Petrology, Sedimentology, and Diagenesis of 

Hemipelagic Limestone and Tuffaeeous Turbidites 

in the Aksitero Formation, Central Luzon, Philippines

Prepared in cooperation with the Bureau of Mines, 

Republic of the Philippines, and the 

U.S. National Science Foundation
Petrology, Sedimentology, and Diagenesis of 

Hemipelagic Limestone and Tuffaceous Turbidites 

in the Aksitero Formation, Central Luzon, Philippines

By ROBERT E. GARRISON, ERNESTO ESPIRITU, LAWRENCE J. HORAN, and 

LAWRENCE E. MACK

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1112

Prepared in cooperation with the Bureau of Mines, 

Republic of the Philippines, and the 

U.S. National Science Foundation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

United States. Geological Survey.

Petrology, sedimentology, and diagenesis of hemipelagic limestone and tuffaceous turbidites

in the Aksitero Formation, central Luzon, Philippines.

(Geological Survey Professional Paper; 1112)

Bibliography: p. 15-16

Supt. of Docs. No.: 119.16:1112

1. Limestone-Philippine Islands-Luzon. 2. Turbidites-Philippine Islands-Luzon. 3. Geology, 

Stratigraphic-Eocene. 4. Geology, Stratigraphic-Oligocene. 5. Geology-Philippine Islands-- 

Luzon. I. Garrison, Robert E. II. United States. Bureau of Mines. III. Philippines 

(Republic) IV. United States. National Science Foundation. V. Title. VI. Series: 

United States. Geological Survey. Professional Paper; 1112.

QE471.15.L5U54 1979 552'.5 79-607993

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03220-1
CONTENTS

Abstract --------------------------------------------------------------------------------------------------------------------

Introduction --—————————-----——————————————————————-———————————————-

Acknowledgments ———-————-——————————-——-————-————————————————-

Field description and stratigraphic relations ————-—-—————————-——————————————— 2

Petrology of hemipelagic limestone ——-——————————-———-———————————————————— 2

luffaceous turbidites —————————————————————-———————————————————————— 7

Sedimentarv structures —-———-———————————--———-——-——-——————————————— 7

Composition and texture ———————-———————--—-——————-——————————————— 8

Diagenetic minerals and textures ——-———————-———————————————————————— 9

Traces of hydrocarbon -------——————————-——————-————-——————————————— 11

Origin and diagenetic histoi \ of the tuffs ———-——-—-——————-——————————————— 11 

Zeolites —————————————————————————————————————— 12

Sedimentological and tectonic setting ————————————————————————————————————— 12 

Significance of h\drocarbon traces in the tuffaceous sediments ———-——————————————— 14 

References cited —————-——--——-————-———————————-——————————————————— 15

ILLUSTRATIONS

I'age

FIGURE 1. Maps showing location of Aksitero River section, central Luzon ————————————————.————————„„.— 2

2. Composite Cenozoic stratigraphic section for central Luzon ——————————————————-—————————————————————— 3

3. Photograph showing outcrop of thin-bedded hemipelagic limestone and tuff, Aksitero Formation ————————————————— 4

4. Photomicrographs of hemipelagic limestone ——————————————————————————————————————————————————— 4

5. Transmission electron micrographs of micritic matrix in hemipelagic limestone ————————————————————————————— 5

6. Transmission electron micrographs of nannofossils and products of diagenesis in hemipelagic limestone ———————————— 6

7. Photograph showing slabbed and polished surface of tuffaceous turbidite bed —————————————————————————————— 7

8. Photomicrographs of turbidite tuff layers ————————————————————————————————————————————————————— 8

9. Photomicrograph of turbidite tuff layer showing volcaniclastic grains embedded in matrix of small glass shards replaced by

mordenite ————————————————————————————————————————-—————————————————————————————— 9

10. Scanning electron micrographs of zeolites in tuffaceous turbidite ——————————————————————————————————————— 10

11. Schematic diagram showing depositional setting of sediments in Aksitero Formation —————————————————————————— 13

12. Sketch showing early Tertiary plate tectonic reconstruction of the northern Philippines and Marianas region————————— 14

III

PETROLOGY, SEDIMENTOLOGY, AND DIAGENESIS OF HEHIPELA^IC

LIMESTONE AND TUFFACEOUS TURBIDITES IN THtE I

i

AKSITERO FORMATION, CENTRAL LUZON, PHILIPPINES |

By ROBERT E. GARRISON, ERNESTO EspiRixu, 1 

LAWRENCE J. HORAN, AND LAWRENCE E. MACK

ABSTRACT

The Aksitero Formation of central Luzon is an upper Eocene and 

lower Oligocene sequence of evenly bedded hemipelagic limestone 

with a few thin interlayers of tuffaceous turbidites. The limestone consists chiefly of planktonic foraminifers and calcareous nannofossils, with 

up to 30 percent of noncarbonate components, chiefly volcaniclastic 

debris. The tuff layers are graded beds, composed mainly of glass 

shards, pumice fragments, crystals, and fine-grained volcanic rock 

fragments. Hydrocarbons migrated into the pores of the tuffaceous 

layers early during diagenesis but they were subsequently flushed out 

and only bitumen remains, chiefly as thin coatings on grains and within 

pumice vesicles. Later during diagenesis, zeolites (mordenite and clinoptilolite) and secondary calcite preferentially replaced glass shards 

and pumice fragments. Deposition of the Aksitero Formation probably occurred at depths of at least 1,000 meters within a subsiding 

basin adjacent to an active island arc system. Submarine ash eruptions 

of silicic composition caused volcaniclastic turbidity currents that occasionally reached the basin floor. The more proximal fades of these 

volcaniclastic deposits may be prospective for hydrocarbons.

