Bathymetry of the eastern Mediterranean Sea

Deep-Sea Research, 1966, Vol. 13, pp. 173 to 192. Pergamon Press Ltd. Printed in Great Britain.

Bathymetry o f the eastern Mediterranean Sea* K. O. EMERY~',BRUCE C. HEEZEN~and T. D. ALLAN§ (Received 13 October 1965) Abstract--About 23,000 k m of continuous precision echo sounding profiles supplemented by about 15,000 k m of less precise profiles were compiled for the eastern Mediterranean Sea. Examination of the profiles reveals the presence of a long sinuous ridge that rises to depths shallower than 2500 m. This feature, the Mediterranean Ridge, is bounded on both sides by steep escarpments that lead down into trenches. The trenches north of the ridge, named Strabo Trench and Pliny Trench, have little fill, owing probably to insufficient supply of sediment. On the south side of the Mediterranean Ridge is the Herodotus Abyssal Plain, about 1200 km long. South of this plain is a broad gentle slope of the Nile Cone that spreads out from the Nile Delta. Thirty-two long piston cores show that the top 10 meters of the Nile Cone consists mostly of gray lutite. In and near the Herodotus Abyssal Plain at the base of the cone are many layers of sandy turbidites. Beyond the abyssal plain and covering most of the Mediterranean Ridge and the area north o f it, the bottom consists of Globigerina ooze having intercalated tephra probably from the active volcano Santorini located northeast of Crete. Most cores also contain numerous sapropelic layers that indicate frequent occurrence of anaerobic conditions during the Late Pleistocene Epoch. The interpretation of the sounding profiles and sediments leads to the conclusion that the Mediterranean Ridge is tectonic in origin and that it constitutes a dam against which abut sediments contributed by the Nile River and carried seaward probably by both general diffusion and turbidity currents.

INTRODUCTION

INFORMATION about depths of the Mediterranean Sea is reported in early literature beginning with Herodotus (CARTER, 1958, Book II, chap. 5), who in 450 B.C. described Egypt as a gift of the Nile, giving as partial evidence the fact that when soundings are made one day's sail from Egypt mud is brought up from a depth of 10 fms. Soundings in much greater depths are implied by Aristotle's (WEBSTER,1923, Book I, chap. 13) statement about 330 B.C. that the Black Sea is unfathomable 10 miles offshore where "fresh water from the Caspian Sea rises to the surface". According to Strabo (HAMILTON and FALCONER, 1854, Book I, chap. 3), Posidonius reported about 85 B.C. that the deepest place in the sea was measured off Sardinia--about 1000 fms. In 77 A.D. Pliny the Elder (BosTocK and RILEY, 5855, Book II, chap. 105) quoted a statement of Fabianus that the greatest depth of the sea (presumably the Mediterranean) is 15 stadia (2850 m). The ability of the ancients to make such deep soundings has been questioned; nevertheless, the reported depths could well be correct at the reported sites of measurement. With greater density of soundings and improved equipment the greatest known depth in the Mediterranean Sea has increased to 5121 m, discovered during 1959 aboard the Chain by J. B. Hersey of the Woods Hole Oceanographic *Contribution No. 1682 of the Woods Hole Oceanographic Institution. Observatory (Columbia University), Contribution 879. "l'Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543. :~Lamont Geological Observatory, Palisades, New York 10964. §North Atlantic Treaty Organization, La Spezia, Italy. 173

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174

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Institution and later verified aboard the Akademik S. Vavilov (OCHAKOVSKY, 1963) about 160 km west of the southern tip of Greece. Soundings have been made by wire and acoustic methods from ships of many countries. Most of the soundings were casual ones, poorly positioned, imprecise, and taken only once each watch. As a consequence, a line of soundings by any particular ship may well fail to record the bottoms of narrow troughs or the tops of steep submarine hills. Casual soundings clearly are useful only for the drawing of generalized wide-interval contours (Monaco charts, Bureau Hydrographique International, 1938 : Goncharov and Mikhailov, 1963). However, they have also been used as a basis for close-interval contours drawn for the eastern Mediterranean Sea on U.S. Navy charts BC 3924 and BC 3925 [U.S. HYDROGRAPHICOFFICE(now U.S. Naval Oceanographic Office), 1959a, 1959b], on British Admiralty chart 2 (HYDROGRAPHICDEPARTMENTOF THE ADMIRALTY, 1958), and on a bathymetric compilation by PEANNENSTIEL (1960). Strange re-entrants and projections, aligned isolated holes, and other weird features which appear on these charts are typical of charts constructed without the aid of continuous precision profiles. In an attempt to learn more about the true topography of the Mediterranean Sea several surveys have recently been made by Soviet ships the Akademik S. Vavilov (UDINTSEV, 196 l) and others (GONCHAROVand MIKHAILOV,1963), but up to the present time only small-scale profiles and maps have been published. In 1959 EMERYand BENTOR (1960) sounded 27 lines across the continental shelf off Israel, but equipment limited the work to less than 350 m in depth. NEW DATA

