28. FORAMINIFERAL STRATIGRAPHY OF CENOZOIC SEDIMENTS OF THEMARIANA TROUGH AND FORE-ARC REGION, DEEP SEA DRILLING PROJECT LEG 601

Valery A. Krasheninnikov, Geological Institute of the USSR Academy of Sciences, Moscow, USSR

ABSTRACT

Deep-water Pliocene and Quaternary turbidites, hemipelagic mudstones, and volcaniclastic sediments of theMariana Trough contain poor assemblages of planktonic foraminifers (Sites 454, 455, and 456); at Site 453 they werealmost completely destroyed by selective dissolution. On the eastern flank of the Mariana island arc, siliceous mud,nannofossil ooze, and chalk with ash layers are characterized by impoverished associations of planktonic foraminifersof Oligocene, Miocene, Pliocene, and Quaternary age (Site 458). They become very scarce in deep-water Oligocene,Miocene, and Quaternary siliceous claystones, volcaniclastic turbidites, siliceous and nannofossil oozes of Site 459,disappearing completely in abyssal sediments of the inner wall of the Mariana Trench (Sites 460 and 461). Quaternarypelagic clays and Cretaceous porcellanites and cherts located on the Pacific plate crust (Site 452) are barren offoraminifers. Comparison is made with the stratigraphic results of previous drilling in the West Mariana Basin (Leg 6)and land section of the Mariana Islands (Guam, Saipan).

INTRODUCTION

Drilling on Leg 60 of the Deep Sea Drilling Projecttook place in the South Philippine Sea and had as itsmain objective to study the structural evolution, back-arc spreading, and subduction processes of the PacificOcean active margin. The area investigated is composedof the following structural elements: the actively spread-ing inter-arc basin—the Mariana Trough; the presentlyactive island arc—the Mariana Ridge, including its fore-arc region; the Mariana Trench; and the Pacific oceanicplate—the northern Mariana Basin. Accordingly, 10sites were selected for the drilling of 17 holes along thetransect at about 18°N latitude (Figure 1): Sites 453through 456 in the Mariana Trough; Site 457 on the axisof the active volcanic arc; Sites 458 and 459 on theeastern flank of the Mariana island arc: Sites 460 and461 on the inner wall of the Mariana Trench; and Site452 on the old oceanic crust of the Pacific. Data on theLeg 60 holes are summarized in Table 1.

A number of samples (474) were washed and in-vestigated for foraminifers. In general, foraminifersare very scarce and assemblages are not diverse in thepredominantly deep-water sediments. Many samplesare barren. The ordinarily high stratigraphic resolutionbased on planktonic foraminifers is therefore not pos-sible.

SITE SUMMARIESThe data on stratigraphic subdivision of Cenozoic

deposits of the South Philippine Sea are presented in thefollowing sections in accordance with the west to eastdisposition of the drill sites—that is, from the MarianaTrough to the Pacific plate.

Site 453This site is located near the western edge of the

Mariana Trough. Coarse, poorly sorted igneous poly-

Initial Reports of the Deep Sea Drilling Project, Volume 60.

mict breccias of the "basement" are overlain by a thickpile of volcanogenic detrital sediments (sands, silts, clays,and silty clays). Based on nannofossils, sediments be-long to the Pliocene-Quaternary. Only three samples con-tain planktonic foraminifers. Very scarce, small Sphaer-oidinella dehiscens, Sphaeroidinellopsis seminulina, Glo-borotalia tumida, Globigerinoides ruber, and Globig-erina sp. were found in Samples 453-37-5, 19-20 cm and453-37-2, 29-31 cm, indicating an early Pliocene age(within the Globorotalia margaritae evoluta Zone). Sam-ple 453-3-1, 50-52 cm is characterized by scarce Sphaer-oidinella dehiscens, Globigerinoides trilobus, G. ruber,and debris of Globorotalia tumida (Pliocene-Quater-nary). The remaining 114 samples are barren of plank-tonic foraminifers.

The sedimentary environment throughout the Plio-cene and Quaternary seems to have been deep-water,similar to the recent depth conditions (4693 m), andplanktonic foraminifers have been dissolved.