INTRODUCTION

This study is a product of a cooperative research program on the origin and distribution of energy resources along technically active plate margins in east 

Asia. One phase of this research has been concerned 

with the sedimentology of basinal sedimentary sequences that are potential source rocks for hydrocarbons. Among rocks of this kind we have concentrated 

mainly on petroliferous diatomaceous deposits of late 

Tertiary age in many parts of the north eircum-Pacific 

area (Garrison, 1975). The limestone and tuffaceous 

turbidites of the early Tertiary Aksitero Formation 

described here are a somewhat different kind of. basinal sequence which appears, however, to have similar potential for the generation of hydrocarbons. TT ""

Tertiary sedimentary rocks exposed along the Aksitero River in the foothills of the Zambales Mountains 

west of the Central Valley of Luzon (fig. 1) form one

'Bureau of Mines, Manila, Republic of the Philippines.

of the standard biostratigraphic reference sectiqns for 

Cenozoic rocks in the Philippines. Among other |workers, Corby and others (1951, p. 105-108), Bandy (B62, 

p. 18-23; 1963, p. 1738-1739), Amato (1965, p. 13-23), 

Divino-Santiago (1963, p. 73), Grey (1967, p. 20), 

Roque, Reyes, and Gonzales (1972, p. 17-19), and Ingle 

(1975, p. 840-848) have referred tojhis section. The 

sedimentary rocks in this sequence consist of tuffaceous mudstone and sandstone of the~Moriones Formation underlain by fine-grained foraminiferal limestone with thin calcareous tuff interlayers (fig. 2). 

These basal calcareous rocks were formerly considered 

part of the Moriones Formation and informally known 

as the Bigbiga limestone (Bandy> 1062, p. 18). Amato 

(1965, p. 13, 14) proposed the name Aksitero Formation for the limestone unit and some bverlying clastic 

rocks. On the basis of rare bent^oriic- foraminifers, he 

suggested that the limestone was deposited in a deep 

bathyal environment, at water depths in excess of 

1,000 m (see also Bandy, 1963, p. 1738- Inglet|[975, p. 

840). Our study of the pe'trolojgy^and sedimentary 

structures in these rocks g@ai©*fc|ly confirms^Ajnato's 

assessment and provides adeKfianaKletails oh trie sedimentological and diagenetic history of these rockSf/fii 

addition, we propose a depositional model tfiat has implications for the prediction of lateral fades relations 

and possibly also for the occurrence of hydrocarbons.

ACKNOWLEDGMENTS

David Bukry identified nannofossil forms in electron 

micrographs of the limestone. We are grateful to 

James Boles, Ray Christopher, R. V. Fisher, : R. S. 

Fiske, Azuma lijima, J. C. Maher, M. J. Terman, and 

A. C. Waters for discussions and suggestions. This report was prepared in cooperation with'the1 Philippine 

Bureau of Mines, and we wish to, acknowledge the help 

of Director Juanito C. F.ernandez, Mr. .Felipe U. Francisco, and Mr. Oscar Crispin. The work was funded in
HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION

130*00' 121*80'

San Fernando 

oBaguio

Manila

121°00'

is* so16*00'

Lingayitn 1

ANVICUN6

IV/ "*"•*,. '

o San Manual

o San Jose

River

section

O

Tarlac

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j_I

FIGVRJS ],.—Location of Aksitero River section, central Luzon, Philippines, A, Index map of central Luzon showing location of Aksitero River, ff, Enlarged view of area near Zambales Mountains 

showing location of section, Dashed line indicates approximate 

boundary of Central Valley.

part by the National Science Foundation and carried 

out in conjunction with the Circum-Pacific Council 

Map Project.

FIELD DESCRIPTION AND STRATIGRAPHIC 

RELATIONS

The Aksitero Formation at its type area along the 

upper Aksitero River is about 200 m thick (fig. 2). On 

the basis of planktonic foraminifers, Amato (1965, p. 

13-14) assigned it a late Eocene and early Oligocene 

age. According to Roque, Reyes, and Gonzales (1972, 

p, 18,19), the lower 148 m of the formation at the type 

area is mainly limestone of Eocene age, and the upper 

52 m is interbedded limestone and tuffaceous sandstone of Oligocene age. The unit lies unconformably 

above highly weathered, poorly exposed, and apparently complexly deformed mafic to intermediate igneous rocks of the Mesozoic to early Tertiary(?) basement complex of the Zambales Mountains (Corby and 

others, 1951, p. 105; Philippines Bureau of Mines, 

1976, p, 63-65), This complex may be in part an ophiolitic sequence (Ingle, 1975, p. 848). Overlying the Aksitero Formation is the Moriones Formation of early 

and middle Miocene age composed of tuffaceous mudstone, sandstone, and conglomerate; the contact between the two units is poorly exposed, but apparently 

it is unconformable and locally may be faulted.

The Aksitero Formation is composed principally of 

thin-bedded pale-yellow to yellowish-white fine-grained 

limestone that ranges from argillaceous and slightly 

fissile to nearly pure, very hard, and conchoidally fractured (fig. 3). A few limestone beds are light greenish 

gray or pink, Interlayered with the limestone are a few 

beds of zeolitized tuffaceous turbidites, most less than 

10 cm thick.

