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Giant burrows in the Quaternary Limestones of Futaysi Island and Al Dabb'iya, Abu Dhabi Emirate
Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 324–331
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Giant burrows in the Quaternary Limestones of Futaysi Island and Al Dabb'iya, Abu Dhabi Emirate Anthony Kirkham a,⁎, Graham Evans b,c a b c
5 Greys Hollow, Rickling Green, Saffron Walden, Essex, UK CB11 3YB, UK Cranford, La Route de la Haule, St. Brelades, Jersey JE3 8BA, UK National Oceanographic Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK
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Article history: Received 6 October 2006 Accepted 24 January 2008 Keywords: Abu Dhabi Pleistocene Carbonate sediments Giant burrows
a b s t r a c t Pleistocene carbonates along the northern coasts of Futaysi Island and Al Dabb'iya tombolo, Abu Dhabi, U.A. E., contain unusual calcirudite-filled structures which are bowl- and funnel-shaped in vertical section; roughly circular in plan view; up to 1 m in diameter and ∼ 1 m deep. The calcirudite infill is dominated by clasts of reworked lithified smaller burrows and what appear to be fragments of beach rock or hardground. These structures were previously interpreted as marking the sites of former trees with holes, left behind by their root balls, infilled by a storm deposit and were considered to indicate an emergent event marking the boundary between two Pleistocene sedimentary sequences represented by the Futaisi (Futaysi) and Dabb’iya Members of the Fuwayrit Formation. However, they are here re-interpreted as giant burrows formed by some, as yet, unknown marine animal probably foraging for food or using them as dwelling sites which contained storm deposits emplaced in shallow water. Thin crusts on top of some giant burrow infills possibly suggest emergence and this would require a repositioning of the sequence boundary between the Futaysi and Dabb’iya Members. The burrows are believed to have been formed at approximately 75–125 Ka. © 2008 Elsevier B.V. All rights reserved.
1. Introduction This paper discusses the origin of some unusual bowl-shaped and funnel-shaped sedimentary/biogenic structures in the Pleistocene (Quaternary) deposits of Futaysi Island and the Al Dabb'iya tombolo which form part of the barrier complex of Abu Dhabi, U.A.E. (Fig. 1). The Pleistocene deposits of the area were described by Kirkham (1998) and Williams and Walkden (2001, 2002). The latter authors proposed a new stratigraphic nomenclature for the Quaternary of the southern Arabian Gulf. Following and extending the work of Hadley et al. (1998), they assigned a Pleistocene calcarenitic aeolianite (miliolite) to the Ghayathi Formation and the overlying marine carbonates exposed in onshore Abu Dhabi to the Fuwayrit Formation. They sub-divided the latter into the Futaisi (Futaysi) and Dabb’iya Members, which they considered represented two sedimentary sequences. The structures in question often appear as isolated bowl- or funnel-shaped columns of calcirudite after removal of the surrounding sediment by erosion and occur within the top of the Futaysi M. (Fig. 2). They are circular in plan view; up to 1 m in diameter in their upper parts; and up to 1 m deep. They have been observed only in the Quaternary sediments on seaward facing promontories at two localities west of Abu Dhabi Island: Futaysi Island and Al Dabb'iya
⁎ Corresponding author. E-mail address: [email protected] (A. Kirkham). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.01.037
tombolo (Fig. 1). On Futaysi Island they occur in the hundreds with sub-regular spacings of 1 to 2 m. They were interpreted by Williams and Walkden (2002) as ‘pits’ and used as evidence of exposure indicating the presence of a sequence boundary between the Futaysi and Dabb'iya Members. 2. Futaysi Island The rock substrate comprises Pleistocene aeolianite beneath the intertidal zone around the northern area of Futaysi Island and the adjacent land area used as a golf course (Fig. 3). The top of the aeolianite is undulating and in places still presents the palaeotopography of the original dune field except where it has been planated by various episodes of Quaternary erosion. This dune field was originally dominated by barchan dunes but later remodelled surficially into seif dunes (Kirkham, 1998). Morphological remnants of the seif dunes are also still evident at other locations such as Al Dabb'iya, Al Aryam, other barrier islands, and on the mainland. At the extreme northern point of the island, the aeolianites form a zeuge (or qassar; Fig. 4), similar to those described by Kirkham (1998). The cross-bedded aeolianite, which constitutes most of the zeuge, is capped by coarse, gastropod-rich, cross-bedded Pleistocene carbonates resembling many of the storm beach deposits along the present Abu Dhabi coastline. Surrounding the zeuge at lower palaeotopographical levels, the cap rock is replaced by beds of less shelly, finer grained calcarenites and calcirudites (probably foreshore and
A. Kirkham, G. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 324–331
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Fig. 1. General location map. Barrier island complex of central Abu Dhabi.
intertidal deposits) which lack cross-bedding. The palaeo-seifs are frequently fringed by cobbles and boulders of aeolianite often exceeding half a metre across and represent scree deposits developed at the bases of palaeo-cliffs (Fig. 5). These deposits were reworked and buried by marine calcarenites during the later Pleistocene marine transgression as it progressively eroded the partly lithified aeolianites. In places, the aeolianite was also bored by Lithophaga sp. Slightly further away from the palaeo-seifs, the basal, transgressive Pleistocene marine sediments overlying the aeolianite comprise about a metre of poorly stratified but highly bioturbated calcarenites containing a range of relatively small crustacean burrows including Thalassinoides and Ophiomorpha. Other burrows have vertical entry lumens, about 3 cm in diameter and lead down into bulbous basal chambers sometimes exceeding 15 cm in diameter and invariably infilled with coarse, vuggy carbonate detritus (Fig. 6). They are possible crab burrows and the vugs are due to leaching of former aragonitic bioclasts. The various burrows often extend into the underlying aeolianite which must have been only slightly indurated at the time of burrowing. These burrowed sediments are similar to submarine sediments in close proximity to correlatable foreshore deposits that accumulated above the palaeo-seif dunes of Al Dabb'iya (Kirkham, 1998). On parts of the old golf course, deflation has removed the upper layers of the marine calcarenite to expose where the ‘pits’ occur, or in
Fig. 2. Low, galleried, beach-cliffs approximately half a metre high with calcirudite columns separated by small caves created by erosion of less resistant, intervening sediment. The beach in front of the low cliff is strewn with clasts eroded from the calcirudite. Futaysi Island.
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Fig. 5. Cobbles and boulders of aeolianite representing a palaeo-scree deposit overlain by marine fossiliferous calcirudites which include clasts of reworked lithified burrows. Futaysi Island.
