Structure and sedimentation in an active caldera, Rabaul, Papua

Structure and sedimentation in an active caldera, Rabaul, Papua

Technical Report No. 39
STRUCTURE AND SEDIMENTATION IN AN ACTIVE CALDERA,
RABAUL, PAPUA NEW GUINEA
by
H. Gary Greene, US Geological Survey, USA
Donald L. Tiffin, CCOP/SOPAC, Suva
Chris O. McKee, Rabaul Volcano Observatory, PNG
TR
39
STRUCTURE AND SEDIMENTATION IN AN ACTIVE CALDERA,
RABAUL, PAPUA NEW GUINEA
by
H.
Gary Greene, Donald L. T i f f i n '
and C h r i s 0. McKee
ABSTRACT
Recent
seismic
and
tectonic
activities i n Rabaul Caldera, Papua New
Guinea, suggest t h a t magma i s moving beneath t h i s p a r t i a l l y submarine
s t r u c t u r e , and t h a t a new volcano may be developing.
Changes i n e l e v a t i o n
noted onshore over t h e p a s t 10 y e a r s i n d i c a t e t h a t r a p i d and l a r g e magnitude
(up t o one meter on south M a t u p i t I s l a n d ) u p l i f t s a r e t a k i n g p l a c e on t h e
s e a f l o o r n e a r t h e c e n t e r of t h e c a l d e r a .
The t r e n d of onshore measurements
s u g g e s t s t h a t even g r e a t e r u p l i f t may have occurred o f f s h o r e .
of c a l d e r a s e i s m i c e v e n t s h a s i n c r e a s e d i n t h e same period.
The frequency
Within t h e
c a l d e r a , s e i s m i c e p i c e n t e r s d e f i n e an e l l i p t i c a l r i n g , t h e f o c a l a r e a of which
l i e s i n t h e same g e n e r a l a r e a a s t h e c e n t e r of t h i s u p l i f t .
A marine g e o p h y s i c a l survey undertaken i n 1982 by t h e S.P. Lee i n Rabaul
Caldera shows t h e development of a s e a f l o o r bulge i n t h e c e n t e r of t h e
caldera.
High- resolution, s e i s m i c r e f l e c t i o n p r o f i l e s show t h a t t h i s bulge
c o n s i s t s of two domal u p l i f t s bounded and s e p a r a t e d by two major north- south
These zones are marked by f o l d e d surficial s e d i m e n t s
t r e n d i n g f a u l t zones.
and y o u t h f u l f a u l t s , s e v e r a l of which c u t t h e s e a f l o o r .
A prominent slump
also f l a n k s t h e upbowed a r e a .
*************************************************************************************
1 D.S. G e o l o g i c a l S u r v e y , H e n l o P a r k , Ca., U.S.A.
2 CCOP/SOPAC, Suva, Fiji
3 Rabaul Volcano Observatory, Rabaul, Papua New Guinea
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F i v e major a c o u s t i c u n i t s have been i d e n t i f i e d i n t h e s e i s m i c r e f l e c t i o n
p r o f i l e s and t h e s e c o n s i s t of an a c o u s t i c basement, and sedimentary u n i t s of
i r r e g u l a r l y - l a y e r e d , c r o s s - l a y e r e d , c o n t o r t e d and w e l l - l a y e r e d sequences. The
s e d i m e n t a r y u n i t s p r i m a r i l y r e p r e s e n t ash laid down io d i f f e r e n t d e p o s i t i o n a l
environments.
The c r o s s - l a y e r e d , i r r e g u l a r l y - l a y e r e d , and c o n t o r t e d u n i t s
appear t o have been d e p o s i t e d i n a dynamic environment s u b j e c t e d t o s t r o n g
c u r r e n t f o r c e s , s e i s m i c i t y and/or mass wasteing w h i l e t h e w e l l - l a y e r e d
sequences were d e p o s i t e d i n a p a s s i v e o r q u i e t - w a t e r environment.
L o c a l l y , an
i n t e r f i n g e r i n g of w e l l - l a y e r e d sequences w i t h t h e o t h e r sedimentary u n i t s
i n d i c a t e d a t r a n s i t i o n a l environment t h a t has a l t e r n a t e d between dynamic and
passive
depositional
processes.
A submarine channel c u t s through most of a l l t h e a c o u s t i c u n i t s and
a p p e a r s t o be t h e conduit f o r sediment t r a n s p o r t o u t of t h e c a l d e r a .
The
channel occupies an o l d e r b u r i e d channel i n t h e n o r t h of t h e c a l d e r a t h a t i s
2
p r e s e n t l y being exhumed, whereas i n t h e south, a c t i v e e r o s i o n of w e l l - l a y e r e d
s e d i m e n t s i s t a k i n g place. S e v e r a l youthful looking v o l c a n i c cones a l s o
interrupt the depositional layers.
We conclude t h a t t h e s e a f l o o r bulge and f a u l t zones r e s u l t from
emplacement of magma a t a shallow depth.
The magma h a s upbowed and d i s p l a c e d
sedimentary l a y e r s beneath t h e s e a f l o o r . Contorted surficial s e d i m e n t s and
slumps a d j a c e n t t o t h e bulge a r e probably t h e r e s u l t of u p l i f t and s e i s m i c
activity.
The p a t t e r n of s e i s m i c i t y appears t o d e f i n e t h e a c c u m u l a t i o n of
magma a t depth beneath t h e s e a f l o o r bulge.
C o n t i n u a t i o n of t h i s a c t i v i t y will
probably c u l m i n a t e i n an e r u p t i o n , p o s s i b l y w i t h t h e c o n s t r u c t i o n of a new
volcano.
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INTRODUCTION
The p o r t c i t y of Rabaul i s s i t u a t e d i n a l a r g e v o l c a n i c c a l d e r a on t h e
n o r t h c o a s t of New B r i t a i n , Papua New Guinea (Fig. 1). The c a l d e r a i s open t o
t h e sea on t h e e a s t e r n s i d e forming an e x c e l l e n t s h e l t e r e d harbor. S e v e r a l
e x t i n c t or dormant s a t e l l i t e volcanoes are s c a t t e r e d around t h e n o r t h e r n and
e a s t e r n r i m of t h e c a l d e r a , and small, a c t i v e volcanoes occur w i t h i n the
c a l d e r a i t s e l f . Eruptions from t h e s e r e l a t i v e l y s m a l l volcanoes have t a k e n
place i n h i s t o r i c a l time.
During one e r u p t i o n i n 1937, 505 l i v e s were l o s t .
The p o p u l a t i o n of Rabaul town a t t h a t t i m e vas about 6000, but has now r i s e n
t o about 15,000, and w i t h i n 1 5 km of t h e c e n t e r of t h e c a l d e r a t h e t o t a l
p o p u l a t i o n i s about 70,000.
any of t h e nearby volcanoes.
A major disaster could occur from t h e e r u p t i o n of
The Rabaul Volcanological Observatory (RVO),
s i t u a t e d on t h e n o r t h e r n r i m of t h e c a l d e r a was e s t a b l i s h e d t o m o n i t o r
v o l c a n i c a c t i v i t y through a network of seismometers. t i l t m e t e r s and o t h e r
t y p e s of s u r v e i l l a n c e i n s t r u m e n t s f o r t h e s p e c i f i c purpose of warning t h e
populace of impending e r u p t ions.
Over t h e l a s t t e n y e a r s , a g e n t l e up-warp h a s occurred within t h e
caldera.
T h i s d e f o r m a t i o n has been measured i n some d e t a i l on land by means
o f o p t i c a l l e v e l l i n g , t i l t m e t e r s , d r y t i l t m e t r y , and r e p e a t e d g r a v i t y
readings.
All measurements have shown
consistent
trends
that
indicate
uplift
of an a r e a around M a t u p i t Harbor (Fig. 2). The l a r g e s t change exceeds one
m e t e r on t h e south end of M a t u p i t I s l a n d . but t h e p a t t e r n of onshore
measurements s u g g e s t s t h a t even g r e a t e r u p l i f t may have occurred s o u t h e a s t of
the
island, in Blanche Bay. The frequency of c a l d e r a s e i s m i c e v e n t s noted on
t h e Rabaul network has a l s o i n c r e a s e d s e v e r a l - f o l d i n the same p e r i o d , and i t
has been found t h a t a r i n g p a t t e r n of s e i s m i c i t y o c c u r s w i t h i n t h e c a l d e r a .
The e l l i p t i c a l l y a n n u l a r p a t t e r n of c a l d e r a s e i s m i c i t y has i t s c e n t r o i d i n t h e
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same g e n e r a l a r e a as t h e f o c u s of this u p l i f t (Fig. 3).
The U.S. Geological Survey‘s (USGS) Research Vessel S.P. Lee made a p o r t
s t o p a t Rabaul w h i l e working under a hydrocarbons r e s o u r c e e v a l u a t i o n program
f o r o t h e r southwest P a c i f i c countries.
This program involved
A u s t r a l i a , New
Zealand, and t h e United S t a t e s (ANZUS), and was coordinated through t h e UNsponsored Committee f o r Coordination of J o i n t P r o s p e c t i n g f o r Mineral
Resources i n South P a c i f i c Offshore Areas (CCOP/SOPAC) I n response t o a
r e q u e s t from t h e Papua New Guinea government through CCOP/SOPAC, t h e USGS
agreed t o c o l l e c t geophysical d a t a i n Blanche Bay and Matupit Harbor t o a s s i s t
RVO i n d e l i n e a t i n g t h e a r e a of suspected u p l i f t .
A m a r i n e g e o p h y s i c a l s t u d y such a s undertaken h e r e i s unique i n t h e
i n v e s t i g a t i o n of t h e development of rubmarine volcanoes. I f indeed t h e bulge
b e l i e v e d t o be developing on t h e s e a f l o o r of Blanche Bay were t o be a s s o c i a t e d
w i t h f u t u r e volcanism, o r i f a new v o l c a n i c cone is formed i n this a r e a , t h e n
t h i s t y p e of study would a l l o w g e o l o g i s t s t o view t h e s e d i m e n t
d e f o r m a t i o n and
d i s r u p t i o n t h a t accompanies t h e development of a s h a l l o w magma chamber p r i o r
t o e r u p t i o n . Marine s e i s m i c r e f l e c t i o n p r o f i l i n g methods g i v e a t h r e e d i m e n s i o n a l p e r s p e c t i v e t o sub- seafloor s t r a t i g r a p h y t h a t can be used to
observe p r o g r e s s i v e s t r u c t u r a l d e f o r m a t i o n a s s o c i a t e d w i t h dynamic t e c t o n i c
e v e n t s , i f monitored on a r e g u l a r b a s i s . The r e s u l t s r e p o r t e d h e r e p r o v i d e
b a s e l i n e i n f o r m a t i o n t h a t may be used t o i n i t i a t e an o f f s h o r e m o n i t o r i n g
program.
