This report documents the geology of seven of the nine emergent salt domes of the UAE exposed
on Delma, Sir Bani Yas, Arzana, Qarnain, Zirku and Sir Bu N’air Islands, along with the only
on-land salt dome cropping out at Jebal Dhanna, near Ruwais. Fieldwork for the study took place
during field seasons 5, 6 and 7 of the main BGS UAE Regional Geological Mapping program
(October to December 2010, January to March 2011 and October 2011). In all cases only a short
period of time (4 man days or less) was spent on the islands. Some 400 observation points were
made in total. All the major rock types were sampled so that petrographic, XRD and U-Pb zircon
analyses could be carried out on selected units. Geological maps at 1: 25 000 scale of each salt dome
are given.
The salt domes range from about 1 km across (the incomplete remnant of Qarnain) to over 6 km
across (Delma). They give rise to dissected hilly topography rising to 140 m above sea-level (Zirku),
typically drained by radial wadi systems. The majority of the salt domes are single intrusions, but
two domes (Delma and Jebal Dhanna) are multiple-phase diapirs.
The rocks of the salt domes belong to the Hormuz Complex, which has been subdivided into the
informal Hormuz breccia, the principal rock-type and its suite of entrained exotic clasts. The
breccia is made up of soft, altered, clay-rich, vari-coloured gypsum-anhydrite-rich matrix, in
which are set sub-angular clasts of all sizes from the mm-scale up to megaclasts several hundreds
of metres across. The Hormuz breccia is extremely heterogeneous in terms of colour, grain size,
alteration, mineralogy, internal structure, fabric and clast content. The heterogeneity is reflected
in the landscape: rounded, deeply eroded, hilly, multi-hued ground, with the larger resistant
clasts often forming upstanding blocks and the highest hills. The breccia may be massive or,
particularly at the margins, strongly layered, with the layering tending to dip centripetally away
from the core. The largest clasts within the breccia are subdivided according to their lithology
into igneous megaclasts (mainly volcanic rocks: basalt, rhyolite, dacite, quartz porphyry; some
gabbro/dolerite) and sedimentary megaclasts (bedded limestone, dolomite and calcareous
shale/siltstone, sandstone, sedimentary breccia, rare metasedimentary rocks). The only other unit
mapped comprises the extensive turbidites which form a semi-continuous carapace on Zirku
island, termed the Zirku Formation. The original halite of the breccia matrix has largely been
dissolved and replaced by anhydrite-gypsum, carbonate, iron and manganese oxides and various
clays and micas. U-Pb dating of volcanic igneous rocks shows them to be Neoproterozoic
(Ediacaran) in age at c.560 Ma. The maximum age of the Zirku Formation is c.560 Ma as
determined by detrital zircon dating, with a layered marble clast from Delma giving an older
maximum depositional age of c.590 Ma. Both detrital spectra show a range of older sources from
Neo- to Palaeoproterozoic and Archaean in age.
The salt domes were emplaced penecontemporaneously with the Miocene country rocks of the
Shuwaihat Formation. The latter are often quite strongly deformed immediately adjacent to the
diapirs margins (tilted and faulted), but locally also contain Hormuz breccia-derived gravel beds
and lenses. Locally, large disoriented blocks of Shuwaihat Formation are found within the salt
domes, suggesting that the Miocene rocks once covered the domes and foundered within the roof
zone due to salt dissolution in the underlying Hormuz Complex. The salt domes are locally
overlain by remnants of Quaternary cover rocks, typically represented by cream-coloured marine
clastic carbonate grainstones and interlayered proximal alluvial fan gravels (derived from the
Hormuz breccia) of the Jebal Dhanna Formation. These deposits are unique to the salt domes.
The salt domes are capped to a greater or lesser extent by an irregular crust, 1 to 3 m thick, of
scoriaceous coarsely crystalline brown gypcrete. Evidence of localised relatively recent
halokinetic reactivation in small dome-like ‘Salt blisters’, typically a few hundreds of metres
across are described from a number of salt islands
