An example from the northern Great Barrier Reef, Australia

24
Tidally incised valleys on tropical carbonate shelves: An example from the northern Great Barrier Reef, Australia Peter T. Harris a, * , Andrew Heap a , Vicki Passlow a , Michael Hughes b , James Daniell a , Mark Hemer a , Ole Anderson c a Marine and Coastal Environment Group, Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia b School of Geosciences, University of Sydney, Sydney NSW 2006, Australia c Kort and Matrikelstyrelsen, Geodetic Division, Rentemestervej 8, DK-2400 Copenhagen, Denmark Received 23 November 2004; received in revised form 19 May 2005; accepted 20 June 2005 Abstract The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia’s Great Barrier Reef, adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than 90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a 120 m, eustatic sea level low-stand, as what occurred on this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position. Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land- bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley. Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal- incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the 0025-3227/$ - see front matter. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2005.06.019 * Corresponding author. E-mail address: [email protected] (P.T. Harris). Marine Geology 220 (2005) 181 – 204 www.elsevier.com/locate/margeo

Transcript of An example from the northern Great Barrier Reef, Australia

Page 1: An example from the northern Great Barrier Reef, Australia

www.elsevier.com/locate/margeo

Marine Geology 220

Tidally incised valleys on tropical carbonate shelves: An example

from the northern Great Barrier Reef, Australia

Peter T. Harris a,*, Andrew Heap a, Vicki Passlow a, Michael Hughes b,

James Daniell a, Mark Hemer a, Ole Anderson c

aMarine and Coastal Environment Group, Geoscience Australia, GPO Box 378, Canberra ACT 2601, AustraliabSchool of Geosciences, University of Sydney, Sydney NSW 2006, Australia

cKort and Matrikelstyrelsen, Geodetic Division, Rentemestervej 8, DK-2400 Copenhagen, Denmark

Received 23 November 2004; received in revised form 19 May 2005; accepted 20 June 2005

Abstract

The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level

conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia’s Great Barrier Reef,

adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than

90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the

maximum depth of fluvial incision that could have occurred under a �120 m, eustatic sea level low-stand, as what occurred on

this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour,

probably in relation to a sea level at around 30–50 m below its present position.

Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are

mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict

fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated

through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest

segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-

bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the

deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are

consistent with a Pleistocene age and relict origin of the valley.

Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal

erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic

pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and

channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal-

incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the

0025-3227/$ - s

doi:10.1016/j.m

* Correspondi

E-mail addre

(2005) 181–204

ee front matter. Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved.

argeo.2005.06.019

ng author.

ss: [email protected] (P.T. Harris).

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P.T. Harris et al. / Marine Geology 220 (2005) 181–204182

adjacent ocean basins during intermediate (rather than last glacial maximum) low-stand, sea level positions. The model may

apply to other rimmed shelves, both modern and ancient.

Crown Copyright D 2005 Published by Elsevier B.V. All rights reserved.

Keywords: Great Barrier Reef; Gulf of Papua; tidal currents; continental shelf; submarine valleys; sea level change; Quaternary

1. Introduction

The creation of continental shelf submarine val-

leys and the accommodation space of most transgres-

sive estuarine systems is generally attributed to the

erosion and incision of the shelf by rivers (or by

glaciers on high latitude shelves) under low sea level

conditions (e.g., Dalrymple et al., 1994). However,

in some cases, shelf valleys are formed by processes

unrelated to rivers. Examples include valleys cut by

shelf-edge slumps and mass flows (e.g., Jansen et al.,

1987) and those eroded by the action of strong tidal

currents (Dalrymple et al., 1994). Valleys cut by

shelf-edge slumps and mass flows are generally con-

fined to the outer shelf and upper slope, although the

heads of some submarine canyons may extend a

considerable distance landward. Examples are the

Scripps and Monterey Canyons in California (She-

pard, 1963) and the bNo GroundQ Channel incised

into the Ganges–Brahmaputra Delta (Kuehl et al.,

1997). These valleys generally become both deeper

and wider in a seaward direction.

Valleys formed purely by tidal erosion processes

are not well documented in the literature, presumably

because the initial valley is usually inferred to have

formed by fluvial or glacial processes during low sea

level stands, followed by further erosion by tidal

currents under transgressive, high-stand conditions.

For example, in his review of the seabed morphology

of the tidally dominated shelf around Great Britain,

Pantin (1991) attributed tidal current scour to the

origin of only a few shelf valleys, such as St. Cathe-

rine Deep in the English Channel. Similarly, most

submarine valleys on the macrotidal Korean shelf

are heterogeneous, with tidal current scour overprint-

ing features that were at least partially formed by

glacial or fluvial processes during the Quaternary

glaciations (Chough et al., 2000, 2002). Tidally

scoured sections of such shelf valleys generally exhi-

bit local depressions with closed bathymetric contours

(see also Harris et al., 1995).

In the case of many continental shelves, signifi-

cant portions of the valleys formed during low sea

level were infilled with transgressive sediment as

sea level rose (e.g., Dalrymple et al., 1994). On

the northeast Australian continental shelf, a combi-

nation of factors appears to have conspired to limit

the overall effect of low sea level fluvial erosion.

These factors include (1) the mostly arid climate

(and thus, low river discharge) that has persisted in

Australia throughout the Quaternary; (2) the conti-

nent’s relatively low relief and, thus, low river

energy (e.g., Woolfe et al., 1998); and (3) although

alpine glaciers formed in southeast Australia and

Tasmania during the Pleistocene ice ages (Williams,

1984), they did not extend down to sea level and

glaciers have not formed in NE Australia at any

time during the Quaternary. Consequently, shelf val-

leys formed by tidal current scour are more easily

recognised on the Australian shelf (because we can

discount glacial processes) and a number are found

where tidal ranges are meso- to macro-tidal (2–4 m

and N4 m, respectively). Examples are the tidally

scoured deeps formed between offshore islands and

the coast within Banks Strait off northeastern Tas-

mania and Van Dieman’s Gulf in the Northern

Territory (Harris, 1994a).

In the Torres Strait region off northeastern Aus-

tralia, the occurrence of tidally scoured valleys was

described by Harris (1994b, 2001). In particular, the

erosion and formation of the deeply incised section

of Missionary Passage (up to 60 m deep locally;

Fig. 1A) was attributed to modern tidal current

scour (Harris, 2001). Further to the east, along the

southern part of the Gulf of Papua, Harris et al.

(1996) describe an bincised valley zoneQ that exhi-

bits a complex, offshore–onshore trending seafloor

topography on the middle to outer shelf between 50

m and 120 m water depth (Fig. 1B). The data

available at that time indicated the presence of a

well-developed shelf valley southeast of Bramble

Cay (Fig. 1A), along the northern margin of the

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P.T. Harris et al. / Marine Geology 220 (2005) 181–204 183

Great Barrier Reef (GBR), forming a system extend-

ing bover a 100 km east–west distance between the

60 m and 120 m isobathsQ (Harris et al., 1996, p.

802). In that study, the shelf valleys were attributed

largely to low sea level fluvial incision and the

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Fig. 1. (A) Bathymetric contour map of the Gulf of Papua, Torres Strait an

data provided by the CSIRO Division of Marine Research and the Royal A

in Fig. 2B is indicated. (B) Enlarged section of (A) showing the locations of

Valley. Shelf valleys are highlighted by dark grey shading; reefs are shad

vertical bars. The positions of Station 21 and 22, listed in Table 2, are sho

bathymetry data as in (A), but with superimposed reef outlines extracted fro

and bBQ are the locations of study areas shown in Figs. 2, 5 and 6). Locatio

Bramble Cay (BC).

deposition of regressive, fluvial–deltaic complexes

during the late Quaternary. Tidal current scour was

thought to have played a secondary role, by over-

deepening some sections of the shelf valleys and

reworking older deposits.

