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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