New evidence of the quaternary history of Abu Dhabi: A pleistocene evaporite deposit near yas Island.
[FIGURE 1 OMITTED]
At Locality 1 (Fig.1; GR24[degrees]29'35"N, 54[degrees]38'04"E), the cross-bedded aeolianite is separated locally from an underlying, flat topped, coarsely bioclastic marine limestone by a 40 cm thick, reddish gypsum layer (Evans and Kirkham, 2005; Kirkham, 2011; Fig. 2). Curiously, the excellent exposures of gypsum in the banks of the canal were not mentioned by Ferrant et al. (2012). In fact, their maps suggest that the aeolianite overlies Miocene strata rather than Pleistocene marine limestone. Evans et al. (1969) recorded Miocene dolomite beneath the Quaternary at Musaffah and Peebles et al. (1994) described Miocene dolomite and evaporites in six boreholes along the Musaffah Channel. Examination of the microfossil assemblage was unable to define the age of the limestone beneath the massive gypsum bed on Jazirat but the general appearance and non-dolomitic mineralogy persuades the present authors that this sub-gypsum layer is Pleistocene.
How far the gypsum layer extends along the Jazirat is impossible to tell. At Locality 2 (Fig. 1; GR24[degrees]30'15"N, 54[degrees]34'22"E) a second canal cutting reveals that the base of the aeolianite dips below average mean sea level and thus the gypsum layer would therefore be obscured even if present. At Locality 1 the gypsum is massive and pinches out north and south towards the flanks of the palaeo-seif at least partly due to aeolian deflation (Fig. 3). Where the gypsum bed is absent beyond the pinchouts, the aeolianite rests directly upon the Pleistocene marine limestone which is capped by a possible hardground or a fossil beach rock with ripples.
[FIGURE 2 OMITTED]
Thin section analysis reveals that the gypsum layer contains irregular patches of carbonate sediment. The gypsum crystals are often poikilitic but with an occasional lozenge-shaped crystalline habit and there are anhydrite relicts within some gypsum crystals. The absence of nodules and banding, combined with its sharp upper and lower bed boundaries tend to argue against a coastal sabkha origin for this gypsum bed and it was most likely deposited as a salina across which the palaeo-seif later migrated. It is probable that the salina originated from sea water impounded behind a coastal barrier. Other Pleistocene gypsiferous deposits were recorded by Williams (1999) in various offshore wells such as at Umm Shaif oilfield and the Zakum oilfield, which he, in contrast, interpreted as peritidal / sabkha in origin.
It is particularly significant that Locality 1 is the only site known to the present authors where a Pleistocene evaporite, in contrast to the Holocene sabkha evaporites, is exposed in the UAE. Elsewhere, the base of the Pleistocene aeolianite is exposed on Marawah Island and succeeds a beach deposit which in turn overlies a reefal limestone dated as 160-280,000 yrs old (Evans et al., 2002; Fig. 4). A temporary excavation on the Abu Dhabi-Jebel Dhanna road near the turning to Al Aryam Island revealed >6 m of Pleistocene brown siliciclastic aeolian sand which is known to underlie the entire Holocene coastal sabkha plain (Evans & Kirkham, 2009a). Shallow pits dug at other localities in the vicinity of Tarif show that the aeolianite overlies this same siliciclastic aeolian deposit (Fig. 4). A similar stratigraphic relationship was once originally exposed in the Musaffah Channel (Kirkham, 1998b). Elsewhere, the base of the aeolianite is sometimes exposed directly overlying Miocene siliciclastic strata inland of the coastal sabkha.
It appears that during the low sea-levels of the penultimate glacial period, a sheet of aeolian carbonate dune sand, derived from the exposed shelf carbonate sediments, extended landwards over a diverse landscape to cover beach sands on Marawah Island and extended landwards to cover both the deflated desert siliciclastic sands which underlie the coastal sabkha plains and Miocene strata further inland. Elsewhere, such as at Locality 1, the advancing aeolianite covered peritidal carbonates and the gypsum deposits of hypersaline shallow-water ponds or hypersaline fringes of the inner parts of lagoons.
The subsequent Pleistocene sea-level rise resulted in the deposition of a shallow marine limestone (Fuwayrit Formation) above the aeolianite over large parts of the present day coastal areas as revealed by scattered deflationary zeugen composed dominantly of aeolianite capped by a shallow water marine limestone dated as approximately 125,000 yrs B P, ie. last interglacial (Stevens et al, 2011; Stevens et al, 2014). This marine transgression created Pleistocene cliff scree deposits around the palaeo-seifs near the northernmost regions of the barrier islands to the west of Abu Dhabi Island such as Al Dabb'iya (Kirkham, 1998a; Evans and Kirkham, 2005) and Al Aryam (Evans and Kirkham, 2009a). However, at Location 1, the palaeo-seif dune shows no evidence of this Pleistocene marine sediment as the capping rock has perhaps been removed by erosion or the dune was too far inland for it to be onlapped or inundated by the marine transgression. These marine sediments are also absent from Belghelam where the Pleistocene aeolianite is even greater in height (Evans and Kirkham, 2009b). However, they were present further north at Ra's Gharab and Locality 3 (Fig. 1; Stevens et al, 2014). This interpretation contrasts markedly to that proposed by Wood et al. (2012) and seemingly supported by Ferrant et al. (2012).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
The flanks of the palaeo-seif are onlapped by evaporitic Holocene carbonates (Kenig, 1991, 2012) which lie directly upon the underlying Pleistocene marine carbonates (Fig. 3). These have a muddy texture and are quite rich in individual gypsum crystals several centimetres in diameter and which typically form slowly beneath the water table under coastal sabkha conditions. Supratidal anhydrite also occurs in these Holocene carbonates in close proximity to Locality 1.
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The authors would like to thank Professor John Murray for reviewing the foraminifera as well as Dr Richard (Dick) Hornby and Veryan Pappin, both of Nautica, for ferrying us through the canals on several occasions during the field work for this paper. Kate L. Davis kindly assisted with the cartography.
by Graham Evans and Anthony Kirkham
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|Author:||Evans, Graham; Kirkham, Anthony|
|Date:||Jan 1, 2016|
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