Fig. 3. Futaysi Island. The rectangle at its northern extremity marks the area where the ‘pits’ occur.
coarse, mouldic calcarenite reminiscent of the abovementioned lithified bulbous crab burrows. Many clasts are encrusted by barnacles and serpulids. Articulated and disarticulated bivalve shells, commonly of Spondylus (often retaining their original pink colouration), together with unfragmented ‘sand dollar’ echinoids, sponges and rhodoliths, are liberally distributed throughout the calcirudite. These clasts and shells have a coarse calcarenitic matrix.
places it has removed completely the marine capping to expose the aeolianite over wide areas (Fig. 7). The ‘pits’ are infilled by a discontinuous calcirudite bed (Figs. 8, 9). The calcirudite includes clasts which vary in their composition but are dominated by fragmented, lithified, small calcarenitic burrow-fills — often with indentations representing relicts of the original burrow lumens. They sometimes resemble rhizoliths but such an origin is discarded because, though they are reworked, they lack signs of regular branching, orientation or petrographic characteristics typical of root systems (Klappa, 1980). Other clasts are nodules comprising
Fig. 6. A bulbous-based burrow with open vertical entry shaft. Futaysi Island.
Fig. 4. A zeuge of Quaternary sediments comprising calcarenitic aeolianite of the Ghayathi Fm. capped by cross-stratified, shelly marine calcarenites of the Dabb'iya M., Fuwayrit Fm.. The Futaysi (Futaisi) M. was never deposited on such relatively high palaeo-topography. The figure is standing on deflated aeolianite. The dark low area behind him is a Holocene wave-cut platform displaying mangrove rhizoliths. Northern extremity of Futaysi Island.
Fig. 7. Cross-bedded Quaternary aeolianite exposed by deflation. Old golf course, Futaysi Island.
A. Kirkham, G. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 324–331
Fig. 8. Bedded calcirudite dominated by clasts of reworked lithified burrows. Futaysi Island.
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Fig. 11. Upstanding erosional remnants of calcirudite-filled ‘pits’ forming column-like features surrounded by a Holocene wave-cut platform with mangrove rhizoliths. Futaysi Island. (Hammer for scale).
bowl- or funnel-shaped calcirudite ‘pit’ fillings are more resistant to erosion than the intervening host sediment. They also sometimes remain as upstanding columns on the beach or wave-cut platform in front of the retreating cliffs to give a ‘stepping stone’ effect when the surrounding sediment has been eroded (Figs. 11, 12). Where the discontinuous calcirudite layer is absent, the tops of many calciruditefilled ‘pits’ are capped by localized, well cemented layers that are restricted to the area of their respective underlying burrows, and sag or tilt slightly due to differential compaction, (Fig. 13). The calcirudite layer which fills the ‘pits’ is, in turn, overlain by about half a metre of shelly Pleistocene calcarenite of which the upper 15 cm forms a continuous but irregular layer of coalesced nodules (a hardground) which seem to have nucleated around small burrows. This unit is part of the Dabb'iya Member of Williams and Walkden (2002) (Fig. 14). Fig. 9. Close-up of numerous clasts of lithified burrow debris comprising the ‘pit’ fills. Futaysi Island.
In vertical section, the ‘pits’ extend downwards below the base of the marine sediments into the underlying aeolianite. They are circular in plan view (Fig. 10) when exposed on possible ‘Stokes surfaces’ that represent deflation surfaces produced at or near a palaeo-water table. They have never been seen to overlap and, where there has been erosion by wave action, often form low galleried cliffs because the
Fig. 10. Numerous filled ‘pits’ exposed on a deflated rocky surface of marine carbonate. Some give the impression of being arranged in rows. Old golf course, Futaysi Island.
Fig. 12. Erosional remnants of calcirudite-filled ‘pits’ forming ‘stepping stones’ on a beach. Futaysi Island.
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Fig. 13. A calcirudite-filled ‘pit’ capped by a tilted calcarenitic crust. The coarse clasts comprise debris of reworked, cemented crab(?) burrows, some of which were encrusted by barnacles before entombment within the ‘pits’. Futaysi Island.
On the old golf course, finer marine calcarenites overlying the aeolianites often show surficial polygonal cracking that is reminiscent of the polygonally cracked microbial mats of modern intertidal settings (Evans and Kirkham, 2005), although they may be merely cracked caliche crusts. The geological setting of the Futaysi M. ‘pits’ is illustrated in Fig. 14. 3. Al Dabb'iya Ra's al-Khaf is the northernmost of two peninsulas of similar trend and dimensions at the extreme northwest corner of Al Dabb'iya Peninsula (Fig. 15). Both are dominated by remnants of Pleistocene palaeo-seif dune aeolianites capped by Pleistocene marine calcarenites and today form zeugen (Kirkham, 1998). As on Futaysi Island, ‘pits’ infilled with calcirudite occur within the marine calcarenites and often extend down into the top of the aeolianite at the northern tip of the southernmost peninsula (Figs. 16, 17). They have similar dimensions and spacings to those on Futaysi Island but are less abundant and occur in a smaller area. The calcirudite clasts of the infills less obviously originated as lithified burrow fragments. Some of them may represent eroded clasts of
Fig. 15. Location of the ‘pits’ at the northern tip of the circled peninsula. Al Dabb’iya.
beach rock or submarine hardgrounds. Discontinuous lines of subvertically orientated (edgewise) calcirudite fragments tend to define the circumferences of these ‘pits’ in plan view. Large disarticulated spondylid bivalves, again often retaining their pink colouration, also commonly contribute to the burrow-fills. Due to erosion of previously overlying sediments, the calcirudite infills cannot be seen to pass upwards into a more extensive calcirudite as on Futaysi Island, although a calcirudite layer occurs at a slightly higher stratigraphic position in a nearby zeuge. 4. Discussion 4.1. Dating the ‘pits’ Prominent wave-cut platforms were cut in the Pleistocene marine deposits and the aeolianites at both Futaysi and Al Dabb’iya and around the peripheries of other coastal zeugen in the area during the Flandrian transgression (Evans and Kirkham, 2005). At Al Dabb’iya, one such platform exposed the 'pits'. Most of these platforms also
Fig. 14. Sketch showing the relationships of the various sedimentary units and their sedimentary characteristics, Futaysi Island. The zeuge represents a relict of a Pleistocene seif dune.
