Petrology and paleogeographic significance of Tertiary nannoplankton-foraminiferal limeston, Guam

Palaeogeography, Palaeoclimatology, Palaeoecology, 17(1975): 49--64 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PETROLOGY AND PALEOGEOGRAPHIC SIGNIFICANCE OF TERTIARY NANNOPLANKTON--FORAMINIFERAL LIMESTONES, GUAM

ROBERT E. GARRISON 1 , SEYMOUR O. SCHLANGER 2 and DANIEL WACHS 1

1Earth Sciences Board, University of California, Santa Cruz, Calif. (U.S.A.) 2Department of Geological Sciences, University of California, Riverside, Calif. (U.S.A.) (Received April 18, 1974; accepted July 26, 1974)

ABSTRACT Garrison, R. E., Schlanger, S. O. and Wachs, D., 1975. Petrology and paleogeographic significance of Tertiary nannoplankton--foraminiferal limestones, Guam. Palaeogeogr., Palaeoclimatol., Palaeoecol., 17: 49--64. The tertiary stratigraphic column on the island of Guam, in the Mariana Island Arc of the western Pacific, is composed largely of volcanic rocks and limestones of shallow-water reef and bank facies. Small amounts of fine-grained limestones, of Eocene, Oligocene and Miocene age, occur interbedded with the volcanic rocks and reef limestones, and consist chiefly of planktonic foraminiferal tests embedded in a matrix of calcareous nannofossils. Although compositionally these pelagic carbonates are nearly identical to nannoplankton-foraminiferal oozes of the deep-sea floor, their deposition probably occurred at comparatively shallow depths -- from a few hundred to perhaps 1000--2000 meters rather than several kilometers. The most favorable paleogeographic settings for pelagic deposition of this kind were probably intra-arc basins and shelf areas within the paleo-Mariana Island Arc during periods of volcanic inactivity. The spatial association of these pelagic limestones with shallow-water reef complexes, or with beds of redeposited, reef-derived coarse skeletal material, may serve to differentiate them from their deep-sea counterparts.

INTRODUCTION I n t e r e s t in t h e g e o l o g y a n d p a l e o n t o l o g y of i s l a n d arcs has b e e n h e i g h t e n e d in r e c e n t y e a r s l a r g e l y b e c a u s e , w i t h i n t h e c o n c e p t u a l f r a m e w o r k o f s e a - f l o o r s p r e a d i n g a n d p l a t e t e c t o n i c s , i s l a n d arcs are v i e w e d as z o n e s o f i n t e r a c t i o n b e t w e e n p l a t e s ( M o b e r l y , 1 9 7 2 ) . F r o m a s e d i m e n t o l o g i c a l p o i n t o f view, t h i s in t u r n has r e d i r e c t e d a t t e n t i o n t o K a y ' s ( 1 9 5 1 ) a n a l o g y b e t w e e n p r e s e n t - d a y v o l c a n i c i s l a n d - a r c s e q u e n c e s a n d a n c i e n t e u g e o s y n c l i n a l a s s e m b l a g e s , a revival o f i n t e r e s t c h a r a c t e r i z e d b y t w o m a j o r t h e m e s . O n e p l a c e s e m p h a s i s on t h e regional aspects of sediment distribution, submarine geomorphology and g e o l o g i c s t r u c t u r e in a n d a r o u n d m o d e r n i s l a n d arcs, as d e t e r m i n e d p r i m a r i l y b y m a r i n e g e o p h y s i c a l m e t h o d s (e.g. Karig, 1 9 7 0 , 1 9 7 1 a , b, 1 9 7 2 ) . T h e other emphasizes the petrogenetic and geometric aspects of sedimentary and

5O

volcanic facies in island-arc systems, both modern and ancient (e.g. Dickinson, 1970, 1974; Mitchell, 1970; Mitchell and Reading, 1971). Because they occur at loci of repeated volcanism, direct volcanic products such as flows, ash falls, hyaloclastites and the diagenetic and weathering products of these (such as zeolites and clays) tend to subdue reef growth and dilute such limestones as do form. Such volcanic loci are in general not very favourable areas for the accumulation of extensive carbonate deposits. Thus limestones, if they form at all, will tend to occur as local structures such as reef complexes that grow around the periodically quiescent volcanic islands and on highs isolated from areas of synchronous volcanism. Although quantitatively they will thus form relatively minor parts of island-arc stratigraphic assemblages, carbonate rocks will be especially informative components of these assemblages because they provide more sensitive indicators of depositional environments than the dominant volcanic sediments. We consider here a distinctive carbonate facies found on the island of Guam in the Mariana Island Arc (Fig.l), and characterized b y abundant calcareous nannoplankton and planktonic foraminifera. Although petrologically and 140°E

