fis
o
Parallel Laminated Deep-Sea Muds and Coupled Gravity
Flow-Hemipelagic Settling in the Mediterranean
^ H
SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES • NUMBER 19
SERIES PUBLICATIONS OF THE SMITHSONIAN INSTITUTION
Emphasis upon publication as a means of "diffusing knowledge" was expressed by the first Secretary of the Smithsonian. In his formal plan for the Institution, Joseph Henry outlined a program that included the following statement: " I t is proposed to publish a series of reports, giving an account of the new discoveries in science, and of the changes made f r o m year to year in all branches of knowledge." This theme of basic research has been adhered to through the years by thousands of titles issued in series publications under the Smithsonian imprint, commencing with Smithsonian Contributions to Know/edge in 1848 and continuing with the following active series:
Smithsonian Contributions to Anthropology Smithsonian Contributions to Astrophysics
Smithsonian Contributions to Botany Smithsonian Contributions to the Earth Sciences Smithsonian Contributions to the Marine Sciences
Smithsonian Contributions to Paleobiology Smithsonian Contributions to Zoology Smithsonian Studies in Air and Space Smithsonian Studies in History and Technology
In these series, the Institution publishes small papers and full-scale monographs that report the research and collections of its various museums and bureaux or of professional colleagues in the world of science and scholarship. The publications are distributed by mailing lists t o libraries, universities, and similar institutions throughout the world.
Papers or monographs submitted for series publication are received by the Smithsonian Institution Press, subject t o its own review for f o r m a t and style, only through departments of the various Smithsonian museums or bureaux, where the manuscripts are given substantive review. Press requirements for manuscript and art preparation are outlined on the inside back cover.
S. Dillon Ripley Secretary
Smithsonian Institution
S M I T H S O N I A N C O N T R I B U T I O N S T O T H E M A R I N E S C I E N C E S * N U M B E R 19
Parallel L a m i n a t e d Deep-Sea M u d s a n d Coupled Gravity Flow-Hemipelagic Settling
in the Mediterranean
Daniel Jean Stanley
SMITHSONIAN INSTITUTION PRESS City of Washington
1983
A B S T R A C T
Daniel J e a n Stanley. Parallel L a m i n a t e d Deep-Sea M u d s a n d C o u p l e d Gravity Flow-Hemipelagic Settling in the M e d i t e r r a n e a n . Smithsonian Contri- butions to the Marine Sciences, n u m b e r 19, 19 pages, 7 figures, 1983.—The origin of fine-grained deep-sea facies is often blurred because of interplay of diverse transport mechanisms: sediment gravity flow, traction related to fluid-driven circulation, a n d pelagic a n d hemipelagic " r a i n " mechanisms. Physical a n d chemical attributes of the M e d i t e r r a n e a n amplify petrologic differences, thus facilitating distinction between m u d types in this sea. I m p o r t a n t attributes include small distances between sediment input a n d depositional site, gener- ally low bottom current velocities in the deep basins, a n d shallow depths that permit preservation of carbonate components, an i m p o r t a n t criterion for m u d facies definition. Particularly i m p o r t a n t in the M e d i t e r r a n e a n are periodic development of intense water mass stratification a n d pycnoclines which act as sediment barriers, i.e., deviation of low concentration sediment gravity flows, a n d temporary retention of particles from turbid layer flows a n d hemipelagic settling. Release and differential settling of terrigenous silt a n d clay floes a n d reworked benthic a n d planktonic (largely coccolith a n d foraminifera) com- ponents from well-marked density interfaces occur in a m a n n e r such that particles are segregated according to size a n d density. T h e resulting varve-like deposits display fine parallel laminae of alternating coccolith- a n d terrigenous- rich layers that show diverse fining-upward trends. Finely laminated sections of this type accumulate more rapidly t h a n hemipelagites a n d are distributed over larger surfaces t h a n m u d turbidites. Analysis of bedform, texture-fabric, composition, geometry, a n d rates of sedimentation help distinguish (1) fine parallel laminated muds derived from coupled sediment gravity flow a n d hemipelagic settling from (2) laminated m u d turbidites, (3) laminated hemi- pelagites, and (4) contourites as commonly defined. Study of m u d lithofacies in small to moderate size seas, such as the M e d i t e r r a n e a n , holds promise for better interpretation of deep-marine fine-grained deposits.
OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: Seascape along the Atlantic coast of eastern North America.
Library of Congress Cataloging in Publication Data Stanley, Daniel J.
Parallel laminated deep-sea muds and coupled gravity flow-hemipelagic settling in the Medi- terranean.
(Smithsonian contributions to the marine sciences ; no. 19) Bibliography: p.
Supt. of Docs. no. : SI 1.41:19
1. Marine sediments—Mediterranean Sea. 2. Sediment transport—Mediterranean Sea. I.
Title. II. Series.
GC389.S8 1983 551.46'083'38 82-600337
Contents
Page
Introduction 1 Acknowledgments 2 Climate, H y d r o g r a p h y , a n d Sedimentation 2
Density Stratification Phases in the M e d i t e r r a n e a n 3 Fine-grained Facies a n d Isothermal H y d r o g r a p h i c Regimes 5
Fine-grained Sedimentation a n d Density Stratification 7
Finely L a m i n a t e d M u d V a r i a n t s 9 Implications a n d Conclusions 14
Literature Cited 17
Dedicated to the memory of
Professor Dr. J o h a n Ferdinand M a u r i t s de R a a f 1902-1982
inspiring leader astute sedimentologist
friend
Parallel L a m i n a t e d Deep-Sea M u d s a n d Coupled Gravity Flow-Hemipelagic Settling
in the Mediterranean
Daniel Jean Stanley
Introduction
Deposition of silt a n d clay lithofacies in the marine environment is attributed to settling of particles from suspension, to transport by fluid- driven processes related to ocean circulation (Heezen a n d Hollister, 1971:335-421; Eittreim a n d Ewing, 1972), a n d to sediment gravity flows (Piper, 1978; Stanley a n d M a l d o n a d o , 1981).
Theoretical a n d experimental studies indicate that suspension settling, traction, a n d gravity- driven high-concentration mass flow a n d low con- centration sediment flow mechanisms each result in characteristic bedforms a n d textural-fabric a n d compositional attributes of fine-grained deposits.
Distinct facies are thus believed to result from pelagic " r a i n , " tractive mechanisms related to bottom current transport (Stow a n d Lovell,
1979), a n d turbidity currents (Stow a n d Bowen, 1980). M u d s from the latter process have received by far the most attention to date.
Conditions in marine settings tend to vary a n d be more complex t h a n theoretical a n d experimen- tal examples. For example, the transit of silt a n d clay floes to the deep m a r i n e floor usually involves two or more processes. Moreover, it is recognized that stratification features in m u d deposits result
Daniel Jean Stanley, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C.
20560.
from mechanisms active just prior to, a n d at, the time of particle e m p l a c e m e n t . It has also become a p p a r e n t t h a t some silty deposits, earlier inter- preted as turbidites, actually have a more com- plex origin.
This p a p e r considers the interplay of transport mechanisms that produces a c o n t i n u u m of m u d types a n d thus blurs the origin of deep-sea fine- grained facies. Specific attention-is paid herein to finely l a m i n a t e d m u d s whose petrology appears to record the combined effects of sediment gravity flow, including turbidity current transport, a n d hemipelagic settling. T h e M e d i t e r r a n e a n Sea is an a p p r o p r i a t e area in which to conduct a study of lamination in t h a t (1) there are suitable n u m - bers of closely spaced core sites, a n d (2) average basin depths a p p r o x i m a t e 3000 m, well above the c a r b o n a t e compensation d e p t h (CCD), such that calcareous components, an i m p o r t a n t criterion of depositional regime, are fortuitously preserved.
Moreover, (3) M e d i t e r r a n e a n basins are consid- erably smaller t h a n those in major world oceans, a n d present diverse temporal a n d spatial facies variations, thus allowing more precise interpre- tation of dispersal a n d transport patterns. It is also recalled that (4) there is strong evidence of large-scale Q u a t e r n a r y to Recent oceanographic changes in the M e d i t e r r a n e a n (Stanley et al., 1975:287-289). These conditions provide an ideal setting in which to assess how climatically in-
SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
duced changes in water mass stratification affect pelagic settling a n d sediment gravity flows of low concentration and, in turn, the formation of lam- inae in m u d deposits. Identification of criteria for laminated m u d types emplaced by combined sed- iment gravity flow-suspension settling processes, a n d distinction of such facies from laminated m u d turbidites a n d hemipelagic end-members, are valuable outgrowths of the study.
