Strait of Sicily depositional rates and patterns, and possible reversal of currents in the late quaternary

Palaeogeography, P a l a e o c l i m a t o l o g y , Palaeoecology , 18(1975): 279--291.

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STRAIT OF SICILY DEPOSITIONAL RATES AND PATTERNS, AND P O S S I B L E R E V E R S A L O F C U R R E N T S IN T H E L A T E Q U A T E R N A R Y

DANIEL JEAN STANLEY 1, ANDRl~S MALDONADO 2 and'ROBERT STUCKENRATH I 1 S m i t h s o n i a n I n s t i t u t i o n , Washington, D.C. (U.S.A.) 2 S e c c i d n de Estratigraf[a, Barcelona (Spain)

(Accepted for publication April 7, 1975)

ABSTRACT Stanley, D. J., Maldonado, A. and Stuckenrath, R., 1975. Strait of Sicily depositional rates and patterns, and possible reversal of currents in the late Quaternary. Palaeogeogr., Palaeoclimatol., Palaeoecol:, 18: 279--291. Late Quaternary sedimentation and lithostratigraphic patterns on the Strait of Sicily platform are readily discernible from those in the adjacent deep Mediterranean basins. Strait cores are unusually uniform, show a high degree of bioturbation and include an important coarse bioclastic fraction. Lithofacies are correlated with depth and proximity to the Strait narrows. Stratification (alternating sand and mud turbidites, hemipelagic mud and ash layers) is best developed in three deep narrow basins in the Strait where benthic populations and rates of bioturbation are significantly lower than on the shallower neritic platform and banks. 14C dates show that the top of some cores are truncated in the early Holocene at a time when black, organic-rich sapropel layers formed in the central and eastern Mediterranean. No sapropels are recovered on the Strait and, further, bioturbation structures occur throughout late Quaternary sections. These and concurrent faunal changes indicate that: (1) the Strait remained ventilated and swept by currents while density stratification and anaerobic conditions prevailed to the east; and (2) circulation was not blocked at the Strait during the last glacial-postglacial evolution. Sedimentological and related observations are best interpreted in terms of short-lived but important water mass fluctuations, including a possible reversal of currents, which took place during the early warming phase of the climatic curve. INTRODUCTION T h e Strait of Sicily, a long, b r o a d t r a p e z o i d - s h a p e d p l a t f o r m b e t w e e n Sicily a n d T u n i s i a , o c c u p i e s a s t r a t e g i c p o s i t i o n , o c e a n o g r a p h i c a l l y s p e a k i n g , in t h a t it separates the d e e p I o n i a n Basin in the eastern M e d i t e r r a n e a n f r o m t h e T y r r h e n i a n a n d B a l e a r i c b a s i n s t o t h e west. T h i s g e o g r a p h i c p r o v i n c e is t o p o g r a p h i c a l l y c o m p l e x a n d i n c l u d e s s h a l l o w b a n k s , ridges, v o l c a n o e s (seam o u n t s and islands), gentle depressions and deep basins (Akal, 1972; Morelli, 1 9 7 2 ) . R e l i e f o f t h e sea f l o o r , l o c a l l y e x c e e d i n g 1 0 0 0 m , is m u c h g r e a t e r t h a n t h a t o f t y p i c a l shelves. A s t u d y was i n i t i a t e d t o d e f i n e t h e m a j o r late Q u a t e r n a r y l i t h o l o g i c facies

