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Research Papers

S E D I M E N T S A N D HISTORY OF THE POSTGLACIAL T R A N S G R E S S I O N

IN THE PERSIAN G U L F A N D N O R T H W E S T G U L F OF O M A N

MICHAEL S A R N T H E I N

Geologische-Paliiontologisches Institut der Univers#iit Kiel, Kiel (Germany)

(Received June 21, 1971)

ABSTRACT

Sarnthein, M., 1972. Sediments and history of the Postglacial transgression in the Persian Gulf and northwest Gulf of Oman. Mar. Geol., 12: 245-266.

Large parts of the Persian Gulf receive little recent sedimentation (see Fig. 2). In these areas the bottom samples contain considerable quantities of parautochthonous relict sediments with radiocarbon ages of 7,000-13,000 years. The relict grains have a shallow water origin. Studies of their locally very differentiated sedimentary facies (see Fig. 5) as well as the morphology help to decipher the palaeogeographic history during the Late Pleistocene-Early Holocene trans- gression. The deepest traces of sub-fossil, water-line sedimentation (oGliths, reef material) are today found at the shelf break in water depths of 105-125 m (see Fig. 10). At the time of this deposition the Persian Gulf was essentially a dry, flat river valley of an ancient Shatt al Arab. Between 100 and 65 m water depth "polymict coquinas", in the shallower part covered by thick unlithified aragonite mud, reflect a rapid transgressive migration of intertidal environments. Behind the Central Swell a temporary lagoon was formed which filled with aragonite mud and some terrigenous deltaic sediments.

Transgression standstill periods are indicated at 64-61 and 53-40 m by coarse, frosted quartz and o/Sid concentrations embedded in lithified aragonite mud. These, together with a fossil ridge and channel system, are interpreted as drowned relicts of strand dunes. Their water depths are analogous to similar occurrences on other shelf regions (see Fig. 8). A possible third standstill lies at about 30 m. The Late Pleistocene climate, as evidenced by the absence of Zagros river sediments, was probably more arid than the present day Persian Gulf climate.

INTRODUCTION

Water depths in the Persian Gu l f a lmost nowhere exceed 100 m (Fig.l) .

Accordingly, the entire area was dry land dur ing the past Ice Age, at which time

sea level was 120 m lower than today (Guilcher, 1969). The purpose of this paper

is to trace the subsequent Late Pleis tocene-Early Holocene transgression based

on evidence found in the sediments and morphology. All data presented here are

derived from observat ions and samples from the 1964-1965 "Meteor" expedit ion

(Dietrich et al., 1966).

The morphology as shown in the bathymetr ic and longi tudinal profile of

the G u l f (see Fig.3) as well as in detailed echogram profiles (both thoroughly

discussed by Seibold and Vollbrecht, 1969) has been interpreted here in the light

of data gained from an analysis of the coarse fraction of grab samples (detailed

Mar. Geol., 12 (1972) 245-266

246 M. SARNTHEIN

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Fig.l. Bathymetry and topographic names in the Persian Gulf (modified after Seibold and Vollbrecht, 1969). M F S - - Mesopotamian shallow shelf; A F S - - Arabian shallow shelf; B S =

Biaban Shelf; W B ~ Western Basin; Z B = Central Basin; H R - - Hormuz region; Z S = Central Swell; E S ~ Eastern Swell; O G - - Gulf of Oman.

data in Sarnthein, 1971). Skeletal and non-skeletal relict grains form a quasi- autochthonous cover in wide areas of the Gulf which receive little recent sedimen- tation (Fig.2). Bioturbation has here resulted in a vertical mixing of Late Pleistocene relict sediment with the overlying modern material over a thickness of more than 2.5 m. For the most part the relict grains are rather brittle, yet they have undergone little modification since their original deposition. Another argument for their quasi-autochthony is found in the composite nature of many gravel and coarse- sand components: in spite of distinct facies changes within short distances they both contain only the local assemblages made up of fine to medium-grained relict sand. Therefore the term "relict sediments" seems justified even in the light of the more restricted definition of Swift et al. (1971).