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METHOD AND PROCEDURES
Data c o l l e c t e d during t h e 8-hour survey (Fig. 4) c o n s i s t s of p r e c i s i o n 12kHz
b a t h y m e t r i c p r o f i l e s , 1,200 j o u l e Uniboom, h i g h - r e s o l u t i o n s e i s m i c r e f l e c t i o n
p r o f i l e s , a sonobuoy r e f r a c t i o n p r o f i l e and s i n g l e channel a i r gun s e i s m i c
r e f l e c t i o n p r o f i l e s ; a s i n g l e l a r g e air gun (500
in) was f i r e d throughout t h e
survey t o test t h e l o c a l s e i s m i c network response from v a r i o u s l o c a t i o n s i n
t h e h a r b o r ( s e e T i f f i n and Vedder, 1982) as well as t o provide an a c o u s t i c
s o u r c e f o r t h e s i n g l e channel s e i s m i c r e f l e c t i o n p r o f i l e . The primary d a t a
used i n t h i s r e p o r t a r e t h e 12kHz sounding p r o f i l e s and t h e h i g h - r e s o l u t i o n
seismic reflection profiles.
The l a t t e r were recorded a t a 0.25 sec. f i r e and
sweep r a t e and passed through a 250Hz low c u t and 1500Hz high c u t f i l t e r . The
Uniboom system aboard t h e Lee has f o u r hull- mounted e l e c t r o m a g n e t i c boomer
p l a t e s a s t h e source and t w i n s h o r t high- resolution hydrophone s t r e a m e r s
boomed abeam a s the r e c e i v e r .
With t h e use of t h i s system we were a b l e t o
o b t a i n good p e n e t r a t i o n of up t o 200 m a t a r e s o l u t i o n of 1 m i n
the
shallow
water depths from 25 t o 300 m i n t h e submarine p a r t of t h e c a l d e r a .
Navigational f i x e s were p l o t t e d a t approximately 5 minute i n t e r v a l s , as
w e l l a s a t t h e t i m e s of t u r n s , u s i n g two high p r e c i s i o n n a v i g a t i o n a l RADAR's
(each one r e a d i n g ranges and b e a r i n g s from s e p a r a t e and independent t a r g e t s )
r e a d simultaneously. Fixes were p l o t t e d d i r e c t l y on U.S. Naval Chart No. 82192
a t a s c a l e of 1:25,000. Accuracy of p o s i t i o n i n g i s e s t i m a t e d t o be better t h a n
80 meters over most of t h e survey area.
TECTONIC SETTING
The Rabaul Caldera dominates t h e lowland of n o r t h e a s t e r n G a z e l l e
Peninsula, New B r i t a i n I s l a n d (Fig. 1).
The c a l d e r a , breached by a 5 km gap
on t h e s o u t h e a s t s i d e , forms Blanche Bay, t h e harbor of Rabaul.
The Rabaul volcano ( F i s h e r , 1939; Fleming, 1974) i s a s h i e l d volcano of
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p r o b a b l e Quaternary age (Macnab, 1970).
It
is
situated n e a r t h e j u n c t u r e of
t h r e e c r u s t a l p l a t e s : t h e Solomon Sea plate, t h e P a c i f i c p l a t e , and t h e South
Bismarck p l a t e (Fig. 5), and is only a few k i l o m e t e r s west of t h e major
t r a n s f o r m f a u l t between t h e P a c i f i c and South Bismarck p l a t e , which p a s s e s
through St.
George's Channel (Johnson,
1979).
South of New B r i t a i n I s l a n d , t h e New B r i t a i n t r e n c h marks t h e a c t i v e
s u b d u c t i o n of t h e Solomon Sea p l a t e under t h e New B r i t a i n arc.
volcano i s t h e e a r t e r n m o s t element of t h e arc.
The
Rabaul
S e v e r a l major a c t i v e f a u l t s
c u t through t h e G a z e l l e P e n i n s u l a (Macnab, 1970), St. George's Channel and
a d j a c e n t s o u t h e r n New I r e l a n d (Hohnen, 1970).
The a r e a i s o f t e n shaken by
l a r g e earthquakes of i n t e r m e d i a t e f o c u s depth along a n o r t h w e s t d i p p i n g
Benioff zone, w h i l e s h a l l o w l a r g e magnitude e v e n t s occur i n St. George's
Channel. The r e g i o n i s one of t h e most s e i s m i c a l l y a c t i v e a r e a s of t h e w o r l d .
CALDERA PHYSIOGRAPHY
The Rabaul Caldera is e l l i p t i c a l i n shape, measuring 14 km from n o r t h t o
s o u t h , and 9 km from e a s t t o w e s t (Fig. 2). This c o l l a p s e d and breached
c a l d e r a i s surrounded by s t e e p w a l l s up t o 450 m high.
The c a l d e r a w a l l s a r e
p a r t of an o l d r i n g f r a c t u r e system t h a t involved t h e c o l l a p s e of t h e summit
of t h e o r i g i n a l s h i e l d volcano.
V e r t i c a l movement a l o n g t h e f r a c t u r e s y s t e m
may have been as much as 600 m.
S e v e r a l e x t i n c t o r dormant b a s a l t i c and b a s a l t i c - a n d e s i t e s a t e l l i t e cones
are
situated
along t h e
northern
and e a s t e r n walls of t h e c a l d e r a (Fig. 6).
These a r e named Tovanumbatir (North Daughter), Kabiu (Mother) and Turangunan
(South Daughter).
The breached cone of Palangiagia s t r a d d l e s the e a s t e r n r i n g
f r a c t u r e w i t h a l a t e r s m a l l e r cone of Rabalanakaia w i t h i n it.
I n s i d e t h e c a l d e r a , a narrow c o a s t a l p l a i n p r o v i d e s a h a b i t a b l e a r e a for
the city of Rabaul,
its a i r p o r t and h a r b o r f a c i l i t i e s .
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cones of Tavurvur on t h e e a s t s i d e , and Vulcan on t h e west, r i s e from t h e
margins of t h e c a l d e r a .
The a c t i v e fissure o f Sulphur Creek is p r e s e n t on t h e
e a s t s i d e n e a r Palangiagia.
Remnants of o t h e r v o l c a n i c c e n t e r s i n c l u d i n g t h e
oddly shaped r o c k s of t h e Dawapia Rocks (Beehives) and Matupit I s l a n d a r e a l s o
found w i t h i n t h e c a l d e r a .
Offshore, b u t w i t h i n t h e c a l d e r a , s e v e r a l bays a r e formed from t h e
m o d i f i c a t i o n of t h e p r i m a r y c a l d e r a by p o s t - c o l l a p s e v o l c a n i c e v e n t s (Fig. 7).
The p r e s e n t day s e a f l o o r topography and morphology r e f l e c t t h e complex
development of t h e bays; t h e p r i m a r y topographic e l e m e n t s are b a s i n s , s h o a l s ,
and cones. I n t h e n o r t h e r n p a r t of t h e c a l d e r a two r e l a t i v e l y narrow bays,
e l o n g a t e d along a north- south a x i s , a r e s e p a r a t e d from each o t h e r by a
p e n i n s u l a a t t h e s o u t h e r n end of which is Matupit I s l a n d . The l a r g e r bay l i e s
t o t h e west and i s known a s Simpson Harbor w h i l e t h e s m a l l e r one t o t h e e a s t
i s known a s M a t u p i t Harbor (Greet Harbor). A r e l a t i v e l y broad embayment known
as Karavia Bays forms t h e s o u t h e r n p a r t of t h e c a l d e r a . All of t h e h a r b o r s and
embayments open t o t h e c e n t r a l and l a r g e s t bay, Blanche Bay, which i n t u r n i s
open t o t h e s e a along its e a s t e r n margin, t h e breached w a l l of t h e c a l d e r a .
Simpson Harbor, in t h e n o r t h e r n p a r t of t h e c a l d e r a , i s a broad,
f l a t t e n e d trough s l o p i n g southward i n t o Blanche Bay.
A submarine channel,
h e r e i n c a l l e d Simpson Channel (Pigs. 2 & 7), heads a d j a c e n t t o t h e Dawapia
Rocks approximately t h r e e - f o u r t h s of t h e way s o u t h from t h e head of t h e
embayment and may extend even f u r t h e r i n t o t h e bay.
North of t h i s a r e a , t h e
h a r b o r f l o o r is r e l a t i v e l y f l a t , s l o p i n g g e n t l y upward and northward from a
d e p t h of 85 m t o about 20 m. Depths i n t h e e n t r a n c e t o Simpson Harbor r e a c h
150 m i n t h e c e n t e r of t h e submarine channel where it e n t e r s Blanche Bay.
Dawapia Rocks appear t o be t h e exposed and eroded remnants of a v o l c a n i c cone
t h a t was b u i l t up from t h e
harbor
floor.
The f l o o r of Blanche Bay g e n e r a l l y s l o p e s t o t h e s o u t h toward Karavia
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Bay, a l t h o u g h it is i n t e r r u p t e d n e a r i t s c e n t e r by a s u b t l e b a t h y m e t r i c
t e r r a c e t h a t extends due e a s t from Vulcan volcano.
This t e r r a c e is b i f u r c a t e d
by an e a s t e r l y - t r e n d i n g , poorly developed rubmarine channel t h a t connects with
Simpson Channel, and which c o n t i n u e s along its s o u t h e r l y t r e n d i n t o Karavia
Bay.
The bay opens t o St. George’s Channel through a 5 km gap i n t h e
s o u t h e a s t e r n c a l d e r a wall.
Two s h o a l a r e a s occur i n t h e mouth of t h e bay
marking t h e l i n e of t h e breached c a l d e r a wall. Mackenzie Shoal, t h e more
n o r t h e r l y s h o a l , comes t o w i t h i n 5 m of the s e a s u r f a c e about 0.5 km o f f s h o r e
immediately s o u t h of Turangunan.
An unnamed knob i n t h e c e n t e r of t h e b r e a c h
r i s e s s t e e p l y from t h e f l o o r of t h e bay t o w i t h i n about 80 m of t h e s u r f a c e .
Karavia Bay forms t h e s o u t h e r n p a r t of t h e c a l d e r a .
It
is a well
developed b a s i n reaching a d e p t h of 275 m and shaped l i k e a half- bowl with
s t e e p s i d e s t o t h e south. Simpson Channel e n t e r s t h e b a s i n from t h e n o r t h ,
then f o l l o w s t h e thalweg of t h e b a s i n eastward i n t o a deep channel n o r t h of
Raluana P o i n t , h e r e c a l l e d Karavia Channel, which connects t h e b a s i n w i t h St.
George’s Channel o u t s i d e t h e harbor through t h e breached sill
o r remnant of
t h e s o u t h e a s t c a l d e r a wall.
Matupit
Harbor is v e r y s h a l l o w and saucer- shaped, with a f l a t - f l o o r at
about 60 m depth. It l i e s between Tavurvur cone and Matupit I s l a n d . A s h a l l o w
sill e x t e n d s eastward from Matupit Island and s e p a r a t e s t h e h a r b o r from
Blanche Bay, making it a p h y s i o g r a p h i c a l l y enclosed b a s i n below t h e sill d e p t h
of 25 m.