A peculiarity of the microfauna at Site 453 is thegeneral absence of benthonic foraminifers. Only in Sam-ple 453-3-1, 50-52 cm were scarce specimens of deep-seaPullenia, Cassidulina, Polymorphina, and Hyperamminaencountered. Absence of representatives of this groupmay be related to the high rate of sedimentation (thick-ness of Pliocene-Quaternary clastic sediments is 455.5m).

Site 454This site is located near the center of the Mariana

Trough, at a shallower depth (3819 m) than Site 453.Hemipelagic sediments (mud, vitric mud, volcanic ash,and siliceous ooze) are more calcareous here, with in-tercalations of nannofossil and siliceous nannofossiloozes. They rest on basaltic rocks with some interbedsof vitric mudstones and silty tuff. Planktonic foramin-ifers were identified in almost all samples, but speci-mens are not numerous and diversity is very low. Itappears that they have been affected by selective dis-solution. The assemblage consists of Globorotalia trun-catulinoides, G. tumida, G. menardii, G. inflata (very

557

V. A. KRASHENINNIKOV

120° 130 140°

# L e g 60 Sites + Leg 58 SitesO Leg 59 Sites If Leg 31 Sites Ridges (all types) Trenches with related subduction

Figure 1. Location of sites drilled during Leg 60.

i Proposed spreading centers: (dashed where no well-defined rift)

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CENOZOIC FORAMINIFERAL STRATIGRAPHY

Table 1. Data on Leg 60 holes.

Hole

45245 2 A453454454A455456456A457458459B460460A461461A

Latitude (N)

17°40.19'17°40.17'17°54.42'18°00.78'18°00.78•17°51.26'17°54.68•17°54.71'17°49.99'17 = 51.84'17=51.75'17=40.14'17'40.02'17°46.05'17°46.01'

Longitude (E)

148°37.73'148°37.75•143°40.95•144°31.92'144°31.92'145°21.48'145°10.77'145°l0.88'145 "49.02'146°56.06•147° 18.09'147°37.92'147°35.16'147°41.18'147-41.26'

Water Depth(m)

5858

46933818.5381934683590.535912637344941156451644370297034

Penetration(m)

28.046.5

60538.5

171.510416915961

465.5691.5

8599.520.515.5

Thickness ofCenozoic

Sediments (m)

28.027.5

455.538.567

10413411461

256.5559

74.57620.515.5

Number ofSamples

2024

1171611151926

443

1172022

910

scarce), G. crassaformis crassaformis, G. crassaformisronda, G. crassaformis oceanica, Sphaeroidinella dehis-cens, Orbulina universa, Globigerinella siphonifera,Pulleniatina obliquiloculata, Neogloboquadrina duter-trei, Globigerinita uvula, Globigerinoides conglobatus,G. trilobus, G. ruber, G. sacculifer, and Globigerinacalida. They indicate the age of the sediments to beQuaternary (undifferentiated).

Benthonic foraminifers, unlike at Site 453, are morenumerous and diverse; they are representative of a deep-sea assemblage (Pullenia, Melonis, Lagena, Cassidu-lina, Eponides, Cibicides, Planulina, Alabamina, Sac-cammina, and Adercotryma).

Interval: from Section 454-3-1, 50-52 cm to Section454-5-6, 30-32 cm and from Section 454A-1-1, 50-52cm to Section 454A-4-3, 50-52 cm.

Site 456Two holes of this site in the eastern part of the

Mariana Trough penetrated Quaternary semi-lithifiedvitric mudstones, nannofossil chalk and tuffs resting onbasalts and passing up-section to unlithified vitric mud,and nannofossil and siliceous oozes with vitric ashlayers.

Planktonic foraminifers here are more common thanat Site 454. Nevertheless, they are not abundant and canbe characterized by the term "scarce" and, in somecases, "common." Quaternary planktonic foraminifersare represented by Globorotalia truncatulinoides, G.tumida, G. menardii, G. crassaformis, G. scitula, G.ungulata, Neogloboquadrina dutertrei, Pulleniatina ob-liquiloculata, Sphaeroidinella dehiscens, Orbulina uni-versa, Hastigerina pelagica, Globigerinella siphonifera,Globorotaloides hexagonus, Candeina nitida, Globi-gerinoides conglobatus, G. ruber, G. sacculifer, G. tri-lobus, Globigerinita glutinata, and Globigerina bul-loides.