PETROLOGY OF HEMIPELAGIC LIMESTONE

The predominant rock in the Aksitero Formation is 

fine-grained foraniiniferal biomicrite with tests of 

planktonic foraminifers dispersed in a nannofossil-rich 

micritic matrix (fig. 4). Pelagic microorganisms are 

thus the most common constituent, but the limestone 

also contains up to 30 percent noncarbonate detrital 

components and is thus best categorized as a hemipelagic deposit. Both thick- and thin-shelled planktonic 

foraminifers are present; compactional crushing commonly has fractured the walls of the thin-shelled forms 

(figs. 4C and 4D). This kind of deformation probably 

also produced the abundant small broken fragments of 

foraminiferal shell walls dispersed in the matrix.

Comparatively rare benthonic foraminifers make up 

less than 5 percent of the foraminiferal population, and
METROLOGY OF HEMIPELAGIC LIMESTONE

AGE

Pleistocene

Pliocene

Late Miocene

Middle Miocene

Early Miocene

Early Oligocene and 

Late Eocene

Mesozoic to 

early Tertiary (?)

GENERALIZED 

AKSITERO RIVER 

SECTION

Tarlac 

Formation

Malinta 

Formation

Moriones 

Formation

Aksitero 

Formation

Basement 

complex, 

Zambales 

Mountains

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COMPOSITE SECTION 

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ESTIMATED PLfifeKtONKS STRATIGRAPHY 

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,

FIGURE 2.—Composite Cenozoic stratigraphic section for central Luzon, with paleobathymetnc curve based on microfatna. Modified from

Ingle (1975, p. 840).

in one specimen we noted very rare echinoderm fragments. Other microfossils present in generally small 

numbers are radiolarians, sponge spicules, and phosphatic fish remains. Very fine grained cristobalite has 

replaced most shells of the siliceous microfossils, and 

secondary calcite fills some radiolarian molds

Nannofossils include spherical shells of Thoracosphaera, coccoliths, and discoasters (figs. 5, 6A, 6C). 

Among the discoasters is one species, Discoaster barTan Sin Hok (figs. 5A and 6A), whose range

of middle to upper Eocene (Bukry, 1973, p. 6£$) Confirms Amato's (1965, p, 13,14) age dating of the Iowapart of the Aksitero Formation. Also present are coccoliths of the genera Coccolithus and FeticulcC^zstra 

(David Bukry, written commun,, 1S7£).

The chambers of the planktonic foramisiifers cdntain 

a variety of different materials. Nannofossil-rich micrite partly or entirely fills some chambers, Others 

contain minerals that either precipitated in void dpace 

or replaced mkrite; these minerals include secondary
•4 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION

FIGURE 3.—Outcrop of thin-bedded hemipelagic limestone and tuff in 

Aksitero Formation, Aksitero River section.

calcite (fig. 6Q, zeolites (probably clinoptilolite), authigenic smectites, optically length-fast chalcedony, 

and a light-brownish very fine grained mineral with 

very low birefringence that X-ray diffraction suggests 

is cristobalite. Very commonly, cristobalite partly replaces the micritic fillings of shell chambers. Apparently the interiors of foraminiferal chambers provide 

a chemical milieu that is conducive to the growth of a 

variety of secondary minerals.

Inorganic detrital components in the limestone appear to be largely of volcanic origin and are present in 

various amounts. Measurements of insoluble residues 

in nine samples indicated a range of 13 to 30 percent, 

with an average of 21 percent. Some of this noncarbonate material consists of clay minerals dispersed in 

the micritic matrix (fig. 5(7). Scattered sand- to coarsesilt-size grains include the following:

of hemipelagic limestpne in Aksitero dant spicules and compactionally flattened foraminiferal tests. D,

Formation. Plane-polarized light. A, Planktonic foraminiferal tests Lamina rich in foraminiferal tests, probably the product of winnowafid volcanic tletritlrs iff ftkiinofossil-ricri micritic matrix. Erribayed ing of micrite by weak bottom current. Many thin-shelled tests are

clast (F^;-Mttef€«i UaSt'ef pumicfe (VAX :B, Planktehic for- broken, whereas thicker shelled ones tend to be intact.

tefetsianxi -matrix, C, .Abun­
PETROLOGY OF HEMIPELAGIC LIMESTONE

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FIGURE 5.—Transmission electron micrographs of micritic matrix in 

hemipelagic limestone, Aksitero Formation. Samples prepared by 

two-stage replica method (Fischer and others, 1967). A, Micrite containing coccoliths and discoasters, including Discoaster barbadiensis Tan Sin Hok, at top. B, Micrite composed largely of coccoliths 

and fragments of coccoliths. Several coccoliths have overgrowths of

secondary calcite crystals (center left). C, Micrite containing coccoliths, authigenic calcite, and clay minerals (dark particles). Wellpreserved coccolith (lower left) is Coccolithus pelagicus (Wallich) 

Schiller. D, Nearly tangential section through shell of Thoracosphaera. Surrounding micrite consists of discoasters, coccolith fragments, authigenic calcite, and clay minerals.
HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES AKSITERO FORMATION

FIGURE 6.—Transmission electron micrographs of nannofossils and 

products of diagenesis in hemipelagic limestone, Aksitero Formation. Samples prepared by two-stage replica method (Fischer and 

others, 1967). A, Discoaster barbadiensis Tan Sin Hok. This species 

ranges from middle to late Eocene. B, Lithoclast of tuffaceous mudstone, largely recrystallized to secondary smectite that shows welldeveloped 001 cleavage faces. C, Micritic matrix has undergone recrystallization causing partial replacement of many nannofossils. 

