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Differentiating reefal ridges from relict coastal ridges: Lessons from the seismic geomorphologic study of buried Miocene buildups (North West Shelf, Australia)

Authors:

Abstract

Linear buildups formed in tropical carbonate environments are often interpreted as bioconstructed reefs. Nevertheless, coastal processes can also form extensive sedimentary ridges exhibiting buildup morphologies. This study investigates two Miocene ridges developed along the Australian North West Shelf using 3D seismic and well data. Ridge 1 is ca. 30 m thick and >60 km long, and it is made of foraminiferal pack‐grainstones. It protects a lagoon with pinnacle morphologies. Ridge 2 is ca. 150 m thick and >80 km long. It is composed of quartz sand forming lobes. Both ridges have a continuous curvilinear front and are in a mid‐shelf setting. They mimic the modern Australian coastline. It is then proposed that Ridge 1 is either: (1) a barrier reef developed on a drowned shoreline, or (2) stacked carbonate aeolianites and beachrocks acting as a barrier. Ridge 2 is interpreted as stacked deltaic sands. This study demonstrates that lithified and buried coastal features of carbonate and siliciclastic nature can form extensive ridges exhibiting buildup morphologies. It is proposed that ridges formed by stacked coastal features are overall continuous with a curvilinear front, while reefal ridges are more discontinuous and exhibit deeper and more stable passes.
Basin Research. 2023;00:1–22.
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INTRODUCTION
Geomorphology, the description and classification of the
Earth surface and of the processes that shaped it, has
traditionally been focused on emerged landforms due
to limited data availability offshore. Over the last cen-
tury, continuous progress in technology- enabled imaging
the seafloor with an increasing level of detail and sup-
ported advancement in the understanding of submarine
landforms (Micallef et al., 2018). Nevertheless, most of
the ocean floor remains poorly surveyed, with large por-
tions of submarine landforms still unobserved and non-
documented (Wölfl et al.,2019). As a result, the origin and
nature of numerous marine sedimentary features are still
uncertain, which has direct consequences for the interpre-
tation of ancient strata that rely on modern analogues.
Tropical shallow- water coral reefs reaching the sea
level have been documented by European scientists since
Received: 20 July 2022
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Revised: 13 April 2023
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Accepted: 24 April 2023
DOI: 10.1111/bre.12774
RESEARCH ARTICLE
Differentiating reefal ridges from relict coastal ridges:
Lessons from the seismic geomorphologic study of buried
Miocene buildups (North West Shelf, Australia)
RosineRiera1,2
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UlysseLebrec1,2
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Simon C.Lang2
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VictorienPaumard2,3
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
© 2023 The Authors. Basin Research published by International Association of Sedimentologists and European Association of Geoscientists and Engineers and John Wiley
& Sons Ltd.
1Norwegian Geotechnical Institute,
Perth, Western Australia, Australia
2Centre for Energy and Climate
Geoscience, School of Earth Sciences,
The University of Western Australia,
Perth, Western Australia, Australia
3UWA Oceans Institute, The University
of Western Australia, Perth, Western
Australia, Australia
Correspondence
Rosine Riera, Norwegian Geotechnical
Institute, 40 St Georges Terrace, Perth,
WA 6000, Australia.
Email: rosine.riera@ngi.no
Funding information
norges geotekniske institutt; University
of Western Australia
Abstract
Linear buildups formed in tropical carbonate environments are often interpreted
as bioconstructed reefs. Nevertheless, coastal processes can also form extensive
sedimentary ridges exhibiting buildup morphologies. This study investigates two
Miocene ridges developed along the Australian North West Shelf using 3D seis-
mic and well data. Ridge 1 is ca. 30 m thick and >60 km long, and it is made of
foraminiferal pack- grainstones. It protects a lagoon with pinnacle morphologies.
Ridge 2 is ca. 150 m thick and >80 km long. It is composed of quartz sand forming
lobes. Both ridges have a continuous curvilinear front and are in a mid- shelf set-
ting. They mimic the modern Australian coastline. It is then proposed that Ridge
1 is either: (1) a barrier reef developed on a drowned shoreline, or (2) stacked
carbonate aeolianites and beachrocks acting as a barrier. Ridge 2 is interpreted as
stacked deltaic sands. This study demonstrates that lithified and buried coastal
features of carbonate and siliciclastic nature can form extensive ridges exhibiting
buildup morphologies. It is proposed that ridges formed by stacked coastal fea-
tures are overall continuous with a curvilinear front, while reefal ridges are more
discontinuous and exhibit deeper and more stable passes.
KEYWORDS
aeolianite, Australia, beach ridges, carbonate buildup, Miocene, palaeoshorelines, reef
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RIERA et al.
Cook's voyage in the Pacific in 1769 (Stoddart,1976). They
have since attracted tremendous research effort and rep-
resent some of the most studied marine features to the
point of relagating to the background other tropical envi-
ronments (Longhurst & Pauly,1987). Today, their geomor-
phology and locations are well- known from the scientific
community through published mapping surveys (e.g.,
Maxwell, 1968; UNEP- WCMC et al., 2021) and satellite
images publicly available online (e.g., Sentinel, Landsat).
Interest for fossil reefs (sensu Lowenstam,1950) emerged
in Europe following the publication of Darwin's treatise
(1842), with for example the term reef reportedly first
used in a geological sense by Murchison(1847) to describe
Silurian strata (Cumings & Shrock,1928). Multiple pub-
lications on ancient reefs dominantly based on a zoologi-
cal approach followed during the late 19th and early 20th
century. The study of ancient reefs has further developed
after the 1950's— and expanded to their study through
seismic reflection and well data— when the petroleum
industry identified their hydrocarbon reservoir properties
(Montaggioni & Braithwaite, 2009). Hence, researchers
working in carbonate sedimentology and stratigraphy are
now well aware of the ability of corals and other organ-
isms to build seafloor ridges.
While it is becoming well documented that non-
reefal accumulations, such as stacked aeolianites and
beachrocks, also have the ability to form bathymetric highs
on the modern seafloor (e.g., Brooke et al.,2017; Bufarale
et al.,2019; Green et al.,2020; Lebrec et al.,2022a, 2022b;
O'Leary et al.,2020; Passos et al.,2019) and can mislead-
ingly exhibit reefal morphologies in seismic- reflection
data (Bubb & Hatlelid,1977; Salzmann et al.,2013), pre-
Quaternary carbonate aeolianites and other relict coastal
features are rarely documented in the geologic literature
(e.g., Abegg & Handford,2001; Dodd et al.,2001; Kindler &
Davaud,2001; Loope & Abegg,2001; McKee & Ward,1983;
Smith et al.,2001), and non- reefal carbonate buildups are
seldom described by seismic interpreters. This is partic-
ularly puzzling given the ability of drowned coastal fea-
tures to exhibit buildup morphologies and to form both
carbonate and siliciclastic barrier complexes— composed
of beachrocks, aeolianites and other coastal sedimentary
deposits preserved through early cementation— forming
seafloor ridges enclosing lagoons, bays or estuaries (e.g.,
Alcántara- Carrió et al.,2013; Brooke et al.,2010; De Falco
et al., 2015; Gardner et al., 2007; Lebrec et al., 2022a;
Locker et al.,1996; Mellett et al.,2012; Passos et al.,2019;
Sade et al.,2006; Wenau et al.,2020). As an example, the
islands of the Bahamas are largely formed by aeolianites
(Carew & Mylroie,2001; Nelson,1853). It is also well doc-
umented that many ancient rimmed shelves do not have
reefs at their shelves, but high energy shoals (James &
Mountjoy,1983). This begs the questions, what is the true
nature of seafloor ridges found in ancient strata and how
can we discriminate reefal ridges from drowned coastal
features?