VOLCANIC
EVENTS
Two major e x p l o s i v e e v e n t s , t h e first 3500 y e a r s B.P. and t h e second 1400
y e a r s B.P., l e d t o t h e development of t h e p r e s e n t c a l d e r a , and covered t h e
surrounding c o u n t r y s i d e by a t h i c k m a n t l e of A i r f a l l pumice and a s h , and
i g n i m b r i t e (Heming, 1974; Heming and Carmichael, 1973; Peterman and Heming,
G4:
rab.gtm
1/5/84
(2)
1/18/85
(3)
8
1974; Walker, e t al., 1981).
The p r i m i t i v e cone of t h e o r i g i n a l volcano
foundered d u r i n g t h e s e e v e n t s along a r i n g f r a c t u r e a t t h e c a l d e r a w a l l s .
dropping t h e c a l d e r a f l o o r approximately 600 m or more t o i t s p r e s e n t
e l e v a t i o n below sea l e v e l . One of t h e a n c i e n t v o l c a n i c events may have t a k e n
p l a c e i n t h e Karavia Bay a r e a of t h e southern c a l d e r a and a n o t h e r was probably
i n t h e n o r t h e r n s e c t i o n (Heming, 1974). Between t h e s e major e r u p t i o n s ,
a d d i t i o n a l v o l c a n i c e v e n t s occurred and are recorded i n t h e s t r a t i g r a p h i c
column.
The s e i s m i c i t y p a t t e r n of Figure 3 probably o u t l i n e s t h e r i n g - f a u l t
developed d u r i n g t h e l a s t c a l d e r a forming e r u p t i o n (McKee, et al., 1983).
Of t h e f o u r youngest v o l c a n i c s t r u c t u r e s . only Rabalanakaia h a s n o t
d e f i n i t e l y been a c t i v e within h i s t o r i c a l time ( F i s h e r , 1939); a r e p o r t by a
French n a v i g a t o r , P h i l i p Carteret, i n 1767 s u g g e s t s t h a t some t y p e of e r u p t i o n
may have been i n p r o g r e s s then a t e i t h e r Rabalanakaia
Rabalanakaia
has
a
or Tavurvur.
well- preserved c r a t e r surrounded by an a s h rampart.
Youthful- looking l a v a f l o w s extend from t h e c r a t e r along t h e lowland as f a r a s
t h e s h o r e of M a t u p i t Harbour n o r t h of Tavurvur.
Sulphur Creek f i s s u r e s were a c t i v e in about 1850.
Tavurvur
erupted
e x p l o s i v e l y i n 1791, 1878, 1937 and i n t e r m i t t e n t l y from 1941 t o 1943.
Vulcan,
which i n earliest h i s t o r i c a l times was a submarine bank, e r u p t e d i n 1878 t o
form an i s l a n d of low relief.
The i s l a n d was welded t o t h e w e s t e r n s h o r e l i n e
d u r i n g t h e 1937 e r u p t i o n of Vulcan i n which 505 people died.
Both Vulcan and Tavurvur were a c t i v e s i m u l t a n e o u s l y i n 1878, even though
t h e y are l o c a t e d on o p p o s i t e s i d e s of t h e harbor ( F i s h e r , 1939).
In
1937,
they were both a c t i v e a t t h e same t i m e again, w i t h Tavurvur e r u p t i n g
s y m p a t h e t i c a l l y f o r one day d u r i n g t h e Vulcan e r u p t i o n (Joycey, undated;
Thomas, 1937; McKee e t al., 1983).
On both o c c a s i o n s when t h e two were
a c t i v e , d i s t u r b a n c e s of the sea or clouds of vapor r i s i n g from t h e bay were
G4:
rab.gtm
1 / 5 / 8 4 (2)
1/18/85
(3)
9
1974; Walker, et al., 1981).
The p r i m i t i v e cone of t h e o r i g i n a l volcano
foundered d u r i n g t h e s e events along a r i n g f r a c t u r e a t t h e
caldera
walls,
dropping t h e c a l d e r a f l o o r approximately 600 m o r more t o i t a p r e s e n t
e l e v a t i o n below sea l e v e l . One of t h e a n c i e n t v o l c a n i c e v e n t s may have t a k e n
p l a c e i n t h e Karavia Bay a r e a of t h e southern c a l d e r a and a n o t h e r was probably
i n t h e n o r t h e r n s e c t i o n (Heming, 1974). Between t h e s e major e r u p t i o n s ,
a d d i t i o n a l v o l c a n i c e v e n t s occurred and a r e recorded i n t h e s t r a t i g r a p h i c
column.
The s e i s m i c i t y p a t t e r n of F i g u r e 3 probably o u t l i n e s t h e r i n g - f a u l t
developed d u r i n g t h e l a s t c a l d e r a forming e r u p t i o n (McKee, et al., 1983).
Of t h e f o u r youngest v o l c a n i c s t r u c t u r e s , only Rabalanakaia has n o t
d e f i n i t e l y been a c t i v e w i t h i n h i s t o r i c a l t i m e ( F i s h e r , 1939); a r e p o r t by a
French
navigator,
Philip
Carteret, i n 1767 s u g g e s t s t h a t some t y p e of e r u p t i o n
may have been i n p r o g r e s s then
Rabalanakaia
has
a
at
e i t h e r Rabalanakaia
or Tavurvur.
well- preserved c r a t e r surrounded by an ash rampart.
Youthful- looking lava f l o w s extend from the c r a t e r along t h e lowland as f a r as
t h e s h o r e of M a t u p i t Harbour n o r t h of Tavurvur.
Sulphur Creek f i s s u r e s were a c t i v e i n about 1850.
Tavurvur
erupted
Vulcan,
e x p l o s i v e l y i n 1791, 1878, 1937 and i n t e r m i t t e n t l y from 1941 t o 1943.
which i n e a r l i e s t h i s t o r i c a l t i m e s w a s a submarine bank, e r u p t e d i n 1878 to
form a n i s l a n d of low r e l i e f .
The i s l a n d v a s welded t o t h e w e s t e r n s h o r e l i n e
d u r i n g t h e 1937 e r u p t i o n of Vulcan i n which 505 people died.
Both Vulcan and Tavurvur were a c t i v e s i m u l t a n e o u s l y i n 1878, even
t h e y are l o c a t e d on o p p o s i t e s i d e s of t h e harbor ( F i s h e r , 1939).
In
though
1937,
they were both a c t i v e a t t h e same time again, w i t h Tavurvur e r u p t i n g
s y m p a t h e t i c a l l y f o r one day d u r i n g t h e Vulcan e r u p t i o n (Joycey, undated;
Thomas, 1937; McKee, et al., 1983).
On
both o c c a s i o n s when t h e two were
a c t i v e , d i s t u r b a n c e s of t h e sea o r clouds of vapor r i s i n g from t h e bay were
G4: rab.gtm
1 / 5 / 8 4 (2)
1 / 1 8 / 8 5 (3)
9
seen i n a d i r e c t l i n e between t h e m , i n d i c a t i n g t h a t a lineament may j o i n t h e
two c e n t e r s , perhaps even c o n t r o l l i n g t h e i r l o c a t i o n s ( F i s h e r . 1939; Johnson,
1981).
H i s t o r i c a l l y , e r u p t i o n s of Vulcan have produced c o n s i d e r a b l e volumes
of e j e c t a , whereas Tavurvur e v e n t s , while e x p l o s i v e i n n a t u r e , have n o t
involved a l a r g e amount of m a t e r i a l (Walker, et
al., 1981).
GENERAL GEOLOGY OF THE CALDERA
Four i n f o r m a l u n i t s (Fig. 6) i n t h e Rabaul v o l c a n i c s e r i e s were
d i s t i n g u i s h e d by Heming (1974).
t h e Karavia
Be termed t h e s e (1) t h e pre-Caldera u n i t , (2)
Bay u n i t , (3) t h e Caldera u n i t , and (4) t h e post- Caldera u n i t . The
b a s a l p a r t of t h e s e i s m i c s t r a t i g r a p h y defined i n our work i s composed
these units.
Pre-Caldera
Of
B r i e f l y , t h e y a r e as f o l l o w s :
Unit
As d e s c r i b e d by Heming (1974), l a v a flows and p y r o c l a s t i c r o c k s make up
t h e pre-Caldera u n i t , which o u t c r o p s on t h e e a s t e r n and n o r t h e r n c a l d e r a w a l l s
and i n an a r e a about 8 km s o u t h of t h e caldera.
The e a r l y volcanism produced
mainly b a s a l t i c l a v a f l o w s and s c o r i a w i t h l a t e r e r u p t i o n s composed
predominantly of a n d e s i t e and d a c i t e p y r o c l a s t i c s .
The b a s a l t and b a s a l t i c -
a n d e s i t e o u t c r o p s a r e m a s s i v e f l o w s up to 20 m t h i c k , i n t e r b e d d e d w i t h t h i n
tuffs.
P y r o c l a s t i c flows were e r u p t e d d u r i n g t h e first c a l d e r a c o l l a p s e 3500
y e a r s B.P., an event t h a t may have ranked with Krakatau i n magnitude and
d e p o s i t e d s e v e r a l t e n s of cubic k i l o m e t r e s of t e p h r a o v e r t h e s u r r o u n d i n g
c o u n t r y s i d e . The s a t e l l i t e v o l c a n o e s Tovanumbatir, Kabiu and Turangunan,
which c o n s i s t of b a s a l t and basaltic-andesite f l o w s and s c o r i a i n t e r b e d s ,
are
p a r t of t h i s u n i t .
Karavia Bay Unit
A n d e s i t e l a v a f l o w s and p y r o c l a s t i c rocks from a s o u r c e i n t h e s o u t h e r n
G4:
rab.gtm
1/5/84
(2)
1/18/85
(3)
10
~
p a r t of t h e c a l d e r a formed a f t e r the first c a l d e r a c o l l a p s e , a c c o r d i n g t o
Heming
(1974).
Andesite lava f l o w s c r o p out on t h e w e s t e r n c a l d e r a wall s o u t h
o f Vulcan while p y r o c l a s t i c rocks appear i n o u t c r o p mainly along t h e s o u t h
These i n c l u d e t u f f s , a s h beds, paleosols and b r e c c i a s .
s h o r e of Karavia Bay.
The b r e c c i a s p r o g r e s s i v e l y i n c r e a s e i n q u a n t i t y i n t h e upper p a r t of t h e
s e c t i o n and e v e n t u a l l y dominate t h e l i t h o l o g y a t t h e t o p of t h e s e c t i o n . The
a s h beds d i p up t o 40' t o t h e south- southvest. The u n i t i s unconformably
o v e r l a i n by p y r o c l a s t i c rocks of t h e Caldera u n i t .
Caldera Unit
The Caldera u n i t i s made up of p y r o c l a s t i c beds r a n g i n g i n l i t h o l o g i e s
from b a s a l t t o r h y o l i t e (Heming, 1974).
The l o v e r beds i n the u n i t a r e mainly
ash- flow d e p o s i t s unconformably o v e r l y i n g t h e Karavia Bay and pre-Caldera
units.
