Paleoenvironments Malampaya Carbonates

Paleoenvironments and high-frequency cyclicity from Cenozoic

South-East Asian shallow-water carbonates: a case study from the

Oligo-Miocene buildups of Malampaya (Offshore Palawan, Philippines)

F. Fournier*, L. Montaggioni, J. Borgomano

Centre de Sedimentologie–Paleontologie, UMR-CNRS 6019, Dynamique des recifs et des plates-formes carbonatees, case 67, Universite de Provence, 3,

place Victor Hugo, F-13331 Marseille cedex 03, France

Received 20 June 2003; received in revised form 21 November 2003; accepted 28 November 2003

Abstract

The combination of core and cuttings analyses and well-log data from the Late Oligocene–Early Miocene carbonate buildup in the

Malampaya gas field (Offshore Palawan, Philippines) allowed the recognition of shelf depositional paleoenvironments and the definition of

high-frequency, metre-scale, subtidal cycles, usually bounded by exposure surfaces. This is amongst the first documentation of short-term

platform-top evolution of Oligo-Miocene carbonates in response to high-frequency relative sea-level change described in South-East Asia.

During the Late Oligocene, the Malampaya carbonate buildup is interpreted as an isolated platform rimmed by a barrier-reef system that has

developed vertically according to the ‘keep-up’ mode, with no significant evidence for retardation in reef development during rises in relative

sea-level. During the Early Miocene, the isolated platform was affected by more open conditions in its inner part due to the inability of the

reef to catch up with relative sea-level rises. Strontium isotope-derived estimations of cycle durations, despite a relatively high degree of

uncertainty in their calculation, are consistent with 4th to 5th-order cyclicity (10–1000 ky).

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Cycles; Carbonate platforms; Paleoenvironment; Diagenesis; Oligocene; Miocene; South-East Asia

1. Introduction

The development of Oligo-Miocene carbonate systems

from South-East Asia has been mostly studied from seismic or

outcrop data, with special reference to 3rd-order depositional

sequences (e.g. Cucci & Clark, 1993; Epting, 1989; Grotsch

& Mercadier, 1999; Kusumastuti, Van Rensbergen, &

Warren, 2002; Saller & Vijaya, 2002; Sun & Esteban, 1994;

Wilson, Bosence, & Limbong, 2000; Wilson, Chambers,

Evans, Moss, & Nas, 1999; Wilson & Evans, 2002). In

contrast, studies on short-term evolution of shallow-water

carbonates in response to 4th or 5th-order relative sea-level

fluctuations (sensu Goldhammer, Dunn, & Hardie, 1990) are

rare, and high-frequency sequences have only been hinted at

in a very few cases (Gibbson-Robinson and Soedirdja, 1986;

Kusumastuti et al., 2002; Park, Matter, & Tonkin, 1995).

In the Early Miocene Batu Raja limestones from the

Sunda basin (South-East Sumatra, Indonesia), Park et al.

(1995) defined wireline log cycles, whose frequency could

be compared with 5th-order cyclicity. In addition, these

authors evoked the existence of repeated exposure events

related to high-frequency cyclicity and emphasized their

significant role on the enhancement of reservoir quality.

Repeated hiatuses associated perhaps with subaerial

exposures are hinted at in other South-East Asian carbonates

of Cenozoic age; some of them could be related to high-

frequency cyclicity, e.g. the Early Miocene carbonate

platform of the Madura Strait, east Java (Kusumastuti

et al., 2002), the Miocene Kais platform, Irian Jaya

(Gibbson-Robinson & Soedirdja, 1986) and the Early

Miocene Gomantong Limestone, eastern Borneo (Noad,

2001). In most cases, however, the regional development of

carbonate reservoirs is thought to be mainly related to 3rd-

order sea-level falls and consequent meteoric diagenesis

(Sun & Esteban, 1994). Often the effect of 4th to 5th-order

cycles on facies and reservoir property distribution has not

been considered in detail or perhaps underestimated.

Indeed, the very few studies on Oligo-Miocene outcrops

0264-8172/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2003.11.012

Marine and Petroleum Geology 21 (2004) 1–21

www.elsevier.com/locate/marpetgeo

* Corresponding author. Tel.: þ33-4911-06178; fax: þ33-4911-08523.

E-mail address: [email protected] (F. Fournier).

http://www.elsevier.com/locate/marpetgeo

are generally not sufficiently detailed to identify such

cycles. Moreover, available core data have generally been

too discontinuous to give a good picture of depositional

processes. In addition, many basins in the area are

subsiding, thus masking the record of such high-frequency

cycles.

The comprehensive database (i.e. cores, side-wall cores,

cuttings, and well-log data) available from the Oligo-

Miocene carbonates of the Malampaya–Camago oil and gas

field was used in this report to provide the high-resolution

stratigraphic and diagenetic framework required for a more

detailed characterization of reservoir flow units (Borgo-

mano, van Konijnenburg, & Jauffred, 2001). In this regard,

our study presents new data for understanding the short-

term evolution of South-East Asian Tertiary shallow-water

carbonate systems. The objectives of this paper are: (1) to

characterize the facies and paleoenvironments of the

Malampaya shelf; (2) to document the existence of high-

frequency sequences deposited on the inner-shelf during the

Late Oligocene and Early Miocene; (3) to identify and

quantify some of the parameters that control cyclic

deposition (sediment accumulation rate, nutrient supply,

water-energy, nature of carbonate producers, periodicity

and amplitude of relative sea-level oscillations); and (4) to

propose a model of short-term, sedimentologic and

diagenetic evolution of the carbonate buildup in response

to relative high-frequency sea-level fluctuations.

2. Location and geological setting of the Malampaya

carbonate buildup

The Malampaya–Camago oil and gas accumulations are

situated within the block SC 38 (Fig. 1), offshore northwest

Palawan Island (Philippines) below 850–2000 m of water

depth (Grotsch & Mercadier, 1999; Neuhaus et al., 2003). In

this area, the presence and overall development of carbonate

buildups are mainly controlled by earlier block faulting. The

Malampaya carbonate buildup is located in the North

Palawan block, along the outer trend of carbonate prospects

that formed along a SW–NE trending extensional fault

zone. The Nido Limestone reservoir has a maximum

thickness of 700 m in the area.

The Malampaya carbonate buildup developed on the

crest of a tilted block (Fig. 2), during the Late Eocene rifting

phase of the South-China Sea (Hall, 2002; Holloway, 1982).

The break-up event related to this rifting phase was dated by

mid-Oligocene magnetic anomaly 11 (Briais, Patriat, &

Tapponnier, 1993). The spreading of the South-China Sea

led to the southward drifting of the Calamian–North

Palawan–North Borneo micro-continent throughout the

Late Oligocene and Early Miocene. During the Late Early

Miocene, collision took place between this micro-continent

and the accretion wedge of the Paleogene subduction zone

of North Cagayan (Schluter, Hinz, & Block, 1996),

promoting the obduction of the collision belt on the North

Palawan block and ceasing seafloor spreading (Briais et al.,

1993). Many carbonate buildups in the area drowned due to

the downwarping of the north-western part of the block and

the important clastic supply from the uplifted Palawan

island (Fulthorpe & Schlanger, 1989). The carbonate

buildups of the Block SC 38 are sealed by the Early to

Middle Miocene basinal Pagasa shale.

The Malampaya buildup was previously studied by

Grotsch and Mercadier (1999) on the basis of three-

dimensional (3D) seismic data and relatively sparse core

and side-wall samples from four wells (MA-1 to MA-4).