280

in the various Strait environments and to compare the sedimentation patterns in this region with those of adjacent, but much deeper Mediterranean basins. The present paper summarizes the rate of deposition, the relative importance of sediment input versus biogenic reworking and the relation between the Strait lithostratigraphy and Quaternary events that affected the region. Of particular interest is the recognition of sedimentological and related data which we believe record paleo-oceanographic changes occurring in this region in the recent geological past. The investigation is based primarily on a detailed X-radiographic examination of 32 cores, on 40 14 C age determinations (bulk samples and on concentrations of sand-sized planktonic foraminifera) and on petrological (including Scanning Electron Microscope) analysis of over 200 samples. B o t t o m photographs and s u b b o t t o m records (30,000 J and 3.5 kHz profiles) have also been examined. A detailed report of all the above is presented elsewhere (Maldonado and Stanley, 1975a). GENERAL SETTING The Strait of Sicily trends NW--SE, is about 740 km long and widens to about 440 km between southeast Sicily and the Gulf of Sirte (Fig.l). Three major morpho-sedimentary environments are recognized: (1) shallow bank, (2) an intermediate depth neritic--bathyal platform and (3) deep basin. Special note also is made of (4) the Strait proper (or narrows, a b o u t 140 km wide between Cap Bon, Tunisia and Marsala, Sicily), and (5) of the slopes bordering the Strait in the Ionian and Balearic--Tyrrhenian basins. Specific features such as seamounts, volcanoes and islands are not considered here. Of the total Strait area (250,000 km 2 ), about 45% of the surface is shallower than 200 m, and less than 3% is occupied by deep ( > 1 0 0 0 m) basins; the intermediate zone, accounting for somewhat more than half of the Strait area includes the sector between 200 and 600 m. This latter environment is topographically the most complex and is completely broken by flats, depressions and highs. Seismic surveys show that most of the Strait has been tectonically displaced by vertical movements into small blocks, and that the predominant fault trend is NW--SE. Structural activity, dated to at least the Miocene, intensified during the Pliocene and appears to have continued until the present (Finetti and Morelli, 1972; Zarudzki, 1972). Recent seismic and geologic interpretations suggest that the Strait and contiguous island of Sicily occupy the leading edge of the North African plate (Akal, 1972, his fig.16; Finetti and Morelli, 1972, thei~ fig.19). OBSERVATIONS Our analyses of Strait of Sicily core material shows several major lithological t y p e s : (1) bioclastic sand (maerl, cf. Caulet, 1972) and gravel inter-

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bedded with mud and sandy lutite deposits; these abound on the shallow banks (Blanc, 1958); (2) predominantly homogeneous (non-stratified, bioturbated) olive gray to dusty yellow mud in which the most important structures are biogenic ones; this type most commonly forms the neritic-bathyal platform section and is also found in some deep basin cores; (3) moderate to well-stratified sand and mud units that consist of gravity sediment flow deposits (sand and mud turbidites, grain flow units) and/or ash layers interbedded with hemipelagic mud; these are typical of the deep basins. Shallow-bank sedimentation

High-resolution 3.5 kHz profiles and sparker records show that the shallow banks, for the most part, are covered by a considerably reduced unconsolidated sediment cover, and that strata are gently tilted, tectonically offset and truncated. Terraces at about 120 m (last eustatic low stand) and deeper are noted

282

on some bank margins. Mapping of the surficial sediment based on b o t t o m photographs and grab samples shows a mosaic distribution that includes mud to coarse-grained, largely bioclastic (calcareous algae, bryozoans, molluscs, etc.) sediment types (cf., Blanc, 1958; Poizat, 1970; Akal, 1972). Two different stratigraphic sequences are defined. The first, an upward-coarsening sequence (Fig.2) shows an upward increase in sand and coarser fractions (largely bioclastic) and shells (largely molluscan). The basal mud of this sequence contains a neritic faunal assemblage, including benthic foraminifera.

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The thickness can exceed 5 m (Fig.3, cores AS 6--7, 8). The second is an upward-fining sequence showing an inversely developed textural and structural evolution (cf. Fig.3, upper part of core AS 6--7). Core AS 6--8 at 93 m in the Gulf of H a m m a m a t shows a rate of sedimentation of 52 cm/1000 years; with the exception of sections of two cores in very deep basins, this is the highest rate measured in the Strait (Fig.5). ~4 C age determinations show that the tops of some cores collected in the shallow banks are truncated before the end of the Pleistocene (cf. core AS 6--8).