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Australia

Holocene DeltaReefShelf valley

East Cay

1B

d northern Great Barrier Reef based on a new compilation of digital

ustralian Navy Hydrographic Office. The location of the area shown

the Great North East Channel (GNEC), Darnley Valley and Bramble

ed light grey and the area less than 30 m in depth is indicated by

wn. (C) Three-dimensional, shaded relief image showing the same

m LANDSAT images of the area. The dashed-line boxes marked bAQns shown include Darnley Island (DI), Missionary Passage (MI) and

Page 4: An example from the northern Great Barrier Reef, Australia

Fig. 1 (continued).

P.T. Harris et al. / Marine Geology 220 (2005) 181–204184

Here we present new detailed multibeam sonar data

to show that one shelf valley located along the northern

edge of the GBR (southern Gulf of Papua) forms a

complex network, extending at least 93 km across the

shelf that contains isolated depressions that are as much

as 220 m deep locally (Fig. 1A and B). Our new data

indicate that large sections of the valleys were eroded

during the late Quaternary by tidal currents and inmany

cases, appear never to have contained significant river

flows. The existence of these valleys raises important

questions regarding the creation of transgressive

estuarine embayments in such carbonate provinces, as

well as the interaction and coexistence of the large

Papua New Guinea rivers with the northern Great

Barrier Reef throughout the late Quaternary. The ques-

tion we pose is: what factors have controlled the for-

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P.T. Harris et al. / Marine Geology 220 (2005) 181–204 185

mation of over-deepened (N200 m deep) shelf valleys

in the northern Great Barrier Reef?

1.1. Study area: the Gulf of Papua and northern Great

Barrier Reef

The Gulf of Papua continental shelf forms a low-

gradient platform typically 50–90 m deep and can be

divided into four geomorphic zones: (1) a low-relief,

inner-shelf, Holocene deltaic zone; (2) a high-relief,

mid- to outer-shelf incised valley zone; (3) a high-

relief, southern reef zone; and (4) a moderate to low-

relief mid- to outer shelf zone in the east and northeast

(Harris et al., 1996). The deltaic zone is an extensive,

flat, shallow surface in 5–30 m water depth (Fig. 1A).

It is bordered by a relatively steep prodelta region in

20–50 m of water that extends along the coast

between the tide-dominated Fly and wave-dominated

Purari River mouths (Fig. 1A; Harris et al., 1993).

Seismic profiles and radiometrically dated core sam-

ples suggest that the Fly Delta is prograding seawards

at an average rate of about 6 m/a (Harris et al., 1993).

Progradation is indicated also by the occurrence of

beach ridges aligned parallel to the coastline (Baker,

1999) and by seismic data that show prodelta muds

downlapping progradationally onto the modern shelf

carbonates that mark the maximum flooding surface

(Harris et al., 1993, 1996; Walsh and Nittrouer, 2003).

Collectively, the Gulf rivers deliver approximately

350 million tonnes/year of sediment and discharge

approximately 13,000 m3/s of water into the coastal

environment (Harris et al., 1996). The Gulf coastline

is dominated by mangrove-forested deltaic islands

having an onshore–offshore orientation (Fig. 1A)

that reflects the influence of tidal processes.

The high-relief, mid- to outer-shelf incised valley

zone has a complex, offshore–onshore-trending sea-

floor topography on the middle to outer shelf between

50 m and 100 m water depth. Two separate, large

shelf-valleys are recognised in this paper; the northern

valley is called the bBramble Valley,Q named for

Bramble Cay located nearby (Fig. 1B). The southern

valley is called the bDarnley ValleyQ in this paper

(named for Darnley Island, located nearby; Fig. 1B).

Available bathymetric data suggest that the Darnley

Valley extends east–west for about 93 km across the

northern limit of the Great Barrier Reef before turning

south-westwards where it extends at least a further 20

km into the Great North East Channel (Fig. 1A–C).

Details of the morphology, seismic character and for-

mative processes of these two valleys, based on newly

collected data as part of this study, are presented and

discussed further below.

The high-relief, southern reef zone is a complex of

barrier and patch reefs in the southern part of the study

area, south of 9830V. The bathymetry is rugged near

steep-sided coral reefs which locally may have vertical

sides. Between the reefs, depths are mostly in the range

of 30–70 m, with some broad shallows less than 20 m

and local depressions up to 100 m deep. The available

bathymetry data compiled for this study shows a branch

of the Darnley Valley extends southwards into the reef

complex east of Darnley Island (Fig. 1B). Water depths

increase rapidly to the east of the barrier reef, which is

located on the shelf margin of the Coral Sea basin (Fig.

1A) in water 120–140 m deep.

The moderate- to low-relief, mid- to outer-shelf

zone lies north of the incised valleys and offshore

from the deltaic zone. Closely spaced, 20–50-m iso-

baths, trending parallel to the coast, delimit the steep

prograding edge of the Holocene deltaic sediments and

contrast with the onshore–offshore trending ridges and

swales in 50–80-m depth. These ridges are flat-topped

and have been described as bmesaQ-like in profile

(Harris et al., 1993). Based on their morphology, seis-

mic character and core samples, including one radio-

carbon date of 33,850 years BP taken from deltaic-

marine sediments (massively bedded gray mud with

scattered shell debris throughout) on the top of one of

these banks, they are interpreted to be relict Pleistocene

deltaic deposits (Fig. 1A). Generally, this zone forms a

low-relief plain, gently dipping towards the shelf break

which is in about 140 m of water.

2. Methods

Bathymetry, seabed sediment samples, seismic,

water column and seabed video data were collected

using the Australian National Research Facility, R/V

Franklin during a cruise carried out in January–Feb-

ruary, 2002 (Geoscience Australia Survey 234,

National Facility Cruise 01/02; see Harris et al.,

2002). We used a Reson model 8101, 240 kHz swath

system for multibeam sonar mapping together with a

Datasonics 3–7 kHz Chirp Sub-bottom profiler (model

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P.T. Harris et al. / Marine Geology 220 (2005) 181–204186

no. DSP 661/66) and model TTV170S tow fish. Survey

data were collected along evenly spaced track lines,

over a total track length of around 1400 km spread

between two survey areas on the middle and outer shelf

(Fig. 1). Navigation was by differential global position-

ing system (DGPS).

Sampling stations occupied during the cruise

included the collection of water column profile data

using a Seabird SBE911 CTD, calibrated by surface

and bottom water samples. Seabed sediment samples

were collected using a Smith Macintyre grab and a 1-

tonne gravity corer, also rigged to work as a piston

corer. An underwater video camera was also deployed

at each station to document the substrate character and

benthic biota.

Fig. 2. Hill-shaded relief images based on a 10-m grid of multibeam sonar d

P3) and submerged reefs; and (B) outer-shelf submarine valleys, platforms

of the images.

Surface sediment grab samples were analysed for

percentage gravel, sand and mud content by the sieve

method, using nested 2-mm and 63-Am analytical

sieves. Carbonate content was determined on the

dried gravel sand and mud fractions separately using

a carbonate bomb (Muller and Gastner, 1971).