A t h i c k , w i d e l y d i s t r i b u t e d paleosol s e p a r a t e s t h e l o v e r and upper
beds and denotes a p e r i o d of i n a c t i v e volcanism p r i o r t o t h e second c a l d e r a
c o l l a p s e 1400 y e a r s B.P. This event e j e c t e d a volume of t e p h r a exceeding
e l e v e n c u b i c k i l o m e t r e s (Walker, et al.,
1981).
The upper beds of t h e Caldera
Unit a r e a l l pumiceous ash d e p o s i t s m a n t l i n g most of t h e c a l d e r a walls and t h e
surrounding countryside.
The lowest upper bed i s a graded dacitic pumice
l a p i l l i and f i n e ash d e p o s i t .
Unconformably o v e r l y i n g t h i s a r e two pumiceous
ash- flow d e p o s i t s , both of which a t t a i n a t h i c k n e s s of up t o 30 m where
d e p o s i t e d i n v a l l e y s ; they a r e s e p a r a t e d by an i n t e r v e n i n g b r e c c i a whose
o r i g i n is u n c e r t a i n .
At one l o c a l i t y on the s o u t h s h o r e of Karavia Bay, t h e
i n t e r v e n i n g b r e c c i a i s r e p l a c e d by a welded t u f f o v e r 6m t h i c k .
Like t h e
l o v e r beds of t h i s u n i t , t h e upper beds also rest unconformably o v e r t h e
Karavia Bay and pre-Caldera u n i t s i n places.
Post- Caldera Unit
The post- Caldera u n i t i s r e p r e s e n t e d by t h e cones of Rabalanakaia,
G4:
rab.gtm
1/5/84
(2)
1/18/85
(3)
11
Sulphur Creek, Tavurvur and Vulcan. Dawapia Rocks and H a t u p i t I s l a n d a r e a l s o
p a r t of t h e post- Caldera u n i t (Heming, 1974).
Rabalanakaia is made up of p r e - h i s t o r i c a l lava flows and ash.
Sulphur
C r e e k c o n s i s t s of p r e - h i s t o r i c a l l a v a f l o w s and ash, and has h i s t o r i c a l l y
e r u p t e d a n d e s i t i c p y r o c l a s t i c s . H i s t o r i c a l l y , Tavurvur has e x p l o s i v e l y e j e c t e d
bombs and ash, but produced no l a v a flows; however,
o l d e r p a r t s of t h e cone.
lava flows a r e p r e s e n t i n
Vulcan, a l s o h i s t o r i c a l l y a c t i v e , i s a pumice and
ash cone of c o n s i d e r a b l e volume.
An e s t i m a t e d 0.3 km of m a t e r i a l was e r u p t e d
i n t h e four-day e r u p t i o n of 1937. Dawapia Rock and Matupit I s l a n d , whose t i m e
of a c t i v i t y are unknown, a r e of s i m i l a r composition t o Vulcan.
DEFORMATION
Onland
AND
SEISMICITY
Deformation
Remarkable d e f o r m a t i o n i n t h e Sulphur Creek-Matupit I s l a n d a r e a has been
monitored c l o s e l y s i n c e 1973, p r i n c i p a l l y by o p t i c a l l e v e l l i n g and g r a v i t y
t e c h n i q u e s (McKee, 1982).
Between 1973 and 1982, t h e southern end of Matupit
I s l a n d v a s u p l i f t e d by almost one meter.
F i g u r e 8 shows t h e r e s u l t s of
o p t i c a l l e v e l l i n g surveys c a r r i e d o u t s i n c e 1973 a l o n g a line from a s t a b l e
bench mark n e a r RVO t o M a t u p i t I s l a n d . McKee et al. (1983) g i v e t h e f o l l o w i n g
d e s c r i p t i o n of deformation: t h e l e v e l l i n g d a t a on s t a t i o n s immediately n o r t h
and 1- 2 km e a s t and s o u t h e a s t of H a t u p i t Harbor i n d i c a t e t h a t over t h e same
t i m e i n t e r v a l e l e v a t i o n changes, a l t h o u g h p o s i t i v e , have been probably l e s s
t h a n 10 cm.
However, s i g n i f i c a n t u p l i f t (about 30 cm o r more) i s b e l i e v e d to
have taken p l a c e a t the s o u t h w e s t e r n c o a s t of Tavurvur volcano.
Partial
c o n t o u r s of t h e s e changes i n d i c a t e t h a t t h e f o c u s of u p l i f t probably l i e s
immediately south of t h e e n t r a n c e t o H a t u p i t
Harbor.
R e s u l t s from d r y
t i l t m e t r y around Matupit Harbor since e a r l y 1981 and measurements from a
s p i r i t l e v e l t i l t m e t e r a t TAV seismometer s i t e (Fig. 8) s i n c e e a r l y 1972
G4: rab.gtm
1 / 5 / 8 4 (2)
1 / 1 8 / 8 5 (3)
12
confirm t h e e x i s t e n c e of an u p l i f t i n g source i n t h i s area.
Dry tilt d a t a
from
a s t a t i o n on t h e s o u t h e r n end of Matupit Inland i n d i c a t e s a c u r r e n t r a t e o f
u p l i f t (July, 1983) o f 4-8 m i c r o r a d i a n s p e r month ( t o s o u t h e a s t ) .
Changes in Bathymetry, 1962
-
1982
Published b a t h y m e t r i c c h a r t s of Rabaul Harbor (US82192, and AUS 680)
p r o v i d e i n s u f f i c i e n t d e t a i l f o r u s e f u l comparison w i t h t r a v e r s e s made by t h e
S.P. Lee
collector
i n 1982.
sheet
However, a d e t a i l e d Royal A u s t r a l i a n Navy (R.A.N.)
(0452/10, 19/10/0) with mainly p a r a l l e l ,
north- south t r a v e r s e s
a t about 20 m i n t e r v a l s covering Matupit Harbor (from which p a r t of c h a r t AUS
680 was p r e p a r e d ) is p u b l i c l y a v a i l a b l e .
This d e t a i l e d coverage, surveyed i n
1962, e x t e n d s t o almost 1 km south of t h e e n t r a n c e t o H a t u p i t Harbor.
Two l i n e s of t h e S.P. Lee’s t r a c k , e n t e r i n g and l e a v i n g H a t u p i t Barbor
(Lines R-6 & R-7), were s e l e c t e d f o r comparison w i t h t h e d e t a i l e d 1962 d a t a of
t h i s area.
Before comparison could be made, a number of n a v i g a t i o n a l and
t i d a l c o r r e c t i o n s t o both s e t s of d a t a were r e q u i r e d t o b r i n g them t o t h e same
datum l e v e l . F i g u r e s 9 and 10 show t h e comparison of 1962 and 1982 bathymetry
a f t e r c o r r e c t i o n s were a p p l i e d . Although t h e s e d a t a show t h e s e a f l o o r t o
g e n e r a l l y be between 3 t o 5 m s h a l l o w e r , d i s c r e p a n c i e s noted below s u g g e s t
t h a t more o f f s h o r e d a t a is needed t o p r o p e r l y e v a l u a t e t h e long term s e a f l o o r
changes.
Comparison of d e p t h s within t h e f l a t f l o o r e d c e n t r a l p a r t of H a t u p i t
Harbor shows a d i f f e r e n c e of about 0.5 m.
This amount of u p l i f t i s i n good
agreement with onshore measurements nearby. This t e n d s t o confirm t h e v a l i d i t y
of t h e c o r r e c t i o n s a p p l i e d . The comparison shows t h a t o u t s i d e o f M a t u p i t
Barbor, t h e r e is a p r o g r e s s i v e d i v e r g e n c e southwards of t h e 1982 and 1962
profiles.
S i g n i f i c a n t u p l i f t of t h e s e a - f l o o r s o u t h of Matupit Harbor is t h u s
i n d i c a t e d between t h e s e times.
G4: rab.gtm
1 / 5 / 8 4 (2)
The g r e a t e s t shown on t h e p r o f i l e s compared i s
1 / 1 8 / 8 5 (3)
13
5-8 m.
For t h e 20 y e a r period, t h i s would t r a n s l a t e i n t o a r a t e of u p l i f t of
25-40 cm p e r y e a r , c o n s i d e r a b l y higher than t h a t recorded from t h e onshore
data.
S e v e r a l o t h e r d i s c r e p a n c i e s a l s o a r e e v i d e n t , however.
The match of 1962
and 1982 d a t a over t h e n o r t h e r n s i d e of t h e sill a t t h e e n t r a n c e t o Matupit
Harbor i s poor.
T h i s may be due t o t h e s t e e p n e s s of t h i s s i d e of t h e s i l l ,
from which diffractive e f f e c t s on sound waves could be expected.
of t h e d a t a has been c a r r i e d out.
No m i g r a t i o n
P a i r match of 1962 and 1982 bathymetry i s
found a t t h e peak of t h e s i l l , but i n g e n e r a l , t h e 1982 p r o f i l e s show t h e s i l l
to be deeper than i t was i n 1962.
This r e s u l t i s anomalous in t h e l i g h t of
evidence of u p l i f t d u r i n g t h e t i m e between t h e two surveys.
Closely spaced
f i x e s n e a r t h e sill i n t h e 1982 survey have a depth d i f f e r e n c e of about 1 m.
Corresponding p o i n t s i n t h e 1962 survey show a d i f f e r e n c e of about 5 m.
This
may r e f l e c t u n c e r t a i n t y in p o s i t i o n i n g r a t h e r than a c t u a l d i f f e r e n c e s i n
soundings.
However, d e p o s i t i o n and e r o s i o n could have had some e f f e c t o v e r
t h e 20 y e a r p e r i o d , p a r t i c u l a r l y i n t h e shallow s i l l a r e a and could account
for some of t h e changes noted.
The r e s u l t s of t i l t m e t r y i n d i c a t e t h a t t h e c e n t e r of u p l i f t i s about 1 km
south- southwest of Matupit Island.
Determination o f p a r a m e t e r s of t h e s o u r c e
of d e f o r m a t i o n , f o l l o w i n g t h e methods of Mogi (1958) and Eaton (1962) f o r a
p o i n t s o u r c e model, y i e l d a f o c a l depth of about 2 km and a volume of about
0.04
km.
C a l c u l a t i o n s i n d i c a t e t h a t t h e maximum u p l i f t i s about 2 m.
v a l u e i s s i g n i f i c a n t l y l e s s than t h e changes i n bathymetry.
This
This discrepency
could r e s u l t from slumping of m a t e r i a l on t h e southern f l a n k of Matupit I s l a n d
( s e e below) which would add a component of apparent u p l i f t .