ACKNOWLEDGMENTS.—The a u t h o r acknowl- edges valuable discussions during the initial phase of the study with P.H. Feldhausen, N U S Corpo- ration; a n d A. M a l d o n a d o , University of Barce- lona; a n d use of coarse fraction mineralogical counts m a d e by R. J. Knight, P e t r o - C a n a d a Ltd.
T h e paper was reviewed by R. D. Flood, L a m o n t - Doherty Geological Observatory; a n d J . W . Pierce, Smithsonian Institution; useful comments were also m a d e by anonymous reviewers. Fund- ing for this investigation, part of the Mediterra- nean Basin (MEDIBA) Project, was provided by Smithsonian Scholarly Studies grant 1233S101.
Climate, Hydrography, and Sedimentation It is recalled that a major control of fine- grained sedimentation in the deep sea is ocean circulation whose driving force is the sun (Lisit- zin, 1972:30-46). Climatic fluctuations, resulting in displacement of t e m p e r a t u r e a n d rainfall belts, modified world ocean circulation in the geologic past (Hay, 1974:1-5). Well documented are the Q u a r t e r n a r y climatic oscillations (Climap Project Members, 1976) that markedly altered circula- tion patterns, even at great depths (Schnitker,
1980). Such changes were of particularly large magnitude in the case of small- to moderate-size basins with silled, restricted passages that limit exchange with adjacent water bodies. T h e Med- iterranean serves as an ideal example in view of its narrow, elongate form between Africa, South- ern Europe, and the M i d d l e East, a n d the re- stricted exchange with Atlantic a n d Black Sea waters at the shallow, narrow Gibraltar a n d Bos- porus-Dardanelles straits.
Moreover, M e d i t e r r a n e a n circulation patterns
have been affected by a complex seafloor config- uration, including shallow ridges t h a t separate this sea into a series of quasi-isolated depressions of highly variable size, shape, a n d d e p t h . T h e Strait of Sicily a n d contiguous Siculo-Tunisian (or Pelagian) Platform (Carter et al., 1972; Bur- ollet et al., 1979) is a long b r o a d sill t h a t effec- tively separates the eastern a n d western basins, a n d thus is particularly i m p o r t a n t in control of M e d i t e r r a n e a n circulation (Allan et al., 1972:11- 25).
Present excess evaporation relative to precipi- tation a n d runoff results in the characteristic M e d i t e r r a n e a n h y d r o g r a p h i c regime involving the eastward flow of a moderately thin layer of less saline Atlantic water above a relatively iso- thermal column of westwardly moving, dense intermediate a n d deep water masses (Wiist, 1961;
L a c o m b e a n d T c h e r n i a , 1972). Circulation pat- terns, however, were markedly altered in the past as a result of somewhat lower t e m p e r a t u r e s a n d higher rainfall a n d runoff t h a n at present. Brad- ley (1938) postulated that basins in the eastern M e d i t e r r a n e a n (Ionian, Aegean, a n d Levantine) were periodically characterized by oceanographic patterns involving Black Sea-type density strati- fication, in contrast to the present isothermal regime.
W h a t would be the sedimentary responses on the deep-sea floor to such Q u a r t e r n a r y climatic- hydrographic oscillations? It can be predicted that gravity-driven, high-concentration mass dis- placement a n d dense sediment gravity flows transporting mixes of coarse silt, sand, a n d coarser particles downslope a n d onto basin plains would not be markedly influenced by development of water mass stratification. In contrast, it is envi- sioned that dispersal a n d the resulting deposi- tional patterns of low concentration flows, carry- ing mostly fine-grained material in suspension, would have been greatly modified by the devel- o p m e n t of a distinctly stratified water c o l u m n of the type that m a y periodically have prevailed in the Q u a t e r n a r y a n d Pliocene. This investigation considers the origin of l a m i n a t e d fine-grained facies, other t h a n sapropels (Ryan, 1972), a n d
NUMBER 19
related organic-rich layers ( M a l d o n a d o a n d Stan- ley, 1976b), that preferentially a c c u m u l a t e d at times when conditions of water mass stratification were clearly different t h a n at present.
Density Stratification Phases in the Mediterranean
In a far-sighted essay referring to modifications of M e d i t e r r a n e a n hydrography induced by Q u a - ternary oscillations, Bradley predicted (1938:377) that
during the last glacial stage, the Mediterranean, because of the different climate, would have had a surface layer of essentially fresh water overlying the oceanic water that filled the deep basins. Furthermore, the oxygenated Atlantic water would have had free access to the western basin of the Mediterranean through the Straits of Gibraltar beneath the outflowing surface stream of fresh water and in consequence the deep water of the western basin would have continued to be at least partially supplied with oxygen. But with a blanket of essentially fresh water covering the Mediterranean and moving westward there appears to be no way in which significantly large volumes of the oxygen-bearing saline wa- ter in the western basin could move across the sill at the Sicily Straits so as to cause circulation and afford an oxygen supply in the deep saline water of the eastern basin. For this reason the water in the eastern deep basin probably would have become stagnant and depleted of oxygen soon after the climatic change permitted fresh water to flood the surface of the Mediterranean.
Indirect, but strong, evidence for such physical oceanographic changes d u r i n g the late Pleisto- cene to Holocene has been provided d u r i n g the past two decades by studies of cores collected in different M e d i t e r r a n e a n basins. R a d i o c a r b o n dating a n d facies analysis of cores establish a stratigraphic base for regional correlation of Late Q u a t e r n a r y sediment sections (Stanley a n d Mal- d o n a d o , 1979). Noteworthy is the remarkable systematic repetition, or cyclicity, of Q u a t e r n a r y sedimentary sequences recorded by means of lith- ofacies ( M a l d o n a d o a n d Stanley, 1976b), geo- chemical a n d isotopic (Vergnaud-Grazzini et al.,
1977; Dominik a n d M a n g i n i , 1979; Luz, 1979), a n d faunal (Mars, 1963; R y a n , 1972; T h u n e l l a n d L o h m a n n , 1979) analyses.
Lithological cyclicity indicates that circulation
patterns in both eastern a n d western basins pe- riodically were altered markedly from those op- erating at present. Particularly i m p o r t a n t evi- dence in this respect is provided by dark, organic- rich fine-grained sapropel layers broadly distrib- uted in the eastern M e d i t e r r a n e a n (Figure 1A).
T h e r e is general agreement that reduced oxygen content favoring anaerobic conditions on the sea- floor resulted from density stratification (Kullen- berg, 1952; Olausson, 1961). T h e distribution of dated sapropels indicates that some stagnation phases resulting from temporarily altered w a t e r mass stratification were basin-wide a n d occurred primarily at depths in excess of 800 to 1000 m (Stanley, 1978). T h e uppermost layer, d a t e d at about 9000 to 7000 year B.P., formed d u r i n g a w a r m i n g phase (Ryan, 1972). T h e two underly- ing sapropel layers are d a t e d at a b o u t 25,000- 23,000 years B.P. (possibly d u r i n g a cooling phase) a n d at >40,000 years B.P. (Stanley a n d M a l d o n - ado, 1979:40-42).
Sapropels are absent in the three deep (to a b o u t 1700 m) Strait of Sicily basins, a n d in the Ligur- ian, T y r r h e n i a n , Algero-Balearic, a n d Alboran seas. However, sections in m a n y cores recovered in these central a n d western M e d i t e r r a n e a n de- pressions, some d a t e d at a b o u t 11,000 to 9,000 years B.P., display dark gray-olive streaked m u d layers that contain pyritized tests a n d burrows (see, for example, core logs in H u a n g a n d Stanley, 1972:536-538; R u p k e a n d Stanley, 1974; 11,25;
M a l d o n a d o a n d Stanley, 1976a:53-56). T h e ra- diocarbon dates of these core sections m a y b e somewhat older t h a n the age of actual sediment emplacement, due to the presence of reworked calcareous microfossil tests. It is likely that the streaked, gray-olive, pyrite-rich m u d sections of western M e d i t e r r a n e a n cores are chronologically equivalent to the sapropel zones of basins east of the Strait of Sicily. T h e reduced, pyrite-rich sec- tions of the western M e d i t e r r a n e a n , like sapro- pels, record phases of density stratification as depicted in oceanographic models presented for the early Holocene (Figure 1B).
