2005, History of sedimentary infilling and faulting in Subic Bay, Philippines revealed in high-resolution seismic reflection profiles. Jour. Asian Earth Science 25 849-858.

History of sedimentary infilling and faulting in Subic Bay, Philippines

revealed in high-resolution seismic reflection profiles

Ma. Edweena Joan A. Cabatoa, Kelvin S. Rodolfoa,b,*, Fernando P. Siringana

aNational Institute of Geological Sciences, University of the Philippines, Diliman QC 1101, PhilippinesbDepartment of Earth and Environmental Sciences (M/C 186), University of Illinois at Chicago, 845 W Taylor Street, Chicago, IL 60607-7059, USA

Received 11 February 2004; revised 3 August 2004; accepted 3 August 2004

Abstract

Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-level curves. The oldest sedimentary packages

are marine sediments subaerially exposed and eroded 20 ka. Fluvio-marine to wholly marine sediments were deposited during the ensuing

transgression, and prograding units were deposited during stillstands or minor sea-level falls. Faults within the bay have three age ranges. The

oldest set cuts through the pre-d18O Stage 2 rock units, O18 ka; a second disrupts 10.2–11.3 ka sediments; and the youngest, which cut the

uppermost sedimentary package, show that movements occurred about every 2 ky, most recently about 3 ka. Northwest–southeast faults that

parallel onshore structures associated with Paleogene emplacement of the Zambales Ophiolite Complex to the west and north likely represent

rejuvenated tectonism. The northern coastline and north–south-trending axial bay islands appear related to a lineament that dissects

Mt Pinatubo farther northeast. A breach in the caldera of Mt Natib is the most likely source of a presumed pyroclastic deposit in the eastern

bay that is associated with sediments about 11.3–18 ka, indicating that a Natib eruption occurred much more recently than previously

documented for this volcano.

q 2004 Elsevier Ltd. All rights reserved.

1. Introduction

Subic Bay in west-central Luzon, Philippines, a north–

south embayment 15 km long and 7 km wide (Figs. 1

and 2), used to be the site of the largest US naval base

overseas. Since the base was vacated in 1992, the bay

region has experienced rapid commercial and industrial

growth under the direction of the Subic Bay Metropolitan

Authority. The bay region is actively seismic and

volcanic, and its burgeoning economy and population

require careful evaluation of the potential for future

earthquakes and volcanism.

Regional faults or lineaments have previously been

identified or postulated. From an INTERA side-looking

airborne radar image, the Philippine National Oil

1367-9120/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/S1367-9120(04)00170-1

* Corresponding author. Address: Department of Earth and Environmen-

tal Sciences (M/C 186), University of Illinois at Chicago, 845 W Taylor

Street, Chicago, IL 60607-7059, USA. Tel.: C1 312 996 3154; fax: C1 312

243 8241.

E-mail address: [email protected] (K.S. Rodolfo).

Corporation (1988) delineated a lineament that trends

northwest through Mt Natib and emerges along the eastern

shore of Subic Bay. Yumul et al. (1990) and Yumul and

Dimalanta (1997) interpreted gravity data and petrographic

discontinuities of rock suites of the Zambales Ophiolite

Complex to postulate a ‘Subic Bay Fault Zone’ trending

northwestward through the bay. From an enhanced Marine

Observation Satellite image, Javelosa (1994) delineated

major lineaments at the head of Subic Bay that trend both

north–northwestward and north–south through Mt Pinatubo.

Using high-resolution seismic-reflection profiles, we

have identified and delineated packages of the sedimentary

bay fill, and faults that deform them. We have combined this

information with onshore data in order to characterize

regional lineaments and structures, identify other structures,

reconstruct the depositional and tectonic history of the bay,

and gain some sense of the frequency and magnitude of the

seismic events it experiences. An unanticipated result of this

work was the identification of volcanic deposits that indicate

a more recent eruption of the adjacent Mt Natib volcano

than previously recognized.

Journal of Asian Earth Sciences 25 (2005) 849–858

www.elsevier.com/locate/jaes

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

Fig. 1. Tectonic map of Luzon, with all earthquake epicenters for 1998.

