Early Miocene deep sea sedimentation at Maewo, Vanuatu (New Hebrides)

177

Tecronoph_vsics. 87 ( 1982) I77- 183 Elsevier Scientific

abashing

Company,

PLATE TECTONIC MIOCENE

Amsterdam-Printed

SIGNIFICANCE

in The Netherlands

OF LATE OLIGOCENE/EARLY

DEEP SEA SEDIMENTATION

AT MAEWO, VANUATU

(NEW HEBRIDES)

G. NEEF Uni~~sit~~ Coliege,

W.S. & L.B. Robinson (Final

version received

Broken

Hill, N.S. U’. ~~u.stru~~a~

25, 198 1)

February

ABSTRACT

Neef, G., 1982. Plate tectonic Maewo,

Vanuatu

Tectonophysics,

Plate Boundaries. Eight lithofacies mechanisms,

representing

are recognised

Oligocene/Early

Miocene

floor on which which indicate

significance

(New Hebrides).

of Late Oligocene/Early

In: G.H. 87:

age, on Maewo

trending,

sections

beyond

deep sea sedimentation

The Evolution

deep sea fan, dominantly

in the lower part

Island,

the deep sea fan prograded deposition

Miocene

(Editor).

at

of the India-Pacific

177- 183.

a westward

in geologic

Packham

Vanuatu,

New Hebrides.

are non-calcareous,

the cafcareous

depth,

from mass flow

Formation,

Also present

red siltstone

compensation

deposited

of the Sarava

and minor

i.e. a depth greater

of Late

representing

the

green siltstone than 4.25 km,

and rare thin airfall ash. Previous Miocene. suggests

proposed the great

that rifting occurred

Pentecost 35-28

workers

However,

Ma (Oligocene).

Miocene

occurred

at which

The ophiolite

reconstruction

day Mariana

Rifting

has a dismembered

suite may have formed

of the Oligocene

Arc System.

the subduction

in the area now occupied

the Late Oligocene/Early

prior to the Late Oligocene.

Island, which lies south of Maewo,

The writer’s present

that rifting depth

Interarc

arc system

by Maewo

Miocene

may have occurred ophiolite

in an interarc

the Mid

were deposited

even earlier because

environment.

of the New Hebrides

westwards

during

suite which ranges in age from

rifting ceased by the Early Miocene

of zone may have migrated

strata

to lie along

is an analogue

and during

of the

the Mid-Late

the Maewo-Pentecost

axis.

INTRODUCTION

Maewo lies in northeastern Vanuatu (Fig. 1). The central part of the archipelago is composed of an eastern belt composed of Early Miocene to Early Pliocene basaltic lavas and volcaniclastic sediment capped by elevated Late Pliocene to Mid Pleistocene limestone. Westwards lies the central chain of active volcanoes, which formed during the Late Pliocene. In the western belt, on the island of Malekula, are thick volcaniclastics of Early Miocene to Late Miocene age, which are overlain by Pliocene sediment and Quaternary raised reefs (Mitchell, 1971). Current plate tectonic models of the New Hebrides have subduction westwards beneath the New Hebrides during the period 23-c. 10 Ma (Carney and Macfarlane, 1978, fig.4). During the Early Miocene a frontal arc and rear arc were extant 0040- 195 1/82,‘0000-0000,‘$02.75

0 1982 Elsevier Scientific

Publishing

Company

MIOCENE

EARLY

/

168.E

Fig. 1. Map of the New Hebrides shows the location of Maewo. Fig. 2. Late Oligocene-Miocene

strata developed

on Maewo island, New Hebrides (after Carney, 1982).

(Carney and Macfarlane, 1978). The North Fiji Basin formed by anti-clockwise rotation of the New Hebrides archipelago relative to Fiji during the last 6 Ma (Falvey, 1978). Subduction of Australian-Indian Plate eastwards beneath the New Hebrides commenced at 5 Ma (Carney and Macfarlane, 1978) (or c. 8 Ma-Carney and Macfarlane, this volume). The geology of Maewo, comprising strata of Late Oligocene, Miocene, Pliocene and Quaternary age, has been mapped by Obelliane (1958), Liggett (1967) and Camey (1982). Stratigraphic nomenclature is by Carney (1982) (Fig. 2). The Sighotara Group, which is at least 6QOm thick west of Sarava Point (Carney, 1982), contains the Sarava- and Wustoro@ra- Formations (Fig. 2). During the writer’s visit to Maewo only the lower part of the Late Oligocene/Early Miocene Sarava Formation

Fig. 3. a. New Hebrides Arc System during the Oligocene symbols

are: 54~ =Maewo;

P = Pentecost;

.S =Espiritu

(after Carney and Macfarlane, Santo; Ma = Malekula;

1978). Island

Vr ==Viti Levu; Vf.. =

Vanua Levu. during the Oligocene-Mid

b. Section of the New

Hebrides

arc system

x =Ohgocene

sequence

on Pentecost

ophiolite

and probably

Miocene deep sea fan (lower part of the Sarava Formation) volcanism mudstone

on Malekula; in northwest

H. D. =Hornblende

diorite

Miocene.

on Maewo;

Symbols

used are:

S= Late Oligocene-Early

on Maewo; C = Eariy Miocene talc alkaline

(18 Ma) on Malekula,

R.M. =?

