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INTRODUCTION
Over the years, Mindoro Island has remained a place of interest
because of its active geologic and tectonic settings, and has
been investigated by numerous workers on various subjects
(e.g., Teves 1954; Caagusan 1966; Andal et al. 1968; Hashimoto
and Sato 1968; Rangin et al. 1985; Karig and Sarewitz 1986;
Bureau of Mines and Geosciences 1982; Aurelio and Peña 2004)
resulting in a better understanding of the geology of the island.
Despite studies on the stratigraphy, paleontology, mineralization
potential, and tectonics of the island (e.g., Hashimoto and Sato
1968; MMAJ-JICA 1983; Karig and Sarewitz 1986; Sarewitz
and Karig 1986; Rangin et al. 1988), calcareous nannofossils in
the area have so far received little attention.
Paleontological analysis of the Middle Jurassic ammonite-
bearing Mansalay Formation in Southeast Mindoro was
carried out by Teves (1954) and several micropaleontological
investigations were conducted by Müller in Marchadier and
Rangin (1990) on the Cretaceous black shales and Eocene
limestone sequences in the area. Moreover, early Middle
Miocene calcareous nannofossils were recorded in the matrix
of a chaotic unit deposited during a compressive tectonic event
in Southwest Mindoro (Rangin et al. 1985). Early Pliocene
calcareous nannofossils were recorded in the thin silt sequences
along the Insulman and Anahawin Rivers in Southwest Mindoro
and Late Pliocene calcareous nannofossils were observed
downstream, along the Caguray River and along the road
between San Jose and Bulalacao (Marchadier and Rangin 1990).
All these paleontological and micropaleontological works were
concentrated in the southeastern and southwestern parts of
Mindoro Island.
In previous studies (Rangin et al. 1985; Sarewitz and Karig
1986), the only sedimentary unit recognized to underlie
northwest Mindoro is the Lasala Formation, an Eocene sequence
of interbedded sandstones and shales with minor conglomerates,
mudstones, limestones and basaltic flows. The age of this
formation was confirmed using calcareous nannofossil analysis
of the red mudstones and shales from the exposures along the
Abra de Ilog, Liwliw, Sinambalan, Mamburao, and Pagbahan
Rivers (Concepcion et al. 2011). Late Eocene to ea rliest Oligocene
calcareous nannofossils were recorded from these fine-grained
clastic units. This formation is overlain by the alternating beds of
calcareous mudstones, very fine- to medium-grained sandstones
of the Balanga Formation, which is proposed in this study to be
the northwest extension of the Balanga Formation reported in
Southeast Mindoro.
A detailed ca lcareous nannofossil study of the Balanga Formation
in Mamburao, Mindoro, Philippines is presented in this paper.
Here, we provide the first detailed calcareous nannofossil
information from the younger Balanga Formation to better
define and verify its stratigraphic relationship with respect to
other formations in the study area. The nannofossil assemblages
of the sequence and the resulting biostratigraphic assignment are
discussed in this paper. We also present the paleoenvironmental
significance of the assemblages as a response to environmental
changes that occurred in this area during the deposition time of
the formation in question.
