Terrace staircases of the River Euphrates in southeast Turkey, northern Syria and western Iraq: evidence for regional surface uplift

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Terrace staircases of the River Euphrates in southeast Turkey, northern Syria and western Iraq: evidence for regional surface uplift Tuncer Demira, Rob Westawayb,, David R. Bridglandc, Ali Seyrekd Department of Geography, Harran University, 63300 S- anlıurfa, Turkey Faculty of Mathematics and Computing, The Open University, Eldon House, Gosforth, Newcastle-upon-Tyne NE3 3PW, UK c Department of Geography, Durham University, South Road, Durham DH1 3LE, UK d Department of Soil Science, Harran University, 63300 S- anlıurfa, Turkey a

b

Received 15 March 2005; accepted 2 July 2007

Abstract We present the first overall synthesis of the terrace deposits of the River Euphrates in SE Turkey, northern Syria, and western Iraq, combining new observations with summaries of data sets from different reaches that had previously been independently studied on a piecemeal basis. The largest number of terraces observed in any reach of the Euphrates is 11, in western Iraq, where this river leaves the uplands of the Arabian Platform. In many other localities not more than 5 or 6 terraces have previously been identified, although we infer that some of these are resolvable into multiple terraces. These terraces are typically formed of gravel, principally consisting of Neotethyan ophiolite and metamorphic lithologies transported from Anatolia. Although older gravels are also evident, most of the Euphrates terrace deposits appear, given the chronologies that have been established for different parts of the study region, to date from the late Early Pleistocene onwards, the cold stages most often represented being interpreted as marine Oxygen Isotope Stages 22, 16, 12, 8, 6 and/or 4, and 2. The formation of this terrace staircase reflects regional uplift of the Arabian Platform. Estimated amounts of uplift since the Middle Pliocene decrease southeastward from almost 300 m in SE Turkey to 150 m in western Iraq. Uplift rates increased in the late Early Pleistocene, the uplift estimated since then decreasing from 110 m in SE Turkey to a minimum of 50 m in the Syria–Iraq border region, then increasing further downstream across western Iraq to 70 m. Numerical modelling of this uplift indicates a relatively thin mobile lower-crustal layer, consistent with the low surface heat flow in the Arabian Platform. r 2007 Elsevier Ltd. All rights reserved.

1. Introduction The Euphrates (Fırat, Al Furat) is the longest river in SW Asia, with a total length of 2800 km. Its source is in the high plateau of NE Turkey, through which it flows generally southwestward for 800 km before passing via a series of gorges through the Taurus mountain range into the uplands of the Arabian Platform. This river then continues SW for 300 km to the Turkey–Syria border, where it is only 150 km from the Mediterranean Sea (Fig. 1). It then turns SE, flowing for 500 km through Syria and 1200 km through Iraq to the Persian Gulf. Corresponding author. Tel.: +44 191 384 4502.

E-mail addresses: [email protected] (T. Demir), [email protected] (R. Westaway), [email protected] (D.R. Bridgland), [email protected] (A. Seyrek). 0277-3791/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2007.07.019

Upstream of the Arabian Platform the Euphrates typically follows gorges that are many hundreds of metres deep (Huntington, 1902), incised in part into metamorphic basement and in part into the beds of a system of large interconnected lakes, which are thought to have existed until the Late Pliocene (e.g., Arger et al., 2000; Westaway and Arger, 2001, Fig. 2). For the first 200 km of its course through the Arabian Platform, the Euphrates crosses the bed of another vast palaeo-lake, which is thought to have existed during the (?) Early Pliocene (Arger et al., 2000, Fig. 2). Although it may have had a drainage outlet to the south, this lake system seems to have acted as a sediment trap, preventing clasts eroded from the north reaching localities further south in the Arabian Platform. This is evident from the reported lack of clasts derived from the Taurides or the Anatolian Plateau in the Upper Miocene– Lower Pliocene of NE Syria, which is made up of lacustrine

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Fig. 1. Regional map showing the Euphrates and other rivers and named localities mentioned in this study. G.Antep and K.Maras- are abbreviations for Gaziantep and Kahramanmaras-, cities in SE Turkey. ARM is Armenia; BG, Bulgaria; CYP, Cyprus; GEO, Georgia, IR, Iran; and KKTC the Turkish Republic of Northern Cyprus. C - u¨ngu¨s- roughly marks the point where the Euphrates passes out of the metamorphic massif of NE Turkey, the former Anatolian continental fragment, and into the Arabian Platform. The River Kebir (Nahr al-Kebir) drains NW Syria west of the Orontes, flowing into the Mediterranean Sea at Latakia.

sediments and evaporites laid down in basins fed by the ancestral river system of this region (e.g., Ponikarov et al., 1966; Besanc- on and Geyer, 2003). The earliest evidence of through-going drainage resembling the modern Euphrates system is provided by the presence of clasts of diverse Tauride lithologies in conglomerates that cap much of the landscape of NE Syria at altitudes of up to 250 m a.s.l. (e.g., Besanc- on and Geyer, 2003). The upland landscape of the Arabian Platform has subsequently been incised by 60 m or more, to create the present Euphrates gorge, which is inset with river terraces. If datable, these can provide a record of the progressive incision by the river. The aim of this study is to summarize the existing literature on this region and assess the incision chronology, which will then be used in numerical modelling aimed at quantifying the surface uplift and its lateral variations across the study region. This study builds upon the brief summary of evidence relating to the Euphrates presented by Demir et al. (2004) and the preliminary modelling study of the associated surface uplift by Arger et al. (2000). Field photographs of key localitiers studied are included in the online supplement, and are cited in the main text and its figures as parts of Figs. S1 and S3–S5. The gorge reach of the Euphrates in the Arabian Platform has a total length of 1000 km: 300 km in Turkey, 500 km in Syria, and 200 km in Iraq (Fig. 1). As previously noted (e.g., Ponikarov et al., 1966; Tyra´cˇek, 1987), because this reach has no major tributaries, much of

its sediment load is transported throughout its length, becoming progressively finer downstream as a result. Thus, the predominance of coarse gravel in SE Turkey gives way to progressively finer gravel and sand across Syria and Iraq. This makes terraces more difficult to identify further downstream, although the progressively greater aridity favours their preservation. Mean annual rainfall exceeds 600 mm along the southern flank of the Taurus mountains in SE Turkey, but decreases to 350 mm at the Syrian border, 200 mm at Raqqa, 150 mm at Deir ez-Zor, and is o100 mm in NW Iraq (e.g., Besanc- on and Sanlaville, 1981). This transition has the effect that Pleistocene fluvial landforms along the Euphrates are relatively rounded and gullied by rain erosion in SE Turkey and northernmost Syria but become progressively less weathered in the more arid reaches. Downstream of Raqqa, terraces attributed to the late Middle Pleistocene (in the chronological scheme of Besanc- on and Geyer, 2003) have extremely fresh morphology in comparison to similar features in more humid areas. Downstream of Deir ez-Zor, some gravels have become calcareously cemented due to carbonate precipitation under arid conditions (e.g., Besanc- on and Geyer, 2003; see also below), to some extent offsetting the difficulties over terrace identification caused by the progressive reduction in grain size. As has been noted previously (e.g., Tyra´cˇek, 1987; Minzoni-Deroche and Sanlaville, 1988), the predominance of clasts derived from the Tauride mountains of eastern Turkey, principally metamorphic

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rocks from Anatolia and ophiolite from the vicinity of the Neotethys suture (Fig. 2) (rather than the less durable limestone typical of the Arabian Platform) gives these gravels a characteristic dark colour.

Downstream of Khan al-Baghdadi, 200 km east of the Syria–Iraq border (Fig. 1) and 200 km west of Baghdad, this river passes out of the uplands of the Arabian Platform and into the Tigris/Euphrates delta plain of central Iraq.

Fig. 2. Map of the reach of the Euphrates in SE Turkey. Light grey shading indicates the former Anatolian continental fragment, the metamorphic basement of which provides the lithologies that characterize the Euphrates gravels, in contrast with the predominant limestone of the Arabian Platform further south (unshaded). Dark grey dots on a white background indicate the Miocene–Pliocene Malatya lake basin; Dark grey dots on a light grey background indicate the Adıyaman lake basin, of similar age (see text). Black-and-white lines indicate the principal active left-lateral fault segments within the EAFZ. The Hazar-S- iro Fault crosses the Euphrates gorge at Dog˘anyol, with the en echelon C - u¨ngu¨s- Fault further downstream. Further west, the faulting splays into the Su¨rgu¨ Fault that heads westward through Nurhak, and the WSW-trending Go¨ksu and Go¨lbas-ı-Tu¨rkog˘lu faults that have been studied in detail by Westaway and Arger (1996) and Westaway et al. (2006). The Aksu and So¨g˘u¨tlu rivers are tributaries of the Ceyhan (Fig. 1). Geological background information is from Tolun and Erento¨z (1962) and Arger et al. (2000). Note that the northernmost part of the Arabian Platform is mountainous, with summits rising to 2500 m a.s.l., similar to those in the metamorphic terrane farther north. This is in part a consequence of prolonged deformation, including folding, during the continental collision that began in the Eocene following the final closure in this region of the Southern Neotethys Ocean. The structural term ‘platform’ thus does not imply ‘flat’. Earlier in this closure process, in the Maastrichtian (latest Cretaceous), oceanic crust formed at the spreading centre within this ocean was obducted onto the northern margin of the Arabian Platform (e.g., Delaloye et al., 1977; Delaloye and Wagner, 1984; al-Riyami et al., 2002), forming the source for another significant constituent of the Euphrates gravels.