These authors dated the carbonates as Late Eocene to the

Early Miocene in age using strontium isotope analyses of

bulk-rock samples. The step-like shape of their Sr-isotope

curve in well MA-3, located in the buildup flank, resulted

from important time-rock gaps (0.5–2 my) in the Nido

sedimentary record. The age of the Nido Limestone is also

constrained by the nannofossil and planktonic dating of the

overlying Pagasa shales (Grotsch & Mercadier, 1999). The

nannofossils in the shales provided a NN5 (Langhian) age in

well MA-1 and a NN3 (Late Burdigalian) age for MA-2,

whereas the planktonic foraminiferal assemblages indicate

Fig. 1. Depth (in metres subsea) of the top Nido Limestone and well

locations, in the Malampaya and Camago gas field (after Grotsch &

Mercadier, 1999) within Block SC 38, offshore western Palawan,

Philippines.

F. Fournier et al. / Marine and Petroleum Geology 21 (2004) 1–212

a Burdigalian age in MA-3. During the Early Oligocene, the

Malampaya carbonate system prograded southward with

buildup aggradation and subsequent backstepping during

the Late Oligocene and Early Miocene (Grotsch &

Mercadier, 1999). The buildup finally drowned during the

Late Early Miocene.

3. Material and methods

Since the initial study by Grotsch and Mercadier

(1999), the Malampaya carbonate buildup has been

penetrated by six additional wells (named MA-5 to MA-

10). It is covered by a 3D seismic survey that has been

porosity-inverted (Neuhaus et al., 2003). The average well

spacing is about 500 m. The present study is based on

new rock material and uses a different analytical approach

than that of Grotsch and Mercadier work. Our work uses a

compilation of various subsurface data from all of the

available wells and seismic lines combined with petro-

graphical, micropaleontological and geochemical analyses

of core and side-wall material from the wells MA-5 and

MA-7, drilled in 2000. Well-log data and cutting samples

were also taken into account to complete the information

in intervals where no core or side-wall samples were

available. The total length of cores in MA-5 is 72 m

distributed into two main intervals (Fig. 3) and 15 m in

MA-7. Thin-sections from core samples were prepared

with an average spacing of 0.50 m.

Detailed microscopic analysis of thin-sections pro-

vided the backbone of this study. Thin-sections were

impregnated with a blue-dyed resin and half-stained with

an alizarine-red S þ potassium ferrocyanide solution for

identification of carbonate minerals. All thin-sections

were point-counted on the basis of 300 points.

The abundance of large coral fragments was estimated

from direct macroscopic examination of the core

sections.

Additional analyses included measurements of carbon,

oxygen and strontium isotope ratios and uranium

concentrations. All of the analyses were made on

selected whole-rock samples. Carbon and oxygen isotope

ratios were measured at the University of Erlangen as

follows: carbonate powders were reacted with 100%

phosphoric acid (density .1.9) at 75 8C in an on-line

carbonate preparation line (Carbo-Kiel-single sample acid

bath) connected to a Finnigan Mat 252 mass spec-

trometer. Results are reported in permil relative to V-

PDB. Reproducibility was checked by replicate analysis

of laboratory standards and is better than ^0.02‰ for

d13C and ^0.03‰ for d18O. Sr-isotope measurements

were performed at the Vrije Universiteit in Amsterdam.

The samples were dissolved in 5N acetic acid and the Sr

was separated using a ion exchange column. The Sr-

isotope ratio was measured by a Finnigan MAT 261

mass spectrometer. The reproducibility of measurements

is very good with two sigma errors between ^6 £ 1026

and ^10 £ 1026.

Petrographical analysis has allowed the recognition of

distinct microfacies that have been interpreted in terms of

depositional environments by reference to modern and

ancient analogues. The identification of diagenetic features

from thin-sections combined with whole-rock d13C and

d18O measurements allowed the reconstruction of the

diagenetic history of the series. The interpretation of the

data in terms of sequence stratigraphy was based finally on

the vertical succession of depositional environments and

diagenetic sequences in cores. Large benthic foraminiferal

biostratigraphy and strontium isotope stratigraphy

were used to constrain the stratigraphic framework of the

Nido Limestone and to help quantify the duration of

sedimentary cycles.

Fig. 2. (a) Regional seismic cross-section showing the structural setting of the Malampaya carbonate buildup, at the crest of a tilted block; (b) cross-section

from the 3D seismic reflectivity data showing the overall morphology of the carbonate buildup, with location of wells MA-1 and MA-5.

F. Fournier et al. / Marine and Petroleum Geology 21 (2004) 1–21 3

4. Results

4.1. New stratigraphical data on the Nido Limestone

The biostratigraphical study of newly collected rock

material from wells MA-5 and MA-7 as well as strontium

isotope measurements on MA-1 allowed the stratigraphic

framework of the Malampaya platform to be clarified.

The Nido Limestone interval was dated on the basis of

benthic foraminiferal stratigraphy and strontium isotope

analyses of bulk-rock samples. The benthic foraminiferal

stratigraphy was based on the East Indian Letter Classifi-

cation (Adams, 1970, 1984; Boudagher-Fadel & Banner,

1999). Biostratigraphic and strontium-derived ages are

presented in Table 1.

The shifts in Sr-isotope ratios are related to major

sequence boundaries delimiting 40–130 m thick sedimen-

tary units: MC1 and MC2 for the Late Oligocene, MM1,

MM2, MM3 and MM4 for the Early Miocene (Fig. 3). Well

correlations are constrained by biostratigraphy, seismic and

well-log data. At the base of the Nido Limestone interval, a

progradational unit of Rupelian age was defined; the top of

this unit corresponds to the lowest exposure surface

observed in the Nido Limestone.

In well MA-1, strontium ages derived from Oslick,

Miller, Feigenson, and Wright (1994) regressions are in

accordance with the biostratigraphy-based ages; however,

the age of the MC2 unit remains uncertain since

biostratigraphy gives a Lower Te age (Chattian) and Sr-

isotope dating yields ages ranging from the latest Chattian to

the earliest Aquitanian. At Eniwetak Atoll, Ludwig, Halley,

Simmons, and Peterman (1988) showed that meteoric

diagenetic alteration did not significantly alter the original

Miocene to the Pleistocene strontium isotope ratios, because

calcite precipitated in soil and fresh-water within the

interval was derived from adjacent depositional carbonates.

For the Nido Limestone, the very narrow range of variation

in strontium-isotope ratios within a given unit and the lack

of correlation with the d13C signal (Fig. 3) similarly suggest

that there was no significant alteration of the original

strontium-isotope signal during meteoric diagenesis. Later

diagenetic alteration of the isotopic signal affecting the

series below the Intra-Nido Marker unconformity (top MC2

unit) is believed to be responsible for the relative dispersion

of the measurements in this interval.

4.2. Facies analysis and paleoenvironmental interpretations

The modal analysis of bioclastic components, the

composition of foraminiferal assemblages and the sedimen-

tological features observed in cores and thin-sections

allowed four Late Oligocene facies and four Early Miocene

Fig. 3. Correlation panel of wells MA-7, MA-1 and MA-5, showing the major sedimentary units of the Malampaya buildup, the location of rock data used and

the main stable isotope results.


facies to be recognized. The bioclastic composition is

expressed as a percentage of the whole non-scleractinian

biota. These facies are interpreted in terms of depositional

environments (Table 2).

4.2.1. Late Oligocene

4.2.1.1. Facies C1a: coralline algal wackestone–packstone

(Fig. 4a). Description. This facies consists of a coarse-

grained, poorly sorted, encrusting coralline algae-rich

(50–70%) wackestone to packstone. Other skeletal con-

stituents include benthic foraminifera (15–25%), echino-

derms (5–20%) and bryozoans (,5%). Thick-layered and

foliose coralline algal growth-forms are dominant. The

foraminiferal assemblage mainly includes arenaceous for-

aminifera, miliolids, amphisteginids and rare alveolinids.

