Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef

Sedimentology (1994) 41, 329-351

Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef

MICHAEL K. GAGAN,* DAVID P. JOHNSON and GABRIEL M. CROWLEYt

Department of Geology, James Cook University, Townsville, Qld. 4811, Australia f Centre f o r Palynology and Palaeoecology, Department of Geography and Environmental Science,

Monash University, Clayton, Vie. 3168, Australia

ABSTRACT

Lithofacies analysis, pollen assemblages and radiocarbon age dates of 20 stratigraphic drill holes are used to develop an evolutionary history for late Quaternary sedimentation in two coastal embayments landward of the central Great Barrier Reef. Different physiographic settings of the embayments result in two contrasting styles of sedimentary sequence: (a) an exposed, moderate energy, beach barrier-lagoon system (Wyvuri Embayment) and (b) a protected, low energy, muddy inlet fill sequence (Mutchero Inlet). Despite sharp contrast in sequence style, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (1) a last interglacial highstand (+2 m; c. 125 000 yr BP) beach barrier (Wyvuri); (2) an early to mid-Holocene (8000-6100 yr BP) transgressive beach barrier- lagoon (Wyvuri) and estuarine infill (Mutchero); and (3) mid-Holocene to present highstand beach barrier (Wyvuri) and estuarine (Mutchero) progradation. Preservation of such cycles in the stratigraphic record would produce a series of vertically stacked and offset linear barrier sands surrounded by lagoonal mud and fine grained shoreface sediment juxtaposed to muddy, estuarine infills.

Sea level elevations are well recorded by the upward transition from Rhizaghoru-dominated intertidal mangrove mud to freshwater swamp (clearly identified by pollen analysis) and by the basal contacts of beach barrier sediments which sharply overlie the upper shoreface. Transgressive sedimentation is interrupted in both embayments by a constructional beach barrier (Wyvuri) and abbreviated progradation (Mutchero) corresponding to a - 5 m pause in relative sea level rise at c. 6800 yr BP. Sea level control of fine scale coastal sedimentation patterns is beginning to be widely recognized and provides an accurate analogue for stacked ancient sequences.

INTRODUCTION

Late Quaternary coastal sequences have been studied world-wide to develop models of late Quaternary sea level change, and also as modern analogues for ancient beach barrier - lagoon systems that are common in the stratigraphic record and are prime petroleum traps (Dickinson et al., 1972; McCubbin, 1982; Reinson, 1984). More recently, the well established sea level histories based on coastal sequences have become import-

*Present address: Research School of Earth Sciences, The Australian National University Canberra, ACT 0200, Australia.

ant in the study and prediction of global climatic, oceanographic and tectonic fluctuations (Pirazzoli, 1988; Shennan, 1989). The purpose of this paper is to present: (1) a detailed late Quaternary evolutionary history for coastal sedimentation in two contrasting embayments (Wyvuri Embayment and Mutchero Inlet) located landward of the central Great Barrier Reef, (2) new stratigraphic evidence for local downwarping of this coastal plain since the last interglacial, (c. 125 000 yr BP), and (3) evidence for a marked - 5 m stillstand near the end of the post-glacial marine transgression (c. 6800 yr BP) in northern Australia.

329

330 M. K. Gagan et al.

REGIONAL SETTING

Physiography

Palaeozoic granitic and metamorphic rocks, with tracts of Tertiary basalt, form the hinterland of the coastal plain between Innisfail and Cairns (Fig. la). Near the coast, Quaternary fluvial and paralic deposits are restricted to narrow corridors flanked by bedrock. Two river catchments supply terrestrial sediment to the coast within the study area. The Russell and Mulgrave Rivers drain a combined catchment (900 km2) containing granitic rocks with up to 1622m relief, and converge near the coast to form Mutchero Inlet. The North and South Johnstone Rivers drain a larger catchment (1 500 km’) of lower elevation, composed primarily of Tertiary basalt, and converge at the coast 40 km south of Mutchero Inlet. The sediment yield of these rivers has not been studied. For the Burdekin River south of Townsville, however, Belperio (1979a) calculated the sediment load to be 15% sand and 85% mud. Mud may form an even greater percentage in the RusselUMulgrave and Johnstone Rivers because of the extremely wet climate and dominance of chemical weathering in the drainage basin. Long term coastal sediment transport is northward, so it is likely that the Johnstone Rivers are the ultimate source of sediment for Wyvuri Embayment (Bird, 1971).

Climate and vegetation

The Innisfail-Cairns coastal plain ranks amongst the wettest in Australia (Fig. lb) with annual rainfall averaging 2500mm over the study area, and maximum recordings of over 7000 mm at the summit of Mt Bellenden Ker (Bone11 & Gilmour, 1980). Over 50% of the annual rainfall is monsoonal and occurs from January to March. Moderate orographic rainfall occurs during the rest of the year and is strongly controlled by elevation and cooling of moisture-laden south-east tradewinds rising over the ranges. The persistent rainfall produces low annual runoff variation for the Russell and Mulgrave Rivers (Pringle, 1991).

Diverse rainforest communities cover the coastal ranges with open eucalypt forest on steeper slopes and Melaleuca swamp forest on poorly drained lowland sites (Tracey, 1982). Mangrove communities line intertidal banks along rivers and colonize mudflats in protected coastal embayments. Extensive areas of the

coastal plain have been cleared for sugar cane cultivation.

Oceanography Tides along the Innisfail-Cairns coastline are pre- dominantly semidiurnal and have a maximum range at springs of 3.3m (Easton, 1970; Pickard, 1977). The coast is protected from open ocean swell by the Great Barrier Reef which lies a minimum of 35 km offshore (Fig. la). As shown by Waverider buoy data (Beach Protection Authority, 1979), from April to November south-east tradewinds blow across the 35 km wide lagoon to produce short period (4-6 s) seas with a significant wave height of 0.5-1.0m. Tradewind-driven seas rarely exceed 2.5 m but wave heights of 4.7 m were recorded during the passage of Cyclone Peter in 1979. Between cyclones, seas from December to March are relatively slight and vary in direction due to the reduced strength of the south- east tradewinds.

METHODS

Drilling

An exploration drill rig (Jacro 350 model) with 13 cm diameter augers was used to collect subsurface samples from 15 sites. The shallow water table (‘5 m) can cause caving and lead to inaccurate sampling of the stratigraphy if material is simply spun up the auger flights. Therefore, the auger-string was carefully corkscrewed into the sediment and the augers were winched from the hole in 3 m sections. Depths to stratigraphic contacts could then be accu- rately logged in the field and representative sediment samples collected at 25-50 cm intervals. Subsequent 3 m sections were drilled by lowering the augers to the bottom of the hole, clearing the hole of any caved sediment, and repeating the corkscrew style drilling. The error in determining the position of stratigraphic contacts using this technique is less than f 15 cm.

The vibracoring equipment and technique for col- lecting continuous 7.6 cm diameter cores from five sites in Wyvuri Embayment has been described by Lankesky et al. (1979). The method is very effective where the sediment is waterlogged and has a high mud content. Core compaction during drilling, calculated by comparing the penetration depth to the length of core recovered, ranged from 27 to 47%. To construct stratigraphic sections, therefore, each sedimentological unit was ‘stretched’ by an amount

Sea level control of stacked coastal sequences 33 1

-- 3 m - - lsohyet (mm) Modern coastal sediment wedge

Late Quaternary marine embayment - f i l l sediment

Coastal plain alluvium

Catchment boundary

40-- lsobath (m) _ _ Reef and soil

0 10 20 . . Bedrock u kilometres

Fig. 1. (A) Physiographical map of the Innisfail-Cairns coastal plain. The rectangular boxes show the location'of the study areas (Wyvuri Embayment and Mutchero Inlet) in relation to the major drainage basins, coastal plain alluvium and late Quaternary marine sediments. (B) Distribution of mean annual rainfall in the Johnstone River and RusselVMulgrave River drainage basins.

equal to the percentage compaction in a given core. Complete drill logs of the auger holes and photo- graphs of the vibracores forming the database for this paper are contained in Gagan (1990).

