Stratigraphy and nature of Taupo Pumice Formation

 

 

New Zealand Journal of Geology and Geophysics

ISSN: 0028-8306 (Print) 1175-8791 (Online) Journal homepage: https://www.tandfonline.com/loi/tnzg20

Stratigraphy and nature of Taupo Pumice
Formation

P. C. Froggatt

To cite this article: P. C. Froggatt (1981) Stratigraphy and nature of Taupo Pumice
Formation, New Zealand Journal of Geology and Geophysics, 24:2, 231-248, DOI:
10.1080/00288306.1981.10422715

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New Zealand Journal of Geology and Geophysics, 1981, Vol. 24: 231-248

231

Stratigraphy and nature of Taupo Pumice Formation

P. C. FROGGATT
Geology Department
Victoria University of Wellington
Private Bag
Wellington, New Zealand

Abstract
Tephras erupted from Taupo Volcanic
Centre, North Island, New Zealand, during the
most recent eruptive event (c. 1820 years ago) are
defined as 4 members of Taupo Pumice Formation.
From youngest to oldest these are Taupo Ignim
brite, Taupo Lapilli, Rotongaio Ash, and Hatepe
Tephra. All are airfall in origin except Taupo
Ignimbrite which is the product of pyroclastic flows.

Three stratigraphic units are here recognised
within Taupo Ignimbrite. The Lower unit, restricted
in occurrence mostly between Taupo and Turangi, is
predominantly reddish brown in colour and has
previously been included within Waitahanui Brec
cia. It is here recommended that use of the name
Waitahanui Breccia be discontinued. The Middle
unit is the most extensive and is discontinuously
underlain by a “lithic lag layer”, rich in accessory
lithics and crystals. The Upper unit is widely
distributed and noted for the large pumice blocks
found at the top of the ignimbrite, as well as
interbedded
layers containing
chalazoidites (accretionary lapilli).

fine ash airfall

Each unit of Taupo Ignimbrite is highly’ variable
in appearance and is described in terms of 3
lithofacies: “valley facies”, “fines-depleted facies”
and “ignimbrite veneer facies”.

Charcoal, from branches and logs, is common
within the Middle unit. Radial orientation of these
logs, as well as grainsize and distribution of
indicates Taupo Pumice
individual members,
Formation was erupted from within the present
Lake Taupo, at Horomatangi Reefs. The total
the eruptive
volume of tephra ejected during
sequence is estimated at about 100 km3
• Approxi
mately 20 km 3 constituted the airfall beds of Hatepe
Tephra, Rotongaio Ash, and Taupo Lapilli. The
total volume represents about 20 km3 of magma.

Keywords
Taupo Pumice Formation; Taupo
Ignimbrite; Taupo Lapilli; Rotongaio Ash; Hatepe
Tephra;
tephra
tephrostratigraphy; chronology;
volumes

Received 17 September 1980, accepted 8 June 1981

INTRODUCTION

Pumice from the most re.cent series of eruptions (c.
1820 years ago) at Taupo is widespread throughout
the central North Island as a primary pyroclastic
deposit and around most of the coastline of New
Zealand as sea-rafted pumice. Such rafted blocks
have been found as far south as Campbell Island (P.
Morris pers. comm. 1978). Consequently Taupo
Pumice has been used extensively as a chronological
marker bed for such studies as shoreline movement
(e.g., Pullar & Selby 1971; Fleming 1972; Gibb
1977), archaeology (Wellman 1962; MacFadgen
1978), erosion rates (Gage & Black 1979), tectonics
(Froggatt & Howorth 1981), and
levels
(Kennedy et al. 1978).

lake

Despite such extensive usage, the Taupo Sub
group (TSG) nomenclature of Healy (1964) has not
been revised. In particular, the uppermost layers
have been variously mapped or described as:
“Taupo Pumice” (e.g., Topping & Kohn 1973;
Vucetich & Pullar 1973), TSG la, b, c, and TSG 2
“Taupo Pumice
(Vucetich &
1964),
Alluvium”
(Grindley
“Waitahanui Breccia”
and
“Ngautuku Block and Ash” (Vucetich & Wells
1978). It is here recommended that use of all these
terms be discontinued in favour of Taupo Pumice
Formation and Taupo Ignimbrite, which are defined
below.

1960; Healy
(Grindley 1960),

Pullar 1964),

“Pumice” is retained in favour of the more
general term “Tephra” in the formation name on
the grounds of historical and current widespread
usage. For similar reasons, the names Taupo Lapilli
and Rotongaio Ash retain size connotations,
although for Rotongaio Ash no material coarser
than ash grade has been found.

