3. CENOZOIC PLANKTONIC-FORAMINIFER BIOSTRATIGRAPHY...

3. CENOZOIC PLANKTONIC-FORAMINIFER BIOSTRATIGRAPHY OF DEEP SEA DRILLINGPROJECT HOLE 462, NAURU BASIN (WESTERN EQUATORIAL PACIFIC),

AND DISTRIBUTION OF THE PELAGIC COMPONENTS1

Isabella Premoli Silva and Donata Violanti, Istituto di Paleontologia, Università di Milano,Piazzale Gorini 15, Milano, Italy

ABSTRACT

The Cenozoic sedimentary sequence recovered at Deep Sea Drilling Project Hole 462, in the Nauru Basin, ischaracterized by rare layers of autochthonous pelagic clay and/or radiolarian ooze, which alternate with much moreconspicuous, allochthonous, carbonate-rich graded layers.

The biostratigraphic signal, based on planktonic foraminifers, is strongly biased by heavy reworking. Nevertheless,most of the biozones of the late Eocene to Pleistocene have been recognized. Boundaries between zones are sometimesonly tentatively drawn. A combination of data based on host and reworked planktonic-foraminifer faunas indicatesthat relatively few biozones are missing in the Cenozoic, namely the Subbotina pseudobulloides, "Morozovella"trinidadenis, M. angulata, and part of the M. pusilla Zones in the Paleocene; the Morozovella formosa throughAcarinina pentacamerata Zones in the early Eocene; most of the Hantkenina aragonensis and Globigerinatheka sub-conglobata Zones in the middle Eocene; and the upper part of Zone N4 through Zone N6 in the early Miocene. Theabsence of Zones N14/N15 in the middle Miocene and the completeness of the Pliocene cannot be proved. The oldestreworked fauna is dated as mid-Cretaceous.

Quantitative analyses of the studied samples distinguish different types of graded layers, characterized primarily byvarying distributions of the main fossiliferous components. Radiolarian-rich turbidites are common in the lower part ofthe sequence (late Eocene to early late Oligocene, and late Pleistocene). Foraminifer-rich turbidites occur primarily inthe upper part of the Oligocene, in the middle Miocene, and in the Pliocene to early Pleistocene.

Accumulation rates per zonal interval have been estimated, the highest values occurring in the latest Eocene and lateOligocene (Zone P22). Relatively high rates are recorded also in the early late Oligocene, latest Oligocene, middleMiocene, and late Pliocene. It is suggested that these high accumulation rate values are related to erosional events datedat ~ 37, ~ 32, ~ 26, -24?, -13 , and possibly 3 m.y. ago, as well as during the Pleistocene. The most important eventsare those dated at -37 (latest Eocene) and at -26 m.y. ago (late Oligocene, Zone P22).

INTRODUCTION

The present depth of the Nauru Basin (westernequatorial pacific) is over 5000 meters, and Hole 462was drilled at a water-depth of 5183 meters below sealevel, on Anomaly M-26, which according to Larsonand Hilde (1975) dates the ocean crust below the NauruBasin at -150 (m.y. old). Although the subsidence pathof the Nauru Basin deviated from the Parson-Sclatercurve (see Schlanger and Premoli Silva, this volume), itsfloor has been below the carbonate compensation depth(CCD) since the Early Cretaceous.

The whole Cenozoic sedimentary sequence was re-covered continuously at Hole 462, at least down to Core39 at a depth of 370 meters sub-bottom; it is character-ized by an alternation of pelagic clay with carbonate-rich and/or radiolarian-rich layers, at first sight verysimilar to pelagic oozes. By analogy with modern sedi-ments, and in agreement with the location of the NauruBasin's bottom below the CCD during the whole Ceno-zoic, the interbedded pelagic clay, which mainly yieldedfish debris and abyssal, non-calcareous agglutinatedforaminifers, must be considered the only indigenoussediments in this location. Consequently, the occurrence

Initial Reports of the Deep Sea Drilling Project, Volume 61.

of carbonates throughout the Cenozoic sequence at Site462 is anomalous and results from mechanical redeposi-tion rather than planktonic fallout.

Analysis by binocular microscope of the washedresidues of the > 63-µm fraction, confirmed by quan-titative analyses, reveals that sediments other than pe-lagic clay are finely graded, and range from coarse fora-minifer and/or radiolarian sand to nannofossil silt.Planktonic foraminifers exceeding 250 µm, almost with-out matrix, are the main components of the bottom ofthe most complete sequences. The sequences grade up-ward, via finer-grained planktonic foraminifer-radio-larian silty sand, to nannofossil silt, eventually toppedby pelagic clay.

Redeposition processes involved a large amount ofreworking. Unlike the Cretaceous sediments, in whichreworking was a minor feature, both in total amountand in terms of ages of the eroded sediments, Cenozoicplanktonic-foraminifer faunas are so mixed that thebiostratigraphic signal may be strongly masked.

The associated calcareous benthic foraminifers showthat the sources of the displaced material were mainlyareas at bathyal depths. Shallow-water skeletal debris,frequently associated with abundant volcanic material,and outer-shelf to upper-bathyal faunas are containedin some very coarse graded layers interbedded epi-sodically in the turbiditic sequence (see Premoli Silva

397

I. PREMOLI SILVA, D. VIOLANTI

and Brusa, this volume). The sources area(s) of thatshallow-water material might be different from those ofthe pelagic turbidites (see Site Summary, this volume).

Because of the heterogeneous character of sedimenta-tion at Site 462 during the Cenozoic, a regular collectionof data necessary for biostratigraphic, paleoecological,and paleogeographical interpretations was prevented. Inan attempt to circumvent this heavy bias, the number ofstudied samples was considerably increased, beyond thenumber necessary from an undisturbed sequence of pe-lagic oozes. Moreover, samples were collected so thatwhole turbiditic sequences were fully represented fromdifferent stratigraphic intervals.

With the aim of separating the biostratigraphic-paleoecologic, and consequently paleooceanographicsignal from the mechanical (size-sorting) overprint, andassessing how this signal, if detectable, evolved through-out the whole Cenozoic, quantitative analyses were per-formed on the collected samples.

Except for most of the core-catcher samples, and afew others, already treated aboard the ship, the follow-ing parameters were calculated for all the other samples:

1) Total weight of original samples, previously driedat constant temperature of 45°C;

2) Weight of the > 63-µm fraction, calculated as per-centage of the total weight;

3) Weight of the > 150-µm and of the >250-µm frac-tions, calculated separately as percentage of the totalweight of the > 63-µm fractions when those latter frac-tions were abundant;

4) Carbonate content, expressed in percentage;5) Percentages of the various biogenic and non-bio-

genic components of the >63-µm fraction, and, whenpresent, of the >150-µm fraction. Those percentageswere calculated on 300 grains and/or specimens;

6) Percentages of reworking of the planktonic-fora-minifer faunas as total amount, and per time intervaland/or biozone, were estimated.

The data and relative curves are plotted in Figures 1to 3. Except for weights and carbonate contents, theparameters were estimated, including those for thesamples treated aboard ship.

FORAMINIFER ASSEMBLAGESAND BIOSTRATIGRAPHY

As mentioned in the previous paragraph, redeposi-tion, size sorting, and reworking strongly affected theplanktonic foraminifer faunas, so they are unevenly dis-tributed in a single sequence, and throughout the wholeCenozoic succession as well. Similar to the biostrati-graphic record from the Cretaceous part of Site 462 (seePremoli Silva and Sliter, this volume), biostratigraphicevents registered by planktonic faunas, even if dis-placed, were expected to occur in a relatively "normal"succession. During the Cenozoic, however, such normalorder is difficult to detect, because (1) the plank-tonic-foraminifer record is largely discontinuous, (2)the youngest forms are largely diluted within heavilyreworked assemblages, or absent in layers too finelygraded for the specific foraminifer size. Nevertheless,with the aim to provide the best visual record, the distri-

bution of the most important species was plotted onroutine range charts (Figs. 4-6). Based on those dis-tributions, some biozones could be distinguished. It isworth mentioning that zonal assignment was frequentlybased on first occurrence of a few diagnostic specieswithin assemblages dominated by reworked faunas.Consequently, zonal boundaries must be consideredonly tentative. In fact, the bulk of planktonic speciescharacteristic of a specific biozone generally occur onlyas reworked faunas in younger (occasionally muchyounger) layers.

Discrepancies are expected among the biostratigraphicrecords based on the three main fossil groups—plank-tonic foraminifers, radiolarians, and calcareous nanno-fossils: all are affected by the same sedimentary proc-esses, which however acted differently on the threegroups because of their different behavior duringmechanical redeposition (difference in size; see follow-ing chapters).

In specific diversity, ignoring their irregular verticaldistribution, the planktonic-foraminifer faunas encoun-tered in Hole 462 are typical of the tropical environmentthroughout the whole Cenozoic. Their preservation isgood to excellent, despite redeposition. Only in a fewcases, mainly confined to the uppermost part of the re-covered succession, are the largest specimens, suchas those belonging to globoquadrinids, mechanicallybroken. Traces of dissolution are absent or very rare,affecting mainly early and middle Miocene assemblages.

The zonal scheme followed in the present paper isshown in the Site Summary (this volume). It is basedmainly on schemes by Hardenbol and Berggren (1978)and Berggren and Van Couvering (1974). The biozonesidentified are as follows (from bottom to top):

Eocene

Zone P6, Morozovella edgari Subzone, Early EoceneOccurrence: Core 44.Relatively abundant, moderately well-preserved plank-

tonic-foraminifer faunas are recorded in Core 44, Sec-tion 1. The assemblage contains keeled morozovellids,and some acarininids. Besides the zonal marker, Acari-nina nitida, "Globorotalia" guatemalensis, Morozo-vella acuta, M. gracilis, "Morozovella" aequa, " M "wilcoxensis, and Subbotina sp. have been identified.They are in general concentrated at the bottom of se-quence (Core 44, Section 1, 7-10 cm), and they decreaseupward. Reworked Late Cretaceous and late Paleocene(Zone P4 = Planorotalites pseudomenardii Zone) as-semblages occur in the same samples.

