The emergence of Miocene reefs in South China Sea and its resilient adaptability under varying eustatic, climatic and oceanographic conditions

Abstract

During the Miocene, extensive carbonate deposition thrived over wide latitudinal ranges in Southeast Asia despite perturbations of the global climate and thermohaline circulation that affected the Asian continent. Nevertheless, the mechanisms of its emergence, adaptability in siliciclastic-dominated margins and demise, especially in southern South China Sea (SCS), are largely speculative and remains enigmatic along with a scarcity of constraints on paleoclimatic and palaeoceanographic conditions. Here we show, through newly acquired high-resolution geophysical data and accurate stratigraphic records based on strontium isotopic dating, the evolution of these platforms from ~15.5–9.5 Ma is initially tied to tectonics and eustasy, and ultimately, after ~9.5 Ma, to changes in the global climate patterns and consequent palaeoceanographic conditions. Our results demonstrate at least two paleodeltas that provided favourable substratum of elevated sand bars, which conditioning the emergence of the buildups that inadvertently mirrored the underlying strata. We show unprecedented evidences for ocean current fluctuations linked to the intensification of the Asian summer monsoon winds resulting in the formation of drifts and moats, which extirpated the platforms through sediment removal and starvation. This work highlights the imperative role of palaeoceanography in creating favourable niches for reefal development that can be applicable to carbonate platforms elsewhere.

Full text

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The emergence of Miocene reefs
in South China Sea and its resilient
adaptability under varying eustatic,
climatic and oceanographic
conditions
Manoj Mathew1 ✉ , Adelya Makhankova2, David Menier3, Benjamin Sautter2,
Christian Betzler4 & Bernard Pierson5
During the Miocene, extensive carbonate deposition thrived over wide latitudinal ranges in Southeast
Asia despite perturbations of the global climate and thermohaline circulation that aected the Asian
continent. Nevertheless, the mechanisms of its emergence, adaptability in siliciclastic-dominated
margins and demise, especially in southern South China Sea (SCS), are largely speculative and remains
enigmatic along with a scarcity of constraints on paleoclimatic and palaeoceanographic conditions.
Here we show, through newly acquired high-resolution geophysical data and accurate stratigraphic
records based on strontium isotopic dating, the evolution of these platforms from ~15.5–9.5 Ma is
initially tied to tectonics and eustasy, and ultimately, after ~9.5 Ma, to changes in the global climate
patterns and consequent palaeoceanographic conditions. Our results demonstrate at least two
paleodeltas that provided favourable substratum of elevated sand bars, which conditioning the
emergence of the buildups that inadvertently mirrored the underlying strata. We show unprecedented
evidences for ocean current uctuations linked to the intensication of the Asian summer monsoon
winds resulting in the formation of drifts and moats, which extirpated the platforms through sediment
removal and starvation. This work highlights the imperative role of palaeoceanography in creating
favourable niches for reefal development that can be applicable to carbonate platforms elsewhere.
A gargantuan modication of the planetary climate system and the thermohaline circulation patterns occurred in
the Cenozoic with the inection point being diused from exceptionally critical tectonic processes that aected
Asia1,2. At ca. 24 Ma, in close proximity to the Oligocene–Miocene boundary, the continent experienced a trans-
formation of climatic processes from a latitudinal zonal circulation pattern to a monsoon-dominated congu-
ration along with the evanescing of an extensively prevalent subtropical-aridity across Asia and the inception
of aridity that was exclusively restricted to the hinterland of eastern Asia3. e Asian continent experienced a
dynamic climate transition that was instigated by the Paleocene–Eocene collision of the Indian Plate with Asia
and the resulting progressive upli of the Tibetan Plateau (TP). is caused an array of feedbacks that signif-
icantly strengthened monsoon-inducing atmospheric circulation with each of its major orogenic pulses4 (i.e.,
at (i) ca. 40–35 Ma; (ii) ca. 25–20 Ma; and (iii) ca. 15–10 Ma [refer to Fig.1 for (ii) and (iii)]). Over a span of
~30 Ma, the stepwise exhumation of the TP governed an accelerated amplication of rstly, the Indian Summer
Monsoon (ISM) and the Somali Jet with the initial pulse of upli; secondly, the Southeast Asian Monsoon, the
East Asian Summer Monsoon (EASM) and the East Asian Winter Monsoon (EAWM) with 40% upli of the TP;
and lastly, the further intensication of the EASM and EAWM with 80% upli of the TP5,6 (Fig.1). is last upli
1Shale Gas Research Group, Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS, 32610, Bandar
Seri Iskandar, Malaysia. 2Department of Geosciences, Universiti Teknologi PETRONAS, 32610, Bandar Seri Iskandar,
Malaysia. 3Laboratoire Géosciences Océan (LGO), Université Bretagne Sud, 56017, Vannes, Cedex, France. 4Institute
of Geology, CEN, University of Hamburg, Bundesstrasse 55, 20146, Hamburg, Germany. 5GEO-Instituut, Campus
Arenberg, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001, Leuven, Heverlee, Belgium. ✉e-mail:
manoj_mathew7@yahoo.com
OPEN
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event concomitantly established the abrupt onset of the vigorous South Asian Monsoon (SAM) circulation winds
at ca. 12.9 Ma7,8. Consequently, the gradational withdrawal and closure of the Paratethys seaway additionally
strengthened atmospheric gradients and seasonal wind velocities in Asia7. Atmospheric circulation could have
been further modied owing to changes in global ice volume (Fig.1) and temperature drops that resulted in
major global glacial events (Mi1–7) in the Miocene as documented in marine δ18O records9,10 (Fig.1) and proxy
estimates, which validated large-scale decline of CO2 in the Oligocene until ~24 Ma11,12. e buildup of organic
carbon-rich sediments during the Early–Middle Miocene Climate Optimum (MMCO) augmented the inorganic
carbon isotopic value of foraminiferal calcite13, resulting in carbon maxima (CM) events (CM1–7) as recorded
by global deep-water benthic δ13C archives (Fig.1). In conjunction with the aforementioned events, the opening
of the South China Sea (SCS) exercised an indispensable control over climatic forcing through the exacerbation
of the north–south contrast of humidity, and particularly in Southeast Asia and Sundaland, where, following a
“ridge-jump” event14, climatic changes were triggered by the inception of seaoor spreading in the southern SCS
at ca. 24 Ma15.