GEOLOGICAL SETTING OF THE STUDY AREA
Mindoro Island is the seventh largest island of the Ph ilippines with
a total land area of 10,572km2. It is situated in the western central
part of the archipelago, southwest of Luzon and separated from
Palawan by the Mindoro Strait and from Panay and Marinduque
by the Tablas Strait (Text-fig. 1A). The basement of Mindoro
is believed to constitute a segment of the North Palawan Shelf-
Mindoro-Cuyo Platform, a microcontinental plate that was once
part of South China (Rangin et al. 1988). Several authors (e.g.,
Rangin et al. 1985; 1988; Karig and Sarewitz 1986; Sarewitz and
Karig 1986) have recognized that there are two major tectonic
elements, which are probably the result of the collision of the
North Palawan Block and the Philippine Mobile Belt. These
are the northwest-trending Central Mindoro divide, also called
the Central Range, and the overlapping system of north to
northwest-trending, east-northeast-dipping thrust faults (Karig
Post-Collision Deposition of Balanga Formation in northwest
Mindoro, Philippines: Calcareous Nannofossil Evidence
Deborah N. Tangunan, Alyssa M. Peleo-Alampay, Jamila B. Abuda, Lara Angeli T. Mambuay,
Camille Regina A. Ramos, Allan Gil S. Fernando, Carla B. Dimalanta, Decibel V. Faustino-Eslava,
Chelo S. Pascua, and Edmundo P. Vargas
National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines 1101
email: dntangunan@gmail.com
AB STRACT: This study investigates the calcareous nannofossil assemblage and composition of 62 field samples collected
from a calcareous sedimentary sequence exposed in Mamburao, Occidental Mindoro, Philippines. The sequence consisting
of foraminifera-rich, alternating beds of mudstones and fine- to medium-grained sandstones was subjected to stratigraphic
logging and detailed calcareous nannofossil analysis. The nannofossils were used to interpret the biostratigraphic assign-
ment of the sequence relative to the identified formations in the study area. Index calcareous nannofossil species reveal a
Late Pliocene to Early Pleistocene age (~1.67 to 4.13 Ma) of the sequence, which suggests it was formed from continuous
sedimentation after the Miocene collision of the Palawan-Mindoro Block with the Philippine Mobile Belt. Based on its li-
thologic description and age, the sequence represents the northwest extension of the Balanga Formation, a sedimentary unit
reported to be extensively distributed in southeast Mindoro.
Stratigraphy, vol. 11, no. 3–4, pp. 235–243, text figures 1–3, table 1, plates 1-2, 2 014
236
Deborah N. Tangunan et al. : Post-Collision Deposition of Balanga Formation in northwest Mindoro, Philippines
and Sarewitz 1986); dividing the island into three geomorphic
provinces: the Eastern Province, the Central Range, and the San
Jose Platform (Rangin et al. 1985).
Rock units identified in the area range in age from Cretaceous
to Recent, the oldest formation being the Halcon Metamorphics.
Recent studies in the northwest part of the island recognize
two ophiolite bodies: the Cretaceous Mangyan Ophiolite and
the Early to Middle Oligocene Amnay Ophiolite distributed
in the vicinity of Amnay River, Sitio Igsoso, near Mamburao
and Lumintao (Yumul et al. 2008). Overlying the Halcon
Metamorphics, the Eocene Lasala Formation consists of
pillow lavas and indurated sandstones, outcropping along
the Alitungan, Amnay, Lasala, Pagbahan, and Talusungan
Rivers (Peña 2008). The Plio-Pleistocene Balanga Formation
composed of a sequence of sandstone, limestone, mudstone
and conglomerate outcrops in the Bongabong River and along
the coast of Colasi Bay (Peña 2008) in southeastern Mindoro.
Recent field campaigns in northwest Mindoro revealed that this
younger sedimentary formation is also represented in the area by
outcrops distributed along the National Highway connecting the
towns of Abra de Ilog and Mamburao and around the vicinity of
Mamburao (Tangunan et al. 2010; Concepcion et al. 2011; Canto
et al. 2012).
MATERIALS AND METHODS
Sixty-two samples were taken from each bed of distinct lithology
and analyzed through stratigraphic logging and calcareous
nannofossils (Text-fig. 1B). The sequence contains a well-
exposed succession of foraminifera-rich layers of mudstones
and fine- to medium-grained sandstones. It has a total thickness
of 13.24 meters and is continuously exposed laterally for ~54
meters. Smear slides were prepared from unprocessed samples
and examined under 1000x magnification using a Carl Zeiss
Axioplan microscope.
