File 1450/1919 ‘Mesopotamia & Kurdistan: Geological Reports on’ [‎190r] (402/522)

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certain as the intervening country has not yet been examined. In the
Bakhtiari country the junction is almost invariably obscured, and though the
gypsum is often not in situ on the Asmari Limestone no actual unconforma-
bility has been proved. In that part of Mesopotamia examined up to the
Diyalah Itiver there is no exposure of Asmari Limestone except towards Hit,
and in that district the Ears Series does not exist.
The Lower Ears Group was deposited in a shallow sea where there was
concentration of the contained salts due to rapid evaporation, in fact under
similar conditions to those of the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. to-day. The salts accumulate until
their point of saturation is reached, when they are precipitated in the reverse
order of their solubility, i.e., the least soluble will be first thrown down. In sea
water gypsum is precipitated after 87 per cent, of the water has been evaporated,
but the evaporation must reach 93 per cent, before the precipitation of salt
takes place. In present North-East Mesopotamia the evaporation of the Ears
sea to a sufficient extent to throw down gypsum alternated with the acquisition
of fresh water which deposited its fine sediment in the form of what are now
clays and shales, while the coarser sediment gave rise to the sandstones. The
limestones are to some extent formed from comminuted shell fragments
cemented by the solution and redeposit of carbonate of lime, though they may
here as in Persia owe their origin also to limestone detritus which has been
likewise compressed and cemented. Anhydrite is likely to be found interstrati-
fied with gypsum, into which it is formed on taking up *263 of its weight of
water; this hydration is accompanied by an increase in volume of 33 per cent,
and may give rise to local uplift and deformation.
Einally a description of this most interesting rock group would be incom
plete without mentioning its extraordinary inability to withstand stress. The
dynamics of the gypsum group of Persia is a science in itself: suffice it here to
say that under stress, if it has no support^ from other rocks, it flows like a
viscuous liquid. This is the more remarkable since as has been observed by the
author again and again in Persia, it applies especially to the lower part of the
Gypsum Group and hardly at all to the top 1,400 feet or so. This is borne out in
North-East Mesopotamia where so far the upper gypsum beds only have
been seen.
(7) The Middle Pars Group .Lower Ears passes conformably up to
the Middle Ears, which is a useful sub-division made by the Anglo-Persian Oil
Company Geological Staff, and comprises a relatively small phase of transition
during which the shallow marine gradually gave place to littoral conditions of
deposition. It is characterised primarily by the rapid diminution of gypsum
upwards, in its place are developed thin-bedded fine-grained usually calcareous
sandstones, and thin limestones and shelly limestones. With these is a consider
able accompaniment of vari-colored clays and shales. The top of the group is
taken as the first thick sandstone band of Upper Ears type. At Naf.t Khana
the thickness of this group is 500 feet, though subsequent mapping and linking
up with other areas may prove lateral variation which might bring the time
horizon of the top of this group as found in Persia well into what is locally
undoubted Upper Ears type. This, however, is a matter rather of academic
interest than of immediate practical importance. Its thickness in the Bakhtiari
country is 1,000 feet, but the reduction here may ,be accounted for by the
thinning out of the whole series against a Miocene shore line approximately
where the Tigris now runs.
( 8 ) The Upper Fars Group -—The Upper Ears rocks which lie conformably
on the Middle Ears, consist of regularly occurring medium coarse-grained
sandstones with intercalated clay shales, the whole being of a characteristic red
color. The sandstones are composed chiefly of fragments of chert the source of
which has been the older rocks of the Persian mountain system, and they, are
remarkably constant as regards the size of their constituent grains.
The thickness of these beds can only be determined by extended mapping,
and in any case is not at present a matter of very great importance. It has
been tentatively placed at 2,000 feet as there appears to be a tendency to thin
from Ahwaz w here the measurement was found to be about 300 feet greater
than this amount.

File 1450/1919 ‘Mesopotamia & Kurdistan: Geological Reports on’ [‎190r] (402/522)

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File 1450/1919 ‘Mesopotamia & Kurdistan: Geological Reports on’ [‎190r] (402/522)

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