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Between here and the Syrian border, the Euphrates terraces were studied by Tyra´cˇek (1987). From that border upstream for 150 km to Deir ez-Zor they have been documented by Besanc- on and Geyer (2003) and Geyer (2003). The 350 km reach from the Turkish border at Jarablus to Deir ez-Zor was documented by Besanc- on and Sanlaville (1981), excluding the 100 km reach upstream of Ath Thawrah (Fig. 1) beneath the Lake Assad reservoir. The 20 km reach of the Euphrates in southernmost Turkey between Birecik and the Syrian border has been investigated by Minzoni-Deroche and Sanlaville (1988) and Kuzucuog˘lu et al. (2004). Upstream of Birecik, virtually the whole length of the river within the Arabian Platform is submerged beneath the Birecik and Atatu¨rk reservoirs (Fig. 2). Regional-scale geological mapping (Tolun and Erento¨z, 1962) revealed extensive spreads of fluvial gravel along this reach and further upstream, which were subjected to some study (e.g., by Erol et al., 1987;

Fig. 3. Map of the reach of the Euphrates within the upland part of the Arabian Platform in western Iraq, adapted from Fig. 1 of Tyra´cˇek (1987), which lists original sources of information.

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Wilkinson, 1990) before the areas were flooded. We report below on the first detailed investigations of these deposits on the short reach of the Euphrates that remains accessible, around Karababa Bridge (Fig. 2), downstream of the Atatu¨rk Dam and upstream of the Birecik Reservoir. 2. The Euphrates in Iraq The reach of the Euphrates within the Arabian platform in Iraq follows the contact between Lower to lower Middle Miocene marine limestone to the south, belonging to the Euphrates (or Asmari) and Jeribe Formations, and the upper Middle Miocene Lower Fars (or Gachsaran) Formation further north (cf. James and Wynd, 1965; Tyra´cˇek, 1987; Alsharhan and Nairn, 1995, Fig. 3). The latter, which comprises interbedded limestone, gypsum and silt, records the transition from marine to terrestrial conditions in the Middle Miocene (e.g., James and Wynd, 1965; Alsharhan and Nairn, 1995), associated with slow regional uplift (e.g., Mitchell and Westaway, 1999; Arger et al., 2000). In the vicinity of Anah (Fig. 3), the Euphrates flows through the relatively resistant Oligocene marine limestone of the Anah Formation, which forms the core of the Anah anticline (Fig. 3). Many similar anticlines are documented in the northern Arabian Platform, representing folding that, according to recent kinematic models (e.g., Westaway and Arger, 1996; Westaway, 2003, 2004b) is considered to pre-date the modern (post-4 Ma) configuration of plate boundaries in this region. The lack of evidence of localized warping of Euphrates terraces across the Anah anticline axis (Fig. 4) suggests that the folding has been inactive during (at least) the Quaternary, although the terraces are not well-developed in this locality. Tyra´cˇek (1987) recognized 10 Euphrates terraces predating the Holocene floodplain, which he called I, II, IIIa, IIIb, IIIc, IVa, IVb, IVc, Va and Vb, I being the oldest (Fig. 4). Some of these have more or less continuous preservation, whereas representation of others is sporadic. For instance, terrace IIIa was only clear around the confluence between the Euphrates and the Hauran, its principal right-bank tributary in Iraq (Fig. 3). As mapped

Fig. 4. Longitudinal profile of the Euphrates terraces between Husaiba and Khan al-Baghdadi in western Iraq (see Fig. 3 for location), adapted from Plate 1 of Tyra´cˇek (1987). Roman numerals indicate the terrace identifiers established by Tyra´cˇek (1987). Other symbols indicate uplift-modelling sites, and the marine Oxygen Isotope Stages in which the deposits forming each terrace are inferred to have aggraded as a result of our later uplift modelling.

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by Tyra´cˇek (1987), these terraces converge gradually upstream across Iraq; for instance, terrace II is 72 m above river level near the downstream limit of the Arabian platform but only 47 m above river level at Husaiba, near the Syrian border (Fig. 4). This convergence causes some terraces to be increasingly difficult to resolve upstream, for instance terraces IVa and IVb are 8 m apart (34 and 26 m above river level) near the downstream limit of the Arabian platform, but cannot be separately recognized near the Syrian border. Subsequent to Tyra´cˇek’s work, much of this reach of the Euphrates has been flooded upstream of the Qadisiyah Dam (Fig. 3), so it is unlikely that his terrace scheme will be superseded (although his chronology is, of course, provisional, subject to confirmation by dating of the terrace deposits). Tyra´cˇek (1987) noted that the two oldest terraces cover broad areas reaching many kilometres away from the river, whereas the staircase of younger terraces is localized within the modern river gorge. Being aware that rivers in central Europe typically switched from incising broad valleys to narrower gorges around the end of the Early Pleistocene, caused by an increase in regional uplift rates (cf. Kukla, 1978), Tyra´cˇek (1987) suggested a similar interpretation for the Euphrates, with terrace II marking this transition. Tyra´cˇek’s (1987) terrace I is at 220 m a.s.l. (56 m above river level) near the Syrian border, suggesting equivalence with the above-mentioned broad spread of gravel of Tauride origin and inferred Late Pliocene age that caps much of the landscape of northern Syria but thins southeastward towards Iraq (cf. Ponikarov et al., 1966, 1967; Besanc- on and Geyer, 2003). This oldest gravel may represent deposition over a substantial span of time, maybe from 3 to 1 Ma (the start of the Late Pliocene to the end of the Early Pleistocene), about which little is known (cf. Tyra´cˇek, 1987). 3. The Euphrates in Syria The modern Euphrates valley is incised into a broad zone of fluvial gravels, up to 12 km wide in NW Syria, around Maskaneh, and up to 20 km wide further downstream, around Mayadin near the Iraqi border (Ponikarov et al., 1967, Fig. 1). The first mapping of the Euphrates terraces in Syria was by Van Liere (1961). A formal terrace scheme was subsequently developed as part of the geological mapping of Syria, sponsored by the former Soviet Union, in the 1960s (e.g., Ponikarov et al., 1966; also see Demir et al., 2004). Terraces were assigned Roman numerals and some very tentative conclusions were reached about their possible ages from the associated archaeology. Subsequent workers (e.g., Besanc- on and Sanlaville, 1981; Besanc- on and Geyer, 2003) have adopted a different terrace correlation scheme for the Euphrates, which its authors have also applied to Syria’s two other main rivers, the Orontes and Kebir (cf. Besanc- on and Sanlaville, 1993; Fig. 1). Age control in support of this scheme is provided by archaeology, biostratigraphy, field

relations with marine terraces, and limited numerical dating (U-series and OSL). Six river terrace levels are recognized, QfV, the oldest, to Qf0, which is considered to be Holocene. In the most recent version of this scheme, by Besanc- on and Geyer (2003), terrace QfI has been thought to represent MIS 6, 4, and/or 2; QfII represents MIS 10 and/or 8, QfIII MIS 14 and/or 12, QfIV MIS 16, and QfV is older. In some localities, a single river terrace has been assigned to each of these intervals of time, but elsewhere stacked fluvial sequences are evident that have been interpreted as spanning much of each interval. This scheme identifies fewer terraces than other workers have found; Tyra´cˇek (1987) observed twice as many Euphrates terraces in Iraq and Bridgland et al. (2003) have reported many more terraces of the Upper Orontes than were identified in the middle reaches of that river by Besanc- on and Sanlaville (1993). Pending any systematic re-mapping of the Euphrates deposits in Syria, we shall work within the existing terrace framework (Figs. 5 and 6, S1), although (as noted in Fig. S1d and elsewhere in this report) it seems probable that more terraces will eventually be recognized. We shall likewise not introduce any new dating evidence; we shall instead tentatively adopt the existing Besanc- on and Geyer (2003) chronology and go on to discuss whether it is consistent with the Tyra´cˇek (1987) chronology for Iraq. We shall review the terraces in reverse order because the bulk of the age control is provided by the archaeology in the youngest terraces. 3.1. Terrace QfI Terrace QfI was originally called the ‘Low Terrace’ by Besanc- on and Sanlaville (1981), and is equivalent to Van Liere’s (1961) ‘Sub-Recent Low Terrace’. The principal dating evidence for it is provided by Middle Palaeolithic archaeology, which led to the age assignment of MIS 6 or 4. In some localities, Besanc- on and Geyer (2003) considered that the uppermost, superimposed, units of this terrace are younger, being assigned to MIS 2. One of the best documented sites in this grouping is Rhiyat, in deposits of terrace QfI, 8 m above the present level of the River Balikh upstream of the Balikh-Euphrates confluence, 5 km NE of Raqqa (Figs. 5(a) and 6(a)). A Palaeolithic assemblage from here (cf. Hours, 1979), consisting of handaxes and Levallois cores and flakes, has been assigned to the ‘Final Acheulean stage’, thought to date from MIS 6 (Besanc- on and Geyer, 2003). Besanc- on and Sanlaville (1981) designated the deposits of terrace QfI (the ‘Wu¨rm’ terrace on their maps) as the Abu Chahri Formation, after a stratotype located 55 km NW of Deir ez-Zor (10 km upstream of Halabiyeh; Fig. 5(b)), where recent incision by a tributary wadi has exposed a 15 m thickness of fluvial sediment, comprising sandy gravel overlain by pale coloured silt and reddish silt. Besanc- on and Geyer (2003) considered that these three superimposed units represent deposition in MIS 6, MIS 4 and MIS 2,

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Fig. 5. Maps of the Euphrates terraces between Raqqa and Deir ez-Zor in northern Syria (Fig. 1), adapted from Besanc- on and Sanlaville (1981), also showing localities illustrated in field photographs in Fig. S1 in the online supplement: (a) The reach around Raqqa. (b) The reach around Halabiyeh. (c) The reach between Bweitiyeh and Deir ez-Zor.