Paleoenvironmental interpretation. The dominance of

well preserved thick-layered and foliose coralline algal

growth forms in the coralline algal facies indicates a

relatively quiet-water environment with stable substrate and

low sedimentation rates (Nebelsick & Bassi, 2000). The

colonisation of the sediment surface by coralline algae is an

important indicator of stabilized bottom substrate (Bosence,

1983a,b). The common presence of miliolids supports the

additional interpretation of a relatively protected environ-

ment, probably the inner part of a platform.

4.2.1.2. Facies C1b: coralline algal–echinoderm wack-

estone–packstone (Fig. 4b). Description. This facies is

similar to facies C1a in containing abundant encrusting

coralline algae (30–50%) but differs by its higher content of

echinoderm debris (20–30%): echinoid spines and plates

are dominant and associated with a few ophiuroid ossicles.

Other components include benthic foraminifera (20–30%),

rare geniculate coralline algae (,5%) and planktonic

foraminifera (,5%). The benthic foraminiferal assemblage

is dominated by hyaline forms (amphisteginids and

rotaliids) associated with occasional occurrences of arenac-

eous foraminifera, miliolids and alveolinids.

Paleoenvironmental interpretation. This facies is also

characterised by the low taxonomic diversity of the

benthonic foraminiferal fauna and a high micritic mud

content. The muddy fabric indicates low energy con-

ditions and the foraminiferal assemblage reflects pro-

tected inner-shelf affinities. The occurrence of planktonic

foraminifera suggests occasional connexions with open

marine environments. Holocene echinoid-rich muddy

facies have been reported from the deeper and inner

parts of the Bahamas Bank (Multer, 1977). The

association is also known in the Oligocene lagoonal

deposits of the Florida Suwanee carbonate system

(Hammes, 1992). This facies coincides with the

uranium-richest intervals of the studied cores. Uranium

concentrations in carbonates are generally related to

oxygen-poor and relatively deep environments (Saller,

Dickson, & Matsuda, 1999). Therefore, facies C1b can

be interpreted as deposited in a relatively deep and

oxygen-deficient protected inner-shelf environment,

occasionally connected with the open sea.

4.2.1.3. Facies C2: coral–coralline algal–foraminiferal

grainstone (Fig. 4c). Description. This facies is a well-

sorted, sand to gravel-sized grainstone dominated by

recrystallized rounded coral debris, geniculate and encrust-

ing red algae (40–50%) and benthic foraminifera (30–40%).

Other constituents include echinoderm (10 – 15%),

molluscan and bryozoan (,5%) debris. The grainstone

forms 0.2–1.50 m thick beds; the intense bioturbation

prevented the identification of sedimentary structures.

The taxonomic diversity of the foraminiferal assemblage

is very high, dominated by robust and rounded-shaped

forms such as alveolinids (Borelis pygmaeus), rotaliids,

amphisteginids, miliolids (including Austrotrillina striata)

and Sphaerogypsina; heterosteginids, soritids and arenac-

eous foraminifera are also present, generally in the form of

broken specimens.

Paleoenvironmental interpretation. The absence of mud

is regarded as indicative of moderate to high bottom current

Table 1

Stratigraphic framework of the Nido Limestone interval based on benthic foraminiferal stratigraphy (Letter Stage Classification) from MA-1 and MA-5 and Sr-

isotope ages obtained in well MA-1

Unit Letter stage age Sr-derived age (my)

Min.a Max.a Age errora Ageb

MM4 Lower Tf1 (Burdigalian to Early Langhian) 19.90 19.97 0.80 Burdigalian

MM3 Upper Te–Lower Tf1 (Aquitanian to Early Langhian) 21.28 21.71 0.77 Aquitanian

MM2 Upper Te–Lower Tf1 (Aquitanian to Early Langhian) 22.06 22.47 0.78 Aquitanian

MM1 Upper Te–Lower Tf1 at top Undifferentiated Te at base (Chattian to Early

Langhian)

22.48 23.00 0.76 Aquitanian

MC2 Undifferentiated Te at top Lower Te (Chattian) at base 23.37 24.61 1.24 Latest Chattian to earliest Aquitanian

MC1 Lower Te (Chattian) 26.24 – 1.20 Chattian

MR Tc–Td (Rupelian) – – – –

Age errors include measurement errors and errors associated with Oslick et al. (1994) regression, considering a 95% confidence interval.a Using Oslick et al. (1994) regressions; min. and max. represent, respectively, the minimal and maximal age calculated for the given interval.b After Berggren, Kent, Swisher, & Aubry (1995).


Table 2

A summary of the main facies from the Late Oligocene and Early Miocene of the Malampaya shelf top: sedimentologic features, bioclastic components and paleoenvironmental interpretation

F.

Fo

urn

ieret

al.

/M

arin

ea

nd

Petro

leum

Geo

log

y2

1(2

00

4)

1–

21

6

conditions, an interpretation that is further supported by the

abundance of robust benthic foraminifera such as alveoli-

nids (Borelis), rotaliids, large miliolids (Austrotrillina),

Sphaerogypsina, amphisteginids. Morever, the Austrotril-

lina–Borelis association has been described by Chaproniere

(1975) in the Australian Oligocene as typical of high-energy

subseagrass environments in sheltered metahaline con-

ditions. Epiphytic forms as Heterostegina borneensis and

the soritids could reflect derivation from adjacent seagrass

communities. The abundant fine-grained coral debris

probably come from a nearby reef rim and/or from inner-

shelf patch reef frameworks. The biotic components and

the grainstone texture of the C2 facies therefore could be

interpreted as reflecting deposition in high-energy sand

shoals migrating across a backreef zone colonized by

seagrass meadows and possibly patch reefs. However, as

suggested by Pomar (2001) grainstone bodies, composed

largely of shallow water-derived components, can also be

deposited in deeper settings by currents. Nevertheless, the

hypothesis of a shallow sand shoal environment is

preferentially retained, because of the complete lack of

foraminiferal deeper-water markers.

4.2.1.4. Facies C3: coral–coralline algal–foraminiferal

packstone/floatstone (Fig. 4d). Description. This facies is a

poorly sorted, coral-rich packstone to floatstone containing

numerous benthic foraminifera (30–40%), geniculate and

encrusting coralline algae (30–50%). Echinoderm frag-

ments (mainly echinoids) are common (10–30%), and

molluscs are occasionally present (,10%). Coral elements

are generally leached and difficult to identify; however,

some of them were identified as faviids.

The foraminiferal assemblage is similar to that of the

facies C2 and is rich in rotaliids (including Neorotalia cf.

mecatepecensis), miliolids, alveolinids, heterosteginids

(mainly H. borneensis), amphisteginids and arenaceous

foraminifera; soritids and lepidocyclinids are more abun-

dant than in C2 while Sphaerogypsina is lacking.

Paleoenvironmental interpretation. Soritids and flat

nummulitids such as H. borneensis are known to be

epiphytic organisms living on the leaves of seagrasses.

The H. borneensis–soritids association was described by

Chaproniere (1975) as indicative of seagrass environments

in sheltered conditions. Rotaliids and amphisteginids are

known to be tolerant to a variety of salinity concentrations;

in particular, they are common in modern seagrass

environments of relatively high salinities (Sen Gupta,

1999). Moreover, high mud content and very poor grain

sorting are common features of seagrass facies. This facies

is therefore regarded as reflecting deposition within or at the

vicinity of seagrass meadows in a protected inner-shelf

setting. The common coral debris may have derived from

adjacent patch reefs but could also have been produced in

situ from isolated Porites colonies that are known to grow in

seagrass environments (Brasier, 1975). Seagrass environ-

ments in inner platforms from Indo-Pacific region have been

reported at water depths lower than 15 m (Brasier, 1975;

Chaproniere, 1975).