e magnitude of tectonic activity in Southeast Asia was pervasive near the Oligocene–Miocene bound-
ary with the indentation of the Australian Plate within the southern Sunda Plate. is inherently pivotal event
resulted in the tectonic restriction and withdrawal of a former deep and wide marine seaway (between 25 Ma and
22 Ma), the Indonesian Gateway, that separated the two continental shelves and culminated in widespread reper-
cussions in terms of paleotopography, paleobathymetry, ecology and the organization of continental exposure2.
ese modications in boundary conditions created a widespread impact to the global thermohaline circulation16
and to ocean gyres that previously channelled the equatorial transfer of water from the West Pacic to the East
Pacic through the Indian, Tethyan and Atlantic Oceans17. is was synchronous with a distinct deep-sea δ13C
Figure 1. Overview of eminent climatic, atmospheric and tectonic events for the Early Oligocene to Late
Miocene. Global deep-sea carbon and oxygen isotope records from 6–32 Ma are graphically depicted in
orange and blue respectively. e isotope data interpreted from10, is an amalgam of more than 40 Deep Sea
Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites and demonstrates several peaks and steps
that corroborate to occurrences of global warming and cooling and the expansion and decay of ice sheets. e
timing of globally-recognized Middle Miocene Climate Optimum (MMCO), Late Oligocene warming, Miocene
glacial events (Mi1–Mi7) and carbon maxima (CM) events based on the analysis of benthic foraminifera are
interpreted from13. ick red line represents the reconstruction of proxy-based partial pressure of atmospheric
carbon dioxide (pCO2) that has been interpreted from63. Bars in light and dark shades of blue and dark blue
arrows indicate the brief history of Northern Hemisphere and Antarctic ice sheets. Black bars marked with
(ii) and (iii) show the temporal upli pulses of the Tibetan Plateau (TP), which is adapted from4. Note that the
initial upli pulse (i) is older (ca. 40–35 Ma) than the time frame illustrated in the abscissa. Also shown are the
timing of Southeast Asia coral reef expansion and intensication of the East Asian Summer Monsoon (EASM)
and East Asian Winter Monsoon (EAWM) in grey bars. Figure labels were added using Adobe Illustrator
version CS5.1 (http://www.adobe.com/products/illustrator.html).
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excursion revealing a Late Oligocene warming18 (Fig.1) and the Mi1 glaciation at ca. 23 Ma. e collisional event
also created bathymetric barriers in the Philippines that restricted the deep to intermediate, nutrient-rich cool
water oceanic through ow from the Pacic to the Indian Ocean, which, at ca. 24–23 Ma, is hypothesized to have
facilitated the acme of Southeast Asian coral reef expansion (Fig.1), of which 70% formed as land-attached fea-
tures, following changes in trophic resources and lateral expansion of the photic zone through the emergence of
newly-exposed littoral zones19. Despite the rarity of Paleocene and Early Eocene carbonates in entire Southeast
Asia, carbonate production in the Cenozoic was observed in the shallow-marine seas of Southeast Asia20 (Fig.2)
with a minor progressive increment in carbonate production in the Late Eocene such as, for example, in east-
ern SE Asia and along the margins of marine extensional basins (e.g.20–22), during the Late Eocene and Early
Oligocene in New Guinea (e.g.23,24) and in the early Late Oligocene in parts of Java and Sumatra (e.g.25) and
the Philippines (e.g.26). However, at the Oligocene–Miocene boundary, there was a profound expansion in the
extent of reef growth that was accompanied by a change from a larger foraminiferal to coral dominance in the
shallow-water realm27. e acme of reefal growth in Southeast Asia (Fig.1) lags the Oligocene coral reefs max-
imum of the Caribbean and Mediterranean, likely due to regional and global controls that include, but is not
Figure 2. Occurrences of Cenozoic carbonates in SE Asia. Red geometries interpreted from20, demonstrates the
distribution of Cenozoic carbonates in and around the shallow seas of Southeast Asia. Solid (50 m isobath) and
dashed (200 m isobath) lines in black and pink colours that is interpreted from45, represent paleobathymetric
reconstructions based on sea level highstands for two (2) time slices, i.e., Langhian (15 Ma) and Serravallian
(12 Ma), respectively. Shades of blue show modern ocean bathymetry, while, black and white arrows indicate the
general direction of present-day wind regime during summer monsoon (north-eastward) and winter monsoon
(south-westward), respectively. Base map is a SRTM (Shuttle Radar Topographic Mission) Digital Elevation
Model (DEM) of 30 m (1-arc second) spatial resolution (SRTM 1 Arc-Second Global elevation data courtesy of
the U.S. Geological Survey, https://lta.cr.usgs.gov/SRTM1Arc) and gridded bathymetric data is from the General
Bathymetric Chart of the Oceans (GEBCO) (15 Arc-Second global ocean and land terrain models courtesy of
the International Hydrographic Organization [IHO] and the Intergovernmental Oceanographic Commission
[IOC] of UNESCO, https://www.gebco.net/data_and_products/gridded_bathymetry_data/). e map was
created using Geographic Information Systems (GIS) soware ESRI ArcGIS version 10.3 (http://www.esri.com/
soware/arcgis/arcgis-for-desktop). Figure labels were added using Adobe Illustrator version CS5.1 (http://
www.adobe.com/products/illustrator.html).
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limited to, changes in atmospheric CO2 levels, pCO2 values that had fallen to pre-industrial levels (Fig.1), oceanic
Ca2+ and Mg/Ca ratios, and reduced salinities in humid equatorial waters.