The presence of scarce specimens of Globigerinacalida calida and Globorotalia tosaensis allows subdivi-sion of the Quaternary deposits.

Interval: from Section 456-1-1, 50-52 cm to Section456-2-3, 62-64 cm—late Quaternary [the Globigerinacalida calida Zone (N23) in Blow's sense].

Interval: from Section 456-2-4, 62-64 cm to Section456-6-2, 50-52 cm—Quaternary undifferentiated.

Interval: from Section 456-6-3, 50-52 cm to Section456-10-3, 63-65 cm—early Quaternary with both Globo-rotalia truncatulinoides and G. tosaensis [the Globo-rotalia truncatulinoides Zone (N22) in Blow's sense].

Interval: from Section 456A-1-1, 50-52 cm to Section456A-9-2, 25-27 cm—Quaternary undifferentiated.

Interval: from Section 456A-10-1, 66-68 cm to Sec-tion 456A-10-4, 32-34 cm—early Quaternary [the Glo-borotalia truncatulinoides Zone (N22) in Blow's sense].

Site 455This is the most easterly site in the Mariana Trough.

Quaternary volcanogenic sediments are composed ofvitric mud, sandy ash, and volcanic sands and gravelwith thin intercalations of vitric nannofossil oozes. Vol-caniclastic material is derived from the nearby islands ofthe active Mariana volcanic arc.

Bionomic conditions apparently were unfavorablefor planktonic foraminifers. The following species arevery scarce in these sediments: Globorotalia truncatu-linoides, G. menardii, G. inflata, G. crassaformis, G.tumida, G. scitula, Globigerinoides conglobatus, G. ru-ber, G. sacculifer, G. trilobus, Orbulina universa, Neo-globoquadrina dutertrei, Sphaeroidinella dehiscens, Glo-bigerinita glutinata, Pulleniatina obliquiloculata, Glo-bigerinella siphonifera, Globorotaloides hexagonus, Glo-bigerina calida, and G. rubescens.

Scarce specimens of Globigerina calida calida in theinterval from Section 455-1-1, 50-52 cm to Section455-1-2, 50-52 cm indicate the late Quaternary Globi-gerina calida calida Zone (N23) in Blow's sense. The in-terval from Section 455-1-3, 50-52 cm to Section 455-3-2, 50-52 cm is assigned to undifferentiated Quater-nary.

Benthonic foraminifers are represented by scarcespecimens of deep-water Cassidulina, Pullenia, Melo-nis, Pyrgo, Laticarinina, and Cibicides.

Site 457This site is situated on the axis of the Mariana island

arc, near Alamagan island. Quaternary sediments con-sist of coarse volcanic sands which appear to alternatewith muddy vitric ash.

In Sample 457-2-1, 23-25 cm, the planktonic fora-miniferal assemblage includes Globorotalia truncatu-linoides, G. menardii, G. scitula, G. tumida, Globi-gerinoides conglobatus, G. trilobus, G. ruber, G. sac-culifer, Neogloboquadrina dutertrei, Orbulina universa,Globorotaloides hexagonus, and Globigerinella siphon-ifera together with white and pink Globigerina rubes-cens and G. calida calida (late Quaternary, the Globi-gerina calida calida Zone in Blow's sense).

In the interval from Section 457-3-1, 29-31 cm downto Section 457-4-2, 40-42 cm, the last two guide fossilsare missing and the poor planktonic foraminiferal as-semblages can be assigned to the Quaternary (undiffer-entiated).

Shallow-water benthonic foraminifers with thick-walled shells are clear indication of displacement fromthe shelf area.