Species identification was based on holotypes, most of them
are illustrated in Perch-Nielsen (1985), Bown (1998) and
Young (1998). The biostratigraphic zones of the sequence were
established using the calcareous nannofossil zonation scheme
of Martini (1971) correlated with the zonation scheme of Okada
and Bukry (1980) (Text-fig. 2). Numerical ages were assigned
TE X T- FIG U R E 1
(A) Philippine map showing Mindoro Island (inset). In gray is the dis-
tribution of the Balanga Formation in northwest Mindoro. (B) The SW
dipping (Strike: N75ºW; Dip: 11ºSW) sedimentary sequence exam-
ined at 13º13.451’ N and 120º36.345’ E is a well-exposed succession
of foraminifera-rich layers of mudstones and fine- to medium-grained
sandstones. The buff to brown beds shows a series of fining upward
sequences. The outcrop has a total thickness of 13.24 meters and is lat-
erally continuous for ~ 54 meters.
TE X T- FIG U R E 2
Stratigraphic log and biostratigraphic scheme based on calcareous nan-
nofossils from the Balanga Formation in NW Mindoro. Numbers (1-62)
in the Stratigraphy column denote sample numbers and their location in
the stratigraphic column.
237
Stratigraphy, vol. 11, nos. 3– 4, 2014
based on the biostratigraphic correlations with the astronomical
time-scale proposed by Lourens et al. (2004).
Semiquantitative analysis was applied for the estimation of
frequencies of the calcareous nannofossil species, with species
identified and counted in 30 fields of view. The calcareous
nannofossil distribution data are presented in a range chart
(Table 1). Micrographs of most species are presented in Plates
1 and 2.
RESULTS AND DISCUSSION
Biostratigraphy
Calcareous nannofossils are common to very abundant in the
finer-grained lithology of the sequence (mudstones and very
fine-grained sandstones), whereas in the coarser-grained
sandstones calcareous nannofossil specimens are few to very
rare. Consequently, calcareous nannofossil specimens in the
finer-grained samples exhibit moderate preservation, whereas
those in the coarser-grained samples are poorly preserved.
The studied sequence corresponds to calcareous nannofossil
Zones NN14 to NN19 of Martini (1971) and CN11 to CN13b of
Okada and Bukry (1980) representing upper Pliocene to lower
Pleistocene deposits, ~1.67 to 4.13 Ma.
A total of 37 species belonging to 20 genera were found in the
samples (Table 1). Reworked Eocene to Miocene forms were
recorded throughout the sequence and reworked Pliocene
Discoaster species were noted in the younger samples. The
oldest species found were Reticulofenestra pseudoumbilicus
(>7µm), Calcidiscus macintyrei, Sphenolithus abies, and
Pseudoemiliania lacunosa. Important indicator species used
in defining boundaries of the calcareous nannofossil zones
belong to the Discoaster group (D. asymmetricus, D. brouweri,
D. pentaradiatus and D. surculus). The occurrences of
Gephyrocapsa caribbeanica and G. ericsonii mark the Early
Pleistocene age of the sequence.
Below are the seven distinct zones established from the
occurrences of index calcareous nannofossils in the analyzed
sequence of the Balanga Formation exposed in the northwestern
part of Mindoro Island.
Sample interval 0–293cm (lower Pleistocene)
The first occurrence (FO) of G. caribbeanica and the rare
to common G. ericsonii in the sample interval 283-284cm
are substantiating evidence for the interval to be assigned to
calcareous nannofossil Zone NN19 or CN13b. Other species
present are Cd. macintyrei, Helicosphaera sellii, P. lacu no sa
and D. triradiatus. Reworked and overgrown D. pentaradiatus
and other Discoaster species were also observed in the samples.
Sample interval 293 –392cm (lower Pleistocene)
The last occurrence (LO) of the marker species, D. brouweri in
the 354-364cm interval was used to define the upper boundary
of the calcareous nannofossil Zone NN18 or CN12c-CN12d
of the sequence. The occurrences of D. pentaradiatus and D.
triradiatus were also used to recognize the z one. Cyclicargolithus
floridanus and Discoaster spp. are common reworked forms
recorded in the samples.
Sample interval 392 –441cm (lower Pleistocene)
The LO of D. pentaradiatus in 392-402cm interval marks the
upper boundary of calcareous nannofossil Zone NN17. Its lower
boundary is defined by the absence of D. surculus.