BASIN

~

(~ DSDP ~SITE 54 15ON

lOON

~

I45°E

I 140°E

150°E

+/+/~31!~.i

,,=z ~)n-.~ E J~.~ ,q,

I~ .1

I 145°E

15°N

150=~- I0 °

Fig.1. Map of Mariana Arc and adjacent parts of the Philippine Sea showing D.S.D.P. drill sites (modified from Karig, 1971b). Generalized depth contours are in kilometers. Crosses indicate recent andesitic volcanoes. Approximate position of the basement high between the trench and frontal arc (see Fig.2) is shown by the dashed line.

51 faunally more akin to pelagic carbonate sediments of the major deep-sea basins, these Tertiary limestones were deposited at relatively moderate depths on shelves and in intra-arc basins associated with an island-arc structure. GEOLOGICAL SETTING The island of Guam lies near the southern end of the Mariana Islands and forms part of what Karig (1971b) has called the frontal arc of the Mariana Island Arc system (Figs.1 and 2). Karig (1971b) outlines the regional THIRD ARC

INTER-ARC BASIN

FRONTAL ARC

0

Intra-Arc Basra

I

5

:

~

0

6 7 8

V. . . . . . . PARECE VELA BASIN

WEST MARIANA RIDGE

MARIANA TROUGH

FRONTALARC SLOPE TRENCH | O (ARC TRENCH I GAP)

25 50 75 IOO Exogg . . . . . . . 30X

MARIANARIDGE

5 C~

LJ MARIANA TRENCH

8

Fig. 2. Composite seismic profile across the Mariana Arc system and adjacent parts of the Philippine Sea, between 15°N and 17°N, north of Guam (from Karig, 1971b). The terminology at the top of the profile is that of Karig (1970, 1971a, 1971b, 1972). structural setting of the Mariana system, and the geology and geological history of Guam is described by Tracey et al. (1964). Table I portrays the general stratigraphy on Guam. The Tertiary section is composed of interbedded volcanic and carbonate rocks. The oldest unit, the Alutom Formation of late Eocene age, is exposed in the central and northern parts of the island; it consists of 600 to over 900 m of interbedded volcaniclastic rocks, lava flows, shales and small amounts of limestone. An argillaceous limestone at the top of the Alutom Formation in southern Guam is assigned to the Mahlac Member. A second, largely volcanic unit, the Umatac Formation of late Oligocene to early Miocene age, forms most of southern Guam and lies unconformably above the Alutom Formation in central Guam. This unit has a m a x i m u m thickness of about 670 m and is composed, like the Alutom Formation, chiefly of volcaniclastic and flow rocks, and locally contains limestone fragments and interbedded limestone in small amounts. Lying u n c o n f o r m a b l y above the Umatac Formation are a series of Miocene to Pleistocene limestones (Bonya, Alifan, Barrigada and Mariana Limestones, Table I) generally of shallow-water reef complex and bank facies. A deeper water facies of late Miocene age, the Janum Formation, apparently represents deposition on shelves along the eastern side of the island. Cenozoic tectonic and volcanic activity in the Mariana island arc appears to have been discontinuous (Karig, 1971b, pp. 341--342). On Guam, episodes

52 TABLE I Stratigraphic units on Guam* Years Epoch (x 106 )

Age

Far East Letter Stage

Formation on Guam

0 Pleistocene

E and L

/ |

Pliocene

E and L

h

5 L g

10 Miocene

f M

20

Mariana Limestone ~._~_ Janum ~'ormation "~ Alifan Limestone .Barrigada Limestone

E

l Bonya Limestone

e5

~

~

O

30

L

el--4

E

d--e

~

.~

Oligocene

35

c

40

L O

45

Eocene

~

50 E

~ ~

~ ¢-,1

55

I

* Modifed from Tracey et al. (1964); time scale after Berggren (1972).