In this context, the North West Shelf (NWS) of Australia,
which is surveyed by >325,000 km2 of high- resolution 3D
seismic data (Paumard, Bourget, Lang, et al.,2019), is an
ideal location to study the origin of seafloor ridges formed
in tropical carbonate environments. Indeed, a ca. 2000 km
long buried seismic reef province, composed of ridges
and circular buildups, respectively, interpreted as barrier
reefs and atolls, developed during the Miocene (Anell &
Wallace,2020; McCaffrey et al.,2020; Ryan et al.,2009).
The scientific objective of this study is to investigate the
nature of two Miocene ridges buried along the Northern
Carnarvon Basin area (southernmost part of the NWS)
using 2D and 3D seismic geomorphology, complemented
by the analysis of available well data and seismic profiles
(Figure1). The results of this study are then utilised to
support a broader discussion on the ability of drowned
coastal features— such as beach ridges, coastal aeolianites
and other geological objects formed along palaeoshore-
lines through wave, tidal, fluvial, and aeolian process-
es— to form continental margin- scale seafloor ridges,
and on their potential morphological similarities to reefal
buildups. A list of criteria for differentiating reefal ridges
from coastal ridges is then presented and discussed.
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GEOLOGICAL SETTING
The North West Shelf (NWS, Purcell & Purcell,1988) is
a ca. 2400 km long passive continental margin located
along the north- western border of Australia, between ca.
11 and ca. 22°S, that has been dominated by carbonate
sedimentation since the Late Eocene (Apthorpe, 1988).
Highlights
Drowned coastal ridges and coral reefs present
geomorphological similarities.
Carbonate coastal features are rarely described
in pre- Quaternary studies.
Coastal aeolianites and stacked shorelines can
build linear ridges 100's km long and 10's m
thick.
Drowned reefs present discontinuous and
patchy morphologies associated with deep
passes.
• Coastal ridges present continuous curvilinear
morphologies, associated with shallow and mo-
bile passes.
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RIERA et al.
The margin is divided in basins and sub- basins formed
through multiple rifting and aborted rifting events be-
tween the Cambrian and the Early Cretaceous (Keep
et al.,2007; Purcell & Purcell,1988; Yeates et al., 1987).
These basins were filled and buried by thick sedimentary
units and have had a limited impact on Cenozoic sedimen-
tation (Apthorpe,1988). Basin and sub- basins names are
however often used in Cenozoic strata studies to designate
geographic areas and are here used accordingly. While the
NWS has been dominantly in a passive state during the
Cenozoic (Apthorpe,1988; Marshall & Lang,2013), local-
ised structural inversion events occurred from ca. 25 Ma
to present, with an apex during the late Miocene (Cathro
et al.,2003; Keep et al.,2007; Keep & Haig,2010; Malcolm
et al.,1991; Saqab et al.,2017).
From the late Oligocene to early Miocene, an extensive
carbonate ramp was covering the NWS (Apthorpe,1988;
Cathro et al., 2003; Moss et al., 2004; Rankey,2017). It
was formed dominantly of micropackstones and foramin-
iferal wacke- packstones (Riera et al., 2022), respectively
designated as Mandu Limestone and Tulki Limestone in
the Northern Carnarvon Basin area (Romine et al.,1997).
Subsequently, at the end of the early Miocene (mid/late
Burdigalian), small aggradational reefal buildups locally
formed along this ramp in the northern part of the NWS
(i.e., Timor Sea and Browse Basin; Belde et al., 2017;
Gorter et al.,2002; Rosleff- Soerensen et al., 2012; Saqab
& Bourget,2016; MioR- 0 on Figure2). During the middle
Miocene, from 16 Ma onward, ridges of inferred reefal or-
igin, locally associated with circular buildups, developed
over ca. 2000 km, thus evolving the ramp into a rimmed
platform (Anell & Wallace, 2020; Belde et al., 2017;
Bradshaw et al., 1988; Collins et al., 2003; Gorter
et al.,2002; Jones, 1973; McCaffrey et al., 2020; Romine
et al., 1997; Rosleff- Soerensen et al., 2012, 2016; Ryan
et al., 2009; Young et al., 2001). These ridges extended
southward to the Cape Range anticline, which was not yet
formed (McCaffrey et al.,2020; Young et al.,2001; Figure1,
MioR- 1 and MioR- 2 on Figure2). The Miocene ridges
present there, which are the focus of this study, are only
documented from 2D seismic lines (McCaffrey et al.,2020;
Young,2001), as 3D seismic geomorphologic studies are
limited to the Browse Basin and Timor Sea, in the north-
ernmost portion of the NWS (Belde et al., 2017; Gorter
et al.,2002; Rankey,2020; Rosleff- Soerensen et al., 2012;
Saqab & Bourget,2016; Thronberens et al.,2022; Van Tuyl
et al.,2018a, 2018b, 2019), where both ridges and circu-
lar buildups are present. The presence of middle Miocene
outcrops of a tropical lagoon with corals in the Cape Range
anticline, designated as Trealla Limestone and adjacent to
the ridges studied here (Figure1), indicates that the envi-
ronment was warm, and possibly favourable to coral reef
development (Riera et al.,2019, 2021).
Miocene reefal ridges are not documented in strata
younger than ca. 10 Ma, but buildup development may
have been locally sustained in the northern part of the
NWS, with Rowley Shoals, Scott Reef, Seringapatam
Reef and Ashmore Reef possibly being modern survi-
vors of the Miocene buildups (McCaffrey et al., 2020;
Ryan et al., 2009). In the southern part of the NWS
FIGURE  Location map presenting
the two Miocene buried ridges in
relation to the modern Australian
coastline. Regional elevation is from
Whiteway(2009), location of the
Ashburton River is based on Crossman
and Li(2015) and extent of the middle
Miocene reef track follows McCaffrey et
al.(2020).
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(Northern Carnarvon Basin area), the shelf was exposed
and karstified at ca. 12 Ma, and an episode of mixed
siliciclastic- carbonate sedimentation established during
the late middle/late Miocene, leading to the deposition of
coastal quartz sandstones locally forming deltas and barri-
ers, that are now partially dolomitised (i.e., Bare Formation
and Pilgramunna Formation; Condon et al.,1955; Heath &
Apthorpe,1984; Hocking et al.,1987; Sanchez et al.,2012;
Tagliaro et al.,2018). Those mixed deposits are overlain
by the detrital carbonates of the Delambre Formation,
that locally interfingers with the siliciclastic intervals of
the Exmouth Sandstone (Hocking et al.,1987). Sustained
siliciclastic influx in the Northern Carnarvon Basin area
ceased at ca. 2.4 Ma (early Pleistocene) possibly due to cli-
matic changes (Tagliaro et al.,2018).