Circular Thoracosphaera shell contains relatively coarse, subhedral 

to euhedral crystals of secondary calcite that have replaced part of 

coccolith filling. D, Volcanic glass shard. Acicular crystals of mordenite and subhedral to euhedral crystals of secondary calcite have 

replaced glass.
TUFFACEOUS TURBIDITES

1. Euhedral to broken or embayed angular 

crystals of plagioclase (fig. 4A).

2. Elongated fragments of altered pumice with 

long tube vesicles (figs. 4A, 4B). Most appear 

to be replaced by fine-grained zeolites or 

coarse secondary calcite.

3. Altered volcanic glass shards, many showing 

round-bubble vesicles. Diverse minerals replace the shards; these include zeolites (fig. 

6D), cristobalite, smectites, chlorite, and 

large crystals of calcite.

4. Lithoclasts of brown mudstone composed of 

well-crystallized secondary smectites that 

have a distinctive "herringbone" pattern 

with two preferred crystal orientations (fig. 

6B). Most of these appear to be altered tuffaceous mudstone, because in some the outlines of glass shards are still visible along 

with radiolarian skeletons. Chlorite is the 

major component of these lithoclasts in the 

light-greenish-gray limestone.

5. Lithoclasts composed of brown to yellow volcanic glass fragments replaced by smectites; 

some lithoclasts contain small plagioclase 

laths.

In addition to the diagenetic effects noted above, 

electron microscopy shows that the micritic fraction 

has undergone some recrystallization. Neomorphic 

microspar has completely replaced some nannofossils 

(figs. 5, 6Q.

The limestone is unlaminated and has indistinct 

mottling probably caused by burrowing. Burrows are 

not conspicuous, however, because the sediment was 

very homogeneous, without the contrasts in color and 

texture that normally accentuate burrow fillings. In 

thin section, the common absence of strongly preferred 

alinement of linear fabric elements (such as spicules) 

parallel to bedding also suggests disruption due to burrowing. Some discontinuous laminae in the limestone 

contain concentrations of foraminiferal tests and sandsize detrital grains. The laminae suggest sporadic winnowing of the sediment by bottom currents (fig. 4D).

TUFFACEOUS TURBIDITES

Thin graded beds of tuff, which we interpret as turbidites, are interbedded within the sequence of pelagic 

limestone. These beds range in thickness from 3 to 15 

cm, and in aggregate they form 5-10 percent of the 

total thickness of the Aksitero Formation.

SEDIMENTARY STRUCTURES

Most individual tuff beds consist of a basal dark, 

sand-size part that grades into an upper part of very 

fine grained, hard, zeolitized, yellowish-white rock. 

The upper part has a pronounced conchoidal fracture 

and resembles porcelanite. A few tuff layers consist 

entirely of fine-grained porcelaneous material; they 

closely resemble the interbedded pelagic limestone in 

outcrop. The fine-grained tuffs commonly contain 

sparse, very faint burrows filled with pelagic limestone. A few beds show complete Bouma sequences, 

with a to e layers (Bouma, 1962, p. 49-51) in which 

fine-grained porcelaneous tuff forms the d interval.

One sample shows a composite of two turbidite layers that form a single bed (fig. 7). The lower layer contains Bouma a, b, and c intervals. The c interval shows 

small faint ripple marks and is erosionally truncated 

by the overlying turbidite layer (see Yamada, 1973, p. 

589). The upper turbidite layer contains less sand-size

FIGURE 7.—Slabbed and polished surface of tuffaceous turbidite bed, 

Aksitero Formation, containing two turbidite layers. Letters a, b, 

and c indicate Bouma intervals. Interval a is massive and b is laminated; both intervals contain numerous pumice fragments (fig. BO. 

Dark color results from staining by bitumen. The c interval contains 

faint small-scale ripple marks and cross-laminations. The upper turbidite layer lies just above the c interval. Poorly sorted basal part 

(x) contains coarse-sand-size crystals and lithic clasts, and changes 

upward rather abruptly into very fine grained zeolitized tuff (y) that 

is very hard and porcelaneous. Top of porcelaneous interval has 

faint burrows, most noticeable at upper right. Upper turbidite layer 

is generally finer grained than lower.
HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION

and more fine-grained volcanic material. The gradation to the upper porcelaneous part of the tuff is relatively abrupt, perhaps due to a sedimentological 

break between the upper low-density and lower highdensity parts of the turbidity flow (Middleton, 1967, p. 

487-490; Yamada, 1973, p. 593).

COMPOSITION AND TEXTURE

Four main components make up the basal sandy 

parts of the turbidites: crystals, altered glass shards, 

altered pumice fragments, and lithoclasts of devitrified 

volcanic glass (fig. 8). The crystals are largely plagioclase; they range from euhedral to rounded to angular 

and broken; some are strongly embayed. Many show

pronounced oscillatory zoning. The glass shards typically show vestiges of round-bubble vesicles. At least 

two types of pumice fragments are present. The most 

abundant type contains rather sparse round-bubble 

vesicles. The other contains long-tube vesicles (see 

Fiske, 1969). Secondary zeolites and calcite have replaced both the shards and pumice fragments, as discussed below.

Fine- to medium-sand-size lithoclasts of fine-grained 

volcanic rock are very abundant in the basal parts of 

the tuffs. Felted masses of feldspar microlites compose 

some of the lithoclasts, but the most abundant fragments have an intersertal texture of feldspar microlites in a devitrified glass matrix. Some lithoclasts are 

flow banded, and others have remnants of spherulitic

FIGURE 8.—Photomicrographs of turbidite tuff layers, Aksitero Formation. Plane-polarized light. A, Fine-grained zeolitized tuff from 

porcelaneous upper part of turbidite layer (fig. 7, y). Outlines of 

small glass shards are faintly visible. Mordenite has replaced 

shards. B, Basal part of tuffaceous turbidite above hemipelagic limestone (L). Contact is irregular and erosional; angular crystals and 

lithic fragments in turbidite are poorly sorted. C, Large grain in 

upper center is highly altered pumice fragment replaced by coarse

calcite and clinoptilolite. Clinoptilolite also fills many vesicles. 