The stratigraphy of Taupo Pumice Formation is
revised and each member is described in relation to
a type section and reference sections. Location of
the source vent, erupted volumes, and the date of
the eruption are discussed.

Exceptional preservation of Taupo Pumice was
probably aided by a dense forest vegetation that was
apparently re-established soon after devastation.
Numerous sections through the tephra sequence are
provided by road cuts, especially in new forest
areas, but few sections expose the full stratigraphy
near the source where
the beds are thickest.
Location and description of a type section for Taupo
Ignimbrite has consequently been difficult, and the
designated type section although visible is now
virtually inaccessible.

232

New Zealand Journal of Geology and Geophysics, 1981, Vol. 24

Previous work

Taupo Pumice was first descibed by early explorers
such as Dieffenbach (1843, p. 128),’ Wakefield
(1845, p. 236), and Taylor (1855, p. 225) and was
believed to have been erupted from Tongariro
volcano.

Many early writers (e.g., Smith 1877) acknow
ledged Tongariro as a possible
source and
considered most of Taupo Pumice to be water lain in
extensive lakes. Evidence of horizontal beds and
extensive, flat terraces were cited. Cotton (1922, p.
204) assumed an alluvial origin, as did Grange
(1927, 1931, 1937) in a series of papers dealing with
the geology and soils of the central North Island.
Grange described Taupo pumice as “sands”
overlying coarser pumice which he distinguished
from the older Taupo showers. He also considered
much of the pumice to be water lain, but as alluvium
redeposited soon after subaerial emplacement.
Grange also examined the charcoal, abundant in
Taupo Pumice, and concluded that charring was by
coking after burial, at a minimum temperature of
250°C (Grange 1927). Grange was the first to place
the source ·vent within the northeast area of Lake
Taupo.

Detailed stratigraphy of Taupo Pumice was first
described by Baumgart (1954) from a now disused
quarry, where he
identified and numbered 8
members and named 3 (Taupo Lapilli, Rotongaio
Ash, and Hatepe Lapilli). Subsequently, Baumgart
& Healy (1956) named a further bed, Rhyolite
Block Member. Healy (1964) retained Baumgart’s
numbers in describing the road section at De Bretts
Hotel (N94/573353*) and placed the beds within
Taupo Subgroup, as part of Arawa Group. He
named the upper 8 beds from oldest to youngest as
Hatepe Lapilli, “putty” coloured ash, Rotongaio
Ash, Taupo LapilIi, Rhyolite Block Member, and
Upper Taupo Pumice Members. The youngest was
regarded as “glowing avalanche” deposits exten
sively redeposited as alluvium, and the rest as airfall
in origin.

In their revision of Taupo Holocene tephras,
Vucetich & Pullar (1973) attempted no further
extension of Healy’s work, concentrating on the
older deposits.

Baumgart (1954) published the first iospach maps
for Taupo LapilIi, Rotongaio Ash, and Hatepe
Lapilli. These maps were extended beyond the
Taupo area by Vucetich & Pullar (1964) who also
for “Upper Taupo Pumice
plotted
Members” and “putty coloured ash”. A map of all
members combined was compiled by Pullar (1973).

isopachs

*Grid references are based on the national thousand-yard
grid of the 1:63 360 topographical map series (NZMS1).

Although much has been published on the
chemistry and mineralogy of Taupo Volcanic Zone
rhyolites, little attention has been paid to the
tephras. Hochstetter (1864) confirmed the presence
of quartz and feldspar in the pumices, and Grange
(1927, 1931, 1937) reported chemical analyses as
related to soil studies.

Ewart (1963) systematically examined the Taupo
sequence (after Baumgart 1954) for mineralogy and
glass chemistry, but the stratigraphic context of his
samples is now in doubt (Vucetich & Pullar 1973, p.
747). Kohn (1970, 1973) and Topping & Kohn
(1973) reported analyses of titanomagnetites, and
later Kohn & Topping (1978) used these analyses to
infer a mechanistic relationship between Holocene
andesite and rhyolite eruptions.

TERMINOLOGY AND DEFINITIONS

Tephra is used to include all primary pyroclastic
deposits of airfall or flow origin as defined by
Howorth (1975). Ash, lapilli, and blocks are used to
describe grainsize, after Fisher (1960). Tephra
formation (rhyolitic) is a bed or sequence of beds
deposited as part of the 1 eruptive event and
separated from enclosing tephras by a paleosol.
These paleosols define the top and base of the
formation.