Neritic--bathyal platform sedimentation Sparker profiles reveal the remarkable vertical structural displacement which has broken this area into a complex net of horsts and grabens. That this play of vertical tectonics has affected even the uppermost Quaternary sections is demonstrated by the high-resolution 3.5-kHz records (Fig.4). The Pliocene--Quaternary cover is variable over much of the Strait, and thicknesses, as noted in our records, can range to as much as 0.5--0.7 sec. (twoway travel time) or about 400--600 m. Sediments have accumulated preferentially in depressions (arrow A, Fig.4) and they are reduced on highs (arrow .B, Fig.4). Volcanism has been most intense in the northern and northwestern sectors of the Strait (Finetti and Morelli, 1972; Zarudzki, 1972).

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284 The most characteristic features of cores are their extremely uniform, homogeneous aspect (even in X-radiographs) and the abundance of bioturbation structures; in a few cases, poorly stratified mud layers are noted. Typical examples of such uniform cores include LY-II-4, 5 and 6A (Fig.3). Another distinction is the low sand fraction (2--7%) in the mud; this consists mostly

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of planktonic foraminifera and pteropods, and an accessory but diverse assemblage of faunal remains. Rates of sedimentation vary from about 16 to 40 cm/1000 years (average of 25 cm/1000 years), and the tops of a number of cores in this environment terminate in the early Holocene (Fig.5). One core (Ges-105), unlike the above, shows a lower sedimentation rate (similar to that of the deep basins) and a continued deposition through much of the Holocene.

Deep-basin sedimentation Three major narrow elongate basins occur in the center of the Strait and all three parallel its NW--SE trending axis: (.1) Malta Trough, 150 km long and 18 km wide; (2) Pantelleria Trough, 90 km long and 30 km wide; (3) Linosa Trough, 75 km long and 17 km wide. Respective depths are about

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Fig.5. Rates of sedimentation based on 14C data from cores in the different environments. A, shallow banks, neritic--bathyal platform and deep slopes bounding the Strait of Sicily; B, deep basins on the Strait platform. 1 7 0 0 , 1 3 0 0 , and 1 6 0 0 m. Other basins such as the o n e w e s t o f M a r i t t i m o Island (core LY-II-6) and o n e rather small deep d e p r e s s i o n just w e s t o f the Strait n a r r o w s ( G e s - 1 2 ) also have b e e n sampled. H o w e v e r , t h e three major trough-like d e p r e s s i o n s stand o u t by their straight, f a u l t - b o u n d e d steep walls apparent in seismic profiles. Sparker records s h o w m o r e than 1-sec pene-

286

tration of the post-Miocene unconsolidated sediment, while 3.5~kHz records illustrate the near-horizontal attitude of distinct reflectors. In contrast to the two earlier described environments, cores in the deep basins present: (1) a more diverse assemblage of sediment types and (2) distinct stratification. Most characteristic of basin cores are their turbidite sequences. These include classic sand and silt turbidites (Figs.2, 3) with the various terms defined by Bouma (1962) and others, as well as mud turbidites, Te (t), as described in detail by Rupke and Stanley (1974). Turbidites are generally about 20 cm thick. The base sometimes includes bioclastic sand, bat more often the turbidite consists almost completely of silt and clay. Compositionally, the bioclastic sand fraction is comparable to sands on the shallow and intermediate platform above the basins. In some cores, particularly in Linosa Trough, tephra (ash) layers are important, and some of these sand-size deposits also present some typical turbidite structures (i.e., graded bedding). Although some volcanic sands have been carried into the basins by turbidity currents, we suggest that graded volcanic layers also can form by lateral wind and water transport of the ash and subsequent settling through the water column. A third sediment type, perhaps accounting for most of the core sections, is hemipelagic mud. This mud is similar in composition to that of the uniform mud cored in the neritic--bathyal platform described earlier; muds c o m m o n l y display bioturbation structures. Evidence of slumping is also noted in basin cores, such as a 70 cm thick contorted unit in core Vema 14-139 at 1703 m in Malta Trough. Rates of sedimentation approximate 20--25 cm/1000 years. The much higher rates in two core sections (lower half of Ges-109 in Malta Basin, and in the upper half of Ges-63 in Linosa Basin) are the result of more frequent turbidite and ash incursions at these two localities. It should be noted that in contrast to cores at shallower depths, deep-basin sediments have accumulated on a continued basis until the present (Fig.5). Rates in core LY-II-6 in the small depression west of Marittimo Island are similar to those of deepbasin cores. S e d i m e n t a t i o n at the S t r a i t n a r r o w s

