins has also been observed, at a rate of
~1.0 m/ka on the western margin and ~0.5 m/ka
on the eastern. Central graben subsidence in
combination with margin uplift has created at
least 550 m of offset in regional stratigraphy
across the graben margins.
Considerations of the graben’s structural
characteristics have been considered by
Mouslopoulou et al. (2007 and 2008), but focus
has been centered on general graben
development and the graben’s eastern margin
leaving a notable void in succinct knowledge
about the western flank.
2. Stratigraphy
The regional stratigraphy of the WWG
represents a complex history of volcanic
eruptions from the TVZ, terrestrial soil
formation processes, as well as fluvial and
marine sedimentation. Problematically, the units
of this sequence are difficult to assess given the
limited extent of exposure due to thick
vegetation and even more difficult to measure
due to drastic variations in layer thickness across
relatively short (100s-1000s m) horizontal
Fig. 2. A locality map of the Matata area labeling the four
locations of stratigraphic data collection. Stratigraphic
columns were created for each of the localities (Fig. 3) and
then correlated (Fig. 3) in order to best appreciate the local
stratigraphic sequence.
distances; however, carefully chosen field sites
including the Awatarariki and Waitepuru
Streams, along an unsealed road, Wilson’s Track,
and on Herepuru Road (Fig. 2) allow for the
recognition and correlation of stratigraphic units
in the region. The following stratigraphic units
will be described from oldest to youngest.
2.1. Awatarariki Stream
The Awatarariki stream stratigraphy (Fig. 3)
is the most complete of all the columns collected,
and represents the compilation of the work of
several geologists along the length of the stream
valley. It was collected over the course of three
years following a May 2005 debris flow, which
created a great deal of spectacular exposure
along the stream as it tore through the stream
valley (Hikuroa, pers. comm., 2008). Since the
debris flow, incredible regrowth of vegetation
and small scale slips have once again entombed
many exposures creating occasional problems in
connecting different segments of the stratigraphy;
these stratigraphic breaks will be explained as
they arise.
The lowest stratigraphic unit considered is a
very light tan to white colored, massively bedded
silt. The unit is well consolidated and is only
exposed at the mouth of this stream where
approximately 1 m is visible above the ground
surface. The light color of the silt suggests that
it is either lacustrian or estuarine in origin.
The next stratigraphic unit considered in this
catchment is a cross bedded layer of greywacke
gravels. Cross beds occur on the scale of 10
cm – 100 cm and indicate unidirectional flowing
during the emplacement of this unit. The dark,
well rounded cobbles range from 1 cm – 6 cm
and are tightly packed with a lighter sand to silt
matrix. The source of these gravels is thought to
be the axial ranges on the eastern flank of the
graben, transported to their current location via
fluvial processes (O’Leary, 2007). The unit is 13
m thick.
A sharp contact separates the greywacke
gravels from the overlying massive siltstone unit
(Lower Silt), which is tan in color with a
weathered red, orange, tan surface coating. This
unit contains the bivalves Paphies australis and
Austrovenus stutchburyi (O’Leary, 2007), and
preserved burrows of varying morphology and
size. This fossil evidence suggests the shallow
marine environment of this unit’s deposition.
This unit is 9.8 m thick.
The lower siltstone grades into a poorly
consolidated sand layer (Lower Sand). The sand
Wilson’s
Track
Fig.3. (previous page) Correlated stratigraphic columns from
the four field sites. Unit descriptions found in text along with
discussion of correlation.
is tan colored and contains abundant quartz.
Fossil evidence from this layer suggests its
origin in a shallow marine environment (O’Leary,
2007). The unit is 3 m thick.
A sharp contact is found separating the Lower
Sand and an overlying siltstone (Upper Silt).
This second silt is surficially blue gray in color,
tan beneath, and is extremely well consolidated.
The presence of nearshore and estuarine bivalves
and other trace fossils indicate that this unit was
deposited in a shallow marine environment
(O’Leary, 2007). The exact thickness of this unit
is impossible to measure given that it’s upper
and basal contacts are not exposed in any one
outcrop; however, individual exposures reveal a
minimum thickness of 40 continuous meters of
siltstone below an interbedded tephra unit
discussed below. This silt is thoroughly jointed
with conjugate sets found ubiquitously on its
weathering surface.