Sample interval 441–590.5cm (upper Pliocene to lower
Pleistocene)
Based on the presence of P. l acunosa, D. asymmetricus, D.
pentaradiatus and D. brouweri, and the absence of D. tamalis,
the sample interval is assigned to calcareous nannofossil Zone
CN12b. The zone is bounded in the upper portion by the LO
of D. surculus which occurs within the 441-444cm interval.
Reworked Cyclicargolithus floridanus and Discoaster spp. were
noted in the samples.
Sample interval 590.5–1049.5cm (lower to upper Pliocene)
The lower sample of this interval is marked by the LO of R.
pseudoumbilicus, dated at 3.79 Ma according to Lourens et al.
(2004). On the other hand, the LO of D. tamalis in the 590.5-
628.5cm interval of the sequence defines the upper limit of the
calcareous nannofossil Zone NN16 or CN12a, corresponding to
2.87 Ma. The occurrences of D. asymmetricus and P. lacunosa also
define the assigned calcareous nannofossil zone. Common reworked
forms are Cy. abisectus, Cy. floridanus and Discoaster spp.
TE X T- FIG U R E 3
Comparative stratigraphy of Mindoro showing the position of the investigated calcareous sedimentary sequence.
238
Deborah N. Tangunan et al. : Post-Collision Deposition of Balanga Formation in northwest Mindoro, Philippines
Sample interval 1049.5–1200cm (lower Pliocene)
The age of the section is assigned to calcareous nannofossil
Zone NN15 or CN11b, ~3.79 to 4.13 Ma, based on the FO of D.
tamalis and the LO of R. pseudoumbilicus. Other species that
were utilized to establish the calcareous nannofossil zone are D.
asymmetricus, P. lacunos a and S. abies. Reworked species of
Coccolithus miopelagicus, Cy. floridanus, Discoaster spp. and
S. heteromorphus were also observed.
Sample interval 1200–1324cm (lower Pliocene)
The occurrences of R. pseudoumbilicus, D. asymmetricus, P.
lacunosa, S. abies, and R. haqii in the lowermost portion of the
sequence (1248-1324cm) indicate that the base of the studied
TABLE 1
Calcareous nannofossil distribution and abundance in the investigated Balanga Formation in Mamburao, Occidental Mindoro, Philippines. (*Re-
worked species; Relative Abundance: VA= very abundant (>25 species/field of view (FOV)); A= abundant (6-25 species/FOV); C= common (1-5
species/ 2-10 FOV); VR= very rare (<1 species/ 50 FOV); Preservation: P = poor; M = moderate)
239
Stratigraphy, vol. 11, nos. 3– 4, 2014
calcareous sedimentary sequence is in calcareous nannofossil
Zone NN14 to NN15, ~4.13 Ma, corresponding to an Early
Pliocene age. The upper boundary is defined by the absence
of D. tamalis at 1200cm. The sample also contains very rare
reworked forms of Cy. floridanus and overgrown Discoaster spp.
Implications on Stratigraphy
Previous studies (Rangin et al. 1985; Sarewitz and Karig 1986)
reported that the youngest sedimentary unit underlying the
northwestern part of Mindoro Island is the Lasala Formation,
an Eocene succession of sandstones and shales with minor
conglomerates, mudstones, limestones, and basaltic flows.
However, detailed calcareous nannofossil study of this sequence
exposed in Mamburao, Occidental Mindoro reveals different
age and lithologic characteristics. Since the studied sequence
was only recently included in the stratigraphy of the study
area (Tangunan et al. 2010; Concepcion et al. 2011, Canto et
al. 2012), its lithologic description and age are compared with
other known sedimentary formations in Mindoro (Text-fig. 3).