Levelling

A standard rise and fall levelling technique was used to relate the elevation of the drill sites to permanent benchmarks installed by the Queensland Department of Mapping and Surveys. Closure errors for the

surveys were less than 2 cm and the overall accuracy of the drill hole elevations relative to Australian Height Datum (AHD; c. mean sea level) is f 5 cm. Hence, by combining the stratigraphic sampling and levelling errors, the maximum uncertainty in the position of the stratigraphic contacts is f 20 cm.

Radiocarbon dating

Eighteen samples have been radiocarbon age dated (Table 1). and are reported as conventional ages,


Table 1. Radiocarbon ages and stable carbon isotope values for Wyvuri Embayment and Mutchero Inlet provided by the Radiocarbon Dating Laboratory, Waikato University.

Depth in Drill hole drill hole Type of 6I3C Conventional

Sample number (m) material (0%) age (yr BPI

Wyvuri Embayment WK1055 JW2 WK1056 JW3 WK1053 JW6 WK799 JW7 WK800 JW7 WK802 JW7 WK803 JW9 WK804 JW9 WK805 JW9 WK806 v w 2 WK807 v w 4 WK1054 v w 5

Mutchero Inlet WK1030 DD 1 WK1028 DDl WK1029 DD1 WK1369 DM 1 WK1370 DMl WK1371 DM1

9.50 13.50 3.60 5.00 6.50

14.25 9.20

12.00 14.75

1.20 1.60 1.25

1.50 4.50 6.60 0.45 4-50 4.65

Coarse organics Whole shells Organic mud

Wood detritus Wood detritus

Coarse organics Whole shells

Shell hash Coarse organics Wood detritus Wood detritus Wood detritus

Wood detritus Coarse organics Coarse organics

Peat Organic mud Organic mud

- 26.9 - 0.7 - 24.0 - 27.3 - 28.2 - 26.2 - 2.9 +0.1 - 27.4 - 27.7 - 27.3 - 26.9

- 28.0 - 28.3 - 28.1 - 29.8 - 31.5 - 32.8

7160 f 70 5050 f 150*

13200 f 260 6300 f 80 6840 f 80 7760 f 80 3100 f 130* 4890 f 130* 7010 f 90 5490 f 60 5950 f 70 6270 f 70

6100 f 140 6600 f 100 7050 f 130 ~

2649 f 70 7140f 120 6700 f 140

*Environment corrected age. Add 450 years to calculate conventional age.

corrected for isotopic fractionation (via S13C) relative to the PBD standard, following the internationally accepted radiocarbon age reporting practice (Stuiver & Polach, 1977). Conventional ages, however, are only valid for those samples that incorporate carbon directly from the atmosphere and a correction must be applied to marine carbonate ages because ocean water is depleted in radiocarbon relative to the atmosphere. The correction adopted in Australia, and the one used for the three marine shell samples in this study, is to subtract 450h35yr from the conventional age (Gillespie & Polach, 1979) to obtain an environment corrected age. To determine the en-vironment corrected age error, the conventional age error is combined with the error ( f 35 yr) for the Gillespie and Polach (1979) seawater correction.

Pollen analysis

One core from Deeral (DD1) and one from Mutchero Inlet (DM1) were collected for pollen analysis using a hand-operated D-section sampler. Pollen was also extracted from vibracore VW4 located in Wyvuri Swamp (Fig. 2). Samples of 1 cm3 were extracted at

intervals through the cores and prepared using standard procedures (Faegri & Iversen, 1975). Pollen grains were identified using light microscopy at x 600 magnification and, where possible, a minimum

of 100 pollen grains were counted. Because samples in the upper part of VW4 were dominated by freshwater wetland taxa, the counts were extended to include a minimum of 50 pollen grains from other species.

Grain size analysis

Eighty-one samples from auger holes JW2-9 in Wyvuri Embayment were divided into gravel (>2.00 mm), sand (0.063-2.00 mm), silt (0,004 0.063 mm) and clay (<0.004 mm) fractions using a combined sieve and pipette technique described by Coventry & Fett (1979). The analytical uncertainty of each size fraction is less than =k 5 wt%. An auto- mated 24 cm diameter settling tube, described by Bryant ef a!. (1 987), was used to determine grain size statistics (Folk & Ward, 1957) for 99 sand samples. The grain size data presented graphically in this paper are tabulated in Gagan (1990).

333 Sea level control of stacked coastal sequences

LAST INTERGLACIAL HOLOCENE Auger drill hole site Freshwater swamp @ Beach barrier

rn Vibracore site ' QUATERNARY Mangroves

Back barrier . . Alluvium A D-section core site

c--l Cross section line PALAEOZOIC

.:.:,.: Beach barrier ridges Granite/metamorphics

LATE QUATERNARY COASTAL SEQUENCES

Models for late Quaternary coastal sedimentation have been developed for a range of physiographic,

oceanographic and climatic settings. This study fills an important gap in our understanding of coastal marine sedimentation in bedrock embayed, low-moderate wave energy, wet-tropical coastlines. For example, the stratigraphic style of the two

bJ

w

P

Tabl

e 2.

Sum

mar

y de

scrip

tion

of li

thof

acie

s co

mpr

isin

g th

e W

yvur

i Em

baym

ent

sedi

men

tary

seq

uenc

e.

Max

imum

A

ppro

xim

ate

Stra

tigra

phic

th

ickn

ess

age

rela

tions

hip

(4

(Y

r BP

I

Dia

gnos

tic

feat

ures

St

ratig

raph

ic

Li th

ofac

ies

Envi

ronm

ent

sequ

ence

de

scrip

tion

inte

rpre

tatio

n

Hol

ocen

e hi

ghst

and

pack

age

(Lag

oon

infil

ling)

Dar

k br

own

woo

d,

root

lets

, and

de

cayi

ng v

ascu

lar

plan

ts

Dar

k br

owni

sh g

rey

mud

with

pre

serv

ed

root

lets

and

coa

rse

orga

nics

Fres

hwat

er s

wam

p pe

at

Abu

ndan

t vas

cula

r pl

ants

V

ery

shar

p lo

wer

11

.5

<550

0 co

ntac

t

Dis

tingu

ishe

d fr

om

unde

rlyin

g tra

nsgr

essi

ve

man

grov

e m

ud b

y co

arse

org

anic

s an

d pr

eser

ved

root

s V

ery

wel

l sor

ted;

fin

e gr

aine

d

Dis

tingu

ishe

d fr

om

beac

h an

d ae

olia

n sa

nd b

y be

ing

wel

l so

rted

and

med

ium

gr

aine

d C

oars

e gr

ain

size

grav

el);

coar

seni

ng

dow

nwar

d tre

nd

Thin

ly b

edde

d an

d la

min

ated

poo

rly

sorte

d ov

eral

l

(up

to 1

5 wt%

Inte

rtida

l m

angr

ove

mud

Lo

wer

con

tact

at

low

wat

er d

atum

2

<630

0 (S

tills

tand

pa

ckag

e: 6

800-

7100

)

1.5 3

< 55

00

550G

6300

Ligh

t br

own,

ver

y w

ell s

orte

d, f

ine

sand

Li

ght b

row

n, w

ell

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d, m

ediu

m

quar

tz s

and

Bac

k-ba

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r ae

olia

n sa

nd

Bac

k-ba

rrie

r w

asho

ver

sand

Shar

p lo

wer

con

tact

% h Sh

arp

low

er c

onta

ct

6 3.5

(Bea

ch b

arrie

r an

d sh

oref

ace

prog

rada

tion)

Ligh

t br

own,

coa

rse

quar

tz s

and

that

is

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ser

and

less

so

rted

dow

nwar

d G

reen

ish

grey

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icac

eous

, thi

nly

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ed a

nd

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inat

ed m

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sa

nd

Dar

k gr

ey s

andy

m

ud t

hat f

ines

do

wnw

ard;

she

lls

com

pris

e 20

-30%

of

th

e se

dim

ent

Bea

ch b

arrie

r sa

nd

Ver

y sh

arp

low

er

cont

act

Upp

er s

hore

face

m

uddy

san

d G

rade

d lo

wer

co

ntac

t <6

300

Low

er s

hore

face

sa

ndy

mud

C

omm

only

has

ar

ticul

ated

Ana

dura

sh

ells

; low

cla

y/si

lt ra

tio (

1:2)

Shar

p or

gra

ded

low

er c

onta

ct

<630

0 6 5

6300

-780

0 H

oloc

ene

trans

gres

sive

pa

ckag

e (la

goon

se

dim

enta

tion)

Dar

k br

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sh t

o gr

eeni

sh g

rey,

cr

udel

y be

dded

mud

w

ith a

bund

ant f

ine

orga

nics

Inte

rtida

l man

grov

e m

ud

Dis

tingu

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d fr

om

over

lyin

g pr

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datio

nal

man

grov

e m

ud b

y fin

er o

rgan

ics;

cru

de

bedd

ing

' Sh

arp

disc

onfo

rmity

m

arks

low

er c

onta

ct

Tabl

e 2.