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V. A. KRASHENINNIKOV

Site 458

This site is located in the fore-arc region midway be-tween the Mariana volcanic arc and the Mariana Trenchaxis. Pillow and massive volcanic flows of the basementare overlain by Oligocene vitric siltstone and sandstone.They are replaced up-section by Oligocene and early andmiddle Miocene nannofossil chalk and siliceous nanno-fossil chalk with intercalations of vitric mudstone andtuff. A hiatus separates these sediments from upperMiocene, Pliocene, and Quaternary siliceous nanno-fossil, nannofossil, and vitric nannofossil oozes withthin beds of silty or sandy ash.

Although the sediments are calcareous, assemblagesof planktonic foraminifers are impoverished. Tests arefew to scarce, and only in some samples are they com-mon. Many zonal species are lacking. Therefore, only arather rough stratigraphic subdivision of the sedimentsis possible.

The interval from Section 458-1-1, 50-52 cm to Sec-tion 458-1-5, 50-52 cm with Globigerina calida calida,Globorotalia truncatulinoides, G. tumida, G. menardii,G. crassaformis crassaformis, G. crassaformis ronda,G. ungulata, Sphaeroidinella dehiscens, Pulleniatinaobliquiloculata, Candeina nitida, Orbulina universa,Globigerinella siphonifera, Globigerinoides congloba-tus, G. trilobus, G. ruber, and G. sacculifer is datedas late Quaternary {Globigerina calida calida Zone inBlow's sense, N23).

The interval from Section 458-2-1, 50-52 cm to Sec-tion 458-2-6, 50-52 cm with more diverse planktonicforaminifers {Globorotalia truncatulinoides, G. tosaen-sis, G. puncticulata, G. menardii, G. tumida, G. scitula,G. crassaformis, Orbulina universa, Globigerinoides tri-lobus, G. ruber, G. sacculifer, G. conglobatus, Pulleni-atina obliquiloculata, Sphaeroidinella dehiscens, Can-deina nitida, Neogloboquadrina dutertrei, Globigeri-nella siphonifera, Globorotaloides hexagonus, and Glo-bigerinita glutinata) is dated as early Quaternary {Glo-borotalia truncatulinoides Zone in Blow's sense, N22).

The interval from Section 458-3-1, 50-52 cm to Sec-tion 458-4-1, 50-52 cm contains very scarce and badlypreserved planktonic foraminifers. A few Globigerinoi-des conglobatus, G. ruber, Globorotalia tumida, Sphaer-oidinella dehiscens are indicative of a Pliocene-Quater-nary age.

The interval from Section 458-5-1, 50-52 cm to Sec-tion 458-5-4, 50-52 cm is characterized by an entirelydifferent microfauna consisting of numerous Globi-gerina nepenthes, Sphaeroidinellopsis seminulina, andscarce specimens of S. subdehiscens, Orbulina universa,Globigerinoides trilobus, and Globorotalia acostaensis.The age of the sediments can be determined only as lateMiocene to early Pliocene.

On the basis of nannofossils and radiolarians (seeEllis and Kling, this volume) the Miocene and Plioceneare separated by a hiatus (between Cores 4 and 5), anduppermost Miocene-lower Pliocene sediments are miss-ing. The sharp change in planktonic foraminiferalassemblages between Cores 4 and 5 indirectly confirmsthis hiatus, but their diversity is too poor to permit

zonation and to define the absence of some foraminif-eral zones.

In the interval from Section 458-6-1, 60-62 cm to Sec-tion 458-7-3, 50-52 cm, planktonic foraminifers arelacking. Radiolarians and sponge spicules and primitiveagglutinated benthonic foraminifers are common.

In the interval where Sample 458-7-4, 50-52 cm wastaken, planktonic foraminifers are represented by scarceSphaeroidinellopsis seminulina, S. subdehiscens, andGlobigerina nepenthes. The last defines the lower agelimit of this interval as the Globigerina nepenthes-Glo-borotalia siakensis Zone (the uppermost middle Mio-cene), but all three of the forementioned species disap-pear in the early Pliocene. Nannofossils (see Ellis, thisvolume) and radiolarians (see Kling, this volume) indi-cate these sediments to be uppermost middle Miocene(the Discoaster hamatus Zone and the Cannartus petter-ssoni Zone, respectively).