Zones P16/P17 Turborotalia cerroazulensis Zone,Late Eocene

Occurrence: Core 38 through Core 34.The relatively diversified assemblages attributed to

these zones contain Turborotalia cerroazulensis cocoa-ensis, rare T. cerroazulensis cerroazulensis, T. pseudo-ampliapertura, Hantkeninae, Catapsydrax unicavus,C. perus, Globorotaloides suteri, Subbotina linaperta,Globoquadrina tripartita, G. galavisi, Pseudohastiger-

398

CENOZOIC PLANKTONIC-FORAMINIFER BIOSTRATIGRAPHY

s,<

XcQ)

g0*i

Ple

i!

?O

0

gCD

uo

α.

α>c

öΦ

I

_>•reΦ

α>

0N

r -

z

(D

en

z

N

Z

CD

0

1

23

4

5

6

7

8

1 0

11

12

13

1 4

16

16

17

tion

J

sC/51234

5

6c c

c c

1

24

6c c

1

4

5

c c

3

46

7

c c

1

2

4

56

c c

1

35

c c

3

4

c c

2

4

6

1

3

4

c cc c

4

5

6

c c1

2

3

4

5

6

2345

c c

1

c c

2

3

4

5

c c

1

iple

rval

m)

Sf ~

34-3677-7978-80

134-13615-1777-79

91-9362-6472-7417-19

101-10345-4659-6078-80

88-8992-9356-57

130-13114 1545-46

127-128140-141102-103146-14796-9757-59

1-384-8591-9256-57

49-50112-113106-107

50-51147-148115-11657-58

79-80117-119

39-40

54-55147-14847-4823-24

85-8685-8685-8685-8685-8685-86

36-37116-11886-8779-81

3-4

55-5640-42107-10841-42

6 - 8

Total

Weight(g)

2.35012.26954.25405.27624.89074 8254

1.94083.50161.52101 93852.25610 88382.24931.4005

0.73841.40740.8250

1.44011.45000.4892

7.3006

1.45781.52841.5738

1.54901.36671. 2854

2.89922.99282.90762.0865

1.5575

2.3780whitebrown1.84601.97351.57642.9182

10.55611. 70981.84051. 76851.61401. 95232.2314

1.28408.3798

10.5989

1.74722.60922.41422.7917

2.8057

ooo

i

i

>63/i

15 30 «

\

{

K

\

>V

>(V(\I

}

i

\

•

Weight

»

)

\

J<\

{

\)

{\\\

}

> '-s‰r

\

I

11

I

\

nO _

oM

7\)

>

> >

\

>

!

1{\)/\

{

)

J

|I\lIi

(

i

]

>63 µm Fraction

Foraminifers

{%\90

\

V/N

>

JK\V

N

~>

sSF/2OS / J 0s / i o F O

S^F

50

M Π / , 0 A Λ <

M-AA/o

A

A/40

MnA/

A

A

A

Mn A

A

A

* / 3

A

A /60

A

A4oSlA/éo

OA

A/,o

oo

A

O.A4O A

O A ^

OA

OA

AOA

Q

S i A ^ oQ A / ßi u.' • " ( •

90

s

-"K—'

"•~--<

(

•

<

<

{t)<

<o'

1>̂

I\

BIOGENIC COMPONENTS•OAMSFDOsW

weighed samplessamples not weigheddeep-water agglutinated foraminiferslarger foraminiferssponge spiculesfish debris

diatomsostracodesshallow-water debris

COLORO: dark to very dark+ : medium dark• : medium grayΔ : light grayD : white

NON-BIOGENIC COMPONENTSM n : manganese

O : opalA : aggregates (lumps)Q : quartzSi : silicates and chert

VOLCAN ICSG : glassR : volcanic-rock fragmentsF/5: 5% of fish debris

Figure 1. Quantitative sedimentological data, DSDP Hole 462, Core 1 to Core 17, Section 1, Pleistocene to early Miocene.

ina micra, and Chiloguembelinae. Rare are representa-tives of the genus Globigerinatheka, possibly reworked.This assemblage is relatively common in Core 37,whereas the overlying cores are almost devoid of plank-tonic foraminifers. Cassigerinella chipolensis, associ-ated with Chiloguembelinae, Tenuitella gemma, T.munda, is recorded in Core 34, Section 1, 14-16 cm, at

the top of which, on the basis of nannofossil and radio-larian records, the Eocene/Oligocene boundary mightbe placed. Rare late Oligocene forms attributable toZone N4 (lower part), found scattered in the discussedinterval, are interpreted as down-hole contaminants.

Planktonic foraminifers are absent in Core 33 and inthe lower part of Core 32.

399


<

iu0

5£

α>co

z

z

g

C\J

α_

z

C•ö

0.

z

o0.

α>O

O

17

18

19

2 0

21

c c

2 2

2 3

2 4

2 5

2 6

2 7

e

tio

S

• r ' '

J/V,J

">\I

{}/\/IV.

J>

</\

1

Biogen'- Non-ic biogen

Components90

S F

S/sDFS

S FF

S

S/,0FSS D

s/.S/aS/,0F

SF/j

s/5S/,0

S/β

S F

S/3F

S

S

S W

sS/13DF

S/jD

S/,,F

S/2DF

S D

S F

S

SF

SF

s/3β/i

s/,S/,FW

S/,OW/SS/5W

S/3FW

S/.D

S/5DW/2s/7 w<sF

SFS/9FJS

s , wS/2FS/,D/5SFWS/,DSF

S

s/2sS F

w

F

SF

SF

S

s

9 0

o/2OA

O S i

0

OA

O

0s,0

OA

OA

0 0

OOSiA

A

OA

OA

OA

MnO/2A^

MnOA

0

S,A

OA

0

O/4 A /3OO/4 A

OA

OA

A/3OA

OA

OA

MnOA

OA

OA/SOA

O / j

OA

MnOA/3

OA

0

MnO/4

MnOA/,

OA/ S0

OS,A/ 3MnOA

OA

OO

MnO,3A

A/s0/3A

OA

MnOA,,

0

OA/ 1 0

A/10

A/soMnOA/,

OOA

OA

O

OA

Volcanics

»0

G

G

G

G

G

G

G

R

R

R

>150µm

Foraminifers

(309 0

0

V

^'

\

\

\

Vx>

.'1/\

*

>

f

\

>

>

P/(P*É)90

J>« A'

r-A

^ ^ • » A *

A M )

λMl

i

*•\

A A >

"̂̂A,

4\1

A)

A,

A.

A,

A

A

A

1

>

AcC

A.

?A

\ A

>

A <

•β

Radio-larians

%9 0

a.

1<

<>

1

<

^ ^

< ^ ' * '^~^~>

>

<

<

>

:ractionBiogen

icNon-

biogenComponents

90

S/2,FE

S F

SF

SFO

aiFO.W

s , 2 0fw/33sF

SFO>WZSFOS

S/,FW/,,s 5 εo.w l 2s/6w/6S/6W/13S/,O.W/jβ

S/7FEW/HS/,FW/2,

S,,0.W/9ijnF^OsW,

SF

S/i,S F O

S/BFW/7S/,F

SW/,

S D F

SDFW

S F W

S

S

o s

SF/,W/2

S / j

9 0

OS, A / l 0A

O

A

OA

OS,

O

A/7

MnOA js

OA

A/30O A ,

OA/32OA

OMnA,,

0 *

J •—

<

\

j

>

>>

r"

\f1\P

\

>

\\j

f

7^»

Figure 2. Quantitative sedimentological data, DSDP Hole 462, Core 17, Section 2, to Core 27,CC, late Oligocene.

Oligocene

Zone P20, Turborotalia ampliaperturaZone, Late Oligocene

Occurrence: Core 32, Section 3, to Core 27, Section3.

Besides the zonal marker, Globorotalia opima opima,Turborotalia pseudoampliapertura, T. increbescens,Globoquadrina tripartita, G. galavisi, Catapsydrax uni-cavus, C. dissimilis, etc. occur, associated with speci-mens attributable to Globoquadrina baroemoenensis, inCore 32, Section 3, 113-115 cm. This assemblagecharacterizes the lower part of Zone P20, or the bound-ary between Zones P19 and P20. This attribution is con-firmed by the occurrence of Globorotalia sp. cf. G.siakensis in Core 29, Section 3, 62-64 cm, which, ac-cording to Blow (1969), would appear in the middle partof Zone P20.

The fine fractions (< 150 µm) are dominated byChiloguembelinae, Pseudohastigerinae, and Tenuitellae(T. gemma and T. munda). The latter forms are con-sidered reworked from early Oligocene layers, alongwith the late Cretaceous through late Eocene assem-blages. Radiolarians are strongly affected by redeposi-tional processes in this interval.

Zone P21, Globorotalia opima opima Zone,Late Oligocene

Occurrence: Core 27, Section 2, to Core 23, Section3.

Above an interval alternately yielding primarily re-worked assemblages (Core 28) or devoid of planktonicforaminifers, Core 27, Section 2, 144-146 cm containsrare forms attributable to Globoquadrina globularis,whose appearance according to Blow (1969) would oc-cur within Zone P21. Among the species here con-

400


!<

I

δ

a>

ëoHI

*

15• "

egΦ

V,a

JS

T I

•o

1

α>

0

N

o

α

σ>

α_

σ>

00

α.

a.

~T•~~

—

α.

.-Q.

