SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES • NUMBER 13

Distinctive Properties of

Turbiditic and Hemipelagic

Mud Layers in the

Algero-Balearic Basin,

Western Mediterranean Sea

Nicolaas A. Rupke

and Daniel Jean Stanley

SMITHSONIAN INSTITUTION PRESS

City of Washington

1974
ABSTRACT

Rupke, Nicolaas A., and Daniel Jean Stanley. Distinctive Properties of Turbiditic

and Hemipelagic Mud Layers in the Algero-Balearic Basin, Western Mediterranean Sea. Smithsonian Contributions to the Earth Sciences, number 13, 40

pages, 21 figures, 8 tables, 1974.—Two types of mud layers alternate in dominantly muddy cores of the southern Balearic Basin. Type A muds (a few cm to

over 50 cm thick), macroscopically homogeneous, occur above turbidite sands

or silt laminae. Type B muds (imperceptible to about 50 cm thick), comparatively coarse due to interspersed microskeletons, occur below turbidite sands or

silt laminae, and lie above type A muds. The two types are distinguished in Xradiographs on the basis of texture and sedimentary structures. Type A and B

mud layers in six cores were sampled at 1 to 8 cm intervals.

Type A muds are distribution graded (upward shift of the entire size distribution to finer sizes), continuing the upward grading of the underlying sand

turbidites. A granulometric change occurs at the boundary with type B muds

which contain sand (to 16 percent), largely tests of forams and pteropod shells.

The sand fraction of type A muds (< 1 percent) differs from that of type B in

the proportion of terrigenous constituents and in remains of pelagic forams

and of pteropods. Type B muds are not graded; their grain-size distribution is

uniform. They have a higher (26 to 46 percent) carbonate content than type A

(16 to 39 percent). In some instances, peak-height ratios of clay minerals change

across the boundary between type A and type B mud layers. It is concluded that

type A muds are turbiditic (deposited instantaneously), while B muds are hemipelagic deposits.

Carbon-14 ages were determined on the carbonate sand fraction of type B

layers. The ages were plotted against the total sediment thickness above the

dated samples in each core. A statistically significant correlation exists. However, when the turbiditic sand and mud layers are omitted and the ages are plotted

only against the combined thicknesses of the hemipelagic type B layers, an even

stronger correlation is obtained. The hemipelagic rate of sedimentation during

the past 16,000 years approximates 10 cm/1000 years. The frequency of turbidity current incursions at a particular core location averages 3 per 2000 years.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded

in the Institution's annual report, Smithsonian Year. SI PRESS NUMRFR 5089. SERIFS COVER DESIGN:

Aerial view of Ulawun Volcano, New Britain.

Library of Congress Cataloging in Publication Data

Rupke, Nicolaas A.

Distinctive properties of turbiditic and hemipelagic mud layers in the Algero-Balearic Basin,

western Mediterranean Sea.

(Smithsonian contributions to the earth sciences, no. 13)

Supt. of Docs, no.: SI 1.26: 13.

1. Marine sediments—Western Mediterranean. 2. Turbidites—Western Mediterranean. I. Stanley, Daniel Jean, joint author. II. Title. III. Series: Smithsonian Institution. Smithsonian

contributions to the earth science, no. 13.

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Contents

Page

Introduction 1

Geographic and Geologic Setting 3

Core Locations 5

Lithologic Description of Cores . • 5

General 5

X-radiographic Identification of Mud Layers 9

Variations within Cores 9

Variations between Cores . 9

Color of Sediment 10

Analysis of Rhythms 11

General 11

Analytical Procedures 12

Grain-Size Distribution 15

Carbonate Content 19

Composition of the Sand Fraction 21

Clay Mineralogy 27

Discussion 28

Types of Depositional Mechanisms . 28

Mud Deposition—General . 30

Turbiditic Mud Deposition—Type A Muds . . 30

Hemipelagic Mud Deposition—Type B Muds 31

Normal Bottom Current Activity 31

Mud Turbidites and Contourites 31

Ancient Flysch and Flysch-like Successions 32

Radiocarbon Ages and Rates of Sedimentation 32

Conclusions 35

Literature Cited 36

i n
Distinctive Properties of

Turbiditic and Hemipelagic

Mud Layers in the

Algero-Balearic Basin,

Western Mediterranean Sea

Nicolaas A. Rupke

and Daniel Jean Stanley

Introduction

Graded sands and sandstones, alternating with

fine-grained layers, form thick accumulations in

modern deep-sea basins and in ancient geosynclinal

belts, where they are called flysch. The deep-sea

sands are generally believed to be the products of

turbidity currents that transport coarse-grained

sediments to the deep marine environment, smooth

out topography, and construct abyssal plains (cf.

summary by Horn, Ewing, and Ewing, 1972). A

compelling amount of evidence points to a turbidity current emplacement of most flysch sandstones as well, in spite of some remaining problems

(cf. review by Kuenen, 1967).

The flysch-type sands and sandstones exhibit a

characteristic assemblage of internal and bedding

plane structures. The internal sedimentary structures occur in a fixed succession of structural intervals or divisions, viz., a basal, graded division (a),

a lower division of parallel lamination (b), a division of current ripple lamination (c), an upper

Nicolaas A. Rupke and Daniel Jean Stanley: Division of Sedimentology, Department of Paleobiology, Smithsonian Institution, Washington, D.C. 20560. Rupke, present address:

Department of Geology and Mineralogy, Oxford University,

Oxford 0X1 3PR, Great Britain.

division of parallel lamination (d), and a largely

structureless, fine-grained or pelitic division (e)

(Bouma, 1962, fig. 8). This succession of sedimentary structures, defined from ancient flysch sandstones, applies to modern deep-sea sands as well

(cf. review by Walker, 1970). The interpretation

of the flysch-type sands and sandstones as turbidity

current deposits or turbidites is to a significant

extent based on these sedimentary structures and

their reproduction in experimental flume studies

(summarized in Middleton, 1965).

In the fine-grained division (variously called

pelite,1 lutite, clay or shale layer, argillite, and

mud2 or mudstone layer), primary sedimentary

structures are virtually absent. Accordingly, the

question of the origin of the pelitic division is still

largely unresolved and has often remained unaddressed in many publications on turbidite sequences.

In most of the early, often classic, papers on turx

 In this study we use the term "pelite," defined as a "sediment or sedimentary rock composed of the finest detritus

(clay- or mud-size particles)" (AGI, Glossary of Geology,

1972).

2

 Mud is defined as "an unconsolidated sediment consisting

of clay and/or silt, together with material of other dimensions (such as sand), mixed with water, without connotation

as to composition" (AGI, Glossary of Geology, 1972).

1
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

bidites it was tacitly assumed, or sometimes argued,

that most or all of the pelitic layers represent the

normal, pelagic settling, interrupted at times by the

instantaneous emplacement of turbidite sands (e.g.,

Sujkowsky, 1957; Dzulynski, Ksiazkiewicz, and

Kuenen, 1959; Gorsline and Emery, 1959; Stanley,

1963). More recent studies of turbidite sequences

have also implied that the pelitic layers represent

normal, pelagic settling (e.g., Cline, 1970; McBride,

1970; Moore, 1971).

However, during the past decade or so the conviction has been growing that at least a portion

of the mud layers in the modern deep sea is deposited by turbidity currents. Van Straaten (1966,

1967, 1970) recognizes turbiditic mud layers in the

southern Adriatic Sea. Van Andel and Komar

(1969) document resedimentation by turbidity

currents of pelagic oozes in valleys on the flanks of

the Mid-Atlantic Ridge. A considerable fraction of

the thick, largely muddy wedges forming base-ofslope deposits are also deposited by turbidity currents (Stanley, 1969, 1970). Piper (1973) describes

silts and muds of turbiditic origin from the Gulf

of Alaska.

Evidence that at least part of the pelitic division

(e) is turbiditic in origin is provided mainly by the

faunal content. Instances have been found in which

only the upper part of the pelitic layer contains a

relatively high percentage of skeletal remains of

pelagic and deep benthonic organisms (Ksiazkiewicz, 1961; Natland, 1963; van Hinte, 1963; Weidmann, 1967; Weiler, 1967; Comas, 1968; Davies,

1968; Tanaka, 1970; and others). This upper part

of the pelite is, in several of these instances, of a

different lithology and color (Ericson, Ewing, and

Heezen, 1952; Bornhold and Pilkey, 1971; Huang

and Pierce, 1971). Graptolites in the pelite have

been shown to be oriented parallel to the sole

markings of the underlying turbidite sandstones

(Moores, 1969). Textural evidence for turbidite

deposition, such as grading in the lower part of the

pelitic layer, and grading throughout the entire

pelite division, have been found as well (Radomski,

1958, 1960; Piper, 1972; Rupke, 1972; Hall and

Stanley, 1973). Other evidence for the turbiditic

origin of pelites comes from various statistical

analyses of bed-thickness properties (Enos, 1969;

Rupke, 1969).

In some publications the pelitic division has been

regarded as almost entirely turbiditic, with little

or no pelagic sediments forming the top part

(Nesteroff, 1961; Bouma, 1962; Nesteroff and

Heezen, 1963; Rusnak and Nesteroff, 1964).

In their reviews of ancient and modern flyschtype sequences, Dzulynski and Smith (1964) and

Kuenen (1964) concluded that in flysch-type sequences the pelites are dominantly turbiditic in

origin, and that the truly pelagic parts are insignificant in volume. In a number of publications it

has simply been assumed that some (lower) parts

of the pelitic layers are turbiditic, while other

(upper) parts are pelagic, without specification of

criteria (Nederlof, 1959; Holtedahl, 1965; Walker,

1967; and others).

In recent years, other depositional mechanisms

for fine-grained sediments have been proposed, in

addition to pelagic settling and turbidity currents.

Ewing and Thorndike (1965) observed a nepheloid

layer of suspended fine-grained sediment on the

continental slope and rise of northeastern America,

which may play a role in resedimentation of muds.

Similar low-density and gravity-assisted flows carrying fine-grained sediments to deep marine environments have been proposed. These can be of

permanent nature or can be seasonally reactivated

during river flooding or scouring of the outer shelf

margin by storms (Moore, 1969; Bartolini and

Gehin, 1970; Stanley, Gehin, and Bartolini, 1970;

Stanley, Sheng, and Pedraza, 1971; Fleischer, 1972;

Huang and Stanley, 1972).

Normal bottom currents, in particular geostrophic contour-following currents (Heezen, Hollister, and Rudiman, 1966), are thought to play

a role as well in deep-sea sediment transport and

deposition. Deposits by these currents, called contourites, are described by Wezel and Ryan (1971)

and Bouma (1972a, 1972b) in ancient flysch

sequences; Bouma (1972a, 1972b) tabulates the differences between turbidites and contourites. These

bottom currents are capable of reworking sands

(Hubert, 1964), but are invoked mainly for the

transport and deposition of very fine sand and silt.

In summary, there is evidence for several depositional mechanisms affecting fine-grained sediments

in both modern and ancient deep marine environments. The question, however, of how much of the

pelitic layers in turbidite sequences is accumulated

by pelagic settling, and how much by turbidity

currents, bottom currents, or other depositional

mechanisms remains largely a matter of doubt and
NUMBER 13

dispute. The present study addresses itself to this

question of the origin of mud layers and constitutes

an attempt to define, from an analysis of different

types of mud, criteria that may indicate a particular

mechanism of deposition. This investigation has

been carried out using sediment cores from the

southern part of the Balearic Abyssal Plain in the

western Mediterranean.