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Fig. 16. Exhumed ‘pits’ infilled with calcirudite exposed on a Flandrian wave-cut platform. Al Dabb’iya.
expose mangrove rhizoliths with remnant pneumatophores and occasional bases of mangrove trunks. These are difficult to date but, at Ra's Al-Khaf, an analogous Holocene wave-cut platform was dated by the authors as older than 2.02 ± 40 Ka, using carbon-dating obtained from accelerated mass spectrometry of shell material from calcarenites deposited directly on the platform. This is, therefore, a minimum age for the ‘pits’. Also, Williams and Walkden (2002) carbon-dated some bioclastic material from the Dabb'iya M. although it is not known if their samples came from the calcirudite fill of the ‘pits’. They obtained ages of ca. 30 Ka although they believed that their samples were diagenetically contaminated and are probably older (Williams and Walkden, 2002). They concluded that they are certainly Pleistocene and, by comparison with sequences elsewhere in the Gulf, were probably deposited during the penultimate interglacial although no definite dates have been obtained as yet. Following attempts at unravelling the Quaternary sequence stratigraphy of the Emirates by Evans et al. (2002) and by Evans and Kirkham (2005), the present authors concur with Williams and Walkden (2002) and assume these Pleistocene marine strata were deposited between 75–125 Ka. 4.2. Origins of the ‘pits’ The following origins have been seriously considered for these large, apparently calcirudite-filled structures: turtle nesting sites as described by Hellyer and Aspinall (2005); karstic solution pits (lapies) representing sites of former tree locations (Williams and Walkden (2002) following descriptions elsewhere by Esteban and Klappa (1983) and Herwitz, 1993); and human excavation for tree plantations
Fig. 17. Marine calcarenite (M) containing three calcirudite-filled (arrowed) ‘pits’. The top of the calcirudite in the ‘pit’ on the right is obscured by surrounding calcarenite. The marine strata overlies aeolianite (A). Al Dabb’iya.
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or for burial sites or fireplaces and hearths which are abundant in the region (King, 1998). Other alternatives are that they may be feeding pits excavated by marine mammals (eg. cetaceans, sea otters), stingrays and other flatfish feeding by either fin-flapping or waterjetting (Gregory et al., 1979; Cadee, 2001; Martinell et al., 2001); or they are shrimp excavations (eg Callianasa-type; Tedesco and Wanless, 1991); or they could have been produced by explosive gas venting (Uchupi et al., 1995). Discussions were held with a number of ichnologists, marine biologists from the Natural History Museum, London, and Middle East specialist archaeologists about these various possible origins. The crude alignment (Fig. 10) of some of the ‘pits’ suggested initially that they were perhaps remnants of tree plantations but the oldest known evidence of human activity in the Emirates was of late Stone Age and dated as ca. 7.5 Ka (Kiesewetter, 2003). Williams and Walkden (2002) conjectured that the root systems of large plants (eg. date palms with associated karstic dissolution) may have occupied these ‘pits’ although no direct evidence of former trees was presented. They further suggested that a tsunami may have been responsible for depositing the calcirudite within the ‘pits’ after it had uprooted the trees. However, mature palm trees may have root-balls exceeding well over a metre across, as illustrated by Williams and Walkden (2002), and some date palms may have tap roots which penetrate downwards tens of metres in search of water in arid environments. Neither of such root systems fit the sizes and shapes of the features described above. They are so numerous and closely spaced; of similar dimensions; and smaller (=/b1 m across and =/b1 m deep) than would be expected from former palm trees. Although the present authors cannot completely refute this hypothesis, the shapes of the ‘pits’ and the lack of any remnant lithified tree stumps or rhizoliths tends to make this explanation unlikely and so suggests the need for an alternative origin. The present authors favour the idea that the ‘pits’ are giant burrows, although admittedly no remains of the burrowing animals have been discovered within them. One such possibility is that they were excavated by giant crabs such as the present day, intertidal ‘mangrove crab’, Scylla serrata. This is the largest of the modern swimming crabs. It may grow carapaces as large as 22–23 cm across and their overall width with their normal limb posture makes them considerably bigger. Indeed, S. serrata itself was previously regarded by the present authors as the potential burrower as it is known to create open burrows with comparable surface apertures and spatial distributions to the giant burrows (Fig. 18; Hogarth and Beech, 2001; Beech and Hogarth, 2002). However, this species was eliminated
Fig. 18. Vacant Scylla serrata burrows on the bank of a tidal channel occupied by mangroves. The surface apertures of these burrows reach up to 60 cm in diameter, but extend for up to 1 m sub-horizontally in the sub-surface. U.A.E. Photograph courtesy of Gary Feulner.
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because on further examination it was realized that its burrows extend for up to a metre in length at a low angle from the horizontal in the sub-surface rather that being entirely vertical as in the case of the fossil giant burrows in question. The actual burrowing organisms must therefore remain a mystery for the time being. 4.3. Origins of the calcirudite burrow-fills As previously discussed, most of the calcirudite clasts in the giant burrows are undoubtedly fragments of lithified smaller burrows (at least at Futaysi Island). Their lithification may have been similar to that of submarine hardground cementation which is so common in the lagoons and shallow shelf areas of the Arabian Gulf. Such lithification is typical today of burrow-fill sediment in the near surface layers of the sea bed. Lithification could also have occurred if the burrowed sediment was exposed at the surface in an intertidal zone. Cementation of burrows is common on the modern intertidal flats of Abu Dhabi, as well as in the shallow sub-tidal areas. The locations of the giant burrows in close proximity to remnant, contemporaneous Pleistocene seif dunes and to the possible polygonally cracked Quaternary microbial mats at least suggests that the burrowers inhabited relatively shallow water environments. Williams and Walkden (2002) regarded the calcirudite as a transgressive lag at the base of the Dabb'iya sequence but it could just as easily have been the result of a storm reworking the sediments and concentrating the coarse lithified burrow clasts as a tempestite layer which filled the burrows in somewhat similar fashion to that which Wanless et al. (1986) described from Caicos Islands. Any