Concurrently, in addition to all the aforesaid tectonic and climatic changes in Southeast Asia, especially in
the southern South China Sea region, large deltas rapidly prograded into the surrounding deep basins at the
Oligo–Miocene boundary2,17,28. e exact paleogeography of these Paleogene–Neogene deltas, however, is only
partly known because of the limited amount of data available29,30 and because tectonic movements could have
overprinted the depositional geometries.
Reef distribution in Southeast Asia is presumed to have declined at the end of the Early Miocene owing to
increased continentality, augmented oceanic ventilation and increased seasonal runo, among others27. Although
the deterioration was critical in the southern realms of the South China Sea, the Middle–Late Miocene Luconia
platforms continued their expansion over a spatial extent31 of ~4.5 ×104 km2 (Fig.3A). However, because of
a severe decit in the availability of continuous data records of carbonates from this region, the theoretical
underpinning of the mechanisms of evolution and demise of the platforms are largely speculative and prevails
as a conundrum along with a dearth of tight constraints of paleoclimatic and palaeoceanographic conditions.
Additionally, in this region, denite evidences of the capability of ancient reefal systems to be opportunistic col-
onizers by conveniently selecting a favourable siliciclastic substratum to grow on and to inadvertently mirror the
morphology of the underlying bedforms that elucidate preceding morphodynamic and oceanographic processes
remains shrouded in mystery. Here, informed by newly acquired high-resolution geophysical data, together with
accurate stratigraphic, sedimentological and geochemical records, we show through clear evidences that the evo-
lution of the Middle Miocene carbonate platforms in the southern South China Sea is explicitly tied initially to the
response of the sedimentary system to regional geodynamics and resultant sea level oscillations, and ultimately
to changes in the global climate patterns. Our data produces evidences for ocean current uctuations and we
demonstrate that one of the most critical factors governing the Late Miocene demise of the buildups was indis-
pensably organized and controlled by the intensication of the Asian summer monsoon winds. Auxiliary to these
ndings, this work highlights the relevance and the imperative role of palaeoceanography in creating favourable
niches and environments for reefal development that can be applicable to other Cenozoic carbonate platforms in
Southeast Asia and other parts of the World.
The emergence and adaptability of Middle Miocene carbonate systems of southern South
China Sea. A detailed analysis of our seismic proles and strontium isotope dating from the southern South
China Sea reveals that the carbonates initiated at ~15.5 Ma (Fig.3C,D) and it can be observed that these Middle
Miocene carbonate platforms overlie distinct progradation features of paleodelta systems (see Supplementary
Fig.S1), which prevailed in this location during the Oligo–Miocene transition2,17,28. e analysis of the base of
carbonates from horizon maps and the evaluation of generic and detailed morphologies of the platforms reveals
two unequivocally distinguishable patterns of platform growth (Fig.3A). e southern set of platforms are man-
ifested in a linear conguration with a predominant N–S orientation, contrasting with the NE-SW carbonate
edices to the north. Our seismic reection-derived horizon surfaces, calibrated to a Middle Miocene age as
constrained through strontium isotopic data, clearly displays the presence of interplatform seaways (Fig.4A)
containing subaqueous dunes (Fig.4A–D). Additionally, the interplatform seaways that are conned between
the carbonate platforms display an internal architecture that demonstrates repeated incisions by possibly tidal
channels and lling phases from above the 15.5 Ma boundary (see Supplementary Fig.S1) because interplatform
seaways predominantly represent passages of amplied tidal current velocity32,33. us, based on our evidences,
the interplatform seaways in our data can be interpreted as corridors characterized by tide-driven hydrodynam-
ics that may have formed the large subaqueous dunes. e subaqueous dunes appear to more or less follow an
orientation that is akin to the regional structural trend, which highlights the fact that the tectonic history of
an area can oen be inherited and control the location of subsequent geomorphic features; however, from our
data, we observe that while the paleotopography could have laid the foundation for initial sediment accretion,
most of the subaqueous dunes appear to be undisturbed and devoid of faulting (Fig.4C,D). Below the 15.5 Ma
boundary, observations can be made of a sedimentary architecture resembling a progradational deltaic system
with numerous channels incepted during progradation and that operated as networks to facilitate the transport of
sediments from continental land masses to deep marine basins. e causative mechanisms of overlying carbonate
development within a deltaic setting in this area are envisaged through a conceptual model (Fig.5) that is pre-
sented herein. During the Oligo–Miocene transitional period, prolonged subsidence and extensional normal fault
systems were ubiquitous in the southern Sundaland following regional crustal stretching and extreme thinning in
the SCS2. Because the balance between deposition and erosion is predominantly inuenced by hydraulic velocity
and sediment ux34, under moderate velocities and exuberant sediment supply, as in the case of the present study
location (e.g.,29,30), along with the subsequently created accommodation space and the onset of rapid sea level
rise could have resulted in the progradation of siliciclastic material along coastlines in the form of deltas with
developed sand bars and linear sand ridges (Fig.5A). Furthermore, the subsequently formed structural dislo-
cations would have been favourable niches for bi-directional tidal channels and the smaller tidal distributaries
(Fig.5A) could have fed the master channels and aided in the export and redistribution of siliciclastic sediments.
