Late Quaternary highstand deposits of the southern Arabian Gulf: a record of sea-level and climate change

Geological Society, London, Special Publications

doi: 10.1144/GSL.SP.2002.195.01.20p371-386.

2002, v.195;Geological Society, London, Special Publications Alun H. Williams and Gordon M. Walkden Arabian Gulf: a record of sea-level and climate changeLate Quaternary highstand deposits of the southern

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Late Quaternary highstand deposits of the southern Arabian Gulf: a record of sea-level and climate change

ALUN H. WILLIAMS' & GORDON M. WALK DEN^ ' ~olithica Geoscience Ltd, 489 Union Street, Aberdeen ABl l 6DB, UK

2~epartment of Geology and Petroleum Geology, University of Aberdeen, King b College, Aberdeen AB24 3UE, UK (e-mail: [email protected])

Abstract: The southern Arabian (Persian) Gulf is at present the site of extensive carbonate sedimentation, as was the case during Pleistocene interglacial marine highstands. During glacial lowstands the basin was subaerially exposed, and aeolian sedimentation predominated. Most of the southern Arabian Gulf floor is underlain by Quaternary carbonates, and scattered outcrops may be found onshore. These belong to three formations: the aeolian Ghayathi Formation, the continental Aradah Formation and the marine Fuwayrit Formation. The Fuwayrit Formation consists of three members, separated by subaerial exposure surfaces. These are, from the base upwards, the shallow marine Futaisi and Dabb'iya Members, and the aeolian A1 Wusayl Member. Offshore, at least six Quaternary sequences are present within the uppermost 50 m of sediment. No reliable direct age dates have been acquired from Pleistocene shallow marine or coastal deposits in the southern Arabian Gulf. It has therefore been necessary to infer the ages of these sediments by a comparison of their stratigraphy and elevation with deposits known from other parts of the world. We regard this approach as valid because the southern Gulf coastline lacks evidence for significant widespread neotectonic uplift, and halotectonic effects are localized. This comparison indicates that the Fuwayrit Formation was deposited during the last interglacial (oxygen isotope substage 5e), as ( I ) these sediments represent the youngest pre-Holocene marine deposits, and (2) they are found at an elevation correlative with many substage 5e deposits from other parts of the globe. Sedimentary evidence reveals two highstands during this period, peaking at around 1.5 m and 6 m above present sea level, respectively. Offshore sediments indicate that sea level did not fall as far as 24 m below present level in the intervening regression. Following the second highstand, sea level fell to more than 23 m below present level, before briefly rising once again (late isotope stage 5). This later highstand probably peaked between 17 and 7 m below present level. The sequence underlying the Fuwayrit Formation was probably deposited during the penultimate interglacial (late oxygen isotope stage 7). It is also likely that the Ghayathi Formation aeolianites were largely sourced fi-om this sequence. Facies analysis of offshore core sediments indicates that sea level reached at least 15 m below present level during this period. Widespread evidence exists for a Holocene sea level higher than at present in the southern Arabian Gulf, indicating that it peaked at 1-2 m above present level, c. 5.5 ka bp. Pleistocene deposits preserved in the southern Arabian Gulf provide a record of changing palaeowinds and palaeoclimates. Currently, the region experiences a hyper-arid to arid climate, with facies patterns dominated by the northwesterly shamal wind. The Ghayathi Formation was originally deposited under an arid climatic regime, which allowed the sediments to remain unconsolidated. The dunefield was later remodelled under conditions of increasing wind speed, with a change in wind direction from NNW to WNW. These changes are thought to reflect the onset of glaciation. Palaeocurrent directions from the A1 Wusayl Member, combined with sedimentary evidence from the Futaisi and Dabb'iya Members, indicate that during the peak of the last interglacial the prevailing wind (the 'palaeo-shamal') blew from the NE. Compelling evidence for a pluvial episode during this period is provided by abundant and widespread dissolution (palaeokarstic) pits found in the top surface of the Futaisi Member, believed to represent the former positions of abundant trees or large plants.

The Arabian Gulf is the marine flooded portion of 90 m. The southern Gulf, from Saudi Arabia to a more extensive foreland basin that forms a the United Arab Emirates (UAE), is generally very northwestern extension of the Arabian Sea, to shallow, with a gentle northwards-sloping floor which it is connected via the Straits of Horrnuz. It (Fig. 1). This area is at present the site of is shallow, with a maximum depth of around extensive carbonate sedimentation, with the pre-

From: CLIFT, PD., KROON, D., GAEDICKE, C. & CRAIG, J. (eds) 2002. The Tectonic and Climatic Evolution of 3 7 1 the Arabian Sea Region. Geological Society, London, Special Publications, 195, 371-386. 0305-87 19/02/$15 .OO Q The Geological Society of London 2002.

at Dalhousie University on November 9, 2014http://sp.lyellcollection.org/Downloaded from


372 A . H . WILLIAMS & G . M. WALKDEN

vailing northwesterly 'shamal' wind acting as one of the major controls on facies patterns. The southern Arabian Gulf has been frequently cited in the literature as a modem example of a carbonate ramp (e.g. Tucker & Wright 1990), although the validity of this model has been questioned (Walkden & Williams 1998). During Pleistocene glacial maxima sea level fell by as much as 120 m below present level, leaving the entire Gulf subaerially exposed. Melting of polar ice masses was associated with repeated flooding of the basin, leading to the deposition of carbonate sediments. During lowstands aeolian sedimentation predominated, under the influence of a strengthened glacial Shamal. Most of the southern Arabian Gulf sea floor is underlain by Quaternary marine carbonates, and scattered outcrops may be found onshore. Quaternary aeolianite deposits are fairly widespread in Abu Dhabi.