preexisting sea bed relief such as bioturbation spoil heaps or burrow depressions could have been obliterated by such storm activity. The sediment surface of shallow water environments following the storm would have been strewn with clasts of lithified burrow fragments just as the modern intertidal zones in proximity to the Futaysi Island are strewn with the same recycled fragments today (Fig. 2). They also litter parts of the shallow lagoon terraces and the surfaces of the tidal deltas (G. Evans, pers. obs.). That some clasts of these Pleistocene calcirudites had accumulated in the littoral zone is evident from the abundant barnacle encrustations of individual clasts entombed within the giant burrow-fills. The authors agree with Williams and Walkden (2002) that the calcirudite could have been deposited as a tempestite layer. However, they differ from the latter authors by believing that the `pits' occurred on a sea bed which was inhabited by the elusive burrowers in question. In addition to infilling open burrows, the calcirudite fragments could have been reworked passively or actively by the burrowers into deeper reaches of the sub-surface in similar fashion to some modern burrowers (Rhoads, 1967). There could therefore have been later mixing of surficial sediments with sub-surface sediments by bioturbation. That the giant burrow-fills are often capped by localized limestone crusts suggests possible intertidal conditions if it is assumed that the crusts formed from ponding of standing water above slightly subsided giant burrow-fills. As such, the ponding would be supportive evidence for emergence and so place a possible sequence boundary at the top of the calcirudite instead of at its base. The calcirudite would then be assigned to the Futaysi M. rather than the Dabb’iya M. Williams and Walkden (2002) placed the sequence boundary at the base of the calcirudite. 5. Conclusions The curious abundant calcirudite-filled ‘pits’ in the Pleistocene marine deposits of Futaysi Island and Al Dabb'iya are here reinterpreted as giant burrows rather than calcirudite-filled holes left by former (uprooted?) trees. Their calcirudite-fills comprise clasts which mainly represent reworked lithified smaller burrows and possibly
reworked beachrock or hardground fragments. The calcirudite forms a discontinuous, bedded storm layer which covered the contemporary sediment surface and infilled the giant burrows. However, it is possible that in those giant burrows that had remained occupied, organisms could have merely drawn some of the calcirudite into them as a result of later bioturbation. The actual burrowing organisms which created such giant burrows remain unidentified but they were active during what appears, on the available evidence, to be during the Pleistocene interglacial period dated as approximately 75–125 Ka. The giant burrows cut into the top of, and sometimes through, the Futaysi (Futaisi) Member of the Fuwayrit Formation. They are sometimes capped by restricted limestone crusts which possibly formed during water-ponding episodes due to preferential compaction within the burrows and provide some support for a sequence boundary between the Futaysi (Futaisi) and Dabb'iya Members at the top rather than the base of the calcirudite. Acknowledgements The authors greatly appreciate the advice and discussions of Gordon Walkden, Alun Williams, Dan Hull and Peter Hellyer who accompanied A. Kirkham in the field, and to Paul Wright, Roland Goldring, Richard Bromley, Richard Sabin and John Pollard for discussions on possible origins of the features in question. Mark Beech, Peter Hogarth and Gary Feulner are thanked for providing information and illustrations of Scylla serrata in the U.A.E., and Christian Velde for organizing a field trip for the authors to examine Scylla serrata burrows in the field. Dan Bosence and two anonymous referees are thanked for their constructive advice on improving the text. Finally, special thanks also go to Sheikh Hamad bin Hamdan Al Nahyan for his hospitality and for kindly allowing access to the outcrops on Futaysi Island. References Beech, M., Hogarth, P., 2002. An archaeological perspective on the development and exploitation of mangroves in the United Arab Emirates. In: Javed, S., de Soyza, A.G. (Eds.), Proceedings of the 2nd International Symposium on Mangrove & Saltmarsh Ecosystems, Environmental Research and Wildlife Development Agency (ERWDA), Abu Dhabi, pp. 196–198. Cadee, G.C., 2001. Sediment dynamics by bioturbating organisms. In: Reise, K. (Ed.), Ecological Comparisons of Sedimentary Shores. Springer, Berlin, pp. 127–148. Esteban, M., Klappa, C.F., 1983. Subaerial exposure environment. In: Scholle, P.A., Bebout, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments. American Association of Petroleum Geologists Memoir, vol. 33, pp. 1–54. Tulsa. Evans, G., Kirkham, A., 2005. The Quaternary deposits. In: Hellyer, P., Aspinall, S. (Eds.), The Emirates: A Natural History. Trident Press Ltd., London, pp. 65–78. Evans, G., Kirkham, A., Carter, R.A., 2002. New evidence on the Quaternary development of the U.A.E. coast from Marawah Island, Abu Dhabi. GeoArabia 7, 441–458. Gregory, M.R., Balance, P.F., Gibson, G.W., Ayling, A.M., 1979. On how some rays (Elasmobranchia) excavate feeding depressions by jetting water. Journal of Sedimentary Petrology 49, 1125–1130. Hadley, D.G., Brouwers, E.M., Brown, T.M.,1998. Quaternary palaeodunes, Arabian coast: age and palaeoenvironmental evolution. In: Alsharhan, A.S., Glennie, K.W., Whittle, G.L., Kendall, C.G.St. C. (Eds.), Quaternary Deserts and Climatic Change. Balkema, Rotterdam, pp. 123–140. Hellyer, P., Aspinall, S., 2005. The Emirates: A Natural History. Trident Press Ltd., London. Herwitz, S.T., 1993. Stemflow influences on the formation of solution pipes in Bermuda eolianite. Geomorphology 6, 253–271. Hogarth, P., Beech, M., 2001. A first modern record of the mangrove crab Scylla serrata in the U.A.E. and southeastern Arabian Gulf. Tribulus 11, 30. Kiesewetter, H., 2003. The Neolithic Population at jebel Buhais 18: remarks and funerary practices. In: Potts, D., Al Naboodah, H., Hellyer, P. (Eds.), Archaeology of the United Arab Emirates. Trident Press Ltd., London, pp. 35–44. King, G.R.D., 1998. Abu Dhabi Islands Archaeological Survey. Publ. Trident Press Ltd., London. Kirkham, A., 1998. Pleistocene carbonate seif dunes and their role in the development of complex past and present coastlines of the U.A.E. GeoArabia 3, 19–31. Klappa, C.F., 1980. Rhizoliths in terrestial carbonates: classification, recognition, genesis and significance. Sedimentology 27, 613–629. Martinell, J., De Gibert, J.M., Domenech, R., Ekdale, A.A., Steen, P.R., 2001. Cretaceous ray traces?: an alternative interpretation for the alleged dinosaur tracks of La Posa, Isona, NE Spain. Palaios 16, 409–416. Rhoads, D.C., 1967. Biogenic reworking of intertidal and subtidal sediments in Barnstable Harbor and Buzzards Bay, Massachusetts. Journal of Geology 75, 461–476.