53 of intensive but intermittent volcanism from eruptive centres west and southwest of the present island produced the A l a t o m and Umatac Formations during late Eocene and late Oligocene--early Miocene times, respectively (Tracey et al., 1964). Late Miocene--early Pliocene time was apparently an interval of tectonic and volcanic quiescence, during which extensive reef complexes developed on the stable submerged platform of Guam. Renewed tectonism and volcanism during the late Pliocene and Pleistocene time resulted in emergence followed by resubmergence of Guam, as well as the addition of mineral assemblages (feldspar--zeolite--montmorillonite) indicative of volcanic ash falls to the shallow-water limestones of these ages (Schlanger, 1964). Karig (1971b, p.341) has correlated opening of the Mariana Trough as an inter-arc basin (see Fig.2) with this period of tectonism and volcanism. Vertical movements, acting in concert with and in part controlling reef growth and volcanism, thus have combined in the evolution of Guam during the Cenozoic Era (Schlanger, 1964). NANNOPLANKTON--FORAMINIFERALLIMESTONES Limestones composed largely or entirely of calcareous nannoplankton and planktonic foraminifera occur in the Alutom, Umatac and Janum Formations (Table I). Alutom Formation General A volcanic breccia containing fossiliferous, upper Eocene limestone fragments outcrops discontinuously in south-central Guam (Tracey et al., 1964, p. A17). These limestone clasts represent a variety of lithologic facies from a late Eocene reef complex which apparently was located west of Guam and was subsequently destroyed by explosive volcanism (Tracey et al., 1964, p.A21; Schlanger, 1964, p.D23). Besides these allochthonous limestone fragments, apparently autochthonous bedded limestones occur interbedded with volcaniclastic rocks near the town of Santa Rita in south-central Guam. These include: (1) limestones rich in broken and abraded fragments of shallow-water foraminiferal and algal limestones; the skeletal debris was probably of reef origin and was redeposited in a fore-reef or basin environment; and (2) nannoplankton--foraminiferal limestones (Fig.3A) of pelagic or basin facies. These limestones apparently are remnants of the intra-arc basin and volcanic-arc fore-reef areas of the late Eocene reef complex which lay to the west (Schlanger, 1964, p.D23; Tracey et al., 1964, p.A58, fig.28). Lithologically somewhat similar to the basin limestones are argillaceous limestones assigned to the Mahlac Member at the top of the Alutom Forma-

54

Fig.3. Photomicrographs of nannoplankton--foraminiferal limestones from Guam. Scale bars indicate 500 u. A. Eocene limestone from the Alutom Formation, with planktonic foraminifera in micritic matrix. (Sample No. EK-7-1.) B. Early Miocene limestone from Maemong Limestone Member of the Umatac Formation. Foraminifera, mostly planktonic, in matrix of micrite. (Sample No. DF-9-1a.) C. Early Miocene limestone from Maemong Limestone Member of the Umatac Formation. On the left are large benthonic foraminifera and other shallow-water skeletal debris, on right planktonic foraminifera in micritic matrix. The shallow-water skeletal material is part of a redeposited layer. (Sample No. Dh-ll-3.) D. Limestone of late Miocene age from the Janum Formation. Densely packed tests of planktonic foraminifera are embedded in a fine-grained matrix of micrite and clay minerals (Sample No. TS-5-7.) t i o n in s o u t h - c e n t r a l G u a m . These c o n t a i n a b u n d a n t p l a n k t o n i c f o r a m i n i f e r a in a fine-grained calcareous--argillaceous matrix, and are e s t i m a t e d to have been d e p o s i t e d at a d e p t h o f a b o u t 2 0 0 m (Schlanger, 1 9 6 4 , p . D 2 3 ) .

Petrology N a n n o p l a n k t o n l i m e s t o n e s in t h e A l u t o m F o r m a t i o n consist o f approxim a t e l y 50% fine-grained m a t r i x (micrite) a n d 50% silt- t o sand-size grains (Fig.3A). A m o n g t h e latter, p l a n k t o n i c f o r a m i n i f e r a d o m i n a t e and o c c u r chiefly as entire tests, b u t in p a r t also as skeletal fragments. O t h e r coarse biogenic c o m p o n e n t s include s c a t t e r e d calcitized sponge spicules, rare b e n t h o n i c f o r a m i n i f e r a , a n d small spherical shells o f t h e incertae sedis