Recognition of the depositional mechanisms

of deep-sea muds is not only of significance for the

understanding of deep marine processes of sediment

distribution, but is also essential for studies of the

chronology and paleoclimatology of deep-sea cores.

ACKNOWLEDGMENTS.—We are indebted to the U.S.

Navy for providing shiptime and facilities aboard

the USNS Lynch on its cruise (LY-II-72) in

March-April 1972, and for furnishing the cores

and 3.5 kHz records used in this study. In particular, we thank the officers and crew of the Lynch,

and scientific personnel including students from

Williams College and the University of Illinois for

their assistance aboard ship. Mr. Harrison Sheng,

Smithsonian Institution, assisted with various laboratory analyses. The paper has been critically read

by Drs. A. G. Fischer, Princeton University, H. G.

Reading, Oxford University, and L.M.J.U. van

Straaten, Rijksuniversiteit, Groningen; we appreciate their constructive criticisms and suggestions.

Funding for this study, part of the ongoing Mediterranean Basin Project, was provided by the

Smithsonian Research Foundation grant (DJS)

FY73-430035. This investigation was undertaken

while Rupke held a postdoctoral fellowship at the

Smithsonian Institution.

Geographic and Geologic Setting

The Balearic Sea, also referred to as the AlgeroProvenc_al Basin, occupies most of the western

Mediterranean. It extends from the Ligurian Sea

in the northeast and the coast of southern France

in the northwest, southward between CorsicaSardinia and the Balearic Islands, down to the

coast of North Africa. Cores examined in this

study were collected in the southern part of the

Balearic Sea, known as the Algero-Balearic Basin

(Figure 1). The present geographic configuration

has been explained in terms of rifting, lateral

spreading, and rotation of microblocks (Iberia, the

Balearic block, and the Corsica-Sardinia block)

(Ritsema, 1969; Hsu, 1971; Vogt, Higgs, and

Johnson, 1971; Auzende, Bonnin, and Olivet, 1973;

Dewey, et al., 1973). In addition to lateral spreading, evidence for vertical tectonics has been presented (Bourcart, 1959; Pannekoek, 1969; van

Bemmelen, 1969).

The Balearic Basin is bounded by narrow

shelves, platforms, and continental slopes that, for

the most part, exceed 5°. These are in many places

incised by submarine canyons (Bourcart, 1959).

They border a large, flat basin floor, called the

Balearic Abyssal Plain, the largest basin plain in

the Mediterranean Sea. The 2600 m bathymetric

contour serves to delineate the abyssal plain (Carter

et al., 1972).

The plain, a depositional feature, forms the

surface of a stratified sedimentary fill, of which

the thickness is estimated from subbottom profiles

at 300 to 450 m (Horn, Ewing, and Ewing, 1972).

The total thickness of the Plio-Quaternary unconsolidated sediment fill reaches to 1200 m in the

vicinity of the Rhone Cone (Menard, Smith, and

Pratt, 1965; Watson and Johnson, 1969; Ryan et al.,

1969). Piston-coring surveys show that the sediment

underlying the plain comprises mostly mud with

some alternating graded and laminated sands and

silts. JOIDES drilling in the Balearic Basin (Leg

13, sites 124, 133, 134) reveals thick deposits of

Quaternary and Pliocene age, underlain by a

Messinian evaporite sequence (Ryan, Hsu, et al.,

1937a,b). For instance, at site 124 situated off the

eastern margin of the Balearic rise (Figure 1), a

thick sequence of rhythmically bedded sediments

has been recovered. Its lower part (200 m), largely

of Pliocene age, contains light- and dark-colored

marl oozes and sand-silt laminae, which form the

bases of the dark oozes. The upper part (160 m), of

Quaternary age, contains alternating graded sandsilt layers and marl oozes. Nesteroff (1973) and

Nesteroff, Wezel, and Pautot (1973) interpret the

lowrer section as contourites, and the upper section

as turbidites (cf. Nesteroff et al., 1972).

The abyssal plain surface has a slight gradient

away from the main source of terrigenous sediment

and toward its deepest part (2800 m) lying west

of Sardinia. The most important terrigenous sediment source is the river Rhone, the mouth and

submarine fan of which are located in the northwestern part of the basin (Menard, Smith, and

Pratt, 1965; Horn, Ewing, and Ewing, 1972). The
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

0 SO 100 KM

0°00' 05°00'

FIGURE 1.—Locations and numbers of seven cores taken from the Balearic Abyssal Plain (hiring

March-April 1972 aboard the USNS Lynch. The isobaths are in meters. The 2600 in isobath

delineates the abyssal plain. JOIDES leg 13 site 124 and core 210 of the Swedish Deep-Sea

Expedition are also shown.

Rhone has been the main source of sediment since

the Pliocene foundering of the northern Mediterranean margin (Bourcart, 1959), and probably

earlier. Another important source of fluvial sediments is the North African margin (Leclaire, 1972a,

1972b; Vita-Finzi, 1972); much of these materials

bypass the narrow shelf and move downslope, in

some cases via canyons, toward the plain surface.

To a lesser degree, sediments are also derived from

the Iberian Peninsula, the Balearic Platform, and

the Strait of Sicily. River input is to a large degree

seasonally controlled; late fall to spring rains and

snow-melt result in increased discharge during these

periods (Semple, 1931). Seasonally shifting wind

patterns also modify the volume of aeolian dust

transported off the North African margin (Eriksson,

1965).

The movement of the upper two water masses

[surface and intermediate or Levantine water

(Wust, 1960; Lacombe and Tchernia, 1972)] are

influential in the movement of fines and planktonic

organisms in suspension. Little is known of bottom

water flow and velocities (Miller 1972). Bottom

photographs made in this region do not indicate

evidence of significant bottom current activity. The

role of large-scale vertical sinking of surface water

in the northern Balearic Sea (Stommel, Voorhis,

and Webb, 1971) as a significant mechanism in

sediment transport to the basin plain has yet to be

evaluated.

Earthquakes play an important role in triggering turbidity currents, slumps, and other mass

gravity failures along the steep, narrow margin of

this region. The area is seismically active (Bara-
NUMBER 13

zangi and Dorman, 1969; McKenzie, 1970); a line of

earthquake epicenters trends from the Azores to

the Strait of Gibraltar eastward across North Africa

to Sicily. The Orleansville earthquake of 1954,

for instance, triggered a large turbidity current in

the southwestern sector of the Algero-Balearic

Basin, an occurrence well documented by cable

breaks (Heezen and Ewing, 1955; Horn, Ewing,

and Ewing, 1972).

The structural mobility of the region also affects

sedimentation in other ways. Numerous salt dome

diapirs (Watson and Johnson, 1969; Ryan et al.,

1969) of Messinian age (Ryan et al., 1973) occur

across much of the abyssal plain. Salt tectonics has

disturbed the Quaternary sediment succession, including the uppermost Holocene strata in many

places, particularly in the vicinity of 2° longitude

(Stanley and Diester-Haas, 1973). Elongate salt

knolls deflect sediment dispersal along the plain

surface and locally act as dams against which stratified deposits are ponded (Figure 2).

Core Locations

During the March-April 1972 cruise of the USNS

Lynch (LY-II-72), seven Ewing piston cores were

retrieved from the Balearic Abyssal Plain at seven

different stations in the Algero-Balearic Basin. The

cores are coded from east to west LY-II-7, -8, -8A,

-9, -9A, -10, -10A (Figure 1). Stations were selected

for deepwater light scattering experiments, and

not with a sedimentary basin analysis in mind. The

cores, therefore, constitute random sediment

samples from the Balearic Abyssal Plain.

TABLE 1.—Location, water depth, and length of

sexien cores obtained from the Balearic Abyssal

Plain on the USNS Lynch March-April 1972 cruise

(see Figure 1)

Core

no.

7

8

8A

9

9A

10

10A

iMtitude

38°02.7'N

37°58.8'N

37°56.0'N

37°47.3'N

37°47.0'N

37°31.FN

37°32.0'N

Longitude

8°09.2'E

6°11.0'E

5°41.0'E

3°43.0'E

3°00.9'E

1°18.9'E

O°33.5'E

Water

(m)

2588

2719

2712

2674

2676

2655

2628

depth

{fm)

(1415)

(1487)

(1483)

(1462)

(1463)

(1452)

(1437)

Core

length

(cm)

342

428

382

513

444

445

461

The ship track, along which almost continuous

12 and 3.5 kHz echo-sounding records were obtained, extends from the western sector of the Strait

of Sicily to the region south of the Balearic Islands

and the southeastern coast of Spain in the vicinity

of the Mar Menor. This east-west trending line

constitutes an almost complete section across the

Algero-Balearic Basin, and is oriented approximately at right angles to the main directions of

sediment supply (except for cores 7 and 10A).

Core location, length, and uncorrected water

depth are listed in Table 1. Total core lengths

range from 342 to 513 cm; the average core length

is 431 cm. The water depth ranges from 2588 to

2719 m. The position of cores 8 to 10A, as indicated on echo-sounding records, is shown on Figure 2.

Core LY-II-7 was collected on the lower fan surface of the Strait of Sicily Fan, just south of a levee

bounding the southern margin of the Strait of

Sicily Canyon.

Core LY-II-8 was retrieved on the almost horizontal basin plain surface, while LY-II-8A was

cored in a wedge of stratified sediment ponded behind an emerged salt dome.

Cores LY-II-9, -9A, and -10 were collected in a

zone of distinctly stratified sediment between salt

domes south of the Balearic block; evidence of salt

tectonism such as rim synclines, arching of strata,

faults, and grabens occur in the immediate vicinity.

Core LY-II-10A was retrieved east of the Mazzaron Escarpment and just beyond the Mar Menor

Fan; the plain surface has a low gradient, and lies

in a structurally active region in the westernmost

Algero-Balearic Basin.

Lithologic Description of Cores

GENERAL

The seven cores consist predominantly of muddy

sediments. Visual examination of split cores shows

that much of the core sections appears homogeneous (Figure 3). Lamont-Doherty cores taken in

the same general area have been interpreted as

consisting largely of homogeneous mud (Horn,

Ewing, and Ewing 1972). In most of the cores, a

few sand-silt and silt intercalations stand out as a

result of textural and color differences (Figure 3).

The cores w

7

ere initially cut in 2 m sections and
s s

3 S

x P

s .

O Q

X S
NUMBER 13

t

8A-18 10-5 10A-19 9A-7

FIGURE 3—Photographs of split-core sections containing typical successions of turbidite sand-silt

layers (8A-18, 10-5) or silt laminae or lamels (10A-19, 9A-7), and two types of mud layers,

A(el) and B (ep). Note the absence of clearly visible layer boundaries within the muds. The

interpretative column on the left is largely based on X-radiographs of these sections.. The first

part of the section numbers refers to a core, the second to a rhythm within a core.
o

o

3

8-7 10-5 8-20 8A-11

FIGURE 4.—X-radiograph prints and interpretative column of five rhythms of turbidite sand-silt

layers or silt laminae, followed by largely homogeneous type A mud (e'), which in turn is

followed by type B mud (ep) exhibiting light speckling. The lower part of type A layers can

be finely laminated (10-5, 8-20, 10A-15). One rhythm (8A-11) contains no type B layer. In

the turbidite sand-silt layer in rhythm 10-5, note current ripple lamination between two co-sets

of lower and upper parallel lamination. Also note in type B mud layer two horizons of extra

light speckled bands. The distinct horizontal lines in 8-7 and 10-5 are artifacts produced by

the joining of two different photographic prints.
NUMBER 13

kept in cold storage, split, color logged, photographed, and X-radiographed before sediment

samples were taken for detailed analysis.