In sum, lithofacies d a t a support the hypothesis that an "estuarine-type" circulation regime a n d
48°-
44°
less dense surface w a t e r
stagnant ( h ^ S - r i c h ) deep water
NUMBER 19
current reversal d o m i n a t e d the M e d i t e r r a n e a n for short periods of time (Bradley, 1938;377;
Mars, 1963;67-7l). T h a t the oceanography in the eastern basins at times of sapropel deposition, when oxygen-deficient conditions prevailed, may be likened to that of the present Black Sea is also indicated by analysis of Sea of M a r m a r a cores (Stanley a n d Blanpied, 1980). It is postulated that excess glacial melt waters moved westward as overflow from the Black Sea across the Bos- porus-Dardanelles portals, a n d then south across the Aegean Sea. After entry into the Levantine a n d Ionian seas, this less saline surface layer flowed westwardly to a n d across the strait of Sicily and, eventually, Gibraltar (Figure 1). Den- ser Atlantic water, flowing eastward below the surface layer, entered the Alboran Sea a n d west- ern M e d i t e r r a n e a n producing some mixing a n d precluding complete anaerobic conditions of the Algero-Balearic, Ligurian, a n d T y r r h e n i a n sea- floor.
Current reversal as outlined above remains within the realm of theory. T h e r e is little doubt, however, that the Q u a t e r n a r y sediment cover, showing cyclothemic development of alternately reduced a n d oxygenated facies across m u c h of the M e d i t e r r a n e a n , records the direct interaction be- tween climatically controlled water circulation fluctuations a n d deep basin sedimentation. Stra-
FIGURE 1.—A, Chart showing distribution of youngest or- ganic-rich sapropel, Si (diagonal lines), deposited in the eastern Mediterranean at about 9000 to 7000 years B.P., a time when hydrographic conditions were markedly different from those at present. (Heavy arrows depict generalized flow patterns of less dense upper water mass; light arrows show fluvial input; Ae = Aegean Sea; B = Balearic Basin; Bo = Bosporus; BS = Black Sea; D = Danube; I = Ionian Sea; Le
= Levantine Sea; Li = Ligurian Sea; N = Nile River input;
P-A = Po-Adriatic Sea; R = Rhone River input; SS = Strait of Sicily; STP = Siculo-Tunisian Platform; T = Tyrrhenian Sea; Ly-II-3 = position of core Lynch cruise II core 3; after Stanley, 1978.) B, Scheme illustrating "estuarine-type" water mass regime in the Mediterranean at times of sapropel and finely laminated mud deposition. Note marked stratification of water masses and development of Black Sea-type anoxic conditions in the eastern basins unlike those presently exist- ing. (Arrows depict possible flow patterns of water masses;
after Stanley et al., 1975.)
tigraphically correlated cyclothemic sequences from eastern a n d western M e d i t e r r a n e a n basins highlight the n o n r a n d o m vertical a n d lateral dis- tributions of different fine-grained sediment types ( M a l d o n a d o a n d Stanley, 1977; Stanley a n d M a l - d o n a d o , 1981). It is useful to c o m p a r e silt a n d clay facies that a c c u m u l a t e in isothermal versus stratified water regimens.
Fine-grained Facies and Isothermal Hydrographic Regimes
M u d types, including the fine silt, clayey silt, a n d silty clay, are most reliably identified as to process of e m p l a c e m e n t on the basis of structures visible in radiographs, size analyses, a n d coarse- fraction composition. Silt a n d sand-sized terrige- nous clasts a n d c a r b o n a t e grains of biogenic (largely coccoliths, foraminifera, a n d shell frag- ments) a n d clastic (reworked limestone a n d do- lomite) origin are valuable criteria for distinguish- ing m u d types.
M u d d y slump, dense m u d flow, a n d m u d d y debris flow deposits a c c u m u l a t e d t h r o u g h o u t the late Q u a t e r n a r y independently of h y d r o g r a p h i c fluctuations (Stanley a n d M a l d o n a d a , 1981:278- 283). These mass flow units are concentrated in areas of marked seafloor relief (slopes, ridge mar- gins, base-of-slope environments) in association with sand a n d m u d turbidites a n d hemipelagic m u d s . In u p p e r Holocene to recent time, m u d t u r b i d i t e - h e m i p e l a g i c m u d assemblages gener- ally prevail on basin margins in areas of low relief, including perched slope basins a n d small depressions in cobblestone topography. In more distal, low relief environments, such as basin a n d trench plains, a distinct m u d turbidite-unifite- hemipelagic m u d assemblage is usually recovered (Figure 2). (Unifite is the descriptive term for uniform, almost structureless m u d , cf. Stanley,
1981; Blanpied a n d Stanley, 1981:2.)
T e m p o r a l a n d spatial distribution p a t t e r n s of this latter distal basin a n d trench plain lithofacies assemblage provides insight on the relation be- tween oceanographic conditions a n d silt a n d clay transport processes in deep m a r i n e settings. Struc-
SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
3cm
NUMBER 19
tureless a n d vaguely laminated unifites (Figure 2E,F) are distinctly restricted to flat basin a n d trench plains where they are associated with fine- grained m u d turbidites (Figure 2 B - D ) : both m u d types are laterally continuous across basins (Fig- ure 2A). Petrologically, unifites a p p e a r similar to nearly structureless m u d s that are referred to as
"homogenites" by some workers (cf. Kastens a n d Cita, 1981). R a d i o c a r b o n d a t i n g of unifite sec- tions records very high rates (>200 c m / 1 0 0 0 years) of accumulation. Both structureless a n d vaguely laminated unifites are somewhat finer grained t h a n turbiditic m u d s with which they are associated. Unifites a n d turbidites fine u p w a r d gradually a n d contain comparable proportions of terrigenous a n d reworked fauna carbonate com- ponents (usually > 3 0 % of the sand-size fraction).
Spatial distribution patterns, rapid sedimentation a n d textural a n d mineralogical attitudes indicate that both fine-grained, almost structureless tur- bidites a n d unifites are released from thick, low density or lower velocity sediment gravity flows (Stanley a n d M a l d o n a d o , 1981:284, 290). Uni- fites a p p e a r to be the dilute end-products from turbidity currents, turbid layer flows, a n d suspen- sate-rich clouds (Stanley, 1981; Blanpied a n d Stanley, 1981:29-35).
Typical M e d i t e r r a n e a n hemipelagic m u d lay- ers, in contrast to fine-grained turbidites a n d unifites, are more commonly bioturbated a n d show speckling as a result of high concentrations
FIGURE 2.—A, Subbottom 3.5 kHz record showing part of the Zakinthos Trench plain in the western Hellenic Arc (arrow points to thick, laterally continuous, acoustically transparent layer, probably a fine-grained turbidite and/or associated unifite). B-D, Radiograph prints of selected core sections showing typical sharp-based, fining-upward lami- nated mud turbidites (t) and hemipelagic mud (He) with speckling. (Note cross-stratification in B and D, and distinct fining-up trend of entire laminated section in c.) E, Faintly laminated unifite. F, Structureless unifite. (B and D, respec- tively, Marsili cores 12 and 18 in the Kithera Trench sector;
F, Trident core 172-34 in the Zakinthos Trench, Hellenic Arc (locations shown in Stanley and Maldonado, 1981); c, Lynch cruise II core 8A in the Algero-Balearic Basin; E, Western Alboran Basin core 91 (locations for c and E, respectively, in Rupke and Stanley, 1974, and Huang and Stanley, 1972).)
of planktonic tests (usually > 7 5 % of sand frac- tion). Moreover, layer thickness a n d rates of ac- c u m u l a t i o n (<10 c m / 1 0 0 0 years; R u p k e a n d Stanley, 1974:33) of hemipelagites tend to be m u c h lower. Structures a n d mineralogical com- position suggest that hemipelagic m u d s a c c u m u - late largely from suspension " r a i n " mechanisms r a t h e r t h a n from sediment gravity flows.