Until the Middle Miocene, owing to further escalated eustatic levels, successive ooding of the lowstand deltaic
features such as tidal sand bars could have provided an auspicious period for reefal communities to establish on
the seaward margin of structural highs and atop strata hosted by the older deltaic siliciclastic deposits that could
be maintained on the structural highs (Fig.5B); thus, mimicking the geometry of the substratal deltaic sand
bodies. is growth pattern can be witnessed in modern analogs of other tropical mixed siliciclastic-carbonate
systems around the world and has been substantially documented in35. e reefal communities would have been
compelled to opportunely chose the inundated tidal sand bars as substratum that could have been located on
structurally-inherited, elevated seaoor morphologies because the height of the water column and photic zone
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Figure 3. e Middle Miocene carbonate platforms of Luconia, southern South China Sea. (A) Bright blue
geometries illustrate subsurface Miocene carbonates of Luconia spread over a spatial extent of ~4.5 × 104
km2 identied through seismic horizon maps of the southern South China Sea and based on31. Note the two
(2) clearly distinguishable patterns of platform growth that can be observed (i.e., linear platforms with a
predominant N–S orientation toward the south and NE-SW directed platforms to the north). Color-coded
continental elevation and bathymetric depth are shown along with major faults indicated with black solid lines
that are based on64. Also shown, in shades of grey, are the modelled Miocene paleogeographic limits adapted
from29. e paleogeography was constrained through sedimentological and stratigraphic evidences contained
within clastic deposits from deep cores and onshore outcrops. Solid black bounding box shows the location
and extent of the dataset used in the study (refer to methods for details of the geophysical dataset). (B) Top
carbonate seismic horizon map of a chronologically-accordant carbonate platform that was substantially
studied herein. Red circles identify locations of drilled wells that penetrated various sequences of the platform
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would have been optimal for biogenic ecosystems to ourish. As seen in our results, colonization does not appear
to exist in most of the interplatform seaways (see Supplementary Fig.S1) that was created following a transition
from a deltaic setting to a marine system. is could illustrate that continental deposits and reef debris could
have been constantly exported down-slope along possible tidal channels contained within these interplatform
seaways that could resemble present-day river network patterns as mentioned in31. Because the sediment load
transferred by the channels is regulated by the inuence of continental landscape dynamics that are orchestrated
by endogenic and exogenic forcing, transiently conveyed sudden spikes of localized sediment ux can inundate
the channels and constrict the pathways that eventually incite an exacerbation of tidal current velocity and tur-
bidity (Fig.5B).
Late miocene shift in carbonate deposition dynamics. The copious accumulation of Mid–Late
Miocene carbonate platforms in southern South China Sea transpired following regional tectonic events that
occurred during the Oligocene to Middle Miocene and involved a network of deep-rooted normal faults36 asso-
ciated with continuous subsidence. e ensuing basement geometry was characterized by dierentially elevated
blocks that could have contained tidal sand bars atop which carbonate platforms grew during the intermediate
Miocene and many of these locations are still presently sites for reefal edices37.
e modern oceanic currents in this region are principally governed by a dynamic wind gradient from the
Pacic to the Indian Ocean and the Southeast Asian and South Asian Monsoon38. ese semi-annually reversing
winds that cause surface current patterns to reciprocally reverse are generally directed north-eastward during
the summer monsoon and south-westward during the winter monsoon (Fig.2), and are integral in the hydro-
logical, chemical, sedimentological and sea-surface circulation dynamics in this part of the planet. Indeed,
counter-clockwise eddies and stronger currents are additionally observed in the western part of the South China
Sea36.
High-resolution geophysical data obtained and interpreted by us in this work and augmented by detailed core
and thin section descriptions shown in39, and strontium isotopic chronology that was integrated and calibrated
with palaeontological records, facilitated the robust constraining of the age and limits of sequence boundaries
(SB) (Figs.3C and 6A) with carbonate packages that reect distinct evolutionary characteristics evidencing the
adaptability of corals to successive periods of climatic and tectonic stress. Because all the carbonate buildups
of this region are stratigraphically well-constrained and documented to be of Middle–Late Miocene age15, we
analysed an ideally representative and chronologically-accordant carbonate platform in this region (Fig.3B,C)
that demonstrates a phase of aggradation and progradation during the inception stage (Fig.6A). is phase can
be associated with global climatic transitions and regional tectonics-induced eustatic uctuations that disrupted
platform development during events of lowstand. Our interpretation of the carbonate system, chronology and
fault architecture of the southern South China Sea suggests that the platform initiated at ~15.5 Ma (Fig.3C,D),
in the later stages of the Middle Miocene Climate Optimum (MMCO) during which the global eustatic levels
were considerably high (Fig.6A), compelling the platform to aggrade on deltaic sand bodies localized on elevated
inverted sedimentary substratum (Fig.3C and S1). Following the aggradation phase, the carbonate banks that
reveal syn-depositional faulting, expanded laterally for ~2 km through progradation in ESE and ENE directions
(Fig.3E) that is a diagnostic feature of platform response under lowered sea levels. Hence, we associate this
expansion with the transition of the planetary climate system from the warm MMCO towards a cooler setting
accompanied by a global eustatic drop during the Mi3b glaciation event owing to the proliferation of the East
Antarctic Ice Sheet at ~13.9 Ma (Figs.1 and 6A). is period of platform extension lasted until ~12.5 Ma marked
by the sequence boundary SB1 (Fig.6A) where the carbonate platform portrays a distinct change in evolutionary
pattern and appears to shrink through backstepping/aggradation as global sea levels climbed (Fig.6A). ere
was exposure of the platform at ~9.5 Ma that is reected in the global eustatic curve as a steep drop in sea level
(Fig.6A), and this coincides with features resembling karsts (Fig.7A) represented by intensively dissolved and
leached limestone in the upper parts of the seismic unit in the cored intervals as described in39, hence character-
izing subaerial exposure and represents the SB2 of the edice. At ~8.8 Ma, as global sea levels rose once again, the
carbonates continued a similar pattern of backstepping/aggradation as seen in the preceding sequence till SB3
and yielded sediment cores which were interpreted in this research. White line marks the position of seismic
line shown in “C”. (C) Interpreted seismic line showing the studied platform in shades of yellow and the basal
substratum in pink. Also shown are stratigraphic ages and important sequence boundaries (SB) constrained
through strontium isotopic dating. Drilled wells are shown in solid red lines. Faults and sense of motion is
shown in black solid lines and adjacent arrows, respectively. (D,E) Spectral decomposition maps for two (2)
time slices, i.e., 15.5 Ma and ~13.9 Ma. Note the individual reefal buildups in “D” followed by coalescing and
progradation of the eastern end of the platform in a predominantly ESE and ENE direction in “E”. Base map
in (A) is a SRTM (Shuttle Radar Topographic Mission) Digital Elevation Model (DEM) of 30 m (1-arc second)
spatial resolution (SRTM data courtesy of the U.S. Geological Survey, https://lta.cr.usgs.gov/SRTM1Arc)
and gridded bathymetric data is from the General Bathymetric Chart of the Oceans (GEBCO) (15 Arc-
Second models courtesy of the International Hydrographic Organization [IHO] and the Intergovernmental
Oceanographic Commission [IOC] of UNESCO, https://www.gebco.net/data_and_products/gridded_
bathymetry_data/). e map was created using Geographic Information Systems (GIS) soware ESRI ArcGIS
version 10.3 (http://www.esri.com/soware/arcgis/arcgis-for-desktop). Base maps and images in (B–E) were
visualized, interpreted and constructed with Eliis PaleoScan soware version 2018.1.0 (http://www.eliis.fr/
products/paleoscan%E2%84%A2-soware). Figure labels were added using Adobe Illustrator version CS5.1
(http://www.adobe.com/products/illustrator.html).