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Fig. 6. Profiles through terraces of the Euphrates system between Raqqa and Deir ez-Zor in northern Syria: (a) The terrace staircase portrayed by Besanc- on and Sanlaville (1981) as of the Balikh tributary near its Euphrates confluence east of Raqqa. Terraces are labelled as designated by Besanc- on and Geyer (2003), with former designations by Besanc- on and Sanlaville (1981) also shown (where different). Terrace QfV has been redrawn to better reflect its disposition capping the Balikh-Euphrates interfluve (Fig. S1 g). This in fact appears to be a composite profile representing terrace QfV at points 20 km north of the Euphrates (where the bedrock is limestone) and lower terraces at sites much further south (Fig. 5(a)), but projected upstream to try to maintain appropriate relative altitudes. The actual altitude of the Balikh is below 240 m a.s.l. at its Euphrates confluence, 250 m at the low terrace sites (Fig. 5(a)) and 270 m adjacent to terrace QfV. Much of the material depicted in these terraces may in fact have been deposited by the Euphrates, not the Balikh. (b) The terraces at Halabiyeh, after Ponikarov et al. (1966, Fig. 32). The original survey of this area reported Euphrates terraces II and IV 9 m and 110 m above the present level of the river. The reported presence of artefacts in terrace IV raised the possibility of human occupation of this area at an early stage in the Pleistocene. (c) Revised terrace staircase at Halabiyeh, adapted from Fig. 11 of Besanc- on and Geyer (2003). Their resurvey places the high terrace at this site, assigned to QfIII, 65 m above present river level; the low terrace being QfI. (d) Cross-section through the Euphrates terraces at Ayash (see Fig. 5(c) for location), adapted from Fig. 12 of Besanc- on and Geyer (2003). Terrace QfII locally consists of a lower gravel member, A, overlain by a sand/silt member, B, as in the adjacent locality depicted in Fig. S1f. Terrace QfI consists of a similar succession, with gravel unit C overlain by silt unit D.

respectively. Besanc- on and Sanlaville (1981) stated that this site has yielded Middle Palaeolithic artefacts, but did not specify from which stratigraphic level. Further downstream, at Ayash, 10 km NW of Deir ez-Zor (Fig. 5(c)), a young basalt flow has capped deposits assigned to terrace QfI, which reach 8–9 m above river level (Fig. 6(b)). These deposits consist of gravel, capped by 6 m of yellow silt (Perves, 1964; Besanc- on and Geyer, 2003, p. 41). ‘Late Levalloiso-Mousterian’ (? MIS 4) artefacts were recovered from the top of the gravel, and ‘Epipalaeolithic’ artefacts (late MIS 2) from the top of the overlying silt. Comparison of the descriptions and reported altitudes by Tyra´cˇek (1987) for Iraqi terraces and by Besanc- on and Sanlaville (1981) and Besanc- on and Geyer (2003) for the Syrian sequence provides a strong indication that Tyra´cˇek’s terrace Vb is the downstream equivalent of terrace QfI in Syria.

3.2. Terrace QfII Terraces QfII and QfIII were not differentiated in the text of Besanc- on and Sanlaville (1981), although they were resolved in their maps into a lower level (QfII, the ‘Riss’ terrace) and an upper level (QfIII, the ‘Mindel’ terrace) into which the lower level is inset. The bulk of these sediments (Fig. S1a, b, e, f) are assigned to terrace QfII, the Abu Jemaa Formation, named after a stratotype near Ayash (Fig. 5(c)) where these deposits are particularly thick (Fig. S1f) and are capped by the basalt flow already mentioned (Fig. 6(b)). Terrace QfII is equivalent to the ‘Main Gravel Terrace’ of the Euphrates, as originally recognized by Van Liere (1961). Many quarries and other sections in these QfII deposits have yielded Acheulean artefacts, considered chacteristically ‘Middle Acheulean’ according to Van Liere (1961) or

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Fig. 7. (a) Idealized transverse profile across the Euphrates downstream of Deir ez-Zor in NE Syria, adapted from Fig. 23 of Besanc- on and Geyer (2003). Note the great thickness of deposits attributed to terrace QfII, which reach more than 20 m below the level of the modern valley floor. (b) Transverse profile across the Euphrates at Rejem el Hijjana in NE Syria, 25 km upstream of the border with Iraq (Fig. 1). Adapted from Fig. 19 of Besanc- on and Geyer (2003).

‘Late Acheulean’ according to Besanc- on and Sanlaville (1981). A notable example is at Tell Bani near El Musallakha (Al Maslakha; 35 km upstream of the border with Iraq), where gravels assigned to QfII locally protrude through the Holocene floodplain (as in Fig. 7(a)) and have yielded artefacts of Middle Acheulean (considered to be reworked) and Late Acheulean (considered in situ) affinity (Besanc- on and Geyer, 2003, p. 38), consistent with an age—inferred by these authors—of MIS 8. This particular deposit is capped by a thin layer of pebbly silt that has reportedly yielded Middle Palaeolithic artefacts, consistent with QfI. The stratotype itself at Abu Jemaa (Fig. 6(b)) consists of a 10 m high section in gravel, rising to 20 m above river level, capped by several metres of silt, the lower part of which is incised into the top of the gravel, then overlain by basalt (Besanc- on and Geyer, 2003, p. 36). Bluffs reaching up to this level, many of them quarried, with gravel exposed, line much of the Euphrates gorge between Raqqa and Deir ez-Zor (Fig. S1b, d, e and f) and were interpreted by Besanc- on and Sanlaville (1981) as marking unit QfII throughout (but see also below). This deposit has also been shown by boreholes to persist to a depth of 30 m beneath present river level, indicating major incision before its deposition; the younger deposits (QfI) that are inset into it do not reach its base (Besanc- on and Sanlaville, 1981, Fig. 7(a)). Near the Syria–Iraq border, gravels attributed by Besanc- on and Geyer (2003) to terrace QfII reach 13 m above present river level, suggesting that it is equivalent to Tyra´cˇek’s (1987) terrace Va in western Iraq, which reaches 11 m above river level.

3.3. Terrace QfIII Terrace QfIII was called the ‘Middle Pleistocene Valley Fill’ by Van Liere (1961). He reported rolled struck flakes from it at one site, Hammam, 25 m above present river level in the right bank of the Euphrates, 20 km west of Raqqa. He also noted that the morphology of this terrace progressively becomes degraded upstream of Raqqa, which he attributed to the increasing importance of slope processes in areas of progressively higher rainfall. From our own observations (such as in Fig. S1a) this upstream increase in degradation affects the other Euphrates terraces also. The only artefact-bearing deposit that Besanc- on and Sanlaville (1981) assigned to terrace QfIII is at Chnineh, north of the Balikh-Euphrates confluence (Fig. 6(a)), in gravel 45 m above the former river. Besanc- on and Sanlaville (1981) thus raised the possibility that this gravel correlates with those at Latamneh (Fig. 1), where gravel at a similar height above the River Orontes has yielded an artefact assemblage described as characteristically Middle Acheulean. At Latamneh, two superimposed gravels are separated by an occupation level in finer-grained deposits, associated with a mammal fauna that is probably indicative of the latest Cromerian complex (MIS 13) or possibly the Holsteinian (MIS 11) (Bridgland et al., 2003). However, from the description of the Chnineh assemblage by Hours (1979), it bears no particular comparison with that from Latamneh; much of it may indeed represent surface rather than in-situ discoveries. At Halabiyeh, about halfway between Raqqa and Deir ez-Zor (Fig. 1), basalt from an adjacent volcanic neck caps

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Euphrates gravel (Figs. 5(b), 6(c), 6(d), S1) in which Ponikarov et al. (1966) reported artefacts (Fig. 6(c)). This gravel was originally reported at 110 m above the present-level of the Euphrates (Fig. 6(c)), but subsequent re-mapping (Besanc- on and Geyer, 2003) places it lower, 65 m above present river level, assigning it to terrace QfIII (Fig. 6(d)). Besanc- on and Geyer (2003) reported, from a quarry 7 km NE of Deir ez-Zor (Fig. 5(c)), a new artefact assemblage that they regarded as comparable to that from Latamneh (although no details were provided). However, this site is in gravel that was previously assigned to terrace QfII, which is indeed only 10 m above the Euphrates. Nonetheless, since it is known that deposition of terrace QfII was preceded by major incision, it is perhaps possible that the source outcrop represents an eroded remnant of QfIII. Near the Syria–Iraq border, gravels attributed by Besanc- on and Geyer (2003) to terrace QfIII reach 25 m above present river level, a similar height to Tyra´cˇek’s (1987) terrace IVa, although he did not record that terrace from near the border. Nevertheless, it is somewhat higher than Tyra´cˇek’s (1987) terrace IVb, which is 21 m above river level in the Syria–Iraq border region. 3.4. Terrace QfIV Gravel attributable to terrace QfIV, which they called the ‘Balikh Upper Conglomerate’, was identified by Besanc- on and Sanlaville (1981) only north of the Balikh-Euphrates confluence (Fig. 6(a)), at 60 m above the present level of the former river. However, Besanc- on and Geyer (2003) assigned the Chnineh gravel in this reach of the Euphrates to terrace QfIV, which would require an adjustment that would assign the ‘Balikh Upper Conglomerate’ to QfV. Besanc- on and Geyer (2003) assigned many sites downstream of Deir ez-Zor to QfIV. Many of these represent erosion surfaces, sometimes cut into the stacked deposits of inferred Late Pliocene age, typically 30 m above the Euphrates just upstream of the Iraqi border. However, in some localities, notably covering an extensive surface at Rejem el Hijjana (25 km upstream of the border with Iraq; Fig. 7(b)), terrace sections have been exposed in excavations; there is typically no more than 1–2 m of fluvial gravel (with clasts from the Taurides) and sand, usually encrusted with gypsum and/or lithified by a distinctive salmon-coloured calcareous cement. At one locality, near Darnaj (15 km downstream of Mayadeen, Fig. 1; 90 km upstream from Iraq), Besanc- on and Geyer (2003) reported 3 m of fluvial sands and gravels at the +30 m level expected for terrace QfIV, overlain by 3 m of silt and underlain by gypsum, beneath which other fluvial deposits (interpreted as much older—Plio-Pleistocene) are exposed. The QfIV terrace level corresponds with that of Tyra´cˇek’s (1987) terrace IIIc, although he did not report this terrace in the immediate vicinity of the Syria–Iraq border. However, the field descriptions of