Con

tinue

d

Stra

tigra

phic

se

quen

ce

(Rew

orke

d be

ach

barr

ier)

Lat

e Pl

eist

ocen

e-H

oloc

ene

back

-bar

rier

infil

ling

Last

inte

rgla

cial

hi

ghst

and

pack

age

(inne

r be

ach

barr

ier)

Late

Ple

isto

cene

su

rfac

e

Max

imum

A

ppro

xim

ate

Lith

ofac

ies

Envi

ronm

ent

Dia

gnos

tic

Stra

tigra

phic

th

ickn

ess

age

desc

riptio

n in

terp

reta

tion

feat

ures

re

latio

nshi

p (

4

(Yr

BP)

Gre

enis

h gr

ey,

mod

erat

ely

wel

l so

rted,

fine

mud

dy

sand

tha

t coa

rsen

s do

wnw

ard

Dar

k br

owni

sh g

rey

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ey m

ud w

ith

abun

dant

fine

or

gani

cs

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ly s

orte

d,

clay

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rkos

ic

grav

el a

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and

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e qu

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dize

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terb

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d w

ell

sorte

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rate

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ch b

arrie

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com

pone

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erat

ely

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l so

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con

tain

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s no

dule

s

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arp

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7800

di

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form

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lian

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nd

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ting

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om

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gr

ain

size

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tled

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ized

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ff cl

ays;

fe

rrug

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s no

dule

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mm

on

Shar

p lo

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tact

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inge

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cont

acts

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ded

low

er

cont

act

Shar

p di

scon

form

ity

mar

ks lo

wer

con

tact

3.5

8.5

13 00

0

< 12

5 000

13 O

OG

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00

125 0

00

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now

n >

125 0

00

Unk

now

n

“i 8 P .-

L

W

W

ul

W

W

o\

Tabl

e 3.

Sum

mar

y de

scrip

tion

of li

thof

acie

s co

mpr

isin

g th

e M

utch

ero

Inle

t sed

imen

tary

seq

uenc

e.

Max

imum

A

ppro

xim

ate

Stra

tigra

phic

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thof

acie

s En

viro

nmen

tal

Dia

gnos

tic

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tigra

phic

th

ickn

ess

age

sequ

ence

de

scrip

tion

inte

rpre

tatio

n fe

atur

es

rela

tions

hip

(4

(Y

r B

PI

Hol

ocen

e hi

ghst

and

pack

age

(Est

uari

ne

prog

rada

tion)

Hol

ocen

e tra

nsgr

essi

ve

pack

age

(Est

uari

ne

sedi

men

tatio

n)

Late

Ple

isto

cene

su

rfac

e

Poor

ly s

orte

d,

arko

sic

grav

el a

nd

sand

with

cla

y m

atri

x D

ark

brow

nish

gre

y m

ud a

nd p

eat

with

w

ood

rem

ains

and

co

arse

org

anic

s

Dar

k br

owni

sh g

rey

mud

with

pre

serv

ed

root

lets

and

coa

rse

orga

nics

Dar

k br

owni

sh t

o gr

eeni

sh g

rey

sand

y m

ud a

nd s

ilty

clay

w

ith a

bund

ant

orga

nics

Pale

gre

y-bl

ue,

fine-

med

ium

cla

yey

sand

(or

gra

vel)

with

fin

e aq

uatic

gr

asse

s

Mot

tled

oxid

ized

, in

terb

edde

d w

ell

sort

ed s

and

and

indu

rate

d cl

ay

Allu

vium

Fres

hwat

er s

wam

p m

ud

Inte

rtid

al m

angr

ove

mud

Inte

rtid

al m

angr

ove

mud

Estu

arin

e cl

ayey

sa

nd a

nd g

rave

l

Coa

stal

pla

in

allu

vium

and

soi

l

Oxi

dize

d co

lour

; di

vers

e m

iner

alog

y;

poor

sor

ting

Dis

tingu

ishe

d fr

om

unde

rlyin

g m

angr

ove

mud

by

grea

ter

abun

danc

e of

rai

nfor

est

and

fres

hwat

er p

olle

n D

istin

guis

hed

from

un

derly

ing

trans

gres

sive

m

angr

ove

mud

by

grea

ter

abun

danc

e of

coa

rse

orga

nics

hoot

s, a

nd

Rhizophora p

olle

n D

istin

guis

hed

from

un

derly

ing

estu

arin

e se

dim

ents

by

incr

ease

d or

gani

c co

nten

t an

d gr

eate

r ab

unda

nce

of

Rhi

zoph

ora

polle

n Pa

le b

lue

colo

ur;

wel

l sor

ted

sand

; aq

uatic

gra

sses

Mot

tled,

oxi

dize

d,

, st

iff c

lays

; fe

rrug

inou

s no

dule

s co

mm

on

Shar

p lo

wer

con

tact

6

40

00

(stil

lsta

nd

pack

age:

670

0)

Shar

p lo

wer

con

tact

1

6100

(stil

lsta

nd

pack

age:

670

0)

Low

er c

onta

ct a

t lo

w w

ater

dat

um

Shar

p lo

wer

con

tact

Shar

p di

scon

form

ity

mar

ks lo

wer

con

tact

Unk

now

n

2 5.5 5

6100

(stil

lsta

nd

pack

age:

670

0)

% h

610C

L700

0

7000

-800

0

Unk

now

n >8

000

Sea level control of stacked coastal sequences 337

embayments is distinctly different, primarily because of their different physiographic settings (Fig. 2). Wyvuri Embayment was originally broad and open to the shelf, and was successively closed by three packages of beach barrier-lagoon sedimentation. In contrast, Mutchero Inlet was a large estuary, protected from the open coast by bedrock headlands, which rapidly filled in during the Holocene transgression and highstand.

Despite the different physiographic settings and sequence styles, similar depositional cycles occur in both embayments in response to late Quaternary sea level fluctuations including: (a) a last interglacial highstand package, (2) an early to mid-Holocene transgressive package punctuated by sedimentation marking a brief pause in sea level rise and (3) a mid- to late Holocene highstand package. The lithofacies forming the three sedimentary packages are defined by their geometry, lithology and, to a lesser degree, pollen assemblages (Tables 2 and 3). Stratigraphic relationships, textural trends and habitats of deposition based on pollen data are summarized in Figs. 3-7.

Last interglacial highstand package

Inner beach barrier

Drill hole JW4 in the 250m wide inner barrier (Wyvuri Embayment) intersected 10 m of oxidized, generally well sorted, fine-medium, clayey quartz sand with a well developed soil profile (Fig. 3). The basal 5 m is coarser and more poorly sorted than the upper 5 m and is interpreted to mark beach barrier followed by aeolian deposition. Stiff, oxidized, interbedded sand and clay which sharply underlie the sand are diagnostic of Pleistocene alluvium in this area. Brownish grey mud interbedded with oxidized, poorly sorted arkosic gravel and sand lie landward (JW6) of the inner barrier. The high clay content (70%) and 6I3C value of organic carbon ( - 24%0) in the mud indicate a freshwater swamp with terrestrial aquatic vegetation.