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(Fig.6A) along with dissolution features (Fig.7B) that could be related to local tectonic events40,41 promoting spo-
radic exposure. It is interesting to note that the sediment accumulation rates (Fig.6A) calculated for the platform
demonstrates a relatively stable trend of slow growth and devoid of sharp peaks before the start of the next seismic
sequence. A dramatic spike in accumulation rate is seen from ~8.8 Ma that terminates with the SB4 at ~8.6 Ma
(Fig.6A). During this period, the platform appears to shrink drastically through a pronounced backstepping/
aggradation of ~13.2 km in an ENE direction. While a perceptible rise in global eustatic levels are identied aer
~9.5 Ma and ~8.8 Ma, large-scale lobate clinoform features are explicitly recognizable in our seismic sections
and horizon map that we interpret as contourites with well-dened dri and moat geometries (Figs.8, 9A,B),
evidencing a dynamic deposition by intense wind-driven currents showing an unambiguous physical record of
the strong inuence of monsoonal winds. With the subsequent rise of eustatic sea levels, strong currents could
have scoured large parts of the Luconia platforms that may have prevented the regeneration of shallow-marine
ecosystems. Along with rising sea levels, this could have been one of the critical factors that forced the platform to
backstep over a large distance (Fig.3C) until its demise at ~8 Ma. is phenomenon marked an eminent change
in depositional dynamics from a system inuenced by global sea level uctuations to an intrinsically organized
framework controlled by wind-driven oceanic currents.
Discussion and implications. Conclusive inferences obtained from our results and interpretation of the
same suggests that the palaeoenvironment of the southern South China Sea was initially controlled by regional
tectonic activity and eustasy in the Early Miocene that saw the widespread progradation of deltas into deeper
marine basins2,17,28. While the existence of some paleodeltas are documented because they are conformant with
present-day shorelines (e.g., Rajang paleodelta42), many remain elusive owing to an absence of accordant current
shorelines, burial by overlying sediments and morphodynamic processes over geological time. e morphody-
namic and oceanographic processes that inuenced these deltas are much less understood, peculiarly, due to a
lack of direct supportive evidences and uncertainties regarding paleoclimates and paleotopography. A few studies
have focused on the sedimentological aspects of prominent paleodeltas in this region using well data, outcrop
descriptions that could be considered as equivalents to oshore geology and by considering regional models
(e.g.30). Our ndings conrm and precisely highlight the presence and classication of at least two paleodeltas by
observing the growth patterns of Middle Miocene carbonate platforms that we demonstrated to have evolved on
a substratum organized by deltaic sand bodies and during growth, inadvertently mimicked the underlying strata
Figure 4. (A) Color-coded seismic reection-derived horizon surface map for the time slice of ~15 Ma. White
dashed-lines distinguish the spatial extent of the carbonate platforms from the interplatform seaways that host
a series of subaqueous dunes. ick black solid lines demonstrate major structural discontinuities aecting the
platform and basement along with the sense of movement that resulted in an array of dierentially elevated
blocks. (B) Zoomed image of the subaqueous dunes shown in “A” and the black line marked X–X’ shows the
position of the seismic line displayed in “C”. ( C,D) Seismic prole across the subaqueous dunes and subsequent
interpretation of the prole. Note that the subaqueous dunes are undisturbed by faults that do not penetrate the
sedimentary structures. Base maps and images in (A–C) were visualized, interpreted and constructed with Eliis
PaleoScan soware version 2018.1.0 (http://www.eliis.fr/products/paleoscan%E2%84%A2-soware). Figure
labels were added using Adobe Illustrator version CS5.1 (http://www.adobe.com/products/illustrator.html).
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Figure 5. (A) e conceptualized evolution of a large Oligo–Miocene delta that formed under conditions
of exuberant sediment supply, prolonged subsidence and accelerated sea level rise that facilitated the rapid
progradation of siliciclastic material to form well-developed tidal deltaic sand bars and linear sand ridges.
Regional crustal stretching and extreme thinning in the SCS2 resulted in wide-spread normal faulting that
created ow paths for deep master channels characterized by bidirectional hydrodynamics. Smaller tidal
distributaries owed on the horsts that shaped the morphology of the sand bars and exported and redistributed
siliciclastic coastal sediments within the delta system. (B) A further rise of eustatic levels in the Middle
Miocene could have led to progressive ooding of the lowstand deltaic sand bars. Since the inundated sand
bars were located on structurally-inherited elevated seaoor morphologies, the depth of the water column and
photic zone would have been auspicious for biogenic ecosystems and these conditions compelled the reefal
communities to establish themselves on the seaward margin of structural highs and along the margins of the
tidal channels. Conversely, under rising sea levels, the structural lows became deeper and no reefs developed
here and within the deep and widened interplatform seaways that could have been continually transporting
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(Fig.5B). e horizon map illustrating the base of carbonates in the Luconia zone of southern South China Sea
clearly shows two distinct growth morphologies/geometries that are predominantly oriented N–S in the south-
ern platforms and NE–SW in the northern platforms, respectively (Fig.3A). e elongated morphologies of the
carbonate platforms located in the south are indicative of developing on wave-dominated delta-derived sand
bars. e direction of the paleoshoreline hosting the delta could have trended roughly in an E–W direction that
is perpendicular to the sand bars and is consistent with the modern coastline of the north-western region of
Borneo Island (Figs.2 and 3A). On the other hand, the platforms in the north portray edices that are sizeably
wider and thicker with denitive appearances of interplatform seaways (Figs.3A and 4A) that can be traced to
the deeper zones in our seismic proles (see Supplementary Fig.S1). Inferences regarding the origin of the sand
bars (Fig.5A) chosen by the northern platforms to establish themselves on suggest that they were deposited by
a tide-dominated delta system and the presence of extensive laterally-channelized sequences in the Oligocene–
Miocene sediments below the ~15.5 Ma boundary, as visualized from our seismic proles (see Supplementary
Fig.S1), further validates our interpretation. e paleoshoreline could have been oriented in a trend that is per-
pendicular to the general NE–SW direction of the platforms, which is congruent to previous estimations of a
NW–SE direction of ancient coastlines in this area with the oshore basins deepening towards the north and
NE (e.g.30,43,44). Modelled paleogeographic locales of the coastal plain (Fig.3A) (e.g.29), changes in subsurface
sedimentology of clastic sequences that predate the carbonate platforms (e.g.30) and paleobathymetries (Fig.2)
(e.g.45), provide additional conrmation for the paleoenvironmental setting that is discussed in this work.