M.E.J.A. Cabato et al. / Journal of Asian Earth Sciences 25 (2005) 849–858850

2. General geology

The Manila Trench (Fig. 1) marks where South China

Sea lithosphere has been subducting eastward beneath

Luzon since the Oligocene or Miocene (Hayes and Lewis,

1984a,b). Low-magnitude earthquakes are common in this

vicinity, and earthquakes of moderate to high magnitude

occur farther offshore. Mt Pinatubo is a volcano associated

with this trench, as are Mts Natib and Mariveles, which

compose the Bataan Peninsula east of Subic Bay. To the

north and west, the bay is bounded by the mountains of the

large, north–south trending Zambales ophiolite of Eocene

age (Rossman et al., 1989; Schweller and Karig, 1982;

Karig, 1983; Fig. 2).

3. Methods and data base

Our primary data consist of high-resolution seismic

profiles (Fig. 2) acquired in collaboration with two

Philippine government agencies: 38 km of sparker profiles

with the Mines and Geosciences Bureau (MGB), and 47 km

of sparker and 40 km of boomer line profiles with the

National Power Corporation (NPC). These profiles reveal

that acoustic basement is overlain by a faulted sedimentary

sequence approximately 120 m thick that we have deli-

neated into seven packages from reflection-termination

patterns, contact relations, internal layering, and variations

in seismic reflectivity. Structure-contour maps were drawn

to show the spatial distribution and thickness of each

package; these and the internal structures in the seismic

profiles were used to reconstruct the depositional paleoen-

vironments and relative ages of each unit.

We estimate the absolute ages of the different structures

and sedimentary packages according to the character of

prominent bounding surfaces and how these correlate with

regional changes in sea level (Fig. 3A), using sea-level

curves from southeast Asia (Pirazzoli and Pluet, 1991;

Pirazzoli, 1996) and other regions (Fairbanks, 1989;

Fig. 3B). Local sea-level curves are available (Berdin

et al., 2004; Maeda et al., 2004), but they do not include data

below present sea level.

The usual practice is to associate the shallowest

unconformity in high-resolution seismic profiles with the

Stage 2 lowstand of about O18 ka BP. During this stage

elsewhere in the world (Fairbanks, 1989), approximately

120 m of sea-level fall caused subaerial erosion and

incision. Within Subic Bay, this lowstand surface occurs

at depths of about 100 m, and may be deeper at the shelf

break. Towards the baymouth the presumed Stage 2

lowstand surface is shallower, at a depth of nearly 80 m,

and is merged with younger and older erosional surfaces.

We estimate the age of a younger, more widespread and

shallower, 55 m-deep ravinement surface by applying the

sea-level curve in the Caribbean Islands of Fairbanks

(1989). This yields an age of about 10.2 ka, which agrees

well with a similarly derived age from sea-level curves for

southeast Asia (Pirazzoli and Pluet, 1991; Pirazzoli, 1996).

We correlate the youngest flooding surface that extends

to the north beyond the seismic data coverage with the

emergence of sea level that is commonly pegged at about

6 ka. Mean 4–6 ka sea-level for southeast Asia is estimated

to have been about 4 m higher than at present (Pirazzoli and

Pluet, 1991; Pirazzoli, 1996) and is consistent with

Philippine data (Berdin et al., 2004; Maeda et al., 2004).

Onshore continuities of faults identified in the seismic

data were inferred based on the lineaments identified in a

1991 synthetic-aperture radar image. Trends of known

structures in the area were examined in the field.

4. Sedimentary packages and their genetic

interpretation

4.1. Package 1

This oldest package, which characteristically displays

well-defined clinoforms, occurs only in the southwestern

portion of the bay (Fig. 4). Its basal layers are horizontal, but

dips increase upwards to 48 at the uppermost, western limit

of the section along line 4A. This package is interpreted as a

prograding delta. The upper, eastern surface of this delta

was incised to a maximum depth of 80 m, probably during

the latest (d18O Stage 2) lowstand in sea level. Such incision

is traceable into the bay to depths of about 100 m. It merges

eastward with an older surface that probably was eroded

during an earlier (Pre-Stage 2) lowstand. We describe how

Fig. 2. Geologic map of Subic Bay (MGB, 1983 Ruaya and Panem, 1990). Tracklines of seismic-reflection profiles used in this study. The north–south and

east–west lines represent the MGB–NIGS data set; the diagonal lines those of the NPC–NIGS survey. The darkened and numbered portions of the tracklines are

sections illustrated in the figures with the same numbers. The bay is bounded to the north and west by Cretaceous to Eocene rocks of the Zambales Ophiolite

Complex, and to the east by Pliocene to Holocene andesites and dacites of the volcanic Bataan Peninsula.