Type section and Type area are defined by the
criteria established by Vucetich & Pullar (1973). A
plinian eruption is loosely defined as “a gas-blast
eruption of great violence” (Lirer et al. 1972, p. 759)
after the type example eruption of Somma-Vesuvius
in A.D. 79 as described by the Roman scholar Pliny
the Younger (see for instance Bullard 1962). The
fall deposit produced by a plinian eruption is
generally coarse, well-sorted pumice
(“plinian
pumice”) arbitrarily classified by Walker (1973) on
size and distribution of the pumice. Ignimbrite is
here defined as all the primary deposits formed from
pyroclastic flows, and is the product of 1 eruptive
sequence commonly consisting of several flows.
Welding of the ignimbrite, when it occurs, is a post
depositional effect and is not implicit in this
definition; it is a logical extension to the general case
of Marshall’s (1934) genetic definition, covering the
possibility of the ignimbrite failing to weld. This
usage of ignimbrite emphasises the pyroclastic flow
origin rather than the welding phenomena and is
now commonly accepted (see, for instance, Sparks
& Wilson 1976; Wright et al. 1980; Walker et al.
1981a). In contrast to the sorted nature of plinian
ignimbrites are commonly
tephras, unwelded
unsorted, matrix supported, and rich in fine ash.

Froggatt-Taupo Pumice Formation

233

GRANGE

BAUMGART

BAUMGART &

HEALY (1964)’

/19 1 1.1937)

(1954)

HEALY

( ]956)

VUCETICII & PULIJR
I] 964. ] 973)

THIS PAPER

TAUpO ASH
SEQUENCE

TAUPO pL’MICE
FOR.’-tATION

TACPO PUMICE FORMAT lOr-:

1 UPPER TAUpO
PTJM[CE

1 CPPER TAUPO PU!’-1ICE

t – – ..

– – – –

– BLOCK ___

—.-

2 RHYOLITE

2 RHYOLITE BLOCK

LITlIIC LAG UlYF.H

t’PPER UNIT

:-tIDDLE U~IT

LOWF:R Ct-;lT

IGNIMBRI TE

} TAUPO

3 TAUPO LAP! LLI

1 TAUPO

l..JPILLI

“3 TAUPO LAPILLI

‘;’,iLPO LAPILL!

4 ROTONGAIO ASH

4

I{OTONGAIO ASH

4 ROTONGA 10 ASH

5 “putty ash”

j{UTONGA 10 ASH

,” “,,” J IlATEPE TEPHRA

6 HATEPE LAPILLI

6 HATEPE

LAPILLI

~I!ATEPE LAPI LLI

lapI.lll. unlt

5

7

8

TAUPO

SHOWER

1

2

5

7

8

Fig. 1 Stratigraphy and nomenclature used in this paper compared to that of previous workers.

TAUPO PUMICE FORMATION STRATIGRAPHY

the

terminology and

The stratigraphy of Taupo Pumice as presented here
is a revision of that of Healy (1964) with some
inclusion and
changes in
definition of Taupo Ignimbrite. From oldest to
youngest
the members are defined as Hatepe
Tephra, Rotongaio Ash, Taupo Lapilli, and Taupo
Ignimbrite. Each of
is described and
designated a type area and type section. Taupo
Ignimbrite is described in terms of 3 units, the
Lower (“pink”) unit, the Middle unit forming the
main body of ignimbrite, and an Upper (“bedded”)
unit. Each unit
to 3
is described
lithofacies. Stratigraphy of this paper is compared to
that of previous workers in Fig. 1.

in relation

these

TAUPO IGNIMBRITE

Type section

Taupo Ignimbrite in the type area consists of all the
primary pyroclastic deposits overlying Taupo lapilli
or those deposits occupying an equivalent stratig
raphic position where Taupo Lapilli is absent. The
ignimbrite comprises all the primary products of the
pyroclastic flows that succeeded eruption of Taupo
Lapilli. The area is a sector from northeast to
southeast within a radius of 10 km of Taupo
is an almost
Borough, and
inacessible cliff beside Hatepe River (Fig. 2,

type section

the

the

full

N103/569230) described in section 1, Appendix 1.
Only 1 accessible section is known that conclusively
demonstrates
stratigraphy of Taupo
Ignimbrite; it is a road cut on Te Heuheu Road
is doubtful.
and preservation
(N103/488161),
Further reference sections are nominated and
described at localities where each unit is best
developed and exposed and can be stratigraphically
correlated to the type section.

