Arkell.1956.Jurassic..

MOBILE BELTS AND GEOSYNCLINES 625
shield. Here any attempt at classification into labile or stable shelves
breaks down when the region as a whole is considered: almost every
imaginable type of terrane is represented. The High and Middle Atlas
if they alone survived would be considered a shelf area; but the Saharan
and Tellian Atlas provide lines of horsts active in the Jurassic and elongated
troughs of truly geosynclinal dimensions, containing up to 3900 m.
of Jurassic sediments (Cornet, 1952, p. 12), which have been converted
in the Tertiary into true fold-mountains, with locally complete overturning
(as in the Ouarsenis: Calembert, 1952). Volcanic activity
alone is lacking. North Africa must be considered to have been part of
a mobile belt in the Jurassic period; it included at least one geosyncline,
but also large tracts occupied by shelf-seas.
Over the rest of the Mediterranean region too much has sunk beneath
the sea to enable safe generalizations to be made about the isolated fragments
that remain accessible. The Pyrenees and most of the Iberian
peninsula had shelf-seas in the Jurassic. So did Sardinia and Sicily,
where the sequence from Bajocian to Oxfordian is usually not more than
10-15 m
- thick. But Sicily had volcanic activity during the Bajocian,
whereas the Atlas geosynclines with their thousands of metres of sediments
did not.
The peculiar lithology and fauna of the Kimeridgian and Tithonian
(fausse breche, Knollenkalk) is not distinctively geosynclinal but rather
Tethyan. It is surely to be ascribed to climatic, chemical and environmental
influences, not tectonic; though the origin of the peculiar breccias
is probably seismic (p. 147).
Except in the Alps and the Atlas there is nothing in the Mediterranean
region comparable to the great geosynclines of western North America.
In Oregon, for instance, the Middle Bajocian alone, with ammonites
throughout, is 1300 m. thick (more than the whole Jurassic at any one
locality in Britain). In California the Tithonian is ?75oo m. thick. Parallel
to these rapidly subsiding geosynclines were long volcanic geanticlinal
ridges or island festoons, which suffered rapid erosion and kept the
geosynclines filled with sediments and volcanic material. These geanticlines
form an integral part of the Pacific mobile belt. Spasms of
exceptional uplift or movement were marked by the deposition of hundreds
of metres of conglomerate. Such conglomerates within the Jurassic
are commonest in North and South America, but occur also in New
Zealand, the Caucasus and Crimea.
SUMMARY
The main shields recognized at the present day were in existence in the
Jurassic and were essentially land areas, undergoing erosion. The edges
of some of them, and more especially the surface of minor shields already
covered by sea in Upper Palaeozoic times, were stable shelves, on which
thin and uniform Jurassics were deposited (e.g. Russian platform). Regions
which had been the site of geosynclinal deposition in Palaeozoic times
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