Through the interest of Sir Edward Bullard arrangements were made for Emery to conduct a bathymetric survey of the eastern Mediterranean Sea aboard the Aragonese in conjunction with a programme of gravity and magnetic field measurements (ALLAN, CHARNOCK,and MORELLI, 1964). The ship was then attached to the La Spezia, Italy, base of the Anti-Submarine Warfare Research Center of the North Atlantic Treaty Organization. She is a 90-meter steamer having a cruising speed of 12 knots and equipped with LORAN-C and RADAR for navigation, a Precision Depth Recorder attached to an EDO sounder, a small Edgerton boomer (for this cruise), an AskaniaGraf gravimeter, and a magnetometer. Precision LORAN and echo sounding devices such as those aboard the ship became available only in the mid-nineteen-fifties; their use provides information far better than that permitted by older devices for sounding and positioning in the deep sea. About 3400 km of continuous sounding profile was run between 31 October and 9 November 1962 aboard the Aragonese (Cruise Corinth). An additional 6200 km of lines made during a geophysical survey of 1961 (Cruise Concrete) aboard the same ship were made available for inclusion in the study (Fig. 1). In addition, 4800 km of precision lines by the Vema (Cruises 10 and 14) were provided by Lamont Geological Observatory. About 7400 km by the Chain (Cruises 7, 21, and 43) and 1300 km by the Atlantis (Cruise 42) were provided by the Woods Hole Oceanographic Institution. Useful profiles of non-precision soundings came from the work of the Soviet ships Ob, Vavilov, and Vityaz (6100 kin), from U.S. Navy ships (4300 km), and from the Atlantis (Cruise 151) (2800 kin), and the Swedish Albatross (1500 kin). Altogether the

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lines of continuous soundings in this small region have a length of about 37,800 kin-almost the circumference of the earth. Contours based upon the sounding lines were drawn at 500-m intervals on Fig. 2 (corrected for the speed of sound according to MATTHEWS(1939)). Old soundings printed on navigational charts were used only where better soundings were lacking, chiefly on continental shelves and areas remote from the ship tracks (Fig. 1). The chief contribution of this report, however, is the delineation of physiographic provinces. The boundaries of these provinces (Fig. 2) are based upon the characteristics of the sounding profiles. PHYSIOGRAPHIC

PROVINCES

Continental shelf The continental shelf around most of the eastern Mediterranean Sea is less than 15km wide. Only off the Nile Delta and off a composite delta of several rivers in the northeastern corner of the Mediterranean (Fig. 2) is the shelf as wide as 70 km and in both areas the feature is of depositional origin--the submarine topset beds of deltas (d'Arrigo in DOEGLAS, 1950). The true continental shelf of the region, as is typical of many other continental shelves of the world, contains numerous rock outcrops; these have been mapped only off Israel (RosENAN, 1937 ; EMERYand BENTOR, 1960) and off Lebanon (BouLos, 1962) where they are partly mantled by relict sediments. Precision sounding lines across the outer edge of the continental shelf are available only for the Nile Delta, because elsewhere the entire shelf lies within claimed territorial limits of the bordering nations. Soundings on the outer edge of the Nile Delta, however, reveal many minor irregularities (Figs. 3 and 4, profiles 11, 12, 13, 14; Fig. 5, profile 1). These irregularities are similar to ones found off Israel by EMERY and BENTOR (1960, Fig. 2G), who suggested that they may be due to rock outcrops, or possibly to exposed kurkar (eolianite) ridges. The presence of this kind of topography off the Nile Delta probably eliminates the possibility of its being due to rock outcrops and is more in agreement with the presence of kurkar. The profiles off the Nile Delta also indicate several terrace levels, most prominent of which are ones at 37 and 70 m. The shelf-break itself occurs mostly at about 95 m, somewhat shallower than the average of 110 m off the south coast of Israel. Continental shelves elsewhere in the region (Fig. 2) are too poorly sounded to depict irregular topography, terraces, or even a reliable depth of the shelf-break. The same is true of the insular shelves around Cyprus, Crete, and the smaller islands of the Aegean Sea.

Continental slope The continental and insular slopes extend from the shelf-break down to intersections with the continental rise, the Nile Cone, or the irregular deep topography of the Aegean Province at depths of 800-2200 m (Fig. 2). A continental slope was not differentiated beyond the shelf-break near the eastern and western parts of the Nile Delta (the Rosetta Fan and Damietta Fan). In these areas deltaic fore-set beds appear to have built up so thickly that they have buried any pre-existing continental slope. The declivity of the continental slope where best known, off Lebanon and Israel (Figs. 3 and 4, profiles 1-9), ranges from 1:7 at the north to 1:21 at the south, with an average of 1 : 14. Less satisfactory information elsewhere in the region shows the same average steepness (between 500-m contours) except off eastern Turkey and off Libya,

179

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Bathymetry of the eastern Mediterranean Sea

181

where deltas may have mantled the continental slope. The average steepness of 1 : 14 (4°05 ' ) is nearly the same as the world-wide average of 1:13.3 (4°17 ' ) obtained by SHEPARD (1963, p. 298) for the upper 200 m of descent. It contrasts with the low declivity of 1:40 (1 °26') of the fore-set beds of the Nile Cone (Fig. 4, profiles 11, 13, 14, 15, 18).