terrace QfIV in Syria and the members of Tyra´cˇek’s (1987) terrace group III in Iraq are quite similar (i.e., thin gravels, o3 m thick; relatively small well-rounded clasts; calcareous cementation), so such an association seems reasonable. 3.5. Terrace QfV Terrace QfV (Besanc- on and Sanlaville, 1981; Besanc- on and Geyer, 2003) is equivalent to the ‘High Gravel of the Euphrates’ of Van Liere (1961). It consists of Euphrates gravel preserved across what are now uplands of the Arabian Platform, above the level of the modern Euphrates gorge. Besanc- on and Sanlaville (1981) did not report deposits assigned to terrace QfV in the reach of the Euphrates between Raqqa and Deir ez-Zor. However, their ‘Balikh Upper Conglomerate’, in the Balikh-Euphrates interfluve near the confluence (Figs. 6(a), S1g), can now be attributed to QfV, given the re-designation by Besanc- on and Geyer (2003) of the Chnineh gravel, at a lower level in this area, to QfIV. The composition of this QfV gravel suggests that it is a deposit of the Euphrates and not of the Balikh (Fig. S1 g); the top of this Euphrates gravel is 315 m a.s.l., 75 m above the modern river at Raqqa. Much less regularly bedded deposits of the Euphrates can also be observed outside its modern gorge 70 m above river level, 60 km downstream of Raqqa (Fig. 5(b), S1 h); we also tentatively attribute these to terrace QfV. From Deir ez-Zor downstream to the Iraqi border, Besanc- on and Geyer (2003) reported the surface of terrace QfV at altitudes of 47–50 m above river level at several localities, where gravel (up to 6 m thick) is overlain in some places by sand and another gravel, again with calcareous cement. The underlying thick gravel was considered by them to be part of the Late Pliocene stacked fluvial sequence, into which the younger fluvial deposits are inset. Besanc- on and Geyer (2003) reported that the upper surface of QfV is 15–20 m below a regional plateau, reached after a gentle upward slope extending many kilometres from the Euphrates. Comparison with Tyra´cˇek’s (1987) results suggests that his Iraqi terrace II is equivalent to the Syrian QfV; his terrace I gravels sit at the level of the regional plateau and thus have no equivalent within the Syrian scheme. Further upstream a similar morphology was reported by Besanc- on and Sanlaville (1981) in NW Syria, just south of the Turkish border, to the east of Manbij (Fig. 2). The Euphrates is locally at 315–320 m a.s.l.; the land surface slopes gently eastward towards it over a distance of 15–25 km from 520–530 to 410 m a.s.l., before dropping abruptly to the river, which occupies a relatively narrow gorge inset with terraces (Fig. S1a). Besanc- on and Sanlaville (1981) reported occasional clasts of durable lithologies on this land surface, plus clearer quarry exposures of gravel including clasts of ophiolite, chert and basalt (the latter now totally rotten due to weathering),

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strongly suggesting that this sloping surface is capped by fluvial deposits of the Euphrates. It presumably represents a succession of river levels, incised at a relatively early stage (between the Late Pliocene and late Early Pleistocene, given the existing chronology), like its more subdued counterparts further downstream. This would be the Syrian equivalent of Tyracek’s pre-Middle Pleistocene plateau in Iraq, and would thus indicate an increase in uplift rate in the early Middle Pleoistocene, a phenomenon recognized widely in Europe (Kukla, 1978; Maddy et al., 2000; Westaway, 2002b) and further afield (Bridgland and Westaway, 2007). 4. The Euphrates in Turkey The reach of the Euphrates in the Arabian Platform that has been most thoroughly investigated is around Birecik (e.g., Minzoni-Deroche and Sanlaville, 1988; Kuzucuog˘lu et al., 2004), just upstream of the Syrian border. Evidence from this area will be summarized, together with the first results from a new study of the terrace staircase immediately downstream of the Atatu¨rk Dam (Fig. 2). These two localities are separated by a downstream distance of 100 km, where the Euphrates is now impounded behind the Birecik Dam and its terrace staircase has thus been inundated. Photographs of reaches of this river that are now flooded, taken before reservoir construction (e.g., Plates 2b, 3 and 4a of Wilkinson, 1990; Figs. 23, 24, 38–40 of Yakar, 1991), indicate a morphology of the valley margins that is similar to what is observed in the remaining free-flowing reach further downstream in NE Syria (Fig. S1b, d).

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of Syria (see Section 3.5 above). Minzoni-Deroche and Sanlaville (1988) also identified an isolated outlier capped with gravel, 130 m above river level, which they called terrace QfVI, located near the inner margin of QfV (Fig. S2). They interpreted this as an older terrace than QfV, but it seems more likely that it represents a level within the multiple QfV range of heights noted above. As illustrated in Fig. S3a–c, the terraced landscape in this reach of the Euphrates is significantly more degraded than its counterpart 50 km further south in NW Syria (Fig. S1a) and vastly more degraded than in the RaqqaDeir ez-Zor reach in NE Syria (Fig. S1b–h). We attribute this progressive effect to a northward and westward increase in the typical rainfall to which these deposits have been subjected since deposition, mimicking the present-day climate (cf. Van Liere, 1961; Besanc- on and Sanlaville, 1981). Notably, the rainfall (and/or rainfall-induced slope processes) in the Birecik area has been sufficient to remove much of the sand and silt that formerly covered the Euphrates gravels, unlike further downstream in Syria. Terrace QfV occurs at the same range of altitude as an upland plateau that can be followed along the Euphrates for many kilometres both upstream and downstream of Birecik, which may therefore represent a fluvial level of comparable age. However, apart from a restricted area close to the confluence with the Nizip tributary (Fig. 4 of Demir et al., 2004), which has incised into the terrace sequence (Fig. S3d), no Euphrates gravels have been reported on this surface. This mimics the situation in the adjacent part of Syria (Section 3.5, above), where the

4.1. The Birecik area Immediately upstream of the Turkish–Syrian border, the main part of the Euphrates terrace staircase is restricted to a zone typically 2–5 km wide on the right bank (Fig. S2), the left bank typically forming a cliff in Palaeogene marine limestone. The terraces within this zone are designated as follows, with heights above present river level (estimated from the limited information provided by MinzoniDeroche and Sanlaville, 1988) in brackets: QfIV (7080 m), QfIII (50–55 m), QfII (30 m), QfI (10 m), and Qf0. The oldest terrace deposits in which Minzoni-Deroche and Sanlaville (1988) reported Lower Palaeolithic artefacts are QfIV, whereas they reported Middle Palaeolithic artefacts in deposits of terrace QfI. Minzoni-Deroche and Sanlaville (1988) thus suggested that terraces QfIV to QfI are of the same ages as their counterparts in Syria, which would now imply MIS 16, 12, 8 and 6 or 4 (respectively), given the Besanc- on and Geyer (2003) dating scheme. Terrace QfV, above the main staircase, occupies a broad zone, 15 km wide, also on the right bank, across which it slopes riverward from 210 to 110 m above river level (Fig. S2), suggesting that it might be multiple. Its overall form is thus very similar to that of its counterpart in the adjacent part

Fig. 8. Schematic transverse profile of the Euphrates terraces in the area that is now flooded, upstream of the Birecik Dam, according to Kuzucuog˘lu et al. (2004), based on their Fig. 4, with bedrock carbonate nomenclature after Terlemez et al. (1997). The original diagram is not fully labelled or explained, but appears to show the oldest terrace as consisting of gravel (A), overlain by silt (B), then an additional deposit (C), then more silt (D) and finally a palaeosol (E). The second-oldest terrace consists of gravel (F) overlain by sand (G) then a final deposit (H). Deposits C and H are described as ‘bedload deposits of the high terraces’ with boulders, and may represent colluvial reworking of the deposits from older terraces onto the surfaces of the younger ones. See text for discussion.