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DATA AND METHODS
Seismic interpretation is based on the analysis of a seismic
volume of ca. 11,000 km2 extracted from the much broader
PGS Carnarvon MegaSurvey (Edwards et al.,2006), and on
the re- interpretation of the regional 2D seismic line s136-
05, previously described in McCaffrey et al. (2020) and
Young et al.(2001). The seismic volume has a spatial reso-
lution of 50 × 50 m, and a vertical sampling rate of 4 ms.
Seismic interpretation was performed in PaleoScan™
software, in two- way time (TWT), and hundreds of seis-
mic horizons representing chronostratigraphic surfaces
were generated following the workflow from Paumard
et al.(2019a).
To complement the seismic data analysis, well cuttings,
side- wall cores (SWC) and thin sections from the offshore
well Ramillies- 1 (Zaunbrecher, 1992) were analysed
both at a macro and micro scale. Carbonate texture de-
scription follows the classification from Dunham(1962),
with the terms dominant, abundant, common, few and
rare indicating that the grains represent respectively
>90%, 50– 90%, 10– 50%, 1– 10% and <1% of the rock vol-
ume. Grain grades, sphericity and sorting follow respec-
tively Wentworth (1922), Powers (1953) and Pettijohn
et al.(1972). Age calibration is based on the review of the
well completion reports and re- analysis of foraminiferal
content of the wells Pyrenees- 1, Macedon- 1 and Macedon- 3
(Riera et al.,2023). In order to integrate well data with
seismic data, well data were loaded in PaleoScan™ and
converted to time domain using publicly available sonic
velocity logs (e.g., Katelis & Hernandianto,1991).
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RESULTS
4.1
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Seismic stratigraphic framework
The two buried ridges are present in a mid- shelf setting.
They are overlying the Oligo- Miocene prograding clino-
forms of the Mandu Limestone and Tulki Limestone that
form an early Miocene distally steepened ramp (Figure 3,
Riera et al.,2022). The ridges are present in two distinct
stratigraphic intervals. The older ridge, Ridge 1, is devel-
oped within the regional seismic sequence Mi5 (Riera
et al.,2023). The identification of Orbulina suturalis and
FIGURE  Chronostratigraphic
framework of the offshore seismic units.
N- zones follow Blow(1969) calibrated
using Wade et al.(2011), Australasian
‘Letter- stages’ follow BouDagher-
Fadel(2018), stratigraphy of the Northern
Carnarvon Basin is modified from Kelman
et al.(2013), seismic sequences follow
Riera et al.(2023), nomenclature and
ages of Miocene reefs along the North
West Shelf follow McCaffrey et al.(2020).
Abbreviations for the sub- basins of
the Northern Carnarvon Basin (from
south- west to north- east): Ba, Barrow; Be,
Beagle; Da, Dampier; Ex, Exmouth.
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Praeorbulina glomerosa at the very base of the sequence
in the well Pyrenees- 1, and of the larger foraminifera
Lepidocyclina (Nephrolepidina) and Flosculinella sp.
around the top of the sequence, respectively from the
wells Macedon- 3 and Ramillies- 1, indicate an accumula-
tion between 15.10 Ma (base of the planktonic foraminif-
eral N9 zone; Blow,1969; Wade et al.,2011) and ca. 13 Ma
(last known occurrence of Flosculinella sp., BouDagher-
Fadel, 2018). Hence, Ridge 1 is time equivalent to the
coral- rich tropical lagoonal limestones outcropping in the
Cape Range anticline (Riera et al.,2021) and to the seismic
barrier reef ‘MioR- 2’ (McCaffrey et al.,2020; Figure2).
Ridge 2 is embedded in the much thicker regional seis-
mic sequence Mi6 (Riera et al., 2023), which reaches a
thickness of ca. 250 m at Ramillies- 1 and is composed of
sub- parallel to chaotic seismic reflectors draping the thin
seismic sequence bearing Ridge 1 (Figure 3). The minimum
age of this sequence is poorly constrained, but it is consid-
ered older than 5.48 Ma because it is overlain by deposits
accumulated during the planktonic foraminiferal N18 zone
in the well Macedon- 1 (Rexilius & Powell,1994). As a result,
Ridge 2 belongs to a seismic sequence time equivalent to the
Bare Formation and Pilgramunna Formation. As with Ridge
1, Ridge 2 appears to be time equivalent to seismic barrier
reef ‘MioR- 2’, but it is younger than the tropical lagoonal
limestones outcropping in the Cape Range anticline.
4.2
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Morphology, seismic facies and
lithology
4.2.1
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Ridge 1
Ridge 1 is a high- amplitude feature associated with un-
derlying velocity anomalies. It is formed by a single
sub- horizontal to undulating seismic reflection that lo-
cally becomes chaotic to transparent (Figure4) and has
a height of ca. 20 to ca. 40 ms TWT. Ramillies- 1 intersects
Ridge 1 between ca. 810 and ca. 840 mKB (metres meas-
ured below Kelly Bushing Height), in an area where Ridge
1 has a height of ca. 30 m. The limited height of Ridge 1
with respect to the seismic vertical resolution prevents any
detailed analysis of its seismic facies from seismic profiles.
Display of seismic amplitudes along the horizons
passing through Ridge 1 reveals that the bright horizon
forming the ridge and time- equivalent landward strata
are covering an area at least 60 km long and 5.5 km wide.
Ridge 1 has a crenulate, continuous front, which is com-
posed of concave, convex, straight and V- shaped features
(Figure5a– c). The front of Ridge 1 in itself does not par-
ticularly stand out from the rest of the structure, but it
is well recognisable as it marks the transition from the
high- amplitude seismic reflectors forming Ridge 1 to the
low- amplitude reflectors present seaward of the ridge.
The seismic character of Ridge 1 is also remarkable as the
feature is covered with small, <100- to- 400- m- wide, high
amplitudes, rounded- to- ovoid, evenly spaced pinnacle
morphologies (Figure5). Examination of seismic profiles
intersecting those features show that they are created
by undulations of the seismic reflectors forming Ridge 1
(Figure5d). The total length of Ridge 1 is unknown, as
it extends beyond the limit of the data towards the north
west and is masked by a seismic artefact from Ridge 2 both
southward and eastward.
One single SWC, Ramillies- 1 810 mKB, is available
from Ridge 1 (Figures4b and 6a– c). It is composed of a
foraminiferal packstone to grainstone with dominantly
fine- to- medium size carbonate bioclasts, and rare granule
to pebble- size carbonate lithoclasts. Quartz grains were
not observed. Bioclasts include porcelaneous foraminifera
FIGURE  Un- interpreted (a) and interpreted (b) seismic line highlighting the general stratigraphic framework (modified from Riera
et al.,2023). Note that Ridge 1 is developed within the sequence Mi5, that is time equivalent to the outcropping tropical lagoon, while Ridge 2
is within the sequence Mi6, that is time equivalent to the Bare Formation (see Figure2). Data courtesy of PGS.
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(few small miliolids, few broken and entire juvenile to adult
Sorites sp.), hyaline foraminifera (common Amphistegina
sp., few undifferentiated small hyaline foraminifera, rare
acervulinids), rare, agglutinated foraminifera, few debris
of coralline algae, few mollusc debris and few echinoderm
debris. A micritic matrix is locally present. No corals or
coralgal crust were observed from well cuttings.