Cores in the narrowest portion of the Strait between Sicily and Tunisia are distinctive in that they show a higher proportion of coarse bioclastic sand. Cores Vema 14-140 at the edge of Adventure Bank (Fig.l) at 166 m and Ges-12 in the deeper (956 m), small, enclosed basin in the center of the Strait ~re interesting in this respect. The former shows a well-marked finingupward sequence (like those in other shallow-bank environments) while the latter shows distinct alternations of thick (to 90 cm) coarse grade sands and bioturbated muds. The sands have been introduced periodically by turbidity currents (graded sand-to-mud units) and mass flow mechanisms (texturally clean grain flow to m u d d y debris flow units). A core (San Pablo 8--7) near

287 the center of the Strait, at intermediate depths (350 m), shows a transitional lithology: a vague fining-upward sequence with intercalations of coarse bioclastic sand layers and muds toward the upper portion of the sequence. 14 C data of core Ges-12 indicate relatively low (15 cm/1000 years) sedimentation rates in the small Strait narrows basin but continuous deposition from the late Pleistocene until recent time (Fig.5).

Basin slopes (Ionian and Algero-Balearic margins) There is a rather sharp lithofacies transition between cores on the Strait platform and those in the deep adjacent eastern and western Mediterranean basins, as can be ascertained by examination of two cores (LY-II-3, 2432 m, on the Ionian margin and LY-II-7, 2588 m, in the Balearic Basin, Fig.3). Average sedimentation rates are respectively 30 cm and 15 cm per 1000 years (Fig.5). Core LY-II-7 consists of alternating mud turbidites, some with thin basal silt layer, and hemipelagic mud (cf. Rupke and Stanley, 1974). The former, LY-II-3, presents an alternation of hemipelagic mud, sand and mud turbidites, and, in addition, distinctive dark gray to black organic-rich sapropel layers. The complete sapropel sequence includes a well-defined vertical transition of sediment types as defined by Maldonado and Stanley (1975b): organic ooze to protosapropel to sapropel to organic ooze to an uppermost oxidized layer. Several such sapropel sequences are observed between the base and the top of core LY-II-3 and each sapropel layer indicates deposition at a time when anaerobic conditions prevailed on the sea floor. Hydrogen sulphide and pyrite in sapropel layers is attributed to the reduction of sulphate by anaerobic bacteria. Stagnation of b o t t o m water is believed to have occurred periodically during the Quaternary as a result of density stratification of water masses which prevented the sinking of oxygenated water, as is the case t o d a y (Van Straaten, 1972; and others). Most workers are of the opinion that the associated stratification--anaerobic conditions developed during the warming phases of Quaternary climatic cycles, and not during the glacial maximum (Ryan, 1972). DISCUSSION The Strait of Sicily late Quaternary lithofacies are differentiated from those in adjacent deep basins by a high degree of bioturbation and the presence of relatively large amounts of coarse bioclastic sediment. Furthermore, sedimentation patterns are closely related with topographic setting (i.e., depth and proximity to the Strait narrows). Four aspects of the sedimentation pattern in the five areas discussed earlier are considered: rate of deposition, uniformity of these rates in time, the age of the sediments at the top of the cores, and the degree of continuity of sedimentation from the Pleistocene to the present. Sedimentation rates {with