3. Results

3.1. Acoustic survey data

3.1.1. Survey Area A (middle shelf)

The survey in the middle-shelf area confirmed the

presence of the Darnley Valley submarine valley sys-

ata showing (A) mid-shelf submarine valleys, platforms (P1, P2 and

(P4 and P5) and submerged reefs. See Fig. 1B and C for the locations

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P.T. Harris et al. / Marine Geology 220 (2005) 181–204 187

tem, trending east–west across the shelf (Figs. 1A–C,

2A and B), as described by Harris et al. (1996).

Multibeam survey results collected in this study

show that the Darnley Valley comprises two separate

limbs, which branch around an unnamed rocky pin-

nacle reef, located in the west-central part of survey

area A (Fig. 2A). The Darnley Valley is as deep as 130

m below sea level south of the pinnacle reef, and shoal

reefs were found in the southernmost section of the

survey area, reaching to within 9 m of the sea surface.

Smaller, low-relief (F10 m) gullies drain into the

Darnley Valley from shallow platforms located

along the valley margins (Fig. 2A).

Fig. 3. Chirper-seismic sections (A–D). The locations of the sections, inclu

Seismic sections show (A) a profile of sediment onlapping onto the souther

reef (Fig. 2A), incised valley floor with sediment wedges (inset A2) and in

sub-horizontally bedded sediment body of Platform P3, exhibiting lateral ac

B1), and an undulating, irregular surface of Platform P2, containing rocky

reef surface adjacent to East Cay, with small sediment mounds in local d

mounds (Inset C2), and reef-studded surface of Platform 4; and (D) low-r

(Inset D1), smooth, subhorizontally bedded sediment body (Inset D2), w

Platform 5.

The seismic survey data collected demonstrates

that the floor of the Darnley Valley is commonly

featureless and acoustically opaque. The two limbs

of the Darnley Valley separate three elevated plat-

forms (P1, P2 and P3, Fig. 2A) each having its own

distinct assemblage of acoustic features (Fig. 3A). The

southernmost platform (P1) is dominated by rocky

(coral) reefs that locally rise up towards the sea sur-

face (Fig. 3A). The largest of these reefs (having the

greatest bathymetric relief) is located adjacent to the

southern walls of the Darnley Valley. Platform P1 also

exhibits localised, infilled channels, erosional notches

and incisions (Fig. 3A). The seabed is acoustically

ding start/stop points of section parts, are shown in Fig. 2A and B.

n side of platform P2 (inset A1), rocky reefs adjacent to the unnamed

cised surface of Platform P1; (B) a profile starting over the smooth,

cretion surfaces and possible slumping on its southern margin (Inset

-reef outcrops (Inset B2) and local incisions; (C) high-relief, rocky-

epressions (Inset C1), incised valley floor with localized sediment

elief valley floor in ~140 m depth with sediment-filled depressions

hich crop out on its southern margin, and reef-studded surface of

Page 8: An example from the northern Great Barrier Reef, Australia

Fig. 3 (continued).

P.T. Harris et al. / Marine Geology 220 (2005) 181–204188

highly reflective and there is no evidence of unconso-

lidated sediment deposits associated with platform P1.

The central platform (P2) includes a large (N2

km across), unnamed patch reef on its western

margin (Fig. 2A). The elevated, middle part of plat-

form P2 exhibits widespread erosional notches and

incisions (Fig. 3A and B), with some rocky reefs

along the eastern part of the surveyed area. Subsur-

face reflectors, in the form of lateral accretion sur-

faces, occur on both sides of platform P2 and

suggest sediment deposition associated with the lat-

eral migration of former river channels or deltaic

distributary channels. These lateral accretion sur-

faces are commonly of low relief, b10 m in overall

amplitude. On the southern flank of platform P2,

foresets dip gently towards the south (Fig. 3, Inset

A1). To the south of the unnamed pinnacle reef

(Fig. 2A), the sediment reaches a maximum thick-

ness of around 20 m above acoustic basement. A

dune field occurs at a depth of between 43 and 70

m on the southern flank of platform P2 (Fig. 2A).

These large-scale, submarine dunes (terminology of

Ashley, 1990) rise about 4–6 m above the level of

surrounding seabed and appear to have their steeper

faces oriented towards the west. They tend to have

sharp crests and no internal stratification was visible

in our seismic records.

The northern platform (P3) exhibits gullies,

notches and incisions, but the most prominent fea-

ture of the northern platform is the widespread

occurrence of subsurface reflectors in the Chirp

seismic data. These are most common in the west

and comprise lateral accretion surfaces, gently dip-

ping towards the south on the southern flank of the

platform (Fig. 3A).

3.1.2. Survey Area B (outer shelf)

On the outer shelf, our survey discovered a seaward

extension of the Darnley submarine valley complex,

which attains a maximum depth of 220 m in the area we

surveyed (Fig. 2B). This is the deepest shelf valley yet

found on the NE Australian shelf. The deepest sections

of the Darnley Valley floor form a series of isolated

depressions having closed contours (Fig. 2B). The

valley is flanked on its southern and eastern margins

by rough, rocky-reef seabed that is acoustically opaque,

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Fig. 3 (continued).

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 189

apart from isolated sediment lenses up to about 10 m in

thickness (Fig. 3C and D).

On the northern side of East Cay is another shelf

valley, called the bBramble ValleyQ in this paper.

Available bathymetric data suggests that the Bramble

Valley extends westwards to the north of Bramble Cay

and as far landwards as the prograding edge of the Fly

River Delta. In the seaward part of the Bramble Valley

mapped in our study (Fig. 2B) seismic sections reveal

only isolated sediment lenses, b5 m in thickness (Fig.

3D). The Bramble Valley attains a maximum depth of

around 120 m north of East Cay (Fig. 2B) and sea-

wards of this it shoals to b100 m and becomes indis-

tinct, where it merges with a broad shelf area, mantled

with large-scale, submarine dunes (terminology of

Ashley, 1990). The dunes rise about 4–6 m above

the level of surrounding seabed and appear to have

their steep faces oriented towards the southeast. They

tend to have well-rounded crests and no internal stra-

tification was visible in our chirp seismic records.

The major geomorphic features of the outer shelf

are submerged platform reefs flanking the Darnley

Valley (P4) and extending eastwards from East Cay

(P5; Fig. 2B). The largest platform (P5) is cut by two

channels and exhibits a sharp eastern face, aligned

north–south over a distance of about 15 km. The

surface of both platforms P4 and P5 is characterised

by a strong acoustic reflector, suggesting a hard, rocky

seabed (Fig. 3C and D).

A hypsometric analysis of the bathymetry data for

the two survey areas (Fig. 2A and B) indicates that the

platforms (P1–P5) are consistently located at a depth

of around 45–55 m below sea level (Fig. 4). Peaks in

the hypsometric curves, corresponding with the broad

platform surfaces, can be correlated between the two

survey areas. Other peaks correspond with the rela-

tively flat and broad valley floors at around 85 m

depth in the mid-shelf area (Figs. 2A and 4) and

with broad outer shelf areas where dune fields occur

at a depth of around 95 m (Figs. 2B and 4).

Page 10: An example from the northern Great Barrier Reef, Australia

Fig. 3 (continued).

00 50 100 150 200 250

Water Depth (m)

Cum

ulta

ive

surf

ace

area

(km

2 )

8000

6000

4000

2000

P1

P2, P3

P4, P5

Broad shelf area(dunes)

Valley floor

Mid-shelf areaOuter-shelf area

Fig. 4. Hypsometric curves for the twomutibeam survey areas shown in

Fig. 2A andB. The depths of submerged platforms P1 to P5 and broad

shelf and valley floor areas are identified as peaks in the two curves.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204190

3.2. Sediment sample and video-observation data

Regional surface sediment distribution maps were

published by Harris et al. (1996). The information

collected in our study includes underwater video

observations of the seabed, which provides details of

the surface sediments found on the deep valley floors

and some insight into the modern sedimentary pro-

cesses in this setting.

