Arkell.1956.Jurassic..

THE ZAGROS RANGES 375
There is no great change in thickness of Jurassic rocks as a whole in
passing from the west to the east side of the Persian Gulf, for while
in Arabia the marine Jurassic is about iooo m. thick, the-thickness in the
normally folded zone is about 800-900 m. or less. But whereas the
earliest Jurassic marine fauna in central Arabia is Toarcian and the latest
Kimeridgian, the earliest on the Persian side (at least in the south) is
Sinemurian and the latest is highest Tithonian, with rich ammonite
faunas providing a passage up into the Berriasian. The facies, moreover,
is very different. In the normally folded zone the dominant Jurassic
rocks are marls, thin-bedded limestones and bituminous shales. In
general shales tend to predominate in the north and limestones in the
south.
In the zone of overthrusting the predominant non-metamorphic facies
consists of radiolarian cherts and calcareous and siliceous shales, which
pass up from Upper Trias to Middle Cretaceous like the 'comprehensive
series' of SW. Anatolia, and they are associated with masses of basic
igneous rocks, both intrusive and extrusive, recalling the ophioloite zone
of the southern Alps.
As in other geosynclinal regions, at least in the Jurassic, however, overgeneralization
must be avoided. Deeps and shallows existed side by side.
For instance, in Bakhtiariland and Kuhgalu the chert-limestone-basic
igneous series seems to be absent and is at least locally replaced by varicoloured
shales and sandstones, sometimes conglomeratic, and by reef
limestones and dolomites. Some extrusive igneous rocks are interbedded
locally with the sandstones. 'The greatest thickness measured seems
to be 900 m.+, but the total may be four or five times as great'
(Wyllie, 1937). This area with the reef limestones is farther north-east
than any of the others and probably represents the edge of, or a
protrusion from, the 'median mass' of the Central Iranian Plateau
(see p. 379).
It is a curious fact also that the Mekran hinterland, where the Lower
Lias is marine, seems to belong to the opposite (eastern) side of the
geosyncline, for the rest of the Jurassic is represented by varicoloured
shales with lavas and ash beds. This region will therefore be described
with the Central Plateau.
The following ammonite faunas have been recognized in the normally
folded zone. Outcrops of Jurassic occur mainly as scattered inliers at
the heart of, or surrounding an older core of, anticlines in Cretaceous
limestones. How small an area they cover, relative to the Cretaceous,
may be seen from the coloured map of the Middle East by Dubertret
(1942). At the same time they are not absent altogether at the surface
except in one locality near Kermanshah as indicated on the coloured map
by Furon (1941), which is so over-generalized as to be misleading (see
Lees, 1946). Some of the inliers are tectonic windows, carved by erosion
through nappes of the radiolarite facies which have been thrust as much as
30 miles south-westwards (Gray, 1950).
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