Figures 3 and 4, many of these lithologic boundaries would go unnoticed without the aid of

X-radiographs.

X-RADIOGRAPHIC IDENTIFICATION OF MUD LAYERS

The X-radiographs of split-core sections reveal

that the sand-silt and silt layers exhibit several of

the structural characteristics associated with turbidites. All layers are base-cutout successions of

Bouma divisions, viz., Tu-c-d, Tc_d, or Td (Figure

4). The lowermost graded a-division is never present. The base of these layers is sharp, and finingupward grading is macroscopically visible in the

split cores. These layers are interpreted as typical

turbidite sands or silts.

In addition to these turbidite sand-silt layers, the

X-radiographs reveal a large number of distinct

mud layers, many of which are very difficult to recognize in the split cores (Figure 3). These more

subtle layers overlie sand-silt turbidites, or they

begin with a dark line on X-radiograph prints,

constituting silt laminae (less than 1 cm thick) or

lamels (less than 1 mm thick). The lower boundary

of the coarser grained lower laminae or lamels is

always sharper defined than the upper. Two kinds

of mud occur above these layers, as well as on top

of the turbidite sand-silt layers. Type A mud, which

appears darker, lies directly on top of the coarser

layer below. Its lower portion can exhibit delicate

lamination. Type B mud appears lighter and is

distinguished by light specks; it lies on top of type

A mud and precedes the coarser layers (Figure 4).

The speckling is produced by interspersed microskeletons of foraminifera and pteropods in the

mud.

The boundary between mud types A and B is

often gradual and in some instances blurred by

mottling and other evidence of bioturbation. The

upper portion of type A mud layers can contain

tubes consisting of FeS2 (Figure 18F). Such pyritized filaments are reported by Hesse and von Rad

(1973) in sediments from the Strait of Otranto.

These, readily apparent in X-radiographs, probably

result from burrowing organisms.

Delicate and sometimes repeated scraping of the

wet sediment surface of the split cores is necessary

to observe the boundaries of the mud layers

identified in X-radiographs. As can be seen from a

comparison of core section 10-5 illustrated in

VARIATIONS WITHIN CORES

Vertical variation within the cores consists of

a rhythmic succession of three members, viz., (1)

the coarser grained turbidite sand-silt layer, or the

silt lamina or lamel, (2) the type A mud layer, and

(3) the type B mud layer. A unit consisting of any

of the three members is herein called a rhythm. In

some rhythms the lower coarser grained member

is absent, or the upper type B mud member is

missing, or both. In general, four types of rhythmic

successions of the three members occur in the cores

(Figure 5).

sand-silt

1

FIGURE 5.—Four types of rhythmic successions of three

member layers (sand-silt, and two types of mud) as found

in seven cores from the Balearic Abyssal Plain (see Figure 6).

The average thickness of turbidite sand-silt

layers is 4.5 cm; of type A mud layers 17.5 cm; and

of type B mud layers 10.5 cm. The maximum

thickness of type A mud is 47.5 cm, and the minimum thickness 1 cm. For type B mud, these values

are respectively 53.5 cm and imperceptibly thin.

The measuring accuracy varies, but is approximately 0.5 cm for the mud layers and can be 0.1

mm for the sand-silt layers.

VARIATIONS BETWEEN CORES

The sand-silt or silt members, type A muds, and

type B muds are depicted in Figure 6; thickness

data are listed in Table 2. The cores contain widely

varying numbers of rhythms, viz., from cores

LY-II-7 to 10A respectively: 25, 23, 22, 12, 18, 13,

and 25. Their average thicknesses vary accordingly

(Table 2; Figure 6).
10 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 2.—Number, thicknesses (in cm), and thickness statistics of sand-silt turbidites (Sa-Si), type A mud layers (interpreted as turbiditic or e

f

, and type B mud

layers (interpreted as hemipelagic or e

p

), in seven cores (7-10A) from the Balearic

Abyssal Plain (the layer succession is listed from left to right, and upward, in

each core column)

Rhythm No.

26

25

24

23

22

21

20

19

18

17

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

Average

thickness

% of total

thickness

Sa-Si

0.2

0.2

0.2

0.4

0.3

0.2

0.2

0.2

0.2

0.5

0.3

0.2

0.2

0.1

0.2

1

7

5

10.5

8.5

6

11

3

4

4

5.5

8

5.5

3.5

10

8.5

5

4

3

1

4.5

3

2

2

2

5.5

2

5

37

(eP)

3.5

5

5.5

2

3

9.5

38

15

23

11

9

2.5

44.5

3.5

16

6

6

1.5

8

11

62

Sa-Si

3.5

16

12

10

13

2.5

1

4

8

0.3

3

8.5

10

2.5

6.7

22

8

(e*)

8

14.5

6

11.5

16.5

7.5

8

6.5

10

6.5

14

9.5

17.5

1

10

13

17.5

1.5

1

13

10

7.5

21

10

54

(e-)

2

8.5

6.5

5

1

3.5

3

4.5

14

7

3

8

14.5

5

3

2.5

11

6

24

Sa-Si

7

8

14

5.5

8

0.7

1.5

10.5

0.7

5.5

18

7

0.7

6.7

23

8A

(el)

19

13.5

9

13.5

21

12

7.5

9.5

8

12

13

10

10.5

12

10.5

16.5

1

11

11.5

1

1.5

1

10.5

59

(ep)

2.5

3.5

7.5

3

3.5

3.5

9

7.5

3.5

6

5

6

1

4.5

2

4

18

Sa-Si

0.7

0.2

3

0.8

7

26

6.3

7

9

(e*)

11.5

16

17

45

41.5

17

18.5

22

45.5

13

47.5

10.5

25.5

60

(e»)

44.5

18

7.5

5

38

12.5

5

3.5

36

19

33

Sa-Si

4

0.5

2

2

0.4

0.3

0.5

1.4

2

9A

(e*)

5

10

9

11

8.5

20.5

45

17

10

4

6

12

21.5

13.5

23

23.5

2

13

14

58

(ep>

16

14.5

25.5

1

23

3

8.5

6.5

17.5

27.5

2.5

2

5.5

4

1

2.5

12

4.5

3

9.5

40

Sa-Si

1.2

11

7.5

9

0.2

4

8.5

2.5

1.5

3

4.6

10

10

(e*)

21

12

11.5

13.5

23.5

35.5

28

24

18.5

10.5

15.5

14

15

18.5

55

(eP)

26

29

1

16

9

4

25.5

23.5

7

5.5

2.5

5

13

35

Sa-Si

2.5

2

5

0.3

0.5

6

33.5

1

2

0.8

0.3

0.4

1

55.2

4.7

12

10A

(e»)

5.5

5.5

9

2

11.5

4

15.5

9.5

3

2

1

5

4

9

1.5

12

2.5

3

18

1.5

7

2.5

2

1

4

5.5

31

(ep)

42.5

0.5

11

12.5

53.5

6.5

3.5

2

5.5

4

3.5

7.5

6.5

16

1.5

3

3.5

7.5

1

23

4.5

31

14

11.5

57

The ratios of the three different members change

markedly from core to core, across the AlgeroBalearic Basin. These changes correlate roughly

with water depth. This is schematically represented

in Figure 7, where the proportions of the different

sediment types (B) are plotted along a bathymetric

profile (A) from station 7 to station 10A. In cores

at greater depth (8 to 10) the proportion of type

A mud is more than 50 percent. In cores at shallower depths (7 and 10A) the percentage of type

B mud is more than half (Table 2).

Another possible correlation is to be noted, viz.,

between lithology and access to fluvial sediment

sources. Cores 8, 8A, 9, and 9A occupy positions

north of the important oueds on the margin of

North Africa (Figure 1).

COLOR OF SEDIMENT

The sand-silt turbidites, when terrigenous in

composition, are olive gray (5Y 4/1) in color. Bioclastic layers are yellowish gray (5Y 7/2). Type A

mud is light olive gray (5Y 5/2) to dusty yellow

(5Y 6/4), whereas type B mud is somewhat lighter

in color, namely, light olive gray (5Y 6/1) to yellowish gray (5Y 7/2). Color differences have been

used to define mud layers (Bornhold and Pilkey,

1971; Davies, 1968; Fleischer, 1972; and others).
NUMBER 13

10 A 10 9A 8A

11

2—

5—'

n Dating Sample

lOpelagic Mud

M u d

 JTurbiditic

Sand-Silt]

5,000 year Isochrons

FIGURE 6.—Distribution of rhythms (numbered on left side of cores) consisting of turbidite

sand-silt layers and two kinds of mud layers, type A (interpreted as turbiditic) and type B

(interpreted as hemipelagic) in seven cores from the Balearic Abyssal Plain. Spacing between

these cores corresponds to the distances between their respective locations. The position and

extent of samples for radiocarbon dating and the corresponding ages are indicated. Isochrons

are drawn between the cores, based on a calculation of the hemipelagic rate of sedimentation.

Note that in core 10 the radiocarbon dates are determined not on type B muds (interpreted as

hemipelagic), but on resedimented turbidite sands, causing a dicrepancy between these dates

and the ages indicated by the isochrons.

However, a comparison of color logs with Xradiography data shows that color is not a reliable

boundary criterion. Color changes in the cores

often do not coincide with layer boundaries, particularly the boundaries between type A and type B

layers. The boundary area can display mottling or

can be uniform in color. A distinct color change is

present within the type A mud member, some 15

cm below its boundary with type B mud in rhythm

18 of core 8A (Figure 3). Such discrepancies may

be the result of burrowing activity.

Analysis of Rhythms

GENERAL

Eight individual rhythms were selected from six

of the cores (7, 8, 8A, 9A, 10, 10A) for detailed

sampling and analysis. The main purpose of the

analysis was to establish and to contrast the properties of mud types A and B in order to gain insight

into their mode of origin. The rhythms are labeled

by the number of the core and the number of the

rhythm within that core. The following rhythms
12 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

100

100 km

B

50-

(hemi)pelagic

mud

10 A 10 9A 7

FIGURE 7.-—A, An east-west schematic bathymetric profile across the Balearic Abyssal Plain from

station 7 to 10A. B, Proportions of total turbidite sand-silt (black), total type A mud

(hatched; interpreted as turbiditic), and total type B mud (white; interpreted as hemipelagic).

Note the rough correlation between water depth and proportion of turbidite sand-silt-mud.

were selected: 7-11, 8-7, 8A-11, 8A-18, 9A-7, 10-5,

10-12, and 10A-19.

The rhythms were selected so as to represent

variety in terms of spread between the cores, spread

within the cores, thickness of the rhythms, thickness

of the rhythm members, and type of rhythm. The

thickness of the sampled rhythms ranges from 11.5

cm (8A-11) to 49 cm (7-11); the average thickness

is 29 cm. The average thickness of type A mud

layers is 13 cm and that of type B layers 10 cm

(Table 3). All four types of rhythms (Figure 5)

are represented. Two have a thick type A mud

member (8A-18, 9A-7), and one has a thick type

B mud member (7-11) (Table 3).