M.E.J.A. Cabato et al. / Journal of Asian Earth Sciences 25 (2005) 849–858 851

these surfaces were identified with the Pre-Stage 2 and

Stage 2 lowstands later in this report. The western end of the

younger surface is truncated by a distinct sub-horizontal

boundary that is interpreted as a ravinement surface because

it also truncates the clinoforms. Such coincidence of the

ravinement surface and the Stage 2 sequence boundary is

common in shelf areas (Suter et al., 1987). Westward, the

deltaic deposit thickens to about 50 m at the offshore limits

of Fig. 4A and B.

4.2. Package 2

The deepest sequence in the inner portions of the bay is

package 2. In most areas it is too thick for its base to be

reached by the seismic returns, but its sub-horizontal

layering distinguishes it from the acoustically homogeneous

basement. Marked by roughly sub-parallel reflectors, its

upper surface is strongly discordant with the overlying unit

and is disrupted by faults, mounds and incisions that are

likely due to subaerial exposure and erosion (Fig. 5). In the

inner portions of the bay, package 2 was eroded during the

lowstand. The succeeding upper units occur as channel fills

that grade upward into hemipelagic deposits of the ensuing

transgression.

4.3. Package 3

Partially capping the irregular surface of package 2,

package 3 consists of separate packets of varying acoustic

expression and drastically different thicknesses that are

Fig. 3. (A) Stratigraphic column for acoustic units in Subic Bay. Ages were estimated using sea-level curves and sedimentation rates. Topmost unit, package 7,

comprises both prograding shoreface units and flat-lying offshore deposits. (B) Top. Sea-level curve derived from d18O. The Stage 2 lowstand that incised to

depths below 100 m occurred approximately 20 ky ago. B, Bottom. The global sea-level curve based on radiocarbon dated Acropora palmata (Fairbanks,

1989). In Subic Bay, the 55-m deep ravinement surface and 20-m deep flooding surfaces in the seismic records are, respectively, 10.2 and 8 ka old.

Fig. 4. Seismic reflection profiles along lines (A) and (B) of Fig. 2. (A) West–east portion. At the baymouth in the southern part of Subic Bay, sedimentary

package 1 overlies acoustic basement and is deltaic, with Stage 2 incisions at the top of its eastern and northern margins. (B) North–south portion. The lowstand

and ravinement surfaces merge towards the southwest. In the inner part of the bay, package 2 may be coeval with package 1.


Fig. 5. Seismic reflection profiles along lines (A) and (B) of Fig. 2. (A) West–east profile. Package 2 is faulted and has incisions suggesting subaerial erosion.

(B) North–south profile, showing internal layering of some portions of package 2.


coeval, based on their positions relative to underlying and

overlying units (Fig. 5). The upper boundaries of the

package are sub-horizontal. The distinct, irregular sequence

boundary between packages 2 and 3 is very likely erosional

in which incised portions of package 2 are filled with

sub-horizontal layers of package 3.

Layers are sub-parallel in the northern (Fig. 5A) and

central portions (Fig. 5B) of the bay, but locally are

hummocky and disrupted. A few reflectors rendered chaotic

by seismic diffraction, probably by gravel, are associated

with narrow depressions. Such coarse materials occur as

channel fill, perhaps channel lag or debris-flow deposits, in

the incised portions of the underlying unit. Widespread,

roughly horizontal layers, on the other hand, probably are

Fig. 6. Seismic reflection profile along line 6 of Fig. 2. Draping deposits, possibly o

adjacent to Mt Natib.

relatively fine-grained. Package 3 has its maximal thickness

of about 50 m in the central depressions of the basin, where

its thickness, presumed coarse to fine grain sizes, and

distribution indicate a subsiding, fluvio-marine environment

fed both by streams and settling.

