Arkell.1956.Jurassic..

CHAPTER 10
NORTH AFRICA
THE NORTHERN FRINGE OF THE AFRICAN SHIELD:
THE SAHARA PLATEAU
Like the present Mediterranean Sea, the European Tethys in the Jurassic
was bounded on the south by the African continent. Where the great
desert plateau now stretches from the Atlantic coast eastwards for 3500
miles across the Sahara, Libya, Egypt and Arabia to the Indian Ocean,
there extended also in Jurassic times a vast landmass. The southern
boundary of the African shield in the Jurassic is unknown. Sea overspread
its eastern edge from 500 to 1000 miles farther than at present, and the
Tethys overspread its northern margin in places 100-250 miles from the
present coast; but in the south and west the present ocean and the ancient
shield, or its more recent sedimentary cover, are everywhere in contact,
without the intervention of any fringe of Jurassic sediments.
The Jurassic fringe along the north coast is broken by the Gulf of
Sidra or Syrtis in Libya. East of the gulf in northern Egypt, Jurassic
rocks exist but are deeply buried under Cretaceous and Tertiary marine
formations. The few occurrences known, from anticlinal folds and borings,
and along the side of the Gulf of Suez rift, will be described in the next
chapter. Westwards, from the Gulf of Syrtis through Tunisia, Algeria,
and Morocco, the Jurassic formations are brought above sea-level and
exposed progressively but intermittently for 1000 miles in the Atlas ranges.
The folded ranges coincide with the Jurassic and lower Cretaceous
rocks, and the southern boundary of these rocks is also the northern
boundary of the Sahara plateau. The projecting land occupied by the
Atlas ranges, known collectively as Barbary, is clearly a part of the
Mesozoic Tethys, of which much has foundered beneath the present
Mediterranean, except in Sicily, southern Italy and Greece.
Both the Sahara plateau and Barbary were intensely folded in the
Variscan orogeny and planed in the Permo-Trias. Differentiation of the
two areas began some time in the Trias with the gentle subsidence of
Barbary and the initiation there of salt lakes elongated east and west.
The Sahara plateau, on the contrary, remained above sea-level, or continued
to rise, supplying clastic sediments to the Jurassic and Lower
Cretaceous seas that invaded and covered up the Triassic lakes of Barbary.
It was not until the early Upper Cretaceous (Albian-Cenomanian) that
the sea spread southwards over the greater part of the Sahara plateau,
probably linking up with the waters of the South Atlantic in the Gulf of
Guinea. The resulting Cretaceous sediments remain horizontal or only
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