Darnley Valley floor sediments from the middle

shelf (Area A) are relatively muddy having a mean

mud content of 20.7% (Stations 32–36, 42–45), by

comparison with sediments deposited on the ele-

vated platforms where samples contain an average

of 9.2% mud (Fig. 5A). This higher mud content is

reflected in the underwater video observations

(Table 1), in which the valley floor is generally

observed to be muddy, flat and featureless with

sparse biota (i.e., little if any biota was observed

Page 11: An example from the northern Great Barrier Reef, Australia

30'

24'

7

52

R3

3

34

9

6

13

6

9

22

11

7

10

14

9

116

18

9

R

54

55

56 57

58

59

60

61

62

63

64

65

66

67

68

69

70

7273

74

4

75

R32'

25'

26'

9°22'S

18' 144° 22' E20'16

80

80

80

6060

60

60

60

120

120

120

120

100

100

100

100

100

160

160

140

180

143° 57' 144° 00'

30'

24'

3

9

10

2612

17

20

22 2

2

13 17

1422

20

2018

11

5

7

29

% Mud content>20%

10-20

<10%

58' 59' 01' 02' E

23'

9° 22' S

25'

26' 29

30

31

3233

34

35

36 37

38

39 17

4142

43

4445

46

47

48

28

60

80

80

80

60

60

60 806040

100100 120

120

80

60

60

A

B

71

Fig. 5. Maps showing distribution of surficial sediment mud content (percentage dry weight) for (A) the mid-shelf and (B) outer shelf study

areas. The station numbers shown correspond with information listed in Table 1. See Fig. 1 for the locations of the maps.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 191

Page 12: An example from the northern Great Barrier Reef, Australia

Table 1

Presence/absence (�=present) of molluscs, crustacea, polychaetes, fish, sponges, corals, echinoids, soft corals, Halimeda, burrows and ripple

marks observed at underwater video camera stations

Station Water

depth

Molluscs Crustacea Polychaetes Fish Sponges Corals Echinoids Soft

coral

Halimeda Burrows Ripple

marks

Middle shelf

28 55 � � � � � �29 46 � � � � �30 54 � � � �31 52 � � � � �32 131.5 �33 50.5 � � �34 90 � � � �35 103 � � � �36 85.5 �37 50 � � �38 55.5 �39 60

40 57.5 � � � � �41 59.5 � � � �42 80.5 � �43 80 � � � � � �44 87 � � � �45 91.5 � � � � � �46 63.5 � � � �47 57.5 � �48 57 � � �

Outer shelf

54 119 � � � � � �55 64.5 � � � � � � �56 29 � � � �57 50 � � �58 104.5 � � � �59 76 � �60 96 � � � �61 92 � � �62 103.5 � � �63 90.5 � � �64 117 � � �65 52.5 � � �66 82.5 � � � � �67 79 � �68 89 � � � �69 109 � � � �70 50.5 � � �72 171.5 � � �73 116 � � �74 90 � � � � �75 220 � � � �For locations of stations, see Fig. 5A (middle shelf stations) and B (outer shelf stations).

P.T. Harris et al. / Marine Geology 220 (2005) 181–204192

at stations in greater than 80 m water depth; Table

1). In contrast, the platforms contain a greater diver-

sity of substrate types, including coral bommies,

limestone rubble and sandy sediments with ripple

marks. Carbonate content did not vary between the

valley floor and platform environments; valley floor

Page 13: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 193

stations (n =8) contain 62.1% carbonate mud and

have a 59.6% total carbonate content, which is

similar to platform stations (n =13) which contain

64.2% carbonate mud and have a 61.2% total car-

bonate content.

Outer shelf (Area B) sediments are generally

more coarse (only 2 out of 20 stations contained

more than 20% mud) and generally higher in car-

bonate mud content than the middle shelf sediments

(Fig. 5A and B). Outer shelf sediment samples

taken from the Darnley Valley floor (54, 58, 72–

75; Fig. 5B) have a mean gravel content of 22.1%,

Un-namedreef

N144° 00'

143° 58'

143° 56'

9° 25'

9° 30'

9° 20'

P2

5

25

50

75

100

125Depth (m)

3 km

C14

C14

C14

51

52

28

53

50

42 41

49

43

48

44

47

Fig. 6. Map showing the location of core samples taken on the middle she

and transgressive fluviodeltaic facies. Inset: core X-radiograph (positive

sediments located on the middle shelf in core from station 50.

which is higher than all other areas sampled in this

study. For example, surface sediments at Station 75

in 220 m water depth comprised gravelly (25%

gravel) calcareous (81% carbonate) coarse olive

sand (5Y 5/3) with abundant granule-pebble shell

hash. The sample included a live sponge, hydroid,

crab, mysid shrimp and polychaete worms (Table 1).

Underwater video at this station showed the seafloor

was relatively flat with rare burrows and pits ~5 cm

in diameter, and sparse biota, apart from some small

rocky outcrops where soft corals, hydroids and

sponges were seen (Table 1).

50 cm

55

60

65

X-ray of Core 50

0

1

2m

144° 02'

Transgressiveterrigenous

mud

HoloceneCarbonate

45

46

lf, and illustrating the occurrence of the Holocene high-stand facies

) showing laminated sediments comprising the transgressive-aged

Page 14: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204194

The other main difference between outer shelf and

middle shelf surface sediments is that the mean car-

bonate mud content is 82.6F3.9% (n =22) on the

outer shelf, compared with 63.4F13.8% (n =21) on

the middle shelf. This difference probably reflects the

greater distance between the major terrigenous sedi-

ment source (i.e., The Fly River) between the outer

shelf area, compared with the middle shelf area.

3.3. Sediment cores and radiocarbon dates

Sediment cores from the middle shelf contained a

0.5–1-m-thick bed of massive, Holocene, carbonate,

Table 2

Radiocarbon ages of peat samples recovered in the Gulf of Papua on R/

Radiocarbon

age (years BP)

Error

(Fyears BP)

Calendar age

(years BP)

8684 60 9711–9547

8969 50 10,204–10,143

8902 60 9711–9547

8697 55 10,204–10,143

11,733 60 13,870–13,487

10,399 60 12,385–12,091

8747 50 9827–9624

8778 50 9909–9692

8687 55 9710–9549

8921 50 10,080–9919

8878 55 10,153–9899

8597 60 9556–9528

9421 50 10,703–10,566

9024 60 10,226–10,174

10,216 55 12,129–11,719

10,785 55 12,940–12,819

11,050 60 13,135–12,925

11,117 55 13,163–13,008

10,865 65 12,983–12,856

10,992 55 13,105–12,899

10,898 55 13,027–12,872

10,625 70 12,888–12,628

13,430 530 16,113–14,970

11,070 330 12,940–11,953

7290 350 8099–7434

16,750 340 19,864–18,893

9280 90 10,150–9811

9280 100 10,148–9814

8600 29 9079–8948

Laboratory reference numbers refer to the University of Sydney (SUA) and

ages are uncorrected in years before present (BP). The uncorrected radioc

ages using the INTCAL-98 calibration curve (Stuiver et al., 1998; see Fig. 7

1C (labelled with underlined station numbers only, i.e., 21 and 22, respe

49GC10 are shown in Fig. 6 (labelled with underlined station numbers o

poorly sorted, muddy gravelly sand sediment which

overlies a fine-grained, terrigenous, laminated to

finely bedded fluvial–deltaic facies. Laminations

and thin beds (1–5 cm thick) of fine sand occur

within the terrigenous mud and are clearly observed

in X-ray photographs (Fig. 6). Radiocarbon dates

from peat layers extracted from within the fine-

grained, laminated to bedded, terrigenous sediment

facies (Table 2) are from around 8000 to about

19,000 years old, which indicates they were depos-

ited mostly during the post-glacial sea level trans-

gression of the Gulf of Papua shelf. The corrected

peat dates are consistent with the published sea

V Franklin Survey 234 (2002) and FR93 (after Harris et al., 1996)