α>0

U

2 8

2 9

3 0

3 1

3 2

3 3

3 4

3 5

3 6

3 7

3 8

3 9

c

:io

Sed

1

2

3

4

5

6

c c

1

3

5c c

2c c

1

2

c c

1

2

3

4

c c

1

2

c c1

c c1

cc123

c c

1

2

3

4

5

6

c c

1

2

3

4

5c c

1

3

5c c

E » gw £ ~~

115-117112-11498-100

103-10590-9294-96

9-11112-11441 -4262 -64142-14465-67

74-76

122-12486-88

121-123

5-1073-7512-14

113-11568-70

75-7784-86115-11715-1722-2428-3067-69

14-1644-4696-98

15-17

86-89141-14313-1583-85

10-1213-15

104- 10644-46141-143128-130

2 - 448-5079-81

100- 102112-11492-94

8-11100-102112-11434-3655-57

vveiyin _

(g)

10.63208. 46058.02506.38527.48389.2219

12.35895.92215 14957.80547. 67039.0077

6. 5027

4.60726.60203 9578

18.005417. 93859. 30297.59106.2544

2.21445.21385. 33055. 00744.31144. 70797.2271

6. 825010.30169. 0926

4. 5023

9. 98089.43359.87787. 5962

10.37916.46107. 17627.91507.0478

12.5278

11.34377.22846. 54368.40724.47558 5636

4.30224.62282.82024.98194.5545

U

15 30 45

r

\

λ\

y\(

\I\

{

\\\\}I

I/

(

I

A/eight

(%)

50

/

1

I

<

>

y)<

\

-

[

CO

8 siU

50

II]\\1

t

I\\)

I

1I

\\\>

<

)y<

>

\

<

/\\4>

(\

\

IN\

>63µm FractionForaminifers

(%)9 0

r

{\V">

N})s1

>

\

{)f

>

1

>

>/

{\\\

>

9 0

I

1

Radio-

larians%9 0

«

I/

/

0

<

— ^ .^ • —

ij

1A >

^ >̂A

A

~̂-•A

.A

A

A

»*.

•

jI\t\

_̂^-"^

y. ""^A

}s(V

><

i

\)

<

<

ii>

i

I///>

><

<

|i

Biogen

ic

-Non-

biogen

Components9 0

SF

S/JDCJ

S

S

F

F

F

S

S/,9

wS/JO

sF

S

S/20D

S

sssS F

8 / j

sS/2F

sssssS/jDS/jF8/4

S/5E

S/j08/j"β / i

s

F

S

8 / j

ssss

SF

SF

S/iβS F

S/,2β / |

S / j

S/,oDS

S/4F8 / J

8 / ,

8 / j

8/,,

9 0

M«A

OA

OA

OA

O

OA

A

O

OA

O

OA

OOA

A/50A

S<

OA

OA

A S i

SlA

OS,/3A

oA/25O

OA

OA

O

OA

OA

MnOA

OA

O

O/jA

O

A / MA

O

O

A

S i

S i A

Si A/,

Si A/4S./jA

Si/3A

OO/s A

O/5A

OA/2O/3A

OA

O/,S,A

O A

A

A

0

OA

VolcanICS

9 0

R

R / 3R

G

G

G / 3

> 1 5 0 µ m

Foraminifers

%9 0

•

V

p*/

N\>

I

Ii\/

1

\\

\K

P/IJ>>B")

9 0

J

A<

>

>

• A

Radio-

larians

9 0

1

1

1

y11

/

•

1

11

J

/

{}/

>

><

1

á

Fraction

Siogen

icComp

90

β / j

osoSF/jSFO

ss0 .

s

swsF

S F

S

SF

f

SFβ / .

S/β*s/,ws^wS F

s

S F

S

S

ssF

S / 5

B/nF

S/50

S F

S

8/2

β/io>=

Non-

biogen>nents

9 0

0

OA jM>A

M.OA/MOA/3S i

OA

OA/,SiA/,0QA/,,

A/,o

OOA

OA/,OA/4S.A/jS i A

OA

A/to

A / 2

0

A

OA/2

*/ioO/3A/5O

O

OA/jOA

A/to

A / 5

OA/4

A / 4AΛOSiA/15

OA/i

OA/,,

A / 2

A/itO/4 A/3

O/j A/,O A

OA/,A

OA

A

VolcarrICS

9 0

β

6/j"/3R

R

R

G

G/jRS / 3

G

G

R

G

R

G

R

G

ReworkFor ami

fers

> 63 µ

9 0

/

<

{\

V

V>,i

I

•—

/

>.̂1

Figure 3. Quantitative sedimentological data, DSDP Hole 462, Core 28 to Core 39.CC, late Oligocene to middle Eocene.

sidered coeval with the mentioned taxon, are Globo-quadhna baroemoenensis, G. galavisi, G. winkleri, G.tripartita, Catapsydrax unicavus, C. dissimilis, Globo-rotaloides suteri, and Cassigerinella chipolensis.

The zonal attribution is confirmed by the occurrencein the upper part of the interval (Core 25, Core 23 lowerpart) of primitive Globoquadrina altispira and Globoro-talia siakensis. The occurrence of the zonal marker isdiscontinuous.

Reworking is considerable and involves also the ra-diolarians, which are clearly size-sorted in most of thestudied samples.

Zone P22, Globigerina angulisuturalis Zone,Late Oligocene

Occurrence: Core 23, Section 2 through Core 19.The beginning of Zone P22 is placed at the ap-

pearance of Globoquadrina praedehiscens in Core 23,Section 2, 39-41 cm. Because of the heavy reworking,the extinction level of Globorotalia opima opima atHole 462 is not biostratigraphically reliable. Within this

interval, rare Globigerinoidesprimordius, Globigerinitajuvenilis, Globorotalia mendacis, G. semivera, andsmall, thin-walled Globorotalia, possibly the ancestralforms of the G. fohsi lineage, also occur, albeit discon-tinuously.

In Core 23, the fine fractions (< 150 µm), previouslydominated by pseudohastigerinids and tenuitellids, be-come composed mainly of Cassigerinella and Chilo-guembelina. Also reworked material from early Oligo-cene layers decreases and is replaced by undifferentiatedmid-Oligocene assemblages.

In Cores 21 through 19, layers rich in planktonicforaminifers are much rarer than in the underlyingCores 23 and 22, while radiolarian-rich layers increase.

Zone N4, Globorotalia kugleri Zone,Latest Oligocene

Occurrence: Core 18,CC to Core 17, Section 3.The appearance of the zonal marker marks the begin-

ning of the zone, although the planktonic-foraminiferfaunas, because of the reworking, appear very similar to

401


Cor

e

111111

Se

ctio

n

Inte

rva

l (c

m)

-1,34-36-2. 77-79-3, 78-804, 134-1365. 15-17-6,77-791,CC

3333

4444

555

σi

»

»1

1»»

•

α

π

αD

D

D

•

•

•

•

•

•

•

•

•

•

•

•

Φ

•

• •

<

(

•

<

<

<

(

• 1

• <

<

<

• »

• 4

<

• »

1

• 4

• 4

• 4

• 4

1

<

i

4

4

4

4

4

4

4

4

1

<

4

• <

»>»»

»>1

»

I11

11»»»»»>»»>»1

I1

»

>»>t

»»»»

»»»•••

Glo

bo

rota

lia p

eri

ph

ero

ron

da

Str

ep

toch

iius

pri

stin

um

•

>

D

•

•

z

Glo

bo

qu

ad

rin

a b

aro

emne

nens

isG

lob

oq

ua

dri

na

obe

saG

Inh

na

ua

dri

na

deh

isce

ns

α

πα

•

•

•

•

•

•

•

αD

α

D

D

ααD

D

ααα

•

•

••

Glo

bo

qu

ad

rin

a d

ehis

cens

ad

vena

Gio

big

eri

no

ide

s tr

ilob

us

ßlo

bige

rinoi

des

quad

rilob

atus

_

Gio

bige

rinoi

des

subq

uadr

atus

Gio

bige

rinoi

des

subs

accu

lifer

Φ

Φ

Φ

α

•

α*

•••

•

••

•

Glo

big

eri

na

nep

enth

esG

lob

ige

rin

a d

rury

iS

ph

aero

idin

ello

psi

s se

min

ulin

aS

ph

aero

idin

ello

psi

s su

bd

ehis

cens

_G

iob

ige

rin

oid

es

sica

nus

Glo

bo

rota

lia p

rae

me

nar

dii

Be

lla b

erm

ud

ezi

'Glo

bo

rota

lia p

rae

scitu

laG

lob

oro

talia

co

nti

nu

osa

Glo

bo

qu

ad

rin

a a

ltis

pir

a g

lob

osa

• G• G•

•

• α• α• α

• G

α

•

Φ Φ

•

•

•

•

•

•

•

•

•

Φ

•

•

•

•

•

D

α

α

αα

•

•

D

σ

αα

α

α

α

Φ

•

ααα

αQ

π

•

•

•

•

•

•

Str

ep

toch

ilus

glof

yige

rum

Gio

big

eri

no

ide

s b

uilo

ide

us

Vela

per

tina?

α

ααα

α

•

•

•Φ

-

•

i

Glo

bo

rota

lia p

seud

op

ach

yde

rma

Pra

eo

rbu

lina

spp

.

•

•

•

•

•

α

ασ

Glo

big

eri

nita

in

cru

sta

Glo

big

eri

na

bu

lbo

saG

lob

ige

rin

a fo

liata

Glo

bo

rota

lia f

oh

siG

lob

oro

talia

pe

rip

he

roac

uta

Glo

bo

rota

lia p

rae

foh

si

••

••

••

••

•

•

-i

α

•

•

•

••

(I

i

4

1

<

4

4

0

4

i

Gio

big

eri

no

ide

s b

olli

iG

lob

oro

talo

ide

s he

×ago

nus

i

»

I

I

» α* α> α

••

»

>

••••

••••

••

•

•

Gio

big

eri

no

ide

s o

biiq

uu

sG

iob

ige

rin

oid

es

rub

er

Φ

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

Glo

big

eri

na

bu

lloid

es

Glo

bo

qu

ad

rin

a l

ang

hia

na

9

•

•

Figure 4. Distribution of selected planktonic foraminifers, DSDP Hole 462, Core 1 through Core 17, Section 1,Pleistocene to early Miocene.

