Arkell.1956.Jurassic..

260 NORTH AFRICA
foreshadowed the Tertiary orogeny as in other regions. A large area in
western Morocco, much of it adjoining the present coast, was never
submerged in the Jurassic, and there are several similar but smaller areas
in central and eastern Morocco. One such horst (though less persistent)
in the mountains of the Ghar Rouban district, straddling the Moroccan-
Algerian border SE. of Oudjda, has been elucidated in detail in a splendid
monograph by Lucas (1942). The core of the horst consists of Palaeozoic
rocks, now bounded by faults with throws up to 1500 m., which bring the
Palaeozoic outcrops up amongst high plateaux of Upper Jurassic limestones
(pis. 11, 12). Lucas shows, however, that the horst was already in existence
at the beginning of the Jurassic and was not entirely submerged until the
Upper Toarcian, while the Bajocian and Bathonian were deposited over it
only in condensed form, the Bathonian in the facies of ironshot oolite
only 3-4 m. thick. In neighbouring areas the Bathonian is represented
by thick grey marls and clays with Posidonia alpina and pyritized
ammonites. With the Callovian such marls extended also over the horst.
During the Upper Jurassic, which is thickly developed and perhaps
complete, though without ammonites and poorly fossiliferous above the
Callovian, erosion on the Sahara plateau was intensified and successive
sheets of clastic sediments (especially sandstones) were pushed out ever
farther across the subsiding area of Barbary. (See fig. 35.)
Extensive volcanic activity took place in western Barbary in the Permo-
Trias and locally may have continued during deposition of the Lower Lias.
In the massifs of Zekkara and Beni Snassen in Morocco diabase tuffs are
interstratified with supposed Lower Lias (Savornin, 1931, p. 256); in the
High Atlas basalts are reported to be interstratified with the base of the
Lias (Moret, 1930, p. 21); and Roch (1939, p. 171) considers the whole
thickness (200 m.) east of Marrakesh to be of Lower Liassic age. In the
Saida area of Algeria, however, limestones reputed to contain Cardinia,
which are interbedded with basalts (Flamand, 1911), have proved to be
Triassic, the pelecypods being not Cardinia but Anoplophora (Lucas, 1952,
p. 55). In the light of this, the other occurrences require re-examination.
SEABOARD OF SOUTH-WEST MOROCCO
From the southern boundary of Barbary at Agadir, where the High
Atlas breaks down near the Atlantic coast, for about 150 miles northwards
there is a series of Upper Jurassic outcrops entirely separated from the
rest by the Moroccan meseta. The Jurassic sediments represent the
remains of an independent basin or bay of the Atlantic, truncated by the
present coast. Inland they change in facies and overlap successively
against the Palaeozoic rocks of the meseta, with the development of
sandstones, reefs and basal conglomerates. The meseta was never submerged
in the Jurassic; it was a rocky promontory extending north from
the Saharan land (Roch, 1930).
The maximum thickness of Jurassic and Berriasian rocks is about
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