Seismicity
Seismic s u r v e i l l a n c e began i n Rabaul i n 1940, but v a s improved i n 1967 by
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t h e e s t a b l i s h m e n t of a network of s e i s m i c s t a t i o n s around t h e n o r t h e r n p a r t of
Blanche Bay (Simpson Harbor). I n t h e e a r l y 1970's.
t h e s e i s m i c network w a s
extended southward t o cover t h e whole of Blanche Bay and now c o n s i s t s of n i n e
s t a t i o n s (Fig. 3). The improved network shows t h a t , a t l e a s t s i n c e 1970, t h e
s e i s m i c i t y o f Rabaul caldera has c o n s i s t e d e s s e n t i a l l y of shallow, shortp e r i o d , v o l c a n o - t e c t o n i c earthquakes o r i g i n a t i n g from d e p t h s o f 10 kms and
l e s s . S i n c e 1977, and p o s s i b l y earlier, earthquake f o c i have been
predominantly i n t h e depth range of 0-2 km (McKee, et al., 1983).
Between 1967 and 1971 s e i s m i c i t y of t h e c a l d e r a was q u i t e low and s t a b l e
and t h e r a t e of occurrence of l o c a l earthquakes v a r i e d from about 20 t o 100
e v e n t s p e r month (McKee, e t al., 1983).
Since l a t e 1971, d i s t i n c t l y h i g h e r
r a t e s of earthquake occurrence have been noted (Fig. 11).
The h i g h e s t monthly
e a r t h q u a k e t o t a l r e g i s t e r e d up t o e a r l y 1983 v a s 1,170, recorded i n J a n u a r y
1982.
The s t r o n g e s t c a l d e r a earthquakes recorded t o e a r l y 1983 were magnitude
(ML) 5.2 and 5.1 e v e n t s i n 1980 and 1982, r e s p e c t i v e l y
(McKee, e t al.,
1983).
The s e i s m i c a l l y a c t i v e a r e a i n Rabaul i s seen t o c o n s i s t e s s e n t i a l l y of
two inward- facing a r c u a t e zones forming an e l l i p t i c a l l y a n n u l a r p a t t e r n w i t h i n
t h e c a l d e r a (Fig. 3).
The shape
of t h e main zones could be l i n k e d t o t h e
caldera- modifying e r u p t i o n about 1400 y e a r s B.P.; t h a t i s , t h e s e i s m i c i t y may
d e f i n e t h e f a u l t s a l o n g which deepening or widening of t h e f o r m e r c a l d e r a took
place.
I n a d d i t i o n , t h e r e a p p e a r s t o be an i n t i m a t e r e l a t i o n s h i p between t h e
most r e c e n t l y a c t i v e volcanoes i n t h e c a l d e r a and some of t h e c o n c e n t r a t i o n s
of s e i s m i c i t y i n d i c a t i n g t h a t c o n d u i t s for t h e i n t r a - c a l d e r a v o l c a n o e s
developed along t h e c a l d e r a f a u l t s ; t h i s is shown on F i g u r e 3 with t h e
volcanoes l y i n g within t h e e l l i p t i c a l p a t t e r n of r e c e n t s e i s m i c i t y .
Apart
from t h e s e r e l a t i o n s h i p s t h e r e appears t o be a s p a t i a l , and perhaps temporal,
connection between t h e s e i s m i c i t y and t h e u p l i f t a t M a t u p i t I s l a n d . Indeed,
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t h e c e n t e r of u p l i f t seems t o be l o c a t e d n e a r t h e c e n t r o i d of s e i s m i c a c t i v i t y
i n Blanche Bay. about 1 km south-southeast o f M a t u p i t I s l a n d .
Volcanic a c t i v i t y is u s u a l l y presaged by i n c r e a s e d s h a l l o w s e i s m i c i t y ,
commonly i n t h e d e p t h r a n g e 20 km t o n e a r surface.
This s e i s m i c i t y o f t e n
c l i m b s t o s h a l l o w e r d e p t h s a s magmatic m a t e r i a l r i s e s towards t h e s u r f a c e ,
t h u s s t i m u l a t i n g harmonic tremors. The r e s u l t s of geophysical s u r v e i l l a n c e i n
Rabaul Caldera s i n c e 1971, p a r t i c u l a r l y t h e r e s u l t s of s e i s m i c and d e f o r m a t i o n
s t u d i e s , have been forming an i n t r i g u i n g s c e n a r i o which can be i n t e r p r e t e d a s
t h e p r e l u d e t o an impending e r u p t i o n .
SEISMIC-REFLECTION
STRATIGRAPHY
The h i g h - r e s o l u t i o n s e i s m i c r e f l e c t i o n p r o f i l e s c o l l e c t e d w i t h i n Rabaul
Caldera a r e of good q u a l i t y and i n d i c a t e a c o n s i d e r a b l e t h i c k n e s s of welll a y e r e d sediments and/or sedimentary rocks beneath t h e h a r b o r s and bays.
A
maximum p e n e t r a t i o n of 200 m i n t h e c e n t r a l p a r t of t h e c a l d e r a was o b t a i n e d ;
we a r e a b l e t o r e s o l v e f e a t u r e s a s s m a l l as 1 m i n t h e s e p r o f i l e s . G e n e r a l l y ,
t h e s e i s m i c r e f l e c t i o n p r o f i l e s show r h y t h m i c a l l y bedded, w e l l - l a y e r e d
reflectors.
F i v e major a c o u s t i c u n i t s a r e i d e n t i f i e d i n t h e s e i s m i c r e f l e c t i o n
profiles:
(1) a c o u s t i c basement, (2) an i r r e g u l a r l y - l a y e r e d sequence, (3) a
c r o s s - l a y e r e d sequence,
(4) a c o n t o r t e d sequence, and (5) a w e l l - l a y e r e d
sequence. Two caldera- wide u n c o n f o r m i t i e s are i d e n t i f i e d :
an a n g u l a r
unconformity s e p a r a t i n g a c o u s t i c basement from t h e o v e r l y i n g s e d i m e n t a r y u n i t s
and
another
unconformity, l o c a l l y a d i s c o n f o r m i t y , which s e p a r a t e s t h e well-
l a y e r e d sequence from t h e u n d e r l y i n g i r r e g u l a r or c o n t o r t e d u n i t s .
Acoustic Basement
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Acoustic basement i s normally opaque t o a c o u s t i c waves o r i s
c h a r a c t e r i z e d by t h e l a c k o f c o r r e l a t a b l e r e f l e c t o r s ; it is not i d e n t i f i e d
everywhere within t h e survey a r e a as t h e s e i s m i c s y s t e m used i n t h e survey was
not s u f f i c i e n t l y powerful t o reach deeply buried basement. Generally, a c o u s t i c
basement i n t h e Rabaul Caldera c o n s i s t s of v o l c a n i c rocks: b a s a l t i c and
a n d e s i t i c lava f l o w s ,
intrusives,
breccias,
agglomerates of t h e Pre-Caldera, Karavia
t h i c k s c o r i a and w e l l - s o l i d i f i e d
Bay, Caldera and Post- Caldera u n i t s
d e s c r i b e d p r e v i o u s l y from onshore exposures.
The a n d e s i t i c l a v a f l o w s of t h e
Karavia Bay u n i t almost c e r t a i n l y a r e t h e a c o u s t i c basement observed near t h e
end of l i n e R-2 and along l i n e R-3 (Figs. 12 & 13), n e a r t h e w e s t e r n and
s o u t h w e s t e r n c a l d e r a wall. Acoustic basement observed on t h e n o r t h e r n p a r t of
l i n e R-2 and along l i n e R-8 (Figs. 12 & 14) probably a r e composed of a n d e s i t e ,
dacite,
and minor b a s a l t i c rocks of t h e Post- Caldera u n i t . Post- Caldera r o c k s
a s s o c i a t e d with Tavurvur and Palangiagia volcanoes a r e a l s o found as a c o u s t i c
basement rimming M a t u p i t Harbor. On one l i n e , an a c o u s t i c a l basement
p r o j e c t i o n above t h e c a l d e r a f l o o r j u s t n o r t h of Vulcan volcano may r e p r e s e n t
a s e p a r a t e v o l c a n i c cone or formed along a north- south t r e n d i n g r i f t (Fig.
14).
It is d i f f i c u l t t o determine t h e l i t h o l o g y and age of t h e s h o a l s that a r e
t h e remnant of t h e e a s t e r n wall of t h e c a l d e r a , b u t it is s p e c u l a t e d t h a t
a c o u s t i c basement i n t h i s a r e a i s composed of rocks of t h e Pre-Caldera and
Caldera units.
Acoustic basement crops out along t h e seaward opening t o Blanche Bay,
along t h e southern and v e s t e r n margin of Karavia Bay, i n n o r t h e r n Matupit
Harbor, and offshore from Vulcan volcano.
Everywhere e l s e i n Rabaul Caldera
a c o u s t i c basement i s unconformably covered w i t h sedimentary d e p o s i t s .
Irregularly-layered
Unit
This u n i t is c h a r a c t e r i z e d by i r r e g u l a r , d i s c o n t i n u o u s r e f l e c t o r s t h a t
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commonly e x h i b i t g e n t l e f o l d i n g (Fig. 12). I n Blanche Bay, K a r a v i a Bay,
Simpson Harbor, and a l o n g t h e upper p a r t s of t h e s h o a l s beneath t h e approach
t o Blanche Bay from the e a s t t h i s u n i t appears t o everywhere lie unconformably
upon t h e a c o u s t i c basement. It ranges i n t h i c k n e s s from 10 to 35 m.
These
d e p o s i t s may r e p r e s e n t w a t e r - l a i d ash deposited during t h e l a t e s t s t a g e of t h e
d e p o s i t i o n of t h e Caldera u n i t .
Folding and f a u l t i n g of t h i s u n i t occurred
a f t e r d e p o s i t i o n from v o l c a n i c and t e c t o n i c a c t i v i t i e s and mass downslope
movement. The age of t h e u n i t i s d i f f i c u l t t o determine, but i s b e l i e v e d t o be
t h e o l d e s t i d e n t i f i a b l e submarine sedimentary u n i t i n Rabaul Caldera. On t h e
o t h e r hand, t h e r e may be o l d e r u n i t s t h a t were n o t d e t e c t e d by our survey.
Cross- layered unit
D i s t i n c t l y , cross- bedded r e f l e c t o r s , well d e f i n e d and continuous, are
i d e n t i f i e d i n s e i s m i c r e f l e c t i o n p r o f i l e s R-6 and R-7 (Figs. 15 & 16) south of
H a t u p i t I s l a n d a s w e l l a s underlying the sill a t t h e mouth of H a t u p i t Barbor.
They appear t o c o n s i s t of w a t e r - l a i d m a t e r i a l d e p o s i t e d i n a complex and
dynamic marine environment. The age of t h i s u n i t i s d i f f i c u l t t o d e t e r m i n e as
i t s b a s a l r e l a t i o n s h i p t o t h e underlying r o c k s i s not seen i n t h e p r o f i l e s .
However, it
is o l d e r than t h e well- layered sedimentary d e p o s i t s found i n
H a t u p i t Harbor and n o r t h e r n Blanche Bay. The t h i c k n e s s of t h i s u n i t v a r i e s
from a t l e a s t 30 m t o a t l e a s t 200 m.
Contorted Unit
Well d e f i n e d t o hashy, d i s c o n t i n u o u s r e f l e c t o r s a s s o c i a t e d w i t h zones of
a c o u s t i c semi- transparency c h a r a c t e r i z e t h i s u n i t .