Black material is mostly bitumen, which lines most of the vesicles. 

Specimen from interval a of turbidite bed shown in figure 7. D, Angular crystals and lithic fragments from same specimen as 8C. Dark 

grains are bitumen-impregnated mudstone lithoclasts. Fractures in 

feldspar crystal (center) also contain bitumen. Irregular mosaic of 

fine-grained calcite crystals cements grains.
TUFFACEOUS TURBIDITES

structures with contraction cracks. Although they are 

too small and fine grained to be identified with certainty, most of the lithoclasts appear to have been derived from extrusive or shallow intrusive rocks of dacitic or trachyandesitic composition.

In addition to the volcanic components, most of the 

tuffs also contain small amounts of pelagic sediment; 

the pelagic sediment suggests erosion, entrainment, 

and redeposition of this sediment during emplacement 

of the turbidites. Redeposited pelagic components are 

present either as micrite and sparse planktonic foraminifers or more rarely as irregularly shaped lithoclasts 

of pelagic limestone. Other minor components include 

radiolarian tests, poorly preserved bryozoans, quartz, 

pyroxene, and amphibole.

Nearly all the sand-size grains in these rocks are 

very angular (figs. 8 and 9). The sandy parts of the 

tuffs range from poorly sorted to rather well sorted. 

The poorly sorted parts have abundant fine-grained 

matrix that commonly is a mixture of small shards and 

micrite. The well-sorted parts, which are most common 

near the base of the turbidite beds, contained appreciable porosity before pore-filling cementation by calcite and zeolites.

The fine-grained porcelaneous upper parts consist 

chiefly of small glass shards that are replaced by the 

zeolite mineral mordenite (fig. 8A). Some small shards 

are parts of the curved walls of vesicles, but most are 

elongated fragments that range from about 30 to 100 

micrometers long and are 5-10 micrometers thick.

Sorting according to density of the grains produced 

reverse size grading within the a intervals of some turbidite beds. The lowest parts of these beds contain 

mainly plagioclase crystals, shards, and felted volcanic 

lithoclasts; these are overlain by abundant large fragments of pumice whose average grain size greatly exceeds that below (see Fiske and Matsuda, 1964, p. 87- 

88; Fiske, 1969, p. 4; Yamada, 1973, p. 590-591; Niem, 

1977, p. 49—61). These pumiceous layers were very porous and, as described in a following section, became 

saturated with hydrocarbons and now stand out as 

dark layers (fig. 7).

DIAGENETIC MINERALS AND TEXTURES

Diagenesis has profoundly altered the composition 

and texture of the tuff layers. The most conspicuous 

diagenetic effects are those produced by growth of zeolites as replacements and as cements. Clinoptilolite 

and mordenite (figs. 8A, and 10) were identified by Xray diffraction; the clinoptilolite identification was 

confirmed by Alietti's (1972) heating test. Mordenite 

(figs. 8A, WC, and WB) completely replaces the small

FIGURE 9.—Photomicrograph of turbidite tuff layer, Aksitero Formation, showing angular sand-size volcaniclastic grains embedded 

in fine-grained matrix of small glass shards replaced by mordenite. 

Coarse secondary calcite has replaced most of coarse volcaniclastic 

grains. Large pumice fragment (P) at upper right has been replaced by several calcite crystals. Single calcite crystals have replaced glass shards, some showing broken edges of round-bubble 

vesicles (lower half). Twinned plagioclase grain (F) remained unaffected. Partly crossed nicols.

glass shards in the upper, porcelaneous parts of the 

turbidite beds and also occurs as a cement between the 

fragments to form a dense and very hard rock. In the 

lower, sandy parts of the turbidite layers, both mordenite and clinoptilolite (figs. 8C and 10) replace glass 

shards and pumice. Clinoptilolite also forms even rims 

of cement crystals that line intergranular pores and 

vesicles within pumice fragments (fig. 10A).

Mordenite, which is much more abundant than clinoptilolite, occurs as very small needlelike length-fast 

fibers less than 2 micrometers in diameter and some 

tens of micrometers long. Clinoptilolite is present most
10 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION

FIGURE 10.—Scanning electron micrographs of zeolites in tuffaceous 

turbidite, Aksitero Formation. Specimen is from basal part of 

graded turbidite (c in fig. 7). A, Euhedral crystals of clinoptilolite 

that formed as intergranular cement. B, Elongate clinoptilolite crystals in altered pumice fragment that in part replaced pumice and

in part grew into vesicle cavity. C, Rosettes of acicular mordenite 

crystals in pumice fragment with long-tube vesicles. Mordenite both 

replaced glass in pumice and filled voids. D, Zeolites filling vesicle 

in pumice fragment. Acicular mordenite at top; euhedral clinoptilolite at bottom.
TUFFACEOUS TURBIDITES 11

commonly as transparent, length-slow, elongated crystals up to hundreds of micrometers long, tens of micrometers wide, and having blunt crystal terminations. In replacing fragments of pumice, individual 

clinoptilolite crystals grew across vesicle walls so that 

they are in part void-filling (see lijima, 1971, p. 334- 

336). Individual mordenite fibers likewise transect the 

edges of the glass shards that they replace.

