Page 81 - Arabian Studies (V)

Water Resources and Agriculture in Qatar 71

From a water resources point of view the most important geolo­
gical structural feature of Qatar is the large number of shallow
depressions which arc the surface expression of collapse structures
at depth. These were formed by sub-surface solution of gypsum
and anhydrite beds within the Rus/Ru’us (lower Eocene) forma­
tion, with the subsequent collapse of the roof being expressed by a
depression at the ground surface. Cavalier (1970) dates the forma­
tion of these collapse structures as being of post-Miocene, and
probably of Plio-Quaternary age. Some 850 of these approximately
circular depressions, ranging from a hundred metres or so to two to
'three kilometres in diameter, are the focus for surface storm water
run-off from their generally small internal catchments. In northern
Qatar these depressions have been infilled to depths of up to 1.5 m.
with colluvium and give rise to a gently rolling terrain with often
only low ridges separating adjacent depressions. In southern Qatar
these depressions are often more crater-like in appearance, with the
floor sometimes as much as 20 m. below the general ground
surface.
The geological succession in Qatar is composed of Tertiary lime­
stones and dolomites with interbedded clays, marls and shales
covered in places by a series of Quaternary and Recent superficial
deposits. The oldest strata exposed are the limestones and dolo­
mites of the Rus formation of lower Eocene age, containing beds of
gypsum and anhydrite that have been subjected to considerable
sub-surface erosion by solution and the subsequent development of
depressions on the surface referred to above. The most widespread
outcrop are the dolomites and crystalling chalky limestones of the
Upper Dammam formation of middle Eocene age. The Tertiary
succession is mainly of marine origin but a change to continental
conditions during the Upper Miocene-Lower Pliocene led to the
widespread deposition of gravels, sands and conglomerates,
brought down by erosion of the Arabian pre-Cambrian shield. The
Quaternary superficial deposits include pseudo-oolitic and con­
glomeratic limestones, beach gravels calcareous sands of marine
origin, continental gravels, silts and muds, and aeolian sands and
sabkhah deposits of marine origin. Sabkhah (pi. sibakh)2 is an
Arabic term for inland or coastal saline mud flats of playas and are
widespread along the coast of Qatar. These are normally saturated
to within a few centimetres of the surface and discharge ground-
water by capillarity and evaporation, leaving a salt-enriched crust
on the surface. Evaporation of groundwater from sibakh along the
coastal margins of adjacent Saudi Arabia has been shown to be
large and important. (Pike, 1971).
The prevailing wind in Qatar is from the north-west and aeolian