Towards the southeast, distinct reflectors either drape

over or closely follow the underlying contours (Fig. 6).

Individual layers are less than 2 m thick, but the total

package thickness is about 40 m. Proximity to the Bataan

Peninsula and the documented Quaternary activity of

Mt Natib and its neighboring volcanoes indicate that

they are most likely pyroclastic flows and tephra-falls, such

as the thick pyroclastic-flow layers that crop out near the

southeast bayshore. Package 3 is missing in the baymouth

f pyroclastic flows and tephra falls, occur in the south-eastern part of the bay,


area farther south, either because it was not deposited, or

because it was eroded during a subsequent transgression.

Package 3, being atop the Stage 2 lowstand surface, we

assign an age of 18–11.3 ka. This estimate considers the fact

that Subic Bay was probably partly subaerial for some time

even as sea-level transgressed starting approximately 20 ka.

From 120 m below present sea-level, which is below the

depth of the shelf break, transgression would not have

inundated the embayment until a few thousand years later.

Hence, the extensive hemipelagic sediments in the bay must

have been deposited much later, although the incisions

would already have had older, fluvial fill. Laterally, this

deposit grades into thinner, flat-lying, normal hemipelagic

layers. The draping pattern is more pronounced in the upper

portion of the package. This might indicate deposition in an

environment that shifted from shallow marine to subaerial,

but sub-parallel drapes also have been recognized in

submarine pyroclastic-flow deposits elsewhere (Catane,

personal communication, 1999).

4.4. Package 4

In the southern part of the bay, package 4 is composed of

deltaic deposits with an uppermost limit at K55 m.

Truncation is evident in other parts of this package

(Fig. 7A). To the north, nearly horizontal and conformable

reflectors characterize this package (Fig. 7B). These deltaic

Fig. 7. Seismic reflection profiles along lines (A) and (B) of Fig. 2. (A) In th

displacements of 5–10 m. (B) Along this profile aligned generally west–east, so

recurrent activity, as evidenced by the inconsistent thicknesses of the packages th

bodies indicate episodic still-stands during a general rise in

sea level (Fig. 7A). They also mark paleo-sea level positions,

and hence, paleo-shorelines. Localized high energy towards

the south may have eroded deposits, thereby removing some

of the transgressive deposits and making erosional surfaces

that merge atop the delta. Fairbank’s (1989) sea-level curve

yields an age of around 10.2 ka for this surface.

4.5. Packages 5 and 6

Both of these packages are nearly horizontal and

conformable, each bounded at its base and top by distinct

surfaces (Figs. 6 and 7). Generally, these deposits are sheet-

like and wedge toward the bay margins. Thicknesses are

commonly about 7 m but range between 5 and 15 m. Less

distinct than the boundaries, internal layering roughly 2 m

thick suggests that the beds have similar lithologies but are

too thin to be resolved by seismic reflection. Divergence of

reflections towards the margin of the bay to form the wedge

is simply due to the tilt of depositional surfaces and

thickening of the sediments towards their source.

4.6. Package 7

Situated near the northeast shore of the bay, package 7

comprises two units that share a boundary marked by

a change in reflector orientation (Fig. 8). The lower unit,

is north–south profile, package 2 is cut by faults with apparent vertical

me of the faults that cut through younger sediment packages experienced

ey disrupt.

Fig. 8. Seismic reflection profile along line 8 of Fig. 2. Shoreface deposits of package 7 consist of a lower sheet deposit that wedges shoreward, and an upper

unit of consistent thickness that progrades out from the shore.


ranging in thickness from 7 m in the basin to 15 m towards

the shore, forms a wedge of clinoforms representing a delta

that onlaps the youngest, w6 ka flooding surface. The

correlative ravinement surface of this flooding surface

would be located landward of the data set. Overlying the

lower unit near the bay margin is a sheet drape, consistently

about 5 m thick, with downlapping, almost parallel-oblique

clinoforms that dip bayward. The clinoforms grade basin-

ward into parallel reflectors that represent present day bay

deposits. Taking the upper and lower units together,

package 7 is approximately 7 m thick in the center of the

bay and thickens to almost 20 m toward some portions of its

margin. This geometry of the two package 7 units is best

explained by deltaic deposition during rising sea level

interrupted by two episodes of stillstand.