METHODS

To carry out the component analysis, the sand fraction (0.063-2.0 ram) from each of 175 bot tom samples was split into five subfractions ( - 1 - 0 , 0-1, 1-2, 2-3, 3-4 ~°). Five hundred to eight hundred grains were counted in each subfraction. The grains were catalogued in up to 40 grain-type categories (e.g., Foraminifera, corals, Bryozoa, molluscs, o6ids, pellets, etc.). Relict components alone occupied 18 categories. The results from each subfraction were then added

M a r . G e o L , 12 (1972) 245-266

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248

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M. SARNTHEIN

Mar. Geol., 12 (1972) 245-266

POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF 249

together after having been weighted to correspond to the actual weight percent

of the sand sub-sample which they represented. The final quantitative result from a particular sand fraction can then be considered as being a mixture of both weight and volume. The gravel fraction was counted separately and the values calculated as weight percent (details in Sarnthein, 197l).

The relict components (see Plate IA), which represent deposition in very shallow water, were separated quantitatively from the recent shelf sediments with the help of the following petrographical criteria: their coarse grain size, often with more than one frequency maximum ("lag sediments"); abundance of "wind quartz" in the terrigenous sand minerals; o6id-like sphaericity and polish on grains and grain lumps; elements of shallow-water fauna mixed with recent outer shelf skeletals; and embedding of grains into partly lithified, partly soft, partly glau- conilised aragonite mud. This last feature is most significant because the presently- forming matrix of all samples consists exclusively of terrigenous calcitic clayey mud ("marl") (Lange and Sarnthein, 1970). Therefore it can be seen that the relict particles show two phases of embedding and deposition. The lithification of the aragonite mud preceded its reworking into the marl and is not a result of present- day submarine lithification, this in contrast to Shinn's (1969) findings. The fossil nature of the relict grains was confirmed by several absolute age determinations of 7,000 to 12,500 years B.P. (Sarnthein, 1971).

RELICT MORPHOLOGICAL FEATURES

The Late Wfirm palaeogeography is reflected in several morphologic forms in the Persian Gulf. One of them is the bottom slope along a longitudinal profile (Fig. 3). It shows an even gradient of approximately 10 m in 100 km, a continuation of the modern Mesopotamian valley of the Shatt al Arab into the Eastern Straits of Hormuz and the Biaban Shelf. It ends with an abrupt steepening at the shelf break in a water depth of 110-120 m (Seibold and Vollbrecht, 1969). At a water depth of approximately 50 m the gentle bottom slope is again interrupted. This negative slope-break may be followed in the morphology around the entire Gulf

PLATE I

A. A group of relict grain types. B. Quartz grains with o61ite shells. C. Oi~-spar-arenite. D. O6-micrite. E. Minero-o6-micrite. F. Thin-section of a brown weathered lithoclast. G. Serpulid-encrusted pebble. H. Relict skeletal. I. Serpulite. J. Corals growing on serpulite.

Mar. GeoL, 12 (1972) 245-266

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POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF 251

and includes the wide platform which forms the top of the Central Swell. A further bottom feature is the horizontal plain in the Western Basin behind the Central Swell. It extends more than 50 km to the northwest in a water depth of 72-73 m, approximately corresponding to the overflow level of the channel through the Central Swell at 67 m. Other, less prominent bottom forms shown on the echo- sounding profiles will be discussed in connection with the distribution of sediments.

DISTRIBUTION OF RELICT SEDIMENTARY FACIES

The different water-depth zones of the Persian Gulf (see Fig. 1) are generally represented by different types of relict calcarenites and calcilutites. They can be characterized on the basis of the distribution of a few dominant, mainly aragonitic relict components: unlithified aragonite mud, o6ids and o~liths, non-molluscan sessile-epibenthos skeletals, other skeletals, faecal pellets, lithoclasts, and non- carbonate terrigenous sand grains (Fig.4A, B). The coarse-fraction composition of several characteristic samples is presented in histogram form (Fig. 5). Unfor- tunately it was technically impossible to evaluate the frequently observed recycled lithified sand-mud grains in an exact coarse-fine ratio for the relict sediments. A few estimations are given in the text.

Relict sediments near the shelf break Close to the shelf break, relict components reflect the lowest Pleistocene sea

level. At a depth of 125 m coarse sands and gravels are found. These consist mainly of brown-weathered, recrystallized aragonite lithoclasts (Plate IF), coral debris, and thick-shelled, weathered, shallow-water coquinas; i.e., mainly grains derived from drowned reef material (Fig.5A and B). Most particles were en- crusted and bored by serpulids, bryozoans and--during the final stage--sessile Foraminifera (Miniaeina miniacea L.). The material was then lithified.