Continental rise At the base of the continental slope off Israel, Lebanon, and part of Syria is a zone of more gently sloping, slightly concave topography (Fig. 4, profiles 1-9). Declivities average about l:91 (0°38'), well within the range of 1:40 to 1 : 1000 which defines a continental rise (HEEZEN, THARP and EWING, 1959, pp. 19--20). Its origin appears to be the same as that of other continental rises, consisting presumably of sediments contributed to the sea by the Nile River and carried northward by the regional Mediterranean current. Some of the sediment is deposited atop the continental shelf, but probably most of it moves seaward to a final resting place at the base of the continental slope. The Nile Cone extends from the continental shelf and slope across the continental rise to a long narrow abyssal plain (Figs. 2 and 6). An oval-shaped area of abyssal hills divides the cone into two parts termed the Rosetta Fan and the Damietta Fan (named for the two presently active distributaries of the Nile River). Together the two fans cover an area of about 90,000 km 2. In a sense, the delta also includes the 14,000 km 2 of the partially covered southern part of the abyssal hills, because deltaic deposits have ponded in depressions between the hills (Fig. 5, profiles 5 and 6). The total area of the submarine par t of the delta thus is about 104,000 km 2, an area which is nearly four times as great as the subaerial part of the delta in Egypt. The surface of the cone has a local relief of less than about 20 m. It is broadly concave with gradients decreasing from about 1 : 40 (1 °26') between 500 and 1000 meters, to 1 : 92 (0037 ') between 1500 and 2000 meters, to 1 : 140 (0°25 ') between 2000 and 2500 m. Such gentle slopes are characteristic of the forset and bottomset beds of most large deltas. As shown by Fig. 4 (profiles 1-13) and Fig. 5 (profile 2), the topography of the upper continental rise is somewhat irregular, much as though it had been dissected by post-depositional or pene-contemporaneous erosion. The most irregular part has been designated as the "dissected fan" on Fig. 2 in the shallower part of the Damietta Fan.

Aegean Province The Aegean Province (Fig. 2) extends south from Greece and western Turkey past Crete to a southeastern boundary at the Pliny and Strabo trenches. Possibly most characteristic is the presence of numerous small islands bounded at the south by the largest ones, Crete and Rhodes. Also present are other hills which rise near enough to sea level to be banks but not islands. Around all of the islands are insular shelves. Continentalslopes are not differentiated as such, because of their smallsize. Maximum depths in the province do not exceed 1500 m except near the trenches to the southeast. The deepest depressions are marked by small abyssal plains, five of which were considered large enough to be shown on Fig. 2. The general aspect of the topography in this province is that of numerous small hills rising above a somewhat irregular plateau. This topography accords with its probable tectonic and volcanic origin only slightly smoothed by later sediments.

182

K . O . EMERY, BRUCE C. HEEZEN and T. D. ALLAN

Mountains and abyssal hills The eastern Mediterranean Sea contains several mountainous areas that rise more than 1000 m above their surroundings (but not above sea level). Two are more than half the length of Cyprus. These will be described from west to east, but only briefly owing to lack of data. All are named in honor of outstanding ancient geographers, mostly Greek, who contributed to knowledge of the countries bordering this part of the Mediterranean Sea. Farthest west and just south of Crete is a complex named Ptolemy Mountains. The shallowest part rises to about 700 m. The block is completely surrounded by two branches of the Pliny Trench, which reach depths of about 3200 m. About half way between Crete and Cyprus are the Anaximander Mountains. Rising less than 1000 m above their surroundings, the shallowest depths are about 1700 m. Abyssal hills adjoining the Anaximander Mountains on the east appear to be related, but they consist of many small isolated hills largely submerged under a blanket of sediment rather than standing boldly above their surroundings as do the Anaximander Mountains. Eratosthenes Seamount, southwest of Cyprus, is a feature somewhat similar to Anaximander Mountains in its relationship to nearby abyssal hills. It differs, however, in being a single massive peak and rising to about 700 meters above an adjacent 3200-m abyssal plain. The abyssal hills which border it to the south split the Nile Cone into its two separate fans. Deltaic sediments have filled the low areas between the hills located farthest south (Fig. 2). It is possible that the area of hills once extended even farther south, but the extension has been completely buried under the cone. Stages of burial are illustrated by profiles 6 and 5 of Fig. 5. Hecataeus Mountains constitute a sort of appendage attached to the south side of Cyprus. Rising sharply above the surroundings at 1500-2000 m, they reach depths of only about 300 m in two peaks on the south side of the mass. These peaks are separated from Cyprus by greater depths. Lastly, a small elongate mass off Syria was termed Mela Mountain. It rises sharply from relatively smooth surroundings at about 1500m to form a sort of plateau at about 1200 m depth. Although the area is small and not very high, it contrasts so greatly with its surroundings as to warrant distinction as a separate physiographic unit.