4.2.2. Early Miocene

4.2.2.1. Facies M1: echinoderm–coralline algal wackes-

tone–packstone (Fig. 4e). Description. This facies is a very

fine to medium-grained, wackestone/packstone, dominated

by small echinoderm (30–80%) and coralline algal frag-

ments (15–50%). Benthic foraminifera (,30%), bryozoans

(,5%) and planktonic foraminifera (,5%) are the main

minor components. Most of the echinoderm debris consists

of ophiuroid ossicle fragments. The foraminiferal assem-

blage is characterized by frequent planktonic foraminifera

and small benthic foraminifera (Bolivina). Large benthic

foraminifera such as Miogypsinoides, heterosteginids,

lepidocyclinids and amphisteginids are occasional

components.

Paleoenvironmental interpretation. The relatively high

planktonic foraminiferal content and the common presence

of small benthic foraminifera such as Bolivina are indicative

of relatively open and/or deep conditions. In modern and

ancient environments, the occurrence of dense populations

of ophiuroids implies the combination of three conditions

(Aronson et al., 1997): low skeleton-crushing predation, low

rates of sediment resuspension and high flux of particulate

organic matter. The scarcity of corals and reef-dwelling

foraminifera suggest deposition in a non-reefal environ-

ment. Facies M1 can be interpreted as deposited on a

relatively open shelf devoid of true reefal environments, in

moderately deep waters. The lack of reliable water-depth

markers makes estimates of paleobathymetry difficult.

4.2.2.2. Facies M2a: coralline algal–foraminiferal–echi-

noderms packstone (Fig. 4f). Description. This facies is a

poorly sorted packstone, enriched in coralline algal frag-

ments (20–50%), benthic foraminifera (20–50%) and

echinoderm debris represented by echinoids (15–30%)

and few ophiuroids. Other components such as bryozoans,

molluscs and planktonic foraminifera are generally rare or

lacking. The foraminiferal assemblage is dominated by

arenaceous forms and miliolids. Small benthic foraminifera

such as Bolivina and diverse discorbids are present along

with lepidocyclinids.

Paleoenvironmental interpretation. This facies contains

a relatively high echinoderm content similar to that

observed in facies M1. However, benthic foraminifera are

more abundant, especially the arenaceous forms. In modern

reefs, arenaceous foraminifera are mostly abundant in the

deeper parts of inner platforms (Hallock & Glenn, 1986;

Montaggioni, 1981). The abundance of miliolids is

additionally indicative of a protected inner-shelf environ-

ment (Hallock & Glenn, 1986). The facies M2a is inferred

to have deposited in a relatively protected inner-shelf

environment. The abundance of arenaceous foraminifera


and miliolids indicates water-depths greater than 20 m

(Montaggioni, 1981).

4.2.2.3. Facies M2b: echinoderm–coralline algal–forami-

niferal packstone (Fig. 4g). Description. This facies is a

poorly sorted packstone, dominated by echinoid and

ophiuroid debris (40–60%), coralline algal fragments

(20–30%) and benthic foraminifers (20–30%). Coral debris

is common in the form of gravels. The M2b facies is

characterized by the dominance of arenaceous foraminifera

and miliolids; few planktonic foraminifera are present.

Paleoenvironmental interpretation. The facies is similar

to facies M2a but contains a higher content in echinoderm

debris. The presence of few planktonic and small benthic

foraminifera such as Bolivina indicate that marine circulation

exchanges with the open sea may have occurred, but toa lesser

extent compared to facies M1. The M2b facies is believed to

have been deposited in a deep and moderately open inner-

shelf environment at water-depth greater than 20 m.

4.2.2.4. Facies M3: coral–coralline algal–foraminiferal

packstone-floatstone (Fig. 4h). Description. This facies is a

poorly sorted packstone to floatstone rich in coral

fragments, encrusting and articulate coralline algae and

benthic foraminifera.

The foraminiferal assemblage is dominated by porcela-

neous benthic foraminifera (mainly soritids and miliolids)

and arenaceous foraminifera. Hyaline forms such as

amphisteginids, miogypsinids (Miogypsina and Miogypsi-

noides) and lepidocyclinids are of common occurrence.

Paleoenvironmental interpretation. The prevalence of

porcelaneous foraminifera strongly suggests deposition in a

protected inner-shelf environment (Hallock & Glenn, 1986)

while the abundance of epiphytic forms such as soritids is

indicative of deposition at the proximity of shallow-water

seagrass meadows. Coral fragments could have derived

from adjacent patch reefs. The M3 facies is therefore

thought to have been deposited in a very shallow and

protected inner-shelf environment colonized by seagrass

meadows and patch reefs, at water depths lower than 15 m.

One of the most conspicuous features regarding the nature

of the carbonate producers at Malampaya is the great scarcity

of the green algae Halimeda in the entire Late Oligocene-

Early Miocene interval. In modern warm sea environments,

Halimeda is known to be widespread, preferentially devel-

oping in nutrient-rich waters (Davies & Marshall, 1985;

Drew & Abel, 1985) and tends to compete spatially with

scleractinian corals (Littler & Littler, 1985). High sedimen-

tation rates are known to be a limiting factor for the

development of Halimeda, a factor that does not play a major

role in the Malampaya shelf (see Section 5.5). A relatively

low-nutrient environment may be a possible explanation for

the lack of Halimeda during the Late Oligocene and Early

Miocene. However, the effect of diverse chemical, physical

and biological disturbances on the development of Halimeda

is not yet well understood (Mankiewicz, 1988).

4.3. Diagenesis

The analysis of diagenetic features, combined with mea-

surements of carbon isotope ratios led to an interpretation of

the diagenetic environments and a delineation of a

diagenetic sequence The striking feature of the diagenetic

history of the Late Oligocene to Early Miocene limestones

from Malampaya is the strong meteoric overprinting that

affected the whole interval. Repeated exposure of the inner

platform carbonates is evidenced by both the recurrent

paleosols observed in cores and successive 13C-depleted

intervals.

4.3.1. Diagenetic environments

4.3.1.1. Early marine diagenesis. Partial micritization of

bioclasts is the most common feature of early marine

diagenesis in the Malampaya inner-shelf. Micritization has

preferentially affected red algae and porcelaneous benthonic

foraminifera. Highly micritized bioclasts often exhibit a

peloid-like appearance and are therefore difficult to identify.

Micritization is a common process in shallow-water

environments and has been interpreted to result from

boring by microorganisms (Bathurst, 1975; MacIntyre,

Prufert-Bebout, & Reid, 2000; Reid & MacIntyre, 2000).

Early marine cements are poorly developed in inner-shelf

environments and mainly occur in the form of thin

isopachous calcite rims (Fig. 5a). They are generally

associated with high-energy facies, particularly the Late

Oligocene C2 grainstone facies.

4.3.1.2. Meteoric vadose diagenesis. The effect of meteoric

vadose diagenesis is mostly inferred from the strong leaching

of bioclasts and matrix and by the development of paleosols.

Aragonite skeletal elements are dissolved or recrystallized.

Pedogenic features are particularly frequent in cores

penetrating both Late Oligocene and Early Miocene deposits.