TABLE 3.—Thickness (in cm) of eight individual

rhythms of sand-silt inrbidites, type A (eJ) and type

B (ei}) muds, which were selected for detailed

sampling and analysis

Core

no.

7

8

8A

8A

9A

10

10

10A

Rhythm

no.

11

7

11

18

7

5

12

19

Average thickness

Sa-Si

0.2

8.5

1.5

14

0

8.5

11

0.5

5.7

A(e')

5

17.5

10

21

21.5

10.5

11.5

9.5

13

B (ep)

44.5

8

0

7.5

5.5

7

1

6.5

10

Toial

49.2

34

11.5

42.5

27

26

23.5

16.5

28.8

ANALYTICAL PROCEDURES

Samples were taken from the split-core halves.

The sample thickness ranges from 3 to 8 mm. The

distance between samples varies from 0.5 to 8 cm.

A total of 59 samples has been analyzed. For the

sttidy of clay mineralogy, four extra samples were

selected.

The selection of exact sample points was based

on the structural and textural details provided by

the X-radiographs of the rhythms. The advantage

of this sampling technique is that no subtle changes

in sediment structure or texture were overlooked.

It also makes possible a more precise correlation

between the sedimentary structures of the sediment

sections and the results of the textural and compositional analyses.

The following analytical procedures were carried out on all samples: (1) a grain-size determination by pipette analysis; (2) a determination of

carbonate percentage by gravimetric ascarite absorption; (3) a compositional analysis of the sand

fraction of all samples by grain-mount counting;

(4) a study of the overall composition, and in particular of the clay mineralogy, by X-ray difrractometry and peak-height analysis.

The results are presented for each rhythm

separately (Figures 8-15). Each figure is composed

of five columns, the first of which provides the

sedimentary structures (Bouma divisions) based on

the X-radiographs. In this column the lower mem-
n 0>

3 cT

rain Size Distrib

y?. Q

Carbonate

Content

lomposition of Sand

O

O CD

r-t9> o

o

o

Q.

Co

7-11

Clay Mineral

Ratios

4 0 -

7-

6-

30-

5-

2 0 -

10-

3-

0

J

2-

1 -

30 6 0 9 0°/oO 30 60°/o 0 3 0 6 0 9 0 %

\i

i

00

o

5

w

o

rr

o"

1

 \

w

\

\

oo

CD

cn

VCD

FIGURIS 8-15—Diagrammatic summaries of the sedimentary structures and textural and compositional properties of eight rhythms from six cores from the Balearic Abyssal Plain. The

coarse-grained material in the top part of rhythm 8A-11 may be due to burrowing from above

rather than representing a separate type B mud layer. Sample numbers marked with "a" and

"b" were used only in the study of clay mineralogy.
14 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

n co

3 0-

20-

10-

Oomposition of Sand

TJ £

o o <* C

Grain Size Distrib

Clay Mineral

Ratios

Carbonate

Content

Mica3-.-.-.piants

30 6 0 9 0%0 30 60% 0 JO 60 90%

FIGURE 9.

ber of each rhythm (turbidite sand-silt layer or

silt lamina) is indicated by Bouma divisions b, c,

and d, while in anticipation of their genetic interpretation the middle (type A mud) and upper

member (type B mud) are indicated by e1 and e"

respectively. In the three following columns, sample

points have been connected by straight lines, except

where they lie on both sides of the mud A-B boundary. Where in X-radiographs this boundary is

clearly denned, the lines are extrapolated from

above and below and display the changes of texture

and composition in the boundary region.
NUMBER 13 15

GRAIN-SIZE DISTRIBUTION

The grain-size distribution of the eight rhythms

has been determined by pipette analysis. Measurements were taken at 1 phi intervals, between 4 phi

and 9 phi (63 and 2 microns respectively). Type A

mud contains less than 1 percent sand; thus, no size

analysis of the coarser than 4 phi fraction was

made. The terms "sand," "silt," and "clay" are used

as grain-size terms without any compositional

connotation.

The results are grouped and plotted in two

different ways. (1) The percentages of sand (larger

than 63 micron), silt, and clay (smaller than 2

micron) (Table 4) are plotted against sample

point positions within rhythms (Figures 8-15).

These percentages also are plotted for all samples

on a ternary sand-silt-clay diagram (Figure 16).

(2) The data are plotted as cumulative curves to

show grain-size distribution on logarithmic probability paper (Figure 17).

AD 1.—The coarse-grained, sand-silt turbidites in

rhythms 8-7 (Figure 9), 8A-18 (Figure 11), 10-5

(Figure 13), and 10-12 (Figure 14) exhibit distinct

fining-upward grading. The grading is reflected by

an upward decrease of sand-size material and an

increase in silt- and clay-size particles.

Eriksson (1965), in an extremely detailed examination of core 210 of the Swedish Deep-Sea Expedition, located approximately 20 km to the west

of core 10 of this study (Figure 1), found two

types of sand-silt turbidites. One type, with a high

percentage of bioclastic material, is distinctly

graded. The second type, with a high percentage

of terrigenous material, has less developed grading.

However, this difference between the sand-silt turTABLE 4.—Percentages of sand-, silt-, and

rhythms analyzed (samples are identified

sample number, respectively)

clay-size material in samples from eight

by core number, rhythm number, and

Sample no.

7-11-1

2

3

4

5

6

7

8

8-7-1

2

3

4

5

6

7

8

9

8A-11-1

2

3

4

5

8A-18-1

2

3

4

5

6

7

8

Sand

3

1

5

6

4

5

6

7

55

32

13

1

-

-

-

11

16

2

-

-

-

1

77

41

40

4

2

-

-

-

Silt

52

46

43

42

46

43

43

49

39

62

68

54

51

41

40

38

31

76

55

38

34

47

18

53

54

89

75

69

46

38

Clay

45

53

52

52

50

52

51

44

6

6

19

45

49

59

60

51

53

22

45

62

66

52

5

6

6

7

23

31

54

62

Sample no.

8A-18-9

10

11

9A-7-1

2

3

4

5

6

7

10-5-1

2

3

4

5

6

9

10-12-1

2

3

4

5

6

7

10A-19-1

2

3

4

5

Sand

1

12

13

-

-

1

-

-

6

9

71

35

26

3

-

-

8

69

39

25

9

-

-

9

1

-

-

6

11

Silt

42

33

30

59

51

52

47

41

45

43

23

53

57

82

51

36

45

18

51

52

77

41

34

36

63

46

43

43

44

Clay

57

55

57

41

49

47

53

59

49

48

6

12

17

15

49

64

47

13

10

23

14

59

66

55

36

54

57

51

45
16 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

Composition of Sand

Clay Mineral

Ratios

Grain Size Distrib

Carbonate

Content

30 6 0 9 0%0 30 60°/.

CD CD CD

FIGURE 10.

bidites of largely terrigenous rhythms (8-7 and 8A18) and largely bioclastic rhythms (10-5 and 10-12)

is not observed.

Interestingly, the upper laminated division d

shows a marked increase in the proportion of siltsize material. In the instance of rhythm 10-12, there

is even a decrease in clay from the current ripple

division c to division d.

Type A mud layers show a distinct fining-upward

grading as well. The grading consists of an upward

decrease of silt and an increase of clay. The shift

in the silt-clay proportion can amount to as much

as 20 percent. Most importantly, the grading of

type A mud layers follows in regular fashion the

grading of the underlying sand-silt turbidites or

silt laminae. The sand-size fraction of type A mud

is too low (less than 1 percent) to be plotted in the

figures.

Type B mud layers exhibit no grading or, if anything, a slight coarsening-upward grading.

A distinct granulometric difference exists between type A and type B mud layers. The former

contain, little, if any, sand, maximally 0.62 percent,

with an average of 0.2 percent. The latter, however, contain a sizable amount of sand-grade material, maximally 16 percent, minimally 4 percent,

with an average of 8 percent (Table 4; Figures

8-15). This sand is not graded as is that of the

turbidite sand layers. In addition, type B mud layers show a lower amount of clay and, in some

instances, an increase in silt content, as compared

to the underlying uppermost part of type A mud

layers.

The granulometric change across the boundary

between type A and type B mud members can be

sharp. Where the change is not abrupt, the boundary area is mottled and can show other signs of

bioturbation.

The absence of a systematic, fining-upward

grading in type B mud layers is shown and underscored by rhythm 7-11, in which type B mud is

sampled over a thickness of 44.5 cm (Figure 8).

The grain-size characteristics of mud types A

and B are difficult to compare with Eriksson's

analysis of the fine-grained layers in core 210.

Eriksson divides the fine-grained layers in several

groups, based on the type of material present and

particularly on what he interprets as wind-borne

components. However, in the diagrammatic representation of his results, several rhythms as found

in our cores can be recognized. Grading in mud

layers following on top of sand-silt turbidites or

silt laminae occurs. Also, on top of graded mud

layers, muds with a higher sand percentage are
NUMBER 13

17

OCD

4 0 -

3 0 -

2 0 -

10

0-

3

CD

z

o

11-

10-

9b9a9-

7 —

6-

5-

4 -

n _

2-

1 -

d

c

omposition of Sand

Grain Size Distrib.

Carbonate

Content

8A-18

Clay Mineral

Ratios

Ul

00

> o

7^

CO

o .IU|

OJ

U1

> o

olorit

e

00

Ul

00

> o

o

rr

o

r-tA

fr\

>o

1 1

r-tCD —

1

/

00

Ul

00

CO

oini r-tcp^

CD

Ul

>o

i—i

—

r-tC D -

10 6 0 -9 0o

/<,0 60% 0 3 0 6 0 9 0 %

FIGURE 11.
18 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

5CD Grain Size Distrib

2 0 -

10-

0

J

4 -

3-

2-

1-

cn n

Carbonate

Content

9A-7

Clay Mineral

Ratios

0 30 6 0 9 0%0 60% 0

omposition of Sand

CO CO CO

ijl Ol 01

i co oo oo

FIGURE 12.

indicated (cf., Eriksson, 1965, pi. 6, between depths

of 484 and 443.5 cm in core 210).

The sand-silt-clay proportions of all 59 samples

are plotted in a ternary diagram (Figure 16). A

textural classification of the different rhythm members can be made, using the textural designations

proposed by Folk (1961). The turbidite sand-silt

layers, comprising the largest textural variety, include silt, sandy silt, silty sand, and, in one instance,

muddy sand. The type A mud layers classify strictly

as mud. The type B mud layers can be mud or

sandy mud.

The diagram also shows differences in sorting between the three rhythm members. The turbidite

sand-silt layers are the least sorted, whereas type

A mud is the best sorted. Importantly, the diagram

illustrates that the three rhythm members, defined

primarily from X-radiographs, can also be recognized on the basis of textural analysis.

AD 2.—The cumulative grain-size curves (Figure

17) show even more distinctly the grading in the

sand-silt turbidites. The lower, coarser grained part

of the sand-silt layers exhibits coarse tail grading

(the coarsest percentiles of the size distribution
NUMBER 13 19

10-

0

J

"O

CD

z

o

5-

4-

3-

2-

1-

Grain Size Distrib

o

CD

Carbonate

Content

Composition of Sand

<?