A quite different grain assemblage is found at a depth of I05 m where the deepest channel opens onto the Biaban Shelf: faecal pellets, fine o6ids and large amounts of fine terrigenous non-carbonate grains (up to 50~ of sand fraction) are contained in a rather weakly-lithified aragonite mud (Fig.5C). These particles were possibly derived from older, reworked deltaic sediments (see Fig. 7B and 8 and discussion on p. 260).

The nearly pure o61itic sediment found on the Biaban Shelf in 101 m depth of water is a third and perhaps the most reliable indicator of fossil shallow-water zones or even fossil strand lines if one compares this sediment with recent o61itic sediments (Rusnak, 1960; Purdy, 1961). More than 80 ~o of their components are o~ids (Md: 0.4 mm), cemented by aragonite spar (Plate IC, Fig.5D). The maximum thickness of the o6id-shells is near 0.2 ram. The nuclei have an average diameter of around 0.1 ram.

Mar. Geol., 12 (1972) 245-266

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POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF

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Mar. Geol., 12 (1972) 245-266

POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF 255

Central Basin

The relict-grain assemblages from the broad plains of the Central Basin, in 65-100 m water depth, are mixtures of variegated components. In contrast to those from the lower Biaban Shelf, they do not form a genetic group and vary in quantity and type within very short distances. A similar facies is found in a few places in the Western Basin (Fig.4). This mixed assemblage of grain types (Fig.5, E-G) is also reflected in the composition of single coarse sand and gravel grains which underwent early lithification. Red or black o6ids (Md: 0.2-0.5 mm; thickness of shells up to 0.3 ram) are the oldest constituents; in some samples they are accompanied by a few coarse relict quartz grains. The o6ids were embedded into either red or black aragonite mud, partly together with similarly-coloured faecal pellets and with molluscs, and then lithified, mostly in the form of coarse crusts (similar to the example in Plate I D).

Overlying this material is a shell-rich sediment (Plate I H). These shells (many of them black), together with a few older, black otiids and the variegated o6micrite were embedded in weakly lithified, white aragonite mud. These sedi- ments were then covered with a thick growth of serpulids, Bryozoa and Foraminif- era (Plate IG). Sometimes this led to the formation of a true serpulite (Plate I J). This surface provided the base for a few, large individual corals (Plate I I); how- ever, as they died they too were quickly crusted over with the above-mentioned organisms.

This total facies community can be brought together under the name "polymict coquina sands". Its formation depends upon quickly changing and intertonguing sedimentation phases such as would result from a rapid rise in sea level. This resulted in the transformation of successive intertidal carbonate areas into deep water areas (see p. 262). The serpulites represent a final "starvation facies". The frequent black or red colouring of grains as well as the glauconitization processes most likely took place in the early, shallow-water stage. This is supported by the fact that different-coloured grains underwent a common lithification process and then played a role as nuclei in o6id formation and in a final stage were polished (Kendall and Skipwith, 1970; Lange and Sarnthein, 1970). The weathering on the surface of the skeletal remains also must have taken place in shallow water; Driscoll (1970) reports that destruction of shell material takes place 150-1,000 times faster in shallow than in deeper low-energy environments.

Also included in the zone between 65 and 100 m water depth are the sedi- ments of a former, roughly 7 m deep, secondary basin within the southeastern Western Basin (see p. 262). In the south and southeast the surface of these deposits consists of white, aragonite mud rich in shell detritus and faecal pellets which are sometimes glauconized. This mud also shows various grades of lithification. Along the northwest (inner) margin a contrasting occurrence of fine-grained sand (Md: 70-90 kt) of terrigenous minerals is found (Fig.5H). In spite of their matrix of

Mar. Geol., 12 (1972) 245-266

256 M. SARNTHEIN

aragonite mud, they are probably related to a fossil river delta (for discussion see Fig.8B and p. 259).

Relict sediments in 61-64 and 40-53 m depth At the 61-64 m and 40-53 m depth intervals, strips of sediment occur which

are characterized by a high percentage of terrigenous minerals and an about equal percentage of o6ids (Fig.5I). Samples from these zones were obtained in the northwest and southwest parts of the Western Basin, on the southern margin of the Central Basin, and from the northern margin of the Strait of Hormuz. The terrigenous sand grains are mostly quartz, although felspars are also present. Their grain sizes tend to be coarse (Md: 120->200 p; see Fig.7A) and occasionally they exhibit double frequency maxima. Quartz grains larger than medium sand are usually rounded and frosted; many have aragonitic o61ite shells (Plate IB). Curiously these relatively coarse grains, together with rough-surfaced o6ids, are embedded in an aragonite mud matrix and this time lithified in clumps to form "minero-o6-micrites" (Plate IE). This probably accounts for a further frequency maximum found in the fine grain size. Biogenic grains are infrequent, those present being mainly fragments of Foraminifera.