Water depth

(m)

Core number Lab reference

number

35 234/22PC06 NZA16780

35 234/22 PC06 NZA16781

35 234/22PC06 NZA16782

35 234/22PC06 NZA16783

81 234/43GC04 NZA16784

80 234/49GC10 NZA16785

30 234/21PC05 NZA16832

30 234/21PC05 NZA16833

30 234/21PC05 NZA16834

30 234/21PC05 NZA16835

30 234.21PC05 NZA16836

30 234/21PC05 NZA16837

30 234/21PC05 NZA16838

30 234/21PC05 NZA16839

59 234/41GC02 NZA16840

59 234/41GC02 NZA16841

59 234/41GC02 NZA16842

59 234/41GC02 NZA16843

59 234/41GC02 NZA16844

59 234/41GC02 NZA16845

59 234/41GC02 NZA16846

59 234/41GC02 NZA16847

77 FR93/15GC4 SUA 3070

47 FR93/24PC5 SUA 3072

64 FR93/38PC12 SUA 3073

93 FR93/54PC20 SUA 3074

40 FR93/13VC8 SUA 2837

40 FR93/13VC8 SUA 2838

29 FR93/20VC15 SUA 3075

Rafter Radiocarbon Laboratory (NZA), New Zealand. Radiocarbon

arbon ages were converted to upper and lower estimates of calendar

). The locations of cores 234/21PC05 234/22PC06 are shown in Fig.

ctively) and locations of cores 234/41GC02, 234/43GC04 and 234/

nly, i.e., 41, 43 and 49, respectively).

Page 15: An example from the northern Great Barrier Reef, Australia

20

0

-20

-40

-60

-80

-100

-120

14 12 10 8 6 4 2 0-140

Corrected calendar age kyr (BP)

Sea

leve

l (m

)

Peat (error bars), this paperShell data from Larcombe et al. (1995)

Depth of highest tidal energy

Tim

e of

hig

hest

tid

al e

nerg

y

Fig. 7. Plot of radiocarbon ages determined from mangrove peat samples in relation to water depth for samples taken in the Gulf of Papua in the

present study plus those reported by Harris et al. (1996). Dates for shell samples are from Larcombe et al. (1995). The sea level curve shown is

from Edwards et al. (1993). The uncorrected radiocarbon ages listed in Table 2 were converted to upper and lower estimates of calendar ages

using the INTCAL-98 calibration curve (Stuiver et al., 1998), prior to plotting on this graph. The timing of meltwater pulses is from Bard et al.

(1996). The hypsometric curve for the Gulf of Papua illustrates the mean depths of the modern coastal plain (CP), Holocene deltas (D) and delta

front (DF) systems and plateaus (PL; probably Pleistocene age; Harris et al., 1996) located on the mid-shelf. During the post-glacial

transgression, shoreface retreat would have been rapid over the broad, low-relief plateaus, limiting the preservation potential for coastal

mangrove peat deposits.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 195

level curves of other workers (e.g., Edwards et al.,

1993; Bard et al., 1996; Fig. 7).

4. Discussion

4.1. Origins of shelf valleys

The Darnley and Bramble Valleys mapped in this

study appear to have been formed by at least two

different processes operating over glacial–interglacial

timescales. The Bramble Valley is less than 120 m in

depth over its entire length. Lateral accretion surfaces

are observed in seismic sections along its walls, which

are documented in sediment cores as transgressive-

aged, terrigenous mud deposits (Harris et al., 1996).

These transgressive-aged deposits are similar in com-

position and primary sedimentary structures to mod-

ern deltaic deposits that have been described from the

nearby Fly River delta (Dalrymple et al., 2003), and

we infer a similar (deltaic) depositional environment

for this facies. Mangrove peat beds are commonly

associated with deltaic islands and intertidal mud

flats in the modern Fly Delta (e.g., Baker et al.,

1995; Baker, 1999; Dalrymple et al., 2003). We con-

clude that the Bramble Valley was probably one of the

main channels (if not the main channel) for the Fly

River during the last glacial maximum, and that it was

formed by fluvial erosion during Quaternary glacial

maxima and modified by fluvial–deltaic sedimenta-

tion during the post-glacial transgression, between

18,000 and 6500 years BP.

The Darnley Valley, located further to the south,

has a more complex morphology and was probably

formed by several different processes operating over

different temporal and spatial scales. Lateral accretion

surfaces are observed in seismic sections which are

also documented in sediment cores as transgressive-

aged, terrigenous mud deposits. Hence, the Darnley

Valley was influenced by fluvial–deltaic depositional

processes during the post-glacial sea level rise. This is

not surprising, given that the modern Gulf of Papua

coast is comprised entirely of mangrove-forested, del-

taic islands that migrate laterally (Harris et al., 1993,

2004; Walsh and Nittrouer, 2003). However, lateral

channel accretion deposits are found along the mar-

Page 16: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204196

gins only of the northern limb of the Darnley Valley

and in patches along the northern wall of the southern

limb. None were found along its southern margin on

the middle shelf (Fig. 2A) or at any location on the

outer shelf (Fig. 2B).

The Darnley Valley is over-deepened locally, to

depths of over 200 m. Erosion of the valley floor to

this depth cannot be explained by fluvial erosion

during late Quaternary, glacial, low sea level periods

even taking into account an inferred subsidence rate of

about 10 cm per 1000 years (Pigram et al., 1989),

because sea level did not drop more than 120–130 m

below its present position. Sedimentary strata under-

lying the Darnley Valley are thought to be comprised

of Cenozoic, interbedded carbonates and fluvio-del-

taic deposits (Pigram et al., 1989) and there is no

seismic profiling evidence for a major basement

fault to explain the position of the valley.

Isolated depressions 150–220 m deep with closed

bathymetric contours at both ends, on the floor of the

Darnley Valley demand an explanation other than

fluvial erosion during glacial maxima. Given their

depth and the closed contours of their morphology,

it is possible that depressions along the length of the

Darnley Valley were the sites of lakes during the last

ice age when Torres Strait formed a land-bridge

between Australia and Papua New Guinea. Given

that the valleys are incised into limestone, karst pro-

cesses may have played a role in forming the depres-

sions. However, there is no indication of former lake

deposits in our cores from the Darnley Valley floor,

nor were any thick, unconsolidated sediment deposits

observed anywhere on the floor of the Darnley Valley

in our seismic records. Karst processes may have

played a role in creating localised depressions in the

study area, perhaps as much as 10–20 m in relief.

However, it seems unlikely that the elongate orienta-

tion of the depressions (that are aligned with the axis

of the valley) or the shear vertical scale of relief

observed for the Darnley Valley could be explained

by Karst processes alone. We conclude that processes

other than (or in addition to) Karst must have formed

the deepest sections of the Darnley Valley.