No Pleistocene marine sediments occur in Mutchero Inlet, even in deeper drill holes such as JR3 (Fig. 4). The basal stiff, oxidized, interbedded sands and clays are interpreted as alluvium. The upper contact of the alluvium is invariably sharp and overlain by pliable, water-laden, grey-green or brown sediments having basal radiocarbon ages 0f<8000 yr BP.

Age of the highstand package

The inner beach barrier sand has not been directly dated, but its location landward and separated from the outer barrier (Fig. 2) suggests it is correlative to other inner barriers deposited during the last interglacial sea level highstand about 125 000 yr BP

(Murray-Wallace & Belperio, 1991). In addition, weathering is much more extensive in the inner barrier compared to the outer barrier. The oxidized yellow-orange colour of the inner barrier sand suggests that weathering has proceeded since the Late Pleistocene (Pye, 1981) in a similar fashion to the weathering that has affected other inner barriers identified in north Queensland (Hopley, 1970; Hopley & Murtha, 1975; Jones, 1985). Moreover, the inner barrier contains up to 15% clay whereas the outer barrier contains no clay (see Fig. 7a). Because the sand of the inner barrier is nearly 100Y0 quartz, while feldspar and mafic minerals remain in the outer barrier, the clay may be diagenetic resulting from long term in situ weathering of other 'components that are unstable in humid envii-onments.

A single radiocarbon date from the clayey mud lying landward of the inner barrier yielded an age of 13 000 yr BP. This mud is thus a younger freshwater deposit, not a lagoonal mud associated with the inner beach barrier.

Holocene transgressive package

Wyvuri Embayment

Reworked beach barrier. The late Pleistocene discon- formity in Wyvuri Embayment is sharply overlain by a muddy sand sheet distinguished by moderately to well sorted sand and medium sand sized ferruginous nodules. The variety of mean grain sizes and sorting coefficients (Fig. 6), and nodules, probably reflect the constituents of different substrates that were weathered before being reworked during the post-glacial marine transgression. The muddy sand sheet is most likely the end-product of poorly preserved beach barrier sand mixed with lagoonal mud as the shoreline moved landward.

Lagoon sedimentation. The transgressive muddy sand sheet overlies organic-rich mud with Rhizophora pollen making up 85% of the total pollen count (Fig. 5). The organic-rich mud represents intertidal mangrove sedimentation established in a protolagoon being overstepped by the reworked beach barrier sand.


A WYVURI EMBAYMENT

-1 Holocene Outer Barrier-Lagoon ___ Last Interglacial 1- Inner Barrier -1.- SSW NNE ] WSW ENE

U I L -14 ' . . . . .

Freshwater swamp peat

Intertidal mangrove mud

g 2 g) Back-barrier aeolian sand

_ox 8 3 3 0 Back-barrier washover sand

Beach barrier sand 2 5?g Upper shoreface muddy sand

Lower shoreface sandy mud

al > g z al g % F Intertidal mangrove mud 0 CnV

L.3

Muddy sand sheet 8 ig I-

- .gz al - ;: u)Y 8 Aeolian sand cap

al cnm Beach barrier sand Y L - 0

= a lug0 2 . E =

h

o : g z Coastal plain alluvium and soil o m

kilometre

1 2 3

*4 *5 '6 7

*8 9

*10 *11 *l2

Radiocarbon Dates (yr BP) 13200 (WK 1053) 6270 (WK 1054) 5950 (WK807) 5490 (WK806) 6300 (WK799) 6840 (WK800) 71 60 (WK 1055) 7760 (WK802) 5050 (WK 1056) 3100 (WK803) 4890 (WK804) 7010 (WK805)


Mutchero Inlet

Estuarine sedimentation Pale blue-grey, clayey sand and gravel overlie late Pleistocene alluvium in Mutchero Inlet. Gravel is more common on the southern (Russell River) side while the northern side is finer with disseminated organic detritus. Pollen data for DD1 (Fig. 5) show this basal unit contains approximately equal mangrove and terrestrial taxa, indicating an estuarine environment marking the initial backfilling of Mutchero Inlet.

The estuarine lithofacies is overlain by a distinctive dark brownish grey to greenish grey sandy mud with high organic content and a greater abundance of mangrove pollen (Fig. 5). The abundance of mangrove pollen in this lithofacies marks the establishment of intertidal mangroves in the estuary during the final stages of the post-glacial marine transgression.

Age of the transgressive package

In Wyvuri Embayment, the oldest radiocarbon age for the basal transgressive mangrove mud is c. 7800 yr BP in JW2 (Fig. 3). and provides a maximum age for the base of the transgressive package. The oldest date from the overlying Holocene highstand package (c. 6300 yr BP in VW5) constrains the age of the Wyvuri transgressive package to c. 6300- 7800 yr BP. The transgressive package in Mutchero Inlet occupies a similar time span. Mangrove mud immediately above the estuarine lithofacies in DD1 is dated at c. 7000 yr BP (Fig. 4), giving a minimum age for the base of the transgressive package. The oldest radiocarbon age from the Holocene highstand package (c. 6100 yr BP in DD1) constrains the age of the Mutchero transgressive package to c. 6100-7000 yr BP.

The stillstand package

Evidence for abbreviated progradation. A final mid- to late Holocene phase of coastal progradation (Holocene highstand package), associated with

present sea level, is described in the next section. It is important to note that an abbreviated version of this highstand progradation (the stillstand package) interrupts the transgressive sedimentary sequence in both embayments (Figs 3 and 4). In Wyvuri Embayment both JW2 and JW7 (Figs 6 and 7b) intersected clean, medium to coarse, moderately- well sorted quartz sand which is very similar in texture, thickness and lateral continuity to the modern highstand beach barrier but c. 5 m lower in elevation. The sediment texture and geometry of this sand body strongly suggest it is a beach barrier constructed during a pause in the post-glacial marine transgression.

The transgressive package in Mutchero Inlet dis- plays further stratigraphic evidence for a brief sea level stillstand, including: (1) a coarsening upward section (c. - 3 to - 4 m AHD) culminating with a lens of alluvium in JR5; (2) a reversal in the general terrestrial to marine pollen sequence marked by mud rich in terrestrial pollen in DM1 (Fig. 5); and (3) oxidized material - 2 to - 4 m -AHD on the northern side of the inlet (JR4) indicating partial subaerial exposure of the sediments. It is unlikely that this temporary progradation could be simply caused by a temporary climate change or series of large floods. The change in the pollen signature in DMl (Fig. 5) is progressive, changing upward from brackish mangrove briefly to Rhizophora-dominated mangrove, followed by freshwater swamp taxa. A series of flood events would rework a variety of terrestrial taxa resulting in a mixed pollen assemblage rather than the systematic succession observed in DMl . Further- more, the top of the oxidized zone in the estuarine sediments (JW4) lies slightly above (+ 1 m) the ab- breyiated progradation sequence and suggests that where the perimeter of the estuary was not being buried by prograding Russel River alluvium it was temporarily exposed and subaerially weathered.

Age of the stillstnnd package. No organic material could be found within the stillstand beach barrier sand in Wyvuri Embayment but the age of the

Fig. 3. (A) Stratigraphic cross section showing the lithofacies in Wyvuri Embayment and (B) a summary cross section of the sea level controlled sedimentary packages producing the stratigraphic sequence. The symbol ‘SP’ inside Holocene highstand package legend boxes highlights those lithofacies that are repeated in the underlying stillstand package. Asterisked radiocarbon dates show the elevation and stratigraphic position of age dates from drill holes (VW2, JW7, JW9) not located on the cross section line. Radiocarbon dates 5 and 6 are from intertidal mangrove mud capping the stillstand barrier intersected by JW7.