Secondary calcite is widespread and abundant in 

these tuffs as intergranular cement and as replacements of glass shards and pumice fragments. In the 

sandy parts of the tuffs, it locally forms a cement of 

irregularly interlocking equigranular crystals between volcaniclastic grains (fig. 8D). Textural relations show that the calcite cement predates the zeolitic 

cement and presumably also the zeolitization of the 

shards and the pumice.

Secondary calcite is most abundant, however, as a 

second, post-zeolite generation of coarse-grained crystals that replaced glass shards and, to a lesser extent, 

crystals of plagioclase and pyroxene. Euhedral calcite 

rhombs, 15-50 micrometers in maximum dimension, 

also locally replace parts of the intergranular clinoptilolite and calcite cements. Commonly crystals of calcite up to 1 mm across preferentially replace shards or 

pumice fragments, although the surrounding finegrained tuffaceous matrix remains unaffected (figs. 8C 

and 9). In some instances the late-generation secondary calcite forms perfect pseudomorphs after shard and 

pumice fragments that had been previously replaced 

by zeolites (fig. 9). Outlines of replaced clinoptilolite 

crystals are visible within the secondary calcite in 

places, clearly indicating that calcitization followed 

the zeolitization of the volcanic fragments.

Other secondary minerals include authigenic smectites, which fill some vesicles in pumiceous fragments, 

and length-slow chalcedony that occurs very locally as 

a replacement of glass shards. Scanning electron micrographs suggest the presence of small amounts of 

cristobalite, but cristobalite could not be detected by 

X-ray diffraction.

TRACES OF HYDROCARBON

The sandy basal parts of some turbidite tuff layers 

have residual patches of bitumen that indicate an earlier stage when the pores were completely saturated 

with hydrocarbons. This bitumen does not fluoresce in 

ultraviolet light, but it does dissolve in carbon tetrachloride, indicating that it is "dead oil."

In thin sections, the bitumen appears as thin coatings on the surface of some grains, in cracks in feldspar 

grains, and on the rims of vesicles in pumice fragments 

(figs. SC and 8D). It also completely fills the vesicles 

of a few pumice fragments, forming a unique type of

grain composed of bitumen-filled vesicles surrounded 

by zeolitized glass or by large crystals of calcite that 

have replaced the zeolitized glass. In one specimen, 

bitumen-impregnated micrite fills a foraminifer shell 

whose pores are also filled with bitumen. Some very 

porous grains, perhaps originally lithoclasts of tuffaceous mudstone, are so thoroughly impregnated with 

bitumen that they are black and opaque in thin section 

(fig. 3D).

The pores in the sandy, relatively well sorted parts 

of the tuffs apparently became saturated with oil early 

during diagenesis, because textures show that the oil 

predated cementation, zeolitization, and calcitization. 

The oil appears to have been flushed out subsequently, 

so that voids were reopened and later filled by calcitic 

and zeolitic cements. Within porous pumiceous and 

mudstone clasts, however, much oil remained trapped 

in vesicles and very small pore spaces. Consequently, 

layers rich in these kinds of clasts, especially pumice, 

tend to be dark because of residual bitumen (fig. 7).

ORIGIN AND DIAGENETIC HISTORY OF THE TUFFS

The sedimentary structures and the entrained pelagic carbonate sediment in the graded tuff layers 

leave little doubt that they were deposited by turbidity 

currents. The major question about the origin of the 

tuffs is whether the constituent grains formed during 

subaerial eruptions with subsequent reworking and 

redeposition by turbidity currents or during submarine ash eruptions of the kinds described by Fiske and 

Matsuda (1964, p. 93-96) and Fernandez (1969, p. 35- 

36).

The composition of the tuffs suggests an entirely 

submarine origin because constituents are absent that 

would indicate source areas in shallow water or on 

land. With the exception of a single poorly preserved 

bryozoan skeleton, no benthonic shallow-water fossils 

were found in the tuffs. There are no wood fragments 

or plant remains. Indications of subaerial eruptions, 

such as accretionary lapilli (see Fiske, 1963, p. 402), 

also are absent. No significant amounts of terrigenous 

clastic sediment are present, and well-rounded grains, 

indicating multicycle sedimentary clasts or earlier 

abrasion in a high-energy environment, are likewise 

absent.

The limited composition and very angular grains in 

this tuff thus strongly suggest that these turbidites 

were fed from underwater eruption columns of pyroclastic debris. The tuffs are similar to dacitic tuffs in 

the Miocene Tokiwa Formation of Japan that Fiske 

and Matsuda (1964, p. 82-105) have shown were products of subaqueous ash flows and turbidity currents. 

They also somewhat resemble Quaternary lacustrine
12 HEMIPELAGIC LIMESTONE AND TUFFACEOLIS TURBIDITES, AKSITERO FORMATION

subaqueous pumice flow deposits in central Japan as 

described by Yamada (1973, p. 587-593), although 

some of the deposits may have been erupted on land 

and flowed into the water where turbidity flows were 

generated. The telescoping of the two turbidite layers 

in the bed illustrated in figure 7 resembles parts of the 

Tokiwa Formation where, subaqueous settling of the 

eruptive cloud around the vent caused repeated small 

surges of sediment on the sea floor. The surges in turn 

generated repeated turbidity currents that deposited 

numerous thin tuffaceous turbidites. Fiske and Matsuda (1964) noted that sequences produced by this kind 

of deposition are doubly graded: grain size decreases 

upward over some tens of meters as well as within the 

individual thin beds. The bed illustrated in figure 7 is 

also doubly graded.