As a result of the physical isolation imposed by the Musandam Peninsula, oceanic influences on the Arabian Gulf are minimal. However, the Gulf exerts a significant influence on the waters of the Arabian Sea, most markedly in the Gulf of Oman. Located in a hyper-arid to semi-arid climate, and with only one permanent inflowing river, the Arabian Gulf suffers a net loss of water through evaporation. This increases the salinity of the water, which sinks and flows back into the Arabian Sea through the Straits of Hormuz. The

outflow increases during the winter, when Gulf waters are often colder than the neighbouring ocean. The bottom current has a high salinity, oxygen content and nutrient content, and its effects can be traced well beyond the continental shelf (Purser & Seibold 1973; Bower et aE. 2000). During glacial periods, when the Gulf is empty, this current, and its influence on the oceanography of the Arabian Sea, is turned off.

Despite the proximity to an active plate margin, we see no unequivocal evidence for late Pleisto- cene uplift of the coastline of the southern Arabian Gulf. Values of up to 190 cm ka-' were reported for Qatar and Saudi Arabia (Vita-Finzi 1978; Ridley & Seeley 1979), but were later disputed (McClure & Vita-Finzi 1982). We believe that these results can be explained by failure to account for a Holocene highstand of 1-2 m above present sea level. By the same token we propose that onshore outcrops of upper Pleistocene marine deposits are also mostly unaffected by tectonism. Study of offshore sediments shows that there is no need to invoke tectonic movements for much of the sea floor in the southern Gulf, such as Urnrn Shaif shoal (Williams 1999). However, active halotectonics means that some offshore shoals and islands do show signs of vertical movements.

This paper is based on work carried out in the UAE and Qatar. It gives an overview of the Quaternary deposits found in the region, and of

Fig. 1. Map showing the location of the southern Arabian Gulf, including bathymetry, localities mentioned in the text, political geography and position of major cities. Depths are in metres.



QUATERNARY HIGHSTAND DEPOSITS, ARABIAN GULF 373

the information they can provide on changing sea levels and climate. The coastline of the southern Arabian Gulf provides a far more reliable indicator of palaeo-sea levels than the coastlines of the Arabian Sea, which have been largely modified by neotectonics. In addition, the deposits provide evidence for palaeoclimatic changes that are likely to have affected the whole region. Our database includes coastal and inland outcrops, man-made exposures and continuously cored boreholes up to 60 m deep in the floor of the southern Arabian Gulf (Williams 1999).

Pleistocene deposits in the southern Arabian Gulf

Scattered outcrops of Pleistocene sediments are found onshore in the southern Arabian Gulf. They belong to three formations: the Ghayathi Forma- tion, the Aradah Formation and the Fuwayrit Formation. The stratigraphy of these deposits is shown in Fig. 2. Although outcrops are rare, they have a widespread distribution, and are found in Abu Dhabi, Qatar, Saudi Arabia, Bahrain and Kuwait. Both the Ghayathi Formation and the Fuwayrit Formation crop out in coastal regions, whereas the Aradah Formation is found inland.

Pleistocene deposits are much more widespread beneath the sea floor of the Arabian Gulf, but have received little study.

Ghayathi Formation

Hadley et al. (1998) proposed the name 'Ghayathi Formation' to cover 'remnant paleodune deposits composed of carbonate and siliciclastic material exposed slightly inland and along the Arabian Gulf coast of Abu Dhabi Emirate'. They are also found in Saudi Arabia. The Ghayathi Formation consists of mixed carbonate-clastic aeolianites, which occur up to 80 km inland in Abu Dhabi, where they are by far the most voluminous Pleistocene sediments (Glennie 1998; Hadley et al. 1998). Near the coast the aeolianites are composed almost entirely of marine-derived carbonate grains, whereas inland they contain up to 89% quartz. Deposits of the Ghayathi Formation are morphologically and chronostratigraphically complex, and have been discussed by Williams (1999) and Williams & Walkden (2001).

Aradah Formation

The Aradah Formation consists of continental sabkha-type deposits overlying the Ghayathi For-

S (inland) (coastal) N

IArada.h F r n l ~

! ! ! ! ! ! ! : : : ! ! i ! : i : : ! ! : ! ~ ~ ! i : : : : ! ! ~ ~ i i : : : : : ~ i 'Fuwayrifl::i

younger

i!!!iiiiiiiiii!!iiiiii!!iiiiiiii %11 i?;u; ;i iili! ii!i ~::::::iiiiii?iiii? iiii ~ ~ ~ o n t i i l ; i

~"-~"~-:~""" "" : : : : : : : :[Ghayathi F°rmali°n]! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! : l ===================== . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

older

Fig. 2. Schematic stratigraphy of the Pleistocene deposits in Abu Dhabi, showing (a) morphological relationship and (b) chronological relationship.



374 A. H. WILLIAMS & G . M. WALKDEN

mation in southern UAE (Hadley et al. 1998). These deposits were not observed in this study, and will not be discussed.