402


inoi

des

"mitr

a"

inoi

des

helic

inus

Glo

bige

rß

lob

ige

r'G

lobi

ger

i

i

4

«• «

»

•

••

»

prim

itive

Pul

leni

atin

aG

lobo

rota

lia j

uana

iG

lobo

rota

lia l

engu

aens

isG

lobo

rota

lia a

cost

aens

is

_O

rbul

ina

univ

ersa

Glo

bige

rinoi

des

ampl

usO

rbul

ina

sutu

ralis

Str

epto

chilu

s la

tum

Glo

boro

talia

lim

bata

Sph

aero

idin

ella

deh

isce

nsG

lobi

gerin

oide

s ob

liquu

s e×

trem

usG

lobo

rota

lia h

umer

osa

Glo

boro

talia

ple

siot

umid

aG

lobo

rota

lia m

edite

rran

ea

•••

•

•

•

•

•

••

••

•

••

••

•

•

•

i

••••••••••

•

••

•

•••••

•

•

•

• •

•

•

•

i

• «

i

•

•

•

•

•

•

•

•

#

•

•

•

•

•

•

alia

cul

trat

aal

ia m

arga

ritae

Glo

boro

i'G

lobo

rot

4

<

4

•

44

41

•

Can

dein

a n

itid

aP

ulle

niat

ina

obliq

uelo

cula

taG

lobi

gerin

a m

icro

stom

a

»

»»»

»•»»»»••••

alia

cib

aoen

sis

alia

anf

ract

aß

loboro

tG

lobo

rot

i f•

•

Glo

boro

talia

tum

ida

fle×u

osa

alia

tum

ida

alia

cra

ssaf

orm

isG

lobo

rot

'Glo

boro

t• l

l<

Pul

leni

atin

a sp

ecta

bilis

ma

prim

alis

~>qu

adrin

a pa

chyd

erm

aP

ulle

niat

Neo

glob

t

•

•

Glo

bige

rinoi

des

fistu

losu

siq

uadr

ina

pseu

dopi

ma

'alia

ron

daN

eogl

obt

Glo

boro

•G

lobo

rota

lia

trun

catu

linoi

des

'alia

pum

ilio

na r

ubes

cens

Glo

boro

Glo

bige

r

••

•

•••

•

•

Glo

bige

rinoi

des

elon

gatu

s

•

• Presence of Species•DReworked specimens

na p

seud

ofol

iata

iqua

drin

a du

tert

rei

Glo

bige

rN

eogl

obt

•

• •

Glo

boro

talia

ung

ulat

aal

ia c

rass

ula

'alia

hirs

uta

.Glo

boro

iG

lobo

roG

lobi

gerin

a ca

lida

Tur

boro

talit

a hu

mili

s

••

•

•

HOLE 462Core 1 through Core 17,

Section 1

Plankton ic-Foraminifer

ZoneAge

(not zoned)

7

(not zoned)

I— ? -p ~>

N17/

N16

N13/N12

N7

01

mid

c1

early

i 1

Ple

isto

ce

ne

o P

lio

ce

ne

or

yo

un

ge

rM

ioc

en

e

Figure 4. (Continued).

403


Cor

e

1717

1818181R181818

119

coco

191O

OO

CM CM

CM

2020

22121212121212121

222-22-22-22-22-

223-23-23-

Sec

tion

cm)

Inte

rval

-2, 40-41-2, 11-227,CC

-2, 130-1323, 70-723,111-1134, 40-424,104-1064,139-1416, 25-278 Cr

2,5,6,6,'9J,3,4,4 , '4,"4,

çy1,1,12,3,53,4,5,6.11,(

140-14226-1283 15

?6 28

:c34-96

1̂ 9—5175-7779-81X2-3J9-8356-6858-60ΛO inQ

30-62125-12737-69

:c1,69-713, 112-1144, 74-765,27 296,92 942,CC1,98 1001,110 11?9 97 9Q

23-2,39 4123-3,98 10023-4,102 10423-5,46 4823-6, 60-6223-6,74 76

23, CC

24-2424

2

Tin

inM

CM

CM

25-25-25-

2

2626-26-26-

227-27-27-77-27-27

2

2, 107-1083,10-113, 78-794, CC

3, "4, (4,5,

7 , :5,2,3, '4,15,.5,S6, (1 <2,3,3,3."5,7, <

71-7354-6636 13835-137

J9—41

:c38-140të-51

32-34"XA QC

:c•9-5144 1462 14

59-7174-76

DC

"Glo

big

erin

a"

ampl

iape

rtur

a

ααπ|α

|σ|G

|α|α

|α|

α

α

•

alia

pse

ud

oa

mp

liap

ert

ura

I

rib

elin

a s

pp

.

O

Tu

rbo

rot

Ch

ilog

ue

iC

assi

gerir

Te

nu

itelli

αGπ

|α|α

iα|α

|α|α

•l l

l l

l l

•α

>l l

G

•

G

- -•

•

•

•

•

T

• α• α• α

• α• α• α

α|α|α|α| [α|α|cG|G| I |

|

|

••••••--•

Te

nu

itella

mu

nd

a

DD

αα

σα

ααπDD

•

•

•

Ca

tap

syd

rax

un

ica

vus

Glo

bo

rota

loid

es

sute

ri•

•

i•

•

•

••

••

• l

l l

l l

l l

l l l l l l l

•

-i

•

•

•

•l

••

••

•

•• l l l l l l l

• l

l l

l l

l !

•l l

na o

ffic

ina

l is

idri

na

αal

avis

iG

lob

ige

riG

lob

oa

ui

•

•

•

••

•

•

m

l l

l l

l l l

Cat

apsy

dra

x d

issi

mili

sG

lob

oq

ua

dri

na

ba

roe

mo

en

en

sis

•

•

•

-

i

•

•

•

Su

bb

otin

a y

egua

ensi

sG

lob

oq

ua

dri

na

tr

ipa

rtit

aG

lob

oα

ua

dri

na

win

kle

ri

•••

l l

l l

l l !

•

•••

•

l l

l l

1 1 1

Glo

bo

qu

ad

rin

a g

lob

ula

ris

Glo

bo

rota

loid

es

vari

ab

ilis

Su

bb

otin

a l

ina

pe

rta

•

•

•

•

1 l l l l l 1

Su

bb

otin

a a

ng

ipo

roid

es

Glo

bo

rota

lia o

pim

a n

an

aG

lob

oq

ua

dri

na

ve

ne

zue

lan

a

•

••

~i

•

•

•

•

•

•

Glo

big

eri

na

pra

eb

ullo

ide

sG

lob

oq

ua

dri

na

alt

isp

ira

Glo

bo

qu

ad

rin

a r

oh

ri

•

•

•

•

•

l l l 1

l l l•

MM

l

l l<

-

•

•


Cor

e

2ü00 00 00

CO 00

CM CM

CM CM

CM

i

29

M M

M M

M

CD

C

D C

D C

O C

O

i

30

CO

CO

CO

3232323232

33333333333333

343434

35

36363636

37373/3/373/

3838383838

3939393939

j

Sec

tio

n

- 1 ,-2,-3,-4,-5,-6,?8^_- 1 ,- 1 ,- 3 , '-3,-3,-5,>9,

(cm

)In

terv

al

115-117112-114

do— 1UU103-105

34-96X3-11112-114π 42

42 14435-67X

-2,74-7630, CC-1,-2,i-2,

π . <-1,-1,-2,-3,-4,32,

122-124

121-123X5-1073-752-14

I 1 "3 1 IK

38-70CC

-1,75-77-1,84-86- 1 , 115-117-2,15-17-2, 22-24-2,28-30-2, 67-69!3,CC- 1 , 14-16-1,44-46-1,96-9834, CC-1, 15-17J5,CC-1,86-89-2, 141-143-3, 13-15-3. 83-8536, CC- 1 ,-2,-3,-4,'•5,

-6,yi,

10-1213-1504-106

14-4641 14628 130

X

-1,48-50-2, 79-81-3, 100-102-3, 112-114-5, 92-94I8.CC

-1,8-111, 100-1021,112-1143, 34-365, 55-579, CC

Cat

apsy

dra×

peru

sG

lobig

erinath

eka

sem

iin v

olu

taT

urb

oro

talia

cerr

oazu

lensi

s I

•

44

4

4

4

4

4

4

4

<

»

4

•

4

Glo

boquadrina

trip

art

itaG

loboquadrina g

ala

visi

>>

»

•

»

t

>

»»

•i

?»

44

• 4

4

4

4

<

4

• 4

• 4

<

<

1

(

»

"Gio

big

erin

a"

gort

anii

>»

»

I

»»»

a ye

guae

nsis

_J

valia

opim

a n

ana

§u

bb

otir

Glo

boro

-

•

•-

••••

•••

Te

nu

itella

gem

ma

Subbotin

a

linapert

a

••••——••••

•

T

•

•

-

••

•

Turb

oro

talia

coco

aensi

sT

urb

oro

talia

sp

seud

oam

p/ia

pert

ura

Pse

udoh

astig

erin

a m

icra

•

•

•

α

D

•

•••

Ha

ntk

en

ina

sp

.T

en

uit

ella

mu

nd

aC

hilo

gu

em

be

lina

sp

p.

^J

•

•

•

• c

•• c• c

• c

• c?• L"

• <• I• 4

• 4

••• ̂• 4• 4•

Pse

ud

oh

ast

ige

rin

a b

arb

ad

oe

nsi

sG

iob

ige

rin

a

off

icin

alis

1 D

3 D] D

] D

] D

»t>>>

»»

Gio

big

eri

na

an

gu

stiu

mb

iiic

ata

Glo

big

eri

na

the

ka

sp

.C

ass

ige

rin

ella

ch

ipo

len

sis

••

••••

•X

Glo

bo

qu

ad

rin

a t

ap

uri

en

sis

Glo

bo

qu

ad

rin

a b

aro

em

oe

ne

nsi

s••

alia

op

ima

op

ima

Ira×

dis

sim

ilis

Glo

boro

t"C

atap

syc

"Gio

big

erina"

am

plia

pert

ura

Glo

boquadrina ro

hri

••

Subbotin

a a

ngip

oro

ides

Turb

oro

talia

incr

ebesc

ens

Glo

boquadrina

ve

nezu

elan

a

•

•••

•

•

•

Glo

boquadrina

pra

saep

isG

loboquadrina

glo

bula

ris

Glo

boro

talo

ides

variabili

sG

loboro

talia

sia

kensi

s

—

Glo

boquadrina

win

kleri

Gio

big

erina p

raebullo

ides

•

Presence of Species

•

—

Reworked Specimens

HOLE 462

Cores 28 through 39

PlanktonicForaminifer

Zone

P20/

P19/

P17

(not zoned)

Age

10

j e

ar

ly

?la

te

st

α>

m i

d d

Eo

ce

ne

•

sj

Oli

go

ce

ne

Figure 6. Distribution of selected planktonic foraminifers, DSDP Hole 462, Core 28 through Core 39.CC, late Oligocene to middleEocene.