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gravels of terrace QfV are only clear in a few quarry sections (Besanc- on and Sanlaville, 1981). However, the Nizip is only 40 km long and is confined to the Arabian Platform (Fig. 2), so it cannot be responsible for the diversity of clasts of Tauride origin found in the fluvial gravels in this region. The terraces of the Euphrates in the area now flooded by the Birecik Reservoir were studied by Kuzucuog˘lu et al. (2004), their investigation concentrating on the latest Pleistocene and Holocene deposits. A notable conclusion from their work was that two low terraces (QfI and Qf0a in the Sanlaville notation) were inundated by deposition of fluvial silt resulting from extreme flooding during the sixth millennium B.C., following an earlier period of stability when a palaeosol developed during the Holocene climatic optimum (Fig. 8). The subsequent increased stability of this river system seems to have been a necessary condition for the establishment of fixed human settlements along it, during the Chalcolithic era (fifth millennium B.C.). In contrast, Kuzucuog˘lu et al. (2004) only summarized the evidence from the older terraces (Fig. 8), which thus seem equivalent to those designated as QfII and QfIII in the area downstream of Birecik Dam. 4.2. The Karababa area The Euphrates is again free-flowing for a short distance downstream of the Atatu¨rk Dam in the vicinity of Karababa Bridge, 100 km upstream of Birecik, where it is crossed by the road (D875) linking the cities of S- anlıurfa and Adıyaman (Fig. 2). Before discussing our own observations in this area we will briefly summarize the previous work, including investigations at sites further upstream, which have been flooded following the construction of this dam in the late 1980s and early 1990s. Erol et al. (1987) summarized what was already known of the geomorphology of this region prior to the decision to proceed with dam construction. They thus reported that the Euphrates in this area has four Pleistocene terraces, known as S1–S4 (the S standing for ‘sekil’, Turkish for ‘terrace’), respectively, at heights of 80–100, 50–70, 30–25

and 15–10 m above pre-dam river level. This conclusion reflected the scientific consensus at the time that all documented rivers in Turkey had precisely four terraces, an idea that is no longer accepted (cf. Demir et al., 2004). In contrast, Wilkinson (1990) reported results of geomorphological studies undertaken in the early 1980s in association with archaeological rescue work, once the decision to build the dam had been made. He reported eight Euphrates terraces, which he called, in declining sequence, terraces Ia, Ib, Ic, II, IIIa, IIIb, IVa and IVb, the last two being Late Holocene (i.e., dating from historical times). The upper surfaces of the gravels forming these terraces were measured, respectively, at approximate heights of 160, 125, 100, 45, 15, 8, 2 and 1 m above the normal level of pre-dam spring flooding. This sequence of heights was supported by detailed observations, but does not match well, overall, the sequence reported by Erol et al. (1987). Our investigations around Karababa indicate that Euphrates terrace deposits are visible in road cutting sections and in adjacent gravel quarries (Figs. 9, S4), at altitudes between the present river level (385 m a.s.l.) and the 500 m a.s.l. level of the adjoining plateau. Heights of deposits noted in this preliminary account, which are based on visual survey and estimation from small-scale topographic maps, will be subject to correction once the sections have been surveyed in detail. About 9 km WNW of Karababa (Fig. 2), the Go¨ksu, the principal right bank tributary of the Euphrates in SE Turkey, joins the main river via a narrow gorge incised 120 m into the Eocene limestone of the plateau, its base at 380 m a.s.l. (Fig. S5). No clear terraces are evident along this gorge, but a thick deposit of gravel is present in the floodplain. As well as local limestone, this contains polymict clasts of Tauride lithologies from the Go¨ksu headwaters (Fig. 2). At Karababa itself the 120 m of incision by the Euphrates occurs over a much broader zone, locally 4 km wide, with terraces evident at estimated heights of 405 m (Fig. S4e), 420 m (Fig. S4c, d), 435–440, 460, 480 and 500 m a.s.l. (Fig. S4a, b). In the left bank the road descends to bridge level through

Fig. 9. Schematic transverse profile across the Euphrates terrace staircase at Karababa. This is based on preliminary fieldwork and will be subject to modification once the terraces have been accurately surveyed. See text for discussion.

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an 800 m long cutting, which exposes two Euphrates terraces, at 435 and 420 m a.s.l. (Fig. 9). The former consists of 30 m of fluvial gravel, on which the village of Kavs- ut is located. A flat is evident at a similar level in the right bank, but no gravel is exposed. The lower terrace appears to be cut into the 30 m thick deposits of the 435 m terrace, such that it is formed in the basal gravel of the latter, the rest of it having evidently been locally eroded. This lower level corresponds approximately to a large active gravel quarry in the right bank, where 8 m of gravel can be observed to be inset to a lower base level, capped by a few metres of fluvial sand (Fig. S4c, d). At a lower level in the right bank, smaller excavations expose carbonate bedrock into which channels have been incised by several metres and later infilled with cross-bedded sand, before being capped with a thin layer of gravel at 405 m a.s.l. (Fig. S4e), indicating complex fluctuations in sedimentary environment that perhaps relate to climate. At higher levels, exposures of a few metres of gravel are evident in road cuts in the right bank at 460, 480 and 500 m a.s.l., the highest (Fig. S4a) seeming to correspond with the upper limit of the 15 m of fluvial gravel exposed in the 300 m long face of Selamet quarry, 2.5 km east of the Euphrates (Fig. S4b). The Atatu¨rk Dam, 150 m high and 1700 m long, now impounds the Euphrates to a level of 550 m a.s.l. at the point where its gorge, which is locally much deeper than at Karababa, cuts through the Bozova anticline (Figs. 10, S4a). To conclude this analysis, our assessment of the Karababa succession does not fully support any of the previously proposed terrace schemes for this reach of the Euphrates. We thus report more terraces than were recognized in either the Erol et al. (1987) or MinzoniDeroche and Sanlaville (1988) schemes. We have only reported six terraces, fewer than the eight identified by Wilkinson (1990). However, we have only looked in this area for terraces in a restricted altitude range, between 20 and 120 m above the Euphrates. Within this range, we tentatively resolve six terraces, compared with three (i.e., S1, S2 and S3) by Erol et al. (1987) and three (i.e., Ib, Ic and II) by Wilkinson (1990). It thus appears that, overall, there are more terraces in this reach of the Euphrates than anyone has previously suggested, with different studies in the past having identified different subsets of the succession. However, we cannot emphasize too strongly that our present conclusions are tentative, pending a detailed survey of the area in order to measure precise heights of the terraces and thus permit more definitive correlations. 5. Discussion 5.1. Early evolution of the upper Euphrates Middle Miocene fluvial deposits (the S- elmo Formation; Terlemez et al., 1997), interbedded with shallow marine sediments, crop out widely in the northern Arabian Platform around Kahraman Maras- (Derman, 1999,

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Fig. 1). This sequence records the transition of this region from a marine to a terrestrial environment, at an early stage in the regional uplift, with evidence of fluctuating global sea levels, presumably due to glacio-eustasy (Arger et al., 2000). The provenance of these sediments is not established, however, making it unclear whether they were derived from what is now the Upper Euphrates catchment or from a more westerly source, such as the upper catchment of the modern River Ceyhan (Fig. 1). Later, probably in the Late Miocene and Pliocene, the area north of the modern course of the Euphrates, around Adıyaman, formed a major lacustrine and fluvial depocentre, with dimensions of 115 km  35 km (Figs. 2 and 10, cf. Tolun and Pamir, 1975; Arger et al., 2000). The initial lacustrine depocentre was bounded to the south by the Bozova anticline and its WNW inline continuation, the Besni (Kozdag˘) anticline (Fig. 10). This lacustrine sediment, comprising whitish siltstone interbedded with sandstone and gypsum, is poorly dated but a Late Miocene age has been suggested from the limited biostratigraphy (Tolun and Pamir, 1975). The lake may have occupied a closed depression (cf. Tolun and Pamir, 1975); if it had an outlet it was most likely located on the Bozova anticline SW of Akpınar (Fig. 10). Lacustrine sedimentation was superseded by fluvial conglomerates, assigned a nominal ‘Plio-Quaternary’ age (Tolun and Erento¨z, 1962; Tolun and Pamir, 1975), which cap much of the landscape in this region, although widely dissected down to the levels of modern rivers (many now submerged as a result of the Atatu¨rk Dam). These gravels have not been studied in detail, but Tolun and Pamir (1975) noted that they contain mostly clasts of local limestone, suggesting that they predate the development of the through-going Euphrates system. Arger et al. (2000) thought that the outlet of this fluvial depocentre was located along the line of the eastern part of the Kızıl Dag˘ anticline, in the vicinity of the modern Go¨ksu course, where the land surface is at only 500 m a.s.l. for several kilometres width; to the east and west the land surface rises hundreds of metres due to the greater structural offsets across the Bozova and Kızıl Dag˘ anticlines. This drainage would thus have flowed across the Bozova anticline SW of Akpınar, where at present there is a broad col at 600 m a.s.l. (Fig. 10). The upper surface of this fluvial and lacustrine sediment is noticeably tilted southward, possibly as a result of lateral variations in subsequent regional uplift (Arger et al., 2000), from 900 m altitude near its north shoreline to as low as 500 m in the south (Fig. 10(b)), although west of Atatu¨rk Dam around Akpınar lacustrine limestone is found up to a few tens of metres above the present reservoir level. This tilting may have caused the drainage outlet across the Bozova anticline to adjust from its deduced former position near Akpınar to its present alignment, across which the Atatu¨rk Dam has been built. East of this dam, the land surface along the Bozova anticline axis rises to a level just above the nominal 550 m a.s.l. reservoir water level, suggesting that there may have been as much as

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Fig. 10. Maps of the Adıyaman palaeo-lake basin (Fig. 2), adapted from Fig. 10 of Arger et al. (2000): (a) Geological map. (b) Structural and topographic map, illustrating the ponding of this lake basin behind the axes of the Bozova, Besni and Kızıldag˘ anticlines. Topography is from US National Imagery and Mapping Agency Tactical Pilotage Chart G-4B, with 2500, 2000, 2500 and 3000 foot contours labelled to the nearest metre. Note the southward decrease in altitude of the palaeo-lake deposits, consistent with subsequent southward tilting of the land surface due to a southward decrease in regional uplift.

