'IRAQ AND THE PERSIAN GULF' [‎391v] (785/862)

APPENDIX A
STRATIGRAPHY
The stratigraphy of Iraq is fairly well known because the rocks are clearly
exposed and can be easily studied in the north-eastern mountains and
because the many borings made in the search for oil have clarified the
position elsewhere. Recent explorations in northern Iraq have revealed
that the systems of rocks exposed date back at least to Carboniferous and
perhaps to Devonian times and have established that the lowest beds here
have enough in common with the Palaeozoic outcrops to the south-east
in Persia to indicate that rather similar conditions existed in both localities.
From that it may perhaps be inferred that the still lower Cambrian beds
which are seen in Persia probably underlie the visible strata here, although,
as far as is known, they are not uncovered in even the deepest defiles of
Kurdistan. To discover rocks deposited still earlier, an excursion must
be made away to the west where, along the shores of the gulf of Akaba,
there are pre-Cambrian schist, gneiss,, and granite. This forms the
archaean foundation of the Arabian platform (fig. 2).
The essential points seem to be that the Arabian platform, where
deserts endured from the Cambrian until the Middle Cretaceous, sloped
down in the east and north to a broad open trough. This provided a
marine girdle around the desert which ran generally from south-east to
north-west as far north as Mosul and then curved round from east to
west in northern Iraq and eastern Syria. Eventually it bent still more
and ran south-westwards in southern Syria and Palestine. Comparatively
regular conditions obtained from end to end of this sea between Trias
and Middle Cretaceous times, but during the Upper Cretaceous local
disturbance in the east set up a ridge, expressed for a while as islands and
shoals running north and south roughly along the line of the Tigris above
Baghdad. To the west of this the floor was surprisingly flat; as a result
the thickness of each deposit tends to be constant over a wide area. East
of it, where the surface was sloping, the conditions changed rapidly across
a strip between 60 and 100 miles wide. In the Late Cretaceous period
this trough was particularly deep and relatively steep on its northern and
eastern sides. It was nearly filled up by very thick deposits, which were
gravelly at first but later became gritty and then sandy as the slope was
reduced by the mounting deposits. The later contracting of the earth’s
crust was accompanied by a shallowing of the sea and its concentration
which resulted in the deposition of a broad deposit of anhydrite, gypsum,
and salt. This was an event with economic results, for it is this salty
mantle which is gas-tight and able to imprison the concentrations of oil
in central Iraq under fairly high pressure.
The following table gives a summary of the stratigraphy of Iraq.