U n d e r conditions of poorly developed w a t e r column stratification, such as that displayed by the present M e d i t e r r a n e a n , gravity-driven flows, such as turbitity currents of moderately low den- sity that carry clay- a n d silt-size floes primarily in suspension, would likely move downslope.
These flows would move relatively u n h a m p e r e d along the bottom, from proximal sites (outer shelf to u p p e r slope, ridge, basin margin) to the base- of-slope a n d well onto basin plains. T h e t u r b u l e n t head of such flows would entrain in a downslope direction pelagic material, settling, or recently deposited, from suspension " r a i n " ; in some cases flows would also resuspend a n d transport basin- ward surfical biogenic a n d clastic material eroded from basin slopes. This process would result in heterogeneous mixtures of displaced planktonic tests, terrigenous grains, a n d older reworked faunas, typically recovered from associated u n i - fites a n d fine-grained turbidites restricted to basin plains.
T h e r e is a decreased proportion of unifites in the eastern M e d i t e r r a n e a n at times of sapropel deposition. This suggests that the flow p a t t e r n s of low density " c l o u d s " were in some m a n n e r m o d - ified d u r i n g downslope movement by a barrier effect resulting from distinct density differences that periodically developed within the water col- u m n .
Fine-grained Sedimentation and Density Stratification
W a t e r masses of different t e m p e r a t u r e , salinity, or both, are separated by a sharp vertical gradient in density, or pycnocline. Theoretical calcula- tions, supported by field observation, show t h a t sharp density gradients m a y temporarily retain
8 SMITHSONIAN CONTRIBUTIONS TO THE MARINE SCIENCES
particles settling through the water column a n d modify the flow p a t h of very low concentration plumes (Pierce, 1976). Pycnoclines would induce detachment of a suspensate-rich flow from the seafloor into the water column, rather t h a n allow- ing a bottom-hugging turbid layer flow of the type necessary to deposit a fine-grained m u d turbidite with typical structures a n d / o r associ- ated structureless unifite. Flow d e t a c h m e n t is ex- pected wherever suspended sediment-enriched water encounters a denser water layer. Off Cali- fornia, for example, Drake et al. (1972:328) have shown that minimal concentrations of 100 m g per liter, a n d generally m u c h higher (~2500 m g / 1 ; cf. Pierce, 1976:441), are needed to overcome density interfaces resulting from t e m p e r a t u r e changes in the water column.
In the Mediterranean, m u c h of the fine-grained sediment presently introduced by rivers (notably the Nile, R h o n e , Po, Ebro, a n d Seyhan) a n d by wind tends to spread out away from the coast along the upper major intermediate a n d deep water (Emelyanov a n d Shimkus, 1972). At times when stratification was better developed, deposi- tional patterns almost certainly were more com- plicated. Particle aggregates settling from the upper pycnocline would reconcentrate along one of several lower density interfaces before finally settling to the bottom. In a multiply stratified column, one envisions a sequence involving (1) detachment of a turbid layer from the bottom, (2) aggregation, (3) particle settling, (4) renewed accumulation along a lower density interface, a n d (5) subsequent settling across still deeper pycno- clines prior to (6) deposition on the seafloor (cf.
Pierce, 1976 Fig. 2).
O p e n ocean suspended sediment studies show that, eventually, particles either become incor- porated in a near-bottom turbid (nepheloid) layer, or settle directly onto the bottom. Still limited investigations of suspended sediment con- centrations in the M e d i t e r r a n e a n show no well- developed nepheloid layers in the western basins at present (Pierce a n d Stanley, 1976; Pierce et al., 1981). Nepheloid layers in this sea, however, con- ceivably may have been i m p o r t a n t in the past. It
is thus possible t h a t m i d w a t e r a n d b o t t o m masses transported silt a n d clay size particles over larger geographical areas t h a n they do today, perhaps in the m a n n e r nepheloid layers in other world oceans presently distribute substantial volumes of material (Eittreim a n d Ewing, 1972).
Some aspects of a generalized model for sus- p e n d e d sediment transport in a stratified system, including d e t a c h m e n t (Figure 3), m a y be applied to the M e d i t e r r a n e a n . For example, detailed lithofacies analyses of Nile cone cores indicate t h a t there exists a r a t h e r direct relation between rates of sedimentation a n d the frequency of lam- inated layers in u p p e r Q u a t e r n a r y sections (Mal- d o n a d o a n d Stanley, 1978, 1979). It has been postulated that at times of sapropel formation, low density t u r b i d flows e m a n a t i n g from the m o u t h of the Nile a n d moving downslope were retained above the cone as detached layers. Con- centration of fine-grained material along pycno- clines on the Nile cone is schematically depicted in Figure 4.
More specific lithologic evidence recording probable influence of density stratification on fine-grained sedimentation is provided by suites of cores recovered from the western Hellenic T r e n c h . These show that the most recent major phase of finely l a m i n a t e d m u d deposition oc- curred from a b o u t 17,000 to 6000 years B.P. This period coincides with rapid eustatic sealevel rise a n d progressive climatic w a r m i n g at the end of the Pleistocene a n d early Holocene. It is recalled that the most recent sapropel, Si, was deposited between a b o u t 9000 to 7000 years B.P. (Ryan, 1972:150). T h e finely l a m i n a t e d m u d facies, as the Si sapropel layer, was broadly distributed areally over highly diverse a n d irregular Hellenic T r e n c h slope a n d basin topography; however, unlike Si which a c c u m u l a t e d in less t h a n 2000 years, finely l a m i n a t e d m u d s were deposited dur- ing a period lasting 10,000 or more years. Finely laminated m u d s d u r i n g this same period also a c c u m u l a t e d over large a n d topographically var- ied areas in other eastern a n d western Mediter- ranean sectors a n d Strait of Sicily basins. T h e i r rate of accumulation is generally lower ( < 2 5 c m /
NUMBER 19
FIGURE 3.—Schematic of suspended sediment transport possibly applicable to the Mediterra- nean at times when water masses were stratified. Concentrations of suspensates (stippled areas) are shown as detached turbid layers along pycnoclines, and as bottom nepheloid layer, driven by bottom current circulation, on slope to basin plain. (Large arrows depict flow of water masses, a pattern similar to off U.S. East Coast, after Pierce, 1976.)
1000 years) t h a n that of other m u d types em- placed by sediment gravity flows, but distinctly higher t h a n that of hemipelagites (<10 c m / 1 0 0 0 years). It is recalled that the proportion of thick unifites, emplaced by low concentration bottom- hugging flows, is reduced d u r i n g the period of finely laminated m u d deposition.
These considerations provide the basis for a generalized depositional model relating finely laminated facies with low concentration flows in a stratified water column (Figure 5). T h e scheme emphasizes introduction of particles from (1) gravity-driven flows entering along basin mar- gins, a n d from (2) hemipelagic settling from sur- face waters. Concentrations of particles spread
broadly on density interfaces in the water c o l u m n as continuous or detached t u r b i d layers. T h e progressive release of silt a n d clay size material over large areas of the seafloor is contrasted to material carried along the b o t t o m by turbidity currents which, morphologically constrained, are more limited in seafloor surface distribution.
Closer examination of finely l a m i n a t e d m u d types elucidates this model.
Finely Laminated M u d Variants Finely l a m i n a t e d facies are most reliably rec- ognized by c o m b i n e d r a d i o g r a p h y a n d petrogra- phy. T h e most diagnostic criterion is closely
10 SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
NORTH
FIGURE 4.—Plexus of sediment processes on the Nile cone during phases of Mediterranean stratification (after Maldonado and Stanley, 1978). Dense turbidity currents (TC) cross pycnoclines as they flow downslope along the bottom, while lower concentration suspensate- rich zones form detached turbid layers (DTL) on pycnoclines. (Other symbols: AC = Alexandria Canyon; HBP = Herodotus Basin plain; RB = Rosetta Branch; RCD = Relict coastal and neritic deposits; SB = stratification barrier; SL = slump; SO = shelf-edge spill-over; SP = sand pods on Nile cone; T D = sediment turbid discharge off the Rosetta Branch of the Nile.)
spaced, parallel (<0.5-5.0 mm) laminae. These are sometimes visible in split cores (Ryan et al., 1973; Hsu et al., 1978), but always m u c h better defined by density differences observed in radio- graphs (Figure 6). Parallel laminae in some Med- iterranean basins are often indistinct, particularly in shallower (<1500 m) settings (Figure 6 B ) . In contrast, they are well developed in deeper basins and particularly those in the Hellenic T r e n c h area (Got et al. 1977, fig. 4; Stanley a n d M a l d o n - ado, 1981, fig. 3).