50 m more uplift around Akpınar than at the dam site since the postulated diversion occurred. The timing of the draining of the Adıyaman lake basin is significant for the overall story of the Euphrates further downstream, because while the lake existed it would have trapped coarse sediment derived from the Taurides,

preventing it from reaching localities in Syria or Iraq. We thus presume that the lake disappeared no later than the age of the oldest Euphrates gravels containing clasts of Tauride origin in Syria and Iraq. A related question concerns the age of the modern incised course of the Euphrates upstream of the Adıyaman palaeo-lake (Fig. 2). This gorge crosses the East Anatolian Fault Zone (EAFZ), the active left-lateral fault zone that forms the boundary between the Arabian and Turkish plates (e.g., S- arog˘lu et al., 1992; Westaway and Arger, 1996, 2001; Westaway et al., 2003, 2004b). This fault zone locally comprises two en echelon left-lateral faults, the Hazar-S- iro fault and the C - u¨ngu¨s- fault, across which the Euphrates gorge is offset by 13 and 5 km, respectively (Fig. 2). Recent kinematic models (Westaway, 2003, 2004b) have predicted that the EAFZ is slipping overall at 8 mm a1, has existed since 4 Ma and has accumulated a total of 33 km of left-lateral slip on faults in the Go¨lbas-ı area, plus up to 4 km of slip on the subparallel Su¨rgu¨ Fault further north (Fig. 2). Following detailed investigations in the area west of Go¨lbas- ı (Fig. 2), Westaway et al. (2006) obtained a revised time-averaged left-lateral slip rate (excluding any contribution from the Su¨rgu¨ Fault) of 8.8570.12 mm a1 and a corresponding revised age for the modern fault system of 3.7370.05 Ma. Because the Euphrates gorge is not offset by the full distance by which the EAFZ has slipped, it cannot be as old as 3.7 Ma; indeed, at the measured rate of slip, the totalled offset of 18 km suggests that this gorge has existed since, at the earliest, 2.0 Ma (18 km/8.85 mm a1). However, the 5 km offset of the river as it crosses the C - u¨ngu¨sfault is the total slip on that fault, which may have developed entirely between initiation of the fault system (4 Ma) and the development of through-going Euphrates drainage across the Hazar-S- iro fault, further upstream. If so, the estimated age of the gorge would adjust to 1.5 Ma (13 km/8.85 mm a1), as noted by Demir et al. (2004), or even younger if a component of slip on the Su¨rgu¨ Fault is accommodated. Since no evidence of a palaeo-delta has been reported where the modern Euphrates gorge enters the area of the Adıyaman palaeo-lake (Fig. 10(a)), and since the fluvial clasts in the conglomerates covering the lacustrine sediments are predominantly local, it would appear that the diversion of the ancestral Euphrates to its present course through the Atatu¨rk Dam reach occurred before its modern upstream course across the EAFZ developed. The creation of the Euphrates gorge through the EAFZ caused the emptying of another very large Pliocene lake basin, the Malatya Basin, located further upstream (Fig. 2, cf. Westaway and Arger, 2001). Before it was drained, it appears that this lake basin had an outlet at its SW end near Dog˘ans- ehir, via a col that leads across the Su¨rgu¨ Fault into the modern headwaters of the Go¨ksu, just north of the main EAFZ strand in this area (Fig. 2). However, it is not clear that the Go¨ksu gorge was already in existence in the Pliocene and, allowing for subsequent left-lateral slip

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on the EAFZ, it is possible instead that the outlet from the Malatya Basin followed the linear valley along the EAFZ eastward to the vicinity of C - elikhan, then southeastward into the Kahta river system (Fig. 2), thus flowing into the large Adıyaman palaeo-lake. The emptying of the Malatya lake can be no earlier than the formation of the modern Euphrates gorge across the Hazar-S- iro Fault. It may have significantly post-dated the formation of this offset gorge if headward incision further upstream was not instantaneous. 5.2. Hydrological and sediment-transport regimes in the Euphrates At present the Euphrates flow is highly regulated. However, before the first large dams were built in the 1970s (forming Lake Assad and at Keban, WNW of Elazıg˘; Fig. 2) its flow was highly variable. In this former unregulated regime (data from Ionides, 1937; Clawson et al., 1971; Kolars and Mitchell, 1991), the peak discharge (typically in April, due to melting of snow in eastern Turkey) reached 7000 m3 s1, with the maximum monthly mean value typically in the range 3000–4000 m3 s1. Conversely, in the early autumns of some years the discharge decreased to minima below 200 m3 s1. Wilkinson (1990) summarized data indicating that at Karababa the typical peak flow was 2500 m3 s1 in April–May and the typical minimum flow was o300 m3 s1 in September–November. Given the narrowness of the Euphrates floodplain within its gorge, these variations in discharge caused significant seasonal variations in water level. Milliman and Syvitski (1992) reported that the combined annual sediment load for the Tigris and Euphrates in their natural state was at least 53 million tonnes. Taking the combined catchment area of these rivers as 1.05  106 km2, and the sediment density as 2000 kg m3, this gives a spatial average erosion rate of 0.03 mm a1. This estimate averages between desert regions with minimal erosion and mountainous regions where the erosion rate must thus be many times this figure, but no more detailed data on this sediment budget appears to exist. In its natural state, the mean annual flow in the Euphrates was 30 km3, or roughly 1000 m3 s1. By treaty commitment, the Turkish government guarantees a minimum discharge of 500 m3 s1 across the Syrian border at Kargamıs-. The modern form of free-flowing reaches of this river, maintaining a near-uniform level of water that is largely sediment-free (the sediment load being largely trapped in reservoirs further upstream) is thus very different from how it behaved in its natural state. The past role of seasonal flooding by the Euphrates in determining Early Holocene human settlement patterns in Syria and SE Turkey (Fig. 8) has been investigated in detail by Geyer and Montchambert (2003) and Kuzucuog˘lu et al. (2004). Of greater relevance to our present study is its potential role in the formation of this river’s Pleistocene terraces. The Euphrates system provides the drainage outlet for mountain ranges in eastern Turkey that have

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been glaciated in the Pleistocene: notably the Malatya range south of Malatya city, the Munzur and Esence ranges SW and NE of Erzincan, the Palando¨ken range SW of Erzurum and the Bingo¨l range north of Mus- (Erinc- , 1978; Demir et al., 2004, Fig. 1). It can be presumed that glacial erosion would have led to major influxes of clastic sediment into the river and that melting of glaciers and permafrost during warming episodes led to discharge peaks. One thus expects the gravels within the Euphrates terraces further downstream to correlate with those cold stages during which there was significant upland glaciation in eastern Turkey, providing an indication of the chronology of these local glaciations (about which little is currently known). The thick sandy and silty deposits that cover some of the Euphrates gravels (notably those of terraces QfI and QfII), which are atypical of Pleistocene river terraces in general, may result from conditions that are unique to the Euphrates, such as dramatic seasonal variations in discharge, even during times of temperate climate, the narrowness of the gorge, which would have ensured that discharge peaks resulted in significant increases in water level, and the local aridity, which minimizes their loss to erosion. Although these deposits presumably represent the combined effects of many seasonal floods, the timing of their accumulation remains problematic. 5.3. Uplift histories and their effect on Euphrates longitudinal profiles We now attempt to calculate the post-Middle Pliocene uplift histories of key localities on the Euphrates in SE Turkey, Syria and Iraq, using the technique of Westaway (2001), Westaway et al. (2002), which has been applied to many other fluvial sequences worldwide. This technique is based on the assumption that the observed surface uplift is the isostatic consequence of net inflow of lower crust to beneath the study region, induced by surface processes. More detailed explanations have been provided elsewhere (e.g., Westaway, 2001, 2002a, b, 2004a; Westaway et al., 2002) and so are not repeated here. Specifically, this technique assumes that the observed uplift is the net isostatic response to repeated cycles of loading and unloading of the crust, for instance by ice sheets or fluctuations in eustatic sea level. Both mechanisms will in general occur together and their individual contributions would be difficult to resolve (Westaway, 2002a). However, as has been noted before (e.g., Westaway et al., 2004), in the eastern Mediterranean region it is more likely that the observed uplift is primarily the isostatic response to increased rates of erosion caused by long-timescale climate change (cf. Westaway, 2002c). Nonetheless, tests (e.g., Westaway, 2002a; Westaway et al., 2004) have established that very similar uplift responses can result from both mechanisms whereby surface processes are coupled to induced lower-crustal flow (i.e., from cyclic surface loading or from increased rates of erosion). The chosen technique,

ARTICLE IN PRESS T. Demir et al. / Quaternary Science Reviews 26 (2007) 2844–2863 Euphrates terraces at Khan al-Baghdadi, Iraq: Uplift history 180 ~3.0Ma

~1.6Ma

140 CLC

Uplift since time t (m)

160 120 100 80

I-O

I-Y

60 40

Khan al-Baghdadi

20

Predicated

0 0.0

1.0

0.5

1.5

2.5

2.0

3.0

3.5

Time before present (Ma) Euphrates terraces at Khan al-Baghdadi, Iraq: Uplift history