Oligocene

red

Malekula.

c. Section of the New Hebrides arc system during the Mid-Late Miocene. Symbol used is: T = Gfobigerina ooze (T~~tmuto

Formation).

179

25 -14 Ma (Late Mid

OligoceneMiocene)

a I

35-14

Ma

Oligocene-Mid

Miocene

b (InactIve)

A

western

Belt (Inactive)

I /

=I-----

L(?

-

I I

/

13-9 Ma (Mid-Late Miocene)

probable westword subduction zone

migration

of the

was

studied.

exposed, upwards Sarava

About

58Om

uninhabited (Carney, Formation age (San

and nannofossils

Jorge

Fauna)

is probably

by diastrophism present

The aim of this paper very deep water-below

Formation

AND ENVIRONMENT

fines

planktic

of thin calcarenite, is not ,known

(Carney

that the Sarava

certain inferences from this new data. the lower part of the Sarava Formation

the well

which

Unfortunately

except for a uniform

the calcareous

along

formation,

The age of the basal part of the

1969).

not far subsurface

is to report

examined

which give a Late Oligocene/Early

in the samples

and the age of the Sarava

strata are little affected

STRATIGRAPHY

inland.

(Coleman,

are lacking

were

of Maewo--the

is from Large Foraminifera

Miocene

Ophiolite

sections

cqast

1982), is not well exposed

aminifera

by the writer,

of geologic

northeastern

compensation

for-

collected

precisely.

The

ca. 30” tilt to the west.

and Macfarlane, Formation depth

1976).

was deposited

(CCD)

and

in

to draw

Full documentation of the sedimentology of is to be made elsewhere (Neef et al, in prep.).

OF DEPOSITION

OF THE SARAVA

FORMATION

The strata comprising the formation contain rudite beds usually ca. 1 m thick (maximum thickness 13.4 m) and graded rudite-arenite/graded arenite beds which range in thickness from 1-15 m. These beds contain clasts of spilite, which have been considered to be of Wainamala type-Eocene 1978), dacite, and Late Eocene and Early Miocene Geochemical

evidence

east from the Vitiaz farlane,

1978)-as

limestone

is strong Arc where

similar

was previously

clasts (Coleman,

calcarenite

that many

of the spilite clasts are derived

rock types are present

proposed

1969). Also present

beds largely composed

in age (Carney and Macfarlane, limestone clasts (Coleman, 1969).

for the origin

(Carney

of the Late

are thin (maximum

of large foraminifera,

from the and MacEocene

thickness

rare Bouma

72 cm)

sequence

beds,

penecontemporaneous breccia beds, and thin air fall ash which is rarely thicker than ca. l-2 cm. Intercalated with these beds are non-calcareous red siltstone-which is thicker,

generally,

in the southern

part of the outcrop

area-and

minor

beds of non

calcareous green siltstone. The red and green siltstone were deposited C.C.D. which was at a depth of 4.25 km during the Late Oligocene/Early (Van

Andel,

1975). Deposition

was probably

significantly

deeper

than

below the Miocene 4.25 km

because red siltstone clasts derived upslope, presumably eroded from the walls of the canyon, are present in the rudite and graded rudite-arenite beds. Other data suggesting deposition in deep water is from trace fossils such as Scoliciu sp., present at the base of a calarenite, which is also known in deep water deposits of Cenozoic age of Spain (Crimes, 1977) and New Zealand (Gregory, 1969). The facies types present in the lower part of the Sarava Formation are consistent with deposition in a deep sea fan environment such as those described by Nelson and Nilsen (1974). The deep sea fan extended eastwards to the surf zone of a Late Oligocene/Early Miocene Vitiaz Arc. Agreement then is with the palaeogeographic reconstruction of Carney and Macfarlane (1978, fig. 4a) (Fig. 3); however, deposi-

tion was in significantly After

deeper water than they proposed.

the deposition

because

of the Sarava

the Late Miocene

1982) contains

benthic

range of 2-3

foraminifera

km (Paltech

Dr. I. Deighton,

Formation

Tafwutmuto

1979/17

shallowed

somewhat

a fossil Globigerina ooze (Carney.

which indicate

report

pers. commun.

the ocean

Formation,

that deposition

to the New Hebrides

was in a depth

Geological

Survey,

1980).