Calcareous nannofossil dating of one sample from the same
study area revealed a Plio-Pleistocene age for the formation
based on the occurrence of Pseudoemiliania lacunosa (Canto et
al. 2012). Results of this detailed calcareous nannofossil study
further refine the age of this sequence to Lower Pliocene to
Lower Pleistocene based on the occurrences of several marker
taxa, R. pseudoumbilicus, D. asymmetricus, D. tamalis, P.
lacunosa, S. abies, D. surculus, D. pentaradiatus, D. brouweri,
G. caribbeanica, and G. ericsonii.
These results show age and characteristics consistent with the
Balanga Formation, a sedimentary unit reported only previously
in the southeastern part of the island (MMAJ-JICA 1983;
Peña 2008). The name Balanga Conglomerate was introduced
by Feliciano and Basco (1947) referring to the sequences
corresponding to the Barubo Sandstone and Famnoan Formation
of Teves (1954). It was then named Balanga Formation by
Peña (2008) for the sedimentary unit consisting of a sequence
of sandstones, limestones, mudstones and conglomerates
outcropping near the Bongabong Bridge along the Bongabong
River and the coast of Colasi Bay, in southeastern Mindoro. The
type locality is found at Balanga Point along the north coast of
Mansalay Bay.
This present study, found the same sedimentary sequence
distributed along the National Highway connecting the towns
of Abra de Ilog and Mamburao, and around the vicinity of
Mamburao. This Plio-Pleistocene sedimentary sequence in
the northwestern part of Mindoro is composed of interbedded
sandstones, siltstones, and mudstones with conglomerates in
some areas. Hence, this study shows that the Balanga Formation
extends to northwest Mindoro where it unconformably overlies
older formations.
Paleoenvironment
The deformation of the underlying rocks during the collision of
the the Palawan-Mindoro Block with the Philippine Mobile belt
during the Middle Miocene probably formed the basin where
the Balanga Formation was deposited (Canto et al. 2012). The
presence of a seaway between the Paluan Peninsula and mainland
Mindoro was further suggested by Canto et al. (2012) during the
Plio-Pleistocene. This study shows that the diverse assemblages
and good preservation of the calcareous nannofossils in the
samples suggest that the sequence was deposited in an open
ocean environment rather than a shallow marine environment
(Canto et al. 2012) as evidenced by the fine-grained sediments,
occurrences of diverse calcareous nannofossil taxa indicative of
shelfal to bathyal environments, and the abundance of planktonic
foraminifera.
Discoaster, a characteristic taxon of warm water and oligo
trophic conditions (Aubry 1992) is rare to abundant in the
samples. Species associated with shelf areas like Helicosphaera
spp. and Pontosphaera spp. (Brand 1994) were frequently
recorded probably suggesting an environment rich in nutrients
as supported by the input of erosion and transport of reworked
taxa. The reworked Eocene to Miocene calcareous nannofossils
may indicate an increase in deposition or erosion, resulting in
the mixing of older and younger forms at the time the Pliocene-
Pleistocene sediments were deposited.
CONCLUSIONS
The calcareous sedimentary sequence of the Balanga Formation
exposed in Mamburao, Occidental Mindoro, Philippines is
composed of foraminifera-rich, alternating beds of mudstones,
siltstones, and fine- to medium-grained sandstones. Calcareous
nannofossil investigation of the sequence reveals common to
very abundant nannofossil species showing poor to moderate
preservation. Better preserved and diverse assemblages are
however recorded in the finer-grained samples.
Several useful marker species present in the samples were
used to define the boundaries and zones of the studied
sedimentary sequence. Among these are R. pseudoumbilicus, D.
asymmetricus, D. tamalis, P. lacunosa, S. abies, D. surculus, D.
pentaradiatus, D. brouweri, G. caribbeanica and G. ericsonii.
The presence of these species in the investigated sequence
indicates a lower Pliocene to lower Pleistocene age.
Seven zones were defined based on the occurrences of the
calcareous nannofossil markers ranging in age from ~1.67 to
4.13 Ma, calcareous nannofossil Zones NN14 to NN19 (Martini,
1971). The resultant age suggests that the sequence was
deposited after the collision of the Palawan-Mindoro Block with
the Philippine Mobile Belt dated as occurring from the late Early
Miocene to early Middle Miocene.
The sequence was probably formed from continuous sedi
mentation in a relatively open sea environment as the blocks
stabilized from the collision and other associated episodes
during the Pliocene to Early Pleistocene and its subsequent
open ocean deposition. The reworked forms may indicate an
environment with increased erosion from terrestrial outcrops
causing the mixing of older and younger forms at the time the
sediments were deposited.