55

nannofossil called Thoracosphaera (Fischer et al., 1967) (also previously referred to as SchizosphaereUa by Jenkyns, 1971). Non-skeletal grains form perhaps 10--15% of these limestones and include angular grains of plagioclase, amphibole, and highly altered volcanic glass as well as scattered iron-oxide granules and finely dispersed clay minerals. Foraminiferal chambers are either e m p t y or partly to entirely filled by micrite; some contain a finely fibrous authigenic mineral with very low refractive index and birefringence, probably cristobalite. Electron micrographs of the fine-grained matrix reveal exceptionally wellpreserved coccoliths (Fig.4) interspersed with a surprisingly high amount of very fine-grained volcanic debris. The limestones appear little altered by diagenesis; drusy calcite infillings are lacking in foraminiferal tests, and the coccoliths are unrecrystallized. Cementation has apparently been effected

Fig.4. T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h o f n a n n o p l a n k t o n - r i c h m a t r i x in l i m e s t o n e f r o m t h e A l u t o m F o r m a t i o n ( E o c e n e ) , G u a m . Scale b a r r e p r e s e n t s 5 u. N o t e well-preserved c o c c o l i t h s w i t h small calcite c e m e n t crystals b e t w e e n t h e m . A t u p p e r left is t h e edge of a n o n - c a r b o n a t e grain, p r o b a b l y a volcanic r o c k f r a g m e n t . ( S a m p l e No. EK-7-1.)

56

by growth of very small calcite crystals on and between the coccoliths (Fig.4). Only the siliceous components appear to have undergone extensive post-depositional alteration, with siliceous sponge spicules having been replaced by calcite; silica released by the alteration of siliceous microfossils and possibly also of volcanic glass may have been the source of the occasional inflllings of (supposed) cristobalite in foraminiferal chambers.

Sedimentology Predominance of pelagic organic components (coccoliths, planktonic foraminifera), and the presence of abundant fine-grained matrix as well as of angular volcanicallyderived grains indicate a quiet-water depositional environment for these limestones,one well below wave base and beyond the sedimentologicinfluence of reef complexes. Volcanism must have been relativelysubduedduring their deposition. Water depths probably exceeded 100 m, and sedimentationrates likely were low, on the order of one or a few centimeters per thousand years. Maemong Limestone Member of the Umatac Formation General Scattered through the predominant volcaniclastic and flow rocks of the Umatac are discontinuous and generally small lenticular masses of limestone which are collectively assigned to the Maemong Limestone Member of the Umatac Formation (Tracey et al., 1964, p.A25). These limestones bodies vary in thickness from a few meters to about 80 m, and they are concentrated in two areas. In the southwestern part of Guam, most of the limestones are of fore-reef, fore-reef transitional, or basin facies (Schlanger, 1964, p.D24--25). The lithologies of the calcareous rocks in this area vary from globigerinid-rich tuffaceous shale to alternating beds, 5--30 cm thick, of fine-grained globigerinid limestone and medium-grained limestones composed largely of benthonic foraminiferal and algal detritus (Tracey et al., 1964, p.A26--A27). The latter were probably redeposited by t ~ d i b i t y and other b o t t o m currents. The second area is the Fena--Mapao region of south-central Guam where the limestones are largely of reef-wall or lagoonal facies, and are apparently younger than the fore-reef and basin limestones to the SW (Schlanger, 1964, p.D24--D25). In addition to these limestone bodies, clasts of reef limestone are found within volcaniclastic rocks in the upper part of the Umatac Formation. Schlanger (1964, p.D25) has postulated the existence of two separate early Miocene centers of reef growth in the region of present-day Guam. One was located on the flanks of a volcanic cone which lay southwest of Guam. The second reef formed somewhat later, in what is now south-central Guam, on a

57

ridge of uplifted and deformed volcanic rocks from the Alutom Formation. Separating the two was a basin (the intra-arc basin of Fig.8) which was normally a receptacle for pelagic carbonate sediments composed of nannoplankton and planktonic foraminifera; at times, however, debris from the adjoining reef complexes was redeposited here.

Fig.5. Transmission electron micrograph of nannofossil-rich, partly recrystallized matrix in a limestone from the Maemong Limestone Member, Umatac Formation, early Miocene. Scale bar is 5 ~. Note variability in preservation of coccoliths, the partly replaced discoaster (center right) and coccoliths, and the large percentage of anhedrai to subhedral secondary carbonate crystals compared to Fig.4.