Mediterranean Ridge The most prominent feature of the eastern Mediterranean Sea is a broad median ridge (the Mediterranean Ridge, Fig. 2). Except for its smaller dimensions, it resembles the Mid-Oceanic Ridge (HEEZEN and EWIN6, 1963). The ridge extends from Italy (RVAN and HEEZEN, 1965), passing between Crete and Libya and curving sinuously northeastward to Cyprus. Cyprus and the shallow area northeast of the island may constitute a shallow continuation of the ridge. The ridge between Italy and Cyprus is about 1600-kin long and averages about 150-km wide. Its top is convex in cross-section, with depths along the crest of 2000-2500 m. The median line is not marked by a rift valley, but it separates areas of differing texture. The surface south of the crest and its continuation south and east of Cyprus (Fig. 2) is coarse textured. Hills having heights of 100-300 m are separated in some areas (Fig. 5, profile 8) and in others they adjoin (Fig. 5, profile 7). In contrast, the surface

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Bathymetry of the eastern Mediterranean Sea

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north of the crest and the crest itself is more finely textured, with numerous closely spaced hills mostly less than 100 m high and forming a very distinctive topography (Figs. 2, 7, and 8). 16oo4~

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Bathymetry of the eastern Mediterranean Sea

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The most striking of these trenches, that bordering the entire south side of the Mediterranean Ridge, is flat floored in cross-section along most of its length. Depths increase from each end toward the middle (Fig. 9). Long (1200-kin) and narrow (averaging 18 km), the Herodotus Abyssal Plain (Fig. 2) separates the irregular topography of the Mediterranean Ridge from the smoother surface of the Nile Cone. The northwestern flank of the Mediterranean Ridge is bounded by two trenches. The Pliny Trench, which divides and rejoins, surrounds the Ptolemy Mountains. West of these mountains the trench is floored by a small abyssal plain, but elsewhere the bottom is sharp with little fill, at least as far as can be determined within the limitations of echo-sounding equipment. Farther east the Mediterranean Ridge is bounded by Strabo Trench, the southwestern end of which cuts into the ridge. The northeastern end diverges from the ridge toward a small circular abyssal plain east of Rhodes. Though small, the Rhodes Abyssal Plain contains the greatest depths of the region (Fig. 2) about 4700 m. This "deep-like depression" was earlier noted by LITTLEHALES(1932). Submarine canyons Superimposed upon the topography of the main physiographic features are smaller ones. Chief of these are submarine canyons. At least four canyons (Fig. 2) indent the shelf-break and extend down to at least 1000 m depth. Largest is Alexandria Canyon off the Egyptian city of that name. Some sounding profiles indicate a relief of at least 200 m and others reveal the presence of several lev6ed channels that may be extensions of the canyon onto the Rosetta Fan. Gaza Canyon is named for the ancient still existing city near its head. A sounding line (Fig. 5, profile 3) crosses its extension on Damietta Fan as a leveed channel. Stratigraphic evidence from wells drilled on land (NEEV, 1960) suggests a landward continuation of the canyon cut into Cretaceous and Eocene strata and filled with Miocene and Pliocene sediments. Another canyon lies just off the Israel-Lebanon boundary, near the former Arab town of Akziv, for which it was named. The walls appear to be steep, possibly of rock, but detailed soundings are not available. Relief probably exceeds 200 m. Beirut Canyon, unlike those off Alexandria and Gaza, has walls steep enough to consist of rock, in accordance with the steep-cliffed shores of Cretaceous dolomitic limestone in that area. Irregularities of the continental rise along part of the eastern end of the Mediterranean have the appearance of dissection topography because the bottoms of the irregularities can be connected with a smoothly concave line approximately parallel to another line connecting their tops. The irregularities are too closely spaced in comparision with the existing sounding profiles to permit the drawing of axial trends. One might infer that the irregularities are similar to those described by MATHEWSand SHEPARD (1962) for the active fronts of the Fraser Delta and the Mississippi Delta; however, these deltaic features mostly appear to extend to depths of only about 70 m, in contrast to 1000-1500 m for the irregularities in the Mediterranean. Possibly the irregularities were formed through erosion by turbidity currents. SEDIMENTS

Small surface samples of the sea floor in the eastern Mediterranean Sea were collected at least as early as 1890 (DE Wl~DT and BERWERTH, 1904), but such samples

186

K . O . EMERY, BRUCE C. HEEZEN and T. D. ALLAN

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Bathymetry of the eastern Mediterranean Sea