Core sections that comprise largely fine lami- nated facies in some of the deeper Hellenic
T r e n c h basins are dark olive gray. Most laminae are varve-like in that they consist of paired alter- n a t i n g layers of different mineralogy; a lamellar pair is termed a couplet. Some individual lami- nae, or couplets, do not clearly display a fining- u p w a r d texture (Figure 6 D ) . C o m m o n l y , however, laminae couplets show diverse fining-upward trends (Figure 6c, E - G ) involving changes from clayey silt to silty clay (textural d a t a from Smith- sonian Sedimentology L a b o r a t o r y d a t a base).
S E M analysis of the silt fraction shows t h a t the layering a p p a r e n t in radiographs consists of alter- nating coccolith- a n d silty clay-enriched oozes.
NUMBER 19 11
PROCESS
' ' ( H P )
ISLUMPING
ii (HI
ISUSPENSiONl
i (HI MUD FLOW TURBIDITY CURRENT
DEBRIS FLOW|-»"rTURBIDITY CURRENT
DEPOSIT LARGELY FROM GRAVITATIVE
PROCESSES: (CHAOTIC MASS)
-F
(DEBRIS FLOW (COMPLETE DEPOSIT) TURBIDITE)1 -\7
j TURBID LAYER FLOW *—
T
ROLE OF STRATIFICATION
• • • • • • • • • • • I
INTERFACE AS BARRIER
I DETACHED TURBID LAYER FLOW"!
DEPOSIT FROM GRAVITATIVE
& SUSPENSION-RELATED PROCESS
rnTrrr"!
WELL-LAMINATED MUD-OCCURRENCE: PRIMARILY AREA OF RELIEF WIDESPREAD DISTRIBUTION IN STUDY AREA
• » » - )
DISTAL BASIN PLAIN
FIGURE 5.—Depositional model showing a basinward transformation continuum of gravity- driven processes, i.e., mechanisms of decreasing concentrations, from slumping to low density turbid layer flows. An important stratification interface in the water column serves as a barrier to particles released from low concentration turbidity currents and turbid layer flows and also from hemipelagic (H.P.) suspension settling. As a result of the interface, particles are distributed over large areas of a basin and eventually settle out as finely laminated layers, often graded.
The nature of the stratification interface controls the degree of grain retention and ultimate release of terrigenous grains and pelagic components particles, which settle according to size and density. Periodic variations in organic productivity and fluctuations in particle density and settling rate give rise to varve-like alternations of coccolith- and terrigenous-rich layers, many of which display fining-upward trends (see Figure 6E-F).
Radiographs reveal no two identical sequences of parallel laminated structures. Variations occur with respect to proportions a n d thicknesses of coccoliths and silty clay-rich layers (compare, for example, Figure 6c a n d D ) . Varved a n d graded finely laminated sections also vary with respect to coarseness of material. Occasionally, large planktonic tests such as pteropod shells are ob- served, a n d these usually are flat-lying a n d buried by fine laminae (Figure 6A). Alternations of coc- coliths a n d clastic silt a n d clay aggregates may in some instances enclose i m p o r t a n t proportions of larger particles, such as disseminated organic m a t t e r (Figure 7A, B) a n d concentrations of plank- tonic tests (usually foraminifera, Figure 7c).
Petrographic examination of the coarse frac- tion (>62 jtim), separated from finely laminated
m u d from different parts of the M e d i t e r r a n e a n , reveals that in almost all instances, the bulk (>70%) of the coarse fraction comprises plank- tonic tests. Foraminifera d o m i n a t e , b u t pteropods may be locally i m p o r t a n t . Benthic shell material usually accounts for less t h a n 10%, while terrige- nous grains generally exceed 10%. Within-core a n d core-to-core variations in the above-cited proportions of these mineralogical c o m p o n e n t s are observed.
O n the basis of structure, texture, a n d c o m p o - sition the M e d i t e r r a n e a n deep-water finely lami- nated facies generally can be distinguished from m u d s of purely pelagic (with little or no terrige- nous components) suspension a n d even from those of essentially hemipelagic (mixed plank- tonic a n d terrigenous components) settling origin.
12 SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
B C D
, » ^ M |
H B 9
-
jfHHr
//^mt^^^^^
•
-'''^MH^^H
'^iiflH
ail
tmm
, '&
W4
m m
0
3cm
NUMBER 19 13
V
0 cm
FIGURE 7.—Selected radiograph prints: A,B, disseminated organic matter (some pyritized); c, concentrations of planktonic foraminiferal tests in finely laminated core sections. (Cores are from the western Hellenic Trench: A, Trident core 172-34, Zakinthos Trench; B, Marsili core 8, Matapan Deep; c, Marsili core 14, Gulf of Laconia, south of Peloponnesus; locations shown in Stanley and Maldonado, 1981.)
T h e a m o u n t of sand fraction in l a m i n a t e d m u d (about 2% by weight) is lower t h a n t h a t in typical M e d i t e r r a n e a n hemipelagic m u d s (to > 5 % by
FIGURE 6.—Selected radiograph prints showing variations of finely laminated muds: A,B, examples of distinct and vague fine parallel laminations; C,D, two different types of varve- like stratification, both showing importance of coccolith (light) layers; individual pairs, or couplet, of laminae show fining-upward trends, but overall the entire laminated se- quence is not graded (unlike turbidite section in Figure 2c);
E-G, thicker fining-upward (arrows) type of finely laminated sections, comprising largely coccoliths and pelagic compo- nents rather than terrigenous silty clay in typical laminated mud turbidites (Figure 2B-D). (Arrow in A points to pteropod buried by finely laminated coccolith ooze; A,D, Trident core 172-34 in Zakinthos Trench basin; c, E-G, Trident core 172- 36 in Strofadhes Trench basin, in western Hellenic Arc (core locations shown in Got et al., 1977).)
weight); sand in t h e latter c o m m o n l y p r o d u c e s speckling observed in r a d i o g r a p h s (He in Figure 2D). Moreover, h e m i p e l a g i t e sections, such as those in t h e Hellenic T r e n c h , generally c o n t a i n higher (>80%) contents of foraminifera a n d pter- opods in t h e sand fraction a n d c o n t a i n coccoliths in t h e silt fraction.
Parallel l a m i n a t e d m u d s also c a n b e distin- guished from some l a m i n a t e d m u d turbidites on the basis of associated structures a n d composition.
Unlike l a m i n a t e d m u d turbidites (Figure 2c; also examples in Stow a n d Bowen, 1980), fine parallel l a m i n a t e d deposits (Figure 2 B - D ) a r e not s h a r p - based nor are they texturally g r a d e d , i.e., showing a progressive decrease in m e a n grain size a n d l a m i n a thickness from t h e base to t h e t o p of t h e l a m i n a t e d bed. Moreover, bedforms c o m m o n l y
14 SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
attributed to traction, such as wave ripples a n d cross-laminations that commonly occur in m u d turbidites (Figure 2 B , D ) , are absent from finely laminated facies. Finely laminated m u d s do not show load structures a n d only rarely do they show bioturbation.
Both laminated m u d turbidites a n d finely lam- inated muds may contain c o m p a r a b l e propor- tions of sand (to about 2% by weight), but sand in turbidites usually contain a higher (to >30%) terrigenous grain content (from mineral count d a t a base, R J . Knight, in litt.). S E M analyses reveal reworked biogenic components, including pre-Pleistocene coccoliths, in both m u d turbidites a n d finely laminated facies. However, there is a higher proportion of coccoliths in the silt fraction and foraminifera in the sand fraction of some finely laminated m u d couplets t h a n in turbidites.