In the interval from Section 458-9-1, 52-54 cm to Sec-tion 458-11-2, 50-52 cm only a few long-ranging speciesof planktonic foraminifers were identified: Sphaeroidi-nellopsis seminulina, Globoquadrina altispira, G. dehis-cens, and Globigerinoides trilobus. They cannot pre-cisely determine the age of the sediments (middle Mio-cene, based on nannofossils and radiolarians) nor con-firm a hiatus within the middle Miocene.

The interval from Section 458-14,CC to Section 458-19-2, 32-35 cm is very poor in planktonic foraminifers,with only a few Globigerinoides trilobus, Globigerinavenezuelana, Globigerinita dissimilis, G. unicava, Glo-borotaloides suteri, Globorotalia nana, and Globo-quadrina altispira present (early Miocene).

The interval from Section 458-21-5, 5-7 cm to Section458-25-1, 53-55 cm is characterized by a scarce micro-fauna of the late Oligocene: Globigerina aff. ciperoensis,G. venezuelana, G. rohri, G. angustiumbilicata, Globig-erinita unicava, and Globorotaloides suteri.

The Oligocene/Miocene boundary cannot be definedat Site 458 with high precision by means of planktonicforaminifers because of an impoverished microfauna.

The lowest specimen from the site (Sample 458-25-2,44-46 cm) contains a few small Globorotalia opimaopima, Globigerina sp. aff. G. ampliapertura, G. rohri,G. praebulloides, G. angustiumbilicata, and Globo-rotaloides suteri; it can be placed approximately at thebase of the Globorotalia opima Zone (Oligocene).

Benthonic foraminifers are scarce in Oligocene, Neo-gene, and Quaternary sediments of Site 458 {Hyper-ammina, Rhizammina, Ammodiscus, Pullenia, Melonis,Cassidulina, Bulimina, Virgulina, Nodosaria, Planulina,Cibicides, Eponides, and Gyroidind). Their species com-position has certainly changed through time but a contin-uing deep-water environment is indicated.

Site 459

This site is located on the eastern margin of theMariana island arc, near the trench-slope break. Hole459 penetrated a few meters of recent sediments; Hole459B reached basalts of the basement. The latter areoverlain by claystones and silicified claystones of themiddle Eocene and siliceous mudstones and vitric tuff

560

CENOZOIC FORAMINIFERAL STRATIGRAPHY

of the early Oligocene. A thick pile (about 475 m) ofdeep-water turbidites (siltstone, sandstone, sandy andsilty vitric tuff, vitric nannofossil chalk) represents thelate Oligocene and Miocene. Pliocene and Quaternarysediments are composed of siliceous and nannofossiloozes, vitric mud, and sandy to muddy ash. Poor as-semblages of planktonic foraminifers were identified inQuaternary, Miocene, and Oligocene sediments; manysamples are barren of planktonic foraminiferal tests.

The interval from Section 459B-1-1, 50-52 cm to"Sec-tion 459B-2-3, 50-52 cm and also Sample 459-1-1, 44-46cm contain scarce Quaternary planktonic foraminifers:Sphaeroidinella dehiscens, Globorotalia tumida, G.menardü, G. crassaformis, Globigerinoides conglob-atus, G. ruber, and Pulleniatina obliquiloculata. Ben-thonic foraminifers {Hyperammina, Valvulina, Pull-enia, Melonis, and Cassiduliná) indicate a deep-waterenvironment.

With the exception of a few specimens of Globigerinanepenthes Sample 459B-11-2, 50-52 cm, the intervalfrom Section 459B-2-4, 50-52 cm to Section 59B-19-1,50-52 cm does not contain foraminifers. The indicatedage is latest middle Miocene-early Pliocene.

The interval from Section 459B-20-1, 50-52 cm toSection 459B-24-2, 91-93 cm, including only scarce ben-thonic foraminifers (Pullenia, Cassiduliná, Bulimina,Cibicides, Anomalina, Nodosaria, Siphonodosaria,Sphaeroidina, Eponides, and Cyclamminá) is of deep-water character. Spicules of siliceous sponges are abun-dant in places.

The interval from Section 459B-25-1, 87-89 cm toSection 459B-29-2, 36-38 cm is barren of foraminifers.