A break in the stratigraphy is present at this
point. Current day exposures are no longer
visible through vegetation and slips, but early
observations in the area recorded a massive,
crystal rich tephra (Tephra A), sandwiched by
Upper Silt. At no point in the stratigraphic
sequence is the Upper Silt visible from its
bottom to Tephra A, so the total thickness of
Upper Silt below this volcanic unit is impossible
to assess. Tephra A is approximately 2 m thick
(O’Leary, 2007), very light in color, and exhibits
continuous horizontal bedding. Based on its
position, within a marine silt unit, it is assumed
that this unit was emplaced in a marine
environment. The unit has been dated to 551 ± 4
ka (Leonard, pers. comm., 2008).
As mentioned above, the Upper Silt continues
directly on top of Tephra A. Further upstream,
and an unknown thickness of Upper Silt above
Tephra A, several meters of marine upper silt
grade into a series of cross bedded sands, some
containing small greywacke pebbles. The
majority of this unit is light colored material, and
very poorly consolidated. The nature of
sedimentary structures suggests in was deposited
via fluvial processes (O’Leary, 2007). At the top
of the sequence is 1 m of parallel bedded sands
containing small greywacke pebbles. The unit is
capped by a dark colored, organic rich paleosol,
a presence which suggests the terrestrial nature
of the sedimentary environment at thus time.
The thickness of the Upper Silt and the terrestrial
packages logged above Tephra A is 10 m.
Sitting on top of the paleosol is another tephra
unit (Tephra B). This light colored to pink unit
consists of small pumice fragments and fine ash
interbedded with dominant biotite and quartz
mineralogy. There is evidence for several pulses
of eruptive activity based on variations between
laminar bedded ash and cross bedded pumice
layers. The whole package is 1.5 m - 3 m thick
and has been dated to 538 ± 5 ka (Leonard, pers.
comm., 2008).
Above Tephra B continued terrestrial deposits
are found in the form of cross bedded siltly
deposits containing greywacke pebbles. These
fluvial deposits are vertically segmented into
distinct packages that might represent some form
of decadal flushing of material from inland;
regardless of the mechanism segmenting these
deposits, it is important to note that the number
of packages and their thicknesses varies a great
deal laterally within the field area, from meter
scale at some points to decimeter scale at others.
Such variation implies difficulty in assuming a
constant thickness for this unit across any
appreciable horizontal distance. Where observed
along the Awatarariki Stream, the deposits are 15
m – 19 m thick.
Near the top of this fluvial sequence we see a
small marine incursion into these terrestrial
deposits of silty material containing fossil
evidence of a shallow marine environment.
Capping 4 m of marine material, another tephra
is found (Below Triplet Tephra), dated to 563 ±
5 ka (Leonard, pers. comm., 2008).
A few meters of terrestrial fluvial packages
above this tephra, the stratigraphy is interbedded
with three closely spaced pumiceous airfalls, the
Triplet, dated at 552 ± 5 ka (Leonard, pers.
comm., 2008). Problematically, the ages of the
Triplet and Below Triplet Tephra are older than
Tephras A and B (551 ± 4 ka and 538 ± 5 ka,
respectively), which they overlie and by the law
of superposition would be expected to be
younger than. This may be a function of the
primary or secondary emplacement of these units
(i.e. the Triplet and Below Triplet Tephra were
erupted and subsequently deposited in this
location after Tephra A and B were emplaced).
Another hypothesis holds that the age dates
errors are simply too large to apply them to
distinguish emplacement times for these units on
the time scale on which they were emplaced
(Wilson, pers. comm., 2008).
Above the Triplet a continuation of the siltly
greywacke cobble terrestrial deposits is found for
a minimum of 18 m, which is followed by a
break in stratigraphy. This break has a minimum
measured thickness of 10-15 m covered by slips
over its exposed surface; however, the gap in
stratigraphy may be much thicker than measured
given the variations in the fluvial deposits
thickness noted above. Where the stratigraphy
resumes 13.8 m of a similar terrestrial deposit are
found (not shown on figure) beneath the topmost
series of units in the sequence.
Resting on these terrestrial fluvial packages is
25 m of massively bedded green, yellow, brown
sand containing shallow marine bivalves.
Overlying these shallow marine deposits is a
sequence of interbedded tephras and soils,
indicating the switch back to a terrestrial
environment. Two of these tephras have been
dated to 367 ± 8 ka and 382 ± 13 ka (Leonard,
pers. comm., 2008). Sitting on these soils and
tephras is approximately 1 m of yellow, green
marine sediments that grades into reworked
Matahina ignimbrite.