Considering the immense volume of basin-lling clastic sediments since Oligocene–Early Miocene from the
hinterland of the coastlines that are perpendicular and adjacent to each other, with a NW–SE trend in the western
side and E–W trend in the southern side of Borneo, it can be deduced that the ux would have to be sourced
from uplied landscapes containing monumentally high relief. ese imposing orogens could have guised as
obstructions that sheltered the southern South China Sea, and as a result, considerably altered wind, ocean-
ographic, morphodynamic and depositional patterns. As mentioned previously, our results based on seismic
reections, core data and supplemented by precise isotopic chronology of Luconia carbonate platforms displays
a good correlation with regional tectonic events and global sea level evolution in the Middle Miocene; however,
in the Late Miocene, a strong shi towards planetary climate system as the primary controlling factor is evidently
observable. e evolutionary pattern of the platform, which initiated at ~15.5 Ma, bears close resemblance to the
growth cycles of carbonate edices in the Maldives7 since the Middle Miocene, in terms of timing of aggradation/
progradation, intermittent drowning and platform backstepping (Fig.6A). While the initial stages of aggradation
reected in our data can be validated through a global eustatic high of the MMCO (Figs.1 and 6A), platform
progradation to the ENE and ESE (Fig.3E) can be attributed to a wind regime that was generally directed to the
eastern side of the platforms, because carbonates favourably tend to prograde towards the leeward side7,46. is
wind pattern is peculiar of the Asian summer monsoon winds that control oceanic current systems and akin to
the present-day climatic circulation experienced in the entire South China Sea38 (Fig.2). Although the summer
monsoon winds aected this region in the Middle Miocene to Late Miocene, its eects during platform inception
could have been weaker, and we base this postulation on two lines of evidences. Firstly, the aforementioned high
relief topographic barriers that surrounded the area could impede on-coming winds from the west, and secondly,
the absence of contourite deposits driven by wind-controlled currents in the deeper sequences from ~15.5 Ma
to ~9.5 Ma. During the later Late Miocene, a sudden initiation of oceanic current controlled sedimentation sur-
rounding the platforms in the form of dri and moat geometries are depicted in the horizon map constructed
herein (Figs.8, 9A and 9B). Currents have the capability to alter the morphology of platform slopes through
sediment removal and starvation, by generating dris around carbonate edices that in seismic proles appear as
convex outward prograding features which onlap and bury sequences of platform carbonates, by forming moats
at the toe of slopes. e application of such a stress on reefal systems can result in backstepping of the bank mar-
gins or slope steepening7,47. We propose that the dri and moat formation in parts of the southern South China
Sea started aer ~9.5 Ma (Figs.8, 9A and 9B) when the platforms backstepped in an ENE direction (Fig.9B)
following morphological modication of the carbonates that could more than likely have been orchestrated by
intense currents generated by the amplication of the Asian summer monsoon winds along with rising sea levels,
resulting in the shrinking of the buildups. Carbonate platforms in the Maldives recorded similar dri bodies and
moat features with the sudden onset of intense Asian summer monsoon winds at ~12.9 Ma and a predated weaker
proto-monsoon7,8,47 (Fig.6A) and this would imply a considerable time lag in the onset of strong summer mon-
soonal winds in southern South China Sea that could be dictated by the highly elevated orogenic barrier. ough
we cannot completely disregard the probable existence of a weaker current system that was incapable of creating
dri sequences prior to later Late Miocene, the abrupt intensication of the northeast-blowing summer monsoon
winds, strong currents and subsequent leeward backstepping aer ~9.5 Ma is likely to be due to a lowering of
topographic elevation of the western orogens that previously protected the area. Indeed, previously documented
paleogeographic reconstructions have testied that the western orogen no longer separated the southwestern and
southern South China Sea in the later Late Miocene45. We present a conceptual sketch (Fig.9C) summarizing the
stratigraphy of the study area. e gure illustrates carbonate growth atop Oligocene–Early Miocene deltaic tidal
sand bars that were located on structural highs and the presence of interplatform seaways that formed in response
to rising eustatic conditions and remained conned in between the growing platforms. e model demonstrates
continental deposits and reef debris down-slope through tidal channels contained within the interplatform
seaways. However, sudden spikes of localized sediment ux could essentially ood the channels and constrict
the pathways that could eventually increase tidal current velocity and turbidity. e conceptual model was
constructed using Adobe Illustrator version CS5.1 (http://www.adobe.com/products/illustrator.html).