Fuwayrit Formation

The Fuwayrit Formation consists of shallow marine and marine-derived aeolian carbonates, preserved in coastal localities (Williams 1999; Williams & Walkden 2001). In Abu Dhabi they mostly overlie Ghayathi Formation aeolianites, and are often preserved as 'zeugen' (erosional remnants; Fig. 3). In Qatar they are found overlying Eocene sediments. Three members have been recognized within the Fuwayrit Formation, separated by subaerial exposure surfaces (Fig. 4). These are, from base upwards: the Futaisi Member and the Dabb'iya Member (shallow marine deposits of up to 6 m thickness in the UAE and Qatar); and the A1 Wusayl Member (marine-derived aeolianites of up to 20 m thickness found only in Qatar).

Offshore deposits

Study of sea-floor cores from six localities in the southern Arabian Gulf indicates that at least six

Quaternary sequences, consisting of marine carbonates and sabkha-derived evaporites, are present within the uppermost 50 m of sediment (Williams 1999). The uppermost Pleistocene sequence represents the offshore equivalent of the Fuwayrit Formation, and has been seen to a maximum thickness of 29 m. Otherwise, there are no correlative submarine deposits onshore, and the stratigraphy of these sequences has not been properly formalized. Only the two uppermost Pleistocene sequences (sequences 2 and 3 of Williams (1999)) will be considered here.

Deposition of the Pleistocene sediments

Ghayathi Formation

The Ghayathi Formation provides a good example of 'regressive aeolianites', deposited through reworking of coastal and shallow marine sediments during and following sea-level fall (Williams & Walkden 2001). The large-scale cross-bedding that is characteristic of these deposits extends below present-day sea level (Fig. 3).

Some of the best exposures of the Ghayathi Formation palaeodunes are where they have been dissected by sea andlor wind, leaving flat-topped

fit

Fig. 3. Zeugen exposure showing Ghayathi Formation aeolianites @re-isotope stage 5) capped by shelly beach facies of the Dabb'iya Member, Fuwayrit Formation (isotope substage 5e), Futaisi Island, Abu Dhabi.



Al Wusayl

Fuwayrit Formation

- - - - - Dabb'lya Member

Futa~s~ Mbr

Dammam Formation (Eocene)

QUATERNARY HIGHSTAND DEPOSITS, ARABIAN GULF

a) Qatar b) Abu Dhabi

10

5

Present sea level

-5

-1 0

Dabb'iya Member F u w a ~ i t

Formation Futaisi Mbr

Ghayathi Formation

- cross-bedd~ng reworked clasts

Pig. 4. Generalized stratigraphy of the Pleistocene deposits subdivisions of the Fuwayrit Fonnation.

and steep-sided zeugen (Fig. 3). Kirkharn (1998) noticed that many of these features are aligned as parallel discontinuous ridges about 150 m apart. Mapping revealed these to be up to 7 km long, and they are visible on satellite images. Compari- son of these ridges with features in the neighbouring siliciclastic sand sea (the Rub a1 Khali) led Kirkham (1998) to conclude that they represent ancient seif dunes. These have an orientation of c. 100" (ESE), parallel to the extensive siliciclastic seif dunes that currently dominate the desert of the eastern Emirates. This implies that they belong to the same dune system. The inland seifs are out of phase with the modem shamal winds, and are believed to be fairly inactive today (K. W. Glen- nie, pers. comm.).

Despite their overall geometry, the primary bedding within the Ghayathi Formation palaeodunes in Abu Dhabi is barchanoid, with dips predomi- nantly to the SE. It is therefore apparent that the original dune morphology was remodelled before lithification (Kirkham 1998). Transverse dunes such as barchans become unstable with increasing wind velocities, especially when accompanied by changes in wind direction. This often leads to the formation of longitudinal dunes, such as seifs

at outcrop in (a) Qatar and (b) Abu Dhabi, showing

(Glennie 1993). Sedimentological evidence from the Ghayathi Formation indicates that an originally barchan dunefield was reworked into seif dunes under conditions of increasing wind speed, accompanied by a change in direction from NNW to WNW. This is illustrated in Fig. 5.

Fuwayrit Fonnation: onshore

The lowermost sediments of the Fuwayrit Forma- tion found onshore belong to the Futaisi Member. These unconformably overlie the Ghayathi Forma- tion in Abu Dhabi and the Eocene Dammam Formation in Qatar (Fig. 4). Reworked blocks of aeolianite indicate that the Ghayathi Formation was indurated by the time it was transgressed. The Futaisi Member consists of littoral and shallow subtidal deposits, which are found to an elevation of 1.5 m above present sea level (Williams 1999). Mangrove root traces (Fig. 6) and abundant crab burrows are found within these deposits, and provide a good indicator of palaeo-sea level.

The Futaisi Member is overlain by the Dabb'iya Member. The contact between the two is com- monly indistinct, but there is abundant evidence in both Qatar and Abu Dhabi that the Futaisi Mem-



A. H. WILLIAMS & G. M. WALKDEN

Continental Marinederived highstand * zliK,ind

d) Last interglachl (substage Se, i25ka)

Fuwayrit Formation

,- Marinederived (carbonate) - Sequence boundary sediment

Fig. 5. Schematic illustration of the depositional model for the Ghayathi Formation in Abu Dhabi (from Williams & Walkden 2001).

ber underwent subaerial exposure and lithification before deposition of the Dabb'iya Member. This evidence includes a lag-covered, pot-holed contact, neptunean dykes, and reworked blocks of marine limestone found within the base of the Dabb'iya Member (Fig. 7 ) . The most striking exposure-related features are found in Abu Dhabi. These are abundant circular pits, up to I m in diameter, and up to 1 m deep (Fig. 8a). We believe that they formed through karstic dissolution, linked to the position of large plants. Similar pits

are created where trees grow in Pleistocene limestone in Florida, and the formation of 'solution pipes' in Bermuda has been attributed to the stemflow influence of trees (Herwitz 1993). Com- parable features are fairly common in cyclic successions in other parts of the geological record (e.g. Walkden & De Matos, 2000).