405


those below. On the basis of the absence of Globigerin-oides and Globoquadrina dehiscens, this interval is stillattributed to the Oligocene.

Layers rich in radiolarians are still common.

Miocene

Zone N7, Early MioceneOccurrence: Core 17, Section 1, through Core 16.On the occurrence of well-developed specimens of

Hastigerina siphoniphera and of Globigerinita glutinatain Core 17, Section 1, 6-8 cm, the top of Core 17 is attri-buted to Zone N7, middle to late early Miocene. Plank-tonic-foraminifer assemblages are dominated by formsfrom Zone N4.

Radiolarian-rich layers are common in this interval.

Zones N12/N13, Middle MioceneOccurrence: Core 15,CC to Core 12, Section 6.The attribution to Zones N12/N13 is based on the oc-

currence of Globigerina druryi and forms transitional toGlobigerina nepenthes, and of Sphaeroidinellopsis sub-dehiscens in the core catcher of Core 15 and in the lowerpart of Core 14—in agreement with the range of thosespecies proposed by Blow (1969). Rare Velapertina seemto occur in this interval. Again, the foraminifer-rich as-semblages are dominated by reworked faunas belongingto the early and early middle Miocene. Such taxa asGlobigerinatella insueta, Beella bermudezi, Globoro-talia archaeomenardii, and G. peripheronda are com-mon.

Reworked forms of early Miocene age display evi-dences of corrosion, perhaps due to dissolution in ag-gressive bottom waters.

Zone N16/N17, Late MioceneOccurrence: Core 12, Section 5, through Core 10.The occurrence of Globorotalia juanai and primitive

forms attributable to Pulleniatina, in the absence ofGloborotalia siakensis and Globigerinoides subquadra-tus, in Core 12, Section 5, 47-48 cm, would place thissample in the late Miocene Zone N16 or Zone N17 (seeStainforth et al., 1975; Brönnimann and Resig, 1971).In the overlying cores, Globigerina nepenthes and Glo-bigerinoides ruber also occur, in agreement with thezonal attribution of the considered interval. Planktonic-foraminifer assemblages are, however, very rich in themore-developed Globorotalia fohsi lineage, consideredreworked from the middle Miocene. Core 9 had norecovery.

Pliocene or YoungerOccurrence: Core 8,CC through Core 4.The occurrence of some well-developed specimens of

Globorotalia margaritae, and possibly Sphaeroidinelladehiscens, in the core-catcher sample of Core 8 and inthe higher levels indicates that the host assemblagebelongs to the Pliocene, although the planktonic-for-aminifer assemblages are dominated by late MioceneZone N17 faunas. The attribution of Core 8 to the Plio-

cene is confirmed by the occurrence of Streptochilustokelauae in Core 8, Section 3, 49-50 cm, which ranges,according to Brönnimann and Resig (1971), from theupper part of the G. margaritae Zone to the lower partof the Pleistocene. The occurrence of Globigerinoidesfistulosus in the core-catcher sample of Core 6 testifiesthat some of the planktonic-foraminifer faunas areattributable to the late Pliocene. However, most of thespecies characterizing the late Pliocene assemblages,such as the large-keeled globorotaliids, are not recordedfrom Hole 462. On the basis of the available data, thecompleteness of the Pliocene record cannot be eitherproved or disproved. A few forms of Pleistocene affin-ities occur from the bottom of Core 8. Taking into ac-count that the highest cores recovered from Hole 462are very disturbed by drilling and soupy, those Pleisto-cene forms are interpreted as down-hole contaminants.However, that the Pliocene assemblages could also bereworked into the Pleistocene, cannot be ruled out.

PleistoceneOccurrence: Core 3 through Core 1.The uppermost three cores yielded a discontinuous

record of planktonic foraminifers attributable to thePleistocene. Although reworking masks the biostrati-graphic signal, it seems that the whole Pleistocene isrepresented in those cores. In particular, the occurrenceof Streptochilus tokelauae, associated with Pulleniatinaobliquiloculata> Truncorotalia truncatulinoides, Sphae-roidinella dehiscens dehiscens in Core 3, would suggestthe presence of taxa attributable to Zone N22, whereasCore 1 is attributable to Zone N23, based on the ap-pearance of Globorotalia hirsuta, Globigerina calida,and Pulleniatina finalis.

Pliocene and early Pleistocene assemblages, still as-sociated with rich late Miocene faunas, are reworked inthe uppermost foraminifer-rich layers (up to Core 1,Section 3).

The uppermost few meters of sediments recoveredfrom Hole 462 are pelagic clay and do not contain anyforaminifers.

Reworked AssemblagesAt Hole 462, several planktonic-foraminifer faunas

not recorded in their correct stratigraphic position onlyoccur reworked in much younger layers. Analysis ofthese reworked assemblages enlarges the biostratigraphicrecord not only for the Cenozoic, but also for the Meso-zoic (see Premoli Silva and Sliter, this volume).

These reworked assemblages are attributable as fol-lows (from older to younger) (Fig. 7):Aptian: Based on the occurrence of a specimen attribut-

able to Hedbergella trocoidea in Core 6, Section 2,140-141 cm.

Albian: Based on the occurrence of Ticinella sp. aff. T.breggiensis (Core 25, Section 4, 136-138 cm), ofTicinellal (Core 22, Section 5, 92-94 cm).

Albian to Cenomanian: Based on the occurrence inseveral samples from Core 27 of rare specimens ofGlobigerinelloides bentonensis, well-preserved Schac-

406


Figure 7. Distribution of reworked planktonic-foraminifer assemblages, plotted against age and/or zones for Hole 462, Core 1 toCore 39. Note that the late Paleocene assemblage includes the late Paleocene Morozovella velascoensis Zone and the earliestEocene Morozovella edgari Zone.

407


koina cenomana, and Hedbergella planispira; theyare generally contained in radiolarian-rich sandy tosilty layers.

Maestrichtian: Maestrichtian and possibly Campanianplanktonic foraminifers are common throughout thewhole Cenozoic sequence, except for Cores 1 and 2.In the coarser layers, the most common species areGlobotruncana area, G. rosetta, G. bulloides, G. lin-neiana, and G. ventricosa. The finest fractions aredominated by Heterohelicidae (Core 29, Section 3,142-144 cm), Pseudoguembelina costulata and P. ex-colata being the most common species; they are asso-ciated with Globigerinelloides volutus, G. asper, andRugoglobigerina rugosa; moreover, Rugoglobigerinahantkeninoides, of early Maestrichtian age, occurs inCore 19, Section 6, 13-15 cm; Globotruncana gag-nebini, of early to mid-Maestrichtian age, occurs inCore 13, Section 5, 85-86 cm. Finally, Racemiguem-belina fructicosa, sometimes associated with Trini-tella and Globotruncana contusa, occurs in Core 38,Section 1, 2-4 cm; Core 21, Section 3, 106-108 cm;Core 14, Section 5, 79-81 cm; and Core 11, Section3, 117-119 cm. The latter forms are characteristic ofthe late Maestrichtian Abathomphalus mayaroensisZone, which was not recorded in the underlyingMesozoic sequence.

Early Paleocene: "Globigerina" eugubina Zone or baseof Subbotina pseudobulloides Zone—Some Wood-ringina attributable to the mentioned zones are re-corded in Core 29, Section 5, 65-67 cm and, in Core29, Section 3, 142-144 cm. "Morozovella" uncinataZone—Based on the occurrence of the zonal markerin Core 29, Section 3, 62-64 cm and, Core 21, Sec-tion 3, 106-108 cm.

Late Paleocene: Planorotalites pseudomenardii Zone(= P4)—Based on the occurrence of the zonalmarker, Morozovella pusilla pusilla, M. laevigata,M. angulata, etc. in Core 38, Section 1, 2-4 cm and,Core 37, Section 4, 44-46 cm.

Late Paleocene/Early Eocene: Morozovella velasco-ensis Zone (= P5) and "Morozovella" edgari Zone(= part of P6a) are considered together because ofthe many forms in common, particularly the morozo-vellids. The latter forms are distributed throughoutthe whole Cenozoic sequence. The most commonspecies attributable to those zones are Morozovellagracilis, M. subbotinae, M. acuta, "M." aequa,"M." wilcoxensis, "Globorotalia" guatemalensis,and the zonal markers.

Middle Eocene: Hantkenina aragonensis/Globigerina-theka subconglobata Zones—Sinistrally coiled speci-mens of Morozovella aragonensis are very rarely re-corded in Core 32, Section 3, 113-115 cm. "Globi-gerinatheka" senni, one of the most common re-worked forms from the middle Eocene, could also bepartially reworked from those zones. Morozovellalehneri Zone through Truncorotaloides rohri Zone(P12-P14)—Planktonic-foraminifer assemblages rep-resentative of these zones are very common through-out the Cenozoic sequence; they mainly occur re-worked all together, and cannot be separated. The

Turborotalia cerroazulensis lineage and several spe-cies of the Globigerinatheka group are well repre-sented, associated with common "Globigerinath-eka" senni.

Late Eocene: Globigerinatheka semiinvoluta Zone (P15)—Based on the occurrence of the zonal marker inCore 32, Section 3, 113-115 cm. Turborotalia cerro-azulensis Zone, topmost part (= P17)—Based on theoccurrence of Turborotalia cerroazulensis cunialensisin Core 32, Section 1, 5-10 cm.

Early Oligocene: Zones P18/P19 (= Pseudohastigerinamicra/Cassigerinella chipolensis Zone)—The recordof these zones is attested by the occurrence of abun-dant Pseudohastigerina micra, P. naguewichiensis,P. barbadoensis, Tenuitella gemma, and T. munda inthe finest fractions of a large number of cores (Core32 through Core 22, and sporadically higher).