Associated s u r f a c e
topography i s i r r e g u l a r and hummocky w h i l e i n t e r n a l f o l d i n g and d e f o r m a t i o n of
r e f l e c t o r s i s common.
This u n i t c o n s i s t s of d e p o s i t s of w a t e r - l a i d a s h and
o t h e r unconsolidated m a t e r i a l t h a t have undergone, o r a r e undergoing, mass
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downslope movement.
T h i s is w e l l e x h i b i t e d i n F i g u r e 16 where t h e u n i t is 40 m
t h i c k and a p p e a r s t o represent
a slumped upper p a r t of t h e c r o s s - l a y e r e d u n i t .
The slump has s l i d southwestward o f f t h e south f l a n k of t h e sill of Matupit
Harbor. Age of slumping i s n o t shown although it may be a s s o c i a t e d with t h e
r e c e n t s e i s m i c i t y and u p l i f t .
No sediments cover t h e hummocky topography i n
this region.
Another a r e a where t h i s u n i t is i d e n t i f i e d i s a t t h e s t a r t of l i n e R-2
(Fig.
12)
where f o l d e d and hashy r e f l e c t o r s cover t h e e a s t e r n and s o u t h e r n
f l a n k s of Vulcan volcano. Here sedimentary d e p o s i t s of unknown l i t h o l o g y
appear t o have moved downslope and now o v e r l i e t h e w e l l - l a y e r e d d e p o s i t s i n
Karavia Bay. T h i s sequence i s contemporaneous and i n t e r f i n g e r s with t h e
uppermost r e f l e c t o r s of t h e w e l l - l a y e r e d u n i t .
It is p o s s i b l e t h a t t h i s
sequence r e p r e s e n t s t h e l a t e s t p y r o c l a s t i c d e p o s i t s from Vulcan.
Well- lavered unit
T h i s u n i t i s c h a r a c t e r i z e d by w e l l d e f i n e d , r h y t h m i c a l l y l a y e r e d ,
In
continuous r e f l e c t o r s t h a t a r e f l a t - l y i n g o r g e n t l y d i p p i n g (homoclinal).
most a r e a s where it o c c u r s t h i s u n i t unconformably o v e r l i e s t h e o l d e r
i r r e g u l a r l y l a y e r e d u n i t o r a c o u s t i c basement.
where
it
is 115 m t h i c k (Fig. 12).
It i s t h i c k e s t i n Karavia Bay
I t i s thinner i n Blanche Bay, r a n g i n g from
1 5 m t o 40 m (Fig. 13) and in Simpson Harbor where it ranges from 20 m
t o 80 m i n t h i c k n e s s (Fig. 17).
T h i s u n i t also o b t a i n s a s i z a b l e
t h i c k n e s s in Matupit Harbor where it i s o v e r 105 m t h i c k (Fig. 15).
It i s
t h e youngest submarine s e d i m e n t a r y u n i t found i n Rabaul Caldera and probably
i s composed of w a t e r - l a i d m a t e r i a l from t h e most r e c e n t Post- Caldera v o l c a n i c
erupt ions.
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STRUCTURE
I n t e r p r e t a t i o n of t h e s e i s m i c r e f l e c t i o n p r o f i l e s show t h a t g e n e r a l l y
structure
w i t h i n Rabaul
Caldera
is
simple, except f o r an a r e a i n Blanche Bay
j u s t s o u t h of Matupit I s l a n d and Harbor (Figs. 18 & 19). Here t h e s e a f l o o r i s
upbowed and hummocky, whereas e l s e w h e r e t h e c a l d e r a f l o o r i s e i t h e r flat o r
g e n t l y sloping.
In Simpson Harbor, Karavia Bay, Matupit Harbor and most of
Blanche Bay t h e s e a f l o o r and s u b s u r f a c e sedimentary l a y e r s r e p r e s e n t
d e p o s i t i o n a l environments t h a t a r e r e l a t i v e l y u n d i s t u r b e d from t e c t o n i c
movements.
In two l o c a t i o n s on t h e c a l d e r a f l o o r , s o u t h e r n Simpson Harbor and
c e n t r a l Blanche Bay, t h e sediments are p i e r c e d and o v e r l a i n by v o l c a n i c cones
and around t h e walls surficial slumping have l o c a l l y taken place.
Nowhere.
except w i t h i n t h e a r e a of upbowing, h e r e c a l l e d t h e bulge, i s f a u l t i n g
identified.
The Bulge - Our d a t a i n d i c a t e s a prominent s e a f l o o r bulge i n t h e a r e a
t h a t h a s been shown from onshore l e v e l l i n g t o be t h e p o i n t of m a x i m u m u p l i f t
(McKee, 1982).
The bulge a p p e a r s t o be composed of two d i s t i n c t , en echelon,
dome- like r i d g e s t h a t e x h i b i t a n t i c l i n a l l y f o l d e d s t r a t a (Fig. 18),
which a r e
b e l i e v e d t o t r e n d g e n e r a l l y i n a north- south d i r e c t i o n and a r e s e p a r a t e d by a
series
of
normal, stepped f a u l t s (Fig. 19). Only two t r a c k l i n e s (R-7 and R-8)
of t h e 1982 & survey c r o s s t h e a r e a of t h e bulge, b u t a l a t e r survey by t h e
Lee i n 1984 also c r o s s e d t h e a r e a and preliminary onboard i n t e r p r e t a t i o n of
t h i s d a t a suggest a g e n e r a l north- south t r e n d f o r t h e s t r u c t u r e s .
U n f o r t u n a t e l y , t h e s e l a t e s t c o l l e c t e d data a r e n o t processed and n o t a v a i l a b l e
f o r i n c l u s i o n i n t h i s paper.
On t h e two dome- like f e a t u r e s t h e western one i s t h e l a r g e s t b e i n g
o v e r 0.5 km vide.
It
i s g e n t l y upbowed with maximum f o l d i n g of strata along
its e a s t e r n boundary well d e f i n e d by a f a u l t .
T h i s f e a t u r e a p p e a r s t o extend
b e n e a t h Matupit I s l a n d and may be d i r e c t l y r e s p o n s i b l e f o r t h e i n c r e a s e i n
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e l e v a t i o n t a k i n g p l a c e there.
The second dome- like f e a t u r e l i e s t o t h e e a s t
of t h e f i r s t . i s much s m a l l e r . being only about 200 m vide.
However, t h e
s u r f a c e of t h i s f e a t u r e is v e r y hummocky and may be s e v e r e l y s l i v e r e d by
f a u l t i n g . Both t h e v e s t and e a s t s i d e s a r e f a u l t e d .
S e p a r a t i n g t h e two dome-
l i k e f e a t u r e s a r e t h r e e well d e f i n e d faults and more may b e p r e s e n t .
Both
f e a t u r e s appear t o plunge t o t h e south.
F a u l t s - As mentioned above, t h e dome- like f e a t u r e s a r e bounded by
f a u l t s (Fig. 19). Four f a u l t s have been i d e n t i f i e d i n t h e s e i s m i c r e f l e c t i o n
p r o f i l e s and more may be p r e s e n t (Fig. 18). These f a u l t s a r e c o n c e n t r a t e d i n
t h e a r e a of t h e bulge and trend north- south.
They a r e a l l normal f a u l t s ,
downthrown on t h e e a s t and a l l have s e a f l o o r expression with s c a r p s ranging i n
h e i g h t from 5-20 m.
Because t h e s e f a u l t s d i s p l a c e t h e s e a f l o o r , o f f s e t t i n g
r e c e n t l y d e p o s i t e d sediments, it is b e l i e v e d t h a t t h e s e a r e a c t i v e f a u l t s t h a t
a r e developing from t e n s i o n a s s o c i a t e d with u p l i f t .
It
is even p o s s i b l e t h a t
g a s e s and steams seen a r i s i n g i n Blanche Bay between Vulcan and Tavurvur
volcanoes d u r i n g t h e i r 1937 e r u p t i o n ( F i s h e r , 1959; Johnson, 1981) o c c u r r e d
along one o r more of t h e s e f a u l t s . Because t h e c r o s s i n g s of t h e f a u l t s a r e
l i m i t e d t o one o r two t r a c k l i n e s i t i s d i f f i c u l t t o p r e c i s e l y d e f i n e t h e
o r i e n t a t i o n and f u r t h e r work i n t h e a r e a may show a d i f f e r e n t t r e n d .
Volcanic Cones - Two v o l c a n i c cones a r e shown t o e x i s t on t h e f l o o r of
Rabaul Caldera (Fig. 19). One cone i s l o c a t e d j u s t n o r t h of Vulcan volcano and
may be a s s o c i a t e d w i t h a north- south t r e n d i n g r i f t o r may even be p a r t of a
v o l c a n i c spur extending o u t from t h e volcano. The second cone l i e s i n t h e
c e n t r a l p a r t of Blanche Bay and v a s discovered i n 1972 by t h e HMS Hydra
(Crick, 1973, p. 3). T h i s cone, 70 m high with 1 5 m h i g h c r a t e r w a l l s i n w a t e r
depth o f 90 m, lies a t t h e southern terminus of t h e bulge a l o n g t h e southward
e x t e n s i o n of t h e e a s t e r n most f a u l t mapped i n t h e a r e a (Fig. 19).
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(1973) s u g g e s t s t h a t t h i s cone and an a d j a c e n t r i d g e a r e c o - l i n e a r w i t h t h e
1878 Vulcan I s l a n d c r a t e r and may be a s s o c i a t e d w i t h an e a s t - w e s t t r e n d i n g
However. we s p e c u l a t e that t h i s cone is a s s o c i a t e d with north- south
fault.
t r e n d i n g f a u l t s of t h e p r e s e n t developing bulge system.
DEPOSITIONAL
ENVIRONMENTS
Based on s e i s m i c s t r a t i g r a p h y t h e marine sediments i n Rabaul Caldera can
be d i v i d e d i n t o three d i s t i n c t d e p o s i t i o n a l environments which r e f l e c t t h e
manner i n which t h e sediments were d e p o s i t e d and t h e p r o c e s s e s t h a t a f f e c t e d
them s i n c e d e p o s i t i o n : (1) a dynamic environment, (2) a p a s s i v e or q u i e t w a t e r
environment, and (3) t r a n s i t i o n a l environment. We assume t h a t t h e m a r i n e
s e d i m e n t s i n t h e c a l d e r a a r e p r i m a r i l y a s h e j e c t e d from the v o l c a n o e s w i t h i n
t h e caldera.
Once these sediments reach t h e s e a f l o o r t h e y a r e a f f e c t e d by
s e v e r a l major p r o c e s s e s :
waves and c u r r e n t s , g r a v i t y and t e c t o n i s m . In
a r e a s where t i d a l or storm g e n e r a t e d waves and c u r r e n t s a r e a b s e n t then only
g r a v i t y and t e c t o n i c f o r c e s w i l l a f f e c t t h e d e p o s i t i o n a l c o n f i g u r a t i o n .