DISCUSSION

Carney and MacFarlane (1978, fig. 4d) proposed that in the latitude of Maewo the sea floor deepened and rifting occurred during the period 13-7 Ma. As evidence for initiation of rifting they show that there are (Mitchell, 1970) and Maewo at this time indicating

unconformities on Malekula a change in tectonic regime.

However, deposited

the great depth at which the lower part of the Sarava Formation was suggests that rifting occurred earlier than Carney and Macfarlane (1978)

thought. Present

day intra-arc

km (Dubois

of the lower part Trough,

troughs

of the New Hebrides

have a maximum

et al., 1978); they are not as deep as the suggested of the Sarava

are 4 km deep (Karig

depth

of deposition

genetic

difference

Formation.

Interarc

1971, fig. 6)-a

depth

of the lower part of the Sarava between

interarc

and intraarc

basins,

of deposition

such as the Mariana

approaching Formation.

basins

depth of 3.3

depth

that of assumed There may be little

because

Carney

and

Mac-

farlane (this volume) show that the New Hebrides “intraarc” basins. such as the Coriolis Trough, are floored by recent lava extrusions suggesting that they are sites of early

interarc

Oligocene ophiolite

extension.

It is likely

(or the Late Eocene), sequence

on Pentecost

that

rifting

occurred

during

the

Early

i.e. before 35 Ma, (the age of the oldest part of the Island,

Mallick

and Neef.

1974). Differences

be-

tween crust formed at mid ocean ridges, and interarc basins are slight (Karig and Moore, 1975). Such a mode of origin, if proved, solves the difficult problem of the kind of plate tectonic environment in which the ophiolite sequence of Pentecost formed

(Carney

formed

in interarc

and Macfarlane, basins

1976; Ravenne

et al., 1976, p. 69). Also, ophiolite

is likely to be considerably

deformed

during

subsequent

periods of diastrophism (Hawkins, 1974). Such deformed ophiolite is found on Pentecost (Mallick and Neef, 1974; Carney and Macfarlane, this volume). The lower part of the Sarava Formation was probably deposited in an extinct interarc environment because the porphyritic hornblende diorite intrusives of Malekula, dated c. 18 Ma (Early Miocene; Gorton, 1974, p. 232) and line volcaniclastics (the Matanui Group; Mitchell, 1970) suggest that a arc (like that of the present day volcanic chain; Mitchell, 1970) was That is igneous rocks should not be formed in a remnant (third) arc Also, proposed

basin

subsidence

elsewhere,

to below

the CCD

interval

following

e.g. the Parece Vela Basin (Karig and Moore,

the calcalkanormal island then present. environment.

rifting

has been

1975). Most of the

182

A

YiTlAL RED B

s

MUDSTONE (MALEWA)

g -

4 2 w

z

i$

;

ARC z 0 ;; 23

1:: z*. -

$zJ

;z

!?a

Fig. 4. A. Bathymetric Oligocene

profile across the Mariana Arc system after Karig (1971). B. Elements

of the

arc system which strongly resembles the Mariana Arc System.

sediment was probably derived from the Vitiaz Arc; Karig and Moore (1975) consider that little sediment is derived from third arc, i.e., sedimentation was asymmetric. Red Mudstone of unknown age is present along the northwest coast of Malekula (Mitchell, 1970). If the red mudstone is pre-Miocene (Mitchell, 1970, p. 205) then Oligocene palaeogeography strongly resembles the present day Mariana Arc System (Fig. 4). The Red Mudstone of Malekula being an analogue of the Parece Vela Basin of the Mariana Arc System. Perhaps the most definitive account of interarc deposits in an oceanic environment is by Karig and Moore (1975). They found that like the deposits of the lower part of the Sarava Formation most of the sedimentation in oceanic interarc basins is from mass flows. The shallowing represented by the Tafwutmuto Formation is probably due to the westward migration of the frontal arc to lie in the longitude of Maewo and Pentecost Islands as submarine tholeiitic volcanism is known at c. 7 Ma on Maewo (Carney, in press) (Fig. 3~). ACKNOWLEDGEMENTS

The New Hebridean Geological Survey is thanked for logistic support, allowing the writer to refer to Dr. Ian Deighton’s report.

and

REFERENCES

Camey, J.N. (in press), Geology Carney, J.N. and Macfarlane,

of Maewo, Reg. Rept. Geol. Surv. New Hebrides.