Results of this study support the recent findings that the Balanga
Formation, previously reported to be extensively distributed only
in southeast Mindoro, extends to the northwest section of the
island.
ACKNOWLEDGMENTS
The authors would like to thank the Geology 170 Class 2009
students and teachers of UP NIGS. We are grateful to Dr. Mike
Garcia and Dr. Rodolfo Tamayo for their valuable comments
on the manuscript. The field mapping activity was supported
by the Department of Science and Technology (DOST).
Sample preparation and analysis was done at the Nannoworks
Laboratory, National Institute of Geological Sciences, University
of the Philippines.
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Deborah N. Tangunan et al. : Post-Collision Deposition of Balanga Formation in northwest Mindoro, Philippines
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PLAT E 1
Photomicrographs of calcareous nannofossils from Mamburao, Occidental Mindoro, Philippines.
Scale bars=2µm; PL-Polarized Light; PH-Phase Contrast Lig ht.
A–B Calcidiscus leptoporus. 80–0cm (PH).
C–D Calcidiscus macintyrei. 1113 –1109cm (C=PL; D=PH).
E Gephyrocapsa caribbeanica. 284–212cm (PL).
F,H Helicosphaera sellii. 439–402cm (F=PH, H=PL).
G Helicosphaera carteri. 80–0cm (PL).
I Coccolithus pelagicus. 897.5 –888.5cm (PL).
J Syracosphaera spp. 897.5–888.5cm (PL).
K Syracosphaera histrica. 444–441cm (PL).
L Reticulofenestra minuta. 392–364cm (PL).
M small Gephyrocapsa. 212–138cm (PL).
N–O Pseudoemiliania lacunosa. 1200–1163cm (PL).
P Umbilicosphaera rotula. 80–0cm (PL).
Q,S Pontosphaera discopora. 402–392cm (PL).
R Pontosphaera japonica. 354–350cm (PL).
T Pontosphaera spp. 1239–1230cm (PL).
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Stratigraphy, vol. 11, nos. 3– 4, 2014
Plate 1Deborah N. Tangunan et al.
stratigraphy, vol. 11, nos. 3–4, 2014
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Deborah N. Tangunan et al. : Post-Collision Deposition of Balanga Formation in northwest Mindoro, Philippines
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PLAT E 2
Photomicrographs of calcareous nannofossils from Mamburao, Occidental Mindoro, Philippines.
Scale bars = 2µm; PL-Polarized Light; PH-Phase Cont rast Light.
A Rhabdosphaera clavigera. 1109.5–1049.5cm (PL).
B Sphenolithus heteromorphus. 1200 –1163cm (PL).
C Sphenolithus abies. 1006.5–982cm (PL).
D Discoaster triradiatus. 350 –293cm (PH).
E-G Discoaster tamalis. E-F, 120 0 –1163cm; G, 1006.5–
982cm (PH).
H, K-L Discoaster asymmetricus. H, 457–444cm; K-L, 897.5–
888.5cm (PL).
I-J Discoaster pentaradiatus. 628.5–590.5cm (PH).
M-P Discoaster brouweri. M, 100 6.5–982cm; N-O, 392–
364cm; P, 628.5-590.5cm (PH).
Q-T Discoaster surculus. Q -R, 1121–1113cm; S, 444–4 41c m;
T, 110 9.5–1049.5cm (PH).
U Discoaster kugleri. 1109.5–1049.5cm (PH).
V Overgrown Discoaster. 1031.5–1024.5cm (PH).
W Florisphaera profunda. 392–364cm (PL).
X Ceratolithus telesmus. 1200-1163cm (PL).
Y Ceratolithus cristatus. 115-80cm (PL).
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Stratigraphy, vol. 11, nos. 3– 4, 2014
Plate 2Deborah N. Tangunan et al.
stratigraphy, vol. 11, nos. 3–4, 2014
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Deborah N. Tangunan et al. : Post-Collision Deposition of Balanga Formation in northwest Mindoro, Philippines