Where the Dabb'iya Member directly overlies the Ghayathi Formation, the contact is highly erosive, and overlain by a lag of aeolianite clasts. Channels with a vertical relief of up to 2 m have




Fig. 6. Mangrove traces, Futaisi Member, Fuwayrit Formation (isotope substage Se), Futaisi Island Abu Dhabi. Field of view along base of frame c. 2.5 m.

been eroded into the aeolianite, and angular blocks of up to 2 m across, interpreted as col- lapsed cliff scree, may be found.

The Dabb'iya Member reaches a maximum elevation of 6 m above present sea level, and consists of shallow subtidal and littoral deposits (Williams 1999). The deposits with the highest

elevation are cross-bedded carbonate sands, deposited in a foreshore setting. These provide a good indication of the highest sea level attained. In Abu Dhabi the Dabb'iya Member is dominated by skeletal grainstones, whereas in northern and eastern Qatar oolites predominate.

The lowermost Dabb'iya Member sediments preserved in Abu Dhabi consist of a rather perplexing facies, which is found infilling the pits within the Futaisi Member (Fig. 8a). It comprises a poorly sorted, structureless conglomerate of marine pebbles and bioclasts, floating in coarse bioclastic sand. The larger clasts are dominated by lithified burrow-eagments, with echinoids, bivalve and gastropod shells, corals and rhodoliths also abundant. Barnacles and serpulids encrust some clasts. Two interpretations for the origins of this conglomerate have been considered (Williams 1999): it represents either a high-energy transgressive lag, formed through a reworking of the nearshore sea bed, or possibly a tsunami deposit. Both hypotheses will be discussed more fully elsewhere.

The Al Wusayl Member is found only in Qatar. It consists of carbonate aeolianites composed mainly of ooids, conformably overlying the uppermost foreshore deposits of the Dabb'iya Member. In places a beach-backshore dune profile can be recognized. The dunes are preserved as coastal ridges up to 20 m high, and form the most voluminous Pleistocene deposits in Qatar. They provide a good example of 'highstand aeolianites' (Fig. 9), and seem to have become rapidly lithified shortly after deposition (Williams & Walkden 2001). The

Fig. 7. Block of reworked Futaisi Member at base of Dabb'iya Member (both isotope substage 5e), Dabb'iya peninsula, Abu Dhabi.



378 A. H. W I L L I A M S & G. M. WALKDEN

Fig. 8. (a) Circular pits in the top surface of the Futaisi Member, filled with conglomerate of the Dabb'iya Member (both isotope substage 5e), Futaisi Island, Abu Dhabi. ( b ) Date palm eroded from beach sands, Abu Dhabi beach. The bowl-shaped depression formed by the roots is of very similar dimensions to many of the pits shown in (a). Note hose for irrigation.

aeolianites of the A1 Wusayl Member have under- submarine carbonate sediments. On Umm Shaif gone extensive meteoric diagenesis, leading to the shoal oolitic and coral reef deposits predominate. creation of up to 35% secondary porosity. They have been found in core at a minimum depth

of 15 m below present sea level, and reach a

Correlation with offshore sediments maximum thickness of 29 m. A subaerial exposure surface has been recognized near the top of the

Offshore, the Fuwayrit Formation (sequence 2 of formation. Facies patterns indicate that this sur- Williams (1999)) consists of a diverse range of face does not correspond to the b o u n d q separat-



QUATERNARY HIGHSTAND DEPOSITS, ARABIAN GULF

A1 Wusayl Member (aeolian)

olian reworking of

Eocene Dammam Formation Futaisi Member (marine)

Key

Aeolian cross-lamination .- Planar cross-lamination

- Wave ripples u Burrowing \ Subaerial erosion surface

Fig. 9. Schematic model for the deposition of the Fuwayrit Formation in Qatar during the peak of the last interglacial (Sangamon; oxygen isotope substage 5e; from Williams & Walkden 2001).

ing the Futaisi and Dabb'iya Members, but occurred later. There is no evidence for subaerial exposure during the peak of the highstand.

Although there are no onshore submarine deposits equating to the sequences underlying the Fuwayrit Formation, the coastal aeolianites of the Ghayathi Formation are dominated by marine- derived sediments. These are likely to have been sourced from the equivalent of offshore sequence 3 of Williams (1999), consisting of submarine carbonates and sabkha evaporites. This sequence has been seen in core to a minimum depth of 27 m below present sea level, and to a maximum thickness of 9 m.