The entire sequence is capped by the Matahina
ignimbrite, aged at 325 ± 6 ka (Leonard, pers.
comm., 2008). This light colored, welded
ignimbrite was erupted from the Haroharo
caldera, Okataina Volcanic Center (Bailey and
Carr, 1994).
2.2. Waitepuru Stream
A great deal of faulting with coincident strike
direction with the stream means that along the
Waitepuru Stream a great deal of stratigraphy is
repeated such that moving upstream does not
reveal the entire local stratigraphy, but rather
shows exposures of the same units repeatedly.
The majority of visible exposures of the
stratigraphy along the stream valley are of the
extremely well consolidated, highly jointed, blue
to gray colored Upper Silt. From repeated
exposures of this unit, a minimum thickness of
approximately 40 m has been assessed (Fig. 3).
Further upstream however, a seemingly out of
place reverse fault reveals a small exposure
where Tephra B and the Triplet are visible in the
same outcrop without evidence of faulting
between them. From this exposure it is possible
to calculate that the thickness of sediments
between these two units, the fluvial packages
also found in the Awatarariki catchment, is 8.8
m- 13 m thick (Fig. 3).
2.3. Wilson’s Track
With the use of on site hand held GPS
elevation data, it is derived here that there is
approximately 64 m of the well consolidated
Upper Silt unit from the road level to where we
find Tephra A deposits. This represents a
minimum thickness to the top of the silt unit at
this site (Fig. 3).
Above Tephra A we find a similar sequence to
that observed at Awatarariki where after 10-12 m
of upper silt, we grade from marine to terrestrial
deposits over 3-5 m. Above this are 15 m of
terrestrial deposits of cross bedded sands and
reworked pumice, the top of which is 4.4 m of
beach sands. This unit is capped by Tephra B
(Fig. 3).
2.4. Herepuru Road
The stratigraphic sequence found at Herepuru
Road (Fig. 3) contains units stratigraphically
beneath those at other locations, as well as units
correlated with the sequence observed at the
other field sites. At the base of this sequence are
at least two, if not three, light colored, very fine
grained, very well sorted tephras (Bottom
Tephra). These units have been dated to 661 ± 6
ka (Leonard, pers. comm., 2008). Above this is
30 cm of crystal rich sand, followed by a
bioturbated sand layer with a great deal of local
variation in thickness, but generally 1 m thick.
Above these units it is possible to correlate this
stratigraphy with what is observed at Awatarariki.
Resting on the bioturbated sands is 0.6 m
thick unit of sand grain sized greywacke grains
mixed with a finer sand to silt matrix. This unit
has been correlated with the greywacke gravel
unit found at Awatarariki Stream; its thinner
nature and smaller greywacke fragments is
associated with variation in location along the
axis of deposition for this unit.
Above this we find the Lower Silt. 1.4 m
from the base of the unit is a horizon of
concretions, and the unit’s total thickness is
approximately 6 m. This unit here contains
fossil gastropods (perhaps Austrofusus) and
Darcinia, both in life position; such fauna is
indicative of a shallow marine environment.
This unit grades into the Lower Sand as at
Awatarariki, which here is 3.5 m thick. The
Lower Sand contains trace fossils associated
with shallow marine depositional environments.
A sharp contact separates the Lower Sand
from the Upper Silt, which here is 5 m thick to
the bottom of Tephra A.
Above Tephra A is a similar sequence to the
one found at Awatarariki where marine silt
grades into terrestrial fluvial deposits, here over
a total thickness of 38.4 m. The first 13 m of this
sequence is essentially a continuation of the
Upper Silt, followed by a marine to terrestrial
gradation into cross bedded units of sands,
greywacke gravels, and reworked pumiceous
material. On top of this sequence Tephra B is
observed.
2.5. Correlation of Stratigraphy
Organizing the field sites by position from
northwest to southeast (Fig. 2), the stratigraphy
has been correlated (Fig. 3) to provide a second
dimensional to the WWG stratigraphic sequence.
Although adequate elevation data was not
collected, and thorough regional normal faulting
has cut these units between sites, exposures of
the same unit in different areas seem to illustrate
that they maintain approximately the same
relative thicknesses from site to site (Fig. 3). In
addition, the systematic thinning of the lowest
marine and fluvial deposits (Greywacke Gravels,
Lower Silt, Lower Sand, Upper Silt), most
notably comparing the Awatarariki Stream work
to the Herepuru Road work, is attributed to an
axis of sedimentation focused around
Awatarariki and fading to the southeast (Fig. 3).