Volcanic e r u p t i o n s cause e p i s o d i c high i n f l u x e s of s e d i m e n t s i n t o Rabaul
Caldera (Fig. 20).
These have c o n s i d e r a b l e impact on t h e s e d i m e n t a r y budget
d u r i n g , and f o r a t i m e a f t e r , e r u p t i o n s when t h e e r u p t e d m a t e r i a l i s washed
i n t o t h e sea from t h e l i m i t e d d r a i n a g e a r e a of t h e c a l d e r a .
n e a r l y enclosed
The c a l d e r a is a
m a r i n e b a s i n , and much of t h e v o l c a n i c m a t e r i a l i s r e t a i n e d
During times o f v o l c a n i c quiescence. normal s e d i m e n t a t i o n r a t e s a r e
i n it.
low, and s e a f l o o r sediments a r e e i t h e r u n d i s t u r b e d or eroded and r e d e p o s i t e d
by dynamic bottom p r o c e s s e s .
Dynamic
Environment
The s e i s m i c r e f l e c t i o n p r o f i l e s i n Blanche Bay o f f the mouth of M a t u p i t
Harbor e x h i b i t c r o s s - s t r a t i f i c a t i o n , f a u l t i n g , and submarine slumps i n t h e
s e d i m e n t a r y u n i t s (Pigs. 16 & 18). This s u g g e s t s t h a t s e d i m e n t s d e p o s i t e d i n
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t h i s r e g i o n n o t only were l a i d down i n a dynamic marine environment, b u t a l s o
i n a r e l a t i v e l y a c t i v e t e c t o n i c environment.
The p r e s e n c e of cross- bedding n e a r Matupit I s l a n d i n d i c a t e s t h a t w h i l e
d e p o s i t i o n was t a k i n g p l a c e t h e s e d i m e n t s were s u b j e c t e d t o s t r o n g currents.
T i d a l and storm c u r r e n t s would i n f l u e n c e , d i s t u r b o r r e a r r a n g e any normal or
q u i e t - w a t e r sedimentary s t r u c t u r e s . As t h i s a r e a i s l o c a t e d n e a r t h e breach of
t h e c a l d e r a , open t o t h e sea, it is s u b j e c t t o c o n s i d e r a b l e wave and t i d a l
energy from St. George’s Channel.
The p r e s e n c e of
slump
structures suggest t h a t sediment d e p o s i t i o n i n
t h i s a r e a has been r a p i d , and on s l o p e s of sufficient g r a d i e n t t h a t downslope
mass movement from t h e f o r c e of g r a v i t y has r e s u l t e d . T h i s h a s probably been
encouraged by e x t e r n a l s t i m u l i such a s wave l o a d i n g and earthquakes.
In
a d d i t i o n , c o n t i n u i n g u p l i f t i n Blanche Bay, a s i n d i c a t e d by t h e measured
u p l i f t of Matupit I s l a n d , and
by t h e changes i n bathymetry, a s w e l l a s
i n f e r r e d movement along s h a l l o w f a u l t s t h a t on our p r o f i l e s appear t o c u t
r e c e n t d e p o s i t s , h a s a s s i s t e d t h e slumping.
The a r e a of slumping i s n e a r t h e
f o c u s of t h e s u s p e c t e d u p l i f t i n d i c a t e d by t h e s e i s m i c i t y p a t t e r n and u p l i f t
c o n t o u r s ( F i g s . 3 & 8).
Passive
or
Quiet-Water Environment
Sediments of t h e w e l l - l a y e r e d u n i t i n both Karavia Bay and Matupit Harbor
e x h i b i t c h a r a c t e r i s t i c s of q u i e t - w a t e r d e p o s i t i o n (Figs. 1 2 & 15) and w i t h t h e
e x c e p t i o n of e r o s i o n i n Karavia Bay t h e r e is no evidence i n our s e i s m i c
r e f l e c t i o n p r o f i l e s of d i s t u r b a n c e or displacement of t h e s e sediments.
In
Matupit Harbor t h e f l a t - l y i n g sediments onlap t h e c o n f i n i n g edges of t h e b a s i n
and a r e n e i t h e r deformed n o r t i l t e d .
I n Karavia Bay an e r o s i o n a l channel
(Karavia Channel) running through t h e south c e n t r a l p a r t of t h e bay may be
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actively carrying sediments out of Rabaul Caldera (Figs. 13 & 15).
Simpson Channel i n t h e northern part of Rabaul Caldera i n t h e southern
most part of Simpson Harbor i s morphologically a r e l a t i v e l y narrow (less than
500 m wide) flat- floored, rounded-edged feature that occupies the course of an
older, f i l l e d channel which is eroded i n t o the irregularly- layered unit (Pig.
17).
The present day channel broadens t o over 1 km wide near the southern
limits of Blanche Bay where it appears t o contain very l i t t l e sediment f i l l
and e x h i b i t s steep, erosional s i d e scarps (Fig. 12). The channel (Karavia
Channel) becomes narrower ( l e s s than 0.5 km wide) and V-shaped near it's e x i t
point along the eastern boundary of the caldera.
Based on the morphology of
Simpson and Karavia Channels it appears t h a t sediment transport alternated
with minor amounts of deposition along the present day Simpson Channel, i n
Simpson Harbor.
Unconsolidated sediments are thereby transported by the
bottom currents through the channel from Simpson Harbor t o Blanche Bay where
they are e i t h e r deposited or carried out of the caldera through Karavia
Channel.
Whether the
sediments carried i n t o Blanche Bay are deposited,
transported o r even remobilized, depends upon the current dynamics and
sediment type a t any one moment in time.
Because the channel changes from a
f i l l e d f e a t u r e in the north t o a d i s t i n c t erosional feature i n the south we
suspect that exhumation of the channel fill
is actively underway.
Also,
since
the o l d e r channel i n the northern part of the caldera appears t o cut the
caldera s t r a t i g r a p h i c u n i t , we would date the origin of the channel at younger
than 1400 y e a r s B.C.
Transitional
Environment
The sedimentary deposits t h a t a r e found i n t h i s environment exhibit
structures t h a t show e f f e c t s of both dynamic and passive depositional
processes.
These deposits, composed of interfingering, contorted and well-
layered strata, are primarily located i n Simpson Harbor and extend i n t o
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Blanche Bay (Figs. 14 & 17). Deposition i n t h i s a r e a is a f f e c t e d by t i d a l
c u r r e n t s which, d u r i n g periods of r a p i d s e d i m e n t a t i o n a s s o c i a t e d w i t h v o l c a n i c
e r u p t i o n s , r e s u l t e d i n cross- bedding. Between e r u p t i v e e v e n t s , w e l l - l a y e r e d
beds were d e p o s i t e d . I n a r e a s of s l o p i n g bottom topography, l o c a l i z e d g r a v i t y
s l i d i n g took p l a c e forming t h e packages of c o n t o r t e d and d i s t u r b e d sedimentary
strata.
SUMMARY AND CONCLUSIONS
S e a f l o o r physiography and s u b s u r f a c e geology i n d i c a t e t h a t Rabaul Caldera
has evolved i n a complex way. Four major basins have been d e f i n e d (Fig. 7) in
which t h i c k s e d i m e n t a r y sequences of t e p h r a have been deposited. I n M a t u p i t
Harbor, Karavia Bay and t h e southern p a r t o f Blanche Bay, s e d i m e n t a t i o n
a p p e a r s t o have been of a normal, q u i e t w a t e r type w i t h l o c a l i z e d slumping
n e a r t h e edges. I n Simpson Harbor a more confused sedimentary r e g i m e e x i s t s ,
w i t h a combination of q u i e t - w a t e r d e p o s i t i o n and c u r r e n t - e f f e c t e d bedding. The
n o r t h e r n p a r t of Blanche Bay e x h i b i t s t h e most complex p i c t u r e of
s e d i m e n t a t i o n w i t h c o n t o r t e d and c h a o t i c a l l y bedded sediments.
Sediment t r a n s p o r t w i t h i n Rabaul Caldera appears t o t a k e p l a c e m a i n l y
a l o n g a submarine channel s y s t e m (Simpson and Karavia Channels) t h a t extend
from t h e head o f Simpson Harbor southward through Blanche Bay t o Karavia Bay
where it loops e a s t t o n o r t h e a s t and l e a v e s t h e c a l d e r a through t h e e a s t e r n
breach. T h i s channel system starts i n Simpson Harbor a s an o b s c u r e d e p r e s s i o n
o v e r l y i n g an o l d e r f i l l e d channel (Simpson Channel) and becomes more d i s t i n c t
as it e n t e r s Blanche Bay. It is a w e l l developed, a l m o s t canyon- like f e a t u r e
i n Karavia Bay (Karavia Channel) where
it a p p e a r s t o be a c t i v e l y e r o d i n g i t s
walls. At t i m e s of volcanism t h e headward r e g i o n of t h e canyon i s choked by
ash
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rab.gtm
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t h e channel and c u r r e n t t r a n s p o r t a b l e sediments e n t e r i n g t h e c a l d e r a are
c a r r i e d o u t t o sea along t h i s channel (Fig. 2).
Three sediment d e p o t a r e a s
(Simpson Harbor, Blanche Bay, and M a t u p i t Harbor) a r e p r e s e n t .
Matupit Harbor
is a r e s t r i c t e d b a s i n with o n l y deposition t a k i n g p l a c e , whereas Simpson Harbor
and Blanche Bay a r e connected by Simpson and Karavia Channels, a sediment
c o n d u i t r e d i s t r i b u t i n g s e d i m e n t s from t h e n o r t h e r n p a r t t o t h e s o u t h e r n p a r t
of t h e c a l d e r a .
Geologic s t r u c t u r e of t h e c a l d e r a f l o o r i s g e n e r a l l y s i m p l e w i t h w e l l
d e f i n e d f a u l t s o c c u r r i n g p r i m a r i l y around the p e r i p h e r y of t h e c a l d e r a . An
e x c e p t i o n t o t h i s i s i n t h e n o r t h e r n p a r t of Blanche Bay where a major
f r a c t u r e zone c o n s i s t i n g of a s e r i e s of r e c e n t f a u l t s e x i s t on t h e f l o o r of
t h e bay. This zone t r e n d s N-S and i s marked by folded surficial s e d i m e n t s and
s e v e r a l f a u l t s that c u t t h e sea f l o o r .
It p a s s e s through a broad dome or
bulge of upbowed sediments s o u t h of M a t u p i t I s l a n d .
Nearby t o t h e n o r t h , a t
t h e e n t r a n c e t o H a t u p i t Barbor, a well developed submarine slump a p p e a r s t o be
in t h e process
of s l i d i n g o f f t h e southern f l a n k of t h e sill t h a t s e p a r a t e s
H a t u p i t Harbor from Blanche Bay.
Recent e l e v a t i o n changes noted on s h o r e i n d i c a t e t h a t r a p i d and l a r g e
i n c r e a s e s of e l e v a t i o n a r e t a k i n g p l a c e south of M a t u p i t I s l a n d . The c o n t o u r s
of e l e v a t i o n change determined from l e v e l l i n g surveys onland show 100 cm of
uplift
parallel t o t h e s o u t h e a s t e r n shoreline of Matupit I s l a n d (Fig. 8).
s e a f l o o r b u l g e d i s c o v e r e d d u r i n g t h e Lee i n v e s t i g a t i o n ,
The
l i e s in t h e same area.