Age of Pleistocene deposits

Only four absolute ages have been reported from Pleistocene sediments in the southern Arabian Gulf (although more dates have been obtained from the unconsolidated clastic dune sands). These were all obtained by luminescence dating of Ghayathi Formation aeolianites. Two of these dates (45 and 130 ka) are from unspecified localities in the Emirates, reported by Glennie & Gokdag (1998). The others (64 and 99 ka) (Juyal et al. 1998) are from a site c. 25 km from the coast of western Abu Dhabi. Attempts to date deposits from nearby sites in Abu Dhabi (Hadley et al. 1998) have indicated that they were depos-

ited before 160 ka. The variability of these dates gives an indication of the chronostratigraphic complexity of the Ghayathi Formation.

Dating of sediments from the Fuwayrit Forma- tion is hampered by the extensive diagenetic alteration they have suffered. As a result of this, most of the original carbonate material has been leached, or has been contaminated by younger material. Luminescence dating may provide possi- bilities, but this is inhibited by the paucity of clastic material present. So far, little work has been published on luminescence dating of marine carbonates. Attempts to gain optically stimulated luminescence (OSL) ages from the Fuwayrit For- mation of Qatar, Abu Dhabi and offshore proved unsuccessfui (Williams 1999).

Five samples were chosen from the Dabb'iya Member for radiocarbon dating (SRR-6248, SRR- 6249, AA-30840, AA-30841, AA-30842). Sam- ples SRR-6248 and SRR-6249 were dated at the NERC Radiocarbon Laboratory in East Kilbride, Scotland. The other samples were prepared to graphite in East Kilbride and then analysed by 14C AMS at the University of Arizona NSF facility. All gave ages of between 29250 k 340 and 33 420 f 550 radiocarbon years (effectively years before 1950) (Table 1). We do not believe that these ages represent the true age of the samples, but are a result of diagenetic contamination and alteration. It has been shown that only a few per



380 A . H. WILLIAMS & G. M. WALKDEN

Table 1. Radiocarbon age dates of Arabian Gulfsamples

Sample Nature Locality Member Age (a bp)

SRR-6248 Coral Futaisi Island Dabb'iya 30 670 f 225 SRR-6249 Red algae Futaisi Island Dabb'iya 30 490 If 260 AA-30840 Barnacle Futaisi Island Dabb'iya 31570f 470 AA-30841 Barnacle Near Doha Dabb'iya 29 250 f 340 AA-30842 Barnacle Near Doha Dabb'iya 33 420 f 550

cent contamination by young material can give a Formation aeolianites were largely sourced from very old sample a radiocarbon age in the range of these sediments. In Bermuda and the Bahamas, 20-45 ka (Olsson 1968; Burr et al. 1992). The deposits from the late stage 7 interglacial are ages do, however, prove that the samples are not found up to 2.5 m above present sea level (Hearty of Holocene age. & Kindler 1995).

Correlation with other stable platform deposits

In the absence of direct age dates, it has been necessary to infer the ages of the Pleistocene sediments in the southern Arabian Gulf, by a comparison of the stratigraphy with deposits described from other parts of the world (Williams 1999).

The Fuwayrit Formation, stratigraphically the youngest pre-Holocene sediments preserved in the region, can be directly correlated with deposits that date from the last interglacial (oxygen isotope substage 5e). In Bermuda and the Bahamas, two sequences were deposited during this time, reaching respective elevations of 4 m and c. 6 m above present sea level (Hearty & Kindler 1995). In the Mediterranean, substage 5e deposits contain two sequences of between 5 and 7 m above present sea level (Hearty 1986; Kindler et al. 1997). Coral reefs dated to substage 5e are found at 2 m and 7.5 m above sea level in the Loyalty Islands (Marshall & Launay 1978). On Hawaii, substage 5e deposits have been correlated with shoreline notches cut in lithified dunes at 6.7 and 8.2 m elevation (Ku et al. 1974; Sherman et al. 1993). Substage 5e deposits from the eastern coast of the USA are found up to 14 m above present sea level, although these may have been uplifted somewhat (Hollin & Hearty 1990; Toscano & York 1992). In Australia, deposits dated to the last interglacial indicate that sea levels were in the range of 2-8 m above present level during substage 5e (Chappell 1987; Nott 1996).

If it is accepted that the Fuwayrit Formation was deposited during the last interglacial, then logic suggests that the underlying sequence (sequence 3 of Williams (1999)) was deposited during the penultimate interglacial (late oxygen isotope stage 7). It is very likely that the Ghayathi

Pleistocene sea levels

All of the above regions show evidence for maximum sea levels of between +2 and +14 m elevation during substage 5e, dating between 130 and 11 7 ka. With the exception of Australia, they all record unequivocal evidence for two highstands, separated by a regression, during which sea level fell below the present level. In all of the localities the second sea-level peak was higher than the first. If the Fuwayrit Formation is correlated with these deposits, sedimentary evidence shows that sea levels in the Arabian Gulf were higher than at present on at least two occasions during the last interglacial, peaking at c. 1.5 and 6 m above present sea level. This is consistent with other low-latitude sites around the world. Umm Shaif shoal remained submerged during this time, indicating that sea level remained higher than 24 m below the present level during exposure of the Futaisi Member.

A large body of evidence exists to suggest that two later highstands occurred in isotope stage 5, during substages 5c and 5a. Of these, the later (substage 5a) is believed to have had the higher sea level, with deposits from Bermuda and the Bahamas (Hearty & Kindler 1995) and Florida (Ludwig et al. 1996) indicating sea levels close to or higher than at present. There is no evidence for sea levels higher than at present during late stage 5 in the southern Arabian Gulf. However, at least one late highstand is recorded in core sediments from offshore Abu Dhabi. An exposure surface near the top of the Fuwayrit Formation indicates that sea level fell to lower than 23 m below the present level on Umm Shaif shoal, before rising to at least 17 m below the present level. Tentative interpretation of sedimentary facies indicates that sea levels may have reached 7 m below present level during this transgression (Williams 1999). It




is not known whether this highstand corresponds to substage 5c or 5a.