On the Central Swell lithified aragonite mud rich in o6ids and shell detritus covers a broad fossil platform in the corresponding depth of about 50 m. Through- out the entire Persian Gulf the depth zones just described are bordered in the direction of deeper water by thick, white, unlithified aragonite mud (Fig.5J). The mud thicknesses can be clearly seen in the sediment echograms. These muds locally contain large percentages of faecal pellets or lumps or, in places, fine sand- sized minerals. A more detailed study of these sediments based on core material has been done by Diester (1971).

Areas less than 40 m deep From areas less than 40 m deep, few samples which contained relict grains

were available. Recent coastal sediments have masked most traces of relict sedi- ment. The distribution of fossil sediments could therefore be analysed only at very

Fig.6. Sediment echogram from the northernmost part of the Persian Gulf, showing a subfossil coquinite bank in 30 m water depth sheltering relict delta-influenced lagoonal sediments. (Linear values are only approximate).

Mar. Geol., 12 (1972) 245-266

POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF 257

few stations (see Fig.2). Only in the far northwest does the sediment, and the sediment-echogram (Fig.6), allow the definition of a distinct facies feature--a bank covered with shell debris (Fig.5K). Behind this "dam" one finds partially lithified and coloured, partially unlithified and white, aragonite mud which contains a significant coarse fraction of shell detritus (>40% Foraminifera: >20% molluscs) and >30% terrigenous minerals of fine-sand size (Fig.7B).

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Mar. Geol., 12 (1972) 245-266

258 M. SARNTHEIN

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Fig.8. Comparison of relict sand-sized mineral content of samples in various water depths in the Persian Gulf and the world-wide post-Glacial sea level rise. A. Curves of post-Glacial sea level rise after Milliman and Emery (1968) shown by solid line, and after Curray (1965): dashed line, compared with water depths of the "quartz zones" on the shelf off Northern Australia (Van Andel et al., 1967). B. Relationship between sand-sized terrigenous minerals and relict biogenic grains. Circles represent fossil delta sediments and dots represent minero-o6-micrites. The water depths of their maxima coincide with those of the quartz zones off Northern Australia.

Mar. Geol., 12 (1972) 245-266

POSIGLACIAL TRANSGRESSION IN THE PERSIAN GULF 259

SOURCE OF RELICT TERRIGENOUS, SAND-SIZED MINERALS

Relict sediments rich in sand-size minerals are limited to a few, relatively narrow depth-zones in the Persian Gulf (Fig.8B). The samples rich in terrigenous material in the fine-sand fraction (Fig.7B, open circles in Fig.8B) can be separated and related to the deposition from a fossil Shatt al Arab. This idea is supported by the fine, well-sorted character of the sand, its composition (mainly detrital quartz, some mica) as well as its occurrence in depressions behind barriers and in deltas. It is, however, noteworthy that even in these relict samples the fluvial portion in the fine fraction (calcareous clayey marl) always remains < 50% (Lange and Sarnthein, 1970). This is in marked contrast to modern fluvial material where marl dominates (Hartmann et al., 1971). This means that the quantities of sediment delivered by ancient rivers were relatively insignificant. In the shallow marine environment the local production of aragonite mud dominated and heavily diluted the fluvial deposits.

The remaining samples rich in terrigenous minerals--the coarse minero-o/5- micrites at 61-64 m and 40-53 m depth--exhibit clearer differences from recent fluvial sediment (Fig.7A). The formation of this unusual facies can hardly be explained by "normal" beach conditions. Equally as curious is the facies-related, subfossil trough and ridge system (Fig. 9). These slightly asymmetrical forms reach

• - ' - - . 2 0 k m

Fig.9. Sediment echogram of relict ridges and troughs interpreted as aeolian dunes. Dark areas represent Recent soft sediments.

a maximum height of 10 m and are spaced at intervals of about 2 to 4 km. Their steep slopes face in the direction of general bottom rise; as in the example shown in Fig.9, to the east. Seibold and Vollbrecht (1969) feel that they may represent fossil tidal ridges. With reference to studies by Kinsman (1964) and Evans et al. (1969) on the present-day southern margin of the Persian Gulf, it seems likely

Mar. GeoL, 12 (1972) 245-266

2 6 0 M. SARNTHEIN

that the forms could be a drowned beach-dune system. This would further explain the mixed character of grain sizes and components in the minero-o6-micrites as a heterogenous mixture of fossil wind- and beach-carbonate sediment.