5. Faults

In the seismic profiles vertically disrupted and displaced

horizons (Fig. 7) document recurrent faulting. The seismic

profiles do not provide any basis for estimating lateral

components of motion, which may have been substantial.

5.1. Faults cutting through package 2

Most of the faults disrupt package 2, the deepest

(Fig. 7A). These generally show deformative episodes

with apparent vertical displacement of about 10 m. In the

northern-central portions of the bay, such faults, which are

closely spaced and cut across north–south seismic lines,

are probably limited in extent, inasmuch as adjacent lines

are not faulted. They are probably en echelon, associated

with a north–south fault. Towards the south, near Grande

Island, the faults trend northwest and northeast.

5.2. Faults cutting through package 5

Younger faults have also disturbed packages 3–5, but

with less vertical displacement, no more than 5 m (Fig. 7B).

Some are only apparent from subtle disruptions in the

layers. These episodes of deformation occurred before

the overlying, undisturbed packages 6 and 7 were deposited,

and are similarly oriented to those that cut package 2.

5.3. Recurrent faults in package 7

Some of the more recent discontinuities extend up

through the topmost package, and run roughly north–south

along both sides of Pequena Island. Faults, however, cannot

be inferred with much confidence from these disruptions,

which may simply represent the abutments of sediments

against the insular mass.

Recurrent faulting deformed the younger sedimentary

packages of the northern portion of the bay (faults A–C in

Fig. 7B). Package 3, the lowest layer affected by Fault B,

has almost equal thickness across the structure. Package 4

thins out towards the apical protrusion of package 3,

however. This means that after package 3 was deformed,

faulting activity paused while continuing deposition of

package 4 restored sea floor horizontality. Package 5, of

uniform thickness, was deposited subsequently. If faulting

had then ceased altogether, packages 5 and 6 would have

remained horizontal. But these upper layers are also

deformed, documenting yet another faulting episode that

post-dated packages 4–6.

Faults A and C, respectively, about 1 km west and 700 m

northeast of Fault B, were also recurrent, as seen in

disrupted layers that thin out towards them from both sides.

Southeast of the bay, near the Bataan Peninsula, traces of a

northwest-trending fault likewise show recurrence. Move-

ment along these faults, moreover, has disturbed even the

base of uppermost package 7.

5.4. Onshore lineaments in satellite images

We attempted to identify onshore extensions of the faults

that have disturbed the bay fill. Structures depicted by

satellite imagery were sought, as well as exposures in the

field.

Three prominent co-sets dominate the lineaments in the

synthetic-aperture radar images (Fig. 9). Most of the

structures can be traced at least 10 km inland from the bay

margin. The major ones can be correlated with the structures

already proposed by previous workers.

Fig. 9. Lineaments (gray lines) in 1991 Intera Technologies synthetic aperture radar imagery of the Subic Bay region and field data of the present study. The

Philippine National Oil Company (1988) described lineament B; together with other subparallel structures, it cuts across Mt Natib and its caldera, which is

breached toward the northwest. Lineament C is the structure dissecting Mt Pinatubo (Javelosa, 1994). Offshore faults are expressed as apparent vertical

displacements in seismic profiles. The solid lines in the bay are probable offshore extensions of onshore structures. The north–south line is a narrow set of

individually short faults; the axial islands probably are associated upthrown blocks.


Several northwest–southeast trending faults may be

related. One northwest of Subic Bay demarcates the rugged

San Antonio Mountains of the Zambales Range and the low-

lying plains to the east (labeled A in Fig. 9), Probably

related to this feature is a similarly oriented lineament to the

southwest. These structures are associated either with the

northward motion of the Zambales Ophiolite Complex

relative to the Central Valley Basin in the Eocene to

Oligocene (Fuller et al., 1982) or with the southward

Miocene translation of the San Antonio massif relative to

the rest of Luzon (Yumul and Dimalanta, 1997; Yumul et

al., 1998). Possibly complementary, northeast–southwest

lineaments cut through the western and southeastern

margins of the bay.