4.2.2
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Ridge 2
Ridge 2 (Figures4 and 7) is formed by undulating seismic
reflectors of high- to- medium amplitudes, and locally pre-
sents a mound- like morphology along 2D seismic profiles,
with bi- directional downlaps on both sides (Figure7e). As
with Ridge 1, Ridge 2 induces velocity pull- ups in under-
lying strata. Overall, Ridge 2 has a height of ca. 50 to ca.
100 ms TWT and is therefore much thicker than Ridge 1.
At Ramillies- 1, where its actual height is measurable, it
extends from 560 to 710 mKB, and therefore has a height
of 150 m (Figure4b).
Seismic amplitude maps along horizons passing through
Ridge 2 reveals a feature at least 80 km long (Figure7). The
front of Ridge 2 is overall curvilinear and continuous, but
the morphology of the ridge front varies between its lower
(Figure7a), middle (Figure7b) and upper (Figure7c) inter-
vals. At its base, Ridge 2 is characterised by the presence of
three asymmetric convex- outward features developing lobes
and cuspate morphologies (Figure7a,d), with the larger one
ca. 5 km long and ca. 10 km wide. Those convex- outward fea-
tures become less and less prominent in younger strata (i.e.,
central and upper part of Ridge 2; Figure7b,c). Smaller high-
amplitude stacked ridges with an overall linear- to- crenulate
morphology are very locally present in Ridge 2 (Figure7f).
Those smaller ridges are locally discontinuous (Figure7g).
The total extent of Ridge 2 is unknown, and as for Ridge 1, it
extends outside the area investigated. Ridge 2 is well imaged
around Ramillies- 1, which intersects it (Figure7).
FIGURE  Un- interpreted and interpreted seismic lines displaying the seismic expression of the two ridges and time- equivalent strata.
(a,b) Crossline extracted from the 3D seismic data. Data courtesy of PGS. (c,d) Regional 2D line 136- 05. Location of the seismic lines are
displayed on Figures5b (Ridge 1) and 7D (Ridge 2). Note the presence of velocity anomaly zones below the two ridges.
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Analysis of the SWC collected at 580 and 650 mKB
(Figures4b and 6d– f), indicates that Ridge 2 is composed
of quartz grains in a dark matrix, which appears locally
dolomitized. Quartz grains are very fine to coarse and are
very poorly to well sorted with sphericity ranging from
sub- angular to well- rounded. Scarce carbonate bioclasts
represented by debris of articulated coralline algae and
echinoids were also identified in the interval. No coral,
coralgal or otherwise bioconstructed crust were observed
from well cuttings.
5
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DISCUSSION
This study has revealed the geomorphology of the two
Miocene ridges, hence allowing their comparison with
analogues present along modern continental shelves. It
is here investigated whether the geomorphology of the
ridges indicates a reefal origin or not. Results are then
utilised to discuss more largely the morphological differ-
ences between reefal ridges and coastal ridges. Lastly, we
conclude with a note on the ambiguity that sometimes ac-
companies the uses of the term reef.
5.1
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Nature of Ridge 1
The main seismic elements characterising Ridge 1 along
seismic profiles are its high- amplitude and the veloc-
ity anomalies underlying it (Figure4), which can be
observed in reefal carbonate buildups (including seis-
mic reefs, sensu Schlager, 2005) but also non- reefal
FIGURE  Seismic geomorphology of the Miocene Ridge 1. (a,b) Un- interpreted and interpreted amplitude maps extracted from the
3D seismic horizon cross- cutting Ridge 1. (c) Close- up view of Ridge 1 and time- equivalent lagoon with pinnacle morphologies. (d) Seismic
cross- sections illustrating the relationship between Ridge 1 and the pinnacles present landward of the ridge. (e) Seismic cross- section
illustrating the relationship between Ridge 1 and Ridge 2, note that Ridge 2 is masking the most landward portion of the lagoon time-
equivalent to Ridge 1. Data courtesy of PGS.
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carbonate accumulations (Bubb & Hatlelid,1977; Marfurt
& Alves, 2015). However, the limited height of the
ridge— it is only identified from one reflector— and the
absence of clear stacking pattern prevents any conclusion
on its nature based on seismic profile observation alone.
The comparison of spatial morphologies with modern
analogues, however, provides material to assess whether
Ridge 1 was bioconstructed or not.
Morphological comparison between Ridge 1 and the
individual coral reefs of the Great Barrier Reef (GBR),
which is the only modern reef province similar in size
to the Miocene seismic reef track buried along the NWS,
highlights several elements that contradict a purely
reefal origin of Ridge 1. First, the GBR is not formed by
ridges, but by ca. 2900 individual reefs (Bridge et al.,2012;
Figure8). Additionally, Ridge 1, like the other ridges of the
Miocene reef track (Anell & Wallace,2020), is located in
a mid- platform setting. This is another dissimilarity with
the GBR, because there the more linear reefs are located
at the shelf edge, whereas only smaller linear and circular
reefs are present in a mid- platform setting (Figure8d,e;
Maxwell,1968). Moreover, even in areas where the reefs
of the GBR have a relatively linear morphology and
form barrier reefs (e.g. Ribbon Reefs area; Figure8e),
barrier reefs are continuous over only a few kilome-
tres, separated by passes hundreds of metres to several
kilometres wide, often >40 m deep, and with curved mar-
gins (Beaman,2017; Hopley,2006). The longest linear in-
dividual reef of the GBR, Ribbon Reef #10, is 28 km long
and surrounded by smaller reefs (Figure8e; Whiteway
et al.,2014). This length is much smaller than the length
of Ridge 1, that has a length of at least 60 km (Figure8a).
The geomorphological elements from the Miocene reef
track that have the more similarities with the GBR are
the ovoid buildups. Those are interpreted as atolls and
are sometimes associated with the Miocene ridges in the
northern part of the NWS (Rosleff- Soerensen et al.,2012,
2016). However, ovoid buildups are absent from the area
investigated here, hence indicating that the reefs of the
GBR are not a possible analogue for Ridge 1.
The linearity of Ridge 1 and its geomorphological sim-
ilarity to the modern NWS coastline (Figure8b,c) might
illustrate an influence of coastal features on its formation.
Given that Ridge 1 is ca. 30 m thick, a thickness docu-
mented for both coral reefs and lithified shoreline ridges
(Salzmann et al.,2013), Ridge 1 could therefore either be:
(1) a bioconstructed reef developed on drowned coastal
features; or (2) a lithified coastal ridge. Some elements are
in favour of a reefal origin, such as the presence of pin-
nacles, which can be interpreted as small patch reefs or
reef knolls within a lagoon (Figure8a). In addition, Ridge
1 is time equivalence to the coral- rich tropical lagoonal
FIGURE  Photomicrographs in plane- polarised light presenting the facies observed in side- wall cores (SWC) from Ramillies- 1. (a– c)
Foraminiferal packstone to grainstone from Ridge 1 (SWC at 810 m), (d– f) Mix of matrix and quartz sand from Ridge 2 (d is from SWC at
650 m and e,f are from SWC at 580 m). Am, Amphistegina sp.; Bi, bivalve debris; Mi, miliolid; Qtz, quartz grain. See Figure4b for location of
SWC.
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RIERA et al.
limestones outcropping in Cape Range anticline (Riera
et al.,2021). The ridge also has geomorphologic similar-
ities to the central part of the Belize Barrier Reef (BBR;
Figure8f), which is composed of a bioconstructed reefal
system developed on beach ridges (Droxler & Jorry,2013).