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are less useful than long cores for the present purpose. Thirty-two cores collected in this area are available for study. Twelve cores were described by OLAUSSON (1960; 1961) with special studies on tephra (volcanic ash) by MELL~S(1954), on mineralogy by DUPLAIX (1958), and on Foraminifera by PARKER (1958). Twenty other long cores collected aboard the Vema were examined and logged at Lamont Geological Observatory. The 32 cores range to l0 m in length and average about 6.5 m. They occupy a broad belt between the Nile Delta and the vicinity of Crete and Rhodes (Fig. 10). A simplified correlation plot was constructed by projecting the core logs to a profile joining Alexandria and eastern Crete (Fig. l l). Core descriptions were generalized in order to present a simple picture of the sediment pattern uncluttered by minor details. Most common of the sediment types is Globigerina ooze, a brown silty calcareous biogenic sediment. It comprises most of the length of cores northwest of Herodotus Abyssal Plain. Globigerina ooze also occurs as layers within most of the cores farther south, occurring particularly commonly at thc tops of the cores. Second most abundant is a gray detrital lutite much of which is laminated and which contains beds of sand and silt. It comprises the bulk of the sediment on the Herodotus Abyssal Plain and on the Nile Cone. This sediment was recognized also in cores Alb. 188 and V10-56 obtained a short distance northwest of the abyssal plain. It is clearly land-derived sediment and its distribution pattern (Figs. l0 and 11) is just what would be expected of Nile-contributed sediment. It probably is a seaward extension of the mud bottom immediately fringing the deltaic shoreline as mapped by D'ARR1GO (1936, PI. l). Four other kinds of sediments found in the cores are quantitatively less important than Globigerina ooze and detrital lutite, but each is of especial interest with respect to sedimentation in the eastern Mediterranean Sea. One of the commonest of these is black sapropelic lutite. This sediment had an odor of hydrogen sulphide when the cores were opened and it now contains pyrite, a high content of organic matter, and abundant pteropods and diatoms. In core V10-59 a real diatom ooze occurs between depths of 235 and 388 cm. The top 20 cm of each layer commonly contains many borings presumably made by worms. Most borings are filled with Globigerina ooze, the usual kind of sediment overlaying the sapropelic lutite. The sapropel apparently accumulated under stagnant conditions at times when density stratification prevented the sinking of oxygenated surface water. Stagnant bottom waters would have permitted reduction of sulphate by anaerobic bacteria to form hydrogen sulphide and pyrite. Absence of benthic animals would have allowed preservation of much of the organic matter from plankton and avoided the destruction of tests of diatoms, radiolarians, pteropods, and fish bones (MENZIES,IMBRIEand HEEZEN,1961). When aerobic conditions were renewed, bottom-living animals obtained a good supply of food by burrowing into the bottom to their depth limitations. Cause of the density stratification has been attributed to excess runoff including that from the Black Sea, perhaps caused by abnormal amounts of glacial meltwater. Conceivably, extreme floods of the Nile River may also have played a part. The uncertainty attached to the cause of stagnant conditions is attested by the recognition of as many as 13 sapropelic layers in core Alb. 187 and 8 or more in several other cores. Layers oftephra were noted in 11 cores, but thinner layers may have been overlooked in other cores. The layers are thickest and most frequent in the most northwesterly

188

K . O . EMERY, BRUCE C. HEEZENand T. D, ALLAN

cores (Fig. 11). Much of the material was derived from volcanic eruptions in the region of Santorini in the Aegean Province (MELLIS, 1954; NINKOV1CH and HEEZEN, 1965). Another sediment type is turbidite sands and silts recognized in 11 cores, particularly in ones near the base of the Nile Cone and associated with the Herodotus Abyssal Plain, Pliny Trench, and the Rhodes Abyssal Plain. In about one-third of these layers grading can be noted by the naked eye; others may be graded but less obviously. All of the sands and silts are dominated by detritial quartz and feldspar. Their abundance in cores near the base of the Nile Cone strongly suggests that the Nile River is a chief source either directly or by marine erosion of the delta (SAID, 1958), and that these turbidites have been instrumental in shaping the cone. Sediment breccia was noted in five cores (Fig. 11). It consists mostly of disarrayed fragments of sediments in layers having a maximum thickness of 200 cm (core V10-64). Above several layers of breccia is a 5-10-cm layer of calcareous sand or silt. Evidently, the breccias were caused by local mass movements which stirred up a cloud of sediment that settled atop the breccia shortly after mass movement ceased. In summary, sediments from the Nile River are believed to be represented by gray detrital lutite located mostly southeast of Herodotus Abyssal Plain. Included within the sequence of lutite are many sandy or silty turbidites. Little of either the gray lutite or of the turbidites reach north of the abyssal plain; instead, the area farther north is characterized by Globigerina ooze. Forming interruptions in both gray lutite and Globigerina ooze are layers of black sapropel and thinner ones of tephra, the latter apparently derived from Santorini north of Crete. STRUCTURE

Measurements of gravity and magnetic fields were made along all the Aragonese tracks (Fig. 1). The gravity meter was the Graf-Askania owned and operated by the Osservatorio Geofisico, Trieste, and the magnetometer was the nuclear-spin model made by Bruce Peebles Ltd., Scotland, and owned by the North Atlantic Treaty Organization. Full details of these measurements are to be published shortly, but a brief summary will be given here. The areas north and south of Crete show marked differences in geophysical properties (Fig. 12). The Aegean Province is characterized by a belt of positive freeair gravity anomalies lying in an area just north of Crete. The magnetic field over most of the Aegean is irregular with sharp local anomalies almost certainly associated with volcanism. Between Crete and the African coast the gravity free-air anomaly is largely negative with a peak of -- 180 milligals over the Pliny Trench. The Ptolemy Mountains produce a small positive anomaly but in general the average gravity anomaly over the whole area is about -- 100milligals. The magnetic field is remarkably smooth. Along 15 north-south profiles from Crete to Africa, each about 300 km long, there was found no variation in the magnetic field apart from the normal gradient. The Mediterranean Ridge evidently lacks important quantities of igneous rocks and instead consists of sedimentary rocks. Further geophysical information about the Mediterranean Ridge is provided by continuous reflection seismic profiles obtained aboard the Chain during 1964. A