It is noted that some fining-upward laminated core sections cannot strictly be a t t r i b u t e d to fine parallel laminated m u d , laminated m u d turbi- dite, or hemipelagic m u d facies. Apparently there exists a petrologic c o n t i n u u m a m o n g these types.
Implications and Conclusions
An association of different laminated end- member m u d types most likely occurs in settings where two or more transport processes are in- volved. A c o n t i n u u m of fine-grained facies a n d a blurring of their origin should be expected where long distance transport increases the possibility of interplay a m o n g turbiditic, traction, a n d hemi- pelagic settling mechanisms. T h e Northwest At- lantic margin off the U.S. East Coast and C a n a - dian Maritime Provinces, where most sedimen- tologic studies of fine-grained deposits have been made, serves as a good example. In this region the possible origins for lamination are reviewed by Stow a n d Bowen (1980). In the case of the m u d turbidite end-member, these authors pro- pose a model involving thick flows of low concen- tration in which laminae formation results from depositional sorting by increased shear near the base of the bottom b o u n d a r y layer.
Theoretical considerations a n d textural-fabric
analysis of l a m i n a t e d turbidite core sections call attention to systematic u p w a r d a n d lateral-down- slope decreases in m o d a l grain size a n d l a m i n a e thickness. In contrast, lamination in typical fine- grained contourites (the term is used here as applied by Heezen a n d Hollister, 1971:420) re- sults from fluctuations in sediment flux a n d ac- c u m u l a t i o n by both traction a n d suspension release mechanisms. Some criteria of m u d con- tourites are sharp base a n d top of b e d d i n g con- tacts, both n o r m a l a n d reverse grading, c o m m o n cross-lamination, a n d preferred grain orientation parallel to the b e d d i n g plane t h r o u g h o u t a bed (Hollister a n d Heezen, 1972; Stow a n d Lovell, 1979). However, review of published radiographs a n d descriptions of laminated m u d core sections in large basins, such as the Atlantic, show that petrologic distinction between fine-grained tur- bidites a n d contourites is by no means clear-cut.
T h e petrology of certain l a m i n a t e d m u d sections interpreted as turbidites suggests a more complex origin involving release from suspension a n d / o r fluid-driven circulation; other sections inter- preted as contourites a p p e a r difficult to distin- guish from low concentration turbidity current or turbid layer deposits.
Physical a n d chemical attributes of the m u c h smaller a n d shallower M e d i t e r r a n e a n serve to amplify petrologic differences that are not so a p p a r e n t a m o n g fine-grained, deep-water depos- its in large ocean basins. I m p o r t a n t Mediterra- nean characteristics include periodic develop- ment of well-stratified water masses, relatively small distance between point of sediment input a n d final depositional site, generally low b o t t o m current velocities in deep basins, a n d preservation of c a r b o n a t e components. D u r i n g specific periods in the recent past, oceanographic conditions, a n d thus sedimentation in the M e d i t e r r a n e a n were somewhat more c o m p a r a b l e to other world oceans. Sediment transport involved (1) periodic injection of sediments a n d their displacement downslope by gravity flows of low concentration, (2) deviation of such flows diagonally or parallel to the margin by current-driven circulation, (3), incorporation, concentration, a n d displacement
NUMBER 19 15
of particles from turbidity current-derived sus- pensates a n d hemipelagic settling on pycnoclines and in near-bottom nepheloid layers, a n d (4) eventual release of these particles through density interfaces a n d from nepheloid layers to the sea- floor. A laminated sequence resulting from this plexus of mechanisms is neither a m u d turbidite, hemipelagite, or contourite in the strict sense.
O n the basis of the petrology of M e d i t e r r a n e a n laminated m u d s , described in the previous sec- tion, it is possible to distinguish (1) laminated m u d turbidite a n d (2) hemipelagic m u d end- members from (3) a deposit originating by cou- pled sediment gravity flow-hemipelagic settling.
T h e multiple-origin deposits, "(3)," are varve-like a n d record episodic fluxes preserved as thin, al- ternating sequences of clastic components that include reworked benthic a n d planktonic faunas mixed with clayey silt a n d layers comprising al- most entirely tests of recent pelagic organisms.
T w o other diagnostic criteria of these finely lam- inated sequences are rates of deposition a n d areal distribution. Core sections show that fine parallel laminated facies, like those of hemipelagites, tend to accumulate over broad areas of a basin a n d its contiguous margins (cf. Stanley a n d M a l d o n a d o , 1981, fig. 7). These muds, recording periodic injection by sediment gravity flows, a c c u m u l a t e at higher rates (>15 c m / 1 0 0 0 years) t h a n hemi- pelagic deposits. In contrast to turbidites a n d unifites that are somewhat richer in terrigenous components a n d accumulate at higher rates, re- gional core surveys show that finely laminated muds display a regionally more consistent thick- ness a n d broader distribution, a n d are less re- stricted to specific depositional environments.
T h e model emphasized here (Figure 5) indi- cates that stratification interfaces in the water column that affect particle retention are impor- tant for interpreting some types of fine-grained lamination. This scheme suggests that distinct pycnoclines modify particle concentration a n d particle release rates through the water column so as to segregate terrigenous silt a n d clay floes from planktonic tests. A well-marked density in- terface amplifies differences in particle release
a n d settling rates through the c o l u m n by density a n d size, thus resulting in (1) more effective sep- aration of coccolith from terrigenous silty clay layers a n d (2) better development of fining-up- ward trends of laminae sets (Figure 6 E - G ) . T h e varve-like configurations in Figure 6 A - D record repetition of depositional events a n d modifying effects of pycnoclines, i.e., injection of material by turbidity currents a n d t u r b i d layers, retention of periodic planktonic blooms, largely coccoliths, a n d their subsequent settling to the seafloor.
Compositional segregation of coccoliths from ter- rigenous clayey silts, as distinct laminae, a p p e a r s less well developed in l a m i n a t e d m u d turbidites than in the fine parallel m u d facies. O n the basis of an averaged depositional rate of 20 c m / 1 0 0 0 years, each laminae couplet is deposited d u r i n g a 2 to 20 year period.
O n the other h a n d , the downslope decrease of grain size a n d laminae thickness measured in m u d turbidites (cf. Stow a n d Bowen, 1980) is less obvious in finely laminated facies. In this respect, the geometry of M e d i t e r r a n e a n u p p e r Pleistocene to lower Holocene finely laminated facies ( H u a n g a n d Stanley, 1972; M a l d o n a d o a n d Stanley,
1976a; Stanley a n d M a l d o n a d o , 1981), recalls the configuration a n d distribution of fine parallel, silt-rich layers m a p p e d over large sectors of the Delaware River Basin of T e x a s - N e w Mexico in middle Permian time. These latter l a m i n a t e d , not conspicuously graded, facies, deposited as a func- tion of density-stratified water mass conditions, show no obvious proximal to distal changes (Harms, 1974). H y d r o g r a p h y a n d sediment grav- ity flows are major factors in b o t h these m o d e r n a n d ancient deposits. Sediment gravity flows that displace at least 1 km' of sediment are not u n c o m - m o n in M e d i t e r r a n e a n basins (Blanpied a n d Stanley, 1981:29). Flows of this m a g n i t u d e intro- duced into the isothermal regime, which has pre- vailed d u r i n g the past 6000 years, are responsible for turbidites a n d unifites r a n g i n g in thickness from 30 cm to over 1000 cm in small basins, such as those of the Hellenic T r e n c h . However, dis- placement of equivalent sediment volumes as low concentration flows, at times of a stratified water
16 SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
regime, would result in m u c h broader distribu- tion of material. Varve-like sets released from about 1 km3 of sediment spread over large parts of basins, such as the Ionian or Algero-Balearic, would likely be less t h a n 1 cm thick (Stanley,
1981:83).