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Figure 6. (A) Regime diagram showing the age of sequence boundaries (SB1–4), important stratigraphic
breaks and changes in platform evolution proposed in this work and sedimentation rate of the studied Luconia
platform. Also shown is the growth cycles of carbonate edices in the Maldives adapted from7, since the
Middle Miocene along with timing of major climatic and oceanographic phenomena observed in the Maldives
record. Global sea level is graphically represented in blue and is interpreted from65. (B) Middle to Late Miocene
(16–8 Ma) benthic foraminiferal δ18O and δ13C records from IODP Site U1443 (Ninetyeast Ridge, Indian Ocean)
and ODP Site 1146 (northern South China Sea), and carbonate mass accumulation rate (CO3 MAR) for ODP
Site 999, Hole 999 A (Colombian Basin, Caribbean) compiled and interpreted from49,51,66 respectively. Grey
shaded bands indicate timing of “Carbonate Crash” event that marks episodes of intense impoverishment in
carbonate accumulation and strong carbonate-dissolution. Teal coloured arrows show main phases of glacial
expansion/deep water cooling. Figure labels were added using Adobe Illustrator version CS5.1 (http://www.
adobe.com/products/illustrator.html). e inset map shown in (B) was created using Geographic Information
Systems (GIS) soware ESRI ArcGIS version 10.3 (http://www.esri.com/soware/arcgis/arcgis-for-desktop).
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the major backstepping events related to sea level uctuations and contourite deposition as a result of strong cur-
rents from the abrupt intensication of the Asian summer monsoon winds aer ~9.5 Ma that formed well-dened
dri bodies and moats.
We expand upon our data and demonstrate that carbonate sediment accumulation rates show generally low-
ered sedimentation from ~15.5–8.8 Ma followed by a sudden spike aer ~8.8 Ma to ~8.6 Ma, following which,
the rates recede until the demise of the platform at ~8 Ma (Fig.6A). Despite having only a few global eustatic
highstands and Miocene glaciation events (Mi2b–Mi7) in the Middle–Late Miocene (Fig.1), a prolonged epi-
sode of reduced carbonate deposition has been witnessed globally between ~13.2 Ma and ~8.7 Ma. is marked
a decline and an intense impoverishment in carbonate accumulation and strong carbonate-dissolution episodes
termed “Carbonate Crash”, that was initially observed in the tropical eastern Pacic Ocean at the ODP site 84648,
and later at ODP sites 999 and 1146 in the Caribbean Sea49 and northern South China Sea50, respectively, and
more recently at IODP site U1443 in the equatorial Indian Ocean51 (Fig.6B). One of the fundamental reasons for
the Carbonate Crash was interpreted to be a feedback to changes in deep-water circulation and shoaling of the
carbonate compensation depth (CCD) and the crash event is essentially followed by a “Biogenic Bloom” that is
an epitome of a major increase in biogenous production and accumulation52. While these phenomena are better
constrained through specialized geochemical analysis, from our results, we posit that aer SB1 (Fig.6A), the low
carbonate sediment accumulation rates reected in our data from the southern South China Sea could likely
be linked to the “Carbonate Crash” because it is coeval with the crash in northern South China Sea at ODP site
1146 (Fig.6B). Moreover, according to the globally accepted cognizance of this event, a “Biogenic Bloom” always
follows the crash49; thus, we centre our inference on the fact that the timing of the end of the crash at ODP site
1146 and IODP site U1443 is synchronous with the large spike in sediment accumulation rates in Luconia aer
~8.8 Ma (Fig.6). Nevertheless, this bloom was short-lived in the southern South China Sea due to the abrupt
onset of the intensied Asian summer monsoon and accompanying vigorous currents that played a vital role
in the extirpation of the carbonate buildups. However, an interesting possibility for the demise of carbonate
buildups that cannot be overlooked is that a sudden abnormal surge in nutrient levels and increased productivity
resulting in higher carbonate mass accumulation rates can lead to the drowning of megabanks as observed in the
Northern Nicaragua Rise (e.g.53). While it is recognized that several factors including, but not limited to, intensity
of chemical weathering, changes in large-scale thermohaline circulation patterns, closure of important seaways
and increased continentalization can combinedly contribute to carbonate deterioration, our work provides con-
vincing evidences for conrming, i) the evolutionary mechanism of carbonates under dierential tectonics in
the southern South China Sea; ii) the adaptability of biogenic ecosystems to resiliently establish themselves on
unfavourable substratum under adverse eustatic uctuations, high turbidity and constant clastic inux in this
region; and iii) one of the crucial, if not the most crucial, factors governing the demise of carbonate platforms in
southern South China Sea through the indispensable domination of monsoonal wind-driven oceanic currents
that is regulated by planetary climatic circulation.
Figure 7. (A,B) Spectral decomposition maps for two (2) time slices, i.e., ~9.5 Ma and ~8.8 Ma showing karsts
as a result of a lowering of global sea levels and exposure of the platform. Base maps in (A,B) were visualized,
interpreted and constructed with Eliis PaleoScan soware version 2018.1.0 (http://www.eliis.fr/products/
paleoscan%E2%84%A2-soware). Figure labels were added using Adobe Illustrator version CS5.1 (http://www.
adobe.com/products/illustrator.html).
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Methods
Geophysical data and interpretation. The interpretation of carbonate sequences and related mor-
pho-structures was supported by a combination of high-resolution 3D seismic proles calibrated with well and
core data. A world-class seismic dataset covering an area of 420 km2 with a bin grid of 12.5 m x 12.5 m and
sampling rate of 4 ms was fully integrated in our regional study. e dataset that includes 1016 inlines and 1357
crosslines with a 25 m step were interpreted using Eliis PaleoScan soware. e data are characterized by normal
SEG polarity, where an increase of acoustic impedance is shown in positive amplitude, and a dominant frequency
of 30 Hz within the platform. Applying an average velocity range of 1965–2547 ms−1, the maximum vertical res-
olution was identied as 16–22 m.
irteen horizons from bottom to top were picked based on the occurrence of acoustic impedance peaks,
using the methodology by54. e seismic cross-sections constructed therein were vertically exaggerated (10×) to
optimize the visualization of carbonate platform morpho-structures.
e outstanding quality of our 3D seismic cube allowed to recognise detailed sedimentary and structural
features such as intra-sequences faults, karsts, channels and reefs among others, with the aid of multiple seismic
attributes. Spectral decomposition was identied as the most appropriate and functional attribute to convey karst
pattern and reef rim. For reef identication two frequency combinations were used; i) 18/32/60 using Short Time
Fourier Transform (STFT), and ii) 23/25/30 Continuous Wavelet Transform with Morlet wavelet (M). Karst pat-
tern were highlighted with a combination of 9/32/60 Hz (STFT) and 15/18/23 Hz (M). Resulting delineations of
reef and karst features were calibrated with core data.
e time-depth relationship, which was used to measure the thicknesses of the carbonate sequences in each
growth phase as well as identify the accurate fault dip and talus angles, was constructed using three check shots
with velocity values to create a brief velocity model for depth cube. e presence of a gas chimney in the middle
of the platform represents a processing artefact that could not be reduced in the current study.