Bathymetryof the eastern MediterraneanSea

189

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portion of the record that crossed most of the Mediterranean Ridge (Fig. 13) shows that the irregular topography of the ridge is underlain by similarly irregular structure. The ridge appears to consist of a broad lens of bedded sediments broken into many small and slightly folded blocks. The middle of the lens required about 1 sec of roundtrip travel time for the acoustic energy, corresponding to about 1-km thickness of sedimentary materials. ORIGIN

OF T H E T O P O G R A P H Y

The Aegean Province and Turkey have long been recognized as belonging to a region of Cenozoic (largely Miocene) tectonism associated with the Alpine-Himalayan belt (UMBGROVE, 1947, pl. 5). Continuing activity of the region is illustrated by the frequency of recorded earthquakes throughout the past two millenia (BALLand BALL, 1953, p. 35), by a broad east-west belt of epicenters (GUTENBERG and RICHTER, 1949, p. 69), and by active volcanism at Santorini north of Crete (FOUQUE, 1879). The Mediterranean Ridge resembles the Mid-Ocean Ridge (HEEZENand EWING, 1963). Its sinuous course through the eastern Mediterranean Sea places it almost equidistant between the African and European shores, especially if one considers that the Aegean Province probably was continental during Pliocene time (BooRCART, 1949, p. 257). Conceivably, the ridge resulted from opening of the eastern Mediterranean

190

K.O. EMERY,BRUCEC. HEEZENand T. D.

Sea perhaps as part of the same movement that opened the Red Sea (WEGENER, 1924; SWARTZ and ARDEN, 1960; DRAKE and GIRDLER, 1964). The Mediterranean Ridge continues northwestward (beyond the limits of Fig. 2) toward Italy as though it were a submarine extension of the Apennine Mountains; it definitely does not continue westward past Sicily to the western Mediterranean Sea. There is a general curved trend from the Apennine Mountains along the Mediterranean Ridge through Cyprus to the eastern Taurus Mountains of Turkey. This trend parallels that exhibited by Greece, Peloponnesus, Crete, Rhodes, and the western Taurus Mountains. Both arcs are convex toward the south around the Aegean Province. The same arcuate trend serves to separate the dominantly east-west structures of central Europe frem the generally north-south ones of Israel and Jordan (PICARD, 1943). Elaborate hypotheses have been proposed during the past for such structural arrangements, but the temptation to develop more of them here will be avoided. It is sufficient to point out that the continuity in trend suggests that the Mediterranean Ridge is of the same age as the mountains at either end, generally considered to be Miocene. The ridge may also be of folded Cenozoic and Mesozoic sedimentary rocks. The sedimentary strata of Cyprus and its prominent northeasterly-trending peninsula supports this inference. The close relationship between structural geology of land and sea floor suggests that at least this part of the Mediterranean Sea is not floored by truly oceanic rocks. After and probably during its tectonic deformation, the sea floor of the eastern Mediterranean Sea underwent modification principally by deposition of sediments. The chief area of these sediments is the Nile Delta. The thickness of the offshore deltaic sediments is not known, but an interpretation of gravity anomalies by HARRXSON (1955) based upon several assumptions led to a maximum of about 3000 m near Alexandria. Harrison's isopach contours (his Fig. 30) indicate a volume of about 140,000 kmL Probably only a small percentage of this volume consists of deltaic sediments, since none of the wells on the land part of the delta penetrated more than 500 m of Plio-Pleistocene strata (SoLIMANand FARIS,1964). The annual rate of contribution of suspended sediments by the Nile River was estimated by SmJKRI (1950) as 57 million tons, corresponding to 0.027 km 3 of sediment compacted to 10 % moisture content by dry weight. At this rate the delta would have required only one or two million years for its construction, a period which is out of line with the known Miocene date of initial cutting of the Nile Valley. However, all figures on which the period is based are too uncertain to warrant further discussion. Sediments elsewhere beyond the continental slopes of the region are too thin to greatly modify the topography, but they are nearly 10-m thick in most of the cores. The bulk of these sediments consist of pelagic Globigerina ooze containing thin beds of tephra (Fig. 11). According to Foraminifera (PARKER, 1958), oxygen isotopes of core Alb. 189 (EMILIANI, 1955, 1958), and salinity of interstitial waters in cores Alb. 187, Alb. 189, and Alb. 190 (KuLLENBERG, 1952), several glacial and interglacial stages are recorded in the cores. A provisional correlation by OLAtJSSON (1961, p. 350) is Postglacial, Wiirm, Riss-Wiirm, Riss (I1?), and Pre-Riss. Probably longer cores would sample earlier glacial ages throughout the entire Pleistocene. Radiocarbon dates supplemented by foraminiferal correlations suggest that an average rate of deposition of about 10 cm per 1000 yrs may be reasonable; although here as elsewhere rates are highly dependent upon local topography. At such an average rate, the

Bathymetry of the eastern Mediterranean Sea

191

thickness of sediments accumulated during the Pleistocene would be about 100 m. Whether this average rate was maintained during the Pleistocene and beyond into Pliocene and Miocene time is uncertain, but the irregular nature of the topography suggests that the sediments cannot be as thick as would be expected from extrapolating this rate back to the Miocene Epoch.