340 M. K. Gugun et a1

A MUTCHERO INLET

I Mutchero 14 Mulgrave River I------ Russell River --

-14 i -16 L

I-

0 L

1 2 I I I I

kilometres

Alluvium

Freshwater swamp mud



Estuarine clayey sand and

Coastal plain alluvium and

,-River Channel Incision

I Late Pleistocene Surface 1

Radiocarbon gravel Dates (yr BP)

1 6100 (WK 1030) 2 6600 (WK1028) 3 7050 (WK 1029) 4 2640 (WK 1369) 5 7140 (WK1370) 6 6700 (WK1371)

soil

Fig. 4. (A) Stratigraphic cross section showing the lithofacies in Mutchero Inlet and (B) a summary cross section of the sea level controlled sedimentary packages producing the stratigraphic sequence. The symbol 'SP' inside Holocene highstand package legend boxes highlights those lithofacies that are repeated in the underlying stillstand package.

barrier is well constrained by datable material directly above and below it. The base of the beach barrier can be no older than the c. 7100 yr BP date from the top of the mangrove mud directly below it in JW2. In W 7 , the base of the mangrove mud draped over the top of the barrier provides a minimum age of c. 6800 yr BP. The brief period of progradation in Mutchero Inlet is very similar in age to the stillstand beach barrier. Radiocarbon ages of c. 6700 yr BP at - 3.5 m AHD in both DD1

and DMl imply the stillstand began at this time since the dates are found at the same elevation in both drill holes and lie within the base of the abbreviated progradation in DMl. The age date of c. 7100 yr BP above the abbreviated progradation package is older than sequential dates around it and indicates reworked organic matter. It is likely that the organic material was not derived from far away, however, since pollen spectra indicate a lowland origin.


2 -

1 -

i D -

-1 -

-2 -

-3 -

AHD

-1

-2 - rn F - E" -3 Y

c 0 ._ c 9 -4 a

-5

-6

-7

Pollen Counts Freshwater: Marine ("G Influence

0 100 High Low

- 5950 yr BP

F Pollen Classes Rhuophora BruguieralCeriops $ Other mangroves b E C t i i l l 2 Other wetland 3 2 8 Terrestrial taxa m u %

P r n u r n s c

Holocene highstand package

Wyvuri Embayment

Beach barrier and shoreface progradation The outer beach barrier of Wyvuri Embayment is 675 m across with eight beach ridges averaging 85 m in width. In the subsurface this lithofacies is easily distinguished by its coarse grain size and coarsening downward trend, having up to 15% gravel near the base (Figs 3 and 7). The basal contact of the beach barrier is very sharp and overlies thinly bedded and laminated, micaceous muddy sand which fines downward to dark grey, sandy mud. The muddy sand forms the upper shoreface where frequent changes in wave energy create alternating fine and coarse grained beds. The underlying sandy mud contains whole Anadara shells and shell hash, especially toward the base, and is interpreted as a lower shoreface deposit.

The three beach barrier packages in Wyvuri Em- bayment have progressively different grain size distri- butions which may be primarily source related. Figure 6 shows mean grain size is finest in the interglacial highstand barrier and becomes progressively coarser in the Holocene transgressive and outer barrier. The interglacial highstand barrier is in a relatively protected part of the embayment but even low energy beaches along the north-east Australian coast, such as those observed in the lee of offshore islands, are composed of coarse sediments if they are available. The finer mean grain size of the interglacial barrier is therefore likely to be due to a finer sediment source, perhaps an offshore source such as that pro- posed for the genesis of south-east Australian barriers (Chapman et al., 1982). The Holocene transgressive barrier is slightly finer grained than the ensuing outer barrier and may reflect a mixing of both shelf and coarser river sediment sources (Pringle, 1991). The grain size and mineralogy of the outer barrier reflects that of modern river sand (Bird, 1971).

Lagoon injilling. Landward of the outer beach barrier is a parallel, back-barrier sand body which is fine grained and well sorted (Figs 6 and 7). The basal

Fig. 5. Summary of pollen counts, habitats of deposition and radiocarbon ages in vibracore VW4 (Wyvuri Embay- ment) and D-section cores DM1 and DDl (Mutchero Inlet). The freshwater to marine influence in the habitats of deposition is estimated using the percentage of Rhizophora mangroves in the total pollen count.


3.5 a,

ii > -3.0

a 2.5

i? g 0.5 0

-0.0

la A

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1 IV Well1 Well I M Well I Moderate I Poor

Sorting

Hol. highstand barrier 0 Back - barrier A Upper shoreface sand sheet + Lower shoreface

Hol. stillstand barrier

0 Transgressive muddy

0 Last interglacial barrier

Fig. 6. Comparison of the grain size attributes in the sandy lithofacies of Wyvuri Embayment. Mean grain size versus sorting differentiates the various depositional environments for each lithofacies with the exception of that for the transgressive muddy sand sheet which contains grains from the underlying late Pleistocene alluvium.

medium sand in the back-barrier is interpreted to be washover sand deposited while the barrier was still narrow, before successive beach ridges were formed during subsequent progradation. The upper portion of the back-barrier sand, however, is finer grained and very well sorted, which suggests it was not deposited by wave action, but may be aeolian. Wind deflation and westward transport of fine sand from the western part of the prograded barrier explains the coarser grain size and improved sorting there (Fig. 7a, JW3 vs. JW5), and the finer, very well sorted sand forming the upper part of the back-barrier. The aeolian lithofacies can be traced further landward where it forms a distinctive, remarkably continuous, 20 cm thick fine sand layer across the entire lagoon.

The bulk of the lagoon infill comprises dark brownish grey, organic mud. The predominance of

Rhizophora pollen (85% of total taxa; Fig. 5) and the proximity of the lagoonal mud to AHD shows it is intertidal mangrove mud deposited at or near present sea level. The transition from transgressive mangrove mud upward into highstand mangrove mud is indis- tinct but is positioned at Australian Low Water Datum to match that documented in modern mangrove swamps (Thom et al., 1975; Grindrod & Rhodes, 1984; Grindrod, 1985). The lagoon infill is capped by 2 m of dark brown freshwater peat distinguished by wood remains, rootlets, decaying vascular plants, and pollen counts dominated by freshwater swamp and terrestrial pollen (Fig. 5). The peat marks an abrupt change from intertidal marine to freshwater swamp sedimentation.

Mutchero Inlet

Estuarine progradatcon. The Holocene highstand package in Mutchero Inlet contains no beach' deposits but is very similar to the lagoonal sequence in Wyvuri Embayment. The basal lithofacies is a dark brownish grey, organic mangrove mud which has lower Rhizophora pollen counts than its Wyvuri equivalent (50-70% vs. 85%). The change from intertidal marine to freshwater swamp conditions is marked by a sharp increase in rainforest and freshwater pollen (Fig. 5 ) . The top unit is yellow-red, poorly sorted arkosic gravel and sand with a clay matrix interpreted to be colluvium interbedded with river channel and overbank deposits. The alluvium thickens southwards and becomes coarser grained. A channel deposit may have been intersected in auger drill hole JR2. near the modern inlet.

Age of the highstand package

The beach barrier in Wyvuri Embayment contains no datable shell material even though shells are common on the modern beach. It is likely that the barrier is continually flushed by heavy rains containing car- bonic acid which dissolves any aragonitic shells. Age dates from lithofacies associated with the barrier, however, can be used to constrain its age and general style of evolution. The oldest date from the highstand mangrove mud, landward of the barrier (Fig. 3), is c. 6300 yr BP, suggesting the barrier was in place and providing protection for the lagoon by that time. The youngest date from the highstand package (c. 3100 yr BP) is situated in the middle of the lower shoreface


sediment. Hence, progradation of the barrier probably began c. 6300 yr BP. and continued to more recently than c. 3100 yr BP. The c. 5500 yr BP age of the aeolian back-barrier sand in VW2 (Fig. 2) suggests the beach barrier was large enough at that time to provide sand for landward transport to the back- barrier and across the lagoon surface. The change from aeolian to freshwater swamp sedimentation in the lagoon could not be dated.