No evidence for stage 7 sea levels higher than at present has been seen in the Arabian Gulf. It is possible, however, that such deposits could have been removed by deflation or erosion. Facies analysis of offshore core sediments indicates that sea level reached at least 15 m below present level during the deposition of sequence 3, and may have been higher.

Evidence for a Holocene highstand in the southern Arabian Gulf

Evidence for Holocene sea levels higher than at present is widespread and compelling in the southern Arabian Gulf (Table 2).

In Qatar, Holocene highstand deposits are generally preserved as beachrock. Taylor & Illing (1969) described cemented beach sands from the northwestern coast, standing between 1.5 and 2.5 m above present sea level, which they dated to between 3930 f 130 and 4340 f 180 a bp. Vita- Finzi (1978) described raised beachrock deposits of up to 2 m elevation from several localities in Qatar. These date from between 4690 & 80 and 5830 f 70 a bp.

In the Abu Dhabi sabkha, beach ridges are found 3-4 km inland from the intertidal algal flats. They stand c. 1-2 m above the adjacent sabkha, and are easily picked out on satellite photographs. These features have been described by several workers (e.g. Evans et al. 1969, 1973; Kendall & Skipwith 1969; Kirkham 1997), and have been dated to between 3465 f 173 and 4191 f 193 a bp (Evans et al. 1973). Kenig

(1991) studied a canal cutting in the sabkha SW of Abu Dhabi island, and was able to map the sediments deposited at the end of the post-glacial transgression. He recorded a transgressive and a regressive phase, bracketed by intertidal algal sediments. At the peak of the transgression 'wash- over fans' of coarse shelly sand were deposited 2 m above present sea level. These were dated at 5400 f 126 and 5 1 10 f 167 a bp. A shell sample from lagoonal sediments gave a date of 4280 f 186 a bp, whereas another shell sample col- lected from the regressive algal layer, at approxi- mately present sea level, was dated at 1580 f 186 a bp. On Zabbut Island, in the west of Abu Dhabi, Williams (1999) observed a stranded mus- sel bed at 1 m above high-tide level. Two in situ specimens of the lower intertidal bivalve Barbatia were dated at 2485 + 50 and 23 15 & 55 a bp.

The above evidence indicates that following the post-glacial (Flandrian) transgression, Holocene sea level in the southern Arabian Gulf peaked at 1-2 m above present level at c. 5500 a bp. It did not return to its present position until after 2300 a bp. As previously argued with regard to the late Pleistocene (oxygen isotope substage 5e) highstand, we see no reason to attribute this Holocene highstand record to local neotectonic change. Amongst a range of variable data, Pirazzoli (1 99 1, 1996) recorded numerous sites in the Arabian Sea, Indian Ocean and NW Pacific where a sea-level high of similar age, magnitude and pattern has been recorded. Elsewhere, well-documented comparable case studies in relatively stable areas include those by Angulo et al. (1999) for southern Brazil (a high of up to 2.1 m c. 5410 a bp) and Beaman et al. (1994) for NE Australia (a high of up to 1.7 m c. 5660 a bp). These data support a

Table 2. Reported ages and elevations of Holocene sediments found above present sea level in the southern Arabian Gulf

Locality Elevation (m) Age (a bp) Reference

NW Qatar NW Qatar NW Qatar Western Qatar NE Qatar Eastern Qatar Abu Dhabi sabkha Abu Dhabi sabkha Abu Dhabi sabkha Mussafah Mussafah Mussafah Mussafah Zabbut Island Zabbut Island

Taylor & Illing (1969) Taylor & Illing (1969) Taylor & Illing (1969) Vita-Finzi (1978) Vita-Finzi (1 978) Vita-Finzi (1978) Evans et al. (1973) Evans et al. (1973) Evans et al. (1973) Kenig (1991) Kenig (1991) Kenig (1991) Kenig (1991) Williams (1 999) Williams (1999)



3 82 A. H. WILLIAMS & G. M. WALKDEN

case that the mid-Holocene highstand in the south- wind direction and the amount of precipitation em Arabian Gulf represents a widespread eustatic may have varied. Both of these are well illustrated effect and that the area is tectonically stable. by the deposits found in Qatar.

One of the most notable differences between deposits of the Fuwayrit Formation and present-

Palaeowinds and palaeoclimates in the day sediments on the coast of Qatar is that the

southern Arabian Gulf Fuwayrit Formation consists largely of ooids. On the present-day Arabian Gulf coastline ooid build-

The Arabian Gulf region is located within the sub- tropical trade wind belt, and is at present dominated by the northwesterly 'shamal' wind, stron- gest during the winter months (Fig. I). This traces a clockwise path across Arabia, a pattern that is reflected in the dune morphologies of the peninsula's desert regions (Glennie 1998). During glacial periods, an increase in the size of the polar climate cells results in an intensification in the strength and duration of the trade winds. This also has the effect of squeezing the lower-latitude climate cells towards the equator, leading to a shift in the direction of the shamal wind. Most of the southern Gulf currently falls within the hyper- arid zone in the UNESCO (1979) classification scheme of the arid lands of the world. This has not always been the case, however. Evidence of pluvial episodes has been reported from the Arabian desert (McClure 1976; Wood & Imes 1994) and from the northern Gulf (Uchupi et al. 1999). It has been hypothesized that increased influence of the SW monsoon during warm interglacial~ may have led to wetter climates in the Arabian Gulf region (Glennie 1998). These palaeoclimatic changes are reflected in the Pleisto- cene deposits preserved in the southern Arabian Gulf.