The interval from Section 459B-29-3, 21-23 cm toSection 459B-43-1, 51-53 cm is characterized by few toscarce Globigerinoides trilobus, G. altiaperturus, Globi-gerina bradyi, G. juvenilis, G. angustiumbilicata, G.foliata, G. woodi, Cassigerinella chipolensis, and Glo-borotalia minutissima. These sediments belong to thelower part of the early Miocene (within the Globig-erinita dissimilis-Globigerinita stainforthi zones). Itshould be noted that in many samples planktonic fora-minifers are missing.

The interval from Section 459B-44-1, 64-66 cm toSection 459B-53-4, 43-45 cm does not contain foramin-ifers, except for some tests of corroded Siphonodosaria,Cibicides, and Eponides.

The interval from Section 459B-54-1, 81-83 cm toSection 459B-54-2, 69-71 cm, with scarce to commonGlobigerina ciperoensis, G. angulisuturalis, G. angusti-umbilicata, G. praebulloides, G. venezuelana, G. oua-chitaensis, and Cassigerinella chipolensis is late Oligo-cene (the Globigerina ciperoensis Zone).

In the interval from Section 459B-54-3, 86-88 cm toSection 459B-56-2, 64-66 cm, the assemblage of plank-tonic foraminifers consists of Globigerina ciperoensis,G. angustiumbilicata, G. angulisuturalis, G. praebul-loides, G. ouachitaensis, Cassigerinella chipolensis, andChiloguembelina cubensis together with scarce Globo-rotalia sp. aff. G. opima opima. Sediments of this inter-val can be placed in the late Oligocene, within the Glo-bigerina ciperoensis-Globorotalia opima zones (prob-ably in the Globorotalia opima Zone).

In the interval from Section 459B-56-3, 10-12 cmto Section 459B-59-2, 46-47 cm foraminifers are absent,except for a few benthonics: Cassiduliná, Pullenia, Gy-roidina, Cibicides, Eponides, and Siphonodosaria. Onthe basis of radiolarians and nannofossils, the sedimentswere subdivided into middle Eocene, late Eocene, andearly Oligocene.

Site 460This site (Holes 460 and 460A) was located on the in-

ner (arc-side) wall of the Mariana Trench. The Quater-nary diatomaceous ooze and siliceous mud are under-lain here by calcareous mud, vitric mud, and conglom-erates with middle and late Eocene and Oligocene nan-nofossils and radiolarians.

In all samples of Hole 460, foraminifers are absent.Most of the samples from Hole 460A are also barren offoraminifers. They were discovered only in three speci-mens: Sample 460A-4-4, 38-40 cm with few Globi-gerinoides ruber, G. trilobus, G. sacculifer, Globigerinasp.; Sample 460A-4-6, 38-40 cm with few Globigerinoi-des ruber and Globigerina sp.; and Sample 460A-5-1,50-52 cm with scarce Globorotalia tosaensis, G. menar-dü, G. scitula, Neogloboquadrina dutertrei, Sphaeroidi-nella dehiscens, Globigerinoides ruber, and G. trilobus.If we exclude the possibility of drilling contamination,the sediments cannot be older than late Pliocene (evenassuming redeposition of microfauna from the shal-lower water area of the fore-arc region).

Site 461Two holes were drilled in the deep area (7034 m) of

the inner wall of the Mariana Trench. They penetratedsiliceous ooze, vitric and siliceous mud, and zeólitic clayof the Quaternary with redeposited Eocene and Oligo-cene nannofossils. Foraminifers are absent from allsamples.

Site 452This site (Holes 452 and 452A) was located in the

northern Mariana Basin, on the deep, old Pacific platecrust. Quaternary pelagic brown clays unconformablyoverlie Cretaceous siliceous mudstone, Porcellanite, andchert. Although 44 samples were examined, foramini-fers are absent.