The coarse quartz grains found with--or as nuclei of--obids could then be thought of (following Sugden, 1963, or Rusnak, 1960) as originating in aeolian dunes which were attacked by beach surf. Shoreward-moving waves, however, replenished the beach with fresh aragonitic sediment which was immediately, and particularly at low tide, formed into new dunes. Fine material picked up by the wind from temporarily dry carbonate-mud areas was also added to the dunes. This would account for the unexpectedly high percentage of fines in the minero-o6- micrites. Lithification of muds into clumps is the rule in such environments (Taylor and Illing, 1969). This also indicates that the mixing process which has been previously described took place very early in the formation of the sediment.

PALAEOGEOGRAPHY AND HISTORY OF THE TRANSGRESSION

A post-Glacial rise in sealevel of at least 110 m has been postulated by several authors based on evidence from various continental shelves (e.g., Curray, 1960, 1961, 1965; Jelgersma, 1966; Van der Harnmen et al., 1967; Milliman and Emery, 1968; see the review of Guilcher, 1969). As shown in Fig.8A, the rise started about 15,000 years ago and reached its present level about 6,000 years B.P. Several studies also show evidence of temporary stillstands or brief reversals of the rise: the Gulf of Mexico (Curray, 1960; Van Andel and Sachs, 1964), the shelf of Florida (Ballard and Uchupi, 1970), the Timor Sea (Van Andel et al., 1967, see Fig.8A), the Gulf of St. Lawrence (Loring and Nota, 1966). Yet the exact time for the start and end of the rise as well as its precise progress are still some- what controversial. It is clear, however, that Postglacial time brought with it a displacement of the Persian Gulf coastline which at times was very rapid. The result has been that the different types of fossil shallow-water facies, which today form the Gulf bottom in an apparently random succession from deeper to shallower water, actually represent a stratigraphic sequence from older to younger sediments.

During the last Glacial stage, the length of the Shatt al Arab was increased some 800 km to the southeast. It reached the shelf margin in the Gulf of Oman which today lies under 110 m of water (Fig.3). Here, the river apparently built a rather elongated estuary or bay (Fig.10). With the exception of a few traces of terrigenous sediment, however, no true delta was formed; this in spite of the fact that sea level probably oscillated back and forth at this low level for some time. This is substantiated by the drowned beginnings of reefs which show up in the present morphology as isolated rises (Seibold and Vollbrecht, 1969). Further evidence for a stillstand is indicated by the relatively coarse-grained obliths (Fig.5D) and the large quantities of aragonite mud. According to Bathurst (1968)

Mar. Geol., 12 (1972) 245-266

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262 M. SARNTHEIN

a period of several thousand years is necessary for the accretion of an o/3id shell layer of several 100 # thickness.

During early Postglacial time the sea transgressed the zone which today lies between approximately 100 and 65 m (11,000-12,000 years B.P. according to Milliman and Emery, 1968) in the Central Basin and outer Western Basin. This corresponds to a coastline displacement of roughly 500 km in 4,000-5,000 years--an average of 100-120 m per year. Similar values were reported by Van Andel et al. (1967) from the Sahul Shelf. This rapid sea level rise left the Gulf bot tom evenly covered with polymict coquina sands, a facies similar to the "transgressive sequence" of Van Andel et al. (1967). During this time the narrow channel through the Central Swell was blocked by sediments with the result that a lagoon formed behind the swell and gradually filled with sediments, some of them being delta deposits from the ancient Shatt al Arab (Fig.3 and 10). No similar sediments, however, were found behind the Eastern Swell. This period also saw several salt domes in the Central Basin become isolated islands.

The minero-o6-micrites, which have been interpreted as a beach-dune mixed sediment and which are now found concentrated in the zones from 61-64 and 40-53 m, most likely represent two stillstands in the transgression or even short regressions. For the building of the dune systems, particularly the widespread dunes of the Western Basin (Fig.11) and the formation of the o6id sands, a longer time period was required than that which, for example, was available for strand- line formation in the Central Basin. The ridges seen in Fig. 11 parallel the dominant wind direction in the Gult, the Shamal. This is typical of "seif" dunes. The areas where dunes occur correspond nearly exactly to areas of minero-o6-micrites (Fig.4). From the same two depth zones, Van Andel et al. (1967) have described two principal zones high in relict quartz on the Sahul Shelf (Fig.8A) which they also relate to litoral deposition during stillstand periods in the sea-level rise. Here, as well as in the Persian Gulf, they may possibly correlate with the Dryas glacial advance.