Uplift since time t (m)

which is simpler to implement, requires fewer model parameters to be specified, and can handle multiple phases of lower-crustal flow forcing (LCFF), is thus used here as an approximation. The parameters to be specified are the magnitude DTe and start time to of each phase of LCFF, the geothermal gradient u and thermal diffusivity k of the lower crust, and Wi, a measure of the thickness of this mobile lower-crustal layer. We assume the same timings of each phase of LCFF as in previous studies (i.e., 3.1 and 0.9 Ma) as these correspond to significant changes in global climate (see, e.g., Westaway, 2001, 2002b; Westaway et al., 2004). We justify the use of fluvial incision as a proxy for regional uplift because the Euphrates terraces are subparallel to each other and to the modern river gradient (cf. Westaway et al., 2002). However, for this particular river, incision cannot be converted directly to uplift, because of significant downstream lengthening of the channel as the coastline has regressed, itself a consequence of the regional uplift. It is well-established that much of what is now Iraq and SW Iran formed an elongated shallow marine embayment, connected to the Persian Gulf and thus to the Indian Ocean, probably until the Middle Pliocene (e.g., James and Wynd, 1965; Ponikarov et al., 1966). It is thus probable that in the Middle Pliocene, the coastline followed the eastern margin of the Arabian Platform uplands, with the contemporaneous mouth of the ancestral Euphrates thus somewhere near Khan al-Baghdadi. The Euphrates is at present 70 m a.s.l. at this point (Fig. 4), showing that its incision since the Middle Pliocene has been less than the accompanying regional uplift by the same amount, 70 m. To correct for this effect we introduce a ‘channel lengthening correction’, which we apply between the Middle Pliocene and late Early Pleistocene, when we believe the coastline to have shifted to approximately its present location. Like other recent studies (e.g., Westaway et al., 2002), we assume that the global ice volume was the same in the Middle Pliocene as at present, consistent with the view that the East Antarctic ice sheet has been stable on this timescale, as is generally accepted (e.g., Denton et al., 1993; Sugden et al., 1995). We begin by modelling the data set from Khan alBaghdadi (Fig. 4), as this has the most terraces and thus offers the greatest possibility of providing tight constraints against the marine Oxygen Isotope timescale. Fig. 11 indicates our preferred solution for this locality. To derive it, it has been assumed (in accordance with earlier discussion) that Tyra´cˇek’s (1987) terrace I marks a prolonged span of time, between nominal limits of 3.0 and 1.6 Ma, during which regional uplift and downstream channel lengthening were occurring together, such that neither significant net incision nor significant net aggradation resulted. The younger terraces are matched as follows: II ¼ MIS 22, IIIa ¼ MIS 18, IIIb ¼ MIS 16, IIIc ¼ MIS 14, IVa ¼ MIS 13b, IVb ¼ MIS 12, IVc ¼ MIS 10, Va ¼ MIS 8 and Vb ¼ MIS 6 and/or 4. This interpretation is broadly consistent with the limited

90 80 70

II 22

IIIa

60 50

IIIb

18

IIIc

40 30 20 10 0

16

IVa

14

IVb IVc 4 Va Vb

13b 12 10

Khan al-Baghdadi

8

Predicated

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Time before present (Ma) Euphrates terraces at Khan al-Baghdadi, Iraq: Predicted Uplift 0.12 Uplift since time t (mm a-1)

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0.10 0.08 0.06 0.04 0.02 0.00 0.0

0.5

1.5 1.0 2.0 2.5 Time before present (Ma)

3.0

3.5

Fig. 11. Modelling solution for the uplift history at Khan al-Baghdadi. Terrace altitudes above present river level are 6 (Vb), 12 (Va), 20 (IVc), 26 (IVb), 34 (IVa), 44 (IIIc), 49 (IIIb), 56 (IIIa), 68 (II) and 80 m (I), after Tyra´cˇek (1987). Terrace I is assumed to represent a substantial span of time, between its old (I-O) and young (I-Y) age bounds, during which the 70 m channel lengthening correction is applied, as discussed in the text. Uplift is measured from a reference level at present river level. Model prediction, using the technique of Westaway et al. (2002) is based on the following parameter values: Wi ¼ 7.5 km, u ¼ 10 1C km1, k ¼ 1.2 mm2 s1, to1 ¼ 18 Ma, DTe1 ¼ 5 1C, to2 ¼ 3.1 Ma, DTe2 ¼ 1.35 1C, to3 ¼ 0.9 Ma and DTe3 ¼ 1.6 1C. This solution predicts 69 m of uplift since 875 ka (MIS 22), 70 m since 950 ka (MIS 25) and 151 m since 3.1 Ma. The maximum predicted uplift rate was 0.087 mm a1 around 2850 ka during the phase of LCFF starting at 3.1 Ma and 0.113 mm a1 around 650 ka during phase starting at 0.9 Ma: (a) Uplift history. (b) Enlarghement of the most recent part of (a). (c) History of variation of uplift rate.

age constraints inferred by Tyra´cˇek (1987). The total number of Euphrates terraces recognized locally is thus 11, including the low-level latest Pleistocene/Holocene deposits

ARTICLE IN PRESS T. Demir et al. / Quaternary Science Reviews 26 (2007) 2844–2863 Euphrates terraces at the Syria - Iraq border: Uplift history 160

~3.0Ma I-O

120 CLC

Uplift since time t (m)

140

100 ~1.6Ma

80

I-Y

60

II

40

36

22

Husaiba / Mari

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Predicated

0 0.0

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1.5

1.0

2.0

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Time before present (Ma) Euphrates terraces at the Syria -Iraq border: Uplift history

Uplift since time t (m)

70 60 50

22

40 16

30

14

IVb

20 10

IVc 10

Va 4

12

6

Husaiba / Mari Predicated

8

Vb

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Time t before present (Ma) Uplift since time t (mm a-1)

Euphrates terraces at the Syria- Iraq border: Predicted Uplift rates 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time t before present (Ma)

Fig. 12. Modelling solution for the uplift history in the vicinity of the Syria–Iraq border based on data at Mari (Syria) and Husaiba (Iraq). Terrace altitudes above present river level are 4 (Vb), 11 (Va), 18 (IVc), 21 (IVb), 31 (III, low facet-IIIL), 36 (III, upper surface) and 47 m (II), after Tyra´cˇek (1987). Terrace I is assumed to represent inset fluvial deposits at relative altitudes between 71 and 56 m above river level, deposited over a substantial span of time, between its old (I-O) and young (I-Y) age bounds, during which the 70 m channel lengthening correction is applied, as discussed in the text. Model prediction uses the same reference level, technique, parameter values and display format as Fig. 11, except: Wi ¼ 8 km, DTe2 ¼ 1.6 1C and DTe3 ¼ 1.05 1C. This solution predicts 48 m of uplift since 875 ka (MIS 22), 49 m since 950 ka (MIS 25) and 141 m since 3.1 Ma. The maximum predicted uplift rate was 0.091 mm a1 around 2825 ka during the phase of LCFF starting at 3.1 Ma and 0.073 mm a1 around 625 ka during phase starting at 0.9 Ma. As discussed in the text, this solution probably significantly underestimates the amount of uplift in this locality during the Late Pliocene and Early Pleistocene.

that Tyra´cˇek (1987) did not include in his numbered scheme—the largest number known on any reach of this river.

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We next consider the data from the Syria–Iraq border region, as represented by Tyra´cˇek’s (1987) data set from Husaiba (Fig. 4) and by the Besanc- on and Geyer (2003) data set from Mari and Rejem el Hijjana, 25 km further upstream (Fig. 7(b)). Between Khan al-Baghdadi and Husaiba, three terraces (IIIa, IIIc and IVa) become indistinct (Tyra´cˇek, 1987, Fig. 4), although terrace IIIc may be equivalent to a low facet of terrace III in the Husaiba area, as noted in Fig. 4. Fig. 12 shows our preferred solution for the Husaiba data set. To achieve it, the channel-lengthening correction has been applied as before. However, we have incorporated the evidence (from Besanc- on and Geyer, 2003) that in adjacent parts of Syria the oldest Euphrates deposits appear to be progressively inset to a depth of 15 m across a broad zone perpendicular to the river. The younger terraces in the Husaiba area are matched to MIS 22 (II), 16 (III), 12 (IVb), 10 (IVc), 8 (Va), and 6 and/or 4 (Vb), with the low facet of terrace III possibly marking MIS 14. Earlier discussion established how the terrace schemes on either side of the Syria–Iraq border match up, from which it can be inferred that QfV ¼ II ¼ MIS 22, QfIV ¼ III ¼ MIS 16 (and possibly also 14), QfIII ¼ IVb ¼ MIS 12, QfII ¼ Va ¼ MIS 8, and QfI ¼ Vb ¼ MIS 6 and/or 4, this being essentially the same chronology as was proposed by Besanc- on and Geyer (2003), although derived from an entirely independent line of reasoning (primarily from the adoption of the chronology based on the results of Tyra´cˇek, 1987). Finally, we model the terrace data set for the Birecik area (see section 4.1 and Demir et al., 2004; and also Fig. S2). The much greater amount of incision and, thus, uplift, inferred from earlier discussion to be post-Middle Pliocene, that has occurred here, compared with in NE Syria and western Iraq (Figs. 11 and 12), means that the channel lengthening correction (which is applied as before) has less relative importance. Trial solution 1 has assumed that the range of levels classified as QfV in this area represents time between 3 Ma and MIS 22, with terrace QfVI placed within this span of time, as previously discussed. This trial solution suggests that, locally, terrace QfI marks MIS 4, QfII marks MIS 8, QfIII marks MIS 12, and the 70 m level of terrace QfIV marks MIS 14, rather than MIS 16 as previously inferred in Syria (Fig. 12). However, above the 70 m facet of terrace QfIV that was used in this trial solution (Fig. 4 in Demir et al., 2004, Fig. S3d), an additional 10 m higher facet is present. We thus consider the possibility that these two facets may mark MIS 16 and 14; as is indicated in Fig. 13, revised solution 2, with a slightly lower value of Wi than solution 1, achieves a more satisfactory fit to this inferred chronology. We thus suggest the notation QfIV-H and QfIV-L (for ‘high’ and ‘low’) to distinguish these facets. Fig. 9 applies this notation to the Karababa section, where these two terrace levels appear well-resolved in our preliminary data set. The best-fitting values of Wi obtained are thus 7.5 km at Khan al-Baghdadi (Fig. 11), 8 km at the Syria–Iraq border (Fig. 12) and 8.5 km at Birecik (Fig. 13). As previous

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325 300 375 250 225 200 175 150 125 100 75 50 25 0 0.0

~2.3Ma QfV-O

CLC

Uplift since time t (m)

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~3.0Ma QfVI QfV-Y 22

36

54

Birecik Predicated 1 Predicated 2

1.5 1.0 2.5 2.0 Time before present (Ma)

0.5

3.0

Euphrates terraces at Birecik: Uplift history Uplift since time t (m)

160 140

QfVI

120

36

QfV-Y

100

22

80 60 40 20 0

14

Qf0

2

QfI

QfII

4

88

16

12

Birecik Predicated 1 Predicated 2

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Time t before present (Ma)