They can be classified in terms of soil maturity, with an

increasing degree of maturity (Wright, 1994) expressed

sequentially as mottles, alveolar septal structures (Fig. 5f–h),

massive micrite layers (Fig. 5e), pisoids (Fig. 5d), peloids

(Fig. 5h) and micritic mottles (Fig. 5e), and 2–3 m thick

Fig. 4. Late Oligocene and Early Miocene microfacies from the Malampaya shelf (well MA-5). Scale bar ¼ 1 mm. (a) Coralline algal crusts (facies C1a). (b)

Coralline algal–echinoderm packstone (facies C1b). (c) Alveolinid-rich grainstone (facies C2). (d) Coral–coralline algal–foraminiferal packstone (facies C3)

with numerous rotaliid foraminifera. (e) Echinoderm–coralline algal packstone (facies M1). (f) Coralline algal–foraminiferal-echinoderm packstone (facies

M2a). (g) Echinoderm–coralline algal–foraminiferal packstone (facies M2b); the micritic matrix is partially neomorphosed into microsparite. (h) Coralline

algal–foraminiferal packstone (facies M3) showing sections of soritid foraminifera. ech, Echinoderm; ca, coralline algae; pf, planktonic foraminifera; ar,

arenaceous foraminifera; sor, soritid; mil, miliolid; b, alveolinid (Borelis).

R


brecciated pisolithic horizons. The brecciated pisolithic

horizons are generally firmly cemented by coarse-grained

sparry calcite and represent very tight beds. Pisoids and

brecciated pisolithic horizons have been encountered only in

the Early Miocene. In a general way paleosols are much more

mature and thicker in the Early Miocene than in the Late

Oligocene cycles. The values of stable isotope ratios

measured on bulk-rock samples range from 21.03 to

26.71‰ PDB for carbon and from 25.74 to 28.38‰

PDB for oxygen. The base of the meteoric vadose zones is

generally associated with a strong shift in the values of

carbon isotope ratios that reach up to þ1‰ PDB downward.

Fig. 5b–h shows various features of a calcrete profile from

the MA-7 core. The calcrete layer forms a brownish crust

topped by a very irregular surface. The observed features are

from base to top: coated pisoids, dark cemented layer rich in

mottled structures and small-sized glaebules, massive

structureless micrite layer, peloid and interval rich in alveolar

septal structures. The whole rock stable isotope ratio

measurements provide highly negative values: 26.71‰

PDB for carbon and 27.92‰ PDB for oxygen.

Subaerial exposure surfaces were recognised on the basis

of the following features occurring below the surface: sharp

negative downward shifts of d13C, presence of pedogenic

structures (alveolar septal structures, mottled structures,

glaebules, pisoids) and intense leaching of bioclasts. The

small interval range between the subaerial exposure

surfaces (3–8 m) suggests that the Malampaya platform

top has been subjected to ‘repeated exposures’. Negative

downward shifts of d18O are not systematically observed

below exposure surfaces.

4.3.1.3. Meteoric phreatic and early burial diagenesis. The

precipitation of non-ferroan drusy calcite spars is the main

cause of porosity reduction in the inner-shelf carbonates

from Malampaya. They postdate the formation of micrite

envelopes and may represent successive phases of meteoric

diagenesis related to the repeated exposure events. Drusy

calcite cements have been reported from meteoric phreatic

as well as shallow burial environments. Aragonitic bioclasts

are commonly replaced by non-ferroan mosaic calcite spars.

Leaching of bioclasts is a common feature of meteoric

phreatic diagenesis in Malampaya. In the grainstone facies

C2, the drusy cements are generally better developed in the

intergranular space than within the moulds derived from

solution of skeletal particles (see Figs. 4c and 5a). This

suggests that the dissolution of bioclasts occurred near

contemporaneously with the precipitation of drusy cements

(Tucker & Wright, 1990; Wilson & Evans, 2002).

The values of stable isotope ratios in intervals affected by

both marine and meteoric phreatic diagenesis range from

þ1.35 to 21.18‰ PDB for carbon and from 25.66 to

26.96‰ PDB for oxygen. These intervals generally exhibit

a regular trend in carbon isotope composition

without noticeable shifts in values (see Fig. 7, interval

3327–3338 m).

4.3.1.4. Late burial diagenesis. Matrix neomorphosis is a

very common event affecting the whole Nido interval: the

micritic matrix is generally transformed into microsparite

and occasionally into mosaic spar. Stylolites and micro-

fractures are commonly observed in the Late Oligocene and

Early Miocene cores. Fractures can be enlarged during late

burial leaching. Dolomitization was not observed in the

platform top of the Malampaya Late Oligocene–Early

Miocene carbonates.

4.3.2. Diagenetic sequences

All the diagenetic sequences begin with early marine

diagenetic features. As a consequence of the repeated

subaerial exposure pattern of the Malampaya top-shelf, the

whole Late Oligocene–Early Miocene interval has been

affected by meteoric diagenesis. Two distinct types of

diagenetic sequences occur. The early marine stage (1) can

be present in both types of sequence.

4.3.2.1. A. Diagenetic sequence from meteoric vadose

zones. (1) Micritization and/or rim cement precipitation

(mostly in high-energy depositional settings) during the

early marine diagenetic stage. (2) Strong leaching of

bioclasts and matrix, formation of vugs and/or development

of paleosoils during earlier meteoric vadose diagenetic

stage. (3) Moderate leaching and partial to complete

occlusion of pores by drusy calcite probably during later

meteoric phreatic phases. (4) Matrix neomorphosis, stylo-

litization and microfracturing during burial. Stages 3 and 4

could have been partly coeval.

4.3.2.2. B. Exclusive meteoric phreatic diagenetic sequence.

(1) Micritization and/or rim cement precipitation (mostly in

high-energy setting) during early marine diagenetic stage.

(2) Moderate leaching and partial to complete occlusion of

pores by drusy calcite during successive meteoric phreatic

phases. (3) Matrix neomorphosis, stylolitization and

Fig. 5. Meteoric diagenetic features from the Late Oligocene and Early Miocene limestones of the Malampaya shelf. (a) Foraminifer-rich grainstone showing

intense leaching of miliolid foraminifera (mil) and drusy calcite cementation (dc) occluding primary intergranular pores (drusy calcite cements are more

developed in the intergranular space than in the biomoulds); a leached mollusk fragment (mol) is recognizable by its residual micrite envelope; (Late

Oligocene, MA-5), scale bar ¼ 1 mm. (b) View of a cored interval (3202.45–3202.13 m) from well MA-7, Early Miocene. (c) Details of a caliche profile; note

the dark colour of the calcrete crust. (d) Coated pisoids (pis), scale bar ¼ 1 mm. (e) Bottom: calcrete mottles (mot) and glaebules (gl); the inter-mottle space is

occluded by drusy calcite cements; top: massive, structureless calcrete crust, scale bar ¼ 1 mm. (f) Alveolar septal structures (alv); the inter-septal space is

partially filled by drusy calcite cements, scale bar ¼ 1 mm. (g) Details of an alveolar septal structure showing the good preservation of needle calcite needles

and local partial recrystallization into smaller micrite-sized crystals, scale bar ¼ 50 mm. (h) Complex network of alveolar septal structures (alv), between

peloids (pel); the inter-septal space is partially filled by drusy calcite cements, scale bar ¼ 1 mm.

Q


microfracturing during burial. Maximum drusy cementation

is often observed just below the water table.

5. Discussion

5.1. High-frequency sedimentary cycles

Based on the vertical succession of facies and diagenetic

features, the Late Oligocene–Early Miocene interval can be

subdivided into 3–10 m thick depositional cycles (Figs. 6

and 7). These high-frequency cycles (or parasequences) are

considered to be the smallest set of genetically related facies

formed during a single sea-level cycle. Some of these cycles

are bounded by unconformities that are, in most cases,

exposure surfaces. In this case, the term ‘high-frequency

sequence’ is applicable (Lehrmann & Goldhammer, 1999;

Van Wagoner et al., 1988).

