Arkell.1956.Jurassic..

262 NORTH AFRICA
Tertiary, with summits rising above 12,000 ft. to a maximum of 13,665 ft.
A beautiful geological map and description by Moret (1930) depict a
strip across this central core, from the alluvial plains of Marrakesh eastwards.
In the SW. corner the last of the Upper Jurassic and Berriasian
deposits described by Roch are seen feathering out upon the folded
Palaeozoics and overlapped by Valanginian and successively higher
members of the Cretaceous.
On the far side of the range the same succession begins again, here
underlain once more by thick Trias with summit basalts, which at Tizimoult
are interstratified with the supposed basal Jurassic limestones.
At first the formation is indivisible, described as a 'complexe jurassique
tres lagunaire', but it soon thickens towards the east and spreads out into
the most extensive Jurassic outcrop in North Africa, the eastern High
Atlas and its easterly continuation in the Saharan Atlas. Upper Jurassic
occurs close to the granite chain in the area of Moret's map, but in the
350 miles of outcrops farther east the sequence stops with the Bathonian,
and by far the greater part of the area consists of Lias. The same applies
to the almost equally large outcrop of Jurassic that forms the adjacent
Middle Atlas and the 'causses' to the NW. of it.
The Lias, and especially the Middle Lias, of this central region exhibits
interesting changes of facies. Along the south, adjoining the Liassic
shoreline of the African shield, or separated from it here and there by
strips of coastal sediments of the 'lagunaire' facies, there runs a fringe of
reef-like limestones, poorly bedded and crowded with thick-shelled pelecypods
and giant gastropods. North of this, forming the High Atlas and
Saharan Atlas, the sediments testify to a deeper trough running parallel
to the coast. Northwards again, in the Middle Atlas and the causses
plateaux, the facies returns to neritic, with great thicknesses of dolomite
and limestones, sometimes recalling the 'reef type of the south and
evidently formed upon shallows covering the shelving edge of the Moroccan
meseta that lies to the west (Fallot & Roch, 1932; Dubar, 1932, 1934,
1943, 1948; Roman & Russo, 1948).
The immense deposits of dolomite in the Middle Atlas and the causses
plateaux bear a strong resemblance to the proved Upper Triassic
(Rhaetian) dolomites of the Rif, which underlie a lumachelle of Pteria
contorta, and they have been claimed as Triassic (Loczy, 1951); but the
few fossils so far found in them point rather to a Lower Lias age, although
the characteristic Megalodon and other heavy pelecypods are said to occur
in both Trias and Lias (Termier, 1936, and discussion of Loczy, 1951,
p. 24). Dolomites, moreover, occur in Morocco and western Algeria
in formations proved to be as late as Upper Pliensbachian and in the
Pre-Rif Bajocian. The remarkable assemblage of heavy-hinged pelecypods,
chiefly giant Opisoma, but comprising also Pachy megalodon,
Pachyrisma, Pachymytilus, Myoconcha and many other interesting forms,
described by Dubar (1948), is proved by ammonites to be Upper Pliensbachian-Lower
Toarcian.
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