Early to Middle Miocene: Zone N8, and possibly ZoneN9—Rare Globigerinoides sicanus and Praeorbulinawould testify that at least a few levels attributable tothose biozones have been deposited in the source area(Cores 14 and 15). Zone N10 to Zone Nil—Thesezones are recognized on the occurrence of all evolu-tionary terms of the Globorotalia fohsi lineage, suchas G. peripheroacuta, G. praefohsi, and G. fohsi,associated with G. archaeomenardii and forms tran-sitional to G. praemenardii. They occur mainly inCore 14 through Core 6.Combining the evidences from all the analyzed plank-

tonic-foraminifer faunas, both surely reworked andhost assemblages, only a few zones are not recorded inHole 462. They are as follows (from older to younger):

Subbotina pseudobulloides Zone and "Morozovella"trinidadensis Zone, early Paleocene.

Morozovella angulata Zone, and at least the lowerpart of the "Morozovella" pusilla Zone, early latePaleocene.

Morozovella formosa Zone through Acarinina penta-camerata Zone, middle to late early Eocene.

Most of the Hantkenina aragonensis and Globiger-inatheka subconglobata Zones, early middle Eocene.

Zone N4 (upper part) through Zone N6, early to mid-dle early Miocene.

Because of the relatively poor characteristics of theplanktonic faunas, the absence of the late middle Mio-cene Zones N14 and N15 cannot be proved. Moreover,few species found in Hole 462 are attributable to thePliocene. However, faunal characteristics are such thatpossible gaps are very difficult to demonstrate.

On the contrary, the late Maestrichtian turns out tobe more complete than is apparent from the record ofthe uppermost Mesozoic cores (Cores 47 and 46), whichcould be attributed only to the Globotruncana gansseriZone (see Premoli Silva and Sliter, this volume).

Finally, it is worth mentioning that in Hole 462 somegenera are poorly represented, although the pertinentsize fractions and/or zonal assemblages are recorded.Among those genera are Globigerinoides, Orbulina,Sphaeroidinella, the Globorotalia crassaformis group,and, generally speaking, the large-keeled globorotaliids,such as G. tumida group, G. cultrata, etc. of Plio-

408


cene-Pleistocene age. Their scarcity or absence is notwell understood.

ABYSSAL AGGLUTINATEDBENTHIC FORAMINIFERS

As mentioned in the introduction, pelagic clay,devoid of carbonate, represents the only autochthonoussediment throughout the Cenozoic sequence of Hole462. This is confirmed by the occurrence throughout thesedimentary succession of agglutinated, totally non-cal-careous, benthic foraminifers, which today inhabitabyssal depths, below the CCD.

At Hole 462, they represent one of the few biogeniccomponents recorded in the pelagic clay, where they areassociated with fish debris, and locally with radiolarians(Core 18, Section 3, 113-115 cm). In constructing thetest, they agglutinate very fine particles, such as opallaminae, volcanic material, or sponge spicules (Plate 5,Fig. 4). At the binocular-microscope scale, it seems thatthey mainly use clay for cementing the particles (noanalysis has been done), which makes their tests veryfragile. Thin films of manganese coat some of them,particularly representatives of the genus Cyclammina,in Cores 18, 20, and 22.

The most common species belong to the generaReophax, Recurvoides, Cyclammina, Paratrochammin-oidesl, Haplophragmoides (Plate 5), all of which are re-corded in Core 18, Section 3, 113-115 cm, constitutingthe largest assemblage recovered at Hole 462. Althoughthey are recorded in the whole sequence, in the upperpart down to Core 17 their occurrence is random andpoor. In Cores 18 and 19, and in the interval from Core22 to Core 27, they are however present in most studiedsamples. Below Core 27, they become again more ran-domly distributed and less abundant than in the inter-mediate portion of the sequence, except in Core 36,where they are relatively common (Figs. 1-3). In manysamples, the deep-water agglutinated forms, evenstrongly diluted, occur along with large amounts of cal-careous material displaced from shallower areas, rarelyincluding shallow-water banks.

DISCUSSION OF DATAFrom the quantitative analyses, the parameters which

turned out to be of some importance are as follows(Figs. 1-3):

1) Weight of the sand fraction (>63 µm), plotted aspercentage of the total weight of the sample dried at45 °C. The values vary largely, from less than 1% tomore than 50%.

The highest values (>30%) occur sporadicallythroughout the sequence and are related to the presenceof volcanic glass (Core 6, Sections 1 and 2), shallow-water skeletal debris and volcanic-rock fragments (Core32, Section 1; Core 34, Section 1), abundant planktonicforaminifers larger than 150 µm, sometimes associatedwith few fragments of shallow-water debris.

In the samples displaying such highest values, theweight of the fractions greater than 150 µm and 250 µmmakes up more than 50% of the weight of the total

residue (>63 µm), and in some cases (Core 32, Section1) almost 90%.

Manganese micronodules, even where recorded inrelatively high percentages, are minor components anddo not specifically increase the weight of the > 63-µmfraction.

The lowest values (63-µm residues are de-void of foraminifers and consist of almost 100% radio-larians; however, the average weights range from 1 to6%. These relatively anomalous high values are relatedto the occurrence of large amounts of cenosphaeres orsimilar forms in the fraction larger than 150-µm, whichare filled with carbonate nannofossil silt.

Between those extreme values (> 10-30%), the gen-eral trend is that the weight per cent increases along withincreasing percentages of the total amount of plank-tonic foraminifers, as well as their average size. Recall-ing the patterns of samples displaying high weight val-ues, as mentioned above, the increase of the weight percent is associated with an increase of the percentages ofthe > 150-µm and >25O-µm fractions over the > 63-µmresidue. Radiolarian distribution is inversely related tothat of planktonic foraminifers: they increase when theforaminifers decrease and vice versa.

The few exceptions to the trend are mainly the conse-quence of poor disaggregation (Core 37), or related tothe occurrence of sparse, but much heavier, shallow-water debris (Core 32).

100

80

~ 60

CD

15 40

20• 1 sample16 samples• 21 samples

10 20 30> 6 3 - µ m Fraction (wt . '

40 50

Figure 8. Variation of foraminifer content with weight of the> 63-µm fraction (= sand fraction). Weight values higher than40% are relative to coarse foraminifer sand (foraminifers > 80%)and very coarse layers with shallow-water debris (foraminifers

409


2) The distribution of CaCO3, as shown in Figure 9,distinguishes two very distinct groups of samples: alarge group displaying >70% carbonates, and a secondgroup in which carbonates are totally absent or car-bonate content is less than 1 %. Plotting the CaCO3 per-centage against weight per cent of the > 63-µm fractionshows that among the samples yielding high carbonatepercentages two subgroups may be distinguished: onecharacterized by less than 10% by weight of the> 63-µm fraction, among which are a great number ofsamples displaying weight values less than 3%; and asecond subgroup, characterized by a weight per cent ofthe > 63-µm fractions higher than 10% (and up to50%). As previously demonstrated, high weight percents of the > 63-µm fraction are strictly related to highpercentages of planktonic foraminifers; in the later sub-group, the carbonate content is related to the planktonicforaminifers. In the first subgroup the high carbonatevalues, because of the low weight per cent must dependupon calcareous nannofossil content (Fig. 10).

3) P/(P + B) (plankton/[plankton + benthos])ratio of foraminifers. The plankton/benthos ratio ofthe foraminifer faunas is strictly related to the distribu-tion of planktonic foraminifers throughout the Cenzoicsequence. Except where foraminifers are minor compo-nents (< 5%) of the > 63-µm residues, the ratio is 100%or close to 100% planktonic faunas. Intermediate valuesare very rare, particularly in the > 63-µm fractions,whereas in the fractions larger than 150 µm, in whichlarge, deep-water agglutinated forms sometimes arecommon, medium values are more common. In thecases of very rare foraminifers, only benthics are found.

4) Among the other biogenic components, the fossilgroups which are of some importance are as follows:

Sponge spicules are present throughout the sequence,only in rare samples are they missing. They are of someimportance in Cores 39 and 38 (1-30%), in Core 35,CCto Core 33,CC (from 2-20%), in Core 25 (1-7%), inCore 23, Section 6, 74-76 cm, to Core 22,CC (2-7%), in

100

10 20 30 40>63-µm Fraction (wt. %)

50

60

•40

20

E3CaCO3< 60%

DCaCO3>60%

0 4 20 40 60Foraminifers (%)

80r-n-

100

Figure 9. Variation of carbonate content with weight of the > 63-µmfraction (= sand fraction). See text.

Figure 10. Distribution of foraminifer content and number of sam-ples per class of foraminifer percentages. Note the large number ofsamples very poor in foraminifers: those belong to the first sub-group described in the text and show that the high carbonate con-tent (>60%) must be dependent on the presence of calcareousnannoplankton.

Core 21,CC to Section 4, 60-62 cm (2-13%), in Core20,CC to Core 18, Section 6, 25-27 cm (1-10%), inCore 15 (5-30%), in Core 4, Section 4, 45-46 cm toCore 3, Section 2, 62-64 cm (l%-8%). High percent-ages of sponge spicules occur sporadically, as in Core31,CC, Core 30,CC, and Core 12,CC (20%), in Core29,CC (15%), and finally in Core 1, Section 2, 77-79 cm(8%). In the above-mentioned samples, foraminifers aregenerally scarce, whereas radiolarians are common toabundant. Moreover, sponge spicules may be abundantin the fractions larger than 150 µm as for instance inCore 23 or in Cores 20 through 19, where up to 48% ofthe > 150-µm residue may be spicules (Core 20, Section4, 75-77 cm).

Shallow-water larger foraminifers and skeletal debris,as described by Premoli Silva and Brusa (this volume),occurs abundantly in few layers, where it can, however,be a very important constituent. This material occurs inCore 34, Section 1 to CC; Core 32, Section 1, 1-79 cm;Core 22, Section 1, 1-75 cm; Core 21, Section 1, 1-10cm; Core 15, Section 1 to CC; and Core 14, Section 5,79-81 cm, in which shallow-water skeletal debris andlarger foraminifers are mainly associated with volcanic-rock fragments and glass. It also occurs in minoramounts at the bottom of coarser turbidites in Core 29,Section 1, 9-11 cm; Core 27, Section 3, 74-76 cm; Core26, Section 3, 49-51 cm; Core 25, Section 4, 136-138 cmand Section 1, 121-123 cm; Core 23; Core 22, Section 5,27-29 cm; Core 20, Section 4, 22-24 cm and Section 3,94-96 cm; Core 19, Section 6 and CC; Core 18,CC andSection 6, 25-27 cm; Core 17, Section 1, 6-8 cm; Core

410


16,CC, Core 11, Section 3, 117-119 cm; and finally inthe topmost part of Core 6. All the samples listed abovedisplay a very high weight of the sand fraction (>63µm). Shallow-water material is always more importantin the fractions larger than 150 µm.