O CD_ CD O

10A-19

Clay Mineral

Ratios

FIGURE 13.

show the grading, particularly well displayed in

rhythm 8A-18). Type A mud layers are distribution graded (the entire size distribution shifts to

finer sizes). Also shown clearly is that type B muds

are not graded, neither internally, nor by a further

size decline following that of the underlying type

A mud.

Most importantly, the grain-size distribution

within B type layers from various depths within the

cores and from six cores at widely separated locations exhibits very little variation and is uniform.

The uniformity within one B layer is particularly

well shown for rhythm 7-11 (Figure 17). Type A

mud layers, on the contrary, exhibit a much greater

variability in grain-size distribution (Figure 17).

The average median diameter, as read off on the

50-percentile line, of type A muds (9.3 phi) is

smaller than that of type B muds (9.1 phi), and

it displays a larger spread (from 7.5 phi to 10.3 phi,

as opposed to 8.4 phi to 9.5 phi).

The inclusive graphic standard deviation, or

sorting, for type A mud is approximately 2, which

is defined as poor to very poor (Folk and Ward

1957). The value for the sorting of type B mud is

approximately 3, i.e., very poorly sorted. This difference in sorting can be clearly seen from the

differences in slope of the cumulative curves

(Figure 17).

The cumulative grain-size curves of type B mud

tend to be straighter (almost perfectly so for some

samples) than for type A muds. This means that

the size distribution of the former approaches normalcy, more so than the latter.

CARBONATE CONTENT

The carbonate content of the eight rhythms was

determined by gravimetric method, using the ascarite absorption technique. A small amount of

sample (0.1 g) is treated with diluted HC1, and

the liberated CO2 is directed through vessels containing agents that remove possible other vapors

to the ascarite tube. The carbonate percentage can

be calculated from the weight increase of the ascarite tube. The accuracy of this method can be of

the order of 1 percent. Results of the analyses are
20 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

=;•(/> in

-" n Co

3 CD ~D_

2 0-

10-

0 -

7-

6a6-

5-

4-

3-

2-

1-

< ?

lam

b

Gram Size Distrib

Carbonate

Content

Composition of Sand

O

O

(D

CO

Co

(Q

n'

~O CD

iMica

10-5

Clay Mineral

Ratios

ui CJI en

W0DC0

>° >o>o

n ^ T^

30 6 0 9 0%0 30 60% 0 30 6 0 9 0 %

FIGURE 14.

detailed in Table 5 and depicted in Figures 8-15.

The turbidite sand-silt layers have the highest

average carbonate content (40 percent) and the

widest spread of values (from 21 to 74 percent).

The overlying type A mud layers have the lowest

average carbonate content (28 percent) and an

intermediate spread (from 16 to 39 percent). The

type B mud layers on top have an intermediate

average carbonate content (35 percent) and the

narrowest spread (from 26 to 46 percent).

In general, the carbonate content decreases with

decreasing grain size. Within the sand-silt turbidites the carbonate percentage shows an upward

decrease. In some of the type A mud layers, for

example those of rhythm 9A-7 and 10-12, the carbonate content shows also an upward decrease by

as much as 6 percent. As such, the A muds are not

only texturally graded, but also compositionally.

This compositional grading follows that of the

underlying sand-silt turbidites.

The continuity from sand-silt turbidites to type

A mud layers above them is illustrated by the fact

that where the former are low in carbonate content

the latter are proportionally low as well (8-7,

8A-18), and the reverse (10-5, 10-12). The carbonate content of the type B mud layers above
NUMBER 13 21

=;(/) Co

n

 E. 3

3 CD T3_

CD

20-

10-

0

J

o

i £

5-

4-

3-

2-

1-

Carbonate

Content

Grain Size Distrib

Composition of Sand 10-12

Clay Mineral

Ratios

ui en en

U) 00 00

30 6 0 9 0 %

these does not change and is independent of that

of the underlying rhythm members.

The carbonate content of the type B layers is

relatively constant. An abrupt change occurs across

the boundary, where the underlying type A muds

have a low carbonate content.

The carbonate materials are almost entirely

biogenic in origin (see following section). Based

on the carbonate content as an indication of the

amount of biogenic components, the sand-silt turbidites fall into two classes, terrigenous (8-7,

8A-18) and biogenic (10-5, 10-12). The type A

muds can be clayey muds (8-7, 8A-11, 8A-15) or

marl muds (10-5, 10-12, 10A-19). The type A muds

of rhythm 7-11 and 9A-7 are somewhat intermediate between terrigenous and biogenic types.

Type B muds classify as marl oozes. Classification

as used here follows that of the Initial Reports of

the Deep Sea Drilling Project (Ryan, Hsu, et al.,

1973c), and is modified after Olausson (1960).

Although only a few rhythms have been examined, it appears that the carbonate admixture of

the sand-silt turbidites and the type A muds

increases toward the west across the Balearic

Abyssal Plain, i.e., from core 8 across the Balearic

Abyssal Plain to core 10 and 10A.

COMPOSITION OF THE SAND FRACTION

The larger than 63-micron fraction was obtained

for all samples by wet sieving. Grain mounts were

prepared for identification of the constituents and

for determination of their relative frequencies.

Relative frequencies were determined by counting
22 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

300 to 400 grains per mount. Due to the very small

percentage of sand-grade material in the samples

from type A mud layers, no technique of point

counting could be practiced. Instead, all grains per

unit area were counted. As a result, bioclastic particles of large size (e.g., pteropod shells of low

effective density) are relatively underrepresented

as compared to smaller particles (e.g., quartz grains

of high effective density).

Constituents were counted and grouped in the

following classes: terrigenous (mica and other, including quartz), bioclastic remains (pelagic forams,

benthonic forams, pteropods, and other), plant

fragments, and other (including ostracode valves,

sponge spicules, unidentifiable fragments, etc.).

The results are given in Table 6 and Figures 8-15,

18, and 19.

TABLE 5.—Carbonate percentages for samples from

eight rhythms studied (samples are identified by

core number, rhythm number, and sample number,

respectively)

Sample no.

7-11-1

2

3

4

5

6

7

8

8-7-1

2

3

4

5

6

7

8

9

8A-11-1

2

3

4

5

8A-18-1

2

3

4

5

6

7

8

CaCO3 %

28

25

28

28

26

28

28

29

33

24

25

21

22

21

22

40

46

25

22

20

22

22

38

23

25

22

21

18

16

17

Sample no.

8A-18-9

10

11

9A-7-1

2

3

4

5

6

7

10-5-1

2

3

4

5

6

7

10-I2-I

2

3

4

5

6

7

10A-19-1

2

3

4

5

CaCOs %

18

35

36

39

37

38

33

32

32

37

74

55

50

49

39

37

38

60

-

_

43

38

35

43

39

38

38

35

43

The widest variety of constituents occurs in the

turbidite sand-silt layers. The terrigenous components are mainly quartz, mica, and feldspars. A

wide spectrum of foram tests, both pelagic and

benthonic, is represented by their microskeletal

remains. Other organic remains include those of

pteropods, crinoids, ostracods, sponges, and plants.

A few siliceous remains from radiolarians and diatoms also occur. Glauconite is present as well.

This association of microfossil and other organic

remains is typical for resedimented deposits containing biogenic remains from various environments (terrestrial, neritic benthonic, bathyal or

abyssal benthonic, and pelagic) (Parker, 1958).

The constituents are very poorly sorted, and the

shelly remains commonly are broken (Figure 18A,C).

The constituents of type A mud layers are largely

the same as those of the underlying sand-silt turbidites; the individual specimens of each constituent, however, are markedly smaller on the average

in the type A mud layers. In the top part of these

layers, large Orbulina specimens can be found.

Their sizes are quite uniform (well sorted), and

the foram tests are only slightly larger than the

quartz grains.

Fecal pellets and remnants of pyritized burrow

tubes occur in type A layers as well, particularly

near the boundary with the overlying type B layers.

Type B layers can contain quartz, mica, and

feldspar, though in small proportions. The organic

remains show less variety. Skeletons of pelagic

forams and pteropods form the main constituents.

Among the rest are sponge spicules, radiolarian,

diatomaceous, and crinoid fragments. The spread

of particle sizes is wide (very poor sorting) (Figure

18B,D).

The quartz grains in the three types of layers

often are of the so-called desert type, i.e., rounded

and stained, or frosted (Eriksson 1965).

In the turbidite sand-silt layers, concentrations

of different constituents are associated with specific

sedimentary structures (Figures 8-15). For example,

the mica flakes and plant fragments occur most

frequently in the d-division. This indicates that the

relative frequencies of the components are determined to a large degree by their hydrodynamic

properties. Vertical sorting appears to have taken

place according to the effective densities of the

components. Light, large, nonspherically (oblately)

shaped particles are therefore concentrated in the
NUMBER 13

SAND

5(5) : sand(sandy)

Z(z) : silt(silty)

M(m): mucKmuddy)

• turbidite sand-silt

° turbidite mud

(hemi)pelagic mud

mixed

CLAY SILT

FIGURE 16.—Ternary diagram in which are plotted the sand-silt-clay proportions of all samples

of eight studied rhythms. Different symbols show turbidite sand-silt layers, type A mud layers

(interpreted as turbiditic), and type B mud layers (interpreted as hemipelagic). The mixed

samples are from boundary regions between two rhythm members.

d-division, while smaller, but spherical, particles

of higher effective densities occur in the lower

structural divisions.

This phenomenon of vertical sorting of different

components according to their hydrodynamic properties continues without a break from the turbidite silt layers into the type A mud layers. Mica

flakes and plant fragments remain relatively frequent in the lower part of type A muds. In the

upper half of these layers, tests of pelagic forams

occur in high relative frequency. The forams

exhibit a small size range (well sorted) (Figure

18E), and the size of the few associated quartz

grains is consistently somewhat smaller.

An abrupt change in relative proportions occurs

across the boundary into type B muds (Figure 19).

The constituents are mainly remains of pelagic

forams and pteropods (Figure 18B.,D). Mica and

a few other terrigenous particles are present as well.

The mica content increases somewhat, as do the

pteropod shells. Pelagic foram tests, on the other

hand, decrease relatively. Plant fragments are

virtually absent (Figure 19). Within the type B

mud layers no systematic change in proportions

or constituents occurs, and apparently no hydrodynamic sorting effective on the sea floor has

influenced their sand composition. The components are of widely varying sizes (very poorly sorted)

(Figure 18B,D).

The composition of sand-grade material in

terrigenous sand-silt turbidites and type A mud

layers differs from sands in bioclastic layers in that

the former have significant proportions of plant

remains and a higher proportion of terrigenous
24 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

C

u

£ _

Z3

E

u

_ 9 5

_ 84

_ 75

_ 5 0

_ 25

_ 16

_ 5

9A-7

99-98

—I 1 1 1 1 1—

QO63 0012 0D04 m m

0031 0.008 QD02

Sand-Silt

Mud

Turbiditic

(Hemi)pelagic Mud

0 0 2

6 7 8 9

Diameter in 0 Units

FIGURE 17.—Cumulative grain-size curves for all samples from the eight studied rhythms. Different lines are used for samples from turbidite sand-silt layers, type A mud layers (interpreted

as turbiditic), and type B muds (interpreted as hcmipelagic). Sample numbers arc indicated

on the curves.
NUMBER 13 25

FIGURE 18.—Photomicrographs of the coarser than 63 micron fraction of turbidite sand layers

(A, C), and type B mud layers (B, D). The corresponding sample numbers from A to D are:

8-7-1, 8-7-9, 10-5-1, and 10-5-7. E = forams in sample 10-5-6. F = pyritized filament in

sample 9A-7-4. The white lines (on background) form squares that measure 3.7 by 3.7 mm.
26 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

TABLE 6.—Percentages of the different components, comprising the sand-size

fraction of eight rhythms studied (samples are identified by core number, rhythm

number, and sample number, respectively)

Sample no.