The northern bay coast continues southeast as a

lineament, labeled B in Fig. 9, that dissects Mt Natib.

Coinciding with one identified in a side-looking airborne

radar image not depicted here (PNOC, 1998), this structure

belongs to a set of sub-parallel lineaments superimposed on

the other structures of Natib volcano, including its caldera.

These lineaments may be due to rejuvenated movement

along a southeast continuation of the older faults that are

oriented northwest–southeast.

Satellite imagery displays several north–south linea-

ments north of the bay (Fig. 9) including one (C) that cuts

through Mt Pinatubo. In the field north of Subic Bay, we

observed sub-vertical faults oriented approximately north–

south coinciding with lineament D. These faults prevalently


cut through the Paleogene ophiolitic rocks, and in fewer

instances through Plio-Pleistocene pyroclastic deposits.

Fresh aphanitic andesites near the northeastern coast of

the bay, probably among the youngest volcanic rocks in the

area, are cut by well-defined, steeply dipping, sub-parallel

faults trending north–northeast (Siringan et al., 2000). The

mylonitized zone along one of these shows reverse move-

ment. North of the bay, extremely macerated and weathered

rocks characterize a low-lying area traversed by the national

road to Castillejos. Along this trend, north–south faulting

was identified only through juxtaposed rocks of contrasting

weathered products. Shear joints are common throughout

the vicinity, but lack apparent general trends and do not lend

themselves to interpretation of specific structures.

6. Discussion

6.1. Correlating faults in the seismic profiles

with onshore structures

Faults in the bay fill cut through Late Pleistocene to

Recent sediments (Fig. 9). These cannot be correlated easily

with more ancient structures, delineated from satellite

images by previous authors and during the present study,

that dissect the much older ophiolite complex.

The northwest-trending faults in the southern part of

the bay probably are associated with similar trending

lineaments that dissect Mt Natib. Lineaments on the

satellite mosaic cut other structures on Mt Natib. Also, the

northern outline of the caldera probably collapsed due to a

northwest-directed explosion that produced flows faintly

evident on the satellite image. If so, the pyroclastic

materials would simply have flowed down the northwest-

trending linear depressions to be deposited in the adjacent

portion of the bay. It is possible, however, that Quaternary

volcanism in the Bataan Peninsula, as well as the sub-

parallel faults in the bay fill, represent rejuvenation of

older tectonic structures.

The structures observed in the field, plotted as

stereonets centered on sites, may be correlated with

lineaments on the satellite images. The north–south faults,

shear zones, and dikes north of the bay may be extensions

of similarly oriented faults that have uplifted the islands

and created the linear northern margins of the bay. Such

tectonic features may also be related with both the

lineament dissecting Mt Pinatubo to the northeast and the

roughly south-trending volcanic chain of Mts Pinatubo,

Natib, and Mariveles.

Other structures identified in the field, however, do not

coincide with the lineaments. This might be because

available outcrops bias field measurements; furthermore,

structures in outcrop scale belong to a three-dimensional

strain field that on the scale of the satellite mosaic is

generalized as a single, major lineament.

6.2. Recurrence of faulting

Faulting has considerably disturbed the units older than

the d18O Stage 2, O18 ka. Younger faults, some with

recurrent activity, have also cut the overlying sedimentary

packages. At least five episodes of faulting since 11.3 ka can

be reconstructed from the seismic data (Fig. 7B). An

average recurrence interval of about 2 ky is estimated for

these faults. The latest movement may have occurred about

3 ka because the fault trace extends halfway to the surface

from the 6 ka horizon.

The lineaments in the satellite images and most of the

faults observed in the field, on the other hand, are apparently

older than those documented in the seismic profiles. Such

structures are commonly associated with the emplacement

of the Zambales Ophiolite and intermittent volcanism in the

Bataan Peninsula before the Pleistocene. However, the

deformation within the bay possibly might be associated

with renewed tectonism along the structures that dissect the

ophiolite complex to the west and the volcanic arc to the

east.