Figure 8. Oceanic current controlled contourites in the southern South China Sea. 3D view of the horizon
pertaining to ~8.8 Ma showing intense Asian summer monsoon wind-driven current deposits identied in this
study with well-dened dri and moat geometries surrounding the carbonate platforms in Luconia. Also shown
are seismic cross-sections of the dri bodies and moat features along with ages of platform sequence boundaries
(SB) recognized and interpreted herein. Base maps and images were visualized, interpreted and constructed
with Eliis PaleoScan soware version 2018.1.0 (http://www.eliis.fr/products/paleoscan%E2%84%A2-soware).
Figure labels were added using Adobe Illustrator version CS5.1 (http://www.adobe.com/products/illustrator.
html).
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Strontium isotopic chronology. Two (2) chemostratigraphic analyses in Luconia were conducted in
previous studies by55 of Shell Sarawak and the Commonwealth Scientic and Industrial Research Organisation
(CSIRO), Australia, in 2009 that were compiled and integrated by us. e latest chronological dataset that was
produced herein involves the measurement of Sr isotope constitution in a VG 354 thermal ionization mass spec-
trometer at the CSIRO radiogenic isotope facility at North Ryde, Australia. e 87Sr/86Sr ratios were denominated
as the average of 54 measurements of ion intensities and normalized to 86Sr/88Sr = 0.1194 using an exponential
correction law. During the course of measurements, strontium standard reference NBS987 resulted in 87Sr/86Sr
values of 0.710235 (2σD error deviation, n = 54). Numerical ages were derived from the look-up table of56,57,
and from the curves of58,59. e earlier study contains an extensive dataset of 87Sr/86Sr values by59 and the isotope
analysis was conducted on 137 samples. All 87Sr/86Sr data were adjusted to a certied NBS987 value of 0.710248.
Both 87Sr/86Sr ratios and age calculations correlate very closely. e obtained data were then integrated with pal-
aeontological records based on the identication of larger foraminifera.
Sediment accumulation rate. Accumulation rates were computed in meters per million years (m.Ma−1)
as this ratio is equivalent to the Bubno unit60. e accumulation rates were estimated from age-depth plots by
drawing best-t lines over successive depth intervals from seismic data. e age-depth relationship was estab-
lished based on biostratigraphic and strontium isotopic ages from core data over averaged thicknesses for each
seismic unit. Compaction adjustment has not been accounted for, due to the absence of core data within the
siliciclastic overburden, thus, they represent minimal values. It must be noted that the rates are from the vertical
dimension, which indicates that they represent only a part of the clinothem accumulation and lateral prograda-
tion rates is estimated to be >100 times greater than vertical accumulation61.
Core description and sedimentary facies. To characterize the evolution of the carbonate platform, a
detailed description of ve (5) wells (Fig.3B,C) that includes 378 m of core was performed (please refer to39) in
terms of lithological changes using 10% HCl, component composition and their abundance, grain size, texture
using Dunham classication62, diagenetic overprints, visual porosity and pore types. Sedimentological dataset
was used for identication of litho-facies types.
Data availability
e datasets generated during and/or analysed during the current study are not publicly available due to condentiality
regulations set by Petroliam Nasional Berhad (PETRONAS), Malaysia. However, data are available from the authors
upon reasonable request and with permission of Petroliam Nasional Berhad (PETRONAS), Malaysia.
Received: 22 January 2020; Accepted: 9 April 2020;
Published: xx xx xxxx
Figure 9. (A,B) Interpreted seismic line showing dri bodies and moat geometries and backstepping of the
platform related to sea level uctuations and contourite deposition following strong currents from the abrupt
intensication of the Asian summer monsoon winds aer ~9.5 Ma. (C) Conceptual sketch summarizing
the stratigraphy of the study area (refer to text for details). Base maps and images in (A,B) were visualized,
interpreted and constructed with Eliis PaleoScan soware version 2018.1.0 (http://www.eliis.fr/products/
paleoscan%E2%84%A2-soware). Figure labels were added using Adobe Illustrator version CS5.1 (http://www.
adobe.com/products/illustrator.html). e conceptual sketch in (C) was constructed using Adobe Illustrator
version CS5.1 (http://www.adobe.com/products/illustrator.html).
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Acknowledgements
A.M., M.M. and B.S. gratefully acknowledge Dr Michael Poppelreiter for facilitating access to geophysical data for
research purposes and for acquiring permission to publish the same.
Author contributions
M.M. conceived and directed the research. Discussions initiated in 2012 between D.M., B.P. and M.M. laid the
foundation for this work. B.S. contributed to this work by providing the geodynamic context of Southeast Asia
and South China Sea. A.M. processed, analysed and interpreted the seismic and sedimentological data under the
guidance of B.S., M.M. and D.M. C.B. aided in the interpretation of the dri and moat sequences and critically
reviewed the manuscript. M.M. wrote the manuscript and modied the text aer revisions from B.S., A.M.,
D.M., C.B. and B.P. All gures were prepared by M.M. and D.M. Base maps and illustration of interpretation in
Figs. 3C,E and 4D was constructed by A.M. All authors discussed, reviewed and commented on the results and
interpretation.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary information is available for this paper at https://doi.org/10.1038/s41598-020-64119-9.
Correspondence and requests for materials should be addressed to M.M.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
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