Acknowledgements Appreciation is due to HENRYCHARNOCKfor his interestland aid during the cruise of the Aragonese. Thanks are expressed also to MAURICEEWlNG and J. B. HERSEYfor data from cruises of the Vema and Chain, respectively. C. MORELLIkindly supplied the gravity data. Assistance in assembling and analyzing sounding and sediment data was given by W. B. F. RYAN. MARIE Tr~ARPand J. L. JOHNSONIII also provided valuable aid in the compilation. REFERENCES ALLANT. D., CHARNOCKH. and MORELLIC. (1964). Magnetic, gravity, and depth surveys in the Mediterranean and Red Sea. Nature, Lond., 204, 1245-1248. BALLM. W. and BALL,DOUGLAS(1953) Oil prospects of Israel. Bull. Am. Assn. PetroL Geol., 37, 1-113. BOSTOCKJOHNand RILEYH. T. (1855) The natural history ofPliny. Bohn, London, 6 vols. BOULOSISMAT(1962) Carte de reconnaissance des c6tes du Liban. Bassile Fr~res, Beyrouth, 1:150,000, 2 sheets. (2nd ed.). BOURCARTJACQUES(1949) Gdographie dufond des reefs. Payot, Paris, 1-307. BUREAUHYDROGRAPHIQUEINTERNATIONAL(1938) Carte G~ndrale Bathymdtrique des Oceans, Sheet A iv. CARTERHARRY(1958) The histories of Herodotus of Halicarnassus. Heritage Press, New York, 2 vols., 1-615. D'ARRIGO AGATINO (1936)Ricerche sul regime dei litorali nel Mediterraneo. Ric. Vat. Spiagge itaL, 14, 1-172. DE WINDT JAN and BERWERTHFRIEDRICH(1904) Untersuchungen von Grundproben des Ostlichen Mittelmeeres, gesammelt auf der I, III, und IV Reise von S. M. Schiff "Pola" in dem Jahren 1890, 1892 und 1893. (Ber. Comm. Erforsch. Ostlichen Mittelmeeres 24) Denkschr. Akad. l,Viss., Wien, 74, 235-294. DOEGLASD. J. (1950) Old beaches in the Mediterranean. 18th Internat. Geol. Congress, Great Britain, 1948, Pt. VIII, Sec. G., 16-20. DRAKE C. L. and GIRDLERR. W. (1964) A geophysical study of the Red Sea. Geophys. J. R. astr. Soc., 8, 473-495. DUPLAIXS. (1958) l~tude min6ralogique des niveaux sableux des carottes prelev6es sur le fond de la M6diterran6e. Rep. Swed. deep-sea Exped., 1947-1948, 8, (2), 137-166. EMERY K. O. and BENTORY. K. (1960) The continental shelf of Israel. Bull., Geol. Surv., Israel, 26, 25-41. EMILIANICESARE(1955) Pleistocene temperature variations in the Mediterranean. Quaternaria, 2, 87-98. EMILIANICESARE(1958) Paleotemperature analysis of Core 280 and Pleistocene correlations: J. Geol. 66, 264-275. FouQuE F. (1879) Santorin et ses eruptions. Masson et Cie., Paris. GONCHAROVV. P. and MIKHAILOVO. V. (1963) New data on the bottom relief of the Mediterranean Sea. (In Russian). Okeanologiia, 3, (6) I056-1061. GUTENBERGB. and RICHTERC. F. (1949) Seismicity of the Earth and Associated Phenomena. Princeton Univ. Press, Princeton, N.J., 1-273. HAMILTONH. C. and FALCONERW. (1854) The geography ofStrabo. Bohn Co., London, 3 vols. HARRISONJ. C. (1955) An interpretation of gravity anomalies in the Eastern Mediterranean. Phil. Trans. R. Soc. (A)248, (947) 283-325. HEEZEN B. C. and EWrNG MAURICE(1963) The Mid-Oceanic Ridge. In: Seas, Ideas and Observations on Progress in the Study of the Seas. M.N. Hill, Editor, Interscience, New York, 3 388-410. HEEZ~N B. C., THAm, MARm and EWlNG MAUmCE (1959) The floors of the oceans: I. The North Atlantic: Spec. Pap. Geol. Soc. Am., 65, 1-122. HERSeY J. B. (1965) Sedimentary basins of the Mediterranean Sea. Colston Papers, Bristol England, 5-9 April 1965, Butterworth, England, No. 17, pp. 75-89.