For the fluvial Greywacke Gravel this is likely a
result of the position of the paleoriver depositing
the unit. The thinning marine deposits may
suggest that the marine environments referred to
represent an estuarine environment rather than
nearshore, where sediment is focused closer to a
tidal inlet, thinner further away.
3. Sea Level
Using a compilation sea level curve for the
last 700 ka (Leonard, pers. comm., 2008) seen in
figure 4, it is possible to fit observed stratigraphy
with changes in world sea level in an attempt to
determine 1) how much of the variations
between marine and terrestrial stratigraphy is
related to changing sea level, 2) how much of the
variations between marine and terrestrial
stratigraphy is related to tectonic activity in the
area, and 3) a first order look at whether sea level
variation in New Zealand appears to match world
variations in relative water level.
This sea level curve shows 18
O values as
preserved in foraminifera as a proxy for sea level
(Leonard, pers. comm., 2008). While 18
O
values do not possess a consistent linear
relationship to a specific meter change in sea
level, we will use the work of Waelbroeck (2002)
to attempt to capture such a relationship. In
Waelbroeck’s work, sea level is assessed to
approximately 400 ka, and a 18
O change of 0.5
units is related to a sea level change of
approximately 65 m. Assuming this linear
relationship and extending it further back to 700
ka, a meter scale has been added to the sea level
curve (Fig. 4).
The addition of a meter change scale allows us
to add a line to the figure representing the level
of the land through the last 700 ka. When this
line lies to the left of the sea level curve, this
specific point on land lies beneath sea level and
represents a marine environment, the opposite
being true when the land line lies to the right of
the curve representing a terrestrial environment.
By plotting the known ages of dated tephras
from throughout the stratigraphy on the curve,
we may then assess where on the curve the land
must lie at any given point to achieve the
observed stratigraphy.
There are essentially an infinite number of
curves that can be fit through the sea level data
that can match the depositional environment and
age constraints from determined stratigraphy;
these may be characterized by three uplift
scenarios (Fig. 4). The ability to produce
multiple plausible uplift histories with the world
sea level curve producing the stratigraphy
observed in this region in itself suggests that no
evidence exists suggesting that New Zealand sea
level has deviated drastically from world sea
level in the last 700 ka.
Each uplift scenario supposes the
emplacement of the Matahina Ignimbrite at sea
level based on its position directly above marine
deposits, as well as coring work that yields
samples identical to those of other welded
ignimbrites deposited in a subaerial environment
(Nairn and Beanland, 1989). This means that
when drawing the land line for each potential
uplift we may pin the top of the land line on the
sea level curve at the Matahina age date, which
on the curve corresponds to a short marine
incursion likely responsible for the 1 m of
marine deposits found below the ignimbrite (Fig.
4)
The first uplift scenario relies on the accurate
dating of Tephras A and B and assumes uniform
uplift rates through time. The slope of this line
yields an uplift rate of 0.7 m/ka between 633 and
322 ka (Fig. 4). Assuming a continued constant
uplift rate from the emplacement of Matahina
Ignimbrite through present, this would place the
volcanic unit 225 m above sea level compared to
~300 m above sea level, where GPS data shows
it is found. Recent increases in tectonic activity
in this region of the TVZ (Nairn and Beanland,
1989) show that this scenario is not unlikely.
Fig. 4. World sea level curve for the last 700 ka and meter
scale in place as extrapolated from Waelbroeck (2002) with
dated tephra units marked in sandwiching the stratigraphic
units they surround. The solid red line represents uplift
scenario 1, the solid green line represents uplift scenario 2
and the dashed green line represents the average uplift rate
for uplift scenario 2. The translucent boxes represent the
hypothetical dates of Tephras A , B, the Below Triplet
Tephra, and the Triplet associated with uplift scenario 3, and
the blue line represents the uplift rate of uplift scenario 3.
Considering how well this land line fits the
sea level curve to match the observe stratigraphy
places some concern of the validity of the model.
Based on the given date for Tephra A (551 ± 4
ka), this scenario shows the emplacement of
Tephra A in a terrestrial environment whereas
stratigraphic evidence places it in a marine
environment. In addition, this model possesses a
marine incursion between approximately 520 and
480 ka, deposits from which are not observed
anywhere in the stratigraphic sequence (Fig. 4).