L a m i n a e preservation also sheds light on dep- ositional origin. Some cores from the Aegean, Siculo-Tunisian (Pelagian) Platform ( M a l d o n a d o a n d Stanley, 1976a; Blanpied, 1978), Corsican T r o u g h (Stanley et al., 1980), Alboran Sea ( H u a n g a n d Stanley, 1972), a n d other areas show indistinct (Figure 6B) or disturbed laminae re- cording the influence of b o t t o m currents, or or- ganisms, or both. In contrast, 10 to 40 cm-thick sections of undisturbed laminated units in the Hellenic T r e n c h region indicate that deposition was rapid, that the influence of b o t t o m currents was negligible and, more importantly, unfavora- ble oxygen-deficient conditions on the seafloor reduced benthic populations, thus minimizing disturbance of surficial sediments. Moreover, it is recalled that the well-preserved finely laminated m u d sections have a basin-wide a n d temporally extensive distribution. It thus appears that u p p e r Pleistocene to Holocene lamination in the Medi- terranean was not produced by an oxygen mini- m u m layer restricted to a specific horizon in the water column as recorded in the present Gulf of California (Calvert, 1964). Preservation of lami- nae is most likely related to the formation of thicker, more extensive columns of oxygen-defi- cient water and to anoxic seafloor conditions of the Black Sea (Arkhangel'skiy, 1927) a n d deep fjord (Strom, 1939) types; the absence of benthic organisms is an important factor in the preser- vation of well-defined parallel laminae, as illus- trated by Miiller a n d Stoffers (1974, fig. 4).
T h e varved aspect calls to mind the relation between sedimentary structures a n d organisms in some stratified basins off Southern California.
T h e r e , it has been shown t h a t small-scale a n d short-term changes in oxygen content can induce rather significant alterations in organic p r o d u c - tivity, including coccolith a n d foraminiferal blooms, a n d modifications of b e n t h i c popula- tions. Physico-chemical changes needed to trans- form hospitable to inhospitable conditions at d e p t h m a y be small; moreover, such changes m a y occur suddenly or gradually a n d , as in the case of lakes, seasonally (cf, Hiilseman a n d Emery,
1961:289). Inhospitable conditions on the sea- floor, such as at times of sapropel formation, result in diminution or absence of b e n t h i c popu- lations a n d e n h a n c e d probability of l a m i n a pres- ervation (Moore a n d Scruton, 1957). Bottom c a m e r a a n d sediment sampling surveys in various M e d i t e r r a n e a n basins d e m o n s t r a t e t h a t , at pres- ent, conditions above the seafloor are highly suit- able for sustaining i m p o r t a n t benthic popula- tions. H i g h concentrations of feeding a n d burrow- ing structures indicate t h a t fine lamination de- veloped in recent time, at least since a b o u t 6000 years B.P., would not likely be preserved.
In s u m m a r y , the study emphasizes the signifi- cance of climatic-oceanographic factors, the in- volvement of two or more processes in the devel- o p m e n t of lamination of fine facies in deep-ma- rine environments, a n d that l a m i n a t e d m u d s are not necessarily turbidites or contourites in the strict sense. Observations m a d e here c o m m o n l y prevail a n d are not u n i q u e to the M e d i t e r r a n e a n . In consequence, m u d deposits of the type de- scribed herein—redeposition of m a r g i n sediments by gravity flows coupled with hemipelagic set- tling through a stratified water c o l u m n — c o u l d probably be recognized in diverse m a r i n e settings.
T h e mechanism deserves to be quantified. Con- tinuing investigation of m u d lithofacies in small to m o d e r a t e seas holds promise for better inter- pretation of fine-grained deposits in the larger world oceans a n d the sedimentary rock record.
Literature Cited
Allan, T.D., T. Akal, and R. Molcard
1972. Oceanography of the Strait of Sicily. Saclanl ASW Research Centre Conference Proceedings, 7:1 -229. La Spezia, Italy.
Arkhangel'skiy, A.D.
1927. On the Black Sea Sediments and Their Signifi- cance in Sedimentology. Moskovskoe Obshchestvo is- pytatelei prirody Biullelen\ Otdel Geologicsku, 5:199-
289.
Blanpied, C.
1978. Structure et sedimentation superficielles en mer pelagienne. 119 pages. Thesis, Universite Pierre et Marie Curie, Paris.
Blanpied, C , and D. J. Stanley
1981. Uniform Mud (Unifite) Deposition in the Hellenic Trench, Eastern Mediterranean. Smithsonian Con- tributions to the Marine Sciences, 13: 40 pages.
Bradley, W.H.
1938. Mediterranean Sediments and Pleistocene Sea Levels. Nature, 88:376-379.
Burollet, P.F., P. Clairefond, and E. Winnock
1979. La mer pelagienne. Geologic Mediterraneene; 4:1-345.
Calvert, S.E.
1964. Factors Affecting Distribution of Laminated Dia- tomaceous Sediments in the Gulf of California. In T.H. van Andel and G.G. Shor, Jr., editors, Ma- rine Geology of the Gulf of California. American Association of Petroleum Geologists Memoir, 3:311-330.
Carter, T.G., J.P. Flanagan, C R . Jones, F.L. Marchant, R.R. Murchison, J.H. Rebman, J.C. Sylvester, and J.C.
Whitney
1972. A New Bathymetric Chart and Physiography of the Mediterranean Sea. In D.J. Stanley, editor, The Mediterranean Sea—A Natural Sedimentation Lab- oratory, pages 1-23. Stroudsburg, Pennsylvania:
Dowden, Hutchinson & Ross.
Climap Project Members
1976. The Surface of the Ice-Age Earth. Science, 191:
1131-1144.
Dominik, J., and A. Mangini
1979. Late Quaternary Sedimentation Rate Variations on the Mediterranean Ridge, as Results from the 230th Method. Sedimentary Geology, 23:95-112.
Drake, D.E., R.L. Kolpak, and P.J. Fischer
1972. Sediment Transport on the Santa Barbara-Ox- nard Shelf, Santa Barbara Channel, California. In D.J.P. Swift, D.B. Duane, and O H . Pilkey, edi-
tors, Shelf Sediment Transport, pages 307-331.
Stroudsburg, Pennsylvania: Dowden, Hutchinson
& Ross.
Eittreim, S., and M. Ewing
1972. Suspended Particulate Matter in the Deep Waters of the North American Basin. In A.L. Gordon, editor, Studies in Physical Oceanography, Georg Wiist
Tribute, 2:123-167. New York: Gordon & Breach.
Emelyanov, E.M.. and K.M. Shimkus
1972. Suspended Matter in the Mediterranean Sea. In D.J. Stanley, The Mediterranean Sea—A Natural Sed- imentation Laboratory, pages 417-439. Stroudsburg, Pennsylvania: Dowden, Hutchinson & Ross.
Got, H., D.J. Stanley, and D. Sorel
1977. Northwestern Hellenic Arc: Concurrent Sedimen- tation and Deformation in a Compressive Setting.
Marine Geology, 24:21-36, 7 figures.
Harms, J.C.
1974. Brushy Canyon Formation, Texas: A Deep-Water Density Current Deposit. Bulletin of the Geological Society of America, 85:1763-1784.
Hay, W.W.
1974. Studies in Paleo-Oceanography. Society of Economic Paleontologists and Mineralogists Special Publication, 20: 218 pages.
Heezen, B.C., and C D . Hollister
1971. The Face of the Deep. 659 pages. New York: Oxford University Press.
Hollister, C D . , and B.C. Heezen
1972. Geologic Effects of Ocean Bottom Currents: West- ern North Atlantic. In A.L. Gordon, editor, Studies in Physical Oceanography, Georg Wiist Tribute, 2:37- 66. New York: Gordon & Breach.
Hsu, J.K., L. Montadert, Shipboard Scientific Party 1978. Initial Reports of the Deep Sea Drilling Project. Volume
42, part 1, 1249 pages. Washington, D.G: Na- tional Science Foundation.
Huang, T . - C , and D.J. Stanley
1972. Western Alboran Sea: Sediment Dispersal, Pond- ing and Reversal of Currents. In D.J. Stanley, editor, The Mediterranean Sea—A Natural Sedimenta- tion Laboratory, pages 521-559. Stroudsburg, Penn- sylvania: Dowden, Hutchinson & Ross.
Hiilsemann, J., and K. O. Emery
1961. Stratification in Recent Sediments of Santa Bar- bara Basin as Controlled by Organisms and Water Characteristics. Journal of Geology, 69:279-290.