6.3. Implications of the pyroclastic deposits in package 3

for Mt Natib volcanism

Inasmuch as this thick packet belongs to package 3, it has

a probable age of about 18–11.3 ka, nearly half that of the

youngest age for Mt Natib rocks previously available, which

is 27 ka for a sample taken from its eastern flank (Newhall,

personal communication, 1999). A breach on the side of the

caldera facing Subic Bay (Fig. 9) is most likely related to the

eruption that produced the presumed pyroclastic materials.

Such an eruption would have collapsed the northwest

portion of the caldera and generated flows into the eastern

portion of the bay. Thus, the presumed pyroclastic materials

are significantly younger than dated samples from the

eastern flank. Moreover, at the caldera and towards the other

side of the volcano, northwest-trending lineaments are

superimposed on other structures. These lineaments most

likely represent the latest deformational episode for this

area.

7. Conclusions

Seismically imaged sedimentary packages in Subic Bay,

all of late Pleistocene to Holocene age, document distinct

periods of emergence followed by fluvio-marine conditions.

The older sedimentary packages consist of incised channel

fill that grade upwards into extensive, nearly horizontal

deposits that form wedges towards the shore. Such sheet-

like units show lobes that indicate shorelines that back-

stepped during transgression. Progression towards fully

marine conditions was punctuated by volcanism in the

Bataan Peninsula, as evidenced by thick, draping deposits

that are best explained as pyroclastic.


Faults within the bay are of three age ranges. Those of the

oldest set cut through the Pre-d18O Stage 2 rock units, hence

are older than 18 ka. The north–south faults in the central

portion of the bay are probably related to the north–south

structures that dissect onshore areas in the north and

northeast. In satellite images, these structures are mani-

fested in the linear northern margin of Subic Bay and

dissecting Mt Pinatubo. Northeast- and northwest-trending

faults of the same age in the south are probably cosets of the

structures associated with volcanism in the Bataan Penin-

sula. Another fault set that disrupts package 5 of this study is

probably 8–10.2 ka old, based on sea-level curves. These

run through the same portions as the older set, and may

represent rejuvenated activity.

The youngest fault set includes the north–south disrup-

tions near Pequena Island. Southwest of this island and

towards the southeast reaches of the bay, fault trends are

difficult to establish, but recurrent activity is apparent. The

age for the latest movement along these faults is about 3 ka.

Northwest-trending lineaments superimposed on other

structures of Mt Natib are most reasonably related to the

draping deposits southeast of the bay. These suggest a latest

eruption about 11.3–18 ka, significantly later than the 27 ka

previously documented for this volcano.

Acknowledgements

Messrs Angel Bravo and Joel Natividad of the Mines and

Geosciences Bureau of the Department of Environment and

Natural Resources provided the set of north–south and east–

west seismic profiles. Messrs Tom Valencia, Ging Lauden-

cia, Benjie Punay, Gener Autor, and Ray Teh San and Ms

Bethel Bogtong provided equipment and assistance in the

field for obtaining the second set of profiles, which used a

generator generously loaned by the Subic Bay Metropolitan

Authority and its then Chairman, Mr Richard J. Gordon. We

thank Dr Christopher Newhall for a thorough, thoughtful

and very useful review. Drs Mario Aurelio, Joseph Foronda,

Eddie Listanco, and Graciano Yumul made many useful

comments. The Office of the Vice Chancellor for Research

and development of the University of the Philippines funded

the later portion of the fieldwork.

References

Berdin, R.D., Siringan, F.P., Maeda, Y., 2004. Holocene sea-level

highstand and its implications for the vertical stability of Panglao

Island, southwest Bohol, Philippines. Quaternary International

115–116, 27–37.

Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record:

influence of glacial melting rates on the Younger Dryas event and deep-

ocean circulation. Nature 342 (250), 637–642.

Fuller, R.M., Williams, I.S., McCabe, R., Encina, R.W., Almasco, J.,

Geary, E.E., Kay, R.W., 1982. Petrological and geochemical docu-

mentation of ocean floor metamorphism in the Zambales Ophiolite,

Philippines. In: Hayes, D.E. (Ed.), The Tectonic and Geologic

Evolution of Southeast Asia Seas and Islands, Part 2 AGU Geophysical

Monograph Series 27, pp. 139–159.