The Ghayathi Formation was deposited during falling sea level, following the (?)penultimate interglacial (late oxygen isotope stage 7). As the sea floor was exposed, carbonate sediments were reworked into a barchan dunefield by winds from the NNW (Fig. 5). An arid climatic regime allowed these dunes to remain unconsolidated, and they were later remodelled into seif dunes. The changing dune morphologies indicate a change in direction of the prevailing wind, from NNW to WNW, accompanied by an increase in maximum wind speeds. These changes are thought to reflect the onset of glaciation (Glennie 1998). The Ghayathi Formation was later lithified before transgression.

The Fuwayrit Formation, exposed on the present-day coastline, consists of coastal deposits from the last interglacial. Thus a useful comparison can be made between the sedimentary environments that existed during the last interglacial and those that are currently found in the region. In general, these are found to be very similar; however, a few notable features indicate that both the

ups are primarily located in tidally dominated settings on windward shorelines. The most striking examples of these are the eastern Abu Dhabi tidal deltas, described by Loreau & Purser (1973). This is in marked contrast to the eastern coast of Qatar today, where the dominant current regime is one of longshore drift (Shinn 1973; Fig. lo), and ooids are not actively forming. Indeed, in many areas sediment production is so low that extensive hard- grounds have formed on the sea floor (Shinn 1969; Williams 1999). During the peak of the last interglacial, ooids were so abundant in eastern Qatar that aeolian dune ridges of 20 m height built up along the coast (the A1 Wusayl Member) (Fig. 9), comparable with those found today on barrier islands adjacent to the Abu Dhabi tidal deltas (Evans et al. 1973).

These dune ridges provide another contrast with the present-day sedimentary regime. The only significant present-day aeolian accumulations found in Qatar are siliciclastic dunes located in the SE of the peninsula (Fig. 10). These are a remnant of the sand sea that covered southern Qatar during the last glacial, and they are at present being deflated into the sea by the northwesterly shamal (Shinn 1973). No significant aeolianites are accumulating today in Qatar. As well as being almost exclusively located on the eastern coast of Qatar (although Vita-Finzi (1978) reported deposits from the eastern side of Ras Abaruk peninsula in the west; Fig. lo), palaeocurrent directions from the A1 Wusayl Member indicate that they were deposited under the influence of northeasterly winds (Fig. 11). The pre- valence of winds from the NE would also account for the abundant ooid production, at present associated with tidal, windward coastlines. Further evidence for a tidally dominated coastline is found near Doha (Fig. I), where coarse tidal bars are found within the Fuwayrit Formation (Williams 1999).

Another interesting feature of the Fuwayrit Formation in Qatar is the in situ preservation of the A1 Wusayl Member palaeodunes. In contrast to the aeolianites of the Ghayathi Formation, deposits of the A1 Wusay-l Member were suffi- ciently well lithified to avoid remobilization and deflation when sea level dropped. This may indicate that they were deposited under more humid conditions than the Ghayathi Formation.



QUATERNARY HIGHSTAN D DEPOSITS, ARABIAN GULF 383

a) Last interglacial (substage 5e. 125 ka)

.:h Ooid ,- . . .? ..... : : : ,&>'yuction ,:Aeolianrte: ::

;\,. .... Arabian

Gulf

.................. .........

.......... -,.. \.-, t i:::::::::.:..::. , .............. . . - ~

i ........... ,* N i:::::::::::. ............. ........... .? ................ > , 5okm , L!_ltLLL:_.-L!.L.L::i

b) Present-day

...............

Ooid production

N where currents : converge

4 4---

Fig. 10. Schematic illustration of the principal differences in the sedimentary environments of eastern Qatar during the last interglacial (substage 5e) compared with those of the present day.

Fig. 11. Foreset measurements from the aeolian A1 Wusayl Member, Fuwayrit Formation, eastern Qatar. Only surfaces of >20° dip have been plotted.

However, the paucity of rootlets within sediments of the A1 Wusayl Member indicates that the climate was semi-arid at best (Williams & Walk- den 2001).

Compelling evidence for a more pluvial episode during the last interglacial is provided by the pits found in the top surface of the Futaisi Member in Abu Dhabi (Fig. 8a). If these pits were formed through the influence of plant stems, they indicate a far higher density of plant life than is suppor- table under the current climate. The regular spacing of the pits is reminiscent of a plantation or mature forest. Similar stands of date palms are found at present in the region (Fig. 8b), but these are artificially irrigated. Such a density of plants would indicate a significantly more humid climate than is found today in the southern Arabian Gulf.

Conclusions

(1) The southern Arabian Gulf is at present the site of extensive carbonate sedimentation, as was the case during Pleistocene interglacial highstands. During glacial lowstands the basin was subaerially exposed, and aeolian sedimentation predominated. Most of the southern Arabian Gulf floor is underlain by Quaternary marine carbonates, and scattered outcrops may be found onshore. Quaternary aeolianite deposits are fairly widespread in Abu Dhabi.