288

some exceptions) generally decrease with depth, i.e., from the shallow banks to the neritic--bathyal platform to the deep basins. With the '4C data at hand (Fig.5), it appears that deposition in all environments, except in the two deep basin cores (Ges-63,109) that have higher ash and turbidite layers, has been relatively constant in the late Quaternary. However, there is a significant difference in the age of sediments at the tops of cores in the different environments. In shallow banks, the tops of some cores are truncated in the late P l e i s t o c e n e to early Holocene; in the neritic--bathyal platform, in the early Holocene; and in the deep basins, sediments have accumulated on a fairly continuous basis from the Pleistocene until the recent (Figs.3, 5). The vertical lithofacies sequences in the shallow-bank and neritic--bathyal environments can be closely related with the Quaternary dynamics. The upward-coarsening and upward-fining sequences in shallow environments clearly reflect b o t t o m conditions related to eustatic oscillations; i.e., shell bank concentrations increased as sea level dropped, banks became shallower and b o t t o m current activity intensified, and vice versa. On some banks, fine-grained sediments have accumulated since the rise of sea level; on others there has been truncation and non-deposition since the last eustatic low stand. At intermediate depths, bioturbation is evident throughout the cores. This homogenization shows that rates of reworking by benthic organisms have been relatively more important than sediment input and accumulation throughout the late Quaternary. Equally significant are the '4C dates which indicate that oceanographic conditions directly affecting the sea floor changed markedly between the late Pleistocene and the early Holocene, and that non-deposition and/or erosion have prevailed since about 10,000 years B.P. In contrast, none of the above changes are noted in the deep Strait basins. Rates of sedimentation approximate those on the neritic platform but a somewhat lower benthic population on the basin floors has resulted in a relatively lower degree of bioturbation and better preservation of stratification. Furthermore, there are no obvious changes in either lithofacies sequence patterns or sedimentation rates between the late Pleistocene and recent, a period of at least 30,000 years. Although the depth of the three deep Strait basin plains (1300--1700 m) is well below that at which sapropel layers are found elsewhere in the central (Adriatic) and eastern (Ionian, Levantine basins) Mediterranean, no sapropels or other distinct evidence of stagnation are noted in the basin cores. On the contrary, structures made by benthic organisms are c o m m o n l y observed indicating that the deep narrow Strait basins remained sufficiently oxygenated to support benthic populations throughout the late Quaternary. Thus, it appears that vertical mixing prevailed on an almost continuing basis as a result of water mass movement across the Strait of Sicily at a time when sapropels were accumulating under stagnant conditions in the adjacent eastern Mediterranean (cf., note sapropel at top of core LY-II-3). In this respect, core LY-II-6A west of the Strait narrows (Fig.3) is of interest. The rate of sedimentation here is higher than in many other sectors. The mud at the top of core LY-II-6A is dated as early Holocene (about 11,000--