As such, corals, or other reef- building organisms, may
have colonised pre- existing seafloor ridges formed
through coastal processes to build=- Ridge 1 (as described
in Droxler & Jorry,2013; Jarrett et al.,2005; Mohana Rao
et al.,2001; Ramsay,1994; Figure9).
Several elements are nevertheless in disfavour of a
reefal origin. Those elements include the absence of
FIGURE  Seismic geomorphology of Ridge 2. (a– c) Un- interpreted amplitude maps derived from three 3D seismic horizons
respectively crossing the base, middle and upper parts of Ridge 2. (d) Interpreted amplitude map derived from the horizon crossing Ridge 2
at its base superimposed with the location of the front of Ridge 2 through time. (e) Seismic cross- sections highlighting reflector terminations
around Ridge 2, note the mounded morphology of the ridge with bi- directional downlaps. (f) Seismic cross- section illustrating the complex
2D morphology of the smaller stacked ridges present in Ridge 2. (g) Close- up view of an un- interpreted amplitudes map derived from a
seismic horizon crossing Ridge 2 in its central part and illustrating the geomorphology of the smaller ridges. Data courtesy of PGS.
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RIERA et al.
FIGURE  Comparison between the seismic geomorphology of Ridge 1 (a; Data courtesy of PGS) and modern geomorphologies. (b,c)
Aerial photographs of the modern coastline of the Australian North West Shelf at the southern extent of Eighty Mile Beach (b) and in the
vicinity of Lagrange, ca. 40 km south- east of Broome (c), aerial photographs are from EarthExplorer. (d,e) Close- up view of the present- day
Great Barrier Reef in its central part, where the reefs are in a mid- platform setting (d) and in its northern part, where reefs have a linear
morphology and are located along the shelf edge (e), note that in both cases the morphology of the individual reefs is clearly discernible;
bathymetry is from Beaman(2017). (f) Aerial photograph of the central portion of the Belize Barrier Reef, where it forms a detached
coral barrier reef with a linear morphology, aerial photography is from EarthExplorer (source: Esri, i- cubed, USDA, USGS, AEX, GeoEye,
Getmapping, Aerogrid, IGN, IGP, UPR- EGP, and the GIS User Community, ESRI).
FIGURE  Conceptual sketches illustrating how both reefs and stacked coastal features can form sedimentary ridges tens of metres
thick. Those ridges can exhibit buildup morphologies when observed from seismic- reflection profiles. The sketch of the non- bioconstructed
carbonate ridge builds on the coastal ridges outcropping along the modern NWS (Lebrec et al.,2022a). The stacked shoreline sketch builds
on the Miocene Bare Formation, a Miocene deltaic deposit locally >500 m thick which accumulated along a carbonate shelf undergoing
a strong subsidence (Tagliaro et al.,2018) and which is locally associated with dolomite causing high- velocity seismic zones (Wallace
et al.,2003).
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smaller reef geomorphologies (e.g., back- reef and atolls),
such as circular buildups. Such geomorphologies are pres-
ent in BBR (Figure8f; James et al.,1976), which is there-
fore not an exact analogue for Ridge 1. Additionally, Ridge
1 is continuous and there are no indicators of sediment
flow, such as inter- platform seaway, reef passes or slope
debris, in contrast to other reefs described from seismic
data (e.g., Courgeon et al.,2016; Posamentier et al.,2010,
2022; Schlager,2005). Drowned reefal ridges described in
the literature tend to be only a few 100's metres or a few ki-
lometres long (e.g., Jorry et al.,2016; Khanna et al.,2017;
Mallarino et al.,2021; Rovere et al.,2018) or to be com-
posed of joint and isolated pinnacles (Abbey et al.,2011),
and as such are dissimilar to Ridge 1. Finally, litholog-
ical data do not fully support a reefal origin, as no bio-
constructed crust, lithified coral conglomerate or other
indicators of reefal bioconstruction were observed along
the time- equivalent outcrops (Riera et al.,2021) or from
well data. Those elements are known not only from mod-
ern reefs (Braga et al.,2019; James et al.,1976; Webster
et al.,2018) but also from fossil ones (James & Jones,2015).
It is hence conceivable that Ridge 1 is composed ex-
clusively of coastal features, similarly to the submerged
sedimentary ridges present along the modern sea-
floor of Western Australia (Brooke et al.,2014; Lebrec
et al.,2022a). Indeed, drowned and cemented wave- and
wind- built beach ridges can form linear ridges composed
of beachrocks and aeolianites reaching heights >30 m
(Salzmann et al.,2013) and lengths >1000 km (Lebrec
et al., 2021, 2022a). Those non- bioconstructed ridges
can form barriers several 100's km long (Dillenburg
et al.,2020), and protect lagoons, hence exhibiting mor-
phologies similar to drowned barrier reefs (Salzmann
et al.,2013; Figure9). In addition, it is well documented
from outcrop studies that coastal deposits, and in par-
ticular aeolianites, can be stacked on top of each other
and reach significant thicknesses (Figure9; Carew &
Mylroie, 2001). For example, shore- parallel barriers
composed of Quaternary stacked dunes reaching thick-
nesses of 200 m above sea level are documented in South
Africa (Bateman et al., 2011). Most of the Bahamian
Islands are built by aeolianites, that can form ridges
up to 63 m high (Carew & Mylroie, 2001). Similarly,
Pleistocene carbonate aeolianites, composed of palaeo-
dunes interbedded with calcretes and palaeosols, form
cliffs up to ca. 80 m high in South Australia (James &
Bone,2015). Quaternary carbonate aeolianites are wide-
spread along the present- day Western Australian coast
(Brooke, 2001), and they can form massive carbon-
ate structures. For example, Shark Bay is protected by
stacked aeolianite islands up to 150 km long and >250 m
thick (Frébourg et al.,2008; Le Guern & Davaud,2005;
Logan et al.,1970; Vimpere et al.,2022).
The marine nature of the sediment forming Ridge 1
does not contradict a formation by coastal features, as
coastal carbonates, including aeolianites and beachrocks,
have a marine provenance (Abegg et al.,2001). As a result,
aeolianites and beachrocks can be undistinguishable from
subtidal carbonate at the thin section scale, due to the
absence of observable sedimentary structures (Frébourg
et al., 2008). As an example, Pleistocene and Holocene
aeolianites composed of coralline algae, corals, mol-
luscs, echinoderms and foraminifera are documented in
Hawaii (Blay & Longman,2001). The present- day Western
Australian coastline is a ‘hot spot’ of beachrock occurrence
(Vousdoukas et al.,2007), and the observation of grains
>4 mm in SWC from Ridge 1 could indicate a formation
by stacked beachrocks. Nevertheless, a coarse grain size
does not necessarily contradict an aeolianite origin, as
carbonate aeolianites are often composed of heterogenous
and coarse- grained material (Frébourg et al., 2008). The
absence of documentation of middle Miocene outcrops of
aeolianites and carbonate beachrocks in Western Australia
is not a proof of their absence, as Miocene outcrops are
largely understudied, and because carbonate coastal fea-
tures are often misinterpreted as shallow- water deposits
(Abegg et al., 2001; Frébourg et al., 2008). In addition,
the absence of observation of shoal morphologies from
3D seismic data raises the question of whether middle
Miocene outcrops interpreted as shoals in Cape Range an-
ticline and Barrow Island (McNamara & Kendrick,1994;
Riera et al.,2021; Figure1) could be formed by beachrocks
and/or coastal aeolianites. As such, Ridge 1 could well be
composed of coastal ridges, which acted as a barrier and
protected a lagoon. In this case, corals might have been
present on the ridge as a veneer, and within the lagoon as
small patch reefs and knolls, but the core of Ridge 1 would
be composed of carbonate coastal features. Aeolianites
colonised by a thin coral veneer (not forming reef) are
for example documented in the Bahamas (Carew &
Mylroie,2001) and in Western Australia (Playford,2004).