During this time the entire Persian Gulf coastal region attained a mor- phological maturity as reflected in the numerous features at - 5 0 m which are described on p. 249 (Fig.3 and 10). This corresponds to the "second off- shore terrace" of Houbolt (1957). The possibility that some of these features represent reactivated, older Pleistocene features cannot be totally eliminated. At this time large quantities of aragonite mud formed in the various shallow-water areas. The mud was carried to a maximum (fossil) water depth of 10 to 25 m and there deposited in large thicknesses over older polymict coquina sands.

At a depth of 30 m there are indications of a third stillstand in the trans- gression (Fig.10). These include the previously mentioned bank of shell detritus with its accompanying lagoon (Fig.6) and the uniform minimum depth of the shallows which could, however, be from an earlier period. Finally, 6,000 years ago, the Persian Gulf attained roughly its present coast line in the north and east.

Mar. Geol., 12 (1972) 245-266

POSTGLACIAL TRANSGRESSION IN THE PERSIAN GULF

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,\

PERSIAN GULF ,~'.~

. . . . . . troughs r i d g e s d i f fe rence in helght;~2rn

~20m isobar hs k~-- 25 m

4 . ' , , %. ,,, ,,,,

• ~ _ ~ - ~

,, ,%

I I J I I

Fig.11. Ridge and trough system interpreted as dunes in the Western Basin southwest of Bushire. Isobaths in meters (modified after Seibold and Vollbrecht, 1969).

Since then the Shatt al Arab as well as most of the Zagros rivers have built very limited deltas. Therefore, the deeper basins and their sediments owe their origin more to earlier events than to present-day processes.

The Persian Gulf lies on the edge ofa tectonically mobile region. Nevertheless, throughout the entire transgression period no discernable evidence for tectonic activity was found. This is supported by the fact that the shelf edge at c. 110 m corresponds well with other shelf margins, the evenness of the bottom slope in a longitudinal profile, the large level surface at 72 in depth, and perhaps also the constant depth of the two zones with high relict-quartz contents which can be correlated with similar zones on the stable north shelf of Australia.

CLIMATE

During the entire transgression period aragonitic sediments always dominated

Mar. Geol., 12 (1972) 245-266

264 M. SARNTHEIN

the northeast parts of the Persian Gulf. Today such carbonates are found only along the arid south and southwest coasts where no rivers enter the Gulf. Along the northeast coast these carbonates are totally suppressed by the calcareous, clayey terrigenous sediment carried in by the Zagros rivers. These rivers must therefore have been much less active during the Late Pleistocene and Early Holocene. It can be concluded that during the transgression period the climate in the Zagros Mountains was considerably dryer than today's climate. This holds true even when the influence of a different plant cover on river regimen is considered (Schumm, 1968). At present these mountains receive the heaviest rainfall in the entire drainage basin (maximum 600-800 mm; see Hartmann et al., 1971).

The ridge and trough system which has been interpreted as a dune field (see p. 262) would also speak for extreme aridity. Even the ancient Shatt al Arab left only unimportant traces in the relict sediment. This could, however, have come about through settling out in lakes along the river's course such as are today found between Bagdad and Basra.

The assumption that an increase in runoff first took place in the Holocene is in agreement with results from pollen analysis (Van Zeist and Wright, 1963; Van Zeist, 1967), geological work on Quaternary sediments of southwest Iran (Butzer, 1958; Bobek, 1963; Vita-Finzi, 1969) as well as climatological theory (Fairbridge, 1965; Lamb, 1965; Flohn, 1969).

ACKNOWLEDGEMENTS

This paper owes much to the informative discussions held with the other members of the Persian Gulf working group of the Geologisch-Pal/iontologisches Institut of the University in Kiel, particularly Professor E. Seibold. Miss L. Diester was kind enough to make information available from her unpublished thesis, H. Lange made available numerous X-ray analyses, M. A. Geyh (Hanover) provided several radiocarbon dates, R. S. Newton helped with the translation and A. Harke assisted in the drafting and photographic work. I would like to express my warmest thanks to all of them. The work was supported by the German Research Society.

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