To account for the absence of lacustrine sediment

deposits within isolated depressions of the Darnley

Valley, we infer that either (1) there were no lakes here

during the ice ages, and hence, no lake deposits exist;

(2) there are lake deposits but they are too thin to be

resolved by our Chirp sub-bottom profiler; or (3) there

were lake deposits but the small volumes of sediment

deposited were exhumed by tidal currents during the

post-glacial sea level rise (and flooding of the shelf).

In the latter case, we might expect to find strong tidal

currents flowing today within the scoured sections of

the valleys. The available current and oceanographic

data, however, indicate that this is not so.

Recent tidal and wind-driven current modelling

work carried out for the region by Hemer et al.

(2004) indicates that tidal and wind-driven currents

over the mid- to outer shelf valleys are relatively

weak. Strong tidal currents do occur in central Torres

Strait and in the 60 m deep valley (Missionary Pas-

sage; Fig. 1) located north of the Warrior Reefs on the

inner shelf (Harris, 2001), but tidal flows are generally

weaker in the eastern patch reef complex. Strong tidal

flows must occur locally to explain the sharp-crested,

active dunes on Platform P2 (Fig. 2A); however, the

large area of dunes in deep water in Area B having

rounded crests (Fig. 2B) that we assume to be mor-

ibund coincides with an area where modeling predicts

weaker currents. Low current energy is also consistent

with the accumulation of muddy sediments on the

Darnley Valley floor on the middle shelf (Fig. 5A).

Oceanographic observations indicate that the Darnley

Valley provides a conduit onto the shelf for cool and

saline, upwelled Coral Sea water (Harris et al., 2004).

Temperature and salinity depth profiles taken within

the valley during the waning spring tide phase, sug-

gest that the water column is stratified above 50 m

depth, although any density-driven circulation is

likely to be sluggish (speeds of b0.1 m s�1). It

appears, therefore, that the modern current regime

could not have created the deep valleys observed on

the mid- to outer shelf. The question is, therefore, how

(and when) were the valleys formed?

4.2. Tidal current modelling of low sea level scenarios

To answer this question, we have carried out a

simple tidal modelling exercise. A brief description

of the tidal model follows; details are documented

elsewhere (Harris et al., 2000; Porter-Smith et al.,

2004). The tidal model has a resolution of 0.0678 in

both latitude and longitude, equal to about 7.4 km.

The bathymetry input was not varied to account for

sediment deposition, reef growth or erosion, which are

Page 17: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 197

considered small within the area of interest (incised

valley zone) in comparison with the model resolution.

The model uses major eight constituents M2, S2, N2,

K2, K1, O1, Q1, P1 and was run for eight different sea

levels at 10-m increments below the present sea level.

The solution was obtained using time stepping on an

Arakawa C grid by applying periodic forcing and time

stepping from homogenous initial conditions. The

solution was achieved in 10,000 time steps using a

step length of 15 s. This corresponds to running the

model for roughly 2 days. Along the open boundaries

around the edges of the model a global ocean tide

model by Andersen et al. (1995; version AG95.1) was

used to provide boundary elevations. The model for-

cing was not varied for the different sea level incre-

ments, which is a potential source of error, but this

must be accepted since we do not have any data to

avoid making the assumption that the tidal forcing

parameters do not vary with changing sea level.

The results (Fig. 8) for the modern sea level are

consistent with available observational data; bottom

stress due to maximum spring tidal currents is gen-

erally low apart from a few localized areas around the

central Darnley Valley (where active dunes also occur;

see above), near Missionary Passage and among the

distributary channels of the Fly Delta. With sea level

at �10 m, the model suggests that bottom stress

maxima occur over the eastern Missionary Passage

area and in small patches around Darnley Island and

other locations north of the island. With sea level at

�20 m, Torres Strait is exposed and the Australian

mainland is joined to Papua New Guinea; the model

suggests that bottom stress maxima increase in overall

intensity and occur in the Great North East Channel,

around the patch reefs adjacent to Darnley Island and

around patch reefs on the outer shelf flanking the

Darnley Valley. With sea level at �30 m, the model

suggests that intense bottom stress maxima occur;

tides are apparently amplified within a broad embay-

ment extending eastwards from Missionary Passage.

Bottom stress bhot spotsQ are also centred over the

Darnley Valley and part of the Bramble Valley. The

pattern is virtually the same for sea level at �40 m,

albeit shifted slightly eastwards. With sea level at �50

m, the Missionary Valley is mostly exposed and the

model suggests that three bottom stress bhot spotsQoccur, centred over the Darnley Valley the Bramble

Valley and an area north of the Bramble Valley. As sea

level is lowered further too �60 and �70 m, the

bottom stress bhot spotsQ become smaller in size as

more shelf area becomes emergent. Bottom stress bhotspotsQ are predicted to occur in different places along

the Darnley Valley at all depths between 30 and 60 m

(see Fig. 8).

In summary, the model (Fig. 8) predicts that max-

imum tidal bottom current stress reached their highest

values over the site of the deepest shelf valleys when

sea level was between 30 and 50 m below its present

position. At these depths, the geomorphology of the

shelf manifests itself as a chain of elongate islands (the

modern barrier reef) separating the open sea from a

shallow, protected lagoon. Currents are accelerated as

they are forced to flow through the constricted channels

between the shelf-edge islands, and in particular as they

flow around the northern promontory of the island

chain. The strongest tidal flows are predicted to occur

at the northern end of the shelf edge barrier (Fig. 8), and

although the 7.4-km resolution of the model is too

coarse to resolve the details of flow over the complex

bathymetry in the area, this general area coincides with

the location of the deepest (N200 m deep) segment of

the Darnley Valley south of East Cay.

The flat-top surfaces of submerged reef platforms

adjacent to the valleys (Fig. 2A and B) also occur in

this 40–50-m depth range, suggesting that these relict

reefs (bgive-up reefsQ using the terminology of Neu-

mann and Macintyre, 1985) developed at times when

sea level was at about 40 m below its present position.

These submerged reefs occur at similar depths to

those reported from other locations in the GBR (Har-

ris and Davies, 1989). When sea level drops below

~40 m, these reefs are emergent and hydraulic tidal

flow is further enhanced.

The overall shape of the Darnley Valley itself

points to processes unrelated to erosion by the Fly

River. It has tributary branches extending southwards

into the northern Great Barrier Reef patch reef pro-

vince, and its western end turns southwestwards, into

the Great North East Channel (and not northwest

towards the Fly River catchment). These features are

more easily explained as being created by tidal water

exchange between the Coral Sea and the protected

lagoon that is formed when sea level drops below

around 30 m (Figs. 1B and 8).

When considering the integrated effect of the

changes in tidal flow associated with different sea

Page 18: An example from the northern Great Barrier Reef, Australia

Fig. 8. Plot of peak (spring) tidal bed stress (N m�2) derived from a tidal model, with outputs derived for sea levels at 0, 10, 20, 30, 40, 50, 60

and 70 m below present position. The locations of modern land (in black) reefs (dark gray), and the 50 m isobath outlines of the Missionary,

Darnley and Bramble Valleys are shown. Locations of Missionary passage (MP), the Great North East Channel (GNEC), Darnley Island (DI),

Bramble Cay (BC) and East Cay (EC) are also shown.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204198

Page 19: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 199

level scenarios, it is important to keep in mind that

the duration of sea level at each point has not been

equal over the last glacial cycle (past 120,000 years;

Fig. 9A and B). The eustatic sea level curve (e.g.,

Chappell and Shackleton, 1986; Fig. 9A) indicates

that over the past 120,000 years, sea level has been

between 30 and 50 m below its present position for

about 38% of the time. Prolonged periods (N10,000

years duration) of sea level within this depth range

occurred during isotope stages 3, 4, 5a and 5c (Fig.