7-11-1

2

3

4

5

6

7

8

8-7-1

2

3

4

5

6

7

8

9

8A-11-1

2

3

4

5

8A-18-1

2

3

4

5

8A-18-6

7

8

9

10

11

9A-7-1

2

3

4

5

6

7

10-5-1

2

3

4

5

6

7

10-12-1

2

3

Terrigenous

Mica

8

12

7

8

4

5

5

9

80

73

45

6

30

71

7

-

1

16

10

3

3

8

-

-

16

33

31

36

6

-

-

-

-

30

10

-

-

-

*

-

5

4

11

24

2

-

5

4

6

6

Other

25

23

25

13

8

11

13

24

5

6

15

44

24

4

-

-

-

4

5

61

15

2

83

82

65

41

13

6

34

8

5

3

3

7

43

48

30

29

22

24

18

11

5

1

32

4

13

19

2

1

Pelagic

forams

43

50

29

37

48

54

43

48

11

13

21

17

16

14

89

35

38

41

53

25

74

84

6

9

10

14

21

21

50

89

93

53

45

42

43

47

68

70

54

51

53

58

70

57

51

90

55

60

63

62

Benthonic

forams

3

2

1

1

1

1

1

_

-

-

-

-

-

-

-

2

2

7

1

-

-

-

1

-

-

-

-

-

-

-

-

1

1

1

1

-

-

-

-

-

2

3

2

_

1

-

1

-

2

3

Pteropods

1

1

27

34

33

24

28

15

3

3

6

-

-

1

1

55

54

13

3

-

-

4

2

2

2

2

2

-

-

-

-

32

40

5

2

_

_

_

18

19

4

9

4

4

1

3

22

13

19

20

Plants

-

-

-

-

-

-

-

-

-

-

12

30

29

5

1

-

-

8

26

7

7

1

-

-

-

-

15

19

-

-

-

-

-

8

-

-

-

-

-

-

-

-

-

-

-

-

_

-

-

_

Other

20

12

11

7

6

5

10

4

1

5

1

3

1

5

2

8

5

11

2

4

1

1

8

7

7

10

18

18

10

3

2

11

11

7

1

5

2

1

6

6

18

15

8

14

13

3

4

4

8

8
NUMBER 13 27

TABLE 6.—Continued

Sample no.

4

5

6

7

10A-19-1

2

3

4

5

Terrigenous

Mica

20

14

-

-

8

4

1

4

3

Other

6

6

3

3

1

13

22

10

9

Pelagic

forams

57

67

94

93

54

51

59

54

53

Benthonic

forams

1

1

-

-

1

1

1

1

-

Pteropods Plants

5

6

- -

- -

1

1

1

20

22

Other

11

6

3

4

35

30

16

11

13

constituents such as quartz and mica. The virtual

absence of plant remains in bioclastic turbidites in

the Caribbean has been reported by Rusnak and

Nesteroff (1964).

Figure 19 summarizes the average relative frequencies of the sand constituents as distributed

through the eight terrigenous and bioclastic

rhythms examined in this study. The boundary

between the lower coarse-grained member and type

A mud is markedly different from the one between

o

^O

0 2

Z>

e

p

d

c

b

ABSOLUTE % 3

=

OF SAND V

i

TERRIGENOUS OTHER P""^^^^^^^^^BBi^^^^^^| m MICA I

1

BIOGENOUS

PLANT

REMAINS iI

V

PTEROPODS H^H|

o p

I o

BENTHONIC

FORAMS

0 30 60%

FIGURE 19.—Summary diagram showing the relative frequencies of the major constituents in the sand-size fraction

of the eight rhythms studied. The columns for mica and

plant fragments are typical for terrigenous turbidites and

type A muds (interpreted as turbiditic, or e*).

mud types A and B. Across the former boundary,

a continuity of components and their gradually

changing proportions is noted. Across the latter

an abrupt change occurs, most clearly indicated by

a small increase in mica, a decrease in pelagic

foram tests, and an increase in pteropod shells.

It should be emphasized that these changes as

schematically depicted are changes in relative

frequencies, not necessarily in absolute numbers.

For example, the relative frequency of pelagic

foram tests is shown to decrease, although the

absolute number, in fact, increases due to the much

higher percentage of sand (indicated in the second

column of Figure 19). In contrast, Weidmann

(1967) in a similar diagram schematically illustrates a relative increase in pelagic foram tests in

the top layer.

An absolute increase in the number of foram

and pteropod remains in the top part of finegrained layers following turbidite sand-silt layers

has also been described by Nesteroff (1961),

Nesteroff and Heezen (1963), and Rusnak and

Nesteroff (1964).

CLAY MINERALOGY

The samples were mineralogically analyzed by

X-ray diffractometry. The instrument used was a

Noralco (Philips) unit, with Cu K-alpha radiation

and Ni filter. Slides were prepared from grain-size

fractions separated by the pipette method. Both

the finer than 4 phi and the finer than 9 phi

fractions were used. Some slides were glycolated

(overnight in glycol vapor).

The minerals present are: quartz, calcite, dolomite, kaolinite, chlorite, illite, plagioclase, K-
28 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

feldspar, and montmorillonite (not always in this

order of relative abundance). The latter mineral

forms no distinct peak. In the finer than 9 phi

fraction the dolomite is absent and feldspars in

most instances are not observed. These minerals

are present in all rhythms but in varying

proportions.

The major clay minerals are illite, chlorite, and

kaolinite; montmorillonite, detected by glycolation,

is present in minor quantities. This confirms earlier

studies of clay mineralogy in the western Mediterranean by Nesteroff, Sabatier, and Heezen (1965).

A study of the relative abundances of the three

clay minerals was made by an analysis of peak

heights. The peaks used are for illite: d-4.95, for

chlorite: cl-3.53, and for kaolinite: d-3.58. All

samples exhibit these peaks quite distinctly, and

TABLE 7.—Peak-height ratios of C (chlorite, d-3.53

A), I (illite, d-4.95 A), and K (kaolinite, d-3.58

A), for samples from eight rhythms analyzed (samples are identified by core number, rhythm number,

and sample number, respectively)

Sample no.

7-11-1

2

3

4

5

6

7

8

8-7-1

2

3

4

5

6

7

8

9

8A-11-1

2

3

4

5

8A-18-1

2

3

4

5

6

7

C/I

1.0

1.1

1.1

1.1

0.8

1.3

0.9

1.0

2.0

2.2

1.4

2.2

1.8

1.4

1.4

1.6

1.4

1.6

1.6

1.8

1.5

1.5

1.5

1.9

1.8

1.7

1.9

2.6

2.5

K/I

2.5

3.3

2.3

2.3

2.4

3.1

2.4

1.8

2.7

2.7

2.1

2.6

2.2

1.8

2.5

1.6

2.1

2.1

2.1

2.4

2.0

2.2

2.4

2.4

2.3

2.8

2.7

3.3

3.9

K/C

2.6

3.0

2.1

2.1

2.9

2.5

2.6

1.8

1.4

1.2

1.5

1.1

1.3

1.3

1.7

1.0

1.5

1.3

1.3

.3

1.3

.5

.6

1.3

1.3

1.7

1.4

1.3

1.6

Sample no.

8A-18-8

9

9a

9b

10

11

9A-1-1

2

3

4

5

5a

6

7

10-5-4

5

6

6a

7

10-12-4

5

6

7

10A-19-1

2

3

4

5

C/I

2.2

2.1

2.3

1.0

1.1

1.1

0.6

0.6

0.6

0.7

0.7

I.I

1.7

1.2

1.0

0.8

0.8

1.1

0.9

0.9

0.9

1.0

1.1

0.9

1.0

0.9

0.7

1.2

K/I

3.3

3.2

3.1

2.0

2.3

2.4

0.7

0.6

0.5

0.6

0.5

1.1

2.9

1.2

0.8

0.7

0.6

1.1

0.8

0.7

0.7

0.9

1.5

0.8

0.8

0.7

0.7

0.8

K/C

1.5

1.5

1.4

2.0

2.1

2.3

1.1

1.0

1.0

0.9

0.8

1.1

1.6

0.9

0.9

0.8

0.8

1.1

0.9

0.8

0.8

0.9

1.4

1.0

0.8

0.8

1.0

0.7

the kaolinite and chlorite peaks can be easily distinguished with a rotation of one-half degree (2

theta) per minute (Figure 20). The results are

listed in Table 7.

The peak-height ratios have been plotted against

the sample points for each rhythm (Figures 8-15).

The ratios are, in most instances, different for the

type A and type B muds of a particular rhythm. A

sharp break in the curves of the clay-mineral ratios

can occur at the boundary between the two kinds of

mud. This mineralogical change is particularly noticeable in rhythms 8A-18 and 9A-7. Extra samples

were taken as near to the boundary as possible to

verify the abruptness of the change (samples marked

by a or b). The change in illite, kaolinite, and

chlorite peaks between the two samples on either

side of the boundary of rhythm 9A-7 is illustrated

in Figure 20.

The change of clay ratios from type A into type

B muds is most pronounced where the A muds are

terrigenous (8-7, 8A-18, 9A-7) and less so where

they are bioclastic (10-5, 10-12, 10A-19). The claymineral suites of terrigenous A mud, bioclastic A

mud, and of type B mud show each a rough

uniformity. The proportion of clay minerals is

different in each mud type.

Discussion

TYPES OF DEPOSITIONAL MECHANISMS

There is evidence for several depositional

mechanisms operating in the abyssal environment

of the western Mediterranean:

1. Normal, pelagic settling of terrigenous particles introduced by rivers and wind, and of

marine microskeietons. Fine-grained sediment layers in cores from the Algero-Balearic Basin have

been attributed to this process (Eriksson, 1965;

Leclaire, 1972a, 1972b).

2. Turbidity currents transporting dominantly

terrigenous materials from shallower environments

onto the abyssal plain. Coarse-grained turbidites

have been identified in a number of cores from

the Balearic Abyssal Plain (Horn, Ewing, and

Ewing, 1972).

3. Normal bottom currents of various types,

eroding, transporting, and redistributing sediments

on the sea floor. The lower section of the Balearic

Basin fill, as recovered at JOIDES drilling site 124,
NUMBER 13 29

FIGURE 20.—Peaks from X-ray diffractograms of 4.95 A illite (I), 3.58 A kaolinite (K), and 3.53

A chlorite, illustrating sharpness of peaks and change in peak-height ratios from sample 9A-7-5

(type A mud) to 9A-7-6 (type B mud).

is attributed to the redepositing activity of contourtype currents ( Nesteroff, 1973). Strong bottom

water movement is noted in shallow straits at the

basin margin (Strait of Sicily and Gibraltar region;

Lacombe, 1971; Kelling and Stanley, 1972a,b). On

the other hand there is little direct evidence

available in support of well-defined bottom currents, although physical oceanographic data suggest

the westerly movement of potentially active but

transient bottom currents in the region off North

Africa and the Balearic-southeastern Iberian platform (Miller, 1972).