General description

Taupo Ignimbrite (= Upper Taupo Pumice Mem
bers of Healy 1964) consists of all the primary
deposits immediately overlying Taupo Lapilli and
found over a roughly circular area of about 80 km
radius, centred on Lake Taupo. Thickness ranges up
to 100 m or more, the deposits always being thickest
in valleys and thinning rapidly up slopes and ridges.
The ignimbrite shows no systematic variation of
thickness with distance from the source, unlike most
other pyroclastic deposits. This deposit exhibits a
remarkably wide variation in nature, which has led
many people to postulate several different sources
and extensive redeposition by mudflow and fluvial
means (Grange 1937; Grindley 1960; Healy 1964).
Three pyroclastic flow units are recognised in
Taupo Ignimbrite. The relationship and distribution
of each of these units is shown schematically on Fig.
3 which also portrays important changes due to
topography and distance from the source.

234

New Zealand Journal of Geology and Geophysics, 1981, Vol. 24

Fig. 2 Type section for Taupo Pumice Formation beside Hatepe River at N103/569230 (section 1). The bedded Upper
unit at the top can be seen overlying the Middle unit, the lithic lag layer, and the Lower unit, which overlies a bedded
sequence of Taupo Lapilli and Rotongaio Ash. Total section thickness shown is about 40 rn.

The Lower unit is restricted in occurrence to a
sector from northeast to southeast of Lake Taupo
and consists of typically pink to red-brown unsorted
pumice lapilli with occasional interbedded layers of
better sorted coarse lapilli. The unit is generally very
firm and massive and stands in vertical sections.
Approximate extent of the Lower unit is shown in
Fig. 3.

The Middle unit forms the main body of the
ignimbrite and is remarkably widespread and of a
highly variable character. Despite
its variable
nature, the unit can be grouped into 3 broad
“lithofacies” described below, but each is distinctive
and they form lateral stratigraphic equivalents. The
facies grade laterally and sometimes vertically into
each other, usually over a short distance. Each is
considered to have been formed as a result of a
particular flow characteristic of the pyroclastic flow.
The vertical and lateral relationships of these facies
within the Middle unit is shown schematically in Fig.
3.

The basal part of the Middle unit is commonly
enriched in denser lithics and crystals relative to the
upper parts. This lithic lag layer or “ground layer”
of Walker et al. (1981a), described below, is the
Rhyolite Block Member of Baumgart & Healy

that formed

(1956) and has formed from the same pyroclastic
the Middle unit of Taupo
flow
Ignimbrite. The layer is considered diagnostic of the
Middle ignimbrite unit, being found only at the base
of this unit, and it is a valuable marker bed when
present.

Overlying the Middle ignimbrite unit is a usually
thin discontinuous sequence of interbedded flow
and airfall material, some of which has subsequently
been removed by erosion or obscured by soil
formation. Thickness is typically 0.3-0.4 m, but
ranges up to 5 m or more (see section 3, Appendix
1). Two notable
the extensive
distribution of chalazoidites (accretionary lapilli) in
the fine ash airfall beds and the occurrence of large
pumice blocks up to 0.4 m diameter in unsorted
units only 1 m thick. This unit corresponds to the
coarse blocks often seen in the top of the ignimbrite,
especially on Kaiangaroa Plateau to the northeast of
Taupo.

features are

A pyroclastic flow origin for Taupo Ignimbrite

Implicit in the term “ignimbrite” is an origin from a
widespread pyroclastic flow. The following features
have been observed in this deposit, are regarded as

LAKE TAUPO

c

D

LAKE

~ +

1

Froggatt-Taupo Pumice Formation

A 1

2

I
~~-g,

3 f~-

I

LAKE
TAUPO

F42?

B

j7

_ .. _11::.- …..

i

8

1

i 11

~J~.;..–:~-=..,,=,:.~.!.~

~

235

B’

c’

—–:..

KEY

UPPER
UNIT

Bedded airlall ash
Pumice blocks and ash

Bedded alriall ash

MIDDLE
UNIT

Unsorted pumice blocks
laPlll1 and ash

Lithic lag layer

Rhyolite hthlcs

LOWER
UNIT

ROTONGAIO
ASH.
TAUPO LAPILLI
and HATEPE
IffJiBA

Unsorted pink pumice
lapilh and ash with interbedded
alriall pumice lenses

Pre-ignirnbnte a.rfall tephras

Merges laterally Into

VENEER DEPOSIT

Shown as

~

Fill. 3 Schematic cross-sections (not to scale) through Taupo Pumice Formation to show the relationship and
distribution of each unit and facies of Taupo Ignimbrite as shown in the key. Thickness of Taupo Ignimbrite is largely
topographically controlled with thick sequences in the valleys and thin “veneer deposits” on the ridges. Restricted
occurrence of pumice alluvium is shown in the Rangitaiki River, Profile A. Stippled area on location map (upper right)
indicates the approximate extent of the Lower unit. The numbered arrows refer to sections figured in the text or described
in Appendix 1.

flow ongm, and are generally
confirming a
inconsistent with other known eruptive mechanisms.
1. Highly variable thickness, grainsize, and sorting
showing no
these
parameters with distance from the source, in
contrast to the well sorted regular nature of
plinian deposits.

regular relationship of

2.