5.2
|
Nature of Ridge 2
Ridge 2 exhibits several seismic characteristics known from
both carbonate bioconstructed and non- bioconstructed
buildups along seismic profile, such as high seismic ampli-
tudes, velocity anomalies underlying it and a mound- like
morphology (Bubb & Hatlelid,1977; Burgess et al.,2013;
Esker et al.,1998; Paumard et al., 2017). However, those
seismic characteristics are not exclusive to carbonate
buildups and can be created by buried coastal sands.
Mound- like morphologies have for example been ob-
served in coastal barriers (Passos et al.,2019). In addition,
the siliciclastic sands of the Miocene Bare Formation,
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RIERA et al.
which are time equivalents to Ridge 2, are also known to
exhibit a mounded geometry along seismic profiles, and
to locally reach thickness >500 m (Tagliaro et al., 2018).
It is furthermore documented that sandstone bodies
present within finer lithologies can cause pull- up effects
(Grasseau et al., 2019). Hence, it appears that aggrading
coastal sands undergoing early cementation and/or early
burial, can form mounded seismic features with apparent
steep slope associated with velocity anomalies (Figure9).
The seismic geomorphology of Ridge 2 clearly differs
from modern bioconstructed reefs. Indeed, Ridge 2 is char-
acterised by the presence of convex- outward features simi-
lar to the deltaic lobes of the present- day Ashburton River
delta coastline, which is composed of active and aban-
doned deltas with well- defined lobes, as well as asymmet-
rical cuspate forelands (Figure10). The location of Ridge 2
in front of the present- day Ashburton River delta complex
further supports a connection between the formation of
the ridge and the palaeo- activity of the Ashburton River
during the Miocene (Figure1). The Miocene asymmetri-
cal cuspate morphologies may indicate the presence of a
palaeo- longshore drift that shaped the front of Ridge 2. A
tidal influence on the formation of Ridge 2 may also be
indicated by the presence of the smaller ridges observed
within Ridge 2 (Figure7f,g). They could represent stacked
linear beach ridges locally developing wave- dominated
barrier complexes incised by tidal channels (i.e., inlet,
Figure10d; also see comparison with Figure2i from
Nyberg & Howell,2016). Those observations indicate that
Ridge 2 contains geomorphologic elements characteristic
of wave processes with a local river input and affected
by tidal processes, leading to the development of wave-
dominated, fluvial- influenced and tide- affected shorelines
(sensu Ainsworth et al.,2011).
The formation of Ridge 2 by mechanical processes, and
not by bioconstruction, is further supported by the mobil-
ity of the lobe and cuspate morphologies through time and
space. Indeed, the front of Ridge 2 is overall transgressive,
while it can be locally regressive (Figure7a– d). The ridge
also appears to be continuous over at least 80 km, except
along the smaller stacked ridges (Figure7g), which is typ-
ical of transgressive non- reefal barrier complexes protect-
ing lagoons or tidal flats (Green et al.,2013; Otvos,2012;
Storms et al.,2008; Wenau et al.,2020). No platform reef
morphologies or deep passes are observed along Ridge 2.
SWC and well cutting analysis further supports the
formation of Ridge 2 by mechanical processes along a pa-
laeoshorelines, as the ridge is dominantly composed of
quartz grains. The siliciclastic nature of Ridge 2 implies a
formation by sediments supplied from rivers, marine cur-
rents and/or wind. The most common bioclasts are debris
of articulated coralline algae, occurring in most marine
environments receiving light (i.e., photic zone, ca. 0– 80 m;
James & Jones,2015), and echinoid debris indicating car-
bonate production in a normal marine environment (i.e.,
neither brackish nor hypersaline; Heckel,1972). Hence,
despite a buildup- like morphology along 2D seismic pro-
files, both the geomorphology and lithology of Ridge 2
point towards a formation driven by the mechanical ac-
cumulation of siliciclastic sediments along an overall
carbonate coast. Modern examples of siliciclastic delta
developed in carbonate environments, that can be used
as analogues, are documented along the NWS (Lebrec
et al.,2023; Semeniuk,1993). Therefore, it is proposed that
Ridge 2 is an accumulation of stacked coastal siliciclastic
sands, possibly related to the palaeo- Ashburton delta.
5.3
|
Differentiating reefal ridges from
coastal ridges
Reefal ridges can, in some instances, be difficult to
differentiate from drowned and/or buried coastal
ridges, as both can form barriers that protect lagoons
FIGURE  Aerial photography of the Ashburton River delta and surrounding shoreline (colour image) compared with selected
close- ups from the Miocene Ridge 2 displayed along envelope attribute maps (grey images), extracted from the 3D seismic volume (Data
courtesy of PGS). Close- ups of the Miocene Ridge 2 are displayed at the same scale than the aerial photograph. Aerial photograph is
from EarthExplorer (source: Esri, i- cubed, USDA, USGS, AEX, GeoEye, Getmapping, Aerogrid, IGN, IGP, UPR- EGP, and the GIS User
Community, ESRI).
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14
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RIERA et al.
(Gardner et al., 2005; Figure9). Examples of drowned
non- reefal ridges that form such structures on the
present- day seafloor are numerous (e.g., Alcántara- Carrió
et al., 2013; Brooke et al., 2010; De Falco et al., 2015;
Lebrec et al., 2022a, 2022b; Mellett et al., 2012; Wenau
et al., 2020), and they should be considered as possible
analogues for seafloor ridges observed along continental
shelves. Additionally, identification of corals or other reef-
building organism on the inside part of a ridge is not infal-
lible evidence of a reefal origin, as loose corals or other
sessile organisms can also be reworked to form ridges
(e.g., Spiske,2016), which may lead to their misinterpre-
tation as a bioconstructed ridge.
It is possible to interpret the origin of a seafloor ridge
when geomorphologic elements characteristic of coastal
or reefal environments are present. Elements character-
istic of reefal development include atoll morphologies
(Khanna et al.,2017), spurs and grooves (Duce et al.,2016;
Gischler,2010; Stoddart,1969) that can be detected from
high- resolution bathymetry (Khanna et al., 2017), but
also knoll morphologies within the lagoon, which can
be formed by pinnacle reefs or coral heads (Kennedy
et al., 2021). Elements characteristic of coastal environ-
ments include prograding beach ridges, tidal or fluvial
channels, recurved spits, blow- outs and washover depos-
its (e.g., Brooke et al.,2017; Lebrec et al.,2022a; Passos
et al., 2019). However, those elements might be visible
only where data resolution is good, and not often observ-
able from seismic- reflection data.