The age of the highstand package in Mutchero Inlet coincides with that in Wyvuri Embayment. The oldest radiocarbon age for the highstand mangrove mud (c. 6100yr BP), associated with the present sea level, is from core DD1 located about 2 km north of the modern inlet. Freshwater swamp mud from DMl, immediately adjacent to the modern inlet, gives an age of c. 2600yr BP. The ages suggest that intertidal sedimentation began at least by 6100 yr BP at DD1 and prograded 2 km seaward to DMl some- time before 2600 yr BP.

RELATIVE SEA LEVEL HISTORY

Coastal sequences may yield accurate sea level histories provided their depositional environments are sensitive to sea level fluctuations. For example, high (or variable) wave energy sequences, and those in macrotidal settings, should be avoided because the elevational resolution of their lithofacies may be too coarse. Furthermore, reconstructing a sea level history can be confounded if the identification of indicator lithofacies is ambiguous or their elevations are poorly determined relative to mean sea level.

Wyvuri Embayment and Mutchero Inlet are well suited for reconstructing a relative sea level history because of their mesotidal, low-moderate wave energy hydraulic regime. The resulting sedimentary sequence contains two sensitive indictors of sea level: (1) the sharp intertidal mangrove mud to freshwater swamp transition clearly identified by pollen analysis and ( 2 ) the clean sand of the beach barriers which sharply overlies the muddy upper shoreface.

Sea level indicators

The transition from Rhizophora-dominated intertidal mud to freshwater swamp is a valuable sea level indicator and occurs at an elevation of +1.0 to + 1 5 m AHD along the mesotidal coasts of north- east Australia (Rhodes, 1982; Belperio, 1983; Chappel & Grindrod, 1984; Grindrod & Rhodes,

1984). The top of he Rhizophora-dominated mud in Wyvuri Embayment (+1.1 m, AHD) has a similar elevation (Figs 3 and 5) . It is important to note, however, that the top of the mangrove mud in Mutchero Inlet becomes progressively lower away from the coast, falling from +0.7 m AHD at DM1 to - 0.2 m AHD at DD1 located further upstream. The decreased elevation is probably related to the high mean annual rainfall of the study area (2500mm yr-') which results in lower salinities towards the head of the estuary. Progressive depres- sion of the upper position of mangrove growth in an upstream direction would be expected in a highly stratified, brackish estuary. Further evidence for the influence of freshwater on Rhizophora growth is provided by a systematic variation in the percentage of Rhizophora pollen versus other mangrove taxa (Fig. 5) . Grindrod & Rhodes (1984) have shown that in the open estuarine conditions of Missionary Bay, sediment accumulating within or immediately adjacent to Rhizophora colonies contains 80-95% Rhizophora pollen. The Rlzizopkora counts for mangrove mud in the Wyvuri lagoon (8.59'0 in VW4) fall exactly into this range. The Rhizophora pollen percentages in Mutchero Inlet, however, decrease in parallel with the depressed mangrove mud heights, from 70% in DMl (+0.7m AHD) to 40% in DDl ( - 0.2 m AHD).

Despite the freshwater-controlled height differences, the transition from intertidal mangrove mud to freshwater swamp is still well constrained and sensitive to slight fluctuations in sea level. In lagoons experiencing limited freshwater input, such as that in Wyvuri Embayment, the mangrove mud to freshwater swamp contact will be about 1 m above mean sea level. In high runoff estuaries, such as ,Mutchero Inlet, this contact will be lower and close to mean sea level away from the coast.

The distinct basal contact of the Holocene highstand barrier is also strictly related to mean sea level and generally occurs at an elevation of 0 to +0.5 m AHD along more protected coasts in north-east Australia (Rhodes, 1982; Belperio, 1983; Grindrod & Rhodes, 1984). The beach barrier to shoreface contact in the protected southern end of Wyvuri Embay- ment (JW9) has a similar elevation of +0.4 m AHD (Fig. 7c) but becomes progressively lower toward the north ( - 0.8 m AHD in JW8). In addition, barrier height also increases northward by about 1 m from +4.5 m AHD (JW9) to +5.5 m AHD (JW8). This south to north increase in thickness (35y0) of the modern beach barrier reflects the northward increase


JW4 (t6.6m AHD) 0 25 50 75 100%

JW2 (+4.7m AHD) 0 25 50 75 100%

JW3 (+5.8m AHD) 0 25 50 75 100%

JW5 (+3.8m AHD) 0 25 50 75 100%

Interglacial Highstand

Barrier 14

15

16

Holocene Stillstand

Barrier

Holocene Highstand Barrier

Holocene Highstand

Barrier

JW2 (+4.?m AHD) B 0 25 50 75 100%

Central 0

15

16

JW7 (+4.7m AHD) 0 25 50 75 100%

Northern

JW9 (+4.6m AHD) C 0 25 50 75 100%

JW5 (+3.8m AHD) '

0 25 50 75 100%

Depositional Environments

Aeolian sand Washover sand Beach barrier sand Upper shoreface muddy sand Lower shoreface sandy mud Muddy sand sheet Intertidal mangrove mud

Grain Size

Gravel Sand Silt Clay

JWB (+5.5m AHD) 0 25 50 75 100%

Central

15] Southern 16

Northern


in mean wave energy along the shoreface. The northern section of the shoreline is more exposed to south-easterly, wind-driven, short period seas which tend to break near shore, especially at high tide. Higher wave surge heights build the beach higher in the north while the increase in mean wave energy allows coarse sand to be transported to deeper water where the finer, upper shoreface sediment is normally deposited. Meanwhile, because the southern end of the embayment is protected, the top of the beach is lower there and coarse sand is restricted to a narrow wave swash zone slightly above AHD.

The basal contact of the Holocene highstand barrier is well constrained throughout the mid-section of Wyvuri Embayment despite the difference in elevation (c. +O.S m) at the southern and northern extremities. The weighted mean height of the barrier is - 0.3 m AHD which accounts for the approximate length of beach represented by each drill hole (JWS, 8, 9). This height is a good estimate for the elevation of the mid-section of the shoreface to barrier contact is used to estimate the elevations of sea levels forming the ancient beach barriers.

The last interglacial highstand

Published evidence shows the relative height of the last interglacial shoreline around Australia to vary by more than 30 m and indicates that north-east Australia has subsided relative to other parts of the continent (Murray-Wallace & Belperio, 1991). For example, the last interglacial shoreline in south-east Australia is reported at + S m AHD, based on the heights of rare intertidal estuarine muds and the general elevation of beach ridges. Further north on the central-east coast, fossil coral elevations imply a lower shoreline height of + 1 to +3 m AHD (Pickett et al., 1989). Although some studies in north-east Australia have identified last interglacial marine sands (Hopley, 1970; Hopley & Murtha, 1975; Jones, 1985; Jones & Stephens, 1986), the related sea levels have not been determined but appear to lie consis- tently below present sea level. Drilling has shown that

late Pleistocene limestones beneath the modern Great Barrier Reef occur at - 3 to - 20 m AHD and never above present sea level (Hopley, 1982), probably due to both shelf subsidence and subaerial weathering.

The base of the last interglacial highstand barrier in Wyvuri Embayment also lies below present sea level, at - 3.6 m (JW4) and - 3.4 m AGD (JW6). Assuming the base of the interglacial barrier extended to 0.3m below mean sea level, like the modern barrier ( - 0.3 m AHD), the corresponding interglacial shoreline now lies at approximately 3.2 m below AHD. The true position of the shoreline, however, may be even lower because the contact between the barrier sand and underlying shoreface was not intersected in the drill holes.

The regional variation in the elevation of the preserved shoreline is due primarily to differential hydroisostatic adjustment of the continental margin (Lambeck & Nakada, 1990; Murray-Wallace & Belperio, 1991). The best estimate for the true elevation of the last interglacial shoreline in Australia comes from the west coast of the Eyre Peninsula, South Australia, where a height of +2 m AHD exists along SO0 km of coastline. It is likely, therefore, that the coastal plain in the general area of Wyvuri Embayment has subsided by about 5 m since the time of the last interglacial sea level highstand (c 12.5 000 yr BP).