Hayes, D.E., Lewis, S.D., 1984a. A geophysical study of the Manila

Trench, Luzon, Philippines, Part 1: Crustal structure, gravity, and

regional tectonic evolution. Journal of Geophysical Research 89, 9171–

9195.

Hayes, D.E., Lewis, S.D., 1984b. A geophysical study of the Manila

Trench, Luzon, Philippines, Part 2: Forearc basin: structural and

stratigraphic evolution, 89, pp. 9196–9214.

Javelosa, R.S., 1994. Active Quaternary environments in the Philippine

Mobile Belt, 20. ITC Publication, pp. 179.

Karig, D.E., 1983. Accreted terranes in the northern part of the Philippine

archipelago. Tectonics 2, 211–236.

Maeda, Y., Siringan, F., Omura, A., Berdin, R., Hosono, Y., Atsumi, S.,

Nakamura, T., 2004. Higher-than-present Holocene mean sea levels in

Ilocos, Palawan and Samar, Philippines. Quaternary International 115–

116, 15–26.

Mines and Geosciences Bureau, 1983. Geologic maps of San Antonio,

Zambales; Dinalupihan, Bataan; and Silanguin Island: 1:50,000; 3

sheets, Geological survey division. Department of Environment and

Natural Resources, Manila, Philippines.

(PNOC) Philippine National Oil Company, 1988. Environmental manage-

ment report for the PNOC geothermal exploration project in Mt Natib,

Bataan 1988.

Pirazzoli, P.A., 1996. Sea level Changes: the Last 20,000 years. Wiley,

West Sussex, England, pp. 211.

Pirazzoli, P.A., Pluet, J., 1991. World Atlas of Holocene Sea level Changes.

Elsevier, Amsterdam, pp. 300.

Rossman, D.L., Castaneda, G.C., Bacuta, G.C., 1989. Geology of the

zambales ophiolite, Luzon, Philippines. Tectonophysics 168, 1–23.

Ruaya, J.R., Panem, C.C., 1990. Mt Natib, Philippines: A geochemical

model of a caldera-hosted geothermal system. Journal of Volcanology

and Geothermal Research 45, 255–265.

Schweller, W.J., Karig, D.E., 1982. Emplacement of the Zambales

Ophiolite into the West Luzon margin. American Association of

Petroleum Geology Memoir 1982;, 34.

Siringan, F.P., Cabato, J.A., Mateo, Z.P., 2000. Marine geological

investigation: sedimentation and tectonics of Subic Bay and vicinity.

Report Submitted for the Subic Bay Protected Areas Management Plan

Project, Woodward Clyde, Inc.

Suter, J.R., Berryhill, H.L., Pentland, S., 1987. Late Quaternary sea level

fluctuations and depositional sequence, southeast Louisiana continental

shelf, In: Nummendal, D. (Ed.), Sea level Fluctuations and Coastal

Evolution, 41. Society of Economic Paleontologists and Mineralogists

Special Publication, pp. 199–219.

Yumul Jr., G.P., Dimalanta, C.B., 1997. Geology of the southern Zambales

Ophiolite Complex (Philippines): Juxtaposed terranes of diverse origin.

Journald of Asian Earth Sciences 15, 413–421.

Yumul Jr., G.P., Datuin, R.T., Manipon, J.C., 1990. Geology and

geochemistry of the Cabangan-San Antonio massifs, Zambales

Ophiolite Complex, Philippines: Tectonically juxtaposed marginal

basin-island arc terranes. Journal of the Geological Society of the

Philippines 45, 69–100.

Yumul Jr., G.P., Dimalanta, C.B., Faustino, D.V., de Jesus, J.V., 1998.

Translation and docking of an arc terrane: geological and geochemical

evidence from the southern Zambales Ophiolite Complex, Philippines.

Tectonophysics 293, 255–272.

2005, History of sedimentary infilling and faulting in Subic Bay, Philippines revealed in high-resolution seismic reflection profiles. Jour. Asian Earth Science 25 849-858.

Documents