Such sequences are usually capped by exposure surfaces

commonly associated with paleosols. Typical intertidal or

supratidal features that locally occur in the Indo-Pacific recent

inner-reef environments (Coudray & Montaggioni, 1986;

Defarge & Trichet, 1990; Montaggioni & Hoang, 1988), such

as early geotropic cementation, beach-rock with associated

fenestrae and microbial laminations, were never observed. In

addition, the subtidal nature of the cycles is identified by the

following features: (1) the cycles generally exhibit a

coarsening-upward trend that is a classic feature of subtidal

cycles (Osleger, 1991); (2) no sharp change is observed in the

uppermost part of the cycles that could represent a subtidal–

intertidal transition; (3) the relative high taxonomic diversity

of organisms found throughout the whole section is an

additional attribute of upper subtidal environments.

Three main mechanisms are generally proposed to

explain the generation of metre-scale carbonate cycles:

autocyclicity (Ginsburg, 1971), episodic tectonism (Cisne,

1986) and sea-level oscillations related to Milankovitch

astronomical rhythms (Goldhammer, Dunn, & Hardie,

1987; Koerschner & Read, 1989). The presence of well-

developed exposure surfaces suggesting prolonged

exposures, associated with several metres-thick meteoric

vadose zones implies a relative sea-level fall and therefore

excludes the possibility of an autocyclic model. Episodic

tectonic events and/or eustatic oscillations more satisfac-

torily explain the prolonged exposure periods, followed by

platform flooding at high-frequency scales.

Figs. 8 and 9 present conceptual subtidal cycles of Late

Oligocene and Early Miocene age, recorded in inner-shelf

areas.

5.1.1. Late Oligocene cycles (Fig. 8)

These display the following features:

(1) gradual upward replacement of coralline algal facies

C1a or coralline algal–echinoderm facies C1b by coral–

Fig. 6. Simplified core log section, biotic zonation, diagenetic features, geochemical results and sequence interpretation of two Late Oligocene cores (in unit

MC1) from the well MA-5.


coralline algal–foraminiferal packstone facies C3. The

grainstone facies C2 is generally interbedded between

C1a (or C1b) and C3 or within the seagrass facies C3;

(2) upward-increase in the amount of framework-derived

elements (corals) and coarsening grain texture upward;

(3) 13C-depleted values and occurrences of scattered

pedogenic structures indicative of exposure events;

most of the Late Oligocene sequences observed in

cores are capped by an exposure surface.

The hypothesis of a permanent coral reef rim or other form

of barrier surrounding the inner-platform area is supported by

the general occurrence of relatively protected environments.

The presence of a reefal barrier is also inferred from the facies

types described in the Late Oligocene interval from the

Malampaya shelf top, the abundance of scleractinian

remains, the composition of foraminiferal assemblages and,

finally, by the overall morphology of the buildup as observed

from the seismic data. However, no true reef frameworks

have been drilled to date. A ‘keep-up’ reef barrier (i.e. coral

growth having kept pace with relative sea-level) and a ‘catch-

up’ inner-platform (i.e. deposition having caught up with

relative sea-level rise) are inferred for the Late Oligocene

time span. The deepening-upward hemi-cycles generally are

limited in thickness and correspond to an ‘empty bucket’

stage in the sense of Schlager (1981), punctuated by

occasional low oxygen conditions as suggested by the higher

uranium content near the base of the cycles (Fig. 6, at 3633.50

and 3630.00 m). The development of shallowing-up stages

during sea-level highstands is enhanced by relatively high

sediment accumulation rates that are considered to be

controlled by carbonate production in seagrass beds and,

possibly, from mid-shelf patch reefs. Seagrass environments

are known to shelter dense populations of carbonate

producers and to trap great quantities of derived particles

(Brasier, 1975). The subtidal nature of all of the facies

encountered indicates that accommodation space has not

been completely filled up during the relative sea-level

highstands; the exposure surfaces that truncate subtidal

deposits at the top of the cycles without intervening intertidal

Fig. 7. Simplified core log section, biotic zonation, diagenetic features, geochemical results and sequence interpretation of an Early Miocene cored interval

from the well MA-5. The base of unit MM3 is characterized by a water deepening event as evidenced by the nature of biological associations, and the strong

shift in C-isotope values in relation to a change of cycle type (exposure-bounded cycles or not).


or subtidal stage suggests a rapid relative sea-level drop.

During the lowstands of relative sea-level, carbonates

deposited at the shelf top were submitted to intense leaching,

cementation and pedogenesis during subaerial exposure. It is

noteworthy that possible accumulation of sediment may have

occurred inter- to supratidally but could have been removed

by erosion during the stage of subaerial exposure or during

the subsequent marine transgression. Subsequent sea-level

rise has resulted in the inundation of the platform once more

and thus favoured the deposition of subtidal material.

Fig. 8. Conceptual model of the Late Oligocene high-frequency cycles from the Malampaya inner-shelf (example of an exposure-capped cycle). (A) Model

describing the idealized cycle controlled by sea-level fluctuations (the effect of sediment compaction is not considered in this model). (B) Growth model of the

Malampaya buildup in response to short-term, relative sea-level fluctuations.


5.1.2. Early Miocene cycles (Fig. 9)

These cycles are characterized by the following features:

(1) gradual upward replacement of echinoderm and

coralline algal facies M1, by coralline algal–

echinoderm-foraminiferal facies M2a and M2b, and

by coral–coralline algal–foraminiferal facies M3

successively;

(2) increasing content of framework-derived elements

(corals) and grain coarsening from base to top;

Fig. 9. Conceptual model of the Early Miocene high-frequency cycles from the Malampaya inner-shelf (case of an exposure-capped cycle). (A) Model

describing the idealized cycle controlled by sea-level fluctuations (the effect of sediment compaction is not considered in this model). (B) Growth model of the

Malampaya buildup in response to short-term relative sea-level fluctuations.


(3) common occurrence of pedogenic structures and/or

highly negative carbon isotope ratios at the top of the

sequences, indicating exposure events.

The Early Miocene sequences are not systematically

overlain by exposure surfaces; the non-exposed sequences

only exhibit a M1–M2 facies succession and the seagrass-

related facies M3 is lacking.

The occurrence of an Early Miocene coral reef barrier

remains unclear in Malampaya. The facies described on

the shelf top indicate relatively shallow water with open

marine environments at the base of the sequence

(deposition of facies M1). This basal facies is interpreted

as deposited in a transgressive system tract or an early

highstand system tract. More restricted conditions seem to

have occurred during deposition of the upper part of the

M1 facies. These conditions were enhanced when the M2

and M3 facies were deposited. The establishment of more

sheltered conditions could be related either to the

development of a reef barrier or sand shoals at the shelf

margin or to local variations in bottom topography.

However, the maximum abundance of scleractinians that

is close to the top of sequence, strongly suggests that a

reef barrier and/or reef patches grew coevally with the

later part of the HST.

The complete cycles (including the M1 –M2– M3

intervals) have been generally exposed at their tops. Similar

to the Late Oligocene deposits, the sediments may not have

aggraded to sea-level. The exposure surface is consequent

upon a rapid sea-level drop, mostly characterized by the

development of 1–5 m thick paleosoils, intense weathering

of subtidal inner-shelf carbonates and the partial occlusion

of pores by low-magnesian drusy calcite.