Petrology Pelagic limestones in the Maemong Limestone Member are generally wellindurated foraminiferal biomicrites, like the Alutom limestones, with tests of planktonic foraminifera dispersed through a micritic matrix which contains numerous nannofossils (Figs.3B and 5). There are, however, slight but significant differences between these Miocene coccolith-rich limestones and the

58

older ones in the A l u t o m Formation. The Maemong limestones contain far less volcanic detritus (feldspars, volcanic rock fragments, etc.) and a larger admixture of benthonic foraminifera. Besides abundant coccoliths, the finegrained matrix contains numerous discoasters, Braarudosphaera plates and spherical Thoracosphaera chambers, as well as non-skeletal carbonate grains of secondary origin (Figs.5 and 6A). Sponge spicules and radiolarian tests are n o t only more abundant compared to the Alutom limestones, but some retain their opaline silica skeletons as well (Figs.6B and 7A).

Fig.6. Scanning electron micrographs of nannoplankton--foraminiferal limestones from Guam. Scale bars represent 10 p. A. Secondary calcite rhombs filling foraminiferal chamber and dispersed through coccolith-rich matrix. Maemong Limestone Member, Umatac Formation, early Miocene. (Specimen No. Fd-3-1.) B. Sponge spicules in coccolith-rich matrix. Maemong Limestone Member, Umatac Formation, early Miocene. (Specimen No. Ec-6-1.) Like the Alutom limestones, foraminiferal chambers in the Maemong limestones may be partly to entirely filled by micrite or by fibrous cristobalite; in addition, however, some have linings of secondary calcite crystals which were apparently precipitated into cavities (Fig.6A). The intensified diagenesis is reflected also in the inferior preservation of the coccolith-rich matrix where many nannofossils are partly recrystallized and where nonskeletal calcite crystals of probable secondary origin are locally abundant (Figs.5 and 6A). The most significant difference from the Alutom pelagic limestones,

59 however, is the presence of a few coarse skeletal calcarenite beds and centimeter-thick laminae interlayered with the pelagic limestones (Fig.3C). These coarse layers consist of large benthonic foraminifera and other shallow-water skeletal material including fragments of calcareous algae, bryozoans and echinoderms mixed with small numbers of planktonic foraminifera and calcitized radiolarian tests.

Sedimentology Being texturally and compositionally similar to the Alutom pelagic limestones, the Maemong limestones were probably deposited under similar conditions of quiet, moderately deep water and at similar low rates of sedimentation. The occasional mixture of predominantly planktonic with predominantly shallowwater benthonic forms, however, suggests a near-reef basin environment of deposition where pelagic sedimentation normally prevailed and shallow-water sediment influxes periodically occurred. The fine-size fraction of pelagic carbonate deposited in such an environment might be somewhat different from that in similar deposits uninfluenced b y shallow-water sediment influxes. In addition to calcareous nannoplankton, the clay and fine-silt fractions also contain an admixture of redeposited shallow-water carbonate muds, composed predominantly of unstable high-Mg calcite and aragonite (Stehli and Hower, 1961). The presence of these unstable components would make the sediment more susceptible to later alteration, perhaps producing the recrystallization and other prominent diagenetic effects noted in the Maemong limestones. Janum Formation General Exposures of this formation are limited to seven small areas on the northeast coast of Guam. The unit varies from 1.3 to 22 m thick and consists of well-stratified, fine-grained limestones which occur in beds 5--30 cm thick. In color they range from grey-white to red-orange, pink, brown and yellow. Most of the limestones consist of planktonic foraminifera in a micritic matrix (Fig.3D), but those in the lower part of the unit contain abundant benthonic as well as planktonic foraminifera. The Janum Formation overlies the Bonya Limestone of shallow-water origin and contains pebbles of the Bonya Limestones; it is overlain unconformably by the Mariana Limestone of reef origin. The Janum is probably correlative with the Alifan Limestone and the Barrigada Limestone, both of shallow-water facies and the Janum apparently is a deeper-water facies deposited around the flanks of the shallow banks and possible reef complexes of the Alifan--Barrigada terraces. The Bonya pebbles in the Janum suggest erosion of the Bonya terrace.