Textural relations in these tuffs indicate the following sequence of postdepositional events:

1. Burial and some compaction.

2. Saturation of pore space in the sandy parts 

of the turbidite layers with hydrocarbons.

3. Flushing of hydrocarbons by water. Some 

residual oil remained attached to grains and 

trapped intragranular pore space.

4. In parts of the sandy layers, precipitation of 

intergranular calcitic cement, possibly from 

the pore water that flushed the hydrocarbons.

5. Zeolitization: replacement of volcanic glass 

in shards and in pumice fragments by mordenite and clinoptilolite, and precipitation of 

these minerals as void-filling cements.

6. Replacement of some zeolitized shards and 

pumice fragments by large crystals of secondary calcite.

Significant cementation of the tuffs must have occurred before the interbedded hemipelagic limestone 

became lithified. Although many foraminiferal tests 

are compactionally crushed in the limestone, foraminiferal shells as well as delicate pumice fragments 

show no signs of compaction within the tuff. One specimen consists of mordenite tuff that contains a small 

vertical dike filled with pelagic limestone. This relation indicates that the tuff was zeolitized, was cemented, and became brittle while the pelagic sediment 

remained plastic and unlithified.

ZEOLITES

The association of clinoptilolite and mordenite is 

common in altered tuffaceous rocks of Tertiary age 

(Hay, 1966, p. 65-76; lijima and Utada, 1972, p. 73- 

75; Seki, 1973, p. 669; Mumpton and Ormsby, 1976, p. 

2, 3). The two minerals are most abundant as replacements of silicic glass or as void-filling cements in tuffs

of rhyolitic to dacitic composition. This kind of alteration may occur either in chemically active alkalisaline lakes, as in the western United States, or during 

burial diagenesis, as in the Aksitero Formation and in 

the Tertiary of Japan.

Hay's data (1966, p. 70-72) suggest that the assemblage clinoptilolite-mordenite is stable at burial 

depths between about 300 and 3,500 m. On the basis 

of a compilation of occurrences in Japan, lijima and 

Utada (1972, p. 74) and lijima (1975, p. 135-138) found 

a similar range of burial depths where clinoptilolite 

and mordenite coexist. They surmise, however, that 

temperature rather than pressure is the principal controlling variable, and that the mineral pair is stable 

at temperatures from about 55°C to less than 90°C 

during the burial diagenesis of marine deposits.

The presence of clinoptilolite and mordenite in tuffs 

of the Aksitero Formation appears to be in general accord with the estimates of burial depths and temperatures noted above. Bandy (1963, p. 1739) and Ingle 

(1975, p. 840) have reconstructed composite Tertiary 

sections for central Luzon that suggest the Aksitero 

Formation was buried beneath a maximum of 3.5-4 

km of younger sediment (see fig. 2). At geothermal gradients of 15°-20°C/km, values derived from a borehole 

measurement in northern Mindanao (Sass and Munroe, 1970, p. 4393), and assuming a surface temperature of 20°C, the maximum temperatures at those burial depths would have been in the range 70° to 100°C.

The Philippine occurrence suggests further that 

slight textural and compositional differences may determine the exact site where the two minerals form. 

Whereas mordenite and clinoptilolite are both present 

in the pumice-rich, basal parts of the tuffaceous turbidites, mordenite is the only zeolite in the upper, finegrained parts of the turbidites that are composed 

largely of very small glass shards. Seki, Oki, Odaka, 

and Ozawa (1972, p. 151) described an occurrence in 

central Japan where laumontite is the main zeolite 

and mordenite is confined to thin interbeds of altered 

fine-grained pumiceous tuff, but the zeolitization in 

that instance resulted from contact metamorphism. 

James Boles (written commun., 1976) has suggested 

the possibility that mordenite in these tuffs formed 

first by rapid alteration of the very fine grained volcanic glass, and that clinoptilolite formed later as an 

alteration of the coarser shards.

SEDIMENTOLOGICAL AND TECTONIC SETTING

We interpret the Aksitero Formation as a lithified 

basinal hemipelagic sediment composed primarily of 

calcareous pelagic microplankton with admixed finegrained volcanic components (fig. 11). As noted above,
SEDIMENTOLOGICAL AND TECTONIC SETTING 13

Sea level

V^/i-mK^ —- Pelagic organism ~—~—

Sea level

Proximal volcaniclastic 

facies

Distal volcaniclastic 

facies

Basinal facies 

(Aksitero Formation)

FIGURE 11.—Depositional setting of sediments in Aksitero Formation and their postulated relation to adjacent facies. A, Explosive submarine eruption at left generated subaqueous eruption cloud and gravity flows; latter includes turbidity currents that moved away from 

vent area (Fiske and Matsuda, 1964). Some turbidity currents reached deep basinal area at right, where predominant sediment is 

calcareous hemipelagic ooze. B, Facies developed as result of sedimentological pattern shown in 11 A.

the basin floor probably lay at water depths of at least 

1,000 m. Turbidity currents laden with volcaniclastic 

sediment flowed sporadically onto the basin floor. These 

resulted from submarine ash eruptions and were fed 

from underwater eruption columns and subaqueous 

pyroclastic debris that had previously accumulated 

upslope (see Fiske and Matsuda, 1964, p. 79-105). If 

our interpretation is correct, the Aksitero Formation 

should grade laterally first into a distal volcaniclastic 

facies dominated by thin-bedded turbidites and then 

into a proximal volcaniclastic facies containing thickbedded gravity-flow deposits (fig. 11).