17
18 SMITHSONIAN CONTRIBUTIONS TO T H E MARINE SCIENCES
K a s t e n s , K.A., a n d M . B . C i t a
1981. T s u n a m i - i n d u c e d S e d i m e n t T r a n s p o r t in t h e Abyssal M e d i t e r r a n e a n Sea. Bulletin of the Geological Society of America, 9 2 : 8 4 5 - 8 5 7 .
K u l l e n b e r g , B.
1952. O n t h e Salinity of t h e W a t e r C o n t a i n e d in M a r i n e S e d i m e n t s . Meddelanden frau Oceanografska Institution Goteborgs, 21:1 - 3 8 .
L a c o m b e , H . , a n d P. T c h e r n i a
1972. C a r a c t e r e s h y d r o l o g i q u e s et c i r c u l a t i o n des e a u x en M e d i t e r r a n e e . In D.J. S t a n l e y , editor, The Med- iterranean Sea—A Natural Sedimentation Laboratory, pages 2 5 - 3 6 . S t r o u d s b u r g , P e n n s y l v a n i a : Dow- d e n , H u t c h i n s o n & Ross.
Lisitzin, A . P .
1972. S e d i m e n t a t i o n in the W o r l d O c e a n . Society of Eco- nomic Paleontologists and Mineralogists Special Publi- cation, 17: 218 pages.
L u z , B.
1979. P a l e o - O c e a n o g r a p h y of the Post-Glacial E a s t e r n M e d i t e r r a n e a n . Nature, 2 7 8 : 8 4 7 - 8 4 8 .
M a l d o n a d o , A., a n d D . J . S t a n l e y
1976a. L a t e Q u a t e r n a r y S e d i m e n t a t i o n a n d S t r a t i g r a p h y in the Strait of Sicily. Smithsonian Contributions to the Earth Sciences, 16: 73 pages.
1976b. T h e Nile C o n e : S u b m a r i n e F a n D e v e l o p m e n t by Cyclic S e d i m e n t a t i o n . Marine Geology, 2 0 : 2 7 - 4 0 . 1977. Lithofacies as a F u n c t i o n of D e p t h in t h e S t r a i t of
Sicily. Geology, 5 : 1 1 1 - 1 1 7 .
1978. Nile C o n e D e p o s i t i o n a l Processes a n d P a t t e r n s in t h e L a t e Q u a t e r n a r y . In D.J. S t a n l e y a n d G.
Kelling, editors, Sedimentation in Submarine Canyons, Fans, and Trenches, pages 2 3 9 - 2 5 7 . S t r o u d s b u r g , P e n n s y l v a n i a : D o w d e n , H u t c h i n s o n & Ross.
1979. Depositional P a t t e r n s a n d L a t e Q u a t e r n a r y Evo- lution of T w o M e d i t e r r a n e a n S u b m a r i n e F a n s : A C o m p a r i s o n . Marine Geology, 3 1 : 2 1 5 - 2 5 0 . M a r s , P.
1963. Les faunes et la s t r a t i g r a p h i e du Q u a t e r n a i r e M e d - i t e r r a n e a n . Recueil des Travaux, Station Maritime d'Endoume, 2 8 : 6 1 - 9 7 .
M o o r e , D.G., a n d P . C . S c r u t o n
1957 M i n o r I n t e r n a l S t r u c t u r e s of S o m e R e c e n t U n c o n - solidated S e d i m e n t s . Bulletin of the American Associ- ation of Petroleum Geologists, 4 1 : 2 7 2 3 - 2 7 5 1 . Miiller, G., a n d P. Stoffers
1974. M i n e r a l o g y a n d Petrology of Black Sea Basin S e d i m e n t s . In E . T . Degens a n d D . Ross, editors, T h e Black S e a — G e o l o g y , C h e m i s t r y , a n d Biology.
American Association of Petroleum Geologists Memoir, 2 0 : 2 0 0 - 2 4 8 , 22 figures.
O l a u s s o n , E.
1961. S t u d i e s of D e e p - S e a Cores. Reports of the Swedish Deep-Sea Expedition, 1947 1948, 8 : 3 5 3 - 3 9 1 .
Pierce, J . W .
1976. S u s p e n d e d S e d i m e n t T r a n s p o r t at t h e Shelf B r e a k a n d over t h e O u t e r M a r g i n . In D . J . S t a n l e y a n d D . J . P . Swift, editors, Marine Sediment Transport and Environmental Management, p a g e s 4 3 7 - 4 5 8 , 8 figures.
N e w York: W i l e y - I n t e r s c i e n c e . Pierce, J . W . , a n d D.J. S t a n l e y
1976. S u s p e n d e d S e d i m e n t C o n c e n t r a t i o n a n d M i n e r a l - ogy in t h e C e n t r a l a n d W e s t e r n M e d i t e r r a n e a n a n d M i n e r a l o g i c C o m p a r i s o n w i t h B o t t o m Sedi- m e n t . Marine Geology, 1 9 : M 1 5 - M 2 5 .
Pierce, J . W . , S. T u c c i , a n d G. Fierro
1981. Assessing V a r i a t i o n s in S u s p e n s a t e s , L i g u r i a n S e a ( N o r t h w e s t e r n M e d i t e r r a n e a n ) . Geo-Manne Letters, 1:149-154.
P i p e r , D . J . W .
1978. T u r b i d i t e M u d s a n d Silts on D e e p s e a F a n s a n d Abyssal Plains. In D.J. S t a n l e y a n d G. K e l l i n g , editors, Sedimentation in Submarine Canyons, Fans, and Trenches, pages 163-176. S t r o u d s b u r g , P e n n s y l v a - nia: D o w d e n , H u t c h i n s o n & Ross.
R u p k e , N.A., a n d D . J . S t a n l e y
1974. Distinctive P r o p e r t i e s of T u r b i d i t i c a n d H e m i p e - lagic M u d Layers in t h e A l g e r o - B a l e a r i c Basin, W e s t e r n M e d i t e r r a n e a n Sea. Smithsonian Contribu- tions lo the Earth Sciences, 13: 40 pages.
R y a n , W . B . F .
1972. S t r a t i g r a p h y of L a t e Q u a t e r n a r y S e d i m e n t s in t h e E a s t e r n M e d i t e r r a n e a n . In D . J . S t a n l e y , e d i t o r , The Mediterranean Sea—A Natural Sedimentation Laborlory, pages 149-169. S t r o u d s b u r g , P e n n s y l - v a n i a : D o w d e n , H u t c h i n s o n & Ross.
R y a n , W . B . F . , J . K . H s u , a n d S h i p b o a r d Scientific P a r t y 1973. Initial Reports of the Deep Sea Drilling Project. V o l u m e
13, p a r t s 1 a n d 2, 1447 pages. W a s h i n g t o n , D . C : N a t i o n a l Science F o u n d a t i o n .
S c h n i t k e r , D .
1980. Q u a t e r n a r y D e e p - S e a B e n t h i c F o r a m i n i f e r s a n d B o t t o m W a t e r Masses. Annual Review, Earth and Planetary Sciences, 8 : 3 4 3 - 3 7 0 .
S t a n l e y , D.J.
1978. I o n i a n Sea S a p r o p e l D i s t r i b u t i o n a n d L a t e Q u a - t e r n a r y P a l e o c e a n o g r a p h i c C o n s i d e r a t i o n s in t h e E a s t e r n M e d i t e r r a n e a n . Nature, 2 7 4 : 1 4 9 - 1 5 2 . 1981. Unifites: S t r u c t u r e l e s s M u d s of G r a v i t y - F l o w O r -
igin in M e d i t e r r a n e a n Basins. Geo-Manne Letters, 1:77-83.
S t a n l e y , D.J., a n d C. B l a n p i e d
1980. L a t e Q u a t e r n a r y W a t e r E x c h a n g e b e t w e e n t h e E a s t e r n M e d i t e r r a n e a n a n d t h e Black Sea. Nature, 2 8 5 : 5 3 7 - 5 4 1 .
S t a n l e y , D.J., a n d A. M a l d o n a d o
1979. L e v a n t i n e S e a — N i l e C o n e L i t h o s t r a t i g r a p h i c Ev- o l u t i o n : Q u a n t i t a t i v e Analysis a n d C o r r e l a t i o n