Fish debris is a minor component of the residuesfrom Hole 462. It is relatively abundant only in threesamples (Core 3, Section 2, 62-64 cm and Section 1,91-93 cm; and Core 1, Section 2, 77-79 cm), in which itrepresents 19, 6, and 14% of the total residues largerthan 63 µm, respectively. Although it is present through-out the Cenozoic sequence, in all other samples, it isnever more abundant than 3% (Core 3 and upper partof Core 4). As a general rule, fish debris occurs with atleast 30% radiolarians.

Ostracodes are a very minor component of the sandfractions from Hole 462. They are never more abundantthan 1 % of the residues and occur with relatively shal-low-water benthic forms.

Diatoms at Hole 462 are very rare (< 1%) and ran-domly distributed, except in Core 25, Section 4, 64-66cm and in the core catcher of Core 28, in which theyrepresent 5 and 3% of the sandy fraction (>63 µm),respectively.

5) Among the non-biogenic components of some im-portance are volcanic ash, volcanic fragments, aggre-gates, and manganese micronodules.

Volcanic ash is common in Core 6, Sections 1 and 2,and in Core 5,CC to Section 7, 45-46 cm. It makes up50% of the sand fraction (>63 µm) as isolated glassfragments and lumps which comprise aggregate plank-tonic foraminifers and radiolarians.

Volcanic-rock fragments occur mainly in the samesamples as the shallow-water debris. Their occurrenceincreases the weight of the sand fractions considerably.

Some lumps are recorded throughout the sequence atHole 462. Their cement is mainly clayey in the upperpart of the sequence, and primarily calcitic in the lowerpart, in relationship with the beginning of diagenesis(Core 22 down).

Manganese micronodules are of some importanceonly in the uppermost cores recovered from Hole 462(Cores 1-4); up to 13% of the >63-µm fraction in Core1, Section 2, 77-79 cm and Section 5, 15-17 cm. In theunderlying cores, manganese micronodules are veryrarely recorded, even in the clayey layers. On the otherhand, manganese can coat some of the deep-dwellingbenthic foraminifers (Cores 18, 22). Where present,manganese micronodules do not significantly increasethe weight of the >63-µm residues.

6) Distributional patterns of the reworked plank-tonic foraminiferal faunas. The rapid evolution dis-played by planktonic foraminifers on which precise bio-stratigraphic scales are constructed enables one to dis-tinguish, among largely mixed assemblages, as in thecase of Hole 462, host faunas from reworked forms.Preliminary qualitative analysis was carried further, inan attempt to quantify not only the amount of reworkedspecies versus the total amount of planktonic foramini-fers per single sample, but also the amount of reworkedspecies per time interval and, where possible, per zone.

The estimated percentages can be considered relativelyreliable where planktonic foraminifers are morphologi-cally well differentiated and/or their range is short, as inthe Cretaceous through Eocene assemblages. Whereplanktonic-foraminifer assemblages are instead mainlycomposed of long-ranging species, such as most of theOligocene and Miocene faunas, estimating reworkedversus host forms is a difficult task; in this case, thecalculated percentages must be considered only tenta-tive. In a few cases, the different states of preservationcould be used to distinguish similar faunas (early Mio-cene from middle Miocene assemblages).

The youngest assemblages for which percentages areestimated are attributable to the late Miocene. Estimateswere not attempted for the Pliocene and Pleistocenefaunas, because, in our view, reliable criteria for distin-guishing them are lacking. In Figure 7, the estimateddistribution of the reworked assemblages per time inter-val and, wherever possible, per zone is plotted againstthe Cenozoic sequence recovered at Hole 462. The per-centages of the oldest reworked assemblages attribut-able to the Aptian through Cenomanian and to theearliest Paleocene "Globigerina" eugubina Zone arenot included in Figure 7, because their values are alwaysless than 1%. The total reworking curve of Figures 1 to3 can be obtained by summing all the values per time in-terval and per zone, as plotted in Figure 7.

As a general rule, when planktonic foraminifers arethe main component of the >63-µm fraction, theamount of reworking is very high: reworked forms canrepresent almost 100% of the planktonic-foraminiferfaunas. Only in a few samples do the values deviatefrom the general trend; these lower values are related tothe difficulties in estimating the reworking, because as-semblages are dominated by long-ranging species, whichin addition display a very uniform state of preservation(Core 23 samples).

The differences in age between reworked and hostfaunas increase in a regular fashion from the early Ter-tiary (from Core 39) to the Pleistocene (Core 1). LateCretaceous through late Eocene faunas, even in verylow percentages, occur as high as Core 3, dated as Pleis-tocene. Exceptions to this trend are related to the occur-rence of reworked forms of mid-Cretaceous age. Themaximum difference in age is registered in Core 6, Sec-tion 2, 140-141 cm, dated as late Pliocene, in which Ap-tian species are recorded (~ 110 m.y.).

Albian to Cenomanian species, always very rare, asmentioned above, occur also in most of the samplesfrom Core 27: in Core 26, Section 5, 94-96 cm; Core 25,Section 4, 136-138 cm; and Core 22, Section 6, 92-94cm.

Late Cretaceous, mainly Maestrichtian, faunas occurthrough most of the sequence. They are a constant, rela-tively important component of the reworked assem-blages in the lowermost cores, with values up to 32% inCore 36. Above Core 36, they become a minor compo-nent, except for three peaks, well visible in Figure 7,corresponding to Cores 29, 21, and 12, where theyrepresent 10 to 24% of the total faunas. Amountshigher than 8% are also present in Cores 32 and 26.

411


Species attributable to the "Globigerina" eugubinaZone, early Paleocene, occur in several samples fromCore 29, Section 5 to Core 25, Section 1, 121-123 cm.

Late Paleocene to early Eocene assemblages (simpli-fied in Fig. 7 as late Paleocene) are important compo-nents of the reworked assemblages up to Core 26; theycan represent more than 70% of the total assemblage(Cores 37 and 34). Above Core 26, their abundancedecreases abruptly to less than 10%, except for threepeaks corresponding to Cores 21 and 12, as registeredby the late Cretaceous faunas—plus the third one inCore 19.

Representatives of the early Paleocene "Morozo-vella" uncinata Zone occur as a minor component(63-µm residues, and particularlythe >150-µm fractions, consist almost exclusively ofcenosphaeres and other forms of similar shape, infilledby calcareous nannoplankton; also, the state of preserv-ation of the radiolarians is not uniform, the host faunabeing better preserved than the supposed allochthonousone. This qualitative information is confirmed by thequantitative analysis of the weight per cent of the>63-µm residues and their carbonate content. As al-ready mentioned above, some radiolarian-rich layers arecharacterized by (1) relatively high weight per centvalues (>l-6%), (2) a carbonate content >70%, and(3) high percentages of radiolarians in the fractionslarger than 150 µm (up to 90% of the total residue; Figs.1-3). These are the layers which yield size-sorted, pos-sibly reworked radiolarian faunas. The samples, whichare inferred to contain only autochthonous radiolarianassemblages, display weight per cent values less than 1,and they are devoid of carbonates. Examples typical ofthese two kinds of layers are shown in Figures 11 and12.

CONCLUSIONSOn the basis of the data obtained from both qualita-

tive and quantitative analyses for Hole 462, the follow-ing conclusions can be drawn.

1) From the qualitative composition and the weightper cent values of the various >63-µm fractions, theweight of the single fossil components can be rankedtentatively in the following increasing order:

Calcareous nannoplankton (>63 µm);Empty radiolarian tests smaller than 150 µm;Medium-sized radiolarians and thin-walled plank-

tonic foraminifers smaller than 150 µm;Radiolarians with nannofossil fills and planktonic

foraminifers not larger than 200 µm, generally with amedium-thick wall;

Thick-walled planktonic foraminifers larger than 150µm, up to more than 250 µm;

Small calcareous benthic foraminifers;Larger benthic foraminifers and shallow-water skel-

etal debris.Deep water agglutinated benthic foraminifers, even

those of very large size (> 500 µm) are much lighter thanthe calcareous benthic forms, and their collective weightis comparable to that of medium-sized radiolarians.

2) On the basis of the distribution of the variouscomponents in a single sequence, throughout the Ceno-zoic succession of Hole 462, different types of gradedlayers can be distinguished. The most-complete se-

412


CORE/Sectioncm | 33-2 I~ rr r

N*

1 0 0 -

50 50 50

F R

Figure 11. Hole 462, Core 33, Section 2, early Oligocene, ZonesP18/P19? The darker layer at 23 cm with poor carbonate contentrepresents autochthonous radiolarian ooze, separating two radio-larian turbidites. Weight of the > 63-µm fraction is very low. Fora-minifers are absent.

CORE/sectioncm | 39-10

50

100"

50 50

Figure 12. Hole 462, Core 39, Section 1, middle Eocene? Darkerlayers, as at 112 cm, are radiolarian oozes devoid of carbonate andforaminifers. Gray layer at 100 cm and whitish one at the top ofthe section represent respectively the finest tail and layer d) of adominantly siliceous turbidites.

413


quences are characterized as follows (from top to bot-tom):

Top (autochthonous): pelagic clay or radiolarian ooze.Layer d), transported: calcareous nannoplankton

with few small radiolarians; carbonate content >70%;weight ~ 1%.

Layer c), transported: calcareous nannoplanktonwith 80 to 30% of radiolarians and 10 to 60% of thin-walled planktonic foraminifers smaller than 150 µm;planktonic foraminifers increase downward; carbonatecontent 90%; weight 5 to 18% (basal); all planktonicforaminifers may be reworked.

Layer b), transported: 100% planktonic foramini-fers, whose average sizes increase downward; carbonatecontent >90%; weight 15 to 45% (basal); almost allplanktonic foraminifers are reworked.