MAIN FEATURES OF THE CENOZOICFORAMINIFERAL STRATIGRAPHY OF THE

LEG 60 AREAOligocene, Neogene, and Quaternary deposits of the

Mariana Trough and Mariana Ridge penetrated by Leg60 holes contain very poor assemblages of planktonicforaminifers. They provide a very vague idea of truetropical Cenozoic assemblages of planktonic foramini-fers known after drilling on the Mid-Pacific mountainsand Caroline Islands. This impoverishment is certain tobe secondary and related to the deep-water character ofsediments. The chief mass of species was destroyed bydissolution. As a rule only resistant species, or specieswith a thickened wall, remain {Globigerina nepenthes,Globorotaloides suteri, Globigerinita unicava, Pulleni-atina obliquiloculata, Globorotalia opima, G. acosta-

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V. A. KRASHENINNIKOV

ensis, G. tumida, Neogloboquadrina dutertrei, Sphae-rodinella dehiscens, and Sphaeroidinellopsis spp.)

A deep-sea environment of sedimentation is also evi-denced by the character of benthonic foraminifers. Note-worthy is that ecological peculiarities of planktonic andbenthonic foraminifers upward in the section remainpractically unchanged; that is, sedimentation in basallayers of the section took place at depths similar to thoseof the present.

Because of impoverishment of planktonic foraminif-eral assemblages, it was not possible to subdivide theCenozoic deposits of Leg 60 on the basis of the zonalscale successfully applied in the tropical, subtropical, andtemperate areas of the Pacific Ocean, as well as in the At-lantic and Indian oceans (Krasheninnikov, 1977, 1978).The Globorotalia opima and Globigerina ciperoensiszones (Sites 458 and 459) can be very tentatively identi-fied in the Oligocene. In the Neogene, only subseries werepractically identified: early, middle, and late Miocene,and early and late Pliocene (Sites 458 and 459). Quater-nary deposits are subdivided into the Globorotalia trun-catulinoides and Globigerina calida calida zones, as inter-preted by Blow (1969), but the boundary between them isobscure (Sites 455, 456, and 458). It appears more expedi-ent to regard these subdivisions as subzones. This roughstratigraphic subdivision does not allow us to evaluatecontinuity of sections nor to fix the presence of strati-graphic breaks.

Redeposited Cretaceous foraminifers were not ob-served in Leg 60 sediments.

Cenozoic deposits penetrated during Leg 60 providegood examples of the greater resistance of calcareousnannofossils to selective dissolution than that of plank-tonic foraminifers. Although it was not possible to usethe foraminiferal zonal scale, as previously mentioned,the nannofossil zonal scale was successfully applied (seeEllis, this volume). Thus, it is necessary to note onceagain the reliability of the methods adopted for strati-graphic studies by the Deep Sea Drilling Project, wherethree of four groups of fossils are analyzed simulta-

neously. This makes the trend of the studies very flex-ible, and the findings rather reliable.

The results of foraminiferal stratigraphy are in ac-cordance with the data on radiolarians and nannofossilson the change in age of basal layers from the axial partof the Mariana Trough toward its peripheral zones (Fig-ure 2).

At the trough axis the basalts are overlain by Quater-nary (Sites 454 and 456). To the west, toward the WestMariana Ridge the basalts are overlain by Pliocene (Site453). East of the Mariana Trench the basalts are over-lapped by Oligocene (Site 458), or middle Eocene (Sites459 and 460). The symmetrical change in age of thebasalt layers testifies to the spreading process in the ax-ial part of the Mariana Trough.

Cenozoic deposits are exposed on some islands of theMariana archipelago, including Guam and Saipan(Todd, 1957, 1966; Todd et al., 1954). On Guam, abovebasalts there occur sandstones, clays, and limestoneswith tuffaceous material. Their basal layers, with Glo-borotalia spinulosa and G. spinuloinflata, are assignedto the upper part of the middle Eocene. The upper Eo-cene contains Globorotalia centralis, Globigerapsis in-dex, Hantkenina alabamensis, and H. inflata. The Olig-ocene is very complete: the presence of Globigerina selliiand Globigerina ampliapertura, together with G. gorta-nii, G. senilis, G. danvillensis, and Chiloguembelina cu-bensis, testifies to the early Oligocene; the appearanceof Globorotalia opima points to the late Oligocene. Thelower Miocene is characterized by Globigerinoides tri-lobus and Globigerinita dissimilis. The undifferentiatedJannum formation contains Globorotalia fohsi, Orbu-lina suturalis, Biorbulina bilobata, Globigerina nepen-thes, Globoquadrina altispira Miocene), as well as Glo-borotalia tumida, Pulleniatina obliquiloculata, Globi-gerinoides conglobatus, G. sacculifer, Pulleniatina ob-liquiloculata, Candeina nitida, and Sphaeroidinella de-hiscens (Pliocene). On Saipan, the section begins alsowith middle-upper Eocene with Globigerapsis index,Globorotalia centralis, Hantkenina sp. No Oligocene