When performing seismic interpretation or work-
ing on low- resolution bathymetry data, larger elements
might help to discriminate between coastal ridges and
reefal ridges. Indeed, coastal ridges are accumulated
along palaeoshorelines, and as such they tend to exhibit
linear morphologies continuous over extensive lengths,
potentially reaching hundreds of kilometres, that re-
produce the shoreline along which they were accumu-
lated (e.g., Brooke et al., 2014; Lebrec et al., 2022a).
Conversely, reefs are organic features that can develop
on any topographic high, and as such, reefal develop-
ment is often not limited to the barrier along modern
continental shelves, and smaller platform reefs with
circular morphologies often develop simultaneously
to the barrier reef (James et al.,1976; Maxwell,1968).
Finally, as coastal features are dynamic objects, the po-
sition of tidal passes is not stable through time, while
the location of reefal passes is relatively stable. It is
hence proposed here that the main elements to differ-
entiate between coastal and reefal ridges using seismic-
reflection data is not the thickness of the ridge in itself,
as both coastal ridges and reefal ridges can reach sig-
nificant thicknesses (Figure9), but: (1) the continuity
of the ridge; (2) the presence or absence of circular
buildups (atolls) associated with the ridge; and (3) the
dimensions of the passes, as the observation of deep
passes along a barrier can be a sign of a stable, biocon-
structed origin.
Reefs that are developed on drowned coastal features
are hybrid sedimentary objects, that can have geomor-
phologic characteristics of both reefs and coastal features.
Present- day coral reefs enhancing the deltaic morpholo-
gies underlying them are well documented, with exam-
ples from the Great Barrier Reef, Belize Barrier Reef and
New Caledonia shelf (Choi & Ginsburg, 1982; Droxler
& Jorry, 2013; Ferro et al., 1999; Le Roy et al., 2019;
Maxwell, 1970). Those hybrid features can be identified
using 3D seismic data by the observations of coastal geo-
morphologies whose thickness has been enhanced by
reefal development (e.g., Mathew et al.,2020). In this case,
the location of coastal features, such as channel levees,
bars and deltaic lobes is stable upward, hence indicat-
ing that those coastal features are colonised by aggrading
reefs.
5.4
|
Note regarding the use of the
term reef
While geologists and most researchers working on mod-
ern coral reefs restrict the use of the term reef to rigid
and wave- resistant structures that are bioconstructed by
frame building, sediment retention and binding, follow-
ing the definition from Lowenstam(1950), this is not the
case of the entire scientific community. Indeed, at least
two other definitions of the term reef exist, which can
sometimes cause confusion. Reef was originally a nauti-
cal term designating a topographic high on the seafloor,
independently of its nature (Cumings,1932). This defini-
tion is still used today by government agencies and re-
searchers working on marine habitat mapping, marine
policies and fishery. The term reef is, for example, de-
fined by the European Commission as a hard substrate of
either bioconstructed or geogenic origin arising from the
seafloor (European Commission,2013, p. 13). As a con-
sequence, the term reef is sometimes used to designate
non- bioconstructed bathymetric highs, such as igneous
rock outcrops (e.g., granite reef; Campbell et al.,2014),
drowned aeolian dunes (e.g., Broken Reef; Beaman
et al., 2005) or undifferentiated bedrock outcrops (e.g.,
rocky and geogenic reefs; Brooke et al., 2014; Diesing
et al.,2009; O'Sullivan et al., 2020). A more restrictive,
yet widely used definition, limits the use of the term reef
to any type of rock lying near or at the surface of the
sea, which can constitute a hazard to surface navigation
(e.g., Harris & Baker,2020). Those two definitions do not
have biological implications, and they contrast with the
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definition from Lowenstam(1950), which explicitly ex-
cludes features not bioconstructed.
Even among geologists, there is a degree of uncer-
tainty on the exact definition of reef. As an example,
Schlager (2005) stats that ‘the question of what is a
reef continues to fuel heated discussions among ge-
ologists’ (Schlager, 2005, p. 115). Indeed, according to
Schlager (2005), seismic features having morphologies
similar to modern coral reefs should be designated as seis-
mic reefs, as those seismic features may contain a significant
portion of non- bioconstructed material. It can indeed be
difficult to prove that a seismic structure is bioconstructed,
even when sedimentary cores are available (Burgess
et al.,2013; Montaggioni & Braithwaite,2009). Debates on
what is a reef are not restricted to seismic reefs, and over
the last century, ambiguity surrounding the term reef in
geological studies has been consistently pointed out, and
repeated attempts were made to homogenise its use (e.g.,
Cumings,1932; Cumings & Shrock,1928; Dunham,1970;
Nelson et al.,1962; Riding,2002; Wilson,1975). For exam-
ple, a debate occurred during the 60's and 70's on whether
the Capitan reef was a reef, even though its structure is
extensively outcropping and has been considerably stud-
ied, making it one of the most famous ancient reefs in the
world (Saller et al.,1999). Identifying a fossil reef based
on the definition of Lowenstam(1950) can be subjective,
and Dunham(1970) advises to differentiate between the
observational term stratigraphic reef, which designates
masses of carbonate sediments either organically or inor-
ganically bound, and the interpretative term ecologic reef,
which designates purely bioconstructed structures (i.e.,
organically bound).
Debates on the definition of reef also concern present-
day reefs, with for example discussion on the minimum
size a structure must have to be designated as a reef
(Montaggioni & Braithwaite,2009), or on the amount of
bioconstruction in modern reefs (Montaggioni, 2001).
Furthermore, when corals and other sessile organisms
colonise topographic highs, they sometimes only form a
veneer of bioconstructed material at their surfaces. This
gives those features the appearance of bioconstructed
reefs from shallow observations (e.g., visual description
based on photographs, surficial sampling), while their
internal structure is non- reefal (e.g., Jarrett et al.,2005;
Mohana Rao et al., 2001; Ramsay, 1994), hence raising
the question of whether or not such features should be re-
garded as reefs. Discoveries of deep water bioconstructed
structures also question whether the term reef should
be restricted to wave- resistant structures (Heckel, 1974;
Schlager,2005), or if it can be used to designate deep water
azooxanthellate coral bioherms (e.g., Roberts et al.,2006)
and coral bioherms living in mesophotic environments
(e.g., Bridge et al.,2012). Hence, it is here recommended
to specify which definition of reef is followed when work-
ing on reefal structures.
6
|
CONCLUSION
Despite the well- known ability of relict coastal features
to build massive structures along present- day carbonate
coasts, those features formed by winds, waves, tides and
currents are not often described in pre- Quaternary strata.
This study investigates the nature of two Miocene ridges
formed along a carbonate shelf that were previously in-
terpreted as reefal ridges based on 2D seismic profiles.
Here, new information derived from 3D seismic volume
and well data highlight the role of coastal processes in the
formation of those ridges.
The older ridge, namely Ridge 1, is a curvilinear car-
bonate feature protecting a lagoon with pinnacles, which
exhibits a geomorphology reminiscent of the modern
Australian coastline. The ridge is time equivalent to the
nearby outcrops of a coral- rich tropical lagoon; however,
no indicators of bioconstruction by coral, algae or mi-
crobial mats were identified from field or well data. The
ridge is not associated with ovoid buildup morphologies
(atolls), and no discontinuities (passes) were observed
along its front. As such, it is proposed that Ridge 1 could
either be: (1) a bioconstructed reef developed on drowned
coastal features, similar to the Belize Barrier Reef; or (2)
stacked aeolianites and/or beachrocks accumulated along
the Miocene palaeoshoreline, similar to the relict coastal
ridges present along the modern Western Australian coast.