Layer a), transported: almost 100% planktonic fora-minifers exceeding 250 µm, little shallow-water debris ofsmall size (Core 11, Section 3, 117-119 cm); fine frac-tion almost absent; carbonate content about 100%;weight 50%; almost all planktonic foraminifers arereworked.

The color of layers a) to d) is whitish, grading upwardto light-gray, whereas pelagic clay and/or radiolarianooze is much darker (Fig. 13). The average thickness ofthe graded portion in this type of turbidite is about 50cm. Frequently, the graded portions are thinner than 50cm; in this case layers b) and a) are missing, and thebasal layer of the sequence corresponds to layer c) (Fig.14). The extreme case is represented by the occurrenceof layer d) alone; this occurs very rarely within theCenozoic sequence, whereas it is relatively common inthe Mesozoic (Fig. 15).

Some cores, such as Core 22 through Core 29, displaya very homogeneous, whitish color for long thickness(Fig. 16); more than one turbidite is clearly represented,but they are characterized by the occurrence of only themedium layers. Detailed sampling is necessary to sepa-rate one sequence from another.

Finally, in very rare cases, radiolarians are absent,and turbidites are entirely composed of carbonate.

The types of graded sequences described above arevery common; the most complete ones occur primarilyin Cores 7, 8, 13, 14, 22, and 23.

A second type of graded layer is characterized by theabsence of planktonic foraminifers, whose place in a se-quence is completely taken by radiolarians. Below thetop layer, which can be either pelagic clay or radiolarianooze, as in the former types, two layers can be dis-tinguished:

Layer b), transported: calcareous nannoplankton andradiolarians of predominantly small size (< 150 µm); car-bonate content >70%; weight 70%; weight >3-6%, and up to 10%.

Both layers are whitish, whereas the radiolarian oozeat their top is much darker (Fig. 11). Where radiolarianooze is not present, only detailed sampling allowsseparation of the discrete sequences. The thickness of acomplete sequence seems comparable to that of the first

CORE/sectioncm | 20-4

0

100

Figure 13. Hole 462, Core 20, Section 4, late Oligocene, Zone P22. At30 to 79 cm, an example of foraminifer-radiolarian turbidite withlayer a) and part of layer b) missing. Darker layer is possibly aradiolarian ooze. Note the changes in weight percent through thegraded layer, whereas carbonate content remains constant.

type (see above). Sometimes sponge spicules occur withradiolarians, also in the > 150-µm fraction.

This second type of graded sequence occurs mainly inthe lower part of the Cenozoic sequence at Hole 462,and in the uppermost cores. This is almost the only typeoccurring from Core 39 to Core 24, and possibly in Core4 to 1.

The very coarse layers, containing large amounts ofshallow-water skeletal debris and volcanic-rock frag-

414


CORE/sectioncm i 4-4 i

0

50

50

'.3

00


CORE/section-il 29-3 J _

-

i

"I

50

50

-i

100 —i

H

150

o s?

50

E .2ro ~a

50

F R

il

Figure 16. Hole 462, Core 29, Section 3, late Oligocene, Zone P20.The slight decrease in foraminifer content from the sample at 63cm to that at 42 cm, and the oppsite trend shown by radiolarians,testifies that the sediments were not homogeneously deposited; thesuccession rather represents a set of successive turbidites.

calibration, the absolute duration of single biozones,based on those dated events, can be calculated.

The absolute ages obtained by these authors havebeen applied to the biostratigraphic events recognized atHole 462 within the Cenozoic sequence, and the ab-solute durations of the recognized biozones, and/orportions of them when they are not fully represented,are tentatively estimated. Then, the thickness of sedi-ments attributable to a single biozone divided by itsduration allows calculation of the accumulation rateduring each interval. The obtained data are plotted inTable 1.

These results show that during the Cenozoic at Site462 the accumulation rate fluctuated consistently. Thehighest values are recorded at the end of the Eocene(Zone PI7) and in the late Oligocene (Zone P22), withvalues up to 47.5 m/m.y. and 41.0 m/m.y., respec-tively. High values also occur in the early late Oligocene(Zone P20), in the latest Oligocene (Zone N4), in themiddle Miocene (Zones N12/N13), and in the Plioceneand Pleistocene. If Cores 1 to 8 are in the future totallyattributed to the Pleistocene, then for the last 1.8 m.y.the accumulation rate will be 36 m/m.y.

In some intervals, pelagic clay and/or radiolarianoozes represent important parts of sedimentary se-quences, as for instance in Cores 19 to 23-2, attributedto the late Oligocene Zone P22 (one-fourth of the totalthickness), or in Cores 1 to 8, of Pliocene-Pleistoceneage, where they are even more important (Table 1). Inthose intervals, the accumulation rate plotted in Table 1corresponds to the mean between the low rate (-10m/m.y.) relative to the radiolarian ooze and/or pelagicclay, and the much higher rate related to the transportedsediments, whose rate of accumulation increased, in thelate Eocene and late Oligocene almost doubling (lastcolumn in Table 1).

High accumulation rates are associated with the oc-currence in the same intervals of (1) common coarserturbidites, (2) coarser graded layers containing shallow-water skeletal debris, (3) a high degree of reworking,and (4) reworked faunas as old as the mid-Cretaceous.

Concerning the low rates of accumulation recordedform Hole 462, the available data—taking into accountthat transported material is still present in the intervalsdisplaying low accumulation rates—leave open thequestion whether (1) the low rates are real, (2) they arethe result of hiatuses, or (3) the absolute duration of theintervals has been overestimated.

4) The high rates of accumulations of siliceous andcalcareous materials at the bottom of the Nauru Basinare suggested to be related to important erosional eventswhich affected topographically more elevated areas, oreven islands, surrounding the deeper basin.

Two main events are recorded at Hole 462: they aredated at:

~ 37 m.y. = end of the Eocene/base of the Oligo-cene;~26 m.y. = late Oligocene.Other, less important erosional events can be dated

at:~32 m.y. = early late Oligocene (Zone P20);~24? m.y. = latest Oligocene/earliest Miocene

416


Table 1. Sedimentological data on Hole 462.

Cores

1-34-8

10-12/512/6-15

16-17/117/2-18

19-23/223/3-27/227/3-32/332/4-33

34-38

Age

PleistocenePliocene?N16/N17N12/N13N7N4P22P21P207P18/P19P17

Duration(m.y.)

1.82.23.01.02.01.01.04.02.02.01.0

Thickness(m)

28.538.026.521.011.017.041.038.048.514.547.5

ClayThickness

(m)

10.6514.152.360.181.783.255.600.250.02

4.80

Accumulation Rate(total)

(m/m.y.)

15.817.38.85

21.005.50

17.0041.009.50

24.257.25

47.50

Accumulation Rate(transported

material)(m/m.y.)

22.4329.358.74

21.245.06

20.5280.389.50

24.367.25

82.05

(Zone N4);~ 13 m.y. = middle Miocene (Zones N12/N13);~3 m.y. = late Pliocene;and during the Pleistocene.It is worth mentioning that those ages may be con-

sidered minimum ages, because they are based on theabsolute ages inferred for the planktonic foraminiferswhich have been submitted to some transport; therecould therefore be some disparity with respect to the ageof the true biostratigraphic events.

Major changes in sea level or in the bottom-currentsystem may have been responsible for these erosionalevents.

ACKNOWLEDGMENTS

The authors would like to thank the Deep Sea Drilling Project forinviting the senior author to participate aboard Glomar Challenger onLeg 61 to the Nauru Basin.

The authors are greatly indebted to G. Spezzi Bottiani of the Cen-tro per la Stratigrafia delle Alpi Centrali (C.N.R.) in Milan for histechnical assistance in preparing for quantitative analysis so largenumber of samples. The authors would like to thank also M. Gnac-colini and A. Malinverno for fruitful discussion, and S. Antico for thedrawings and technical help, A. Rizzi for operating the scanning elec-tron microscope of the Centro di Stratigrafia delle Alpi Centrali(C.N.R.) in Milan.

The present paper was critically revised by H. M. Bolli, Institute ofGeology, Federal Polytechnical School, Zurich, Switzerland, and byH. Jenkins, Department of Geology, University of Oxford, U.K.,both of whom are warmly thanked.

Financial support was provided by Consiglio Nacionale delleRicerche, through Grant No. 325071/79 to Prof. C. Rossi Ronchetti,Director of the Paleontological Institute, University of Milan.

REFERENCES

Berggren, W. A., and Van Couvering, J. A., 1974. Biostratigraphy,geochronology and paleoclimatology of the last 15 million years inmarine and continental sequences. Palaeogeogr., Palaeoclimatoi,Paleoecol., 16:1-216.

Blow, W. H., 1969. Late middle Eocene to Recent planktonic fora-miniferal biostratigraphy. Proceedings of the First InternationalConference on Planktonic Microfossils: Geneva, 1967 (Vol. 1),199-422.

Brönnimann, P., and Resig, J., 1971. A Neogene globigerinacean bio-chronologic time-scale of the Southwestern Pacific. In Winterer,E. L., Riedel, W. R., et al., Init.Repts. DSDP, 1, Pt. 2: Washing-ton (U.S. Govt. Printing Office), 1235-1469.

Hardenbol, J., and Berggren, W. A., 1978. A new Paleogene nu-merical time scale. Am. Assoc. Petrol. Geoi, Stud. Geol., 6:213-234.

Larson, R. L., and Hilde, T. W., 1975. A revised time scale ofmagnetic reversal for the Early Cretaceous and Late Jurassic. / .Geophys. Res., 80:2586-2594.

Stainforth, R. M., 1975. Cenozoic planktonic foraminiferal zonationand characteristics of index forms. Univ. Kansas Paleont. Contr.,62(1, 2).

417


i"

w1

J

* • S i *

5

I

v

ST.* -.

'-if-

•

i

%

jP

!

1

• •4

P •

- •

„

»** * ..**

2

L

i

;'

;

*

1

||

3. CENOZOIC PLANKTONIC-FORAMINIFER BIOSTRATIGRAPHY...

Documents

Transcript of 3. CENOZOIC PLANKTONIC-FORAMINIFER BIOSTRATIGRAPHY...