60

Pet ro logy Pelagic limestones from the upper and middle parts of the Janum Formation are friable, porous and argillaceous foraminiferal biomicrites (Fig.3D). They are similar to those in the Alutom and Umatac Formations in consisting of planktonic foraminifera and coccolith-rich matrix (Fig.7B) but differ from t h e m in having a more impure, clay-rich matrix. The upper limestones contain a substantial admixture of volcanic detritus as well. Compared to most of the older pelagic limestones considered here, especially those in the Umatac Formation, the Janum Limestones are little affected by post-depositional 'alteration. Sedimentology Because limestones of the Janum Formation are apparently in part contemporaneous with late Miocene bank and reef carbonates, they probably represent frontal arc, bank or shelf-facies sediments. Sedimentation depths of a b o u t 80--200 m are suggested for lower beds of the Janum Formation that contain mixed benthonic--planktonic faunal elements, while the middle and upper parts of the unit are postulated to have been deposited between about 200 and 3000 m (R. Todd quoted in Schlanger, 1964, p.D32).

Fig.7. Scanning electron micrographs of nannoplankton--foraminiferal limestones from Guam. Scale bars represent 10 ~. A. Radiolarian test in coccolith-rich matrix. Maemong Limestone Member, Umatac Formation, early Miocene. (Specimen No. Ec-6-1.) ]3. Coccoliths dispersed through clay-rich matrix of marly limestone from the Janum Formation, late Miocene. (Specimen No. Ts-5-7.)

61 PALEOGEOGRAPHIC SIGNIFICANCE OF THE NANNOPLANKTON--FORAMINIFERAL LIMESTONES EXPOSED ON GUAM

In any discussion of the paleogeography of the Alutom, Umatac and Janum Formations, the paleogeography of the Mariana Arc system must be discussed. Karig (1971b) has shown that prior to Pliocene time the Mariana Trough did not exist and the West Mariana Ridge abutted on the Mariana Ridge (Figs.1 and 2). With the opening of the Mariana Trough, the West Mariana Ridge separated from the Mariana Ridge (and also apparently subsided one kilometer) and migrated relatively west. Fig.8 is a diagrammatic cross-section through the Mariana Arc prior to Pliocene time; it can serve as well as a model for any arc developing in a reef-sustaining latitude. The zonation scheme for limestone facies is based on studies of the limestones on the islands of the frontal arc (Schlanger, 1964; Cloud et al., 1956), studies of the sediments immediately surrounding Guam (Emery, 1962), marine geologic and geophysical studies (Karig, 1971b) and results of the Deep Sea Drilling Project at Sites 53, 54 and 60 (Fischer et al., 1971). Within an active volcanic island-arc system like the Mariana Arc, accumulation of relatively pure nannoplankton--foraminiferal oozes that formed the pelagic limestones (here described from the Alutom, Umatac and Janum Formations) most likely would have occurred in intra-arc basins or on shelves associated with the frontal arc, as shown in Fig.8. The frontal arc itself, however, was constructed of volcanic piles on the sea floor. These very often I

Carbonote production

(0 lO00m}

I

~

Frontal

~

.

~

;

~

,

Arc

~,~'

~nd sem,ng ~-

b-

j

(o sooo+m)

-~

{45 5Kin)

DSD~ Site 60 pOllmSpost,c Iocohon of DSDP Site 54

e-

vo¢c~nos a#d n m t t a - a r c #asPn5

~o,m~t,o# o~er Sony~ {,me$?one~

Vectlcol e~o~geraf,om

~

-~

, TO

/~ lrench

~

2 5X

_ E

-U

Fig.8. Distribution of limestone facies in the paleo-Mariana Arc system. Bathymetric,

sedimentologic and geologic data from Emery (1962), Schlanger (1964), Tracey et al. (1964), Berger (1971) and Karig (1971b). Configuration is generalized for Eocene through Miocene time along a section through D.S.D.P. Sites 54 and 60, prior to the formation of the Mariana Trough and the Mariana Ridge (Karig, 197 l b ) in the Pliocene.