A. Kirkham, G. Evans / Palaeogeography, Palaeoclimatology, Palaeoecology 270 (2008) 324–331 Tedesco, L.P., Wanless, H.R., 1991. Generation of sedimentary fabrics and facies by repetitive excavation and storm infilling of burrow networks, Holocene of South Florida and Caicos Platform, B.W.I. Palaios 6, 326–343. Uchupi, E., Swift, S.A., Ross, D.A., 1995. Gas venting and late Quaternary sedimentation in the Persian Gulf. Marine Geology 129, 237–269. Wanless, H.R., Tedesco, L.P., Tyrrell, K.M., 1986. Production of subtidal tubular and surficial tempestites by Hurrican Kate, Caicos Platform, British West Indies. Journal of Sedimentary Petrology 58, 739–750. Williams, A.H., Walkden, G.M., 2001. Carbonate eolianites from a eustatically influenced ramp-like setting: the Quaternary of the southern Arabian Gulf. In: Abegg, F. (Ed.),
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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o
Giant burrows in the Quaternary Limestones of Futaysi Island and Al Dabb'iya, Abu Dhabi Emirate Anthony Kirkham a,⁎, Graham Evans b,c a b c
5 Greys Hollow, Rickling Green, Saffron Walden, Essex, UK CB11 3YB, UK Cranford, La Route de la Haule, St. Brelades, Jersey JE3 8BA, UK National Oceanographic Centre, University of Southampton, European Way, Southampton, SO14 3ZH, UK
a r t i c l e
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Article history: Received 6 October 2006 Accepted 24 January 2008 Keywords: Abu Dhabi Pleistocene Carbonate sediments Giant burrows
a b s t r a c t Pleistocene carbonates along the northern coasts of Futaysi Island and Al Dabb'iya tombolo, Abu Dhabi, U.A. E., contain unusual calcirudite-filled structures which are bowl- and funnel-shaped in vertical section; roughly circular in plan view; up to 1 m in diameter and ∼ 1 m deep. The calcirudite infill is dominated by clasts of reworked lithified smaller burrows and what appear to be fragments of beach rock or hardground. These structures were previously interpreted as marking the sites of former trees with holes, left behind by their root balls, infilled by a storm deposit and were considered to indicate an emergent event marking the boundary between two Pleistocene sedimentary sequences represented by the Futaisi (Futaysi) and Dabb’iya Members of the Fuwayrit Formation. However, they are here re-interpreted as giant burrows formed by some, as yet, unknown marine animal probably foraging for food or using them as dwelling sites which contained storm deposits emplaced in shallow water. Thin crusts on top of some giant burrow infills possibly suggest emergence and this would require a repositioning of the sequence boundary between the Futaysi and Dabb’iya Members. The burrows are believed to have been formed at approximately 75–125 Ka. © 2008 Elsevier B.V. All rights reserved.
1. Introduction This paper discusses the origin of some unusual bowl-shaped and funnel-shaped sedimentary/biogenic structures in the Pleistocene (Quaternary) deposits of Futaysi Island and the Al Dabb'iya tombolo which form part of the barrier complex of Abu Dhabi, U.A.E. (Fig. 1). The Pleistocene deposits of the area were described by Kirkham (1998) and Williams and Walkden (2001, 2002). The latter authors proposed a new stratigraphic nomenclature for the Quaternary of the southern Arabian Gulf. Following and extending the work of Hadley et al. (1998), they assigned a Pleistocene calcarenitic aeolianite (miliolite) to the Ghayathi Formation and the overlying marine carbonates exposed in onshore Abu Dhabi to the Fuwayrit Formation. They sub-divided the latter into the Futaisi (Futaysi) and Dabb’iya Members, which they considered represented two sedimentary sequences. The structures in question often appear as isolated bowl- or funnel-shaped columns of calcirudite after removal of the surrounding sediment by erosion and occur within the top of the Futaysi M. (Fig. 2). They are circular in plan view; up to 1 m in diameter in their upper parts; and up to 1 m deep. They have been observed only in the Quaternary sediments on seaward facing promontories at two localities west of Abu Dhabi Island: Futaysi Island and Al Dabb'iya
⁎ Corresponding author. E-mail address: [email protected] (A. Kirkham). 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.01.037
tombolo (Fig. 1). On Futaysi Island they occur in the hundreds with sub-regular spacings of 1 to 2 m. They were interpreted by Williams and Walkden (2002) as ‘pits’ and used as evidence of exposure indicating the presence of a sequence boundary between the Futaysi and Dabb'iya Members. 2. Futaysi Island The rock substrate comprises Pleistocene aeolianite beneath the intertidal zone around the northern area of Futaysi Island and the adjacent land area used as a golf course (Fig. 3). The top of the aeolianite is undulating and in places still presents the palaeotopography of the original dune field except where it has been planated by various episodes of Quaternary erosion. This dune field was originally dominated by barchan dunes but later remodelled surficially into seif dunes (Kirkham, 1998). Morphological remnants of the seif dunes are also still evident at other locations such as Al Dabb'iya, Al Aryam, other barrier islands, and on the mainland. At the extreme northern point of the island, the aeolianites form a zeuge (or qassar; Fig. 4), similar to those described by Kirkham (1998). The cross-bedded aeolianite, which constitutes most of the zeuge, is capped by coarse, gastropod-rich, cross-bedded Pleistocene carbonates resembling many of the storm beach deposits along the present Abu Dhabi coastline. Surrounding the zeuge at lower palaeotopographical levels, the cap rock is replaced by beds of less shelly, finer grained calcarenites and calcirudites (probably foreshore and
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Fig. 1. General location map. Barrier island complex of central Abu Dhabi.
intertidal deposits) which lack cross-bedding. The palaeo-seifs are frequently fringed by cobbles and boulders of aeolianite often exceeding half a metre across and represent scree deposits developed at the bases of palaeo-cliffs (Fig. 5). These deposits were reworked and buried by marine calcarenites during the later Pleistocene marine transgression as it progressively eroded the partly lithified aeolianites. In places, the aeolianite was also bored by Lithophaga sp. Slightly further away from the palaeo-seifs, the basal, transgressive Pleistocene marine sediments overlying the aeolianite comprise about a metre of poorly stratified but highly bioturbated calcarenites containing a range of relatively small crustacean burrows including Thalassinoides and Ophiomorpha. Other burrows have vertical entry lumens, about 3 cm in diameter and lead down into bulbous basal chambers sometimes exceeding 15 cm in diameter and invariably infilled with coarse, vuggy carbonate detritus (Fig. 6). They are possible crab burrows and the vugs are due to leaching of former aragonitic bioclasts. The various burrows often extend into the underlying aeolianite which must have been only slightly indurated at the time of burrowing. These burrowed sediments are similar to submarine sediments in close proximity to correlatable foreshore deposits that accumulated above the palaeo-seif dunes of Al Dabb'iya (Kirkham, 1998). On parts of the old golf course, deflation has removed the upper layers of the marine calcarenite to expose where the ‘pits’ occur, or in
Fig. 2. Low, galleried, beach-cliffs approximately half a metre high with calcirudite columns separated by small caves created by erosion of less resistant, intervening sediment. The beach in front of the low cliff is strewn with clasts eroded from the calcirudite. Futaysi Island.