Site 4534693 m

Site 4543818.5 m

Site 4563591 m

Site 4553468 m

100 •

Q

N2

z--yz-z-i

-__—-_s~— - r_r^

rv:. _̂ _~

;»:ò'>o.

r --------_.—^_—

r 7 - ~ — ~

Q

- • - - L . • -

- _ — J_—_Q

r< .YV^β.ö'*

Site 458 Site 4593449 m 4120 m

Q•

upv

Site 4603451.5 m

Q

— 7

Site 4617034 m

Q

K2

Figure 2. Correlation of sediments penetrated by the Leg 60 holes. Legend: 1 = clay, 2 = silt, 3 = sand, 4 = sandy clay, 5 = silty clay, 6 = mud-stone, 7 = breccia and conglomerates, 8 = radiolarian ooze, 9 = diatom ooze, 10 = nannofossil ooze, 11 = nannofossil chalk, 12 = calcareousclaystone, 13 = chert, 14 = tuff, 15 = coarse gabbro-metabasalt breccia, 16 = basalts; Q = Quaternary, N2 = Pliocene, Nj = late Miocene,N^ = middle Miocene, NJ = early Miocene, P2 = Oligocene, P2 = Eocene, K2 = Late Cretaceous.

562

CENOZOIC FORAMINIFERAL STRATIGRAPHY

has been established. The lower Miocene contains Glo-bigerinatella insueta, Globigerinoides trilobus, G. bi-sphaerica, Globoquadrina altispira, and G. dehiscens.Middle and upper Miocene have been identified withOrbulina suturalis, O. universa, Globorotalia menardii,Sphaeroidinellopsis seminulina, and Pliocene with Pul-leniatina obliquiloculata, Sphaeroidinella dehiscens, Glo-bigerinoides ruber, and G. conglobatus.

Thus, the age of basal layers of the sections on Guamand Saipan is the same as in Sites 459 and 460 on the east-ern slope of the Mariana Ridge (middle-upper Eocene).

ACKNOWLEDGMENTS

The author is deeply grateful to Dr. A. Sharaskin (Institute ofGeochemistry and Analytical chemistry of the USSR Academy of Sci-ences, Moscow), for valuable advice and comments, and to Dr. T. P.Bondareva (Geological Institute of the USSR Academy of Sciences,Moscow) for reading the manuscript and suggesting further improve-ments.

REFERENCES

Blow, W. H., 1969. Late Middle Eocene to Recent planktonic fora-miniferal biostratigraphy. Proceedings of the First InternationalConference on Planktonic Microfossils (Vol. 1): Leiden (E. J.Brill), 199-422.

Krasheninnikov, V. A., 1977. Importance of oceanic deposits forelaboration of the Mesozoic and Cenozoic stratigraphic scale (In-dian Ocean). Questions of Micropaleontology, N 19: 124-227 (inRussian).

, 1978. Importance of oceanic deposits for elaboration of theMesozoic and Cenozoic stratigraphic scale (Pacific and AtlanticOceans). Ibid., N 21: 42-161 (in Russian).

Todd, R., 1957. Geology of Saipan, Part 3. Paleontology, smallerforaminifera. U.S. Geological Survey, Professional Paper, N290-H, 265-320.

, 1966. Smaller foraminifera from Guam. U.S. GeologicalSurvey Professional Paper, N 403-1, 1-41.

Todd, R., Cloud, P.E., Low, D., and Schmidt, R. D., 1954. Probableoccurrence of Oligocene on Saipan. Amer. J. Sci., v. 252,673-682.

563