(2) Pleistocene sediments found onshore in the southern Arabian Gulf belong to three formations: the Ghayathi Formation, the Aradah Formation, and the Fuwayrit Formation. Offshore, at least six Quaternary sequences are present within the uppermost 50m of sediment. The uppermost Pleistocene sequence represents the offshore equivalent of the Fuwayrit Formation. Otherwise, there are no correlative submarine deposits preserved onshore.

(3) The Ghayathi Formation consists of mixed carbonate-clastic aeolianites, found in Abu Dhabi and Saudi Arabia. They occur up to 80 km inland in Abu Dhabi, where they are by far the most voluminous Pleistocene sediments. The Ghayathi Formation provides a good example of 'regressive aeolianites', deposited through reworking of coastal and shallow marine sediments during and after sea-level fall. They were originally deposited as barchan dunes, but were later reworked into seif dunes under conditions of increasing wind speed, accompanied by a change in wind direction from NNW to W1VW. The Aradah Formation consists of continental sabkha-type deposits found overlying the Ghayathi Formation in southern Abu Dhabi.

(4) The Fuwayrit Formation consists of shallow marine and marine-derived aeolian carbonates, preserved in coastal localities in Abu Dhabi,



3 84 A. H. WILLIAMS & G. M. WALKDEN

Qatar, Bahrain, Saudi Arabian and Kuwait. On- Holocene sea level higher than at present in the shore, it is composed of three members, separated southern Arabian ~ u l f . Elevated Holocene sedi- by subaerial exposure surfaces. These are, from base upwards: the Futaisi Member, shallow marine deposits preserved to a maximum elevation of 1.5 m above present sea level; the Dabb'iya Member, shallow marine deposits preserved to 6 m above present sea level; and the A1 Wusayl Member, oolitic aeolianites found only in Qatar, to a maximum elevation of 20 m above present sea level. Offshore, on Umm Shaif shoal, the Fuwayrit Formation has a thickness of 29 m. Here it is dominated by oolitic and coral reef sedi-

ments indicate that sea level peaked at 1-2 m above present level at c. 5500 a bp. It did not return to its current position until after 2300 a bp.

(9) Pleistocene deposits preserved in the southern Arabian Gulf provide a record of changing palaeowinds and palaeoclimates. Currently the region experiences a hyper-arid to arid climate, with facies patterns dominated by the northwesterly shamal wind. The Ghayathi Formation was originally deposited under an arid climatic regime, which allowed the sediments to remain unconsoli-

ments. dated. The dunefield was later remodelled under (5) No direct age dates have been acquired from

Pleistocene marine or coastal deposits in the southern Arabian Gulf. It has therefore been necessary to infer the ages of these sediments by a comparison of their elevation and stratigraphy with deposits described from other parts of the world. This approach is valid in the absence of unequivocal or independent evidence for significant late Quaternary neotectonic change along the coastline, with the exception of local halotectonics. We propose that late Quaternary onshore deposits, and much of the offshore deposits, are in place and largely unaffected by tectonism.

(6) Comparison with deposits from other appar- ently stable platforms indicates that the Fuwayrit Formation was deposited during the last interglacial (oxygen isotope substage 5e). We believe that this is a valid assumption as (a) these represent the youngest pre-Holocene marine deposits, and (b) they are found at an elevation correlative with substage 5e deposits from other parts of the globe. Sedimentary evidence reveals that there were two highstands in the southern Gulf during this period. The first peaked at around 1.5 m above present sea level, with the second reaching 6 m. Offshore sediments indicate that sea level did not fall as far as 24 m below present level in the intervening regression. Following the second highstand, sea level fell to below 23 m below present level, before briefly rising again (late oxygen isotope stage 5). This later highstand probably peaked between 17 and 7 m below present level.

(7) The sequence underlying the Fuwayrit For- mation was probably deposited during the penultimate interglacial (late oxygen isotope stage 7). It is also likely that the Ghayathi Formation aeolianites were largely sourced from this sequence. No evidence for stage 7 sea levels higher than present level has been seen in the Arabian Gulf. Facies analysis of offshore core sediments indicates that sea level reached at least 15 m below present level during this period.

(8) Given our case for minimal neotectonic change after c. 120 ka, we further invoke a mid-

conditions of increasing wind speed, with a change in wind direction from NNW to WNW. These changes are thought to reflect the onset of glaciation.

(10) Palaeocurrent directions from the A1 Wu- say1 Member, combined with sedimentary evidence from the Futaisi and Dabb'iya Members, indicate that during the peak of the last interglacial the prevailing wind (the 'palaeo-shamal') blew from the NE. Compelling evidence for a pluvial episode during this period is provided by palaeokarstic pits found in the top surface of the Futaisi Member. These pits probably correspond to the former positions of trees or other large plants.

The authors would like to thank K. Glennie, A. Kirkham and M. Simmons for guidance in the field and stimulat- ing discussion. A. Williams was in receipt of grant GT4/ 95/2/E from the Natural Environment Research Council. G. Walkden thanks the NERC Radiocarbon Laboratory under allocation 71 9.1297 for undertaking the dating of samples. We also thank ADMA for the generous avail- ability of Pleistocene cores and data.

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Late Quaternary highstand deposits of the southern Arabian Gulf: a record of sea-level and climate change