62 must have formed emergent islands or very shallow platforms where reef complexes became established to produce distinctive shallow-water carbonate facies (e.g. Bonya, Alifan, Barrigada and Mariana Limestones on Guam; see Schlanger, 1964, for lithologic descriptions of reef-wall, lagoon, fore-reef and fore-reef transitional facies). At the other extreme of water depths in the inter-arc basin and on the frontal-arc slope (Fig.8), carbonate sediments would, of course, n o t normally accumulate below the regional carbonate compensation depth. In and around an island-arc system, such abyssal environments would be encountered most c o m m o n l y in the trench itself, on the outer frontal-arc slope and in the inter-arc basin (e.g. the Parece Vela Basin; see Fig.2). D.S.D.P. Site 60 cored typical frontal-arc slope facies, whereas D.S.D.P. Sites 53 and 54 sampled pelagic facies from an inter-arc basin. Only at water depths below the level of very abundant benthonic life and above the carbonate compensation level, at depths roughly below 100--200 m (Wells, 1967) and above 3500--4000 m (Berger, 1971), and in areas geographically b e y o n d the reach of downslope transport of shallow-water reef debris, would relatively pure coccolith--foraminiferal oozes accumulate. Detritusfree settings of this sort would be most c o m m o n l y found on the shelves or in intra-arc basins. DIFFERENTIATION BETWEEN NANNOPLANKTON--FORAMINIFERAL LIMESTONES FORMED IN ISLAND-ARC AND IN DEEP-SEA ENVIRONMENTS The coccolith-rich sediments from Guam, deposited within the Mariana Arc or on shelves adjacent to it, are petrologically and faunally similar to nannoplankton--foraminiferal oozes of the deep Pacific Basin that lies within the Circum-pacific arc belt. Inasmuch as the island arc--oceanic plate junction may become incorporated into an alpine belt, it becomes useful to attempt a distinction between two categories of pelagic limestones, viz., those formed in an arc--marginal sea realm, and those formed on an oceanic plate. For the geologic record, this is best accomplished by employing the full range of sedimentologic, paleontologic and geochemical criteria normally utilized by geologists to make paleogeographic and paleobathymetric interpretations (Hallam, 1967). Among these criteria, the lithologies of the associated sedimentary rocks may provide the most critical evidence. Thus, the close spatial association of the pelagic limestones with limestones composed predominantly of shallowwater skeletal material -- either as reef complexes (Alifan Limestone, etc.) or as reef-derived turbidite beds of coarse skeletal material -- points to their genesis in relatively shallow-water environments of the kinds discussed previously. Truly deep-sea nannofossil limestones, on the other hand, more

63 likely would have as associated rocks radiolarian cherts and iron/manganeserich clays of abyssal environments. For limestones associated with pillow lavas, another potential paleobathymetric indicator exists in the size, density and arrangement of vesicles in chilled pillow margins. Recent work (Moore, 1965; Jones, 1969) has suggested that these properties may be depth-dependent, although uncertainties remain regarding correlation of these properties in different types of lava. We have made no measurements on the appropriate volcanic rocks on Guam, but recent studies (e.g., Kanmera, 1974; Matthews and Wachs, 1973) have employed this m e t h o d to suggest that some pelagic sediments associated with volcanic rocks in ancient eugeosynclinal assemblages were deposited at relatively shallow depths. SUMMARY AND CONCLUSIONS Limestones composed largely of calcareous nannoplankton and planktonic foraminifera formed in relatively small amounts within the area of presentday Guam during Eocene, Oligocene and Miocene times. Compositionally these limestones resemble deep-sea calcareous oozes, but their spatial association with limestones of reef-complex facies suggests deposition at moderate water depths, below levels of active reef growth b e y o n d the reach of downslope transport of reef debris, but above depths of extensive carbonate dissolution. In addition, these limestones signify periods of volcanic quiescence, and detritus-free depositional environments on banks and in basin areas of reef complexes. ACKNOWLEDGEMENTS Grants from the Petroleum Research Fund administered by the American Chemical Society (PRF515-62 and PRF5962-AC2) and from the Research Corporation supported the petrologic and electron-microscope work; grateful acknowledgement is made to the donors of these funds. We are indebted to Joshua I. Tracey who generously made samples available to us for study. REFERENCES Berger, W. H., 1971. Sedimentation of planktonic Foraminifera. Mar. Geol., 11: 325--358. Berggren, W. A., 1972. A Cenozoic time scale -- some implications for regional geology and paleobiogeography. Lethaia, 5 : 195--215. Cloud, P. E., Schmidt, R. G. and Burke, H. W., 1956. Geology of Saipan, Mariana Islands. U.S. Geol. Surv. Prof. Pap. 280-A, 126 pp. Dickinson, W. R., 1970. Relations of andesites, granites and derivative sandstones to arc-trench tectonics. Rev. Geophys. Space Phys., 8: 813--860.

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