4. Certain types of low density, gravity-assisted

currents transporting largely terrigenous clay-size

particles down the continental slope-rise. Muds in

the Alboran Sea are attributed to this mechanism

(Bartolini and Gehin, 1970; Stanley, Gehin, and

Bartolini, 1970; Huang and Stanley, 1972).
30 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

MUD DEPOSITION—GENERAL

Nesteroff (1961; also Nesteroff and Heezen, 1963;

and Rusnak and Nesteroff, 1964) describes the

typical sequence of structures in oceanic turbidites,

and the distribution of foram and pteropod skeletons in these. His typical sequence ("sequence type")

is quite similar to the type 1 rhythm of this study:

a sand-silt layer is followed by a mud layer containing very little sand-grade material, which in turn

is followed by a layer of mud with a high concentration of foram test and pteropod shell material.

The explanation offered is that the entire sequence

formed by a turbidity current, the tail of which

carried skeletal material of extremely low effective

densities that became incorporated in the upper

part of the deposit. Pelagic sediments are thought

to have been eroded by turbidity currents. Kuenen

(1964) interprets the upper layer of shell material

as winnowed concentrates from a pelagic stratum

by the turbidity current that deposited the covering

turbidite, though in some instances it may represent the pelagic stratum.

From the foregoing analysis, however, it appears

that type A and B mud layers formed from different depositional mechanisms. The break in texture and composition across the boundary between

them excludes the explanation of continuous deposition from a turbidity current for both mud layers

combined. Also the relatively high clay content and

the poor sorting of the B muds exclude winnowing

bottom currents as a mechanism.

TURBIDITIC MUD DEPOSITION—TYPE A MUDS

The assemblage of characteristics of the type A

mud layers can be best explained by deposition

from turbidity currents. The following features

support this interpretation.

STRUCTURAL.—On X-radiographs the lower portion of several type A mud layers exhibits delicate

lamination (8A-11, 8A-18, 10-5, 10-12). As such,

a structural continuity exists from the upper laminated d-division into type A mud layers (Figure 4).

Normal pelagic settling does not produce lamination due to the continuous organogenic disturbance of the slowly accumulating particles.

Laminations can form in stagnant water of which

the lower portion has become anaerobic (van

Straaten, 1967). The presence of benthonic life,

however, is evident from burrow tubes. Bottom

photographs also show that the modern plain surface is affected by the activity of animals (Heezen

and Hollister, 1971).

TEXTURAL.—Type A mud layers show distribution grading following the coarse tail grading of

the lower part of the sand-silt turbidites where

these are present. Experiments indicate that coarse

tail grading is produced by high density turbidity

currents, whereas distribution grading results from

relatively dilute flows (Middleton, 1967). Also, the

sorting of type A muds is better developed than

that of type B mud layers, which can be interpreted

as the effect of hydrodynamic selection during current transportation. The fact that type A muds are

poorly sorted nevertheless excludes normal bottom

currents as depositional mechanism. Contourites,

for example, exhibit good to very good sorting

(Bouma, 1972a, 1972b).

COMPOSITIONAL.—The carbonate content of some

type A mud layers shows an upward decrease and,

as such, a compositional grading which follows

that of the underlying sand-silt turbidites where

present. The overall carbonate content of type A

muds varies with that of the underlying turbidite

sand-silt layers.

Such compositional grading as indicated by the

carbonate content is documented as well for very

thick turbidite marlstone layers following on top of

very thick turbidite calcarenites in the Eocene frysch

of the southwestern Pyrenees (Rupke, 1972).

The individual components of the sand-grade

fraction, in particular the mica and the plant

particles, are concentrated in the upper d-division

of the turbidite sand-silt layers and in the lower

portion of the type A muds. This continuity of

composition cannot be attributed to biogenic homogenization of the boundary area, otherwise the

delicate lamination would have been obliterated.

In general, structural, textural, and compositional continuity from the sand-silt turbidites into

the type A mud layers suggests that they formed

from a single turbidity current. The genetic connection between coarse-grained turbidites and their

overlying type A muds excludes permanent or

semipermanent low-density suspension currents as

the depositional mechanism of the type A muds,

and indicates emplacement by the tail of a normal,

high-density turbidity current (Kuenen, 1967). In

short, type A muds are the turbiditic portion of the
NUMBER 13 31

pelitic e-division, symbolized as e

x

 (Figures 8-15).

Where only thin silt laminae or lamels form the

beginning of a rhythm, the type A mud layers are

likely to be the distal portion of a turbidite body,

deposited by the slowly moving, or ponded, less

dense tail of a normal, high-density turbidity current. Such layers can be designated as mud

turbidites. However, the depositional mechanism

for mud turbidites can also be an originally lowdensity turbidity current, triggered, for example,

by stirred up fine sediment on the shelf-break

during storms (Stanley, 1969, 1970).

Further evidence for deposition by gravitypropelled density currents of type A muds, rather

than by normal pelagic settling, comes from the

fact that the proportion of type A mud, as well

as that of turbidite sand-silt, is higher in the cores

from greater water depth (Figure 6). This is to be

expected where turbidity currents seek the lowest

parts of the basin plain. In contrast, the proportions of type B muds show an inverse relationship

with the water depth of the cores.

HEMIPELAGIC3 MUD DEPOSITION—-TYPE B MUDS

The properties of type B mud layers fit best the

depositional process of pelagic settling. Evidence

for this comes from the following observations.

STRUCTURAL.—All type B mud layers lack any

kind of primary sedimentary structure, including

fine lamination. Pelagic deepwater deposits do not

become laminated due to the fact that the slowly

accumulating particles are constantly being rearranged by benthonic organisms. Accordingly,

burrowing structures are present in type B muds.

TEXTURAL.—Type B muds exhibit no systematic

changes in grain-size distribution. In addition, the

grain-size distribution of all type B layers is vir3

 We have opted for the term "hcmipclagic," as opposed

to "pelagic," in describing the type B (ep) muds. Pelagic sediments have been defined as "Deep-sea sediments without terrigenous material; they are either inorganic red clay or

organic ooze" (AGI Glossary of Geology, 1972). Hemipelagic

sediments are defined as "Deep-sea sediments containing a

small amount of terrigenous material as well as remains of

pelagic organisms" (AGI Glossary of Geology, 1972). Authors

differ on the amount and si/e of terrigenous matter. Kuenen

(1950) defines hemipelagic sediments as those with mainly

pelagic but little terrigenous matter, while Kukal (1971)

defines these as containing more than 20 percent of terrigenous material of silt and sand grade.

tually uniform throughout the basin area examined

and is constant through time as well. This uniformity of type B mud layers is a typical feature of

pelagic deposition and cannot be explained by

other depositional processes.

The high clay content and very poor sorting of

the type B muds excludes any normal bottom

current emplacement.

COMPOSITIONAL.—The composition of the type B

muds is typical of autochthonous deepwater accumulations. The components of the coarser fraction are mainly from pelagic organisms, dominantly

forams and pteropods. The shelly remains are

largely unbroken and unworn, even the delicate

pteropod skeletons. In addition, the composition

of type B layers varies little from one to the other,

unlike type A muds.

The interpretation of type B muds as hemipelagic deposits is significantly strengthened by an

analysis of radiocarbon dates determined on their

carbonate sands. In summary, type B muds constitute the hemipelagic upper part of the pelitic

<?-division, or the <?' (Figures 8-15).

NORMAL BOTTOM CURRENT ACTIVITY

In addition to evidence for turbidity currents

and pelagic settling, evidence for occasional bottom

current activity exists. Within type B mud layers,

concentrations of coarse skeletal remains can occur

with a low pelite content (for example two of these

accumulations are observed in Figure 4:8-20). Van

Straaten (1967) found pteropod shell layers in the

Adriatic on top of ash layers and interpreted their

origin to mass death caused by a natural clamity

such as a volcanic eruption. No ash occurs in the

type B mud layers in question, and the most logical

explanation would seem to be a winnowed concentrate formed by normal bottom currents. It is

also possible, however, that such skeletal remains

accumulate as a result of periods of particularly

high productivity (Huang, Stanley, and Stuckenrath, 1972). Such shell concentrates only occur in

type B layers, never in type A muds. This is to be

expected where the latter formed instantaneously.

MUD TURBIDITES AND CONTOURITES

In the lower sediment sequence of site 124 of

the JOIDES drilling, rhythmically interbedded

light- and dark-colored marl oozes occur (see
32 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

"Geographic and Geologic Setting"). The darkcolored oozes can start with sand-silt laminae and

are more terrigenous in composition. The lightcolored oozes contain predominantly particles of

pelagic origin. The lower limit of the dark-colored

layers can be sharp, or can be diffuse due to burrowing. These rhythmically interbedded sediments

are interpreted by Ryan, Hsu, et al. (1973a) as

contourites, formed by currents on the sea floor

associated with a strong thermohaline circulation.

The alternating fine-grained layers are quite similar to the type 3 rhythm of this study (Figure 4),

in which a turbiditic mud layer, beginning with

a thin coarse-grained lamina or lamel, is followed

by a hemipelagic mud layer. From the description

of these rhythmically interbedded fine-grained

layers in the initial report of Leg XIII drilling

(Ryan, Hsu, et al., 1973c) no convincing evidence

appears that these deposits are contourites. The

description suggests that they resemble closely an

alternation of mud turbidites and pelagic deposits.

However, the evidence for bottom current activity

provided by coarse foram test concentrations, also

reported in this sequence, is not disputed here.

These foram test pavements are similar to those

interpreted in this study as winnowed concentrates.

ANCIENT FLYSCH AND FLYSCH-LIKE SUCCESSIONS

The results of this study should be tested in

other deep-sea basins. It is important that cores be

examined by X-radiography prior to hardening,

since it provides information needed to identify

mud types.

The distinctive properties of the turbiditic and

hemipelagic mud layers in the Algero-Balearic

Basin could prove useful in the study of flysch

shales (see "Introduction"). Also, the sedimentology and paleogeography of the numerous, sometimes thick, shaly, flysch-like successions in ancient

geosynclinal belts (flysch noire, Helminthoides

flysch, argile scagliose, blue marl, etc.) remain

largely enigmas. The combined radiographicgranulometric-compositional method used in this

study could best be applied to relatively fresh,

unbroken shale slabs (Hall and Stanley, 1973)

or core sections. The identification of possible

rhythms in these shale samples would be a first

step in determining the depositional history of

these flysch-like formations.

Radiocarbon Ages and Rates of Sedimentation

Ages of ten samples from five different cores (7,

9, 9A, 10, 10A) were obtained by radiocarbon

determinations (Figure 6). The samples selected

for C14 dating consisted of carbonate sand (predominantly pteropod shells and foram tests) sieved

from hemipelagic layers. In addition, two samples

from core 10 consisted of carbonate sand from

two sand turbidite layers. Lack of sufficient concentrations of carbonate sand in other layers prevented

taking more than twelve samples for dating purposes. The position and the extent of the sampled

mud and sand layers are indicated in Figure 6. The

ages are given in Table 8.