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HYDROGRAPHIC DEPARTMENT OF THE ADMIRALTY (1958) Eastern Mediterranean--sheet 2,

Crete to Port Said. Experimental Depth Contour Chart. Contour interval 100 fathoms, C. 6192. KULLENBERGB. (1952) On the salinity of the water contained in marine sediments. Meddm Oceanogr. Inst., G6teborg, 21, 1-38. LITTLEHALESG. W. (1932) Configuration of the oceanic basins: Physics of the Earth--V, Oceanography. Bull. natn. Res. Coun., Washington 85, 13-46. MATHEWS W. H. and SHEPARD F. P. (t962) Sedimentation of Fraser River Delta, British Columbia. Bull. Am. Assn. Petrol. Geol., 46, 1416-1443. MATTHEWS D. J. (1939) Tables of the velocity of sound in pure water and sea water for use in echo-sounding and echo-ranging. Hydrographic Department, Admiralty, London, H. D. 282, 1-52. MELLIS OTTO (1954) Volcanic ash-horizons in deep-sea sediments from the eastern Mediterranean. Deep-Sea Res. 2, 89-92. MENZIES R, J., IMBRIE JOHN a n d HEEZEN B. C. (1961) Further considerations regarding the antiquity of the abyssal fauna with evidence for a changing abyssal environment. DeepSea Res., 8, 79-94. NEEV DAVID (1960) A pre-Neogene erosion channel in the southern coastal plain of Israel. Bull. Geol. Surv. Israel, 25, 1-21. NrNKOVICH D. and HEEZEN B. C. (1965) Santorini tephra: Colston Papers, Bristol, England, 5-9 April 1965, Butterworth, England, No. 17, 413-453. OCHAKOVS~ZYYC E. (1963) The fourth Mediterranean Expedition aboard Research Vessel Akademik S. Vavilov. (In Russian). Okeanologiia, 3, (3), 550-554. OLAUSSON ERIC (1960) Description of sediment cores from the Mediterranean and the Red Sea. Rep. Swed. deep Sea Exped. 8, (3), 285-334. OLAUSSONERIC (1961) Studies of deep-sea cores. Rep. Swed. deep Sea Exped. 8 (4) 335-391. PARKER F. L. (1958) Eastern Mediterranean foraminifera: Rep. Swed. deep Sea Exped. 8 (2) (4) 217-283. PFANNENSTIEL MAX (1960) Erlauterungen zu den bathymetrischen Karten des Ostlichen Mittelmeeres: Bull. Inst. Oedanogr., Monaco, (1192), 1-60. PICARD LEO (1943) Structure and evolution of Palestine, with comparative notes on neighbouring countries. Publ. geol. Inst., Israel, 84, 1-187. ROSENAN E. (1937) Fisherman's Chart--1:100,000 scale in four sheets. Gov't. Palestine, Dept. Agric., Fish. Serv. RYAN W. B. F. and HEEZENB. C. (1965) Ionian Sea submarine canyons and the 1908 Messina Turbidity Current. Bull. geol. Soe. Am., 76, 915-932. SAID RUSHDI (1958) Remarks on the geo,morphology of the deltaic coast between Rosetta and Port Said. Bull. Soc. Gdogr. 'Egypte, 31, 115-125. S~IEPARDF. P. (1963) Submarine Geology. Harper and Row, New York, 557 pp., (2nd. ed.). SHUKRI N. M. (1950) The mineralogy of some Nile sediments. Q. Jl. geol. Soc. Lond., 105, 511-534; 106, 466-467. SOLrMANS. M. and FARISM. I. (1964) General geologic setting of the Nile Delta Province, and its evaluation for petroleum prospecting. Fourth Arab Petr. Congr., Beirut, 23 (B-3), 1-11. SWARTZ D. H. and ARDEN D. D., JR. (1960) Geologic history of Red Sea area. Bull. Am. Assn. Petrol. Geol., 44, 1621-1637. UDrNTSEVG. B. Editor (1961) Data of oceanological investigations, Research Vessel Akademik S. Vavilov, the First Mediterranean Sea Cruise, 1959. Bottom relief (In Russian). Mezhd. Geofiz. God. 1957-1958-1959, Inst. Okeanol., Akad. Nauk, S.S.S.R, Moscow, 1-58. UMBGROVEJ. H. F. (1947) The Pulse of the Earth. Martinus Nijhoff, The Hague, Netherlands, 1-358. (2nd. ed.). U.S. HYDROGRAPHICOFFICE (1959a) Mediterranean Sea--Antalya K.orfazi to Alexandria. B.C. 3924N (2nd. ed.), scale 1 : 817,635. Contour interval 100 fathoms. U.S. HYDROGRAPHICOFFICE (1959b) Mediterranean Sea--Tobruck to Alexandria. B.C. 3925N (2nd. ed.), scale 1 : 817,635. Contour interval 100 fathoms. WEBSTERE. W. 0923) The Works of Aristotle. Clarendon Press, Oxford, 11 vols. WEGENER A. L. (1924) The origin of continents and oceans. Methuen, London.