In most sections the ignimbrite is completely
unsorted with an abundance of pumiceous ash
and large pumice clasts (i.e., matrix sup
ported).

3. The ignimbrite does not conformably mantle
the landscape, but smooths the topography by
being thickest in valleys and rapidly thinning up

Sig. 7

236

New Zealand Journal of Geology and Geophysics, 1981, Vol. 24

Ignimbrite

Fig. 4
thins rapidly
up slopes to maintain a near-level
upper surface, shown here cover
ing a ridge composed of older
tephras
11 ,
(NI02/42S080). Section thickness
is about 3 m.

section

at

4.

slopes (Fig. 4) , often with internal layering
conformable with the base when on higher
topography .
In most valleys the ignimbrite infiIIs the valley
floor forming a flat-topped aggradation surface
of primary pyroclastic material (Fig. 5) .
5. Near the source, occasional cross bedding and
weakly developed duneforms within the ignim
brite on flat topography attest to deposition
from a high velocity flow regime.
regular

sloping basal contact
consequent on widespread erosion and trunca
tion of underlying beds (Fig. 6) . This is not
formed by wind or fluvial action , because the
erosive products are not seen, except within the
overlying bed (see 7 below) .

smooth

6. A

7. Frequent incorporation of rounded clasts of
older beds recognisable as Rotongaio Ash (Fig.
7) or older paleosols within the deposit (rip-up
clasts) , as evidence of widespread erosion.
8. Common occurrence of charcoal throughout the
ignimbrite and not just at the base , charcoal
being rare within plinian deposits.

9. Radial orientation of the charcoaled logs close
to the source vent, and alignment parallel to the
enclosing valley further away from the source
(Froggatt 1980; Froggatt et al. 1981).

10. Marked segregation of dense lithic material
towards the base of the Middle unit, forming
the lithic lag layer.

11. Discontinuous nature of the basal lithic lag
layer which is thickest in valleys and hollows
and which can be best explained by deposition
from a pyroclastic flow.

12. Vertical “pipes” composed of lithics and free
crystals , which are features typically resulting
from fluidisation (Wilson 1980) .

Ignimbrite facies

Each unit of Taupo Ignimbrite is highly variable in
thickness, grainsize, and sorting, with no apparent
relationship of these parameters to distance from
the source. However, there is a marked relatioD”hip
with topography such that the distribution and
appearance is almost totally topographically control
led. Apparently the pyroclastic flows which formed
Taupo Ignimbrite were of high fluidity and high
velocity so even small changes in the ground surface,
or even forest cover, affected deposition from the
flows.

Three lithofacies are used to describe the differing
appearance of Taupo Ignimbrite: “valley facies ”
(VF), “fines-depleted facies ” (FDF) and “ignim
brite veneer facies”
(IVF) . These facies are
exceptionally well developed within the Middle
ignimbrite unit, but the Upper and Lower units also
show similar facies development.

Each facies is designated a reference section
where the characteristics of the facies are particu
larly well developed and exposed.

Valley facies (VF)
VF is the typical unwelded, unsorted mixture of
pumice ash, lapilli, and blocks found in valleys over
a wide area of the central North Island. Reference
section for VF is defined at Waitahanui River
(N103/592177) where the deposit is 50 m thick
(section 8 Appendix 1) . The deposit is matrix
supported due
to an abundance of fine ash.
Maximum grainsize varies markedly between
sections and ranges from 0.01 m to 1 m. The full
thickness of VF is seldom exposed, and the deposit
is thickest in valleys and thins rapidly upslope to
IVF. Maximum exposed
grade

laterally

into

Froggatt-Taupo Pumice Formation

237

Fig. S A valley eroded into Taupo Ignimbrite at Maroa (N85/481625) accentuating the flat upper surface and the typical
“box canyon” type of valley cut into this deposit.

Fig.6 Section 5 on Rotoakui Road (NI03/662208) with thin “Ignimbrite veneer facies” (IVF) overlying Taupo Lapilli,
Rotongaio Ash, and Hatepe Tephra. Note the strong erosion at the base of Taupo Ignimbrite and typical “rill” erosion in
Hatepe Tephra mantled by Rotongaio Ash. Spade 1 m long.

7′