The younger ridge, namely Ridge 2, is curvilinear and
contains several lobes and cuspate landforms. Those
features appear mobile, as their morphology evolves
throughout the different stratigraphic intervals. Overall,
Ridge 2 has striking morphological similarities to the
modern Ashburton River delta complex, hence pointing
towards a formation by the accumulation of coastal sand.
This interpretation is further supported by the abundance
of quartz grains within the ridge. Hence, Ridge 2 is here
re- interpreted as a drowned siliciclastic coastline devel-
oped in an overall carbonate environment. As such, it is
proposed that this ridge belongs to the palaeo- Ashburton
River delta complex.
Those observations illustrate that corals and other
reef- building organisms are not the only builders of sed-
imentary seafloor ridges in tropical environments, and
that aeolianites and other coastal features are capable of
creating massive structures along carbonate coasts. As
such, their scarcity of documentation in pre- Quaternary
strata might be a description bias, as ridges formed by
stacked coastal features can be misinterpreted as biocon-
structed reefs. Indeed, coastal features subject to early
13652117, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1111/bre.12774 by Dokumentsenteret Norges Geotekniske Institutt, Wiley Online Library on [22/05/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
16
|
EAGE
RIERA et al.
cementation and/or rapid burial are capable of creating
high- velocity seismic ridges 10's of metres thick. It is
proposed that coastal ridges can be identified from their
continuous front and from the presence of coastal geo-
morphologies. When several coastal ridges are stacked
on top of each other, the location of finer- scale geomor-
phologic elements present within the ridges, such as
lobes or channels, is expected to evolve upward. In con-
trast, it is proposed that reefal ridges are more discontin-
uous, with deeper and more stable passes. Reefal ridges
developed on drowned coastal features might contain
geomorphologic elements characteristic of coastal en-
vironments, whose thickness has been enhanced by
reefal development, and whose location remains stable
upward.
ACKNO WLE DGE MENTS
The authors would like to thank the Centre for Energy
Geoscience (University of Western Australia) and the
Norwegian Geotechnical Institute for providing finan-
cial support to the project. The authors are also obliged to
PGS and Eliis for providing access to the PGS Carnarvon
MegaSurvey and PaleoScan™ software, respectively.
The Department of Mines, Industry Regulation and
Safety, Western Australia is also acknowledged for
providing access to offshore well material (sampling
approval N00832) and archived thin sections. The au-
thors are grateful to the School Earth Science (UWA) for
providing equipment access. Two anonymous review-
ers, Stephan J. Jorry and Prof. Peter Burgess are greatly
thanked for their very constructive feedbacks on the
manuscript.
CONFLICT OF INTEREST STATEMENT
The authors are not aware of any conflicts of interest relat-
ing to this work.
DATA AVAILABILITY STATEMENT
3D reflection seismic data used for this research consist of
public and PGS- owned seismic- reflection data combined
in a regional dataset by PGS. 2D seismic lines are avail-
able through the Australian National Offshore Petroleum
Information Management System. SWC and well cuttings
can be accessed through the Perth Core Library.
ORCID
Rosine Riera https://orcid.org/0000-0002-9179-4330
Victorien Paumard https://orcid.
org/0000-0001-7606-7293
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nopims
How to cite this article: Riera, R., Lebrec, U.,
Lang, S. C., & Paumard, V. (2023). Differentiating
reefal ridges from relict coastal ridges: Lessons
from the seismic geomorphologic study of buried
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... They form unique depositional environments and are renowned biodiversity hotspots, in particular due to the presence of coral reefs (Wilson, 2013). Coral reefs and other bioconstructed reefs have been extensively studied, sometimes overshadowing the importance of other sedimentary features, and leading to the view that reefs dominate tropical-shelf sedimentary systems (Droxler and Jorry, 2013;Longhurst et al., 1987;Riera et al., 2023). In reality, reefs such as the Australian Great Barrier Reef, Belize Barrier Reef, Florida Keys Reef Tract and New Caledonian barrier reefs consist of relatively thin late-Quaternary deposits overlying antecedent sedimentary substrate (Droxler and Jorry, 2013;Montaggioni et al., 2011;Multer et al., 2002;Purkis et al., 2014;Webster and Davies, 2003). ...
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Published in 1842, this important monograph by Charles Darwin (1809–82) formed the first part of a trilogy of geological studies based on observations made during the celebrated second voyage of the Beagle. Influenced by Charles Lyell's Principles of Geology, Darwin drew in particular on data from the survey of the Keeling Islands in the Indian Ocean to support his theory that subsidence of the ocean floor can account for the formation of coral atolls. He first presented his findings in a paper for the Geological Society of London in 1837, but a heavy workload and illness delayed the appearance of this elegantly argued and illustrated study. For this and his work on barnacles, Darwin would receive the Royal Society's royal medal in 1853. The other studies in the trilogy, Geological Observations on the Volcanic Islands (1844) and Geological Observations on South America (1846), are also reissued in this series.
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With the advent of widely available 3D seismic data, numerous workflows focused on extracting subsurface stratigraphic information have been developed. We present here tools and rules that can maximize the amount of geologic information that can be extracted from seismic volumes. The fundamental principle enhancing the value of geologic insights extracted from seismic data is the integration of two disciplines: (1) seismic stratigraphy (i.e., images derived from section views that yield insights into stratigraphic architecture); and (2) seismic geomorphology (i.e., images derived from plan views that yield insights into paleo landscapes). Both disciplines leverage the possibility of generating subsurface images in 3D with the objective of interpreting depositional environments and better predict lithofacies distribution both spatially and temporally. A second principle is that interpretation of geologically meaningful patterns derived from these two disciplines must be in agreement with each other. That is, interpretations based on observations from the geomorphological domain must be corroborated by interpretations based on observations from the stratigraphic domain, and vice versa. To achieve this, the workflows presented here illustrate multiple analytical approaches, which include: (1) initial reconnaissance through 3D volumes in several observational domains (e.g., section, plan and perspective views) with various slicing techniques and animation tools; (2) detailed focus on features of geologic interest that have been identified through reconnaissance, with further investigation through a combination of detailed slicing, horizon picking and seismic attributes calculation; and (3) comprehensive integration of seismic geomorphologic with seismic stratigraphic analyses to ensure consistent and reasonable interpretation of depositional environments and lithofacies. Context is highlighted as a critical underlying aspect of these workflows to help differentiate between reasonable and unreasonable interpretations, which constitutes a third principle of seismic stratigraphy and seismic geomorphology. It is not uncommon that a pattern observed in section or map view might be a nonunique indicator of a depositional environment; hence, knowing the context can be critical in making the correct geological interpretation of these non-unique patterns. From past to future, seismic stratigraphy and seismic geomorphology are two disciplines that, when integrated, play a fundamental role in resource exploration / production (e.g., hydrocarbon, water, hydrogen) and carbon storage, as well as providing enhanced understanding of the past evolution of Earth and prediction of its future (e.g., sea level and climate). Hence, awareness of the broad range of workflows available to extract geologic insights from seismic data is critical for both applied and fundamental sciences.