"العراق والخليج الفارسي" [و‎‎٢‎٢] (٨٦٢/٤٨)

GEOLOGY
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and by Lower Cretaceous times formed a wedge level with the surface
of the north-east shoulder of the basement block. The Nubian sand
stone to the west and these limestones to the east then formed a wide
submarine plain evenly covered by a shallow sea in which a fairly thin
sheet of sandstone was deposited. In the Middle Cretaceous the sea
ebbed, and a low flat land occupied western Iraq much as it does to
day, only to be covered again by the sea in Upper Cretaceous times.
This sea ebbed and flowed during Eocene, Oligocene, and Miocene
times, in each of which a thin layer of limestone was produced.
These limy veneers find contemporary counterparts across Meso
potamia in the Kurdish foothills, while between the two a calcareous
sludge settled in the sea. Torrid conditions in the Miocene effected
the concentration of salts in the sea-water, and these have been re
corded by a wide apron of red clays interbedded with anhydrites.
This stage was succeeded by a period when lagoons of fresh or
brackish water were formed as the sea-bed filled up with sediments,
leaving a fairly thin deposit of pebbly red marls and clays along a
narrow fringe. The latest phase in the history of this western region
is the recurrence of desert conditions, such as are evident to-day,
which have made their mark by leaving a strip of dunes along the
Euphrates edge of the southern desert.
Like many of the plateaux of western and central Asia, this western
platform of Iraq is pitted by a few late shallow depressions of per
plexing origin. Perhaps they are the result of sagging—more pro
bably of local faulting. They were filled with water, and their lake-
deposits contain freshwater shells. They immediately recall the local
depressions (ovas') of the Anatolian plateau and the larger basins, such
as the Turfan depression, of central Asia.
(ii) The Mountain Region of the North and East
The history of this region is much more complicated. It is, how
ever, important, for, as mentioned in the Introduction, this region
has been found to contain mineral oil, and the search for oil is very
closely connected with the nature, composition, and history of the
sedimentary layers. Mineral oil is formed under certain conditions
by biological and chemical processes in sedimentary strata; it collects
in bulk when the strata are folded into gentle arched structures or
anticlines under suitable cover. Seepages occur where the cover is
thin or where there is a fracture. If the anticline has been badly
broken, all or most of the oil may have been drained away and lost. It
is therefore of the greatest importance for the oil geologist to know