Eocene volcanic activity contemporaneous with deposition of limestone in the Aksitero Formation is widespread in the Philippines and the products of this volcanism are present as andesitic, dacitic and keratophyric 

flows, tuffs, and agglomerates (Philippine Bureau of 

Mines, 1963; Gervasio, 1973, p. 313). Most reconstructions of early Tertiary tectonic plates of the northern 

Philippines show Luzon on an east-facing arc, lying 

behind and above a west-dipping subduction zone (fig. 

12; Moberly, 1972, p. 43-45; Karig, 1973, p. 164; Karig, 1975, p. 870-876; Hilde and others, 1976, p. 13). 

Thus the Eocene volcanism was probably associated 

with an island arc-trench complex of early Tertiary 

age that lay along the east edge of the Philippines. At
14 HEMIPELAGIC LIMESTONE AND TUFFACEOUS TURBIDITES, AKSITERO FORMATION

120°

25° N 4

125° 130° 135° 140° 145 E

10

5° -

EXPLANATION

. ? Subduction zone - Queried where uncertain, sawteeth

point in direction of dip ofsubduction

FIGURE 12.—Early Tertiary plate tectonic reconstruction of the 

northern Philippines and Marianas region according to Karig 

(1973, 1975). Present-day latitude and longitude are used.

about the same time, Karig suggests that the South 

China Basin to the west of Luzon was apparently being 

opened as an extension marginal basin (Karig, 1973, 

p. 164).2

The Aksitero Formation, therefore, appears to have 

accumulated behind or to the west of an island arc-subduction complex in the eastern Philippines and on the 

eastern transitional edge of a major marginal basin. 

It directly overlies a complex ophiolitic basement sequence that has been interpreted as a remnant of an 

arc-trench system of Paleogene age (Ingle, 1975, p. 

848). The Aksitero Formation may have been deposited on the flanks of a back-arc or interarc basin formed 

by extensional rifting. Karig and Glassley (1970, p. 

2143-2146) have described an apparently similar setting of Pliocene age just west of the present Mariana 

Island arc. Dredging on the volcaniclastic aprons of 

this region recovered vesicular dacitic lava and foraminiferal sandstone containing dacitic glass. Karig and 

Glassley correlate submarine eruption of the dacitic 

lava with Pliocene extensional rifting of the Mariana

2Karig also believes that the West Philippine Basin, to the east of Luzon, opened as an 

extensional basin behind the Mariana Island arc system during middle Eocene to early 

Oligocene time (fig. 12; Karig, 1975, p. 876-878). This episode of extensional tectonics, 

which was contemporaneous with deposition of the Aksitero Formation, was accompanied 

by large-scale volcanism in the Marianas.

Island arc system. Redeposition of volcaniclastic sediment created extensive sediment aprons composed of 

turbidites. These volcanic aprons grade into calcareous 

pelagic sediment farther to the west (Karig and Moore, 

1975, p. 236; Mitchell, 1970, p. 236-237) resembling 

the facies changes we envisage for the Aksitero Formation (fig. 11).

Ingle's paleobathymetric curve for the Tertiary of 

Luzon, reproduced in figure 2 of this report, suggests 

further that deposition of the Aksitero Formation occurred during a period of subsidence (Ingle, 1975, p. 

840). This subsidence was perhaps a consequence of 

interarc rifting and lateral plate motion away from a 

spreading center within a back-arc region. In the Mariana Island arc system, Karig (1971, p. 342) has shown 

that substantial vertical subsidence accompanied 

large-scale horizontal movements in basins behind island arcs.

SIGNIFICANCE OF HYDROCARBON TRACES IN 

THE TUFFACEOUS SEDIMENTS

Several workers have postulated that scarcity of 

suitable reservoir rocks may have limited the accumulation of commercial deposits of hydrocarbons in 

the Philippines (Merrill, 1965, pi. 65; Mainguy, 1970, 

p. 103). In part, the reasoning behind this assertion is 

that silicic to intermediate plutonic rocks, which would 

yield porous quartz-rich sandstones, are relatively rare 

in this region.

Volcaniclastic deposits such as the turbidites in the 

Aksitero Formation, however, may contain considerable initial porosity hr intergranular voids as well as 

in intragranular voids such as vesicles. These kinds of 

deposits, especially those containing appreciable 

amounts of volcanic glass, tend to be especially susceptible to diagenetic processes that occlude pore 

space. Among those processes is the growth of authigenic minerals. However, if the hydrocarbons migrate 

into the pore space before the onset of burial diagenesis 

(as was the case in the Aksitero tuffs) and are not subsequently flushed, then the hydrocarbons would probably serve to inhibit pore-filling diagenetic reactions 

(Fiichtbauer, 1961).

From the above considerations, we conclude that tuffaceous sandstone like that in the Aksitero Formation 

is a potential oil and gas reservoir. Similar volcaniclastic rocks of Neogene age contain important hydrocarbon reserves in Japan (Ikebe and others, 1967, p. 

231; Katahira and Ukai, 1976). The turbidite tuffs in 

the Aksitero River section are of course too thin and 

too few to have much reservoir capacity. However, in 

parts of the depositional system nearer the source 

vents, thicker tuffaceous sand units may occur (fig. 11).
REFERENCES CITED 15

For example, in the Miocene subaqueous tuffs described by Fiske and Matsuda in Japan (1964, p. 82- 

100), tuffaceous sands are present in sequences that 

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594). This kind of proximal volcaniclastic facies could 

contain hydrocarbons where it interfingers with organic-rich rocks of the basinal facies, such as the limestones of the Aksitero Formation. Amato's work (1965, 

p. 6, 7) suggests that such proximal facies might occur 

southeast of the Aksitero River section, where he detected a transition across a basin margin.

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