The mid-Holocene - 5 m stillstand

The exact nature of the post-glacial marine transgression is actively debated because it is difficult to identify unequivocally and to date submergent shorelines. Transgressive sand sheets on shelves have been cited as evidence for a continuous transgression (Swift et al., 1972; Field & Duane, 1976) while the positions of submerged shoreline features have been used to infer episodicity in sea level rise (Sanders & Kumar, 1975; Carter et al., 1986). Although submerged shoreline features have been documented on the central Great Barrier Reef shelf (Carter &

Fig. 7. Comparison of the lateral and vertical distribution of gravel, sand, silt and clay in Wyvuri Embayment depositional environments. (A) Shore-normal changes in grain size from the last interglacial highstand barrier (JW4) seaward to the front of the Holocene highstand progradational barrier (JW5). (B) The thickness, bulk grain size and northward shore-parallel coarsening of the Holocene stillstand barrier which are similar to the younger highstand barrier. (C) The northward shore-parallel thickening and coarsening of the modern beach and upper shoreface which reflects the greater exposure of the northern end of the embayment to the mean wave climate. The JW5 drill hole elevation (+3.8 m AHD) is lower than that for JW8 and JW9 because it was sited atop a recently formed, incipient beach ridge.

346 M. K Gagan et al.

Johnson, 1986), the exact age and origin of these features are not known and hence their relationship with the post-glacial marine transgression is still not clear (Harris & Davies, 1989).

This study presents the first evidence for a - 5 m stillstand during the post-glacial marine transgression in north-east Australia. Two lines of evidence support this view: first, the beach barrier beneath the Holocene highstand package in Wyvuri Embayment suggests sea level stabilized and a temporary shoreline was constructed; second, the abbreviated period of progradation interrupts the transgressive sequence at a similar elevation in Mutchero Inlet. The elevation of the sea level stillstand is well constrained in both Wyvuri Embayment and Mutchero Inlet. The sharp basal contact of the stillstand beach barrier with the underlying intertidal mangrove mud is interpreted as the base of the beach barrier (Fig. 3). In JW2 the base of the beach barrier is at - 4.6 m AHD while in JW7 it is lower at - 6.1 m AHD (Fig. 7b). Assuming the beach extends to - 0.3 m AHD as for the modern beach, the associate stillstand elevation lies between -4.3 m and - 5.8 m AHD, with a mean value of c. - 5 m AHD. A similar shoreline elevation is reflected by the elevation of the Rhizophoralfreshwater swamp contact ( - 3.3 m AHD, DMI) in the stillstand package buried in Mutchero Inlet. Based on the modern Rhizophoral freshwater swamp contact being at +0.7 m AHD in DMl (Fig. 5), the elevation of the associated stillstand corresponds to c. - 4 m AHD. The 1 m difference between the two calculated elevations could be narrowed if the elevation at which mangroves grew during the transgression in Mutchero Inlet was not depressed by 1 m, as it appears to be in the modern inlet. Nevertheless, the elevations from both Wyvuri Embayment and Mutchero Inlet are surprisingly similar and a mean elevation toward c. - 5 m AHD is estimated for the stillstand.

The age of the stillstand is also well constrained. Radiocarbon dates (6800-7100 yr BP) from immediately below and above the stillstand beach barrier match the age from the base of the stillstand package (6700yr BP) in Mutchero Inlet. The difference between the mean age for the beach barrier (6950 yr BP) and the 6700 yr BP date in Mutchero Inlet is not statistically significant, and therefore the approximate mean age for the stillstand is estimated to be c. 6800 yr BP.

The extent of the - 5 m stillstand is difficult to determine because it could have easily gone undetec- ted in other coastal sequences. For example, the el-

evation and age of the - 5 m stillstand fits well within the post-glacial sea level envelope for east Australia which has been summarized by Thom & Roy (1985) and refined using the elevations and ages of transgressive mangrove mud (Grindrod & Rhodes, 1985; Clark & Guppy, 1988; Crowley et al., 1990). That the stillstand falls within the bounds of the published envelope is not surprising, however, since the envelope boundaries are not well constrained. The elevation of the stillstand could vary by 10 m (from - 9 m to + I m AHD) and range in age from 8000 to 6500yr BP and still be within the agedepth uncertainty of the present east Australia sea level envelope.

It is important to note that the initiation of the northern Australian ‘Big Swamp’ period of mangrove sedimentation (Woodroffe et a!., 1985, 1989) corresponds well with the timing and elevation of the - 5 m stillstand. Mangroves proliferated during the ‘Big Swamp’ period which began c. 6800 yr BP at - 2 to - 5 m AHD as the rate of sea level rise began to slow. This period of sedimentation may have been triggered by the - 5 m stillstand which would have allowed estuarine sediments to fill partially coastal basins and provide shallow sites for colonization by mangroves which dominated once the transgression resumed. Future sea level studies may show that the - 5 m stillstand is a general feature that is detectable in coastal sequences throughout northern Australia.

Recent age-depth data from submerged coral reefs in the Caribbean (Fairbanks, 1989) and at Huon Peninsula, Papua New Guinea (Chappell & Polach, 1991) provide the most detailed records of glacio- eustatic sea level change during the post-glacial marine transgression. Both studies show that the rate of sea level rise was not constant. In the Caribbean, rapid sea level rise coincides with northern hemi- sphere glacial meltwater peaks centred on 12 000 yr BP and 9500yr BP. The Huon Peninsula data also show sea level rising rapidly from 9000 to 10 000 yr BP, during the time of the Younger Dryas. However, neither record is capable of discerning century scale pauses in sea level rise, particularly toward the end of the transgression (<8000 yr BP) when the rate of rise slowed significantly.

Sedimentary sequences in deep coastal enibay- ments may provide the best prospect for developing continuous, high resolution sea level histories for the later stages of the post-glacial marine transgression. Intertidal sediment accumulation in protected settings commonly mimics the rate of sea level rise. Such is the case in the Mutchero Inlet and the Wyvuri Embayment lagoon where mean vertical


sedimentation rates for the transgressive mangrove muds coincide at 5.4 and 5.1 mm yr - ', respectively. The predicted rate of sea level rise from 8000 to 6500 yr BP in north-east Australia is 4.5 mm yr ~ I ,

nearly matching the mangrove mud sedimentation rate. Similar sea level control of intertidal sediment accumulation has been confirmed in a study of 22 lagoons along the Gulf of Mexico and Atlantic coasts (Nichols, 1989). Sea level has continued to rise relative to the eastern US during the Holocene and long term vertical accumulation rates closely paralleled the sea level rise in 71% of the lagoons.

Of additional importance is that some of the organic matter preserved in protected intertidal to supratidal deposits is generally produced and buried in situ thus greatly improving its value for radiocarbon dating. Perhaps the weakest links in previous sea level studies are chronologies based upon bulk radiocarbon analysis of organic material having a variety of ages. Accelerator mass spectrometric radiocarbon dating now makes it possible to date selectively even small quantities of organic matter preserved in situ and should produce tightly constrained chronologies in the future. If the elevations of the sea level indicators are also well constrained the age-depth curve should be sensitive to subtle sea level movements. Recent studies of transgressive facies relationships in such settings are beginning to reveal century scale fluctuations in Holocene sea level. Recorded in the Wolfe Glade tidal wetlands, for example, are five short term episodes of rapid sea level rise resulting from surges and relaxations in the Atlantic Gulf Stream during the Holocene (Fletcher et al., 1993).