In Figure 21 the sample dates (ages) are plotted

(1) versus the total sediment thickness (depth in

cores) and (2) versus the combined thicknesses

(depth in cores) of the hemipelagic layers only,

leaving out the turbiditic intercalations. For both

plots the correlation coefficients between age and

thickness of the sediments plus the regression lines

are indicated. For the total sediment thickness the

values are: r = 0.93, and y = 22.7X51.3. For the hemipelagic sediment thickness these are : r=0.98, and

y = 9.7X8.1. Both correlations are statistically

significant at the 0.05 level of confidence.

As might be expected, the correlation for the

combined hemipelagic thickness only is stronger

than that for total sediment (hemipelagic plus

turbiditic) thicknesss; the latter includes a considerable number of instantaneously deposited

TABLE 8.—Radiocarbon dates determined on the

carbonate sand fraction of ten hemipelagic mud

layers and two turbidite sand layers

Core

no.

7

7

9

9

9

9A

9A

10

10

10A

10A

10A

Rhythm

no.

17

11

12

7

3

17

10

12

5

26

20

2

Carbon

dating

code

SI-1429

SI-1430

SI-1431

SI-1432

SI-1433

SI-1434

SI-1435

SI-1436

SI-1437

SI-1438

SI-1439

SI-1440

Radiocarbon

date

6215 ± 180

14,810 ± 275

2780 ± 50

11,915 ±200

16,165 ±310

6135 ± 125

12,605 ± 190

21,205 ± 330

29,650 ± 1180

2590 ± 75

12,460 ± 230

> 41,000

Carbonate

sand

sieved from

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud

Turbidite sand

Turbidite sand

Hemipelagic mud

Hemipelagic mud

Hemipelagic mud
NUMBER 13

I I » »

AGE in 10 years

8 12 16

J 1 1 I I I I I I I I I

100 —

200-

300

TOTAL SEDIMENT THICKNESS

A THICKNESS OF (HEMI)PELAGIC LAYERS ONLY

400 —

FIGURK 21.—Graph of ten radiocarbon dates from carbonate sands of hemipelagic (type B mud)

layers versus (1) total depth in the core of the respective layers (solid line), and (2) the combined thicknesses of hemipelagic mud layers only, omitting the turbiditic sections (dashed line).

mud, silt, and sand-turbidite layers distributed

irregularly throughout the cores examined (Figure

6). The virtually perfect correlation for the hemipelagic layers only is in accordance with a

continuous pelagic settling process. The sedimentation rate for these layers is almost exactly the

same in all cores throughout the Abyssal Plain.

In contrast, the total sedimentation rate differs

markedly from core to core.

The following sedimentation rates are inferred

from the plots in Figure 21: total sediment thickness, 23 cm/1000 years (this value is only an
SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

average for all cores combined); hemipelagic

sediment thickness, 10 cm/1000 years (virtually

a constant value in all cores, confirming the interpretation of their hemipelagic origin). These rates

correspond to 230 Bubnoff units and 100 B respectively (Fischer 1969). The core-to-core variation

in sedimentation rates is illustrated in Figure 6,

where 5000-year isochrons are drawn between the

cores. These isochrons have been calculated based

on the sedimentation rate for the hemipelagic layers of 10 cm/1000 years.

In core 10, the ages of the turbidite carbonates

are much older than those indicated by the isochrons. This fits the theory that the sampled

layers include much older materials that were

redeposited by turbidity currents. By interpolation

between the two turbidite dates, it is possible to

calculate the following sedimentation rates: for

the total sediment thickness, 33 cm/1000 years

(331 B), and for the pelagic layers, 10 cm/1000

years (104 B). The latter value approximates the

pelagic rate as determined from the other samples

(Figure 21).

The almost perfect correlation between age and

depth (thickness) of the hemipelagic layers indicates that no significant erosion of these layers by

bottom or turbidity currents has occurred. Little

or no erosion by the turbidity currents is to be

expected in a distal setting where the majority of

the turbidity currents deposit mud turbidites.

The negative intercept in the regression lines

in Figure 21 could imply a reduction in sedimentation rate during the past several thousand years.

Another possible explanation, however, is compaction: with increasing depth, the compaction

increases and the age-depth relationship changes

from a straight to a curved line. In any case, it is

known that total sedimentation rate in the western

Mediterranean has not been constant during much

of Quaternary time, due in large part to climatic

and eustatic changes affecting the circumMediterranean region (Mars, 1963). Rates have

decreased considerably since the end of the

Pleistocene (Bartolini and Gehin, 1970; Leclaire,

1972a; Huang, Stanley, and Stuckenrath, 1972).

The isochrons in Figure 6 show that, except for

cores 8 and 8A, all cores extend backward from

the postglacial (11,000 B.P., Leclaire, 1972a) into

the Late Wiirm. Cores 8 and 8A, lying at the

greatest water depths, display the highest overall

sedimentation rate. The area which they occupy

probably has had a greater access to sediments

derived from the important oueds (Soummam,

el Kebir, etc.) on the eastern Algerian margin;

turbidity currents redeposit these materials downslope, as they flow toward the deepest basin plain

off western Sardinia. Turbidity currents have been

triggered by earthquakes (Heezen and Ewing,

1955) and perhaps also by storms and phases of

high-river discharge, as in the northern Balearic

Basin (Gennesseaux, Guibout, and Lacombe,

1971). Although cores 7, 10, and 10A lie closer to

the base of fans, the relatively low proportion of

sand and mud turbidites suggest that they probably

occupy sectors of the sea floor somewhat more

shielded from sediment sources by topographic

features such as levees (Figure 1).

Other studies indicate that the average sedimentation rate for the upper 300 m of section (lower

Pliocene-Pleistocene) at JOIDES site 124 is 9

cm/1000 years, and that of the Quaternary section

at site 133 exceeds 13 cm/1000 years (Ryan, Hsu,

et al., 1973b). Rates of 12 to 100 cm/1000 years

are calculated for deposits at the base of the slope

off Algeria (Leclaire, 1972a). However, the sedimentation rates of our study are of the same order

of magnitude as those calculated by others for the

Late Pleistocene-Holocene in the western Mediterranean in its deep parts. Gennesseaux and

Thommeret (1968) calculate rates of 30 cm/1000

years in the northern part of the Balearic Basin.

Eriksson (1965) puts the total sedimentation rate

of core 210 southeast of our core 10A at 35

cm/1000 years, and he estimates the pelagic rate

at between 14 cm to 29 cm/1000 years. The latter

values are higher than ours, however, inasmuch

as Eriksson was unable to make a precise distinction between turbiditic and pelagic mud layers,

and he included part of the former for his pelagic

rate calculations. In the neighboring Alboran Sea,

Vergnaud-Grazzini and Bartolini (1970) calculate

rates of 26 to 30 cm/1000 years and Huang and

Stanley (1972) calculate a rate of sedimentation

during the past 10,000 years of 30 to 40 cm/1000

years in the basin plain and about 20 cm/1000

years on slopes. These somewhat higher rates

reflect the closer proximity to the North African

and Iberian Margin and greater access to sediments

transported by turbidity currents and gravityassisted flows; these studies also disregard the dis-
NUMBER 13 35

tinction between turbiditic and pelagic muds.

The frequency of turbidity currents has been

calculated on the basis of radiocarbon ages and

the isochrons computed with the aid of these. The

average frequency is 3 turbidity currents per 2000

years; the turbidity current frequency differs

greatly from core to core across the Algero-Balearic

Basin. From core 7 to core 10A these frequencies

number as follows: 12, 30, 30, 8, 18, 9, and 12 per

12,000 years. The average frequency of 3 turbidity

currents per 2000 years applies to the seven localities examined within the Abyssal Plain. It should

be pointed out that single turbidites were not

correlated between cores, and it is unlikely that

their individual beds extend over most of the basin

floor covered by the cores. Thus, the 3 turbidites

per 2000 years reflect different turbidity currents

at widely separated core localities. In other words,

the basin plain as a whole received considerably

more than 3 incursions per 2000 years.

In other basins, such as the middle of the San

Pedro and Santa Monica basins off California,

the frequency of turbidity currents is estimated at

1 every 400 years (Gorsline and Emery, 1959).

Most other frequency estimates are significantly

lower. In the Bahamas region, for instance, the

frequency varies from 1 per 500 years to 1 per

10,000 years in the Tongue of the Ocean (Rusnak

and Nesteroff, 1964), and I per 3000 to 6000 years

in the Columbus Basin (Bornhold and Pilkey,

1971). Estimates for ancient flysch occurrences are

1 turbidite per 2000 years (Moore, 1971), or per

4000 years (Sujkowski, 1957). The much higher

average turbidity current frequency in the AlgeroBalearic Basin (more than 3 incursions per 2000

years) is in part based on the fact that mud turbidites can be recognized in this study.

Conclusions

1. Sediment cores from the Algero-Balearic

Abyssal Plain, western Mediterranean, contain

large sections of mud, seemingly homogeneous.

Use of X-radiographs of split cores enables the

distinction between two different types of mud,

and the recognition of a large number of mud

layers, alternating in a regular fashion.

2. The color of the sediment is not a reliable

criterion to distinguish between different mud

layers and to determine their boundaries.

3. Structural, textural, and compositional properties of the two mud types indicate that one type

is deposited by turbidity currents, and the other

by pelagic settling. Incidental reworking of hemipelagic muds by bottom currents has occurred.

4. Among the many properties of the turbiditic

muds, the most characteristic are: structural,

textural, and compositional continuity between

underlying sand-silt turbidites and the muds,

grading within the mud layers, and a very small

amount of sand-size material (^ 1 percent).

5. The most distinctive properties of the hemipelagic muds are: they are entirely unlaminated,

display exclusively burrow structures, a sizable

amount of sand-size material is present (an average of 8 percent), the coarse components are dominantly made up of pelagic foram tests and pteropod

shells, their grain-size distribution is nearly normal

with a median around 9 phi, they are very poorly

sorted, and in general their properties are virtually

uniform between muds from different sediment

depths and core locations.

6. The boundary between the two mud types is

most distinctly marked by changes in the amount

of sand-grade material, the proportion of remains

of pteropods and pelagic forams, and the proportion of mica.

7. When the mud turbidite is markedly terrigenous in composition, the ratios of illite, chlorite,

and kaolinite differ between a turbiditic mud and

the overlying hemipelagic mud. This observation

is of consequence to studies of paleoclimatological

change based on clay mineralogy.

8. The distinction between turbiditic and hemipelagic muds in the cores enables an accurate

estimate of pelagic sedimentation rate from radiocarbon dates on pelagic carbonate sands. During

the past 16,000 years this rate has been 10 cm/1000

years.

9. This distinction coupled with the radiocarbon

dates also makes possible an accurate estimate of

the frequency of turbidity current incursions in

the Balearic Basin. The average frequency for

the past 20,000 years or so has been 3 incursions

per 2000 years in a particular core locality. The

number for the basin as a whole should be higher.

10. The seven analyzed cores from across the

southern Balearic Abyssal Plain show a facies of

alternating sand-silt and silt turbidites, mud turbidites (locally more than half of the core section),
36 SMITHSONIAN CONTRIBUTIONS TO THE EARTH SCIENCES

and hemipelagic muds (more than half in some

cores). The two mud types have as yet not been distinguished to this degree in ancient deep marine

successions. Largely shaly or marly flysch facies are

known, and these may bear some analogies with

the muddy turbiditic-hemipelagic sequences of the

southern Balearic Abyssal Plain examined in this

study. Comparison of similar ancient and modern

deepwater, mud-rich deposits warrants further

analysis.

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