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Fig. 5. Cobbles and boulders of aeolianite representing a palaeo-scree deposit overlain by marine fossiliferous calcirudites which include clasts of reworked lithified burrows. Futaysi Island.
Fig. 3. Futaysi Island. The rectangle at its northern extremity marks the area where the ‘pits’ occur.
coarse, mouldic calcarenite reminiscent of the abovementioned lithified bulbous crab burrows. Many clasts are encrusted by barnacles and serpulids. Articulated and disarticulated bivalve shells, commonly of Spondylus (often retaining their original pink colouration), together with unfragmented ‘sand dollar’ echinoids, sponges and rhodoliths, are liberally distributed throughout the calcirudite. These clasts and shells have a coarse calcarenitic matrix.
places it has removed completely the marine capping to expose the aeolianite over wide areas (Fig. 7). The ‘pits’ are infilled by a discontinuous calcirudite bed (Figs. 8, 9). The calcirudite includes clasts which vary in their composition but are dominated by fragmented, lithified, small calcarenitic burrow-fills — often with indentations representing relicts of the original burrow lumens. They sometimes resemble rhizoliths but such an origin is discarded because, though they are reworked, they lack signs of regular branching, orientation or petrographic characteristics typical of root systems (Klappa, 1980). Other clasts are nodules comprising
Fig. 6. A bulbous-based burrow with open vertical entry shaft. Futaysi Island.
Fig. 4. A zeuge of Quaternary sediments comprising calcarenitic aeolianite of the Ghayathi Fm. capped by cross-stratified, shelly marine calcarenites of the Dabb'iya M., Fuwayrit Fm.. The Futaysi (Futaisi) M. was never deposited on such relatively high palaeo-topography. The figure is standing on deflated aeolianite. The dark low area behind him is a Holocene wave-cut platform displaying mangrove rhizoliths. Northern extremity of Futaysi Island.
Fig. 7. Cross-bedded Quaternary aeolianite exposed by deflation. Old golf course, Futaysi Island.
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Fig. 8. Bedded calcirudite dominated by clasts of reworked lithified burrows. Futaysi Island.
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Fig. 11. Upstanding erosional remnants of calcirudite-filled ‘pits’ forming column-like features surrounded by a Holocene wave-cut platform with mangrove rhizoliths. Futaysi Island. (Hammer for scale).
bowl- or funnel-shaped calcirudite ‘pit’ fillings are more resistant to erosion than the intervening host sediment. They also sometimes remain as upstanding columns on the beach or wave-cut platform in front of the retreating cliffs to give a ‘stepping stone’ effect when the surrounding sediment has been eroded (Figs. 11, 12). Where the discontinuous calcirudite layer is absent, the tops of many calciruditefilled ‘pits’ are capped by localized, well cemented layers that are restricted to the area of their respective underlying burrows, and sag or tilt slightly due to differential compaction, (Fig. 13). The calcirudite layer which fills the ‘pits’ is, in turn, overlain by about half a metre of shelly Pleistocene calcarenite of which the upper 15 cm forms a continuous but irregular layer of coalesced nodules (a hardground) which seem to have nucleated around small burrows. This unit is part of the Dabb'iya Member of Williams and Walkden (2002) (Fig. 14). Fig. 9. Close-up of numerous clasts of lithified burrow debris comprising the ‘pit’ fills. Futaysi Island.
In vertical section, the ‘pits’ extend downwards below the base of the marine sediments into the underlying aeolianite. They are circular in plan view (Fig. 10) when exposed on possible ‘Stokes surfaces’ that represent deflation surfaces produced at or near a palaeo-water table. They have never been seen to overlap and, where there has been erosion by wave action, often form low galleried cliffs because the
Fig. 10. Numerous filled ‘pits’ exposed on a deflated rocky surface of marine carbonate. Some give the impression of being arranged in rows. Old golf course, Futaysi Island.
Fig. 12. Erosional remnants of calcirudite-filled ‘pits’ forming ‘stepping stones’ on a beach. Futaysi Island.
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Fig. 13. A calcirudite-filled ‘pit’ capped by a tilted calcarenitic crust. The coarse clasts comprise debris of reworked, cemented crab(?) burrows, some of which were encrusted by barnacles before entombment within the ‘pits’. Futaysi Island.
On the old golf course, finer marine calcarenites overlying the aeolianites often show surficial polygonal cracking that is reminiscent of the polygonally cracked microbial mats of modern intertidal settings (Evans and Kirkham, 2005), although they may be merely cracked caliche crusts. The geological setting of the Futaysi M. ‘pits’ is illustrated in Fig. 14. 3. Al Dabb'iya Ra's al-Khaf is the northernmost of two peninsulas of similar trend and dimensions at the extreme northwest corner of Al Dabb'iya Peninsula (Fig. 15). Both are dominated by remnants of Pleistocene palaeo-seif dune aeolianites capped by Pleistocene marine calcarenites and today form zeugen (Kirkham, 1998). As on Futaysi Island, ‘pits’ infilled with calcirudite occur within the marine calcarenites and often extend down into the top of the aeolianite at the northern tip of the southernmost peninsula (Figs. 16, 17). They have similar dimensions and spacings to those on Futaysi Island but are less abundant and occur in a smaller area. The calcirudite clasts of the infills less obviously originated as lithified burrow fragments. Some of them may represent eroded clasts of
Fig. 15. Location of the ‘pits’ at the northern tip of the circled peninsula. Al Dabb’iya.
beach rock or submarine hardgrounds. Discontinuous lines of subvertically orientated (edgewise) calcirudite fragments tend to define the circumferences of these ‘pits’ in plan view. Large disarticulated spondylid bivalves, again often retaining their pink colouration, also commonly contribute to the burrow-fills. Due to erosion of previously overlying sediments, the calcirudite infills cannot be seen to pass upwards into a more extensive calcirudite as on Futaysi Island, although a calcirudite layer occurs at a slightly higher stratigraphic position in a nearby zeuge. 4. Discussion 4.1. Dating the ‘pits’ Prominent wave-cut platforms were cut in the Pleistocene marine deposits and the aeolianites at both Futaysi and Al Dabb’iya and around the peripheries of other coastal zeugen in the area during the Flandrian transgression (Evans and Kirkham, 2005). At Al Dabb’iya, one such platform exposed the 'pits'. Most of these platforms also
Fig. 14. Sketch showing the relationships of the various sedimentary units and their sedimentary characteristics, Futaysi Island. The zeuge represents a relict of a Pleistocene seif dune.