In the cycles devoid of subaerial signatures, the lowstand

or stillstand deposits cannot be distinguished from the

highstand deposits and are generally represented by the

coralline algal–foraminiferal-echinoderm facies M2b. In

this case, there is no evidence of sea-level drop. At the top of

the cycle, the sequence boundary is marked by a sudden

change in depositional environments recorded by the

recurrence of the M1 facies.

5.2. Subtidal nature of the cycles

Antecedent topography features, combined with water-

energy factors, probably played a key-role in the control of

sediment deposition. Three scenarios can be proposed to

support the apparent absence of inter- or supratidal deposits

in the upper parts of the Late Oligocene to Early Miocene

cycles of the Malamapaya buildups. (1) The deposition of

purely subtidal facies might result from the existence of an

energy threshold that has prevented the sediment to aggrade

above this limit and, consequently, prevented the total infill

of the available accommodation space (Osleger, 1991). The

subtidal nature of high-frequency cycles from Malampaya

therefore could reflect current control on the transport

and deposition of carbonate particles, as suggested by

Grotsch and Mercadier (1999). These authors pointed out

that the western margin of the buildup had been subjected to

strong currents and documented the existence of prevailing

winds from the North-East sector. Important offbank

transport from the buildup top to the leeward flank and the

subsequent formation of progradational lobes have been

advocated by these authors for the Oligocene interval.

(2) The absence of inter- and supratidal structures at the top

of cycles could have resulted from a fast sea-level drop that

did not allow beach-rocks or microbial mats to develop.

(3) The third alternative is that tidal deposits may have been

removed by subaerial erosion during the phase of platform

exposition and/or by marine erosion during the subsequent

phase of flooding.

5.3. High-frequency cycle stacking pattern and recognition

of composite depositional sequences in the Early Miocene

interval

Through analysis of the vertical cycle succession in the

Early Miocene interval, the high-frequency cycles have

been grouped into composite depositional sequences,

correlatable from well to well, within the inner-platform

areas. The composite depositional sequences are com-

posed of at least three high-frequency cycles and are

bounded by exposure surfaces. These sequences are

defined by sharp changes in facies and cycle types: their

lower parts are composed of non-exposed cycles, and

comprise the M1–M2 facies succession only. A marked

rise in relative sea-level is thought to be responsible for

the sharp changes in facies and cycle type. Fig. 7 shows

the base of the sequence MM3-1, at 3338 m deep, that is

also the base of unit MM3. Sedimentary units group up to

three sequences. The unit boundaries are clearly sequence

boundaries and correspond to major time-rock gaps in the

sedimentary record, as evidenced by Sr-isotope measure-

ments (Fig. 3).

Fig. 10 presents the cycle thickness evolution and the

vertical facies distribution in the upper part of well Ma-5

(upper unit MM2 and unit MM3). These plots show that the

bases of sequences can display sharp changes in cycle

thickness. However, there is no clear thinning upward

evolution and no correlation between cycle thickness, the

proportion of shallower facies (M3) versus deeper facies

(M1, M2a, M2b) in the cycle and the type of sequence top

(occurrence of an exposure surface or not). The lack of

correlation between cycle thickness and accommodation in

subtidal cycles has been published previously by Boss and

Rasmunssen (1995). Since sediments have not aggraded up

to sea-level. Variations in cycle thickness are therefore more

likely to reflect variations in the rate of carbonate

accumulation rather than variations in the accommodation

space driven by eustacy.


5.4. Period and amplitude of the high-frequency relative

sea-level cycles (Table 3)

Strontium-derived ages were calculated from the MA-1

well where there is a paucity of paleontologic control, in

particular in the Early Miocene interval. Duration of

stratigraphic intervals was estimated for units MC2, MM1,

MM2 and MM3 (Table 1) on the basis of Sr-isotope ratios

within the given interval using the regressions of Oslick et al.

(1994). The power of resolution of the 87Sr/86Sr regressions

is relatively low (^640 ky for the Early Miocene and

^1080 ky for the Late Oligocene with a 95% confidence

interval) and consequently only permits rough estimates of

time durations. The duration of the Early Miocene sedimen-

tary units using Oslick et al. (1994) regressions at site 747A

ranges from 406 ^ 1570 to 521 ^ 1526 ky. The age errors

take into account the internal error on 87Sr/86Sr measure-

ments and the error associated with Oslick et al. (1994)

regressions. By dividing the maximum calculated duration of

the unit by the number of high-frequency sequences, the

cycle periods were estimated as averaging 35 ^ 102 to

48 ^ 177 ky. In spite of the large range of uncertainty of

these estimations, the averages are compatible with Milan-

kovitch frequency bands. Orbital forcing is now known to be

important in dictating climate changes during the last 60 my

(Palike & Shackleton, 2000). The importance of eccentricity

and obliquity cycles has also been demonstrated for the Late

Oligocene and Early Miocene (Zachos, Shackleton, Reve-

naugh, Palike, & Flower, 2001). Nonetheless, episodic

tectonic control of these cycles cannot be ruled out.

Moreover, some cycles may not have been recorded, in

particular during short-term lowstands. This could result in

minimizing the number of cycles per unit, thus leading to an

overestimation of the cycle period.

The relative scatter of Sr isotopic ratios in the Late

Oligocene below the intra-Nido marker unconformity, in

particular, makes the duration of the composite sequence

difficult to determine. In this regard, the duration of

1235 ^ 2493 ky obtained for MC2 by considering the

maximal range of Sr-ages calculated for this interval, is

probably overestimated.

Due to the high uncertainty in paleo-water-depths

estimations in the basal facies (M1, C1a and C1b),

amplitudes of relative sea-level cycles are difficult to

evaluate. For the Late Oligocene, the maximal cycle

thickness of 10 m is suggested as a minimum value for

the relative sea-level cycle amplitude. For the Early

Miocene, the deepest facies (M1 or M2a–b) was probably

deposited in water-depth greater than 20 m, thus indicating a

relative sea-level cycle amplitude greater than 20 m.

5.5. Rates of accumulation

Considering the mid-point value of unit durations,

accumulation rates are estimated as averaging

0.10–0.19 m/ky for the Late Oligocene and Early Miocene

(Table 3). In Neogene carbonates from South-East Asia,

calculated accumulation rates range from 0.3 to 1 m/ky

(Wilson, 2002). For Holocene lagoons, a wide range of

values is in evidence. For example, in the Mayotte lagoon

(Western Indian Ocean), sedimentation rates vary from 0.12

to 4.39 m/ky (Zinke, Reijmer, & Thomassin, 2003). In the

shallowing-upward cycles of Davies lagoon (Eastern Aus-

tralia), rates range from 1.4 to 3.4 m/ky (Tudhope, 1989).

Fig. 10. Plot of cycle thickness and facies proportion, from the upper unit MM2 to the top of unit MM3, in the well Ma-5. Bases of composite sequences are

characterized by high-frequency cycles not bounded by an exposure surface: they also correspond to jumps in cycle thickness. However, there is no clear

correlation between cycle thickness, the nature of the sequence boundary (occurrence of an exposure surface or not) and the proportion of shallower facies

(M3) versus deeper facies (M1, M2a, M2b) in the cycle.