The b a t h y m e t r i c changes noted between 1962 and 1982 amount t o a b o u t 58 m about 1 km south of t h e i s l a n d g i v i n g a mean r a t e of u p l i f t of 25-40
cm/yr.
I n comparing t h e l o c a t i o n of t h e bulge w i t h t h e p a t t e r n of s e i s m i c i t y in
Rabaul Caldera (Fig. 21) it is seen t h a t t h e s e i s m i c i t y s u r r o u n d s t h e bulge in
an e l l i p t i c a l shape w i t h t h e long a x i s extending n e a r l y north- south. The
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f o c u s of t h e bulge and t h e f r a c t u r e zone on its n o r t h e r n f l a n k l i e w i t h i n t h e
a
seismic a r e a .
We conclude t h a t t h e s e a f l o o r bulge south of Matupit I s l a n d i s t h e
r e s u l t of emplacement of a magma body a t shallow depth, which has upbowed
s e d i m e n t a r y l a y e r s beneath t h e s e a f l o o r .
The f r a c t u r e zone may be a s s o c i a t e d
with t h e bulge and may have been t h e f e a t u r e along which t h e r e p o r t e d
d i s t u r b a n c e s of t h e sea o r clouds of vapor r i s i n g from t h e bay were seen
between Vulcan and Tavurvur d u r i n g t h e i r 1878 and 1937 e r u p t i o n s .
contorted
surficial
The
s e d i m e n t s i n t h e g e n e r a l a r e a of t h e bulge a r e i n t e r p r e t e d
a s slumps. The slumping may have been s t i m u l a t e d by s e i s m i c e v e n t s a s s o c i a t e d
with t h e development of t h e bulge. However, t h e c r o s s - s t r a t i f i e d s e d i m e n t s i n
t h e area a r e t h e r e s u l t of c u r r e n t e f f e c t s from t i d a l f l o w i n and o u t of t h e
caldera.
We b e l i e v e t h a t t h e p a t t e r n of s e i s m i c i t y d e f i n e s a c t i v i t y along r i n g
f a u l t s of t h e c a l d e r a . T h i s a c t i v i t y probably r e s u l t s from stresses a p p l i e d by
t h e a c c u m u l a t i o n of magma a t shallow depth below t h e apex of t h e s e a f l o o r
bulge. C o n t i n u a t i o n of t h i s a c t i v i t y w i l l almost c e r t a i n l y c u l m i n a t e i n a n
e r u p t i o n , p o s s i b l y w i t h t h e c o n s t r u c t i o n of a new volcano.
ACKNOWLEDGEMENTS
We wish t o thank P e t e r Lowenstein, D i r e c t o r of t h e Rabaul Volcano
Observatory (RVO), f o r h i s encouragement and guidance i n t h i s study.
C o n s t r u c t i v e c o n v e r s a t i o n s with Patrich de S a i n t O u r s of RVO and a c r i t i c a l
review by Donald Hussong of t h e Hawaiian I n s t i t u t e o f Geophysics l e d to
c o n s i d e r a b l e improvement of t h e manuscript.
We a p p r e c i a t e t h e e f f o r t t h a t
Wally Johnson of t h e A u s t r a l i a n Bureau o f Mineral Resources (BMR) went through
t o supply us w i t h photos of t h e Vulcan 1937 e r u p t i o n .
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We are g r a t e f u l t o t h e captain and crew of the S.P. Lee for t h e i r hard
work and cheerful support given during t h e survey.
Special thanks go t o Jack
Vedder of the USGS and N e v i l l e Exon of Australia’s BMR for a s s i s t a n c e i n the
c o l l e c t i o n of the data.
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T.J.
Sprod,
and H.R.
M a j o r E r u p t i o n s o f Rabaul Volcano. In
Walker,
1981. L a t e s t
R.W. J o h n s o n . ed., Cooke-Ravin
Volume of V o l c a n o l o g i c a l Papers: Geological Survey of Papua New Guinea
Memoir,
G4: rab.gtm
10,
181-193.
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FIGURE CAPTIONS
F i g u r e 1. Index map shoving l o c a t i o n of study a r e a .
F i g u r e 2. Bathymetric map o f Rabaul Caldera shoving l o c a t i o n o f Simpson and
Karavia Channels and major onshore geomorphic f e a t u r e s . Onland
c o n t o u r s i n m e t e r s , i s o b a t h s i n fathoms (about 36 m i n t e r v a l s ) .
Bathymetry from A u s t r a l i a n Department of National Development Map
(1970). S e i s m i c d a t a from Denham (1969). P l a t e boundaries and
r e l a t i v e motions of c r u s t a l p l a t e s from Taylor (1979) and Johnson
(1979).
F i g u r e 3. P a t t e r n of s e i s m i c i t y in Rabaul Caldera from February 1977 t o J u n e
1982.
Modified a f t e r McKee (1982).
F i g u r e 4. T r a c k l i n e s o f
t h e S.P. Lee 1982
geophysical
survey i n
Rabaul
Caldera.
F i g u r e 5. T e c t o n i c s t r u c t u r e map of t h e Papua New Guinea region.
contours i n meters.
Bathymetric
Modified a f t e r Heming (1974).
F i g u r e 6. Geological map of Rabaul Caldera. Modified
after
Heming (1974).
F i g u r e 7. Oblique map of Rabaul Caldera showing submarine physiography.
After
Alpha and Greene (1985).
F i g u r e 8. E l e v a t i o n changes between 1973 and 1983 determined from s t a t i o n s of
t h e Rabaul Caldera g r a v i t y network from Bench Mark (BM) 21 to
Matupit Island.
F i l l e d c i r c l e s r e f e r t o g r a v i t y network s t a t i o n s
t h a t a r c a l s o used as l e v e l l i n g s i t e s . Dashed l i n e s a r e c o n t o u r s
e l e v a t i o n changes r e l a t i v e t o BM 21.
Of
Modified a f t e r McKee e t al.
(1983).
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F i g u r e 9. Bathymetric p r o f i l e s c o n s t r u c t e d from t h e Royal A u s t r a l i a n Navy
c o l l e c t o r s h e e t 0425/10, 19/10/0
(dot i n c i r c l e ) developed from a
1962 b a t h y m e t r i c survey of Matupit Harbor and c o n s t r u c t e d from t h e
1982 S.P. Lee Survey ( s o l i d l i n e ) Line R-6 i n t h e same area. Along
t h e o u t e r ( s o u t h e r n ) f l a n k of t h e Matupit Harbor sill t h e r e i s a 35 m d i f f e r e n c e in d e p t h s t h a t may r e l a t e t o t h e p r e s e n t u p l i f t .
F i g u r e 10. Bathymetric p r o f i l e s c o n s t r u c t e d from t h e Royal A u s t r a l i a n Navy
C o l l e c t o r s h e e t 0425/10, 19/10/0 (dot i n c i r c l e ) and developed from
a 1962 hydrographic survey i n t h e Matupit Harbor a r e a compared with
t h e 1982 S.P. Lee Survey ( s o l i d l i n e ) Line R-7 i n t h e same area.
S i m i l a r amounts of u p l i f t t o Lee Line R-6 a r e seen h e r e .
F i g u r e 11. S e i s m i c i t y of Rabaul Caldera showing monthly number of e a r t h q u a k e s
between 1967 and 1982.
Modified a f t e r McKee (1982).
F i g u r e 12. S e i s m i c - r e f l e c t i o n (Uniboom) p r o f i l e taken along a p a r t o f S.P. Lee
L i n e R-2 and shoving t h e well- layered sequence i n Blanche Bay.
V e r t i c a l Exaggeration
5.86x.
F i g u r e 13. Seismic- reflection (Uniboom) p r o f i l e taken along S.P. Lee L i n e R-3
s h o v i n g t h e w e l l - l a y e r e d sequence i n Karavia Bay. V e r t i c a l
Exaggeration
2.8x.
F i g u r e 14. S e i s m i c r e f l e c t i o n (Uniboom) p r o f i l e taken along a p o r t i o n of
S.P. Lee L i n e R-8 shoving t h e v o l c a n i c cone or e x t e n s i o n of a
v o l c a n i c r i d g e n o r t h e a s t of Vulcan i n Simpson Harbor.
Exaggeration
F i g u r e 15. S e i s m i c
Vertical
5.25x.
r e f l e c t i o n (Uniboom) p r o f i l e
along S.P. Lee L i n e R-6
s h o v i n g well l a y e r e d u n i t i n Matupit Harbor, c r o s s - s t r a t i f i e d u n i t ,
and Simpson Channel i n Blanche Bay. V e r t i c a l E x a g g e r a t i o n 4x.
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F i g u r e 16. S e i s m i c r e f l e c t i o n (Uniboom) p r o f i l e along a p a r t of S.P. Lee Line
R-7 shoving c r o s s - s t r a t i f i e d u n i t and slump n e a r s i l l t o Matupit
Harbor.
V e r t i c a l Exaggeration 3.2x.
F i g u r e 17. S e i s m i c r e f l e c t i o n (Uniboom) p r o f i l e along S.P. Lee L i n e R-9 showing
i r r e g u l a r l y layered u n i t and Simpson Channel ( i n b u r i e d channel) i n
Simpson Harbor.
V e r t i c a l Exaggeration 6.4x.
F i g u r e 18. S e i s m i c r e f l e c t i o n (Uniboom) p r o f i l e along a p a r t of S.P. Lee Line
R-8 showing f a u l t i n g and sediment upbowing i n Blanche Bay j u s t
s o u t h of Matupit I s l a n d .
V e r t i c a l Exaggeration 3.46x.
F i g u r e 19. Offshore g e o l o g i c s t r u c t u r e s k e t c h map of t h e c e n t r a l p a r t of
Rabaul Caldera.
Map c o n s t r u c t e d from s e i s m i c r e f l e c t i o n (Uniboom)
p r o f i l e s c o l l e c t e d by t h e S.P. Lee d u r i n g t h e 1982 g e o p h y s i c a l
survey.
F i g u r e 20. Photo of damage c r e a t e d from ash f a l l d u r i n g t h e 1937 e r u p t i o n of
V u l c a n v o l c a n o . (a) view o f Vulcan f r o m t h e north,
(b) t h e S.S.
Durour i n Karavia Bay, and (c) a p a r t i a l l y b u r i e d automobile.
Photos c o u r t e s y of t h e A u s t r a l i a n Bureau of Mineral Resources.
F i g u r e 21. O f f s h o r e g e o l o g i c s t r u c t u r e superimposed on t h e s e i s m i c i t y p a t t e r n
t o show t h a t t h e g r e a t e s t amount of sediment d i s r u p t i o n and
seafloor
distrurbance
is
t a k i n g p l a c e within t h e
eleptical pattern,
i n t h e a seismic region.
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