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Fig. 16. Exhumed ‘pits’ infilled with calcirudite exposed on a Flandrian wave-cut platform. Al Dabb’iya.
expose mangrove rhizoliths with remnant pneumatophores and occasional bases of mangrove trunks. These are difficult to date but, at Ra's Al-Khaf, an analogous Holocene wave-cut platform was dated by the authors as older than 2.02 ± 40 Ka, using carbon-dating obtained from accelerated mass spectrometry of shell material from calcarenites deposited directly on the platform. This is, therefore, a minimum age for the ‘pits’. Also, Williams and Walkden (2002) carbon-dated some bioclastic material from the Dabb'iya M. although it is not known if their samples came from the calcirudite fill of the ‘pits’. They obtained ages of ca. 30 Ka although they believed that their samples were diagenetically contaminated and are probably older (Williams and Walkden, 2002). They concluded that they are certainly Pleistocene and, by comparison with sequences elsewhere in the Gulf, were probably deposited during the penultimate interglacial although no definite dates have been obtained as yet. Following attempts at unravelling the Quaternary sequence stratigraphy of the Emirates by Evans et al. (2002) and by Evans and Kirkham (2005), the present authors concur with Williams and Walkden (2002) and assume these Pleistocene marine strata were deposited between 75–125 Ka. 4.2. Origins of the ‘pits’ The following origins have been seriously considered for these large, apparently calcirudite-filled structures: turtle nesting sites as described by Hellyer and Aspinall (2005); karstic solution pits (lapies) representing sites of former tree locations (Williams and Walkden (2002) following descriptions elsewhere by Esteban and Klappa (1983) and Herwitz, 1993); and human excavation for tree plantations
Fig. 17. Marine calcarenite (M) containing three calcirudite-filled (arrowed) ‘pits’. The top of the calcirudite in the ‘pit’ on the right is obscured by surrounding calcarenite. The marine strata overlies aeolianite (A). Al Dabb’iya.
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or for burial sites or fireplaces and hearths which are abundant in the region (King, 1998). Other alternatives are that they may be feeding pits excavated by marine mammals (eg. cetaceans, sea otters), stingrays and other flatfish feeding by either fin-flapping or waterjetting (Gregory et al., 1979; Cadee, 2001; Martinell et al., 2001); or they are shrimp excavations (eg Callianasa-type; Tedesco and Wanless, 1991); or they could have been produced by explosive gas venting (Uchupi et al., 1995). Discussions were held with a number of ichnologists, marine biologists from the Natural History Museum, London, and Middle East specialist archaeologists about these various possible origins. The crude alignment (Fig. 10) of some of the ‘pits’ suggested initially that they were perhaps remnants of tree plantations but the oldest known evidence of human activity in the Emirates was of late Stone Age and dated as ca. 7.5 Ka (Kiesewetter, 2003). Williams and Walkden (2002) conjectured that the root systems of large plants (eg. date palms with associated karstic dissolution) may have occupied these ‘pits’ although no direct evidence of former trees was presented. They further suggested that a tsunami may have been responsible for depositing the calcirudite within the ‘pits’ after it had uprooted the trees. However, mature palm trees may have root-balls exceeding well over a metre across, as illustrated by Williams and Walkden (2002), and some date palms may have tap roots which penetrate downwards tens of metres in search of water in arid environments. Neither of such root systems fit the sizes and shapes of the features described above. They are so numerous and closely spaced; of similar dimensions; and smaller (=/b1 m across and =/b1 m deep) than would be expected from former palm trees. Although the present authors cannot completely refute this hypothesis, the shapes of the ‘pits’ and the lack of any remnant lithified tree stumps or rhizoliths tends to make this explanation unlikely and so suggests the need for an alternative origin. The present authors favour the idea that the ‘pits’ are giant burrows, although admittedly no remains of the burrowing animals have been discovered within them. One such possibility is that they were excavated by giant crabs such as the present day, intertidal ‘mangrove crab’, Scylla serrata. This is the largest of the modern swimming crabs. It may grow carapaces as large as 22–23 cm across and their overall width with their normal limb posture makes them considerably bigger. Indeed, S. serrata itself was previously regarded by the present authors as the potential burrower as it is known to create open burrows with comparable surface apertures and spatial distributions to the giant burrows (Fig. 18; Hogarth and Beech, 2001; Beech and Hogarth, 2002). However, this species was eliminated
Fig. 18. Vacant Scylla serrata burrows on the bank of a tidal channel occupied by mangroves. The surface apertures of these burrows reach up to 60 cm in diameter, but extend for up to 1 m sub-horizontally in the sub-surface. U.A.E. Photograph courtesy of Gary Feulner.