9A). Isotope stage 3, in particular, was a time span

of around 22,000 years during which sea level was

positioned at between 30 and 50 m below present.

It is not just the depth where maximum tidal bed

stress occurs that is important, but also the duration

0 50 100 150

0m

50

100

150

Kyr before present

DepthBelow

PresentSea-level

1 2 3 4 5a b c d e 6Oxygen isotope stageA

B

0

5

10

15

20

25

Per

cent

of t

ime

(last

120

kyr

)

10 20 30 40 50 60 70 80 90 100 110 120 130 140

Depth below present sea level (m)

30-50 m

20-60 m

Fig. 9. (A) Global eustatic sea level curve for the last 150,000 years,

modified from Chappell and Shackleton (1986) to illustrate times of

sea level positioned at between 30 and 50 m below present (grey-

shaded areas). Isotope stages are after Martinson et al. (1987). (B)

Histogram showing percentage of time that sea level has been

within 10-m depth bands (i.e., 0–10 m, 10–20 m, etc.) over that

past 120,000 years (isotope stages 1 to 5d, inclusive), based on the

curve shown in bAQ. The graphs show that sea level was within the

30–50-m depth range for approximately 38% of the time (46,400

years) over the past 120,000 years. For comparison, sea level has

been within the 20–60-m depth range for approximately 60% of the

time (74,500 years) and in the 0–10-m range for only 12.8% of the

time (15,500 years).

of sea level within the 30–50-m depth range (equal

to 38% of the last 120 ky; Fig. 9B) that must be

considered in considering the development of the

shelf’s geomorphology. It is logical that, all things

being equal, current erosion features would form at

depths corresponding with the long-lived and strong

tidal flow conditions related to sea level at 30–50 m

below present. Although it is difficult to accurately

determine the age of any geomorphic feature

formed by erosion, we conclude that the over-dee-

pened Darnley Valley is a relict feature, whose

origin is related mainly to tidal current scour during

the late Pleistocene when sea level was ~30–50 m

below its present position.

An important consequence of these valley-erosion

processes is to halt the southward advance of terri-

genous sediment, supplied to the Gulf of Papua

from the island of New Guinea, into the Great

Barrier Reef province. During interglacial, high sea

level stands, such as at present, fluvial deltaic

deposition is focused along the coast with the pro-

grading delta extending offshore into waters less

than around 40 m deep (Harris et al., 1993). The

Darnley Valley tends to trap only small amounts of

fine-grained terrigenous sediment (Fig. 5A and B).

During intermediate sea level positions (isotope

stages 3, 4, 5a and 5c; Fig. 9) strong tidal flows

scour the valley clear of unconsolidated sediment.

The integrated result, over several glacial/interglacial

sea level cycles, is that the valley system is main-

tained as it is seen today.

4.3. Valley formation on tropical carbonate shelves by

tidal scour and reef growth

The formation of shelf valleys by tidal current

scour and sea level change in the northern Great

Barrier Reef implies a conceptual model for under-

standing the evolution of tidally incised shelf valley

systems (Fig. 10), which can be contrasted with the

conventional model of fluvially incised valley for-

mation. For both types, the changes caused by a rise

or fall of relative sea level are of fundamental impor-

tance. However, whereas a lowering of sea level

results in fluvial erosion, entrenchment and the for-

mation of fluvially incised valleys (a sequence

boundary; e.g., Vail et al., 1977; Van Waggoner et

al., 1990), the formation of a tidally incised valley

Page 20: An example from the northern Great Barrier Reef, Australia

A. Tidal erosionreef growth

sediment export

B. Fluvial erosionshelf-perched

lakes

C. Coastal progradation

shelf depositionreef growth

100km

100km

Fly Rivermeander zone

Valleys

Reefs

Reefs

0 50 100 150Kyr before present

Dep

th B

elow

Pre

sent

Sea

-leve

l

1 2 3 4 5a b d e 6Oxygen isotope stage

0m

50

100

150

c

Erosional unconformityLacustrine sediments

Calcareous marine sediments

Incised older shelf deposits

}

}

}

Erosional unconformity

Fig. 10. Conceptual model showing the evolution of tidally incised shelf valleys and inferred facies succession. (A) bIntermediateQ sea level

conditions dominate the late Quaternary and give rise to valleys that are tidally incised into broad platform reefs on rimmed, tropical carbonate

shelves. The occurrence of a brimmedQ outer shelf establishes a tidally varying hydraulic pressure gradient between the shelf-lagoon and the

adjacent ocean basin, which reaches a maximum tidal energy under certain sea level conditions. Tidally incised channels may be further

increased in relief by framework coral growth along the valley margins, akin to levy banks, through the late Quaternary to produce steep-sided

(locally vertical) valley walls. Tidal erosion liberates shelf sediment for export to the adjacent slope and forms a regional erosional

unconformity. (B) During glacial, low sea level conditions, the shelf valleys may be transformed into lakes perched on the outer shelf, trapping

sediments and limiting sediment export to the adjacent slope. Post-glacial sea level transgression of the shelf results in erosion and localized

exhumation of the valleys. (C) High-stand valleys (e.g., Fig. 1) contain weak currents and may be locally depositional, containing muddy

calcareous marine sediments. Elsewhere, deposits of relict carbonate gravels dominate valley floor deposits.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204200

relies on the occurrence of a brimmedQ outer shelf toestablish a hydraulic pressure gradient between the

shelf-lagoon and the adjacent ocean basin (Fig. 10).

A key factor is the relative tidal range, which must

be large enough to generate the strong tidal flows

necessary to erode the seabed and form the shelf

valleys. The tidal flows attain a maximum value

when the largest hydraulic pressure gradient is forced

through the smallest channel cross section, or as in

the case of the Darnley Valley, around the end of a

promontory (Fig. 10).

The generalised stratigraphic succession produced

by the transgression of tidally incised shelf valleys

will also differ from the conventional, fluvially

incised valley-fill model. The facies succession of

an infilled estuary, in relation to a rise and stabilisation

of relative sea level, consists of an erosional uncon-

formity overlain by alluvial channel deposits (low-

stand systems tract, LST) followed by estuarine/mar-

ine units (transgressive systems tract, TST), which

vary in character depending upon sediment supply

and the relative influence of tides and waves (Dal-

rymple et al., 1994). High-stand systems tracts (HST)

marine sediments drape over the TST deposits, com-

pleting the succession.

The facies succession within tidally incised shelf

valleys mapped in the present study includes an

erosional unconformity that is not necessarily over-

Page 21: An example from the northern Great Barrier Reef, Australia

Fig. 11. False colour bathymetyric map of the Timor Sea region o

northern Australia showing shelf valleys (tidally?) incised into an

800-km-long, shelf-edge, raised carbonate rim.

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 201

lain by any unconsolidated deposits. LST deposits

may contain lacustrine facies (NB the valleys in our

study were scoured clear of any such deposits). The

sequence boundary may be overlain by thin beds of

coarse-grained, transgressive marine sediments,

grading upward into lower energy, muddy gravelly

sands (HST marine sediments; Fig. 10). A likely

scenario is that, as infilling of the lagoon leads to

reduced tidal flows in the valleys (due to the dimin-

ishing size of the tidal prism), the preservation

potential of LST lacustrine and TST marine deposits

is enhanced.