These fitting imperfections may be related to
poor dating of tephra units, small deviations in
New Zealand sea level from world sea level, or
slight variations in uniform uplift rates through
time although data possessed at this time can not
rule out any of these possibilities.
In the second uplift scenario, the third
potential source of error noted above is
considered, and a land line fitted to the sea level
curve is inserted that considers varying uplift
rates through time (Fig. 4). This line is designed
to perfectly match the stratigraphy and dated
units to the depositional environments dictated
by the sea level curve, but does so without
evidence suggesting the claimed uplift rate
variations. The average uplift rate for this period
in this scenario is 1.0 m/ka between 611 and 322
ka (Fig. 4), which, if continued, would place that
Matahina Ignimbrite within 22 m of the
elevation it is found at present day.
The third scenario for uplift considers the
concerns with the disagreement between the
dated ages of the tephra units and their
stratigraphic relationship. If we presume to
correct these dates in a manner that orders them
as found in the stratigraphic sequence and that
makes sense with the sea level curve, we can fit a
third land line to the sea level curve (Fig. 4).
This scenario yields a uniform uplift rate of 0.4
m/ka between 633 and 322 ka. Such a rate, if
continued, would place that Matahina Ignimbrite
at 129 m elevation today, and would require a
133% increase in uplift rate in the last 322 ka.
4. Deformation
The stratigraphy present in these three
catchments has experienced a great deal of brittle
deformation, and throughout all of the described
stratigraphic units both faulting and jointing was
observed and measured. Both southeast and
northwest dipping faults and joints were present,
all trending in the same general direction –
N28°E ± 14°, and dipping 66° ± 9° (Fig. 5).
Such orientations are closely in line with the
orientation of the TVZ in this location; an
expected connection assuming that TVZ
orientation possesses control on the Whakatane
Graben formation.
Fig. 5. Stereonet representation of recorded attitudes of faults
and joint in the Awatarariki and Waitepuru catchments.
Great curcles are shown for the averaged values of the
measurements.
The style of faulting in the region is normal,
and the scale varies widely from a few
centimeters to tens of meters of apparent offset.
In the Waitepuru catchment approximately 20 m
of total offset was observed on small faults along
the stream; however, much larger scale faulting
is evident given the continued presence of the
upper silt layer at stream level despite walking
up stream and gaining 10s of meters of elevation.
Similar offsets are found along the Awatarariki
Stream, where smaller faults account for small
amounts of offset, but larger faulting often
juxtaposes stratigraphic units along the stream.
Cross sectional exposures of the mouth of the
Awatarariki Stream reveal three large faults
trending with the stream, cumulatively
accounting for nearly 30 m of offset.
Further work will be required mapping fault in
this region, specifically placing known locations
of fault measurements on a map in an attempt to
observe whether the numerous fault
measurements represent of major fault running
the length of the stream and perhaps offshore, or
are rather numerous smaller faults with similar
orientations.
5. Conclusion
This work is only another step on the way to
fully understanding the existence of the
Whakatane Graben in the northeast most onshore
portion of the TVZ. From the stratigraphic work
performed in this area the complex terrestrial and
marine paleoenvironment in this location has
been clarified, especially considering the period
between 700 and 322 ka. During this same
period an uplift rate has also been determine,
placing the uplift of the western margin of the
graben between 0.4 and 1.0 m/ka, and in so
doing implying a recent increase in tectonic
activity to raise the local stratigraphy to the
elevations it is observed at today. A small
amount of work has also gone into characterizing
the graben structurally, although especially along
the western margin this is still an area of work
remaining wide open for future projects.
Particular attention should be paid to mapping
faults in attempt to gauge the character of the
graben faulting as localized faulting or as a zone
of faulting along each margin.
Acknowledgments
This work is the beholden to the countless
hours of work performed by Darren Gravley,
Dan Hikuroa, Max Borella, and countless other
players spending hours organizing a program to
bring students to the Bay of Plenty region to
conduct geologic and social projects within areas
of passion working for the betterment of the area.
Special thanks also the Colin Wilson for his
wonderful variety of encouragement and aid in
the understanding of the history of this incredible
complicated region. Also thanks to Graham
Leonard, celebrity volcanologist, for providing
age dating of tephras and sea level curves that
were essential to the completion of this project.
Endless thanks must also be given to Anthony
Olson and Ngati Umutahi for introducing us to
this sacred land, guiding us through its nooks
and crannies, and for helping us learn the value
and special quality this area possesses.
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