The mid- to late Holocene highstand Our estimate for the end of the post-glacial marine transgression is 6300-6100 yr BP, which agrees well with previous estimates of 6500-6000 yr BP (Chappell et al., 1983; Grindrod & Rhodes, 1984; Thom & Roy, 1985). Whether sea level stabilized at this time, or rose slightly higher before falling to its present position is uncertain. Convincing evidence for sea level being 1 m higher about 6000 yr BP is provided by Chappell et al. (1983) where fringing reefs from 11 offshore Great Barrier Reef islands were carefully surveyed and radiocarbon dated to show how sea level fell from + 1 m AHD to its present position. In contrast, sea level studies involving Holocene marine sediments in northern Australia have not discerned the + 1 m shoreline (Belperio, 1979b, 1983; Grindrod & Rhodes, 1984; Chappell & Grindrod, 1984; Jones, 1985; Woodroffe et al., 1989). The only exception is

in the southern Gulf of Carpentaria where the basal elevations of cheniers were used to show about 2.2 m of sea level fall over the last 6000 years (Rhodes, 1982).

Differences in the magnitude of sea level fall are to be expected throughout northern Australia due to different degrees of hydroisostatic adjustment of the continental margin to post-glacial sea level rise. The hydroisostatic model of Chappell et al. (1982) suggests that the Innisfail-Cairns coastal plain should have experienced a sea level fall of about 1.2 m during the last 6000 years. There is no evidence for a higher sea level in Mutchero Inlet, but this may not be a good site from which to judge the matter because post-depositional compaction may depress muddy sequences, as suggested by Chappell & Grindrod (1984). There is, however, tentative support for a +1 m sea level in Wyvuri Embayment where the base of the beach barrier on the landward side of the prograded barrier sequence (intersected in JW3) is about 0.9 m higher (+0.6 m AHD) than the mean height for the modem beach barrier ( - 0.3 m AHD). The good match of this height with the predicted height may be coincidental, however, and more drill holes are needed to prove the point.

STRATIGRAPHIC RECORD OF AN EMBAYED TROPICAL COAST

The embayed nature of the Innisfail-Cairns coastline and sea level control of marine sedimentation both influence the stratigraphic style of the preserved coastal sequences. The influence of coastal physiography is manifested in two contrasting styles of sedimentary sequence: (1) an exposed, moderate energy, beach barrier-lagoon system and (2) a protected, low energy, muddy inlet fill sequence. How- ever, the same depositional cycles occur in both embayments and are strictly controlled by late Quaternary sea level fluctuations. Figure 8 illustrates four key depositional stages common to the late Quaternary coastal evolution of both embayments: 1 an inner beach barrier (Wyvuri Embayment) and

an open estuary (Mutchero Inlet) during the last interglacial sea level highstand (I25 000 yr BP);

2 partial preservation of a beach barrier-lagoon sequence (Wyvuri Embayment) and an abbreviated period of estuarine progradation (Mutchero Inlet) associated with a - 5 m mid-Holocene sea level stillstand (6950-6700 yr BP);

3 initiation of the progradational beach barrier and lagoon (Wyvuri Embayment) and extensive

348 M. K. Gagan et a1

Ap 125000yr BP

0 2 2 kilometres

+2m Highstand

jouthwest Northeaz

+2m Highstand

iorth souti

6950yr BP c Establishment

beach barrier

-5m Stillstand

-5m Stillstand

6300yr BP

+1 m Highstand

+1 m Highstand i

To Present

I To Present

Fig. 8. Summary maps and cross sections showing four successive stages in the late Quaternary coastal evolution of (A) Wyvuri Embayment and (B) Mutchcro Inlet.


4

estuarine mangroves (Mutchero Inlet) at the mid- Holocene sea level highstand (6300-6100 yr BP);

and mid- to late Holocene coastal progradation in both embayments as sea level stabilized near its present position. Aeolian activity marked the early period of beach barrier progradation in Wy- vuri Embayment resulting in the burial of the lagoonal mangrove mud which was followed by contraction of mangroves to the northernmost corner of the lagoon. Progradation of alluvium and freshwater swamp lithologies over the estuarine mangrove mud in Mutchero Inlet was followed by downcutting of the Russell/Mulgrave River channels.

Partially preserved beach barrier-lagoon and inlet fill sequences will be closely juxtaposed in ancient bedrock-embayed coastal settings and their relative positions should be useful for reconstructing palaeoshorelines. The barrier-lagoon systems will be the most important, volumetrically, and contain additional information related to the nearshore palaeohydraulic regime. For example, the bedrock- embayed, barrier-lagoon sequences of south-east Australia are sand dominated and up to 50 m thick because of the high wave energy hydraulic regime (Roy et al., 1980; Thom et al., 1981; Chapman et al., 1982; Thom, 1983; Melville, 1984). The Wyvuri barrier-lagoon sequence is much thinner (c. 15 m) due to the low-moderate wave energy setting and is also muddier because of the abundance of wet- tropical, chemical weathering products.

Given similar coastal hydraulic regimes, the most notable difference between the Wyvuri beach barriers and those from non-embayed coasts is the lack of tidal inlet sedimentation, which appears to be bedrock controlled. Tidal inlet deposits are commonly preserved in beach barrier systems fringing the Gulf of Mexico and Atlantic coast of the United States such as Matagorda Island (Wilkinson, 1975), Galveston (Bernard & Le Blanc, 1965) and Core Banks in North Carolina (Moslow & Heron, 1978). On non-embayed coasts such as these, tidal inlet migration forms sandy, cross bedded inlet migration sequences stratigraphically below the beach barrier (Kumdr & Sanders, 1974; McCubbin, 1982; Reinson, 1984). There is no geomorphological or stratigraphic expression of abandoned tidal inlets in Wyvuri Embayment because long term northward sediment transport forces the tidal inlet to be positioned at the northern end of the embayment. Tidal inlet sedi-

mentation is therefore restricted, not only on the Innisfail-Cairns coastline but also on embayed coastlines, in general, where longshore sediment transport has a preferred direction, such as in south-east Australia (Thom, 1983).

Numerous studies of ancient barrier sequences have highlighted the relationship between sea level and the formation and preservation of beach barriers (Campbell, 1971; Dickinson et al., 1972; Bridges, 1976; Ryer, 1977; Carter, 1978; McCubbin, 1982; Reinson, 1984). An ancient analogue for the Wyvuri Embayment beach barriers would be one having stacked linear sand bodies surrounded by lagoonal mud and fine grained shoreface sediments. Such a sequence has been described by Donselaar (1989) in the Upper Cretaceous Cliff House Sandstone of New Mexico where there was relative sea level rise punctuated with the accumulation of barrier sands during brief regressive intervals. The subsequent transgres- sions erode the upper part of each barrier while the low lying shoreface, tidal inlet and lagoonal units are preserved. The result is a series bf stacked sandstone bodies, slightly offset due to minor lateral shifts of the coastline. The - 5 m Holocene stillstand barrier and the subsequent progradational highstand barrier in Wyvuri Embayment represent one couplet of barriers in such a stacked sequence.

ACKNOWLEDGMENTS

Our appreciation is extended to all who made this study possible, in particular Neil Mockett who per- fected his drilling skills during many weeks in the field. The help of Bramston Beach locals Keith Sellers, and Glen and Mike Dawes, was fundamental to the success of the drilling. Doug Haywick com- petently assisted with lengthy drill hole levelling transects. Peter Kershaw, Phil Anderson, Peter Edney, and Kathy Strickland are warmly thanked for assisting in the field and with the pollen work. The bulk grain size analyses were completed in the Davies Laboratory of the Commonwealth Scientific and Industrial Research Organisation and sincere thanks go to Graham Murtha, Ross Coventry, Mike Cannon, and especially Kim Dent for fitting M.K.G. into their laboratory routine. Bob Carter provided generous support during all phases of the project. Michael Field and Lionel Carter are thanked for critical appraisal of an early manuscript and valuable reviews by Elana Leithold and Colin Woodroffe are gratefully acknowledged. Financial support for the project was provided by the Marine Science


and Technologies Grant Scheme and James Cook University Special Research Grants. G.M.C. was supported by a National Research Fellowship.

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(Manuscript received 10 February 1993; revision accepted 16 September 1993)

Sea level control of stacked late Quaternary coastal sequences, central Great Barrier Reef

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