A., 1976. Volcano-tectonic

New Hebrides. In: Geodynamics

in South-West

events and Pre-Pliocene

crustalextension

in the

Pacific. Editions Technip, Paris, pp. 91-104.

Carney, J.N. and Macfarlane,. A., 1978. Lower to middle Miocene sediments on Maewo. New Hebrides, and their relevance Geophys.,

to the development

9: 123-130.

of the outer Melanesian

Arc System.

Bull. Aust. Explor.

183

Camey,

J.N. and Macfarlane,

evolution

Plate Boundaries. Coleman,

P.J.,

Dubois,

1977. Trace

(Editors),

J., Dugas, D.A.,

Gorton,

M.P.,

F., Lapouille,

Karig,

1969. Trace

Auckland.

76: 2542-256 D.E. and

Mallick,

Can. J. Earth data

from

New Hebrides,

and their

for 1967: pp. 36-37. Spain.

and J.C.

In: T.P. Crimes

Press, Liverpool,

pp. 71-90.

at the rear of the New Hebrides

Sci., 15: 351-360. the New

Bull. Aust.

Hebrides.

Explor.

of the New Hebrides.

Ph.D.

Thesis

Australian

mimeographed. facies of the Waitematea

Group,

Whangaparaoa

R. Sot. N.Z. Earth Sci., 7: I-20.

of the Lau Basin, a marginal of Continental

and development

sea behind

Margins.

of marginal

basins

the Tonga Arc. In: C.A. Burk and

Springer,

Berlin, pp. 505-520.

in the western

Pacific.

J. Geophys.

Res.,

1. Moore,

C.F.,

K.A.,

1975. Tectonically

1967. Maewo.

controlled

sedimentation

Ann. Rep. Geol. Surv. New Hebrides

D.I.J. and Neef, G., 1974. Geology A.H.G.,

1970. Facies

Sedimentology,

14: 201-243.

Mitchell,

A.H.G.,

197 1. Geology

C.H. and Nilson, T.H.,

of Pentecost.

of an Early

Miocene

of northern

Malekula.

19’74. Depositional

Dott Jr. and R.H.

Shaver

Paleontol.

Spec. Publ. 19, pp. 69-105.

Obellianne,

Mineral. J.M.,

(Editors),

1958. Contribution

(Iles Vat& Pentecote Ravenne,

to Recent structural of the India-Pacific

in marginal

basins.

Earth

Sci. Lett., 26: 233-238.

Mitchell,

Nelson,

fan, northern

and geochronology A.C.T.,

The Geology

D.E., 1971. Origin

Liggett,

eastern

R., 1978. The troughs

fossils from the turbidite

Trans.

J.R., 1974. Geology

Planet.

on Maewo,

deep-sea

of palaeomagnetic

Canberra,

Drake (Editors),

Karig,

foraminifera

of formation.

1974. Geochemistry

M.R.,

CL.

large

A. and Louat,

mechanisms

University,

Peninsula, Hawkins,

The Evolution

9: 117- 123.

National Gregory,

on the Miocene

Ann. Rep. Geol. Surv. New Hebrides

fossils of an Eocene

1978. Analysis

Geophys.,

bearing (Editor),

Trace Fossils 2. Geol. J. Spec. Issue 9. Steel House

Island arc: possible Falvey,

Packham

87: 147-176.

Eocene

Pacific implications.

T.P.,

Harper

evidence

Arc. In: G.H.

Tectonophysics,

1969. Derived

south-west Crimes,

A., 1982. Geofogicai

of the New Hebrides

Maewo,

Volcanic

6(3-4):

F. and Montadert,

and ancient

Geosynclinal

geologique

arc. In: Geodynamics

Malekula

Island,

New Hebrides.

Reg. Rep. Geol. Surv. New Hebrides.

and Ancient

&‘la connaissance

J., Dugas.

island

Arc,

trends of modern

Santo). Sci Terre, Nancy,

C., Pascal, G., Dubois,

arc: the New Hebrides

Modern

for 1965, pp. 8- 12.

Reg. Rep. Geol. Surv. New Hebrides.

deep sea fans. In: R.H.

Sedimentation.

de l’archipel

Sot.

des Nouvelles

Econ.

Hebrides

139-369.

L., 1976. Model of a young

in South-West

Pacific.

Editions

intra-oceanic

Technip,

Paris,

pp. 63-78. Van Andel, calcareous

T.H.,

1975. Mesozoic/Cenozoic

sediments.

Earth

Planet.

calcite

compensation

Sci. Lett., 26: 187-194.

depth

and the global

distribution

of