Uplift since time t (mm-1)

Euphrates terraces at Birecik: Predicated uplift rates 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time t before present (Ma)

Fig. 13. Modelling solutions for the uplift history at Birecik. Terrace altitudes above present river level are estimated as 5 (Qf0), 10 (QfI), 30 (QfII), 55 (QfIII), 70 (QfIV, low facet—QfIV-L), 80 (QfIV, high facet— QfIV-H) and 130 m (QfVI), based on information from Minzoni-Deroche and Sanlaville (1988). Terrace QfV is assumed to represent inset fluvial deposits at relative altitudes between 210 and 110 m above river level, deposited over a substantial span of time, between its old (I-O) and young (I-Y) age bounds, during which the 70 m channel lengthening correction is applied, as discussed in the text. Uplift is measured from a reference level 3 m above present river level. Model predictions use the same technique, parameter values and display format as Fig. 11, except for the following. Trial solution 1: Wi ¼ 9 km, DTe2 ¼ 3.4 1C and DTe3 ¼ 2.9 1C. Preferred solution 2: Wi ¼ 8.5 km, DTe2 ¼ 3.15 1C and DTe3 ¼ 2.65 1C. The latter solution predicts 105 m of uplift since 875 ka (MIS 22), 107 m since 950 ka (MIS 25) and 275 m since 3.1 Ma. The maximum predicted uplift rate was 0.158 mm a1 around 2775 ka during the phase of LCFF starting at 3.1 Ma and 0.158 mm a1 again around 575 ka during phase starting at 0.9 Ma.

studies (e.g., Arger et al., 2000) have noted, the crust in this part of the Arabian Platform is of Pan-African (i.e., latest Precambrian; 600 Ma) age. For comparison, modelling

by Westaway et al. (2002) and Westaway (2002a, b), using the same technique as in the present study, has established that Wi for the crust of the London Platform in SE England (also latest Precambrian) is 8–9 km.This both similar to the Euphrates figures and is consistent with the view (e.g., Westaway, 2001) that the thermal state of the continental crust, which depends mainly on its age, is the main factor governing how it responds under this type of model, by controlling the thickness of the mobile lowercrustal layer. The surface heat flow is also quite similar in both the Arabian Platform and the London Platform, being as low as 45 mW m2 in parts of the former (e.g., Medaris and Syada, 1999; al-Mishwat and Nasir, 2004) and 50 mW m2 in the latter (Westaway et al., 2002). One significant difference between these platform regions is that, from geobarometric, seismic and gravity studies, the crust is thicker (40 km against 3035 km) in the Arabian Platform (e.g., Best et al., 1990; Nasir and Safarjalani, 2000). However, the basal Arabian Platform crust is thought to include a layer of mafic underplating at least 10 km thick (e.g., Nasir and Safarjalani, 2000; alMishwat and Nasir, 2004), which will not flow and thus forms the lower boundary to the overlying mobile layer (e.g., Westaway, 2001). In general, the thickness of the mobile lower crust in any region can be estimated as 10 Wi/9 (e.g., Westaway, 1998); from the uplift modelling above, it thus varies between 8 and 9 km in our study region. We thus infer that the 40 km thickness of crust of the Arabian Platform typically consists of a 20 km upper brittle layer, a 9 km mobile layer, and a 11 km mafic basal layer. The resulting relatively low temperature at the base of this mobile layer can be expected to cause a relatively high effective viscosity; along with the thinness of this layer, this high viscosity will restrict rates of horizontal flow. The relatively low amounts and rates of uplift away from the margins of the Arabian Platform, notably in NE Syria (Fig. 12), can thus be readily explained. Much more uplift has typically occurred on the same timescale in parts of Turkey that lie within the former Anatolian continental fragment, where the crust appears to lack mafic underplating and so its lower layer is typically thicker and hotter (and thus more mobile, i.e., less viscous) than in the interior of the Arabian Platform (e.g., Demir et al., 2004; Westaway et al., 2004). The field observations and uplift modelling (Fig. 13) indicate that 105–115 m of uplift has occurred along the Euphrates in SE Turkey since MIS 22, slightly more than the 95 m estimated on the same timescale in the Orontes in NW Syria (Bridgland et al., 2003; Bridgland and Westaway, 2007) but significantly more than the 50 m typical of NE Syria (Fig. 12). One cannot thus correlate terraces by altitude above river level throughout these regions. It is fortuitous that similar amounts of uplift appear to have occurred on similar timescales in the Orontes in NW Syria and on the Euphrates in SE Turkey; thus, terrace schemes for these reaches (e.g., Besanc- on et al., 1978; Minzoni-Deroche and Sanlaville,

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Fig. 14. Schematic longitudinal profile of the Euphrates in the study region, indicating how its altitude and gradient may have varied over time. The true present-day gradient profile falls below BC across Syria, being steeper in NW Syria and flatter in NE Syria, as noted in the text. See text for discussion.

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as much as 50 m lower than the alternative prediction in the vicinity of Husaiba. If such a concave-upward gradient profile still existed by 0.9 Ma, as AKCD in Fig. 14, it would imply that the Husaiba area had subsequently uplifted 50 m further than the 48 m prediction in Fig. 12. However, there is no evidence to support this view; notably, there is no evidence that the Euphrates terraces diverge downstream towards Husaiba (see earlier discussion). We thus infer instead that the present ‘humped’ gradient profile in this region developed earlier, during the Late Pliocene and Early Pleistocene. If so, the oldest Euphrates gravels in this region have uplifted from something like 50 m a.s.l. to their present 241 m, a total of 191 m, rather than the 141 m estimated in Fig. 12, with 143 m of this uplift before 0.9 Ma and the remaining 48 m since. The long profile of this river thus became ‘back-tilted’ in the Late Pliocene and Early Pleistocene and has remained so since. A possible explanation for this effect is the difficulty in incising through the relatively lithified rocks forming the axis of the Anah anticline (Fig. 3). Westaway (2001) suggested a similar explanation for the observed complexity of the long profile of the River Meuse as it flows through erosionresistant metamorphic basement in the Ardennes region of Belgium. 6. Conclusions

1988; Besanc- on and Sanlaville, 1993), which were established largely on the basis of altitude, appear to be roughly consistent with each other. Overall, in the 800 km from Karababa to Khan alBaghdadi (Fig. 14), the Euphrates descends from 385 to 70 m a.s.l., at a mean gradient of 0.4 m km1. Its local gradient is slightly steeper than this in SE Turkey, where it descends from 385 to 340 m a.s.l. in the 100 km from Karababa to Birecik, at a gradient of 0.45 m km1, and in western Iraq, where it descends from 170 m to 70 m a.s.l. in the 200 km from Husaiba to Khan al-Baghdadi, again at 0.45 m km1. Conversely, it has a rather gentler gradient across NE Syria, where it descends from 240 to 200 m a.s.l. in the 130 km between Raqqa and Deir ez-Zor, at a gradient of 0.3 m km1, and to 170 m a.s.l. in the 150 km thence to the Iraqi border, at a gradient of 0.2 m km1. This present-day gradient profile is shown schematically as ABCD in Fig. 14. The uplift modelling has assumed that the same gradient profile has been maintained over time, although the altitude of the river at each point has varied as a result of the use of the channel lengthening correction. The modelling thus implies that the river’s gradient profile was EFGH (Fig. 14) at 3 Ma. It seems probable, however, that the present low downstream gradient in NE Syria results from the fact that the river is flowing towards a region that is uplifting faster (compare Figs. 11 and 12), rather than having been present throughout the existence of this river. If so, then the gradient profile at 3 Ma may instead have been something like EJGH in Fig. 14, possibly

We have presented the first overall synthesis of the Euphrates terrace deposits from SE Turkey to western Iraq, combining new observations with summaries of earlier localized data sets. The largest number terraces observed in any reach of this river is 11, in the vicinity of Khan al-Baghdadi in western Iraq (cf. Tyra´cˇek, 1987), where the Euphrates leaves the uplands of the Arabian Platform (Fig. 1). These terraces are typically formed of gravel, of characteristic dark colour, consisting principally of Neotethyan ophiolite and metamorphic lithologies transported from Anatolia. Individual gravels locally reach thicknesses of 10 m or more and are sometimes overlain by greater thicknesses of fluvial sand and silt, although these fine-grained deposits are typically much better preserved in the more arid reaches further downstream, where postdepositional degradation has been less. Although older gravels are also evident, most of the terrace deposits are interpreted (using chronologies based on indirect arguments, not directly supported by dating evidence, as already noted) as dating from the late Early Pleistocene onwards, the cold stages most often represented being MIS 22, 16, 12, 8, 6 and/or 4, and 2. The formation of this terrace staircase is a response to regional uplift of the Arabian Platform. Estimated amounts of uplift since the Middle Pliocene decrease southeastward from almost 300 m in SE Turkey to 150 m in western Iraq. Uplift rates are inferred to have increased in the late Early Pleistocene, the uplift estimated since then decreasing from 110 m in SE Turkey to 75 m at Raqqa and then to

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a minimum of 50 m in the Syria–Iraq border region, then increasing further downstream across western Iraq to 70 m. Numerical modelling of this uplift indicates a relatively thin mobile lower-crustal layer, consistent with the low surface heat flow in the Arabian Platform. Acknowledgements This paper is a contribution to IGCP-449 ‘Global Correlation of Late Cenozoic fluvial deposits’, to IGCP 518 ‘Fluvial deposits as evidence for climate change and landscape evolution in the Late Cenozoic’ and to FLAG Focus 1. We thank the Council for British Research in the Levant for funding for fieldwork in Syria. Appendix A. Supplementary materials Supplementary data associated with this article can be found in the online version at doi:10.1016/j.quascirev. 2007.07.019.

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