Similar ranges of accumulation rates are given for lagoonal

environments in different locations: 0.65 m/ky in Pacific

lagoon atolls (Yamano, Kayanne, Matsuda, & Tsuji, 2002),

0.5–0.6 m/ky in Belize atoll lagoons (Gischler, 2003),

0.7–2.1 m/ky in the Great Barrier Reef (Smith, Frankel, &

Jell, 1998). In the Malampaya inner-shelf, changes in

carbonate producers from the base to the top of sequence

suggest changes in production/accumulation rates during a

given cycle. The basal transgressive echinoderm–coralline

algal facies is probably related to lower production/

accumulation rates than the sequence top seagrass facies

dominated by benthic foraminifera, corals and coralline

algae. Indeed, in the modern reef environments, the

respective potential carbonate production of these organisms

is as follows: up to 9000 g m22 yr21 for corals (Heiss, 1995),

up to 3300 g m22 yr21 for coralline algae (Chisholm, 2000),

up to 900 g m22 yr21 for benthic foraminifers (Harney,

Hallock, Fletcher, & Richmond, 1999). Very few studies are

available concerning the potential carbonate production by

echinoderms. For the echinoid genus Diadema, according to

the weight and the age of adults, carbonate production rates

can be estimated to about 10 g yr21 per individual (Bauer,

1976), their density averaging commonly 4–5 individuals

per m2 in modern reef environments (Peyrot-Clausade, pers.

comm.). Values of 40–50 g yr21 m22 can be considered

reasonable estimates for potential carbonate production of

Diadema. Although such rates are probably strongly

dependent on echinoderm taxa, echinoderm–coralline

algal dominated facies are inferred to have lower potential

carbonate production than that of the coral–foraminiferal–

coralline algal facies.

These accumulation rates may be affected: (1) by the

relatively low resolution of Sr-isotope stratigraphy that

biases an estimation of cycle durations; and (2) by a period

of cycle formation that does not coincide with the period of

active sedimentation. For example, cycle durations include

the duration of exposure during sea-level lowstands.

Additionally, Grotsch and Mercadier (1999) suggested

that a large proportion of the sediment produced in the

platform top was probably exported basinward and

deposited in the form of leeward progradational wedges.

In brief, the mean rates obtained from cycles are likely to

underestimate the actual accumulation rates during the

interval of active sedimentation.

5.6. Comparison with ancient and modern analogues

The Upper Oligocene Malampaya sequence differs from

that of the Lower Miocene through the apparent absence of

attributes characterizing deposition in an open inner-

platform during sea-level transgressive and early highstand

stages. In modern reef environments, sequences related to

both stages are present and many authors have defined the

timing and development patterns during postglacial time

(last 23,000 yr).

For example, Tudhope (1989) described Holocene

shallowing-upward sedimentation in the Davies Reef

lagoon (Great Barrier Reef, Australia), as partly subject

to open marine conditions at the base (7500 yr BP) and

followed by restricted lagoonal sedimentation afterwards.

The ‘high-energy window’ evidenced at the base of the

sequence was interpreted as resulting from a rapid sea-

level rise that outstripped the capacity of the reef to

catch-up (Hopley, 1984). The onset of sheltered inner-

platform conditions occurred as soon as the open

circulation ‘window’ was closed by barrier reef accretion.

Such a scenario can be applied to the upward M1–M2

facies transition from open to protected inner-shelf

environments. The starting growth phase of the outer

reef rim may have been postponed due to a variety of

environmental constraints. Such a retardation in reef

initiation was also reported from the Holocene fringing

reefs in New Caledonia (Cabioch, Montaggioni, & Faure,

1995).

Similar to modern shallow-water carbonate environ-

ments, the differences in the depositional patterns between

the Malampaya Late Oligocene and Early Miocene

sequences might reflect differences in water quality and

conditions of larval recruitment (Cabioch, Camoin, &

Montaggioni, 1999; Montaggioni, 2000). But, the open

platform stage in the Early Miocene cycles could have

resulted from faster changes in relative sea-level rises,

possibly in conjunction with episodic tectonic events or with

higher-frequency eustatic oscillations. However, the

Table 3

Ranges of sedimentary unit durations, cycle periods and accumulation rates for units MC2, MM1, MM2 and MM3 in well MA-5

Unit Number

of cycles

Calculated deposition

ime of units using

Oslick et al. (1994)

regressions (ky)

Duration of

cycles (ky)

Average cycle

thickness (m)

Average accumulation

rate (m/ky)

MM3 9 430 ^ 1540a 48 ^ 177a 5.50 0.02b 0.11c

MM2 10 410 ^ 1570a 41 ^ 157a 4.00 0.02b 0.1c

MM1 15 520 ^ 1520a 35 ^ 102a 5.30 0.04b 0.15c

MC2 14 1240 ^ 2490a 88 ^ 178a 8.50 0.003b 0.1c

a Age errors include measurement errors and errors associated with Oslick et al. (1994) regression, considering a 95% confidence interval.b Value calculated using the maximal expected value of cycle duration.c Value calculated using the average expected value of cycle duration.


difference between Oligocene and Miocene cycles could be

related to the development of the East Asian Monsoon in the

earliest Miocene (Guo et al., 2002). The increased

storminess may have affected the nature and distribution

of barriers along the platform margins and caused

significant variations in the protected versus open signatures

in the Malampaya inner-shelf.

The most common feature of the Late Oligocene and

Early Miocene cycles is that they probably have all

developed subtidally and are frequently bounded by an

exposure surface at the top. In ancient carbonate systems,

diverse examples of such subtidal cycles have been

described: Triassic of Latemar, Italian Alps (Goldhammer

et al., 1987, 1990), Oligocene of Suwanee, Florida

(Hammes, 1992), Pleistocene of South Florida (Perkins,

1977). All of them exhibit shallowing-upward trends, and

caliche beds at the top of sequences. Such cycles are thought

to have been driven by high-frequency changes in

accommodation space, with rapid falls in sea-level.

6. Conclusions

Petrographical and geochemical analyses of subsurface

material from the Malampaya buildup provide new

information about the depositional patterns of Oligo-

Miocene carbonates from South-East Asia, particularly

with regard to the short-term evolutionary history of isolated

shallow-water carbonate systems in response to high-

frequency variations in relative sea-level.

– The vertical and lateral distribution of facies appears

to be strongly controlled by short-term relative sea-

level changes. In the inner-shelf area, the main types

of carbonate producer have varied throughout a

cycle in response to changes in water-depth and to

the degree of marine circulation exchanges with the

open sea. Connectivity to the open sea probably has

been mainly dependent on sea-level control of reef

development at the shelf margin. Relative sea-level

falls caused subaerial exposure and the consequent

intense alteration of cycle top carbonates. In this

way, short-term changes in accommodation space

controlled the reservoir properties of the Nido

Limestone.

– Despite a relatively high degree of uncertainty, the

cycles have the same time duration range as that of

orbital forcing events; however, episodic tectonic

controls could have mimiked Milankovitch-band

frequency cycles.

– The response of Malampaya Oligo-Miocene, reef-

bearing buildups to high-frequency variations in sea-

level is partly similar to that observed in Recent reef

systems, particularly in the timing of reef growth

relative to flooding of the antecedent substrate.

Acknowledgements

This work was funded by Shell Philippines Explora-

tion B.V. (SPEX). Their support and approval to publish

this paper are gratefully acknowledged. We especially

thank D. Neuhaus (SPEX) and J. Borgomano (Shell

Research International B.V., Rijwijk, The Netherland) for

the initiation of this project. This paper largely benefited

from the experience of F. Abbots-Guardiola (Shell

International, Houston), C. Mercadier, P. Cassidy and

G. Warrlich from the Shell Carbonate Team (Rijwijk,

The Netherland). We are also grateful to D. Bosence, L.

Pomar, M. Wilson, G. Conesa, J.P. Margerel and J.P.

Masse, for very helpful discussions. We thank

M. Joachimski (Erlangen University) for carbon and

oxygen isotope analyses and the Laboratory of Isotope

Geochemistry at the Vrije Universiteit (Amsterdam) for

Sr-isotope ratio measurements.

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