A common feature of the channels in the north-

ern Great Barrier Reef is the growth of coral reefs,

like river levy banks, on the margins of channels.

Current control over reef growth and reef morphol-

ogy is well established in the literature. For exam-

ple, the modern barrier reef in the study area was

described by Veron (1978) who noted that some

shelf-edge, barrier reefs appear to have a bdeltaicQshape in plan view (this term was first used to

describe a specific reef type by Maxwell, 1968).

Strong tidal flows up to 3.8 m/s characterize the

narrow, inter-reef channels, which are about 33 m in

water depth. Veron (1978) explained the deltaic

morphology as a reef-growth response to the strong

tidal flows characteristic of the Torres Strait region.

The elongate reefs in central Torres Strait have the

appearance of tidal current sand banks in plan view,

as first noted by Off (1963) and later studied by

Jones (1995) using seismic profiling.

Reefs growing preferentially along the margins

of tidally scoured channels act as a positive feed-

back, by further constricting the channel width and

accelerating the tidal flow. The vertical scour and

over-deepening of channels by tidal currents reaches

a maximum in the most narrow part of the channel,

at the point of greatest flow constriction, and tidal

energy drops away along the channel axis in both a

landward and seaward direction, which explains the

closed bathymetric contours of tidally scoured chan-

nels. Missionary Passage at the northern end of the

Warrior Reefs (Fig. 1) is a modern example of a

tidally scoured channel that exhibits these morpho-

logical features. Tectonic subsidence of the Gulf of

Papua foreland basin at rates of around 10 cm per

1000 years (Pigram et al., 1989) may also be a

factor for the evolution of channels.

Our model of tidally incised valley formation

may be applied to other rimmed shelves in Australia

and elsewhere. In particular, the modern valleys

adjacent to the Sahul Shoals in the Timor Sea

(e.g., the Malita Shelf valley) have a similar geo-

morphology, exhibiting closed isobaths and a sig-

nificant basin (Bonaparte Depression) located

landward of the valley complex (Lavering, 1993;

Fig. 11). These incised channels have closed bathy-

metric contours, contain silty clays and molluscan

debris (Lavering, 1993) and currents measured in

the channels are mainly weak and variable (Marshall

et al., 1994); it may be concluded that they were not

created by modern processes. Further tidal current

modelling, under different sea level scenarios, is

needed to determine the depth range at which tidal

currents are at a maximum, when the emergent

Sahul banks produced accelerated tidal flows

through interbank, incised channels. Other arid, tro-

pical carbonate shelves may exhibit similar, outer-

shelf incised valley systems, both in the modern

world and in the rock record.

4.4. GBR transgressive sediment export to slope

explained?

In a study of sediment mass accumulation rates on

the slope adjacent to the Great Barrier Reef, Page et

f

Page 22: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204202

al. (2003) have noted that peaks in sediment accumu-

lation rate do not conform to the prediction of the

conventional model, which calls for maximum sedi-

ment export during sea level low-stands. Over the last

glacial cycle, the conventional model predicts that

maximum export to the slope would have occurred

around 18 ky BP, but instead Page et al. (2003)

measured peak export to occur from between 12 and

7 ky BP, during the rising phase of sea level. Page et

al. (2003) suggest that the reason for this low sedi-

ment discharge during the last glacial maximum was

because sediment is deposited on the low-gradient

shelf behind the rimmed profile. Flooding of the

shelf during sea level rise liberates the sediment stored

behind this rim resulting in an increased sediment

supply to the adjacent slope.

Our model for tidal incision of rimmed shelves

provides a mechanism to explain how the sediment

is mobilized during sea level rise. Our model pre-

dicts a phase of strong, tidally induced, bed stress

on the outer shelf when sea level is between 30 and

50 m below its present position. Available sea level

curves indicate this depth range corresponds to a

time of from between 11 and 9 ky BP (Fig. 7),

which is within the time range of 12–7 ky BP

specified by Page et al. (2003) for maximum sedi-

ment export from shelf to slope. The ebb-orientation

of the large field of moribund dunes on the outer

Gulf of Papua shelf (Fig. 2B) also conforms with

the concept of sediment export from the shelf

caused by a pre-Holocene phase of enhanced tidal

current activity.

5. Conclusions

Shelf valleys at the northern end of the Great

Barrier Reef may be divided into types having two

different origins. The Bramble Valley in the north is

clearly a relict fluvial valley, exhibiting lateral accre-

tion surfaces observed in seismic profiles and

incised margins that intersect and truncate under-

lying strata. The over-deepened Darnley Valley,

located among reefs in the northernmost GBR, how-

ever, appears to have formed by a combination of

reef growth and tidal current scour. During sea level

transgression of the shelf, the Bramble Valley was

an estuary where fluvial deltaic and marine sedi-

ments were deposited, as documented by cores and

seismic data. The Darnley Valley, however, would

have formed an estuarine embayment, whose origin

is not reliant upon fluvial erosion processes, and

where cores and seismic data indicate minimal ter-

rigenous deposition occurred.

The tidally incised Darnley Valley is up to 220 m

deep locally and extends for 93 km across the con-

tinental shelf. Depressions on the valley floor exhibit

closed bathymetric contours at both ends and are

floored with well-sorted carbonate gravely sand con-

taining a large relict fraction and up to 20% mud,

locally. The deepest segments of the Darnley Valley

were probably the sites of lakes during the last ice

age, when Torres Strait formed an emergent land-

bridge between Australia and Papua New Guinea.

Current modeling and observations suggest that mod-

ern tidal flows are weak and insufficient to have

eroded the valley. Numerical modelling further pre-

dicts that the strongest tidal currents occur over the

deepest, outer-shelf segment of the valleys when sea

level is about 30–50 m below its present position.

This depth range corresponds to a brief period of

around 9–7 ky BP during the post-glacial sea level

rise, but more importantly also to several prolonged

phases of sea level during oxygen isotope stages 3, 4,

5a and 5c (Fig. 9). These results are consistent with a

Pleistocene age and relict origin of the over-deepened

portions of the valley floor. Tidal erosion and mobi-

lisation of sediments could explain the peak in export

of shelf sediments to the adjacent slope during the

post-glacial sea level rise as reported by Page et al.

(2003).

We propose a conceptual model to explain the

evolution of shelf valleys that are tidally incised into

broad platform reefs on rimmed, tropical carbonate

shelves. The occurrence of a rim on the outer shelf

establishes a tidally varying hydraulic pressure gradi-

ent between the shelf-lagoon and the adjacent ocean

basin, which reaches a maximum under certain sea

level conditions (Fig. 10). Tidally incised channels

may be further increased in relief by framework

coral growth along the valley margins, akin to levy

banks, through the late Quaternary to produce steep-

sided (locally vertical) valley walls. This model may

explain the evolution of valleys incised into the Sahul

shelf of northern Australia as well as other rimmed

carbonate shelves.

Page 23: An example from the northern Great Barrier Reef, Australia

P.T. Harris et al. / Marine Geology 220 (2005) 181–204 203

Acknowledgements

Ship time aboard R/V Franklin was granted to

PTH by the Australian National Facility Steering

Committee. Additional bathymetric images of the

reefs and seabed photographs can be viewed on-

line at this address:www.ga.gov.au/oceans/projects/

20010917_12.jsp. The paper was improved by the

critical reviews of Drs. Jim Colwell, Phil O’Brien,

John Marshall, Gavin Dunbar and an anonymous

reviewer. Published with permission of the Executive

Director, Geoscience Australia.

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