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because on further examination it was realized that its burrows extend for up to a metre in length at a low angle from the horizontal in the sub-surface rather that being entirely vertical as in the case of the fossil giant burrows in question. The actual burrowing organisms must therefore remain a mystery for the time being. 4.3. Origins of the calcirudite burrow-fills As previously discussed, most of the calcirudite clasts in the giant burrows are undoubtedly fragments of lithified smaller burrows (at least at Futaysi Island). Their lithification may have been similar to that of submarine hardground cementation which is so common in the lagoons and shallow shelf areas of the Arabian Gulf. Such lithification is typical today of burrow-fill sediment in the near surface layers of the sea bed. Lithification could also have occurred if the burrowed sediment was exposed at the surface in an intertidal zone. Cementation of burrows is common on the modern intertidal flats of Abu Dhabi, as well as in the shallow sub-tidal areas. The locations of the giant burrows in close proximity to remnant, contemporaneous Pleistocene seif dunes and to the possible polygonally cracked Quaternary microbial mats at least suggests that the burrowers inhabited relatively shallow water environments. Williams and Walkden (2002) regarded the calcirudite as a transgressive lag at the base of the Dabb'iya sequence but it could just as easily have been the result of a storm reworking the sediments and concentrating the coarse lithified burrow clasts as a tempestite layer which filled the burrows in somewhat similar fashion to that which Wanless et al. (1986) described from Caicos Islands. Any preexisting sea bed relief such as bioturbation spoil heaps or burrow depressions could have been obliterated by such storm activity. The sediment surface of shallow water environments following the storm would have been strewn with clasts of lithified burrow fragments just as the modern intertidal zones in proximity to the Futaysi Island are strewn with the same recycled fragments today (Fig. 2). They also litter parts of the shallow lagoon terraces and the surfaces of the tidal deltas (G. Evans, pers. obs.). That some clasts of these Pleistocene calcirudites had accumulated in the littoral zone is evident from the abundant barnacle encrustations of individual clasts entombed within the giant burrow-fills. The authors agree with Williams and Walkden (2002) that the calcirudite could have been deposited as a tempestite layer. However, they differ from the latter authors by believing that the `pits' occurred on a sea bed which was inhabited by the elusive burrowers in question. In addition to infilling open burrows, the calcirudite fragments could have been reworked passively or actively by the burrowers into deeper reaches of the sub-surface in similar fashion to some modern burrowers (Rhoads, 1967). There could therefore have been later mixing of surficial sediments with sub-surface sediments by bioturbation. That the giant burrow-fills are often capped by localized limestone crusts suggests possible intertidal conditions if it is assumed that the crusts formed from ponding of standing water above slightly subsided giant burrow-fills. As such, the ponding would be supportive evidence for emergence and so place a possible sequence boundary at the top of the calcirudite instead of at its base. The calcirudite would then be assigned to the Futaysi M. rather than the Dabb’iya M. Williams and Walkden (2002) placed the sequence boundary at the base of the calcirudite. 5. Conclusions The curious abundant calcirudite-filled ‘pits’ in the Pleistocene marine deposits of Futaysi Island and Al Dabb'iya are here reinterpreted as giant burrows rather than calcirudite-filled holes left by former (uprooted?) trees. Their calcirudite-fills comprise clasts which mainly represent reworked lithified smaller burrows and possibly
reworked beachrock or hardground fragments. The calcirudite forms a discontinuous, bedded storm layer which covered the contemporary sediment surface and infilled the giant burrows. However, it is possible that in those giant burrows that had remained occupied, organisms could have merely drawn some of the calcirudite into them as a result of later bioturbation. The actual burrowing organisms which created such giant burrows remain unidentified but they were active during what appears, on the available evidence, to be during the Pleistocene interglacial period dated as approximately 75–125 Ka. The giant burrows cut into the top of, and sometimes through, the Futaysi (Futaisi) Member of the Fuwayrit Formation. They are sometimes capped by restricted limestone crusts which possibly formed during water-ponding episodes due to preferential compaction within the burrows and provide some support for a sequence boundary between the Futaysi (Futaisi) and Dabb'iya Members at the top rather than the base of the calcirudite. Acknowledgements The authors greatly appreciate the advice and discussions of Gordon Walkden, Alun Williams, Dan Hull and Peter Hellyer who accompanied A. Kirkham in the field, and to Paul Wright, Roland Goldring, Richard Bromley, Richard Sabin and John Pollard for discussions on possible origins of the features in question. Mark Beech, Peter Hogarth and Gary Feulner are thanked for providing information and illustrations of Scylla serrata in the U.A.E., and Christian Velde for organizing a field trip for the authors to examine Scylla serrata burrows in the field. Dan Bosence and two anonymous referees are thanked for their constructive advice on improving the text. Finally, special thanks also go to Sheikh Hamad bin Hamdan Al Nahyan for his hospitality and for kindly allowing access to the outcrops on Futaysi Island. References Beech, M., Hogarth, P., 2002. An archaeological perspective on the development and exploitation of mangroves in the United Arab Emirates. In: Javed, S., de Soyza, A.G. (Eds.), Proceedings of the 2nd International Symposium on Mangrove & Saltmarsh Ecosystems, Environmental Research and Wildlife Development Agency (ERWDA), Abu Dhabi, pp. 196–198. Cadee, G.C., 2001. Sediment dynamics by bioturbating organisms. In: Reise, K. (Ed.), Ecological Comparisons of Sedimentary Shores. Springer, Berlin, pp. 127–148. Esteban, M., Klappa, C.F., 1983. Subaerial exposure environment. In: Scholle, P.A., Bebout, D.G., Moore, C.H. (Eds.), Carbonate Depositional Environments. American Association of Petroleum Geologists Memoir, vol. 33, pp. 1–54. Tulsa. Evans, G., Kirkham, A., 2005. The Quaternary deposits. In: Hellyer, P., Aspinall, S. (Eds.), The Emirates: A Natural History. Trident Press Ltd., London, pp. 65–78. Evans, G., Kirkham, A., Carter, R.A., 2002. New evidence on the Quaternary development of the U.A.E. coast from Marawah Island, Abu Dhabi. GeoArabia 7, 441–458. Gregory, M.R., Balance, P.F., Gibson, G.W., Ayling, A.M., 1979. On how some rays (Elasmobranchia) excavate feeding depressions by jetting water. Journal of Sedimentary Petrology 49, 1125–1130. Hadley, D.G., Brouwers, E.M., Brown, T.M.,1998. Quaternary palaeodunes, Arabian coast: age and palaeoenvironmental evolution. In: Alsharhan, A.S., Glennie, K.W., Whittle, G.L., Kendall, C.G.St. C. (Eds.), Quaternary Deserts and Climatic Change. Balkema, Rotterdam, pp. 123–140. Hellyer, P., Aspinall, S., 2005. The Emirates: A Natural History. Trident Press Ltd., London. Herwitz, S.T., 1993. Stemflow influences on the formation of solution pipes in Bermuda eolianite. Geomorphology 6, 253–271. Hogarth, P., Beech, M., 2001. A first modern record of the mangrove crab Scylla serrata in the U.A.E. and southeastern Arabian Gulf. Tribulus 11, 30. Kiesewetter, H., 2003. The Neolithic Population at jebel Buhais 18: remarks and funerary practices. In: Potts, D., Al Naboodah, H., Hellyer, P. (Eds.), Archaeology of the United Arab Emirates. Trident Press Ltd., London, pp. 35–44. King, G.R.D., 1998. Abu Dhabi Islands Archaeological Survey. Publ. Trident Press Ltd., London. Kirkham, A., 1998. Pleistocene carbonate seif dunes and their role in the development of complex past and present coastlines of the U.A.E. GeoArabia 3, 19–31. Klappa, C.F., 1980. Rhizoliths in terrestial carbonates: classification, recognition, genesis and significance. Sedimentology 27, 613–629. Martinell, J., De Gibert, J.M., Domenech, R., Ekdale, A.A., Steen, P.R., 2001. Cretaceous ray traces?: an alternative interpretation for the alleged dinosaur tracks of La Posa, Isona, NE Spain. Palaios 16, 409–416. Rhoads, D.C., 1967. Biogenic reworking of intertidal and subtidal sediments in Barnstable Harbor and Buzzards Bay, Massachusetts. Journal of Geology 75, 461–476.
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