289

10,000 years B.P.), or well after sea level had begun to rise. Inasmuch as this core lies at a depth of 755 m, the eustatic oscillation alone is not believed to be the primary factor for erosion or non-deposition in this sector. The region just west of the Strait narrows may be critical for interpreting Quaternary oceanographic fluctuations since it occupies a zone of particularly strong current regime. Currents accelerate in the constricted narrows and decelerate as the Strait widens with a probable increase in deposition away from the narrows. Thus, we would expect that cores collected in the vicinity of the narrows would provide the best record of water mass--bottom current fluctuations in the recent geological past. It is probably not accidental that there is an apparent correlation between the time of truncation of core tops on the Strait platform and that of protosapropel and sapropel formation (sapropels are dated at about 9000--7500 years B.P.; cf., Ryan, 1972; Van Straaten, 1972) in the eastern and central Mediterranean. Independently, other workers (Colantoni and Borsetti, 1973) record microfaunal changes in the Linosa and Malta basins at about this period. One possible explanation for these early Holocene depositional and faunal changes is a temporary short-term reversal of surface and deeper water flow (Olausson, 1961; Mars, 1963; Huang et al., 1972; Mfiller, 1973; Nesteroff, 1973; Huang and Stanley, 1974; and others). At present, less dense water flows (>30 cm/sec) southeastward above northwestward-flowing (32 cm/sec) Levantine water (Molcard, 1972). We illustrate an early Holocene current reversal model that depicts the northwestward movement of less dense surface water in response to the early Holocene climatic evolution (Fig.6). Surface water salinity and temperature conditions (Farrand, 1971) certainly were altered in the Mediterranean during the warming phase of the climatic curve, but the degree of stratification resulting from this remains a point of conjecture (Letolle and Vergnaud-Grazzini, 1974). Nevertheless, our study does show (1) that the sea floor of the Strait of Sicily remained ventilated and swept by currents at a time when anaerobic conditions prevailed in the Ionian--Levantine basins east of the Strait, and (2) that the Strait did n o t completely block circulation between the eastern and western Mediterranean basins. We conclude that our observations are best explained in terms of early Holocene paleo-oceanographic changes including possible reversal of currents. The latter concept requires further testing and we suggest that the Strait of Sicily, the major sill separating sapropel-rich eastern Mediterranean basins from non-sapropel basins in the west, is clearly one of the key sites in which to investigate this problem. ACKNOWLEDGEMENTS Cores used in this study were generously provided by the U.S. Navy (LYNCH-II-1972; ATLANTIC SEAL 6, 1965--1967), the Laboratoire de G~odynamique Marine, Villefranche-sur-Mer (GESITE-KS-1973; also one 14C date on core Ges-69), Lamont-Doherty Geological Observatory (VEMA-

290

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Fig.6. Schematic showing possible early Holocene water mass changes in the Mediterranean. The model depicts stratification and reversal of currents during the warming phase of the climatic curve. The present study indicates that the Strait of Sicily did not completely block circulation and that deep Strait basins remained ventilated at the time that sapropel layers accumulated in the eastern Mediterranean. Topographic base after Wrist (1961).

14; SAN PABLO-8), the University of Miami (PILLSBURY-6510), and the Woods Hole Oceanographic Institution (CHAIN-61). J. W. Pierce, Smithsonian Institution, and A. R. Miller, Woods Hole Oceanographic Institution, reviewed the paper. This investigation, part of the Mediterranean Basin (MEDIBA) Project, is supported by Smithsonian Research Foundation grant 430035 (D.J.S.); support has also been given by the Program of Cultural Cooperation between the U.S.A. and Spain (A.M.). REFERENCES Akal, T., 1972. The general geophysics and geology of the Strait of Sicily. In: T. D. Allan, T. Akal and R. Molcard (Editors), Oceanography of the Strait of Sicily. SACLANTCEN Conf. Proc., La Spezia, 7: 177--192. Blanc, J. J., 1958. S~dimentologie sous-marine du d~troit siculo-tunisien. Campagne du "Calypso" (Aof~t--Sept.). R~sultats scientifiques. Inst. Oc~anogr., Monaco, pp. 92--126. Bouma, A. H., 1962. Sedimentation of Some Flysch Deposits: a Graphic Approach to Facies Interpretation. Elsevier, Amsterdam, 168 pp. Caulet, J. P., 1972. Les s~diments organog~nes du pr~continent alg~rien. Mem. Mus. Paris, 2 5 : 2 8 9 pp. Colantoni, P. and Borsetti, A. M., 1973. Some notes on geology and stratigraphy of the Strait of Sicily. Bull. Geol. Soc. Greece, 10: 31--32. Farrand, W. R., 1971. Late Quaternary paleoclimates of the eastern Mediterranean area.

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